Non-Energy Applications of Coal-Based Products

ABSTRACT

Coal is used predominantly for power generation, steel and chemicals production. The chemicals are obtained from coal tar pitch and by gasification to produce methanol-to-olefins. The coal-to-chemicals industry was overtaken in the 1950s by petroleum feedstocks which offered simpler and cheaper production methods, since challenged by fluctuating oil prices. The markets for products derived from coal tar pitch, such as dyes, preservatives and fungicides, are growing slowly but are mature, while coal gasification to chemicals has seen recent expansion in China against a background of growing environmental challenges.

A number of new carbon-based industries and technologies are emerging in energy storage, aerospace and composite materials to take advantage of the particular properties of carbon. The declining use of coal power generation in western economies may release this resource for other purposes. The topics covered by this report include: rare earth element extraction from coal, activated carbon products, carbon electrodes, carbon fibre and composite production; carbon nanotubes and graphene; and the production of humates agrichemicals from lignite.

Electrification and the growth in renewable energy technologies is leading to an anticipated increase in demand for rare earth elements and new recovery technologies are under development to extract directly these minerals from coal. There is growth in demand for activated carbon used in water purification, mercury adsorption and potentially carbon capture. The electrodes market is mature but a surge in graphite costs may lead to enhanced use of coal products for steel, silicon and aluminium production. Products that might be described as ‘high value – low volume’ include carbon nanotubes and the promising use of graphene, both may be sourced from coal feedstocks and are the basis of new composite materials.

The emergence of new technologies using coal as a feedstock is linked to the development of green energy generation, electrification, energy storage, carbon sequestration and low emission energy generation that complement the Paris Accord on greenhouse gas emissions.

ACRONYMS AND ABBREVIATIONS

• AMD - acid mine drainage
• bbl - barrel of oil (42 US gallons/159 litres)
• CO2RR - electrochemical carbon dioxide reduction reaction
• CCUS/CCS - carbon capture utilisation and storage
• CF - carbon fibre
• CNT - carbon nanotubes
• COP - Conference of the Parties to the UN Framework Convention on Climate Change, COP21 Paris meeting 2015
• CTC - coal-to-chemicals
• CTO - coal-to-olefins
• CVD - chemical vapour deposition
• DME - dimethyl ether
• DMSO - dimethyl  sulfoxide
• DOE - Department of Energy, USA
• EDC - ethylene dichloride
• EOR - enhanced oil recovery
• GQD - graphene quantum dot
• FLG - few layers graphene
• HCPF - hierarchical porous carbon fibre
• HDPE - high density polyethylene
• HHS - hydrophobic hydrophilic separation
• HIMS - high intensity magnetic separation
• HREE - heavy REE
• HS - humic substances
• LDPE - low density polyethylene
• LIMS - low intensity magnetic separation
• LLDPE - linear low-density polyethylene
• LPG - light petroleum gas (propane and butane)
• LREE - light REE
• MCC - microbial coal conversion
• MLG - multi-layer graphene
• MTO - methanol-to-olefins
• MTP - methanol-to-propylene
• MWCNT - multi-walled carbon nanotubes
• NCM - nickel cobalt magnesium cathodes
• NETL - National Energy Technology Laboratory, USA
• PAN - polyacrylonitrile
• PSA - pressure swing adsorption
• REE - rare earth elements
• REO - rare earth oxides
• SWCNT - single walled carbon nanotubes
• SOC - soil organic carbon
• TREE - transition rare earth elements
• UCG - underground coal gasification

Rare Earth Elements

‘Light (LREE)’: Lanthanum (La), Cerium (Ce), Praseodymium (Pr), Neodymium (Nd), Promethium (Pm), Samarium (Sm) and Europium (Eu)

‘Heavy (HREE)’: Gadolinium (Gd), Terbium (Tb), Dysprosium, (Dy), Holmium (Ho), Erbium (Er), Thulium (Tm), Ytterbium, (Yb) and Lutetium (Lu) Lanthanum group: Yttrium (Y) and Scandium (Sc)

EXECUTIVE SUMMARY

The combustion of coal for power and heat generation is predicted to gradually reduce as nations seek to lower CO2 emissions. This study examines alternative uses for coal that range from large scale processes treating raw lignite to novel small-scale applications associated with high tech industries.

The coal to chemicals industry is the fourth largest consumer of coal, after the power, steel and cement sectors. Coal tar pitch and coal gasification yield a wide range of established chemical products. Other commercial uses for coal include the synthesis of activated carbon, carbon fibre, composite materials, and carbon electrodes. Recent innovative technologies include the development of high value nanomaterials, the novel extraction of key rare earth elements from coal and lignite modified for agricultural purposes.

COAL TO CHEMICALS

The coal tar industry operates worldwide and converts the by-products of coal coking to a host of common chemicals, pharmaceuticals, dyes and preservatives. Although the demand for them is increasing, the feedstock supply is shrinking due to contraction in blast steel manufacture. Also, the tar industry must respond to growing concern over the environmental impact of polyaromatic products.

Coal gasification to polymers, based in China, has an increasing share of the overall polymer market. The industry benefits from low feed cost but has more complex production methods. The process has greater CO2 emissions than equivalent gas polymerisation, and so, coal-to-chemicals looks to be an early adopter of carbon capture (CCUS).

MINERALS FROM COAL

Growing demand for rare earth elements (REE) which are critical to the deployment of renewable energy and transport electrification has led to a new initiative to extract REE minerals directly from coal. Unique to the coal industry, waste streams and lignite resources can be relatively rich in REE and offer an alternative resource to the restricted supply from conventional ore mining. Initial REE recovery targets in the US funded programme have been exceeded. The removal of heavy metals from coal waste streams may offer substantial environmental benefits at coal mining sites.

PITCH CARBON FIBRE

Carbon fibre is a high performance, weight saving structural material with properties superior to either aluminium or specialised steels. Although expensive, carbon fibre is ideally suited to specialised engineering applications within the aviation, aerospace, and motor sport sectors. Carbon fibre produced from coal pitch is an alternative to the more common polyacrylonitrile (PAN) fibre derived from petroleum feedstocks. Production of pitch derived fibre is technically harder, but manufacture in the USA and Japan has recently doubled to 10,000 t/y. High quality pitch fibre is deployed in space craft materials and can exhibit exceptional thermal conductivity properties.

ALTERNATE USES FOR LIGNITE

The international trend to withdraw from lignite power generation is due to its relatively high CO2 emissions. The impact on countries such as Australia may mean that there is a risk that lignite could become a stranded energy resource.

Desertification and rising demand for food may increase demand for agricultural products. New lignite processing techniques involving air or microbial treatment can provide humic products capable of enhancing soil fertility. A substantial application rate required, making this a potential high-volume lignite market.

Gasification of lignite to hydrogen gas transforms low quality coal into a carbon-free fuel. Initial technical developments in Australia are testing oxidation, shift and capture technology, ultimately intended to form part of Japan’s hydrogen economy transition. The project will also examine the safe and economical transport of hydrogen, either cryogenically or using ammonia as an intermediate hydrogen carrier.

ACTIVATED CARBON FOR GAS AND LIQUID PURIFICATION

A rising demand for activated carbon products is due to the role it plays in water recycling, natural gas purification, mercury emission control, together with the potential to act as a CO2 capture agent. Activated carbon is produced in a mild coking process, with production currently over 1 Mt/y.

NANOMATERIALS

Synthesis of Carbon Nanomaterials

An increasingly important if low-volume use of coal carbon is the synthesis of nanomaterials to form polymer composites, energy storage devices, novel electrodes, catalysts, and specialist coatings. Carbon nanotubes (CNT) and graphene are typically produced by methane vapour deposition or graphite exfoliation techniques. However, a new electrochemical method can directly convert raw coal to produce graphene sheets, and selected coals can be processed to extract graphene dots for the latest display technologies.

FUTURE FOR COAL PRODUCTS

Non-energy uses of coal are growing in all sectors and cumulatively the total requirement for coal feedstocks exceeds 100 Mt/y. The increasing electrification of energy and transport depends upon specialist products (carbon fibre, nanomaterials, REE) which can be obtained from coal. The nanomaterials sector is a valuable niche market that is rapidly expanding. The manufacture of graphene directly from coal is potentially a breakthrough technology providing coal sourced materials for the latest IT applications.

Rare earth elements are a valuable commodity, essential to aerospace development, but there is growing concern over limited economical supplies that are predominantly obtained from China. The US coal to REE programme offers an alternate source with positive environmental benefits.

Lignite resources, formerly used in local power plants, may be harnessed for agricultural humic products to counter the increasing crisis of land desertification. Alternately, lignite is under investigation as a source of carbon free fuel in pursuit of the Japanese hydrogen economy.

1. INTRODUCTION

Coal is used predominantly to generate electrical power and in the production of steel from iron ore. In addition to these industries, there is growing demand for carbon-based products, which can range from traditional chemicals to novel materials in fields such as: aerospace, 3-D printing, batteries, electric ground transport, pollution control, and renewable energy.

This report examines a range of alternative coal products such as chemicals from coal tar and coal gasification which require tens of millions of tonnes of coal feedstock, agriculture products derived from lignite, activated carbon and carbon fibre, and ‘high value – low volume’ carbon-based nanoscale graphitised products. Carbon-derived materials are proving to  be  essential  across  industry  and  frequently the key question for the development of  new processes is whether  coal can be used as a  suitable carbon source. Applications aimed at processing coal waste streams may prove to be a valuable mineral resource of rare earth elements, essential for  a host of aerospace applications. Where coal can      be used as a feedstock for these new materials, then the key benefit is the lower feed cost. The breadth      of this subject confirms that coal has much more potential than simply to be consumed as a fossil fuel.

Coal-to-chemicals

The long-established coal-to-chemicals (CTC) industry directly processes coal tar obtained from coking plants which is then used to manufacture a host of everyday products such as pharmaceuticals, dyes and preservatives. This report examines the breadth of the product range and explores issues affecting the growth of the industry such as concerns over the safe use of some chemicals.

Coal gasification involves partial combustion of coal with pure oxygen to form a synthetic gas (CO/H2) which is subsequently converted to methanol. This expanding industry, primarily located in China, produces intermediate chemicals for the manufacture of polymers, in direct competition with plastics obtained from oil feedstocks. The production of chemicals by this method is examined, updating an earlier IEA CCC report, highlighting environmental challenges facing the industry (Nalbandian, 2014).

Extraction of rare earth elements

A proportion of coal deposits are naturally rich in rare earth elements (REE), essential for the construction of batteries and electromagnetic motors. The extraction of REE from raw coal or coal by-products (tailings, ash and aqueous effluent) is viewed as an important method to secure the industrial supply of critical elements. Coal as a source of minerals has a history dating back to the 1940s, when uranium was first extracted from coal seams. For this new REE-coal initiative the presence of a pre-existing mine operation and material handling systems may make the industry competitive with conventional ore mining. A review of these techniques describes the three most promising options to obtain REE from coal resources and outlines the environmental benefit of removing REE.

Carbon fibre and composites

The incorporation of carbon fibre products in the motor and aerospace industries is of increasing importance. Currently the fibre is primarily produced from acrylonitrile (PAN), which is derived from propene. The melt spinning method used for coal tar pitch is compared to the PAN production process. Carbon fibre technologies using lower cost coal have the potential to broaden the use of fibre, opening markets to a host of new carbon fibre products that include furniture, sports equipment, electric vehicle parts, submarine hull construction and lightweight insulation. Fibre and composite products also lend themselves to new 3-D printing techniques; these use short fibre filaments, a technology that is gaining importance in manufacture.

Activated carbon

The production of activated carbon from coal creates a microporous carbon material that adsorbs gas or liquids onto the expanded surface. Activated carbon is mainly used to remove contaminants in water filtration, gas purification and pharmaceuticals synthesis. A significant application involves the removal of mercury from coal power plants, and activated carbon is also under development as a carbon dioxide capture agent in temperature swing adsorption plants. The production of activated carbon may also yield coal pitch as a by-product for use in other graphitic carbon products, providing an alternative to the conventional coking industry.

Carbon electrodes

The market for steel and aluminium is volatile and currently there is a shortfall in the supply of carbon electrodes necessary for steel electric arc furnaces, aluminium and silicon production. While graphite or graphitised petroleum needle coke is the preferred electrode material, the recent upsurge in costs may make lower cost graphitised coal a suitable option. Coal pitch is the preferred graphite binder agent, forming about 20% of the electrode mass. Carbon is also essential in the production of lithium ion batteries and supercapacitors, significant consumers of raw carbonised materials. Carbon materials such as graphene may be introduced to these devices to improve charging rates and battery lifespan.

Agrichemicals

Lignite mines are under pressure to close in OECD countries following a retreat from lignite-based power generation, potentially leaving a stranded resource. However, there is the potential to deploy oxidised forms of lignite as a fertiliser or soil improvement compound – a synthetic humate fertiliser. The addition of humate benefits regions where soil conditions are deteriorating due to desertification arising from the changing climate or poor land management.

Reversing the lignite coalification process forms a product high in humic acid. Methods currently in operation to partially oxidise lignite are chemical based; recent developments involve low-energy oxidative ammonolysis and a bacterial method that can be applied to a wider range of coal feeds.

Application for carbon nanotubes and graphene

The direct conversion of coal products to carbon nanotubes (CNT) is under investigation but requires the use of higher energy arc apparatus. CNT are produced from chemical or gaseous feedstocks and the option to use coal may be limited to the use of by-products such as carbon monoxide as a reagent for chemical vapour deposition. Numerous CNT applications are emerging, most recently as an additive to nanocomposite materials.

Since its discovery in 2004 there has been excitement over the potential applications for graphene. Early developments utilise graphene’s unique properties: high thermal conductivity enables brighter LED lighting and graphene’s water hydrophobicity has resulted in novel paint coatings. The direct production of graphene from coal has proved to be problematical, however a new electrochemical production technique is described which produces a graphene sheet from bituminous coal. Additionally, new nanographene products are more easily formed from coal, taking advantage of the reactivity difference of amorphous and crystalline carbon that constitutes the structure of bituminous coal.

Further recent developments

Japan is considering the development of a hydrogen economy to replace an energy sector currently dependent on coal and oil. Previously hydrogen has been generated from the partial oxidation of natural gas or by electrolysis of water. A new concept is to gasify lignite fuels to create a pressurised hydrogen stream accompanied by sequestration of the carbon dioxide by-product in permanent geological storage.

The low energy conversion of CO2 to carbon monoxide would revolutionise the coal industry by allowing a fuel recycle. Previously, the energy barrier for this reaction has been too great. New research indicates that a lower energy electrochemical route is available and has the potential to recycle CO2 into CO, offering a novel energy storage technology.

Carbon and silicon share the same valency group in the periodic table, and researchers are exploring the substitution of silicon wafers by thin coal films that possess the same electrical properties in the field of microelectronics.

For specialist products, coal is not often the first choice of feedstock but can offer an alternate and often lower cost production route. In addition to describing the current main uses for coal in manufacturing, more recent developments in specialist carbon derived products and the potential for coal feedstocks are reviewed.

2. COAL-TO-CHEMICALS: COAL TAR INDUSTRY AND COAL GASIFICATION

The coal-to-chemicals (CTC) industry has two distinct branches: the production of chemicals from coal tar, itself a by-product of coal coking; and the gasification of coal to produce polymers employing methanol-to-olefins (CTO/MTO) technology. Power generation and steel manufacture are the largest consumers of coal, followed by the chemical production industry which yields over 40 million tonnes (Mt) of product each year. The following sections provide a brief overview of these two sectors.

2.1 CHEMICALS DERIVED FROM COAL TAR

Coal tar is the most significant feedstock in the production of complex aromatic chemicals, including bi- and tri- aromatic forms. Prominent manufacturers of such coal tar-based products include international companies such as Koppers Inc, BASF SE and the Rutgers Group. In recent years production of coke and hence coal tar has increased in China due to its association with the steel industry. A comprehensive range of chemicals are produced from coal tar, with global production in excess of 23 Mt. Key products in the coal tar derived range are listed in Table 1, and include: dyes, preservatives, solvents, plastics, fibres (rayon and nylon), medicated shampoos and perfume additives.

The table indicates a range of familiar everyday products derived from coal tar or its gaseous by-products: soda water and baking powder from carbon dioxide; and ammonia obtained as a by-product of the coking process.

Raw coal tar consists of a complex mixture with hundreds of aromatic compounds (benzene, phenol, naphthalene, and creosote oil) comprising the lighter fraction and the heavier aromatic tar pitch tends to be used directly, for example as in roadway sealants and as a binder in the manufacture of industrial electrodes.

In addition to the chemicals summarised in Table 1, the manufacture of coal tar is also important as a feedstock for carbonised products such as carbon fibre that can be obtained from pitch (Chapter 4 and Chapter 6).

2.1.1 Manufacture of coal tar for chemicals and pitch products

Coal tar is generally produced as a by-product of a coal carbonisation process to make metallurgical coke to be used as a reductant in iron ore smelting. Coke production exceeds 600 Mt and the coal tar by-product yield is approximately 45 Mt/y. Coal coking yields coke oven gas, ammonia liquor, light oils, tar and coke; half the tar products are converted to chemicals and the remainder processed for fuel oil.

The quantity and chemical composition of tar is dependent upon the source coal and the coking temperature and conditions, as shown in Table 2.

Low temperature coking results in a higher proportion of light oils and tar in the product, while the more commonly applied high severity conditions maximise coke yield and produces tar that is more suitable for chemicals production (Owen, 1979; Speight, 1994).

Figure 1 By-products from a metallurgical coking plant (Satyendra, 2014)

The flow chart shown in Figure 1 illustrates the coal coking process and the processing of tar and light distillate components released from coal when roasted in a coke oven. Volatile components are transferred to a gas condensation plant where the tar is collected; lighter liquids pass to ammonia recovery and a benzene distillation plant where the coke oven gas is separated. Typically, a tonne of coal will yield between 30–45 litres of tar. The tar composition varies but contains approximately 50% pitch, the remaining light tar fraction has over 300 components principally phenolic compounds, light aromatic oils (2%), naphthalene (10%) and creosote (33%). The heavier pitch contains a toluene insoluble fraction (73%), beta resins (13%) and residue classed as quinoline insoluble matter.

Lower temperature processing of coal yields greater quantities of tar (Table 2) and is the focus of new technological developments in this sector. These new methods aim to improve energy efficiency in the preparation of low volatile coal and coke, while maximising the production of coal liquid products.

The Clean Coal Technologies’ Pristine-M™ process devolatilises coal to produce a more stable coal containing fewer contaminants. During this process the gas released is recycled to fuel the beneficiation process and heavy tar components are used to seal the coal particle surface (Clean Coal Inc, 2016).

In Carbon Technology Co’s (CTC) mild gasification process the coal feedstock is thermally treated to around 750°C in the absence of air to produce a range of carbon and gas products: char, coal oil liquids and a synthesis gas. Energy is saved by only carbonising the separated char at high temperature to produce the coke product (Wolfe, 2018).

2.1.2 Challenges to the tar and pitch chemicals industry

Coal tar availability is affected by the contraction of the coking industry occurring in developed countries, with feedstock shortages as a consequence; this trend is linked to the changing steel market which currently favours electric arc production over blast furnaces. China is the largest tar producing country with tar to chemicals output exceeding 22 Mt in 2016; a reduction in volume is predicted for the next 5-year period as a result of environmental legislation designed to eradicate illegal polluters (Cision, 2017).

Overall, demand for tar chemicals has experienced modest growth in recent years, with increasing requirements for tar feedstocks against a background of falling production. For example, the product range of naphthalene derivatives used in construction, textiles, agrochemicals and detergent manufacture exhibits steady growth in demand; naphthalene derivatives include naphthalene sulphonic acid, naphthalene-formaldehyde condensates, alkyl naphthalene sulphonates and naphthols. The sales of such derivatives are expected to rise from 630 to 740 kt by 2021; a substantial increase equivalent to $125 million per year (Markets and Markets, 2017).

A major challenge to the tar chemicals industry is increasing disquiet over the safe use of coal tar compounds in regard to potential carcinogenicity or combustibility. Moth balls made from

naphthalene are no longer legal in the European Union (EU) due to concerns over flammability and toxicity. Coal tar-based driveway sealants are likely to be banned in the USA due to worries over the levels of polyaromatic hydrocarbon (PAH) present in sediments formed as a result of product erosion.

European Union legislation on PAH is directed at achieving safe levels of benzanthracene, benzopyrene, benzofluoranthene and chrysene; components which are either present in coal tar pitch or in the smoke formed on burning pitch. The legal exposure limit for these components is steadily reducing; the USA airborne limit of benzene soluble pitch volatiles for an 8-hour exposure is currently 0.2mg/m3, and there is to be further examination of the relative toxicity of individual components (Pohanish, 2017).

The primary concern in the medium term must be to ensure the safe use of an aromatic based product range. The industry must also respond to a potential tar feedstock supply shortfall caused by the contraction of the coking industry.

2.2 COAL GASIFICATION TO POLYMERS AND LIQUID FUELS

The industry converting coal to polymers has expanded significantly over the last decade. The gasification industry is located largely in China, providing an alternative indigenous feedstock for a growing chemicals industry. The option to use coal rather than oil or gas circumvents a potential risk posed by high import costs. The production of polymers and synthetic fuels from coal currently exceeds 10 Mt/y, approaching the scale of the more mature tar to chemicals industry (Amghizar and others, 2017). The manufacture of polymers represents a significant, high volume use of coal and is the subject of a recent IEA CCC review (Nalbandian, 2014). This report highlights current developments and examines emerging industry issues.

Olefinic chemicals are derived via coal gasification to form a synthetic gas intermediate, used to manufacture methanol; methanol feedstock is catalysed in the methanol-to-olefins conversion reactors. Olefin intermediates are subsequently polymerised to yield polyethylene and polypropylene. Figure 2 depicts a flexible flow chart of a coal-to-chemicals facility showing the main process steps and optional product streams.

Figure 2 Products obtained from coal gasification and liquefaction to oil and chemicals (Nalbandian, 2014)

Coal is first converted to a mixture of carbon monoxide and hydrogen gas at elevated pressure using pure oxygen; the resulting syngas product may then be converted to ammonia, ethylene glycol, or methanol. Alternatively, the syngas can be used to supply an IGCC gas power plant. Methanol is the desired intermediate product which is then processed to create olefinic polymers. Coal-to-chemicals technologies such as methanol-to-olefins (CTO/MTO) and methanol-to-propylene (CTO/MTP) compete directly with the petrochemical polymer industry, while dimethyl ether (DME) produced from methanol provides an alternative to propane gas.

Intermediates, commercial products and polymers that may be produced by a coal gasification facility are summarised in Table 3.

The wide range of intermediates listed demonstrates the flexibility of a syngas-based manufacturing plant. Direct liquefaction of coal to liquid fuels, either by hydrogenation, or by the Fischer-Tropsch process to make naphtha and diesel, offers a substitute to petroleum products. The gasification method may be configured to produce the desired chemical or fuel products.

The additional process units required for a coal gasification facility results in significantly higher capital investment than that for an equivalent petrochemicals plant. Gasification is more complex than steam cracker technologies and the contaminants present in coal also require additional purification stages which lead to increased operating costs. To compete with oil and gas processes, the advantage of low coal feedstock price must be sufficient to offset the higher running costs.

2.2.1 Industry expansion and influence of oil price

During the period 2010 to 2014 sustained high oil prices led to raised costs for petrochemical feedstocks; against this favourable economic background a number of new coal to polymer facilities were constructed. Thereafter, the financial environment changed the price of oil from a peak of 115 $/bbl to just 30 $/bbl, thus making coal gasification processes much less competitive.

The significant oscillation in oil prices has had a profound effect on the industry but does support the role of coal underpinning chemicals production during periods of high oil price. The current price of oil continues to fluctuate but is approximately 75 $/bbl. of crude oil (Jun 2018), which is 600 $/t compared to Australian thermal coal imports at 90–100 $/t, rendering gasification facilities increasingly competitive with petroleum-based feedstocks (Oilprice, 2018). The latest IEA analysis of oil production indicates that a lack of investment in oil exploration is likely to result in varying oil prices until 2023; this analysis assumes increased oil production in the USA which is set to be the largest oil producer (Birol, 2018). Meanwhile, BP is planning on the basis that oil prices will be typically between 50 and 60 $/bbl. for the next decade (Jones, 2018). A sustained high oil to coal price ratio would have the knock-on effect of raising the profitability of the coal-to-olefins (CTO) industry.

Table 4 lists newest CTO facilities; the additional capacity provides a total of 0.8 Mt of olefins, resulting in a total production close to 11 Mt of polymers from the coal gasification sector in China.

To place this recent additional polymer production in context, coal derived products of 800,000 t represents about one tenth of the new global polyolefin produced from naphtha and gas cracking; in addition, there are several natural gas-based MTO facilities (Chang, 2016).

More recently, Zhongtu Synergetic Energy Co has announced the installation of a major new CTO facility to be located at Ordos in Inner Mongolia. The plan for the facility is the large-scale gasification of coal with oxygen to produce syngas for methanol (3.6 Mt) sufficient for a production capacity of 1.3 Mt/y of plastics. The Ordos gasification and polymer facility has an overall plant cost estimated in excess of $9 billion (Meichen, 2018).

2.2.2 Greenhouse gas emissions

In comparison to petrochemicals, the major environmental issue arising in the CTO sector is the elevated level of CO2 emissions, while the more complex plant also leads to higher water consumption and aqueous waste disposal.

Greenhouse gas emissions are dependent on the quality of the coal feedstock and process technology that comprises multiple reactor units. The production of CO2 in coal-to-chemicals plants ranges from 6–10 t per tonne of polymer product, which is at least six times more than the 1 tCO2 per tonne of polymer obtained by steam cracking of hydrocarbons (Amghizar and others, 2017). Coal gasification  is more energy intensive and less selective than petroleum methods, which is a major drawback as countries seek to reduce overall CO2 emission.

To counteract the higher CO2 profile of the coal gasification industry, CO2 obtained from the high-pressure gasification plant may be collected for use in enhanced oil recovery (EOR) operations. Carbon capture and utilisation (CCU) schemes have been primarily applied in the power sector, but a new 400,000 tCO2 CCUS scheme under construction at Yulin, China, intends to capture CO2 from an industrial facility in order to mitigate the impact of coal chemicals production (Peplow, 2018). The industry will closely monitor the implementation, technical success and economics of this initial installation.

2.2.3 Single use plastic

Plastic waste has become an environmental issue of global significance as mismanaged waste has been detected in all regions of the world’s oceans. Within the United Nations, countries have agreed the need to halt plastic waste from entering the oceans, although no legally binding international measures have yet been set. The proliferation of single use plastics, their long-term persistence in the environment and growing awareness of micro-plastics entering the food chain has prompted governments to evaluate and introduce new legislation on the use and disposal of plastic materials.

The EU has instigated a ‘clean-up’  plan to make all  packaging reusable or recyclable by 2030. The scheme aims to reduce single use plastic consumption, to ban micro-plastics from personal care products, and discourage the use of bottled water. European countries are introducing plastic deposit schemes and encouraging the use of biodegradable plastics. The EU initiative follows an announcement by China of a ban on imports of foreign recyclable materials; this environmental initiative was followed by prohibitions from other major plastic waste importers. The loss of an export market has huge significance for the European plastics recycling industry, essentially making plastic waste a worthless commodity (Boffey, 2018).

The fundamental changes proposed for plastic waste will affect the future of the polymers industry and influence the selection of feedstock, polymer production, fabrication and demand. The unique properties of polymeric materials have led to plastic becoming an essential constituent of many everyday products; the use of polymers is significant in medical or health products, and food and beverage packaging. Plastic products may have to be reformulated to assist recycling; multi-packaging with mixed polymer content has proved to be technically difficult to recycle.

Although initial measures to control the use and safe disposal of plastic products are not yet internationally coordinated, the growing environmental crisis may cause a substantial change to the whole polymers industry, including the coal-to-chemicals sector. While the expected effect of enhanced recycling would be to reduce demand, economic theory suggests that increasing the supply of recycled material lowers prices and raises demand for virgin plastic making predictions on plastic consumption more challenging (Irwin, 2018).

2.3 CLOSING COMMENTS

Tar and gasification chemicals production both face significant challenges but the demand for the majority of products is set to increase. While there are safety concerns over polyaromatic tar products the main issue is the impact on production from declining coke production as steel manufacture switches toward electric arc methods. Gasification of coal is favoured in a high oil price environment, and a number of new coal gasification facilities are in the pipeline. To protect the industry in the future the emissions of CO2 must be limited. A broader issue affecting the whole polymers industry is to ensure that products are constituted of recyclable materials and to encourage a switch away from single use plastics; a move likely to require reformulation of polymer products.

3. EXTRACTING RARE EARTH ELEMENTS (REE) FROM COAL

Rare earth elements (REE) are in large demand in technical fields such as renewable energy, aerospace and computer technology. REE are currently obtained from mineral ore deposits concentrated in a restricted number of countries. China controls over 85% of REE production, the remainder is mined in Australia and Mongolia (British Geological Survey, 2017). Although there are known mineral reserves in 34 other countries these are problematic: US reserves are currently uneconomic; deposits may be geographically inaccessible; or mining may raise environmental concerns as in off-shore deposits recently discovered in Japan.

Global consumption of REE is around 130,000 t/y from total reserves of 120 Mt, of which 50% are located in China (Timperley, 2018). Thus, there are significant reserves of REE, but current supply issues are exacerbated by low recycling rates, the majority of REE are exported from China as finished goods.

The heavily restricted supply of REE has instigated a search for a substitute source of these valuable metals. For example, a limited number of coal seams in the USA are known to contain naturally high levels of REE, although the percentage mineral content was considered too low for conventional mining techniques. New methods have been developed to concentrate the REE from coal waste in order to compete with traditional ore mining.

Rare earth elements and their application

The rare earth elements are in the lanthanide series of the periodic table and are classed as either light (L) or heavy (H) according to atomic weight. Although described as rare elements, there is a general prevalence in the earth’s crust of about 9 ppm, with cerium the most abundant of the REE. The REE are obtained from minerals such as: Bastnasite (LREE: Ce, La, Nd), Monazite (LREE: Ce, La, Nd) and Xenotime (HREE: Y, Dy, Er, Yb and Ho) (British Geological Survey, 2017).

Figure 3 Rare earth elements, the lanthanide series with Sc and Y showing LREE and HREE

The REE shown in Figure 3 comprise the lanthanide series together with Yttrium (Y) 4200 $/kg and Scandium (Sc) 15000 $/kg, which occupy the same group in the periodic table as Lanthanum:

• ‘light’ (LREE): Lanthanum (La) 7 $/kg, Cerium (Ce) 7 $/kg, Praseodymium (Pr) 85 $/kg, Neodymium (Nd) 60 $/kg, Promethium (Pm), Samarium (Sm) 7 $/kg, Europium (Eu);
• ‘heavy’ (HREE): Gadolinium (Gd) $55/kg Terbium (Tb) 550 $/kg, Dysprosium, (Dy) 350 $/kg, Holmium (Ho), Erbium (Er) 95 $/kg, Thulium (Tm), Ytterbium, (Yb) and Lutetium (Lu).

Last quarter 2017 metal prices for the more expensive REE are shown in italics to highlight relative values; intermediate metal oxides are of lower value (Mineral Prices, 2017). The natural abundance of LREE is greater, reflected in the lower prices. The REE market is quite volatile: neodymium and praseodymium, normally priced together, temporarily fell to a minimum of 39 $/kg in 2016 but recovered to a more typical value of 73 $/kg reflecting the anticipated rise in demand associated with a global return to economic growth (Financial Times, 2017).

Manufacturers using REE anticipate a substantial increase in demand with the proliferation of electrical devices and an associated rise in electricity’s share of total energy consumption (Mansoor, 2017). An outline of the industrial importance of REE and their widespread application is presented in Figure 4 and demonstrates the intrinsic importance of REE across hi-tech industries (Alvin, 2017).

Figure 4 Application of REE in aerospace, energy storage, industry and transport, highlighting associated elements (Alvin, 2017)

Figure 4 illustrates the varied uses of REE and the elements relevant to each industrial sector. Within the automotive industry, major manufacturers are developing hybrid or electric powered cars to reduce CO2 emissions and urban air pollution. Electric motors require neodymium (Nd) to maintain magnetic properties of ferromagnetic alloys such as Nd2Fe14B. The alloys are susceptible to randomisation of the magnetic field direction, the presence of Nd preserves magnetic anisotropy preventing deterioration of the motor (Extance, 2018). Currently, neodymium is the only additive known to perform this role.

A subset of REE comprise the ‘critical list’ which includes those elements considered vital to hi-tech products, including neodymium, holmium, thulium, ytterbium and lutetium (Seredin and others, 2012).

3.1 SUITABLE COAL RESOURCES FOR REE RECOVERY

A typical carbonatite from the Maoniuping deposit located in the Sichuan Province of China has an average content of 2.89% rare earth oxide (REO) in the mined ore, and veins within the ore may contain up to 15% REE.

REE may also be deposited in coal seams and surrounding rock strata; the average level in coal is typically just 35 ppm, which is insufficient for economic extraction from most coal reserves. The concentration of REE in and around coal seams is thought to be a result of a sedimentation mechanism involving physical and chemical rock weathering, followed by mobilisation of REE with plant matter during coalification and diagenesis (Bank and others, 2017).

In individual sites the REE concentration can exceed 35 ppm. The US Department of Energy (US DOE) described samples from coal seams containing more than 300 ppm REE located in Illinois, Northern Appalachian, Central Appalachian, Rocky Mountain Coal Basins, and Pennsylvanian Anthracite (US DOE, 2017). Techniques currently being developed in the USA for REE recovery from such coals may have global implications as suitable levels of REE are also present in coal deposits located in China and Turkey, among others (Zhang and others, 2015).

The targeted coal REE resource is either a mine waste product or lignite deposit rich in REE. Processing coal tailings to recover REE, or extraction from acid mine waste fluids, could form part of an overall waste management programme to reduce contamination from coal refuse.

Extraction from post-combustion fly ash from a power plant is an alternative method to obtain REE from coal, as the organic matter is removed during combustion thereby concentrating the rare earths. Recovery of REE from fly ash is the subject of a separate report to be published by the IEA Clean Coal Centre (Carpenter, 2019). This method may require intensive leaching treatment due to the physical and chemical composition of the anions after undergoing high temperature treatment.

Factors determining extraction from waste coal feedstocks rather than post combustion fly ash include:

• A focus solely on fly ash will reject much of the REE content of the original mined coal to waste or refuse. The majority of the REE content in coal (75%) is expected to be associated with mineral matter rather than the organic fraction of coal and would be removed at the preparation plant.
• The coal mining operation may be optimised for REE recovery, by including rock layers immediately above the coal seam which has been shown to be rich in REE deposits, material that would not normally be processed and transported from the mine to a power plant.
• An additional benefit gained by removing REE from coal tailings is a resultant reduction in contaminant heavy elements from aqueous waste streams; additionally, in acid mine waste (AMD) extraction neutralising the acidity of waste improves the quality of water entering river systems.
• The REE in fly ash have been subjected to high temperatures in the presence of a vitiated oxygen atmosphere and this thermal process separates REE elements from their original co-anions, potentially affecting the ease of REE acid recovery. REE bearing minerals are dispersed through the fly ash particle's alumino-silicate matrix which resists acid leaching (Ziemkiewicz, 2018).

HREE/LREE distribution

The distribution of REE between mineral and organic matter may determine the best method of extraction of REE from coal feedstocks. Heavy REE are the more valuable elements, and if predominantly contained within the organic fraction this may influence the processing of the coal and coal waste, potentially favouring fly ash treatment. For example, a recent analysis of coal samples from China showed that while total REE are concentrated within the mineral fraction, the HREE/LREE ratio is higher in the organic component (Cheng and others, 2018); furthermore, Zhang inferred a poor correlation between HREE content and typical ash metal components (Si, Al and Fe). A general view is that HREE preferentially bond with organic matter, LREE are more abundant than HREE in raw coal, and LREE have a greater affinity for mineral matter (Zhang and others, 2015).

Some US coal resources do show LREE to dominate coal feed stocks supporting this interpretation but analyses of REE extraction from US acid mine waste samples derived from bituminous coal showed almost half of the rare earth elements to be the more valuable HREE, and in that case the organic component had largely been removed (Franus and others, 2015). This indicates that the HREE was largely present in the mineral fraction.

Lignite feedstocks analyses at two mines showed a wide range of REE content and distribution between the mineral and carbon fractions in a range of samples (Benson, 2017). One interpretation alluded to earlier is that chemical bonding existing between REE ions and the coal may depend on the presence of carbon-oxygen bonding. The propensity to form weak coordination bonds favours HREE content in the carbon component of immature coals, and within the ash for mature coal wastes.

3.2 RARE EARTH EXTRACTION FROM COAL

New methods are under development to extract REE present in enrichened coal or waste products from coal beneficiation plants or mine drainage systems. A new initiative involves direct extraction from lignite feedstocks. These methods must concentrate a lower REE content than that of post combustion fly ash or mineral ores. Compared to conventional ore mining the methods benefit from a feedstock waste material in fine particulate or liquid form and avoids the formation of acid resistant glasses obtained during fly ash extraction. The separation system used for mineral ores is discussed in Appendix 1. Physical treatments that include magnetic, gravimetric and hydrodynamic separation are not relevant to coal wastes that are the products of a beneficiation step, while the smelting plant for ores is adapted in the following cases for coal derived feedstocks.

3.2.1 Rare earth elements from coal tailings

Coal beneficiation plants produce a number of potential coal waste streams, the most appropriate are coal tailings where the majority of carbon has been separated and the remaining coal residue is presented as finely divided small particles, including free mineral matter. A typical USA beneficiation plant is outlined in Figure 5, showing four coal separation circuits including an optional froth flotation stage. Coarse coal is subject to gravity separation in a dense media vessel accompanied by drain and rinse screens. Small coal and rock fragments obtained from the first separation process then progress to second and third stages, comprising cyclone and spiral separators, together with screening and centrifuges to remove water.

Figure 5 Flowsheet for a typical US coal preparation plant (Luttrell, 2009)

The majority of coal is recovered from the first stage, then each subsequent circuit adds a smaller proportion of the final coal product that would be delivered to a power plant. In the final circuit coal and ash are normally separated by froth flotation, if available, and waste product from the plant seems the most promising for REE recovery from raw coal. Froth flotation can be replaced by a bespoke REE technology.

Utilised coal tailings consist of finely divided particles suspended in solution; the small particle size avoids the need for grinding and crushing. Normally tailings would be sent to a thickener to improve waste water quality, but it may be possible to further process this refuse stream for REE.

Figure 6 shows the conceptual flow sheet for a process developed by the University of Kentucky (Honaker, 2018). The flowchart depicts a feedstock handling step that may be varied depending on the nature of the waste material; there is provision for Drijet™ X-ray sorting tailored to select REE and coal rich elements of the feed (Kiser and others, 2013). Crushing and grinding may be performed but it is preferable for a plant to operate in conjunction with an existing coal preparation facility. The raw feed may be processed in a hydrophilic-hydrophobic separator (HHS) to remove the coal component, producing an REE rich stream. As Central Appalachian Basin and Illinois Basin feedstocks are mature coals then the REE are expected to be associated with the mineral fraction of the waste matter.

Figure 6 Conceptual flowsheet for the extraction of REE from coal wastes (Honaker, 2016)

Hydrophobic hydrophilic separation (HHS), a solvent extraction process, effects the separation of ultra-fine particles of raw coal, and by default collects a mineral stream enriched in REE. The coal produced is a high quality, dry, low ash coal, and so a valuable co-product from the process.

Figure 7 shows the HHS process, exhibiting a recycle hydrocarbon solvent loop on the left, and the modified apparatus for REE recovery on the right. Raw coal containing 340ppm REE is fed to a 2-stage HHS process to concentrate minerals containing REE. The organic fraction is extracted by a hydrophobic solvent to provide a dry coal stream, and a mineral stream with REE concentrated by a factor of 30 (Honaker, 2018).

Figure 7 Separation of organic carbon from the REE stream using 2-stage HHS (Honaker, 2016)

Following the initial concentration stage shown in Figure 7, the product is then passed to the main purification plant. Figure 8 shows the multistage process in detail.

Figure 8 Hydrometallurgy circuit for the concentration of REE from coal wastes (Honaker, 2016)

The mineral product from the initial HHS treatment is processed in an acid – solvent extraction system (Honaker, 2018). The first step involves leaching metals using a weak acid followed by a sulphuric acid extraction stage; this process is termed scrubbing and stripping. The mineral rich stream is then neutralised with ammonium hydroxide before passing through a multi-stage solvent extraction process. Following a final treatment by strong acid in an organic/aqueous separator the process stream is again neutralised and undergoes oxalate precipitation to obtain the final REE product. This concentrated product would then be refined to obtain individual REE.

The latest test results from an optimised bench scale apparatus, report total REE concentration of 90%, with an overall recovery exceeding 80%, which is suitable for the final refining stage.

There are plans to implement and test the HHS/REE proprietary technology at multiple field locations in Kentucky, USA including the Dokiti mine site (Harder, 2017). Under a rapid development programme, a second pilot-scale facility using the HHS-REE technology is being designed and constructed by Physical Sciences Inc to scale-up and optimise the process.

3.2.2 Acid mine drainage: REE extraction

Acid mine drainage (AMD) is the term for the outflow of acidic water from a mining site, produced either from a coal preparation process where the waste exits via a coal thickener or by the action of rainwater passing through mine waste rock piles and ground water action in the mine itself. Typically, water is acidified by the oxidation of iron sulphide present in waste coal, this is more significant for coals with a high sulphur content and can release rare earth and other heavy elements from the waste. The environmental damage arising from acid mine drainage is worsened when heavy elements are extracted from rock spoil.

The low pH of the acidified mine water slowly extracts REE from their salts, and normally this uncontrolled process is undesirable as it causes contamination of rivers downstream. Acid mine drainage can impact aquatic species, tourism and drinking water supplies. The introduction of a process to extract heavy elements and neutralise the mine waste water would be highly beneficial, and in the USA such a process is being considered to remediate old mine workings.

Figure 9 shows the processes affecting the pH of the water run-off from a mining site, which can enhance or moderate the impact of sulphuric acid production dependent on factors that include: mineralogy (sulphide/carbonate); rainfall and infiltration; temperature; rock structure including porosity; and micro-fauna effects.

Figure 9 Formation of acid mine drainage and factors affecting pH (Tremblay and others, 2017)

Examining the action of water and oxygen on a waste rock pile containing sulphidic minerals, the materials micro-porosity controls the rate of ion sulphide reaction with oxygen and water to form sulphuric acid. The resultant solution pH is typically between 2.5 and 6 while acidities range from hundreds to several thousand mg/L. The acid slowly leaches heavy metals present in rock spoil such as copper, lead and mercury (Sweeny, 2017). In addition to the bulk heavy metals the rare earths are also slowly drawn out. A similar process takes place in a tailings stream from the coal preparation plant. Each site possesses an optimum ‘take off’ point to maximise the action of acidified water on the REE containing coal and rock before the effluent reaches main water systems. This natural process provides an opportunity to concentrate and remove REE and other heavy elements, beneficially reducing the environmental impact of mine waste water ((Ziemkiewicz and others, 2017).

A proposed 5 step AMD concentration process is outlined in Figure 10; acid mine drainage streams are collected from coal mine sites then converted into an REE rich ‘sludge’ following neutralisation by lime application.

Figure 10 Conceptual process flowsheet showing an AMD treatment process and separation of sludge containing REE (Noble and Ziemkiewica, 2018)

The addition of alkali (lime) for neutralisation results in precipitation of metal oxides, and this solid AMD ‘sludge’ product contains the desired REE. The concentration process first dewaters the sludge, then treatment with a range of acids including hydrochloric, nitric and sulphuric acid separates out crude REE. Solvent extraction and stripping and finally neutralisation of the acidic REE concentrate is followed by precipitation of a final concentrate. This plant design is likely to involve significant waste stream recycling to maximise recovery. Initial neutralisation to form the REE rich sludge is likely to take place locally to the mine, followed by sludge processing at a centralised bespoke plant.

A facility for treating AMD from the Northern and Central Appalachian Mountains is centered at West Virginia University (WVU). Initial laboratory trials have shown 5% REE concentration at greater than 90% REE recovery from an initial concentration of less than 300 ppm REE in raw waste coal feedstocks (NETL, 2018). The first bench scale test unit to concentrate REE is due to commence trials during 2018.

The conceptual plant depicted in Figure 10 would be expected to substantially increase concentration levels compared to those obtained at laboratory scale. Of particular significance for the AMD method, the samples treated show 45% of the REE recovered was classified as valuable HREE grade. This high value content rises to over 60% when the ‘critical list’ are included (Griffith, 2018).

3.2.3 Extraction from lignite

In the USA lignite is under review as a potential source of rare earth minerals. Geological surveys indicate that suitable deposits are to be found in the Harmon-Hansen and Hagel fields located in North Dakota.

Lignite, as a low calorific value fuel, is normally used in its raw state hence there is no waste material to collect and upgrade, it is not readily available in a finely divided form, and therefore raw feedstock would require crushing and grinding prior to acid treatment.

The REE content of North Dakota lignite depends on both the site and mined depth but is associated predominantly with the carbon rather than the mineral fraction; analysed values of REE range from 35 ppm up to a peak of 642 ppm. The heavy/light REE ratio can exceed 2, which is higher than normal making these deposits attractive for extraction (Benson, 2018). Following a flotation analysis, the heavy REE were found predominantly in the organic fraction, which was attributed to the higher oxygen content of these immature coals. The HREE are thought to form weak bonds within coordination complexes with the carbon matter.

As lignite is generally burned in its raw state it may be considered preferable to extract REE after combustion as the resultant fly ash is free of both carbon and water. However, initial acid leaching experiments show that rare earth removal is more easily achieved from raw samples, as in the fly ash REE anions are altered by high temperature oxidation processes. Bonding of REE to the humic acid content of lignite means that separation may be performed under milder leaching conditions than that of sintered fly ash.

Lignite comprises up to 35 wt% moisture which affects efficient combustion of the fuel, but also lowers the REE concentration, and may dilute the leaching fluid. Fluidised bed processes have been developed that partly remove lignite moisture, such as DryFining™ developed by Great River Energy. A lignite dryer attached to a power plant may be configured to perform an initial REE concentration step of raw lignite by partially removing water and unwanted minerals; this could be arranged as a side stream taken prior to admitting feedstock to a combustor (Bullinger, 2015).

Reporting initial extraction results from North Dakota lignite coal, the first project targets have been achieved obtaining a 2 wt% REE concentrate that represents 35% recovery of total REE present in the sample (Benson, 2017). A more advanced laboratory test unit is under construction intended to form part of a modular device processing 10–20 kg/h of lignite feedstock (NETL, 2018). Longer term the project will upscale in order to process two tonne batches of lignite in a redesigned pilot-scale plant.

3.3 PROSPECTS FOR RARE EARTH ELEMENTS FROM COAL

All the three methods outlined to obtain rare earth elements from coal feedstocks (coal tailings, acid mine drainage and raw lignite) are promising alternative sources of these valuable elements that may compete with traditional mined ores. Given the initial success of trial projects, the US (DOE)’s Office of Fossil Energy in combination with the National Energy Technology Laboratory (NETL) is supporting the next phase of commercial development.

Initial targets set for the programme of REE extraction via the three diverse technologies aimed to achieve 2 wt% concentrate of rare earth elements from coal products. The original 2 wt% target represents a substantial enrichment from original levels of about 300 ppm and is comparable to raw REE mined ore in China (Maoniuping (2.89 wt%), Bayan (6.0 wt% REO) and Mianning (3.7 wt%) (Zhi, 2014). The projects have exceeded the set target using non-optimised bench scale apparatus (NETL, 2018):

• Processing coal tailings from beneficiation plants in the Central Appalachian and Illinois coal basin, the University of Kentucky achieved a concentrate of greater than 90 wt% REE recovering over 75% of the target metals.
• Extraction from acid mine drainage examined by the University of West Virginia provided a concentrate of 5 wt% REE at high recovery exceeding 90%.
• Early results from the University of North Dakota showed 2 wt% REE concentrate from a lignite feed, recovering over a third of the potential REE content.

Given the promising results obtained, two pre-commercial REE pilot-scale facilities began construction in 2017 as part of the overall NETL project aim to develop small-scale, recovery systems. These include Inventure Renewables using feedstock from the Eastern Pennsylvania Anthracite Region and Marshall Miller & Associates using Northern Appalachian Upper Freeport bituminous coal preparation plant middlings refuse.

3.4 ENVIRONMENTAL BENEFITS

Coal mining and beneficiation activities generate aqueous waste and spoil from rejected material which can accumulate and pose environmental problems to local river systems. The addition of an REE extraction technology may be potentially beneficial to the local environment by reducing the emission of water contaminants. Smelting is required to concentrate REE and it involves hydrological, acidic and thermic process stages. REE mineral ore processing in China resulted in extensive pollution of the region due to inadequate waste control which would be unacceptable by current standards. Coal deposits generally possess a limited content of radioactive elements, however the REE concentration process raises alpha radiation which requires protective control measures.

3.4.1 Site remediation

A typical coal beneficiation process releases several waste streams that range from larger rock particles to fine rock and coal particles suspended in water, and safe disposal of these acidic tailings streams and resultant tailings ponds is challenging. A heavy metal removal stage introduced to the coal process could improve waste water quality reducing pollution of ground waters.

In the USA remediation work is underway at many older mine sites that possess existing waste piles and ponds. REE recovery may have a role as part of the reclamation process to restore the land closer to its original state by reducing the heavy metal content (Mills, 2018). The waste from older workings often contains a significant content of fine coal particles amid the spoil. In many cases this material can be recovered as ‘waste coal’ for combustion in circulating fluidised bed combustors. The recycled coal can often be of lower cost than freshly mined supplies. Coal waste sufficiently rich in REE is likely to have retained the desired metals trapped in the particles.

A controversial feature of mining in the Appalachian region is the technique of ‘mountaintop removal’ where the rock above the coal deposit is removed and the crushed rock deposited in adjacent valleys as headwater valley fills. While the most noticeable effect of this mining technique is the change to the landscape, and the interim loss of flora and fauna, the longer-term impact is on water quality that passes through the detritus from the mining operation, where metals are leached from the spoil (Nippgen and others, 2017). The recovery of REE from AMD waste streams could play an important role in the remediation of these sites.

3.4.2 REE smelting in China

After initial processing the rare earth elements separated then require further refining to remove impurities and obtain individual elements in a smelting facility. The refining process is common to both traditional mined ores and the purification of REE concentrates derived from coal.

The refining industry centred within China has struggled to enforce acceptable environmental standards in the mining and smelting process for rare earths. Of particular concern has been unacceptable practices associated with a large number of unregulated ‘wildcat miners’ operating in the Batou region. Untreated acidic effluent from hydro and acid-bath processes has been illegally discharged forming a massive aqueous waste lake (>10 km2) located near Batou in Inner Mongolia; this practice has continued despite international reports of severe environmental impact to the surrounding region which dates back over a period of five years (Bontron, 2012).

The Chinese government has acted to impose stricter pollution standards on the industry leading to closure of a significant number of unregulated REE mining projects in the Batou region. In addition, up to one thousand iron ore facilities have closed due to environmental infringements. Prior to the regulator’s action REE production by unregulated miners is thought to have accounted for one third of global supplies (Fanbin, 2017).

The National Rare Earth Office in China has sought to stabilise the REE market following the loss of production caused by the removal of facilities. Given the refining experience in China, performance of a new REE smelting industry will be under intense scrutiny by environmental agencies.

3.4.3 Radioactive elements present in coal seams

Coal seam deposits of rare earths may also contain radioactive elements, most notably uranium and thorium, and their decomposition products radium and radon gas (USGS, 1997). In the USA which is committed to the recovery of REE from coal, a content of uranium and thorium is considered low at less than 20 ppm, in practice the values found are typically <4 ppm.

The REE concentration process may also enhance the level of radioactive elements by over 3000-fold. Where the radioactive level is significant then additional precautions will be required to protect workers during the refining process; the University of Kentucky HHS-REE method includes removal of thorium. Radioactivity is a more significant issue for coal REE applications than for mineral ore extraction due to the greater degree of concentration involved.

3.4.4 REE from coal wastes

The extraction of REE from coal is set to enter the demonstration phase having met key targets to produce a 2% concentrate, a qualifying limit exceeded by projects discussed here. Enriched coal seams contain 300 ppm of REE compared to 2–6 % for mineral ore, but this has proved sufficient as a basis for extraction. The next phase involves development of plant to remove contaminants and provide REO concentrates in excess of 90%. The technologies required are an adaptation of established methods for mineral ores and offer an opportunity to optimise the concentration process.

3.5 CLOSING COMMENTS

The background to this initiative, the constrained supply of REE, has if anything intensified recently with the closure of illegal producers in China which has reduced the main supply of REE, during a period of rising demand. The extraction of REE from coal streams is attractive simply as an alternate economical supply of critical elements. However, technologies described here which remove heavy metals from coal effluent streams that enter river systems may also offer significant environmental benefit, supporting mine remediation programmes already initiated in the USA.

4. CARBON FIBRE

Carbon fibre is a long, thin strand of material approximately 0.01 mm in diameter composed mostly of carbon atoms. The carbon atoms are bonded together in microscopic crystals aligned parallel to the long axis of the fibre. The crystal alignment results in a high strength fibre that has exceptional thermal properties.

About 90% of the carbon fibre produced is made from polyacrylonitrile (PAN). The remaining 10% is made from coal tar pitch or petroleum heavy oil feedstocks. The complex production method and costly acrylonitrile feedstock mean that carbon fibre is currently limited to high value or luxury goods. During 2017 the price of acrylonitrile peaked at approximately 1700 $/t, with higher costs predicted for 2018 (Sumayao, 2017). The use of lower cost coal tar pitch feedstock may be a route to bring carbon fibre into more general use.

Figure 11 Carbon fibre market sectors in 2015 (Kuhnel and Kraus, 2016)

Figure 11 shows the importance of carbon fibre to the aerospace industry to replace aluminium in aeroplane fuselage and wings. Aluminium and carbon fibre possess similar density, but the strength of carbon fibre-reinforced plastic (CFRP) may be three times greater than that of aluminium, thereby reducing the weight of an aircraft to save fuel.

Figure 12 Woven carbon fibres (left) and an automobile application (right) (Bigstock Image IDs: 27604985; 214366633)

As the price of carbon fibre falls then the automotive sector may extend its use beyond that of luxury vehicles. For electric vehicles, carbon fibre has a high strength to weight ratio which will compensate for any additional weight of a battery, which limits vehicle range. Figure 12 shows woven carbon fibre, and a carbon fibre vehicle hood as an example of how carbon fibre is expected to be used in the general automotive sector.

Developed as a New-Generation Truck, “efWING” uses CFRP (Carbon Fiber Reinforced Plastic) leaf springs to bring new innovation to rolling stock technology. The weight saving properties of CFRP offer numerous performance advantages, including increased safety and ride comfort. Additional benefits from the resultant low energy requirements include environmental friendliness and reduced running costs. The innovative “efWING” will accelerate the evolution of rolling stock truck design. - Kawasaki

4.1 PAN CARBON FIBRE PROCESS

The PAN method using acrylonitrile is one of the most expensive production processes in the chemical industry. Each manufacturing stage involves a high cost chemical plant with a steam cracker, a fluidised bed oxidation reactor, a solvent based polymerisation reactor and a fibre carbonisation plant.

The complex sequence of reaction steps begins with propane gas followed by:

• steam cracking of propane to propene;
• ammoxidation of propene to acrylonitrile;
• polymerisation of acrylonitrile; and
• PAN coagulation, wet spinning and carbonisation.

Figure 13 depicts carbon fibre wet spinning in more detail, showing the sequence from coagulation of the wet polymer raw feedstock through carbonisation furnaces to winding of the product fibre.

Figure 13 PAN carbon fibre production process (Ying and others, 2015)

The PAN carbonisation process (Figure 13) involves coagulation and wet spinning of polyacrylonitrile using a 60% dimethyl sulfoxide solution (DMSO), followed by washing in water to remove DMSO, to form a macro-porous PAN fibre. The fibre is stretched to five times the original length and progresses through a pre-oxidation oven before transfer to a carbonisation furnace prior to fibre winding. The fibre can be further activated in an additional furnace heating step to produce hierarchical porous carbon fibre (HPCF) for use in the construction of supercapacitor electrodes (Ying and others, 2015).

The highest quality carbon fibre possessing high tensile strength, is currently synthesised using the PAN process which benefits from a simple molecular feed stock that behaves uniformly under the processing conditions.

4.2 PRODUCTION OF PITCH FIBRE

There are a number of initiatives to simplify the carbon fibre manufacturing process and lower production costs. The successful substitution of polyacrylonitrile by coal tar pitch (pitch process) is believed to be a more economical route to carbon fibre which would make it more attractive to the materials science industry. A pitch process eliminates the need for large-scale chemical plants required at each step in the synthesis of polyacrylonitrile and avoids dependence on oil-based feedstocks.

Coal tar pitch is obtained as a by-product from the carbonisation of coal utilised in the steel industry. Coal is heated to between 500°C and 1200°C, with lower severity coking preferred to maximise pitch production. A tonne of bituminous coal may yield up to 70 kg of aromatic liquid product, with up to half that mixture classed as tar pitch. The tar pitch is comprised of many thousands of different molecules, but the majority are polyaromatic ring structures. The fact that the pitch composition is variable and dependent on the source coal means that tight process control is required in the handling of pitch during graphitisation, compared to the acrylonitrile route.

The typical pitch fibre manufacturing process is similar to that of PAN; the main difference is that the fibre  is  prepared  by  a  ‘melt  spinning’  technique  rather  than  ‘wet  spinning’.  The  tar  pitch  process involves the following steps and reactor conditions:

• Pitch preparation: the pitch is first distilled under vigorous nitrogen bubbling to collect a suitable fraction at up to 370°C, followed by heat treatment at between 200–360°C to produce a pitch with a high softening point of approximately 295°C. This procedure can also remove unwanted insoluble components from the pitch which can otherwise mar the final product (Kim and others, 2016).
• Spinning: the prepared hot pitch is rapidly cooled forming a meso-phase pitch which is a highly oriented form. The meso-phase pitch can be melt spun into raw fibre under tightly controlled thermal conditions. The resultant fibres are then washed and stretched.
• Stabilisation (or oxidation/pre-carbonisation): the fibres are treated in an air atmosphere at approximately 320°C to achieve a degree of crosslinking which prevents fibres fusing together.
• Carbonisation: the stabilised fibres are heated to high temperature in the absence of oxygen to remove any impurities in the filament, leaving only carbon.
• Graphitisation: the carbonised fibres are heated at a higher temperature to form tightly bonded carbon crystals.
• Final oxidation: the fibre surface is partially oxidised to improve bonding properties.

The first stages are markedly different from the PAN process: the pitch is devolatilised under nitrogen to prepare a more uniform feedstock; instead of wet spinning, pitch undergoes melt spinning that involves processing hot pitch through circular nozzles in a monofilament apparatus and then stabilisation in an oven in the presence of air. The pitch fibre product has high meso-phase content, a circular core and possesses side aliphatic chains that assist the spinning process. After spinning the processing stages are similar to those of PAN fibre production.

Following graphitisation in the wind-up stage, carbon fibre filaments are bundled into ‘rovings’ which are subsequently wound onto spools. The rovings are typically prepared in filament bundles of varying sizes: 1000 (1K)/3000 (3K) /6000 (6K)/ and 12000 (12K). These carbon fibre spools can then be woven into fabric sheets arranged in a plain or twill weave pattern.

Mitsubishi Chemical Holdings Corporation (MCHC) possesses proprietary technology for converting coal tar pitch into carbon fibre (MCHC, 2018). The technical advances made to raise the quality of pitch  fibre  are  illustrated  by  the  use  of  the  new  highest-grade  fibre  ‘Dialead™’  in  satellite  space applications. The pitch process is more sensitive to the processing conditions, but product quality is steadily improving, and the products are already widely used: one advantage of coal pitch over PAN fibre is that it already possesses a high carbon content offering the potential of less filament change during the graphitisation process.

Optimisation and uniformity of the process temperatures during pitch softening, spinning and baking allows specific graphite crystal dimensions to form from raw pitch. As a result, it may now be more straightforward to achieve high thermal conductivity with pitch rather than PAN fibres for thermal dissipation applications (Yamamoto and others, 1998). The pitch process has the potential for lower

production cost, and although the more complex composition of pitch means that it is more difficult to achieve high quality fibre, new pitch-based products are emerging with superior conductivity properties.

4.2.1 Pitch carbon fibre properties

The lower tensile strength of pitch fibre compared to PAN is attributed to the presence of flaws in carbon filaments that are potentially caused by insoluble components in the tar pitch. The use of coal pitch also tends to result in more slender filaments. A recent research study in Korea has demonstrated that the quality improves if contaminants are removed from the pitch during the feed preparation step which would address concerns over fibre strength and formability (Kim and others, 2016).

Figure 14 Pitch fibre tensile modulus and strength (MCHC, 2018)

The leading pitch fibre product is produced by the Kaiteki Company, Mitsubishi HPC; Figure 14 shows the range of properties of carbon fibre in the Dialead™ range. PAN and pitch fibre have comparable thermal and electrical properties. Main differences lie in the strength and elasticity of the pitch products compared to PAN fibre. Tensile strength is a measure of the resistance of a material to break under tension, while tensile modulus is a measure of the resistance to deformation. Pitch fibre properties can approach those of PAN fibre but tend to be lower, a wider range of tensile modulus is observed for pitch attributed to variable feedstock and preparation conditions. The strength property of pitch carbon fibre is clearly far superior to steel and other alternate materials.

4.2.2 Technical challenges in pitch fibre manufacture

A number of technical challenges are encountered in the production of pitch fibre related to the process and life cycle of carbon fibre (ThoughtCo, 2018):

• Pitch fibre is prone to pitting of the surface which affects the strength and other properties of the material, and is attributed to insoluble components in the pitch and processing conditions. The selection and treatment of pitch is an essential factor in the manufacture of high quality fibres.
• The pitch process requires tighter temperature control than the PAN process, especially during air treatment (pre-oxidation), preparing the fibres for graphitisation.
• During processing of the fibres, the high electro-conductivity can increase the risk of arcing in the equipment, which is a potential source of ignition.
• The recovery, repair and recycling of carbon fibre is expensive and at an early stage of development. Recycling involves thermolysis to remove epoxy resins and new full-scale facilities are under construction to process waste materials (Francis, 2018).

The development of the health and safety framework for carbon fibre is on-going, with the current focus on skin and breathing irritation during manufacturing.

4.3 PITCH CARBON FIBRE ISSUES AND MARKETS

The market for carbon fibre is growing rapidly with notable carbon fibre products in sporting goods, automotive, aerospace and wind turbine blades. The demand for lightweight carbon fibre is set to increase dramatically in the next decades, as it is an important contributor to improved energy efficiency in transport. Carbon fibre is both lighter and stronger than steel or aluminium which makes it an attractive alternative material to reduce the weight of all types of vehicles with the advent of transport electrification and applications to aerospace industries. The use of carbon fibre is already established in the aerospace industry, as the Boeing Dreamliner 787 and Airbus A350-1000 aircraft make extensive use of carbon fibre as laminate and composite materials for the fuselage and wings (Boeing, 2017; Composites World, 2015).

The prospective deployment of electric vehicles requires a lightweight body to compensate for additional battery weight, and carbon fibre offers superior properties to alternatives such as aluminium, magnesium, super steel alloys and lower strength fibre glass. For example, an 85 kWh battery pack, consisting of 7104 lithium-ion battery cells adds half a tonne out of a total vehicle weight of two tonnes, which is a significant factor affecting vehicle range (Desjardins, 2016).

The global market for carbon fibre now exceeds 100,000 t. To put the current carbon fibre production in context, carbon fibre is a niche sector compared to traditionally used materials: aluminium has an annual output of 50 Mt; plastics of 300 million Mt; and the market leader is steel at 1580 Mt. This market position is almost entirely attributed to the manufacturing cost of carbon fibre production which leads to wholesale prices ranging from 80 rising to 300 $/kg for high specification material used for aerospace applications. For comparison, at the beginning of 2018, aluminium was priced at 22 $/kg. Given the special properties of carbon fibre annual production is expected to more than double over the next decade (Kuhnel and Kraus, 2016).

5. ACTIVATED CARBON FROM COAL: GAS ABSORPTION AND WATER PURIFICATION

Activated carbon is a highly porous material possessing a disordered layered structure of carbon atoms obtained from thermal and steam treatment of a source feedstock (Haycarb, 2018). It can be made from a variety of carbonaceous materials such as coconut shells, wood, peat, lignite and bituminous coal. Activated carbon is commonly derived from a coal carbonisation method, which produces an open structure that traps impurities onto the large surface area.

Activated carbon is available in a number of grades. Material properties such as: hardness, density, porosity, pore size or radius, shape, particle size, and surface area (typically 500–1500 m2/g) may be adjusted to suit individual applications. Commercial products include granulated, powdered and extruded forms. Granulated carbon is applicable to liquid and gas systems and is normally regenerated for reuse. Powdered carbon is normally single use and is added in batch processes to absorb impurities present in liquids; they can be removed later by sedimentation with the contaminated carbon for disposal. Extruded pellets are more robust and used for gas purification and automotive emissions reduction.

The target molecules or ions for trapping impurities by adsorption are held on the surface partly by morphology but also by forming weak chemical bonds, such as Van Der Waals bonds. Adsorption may be reversed by raising the temperature or by counterflow techniques. If chemisorption occurs, stronger bonds are formed to the carbon framework and produce permanent adsorbates; in this case the activated carbon is for single use only.

The most common usage of activated carbon is in the removal of contaminants in water treatment and the purification of organic solvents. In the coal power industry activated carbon removes mercury from exhaust streams, to comply with national regulations arising from the Minamata Convention. Some new developments in carbon capture utilise activated carbon as the CO2 adsorption agent.

Carbon molecular sieves are thermally processed activated carbon materials that possess a nanometre scale structure. Suitable precursors for the synthesis of carbon molecular sieves include coal granules, coal tar pitch and carbon fibres. Carbon molecular sieves are used for nitrogen production by pressure swing adsorption and in the petroleum industry to dry gas streams. In the liquid natural gas (LNG) industry, carbon molecular sieves are used to reduce the water content of the gas to less than 1 ppmv to prevent ice forming.

5.1 ACTIVATED CARBON PRODUCTION FROM COAL

The preparation of activated carbon from coal is generally a 2-step process: in the first stage a prepared coal (beneficiated to remove ash) is carbonised at medium heat to remove all volatile matter; the second activation stage opens the structure of the coal to provide a high porosity substrate (Haycarb, 2018).

Carbonisation for tar and pitch extraction is similar to the low temperature coking process required for  activated  carbon.  The  carbonisation  typically  takes  place  at  under  700°C  in  an  oxygen  free atmosphere. The carbonised material has an open structure with many pores, is dry and free of organic oils and tars.

The carbonised product then undergoes activation by steam treatment at the higher temperature of 900°C to 1100°C. The steam slowly combines with carbon in a steam reforming reaction; this partially removes carbon and creates a high porosity surface within individual pores. By adjusting the exposure time and process conditions the porosity may be modified to synthesise the required grade. The resultant activated carbon is crushed and screened to the required particle size or milled to provide powdered activated carbon.

5.2 APPLICATIONS FOR ACTIVATED CARBON

Activated carbon is used extensively in water purification to remove organic constituents and disinfectants. The removal of these contaminants protects osmosis membranes and ion exchange resins within the water treatment system (DeSilva, 2000). The molecular size and the pH of the activated carbon are selected to promote catalytic reduction converting chlorine compounds to chloride, eliminating contaminant disinfectants. Additional liquid phase applications for activated carbon include purification of aqueous solutions and organic liquids, and the purification of cane sugar liquor (Chemviron, 2017).

The Minamata Convention, ratified by 95 countries, is an initiative to reduce industrial mercury emissions; mercury is a fossil fuel constituent which gradually accumulates and persists in the environment (Minamata Convention, 2016). The inclusion of a dry sorbent system following on from an existing ash treatment plant is one of the practical retrofit options to remove mercury, and involves the use of powdered, brominated activated carbon (Fischer, 2012).

International targets to reduce CO2 emission have led to the development of carbon capture utilisation and storage (CCUS) technologies. Research on activated carbon adsorbents shows that CO2 may be removed at higher temperature than amine solvent-based systems, thereby reducing heat transfer demand and making them suited to pre-combustion capture systems. The higher partial pressure of CO2 associated with pre-combustion capture leads to more efficient adsorption when applied in pressure swing adsorption (PSA) technologies using activated carbon (Lockwood, 2016). In one example, initial trials recovering CO2 from a high-pressure syngas stream demonstrated a 96% CO2 capture rate with a capacity of 4 wt% of the adsorption bed. Such performance indicates significant cost savings compared to the Selexol process (Alptekin and others, 2015).

Global production of activated carbon exceeded 1.7 Mt during 2017. The industry is well established and leading producers of activated carbons include: Calgon; Cabot Norit, MeadWestvaco, Kuraray, Osaka Gas Chemicals Group, and CECA (Tiwari, 2015). Prices for activated carbon are approximately 0.5–5 $/kg depending on the degree of preparation required (Alibaba, 2018). A group of suppliers based in China (Xinhua Chemical, Huahui Activated Carbon and Taixi Coal, Xinhua Chemical and Datong Coal Jinding Activated Carbon) account for almost half the global production of activated carbon, of which approximately half is derived from coal.

5.3 A NEW COAL FACILITY CONCEPT

Typically coal preparation involves upgrading the coal in a beneficiation plant and sending the product to a power station to generate electricity. Alternatively, metallurgical coal is carbonised at a coking facility for use in a blast furnace, with volatile components used for coal tar derivatives.

The growing demand for alternative coal products such as activated carbon products or new pitch derived carbon fibre have led to the development of a new concept coal plant. The coal processing facility, proposed by Ramako Carbon integrates a number of separate technologies making high value carbon products, outlined in Figure 15 (Ramako Carbon, 2017).

Figure 15 Production facility converting coal to a range of carbon-based products based on the Ramako Brook mine concept (Ramako Carbon, 2017)

The Ramako coal factory concept aims to maximise the production of high value products, while retaining a smaller portion of prepared coal for direct sale to a power plant. At the heart of the facility is a coal processor that removes volatile coal components: devolatilised coal then progresses to an activated carbon plant, while the tar product is partitioned into suitable components. Pitch conversion to carbon fibre would be the main pitch product from the plant. The Ramako Carbon concept plant would produce carbon fibre and activated carbon initially but has plans to add to the product range as new products emerge from research that may include: resins, carbon fibre reinforced plastics (nanocomposites), synthetic graphite, graphene, and carbon nanotubes. One additional option, if appropriate, is to extract rare earth elements from the coal preparation section of the plant. This coal treatment concept would provide a high degree of product flexibility to respond to the future demands on the coal industry.

6. INDUSTRIAL ELECTRODES,BATTERY ANODES AND CAPACITORS

The industrial electrodes market is currently experiencing a marked increase in demand, partly due to a reduction in electrode production capacity and a resumption of global economic growth. The emergence of new markets for graphite beyond industrial electrode materials is contributing to higher electrode prices. These include: lithium ion batteries for personal electronics, large scale battery packs and carbon electrode materials for electrified transport. The rising demand for graphite or synthetic graphite that is produced from petroleum or coal pitch affects demand for anodes in the silicon and aluminium sectors, but more significantly in the high consumption electric arc steel furnace sector. Although the market may stabilise, current industrial carbon electrode prices have risen fivefold over the last year and this raises the potential for an increased use of coal derived products that are available at lower cost.

6.1 INDUSTRIAL ELECTRODES

The favoured form of carbon for industrial electrodes is graphite or graphitised petroleum needle coke that offers high mechanical strength, low thermal expansion, high purity, vibration resistance, and chemical inertness. Carbon electrodes possess high thermal and electrical conductivity suitable for tip temperatures  that  approach  10,000°C  in  steel  making.  These  electrodes  are  used  in  a  number  of industrial applications covering: electric furnace steel production; refining furnaces; ferroalloy production, industrial silicon manufacture; yellow phosphorus; corundum; aluminium; submerged arc furnaces and other electric arc smelting furnaces, and nuclear reactor engineering. Compared to carbon electrodes, graphitised electrodes have a lower coefficient of thermal expansion, and significantly higher flexural strength for heavy duty uses.

The majority of coal sourced needle (or crystallised) coke for synthetic graphite electrodes is produced in Japan and is typically utilised for ordinary grade electrodes (RP), while graphite or petroleum needle coke are preferred for high power (HP) and ultra-high power (UHP) applications. The demand for needle coke is anticipated to rise to over $4 billion by 2025; although petroleum needle coke is preferred due to its high severity properties, coal pitch products possess the advantage of lower feed cost (Transparency Market Research, 2016).

Needle cokes are classed according to sulphur content which is a significant factor in selecting coal pitch products. The highest grade is super premium needle coke with less than 0.4% sulphur, associated with a thermal expansion coefficient of less than 0.4 K-1. Recently needle coke prices have substantially risen from 400 to 3200 $/t during 2017, and if this high price is sustained it may make coal needle coke more economically viable for synthetic graphite products. The corresponding graphite electrode cost price has peaked at 8900 $/t.

Figure 16 shows a typical industrial scale electrode used in an electric arc furnace which can weigh up to 2 t depending on the scale factor.

Figure 16 Industrial graphite electrodes for the electric arc industry (Flickr Image ID: Sheldon, 2018) High power (HP) graphite electrode mainly used in electric furnace and ladle furnaces

The electrodes consist of graphite or graphitised needle coke formed with a pitch binder that is usually obtained from coal pitch and forms about 20% of the finished item. The tips are threaded to allow for ease of change over as the electrode is consumed over a few hours. The synthetic graphite electrodes are produced by the following well-established process sequence:

• coke grinding (normally sourced from petroleum);
• mixing with binder pitch (from metallurgical coal coking);
• mining and extrusion;
• baking and pitch impregnation;
• graphitisation; and
• machining.

The electrodes operate with a tip temperature from 3000°C up to 10,000°C in UHP furnaces and are expected to last for about eight hours normal use given the severe process conditions. Electric arc steel processing at 630 Mt/y of scrap steel is a major carbon market, with at least 2.5 kg of graphitised electrodes consumed per tonne of steel, associated with 0.5 kg of coal pitch binder. For comparison, in aluminium applications the consumption is much less as the electrodes have a considerably longer life of between 5 and 10 years, requiring the equivalent of 30 kg/t of aluminium capacity (Perruchoud and Fischer, 2012).

6.2 CARBON CATHODES IN BATTERIES AND SUPERCAPACITORS

The limitation of lithium ion batteries is well known, smart phone charges tend to last only a day or two and batteries possess a limited lifespan; laptops and electric car batteries are all based on this decades-old technology. Multiple research programmes are developing new formulations for lithium ion batteries incorporating advanced materials and alternatives such as aluminium oxide, aluminium air, lithium air, lithium sulphur and devices such as vanadium flow batteries aimed at large-scale electrical storage.

Improvements to lithium cell electrodes in the near term seek to reduce the anode cobalt content and thus dependence on cobalt. There are ethical concerns over the source of cobalt mined in the Democratic Republic of Congo and economic issues as cobalt comprises approximately 60% of the anode electrode mass, which translates to almost 20 kg cobalt in an electric car battery assembly, with a metal price exceeding 75,000 $/t (King, 2018).

To enhance battery performance, carbon cathode components are undergoing substantial revision to improve robustness during fast charging cycles and to incorporate new forms of carbon to raise battery capacity. The production of carbon-based electrodes for personal electronics and new battery storage devices associated with renewable energy are areas where coal carbon products may become more relevant; in these low temperature applications synthetic graphite from coal pitch may perform as well as graphite.

6.2.1 Batteries: graphene, CNT and porous carbon

The electrical and thermal properties of graphite make it a key component of lithium ion battery cathodes, permitting rapid charging while avoiding over-heating and mechanical distortion. The graphite cathode is arranged in layers contained in an electrolyte. The battery functions by the migration of lithium ions to the carbon cathode during discharge and the ions are restored to the anode on recharge. Thus, ideally it is a fully reversible process.

Graphite is mined in a number of countries but the largest producer by far is China, followed by India. The use of lithium ion batteries continues to increase and although the amount of graphite in each battery is small, hundreds of millions are produced each year. Given production constraints, graphite may need to be substituted by petroleum or coal tar pitch based synthetic graphite. It is possible to produce synthetic graphite from coal pitch which in many applications can perform as well as mined graphite; soft and hard coke products represent an attractive market for coal distillers (Perruchoud and Fischer, 2012).

The properties of carbon present in lithium ion batteries can be improved by utilising graphene that raises the conductivity and thermal distribution within graphite layers of the cathode. It is also possible to form an anode from graphene with CNT grafted to the surface to increase capacity by limiting dendritic growth of lithium from the anode into the electrolyte. This is more suitable for enhanced charging systems and battery life is extended. Hybrids of graphene with vanadium oxide are under development to improve battery charge cycling that also raise overall energy capacity (Tour, 2017b).

Recent reports describe electrode research examining carbon-silicon precursors that may provide an alternative to the use of graphite in high end battery development, as in aerospace and satellite technologies. A new form of porous carbon (OSPC-1) has been discovered with unique properties shown to adsorb double the lithium content of graphite electrodes without the formation of lithium tendrils that pose the risk of short circuiting the battery. This new carbon form possesses exceptionally high conductivity avoiding the expansion-contraction mechanical issues that limit current battery capacity and lifespan (Zhao and others, 2018). Deposition of graphene balls onto an electrode surface is a hybrid technique to improve high temperature performance under fast charging, as the graphene – silicon assemblies form an anode coating (Son and others, 2017).

The use of coal feedstocks in new battery technology is evolving; the first step is likely to be synthetic graphite produced from coal tar pitch to replace mined graphite. Specialised electrode materials such as carbon nanotubes and graphene are currently synthesised from methane, but in the longer-term these materials may be derived from lower cost coal feedstocks.

6.2.2 Supercapacitors

A supercapacitor is an energy storage device similar in construction to a normal 2 plate capacitor, but the area of the plates is much larger and the gap between plates narrower; metallic plates are contained in an electrolyte and are separated by a thin insulator. As the plates charge, an electrical double layer is formed as opposite charges form on both sides of the insulator. The supercapacitor possesses a large capacitance that can be charged and discharged many more times than a battery with minimal deterioration, but over shorter periods of less than one minute.

The main disadvantage of supercapacitors is the low specific energy. The energy stored per mass of the device is approximately 5 Wh/kg compared to a lithium ion battery at 100–200 Wh/kg. The supercapacitor would weigh 20–40 times as much to produce the same energy as the lithium ion battery. Although now applied in automotive start-stop technology, supercapacitors are suited to stationary energy storage including consumer electronic applications (Capacitor Guide, 2018).

The most common carbon materials used for supercapacitors are carbon aerogel, carbon fibre cloth, and activated carbon; new electrode designs utilise graphene and carbon nanotubes. With the exception of aerogel, these materials may be sourced from coal feedstocks and the manufacture of activated carbon, carbon pitch fibre, carbon nanotubes and graphene are described in this report.

7. CARBON NANOTUBES AND GRAPHENE

Carbon nanotubes and graphene are new forms of carbon that may be obtained from raw coal or products obtained from coal. The preferred feedstock for carbon nanotubes are simple gases such as methane and carbon monoxide which can be obtained from coal processing. The amorphous, semi-crystalline structure of coal and the presence of contaminants makes its use more challenging, but actually may be advantageous for certain synthesis methods for graphene.

Sales of carbon nanotubes are already substantial, while graphene applications are at an earlier stage of development, but first products are now appearing. Eventually, graphene may prove to be a competitor material to carbon nanotubes. Currently, this burgeoning sector may be characterised as low volume/high value. However, as products gain acceptance and commercial volumes increase then the option to use a coal derived feedstock will become more relevant to drive down costs.

7.1 CARBON NANOTUBES

Carbon nanotubes (CNT) are linear molecules of pure carbon that are one dimensional in that they are long and  thin,  shaped like  miniature pipes or  tubes,  of only 1-3 nanometers in diameter     (1 nm = 1 billionth of a metre), and are typically hundreds to thousands of nanometers long.

Figure 17 A wrapped carbon nanotube showing the 3-D aromatic ring base structure (Bigstock ID 222800983)

Figure 17 gives an impression of the scale of a nanotube relative to a benzene aromatic ring which is the unit structure of CNT, graphene and graphite, and is of course the dominant structure in most coals. As individual molecules, nanotubes are 50 times stronger than the equivalent structure made from steel and as carbon is a lighter element, only one-sixth of its’ weight. As well as strength, CNT show similar properties to metals, exhibiting extremely efficient conduction of electricity and heat. They may also be modified to simulate semi-conductor behavior (Harris, 2017).

CNT cannot normally be used in their bulk form as a powder or film as their properties are not translated in the macroscopic form. Rather, CNT are used as alloys, blends, composites or hybrid materials where the CNT enhances the strength of a base material (Khan and others, 2016). A major breakthrough in employing CNT has been their use as fillers for thermoplastic, thermoset polymers and elastomers forming CNT/polymer nanocomposites. These materials exhibit outstanding mechanical, thermal and electrical properties that could not be achieved in the absence of CNT. Long length CNT exhibit enhanced mechanical properties and prevent crack formation, behaving like a fibre, while short length CNT add stiffness as they act like miniature rods in the polymer matrix.

In a composite material single-walled CNT have a higher interfacial surface area with a polymer substrate, preventing crack formation, while multi-walled CNT show improved mechanical and electrical properties due to the increase in tubular layers. In general, a much lower content of CNT is needed to achieve desired mechanical and electrical properties than alternatives using carbon fibre or stainless steel.

There is considered to be a huge potential market for CNT that hinges on industry’s ability to produce large quantities of CNT more cheaply and uniformly; output is growing rapidly and exceeded a value of $4 billion in 2017 (Industry Today, 2018). CNT are produced from gaseous feedstocks which may be obtained from coal; although it is possible to make nanotubes from coal tar this method involves an additional pyrolysis step and remains at the research stage.

Carbon nanotube toxicology

Hazards associated with using ultra-fine particulate fibres are well known and the process requires protective control measures to prevent workers’ exposure. Toxicological studies indicate that there is reason to be concerned about the direct handling of CNT during the manufacturing process. A recent study analysing the effect of fibre length found that short fibres appeared harmless, while the body’s scavenger cells had difficulty in handling longer fibres. The issue is significant for the manufacturing workplace as once nanotubes are embedded within products the risk to consumers is considered minimal (Donaldson, 2016).

7.1.1 Synthesising CNT

CNT were originally discovered as a natural byproduct from candle flames. Since then three commercial production methods have been developed for CNT that include chemical vapour deposition, arc discharge, and laser ablation.

Chemical vapour deposition (CVD)

In chemical vapour deposition, the dominant manufacturing technique, CNT are grown from metal nanoparticle seeds sprinkled on a substrate heated to about 700°C. Figure 18 shows a schematic of the process where a carbon gas feed source is admitted to a tubular furnace together with a suitable catalyst (Khan and others, 2016).

Figure 18 Schematic of a chemical vapour deposition apparatus for CNT (Khan and others, 2016)

To avoid interaction of the metal substrate with electric components Zirconium oxide is the seed metal, along with catalytic components selected from nickel, cobalt, iron or a combination of them. Two gases are used: a carrier dilution gas that can be nitrogen or argon; and a carbon source material that may be methane, acetylene or carbon monoxide (Dai, 1996). Under suitable reaction conditions nanotubes commence formation and deposit on the walls of the furnace. Chemical vapour deposition using simple feeds is preferred as CNT are then formed largely in the absence of amorphous matter. Both single walled CNT and multi-walled CNT can be produced by adjusting temperature with the single walled ones the higher temperature product. The CVD process is more energy efficient than alternatives, requiring less power and using milder temperatures that yield high purity CNT.

Arc discharge

The original method to make CNT used an arc discharge technique. Two carbon rods placed end-to-end, about 1 mm apart, are arc vapourised applying a current of 50 to 100 amps using a potential difference of 20 V. The apparatus is contained in an inert helium atmosphere and CNT form as a rod-shaped deposit on the end of a graphite cathode. While this is a relatively simple method, the CNT must be further separated from the vapour and soot, and other carbon products such as fullerenes. The yield of CNT is a function of the plasma uniformity and the carbon deposit temperature (Kratschmer and others, 1990). Saha (2017) reports on arc discharge production of single walled CNT from thermal coal fines observing the advantage that the coal may be more easily fragmented by the arc discharge than graphite due to the weakly bonded macromolecular structure. However, while low costs are claimed due to the apparatus and feedstock, CNT separation from other carbon materials and metals remains an issue, and the electric power demand means that arc discharge is unlikely to compete with the CVD process (Saha, 2017).

Laser ablation of graphite

Laser ablation was first used to synthesise fullerene in 1985, and later this method was applied to form CNT (Guo, 1995). A graphite or synthetic graphite target is vapourised by an Nd/YAG laser in an inert argon atmosphere at high temperature (1200°C).  The pulsed laser dislodges large carbon particles from carbon graphite that are then fragmented by a second pulse; the resultant fine particles coalesce to form CNT. The CNT condense onto a water-cooled collector located downstream from the target carbon source. The products are obtained in the form of interlaced CNT ropes comprised of bundles of single walled CNT. The nanotube dimensions can be modified by altering the temperature and catalyst composition which is typically a 50:50 mixture of cobalt and nickel. However, as for the arc discharge method, CNT produced by the laser ablation have the disadvantage of requiring further purification.

7.1.2 Industrial applications for carbon nanotubes

CNT are currently one of the most sought-after materials and there is a proliferation of applications for nanomaterials across industry. The unique properties of CNT have led to their introduction in applications that touch nearly every industry, including aerospace, electronics, medicine, defence, automotive, energy, construction, and even fashion. CNT are of particular import in the development of sensors for medical applications, electronic and optical devices, catalysts, batteries, fuel cells, solar cells and accurate drug delivery systems. Table 5 lists a further selection of applications currently under development.

Of the many applications listed in Table 5 perhaps the most significant to the electrification of cities are those uses associated with energy storage and the replacement of copper to reduce electrical energy losses and transmission costs. The rapid development of battery technology is likely to require CNT intended for high efficiency electrodes. Research based in Japan is pioneering a hydrogen powered economy and the development of CNT hydrogen storage is an integral feature for hydrogen- based transport.

Demand for CNT is predicted to substantially increase over the next decade; cheaper manufacturing processes would accelerate development within the industry. Current nanotube prices vary markedly dependent on functionalisation, ranging from 30 $/g for multi-walled CNT to 3000 $/g for functionalised single-walled CNT (Nanocs, 2018). Composite materials containing multi-walled CNT are increasingly available, while single walled CNT are considered to be more appropriate to specialist products such as transparent conductive films, transistors, sensors and computer data storage. The main competitor to the expansion of the CNT market is potentially graphene.

7.2 GRAPHENE

Graphene is a naturally occurring form of carbon existing as a two-dimensional monolayer of sp2 hybridised carbon atoms arranged in a honeycomb-like network of six carbon atom rings. It was discovered in 2004 by Geim and Novosolev at the University of Manchester, UK and was originally obtained by the repeated scotch tape exfoliation of graphite. The prodigious interest in graphene is due to its unique properties of high strength, flexibility, transparency and exceptional thermal and electrical conductivity.

Graphene is one of the most promising areas of research in materials science, resulting in over 50,000 patents worldwide on graphene technology, the majority of which originate from China or the USA (Zhuan, 2017). Figure 19 highlights the 2-D planar geometry of graphene.

Figure 19 3-D illustration of a graphene layer showing hexagonal geometric form (Bigstock Photo ID 219561901)

Graphene can exist in a number of physical forms (National Graphene Institute, 2018), providing a wide range of material properties and applications. A single sheet is the most expensive form of graphene while multi-layered materials are cheaper. The different types of graphene are summarised below:

• Graphene: single atom thick layer of hexagonally arranged, bonded carbon atoms that can be freely suspended or attached to a substrate, suitable for electronics
• Graphene oxide: graphene formed by oxidative exfoliation is a monolayer material with a relatively high oxygen content relevant to liquid – gas separation
• Graphite oxide: formed by oxidation of bulk graphite is a precursor for graphene oxide
• Graphite nanoplatelets, nanosheets and nanoflakes: 2-D graphite materials less than 100 nm thick used in electrical composites
• Few layer (FLG) or multilayer graphene (MLG): 2-D sheet consisting of between two and ten layers of graphene, used to add strength to composite materials.

7.3 GRAPHENE FABRICATION: CVD, EXFOLIATION AND ELECTROLYSIS

There are several established routes to manufacture graphene using precursors that include methane gas and graphite, but more novel methods can directly utilise coal as a substrate. Graphite exfoliation and chemical vapour deposition of methane are typical methods; a more recent methanol solvothermal route proceeds via metal alkoxide decomposition. The direct conversion of bituminous coal to graphene sheets has been achieved for the first time using an electrolytic method. In a separate development acidic oxidation of coal generates graphene quantum dots for specialised usage.

7.3.1 Established manufacturing methods

Exfoliation of graphite

The original discovery of graphene involved the exfoliation of graphite with scotch tape; single layers of graphene were removed from the graphite surface. Chemical exfoliation techniques are now deployed to separate individual layers of graphene from a graphite substrate. However, the international availability of graphite is problematic leading to a rise in the manufacturing cost.

Another exfoliation method involves sheets of graphene oxide produced from coke oxides which may then be separated by ultrasound (Sierra and others, 2016). Initial studies show the graphene oxide produced in this way shows similar characteristics to that obtained from natural graphite. The yield and sheet size of graphene oxide is dependent upon the original crystalline structure on the surface layers of the coke oxide.

The molecular arrangement of graphite that lends itself to exfoliation is absent from raw bituminous coal supplies, which generally possess carbon crystals in an amorphous matrix, rather than in continuous regular layers. Exfoliation is considered unsuitable for the direct conversion of bituminous coals.

Chemical vapour deposition (CVD) of methane

Carbon vapour deposition (CVD) of methane is the normal commercial route to graphene; production of large scale graphene films with wafer sizes of up to 760 mm were first obtained by this technique in 2009 (Kim and others, 2009). Chemical vapour deposition of carbon in a vitiated methane atmosphere adsorbs carbon atoms onto metal substrate layers to create sheets of graphene. Sheets of nickel or copper provide suitable substrates to adsorb precursor carbon materials. During 2017 there was a significant manufacturing advance when, graphene wafers were commercially produced on 200 mm CVD lines.

Coal tar volatiles can replace methane in CVD, but there is scepticism over the quality of the graphene sheets produced from these feedstocks (Buckley, 2018). Using aromatic molecules may be considered to offer an advantage as the unit cell of graphene is already present, but this has not been translated into a high-quality product. Additional coal tar preparation steps increase the cost of the CVD method.

Metal alkoxide route (solvothermal)

In this new commercial method, an alkali metal oxide is produced in the first stage by reacting ethanol or methanol with a metal such as sodium or magnesium; methanol is readily available as a by-product from coal gasification processes. The resultant solution, along with an inert gas carrier, is subsequently sprayed as an aerosol or mist into a tube furnace at a temperature exceeding 350°C, where the metal alkoxide decomposes to form graphene. As in the CVD method, graphene forms on cooler surfaces or on a metallic substrate, but in this case a non-catalytic metal such as gold or silver is preferred. The

graphene powder product is then washed to remove contaminants (sodium carbonate and sodium hydroxide), centrifuge dried and finally annealed to enhance crystallinity. This method has the advantage of being a continuous process unlike exfoliation (Coleman, 2018).

7.3.2 Electrochemical production from coal

A new coal to graphene (termed C2G) process is under development at the University of Ohio where coal is treated electrochemically to yield graphene (Botte, 2017). The patented process involves the electrolytic breakdown of coal particles which occurs in the presence of hydrogen obtained from the electrolyte solution (Botte, 2012). The addition of hydrogen as a reductant converts coal carbon to an aromatic form while also forming some heavy matter that separates as waste, while CO2 is formed as a by-product. A simplified diagram of the process is shown as Figure 20 that depicts the overall scheme and an electrolytic cell.

Figure 20 Simplified process flow diagram for electrolytic conversion (Botte, 2017) and electrolytic cell schematic of coal to graphene

Milled coal and water are fed to the electrolytic cell where the electrolyte is a mixture of a strong acid and a catalytic salt such as iron. The anode can be a noble metal or carbon, while the cathode consists of a noble metal electroplated onto a support material. The conditions in the electrolytic cell are as follows:  a  current  density  of  between  90  and  200  mA/cm2;  atmospheric  pressure;  and  a  reaction temperature  of  approximately  80°C.  During  the  course  of  the  reaction  hydrogen  is  released  from solution to be used later to hydrogenate fragmented carbon compounds.

Electrolysis produces light aromatic hydrocarbons and nano-sized coal crystals; the aromatic molecules then coat the coal crystals forming an aromatic gelatinous film over coal particles. Within the molecular structure of bituminous coal, graphite crystals are surrounded by amorphous carbon that reacts more readily to form aromatics leaving discrete nanocrystals. Once hydrogen production has slowed, this indicates that the batch reaction is near completion.

After electrolysis, the char slurry or syrup of graphene particles and aromatics is separated from the electrolyte which can be recycled. Coated coal particles can then be combined with a solvent and the resultant slurry sprayed onto a substrate, such as a copper foil, forming a thin graphene precursor layer on  a  copper  base.  Finally,  the  coated  foil  is  processed  at  high  temperature,  800–1000°C,  in  the presence of a hydrogen/argon gas mixture for a period of about 30 minutes to facilitate graphene sheet formation.

This process is a direct competitor to graphite exfoliation and also exhibits a number of potential advantages over current CVD methods:

• reduced need for certain transition metal catalysts (Co, Ni, Pt, Ir, Ru);
• methane gas replaced by lower cost coal;
• milder reaction conditions;
• E-beam evaporation not required;
• vacuum not required; and
• expensive graphene substrates replaced by copper foil.

The C2G graphene product is currently undergoing tests with partner companies for comparison with conventionally produced graphene, and the University of Ohio are further developing a design for a full-scale pilot plant (Botte, 2018).

7.3.3 Graphene quantum dots from coal

The discovery that bituminous coal may be used to synthesise nano-sized graphene quantum dots (GQD), carbon crystals of nanometer dimensions, is an exciting development in the electronics industry (Tour, 2017a). Quantum dots possess optical and electronic properties that vary according to particle size.

Quantum dots are small assemblages of approximately 50 carbon atoms, giving a graphene dot of 2 to 10 nm diameter. The size of the dot determines which colour is observed; at one end of the range blue colouration is associated with small sizes, while a red appearance is obtained from larger dimensions. In manufacturing GQD from coal a key issue, apart from releasing graphene, is to form a specific size range of dots. The presence of discrete crystals linked by amorphous carbon in coal is an advantage in making GQDs, so specific coals can be selected for a particular set of quantum dot size.

Bituminous or anthracite coal and coke are particularly suited for GQD synthesis as they offer more crystalline carbon content than lower grades; bituminous sources supply smaller crystal sizes of approximately 2 nm. The preparation method involves partial oxidation of coal at 100–150°C using a combination of acids and oxidants that are selected from: nitric, sulphuric and hypophosphoric acid; potassium, sodium and ammonium permanganate, potassium chlorate; and hydrogen peroxide. The acid acts to break the amorphous carbon links to the crystals releasing the GQD which can then be filtered from a neutralising alkaline solution (Tour and others, 2013).

Quantum dot technology is extremely specialised for use in the latest display screens of high dynamic (HD) range televisions: the latest ultra-HD premium standard offers increased peak brightness (quantum yield) and supports the Dolby Vision film standard. Quantum dots are used to replace white LED backlighting and colour filters which reduces the power consumption of the display and is especially important in portable electronics. A wider range of applications is envisaged that also includes solar cells and biomedical imaging (Dotz Nano, 2016)

These products are more easily made from coal than by graphite exfoliation. Apart from the technical advantages, coal is a cheaper resource priced at about 100 $/t compared to over 1200 $/t of graphite, realising a substantial cost benefit.

7.4 COMMERCIALISATION

Initial commercial uses for graphene harness the special properties of conductivity, hydrophobicity and high strength. Exciting recent products include the development of water impermeable coatings, membranes for water desalination, and hi-tech use of graphene layers to dissipate hotspots in space mirrors preventing warping (Hurley, 2018). The first commercial LED graphene light bulb arising from research at the Graphene Institute uses thermal conductivity properties to limit temperature which extends bulb lifetime (Graphene Lighting, 2017). In the future, it is anticipated that graphene will be used to transform innovations in flexible computer display screens, medical sensors, supercapacitors, graphene-silicon solar cells, and novel lithium battery electrodes.

As the number of graphene products increases then the cost of the manufacturing process and reactants will become increasingly important. Most graphene is currently produced from methane or graphite precursors, resources that are at least ten times more costly than bituminous coal. Existing methods which work for graphite have proved to be unsuitable for coal, but two new processes seek to form graphene directly from bituminous coal.

The manufacture of quantum dots for next generation screen displays is one example of how the amorphous-crystalline structure of coal may be an advantage in synthesising specialist graphene materials. Selected coal directly provides the required nano-size range, which is harder to achieve by conventional methods.

In the C2G process, the synthesis of graphene sheets of up to 80 cm2 is a breakthrough in the industrial direct conversion of coal suitable for more general graphene developments. Coal offers the major benefit that at less than 100 $/t the precursor cost is a fraction of that of graphite, which currently retails in excess of 1000 $/t. The C2G electrolytic process competes directly with methane CVD and solvent based graphite exfoliation, but with potentially lower fabrication costs (Botte, 2018).

Harnessing coal in the manufacture of graphene is an exciting new field for coal, where the extracted graphene nanocrystals, already present in coal, can become integral parts of the latest hi-tech devices.

7.5 THIN COAL FILMS

Graphene was expected to be a suitable alternative to silicon as a platform for electronic transistors,  but this is beginning to look unlikely. The attraction of using graphene in computer transistors was due to its two-dimensional structure, as it is only one atom thick, allowing a greater density of transistors  on a microchip, which is an enabling technology for the development of quantum computers. However, the high conductivity property of a graphene layer, which is so valuable in other applications, is proving to be an obstacle as a single layer of graphene doesn’t possess the ‘band gap’ present in silicon wafers (1.1 eV band gap).

Researchers are examining other two-dimensional materials as well as modified graphene. For example, studies using narrow graphene ribbons (10 nm wide) show semi-conductor behaviour and are a promising area of research for graphene nanoconstructs (Bennett, 2018). A novel form of carbon has been discovered at the Massachusetts Institute of Technology (MIT) that possesses unique electrical properties and could replace silicon in the field of electronics. This alternative material consists of thin films or wafers of raw coal.

By careful selection of raw coal substrates, it is possible to create wafers of coal with a large range of electrical conductivities spanning seven orders of magnitude (a ten million scale). This means that a specific coal could be converted into a coal wafer that inherently provides the electrical properties needed for a particular component without additional refining. Such a procedure compares with the preparation of a pure silicon wafer that necessitates manufacture under ultra-clean room conditions and subsequently requires a second step to distribute pure metals on the surface to achieve the desired properties (Keller and others, 2016).

Thin coal films are prepared using ball milled samples of coal to provide powdered carbon held in suspension. Centrifugation then separates the suspension in order to select the required particle size (<100 nm). The particles are then spin coated to form a thin coal film before a final annealing stage. Thermal evaporation is used to deposit gold segments on the film surface to form the electrical contacts.

To date, the MIT team have characterised chemical, electrical, and optical properties of thin films of several coals covering a range of grades that include: lignite, two bituminous samples and anthracite. The MIT team has prepared a first product using transparent thin coal films that form a planar Joule heating device that can sustain temperatures of up to 300°C. A first application for the transparent thin film heater may be for car window defrosters, airplane wings, or as part of a biomedical implant.

The largest markets for this technology are considered to be new solar panel technology and novel battery development. The major advantage over silicon is that the source material is very low cost and relatively simple to manufacture compared to the requirement for silicon to be 99.999% pure. Thin coal films have the potential to compete with graphene where electro-thermal transparent products are also in development.

These nanocarbon products are of high value, priced in $/g rather than $/t and require low volumes of carbon or coal precursors. Even a device using a large area of material such as transparent car windscreen heaters will require little material as the thickness is so small. Although the demand for CNT, graphene and other specialised nanocarbon forms is increasing rapidly, and sales are already counted in billions of dollars, the quantity of carbon will remain low.

8. LIGNITE AND AGRICHEMICALS

Lignite is the lowest rank of coal, often referred to as brown coal, and is used almost exclusively as fuel for steam-electric power generation. Brownish-black in colour it has high inherent moisture content, sometimes as much as 70% (German lignite). The countries possessing the most significant reserves  of lignite include Russia, Germany, Australia and USA.

Lignite fuels characteristically have low calorific value (CV of 8000 to 15000 kJ/kg) due to the high moisture and ash content which combine to result in low efficiency thermal power generation. An indigenous low-cost lignite feedstock offers the benefit of relative ease of extraction via open cast mining and provides a dependable alternative to imported coals. Thermal power stations are generally located close to lignite mines to avoid transporting chemically reactive and low CV fuel that is typically burned in its raw state. To meet regulators’ uprated plant heat rate targets, lignite may be beneficiated and processed using advanced combustion technologies to substantially reduce CO2 emissions, at the same time improving boiler reliability and reducing plant and effluent treatment costs (Reid, 2016).

International environmental pressure seeks to reduce lignite combustion for power generation in order to limit CO2 emissions. Australia and Germany are accelerating the phase out of lignite-fired power; 60% of German electricity generation capacity is currently dependent on coal and lignite. The reliance on solid fossil fuels is reducing in accordance with Germany’s Energiewende (energy transition), a key component of the new German coalition government agreement (Amelang and others, 2018).

Against a background of downscaling lignite power generation, there is a risk of changing a useful fuel into a stranded resource. Lignite may have a future as a soil additive; discarded surface mine waste deposits gradually transform into the mineral Leonardite which has been collected since the 19th century for agricultural use. Leonardite, a soft dark brown vitreous mineral, is produced by a weathering process involving the natural partial oxidation of lignite where humic acid forms via a slow ageing process that incorporates nitrogen. Such humic products have been used to enhance soil since the first extraction of humic acid from peat by Achard recorded in 1786 (Ganesa and others, 2007).

Lignite has a long history of use as a fertiliser; the applicability of this niche scale activity is being examined to assess if it can become a large-scale viable alternative to nitrogenous fertilisers and simultaneously help to counter the problem of desertification. If upgraded lignite performs as an effective agricultural additive, then this could be a significant alternate high-volume use of a valuable resource.

Lignite products could play an important role in counteracting the deterioration of fertile land, a global problem caused by intensive farming practice, erosion and drought caused by changing climate conditions. The 2015 UN declaration of an ‘International year of soils’ aimed to increase worldwide recognition of extensive land desertification and the associated decline in food production. This initiative is an outcome of the 2015 United Nations Climate Change Conference in Paris (COP21) and resulted in the establishment of the ‘4 pour 1000’ soil organic carbon (SOC) programme, to organise efforts to improve soil quality.

Soil degradation poses a threat to more than 40% of the Earth’s land surface and the rate of degradation is accelerating. The situation in North Africa typifies an increasing agricultural crisis, where over the last century the Sahara Desert has extended northwards covering an area equivalent to that of France. The IEA Clean Coal Centre has previously reported on the impact of drought leading to fresh water shortage and describes the efforts the power industry has made to utilise waste water and saline supplies (Carpenter, 2017). It is anticipated that concerns over food and water security will intensify  as the world population is predicted to increase by 2 billion towards 9.5 billion people by 2050      (Lal, 2017).

Compared to chemical fertilisers, lignite additives have characteristics close to those of existing soils and may help to rebuild the soil structure providing a long-term benefit to the soil. Lignite humate products are prepared either by chemical techniques utilising nitric acid, hydrogen peroxide or ozone, or by novel methods that avoid the use of chemicals, using either using air in an oxidative ammonolysis conversion, or a microbial technique that converts lignite to humates and methane gas.

8.1 CHEMICAL CONVERSION OF LIGNITE

The Brown Coal Innovation Australia (BCIA) recently reported on the potential of chemically treated lignite as a soil improvement agent, assessing the benefit of applying lignite sourced humates (McManus, 2016). The report examines the impact of lignite on soil organic carbon uptake; and compares the efficacy of the lignite product to alternatives based on manure and compost. Initial analysis is positive, and the report recommends implementation of a substantial field trial programme within Australia.

Table 6 lists current methods to convert lignite to humate products; of these the leading process upgrades lignite using nitric acid and ammonia to produce nitro humic products.

Current commercial production of nitro humates demonstrates both the suitability and commercial acceptability of a modified lignite humate product. The nitric acid method exhibits enhanced nitrogen uptake in the humate product, but nitric acid production requires an expensive ammonia facility which raises costs.

The treatment of lignite with potassium hydroxide provides a soluble humate product with up to 55% humate content (Ganesa and others, 2007). The alkaline digestion of lignite requires a relatively long period of eight hours in a batch process. This method, and the nitric acid route, provides a soluble product which will be less effective at improving the physical soil structure.

The expensive ozonolysis of lignite has shown initial positive product results but has not progressed commercially; ozonolysis is a high energy consumer and may require a low-cost source of electricity.

The more expensive chemical processes described may not be suitable for severely affected regions in developing countries (Hakli and others, 2010). Lignite technologies capable of producing high tonnages of low cost humate products, while avoiding the use of chemical reagents and expensive equipment, are being explored.

8.2 OXIDATIVE AMMONOLYSIS OF LIGNITE

A novel, mild oxidative ammonolysis process converts lignite to humate fertiliser using pressurised air that introduces nitrogen and oxygen into the lignite structure. The resultant Novihum® product is a soil conditioning agent that replaces natural humus lost from degraded and arid soils (Novihum, 2015a). The material may be applied as a deep layer onto impoverished soil as an alternative to organic waste materials such as manure and mineral fertilisers, providing high-grade humus to improve soil development and plant growth.

The biological plant life cycle produces fertile humus which gradually accumulates over time. In lignite formation, oxygen is gradually depleted from the humus, accompanied by a loss of nutrients and humic characteristics, eventually forming brown coal. The action of oxidative ammonolysis partially reverses the natural decomposition that converts vegetation to lignite (Novihum, 2015b).

The  lignite  reaction  conditions  require  a  moderate  temperature  of  130°C  and  an  oxygen  partial pressure of several bars, in order to incorporate nitrogen and oxygen, which is accompanied by the partial release of carbon dioxide. Figure 21 shows the main components of the humic product which comprises: lightweight fulvic acids that are readily incorporated by plants; humic acids suitable as long-term fertilisers; and humins which are an insoluble residue that act to improve soil aggregation (Ganesa and others, 2006). Humic acid is a complex oxygenated hydrocarbon with a structure as shown in Figure 21, forming the principal component of humic substances found in soil and peat.

Figure 21 Approximate composition of Novihum® converted lignite product (Novihum, 2015b) and a typical humic acid molecular structure

Novihum describes a number of enhanced soil properties following the application of lignite humate product:

• improved physical and chemical characteristics of soils; applicable to complex sites such as eroded soils, semi-deserts, landfills and re-cultivation sites;
• improved ability of soils to retain nutrients, water and carbon dioxide;
• improved plant growth, stimulated development of blossom, chlorophyll and superior fruit flavour; and
• promoted mycorrhization, the incorporation of fungi on plant roots, and supported microbial soil-life.

Novihum® product is supplied as granules and typically added at up to 1.5 kg/m2 on the surface of the soil (a layer of up to 15 cm) that is equivalent to 15 tonnes per hectare (t/ha). In a programme encompassing over a 200 European field trials, the medium-term results show a number of fruiting crops (peppers, strawberries, grapes) with substantially increased yields of 10% to 25%. Longer-term trials over a 15-year period show increased productivity from intensively rotating cereal crops.

Novihum® technology reports that the demonstration phase is complete, and the company has received further EU investment; enabling scale-up from a test production rate of 1000 t/y to an industrial production line on a 200 ha facility located in Germany.

8.3 ANAEROBIC DIGESTION OF LIGNITE (MICROBIAL REFINERY)

The anaerobic digestion of lignite is an innovative method which generates humic acid material and natural gas (Walia and Yurek, 2014). The structure of brown lignite coal is close to that of natural humates rendering it suitable for conversion by microbial techniques. The degree of difficulty and digestion time required increases for more mature coal grades.

In this method coal must first be milled to the required size before bacterial bioconversion involving hydrolysis and fermentation of lignite. Several culture media are suitable for the maintenance of anaerobic cultures and bioconversion of low rank coals; appropriate bacterial cultures are from the zootermopsis dampwood termites’ (Isoptera) digestion system (Sheftone New Termite Medium (SNTM), or a variant containing citrate and methanol (SNTM-CM)) (Srivastava and Walia, 1996). Cultures are accompanied by a low dosage nutrient package consisting of yeast extract with trypticase soy broth (YE/TSB) solution, nitrogen compounds, chelators, and vitamins.

The bacterium cultures convert coal into a complex mixture of oxygenated organic molecules that include: methane, volatile fatty acids, lower alcohols and humic products. A conceptual bio-refinery shown in Figure 22 outlines the flow of material through a microbial plant digesting raw lignite into gaseous and humic products.

Figure 22 Conceptual flow diagram of coal microbiological conversion to humic acid products (Walia and Yurek, 2014)

The process is operated at pilot scale in Turkey; in step 1 ground lignite is fed to the first anaerobic digester to form intermediate oxygenated compounds (MicAN1). Step 2 converts these oxygenated intermediates into gas utilising a microbial methanogen, a methane generating organism (MicAN2). Methane gas is collected, and the residue converted into humate products in a third chemical extraction to produce the commercial products: Humaxx; Actosol; Humasorb and Actodemil. The humate products can be used in agriculture, employed for environmental restoration to remove contaminants and operate as filters for waste water recycle. The Actosol fertiliser is now sold in a number of countries that include Egypt, Turkey and China. A humate based ion – exchange resin product shows metals recovery in excess of 93% for the following elements: boron, cadmium, chromium, copper, iron, selenium and zinc (Walia and Yurek, 2014).

Anaerobic digestion of coal, in a ratio of 1 t of lignite per tonne of water, is processed in a bioreactor for 35 days to yield 60 m3 of methane rich biogas (>40 kg), with the remainder of the coal converted to humic acid.

Laboratory trials of anaerobic digestion have successfully confirmed gas and humate production from over one hundred different lignite and sub-bituminous coals; the quantity of gas produced per tonne of coal reflects the carbon content of the feedstock. The bio-refinery concept has been proven at pilot scale and a number of humate products are commercially available.

A novel option is to perform an in-situ injection of microbial solution into an otherwise inaccessible coal seam. When contrasted with the better-known underground coal gasification (UCG), the in-situ microbial coal conversion (MCC) technology forms a more benign product and avoids subsidence and the formation of harmful aromatic by-products. This underground in situ bio-gasification method has so far only been demonstrated in simulated laboratory conditions.

8.4 FUTURE OF LIGNITE AGRICHEMICALS

The future market for humate products is predicted to rise substantially amid increasing concern over global food production and the effect of soil erosion. Novihum estimate that over 3.5 million km2 of land requires urgent remedial action across Europe, USA, and Middle East. The economic value of the industry depends on the adoption of humate technology to replace chemical fertilisers, but a forecast of projected sales estimates over €300 million per year (Novihum, 2015a). The value of the humates industry depends on demonstration of proven long-term fertility benefits.

For comparison, mineral humate deposits retail in the USA at approximately 600 $/t (Mesa Verde, 2016; Harvest Grow, 2016). Potassium nitro humates, manufactured by the Ostwald nitric acid process, using ammonia, show product prices of between 300 and 500 $/t in China, dependent on the volume purchased (Shijiazhuang Han Hao Trade Co, 2018).

The Brown Coal Innovation Australia (BCIA) review found that Australian soil quality is deteriorating in the face of extended periods of drought, and the availability of manure and compost is considered insufficient to improve soil organic carbon levels adequately. Indigenous Victorian brown coal resources are available which could form humate precursors in sufficient volume to meet future demand to counteract a serious decline in soil quality (McManus, 2016).

A number of environmental issues have been linked to the use of chemical fertilisers and manure. Problems associated with such fertiliser use include: loss of habitat and carbon sinks with respect to peat compost extraction; high ammonia emissions arising from intensive farming methods; and the nitrate contamination of river systems (Misselbrook and others, 2018; Mehta, 2018a). A humate product sidesteps these issues and may be a more suitable environmentally acceptable alternative.

Numerous studies have examined the link between soil, peat and lake sediments and mechanisms of desertification (Gong and others, 2018). The addition of humate matter improves the capability of soil to act as a carbon sink to adsorb CO2. Soil carbon retention remains a significant factor in climate models to assess carbon budgets and predict CO2 growth rates; the cycling rate of CO2 adsorption/desorption in soil is slower than originally perceived, operating over a period of several years, therefore it will take longer to measure the benefit of improving soil’s maximum carbon take-up (Yujie and others, 2016).

Examination of the agricultural benefit of humate usage showed that some products are promoted at application levels of less than 20 kg/ha while a level in excess of 400 kg/ha may be required to obtain lasting benefits (Lyon and Genc, 2016). Laboratory trials of commercial lignite based humates confirm plant growth improvement but display variable results for other soil indicators such as mycorrhizal colonisation and biomass carbon accumulation (Little and others, 2014). While there are a range of products of differing efficacy, conclusions seem to support the Novihum recommended application level of 15 t/ha, equivalent to 15 cm of material deposited as a topsoil improver.

The current status of recent lignite conversion technologies is set out in Table 7. The mild oxidative ammonolysis of lignite to humic acid has completed pilot trials and progressed to a significant demonstration facility supporting an extensive field trial programme.

The microbial technology is already commercialised, and products are sold to an increasing number of countries, with parts of the microbial refinery demonstrated at pilot scale.

The down-scaling of lignite coal power is underway in developed countries and the preparation of lignite based humate products offers an alternate use for readily available mined brown coals. The addition of lignite converted to humates is shown to improve crop yields and the quality of plants. Claims for low application rates have raised doubts over the efficacy of humates, but the latest recommendations are for loadings that comply with recent scientific studies and equate to a substantial use for lignite. The long-term benefits reported are of increasing import, given recent data on the turnover of CO2 retention in soil. In overall environmental terms, lignite humate may be used to absorb carbon dioxide rather than generate CO2 by burning fuel.

9. DEVELOPMENTS IN DECARBONISED FUELS; H2 FROM LIGNITE AND CO2 REDUCTION

Hydrogen production from lignite

In certain regions, the use of lignite for power generation is in decline due to its carbon intensity, that  is a relatively high rate of CO2 emission per unit of power. Japan is pursuing a hydrogen-based economy, ‘Basic Hydrogen Strategy’, and lignite is a candidate to be used as the hydrogen source. Lignite fuels may be decarbonised by a combination of gasification and catalytic conversion, providing hydrogen fuel and CO2 separated for storage.

A proposed lignite gasification project in Australia would use established high pressure partial oxidation methods to convert lignite to syngas. The carbon rich syngas would be treated subsequently using high and low temperature catalytic shift chemistry to maximise the hydrogen content of the gas (Kawazoe, 2018). The CO2 by-product would be separated from the hydrogen stream and piped into storage as part of a CCS project.

In this Australian/Japanese programme, the intent is then to compress the produced hydrogen for transport from Australia to Japan. The programme faces challenging technical issues concerning the storage, loading and shipping of high pressure hydrogen. Hydrogen is more volatile and explosive than natural gas which increases the handling hazard, and is prone to higher leakage rates. The proposed programme aims to produce 10 Mt/y of hydrogen by 2030, which would require 200 Mt of Australian lignite. This programme may compete with the generation of hydrogen by water electrolysis which is the subject of numerous catalytic studies aiming to reduce the necessary energy input (Mills, 2013).

The development of a hydrogen fueled economy has been mooted for many years as hydrogen combustion is carbon free. The production of hydrogen by reforming is still considered to be more cost effective than water electrolysis by a factor of 2, but of course this route requires carbon sequestration which also increases costs. A major programme is underway in Japan, with technical developments to be highlighted at the Tokyo Olympics in 2020. Currently plastic lined carbon fibre tanks are the preferred method for hydrogen transport and storage, but one option is to convert hydrogen to ammonia for shipping, and then crack it back to hydrogen at its destination (Mehta, 2018b).

Carbon dioxide reduction to carbon monoxide

The coal power industry is adopting a number of measures to reduce emissions of CO2 and to capture, use, store or convert CO2 into other products. High efficiency low emissions (HELE) technologies, the next generation of clean coal power plants, require less fuel to generate power leading to reduced emission of CO2. CCS technologies collect CO2 for use in enhanced oil recovery or for permanent geological storage, methods which are to be implemented in both energy and manufacturing industries.

A chemical industry initiative aims to reduce CO2 emissions by the synthesis of compounds harnessing CO2; one method involves water electrolysis to supply hydrogen for catalytic conversion of CO2 to methanol and fuels (Zhu, 2018).

A novel carbon dioxide reduction method is currently under development which forms carbon monoxide within an electrochemical cell. Protons generated from aqueous solution break down the CO2 to CO. Unlike the use of separate water electrolysis to generate hydrogen, reaction occurs in a single step thereby eliminating an extra process vessel (Su and others, 2018). The technical barrier to electrochemical CO2 reduction processes is a result of the relatively low faradaic efficiency and high over-potential, in other words much more energy is consumed than can be generated from the CO product. The electro-catalytic reduction method (CO2RR) exhibits enhanced performance using a range of low cost, common elements that had previously been rejected as unsuitable in other metal complexes. The formation of covalent triazine frameworks (CTF) containing cobalt, nickel and copper have shown high faradaic efficiency of the order of 90% for a Ni-CTF complex. Catalysis is thought to stabilise a COOH species adsorbed on nickel, lowering the free energy barrier to form desorbed CO.

In a separate electrocatalytic study, isolated nickel atoms distributed on a graphene sheet have also shown high efficiency carbon dioxide reduction (Jiang and others, 2018). In this case the uniform distribution of the nickel atoms appears to facilitate the conversion reaction rather than the competing water splitting reaction which may require adjacent catalyst sites.

The capture and reduction of CO2 to CO by CO2RR methods provides an opportunity to feed carbon monoxide into a coal plant boiler, effectively returning CO2 as a recycled carbon fuel. The CO could alternatively be used in the chemical industry. The production of CO may form part of a scheme involving optimisation of flexible power generation, where CO collection forms part of an energy storage system.

10. CONCLUSIONS

Worldwide, coal has underpinned the power, steel and cement industries leading to annual consumption of over five billion tonnes. Other important coal users include aluminium refineries, paper mills, and chemical and pharmaceutical industries. A fraction of coal usage is converted into commercial carbon products, but this still requires in excess of 100 Mt. The long-established coal tar chemical industry synthesises thousands of familiar products that include soap, solvents, dyes, plastics and fibres, such as rayon and nylon. The petrochemical industry manufactures 150 Mt/y of polymers; in comparison the developing coal gasification industry in China has captured a 6% global market share of polyethylene and propylene synthesis. Carbon-based materials are furthering the development of many new products including activated carbon adsorbents, carbon fibre materials, and devices that require new physical forms of carbon such as graphene and carbon nanotubes.

Exciting new alternative technical uses for coal range from rare earth element extraction, chemical synthesis, agrichemicals, and products such as quantum dots that exploit the inherent structural properties of raw coal for the latest hi-tech screens. The application of coal carbon-based technologies may become essential to protect the environment: activated carbon for water purification and mercury emission control, lightweight carbon fibre materials for energy efficiency in aerospace and transport, humates to reverse desertification and reduce nitrogen pollution and specialist carbon materials to revolutionise battery development associated with renewable power.

Coal tar chemicals

The contraction of blast steel manufacture has resulted in a corresponding reduction in the associated coking industry which supplies coal tar refiners. Thousands of products are obtained from refined coal tar and demand for these continues to expand heightening feedstock shortages; for example, naphthalene  derivative  sales  are  currently  increasing  at  3%/y,  which  would  correspond  to  an additional 110,000 t of derivatives over the next five years. Mild pyrolytic methods of coal beneficiation are under development, offering an alternate route to provide tar volatiles as coal coking decreases. Regulation of the handling of coal tar polyaromatic compounds (PAH) is tightening to reduce harmful levels of exposure and PAH leaching that results in contamination of water systems; manufacturers may find it necessary to shift production to more environmentally acceptable products.

Coal gasification

The gasification of coal-to-chemicals in China is currently highly competitive with petrochemicals due to rising oil prices, and production is set to increase substantially in the next five-year period. In addition to new coal to polymers production in China, there is also upgraded petrochemicals capacity in the Middle East utilising cheap gas supplies. In Western Europe, Ineos Olefins and Polymers Ltd have announced a new world scale propane dehydrogenation petrochemical facility, a first for many years. The indications are that the polymer industry expects sales of plastics to rise significantly over the next 10 years.

Plastics packaging has been adopted for almost all everyday products, however, international concern over the disposal of single use waste plastic has not yet significantly affected polymer production or plastics fabrication. Compared to oil and gas polymer production, the coal gasification industry possesses greater carbon intensity for equivalent products obtained. Against this background, a new chemical facility in China will introduce and evaluate carbon capture for enhanced oil recovery to offset 400,000 t of CO2 emissions from gasification.

Rare earth element extraction

Conventional ore mining is experiencing increased competition from the extraction of valuable elements present in industrial waste, for example new micro refineries recycle electronic waste to recover gold. Unique to the coal industry, waste coal tailings, acid mine drainage (AMD) and lignite are under evaluation as future resources of rare earth elements (REE). A new source of REE avoids dependence on a tightly controlled supply of mineral ore metals. Initial REE concentration targets in the waste coal development programme are exceeded and bespoke waste coal processing schemes are specifically designed to concentrate REE from coal feedstocks. Bench-scale results obtained by processing coal tailings show that a 90% REE concentrate can be produced with efficient metals recovery; these preliminary results are comparable to the performance of commercial smelting facilities. The removal of heavy metals from coal tailings and neutralisation of AMD streams may offer substantial environmental benefits and feature as an integral part of remedial activities at both current and aged mine workings.

Carbon fibre

Carbon fibre is renowned as a high-performance, weight-saving structural material of choice in motor sport, aerospace and, increasingly, turbine blades and automotive parts. The desirable qualities of carbon fibre, exceptionally high strength and stiffness to weight ratio, are superior to that of aluminium and specialised steels. Although the relative price of carbon fibre composites is gradually falling, the highly expensive manufacturing process has limited its widespread use. Carbon fibre obtained by ‘melt spinning’ of coal pitch is the most promising route to lower the cost of production. Production of pitch fibre has recently doubled, lifting market share to 10% equivalent to 10,000 t, while the best-selling PAN fibre may be impacted by increased petroleum feed costs. Mitsubishi are at the forefront in developing pitch fibre products that require coal tar pitch pre-treatment and restricted operating conditions during the fabrication process; pitch fibre product quality is sensitive to temperature control in the preliminary heat treatment. Achieving high quality of fibre derived from coal pitch is a major challenge for manufacturers, but pitch fibre can be produced to a standard similar to that of PAN fibre, furthermore the pitch fibre exhibits superior thermal conductivity. Pitch fibre has been successfully deployed in satellite applications confirming the long-term reliability of such material properties.

Activated carbon

Activated carbon is commonly used in the purification of water and organic solvents; it has a well-established market that is expanding as water scarcity leads to increased recycling. The recent introduction of mercury emission limits within coal power generation has increased demand for ‘single use’  activated  carbon  in  waste  stream  treatment  systems.  In  the  longer  term,  the  use  of  activated carbon for carbon capture is a promising method to replace expensive amine systems  in  the  environmental reduction of CO2 emissions.

Carbon in Electrodes

New battery powered devices are integral to a modern society based on personal communication and computing. Graphitic carbon is used in the manufacture of the battery electrodes for hundreds of millions of these devices. Renewed demand exists for industrial graphitic carbon electrodes required in electric arc steel manufacture. The restricted supply of graphitic carbon has substantially raised the cost of industrial electrodes. Petroleum coke is the preferred material to produce synthetic graphitic carbon. However, pet coke may be substituted by needle coke produced from coal tar pitch to form lower cost synthetic graphite suitable for use as industrial electrodes. The major quality issue preventing widespread use of coal-based electrodes is due to the sulphur content that affects durability at high temperatures; electrode tips can reach 10,000°C in the latest furnaces. In small scale devices, it is likely that new carbon materials such as graphene will be deployed in the production of electrodes for novel batteries and supercapacitors as part of a drive to achieve rapid charge and discharge at higher temperatures. New carbon materials are introduced to the carbon electrodes to prevent mechanical distortion during charging which is the main parameter affecting battery life.

Agrichemicals

An alternate use for lignite is emerging in the manufacture of synthetic humates. Many countries with substantial lignite resources now face the reduced demand for lignite thermal power. Lignite resources may find a new market in the production of agricultural products capable of improving soil quality and counteracting the increasing problem of desertification. Methods are emerging to produce humates from lignite in the absence of chemical reagents. One method employs microbial conversion of lignite to humic products, and the other oxidative ammonolysis method reintroduces oxygen and nitrogen into the lignite internal structure. The large-scale deployment of lignite humates provides a means to significantly improve land reclamation, regenerating the fertility of soils.

Decarbonisation

Carbon capture, utilisation and storage (CCUS) can abate the carbon intensity of solid fuels. Coal gasification accompanied by pre-combustion carbon capture enables adsorption at high pressure in order to improve CCS efficiency; the conversion of lignite to hydrogen employs this technology to produce hydrogen suitable for high efficiency fuel cells. Research studies indicate that future improvements to coal plants operating a CO2-capture system would benefit from the electrochemical reduction of CO2 to produce carbon monoxide fuel, which would act as a carbon recycle technology, enabling optimisation of flexible generation using CO as an energy store.

CNT and graphene

Carbon nanotubes (CNT) have unique properties of aspect ratio, mechanical strength, and high electrical and thermal conductivity. New applications emerging in this $4 billion industry involve the inclusion of CNT in polymer composites, biomedical devices, energy storage and catalysts. The chemical vapour deposition (CVD) production method for CNT requires a simple gaseous feed; a coal tar feedstock would require much higher energy input, but gases such as carbon monoxide derived from coal gasification are suitable precursors for CNT.

Graphene is a relatively new single atom thick planar material of exceptional strength and conductivity. Graphene is under intensive research for new battery electrode designs, but recent commercial uses include water impermeable coatings, and incorporation within LED bulbs to dissipate heat and lining of satellite mirrors to reduce distortion. A graphene sheet has been prepared from raw coal using an electrochemical process. Coal possesses an amorphous-crystalline structure which has been exploited in the manufacture of graphene quantum dots, changing the coal feed to adjust the required size range. The synthesis of graphene directly from a coal substrate is a brand-new direction in the development of graphene.

Non-energy sectors for coal feedstocks are experiencing a period of sustained growth covering large- scale users through to specialist applications. Chemicals manufacture is set to continue to be the greatest consumer of coal outside of power, steel, and concrete with demand continuing to rise even though there are environmental challenges in the production, use and disposal of products. Contraction of the blast steel sector means feedstock constraints for tar-based products, while the gasification industry is to come under increasing scrutiny by regulators in China due to recent initiatives to lower industrial emissions. There is a history of coal as a mineral resource, but that has gained impetus recently with new techniques to extract REE from coal and coal wastes, at a time of constrained supply of critical metals. There is an increase in uses for a diverse range of activated carbons for the purification and adsorption of contaminants, and pitch carbon may have a more significant role in industrial electrodes. The application of lignite humates in agriculture may become a significant alternate use for lignite at a time of decline in demand for thermal power. Enhancing depleted soils may become more urgent over the next few years amid predictions of increasing global temperatures (Sevellec and Drijfhout, 2018). In the specialised carbon sector, the deployment of pitch fibre is set to increase, partially replacing steel and aluminium in ground transportation adding to established applications in aerospace. The molecular structure of coal has advantages in synthesising certain nanomaterials, and the drive to reduce costs is raising interest in coal as a suitable feedstock in a rapidly growing industry.

12. APPENDIX: SEPARATION OF RARE EARTH ELEMENTS IN MINERAL ORES

Ore processing in a conventional REE mining operation consists of physical separation followed by smelting. The main physical separation units designed for an REE content of 2% to 5% involve magnetic and physical/gravimetric processes. The processing plant represents a markedly different strategy to that adopted for REE extraction from coal that possesses a lower initial content of rare earth elements (Hagen, 2016).

Figure 23 REO beneficiation plant flow sheet, Bayan Obo ore deposit (Zhi, 2014)

The REE ore beneficiation plant at Bayan Obo mine (Figure 23) shows that REO are concentrated using a hydro-dynamic separation strategy. The processing of REO ore has some similarity to a coal beneficiation plant. Following crushing and grinding the REE ore passes through a low intensity magnetic separator (LIMS) to remove magnetic gangue, and then high intensity (HIMS) to concentrate paramagnetic REE. The REE rich portion is then passed to a series of flotation separators using selective collection and depressant chemicals to raise the concentration of REE, a process that incorporates hot flotation conditions. The concentrate obtained from this treatment plant has between 30% and 60% REO content.

The intermediate rare earth concentrate is then purified using acid and solvent leaching methods. Within the smelter the process stream is first treated with hydrochloric acid (HCl) to dissolve out calcite (CaCO3). The insoluble residue containing REE is then roasted: for example, sulphuric acid treatment at 300°C to form sulphates or alternatively by ammonium chloride at 500°C to form REE chlorides. After cooling, the material is leached with hydrochloric acid to dissolve trivalent rare earths (La, Pr, Nd, Sa, Eu, and Gd), leaving behind a cerium concentrate which can be refined. Europium can be separated from other lanthanides by reduction to its divalent form, and the remaining dissolved lanthanides are subsequently separated by solvent extraction techniques. A more detailed description of the typical mineral ore purification process options and relevance to coal REE extraction is provided by Zhang and others (2015), with the relevant recommendation that roasting techniques are more appropriate for coal REE than magnetic separation and flotation techniques (Zhang and others, 2015).

Source: Dr Ian Reid - IEA Clean Coal Centre

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