The financial uncertainties inherent in establishing capital-intensive coal-to-chemicals plants, the products of which can be vulnerable to those from competing processes based on the oil and/or natural gas feedstocks, has meant that the take-up has been limited. This provides a strong example of the conflict between strategic longer-term planning and short-term expediency. The early technology champions were South Africa and the USA and their two commercial-scale gasification-based coal conversion projects are described below. Subsequently, China has taken a major initiative to establish a massive coal-to-chemicals industrial programme. The description and assessment of this initiative is provided separately in Section 3, in recognition of its unique approach.
2.1 EARLY TECHNOLOGY CHAMPIONS
2.1.1 South Africa
The original extensive commercial deployment was in South Africa, arising from the period when imports of oil by that country were politically problematical. Currently, South Africa, through Sasol, operates the world’s only gasification-based commercial coal-to-liquid (CTL) facility at Secunda with an output capacity of 160,000 bbl/d of oil equivalent (see Figure 2). All the synthetic fuels are used to meet growing domestic demand for petroleum products, with about 30% of South Africa’s petrol and diesel needs being met through coal conversion (Sasol, 2010).
Figure 2 Schematic of the Sasol coal-to-liquids commercial scale plant (NETL, 2011a)
At the two units in Secunda, which began operation in the early 1980s, pressurised Sasol/Lurgi fixed bed dry bottom gasifiers are used to produce syngas from high ash content, high ash melting point coal in the presence of steam and oxygen (Van Nierop and others, 2000). The average syngas production rate is 1.5 million m3/h, with a typical composition of 58% H2, 29% CO, 11% CH4, 1% CO2. After cooling, the various condensates that are removed from the syngas provide co-products such as tars, oils and pitches, together with ammonia, sulphur, cresols and phenols, with the pitch being converted into coke in an anode coke plant.
Once purified, the syngas is sent to a suite of nine Sasol Fischer-Tropsch (FT) Advanced Synthol (SAS) reactors where it is reacted in the presence of a fluidised iron-based catalyst at elevated pressure (~2.5 MPa) and a temperature of about 350°C (Dry, 2002). This produces further by-products, namely reaction water and oxygenated hydrocarbons, together with a wide range of hydrocarbons in the C1-C20 range (Gibson, 2007). These hydrocarbons are cooled in the plant until most components become liquefied. Differences in boiling points are utilised to yield separate hydrocarbon-rich fractions and methane-rich gas. Some of the methane-rich gas (C1) is sold as pipeline fuel gas, while the rest is sent to a reforming unit, where it is converted back to syngas and re-routed to the reactors. The C2-rich stream is split into ethylene and ethane. The ethane is cracked in a high-temperature furnace yielding ethylene, which is then purified. Propylene from the light hydrocarbon gases is purified and used in the production of polypropylene. Within this stream there are also large quantities of olefins in the C5 to C11 range. Most of this oil stream is routed to a refinery where liquefied petroleum gas, propane, butane, fuel oil, paraffin, petrol and diesel are produced. The oxygenates in the aqueous stream from the synthesis process are separated and purified in the chemical work-up plant to produce alcohols, acetic acid and ketones including acetone, methyl ethyl ketone and methyl iso-butyl ketone. These oxygenate chemicals are either recovered for chemical value or are processed to become fuel components. Of the olefins, ethylene, propylene, pentene-1 and hexene-1 are recovered and sold into the polymer industry. Surplus olefins are converted into diesel to maintain a gasoline-diesel ratio to match market demand. The annual synfuels output from these High Temperature FT plants in 2002 was about 8 Mt.
At Sasolburg, from 1955 when it began operation until 2004, the coal-based synthesis feed gas was reacted in the Sasol slurry phase distillate (SSPD) reactors at a lower temperature than is the case in the SAS reactors, primarily producing linear-chained hydrocarbon waxes and various liquid products (Dry, 2002). Residual gas was sold as pipeline gas, while lighter hydrocarbons were hydro-treated to produce either pure kerosene or paraffin fractions. Ammonia was also produced and either sold directly or utilised downstream to produce explosives and fertilisers. Around 40 Mt/y of low-grade coal were converted into liquid fuels, gas, and other products.
The SSPD technology is also the technology favoured by Sasol for the commercial conversion of natural gas to synfuels. It produces a less complex product stream than the SAS technology and products can readily be converted to high quality diesel. In 2004, Sasol switched feedstock at Sasolburg to natural gas imported from Mozambique (Sasol, 2010).
2.1.2 USA
The second commercial-scale operation, the Great Plains Synfuels Plant, was established in Beulah, North Dakota and has been in operation producing synthetic natural gas (SNG) from lignite for 35 years. It remains the only coal-to-SNG (CTSNG) facility in the USA (NETL, 2011b). The plant also produces high purity CO2, which is distributed through a pipeline to end users in Canada for enhanced oil recovery (EOR) operations. Other products include anhydrous ammonia, ammonium sulphate, krypton, xenon, de-phenolised cresylic acid, liquid nitrogen, phenol, and naphtha, the latter being burned as fuel in plant boilers.
The plant began operation in 1984. However, after the facility was built, natural gas prices continued to drop impacting on the profitability of the plant, which led the US DOE to purchase the plant for US$1 billion in 1986. The US DOE sold the plant in 1988 to Basin Electric Power Cooperative, which owned the adjacent power plant and has since operated the facility through its Dakota Gasification Company subsidiary. Over the years, various studies to consider an expansion of this technological base were undertaken but none were ever implemented. The recent and rapid development of the shale oil fracking approach from which shale gas is a plentiful low cost by-product, suggests that there is little likelihood of expanding the Grand Plains SNG project.
Figure 3 Simplified plant process of the Great Plains Synfuels Plant (NETL, 2011b)
Figure 3 provides a schematic of the overall process. Some 14,500 tonnes per day (t/d) of lignite are gasified with oxygen and steam in 14 Lurgi Mark IV gasifiers to produce a wide range of gaseous raw products. These exit the gasifiers and are then cooled, removing tar, oils, phenols, ammonia and water via condensation from the gas stream. These products are then purified, transported, or stored for later use as a fuel for steam generation. After cooling, the gas is further treated to remove impurities, then sent to a methanation unit where CO and most of the remaining CO2 is reacted over a nickel catalyst with free H2 to form CH4, which is then further cooled, dried, compressed and transported by pipeline to the eastern United States.
2.2 OPPORTUNITIES FOR AUSTRALIA
Australia is a leading international coal exporter, supplying high quality steam and metallurgical coal, particularly to Asia. At the same time, there should be great potential for upgrading their low-quality brown coal, which represents an enormous energy resource but in its current form is almost unsaleable. There have been ongoing discussions with companies from both Japan and China to use gasification-based conversion technologies to produce high-grade high-value products, to be shipped back to these two target customers, although no commercial deals are yet in place. The latest possible venture is the Kawasaki Hydrogen Road, for which the plan is to produce hydrogen from mined brown coal and send it in liquefied form to Japan in custom-made ships. It would then be used for a variety of purposes via conversion into electricity and thermal energy. The CO2 emitted in the brown coal conversion process would be captured and stored in geological formations in Australia. Kawasaki, Iwatani, J-Power and Shell Japan are backing the project, with the Victorian and Commonwealth governments committing A$1 million and A$2 million respectively to the front-end engineering design (FEED) study (The Australian, 2016).
The 10 largest coal producers and exporters in the Indonesia:
- Bumi Resouces
- Adaro Energy
- Indo Tambangraya Megah
- Berau Coal
- Bukit Asam
- Baramulti Sukses Sarana
- Harum Energy
- Mitrabara Adiperdana
- Samindo Resources
- United Tractors
Source: IEA Clean Coal Centre
