Fly ash is the fine particles carried in the output of the furnace in a power station. Fly ash is removed by precipitation before the chimney stack. Fly ash consists mainly of minerals and generally has a large percentage of aluminium and silica compounds, as well as small concentrations of rare earths. This article is compiled from information presented at a recent conference hosted by the Fossil Fuel Foundation.
During the 2014/15 financial year, 119,2-million ton of coal was consumed, producing 34,4-million t (28,9%) of ash. Only about 7% of the Eskom (South African electricity public utility) ash is sold from six of the 13 Eskom coal-fired power stations, compared to 50 to 60% in other countries. Usage or commercial application is restricted by several factors, the main being legislation, and others being lack of knowledge of the use of fly ash. The high percentage of aluminium and silica compounds give fly ash its useful properties. Fly ash is distinct from bottom ash which consists of large lumps of clinker formed when the combustion products deposit on the surface of the furnace, or when fly ash conglomerates. The composition of a typical South African fly ash is shown in Table 1 (Matla power station).
Eskom’s policy on fly ash
In 2015, South African authorities escalated the use of ash to a strategic priority. It was envisaged that the sale of ash could allow for improved business development, increased infrastructure development, job creation, create social uplift and allow for skills development.
Power station systems may impact the availability of ash. Eskom power stations operate on a zero liquid effluent discharge policy. This means that all water that enters an Eskom site is retained on site, through an ever cascading quality. Saline effluents and the poorest quality water are disposed of on the ash handling facility, where the ash acts as a salt sink for the effluents. It is estimated that 74% of the fresh ash produced is required for effluent treatment. If this sink capacity is not available, leaching may occur. Thus approximately 19% of the fresh ash produced is available for reuse. If larger volumes of ash are required alternative water treatment technologies will be necessary.
In order to manage the increased utilisation of ash, Eskom has developed an ash strategy, which includes:
- The adoption of the cost avoidance rather than the revenue recovery model
- The development of an ash strategy per power station
- Consultation with the Department of Environmental Affairs (DEA) and relative government departments, in order to change the ash classification
- Submission of exclusion documents to government, on behalf of industry, to ease the legislative constraints on users
- Unlock legislative constraints
- Explore all possible ash utilisation technologies and applications
- Collaboration with industrial stakeholders
Eskom’s ash strategy aims to use ash to benefit from an associated avoided cost linked to the handling and storage of the ash. In order to permit ash utilisation a Regulation 9 submission has been handed to DEA to allow for the exclusion of the hazardous classification of ash, when used in brick making, cement, road construction, soil amelioration and mine backfilling.
Fig. 1: Fly ash dump in India.
Incentive and policies
In 2008, the DEA declared fly ash to be a “hazardous waste”, placing severe restrictions on the use of fly ash and the development of industries based on fly ash. This classification posed enormous problems for users of fly ash. The hazardous classification of ash in South Africa is unique worldwide and the waste type classification is the strictest in the world. Internationally ash is classified as a by-product or a resource – depending on the country.
The legislation was later changed to exclude ash used as a product from the definition of waste, which allowed the industry to develop. Further consultation is under way to relax the more stringent sectors of the legislation to allow a wider range of uses for fly ash, without the necessity to comply with waste handling restrictions.
Uses of fly ash
The advantage of fly ash is that it has already been mined and only needs to be transported to the site of use. There are 150 possible uses for fly ash. Some of them are being put to use already, or being considered in South Africa, are described below, as well as emerging potential uses.
Cement, concrete and brickmaking
This is the most well-known application of fly ash in this country, and an industry that has been thriving for many years. Bottom ash is used directly in the manufacture of “clinker” bricks, and bricks with a content of up to 70% bottom ash are common.
Fig. 2: Bricks made from fly ash.
Fly ash is not a hydraulic material and, strictly speaking, it does not hydrate to form cementitious hydrates. It is a pozzolan, which is defined as a siliceous or a siliceous material which, though itself possessing little or no cementitious value, will, in a finely divided form and in the presence of moisture, react chemically with calcium hydroxide (Ca(OH)2) at ambient temperature to form compounds with cementitious properties. In concrete, the calcium hydroxide for the reaction is produced by the portland cement from which it is liberated during hydration.
Fly ash is used extensively in cement and in concrete in this country, the general cement mixture comprising up to 45% fly ash by weight. Fly ash may be added to the final product or ground together in the cement mill. Fly ash can constitute between 15 to 35% by weight of the cement, although higher percentages can be used in bulk concrete placements.
Geopolymers
Geoploymers are formed by an alkaline liquid reacting with the silicon (Si) and the aluminium (Al) in a source material of geological origin or in by-product materials such as low-calcium fly ash to produce binders. Because the chemical reaction that takes place in this case is a polymerisation process, the term “geopolymer” was coined to represent these binders. What happens when you mix a batch of geopolymer cement is an alkali activator breaks down the chemicals of an alumino-silicate fly ash material then rebuilds it in long polymer chains, basically stone polymer. When it sets it is as hard and strong as a good concrete, and much more flexible than most concrete, making it crack resistant.
Water is released during the chemical reaction that occurs in the formation of geopolymers. This water, expelled from the geopolymer matrix during the curing and further drying periods, leaves behind nano-pores in the matrix, which provide benefits to performance. Geopolymers are also resistant to corrosion in the seawater environment, as they do not contain calcium. One of the big advantages of geopolymers is the saving of energy (and CO2 emissions) in the production of the component material.
Geopolymer cement is about the same weight as conventional cement and can also be used as a component of aerated concrete, a lightweight product (quarter of the weight of normal concrete) with high thermal insulation, fire resistance and sound insulation properties. It is produced as blocks, panels and precast units, or composite boards such as ceiling board or dry wall panels. The material has been used for pre-fabricated building components, producing structures at a much lower cost and shorter assembly time than conventional structures, but with the same strength. The market is unfortunately developing slowly. One of the biggest reasons is the innate conservatism of engineers.
The industry has a lot of experience with concrete, and geopolymers are fairly new. The standards on pure geopolymer concrete only came out recently.
Zeolites
Zeolites are crystalline solids structures made of silicon, aluminium and oxygen that form a framework with cavities and channels inside where cations, water and/or small molecules may reside. They are often also referred to as molecular sieves. Because of their unique porous properties, zeolites are used now in a variety of applications. They are used in petrochemical cracking, water softening and purification, in the separation and removal of gases and solvents, agriculture, animal husbandry and construction.
They are particularly useful in the removal of heavy metals from water. It has been shown that South African fly ash can be used as a feedstock for zeolite synthesis due to the dominance of aluminosilicate and silicate phases in the ash. Production has been demonstrated at laboratory level.
Acid mine drainage (AMD) treatment and mine backfilling
Acid mine water is caused by the action of oxygen and water on iron pyrites in old mine shafts. It has been shown that fly ash can be used to remove AMD contaminants from mine water. In addition, the insoluble solid residue from the process has also been shown to have the necessary strength to be useable as a backfill material for mines, providing a solution to disposal of fly ash. This could provide an integrated solution to the rehabilitation of old mines.
Agricultural use
Fly ash has proved useful in the amelioration of poor quality agricultural land and the rehabilitation of mine land. The success depends on the composition of the soil. Fly ash has great potential in agriculture due to its efficacy in modification of soil health and crop performance. The high concentration of elements (K, Na, Zn, Ca, Mg and Fe) in fly ash increases the yield of many agricultural crops. Fly ash can be used as a substitute for agricultural lime for treating acid soils.
Fillers and road surface applications
Fly ash is being used as a component of both underlying layers of tar roads and road embankments, as well as a stabilising component of un-tarred roads. Investigation into the use of fly ash as a filler for asphalt is ongoing.
Mining of coal ash for metals and rare earths
The chemical composition of fly ash is similar to bauxite ore. Alumina (Al2O3) is present as the second major constituent in fly ash, after silica (SiO2) and is therefore amenable to metallurgical and chemical processes of recovery. Fly ash commonly contains approximately 30% aluminates as a component, and at a production of 34 Mt of fly ash annually, this gives some 13 Mt of aluminium ore which could be processed to yield aluminium metal. South Africa does not have a natural aluminium ore deposit. Studies have shown that it is possible to extract up to 85% of the aluminium contained in fly ash using commercially realisable processes. It is possible to produce aluminium directly by a leaching process – i.e. no further processing.
Fly ash also contains small quantities of rare earths, hard coal fly ashes (CFAs) have been estimated to contain 445 ppm rare earths on an average global basis and it has proved to be possible to extract these by metallurgical processes at a reasonable cost. The US has found fly ash sources with REE concentrations of >1000 ppm which are considered suitable for extraction. Fly ash is considered as a strategic source of RE in the US.
Rubber and plastic fillers
The spherical shape of the aluminium and silica compounds which form the majority of fly ash components can be used for fillers and stiffeners in rubber products. Carbon black is commonly used as a filler, but has a disadvantage of affecting the colour, whereas fly ash components have little or no effect on the final colour of the product. In addition to increasing strength, fly ash increases the elasticity of the product, which aids moulding and curing. Fly ash used in silicone rubber composite insulators improves the dielectric strength, resistivity and tensile strength, while retaining the hydrophobic properties.
The spheres of fly ash add stiffness and compressive strength to plastic compounds, but at the expense of increased brittleness. On the other hand, some processors who use ash say the spheres act like small ball bearings to improve throughput. Fly ash can be used as an additive in recycled polyethylene in the manufacture of building products.
The 10 largest coal producers and exporters in Indonesia:
- Bumi Resouces
- Adaro Energy
- Indo Tambangraya Megah
- Berau Coal
- Bukit Asam
- Baramulti Sukses Sarana
- Harum Energy
- Mitrabara Adiperdana
- Samindo Resources
- United Tractors
Source: Mike Rycroft, EE Publishers
