Increasing use of coal as boiler fuel means increasing amounts of coal combustion by-products (CCB). The disposal or use of the by-products is increasingly important in environmental and financial terms.
Coal combustion products are generally grouped into four main classes:
- Fly and Bottom Ash
- Boiler Slag
- Fluidized Bed Combustion (FBC)
- Flue Gas Desulphurisation (FGD)
Fly and bottom ashes are sometimes classified as Class F or C ashes. They are distinguished by the free lime content (CaO%). Class F have less than 10% CaO and Class C has more than 10% CaO.
FBC ash and FGD solid by-products result from clean coal technologies. Both use limestone to generate CaO to capture SO2.
FBC ash is produced when coal and coal waste materials are burned with limestone in a fluidized bed boiler. Sulphur oxide is precipitated as gypsum (CaSO4) along with unreacted lime in a strongly alkaline ash.
FGD solids or sludges are produced when lime or limestone slurries are injected into the exhaust gas downstream of the boiler. Sulphur oxides are precipitated as either gypsum or calcium sulfite (CaSO3).
• Replacement for Portland Cement
The most conventional use of coal ash is as a partial replacement for Portland cement in the manufacture of concrete.
Fly ash is added to cement to produce the concrete mixture.
Fly ash improves the quality of concrete and cost effectiveness.
- Compensates for fines not found in some sands and enhances pumpability and finishing.
- Results in a more predictable and consistent product.
- Improves flowability.
- Enables custom design mixtures to meet end use requirements.
The fly ash needs to be high quality. In general it must have chemical and physical characteristics as described below.
- High proportion of reactive oxides of silica and alumina
- Low percentage of carbon (<3%)
- Low percentage of magnesium oxide
- Low percentage of alkalis, potassium and sodium oxide
- Low percentage of phosphorus
- Low percentage of sulphur
- Greater than 75% passing 45 micron
- High percentage of spherical shape
- Acceptable color
• Landfill
Examples of landfill uses include:
- Filling of underground mine voids
- Subsidence control in underground mines
- Pit filling to achieve approximate original contour in surface mines
- Barrier to acid mine drainage formation and transport
- Alkaline amendment to neutralise acid-producing rock
- Soil substitute or additive
The suitability of by-products depends on their physical and chemical characteristics and the intended landfill conditions. Geological and hydrological conditions affect the type of by-product that can be used in different environment.
Example is by-products containing significant amounts of soluble salts are not placed in areas where significant groundwater flux occurs.
By-products have permeable or impermeable properties. Bottom ashes tend to have hydraulic conductivity of gravel while most fly ashes have the hydraulic conductivity of soil-like material. Class F ashes tend to be more permeable than class C ashes due to texture differences. FGD solids and FBC ashes have very low hydraulic conductivity and are virtually impermeable like strong cement.
The trace element concentration is of particular importance with respect to environment. This is both in the by-product and leachate concentrations.
• Other uses
There are many other cited uses of CCB’s.
- Road base
- Asphalt filler
- Blasting grit
- Grouting
- Wallboard
- Waste stabilisation
- Paints and coatings additive
- Fertilizer manufacture