Coal ash includes a number of by-products produced from burning coal, including:
- Fly Ash, a very fine, powdery material composed mostly of silica made from the burning of finely ground coal in a boiler.
- Bottom Ash, a coarse, angular ash particle that is too large to be carried up into the smoke stacks so it forms in the bottom of the coal furnace.
- Boiler Slag, molten bottom ash from slag tap and cyclone type furnaces that turns into pellets that have a smooth glassy appearance after it is cooled with water.
- Flue Gas Desulfurization Material, a material leftover from the process of reducing sulfur dioxide emissions from a coal-fired boiler that can be a wet sludge consisting of calcium sulfite or calcium sulfate or a dry powered material that is a mixture of sulfites and sulfates.
Other types of by-products are:
- fluidized bed combustion ash,
- cenospheres, and
- scrubber residues.