What is Coal Ash?

What is Coal Ash?

Coal combustion byproducts (CCBs) are considered to be four distinct and extremely different materials.

FLY ASH

Fly ash is the finest of coal ash particles. It is called "fly" ash because it is transported from the combustion chamber by exhaust gases. Fly ash is the fine powder formed from the mineral matter in coal, consisting of the noncombustible matter in coal plus a small amount of carbon that remains from incomplete combustion. Fly ash is generally light tan in color and consists mostly of silt-sized and clay-sized glassy spheres. This gives fly ash a consistency somewhat like talcum powder. Properties of fly ash vary significantly with coal composition and plant-operating conditions.

Fly ash can be referred to as either cementitious or pozzolanic. A cementitious material is one that hardens when mixed with water. A pozzolanic material will also harden with water but only after activation with an alkaline substance such as lime. These cementitious and pozzolanic properties are what make some fly ashes useful for cement replacement in concrete and many other building applications.

BOTTOM ASH

Coal bottom ash and fly ash are quite different physically, mineralogically, and chemically. Bottom ash is a coarse, granular, incombustible byproduct that is collected from the bottom of furnaces that burn coal for the generation of steam, the production of electric power, or both. Bottom ash is coarser than fly ash, with grain sizes spanning from fine sand to fine gravel. The type of byproduct produced depends on the type of furnace used to burn the coal.

BOILER SLAG

Boiler slag is coarser than conventional fly ash and is formed in cyclone boilers, which produce a molten ash that is cooled with water. Boiler slag is generally a black granular material with numerous engineering uses.

FGD GYPSUM

Flue gas desulfurization (FGD) gypsum is also known as scrubber gypsum. FGD gypsum is the byproduct of an air pollution control system that removes sulfur from the flue gas in calcium-based scrubbing systems. It is produced by employing forced oxidation in the scrubber and is composed mostly of calcium sulfate. FGD gypsum is most commonly used for agricultural purposes and for wallboard production.

More details: What is coal ash? (source)

Video - How a coal power station works?

Video - How a coal power station works?

Coal shipment from coal jetty transfered to the coal stockyard. There is a large machine called stacker reclaimer, arranged the coal into the storage piles. A series of conveyor belt transport the coal to the generating plant where it goes to the bunker hopper for temporary storage before it was introduced to bowl milling. Coal will be pulverized or; coal is grinded to a fine powder more less 70 microns prior to burning. The pulverized coal is mix with air and is feeded to the furnace combustion that is surrounding by boiler tube filled with purified water. The burning coal heats the purified water inside the boiler tube to the steam. The steam is transfered under high pressure and high speed throughout the pipe turbine. This pressure flow pushes the blade of turbine to spin. Turbine is connected to the generator where the spinning of the turbine will causes a shaft to turn inside generator and create electricity. The producing of the electricity that can be step up voltage through the station transformer and send from the station across transmission line. The steam from the turbine (exhausted) condense back to the purified water using cooling water from the forebay or seawater and pump back to the boiler where it's reheated back to continue the process again.

How a coal-fired power plant works
General coal-fired power plant view