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What are Coal Combustion By-Products (CCBs)?


Synthetic Gypsum is a by-product of the flue gas desulfurization (FGD) process, commonly known as "scrubbing." In a scrubber, sulfur oxides are removed from combustion gases by mixing the gases with finely-ground sorbents, usually limestone (calcium carbonate) or lime (calcium oxide).

Gypsum Pond
Gypsum storage site - Enlarge Photo

Gypsum material
Gypsum material - Enlarge Photo

Gypsum Sample from SEM
Gypsum crystals and scrubber solids as viewed via SEM at 2000x magnification - Enlarge Photo

There are a number of different types of scrubbers; an excellent description of each can be found within the IEA Clean Coal Technologies database. However, all of the different scrubber types are designed to maximize contact between sorbent particles (e.g. limestone) and the gaseous sulfur oxides within the flue gas. The desulfurization reactions that take place in the scrubber mainly form calcium sulfite hydrate, and calcium sulfate hydrate (gypsum). If the by-product comprises mainly calcium sulfite hydrate with some gypsum present, it is known as "scrubber sludge." If the byproduct is mainly gypsum, it is referred to as "FGD gypsum" or "synthetic gypsum," and is produced by forced oxidation of calcium sulfite hydrate.

Fluidized bed combustion (FBC) boilers burn coal in a fluidized bed of limestone, which captures most of the sulfur in the flue gas as calcium sulfate. The bed material is continuously removed from the boiler and mainly comprises sand-sized particles of calcium sulfate and lime. This byproduct is referred to as "spent bed". The FBC unit also produces a finely divided byproduct similar to fly ash that comprises small particles of spent bed and ash from the coal.

FBC Ash - Spent Bed
FBC Spent Bed Material - Enlarge Photo


The by-products are scrubber solids (gypsum) which are then collected ...

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Products from Gypsum are used in:
manufacturing of wallboard - agricultural uses ...

Products include ...

"Gypsum, one of the most widely used minerals in the world, literally surrounds us every day. Most gypsum in the United States is used to make wallboard for homes, offices, and commercial buildings; a typical new American home contains more than 7 metric tons of gypsum alone. Moreover, gypsum is used worldwide in concrete for highways, bridges, buildings, and many other structures that are part of our everyday life. Gypsum also is used extensively as a soil conditioner on large tracts of land in suburban areas, as well as in agricultural regions. (Courtesy of USGS)".

  • Tell Me More About Gypsum - Statistics, Publications and Information from the USGS
  • FGD Products.org - A web site "expanding awareness of the beneficial use of FGD Gypsum" developed by the ACAA Educational Foundation

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