Coal combustion by-products
(CCBs) include several types of materials which are left
over from the burning of coal: fly ash, bottom ash, boiler
slag and flue gas desulfurization (FGD) materials (either
wet or dry). In the past, CCBs were treated largely as a
waste. However, since the 1960's, many applications have
become identified such as using CCBs as a substitute for
portland cement, in cement manufacturing, in roofing tiles,
structural fills, sheetrock, and fertilizers to name a few,
which placed these residues into the category of a "product"
and the term has become commonly used as coal combustion
products or "CCPs."
CCPs contain basic mineral
elements which make them similar to the earth's crust:
silicon dioxide, aluminum oxide, iron oxide, calcium oxide
and trace amounts of sulfur trioxide, sodium oxide and potassium
oxide. Thus, they are an excellent replacement for natural
materials.
While the term "by-product" may include residues from other
combustion processes, CCPs do not include products
derived from the burning of waste; municipal, industrial,
or commercial garbage; sewage sludge or other refuse, or
both. It does not include ash from derived fuels; wood;
wood waste products; rice hulls; agricultural waste; or
other non-coal fuels or other fuels blended with coal or
some combination thereof.
CCPs are catagorized by the process
in which they are generated in the coal plant and their
application.
Fly ash
- Fly ash is the very small particle mineral residue that
results from the burning of powdered coal in utility boilers.
The individual particles are very small, like talcum powder,
and are carried up and out of the boiler in the flow of
exhaust gases leaving the boiler after the coal is consumed.
Hence the term " Fly Ash". The Fly Ash particles are removed
from the stack gasses using Electrostatic Precipitators,
FGD systems or Bag Houses and are collected and stored dry
for recycling. Fly Ash is a pozzolan: a silica, alumina,
and calcium based material which, in the presence of water,
will chemically combine with the free lime contained in
the fly ash and produces a cementitious material with excellent
structural properties. Some Fly Ash contains enough calcium
compounds to be self-hardening when mixed with water. Applications:
All Fly Ash can be used as a direct replacement for portland
cement in making concrete, in addition to many other applications.
Bottom Ash
- Bottom Ash is the coarse, solid mineral residue that results
from the burning of coal in utility boilers. The individual
particles are much larger than Fly Ash particles and fall
down through the air flow to the bottom of the boiler. Hence
the term " Bottom Ash". The material is removed from the
bottom of the boilers either in a wet or dry state and is
transported to handling areas by conveyor or pipe. Bottom
Ash has a similar chemical composition to Fly Ash, but is
produced in size grades ranging from fine sand to medium
gravel. Because of the larger sizes, it does not have any
cementitious properties. It is utilized in a variety of
markets as an aggregate or filler material in construction
projects and building products. One other product, Boiler
Slag, is Bottom Ash produced by some older boilers, where
the material is actually melted in the boiler, quenched
in the bottom hoppers, and becomes hard and glassy. Applications:
This material has found an excellent market as a low silica
blasting grit.
Flue gas desulfurization
(FGD) material - FGD
Material is the solid material resulting from the removal
of Sulfur Dioxide gas from the utility boiler stack gasses
in the Flue Gas Desulfurization process. The material is
produced in the flue gas scrubbers by reacting slurried
limestone or lime with the gaseous Sulfur Dioxide to produce
Calcium Sulfite. Some utility plants further oxidize the
Calcium Sulfite to Calcium Sulfate (which is the same thing
as natural Gypsum). The Calcium Sulfite Product (" Gypsite
") has shown limited market potential at this time and is
usually disposed. FGD " Gypsite " can be stabilized with
Fly Ash and is used for roadbase material. FGD Gypsum, however,
is 95% pure Calcium Sulfate and has found many more uses.
Both materials are produced wet in the scrubbers and are
then thickened and dried for handling and/or recycling.
Applications: FGD Gypsum is used in many parts of
the country in any application involving natural Gypsum,
including sheetrock production, agricultural fertilizer,
soil amendments, and Portland cement production.
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