Coal Combustion Waste Management at - DOE - Fossil Energy ...

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1 INTRODUCTION
1.1 DESCRIPTION OF COAL COMBUSTION WASTES
Coal is fired in boilers to heat water in order to generate high-pressure steam, which in
turn drives generators that produce electricity. Electric utilities generally use pulverized coal
boilers, where the coal is crushed to fine particles and blown into the boiler furnace for
combustion. Another type of furnace, used mostly by smaller power generators, is the fluidized
bed, which can burn coal of poorer quality or grade and be used for the economical production of
electricity. The uncombusted residue is called coal combustion waste (CCW), coal combustion
by-product (CCB), or coal combustion residue (CCR).
CCWs can be classified as large-volume CCWs and low-volume CCWs. The largevolume
CCWs include fly ash, bottom ash, boiler slag, and flue gas emission control (or flue gas
desulfurization [FGD]) waste. Fly ash is a silt-sized residue (typically between 10 and 100 μm),
composed mainly of amorphous spherical particles, that is transported from the combustion
chamber by exhaust gases and collected by particulate emission control devices before entering
the boiler stack. Bottom ash is a dry, coarse material with sintered and agglomerated amorphous
particles taken from the bottom of the boiler furnace either in its dry form or as a slurry (via the
addition of water). Boiler slag is also taken from the bottom of the furnace, but emerges in a
molten form; it is usually quenched immediately in water and forms large, glassy pellets. Flue
gas desulfurization residues are generated by units that remove sulfur dioxide (SO2) from flue
gas. “Wet” FGD systems are found most frequently at large coal-burning utilities; these systems
are installed downstream of the particulate control devices and produce residues that consist
mainly of calcium sulfate or calcium sulfite salts. However, a small but growing percentage of
FGD residues come from “dry” FGD systems or fluidized bed combustion (FBC) systems that
remove sulfur upstream of the particulate control devices; therefore, these “FGD residues” are
inseparable mixtures of fly ash, bottom ash, and calcium sulfates/sulfites. Fly ash and “wet”
FGD comprise the largest quantity of these four waste types.
Low-volume CCWs result from supporting processes that are ancillary to the combustion
and power generation processes. They include the following:
Coal pile runoff,
Coal mill rejects/pyrites,
Boiler blowdown,
Cooling tower blowdown and sludge,
Water treatment sludge,
Regeneration waste streams,