69
3
Coal Combustion
Waste Materials
J. Groppo
CONTENTS
3.1 Coal Combustion ............................................................................................70
3.1.1 Power Production from Coal Combustion ..........................................70
3.1.2 Coal Mineralogy and Chemistry ........................................................ 70
3.1.3 Coal Utilization .................................................................................. 71
3.2 Pulverized Coal Combustion By-Products ..................................................... 74
3.2.1 Fly Ash and Bottom Ash .................................................................... 76
3.2.1.1 Fly Ash Mineralogy and Chemistry .................................... 76
3.2.1.2 Fly Ash Production and Utilization ..................................... 78
3.2.2 Wet Flue Gas Desulfurization (FGD) .................................................85
3.2.2.1 Wet FGD By-Product Mineralogy and Chemistry ..............85
3.2.2.2 Wet FGD By-Product Utilization .........................................86
3.2.3 Dry FGD ............................................................................................. 89
3.2.3.1 Dry FGD By-Product Mineralogy and Chemistry ..............89
3.2.3.2 Dry FGD By-Product Utilization .........................................89
3.3 Fluidized Bed Combustion (FBC) .................................................................. 92
3.3.1 Power Production with FBC ...............................................................93
3.3.2 FBC Mineralogy and Chemistry ........................................................ 93
3.3.3 FBC Ash Utilization ...........................................................................95
3.3.3.1 Mining Applications ............................................................95
3.3.3.2 Waste Stabilization ..............................................................95
3.3.3.3 Structural Fill/Road Base ....................................................95
3.3.3.4 Agriculture ........................................................................... 96
3.3.3.5 Cement Manufacture............................................................96
3.3.3.6 Biomass Ash ........................................................................97
3.3.3.7 Utilization Statistics ............................................................. 97
References ................................................................................................................97
70 Conversion of Large Scale Wastes into Value-added Products
3.1 COAL COMBUSTION
3.1.1 PoWer ProDuction from coal combustion
Coal is the most widely used fuel for electrical generation in the world, responsible
for more than 40% of the worlds electricity generation (IEA, 2011). Although the
technologies used to accomplish this task are varied, the principles are basically
similar. Coal is burned in a boiler or furnace to generate heat in order to produce
steam. The steam is generated by circulating water through a network of tubes located
throughout the boiler. Superheated steam is then directed into a turbine where it
expands to turn the turbine shaft, which is coupled to the shaft of a generator, which
turns to produce the electrical current. Condensed water and low-pressure steam are
recirculated back to the boiler where they are reheated to produce additional steam
in a continuous closed heating loop.
3.1.2 coal mineralogy anD chemistry
Since coal is the primary component of most of the by-products produced during
combustion, some understanding of coal mineralogy and chemistry is a prerequisite
to understanding the characteristics of coal ash. Coal is the general term applied to
numerous organic minerals of varying composition and properties; however, all of
these minerals are enriched in carbon and are generally black in color. All coals
originate from the slow decomposition and chemical conversion of immense masses
of organic material (Hower and Parekh, 1991). Coal formation is a continuous
process, and as the chemical conversion proceeds with time, physical and chemical
properties of the organic matter change with a general increase in carbon content,
referred to as rank.
Specic coal rank classications vary between countries, but in general, formation
begins with peat, and sequentially progresses to lignite, subbituminous coal,
bituminous coal, and anthracite. As this progression proceeds, several important
compositional transformations occur. As previously mentioned, carbon content
increases with coal rank. This increase occurs as volatile matter is converted into
carbon; thus, volatile matter decreases with rank. Another important change is that
moisture content decreases with rank. This moisture is not free or surface water,
but rather it is moisture within the chemical structure of the coal itself. Peat can
contain as much as 75% chemically bound water, which decreases to 35% in lignite,
further decreasing to 25% in subbituminous coal, and to less than 10% moisture in
bituminous coal (Teichmϋller and Teichmϋller, 1982). Moisture content is particu-
larly important when considering thermal properties of coal because higher moisture
content results in lower heating value.
In addition to carbon, volatile matter, and moisture, inorganic material is also
found in coal. The source of the inorganic material is weathering and erosion
of associated minerals that are deposited along with organic matter through-
out the coal formation process. A variety of minerals can be found in coal in
varying concentrations, depending on the depositional environment, as shown in
Table3.1. Similarly, trace elements are also found in coal in varying concentrations
(Table3.2). Coalpreparation or cleaning with physical beneciation methods can

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