POLLUTION AND ITS CONTROL 427as much as 10% S by weight. The sulfur is contained in both the organic and inorganic fractions of the
coal. It is possible to remove some of the sulfur containing ‘rocks’ found with the coal using gravity
separation methods. Processes have been studied for removing additional amounts of S from coal; how-
ever, these processes are not widely used commercially.
Coal contains about 0.5 to 1.5% by weight N bound into the organic structure of the coal. When
the coal is heated, much of the N is released during the devolatilization stage as hydrogen cyanide
(HCN), other cyano species, and as ammonia (NH 3 ). These species are rapidly converted into NO or N 2 ,
and thus are not emitted directly from the combustor.
Some coals (e.g., British coals) contain chlorine, which upon combustion can be emitted as
hydrogen chloride (HCl).
Typically, coal has about 10% by weight mineral matter. This mineral matter is high in Si, Al, Fe,
and Ca and these are the primary elements found in the ‘big’ flyash particles. However, many other
inorganic elements are associated with the mineral matter of coal, including Mg, Na, and K. Toxic
metals are found in trace amounts in coal, including Ni, Pb, Cd, Cr, As, Se, and Hg. Generally, the toxic
metals are found associated with the sub-micron flyash, though Hg will be emitted as a gas. Radioactive
isotopes are also emitted from coal combustors. It has been estimated that more radioactivity is emitted
from a coal combustion power plant than from a nuclear power plant. Emission control is practiced as
follows for Pulverized Coal Furnaces:
A significant fraction of the installed cost of a coal-fired electrical generating station is devoted
to environmental control — about 30 to 40%. Also, a significant fraction of the operating and mainte-
nance cost is devoted to environmental control. Sulfur (i.e., SO 2 emission) is controlled by burning a
low sulfur coal, by pre-combustion coal cleaning, and by flue gas desulfurization.
NOx is controlled by the use of low-NOx burners, by SNCR, and by SCR. Note that NOx is
formed from both the air nitrogen and the fuel-bound nitrogen.
Flyash is controlled by stack gas particulate removal using electrostatic precipitators (ESPs) or
baghouses (i.e., the stack gas is filtered).
Coal is also burned by gasifying it and burning the gases in a gas turbine engine, which is part of
a combined cycle. There are about five integrated gasification combined cycle (IGCC) power plants
operating in the US. The combustible compounds present in the gas (i.e., the synthetic gas) are mainly
H 2 and CO, though in some cases CH 4 is also present in significant amounts. Other gases present are
CO 2 , H 2 O, and N 2. If the gasifier is ‘air blown’ there is a lot of N 2 present (about 30 to 50% by volume),
and thus the gas has a low heating value (of about 100 to 200 Btu/scf). If an oxygen-blow gasifier is
used, the heating value is in the medium Btu range (about 1/4 to 1/2 that of natural gas), though the
power-plant now must also include an air separation plant for making O 2 — actually, oxygen enriched
air is produced. The gas is cleaned to remove sulfur (as H 2 S) and particulate matter before entering the
gas turbine. Some gasifier product streams carry several thousand ppms of NH 2 (formed from the coal-
bound nitrogen), which needs be controlled either by scrubbing or by special combustion methods, so
that it does not react to NOx in the combustor. However, other gasifiers emit only about 200 ppm ammo-
nia, and when this gas is burned in a gas turbine, the sum of the fuel-NOx and thermal NOx leaving the
burner is only about 30 to 40 ppm. For reference, when natural gas is used as the fuel in a gas turbine
engine equipped with state-of-the-art, commercially available, lean premixed combustors, the NOx emis-
sion is about 9 to 25 ppm (depending on the make and model of the engine.) The coal IGCC system has
an overall efficiency in the low 40% range, whereas the natural gas fired combined cycle is about 50 to
58% efficient (as discussed earlier). The IGCC system has higher CO 2 per kW-hr than the natural gas
fired system.