Power Plant Engineering

x
is controlled by the use of low-NO
x
burners, by SNCR, and by SCR. Note that NO
x
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 NO
x
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-NO
x
and thermal NO
x
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 NO
x
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.

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POWER PLANT ENGINEERING