Environmental Engineering Reference
In-Depth Information
combined cycle power plants may require a NO
x
control system because in the gas turbine copious
quantities of thermal NO
x
may be created. Of course, a condenser and cooling tower are also
required for the steam portion of the combined cycle.
5.3.2
Coal Gasification Combined Cycle
A combined cycle power plant can be fueled by coal, but then the coal needs first to be gasified. The
gasified coal (syngas) propels the topping cycle gas turbine. A power plant using coal gasification
and combined cycle is called an integrated gasification combined cycle plant (IGCC).
Various coal gasification methods were developed already in the nineteenth century for pro-
viding piped gas for home heating, cooking, and lighting. This was called
city gas
and was used
before natural gas became widely available in the second half of the twentieth century. In the world
wars, Germany used gasified coal as a fuel for automobiles, trucks, and military vehicles.
Coal can be gasified to low-, medium- and high-heating value syngas. The processes differ
depending upon whether air or pure oxygen is used for gasification and whether the product gas is
rich or devoid of CO
2
. For an IGCC plant, high-heating value syngas is preferred.
The process starts with crushing the coal. If the coal has a tendency for caking, preoxidation
my be necessary to render its surface more porous. The crushed coal is fed to a retort, where, in
the presence of catalysts, it is exposed to pure oxygen and steam:
3C
+
O
2
+
H
2
O
→
3CO
+
H
2
(5.10)
Pure oxygen (99%
) is supplied from a special
air separation unit
constructed at the plant site.
The resulting gaseous mixture contains a host of higher-molecular-weight organic compounds, as
well as hydrogen sulfide. The next step is called
quenching,
in which the heavy oils and tar are
removed from the mixture by condensation and aqueous solution scrubbing. This is followed by
removal of H
2
S by the above-described Claus or similar process.
The syngas resulting from reaction (5.10) has a relatively low heating value of 250-500 Btu
per standard cubic foot (sfc), which equals 9.1-18.2 MJ m
−
3
. This gas would not be suitable as a
fuel for an IGCC. To increase the heating value, the carbon monoxide and hydrogen is passed over
a catalyst at about 400
◦
C to form principally methane, a process called
methanation:
+
3H
2
+
CO
→
CH
4
+
H
2
O
(5.11)
The resulting syngas has a heating value of 950-1000 Btu/scf (36-38 MJ m
−
3
), similar to methane.
For this syngas, a conventional gas turbine can be used. An alternative is the production of hydrogen
in the
water gas shift reaction:
+
→
CO
2
+
CO
H
2
O
H
2
(5.12)
The resulting hydrogen can be separated from CO
2
by membrane separation and used in high-
efficiency H
2
/O
2
fuel cells (see Section 3.12).
The estimated thermal efficiency of a coal gasification combined cycle plant is on the order
of 40-45%, including the energy required for air separation and coal gasification. This efficiency
exceeds that of a pulverized coal-fired steam plant with emission controls, having an efficiency of
36-38%. However, the capital and operating costs of such a plant would be considerably higher
