Page 142 - Advanced Gas Turbine Cycles

114 Advanced gas turbine cycles

(7.12b)

(7.12~)

Expression (7.12a) for overall efficiency is similar to that for the combined doubly
cyclic plant; the term %[Hps - Hw]/F corresponds to the ‘heat loss’ term of Section 7.3.
The extent of this reduction in overall efficiency depends on how much exhaust gases can
be cooled and could theoretically be zero if they emerged from the HRSG at the (ambient)
temperature of the reactants. In practice this is not possible, as corrosion may take place on
the tubes of the HRSG if the dew point temperature of the exhaust gases is above the feed
water temperature. We shall find that there may be little or no advantage in using feed
heating in the steam cycle of the CCGT plant.

7.4.2. The integrated coal gasijication combined cycle plant (IGCC)

A current development of the exhaust heated plant (unfired) is the integrated coal
gasification combined cycle (IGCC) plant. One of the earliest of these IGCCs was the Cool
Water pilot plant built by the General Electric company, using a Texaco gasifier. This
complex plant is shown in Fig. 7.4, after Plumley [4]. The gas turbine, HRSG and steam
turbine components were standard so it was the performance of the gasifier which was
critical for new development and close integration between the gasifier and the HRSG was
important.
In the plant, coal is ground and mixed with water to form a slurry and this is fed to the
gasifier through a burner, in which partial combustion takes place with oxygen (supplied
from a separate plant). During gasification the coal ash is melted into a slag, quenched with
water and removed as a solid.
Following the high temperature reactions of coal and water with oxygen, the raw
synthetic gas (syngas), consisting mainly of hydrogen and carbon monoxide (about
40% each by molal concentration) is water-cooled in radiant and convection coolers,
generating saturated steam. The gas is then passed through a particulate scrubber, further
cooled to near ambient temperature prior to sulphur removal, and then saturated to reduce
the subsequent combustion temperature and NO, production.
The syngas then enters the conventional exhaust heated CCGT plant, being burnt in the
gas turbine combustion chamber with air from the compressor. The combustion gas
supplies the gas turbine, driving the compressor and a generator, and then exhausts into the
HRSG (unfired), which raises superheated steam. By-product steam from the gasifier
coolers (some 40% of the total steam supply) is also superheated in the HRSG and the two
streams of steam enter the steam turbine which drives its own generator.
Some 20 IGCC plants, in various forms, some with other gasifiers but most using
oxygen, are now operating or are in the process of construction. Modifications of the IGCC
plant to sequestrate the carbon dioxide produced will be discussed in Chapter 8.