Combined cycle power plant incorporating coal gasification

A combined cycle power plant incorporating a coal gasifier as the energy source. The gases leaving the coal gasifier pass through a liquid couplant heat exchanger before being used to drive a gas turbine. The exhaust gases of the gas turbine are used to generate both high pressure and low pressure steam for driving a steam turbine, before being exhausted to the atmosphere.

BACKGROUND OF THE INVENTION 
One means of using coal in a relatively pollution-free manner to generate 
power is to burn the coal in a gasifier, producing a low BTU gas which can 
be used in a combined cycle, or a gas turbine and a steam generator, both 
of which drive electrical generators. In order to maximize efficiency of a 
combined cycle, it is necessary to utilize as much of the heat at the 
highest thermal level in the flue gases leaving the gasifier, steam 
generator, and the gas turbine as possible. 
SUMMARY OF THE INVENTION 
In accordance with the invention, a combined cycle power plant is provided 
having a coal gasifier as the energy course. The gases leaving the coal 
gasifier pass through a liquid couplant heat exchanger before being used 
to drive a gas turbine. The exhaust gases of the gas turbine are used to 
generate both high pressure and low pressure steam for driving a steam 
turbine, before being exhausted to the atmosphere.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Looking now to the drawing, numeral 10 designates a coal gasifier producing 
a low BTU gaseous fuel. In the gasifier, pulverized coal and air are 
reacted, at a high temperature, to form a fuel gas containing primarily 
carbon monoxide and hydrogen as combustibles, having a BTU value of 
100-125 BTU cubic feet. The gas produced in the gasifier passes through a 
low temperature superheater 12, a low temperature reheater 14, and a 
liquid couplant heat exchanger 16, before being cleaned of particulate 
matter in the gas cleanup complex 18. The gas leaves the gasifier at 
approximately 1700.degree. F. and after giving up heat in the heat 
exchangers, it enters the gas cleanup complex 18 at 300.degree. F. 
In addition to removal of solids, the complex 18 removes sulphur compounds, 
such as hydrogen sulphide, by means of any of several well-known processes 
such as solvent absorption, or a dry absorption process. The low BTU gas 
leaving the cleanup complex passes through a booster fan inlet heater 20, 
and a booster fan 22 before entering a fuel compressor 24, at 
approximately 150.degree. F. and 15 psia. The compressed gas flows to 
combustor 28 at 775.degree. F. and 260 psia. 
Air is also supplied to combustor 28 by compressor 30, to support 
combustion of the gas. The hot gases pass through a gas turbine 32, which 
drives an electric generator 34. The turbine also drives the fuel and air 
compressors. 
The exhaust from the gas turbine at approximately 1085.degree. F. and 15 
psia, flows through duct 36 to a heat recovery steam generator 38. The 
gases flow through two parallel passes 40 and 42 in the boiler 38, before 
being exhausted to the atmosphere through duct 44. 
The liquid and steam circuits will now be explained. Steam from the 
generator passes through steam turbine 50, which drives the electrical 
generator 52. The steam then is condensed in condensor 54 and flows to the 
feedwater heater 56 and deaerator 58. Part of the flow then passes through 
the low pressure boiler feed pump 60, onto the low pressure boiler system 
contained in the right-hand pass 42 of the steam generator 38. The fluid 
passes through economizer 62, evaporator 64, and into drum 66. The low 
pressure steam from drum 66 flows through superheater 68, and then goes to 
the low-pressure stage of the turbine 50. Part of the steam from drum 66 
flows through line 70 to the booster fan inlet heater 20, for heating the 
low BTU gas coming from the gas cleanup complex 18. The fluid then returns 
through line 72 to the economizer 62. 
Another portion of the fluid from the dearator 58 passes through line 74, a 
high pressure boiler feed pump 76 and line 78 to the liquid couplant heat 
exchanger 16. The heated fluid from 16 flows through line 80 to drum 82. 
This drum feeds evaporator surface (not shown) in the gasifier 10 through 
line 84 and header 86. Steam flows from drum 82 through line 114 to the 
low temperature superheater 12. 
Hot water also flows from drum 82 through lines 84 and 90 to the primary 
air heater 92 and secondary air heater 94, to heat the air used for 
combustion of the coal in the gasifier 10. Line 96 introduces coal to the 
gasifier 10. The heating fluid discharged from the air heaters flows back 
to the heat exchanger 16 through lines 98 and 78. 
Looking again to the heat recovery steam generator 38, water is supplied to 
a high pressure economizer 100 through line 102 from the deaertor 58. Part 
of this discharges into drum 104, with the water therefrom flowing to and 
through the evaporator 106, which extends across both of the gas passes 
40, 42. The steam and water mixture then returns to drum 104 with the 
steam flowing onto superheaters 12 and 108, and then to the turbine 50. 
Steam is supplied to 12 from drum 104 through line 112 and also from the 
gasifier drum 82 through line 114. 
In the right-hand gas pass 42 of the waste heat boiler 38, a reheater 116 
receives steam from the low temperature reheater 14 through line 118, and 
discharges it through line 120 to the intermediate state of turbine 50. 
Reheater 14 receives steam from the discharge of the first stage of 
turbine 50 through line 122. 
The steam from superheater 108 flows through turbine 50 at 1000.degree. F. 
and 2535 psia. and the steam leaves reheater 118 at 1000.degree. F. and 
450+psia. This is very good, considering the gas entering the heat 
recovery steam generator is at 1085.degree. F. and 15+psia. This is made 
possible because of the efficient usage of the available heat in the 
system, particularly the liquid couplant heat exchanger 16, the heat 
exchange surface in gasifier 10, and the low temperature boiler, part of 
the steam from which is used to heat the low BTU gas in heater 20.