Method of low temperature heat utilization for atmospheric pressure coal gasification

A combined cycle power plant having a steam generator and a gas turbine which makes use of a high temperature liquid couplant, or heat exchange arrangement, and a low temperature liquid couplant, or heat exchange arrangement, for maximizing use of the heat available in the power plant. The high temperature liquid couplant extracts heat from the gases leaving a coal gasifier, and is used in (1) a high temperature air heater for the gasifier, and (2) a high temperature fuel preheater of a steam generator. The low temperature liquid couplant extracts heat from the exhaust gases of the steam generator, and is used as (1) preheat for the high temperature liquid couplant, (2) low temperature air preheater for the gasifier, (3) a booster fan inlet heater, and (4) a low temperature fuel preheater for the steam generator.

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 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 
which incorporates a high temperature liquid couplant or heat exchange 
arrangement, and a low temperature liquid couplant, or a heat exchange 
arrangement. Fluid is heated in a high temperature heat exchanger by the 
gases leaving a coal gasifier and the heat is used in a high temperature 
air heater for the gasifier, and a high temperature fuel preheater of a 
steam generator. A low temperature liquid couplant obtains heat from the 
exhaust gases of the steam generator, and is used in (1) preheat for the 
high temperature liquid couplant, (2) low temperature air preheater for 
the gasifier, (3) a booster fan inlet heater for the fuel, and (4) a low 
temperature fuel preheater for the steam generator. 
BRIEF DESCRIPTION OF THE DRAWING 
The drawing is a schematic of a combined cycle having a gas turbine, and a 
steam generator, incorporating both high and low liquid couplants in 
accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Looking now to the drawing, numeral 10 designates a coal gasifier for 
producing a low BTU gaseous fuel. In the gasifier, pulverized coal, air 
and steam are reacted, at a high temperature, to form a fuel gas 
containing primarily carbon monoxide and hydrogen, having a BTU value of 
100-125 BTU/cubic feet. The gas produced in the gasifier passes through a 
high temperature liquid couplant heat exchanger 12, before being cleaned 
of particulate matter in the gas cleanup complex 14. Sulphur compounds, 
such as hydrogen sulphide, are also removed here by means of any of 
several well-known processes such as solvent absorption, or a dry 
absorption process. The low BTU gas exiting from the cleanup complex 14 
passes through a booster fan inlet heater 16, where it is heated, before 
flowing to the booster fan 18. The gas flow then splits, with part of the 
gas flowing to the fuel compressor 20, and another portion going to a low 
temperature fuel preheater 22. The compressed fuel is burned in a 
combustor 24, which is supplied with compressed air through duct 26. The 
hot combustion gases drive a gas turbine 27, which in turn drives an 
electrical generator (not shown). 
The exhaust from turbine 27, which is rich in oxygen and also contains 
considerable residual heat, can be used as combustion air in the steam 
boiler 28. Boiler 28 is supplied with low BTU gas from a high temperature 
heat exchanger 30. Additional combustion air 32 can be supplied to the 
boiler as needed. 
The steam produced in generator 28 is used to drive an electrical generator 
(not shown). The exhaust gases from the boiler 28 are used to heat liquid 
in a low temperature heat exchanger 34, before being discharged to the 
atmosphere. 
The low temperature liquid from heat exchanger 34 is used for a number of 
purposes. A first portion flows through line 36 to the fuel preheater 22. 
A second portion flows through line 38 to the booster fan inlet heat 
exchanger 16. A third portion flows through line 40 to a low temperature 
air preheater 42. A high temperature air preheater 44 receives heat from 
the heat exchanger 12 through line 46. Liquid also flows from heat 
exchanger 12 to the high temperature fuel preheater 30 through line 50. 
The high temperature liquid, after giving up heat in heat exchanger 30, 
flows through the low temperature fuel preheater 22, before returning 
through line 52 to the low temperature liquid couplant 34. Likewise, the 
liquid leaving high temperature air preheater 44 flows through the low 
temperature air preheater 42, before returning through line 60 to the 
liquid couplant 34. 
The air heater discharges secondary air, which flows through duct 70 to the 
coal gasifier 10. Primary air flows from the air heater to a coal 
pulverizer 72 and then carries the pulverized coal through duct 74 to the 
gasifier. 
From the above, it can be seen that a combined cycle power plant has been 
provided which makes use of all of the heat available, thus optimizing the 
efficiency of the unit.