Power plants – Combustion products used as motive fluid
Patent
1993-09-13
1998-02-10
Casaregola, Louis J.
Power plants
Combustion products used as motive fluid
60 3912, F02C 328
Patent
active
057156719
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to an integrated gasification combined cycle (IGCC) process. More specifically, the present invention relates to an IGCC process having an improvement in overall thermodynamic efficiency by increasing the proportion of power generated by gas turbines to that generated by steam turbines. This is achieved by placing an exothermic catalytic reactor downstream of the gasification step to preheat the gas turbine fuel gas, which is supplemented by the addition of noncombustible gas, and expanding the cooled shifted gas stream.
BACKGROUND
Integrated Gasification Combined Cycle plants generate power in gas turbine(s) and steam turbine(s) wherein the steam for the steam turbine(s) is raised from the heat in the exhaust gas from the gas turbine(s) and, optionally, waste heat from the gasification step. Optionally, further power may be generated in a fuel gas expander situated between the gasification step and the gas turbine.
The thermodynamic efficiencies of gas turbines are generally higher than those of steam turbines whether the latter be of the back pressure or condensing type. Hence the greater the proportion of the IGCC power generated by the gas turbine(s) in relation to the proportion generated by the steam turbine(s), the higher the overall thermodynamic efficiency of the IGCC process. If the power generated by the gas turbine(s) is Pg and the power generated by the steam turbine(s) is Ps then the higher the value of Pg/Ps, the higher the efficiency of the IGCC process for the specified total power of (Pg+Ps).
The two major sources of heat from which steam can be raised are the waste heat from the gasification step and the exhaust gases from the gas turbine. The more efficient the gasification step i.e. the lower the quantity of energy converted to heat in order to carry out gasification, the lower the quantity of heat available from the gasification step to raise steam. The more efficient the gas turbine i.e. the higher the proportion of energy in the fuel gas converted to net shaft power, the lower the heat available in the exhaust gases to raise steam.
For higher IGCC thermodynamic efficiency, the gasification and gas turbine efficiencies should be preferably as high as possible. Furthermore, at high gasification and gas turbine efficiencies, an increase in the proportion of total IGCC power generated in the gas turbine becomes more significant in raising the overall thermodynamic efficiency of the IGCC.
The present invention increases IGCC efficiency by increasing the sensible heat carried by gas into the gas turbine and thereby increasing the proportion of IGCC power generated in the gas turbine.
It is well known to those versed in the art that using the gas turbine fuel, preferably supplemented with a non-combustible gas heat carrier, to carry waste heat into the gas turbine will increase gas turbine efficiency. Such non-combustible heat carrier can be a gas such as nitrogen or carbon dioxide, or water vapour either evaporated into the fuel gas in a saturator device or injected directly as steam. The additional non-combustible gas permits the mixture of combustible fuel gas and non-combustible gas to contain more sensible heat at a given temperature and hence to carry more heat into the gas turbine to improve efficiency. The non-combustible gas also provides other benefits such as a lowering of flame temperature and hence NO.sub.x formation and a reduction of the quantity of cooling air required for the gas turbine expander,
Nitrogen addition to improve IGCC thermodynamic efficiency is discussed in a paper entitled "Air separation integration for IGCC Plants" by Union Carbide, General Electric and Texaco at the 10th EPRI Gasification Conference in October 1991.
EP-A-034781 discloses the use of a deliberate fuel gas pressure drop to assist in adding non-combustible water vapour to the fuel gas in a saturator.
However, these schemes use mainly waste heat from the gasifier to preheat a mixture of a fuel and a non-combustible gas stream before passing to
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Casaregola Louis J.
Jacobs Engineering Limited
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