Power plants – Combustion products used as motive fluid
Reexamination Certificate
1999-03-19
2001-06-12
Thorpe, Timothy S. (Department: 3746)
Power plants
Combustion products used as motive fluid
C060S039182
Reexamination Certificate
active
06244033
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to generation of electric power in large stationary power plants.
BACKGROUND OF THE INVENTION
Modern electric power generating stations include steam turbine power generating systems comprised of a boiler, steam turbines, and an electric generator. The boiler produces high pressure saturated steam that is usually superheated in tubes fired in the boiler. The steam turbines are connected in series trains with steam flowing from turbine to turbine. The high pressure superheated stream from the boiler is fed into the inlet of the upstream steam turbine. Steam pressure and temperature decrease as the steam moves downstream through the turbine train. The steam turbines are connected to a drive shaft that turns the electric generator producing electric power. The steam boiler is fired with a fossil fuel, e.g. natural gas, coal or lignite, or is heated by a nuclear reactor. Modern fossil fuel fired boilers typically operate at pressures between 1800 and 2400 psig and some operate above the critical pressure of water, which is 3206 psia. Nuclear powered boilers typically operate at much lower pressures, about 600 psig.
Typically, a steam side stream is extracted from one of the intermediate downstream turbines in the turbine train at a pressure and temperature significantly lower than the high pressure steam raised in the boiler. The extracted steam side stream is reheated and then fed back into the steam turbine train at a point downstream of the extraction point.
The exhaust steam from the steam turbines is condensed. The steam condensate is preheated, and then recycled to the steam boiler. Treated boiler feed water is added to the steam condensate to makeup losses. Typically, the boiler feed water stream is preheated with steam extracted from an intermediate point on the steam turbine train.
The capital cost of steam turbine generating systems per KWH generating capacity is high. But they are thermally efficient and have low fuel cost. Accordingly, steam turbine systems are cost effective when operated continuously to provide base load power.
Modern power stations also typically include gas fired turbine units that drive electric generators. Gas turbines cost less than steam turbine units per KWH of power capacity but they are less energy efficient than steam turbine units. Accordingly, gas turbine generators are best suited for intermittent operation to meet peak power duties.
Combined cycle units are an increasingly important component of modern power generating stations. Combined cycles are comprised of a gas turbine-generator unit wherein the hot exhaust gas from the turbine is fed into a boiler to raise steam. The steam powers a condensing steam turbine that drives a power. Alternatively, the steam is used for process heating.
Firing temperatures of gas turbines are being increased as turbine construction materials are improved to withstand higher operating temperatures. Increasing firing temperature increases gas turbine fuel efficiency. Accordingly, combined cycles are now competitive against steam cycles for base load power generation.
Minimizing fuel consumption is a key objective in design and operation of electric power generating stations. Reducing fuel consumption reduces fuel cost and reduces the amount of carbon dioxide and other pollutants dispersed into the atmosphere, or in the case of nuclear reactors, reduces nuclear fuel cost and nuclear wastes to be disposed. Fuel efficiency of a generating system in the power industry is commonly expressed as the heat rate for the system which in English units is defined as the BTU's (British Thermal Units) of heat from combustion of fuel required per KWH (kilowatt hour) of electricity produced. The heat rate can be expressed either at the lower heating value (LHV) which means that water vapor produced by total combustion of the fuel is not condensed or at the higher heating value (HHV) which means that the combustion water is condensed.
SUMMARY OF INVENTION
The process of the present invention includes a steam power generator system comprised of a steam boiler, steam turbines, and a generator, and a gas turbine-generator system. In the steam generator system: 1) the high pressure saturated steam stream produced by the boiler is superheated, 2) an intermediate steam side stream is extracted from the steam turbine train which is reheated and inserted back into a steam turbine at a downstream location, and 3) the steam turbine exhaust steam stream the downstream steam turbine is condensed, preheated and recycled back to the steam boiler. The gas turbine exhaust gas is utilized to:
superheat the high pressure steam stream; and/or
reheat the intermediate steam side stream; and/or
preheat the steam condensate stream.
The preferable embodiment of this invention from the standpoint of maximizing thermal efficiency is to use the gas turbine exhaust gas stream to heat all three streams. Processes wherein the gas turbine exhaust gas stream heats one or two of the streams are alternate but less thermally efficient embodiments of this invention.
It is also preferable in order to maximize thermal efficiency that the gas turbine exhaust gas stream first superheat the high pressure steam and/or reheat the intermediate steam stream after which it preheats the steam condensate stream.
Other embodiments of the present invention include firing supplemental fuel to increase high pressure steam superheat, and/or intermediate pressure steam reheat, and/or condensate preheat above heat duties that can be obtained using gas turbine exhaust gas as the only heat source for superheat, reheat, and preheat. A preferred mode is to fire the supplemental fuel only during periods of peak power demand to temporarily increase power output of the system.
It will become apparent from the forthcoming discussion that the power cycles of the present invention exhibit heat rates that are significantly lower than heat rates obtained using the conventional power cycles.
REFERENCES:
patent: 4272953 (1981-06-01), Rice
patent: 5526386 (1996-06-01), Tsiklauri
patent: 5628183 (1997-05-01), Rice
patent: 5727379 (1998-03-01), Cohn
patent: 5793831 (1998-08-01), Tsiklauri
Gray Michael K.
Nadler Murray
Thorpe Timothy S.
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