Power plants – Combustion products used as motive fluid
Reexamination Certificate
2000-06-29
2002-08-13
Freay, Charles G. (Department: 3746)
Power plants
Combustion products used as motive fluid
C060S039120, C060S039182, C122S00700A
Reexamination Certificate
active
06430914
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a combined cycle power generation plant, and a method of operating such a plant, the plant comprising a boiler with a furnace for combusting a first fuel to produce steam, a steam generator driven by the steam to generate power and a combustor for combusting a second fuel to produce exhaust gas, which is expanded in a gas turbine to generate power and passed as a process gas to the furnace. The boiler is especially designed by taking into account the characteristics of the exhaust gas from the gas turbine as a process gas in order to render possible effective steam production with low emissions. The present invention addresses a problem of maintaining high performance of the boiler under varying operating conditions of the gas turbine combustor, including an operating mode in which the gas turbine combustor is not in use.
The combined cycle power generation plant may also comprise a gasifier to produce fuel gas and combustible char. The char may be used as the first fuel in the furnace of the boiler and the fuel gas as the second fuel in the gas turbine combustor. Thus, the plant can comprise a plurality of, such as two or three, separate systems, e.g., a boiler with a furnace, a gas turbine with a combustor and possibly a gasifier, which all are normally used together as an integrated system. The boiler is preferably a fluidized bed boiler, but it can also be a suspension boiler or some other type of boiler. The gasifier is preferably a pressurized fluidized bed gasifier, but it can also be of some other type. Both the gasifier and the combustor are preferably operated as circulating fluidized bed (CFB) systems.
U.S. Pat. No. 3,986,348 and No. 4,470,255 and Great Britain Patent No. 2,095,762 disclose combined cycle power plants, in which a pressurized gasifier, a gas turbine and a fluidized bed boiler are integrated so that fuel gas produced in the gasifier is combusted in a combustor of the gas turbine and the exhaust gases from the gas turbine are led to the fluidized bed boiler to function as a process gas, and char produced in the gasifier is combusted in the furnace of the fluidized bed boiler. In these types of plants, the particles in the product gas of the gasifier have to be removed before the gas is passed to the gas turbine, but most of the gaseous emissions can be removed at or downstream of the furnace of the CFB boiler, which renders possible cost-effective manufacturing and operation of the system.
In order to keep the emission level low, the amount of oxygen in the process gas has to be closely connected to the fuel feed rate. When compared to using air as combustion gas, the gas turbine exhaust gas is lean, including typically about 10 to about 15% by volume of oxygen, and hot, having a typical temperature of about 500 to about 600° C. Thus, when using the gas turbine exhaust gas as combustion gas, the flow rate of the combustion gas is high, which has to be taken into account when designing the boiler. Generally, the cross-sectional area of the furnace has to be large, the means for supplying process gas, e.g., the grid of a fluidized bed boiler, has to allow a high gas flow rate, and more heat transfer surfaces than normal have to be located in the back-pass of the boiler.
In these kinds of systems, the quantity and quality of the exhaust gas may strongly depend on the operating conditions of the gas turbine combustor. Thus, without special precautions, the performance of the boiler may vary under different operating conditions of the gas turbine combustor, and the efficiency of the system and the emissions released to the environment may, in some conditions, be far from optimal.
There may be a need to run the system in different operating modes, e.g., when having the gasifier down because of regular maintenance. The power should then be generated, e.g., by means of the boiler system alone, without having gas turbine exhaust gas available. If, under such operating conditions, fresh air is used as the process gas in the boiler, it may be impossible to gain optimal or even acceptable performance. By using optimal process gas flow, good bed temperature can be achieved, but, on the other hand, high excess air is produced, which results in a low boiler efficiency and high NOx emissions. Another alternative would be to use low excess air, but that would in turn lead to too high a bed temperature and very high SO
2
emissions.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a combined cycle power generation plant, including a gas turbine combustor and a boiler, which can provide high performance such as high efficiency and low emissions under (i) varying operating conditions of the combustor or (ii) an operating condition in which the combustor is not in use. It is also an object of the present invention to provide a method of using such a power generation plant.
In order to achieve these and other objects of the present invention, a combined cycle power generation plant and a method of using such a power generation plant are provided, as described in the independent claims.
In one aspect, the present invention provides a method of operating a combined cycle power generation plant that includes providing a boiler having a furnace for combusting a first fuel at a first temperature to produce flue gas and for producing steam, the boiler having an optimal performance in terms of steam production and emissions to the environment, wherein the first temperature provides an optimal temperature, conducting the flue gas through a flue gas duct to the environment, supplying process gas to the furnace at a first mass flow rate, the first mass flow rate providing an optimal mass flow rate, driving a steam turbine by the steam to generate power, combusting, in a combustor, a second fuel to produce exhaust, expanding the produced exhaust gas in a gas turbine to generate power, passing the exhaust gas from the gas turbine to the process gas supply, recirculating a portion of the flue gas from the flue gas duct to the process gas supply, controlling the rate of recirculation of the flue gas by a first controller, supplying fresh air to the process gas supply, controlling the rate of fresh air supply by a second controller, supplying a selected amount of first fuel to the furnace, supplying a selected amount of second fuel to the combustor, and the boiler having, in first operating conditions of the combustor, the optimal performance when the first and second controllers minimize the rate of flue gas recirculation and fresh air supply, respectively, and the exhaust gas is used alone or as a major portion of the process gas, and controlling in conditions other than the first operating conditions of the combustor, the first and second controllers to obtain at least nearly the optimal performance of the boiler.
In another aspect, the present invention provides a combined cycle power generation plant that includes a boiler for producing steam, the boiler including a furnace for combusting a first fuel to produce flue gas, a back-pass for receiving the produced flue gas and a flue gas duct for passing the flue gas from the back-pass to the environment, a process gas supply for supplying the process gas to the furnace, a steam turbine for receiving and being driven by the steam to generate power, a combustor for combusting a second fuel to produce exhaust gas, a gas turbine for expanding the exhaust gas from the combustor to generate power and for passing the exhaust gas to the process gas supply, a return line for recirculating a portion of the flue gas from the flue gas duct to the process gas supply, a first controller for controlling a rate at which the flue gas is recirculated in the return line, a supply for supplying fresh air to the process gas supply, a second controller for controlling a rate at which the fresh air is supplied by the supply, and a controller for controlling the first and second controllers so as to maintain at least nearly optimal performance of the boiler under differen
Goidich Stephen J.
Rainio Aku
Fitzpatrick ,Cella, Harper & Scinto
Foster Wheeler Energy Corporation
Freay Charles G.
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