Patent Application
20070256424
This invention relates to electric power generation, especially to combined cycle power generation using a gas turbine engine in a first power cycle that produces waste exhaust heat, and a waste heat recovery system driving a second power cycle.
Electric power plants commonly use F-class gas turbine technology, which is distinguished by firing temperatures of about 1,300° C. and exhaust temperatures of over 580° C. A strong demand exists for turbine power plants with nitrogen oxide (NOx) emissions low enough to meet increasingly strict environmental regulations. Since gas turbines themselves do not achieve the required low emissions, NOx removal technology must be applied to the combustion exhaust gas. There are currently two main commercial alternatives for this: 1) Hot selective catalytic reduction (SCR), which can operate at the gas turbine exhaust temperature; and 2) Conventional SCR, which must operate at temperatures far below the gas turbine exhaust temperature, such as 232° C. to 370° C. Conventional SCR is preferable, due to its higher efficiency, reliability, and lower cost. Thus, technologies have been developed to reduce exhaust gas temperature to the operating range of conventional SCR. These include mixing the exhaust with ambient air, or using the hot exhaust gas in a heat recovery system that powers a subsequent power cycle such as a steam turbine.
The invention is explained in following description in view of the drawings that show:
Gas turbine engines operate on a thermodynamic Brayton cycle, in which ambient air is drawn into a compressor and pressurized. The compressed air is heated in a generally constant-pressure process in a heating chamber that is open to both inflow and outflow. This is normally done by burning fuel in the compressed air in a combustion chamber, producing a hot working gas comprising combustion gasses. The heated air is then expanded through a turbine to extract energy in the form of shaft power.
In accordance with an aspect of the invention
A second Brayton cycle 50 may comprise a heat recovery gas turbine engine (HRGT) 51 comprising an air inlet 52, an air compressor 54, a compressed airflow 56, a heat exchanger 58, a compressed heated airflow 62, a hot air turbine 64, and an exhaust airflow 66. The hot air turbine 64 drives a power shaft 68 that drives the air compressor 54 and a generator 70, producing electrical power 71. The heat exchanger 58 transfers heat from the exhaust combustion gas flow 36 to the compressed airflow 56, providing heat energy for the second Brayton cycle. This recovers waste heat from the first Brayton cycle, and reduces the temperature of the exhaust combustion gas flow 36 to the operating range of a conventional selective catalytic reduction unit 80. The electrical power outputs 41 and 71 may be combined to supply the plant load 72.
In an aspect of the present invention, the heat recovery gas turbine 51 comprises a heat exchanger 58 instead of a combustion chamber 28 heating the compressed air in the generally constant-pressure process. The heat exchanger 58 transfers waste heat from the first Brayton cycle 20 to the second compressed airflow 56, producing heated compressed air 62 as the working gas. The term “gas turbine” is used generically herein for gas turbine engines with either type of heating; i.e. combustion or heat exchange, while “combustion gas turbine” is used to denote a gas turbine engine in which combustion occurs in the working gas. In either case, the compressed and heated working gas, comprising either combustion gas or air, then transfers some of its energy to shaft power by expanding through a turbine or series of turbines. Some of the shaft power extracted by the turbine is used to drive the compressor.
In accordance with another aspect of the invention,
An important factor in the efficiency of the present invention is the HRGT compression ratio; i.e. the ratio between the outlet and inlet pressures of the HRGT compressor 54.
A cost-effective means to produce an HRGT for the present invention is to use standard equipment wherever possible. An existing combustion gas turbine engine design can be modified for this purpose by replacing the combustor with a heat exchanger. Some combustion gas turbine engines have a combustion chamber in a silo connected by ducts to the gas flow of the engine. It is generally easier to replace this type of combustion chamber with a heat exchanger than to replace a can-style combustor. Typical commercially available combustion gas turbine engines have a compression ratio of over 10. One or more stages at the compressor outlet and one or more stages at the inlet of the turbine section may be removed to reduce the compression ratio of an existing gas turbine engine to a desired range for an HRGT application.
As an illustrative example of this type of implementation of the invention, a primary combustion gas turbine generator such as Siemens SGT6-5000F may be enhanced by adding a heat recovery gas turbine made by modifying a second combustion gas turbine such as Siemens SGT5-2000F. The combustion chamber of the second gas turbine may be replaced with a heat exchanger. The last 4 stages of the compressor and the first stage of the turbine section of the secondary gas turbine may be removed to achieve a pressure ratio of approximately 6. Ducting the combustion exhaust from the primary gas turbine through the heat exchanger, and operating the second gas turbine as described herein, will bring the combustion exhaust within range of conventional SCR units.
While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.
