Patent Application
20150192036
The invention relates to an arrangement for heating/preheating a component of a combined cycle power plant using heat present in exhaust from an auxiliary steam boiler
During operation of a combined cycle plant a gas turbine engine produces all the heat necessary to generate all the steam that is required to run the plant. This includes steam required to operate the steam turbine as well as the steam required to supply the plant's peripheral processes When the gas turbine engine is not operating some of the peripheral plant processes still require steam Conventionally this steam is generated by an auxiliary steam generation system that is a miniature, self-contained boiler system that may include a gas or oil fired burner, a water feed pump, chemical treatment equipment etc. This auxiliary steam may be used for any or all of the steam turbine gland seals, low pressure sparging of condenser hotwell, pegging of the deaerator tank (DA tank), or other purposes. Providing this steam is viewed as essential prior to plant startup and sometimes the auxiliary boiler is sized to accommodate all the essential functions simultaneously. To generate the auxiliary steam a fossil fuel is burned in a combustion chamber and much of the heat from the combustion is transferred to water inside a boiler Byproducts of the combustion, including heated gases, are then discarded by way of an exhaust stack Typically there is no recovery of the energy exhausted into the stack Once the primary source of heat for the steam in the combined cycle plant is operational, such as a gas turbine engine, the auxiliary steam generation plant is shut down until needed again for auxiliary steam
The invention is explained in the following description in view of the drawings that show:
The present inventors have devised a way to decrease the startup time and potentially increase the efficiency of a power generation station that includes a combined cycle plant and an auxiliary boiler. Specifically, the inventors propose to utilize heat present in exhaust from the auxiliary boiler to heat/preheat another component in the power generation station In an exemplary embodiment the heat present in the auxiliary boiler exhaust will be transferred to the combined cycle plant to heat a component during a short shutdown or to preheat the component after a long shutdown. Heating/preheating in this manner will shorten startup times and reduce wear and tear on the component(s) due to thermal cycling. In addition, utilizing energy previously lost to the environment from the power generation station will increase the operating efficiency of the power generation station in some of the embodiments. The arrangement proposed herein can be applied to new power generation stations or retrofit into existing power generation arrangements
As shown in this exemplary embodiment the auxiliary boiler exhaust heat transfer arrangement 16 includes an optional auxiliary exhaust heat exchanger 50 configured to receive auxiliary boiler exhaust 52 and transfer heat from the auxiliary boiler exhaust 52 to clean air 54 As used herein the term clean air denotes air that is not exhaust from a combustion process. Instead, clean air 54 may be unfiltered, atmospheric air or the like. The auxiliary exhaust heat exchanger 50 can be installed in a new power generation station 10 or retrofit into an existing power generation station 10. The clean air may be supplied by an air source 56 that may be a low pressure blower or a compressor. Units that would perform satisfactorily in this role are commercially available Heated clean air 58 exits the auxiliary exhaust heat exchanger 50 and can be utilized by any or all of several plant operations. Whether or not the auxiliary exhaust heat exchanger 50 is present, the auxiliary boiler exhaust 52 can be directed into the HRSG 30 which is then heated/preheated by the heat present in the auxiliary boiler exhaust 52 Heating/preheating the HRSG 30 may reduce startup times and may reduce unwanted emissions associated with startup Additionally, there may be a cost savings associated with not needed to purchase, install, and maintain a stack dedicated to the auxiliary boiler 14, because the auxiliary boiler exhaust 52 would exit a stack associated with the HRSG 30.
The heated clean air 58 can be directed toward any component of the power generation system 10 including any component of the combined cycle plant 12. For example, the heated clean air 58 can be directed to the gas turbine engine 20 and may be used to heat/preheat any component therein, such as a rotor (not shown) This could decrease startup time and reduce stress on the metal components within the turbine 24 by moving them from cold to warm before the unit is ever started, or heated during short shutdowns In addition, or alternately, during operation the heated clean air 58 can be used as a mix with clean air 96 to cool components of the gas turbine as required In another exemplary embodiment the heated clean air 58 can be directed into the HRSG 30 in addition to or in place of the auxiliary boiler exhaust 52 Within the HRSG 30, the heated clean air 58 can be used to heat/preheat the superheater 32 and/or the SCR catalyst 34, and/or the cooler end 36 of the HRSG 30. The same advantages experienced by heating/preheating the turbine 24 would apply to heating/preheating the HRSG 30 and/or individual components therein
In another exemplary embodiment the heated clean air 58 can be directed to one or both of the steam turbines 38, 40 to heat/preheat any component, including a steam turbine rotor (not shown). Likewise, heating/preheating may lead to shorter startup times and less stress on the components. In still another exemplary embodiment the heated clean air 58 can be used to heat/preheat plumbing in the power generation station 10, including piping 70 and/or valves 72. This may be done by the heated clean air 58 passing through, or alternately by heating trace lines disposed adjacent to the plumbing
In another exemplary embodiment the heated clean air 58 may be directed to an air/HRSG feedwater heat exchanger 74 configured to receive HRSG feedwater 76 from, for example, the boiler feed pump (not shown) and transfer heat from the heated clean air 58 to the HRSG feedwater 76. In a variation of HRSG feedwater heating, the auxiliary boiler exhaust 52 may be directed into an exhaust/HRSG feedwater heat exchanger 80 that is configured to receive HRSG feedwater 76 and transfer heat directly from the auxiliary boiler exhaust 52 to the HRSG feedwater 76 This would speed up initial steam production and provide some warm up to the HRSG feedwater 76 system piping and valves. Note that the feedwater could be in various pressure levels depending upon the combined cycle plant design.
In an exemplary embodiment where the air source 56 is a compressor, compressed air 90 may be directed to a blade clearance control arrangement 92 to thermally regulate blade clearance in the turbine 24 of the gas turbine engine 20. Alternately, the compressed air 90 may be directed to plant instrumentation that requires compressed air 90 to operate After the heated clean air 58 has performed its intended function, the spent heated clean air 94 can be returned as inlet air 96 for the air source 56 and/or inlet air 98 for the auxiliary boiler 14 where it can be used for combustion, which may boost operating efficiency In an exemplary embodiment where the compressor is an interstage cooled compressor, boiler feedwater 100 can be used to cool the interstage cooled compressor, and the preheated boiler feedwater 102 can be used to supply the auxiliary boiler 14.
From the foregoing it can be seen that the inventors have devised a new arrangement that takes advantage of heat previously wasted. The new arrangement will increase operating efficiency and can further be incorporated into many of the plant's other systems, allowing for a more consolidated power generation station. Consequently, the proposed arrangement represents an improvement in the art
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.
