Patent Application
20110162386
The invention relates to flow circuitry in a gas turbine to augment turbine performance and, more particularly, to using the overboard bleed flow to run an ejector and communicating ejector outlet flow to the turbine for stator and rotor cooling.
In gas turbines, it is typical for a portion of the total air flow from the compressor inlet to be diverted to various turbine components for various purposes, including cooling those components. The diverted air can consume a large proportion of the total air flow through the compressor. The management and control of these parasitic flows can dramatically increase the performance of the turbine. Typically, air under pressure is extracted from the compressor and bypasses the combustion system of the turbine for use as a cooling flow for various turbine components.
Blast furnace gas (BFG) is a low calorific value (BTU) byproduct of a steel mill, and it is desirable to use BFG as a fuel in the combustor of a gas turbine. Such use, however, involves several challenges. For example, high fuel mass flows are required to attain decent firing temperatures. Conversely, the air flows are reduced so that the air/fuel ratio is lesser, and a higher flame temperature can be attained. The gas turbine compressor, however, needs to run at a minimum volumetric flow (to avoid surge). To reduce the mass flow to the combustor, some amount of air is bled after the last stage of the compressor. This bleed flow is referred to as overboard bleed (OBB).
Pressurized air can be extracted from the compressor 2 via line 8 to bypass the combustor 3. This pressurized air can be input to the turbine 7 for cooling purposes, such as for cooling the third expander stator of the turbine 7. The first and second stage expanders of the turbine (stator, forward wheel space and the rotors) and the third stage of the rotor can be cooled using, for example, 17th stage air from the compressor 2 via line 9. The rest of the final stage extraction is the OBB flow which is input to a heat recovery steam generator (HRSG) 11.
It would be desirable to reduce the flow rate through the compressor 2 by making optimal use of the OBB flow.
Additionally, low BTU fuel run gas turbine units need to have a lesser air flow, but because of surge requirements, the air flow through the compressor is much higher than required. This leads to loss of valuable power in the form of compressor work. It would also be desirable to reduce this loss of power.
In an exemplary embodiment, a gas turbine includes a compressor including a plurality of stages, a combustor, and a turbine. Each stage of the compressor compresses input air to higher pressures and temperatures. The combustor receives pressurized air from the compressor and mixes the pressurized air with fuel to output hot gasses of combustion. The turbine receives the hot gasses of combustion from the combustor. An over board bleed (OBB) circuit bleeds air from a last stage of the compressor, and an extracted air circuit extracts air from an upstream stage of the compressor. An ejector receives input from the OBB circuit and the extracted air circuit, and ejector outlet flow from the ejector is communicated to the turbine.
In another exemplary embodiment, a cooling circuit in a gas turbine includes an over board bleed (OBB) circuit bleeding air from a last stage of the compressor, an extracted air circuit extracting air from an upstream stage of the compressor, and an ejector receiving input from the OBB circuit and the extracted air circuit, where ejector output (outlet flow) from the ejector is communicated to the turbine for rotor and stator cooling.
In yet another exemplary embodiment, a method of operating a gas turbine includes the steps of compressing input air across a plurality of compressor stages to higher pressures and temperatures; mixing the pressurized air with fuel in a combustor and outputting hot gasses of combustion to a turbine; bleeding air via an over board bleed (OBB) circuit from a last stage of the compressor; extracting air via an extracted air circuit from an upstream compressor stage; communicating output from the OBB circuit and from the extracted air circuit to an ejector; and communicating ejector outlet flow to the turbine.
With reference to
Bleed air can be removed from stages of the compressor 22 for use as cooling/purge air flow in the turbine. A portion of the compressor air flow is diverted from flow through the combustor 28 for these other purposes. In the exemplary scheme shown in
With continued reference to
The OBB flow is thus used to pressurize a compressor upstream stage extraction in the ejector to reduce the higher stage extractions and hence reduce the compressor air flow and the power consumption. This also has an advantageous result of removing the limit of 730° F. on the compressor discharge temperature, which was established because there is a maximum cooling air temperature that can be used for the forward wheel space cooling.
An exemplary ejector 35 is shown in
The described system reduces the flow rate through the compressor by making optimal use of the OBB. An ejector used in conjunction with the OBB and an upstream compressor stage extraction reduces the last stage extraction used for cooling the rotors, stators and the forward wheel spacing of the gas turbine expanders. Thermodynamic models have been built, and substantial efficiency increases can be achieved over the presently available combined cycle systems.
While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.
