Patent Application
20110265486
The present invention relates generally to gas turbine engines and, more particularly, to providing additional turndown capability during part load operation of a gas turbine engine for reduction of CO emissions.
Gas turbine power plant operators are often faced with an economic dilemma of whether or not to operate their plants during low power demand times. By operating continuously, the plant will be available to quickly produce base load power when the power demand becomes high. The plant maintenance cost will also be reduced with fewer plant starts and stops. However, operation during these low power demand times often results in negative profit margins, or losses, for the plant operator because the low cost of power does not offset the cost of fuel.
The logical solution for the plants that choose to operate continuously is to minimize the losses by minimizing fuel consumption during operation at minimum power demand. Industrial gas turbine engines are designed to operate at a constant design turbine inlet temperature under any ambient air temperature (i.e., the compressor inlet temperature). This design turbine inlet temperature allows the engine to produce maximum possible power, known as base load. Any reduction from the maximum possible base load power, such during a plant turn down, is referred to as part load operation. That is, part load entails all engine operation from 0% to 99.9% of base load power. However, operation of the plant is restricted by its exhaust gas emissions permit. Since emissions such as nitrous oxides (NOx) and carbon monoxide (CO) typically increase on a volumetric basis as the gas turbine power decreases, this limits how much the plant can turn down, or reduce power, during the low power demand times.
In particular, part load operation may result in the production of high levels of carbon monoxide (CO) during combustion. One known method for reducing part load CO emissions is to bring the combustor exit temperature or the turbine inlet temperature near that of the base load design temperature. It should be noted that, for purposes of this disclosure, the terms combustor exit temperature and turbine inlet temperature are used interchangeably. In actuality, there can be from about 30 to about 80 degrees Fahrenheit difference between these two temperatures due to, among other things, cooling and leakage effects occurring at the transition/turbine junction. However, with respect to aspects of the present invention, this temperature difference is insubstantial.
To bring the combustor exit temperature closer to the base load design temperature, the mass flow of air through a turbine engine can be restricted by closing compressor inlet guide vanes (IGV), which act as a throttle at the inlet of a compressor for the gas turbine engine. When the IGVs are closed, the trailing edges of each of the vanes rotate closer to the surface of an adjacent vane, thereby effectively reducing the available throat area. Reducing the throat area reduces the flow of air which the first row of rotating blades can draw into the compressor. Lower flow to the compressor leads to a lower compressor pressure ratio being established in the turbine section of the engine. Consequently, the compressor exit temperature decreases because the compressor does not input as much energy into the incoming air. Also, the mass flow of air through the turbine decreases, and the combustor exit temperature increases.
While controlling emissions during plant turn down is effectively controlled by closing the IGVs, this has limited capability. Constant speed compressors, such as those used for industrial gas turbines, have limitations on the amount that the mass air flow into the compressor may be reduced using the IGVs before running into structural and/or aerodynamic issues. Further, CO emissions increase rapidly as gas turbine engine load is reduced below approximately 60%. Once the IGVs have been closed to their limit, and the engine's exhaust temperature limit has been reached, then power typically may be reduced only by decreasing turbine inlet temperature. Turbine inlet temperature reduction corresponds to a decrease in the combustion system's primary zone temperature (T_PZ), resulting in CO and unburned hydrocarbons (UHC) being produced due to quenching of the combustion reactions in the turbine hot gas path. To prevent CO from increasing as engine load decreases, the T_PZ must be maintained at a high level.
In accordance with an aspect of the invention, a turbine engine assembly for a generator is provided. The assembly comprises a turbine engine having a compressor section, a combustor section and a turbine section, the combustor section having a primary zone temperature (T_PZ) and the turbine engine having a base load. The combustor section includes a combustor and a combustor shell, the combustor shell receiving air from an exit of the compressor section. A variable flow restrictor is located in a flow path between the combustor shell and an inlet to the combustor. The variable flow restrictor effects an increase in a pressure drop of shell air flowing from the combustor shell to the combustor. A controller is provided for operating the variable flow restrictor to increase a pressure drop across the variable flow restrictor when an operating load is less than the base load, wherein a flow of shell air passing from the combustor shell to the combustor is decreased thereby reducing power output from the turbine engine while maintaining the T_PZ above a T_PZ lower limit.
In accordance with another aspect of the invention, a turbine engine assembly for a generator is provided. The assembly comprises a turbine engine having a compressor section, a combustor section and a turbine section, and the turbine engine having a base load. The combustor section includes a combustor and a combustor shell, the combustor shell receiving air from an exit of the compressor section. A flow control device is located in a flow path between the combustor shell and an inlet to the combustor. The flow control device effects an increase in a pressure drop of shell air flowing from the combustor shell to the combustor. A controller is provided for operating the flow control device to change a pressure drop across the flow control device, wherein an increase in the pressure drop across the flow control device results in a corresponding reduction in mass flow through the combustor for effecting a reduction in power output from the turbine engine during a reduction in an operating load to less than the base load.
In accordance with a further aspect of the invention, a method of operating a turbine engine assembly is provided comprising: sensing a load on a turbine engine for a reduced operating load; and increasing a pressure drop between a supply of shell air provided in a combustor shell and an inlet to a combustor receiving the shell air for combustion to reduce a mass flow rate of hot gases from the combustor to a turbine section of the turbine engine while maintaining a primary zone temperature (T_PZ) of the combustor above a T_PZ lower limit.
While the specification concludes with claims particularly pointing out and distinctly claiming the present invention, it is believed that the present invention will be better understood from the following description in conjunction with the accompanying Drawing Figures, in which like reference numerals identify like elements, and wherein:
In the following detailed description of the preferred embodiment, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration, and not by way of limitation, a specific preferred embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized and that changes may be made without departing from the spirit and scope of the present invention.
Referring initially to
The combustor section 30 includes a combustor shell 18 (
Referring to
The flow control device 46 may comprise any mechanism that implements a restriction or blockage on air entering the combustor 42 for passage to the combustion zone 55. As illustrated diagrammatically in
Referring to
Other embodiments of the flow control device 46 may be implemented, depending on the particular design of the engine 10, and it should be understood that the present invention is not limited to a particular form of flow control device 46, as described herein.
A portion of the compressor exit air is provided as cooling air to turbine components, which air bypasses the combustor 42, as represented by line 56 in
In the combustor section 30, air is mixed with fuel, such as may be provided through one or more fuel lines 62, and combusted to produce hot, high pressure gas and reaction products including unburned hydrocarbons (UHC) and CO. It is desirable to keep CO emissions low, preferably less than 10 ppmvd at 15% O2. As the load on the turbine engine assembly 10 is reduced, the fuel supply to the combustors is reduced. The IGVs 40 can be closed to a limit position as the fuel supply is reduced to limit the mass flow of air into the combustor section 30. Further reductions in power typically would require a reduction in the T_PZ, which would result in increased UHC and CO production (
During low load operation and turndown of the turbine engine assembly 10, a CO reduction process may be implemented, including operating the flow control device 46 to restrict the flow of shell air As to the combustion zone 55 of the combustor 42. In particular, the flow control device 46 may at least partially close or restrict the air passage through the annular area 48, increasing a pressure drop between the air in the combustor shell 18 and the air passing into the combustion zone 55 of the combustor 42. The restriction of airflow to the combustion zone 55 coincides with a reduction in the flow rate of fuel to the combustor 42 such that the air/fuel ratio is not substantially increased during low load operation and the T_PZ is maintained above a lower limit to thereby maintain the CO production at a low level, i.e., less than about 10 ppmvd at 15% O2.
The increased pressure drop in the combustor section 30 between the combustor shell 18 and the inlet 54 to the combustor 42, resulting from the additional restriction provided by the flow control device 46 during a turndown operation, further operates to increase the shell air pressure within the combustor shell 18. As a result of the increased shell air pressure, additional work is required from the compressor section 12 to compress the air provided to the compressor exit 41 and discharged into the combustor shell 18. Accordingly, the output power to the generator 68 is further reduced during low load operation by an amount that the restriction provided by the flow control device 46 increases the load on the compressor section 12.
As noted above, a portion of the compressor exit air is provided as cooling air to stationary vanes 29a in the first stage 28a of the turbine section 14 and to the rotor 58 and associated rows of blades 22a-d, and to the transition section 44. As a result of the increased pressure of the shell air as the flow control device 46 reduces the airflow As to the combustor 42, the cooling airflow is increased to stationary vanes 29a in the first stage 28a, the rotor 58 and to the transition 44, providing additional convective cooling to these components. The additional cooling, i.e., over-cooling, provided by the increased air pressure to the turbine components is further beneficial to the present low load operation of the turbine engine 10 in that, since the T_PZ is maintained at a high temperature with the reduced flow of hot gases through the turbine section 14, the additional cooling protects the components in the hot gas path 45 to improve part life and reduce fallout.
Further, as the flow of cooling air to the components in the hot gas path 45 increases during turndown, additional cool air is discharged into the hot gas path 45 after passing through and performing convective cooling in the components. That is, additional cool air may be discharged at different locations along the hot gas path including at the transition section 44, at the stationary vanes of the first stage 28a of the turbine section 14 and at one or more stages of the blades 22a-d within the turbine section 14. The additional cool air mixes with the hot gases, providing additional quenching of the hot gases. Hence, the work performed by the hot gases is reduced, thereby further reducing the power output to the generator 68 and additionally reducing the hot gas temperature that the components are exposed to, and reducing the exhaust gas exit temperature TEXIT, during low load operation.
In accordance with a particular example of the invention, in a turbine engine 10 incorporating the flow control device 46, where an unrestricted pressure drop through the combustor 42, i.e., between the combustor shell 18 and the inlet 54 to the combustion zone 55, is about 4%, the flow control device 46 may increase the pressure drop to 14%, corresponding to about a four-fold pressure drop from the unrestricted flow through the combustor 42. Further, the increased pressure in the combustor shell 18 resulting from the restricted flow to the combustor 42 may provide about a 30% increase in the cooling air flow to the transition section 44 and the first row of turbine vanes 29a, and about an 8% increase in cooling air flow to the rotor 58.
As a result of implementing the flow control device 46 to provide the controlled pressure drop through the combustor section 30, it is believed that the power output at low load operation may be reduced 15% below that available when relying solely on the closure of the IGVs 40, corresponding to a 7% reduction relative to baseload power output. In addition, the fuel consumption may be reduced 8% below that available when relying solely on the closure of the IGVs 40 at low load operation, corresponding to a 5% reduction relative to fuel consumption at baseload operation.
The flow control device 46 may be controlled by the controller 50 to progressively close, decreasing the flow rate of shell air As to the combustor 42 as the operating load is decreased, and thereby reducing power delivered to the generator 68 (
While particular embodiments of the present invention have been illustrated and described, it would be obvious to those skilled in the art that various other changes and modifications can be made without departing from the spirit and scope of the invention. It is therefore intended to cover in the appended claims all such changes and modifications that are within the scope of this invention.
