Patent Application
20130255267
The disclosure relates generally to emission reduction and more particularly, to emission reduction in gas turbine engines.
Oxides of Nitrogen (NOx) are major pollutants found inherently in an exhaust gas stream of combustion engines. They are known to cause acid rains that are harmful to living organisms. Several emission reduction technologies, such as, but not limited to, dry low NOx (DLN) combustion when operated at fully premixed mode, exhaust gas recirculation (EGR), steam addition in diffusion combustion, reheat combustion and selective catalytic reduction (SCR) have been employed to reduce NOx emissions.
In dry low NOx (DLN) premixed combustion, for example, a feed oxidant stream is mixed with a fuel prior to being introduced into a combustor. In such a case, the fuel is uniformly mixed with combustion air and excess air available helps to keep the flame temperatures low. Low flame temperatures in turn reduce NOx formation.
Exhaust Gas Recirculation (EGR) has been an attractive method for NOx reduction especially in internal combustion engines or furnace applications where fuel and air are usually burned in diffusion or partially premixed mode. In exhaust gas recirculation (EGR), a part of an exhaust gas stream is re-circulated back into the feed oxidant stream, effectively reducing oxygen concentration in the feed oxidant stream. Lack of excess oxygen in the combustor reduces the formation of NOx.
Steam addition into a diffusion flame quenches the diffusion flame temperatures to a desirable limit, thus reducing the formation of NOx. In Selective Catalytic Reduction (SCR), a reduction agent like ammonia, for example, is employed to reduce the oxides of nitrogen in exhaust gas stream into elemental nitrogen.
However, employing the aforementioned emission reduction technologies reduce NOx concentration in exhaust gas stream. With the growing concern for cleaner environment and stricter emission regulations, further reduction of NOx concentration in exhaust gas streams of combustion engines is highly desirable.
Therefore, there is a need for improved emission reduction technologies that addresses one or more of the aforementioned issues.
In accordance with an embodiment, a power generation system is provided. The system includes a gas turbine system and an exhaust gas recirculation system. The gas turbine system including a combustion chamber, a compressor, and an expander. The combustion chamber is configured to combust a fuel stream. The compressor is configured to receive a feed oxidant stream and supply a compressed oxidant stream to the combustion chamber. The expander is configured to receive a discharge stream from the combustion chamber and generate an exhaust stream comprising carbon dioxide and electrical energy. The exhaust gas recirculation system including a heat recovery steam generator and an exhaust gas recirculation loop. The heat recovery steam generator is configured to receive the exhaust stream and generate a recycle stream, wherein the recycle stream is mixed with a fresh oxidant to generate the feed oxidant stream. The exhaust gas recirculation control loop is configured to receive feedback related to at least one of a load pressure, a percentage of exhaust gas recirculation, emissions of NOx and CO2 at an exit of the combustor and a firing temperature in the combustion chamber to control a pilot ratio of the feed oxidant stream.
In accordance with another embodiment, a power generation system is provided. The system includes a gas turbine system and an exhaust gas recirculation control loop. The gas turbine system including a combustion chamber, and a compressor, and expander. The combustion chamber is configured to combust a fuel stream. The compressor is configured to receive a feed oxidant stream and supply a compressed oxidant stream to the combustion chamber. The expander is configured to receive a discharge stream from the combustion chamber and generate an exhaust stream comprising carbon dioxide and electrical energy. The exhaust gas recirculation system including a splitter, a heat recovery steam generator and an exhaust gas recirculation control loop. The splitter is configured to split the exhaust stream into a first split stream and a second split stream. The heat recovery steam generator is configured to receive the first split stream and generate a cooled first split stream, wherein a portion of the cooled first split stream is mixed with the second split stream to generate a recycle stream, and wherein the recycle stream is mixed with the fresh oxidant to generate the feed oxidant stream. The exhaust gas recirculation control loop is configured to receive feedback related to at least one of a load pressure, a percentage of exhaust gas recirculation, emissions of NOx and CO2 at an exit of the combustor and a firing temperature in the combustion chamber to control a pilot ratio of the feed oxidant stream.
In accordance with another embodiment, a method of generating power with reduced NOx emission is provided. The method includes compressing a feed oxidant and generating a compressed oxidant stream in a compressor, combusting a fuel stream and the compressed oxidant stream in a combustion chamber and generating a discharge stream, expanding the discharge stream from the combustion chamber and generating an exhaust stream comprising carbon dioxide and electrical energy, recovering heat from the exhaust stream and generating a recycle stream, mixing the recycle stream with a fresh oxidant and generating the feed oxidant stream and controlling a pilot ratio based on feedback regarding a plurality of combustor parameters received by a model based control of an exhaust gas recirculation control loop.
Other objects and advantages of the present disclosure will become apparent upon reading the following detailed description and the appended claims with reference to the accompanying drawings. These and other features and improvements of the present application will become apparent to one of ordinary skill in the art upon review of the following detailed description when taken in conjunction with the several drawings and the appended claims.
The above and other features, aspects, and advantages of the present disclosure will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
As described in detail below, embodiments of the present disclosure provide a system for improving emission performance and a method for operating a gas turbine to reduce oxides of nitrogen (NOx) emission in an exhaust gas stream of a gas turbine. The term “improving emission performance” used herein refers to reduction of NOx concentration in the exhaust gas stream of the gas turbine. The term “EGR” refers to exhaust gas recirculation in a gas turbine engine. In exhaust gas recirculation, a portion of the exhaust from the gas turbine is re-circulated into the inlet of the turbine, which portion of the exhaust is blended with fresh oxidant such as air before being introduced to the combustion chamber of the turbine system. As a result the blended oxidant used for combustion has lower oxygen content when compared to a conventional oxidant and results in low NOx emission in the discharge from the combustion chamber. Additionally, the carbon dioxide generated in the combustion process is concentrated as a portion of the exhaust gas is re-circulated back to the turbine system which recirculation enhances the CO2 separation processes downstream.
As previously stated, exhaust gas recirculation has been an attractive method for NOx reduction in applications where fuel and air are usually burned in diffusion or partially premixed modes. Exhaust gas recirculation has now been determined as a short term path for CO2 capture power plants. Utilizing exhaust gas recirculation in a gas turbine application has been found to increase CO2 concentration at the stack leading to a reduced separation cost and smaller separation equipment. On the other hand, gas turbines utilizing DLN combustion systems are well known for their low emissions especially when operated at fully premixed mode. As disclosed herein, integration of exhaust gas recirculation with a DLN combustion system may lead to further NOx reduction.
The present system provides a combined DLN/EGR system with the addition of an exhaust gas recirculation control loop to recirculate a portion of an exhaust gas stream back into a compressor inlet of the gas turbine, wherein the exhaust gas recirculation control loop controls pilot ratios based on received feedback of EGR %, firing temperatures and load pressures. The ability to control these factors provides improved flame stability and maintaining NOx emissions to minimum values.
Turning now to the drawings,
It is understood that the compressed oxidant stream 18 from the compressor 16 may comprise any suitable gas containing oxygen, such as for example, air, oxygen rich air and oxygen-depleted air. The combustion process in the combustion chamber 20 generates the discharge stream 22.
As illustrated, the discharge stream 22 from the combustion chamber 20 may be introduced into the expander 24. As indicated, the power generation system 10 includes a generator 28 attached to the turbine system 15. The thermodynamic expansion of the hot discharge stream 22 fed into the expander 24 produces power to drive the gas turbine system 15, which, in turn, generates electricity through the generator 28. In this embodiment, electricity from the generator 28 may be converted to an appropriate form and is provided to a distribution power supply network grid (not shown). The expanded exhaust stream 30 from the expander 24 may be fed into the splitter 32. In one embodiment, the splitter 32 is a Coanda ejector enhancing the splitting of the exhaust stream into the first split stream 34 and the second split stream 36. The HRSG 38 is used for recovering the heat content of the first split stream 34 to generate steam. The temperature of the exhaust stream 30 is at about 700° F. to about 1100° F. and the cooled first split stream 40 is at a temperature of about 60° F. to about 200° F.
In one embodiment, the exhaust gas recirculation system 54 further includes a first control valve 42 configured to release a portion of the cooled first split stream 40 as an output stream 44. More particularly, the HRSG 38 extracts heat from the first split stream 34 of the exhaust stream 30 to generate steam as the output stream 44. In one embodiment the output stream 44 is released to atmosphere and in another embodiment, the output stream 44 is sent to a CO2 separation unit (not shown) to separate CO2 before being released to atmosphere. A remaining portion 46 of the cooled first split stream 40 is mixed with the second split stream 36 to generate a mixed exhaust stream 48. The exhaust gas recirculation system 54 recirculates a portion of the exhaust stream 30, and more particularly circulates the mixed exhaust stream 48, to the compressor 16 of the gas turbine 15, to reduce concentration of oxygen by about 5% in the compressed feed oxidant stream 18 into the combustor 20 of the gas turbine 15. In one embodiment, the exhaust gas recirculation system 54 recirculates less than about 50% of the exhaust stream 30. In an embodiment, the exhaust gas recirculation system 54 may further include a cooler (not shown) to cool the mixed exhaust stream 48. Water present in the mixed exhaust stream 48 is condensed in the cooler by a reduction in temperature of the mixed exhaust stream 48. The term “HRSG” used herein refers to heat recovery steam generator 38 that recovers heat from the exhaust stream 30 to generate steam as the output stream 44. The steam is typically directed to a steam turbine (not shown) to extract additional work.
Prior to recirculation into the compressor 16, the mixed exhaust stream 48 may be sent to the purification unit 50. The purification unit 50 is configured to remove contaminants such as moisture, particulates and acid gases from the mixed exhaust stream 48 before recycling it back to the inlet of the compressor 16 as a recycle stream 52. Impurities and moisture content in the exhaust gas 30 prevent utilization of a simple recirculation loop to accomplish the CO2 concentration. Direct recirculation of a portion of an exhaust from a turbine system may result in turbine failing and accelerated wear of internal components due to the presence of certain impurities such as particles and acid gases in an exhaust stream. Therefore the presence of the purification unit 50 may enhance the removal of contaminants such as water vapor, acid gases, aldehydes and hydrocarbons and reduces chances of accelerated corrosion and fouling in the internal components of the gas turbine system 15. As shown in
In a perfectly premixed combustion, the exhaust gas recirculation system 54 is not always working for the benefit of NOx emission reduction and regions exist where an optimization analysis is needed. Accordingly, an exhaust gas recirculation control loop 60 is included in the system 10 to further reduce NOx emissions. The inclusion of the exhaust gas recirculation control loop 60 will better ensure that when an exhaust gas recirculation system is integrated into the DLN system, the NOx emissions will be kept at minimum levels at part loads. In an embodiment, the exhaust gas recirculation control loop 60 comprises a model based control (MBC) 64 in feedback communication 62 with the combustor 20, wherein the model based control 64 is configured to receive feedback with regard to combustion dynamics and combustion instabilities in the combustor 20. The model based control 64 is further configured to control (adapt) the EGR % in response to the received feedback to provide lower NOx emissions and/or avoid out of compliance conditions.
Experimental results show that NOx production is sensitive to the CO2 percentage in the recycle stream 52, as well as the pressure in the combustion chamber 20. It has been found that by incorporating the exhaust gas recirculation control loop 60 into the power generation system 10, further NOx reductions may be obtained at full load conditions with the power generation system 10 operation at higher than 150 psi. The exhaust gas recirculation control loop 60 further improves flame stability margins due to lower O2 concentrations in lean premixed systems, decreasing the probability of lean blowout (LBO) at part loads. In an embodiment, the exhaust gas recirculation control loop 60, and more particularly the model based control 64, is configured to receive feedback of at least one combustion parameter, and in a preferred embodiment to receive feedback of a plurality of combustion parameters. More specifically, the exhaust gas recirculation control loop 60 is configured to control pilot ratios (diffusion to total fuel ratio) based on received feedback related to EGR %, firing temperatures, emissions of NOx and CO2 at exit of the combustor and load pressure within the combustion chamber 20, thereby improving the flame stability and keeping NOx emissions to minimum values. The exhaust gas recirculation control loop 60 utilizes combustion information regarding pressure, EGR %, emissions of NOx and CO2 at exit of the combustor and firing temperature and determines the pilot ratio required to keep the NOx emission in its minimum ratios with improved flame stabilization.
The exhaust gas recirculation system 54 may further utilize an additional integrated control system (not shown). The control system integrates the operation of each unit in the exhaust gas recirculation system 54 to further achieve optimum performance of the power generation system 10. In one embodiment, the control system may be driven by a continuous monitoring system (not shown in Figures). The continuous monitoring system measures the constituents in the recycle stream 52 and provides feedback to the control system. Based on this feedback, the control system may further adjust parameters including but not limiting to quench recirculation rate, sorbent injection rate in quench unit, and gas temperatures at several location in the exhaust gas recirculation system 54. The control system may further optimize gas temperature at key locations in the exhaust gas recirculation system 54 to ensure most efficient cooling. In an embodiment, the exhaust gas recirculation control loop 60 may provide the control of these additional parameters in the exhaust gas recirculation system 54 to achieve an optimum performance of the power generation system 10.
In operation, NOx formation increases exponentially with the flame temperature and proportionally with the availability of oxygen in the combustor 20. The exhaust gas recirculation system 54 recirculates a portion of the exhaust stream 30 as the recycle stream 52 into the compressor 16 to reduce oxygen content in the feed oxidant stream 14 by about 5%. Due to combustion of the fuel stream 13 and the compressed oxidant stream 18 in the combustor 20 the oxygen content is depleted in the exhaust stream 30. Once the mixed exhaust stream 48 is mixed with the feed oxidant stream 14, the oxygen content in the mixture is lower in comparison to the oxygen content in a plain feed oxidant stream. This reduction in the oxygen content helps to reduce the formation of NOx in the combustor 20, for example by between about 70% to about 80%.
The use of the exhaust gas recirculation control loop 60 may provide further reduction in the NOx emissions and may increase the concentration of carbon dioxide in the recycle stream 52. In a particular embodiment, the recirculation of the exhaust gas 30 increases the concentration of carbon dioxide by about 10%. In carbon capture and sequestration the carbon dioxide from the exhaust stream 30 is separated and is either stored in geological formations, deep in the oceans or converted into mineral carbonates. Carbon capture and sequestration techniques are more efficient and cost effective with increase in carbon dioxide concentration in the exhaust stream 30. In an exemplary embodiment, as illustrated herein, the exhaust gas stream 40 from the outlet of the HRSG 38 may be passed through a carbon capture system (not shown) to reduce the amount of carbon dioxide rejected with the exhaust gas stream 44 into the atmosphere. In another exemplary embodiment, an exhaust gas recirculation mixer (not shown) may be provided to mix the recycle stream 52 with the feed oxidant stream 14.
The fuel stream 13 may include any suitable hydrocarbon gas or liquid, such as natural gas, methane, naphtha, butane, propane, syngas, diesel, kerosene, aviation fuel, coal derived fuel, bio-fuel, oxygenated hydrocarbon feedstock, and mixtures thereof, and so forth. In one embodiment, the fuel is primarily natural gas (NG) and, therefore, the discharge stream 22 from the combustion chamber 20 may include water, carbon dioxide (CO2), carbon monoxide (CO), nitrogen (N2) Nitrogen oxides (NOx), unburned fuel, and other compounds.
The exhaust gas recirculation system 54, including the exhaust gas recirculation control loop 60, may be configured as a retrofitable unit for retrofitting into any existing gas turbines to achieve higher concentration of CO2 in the working fluid in the turbine system and also to lower NOx emissions. Reduced NOx emission from the combustion chamber 20 is achieved due to several factors: (i) decrease in oxygen content in the compressed oxidant 18 as fresh air 12 is mixed with the recycle stream 52 comprising depleted oxygen levels; and (ii) adjustments to the pilot ratio based on feedback regarding combustion load pressure, EGR % and firing temperatures received by the exhaust gas recirculation control loop 60.
Exhaust gas recirculation (EGR) in combination with the exhaust gas recirculation control loop is effectively used in the present techniques to increase CO2 level in the exhaust and reduce NOx at the same time by adjusting pilot ratios based on combustion parameters. NOx treatment in any combustion process is typically achieved by selective catalytic reduction (SCR) and/or using a pre-mixed combustion process. The present techniques provide slight modifications to the gas turbines including an exhaust gas recirculation system that may be applied to existing systems as a retrofit and minor modification of the combustion nozzles to allow a more flexible operation and controlling of multiple combustion parameters to achieve low NOx formation and higher CO2 level in the exhaust. In one embodiment, as stated earlier, the portion of the output stream 44 is directed to the CO2 separation unit (not shown). Any CO2 separation technology may be involved (for example amine treatment, PSA, membrane, etc.). After separating the CO2 rich stream may be directed to a CO2 conditioning system, including a CO2 compression system. The increase in CO2 concentration in the exhaust stream from the turbine system enhances the efficiency of the CO2 separation process.
Data compiled in a combined DLN/EGR system, utilizing a DLN nozzle with capability to operate in both a perfectly premix mode and a partially premix mode, and including an exhaust gas recirculation control loop as described herein, is provided. Exhaust gas recirculation was produced using a fuel blending station. Data results show that in a perfectly premix combustion, exhaust gas recirculation is affected by the following parameters: (i) percentage of CO2 in the oxidizer; (ii) pressure on the exhaust gas recirculation process; (iii) fully premixed combustion vs pilot (1-3% diffusion) affecting lean blow-out; (iv) nozzle mixing; and (v) oxygen content in the exhaust. The inclusion of the exhaust gas recirculation control loop in the power generation system as disclosed herein provides the ability to determine the optimum regions of operation and control the pilot ratio in response to feedback regarding such combustion parameters, and thereby achieve low NOx emissions. In an attempt to determine the optimum regions of operation via the exhaust gas recirculation control loop in the exhaust gas recirculation process, there is a need to map exhaust gas recirculation at varying times, pressures and pilot ratios.
Referring more specifically to
During an initial atmospheric pressure experiment to validate the effect of CO2 on NOx production in a perfectly premixed combustion a similar trend was obtained as depicted in
Additionally, the effect of pressure at fixed CO2 percentage in the oxidizer was studied and results showed that pressure plays an important role. It has been known that pressure has a relation to NOx production in the form NOx˜(P2/P1)̂m where m is in the range of 0.4.
When CO2 is present, increasing combustion pressure in a perfect premix mode has an opposite effect on NOx production. This phenomenon could be explained as increasing pressure suppresses the CO2 kinetics with NOx production.
The power generation systems described herein include an exhaust gas recirculation unit, including an exhaust gas recirculation control loop, which may be implemented as a retrofit for turbines that need to comply with lower levels of NOx formation, as well as allowing more effective CO2 separation, in situations where CO2 separation is required. The exhaust gas recirculation system disclosed herein is an option for power generation with exhaust that is CO2-lean and has a lower NOx level than NOx levels typically observed in a combustion process. The exhaust gas recirculation control loop provided in all the embodiments described herein provides optimization analysis to minimize NOx production and improve flame stability at part load conditions. In addition, the exhaust gas recirculation control loop addresses the effect of CO2 on NOx production. Through the understanding of CO2 effects and its dependence on combustion pressure on either enhancing or suppressing its role on NOx production, the exhaust gas recirculation control loop disclosed herein receives feedback related to pressure, EGR % and firing temperature and decides the pilot ratio required to keep NOx in its minimum ratios with improved flame stability. The inclusion of the exhaust gas recirculation system, including the control loop disclosed herein, with a gas turbine system in a power generation system, provides significant simplification of the system in achieving lower NOx emissions, such as in the form of cost reduction and increased reliability.
While only certain features of the disclosure have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the disclosure.
