Systems and methods for controlling aftertreatment systems

A system includes a nitrogen oxide reduction catalyst fluidly coupled to an exhaust conduit of an engine system. The nitrogen oxide reduction catalyst is configured to reduce nitrogen oxides in an engine exhaust. The system also includes an ammonia oxidation catalyst fluidly coupled to the exhaust conduit downstream of the nitrogen oxide reduction catalyst and configured to reduce ammonia in the engine exhaust. Further, the system includes a reductant injection control system configured to control an injection of reductant into the exhaust conduit, determine a first nitrogen oxide conversion rate of the nitrogen oxide reduction catalyst, determine an ammonia storage value of the nitrogen oxide reduction catalyst, and determine a first temperature of the engine exhaust upstream of the ammonia oxidation catalyst. The reductant injection control system is also configured to increase or decrease the injection of reductant based on the first nitrogen oxide conversion, the ammonia storage value, and the first temperature.

BACKGROUND

The subject matter disclosed herein relates to power generation systems. Specifically, the embodiments described herein relate to improving aftertreatment systems within power generation systems.

Many power generation systems utilize an aftertreatment system to condition the exhaust gases generated by the power generation system. In particular, aftertreatment systems may be used to reduce certain types of emissions by converting exhaust gases produced by the power generation system into other types of gases or liquids. For example, aftertreatment systems may be used to reduce the amount of nitrogen oxides within the exhaust gases.

To reduce the amount of nitrogen oxides in the exhaust gases, an aftertreatment system may include a nitrogen oxide (NOx) reduction catalyst and an ammonia oxidation catalyst, which reduce the amount of nitrogen oxides and ammonia in the exhaust gases, respectively. Further, the aftertreatment system may also inject a fluid, such as urea, into the exhaust gases to facilitate the reduction of the nitrogen oxides and ammonia. It would be beneficial to improve the NOxconversion rate across an aftertreatment system for a power generation system.

BRIEF DESCRIPTION

In a first embodiment, a system includes a nitrogen oxide reduction catalyst fluidly coupled to an exhaust conduit of an engine system and configured to reduce nitrogen oxides in the engine exhaust, and an ammonia oxidation catalyst fluidly coupled to the exhaust conduit downstream of the nitrogen oxide reduction catalyst and configured to reduce an ammonia in the engine exhaust. Further, the system includes a reductant injection control system configured to control an injection of reductant into the exhaust conduit, determine a nitrogen oxide conversion rate of the nitrogen oxide reduction catalyst, determine an ammonia storage value of the nitrogen oxide reduction catalyst, and determine a first temperature of the engine exhaust upstream of the ammonia oxidation catalyst. The reductant injection control system is also configured to increase, decrease, or a combination thereof, the injection of reductant based on the nitrogen oxide conversion rate, the ammonia storage value, and the first temperature.

In a second embodiment, a method includes controlling a reductant injection into an engine exhaust and determining a nitrogen oxide conversion rate and an ammonia storage value of a nitrogen oxide reduction catalyst configured to receive the engine exhaust and reduce nitrogen oxides in the engine exhaust. The method also includes receiving a first input corresponding to a first temperature upstream of an ammonia oxidation catalyst fluidly coupled to the nitrogen oxide catalyst, wherein the ammonia oxidation catalyst is downstream of the nitrogen oxide catalyst and configured to reduce ammonia in the engine exhaust. Further, the method includes increasing or decreasing the reductant injection based on the nitrogen oxide conversion rate, the ammonia storage value, and the first temperature.

In a third embodiment, a non-transitory computer-readable medium includes computer executable code. The computer executable code includes instructions configured to control a reductant injection into an engine exhaust and determine a nitrogen oxide conversion rate and an ammonia storage value of a nitrogen oxide catalyst configured to receive the engine exhaust and reduce nitrogen oxides in the engine exhaust. The computer executable code also includes instructions configured to receive a first input corresponding to a first temperature upstream of an ammonia oxidation catalyst fluidly coupled to the nitrogen oxide catalyst, wherein the ammonia oxidation catalyst is downstream of the nitrogen oxide catalyst and configured to reduce ammonia in the engine exhaust. Further, the computer executable code includes instructions configured to increase or decrease the reductant injection based on the nitrogen oxide conversion rate, the ammonia storage value, and the first temperature.

DETAILED DESCRIPTION

Many power generation systems use an aftertreatment system to condition the exhaust gases generated by the power generation system. For instance, certain power generation systems utilize aftertreatment systems that are designed to reduce the amount of nitrogen oxides in the exhaust gases. These aftertreatment systems may include a nitrogen oxide (NOx) reduction catalyst and an ammonia oxidation catalyst. Prior to entering the catalysts, the exhaust gases may be mixed with urea, or some other type of fluid that prompts the desired chemical reactions. The exhaust gas-urea mixture then enters and reacts with the catalysts to generate the desired conversions (i.e., reducing nitrogen oxides and ammonia to carbon dioxide, water, etc.).

To improve the conversion rates of the catalysts, present embodiments of the aftertreatment system include a urea injection control system. The urea injection control system evaluates the operating characteristics (e.g., current conversion rate, current temperature at one or more locations, flow rates, etc.) of the catalysts and adjusts the amount of urea injected into the exhaust gases based on the operating characteristics of the catalysts and a desired conversion rate for the aftertreatment system. The urea injection control system also controls the operating window for certain characteristics of the catalysts based on the desired conversion rate. Further, in certain embodiments, the data collected by the urea injection control system may be used to perform diagnostic evaluations of the various components of the aftertreatment system and execute various actions (e.g., alarms, alerts, corrective actions) if necessary.

With the foregoing in mind,FIG. 1depicts a power generation system10that may be used to provide power to a load, such as an electric generator, a mechanical load, and the like. The power generation system10includes a fuel supply system12, which in turn includes a fuel repository14and a throttle16that controls the fuel flow from the fuel repository14and into the power generation system10. The power generation system10also includes an engine system18which includes a compressor20, a combustor22, and a gas engine24. Further, the power generation system10includes an aftertreatment system26, which is described in further detail below.

The power generation system10also includes a control system28which monitors various aspects of the operation of the power generation system10. In particular, the control system28may work in conjunction with sensors30and actuators32to monitor and adjust the operation of the power generation system10. For instance, various types of sensors30, such as temperature sensors, oxygen sensors, fluid flow sensors, mass flow sensors, fluid composition sensors, and/or pressure sensors may be disposed on or in the components of the power generation system10, and the throttle16is a specific actuator32. Although the power generation system10is described as a gas engine system, it should be appreciated that other types of power generation systems (e.g., turbines, cold-day systems, combined cycle systems, co-generation systems, etc.) may be used and include the control system28, aftertreatment system26, and urea injection control system34.

During operation, the fuel supply system12may provide fuel to the engine system18and, specifically, the combustor22, via the throttle16. Concurrently, the compressor20may intake a fluid (e.g., air or other oxidant), which is compressed before it is sent to the combustor22. Within the combustor22, the received fuel mixes with the compressed fluid to create a fluid-fuel mixture which then combusts before flowing into the gas engine24. The combusted fluid-fuel mixture drives the gas engine24, which in turn produces power for suitable for driving a load. For example, the gas engine24may in turn drive a shaft connected to the load, such as a generator for producing energy. It is to be understood that the gas engine24may include internal combustion engines, gas turbine engines, and the like.

The combustion gases produced by the gas engine24exit the engine and vent as exhaust gases into the aftertreatment system26. In present embodiments, the exhaust gases pass through one or more catalytic converter systems, which will be described in further detail below. In some embodiments, the exhaust gases may also pass through a heat recovery steam generator (HRSG), which may recover the heat from the exhaust gases to produce steam. To monitor and adjust the performance of the aftertreatment system26, the power generation system10includes a urea injection control system34, which is described in further detail below.

As mentioned earlier, the control system28oversees the operation of the power generation system10. The control system28includes a processor36, memory38, and a hardware interface40, as shown inFIG. 2. As depicted, the processor36and/or other data processing circuitry may be operably coupled to memory38to retrieve and execute instructions for managing the power generation system10. For example, these instructions may be encoded in programs that are stored in memory38, and the memory38may be an example of a tangible, non-transitory computer-readable medium. The instructions or code may be accessed and executed by the processor36to allow for the presently disclosed techniques to be executed. The memory38may be a mass storage device, a FLASH memory device, removable memory, or any other non-transitory computer-readable medium suitable for storing executable instructions or code. Additionally and/or alternatively, the instructions may be stored in an additional suitable article of manufacture that includes at least one tangible, non-transitory computer-readable medium that at least collectively stores these instructions or routines in a manner similar to the memory38as described above. The control system28may also communicate with the sensors30and the actuators32via the hardware interface40. In some embodiments, the control system28may also include a display42and a user input device44to allow an operator to interact with the control system28.

In some embodiments, the control system28may be a distributed control system (DCS) or similar multiple controller systems, such that each component (e.g., gas engine24, aftertreatment system26, urea injection control system34) or group of components in the power generation system10includes or is associated with a controller for controlling the specific component(s). In these embodiments, each controller includes a processor, memory, and a hardware interface similar to the processor36, the memory38, and the hardware interface40described above. Each controller may also include a communicative link to communicate with the other controllers.

Turning now toFIG. 3, the aftertreatment system26includes a selective catalytic reduction (SCR) catalyst46and an ammonia slip catalyst (ASC)48that receive and condition an exhaust gas stream50exiting the gas engine24. BecauseFIG. 3includes like elements toFIGS. 1 and 2, the like elements are depicted with like numbers. Although the depicted embodiment depicts an SCR catalyst46and an ASC48, it should be appreciated that the aftertreatment system26may include any type of NOxreduction catalyst and ammonia oxidation catalyst, as well as other catalytic converter systems and other components, such as the HRSG mentioned above.

The SCR catalyst46is a particular type of exhaust catalyst used to convert nitrogen oxides into diatomic nitrogen (N2) and water. In addition to being used in the gas engine system24, SCR catalysts46may also be used in utility boilers, industrial boilers, municipal solid waste boilers, diesel engines, diesel locomotives, gas turbines, and automobiles. The SCR catalyst46may use ammonia to help trigger the reaction that converts the nitric oxides in the exhaust gases to N2and water. However, some ammonia may remain within the exhaust gas stream50that is not consumed in the chemical reaction. To convert the remaining ammonia to N2, the aftertreatment system includes the ASC48. The ASC48may be a zeolite style catalyst, in that it may use microporous, aluminosilicate minerals to provide the chemical reactions that oxidize the ammonia within the exhaust gases.

To cause the desired reactions within the SCR catalyst46and the ASC48, urea is injected into the exhaust gas stream50upstream of the SCR catalyst46. The injection may be continuous or discrete, and may be controlled by either the control system28and/or the urea injection control system34, as will be described in further detail below. Further, while the embodiments described herein describe an injection of urea into the exhaust gas stream50, it should be appreciated that the embodiments can be modified for any suitable gaseous reductant, such as anhydrous ammonia and aqueous ammonia. Additionally, the amount of urea injected into the exhaust gas stream50may be based on the volume of the urea, the mass of the urea, or the potential of chemical reduction due to the urea injection. Once the urea is injected into the exhaust gas stream50, the exhaust gas stream50enters the SCR catalyst46and then the ASC catalyst48, which convert the nitrogen oxides and ammonia within the exhaust gases to N2and water as described above.

As stated above, the urea injection control system34monitors the performance of the aftertreatment system26. In particular, the urea injection control system34may determine the appropriate amount of urea to inject into the exhaust gas stream50based on the current NOxconversion rates of the catalysts, the operating characteristics (e.g., temperature, fluid flow, pressure, urea type) of the catalysts, and the desired conversion rates for the aftertreatment system26. This, in turn, may reduce the amount of NOxthat remains in the exhaust gases, enabling the power generation system10to achieve lower NOxemission values, particularly for power generation systems10that utilize lean-burn engines. The urea injection control system34may also determine the appropriate operating window for particular characteristics of the ASC48to maximize the selectivity of ammonia being converted to N2. Further, the urea injection control system34may prompt diagnostic evaluations of and certain action (e.g., alarms, alerts, corrective actions) for the aftertreatment system26.

The urea injection control system34, as shown inFIG. 3, may be separate from the control system28, and may contain a processor, memory, and a hardware interface similar to those of the control system28. In other embodiments, the urea injection control system34may be part of the control system28. For example, the urea injection control system34may reside in one of multiple controllers within a distributed control system, as described above, or may be provided as computer instructions executable via the control system28.

In one example, the urea injection control system34may use the data collected by sensors30to determine the temperature of the exhaust gases after exiting the SCR catalyst46and the amount of NOxin the exhaust gases in an area or areas between the SCR catalyst46and the ASC catalyst48. In other embodiments, the urea injection control system34may determine the measurements using virtual measurements derived from models (e.g., first principle models such as kinetic models, statistical models, neural networks, genetic algorithms, and/or data mining models) of the aftertreatment system26and its components, as well as models of the engine24and the engine system18as a whole.

The urea injection control system34may adjust the amount of urea injected into the exhaust gas stream50based on whether the temperature of the exhaust gas stream50after exiting the SCR catalyst46falls within a desired operating window. Similarly, the urea injection control system34may adjust the amount of urea injected into the exhaust gas stream50based on whether the NO values of the exhaust gases in areas between the SCR catalyst46and the ASC48are less (or more) than set reference values. The operating window and the reference values may be derived by using bench reaction experiments. In one embodiment, the bench reaction experiments are performed in the lab, and the results included in a table or other data structure stored in memory. In another embodiment, the bench reaction experiments may be performed in the field, and may additionally be performed in real-time to provide for real-time inclusion of the results.

Alternately or additionally, the operating window and the reference values may be derived using models of the aftertreatment system26and its components or the engine system18and its components (e.g., first principle models such as kinetic models, statistical models, neural networks, genetic algorithms, and/or data mining models). Although the present urea injection control system34adjusts the amount of urea injected into the exhaust gas stream50based on the temperature of the exhaust gas stream50after exiting the SCR catalyst and the NO values of the exhaust gas stream50in areas between the SCR catalyst46and the ASC48, it should be appreciated that the urea injection control system34may be configured to adjust the amount of urea injected into the exhaust gas stream50based on other operating characteristics of the aftertreatment system26, e.g., observed pressures, fluid flows, and so on.

In addition to adjusting the urea injection, the urea injection control system34may also adjust the ASC48operating temperature window to maximize ammonia selectivity to N2, as mentioned above. As noted above, the ASC48converts ammonia to N2; however, the ASC48may also convert ammonia to NOx. Therefore, maximizing the ammonia selectivity to N2entails increasing the probability that the ASC48will convert the ammonia to N2. Further, there may be a relationship between the operating temperature window of the ASC48and the ammonia selectivity to N2. That is, the inlet temperature of the ASC48may affect the efficiency of the chemical reactions that occur within the ASC48. For example, in some ASCs48that contain zeolite formulations, the ammonia selectivity to N2is maximized when the inlet temperature of the ASC48is between 400-510° C.

To maximize or otherwise improve the ammonia selectivity to N2, the urea injection control system34may use the following formula:

Further, as noted above, the urea injection control system34may perform or start diagnostic evaluations of the aftertreatment system26and its components based on the data collected. For example, as described below, the urea injection control system34may start a diagnostic evaluation of the ASC48if the NOxvalue of the exhaust gases after exiting the ASC48is less than a set reference value. Either the urea injection control system34or the control system28may perform the diagnostic evaluations. In some embodiments, the urea injection control system34may also prompt corrective action (e.g., a warning to an operator to schedule maintenance) based on the results of the diagnostic evaluations. Alternately, the control system32may prompt corrective action.

Indeed, the techniques described herein exploit the aspect of ASC48temperature window for maximum NH3 selectivity to N2 and commands extra urea injection when optimal NOx conversion across SCR catalysts is not achieved in an SCR-ASC aftertreatment network. This may be implemented by reading the temperature at SCR catalyst out, computing NOx conversion across SCR catalyst through sensing NOx before and after the SCR catalyst46, comparing NH3 storage profiles, reading NOx sensor30at ASC out, and then commanding extra urea for more optimal NOx reduction if a certain logic is met, as described below with respect toFIG. 4.

Turning now toFIG. 4, the figure is a flow chart of an embodiment of a process60suitable for execution by the urea injection control system34for controlling the aftertreatment system26. Although the process60is described below in detail, the process60may include other steps not shown inFIG. 4. Additionally, the steps illustrated may be performed concurrently or in a different order. The process60may be implemented as computer instructions or executable code stored in the memory38and executed by the processor36, as described above.

Beginning at block62, the urea injection control system34determines the temperature of the exhaust gases after exiting the SCR catalyst46, which is referred to below as the SCR exit temperature64and illustrated inFIG. 4as SCREXIT. As mentioned above, the SCR exit temperature64may be determined based on readings from a temperature sensor30, or from virtual measurements derived from a model of the aftertreatment system26and its components and the engine system18and its components.

At block66, the urea injection control system34determines whether the SCR exit temperature64is between a lower limit and an upper limit. The temperature limits may be determined via bench reaction experiments either in real-time or offline, as described above, and may be stored on the memory38. If the urea injection control system34determines that the SCR exit temperature64is not within the lower and upper limits, then it may proceed to block68. At block68, the process60(e.g., the urea injection control system34) determines whether the SCR exit temperature64is below the lower limit or above the upper limit. Based on the determination at block56, the urea injection control system34adjusts the amount of urea injected into the exhaust gas stream at block70. The urea injection control system may then return to the beginning of the process60at block62.

If the urea injection control system34determines that the SCR exit temperature64is within the lower and upper limits, then at block72, the urea injection control system34determines the NOxvalue of the exhaust gases after exiting the SCR catalyst, which is referred to below as the SCR exit NOxvalue74and labeled inFIG. 4as NOx,SCR. As stated above, the SCR exit NOxvalue74may be determined via readings from a sensor30, such as gas analyzer, or from virtual measurements derived from a model of the aftertreatment system26and its components and the engine system18and its components.

Using the SCR exit NOxvalue74, at block76, the urea injection control system34computes an estimate of the NOxconversion rate of the SCR catalyst46, referred to below as the estimated SCR NOxconversion rate78and labeled as estimated SCR_NOx. At block80, the urea injection control system34determines whether the estimated SCR NOxconversion rate is less than a target SCR NOxconversion rate, illustrated inFIG. 4as target SCR_NOx. If not, then the urea injection control system34then returns to determining the SCR exit temperature64at block62.

If the urea injection control system34determines that the estimated SCR NOxconversion rate is less than the target SCR NOxconversion rate, then, at block82the urea injection control system34determines the amount of ammonia stored in the SCR catalyst46, referred to below as the ammonia storage value84and labeled inFIG. 4as NH3,SCR. As mentioned above, the SCR catalyst46uses ammonia to trigger the chemical reaction of converting nitrogen oxides to N2and water. Therefore, if a large amount of ammonia remains within the SCR catalyst46after the exhaust gases pass through the SCR catalyst46, then it may be an indication that the SCR catalyst46is not functioning as desired. Accordingly, at block86, the urea injection control system34determines whether the ammonia storage value is less than an ammonia storage reference value, labeled inFIG. 4as NH3,STORAGE. If so, then the urea injection control system34may begin a diagnostic evaluation of the SCR catalyst46at block88. The diagnostic evaluation of the SCR catalyst46may be performed by either the urea injection control system34or the control system28. Further, either the urea injection control system34or the control system28may prompt corrective action (e.g., an operator warning to schedule maintenance) based on the results of the diagnostic evaluation, as mentioned above.

If the urea injection control system34determines that the ammonia storage value is not less than an ammonia storage reference value, then, at block90, the urea injection control system34determines the NOxvalue of the exhaust gases after exiting the ASC48, which is referred to below as the ASC exit NOxvalue92and labeled inFIG. 4as NOx,ASC. As mentioned above, the ASC exit NOxvalue92may be derived from readings by a sensor30, such as a gas analyzer, or from models of the aftertreatment system26and its components and the engine system18and its components.

At block94, the urea injection control system34determines whether the ASC exit NOxvalue92is less than the system NOxreference value, which may be representative of the desired amount of NOxpresent in the exhaust gases after passing through the aftertreatment system26and is illustrated inFIG. 4as NOx,SYSTEM. If it is, then the urea injection control system34may begin a diagnostic evaluation of the ASC48at block96. As mentioned above, either the urea injection control system34or the control system28may perform diagnostic evaluations, and either may prompt corrective action based on the results of the diagnostic evaluation.

If the urea injection control system34determines that the ASC exit NOxvalue is not less than the system NOxreference value, then the urea injection control system34may increase the amount of urea injected into the exhaust gases at block98. The increase may enable the SCR catalyst46to achieve the desired conversion rate while still allowing the ASC48to oxidize any remaining ammonia in the exhaust gases downstream of the SCR catalyst46. The urea injection control system34then returns to determining the SCR exit NOxvalue74at block72.

Technical effects of the invention include monitoring and adjusting the operation of an aftertreatment system of a power generation system. Certain embodiments enable improving the performance of an aftertreatment system by adjusting the amount of urea injected into exhaust gases prior to treatment by the aftertreatment system based on operating characteristics of the aftertreatment system. For example, the present urea injection control system may adjust the amount of urea injected into exhaust gases based on the temperature of the exhaust gases after exiting an SCR catalyst as well as the nitrogen oxide conversion rates of an SCR catalyst and an ASC. Other embodiments enable adjusting operating characteristics of the aftertreatment system to improve the conversion rates of various components. For instance, the present urea injection control system may determine the probability that an ASC converts ammonia to N2and may adjust the operating temperature window of the ASC to increase the probability. The technical effects and technical problems in the specification are exemplary and not limiting. It should be noted that the embodiments described in the specification may have other technical effects and can solve other technical problems.