Patent Application
20170167341
The invention relates generally to systems for emission control in power plants, and more particularly to systems for controlling flow of reductant to control emissions in the power plants.
Industrial emanations such as nitrogen oxides and sulphur dioxide create environment pollution. Environment pollution is regulated in most industries. Stringent regulation requirements are being adopted by governments and standard bodies in order to minimize the discharge of noxious gases into the atmosphere by industrial facilities.
Analysis of emanations from exhaust is performed to determine the amount of emissions for the purpose of complying with the regulation requirements. Emission analysis may be performed continuously by using a gas analyzer installed in the exhaust stack. Alternatively, the emission analysis may be performed using the gas analyzer connected to the exhaust stack through an extractive system. However, continuous analysis is expensive due to installation cost, maintenance and calibration requirements. A computer based model may be used to predict emissions such as nitrogen oxide (NOx) emission in order to reduce the cost of analysis of emanations. A number of predictive parameters associated with the fuel conversion process such as temperature, pressure, are used by the computer based model to determine an estimate of the amount of the emissions.
Methodologies used in the past include nonlinear statistical, neural network, eigenvalue, stochastic, and other methods of processing the input parameters from available field devices and to predict process emission rates and combustion or process efficiency.
A reduction reactor may be used in the exhaust system of power plant and engine systems to treat emanations to reduce emissions. Specifically, a reductant such as ammonia is injected into the exhaust gas stream entering the reduction reactor to remove emissions such as NOx from the exhaust gas stream. A portion of the injected reductant may remain unreacted with the emissions and may come out of the reduction reactor along with the exhaust gas combined with unreduced emissions.
In accordance with one aspect of present specification, a method of emission control is disclosed. The method includes receiving a slip set-point and a residual set-point corresponding to a reductant from a selective catalyst reduction (SCR) reactor. The method further includes receiving a plurality of inlet parameters of the SCR reactor, wherein the plurality of inlet parameters comprises a concentration of emission gas. The method also includes receiving a slip value corresponding to the reductant from outlet of the SCR reactor. The method includes generating a feedback signal value using a gain scheduling approach based on the slip set-point and the slip value. The method further includes generating a feedforward signal value using a gain scheduling approach based on a residual value of the reductant on a catalyst surface within the SCR reactor and the plurality of inlet parameters using a time-varying kinetic model. The method also includes determining a flow set-point corresponding to the reductant based on the feedback signal value and the feedforward signal value. The method includes regulating injection of the reductant into the SCR reactor based on the flow set-point.
In accordance with another aspect of present specification, a system for emission control is disclosed. The system includes a selective catalyst reduction (SCR) reactor having an inlet, an outlet and a catalyst disposed in the SCR reactor. The system further includes a signal acquisition unit configured to acquire a slip set-point and a residual set-point from the selective catalyst reduction (SCR) reactor and measure a plurality of inlet parameters of the SCR reactor and a slip value from outlet of the SCR reactor. The plurality of inlet parameters comprises a concentration value of emission gas. The system further includes an injector unit coupled to the SCR reactor configured to inject a reductant into the SCR reactor. The system also includes a regulator unit coupled to the signal acquisition unit and the injector unit and configured to generate a feedback signal value using a gain scheduling approach based on the slip set-point and the slip value. The regulator unit is further configured to generate a feedforward signal value using a gain scheduling approach based on a residual value on a catalyst surface within the SCR reactor and the plurality of inlet parameters using a time-varying kinetic model. The regulator unit is also configured to determine a flow set-point based on the feedback signal value and the feedforward signal value. The regulator unit is configured to regulate injection of the reductant into the SCR reactor based on the flow set-point.
In accordance with one aspect of present specification, a non-transitory computer readable medium having a program is disclosed. The program instructs at least one processor to receive a slip set-point and a residual set-point corresponding to a reductant from a selective catalyst reduction (SCR) reactor. The program further instructs the at least one processor to receive a plurality of inlet parameters of the SCR reactor and receive a slip value from outlet of the SCR reactor. The program also instructs the at least one processor to generate a feedback signal value using a gain scheduling approach based on the slip set-point and the slip value. The program instructs the at least one processor to generate a feedforward signal value using a gain scheduling approach based on a residual value on a catalyst surface within the SCR reactor and the plurality of inlet parameters using a time-varying kinetic model. The program further instructs the at least one processor to determine a flow set-point based on the feedback signal value and the feedforward signal value. The program also instructs the at least one processor to regulate injection of the reductant into the SCR reactor based on the flow set-point.
These and other features and aspects of embodiments of the present invention 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:
Embodiments of methods and systems for emission control in power plants include receiving a slip set-point value and a residual set-point value from a selective catalyst reduction (SCR) reactor and measuring a plurality of inlet parameters of the SCR reactor. A slip value from outlet of the SCR reactor is also measured. A feedback signal value is generated using a time-varying proportional-integral (PI) controller based on the slip set-point and the slip value. A feedforward signal value representative of the reductant on catalyst surface within the SCR reactor is estimated based on the plurality of input parameters using a time-varying kinetic model. A flow set-point is determined based on the feedback signal value and the feedforward signal value. The flow of the reductant into the SCR reactor is regulated based on flow set-point.
The term emission refers to hazardous chemical components present in exhaust emanations such as nitrogen oxide emissions measured in parts per million (ppm). The term Selective Catalytic Reduction (SCR) refers to a chemical reaction that converts nitrogen oxides into diatomic nitrogen, and water with the aid of a catalyst. The term ‘SCR reactor’ refers to a chamber where the selective catalytic reduction reaction occurs. The term ‘reductant’ generally refers to a chemical element or a compound that loses an electron in a chemical reaction to another chemical element, or a compound. The term ‘slip set-point’ refers to a design parameter indicative of a reference amount of reductant that comes out of outlet of the SCR reactor. The term ‘residual set-point’ refers to a design parameter indicative of a reference amount of reductant residual that is deposited on a catalyst within the SCR reactor. The term ‘slip value’ refers to an amount of the reductant at the outlet of the SCR reactor. The term ‘residual value’ refers to an estimated amount of reductant deposited on a catalyst within the SCR reactor. The term ‘reductant estimate’ refers to an estimate of the slip value of the reductant. The term ‘flow set-point’ refers to a reference amount of the reductant to be introduced into the SCR reactor for reduction of nitrogen oxides. The term ‘feedback signal’ refers to a component of the flow set-point corresponding to the slip value. The term ‘feedforward signal’ refers to a component of the flow set-point corresponding to the residual value.
The signal acquisition unit 118 is communicatively coupled to the power plant 102 and configured to receive the plurality of parameters 104. In one embodiment, the plurality of parameters includes a plurality of inlet parameters and a slip value measured at the outlet of the SCR reactor 108. The plurality of inlet parameters include, but not limited to, ammonia (NH3), oxygen (O2), nitrogen monoxide (NO), nitrogen dioxide (NO2), and combinations thereof. The signal acquisition unit 118 is also configured to retrieve a slip set-point from a predetermined memory location. In one embodiment, the slip set-point is determined by offline experiments and is stored in a memory location accessible by the signal acquisition unit 118.
The injector unit 120 is coupled to the power plant 102 and configured to introduce a reductant into the SCR reactor 108 through the injector 116 in a controlled manner to optimally reduce the emissions from the outlet 110 of the SCR reactor 108. In one embodiment, the injector unit 120 receives a flow set-point value determined based on the plurality of inlet parameters and the slip set-point. The injector unit 120 determines a rate of flow of the reductant into the SCR reactor based on the flow set-point.
The regulator unit 122 is communicatively coupled to the signal acquisition unit 118 and the injector unit 120. Further, the regulator unit 122 is configured to receive a plurality of inlet parameters, and the slip value from the outlet 110. In one embodiment, the regulator unit 122 is configured to receive a reductant measurement from a reductant sensor disposed at the outlet of the SCR reactor 108. However, there is a time delay associated with the reductant measurement from the reductant sensor. Advantageously, a time-varying kinetic model is used to provide an estimate of the reduction slip value based on the reductant measurement. Using the time-varying kinetic model at least in part compensates the effect of a time delay otherwise caused in such measurements. In some embodiments, a feedback gain may be applied by the regulator unit 122 to the slip value provided by the time-varying kinetic model. Further, the regulator unit 122 is configured to estimate a feedforward signal value representative of the reductant on catalyst surface within the SCR reactor based on the plurality of inlet parameters using the time-varying kinetic model. The regulator unit 122 is also configured to generate a feedback signal value using a time-varying proportional-integral (PI) controller based on the slip set-point and the slip value. In one embodiment, a difference between the slip value and the slip set-point to generate a slip difference value. The slip difference value is used to generate the feedback signal. The regulator unit 122 is also configured to determine a flow set-point based on the feedback signal value and the feedforward signal value. In one embodiment, the flow set-point is determined based on the amount of emissions from the outlet of the SCR reactor. In another embodiment, the regulator unit 122 is configured to determine the flow set-point based on a gain value corresponding to the nitrogen oxide emissions. The flow set-point is estimated using a time-varying kinetic model using a complimentary filtering technique. The regulator unit 122 is also configured to regulate the injection of the reductant into the SCR reactor 108 based on the flow set-point.
The regulator unit 122 disclosed herein is configured to determine a trade-off between the amount of emission and the slip value. For a given regulatory specification, the feedforward gain and feedback gains of the regulator unit 122 may be used to maintain both the emissions such as NOx and reductant residual such as ammonia within the acceptable limits. In one embodiment, the feedforward gain and the feedback gains are determined based on a gain scheduling approach. The gain scheduling approach includes determining the feedforward gain and the feedback gain dynamically based on at least one of the amount of emissions at the inlet of the SCR, temperature and flow of emissions at the inlet of the SCR. In some embodiments, the regulator unit 122 is configured to retrieve at least one of the feedback gain and the feedforward gain from a memory. In some of these embodiments, the gain values may be computed apriori based on offline experiments, and subsequently stored in the memory. In other embodiments, at least one of the feedforward gain and the feedback gain are provided by a user.
The processor unit 124 is communicatively coupled to the communication bus 132 and may include at least one arithmetic logic unit, a microprocessor, a general purpose controller or a processor array to perform the desired computations or run the computer program. In one embodiment, functionality of the processor unit 124 may be limited to tasks performed by the signal acquisition unit 118. In another embodiment, the functionality of the processor unit 124 may be dependent upon the functions performed by the injector unit 120. In another embodiment, the functionality of the processor unit 124 may be dependent upon the functions performed by the regulator unit 122. While the processor unit 124 is shown as a single unit, in exemplary embodiments, the emission control system 100 may include two or more processor units. Further, the single or plurality of processor units may have the functionality of one or more of the signal acquisition unit 118, the injector unit 120, and the regulator unit 122. Although the system 100 is shown as a different unit from the power plant 102, in some embodiments, the system 100 may be integrated with the power plant 102.
The memory unit 126 is communicatively coupled to the processor unit 124 and is configured to be accessed by at least one of the units 118, 120 and 122. In an exemplary embodiment, the memory unit 126 may refer to one or more of memory modules. The memory unit 126 may be a non-transitory storage medium. For example, the memory may be a dynamic random access memory (DRAM) device, a static random access memory (SRAM) device, flash memory or other memory devices. In one embodiment, the memory may include a non-volatile memory or similar permanent storage device, media such as a hard disk drive, a floppy disk drive, a compact disc read only memory (CD-ROM) device, a digital versatile disc read only memory (DVD-ROM) device, a digital versatile disc random access memory (DVD-RAM) device, a digital versatile disc rewritable (DVD-RW) device, a flash memory device, or other non-volatile storage devices. In one specific embodiment, a non-transitory computer readable medium may be encoded with a program to instruct at least one processor to perform functions of one or more of the signal acquisition unit 118, the injector unit 120, and the regulator unit 122.
The regulator unit 200 is configured to receive a feedback input, generally represented by reference numeral 204. The feedback input is representative of the slip value obtained from measurements. In one embodiment, a measurement value 228 is the reductant measurement representative of slip value obtained from a reductant sensor disposed in the exhaust of the SCR reactor 108. It should be noted that the measurement value 228 is a delayed response of the slip value. In one embodiment, a complimentary filtering technique is applied to the measurement value 228 to obtain the feedback input 204. Specifically, the complimentary filtering generates the feedback input 204 based on a reductant estimate generated by an SCR model 224 representative of the time-varying kinetic model based on the reductant measurement.
In one embodiment, the time-varying kinetic model is a mathematical model representative of chemical reactions that take place in the SCR reactor. In the SCR reactor, ammonia is injected from the injector into the exhaust gas stream and may react, in the presence of the catalyst, with NOx to produce nitrogen (N2) and water (H2O). The chemical reactions include, but are not limited to, ammonia adsorption and desorption reaction with the catalyst, ammonia oxidation reaction, standard SCR reaction, fast SCR reaction, NO2 SCR reaction, and NO oxidation reaction. The mathematical model includes a set of algebraic equations and a set of ordinary differential equations characterizing reactions within the SCR reactor. The set of algebraic equations includes a plurality of rate equations describing characteristics of the individual reactions such as concentration change of each chemical reactant or product. The set of algebraic equations also include equations for catalyst temperature, mass balance equation on ammonia, nitrogen oxide, and nitrogen dioxide. The set of ordinary differential equations include equations for mass balance on ammonia surface coverage. The set of algebraic equations and the set of ordinary differential equations of the mathematical model have the plurality of parameters such as chemical composition and concentration of each chemical reactant or product and the coverage ratio of ammonia on the catalyst.
An error value 206 is generated based on the reference input 202 and the feedback input 204 using a subtractor 208. The subtractor 208 is used to determine a difference value between the reference input 202 and the feedback input 204. A time-varying proportional integral controller 212 is used to generate a feedback signal value 210 based on the error value 206. A feedback gain 238 is applied in the controller 212 while generating the feedback signal value 210. In one embodiment, the SCR model 224 may also generate the feedback gain 238 based on at least one of temperature, flow and amount of emission gas measured at the inlet of the SCR. The feedback signal value 210 is modified by a feedforward signal value 216 to generate a flow set-point 214. In the illustrated embodiment, the feedforward signal value 216 is determined based on a residual error signal 234 generated as a difference between a residual set-point 232 and a residual value 236. It may be noted that the residual value 236 is representative of an amount of reductant deposited on a catalyst of the SCR reactor 230. In one embodiment, the residual value 236 is determined by the SCR model 224 based on a plurality of SCR inlet parameters 226. The plurality of SCR inlet parameters 226 include, but are not limited to flow rate of ammonia (NH3), percentage concentration of oxygen (O2), concentration values of emissions measured at the SCR inlet, temperature of a gaseous component at inlet of the SCR (represented by symbol Tg), pressure of a gaseous component measured at inlet of SCR (represented by symbol Pg), flow of a gaseous component at the inlet of SCR (represented by symbol Fg), a ratio of NH3 to NOx. The concentration values are measured in parts per million (ppm) units. The flow rate is measured in pounds per hour (lb/hr) units. The emissions include nitrogen monoxide (NO) and nitrogen dioxide (NO2). A feedforward gain 222 is applied to the residual error signal 234 to generate the feedforward signal value 216. The feedforward gain includes a first component corresponding to the residual value 236. Further, the feedforward gain also includes a second component associated with NOx signal 218 corresponding to an amount of emission 220 entering the SCR reactor.
In one embodiment, the complimentary filtering technique compensates the time delay that is inherent in the slip measurement value 228 acquired from the SCR reactor 108. The complimentary filtering disclosed herein is performed using a complimentary filter having a low pass filter circuit coupled to a time delay compensating circuit. In one embodiment, the time delay compensating circuit has a first time constant and the low pass filter circuit has a second time constant matching the first time constant. An output of the low pass is a time delayed signal representative of the slip measurement value 228. In another embodiment, the feedback input 204, representative of the slip value, is determined using a complimentary filtering technique.
In step 608, a time-varying feedforward gain is determined based on a plurality of inlet parameters of the SCR reactor. Further, in the same step, a feedback gain is also determined based on the plurality of inlet parameters of the SCR reactor. In one embodiment, the feedforward gain and the feedback gains are determined using a gain scheduling approach. In another embodiment, the feedforward gain and the feedback gain are retrieved from the memory. The feedforward gain and the feedback gains are used to simultaneously optimize the emission levels and the slip values at the outlet of the SCR reactor.
A feedback signal value is generated using a time-varying proportional-integral (PI) controller based on the slip set-point and the slip value in step 610. In one embodiment, a gain scheduling approach is used in the PI controller for determining the feedback signal value. The gain scheduling approach includes applying a feedback gain applied to the slip value. In one embodiment, a time-varying feedback gain is determined based on concentration of emission gases, a temperature of exhaust gas, and a flow rate value of the reductant. In step 612, a feedforward signal value representative of the reductant on catalyst surface within the SCR reactor is generated. The generation of feedforward signal value is based on a residual difference value based on a residual value and the residual set-point value. The residual value is estimated by the SCR model and is representative of concentration of reductant on the catalyst surface with the SCR reactor. In one embodiment, a gain scheduling approach is used for determining the feedforward signal value. In one embodiment, a time-varying feedforward gain is determined based on one or more combinations of concentration of emission gases, temperature values of exhaust gases, and the flow rate value of the reductant. In other embodiments, other inlet parameters may also be used to determine the feedforward gain value. In one embodiment, at least one of the feedforward gain and the feedback gain are determined apriori based on offline experiments.
The method also includes determining a flow set-point based on the feedback signal value and the feedforward signal value in step 614. The step 614 of determining the flow set-point includes incorporating an estimate of the amount of emission gas. In step 616, injection of the reductant flow is regulated based on the flow set-point. It should be noted herein that the step 614 determining the flow set-point provides a trade-off between the amount of emission and the slip value. The method of the flow chart 600 also includes regulating injection of the reductant into the SCR reactor based on the flow set-point in step 616
It is to be understood that not necessarily all such objects or advantages described above may be achieved in accordance with any particular embodiment. Thus, for example, those skilled in the art will recognize that the systems and techniques described herein may be embodied or carried out in a manner that achieves or improves one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein.
While the technology has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the specification is not limited to such disclosed embodiments. Rather, the technology can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the claims. Additionally, while various embodiments of the technology have been described, it is to be understood that aspects of the specification may include only some of the described embodiments. Accordingly, the specification is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 6606/CHE/2015 | Dec 2015 | IN | national |
