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Show The U N 0 X " controller treats the predicted N O x just as a conventional feedback controller would treat measured NOx . Based on the difference between the predicted N O x and the setpoint, the controller cascades onto an F G R controller to adjust the F G R fan speed. Feedforward/Feedback NOx Control In many cases, regulatory districts require a N O x analyzer. When this is the case, it makes sense to use this information in a feedforward-feedback loop. Figure 5 depicts the overall program logic for the M S W facility given earlier. In the feedforward/feedback case we make use of M R S M to generate advance information. It outputs to a summer, where it combines with the actual process value (PV) measured from the N O x analyzer. By adjusting the summer gains one can weight the actual signal with the predicted (advanced) value. NH3 Flow Nozzle Firing (PV) Pressure (PV) Rqte 3' >r yr >F MRSM NOx PV NOx SP ^ 2. Summer NOx Controller ^ Estimate NH3 Slip Pressure SP PV NH3SP 1 Pressure Controller NH3 Flow Controller NH3 Control Valve Carrier Air Valve Figure 5, Feedforward/Feedback N O x Control This adjusted signal cascades onto the N O x controller. Since predicted N O x weights the measured value, M R S M ameliorates measurement lag. The result is snappy, robust control. Other Experience with M R SM The author has applied the method to other combustion-related emissions with good results. Such processes include fluidized beds, boilers, and process heaters using natural gas, light and heavy fuel oils, M S W , and coal fuels. So M R S M is a validated semi-empirical model for N O x. RSM-Only Algorithms For control of other emissions of interest, a purely empirical approach is often effective because second-order polynomials approximate complex kinetic relations well over some interval of interest, albeit with more parameters. Such polynomials function as truncated Taylor series. Since all analytic functions have Taylor series representations, the method has wide applicability to control of complex processes. However, it is always more 5 |