Optimal control strategy for minimization of exergy destruction in boiler superheater

► Optimal controller using LQR approach proposed for SH steam temperature control. ► Exergy destructions in SH evaluated for steady state and transient conditions. ► A rational basis for spray flow distribution between attemperators is established. ► Exergy destruction studied with optimal controllers of varying weighing matrices. ► The model can effectively guide controller tuning to minimize exergy destruction. Steam temperatures in large capacity boilers of modern electric power stations are maintained closely around the design specification by spraying water in the superheater (SH) attemperator to ensure safe and efficient operation and long plant life. Although the process of attemperation involves exergy destruction, and optimal controllers have previously been proposed for steam temperature control, prior studies on such controllers have not considered exergy as an important parameter. Exergy analysis of a two-stage SH attemperator with real time operation parameters in a 500MWe pulverized fuel fired power plant pinpoints the avenues for optimization that is beyond the scope of the traditional First-Law based analysis. Strategies to minimize exergy destruction by suitably varying the proportions of stage I and stage II spray flows are established. Further, a MATLAB-SIMULINK-based model is developed and optimal control strategies are devised for SH steam temperature control following a Linear Quadratic Regulator (LQR) approach. Variation of the process parameters and the exergy destructions during the transient operations of the attemperator under stipulated disturbances have been analyzed using the model, with different values of the controller parameters. Guidelines are formulated for the spray flow controller tuning so that the total exergy destructions during the system transients are minimized.