Dynamic control of a power boiler

Abstract

A dynamic model of a 300000lb/h natural-circulation boiler, operating with a stop-valve pressure and temperature of 950lbf/in2 gauge and 496°C, respectively, is used to investigate possible alternative methods of boiler control. First, conventional analogue controllers incorporating proportional plus integral action are considered. The controller-gain constants are adjusted by informed trial and error, to obtain, in a practical manner, the ‘best’ closed-loop transient behaviour of the system. A digital computer is then introduced, to effect online control of the boiler unit. Initially, this computer simulates the action of the conventional analogue controllers, and the gain constants of this system are also adjusted by trial and error to produce the ‘best’ closed-loop responses for this direct-digital-control scheme. The digital computer is then used online to produce closed-loop-system responses, which may be either nonoptimal or optimal with respect to a specified scalar performance index. The nonoptimal and optimal controller matrices are computed using Kalman's formulation of Bellman's dynamic-programming principle. A comparison is made of the closed-loop-system responses obtained using the alternative methods of control. As a result of this comparison, it is believed that integrated optimal schemes of control would be unlikely, in practice, to significantly improve the dynamic performance of the particular type of single-unite power-boiler process considered. This is not in accordance with the results of previously published work on power boilers, and an attempt is made to explain why this is so. Finally, the results may be expected to provide a more realistic assessment of the gains which could be expected from the introduction of integrated optimal schemes of control to the single-unit power-boiler process.