Determination of mill and boiler characteristics and their effect on steam pressure control

Abstract In the Midlands Region of the C.E.G.B., the conventional automatic control of boiler outlet steam pressure by means of the demand to the pulverized fuel mills has been found to be unstable on some coal-fired boiler/turbine units. This paper describes an investigation on a 120 MW unit at Rugeley “A” power station. The frequency responses of mills and boiler are obtained from tests in which the mill demand was perturbed with single frequency sinusoids. It is impracticable to measure the fuel output from the mills directly, but this is inferred from an oxygen in flue gas measurement at constant boiler air flow. To obtain a sufficiently fast measurement response, direct-in-gas zirconia probes were used to measure oxygen concentration. The boiler response is also inferred from this measurement and a boiler outlet steam pressure measurement. A low-order, linear mathematical model is derived which is in good agreement with the experimental data. The milling plant is described by two parallel processes. The one provides a very rapid response but is limited both in magnitude and duration. The other is much slower and provides a sustained response. The boiler response approximates to a first order lag with a time delay. This model is used to analyse plant stability and accounts for two different cases of instability which have arisen when controlling the plant.