Simulation of dynamics of a boiler steam superheater with an attemperator

Abstract

This paper presents a numerical method to determine the proper rate of spray water mass flow to be supplied to an injection attemperator. The basic aim of this method is to keep the steam temperature of the superheater outlet at a constant (assumed) level, when the parameters of steam at the inlet to the attemperator and combustion gases at the inlet to the superheater vary. The presented method is suggested for digital control of superheated steam temperature. A crucial condition for correct control is setting up a precise numerical model of the superheater, which would provide fast and accurate determination of the cooling water mass flow supplied to the injection attemperator. For that purpose, a model of a superheater with distributed parameters was developed, which comprises solving equations describing the laws of mass, momentum, and energy conservation. The proposed method to determine the mass flowrate of spray water is based on computed steam temperatures at the inlet and outlet of the analysed superheater stage. The so formulated problem, being an inverse one, was solved using the Regula Falsi method. A computational test verifying the proposed solution is presented. A comparison of numerical calculation results and measurements, taken on the platen superheater of a steam boiler type OP-210 with 210 × 103 kg/h capacity, is also presented. In both cases, a fully satisfactory agreement of the computed steam temperature at the superheater outlet with the assumed value was achieved. The suggested solution to the inverse problem, combined with the proposed model of superheater dynamics, enables one to determine the proper mass flow of spray water in on-line mode and improve the control quality.