Characteristic Analysis On Working Medium-Energy Balance Of Once-through Boiler

Supercritical once-through boiler is the inevitable choice of large capacity, high parameter thermal power units, and the building of its simplified nonlinear dynamic model is the basis of designing good control system and effective control of the key operation parameters. Once through boiler can hardly proceed to use the model of traditional drum boiler mainly because of their big difference in steam water system dynamic characteristics. The core of modeling lies in analyzing the balance between energy and material flow of the once through boiler( steam water system). Then the differential equation described, with main parameters having clear physical meaning’s dynamic model is established, and it can reflect the object main dynamic and nonlinear properties. Based on design and experimental data of ZX1000 MW, JB1000 MW, ND660 MW, the working medium-energy balance characteristics of once through boiler was studied by combining mechanism, data and simulation analysis.Whether superheated steam temperature control system is in the automatic or not can make a major difference in the character of working medium-energy balance. When it is in the automatic, intermediate point enthalpy changing within a narrow range, the control system can maintain integral constant of coal to water ratio by regulating desuperheating water flow. That is to say, in a small scale intermediate point enthalpy can be a variable when coal to water ratio is fixed, and that is described as the multi equilibrium point phenomenon of intermediate point enthalpy. Then utilizing design data of the units conducted quantitative analysis on the phenomenon and elaborated its main character in practical work.In the study of regenerative characteristic, the changing process of phenomenon was simplified into two ideal phases, constant volume heat and adiabatic expansion. The result indicates that the regenerative and load changes exist monotonic relationship, and the former one is mainly the change of metal heat storage.The scheme control of intermediate point enthalpy includes two type, water following coal control and coal following water control. By establishing simulation model and testifying field data, it shows that the scheme of water to coal is more stable and with smaller error comparing to the other.