Modeling And Simulation Study On DG1900/25.4 Boiler System

The dynamic characteristics of supercritical unit are very complex, and its stability and economy of operation strongly depend on high performance of control system. It has important practical significance to study and master the dynamic characteristics of supercritical units and explore control technology which can improve the performance of the units.This thesis takes DG1900/25.4-â…¡1 supercritical pressure once-through boiler of East Boiler Company as the object of study. On the basis of analyzing the structure and characteristic of unit, the mathematical model is built through studying the mechanism of unit and dynamic characteristics is researched by simulation experiment. The main contributions of the thesis are as follows:â‘ The mathematical model of supercritical pressure once-through boiler system are obtained, including the models of water wall, steam-water separator, single-phase heat-exchanger, spray desuperheater, bypass system, furnace ,waste gas of the exchanger and steam turbine. The steam generator system composed of water wall and steam-water separator is the key of modeling. Four models of water wall under different way of operation are built through the method of nonlinear lumped parameter moving boundary. The model of steam-water separator is established by lumped-parameter, including dry and wet models. The works above provide the necessary conditions for establishing mathematical model of full scope and large disturbance of the entire boiler. During the process of modeling, the thesis not only provides the basic conservation equation, but also introduces the way of how to deal with the equations in detail, and finally present the complete equations.â‘¡To avoid the problem of calculation overflow and model stiff when switching models, the transition model and preparing transition model are added in this thesis. In preparing transition model, the influence of the change of working fluid mass in pipe is neglected, the flow in the whole pipe is considered as a constant, and outlet working fluid pressure is calculated based on the principle of drop distribution, so the problem of model stiff can be avoided; To avoid calculation overflow, flow and pressure is calculated by using the preparing transition model in transition model, the outlet working fluid enthalpy is calculated by the static equation, the metal temperature is considered equal to nearby point, arithmetic average value of the inlet and outlet is chosen as the parameters of working fluid in representative points. The method successfully can solve the problems when switching between different operation ways of supercritical unit, and make model switch smoothly.â‘¢Based on the modular modeling techniques and variable step Euler algorithm, the model of supercritical pressure once-through boiler system is established. Subsequently, the dynamic simulation experiments at 20%, 50%, 75%MCR with single parameter (fuel quantity, the flow of feed-water, feed-water temperature) step disturbances and variable load experiments are carried out, the main dynamic characteristics of unit under different loads are obtained, and the validity of the simulation model is verified.Using the simulation model established in the thesis, the main dynamic characteristics of supercritical pressure once-through boiler system can be examined and analyzed comprehensively. The simulation model can provide the necessary foundation for mastering the operation characteristics of whole unit and improving the control system and control scheme.