Online Monitoring Of The Key State Variables And Thermal Economy Optimization Based On The Whole Process Mechanism Modeling For The Coal-fired Power Plants

The thermal power generation,especially coal-fired power generation,is the main source of electricity supply in the current and foreseeable future for China.The coal-fired unit is a time-variant and nonlinear system with large time lag,strong coupling and multi-variables.Due to the incomplete measurements,there still remain many problems in the unit performance monitoring and operation optimization.With the commissioning of a large number of high-parameter and large-capacity coal-fired units,the need for the state monitoring,performance evaluation and operation optimization has become particularly urgent.In this context,this dissertation develops the whole-process mechanism modeling,on-line monitoring of critical state parameters,and operation optimization covering the boiler side and the turbine side.The main innovative research results are as follows:(1)A whole-process mechanism model for coal-fired units covering the boiler side and the turbine side is established.Based on the MATLAB programming environment,a real-time simulation platform with certain versatility and scalability for both subcritical and ultra-supercritical units is developed.(2)Taking advantage of the evaporation system model,heat exchanger system model and flue gas mass flow model,the flue gas temperature at the furnace outlet is estimateed.A dynamic heat transfer model of the semiradiative heat exchanger is established.According to the energy balance,the identification of the flue gas temperature is converted into the optimization problem with the flue gas temperature as the optimized variable.The online evaluation of the heat transfer performance of the heat exchanger is realized based on the estimation results of the flue gas temperature in the horizontal flue.(3)A direct air leakage estimation method based on the temperature distribution of rotary air preheater is established.The correction coefficients are introduced to compensate for the impact of unstable heat transfer on the axial and circumferential temperature distribution of the air preheater.Based on the temperature distribution of the air preheater,the optimization problems which take the direct leakage of the primary air leakage and the secondary air as the optimization variables are obtained.The simulation results of the direct air leakage and air leakage area in the time scale of one day and one month are given.(4)The effect of the regenerative extraction system on the thermal economy of the unit is studied.A model for the target value of the terminal temperature difference(TTD)and the drain subcooler approach(DCA)is established.The steam-water distribution matrix equation of the regenerative extraction system is used to calculate the relative variation of the turbine efficiency.The method is extended to full operation conditions by the rolling update of the the steady states.According to the simulation results,the following conclusions are drawn: High-pressure regenerative heaters have greater impact on unit thermal economy than low-pressure heaters,and the turbine efficiency is more sensitive to variations of the TTD than DCA.(5)The optimization of the condenser pressure based on the fixed-speed and the variable-speed pumps is studied.The thermodynamic characteristics of condensers under varying working conditions are established.For units equipped with dual-speed pumps,the condenser pressure optimization is simplified into an integer programming problem with a finite number of feasible solutions.For the unit equipped with a variable-speed pump,the unit net power is selected as the objective function of the condenser pressure optimization.Considering the dynamic process of circulating water regulation and other factors,the holding time is introduced to impose constraints on the manipulated variables.(6)The heat transfer mechanism of the turbine & cold-end coupling system is analyzed by using the main steam pressure,the pressure and the mass flow rate of the exhaust steam of the low-pressure cylinder as the coupling variables.Under the premise of appropriate assumptions and simplification,the thermodynamic characteristics model of the turbine & cold-end coupling system under varying working conditions is established.The power gain of the unit is chosen as the evaluation index of the thermal economy of the coupling system,and the increment of the unit power gain is taken as the objective function of the coordinated optimization.The simulation results show that the main steam pressure regulation is dominant and the turbine efficiency is improved after the optimization.Under the same load,the optimized main steam pressure is higher than the actual main steam pressure as well as the reference value of the main steam pressure for the sliding pressure operation.