Modeling And Energy-saving Optimization Of The Feed-water Control System In A Once-through Boiler Of The Supercritical Unit

With the continuous expansion of the unit capacity,more and more supercritical or ultra-supercritical unit are gradually put into operation in China,and researchers and scholars pay more attention to the safety and economy operation of the power plant unit.It is important of the feed-water control system in once-through boiler for the safety and economy operation of the unit.In this paper,the mechanism model of once-through boiler and the model of feed-water control system in once-through boiler are built firstly,and make some optimized designs with taking intermediate point enthalpy control as the core of the feed-water control system.Specific contents as follows:Firstly,aiming at the problem of over-simplication of traditional control object model which is identification model,a mechanism model of the once-through boiler in a 600 MW supercritical unit is modified.The higher computing accuracy and precision is obtained by using mechanism model instead of identification model of the once-through boiler as control object,and the simulation results are improved closer to the actual control process.So that,the simulation results of the model built in this paper have higher reference value and a more persuasive model is provided for the feed-water control system optimization came up in the later section.Secondly,on the basis of the mechanism model of once-through boiler in a 600 MW supercritical unit,the model of feed-water control system is built.Superheated steam temperature,reheated steam temperature and intermediate point enthalpy are chosen as controlled parameters,and complete the control task of the feed-water control system by adjusting quantity of feed-water and coal with fine-tuning of desuperheating water.In the meantime,an improved feed-water control system is came up,it combines the two traditional feed-water control system such as “the scheme of amendments to feed-water quantity”(i.e.water followed)and “the scheme of amendments to coal quantity”(i.e.coal followed)by using fuzzy control.The simulation results show that this improved feed-water control system model can combine the advantages of two traditional feed-water control systems,the overshoot is effectively shortened and the adjusting time is evidently reduced of the control process,in the other hand,the safety and economy of control process are guaranteed in the same time by applying this improved feed-water control system.Then,on the basis of the improved feed-water control system model and the mechanism model of once-through boiler model mentioned above,load-varying model is added to simulate control process of load-varying.Set the load changing rate at +5MW/s,+10MW/s and +15MW/s to fulfill the simulation,results show that the improved control model performs well under the circumstances of fixing load-varying rate at 10MW/s.Besides,computing model of the once-through boiler’s thermal efficiency by means of “positive energy balance method” is added too,to quantitatively analysis the optimized control effect of the improved feed-water control system model.Simulation results show that during the load-varying process of 75%THA to THA to BMCR,the overshoot and adjusting time are optimized,and in the meantime,the boiler thermal efficiency is increased,the computational accuracy of superheated steam temperature and the reheated temperature is improved too.Finally,the conclusion is obtained: the improved feedwater control system model can make the overshoot and the adjusting time optimized under several conditions,make the boiler thermal efficiency higher in the condition of load-varying,and make the computational accuracy of some main-steam parameters improved.