Research On Key Process Modeling And Predictive Control For Wet Desulfurization System In Coal-fired Unit

China is a large coal-consuming country,and coal combustion produces pollutants,which is one of the important sources of air pollution in China.In recent years,driven by major national needs to win the defense of the blue sky and build an ecological civilization,significant progress had been made in the emission reduction technology of coal-fired pollutants.The emission concentration of major pollutants in flue gas had been significantly reduced through the application and promotion of ultra-low emission technologies for coal-fired flue gas pollutants.However,with the increasingly strict emission standards of air pollutants and the continuous improvement of ultra-low emission technology of coal-fired power plants,the operation control level of environmental protection devices is difficult to meet the demand.How to realized that the system remains stable,economical and efficient under the conditions of variable-load has become an urgent problem.Therefore,this paper took WFGD in the ultra-low emission system as the research object,carried out the modeling research of the key process and key equipment,as well as the predictive control research and then had realized the industrial application.Firstly,the modeling of the key process and key equipment of WFGD system were researched.Aiming at the sulfur dioxide removal process in the absorption tower,a method for determining the relative lag time the relative lag time between inlet and outlet SO₂ concentrations was proposed,and it was found that the model error of the outlet concentration prediction model considering the lag time correction was significantly reduced,which was 12.3%lower than before the correction.Based on the mechanism of sulfur dioxide removal-conversion,the key influencing factors on the sulfur dioxide absorption process was determined,and then the outlet concentration prediction models based on the mechanism method,data method,and mechanism-data coupling method were established.By comparing the prediction performance of different models on the validation set,it was found that the mechanism and data hybrid model had the best prediction performance,and the RMSE was 3.778.Based on the mechanism of sulfite oxidation,the key influencing factors of the process were determined,and then the oxidation rate model of the slurry droplet during drop process was established.With the model,the influences of temperature,pH value,sulfite concentration and slurry droplet diameter on the oxidation process of sulfite were obtained.Meanwhile,through the establishment of oxidation air volume model of desulfurization system,the recommended oxidizing air volume under different operating conditions was obtained.What’s more,the characteristics of the key equipment in the wet desulfurization system were studied and the relationship between equipment parameters and operating parameters was obtained.Based on the characteristic curve of pump and similarity law,the relationship between the frequency of the slurry circulation pump and the spray flow was obtained.The establishment of key processes and key equipment models for wet desulfurization can provide guidance for the operation of the desulfurization system,and then beneficial to fully explore the energy-saving potential of the subsystems and equipment in the desulfurization system.It also provides important model support for predictive control research.Secondly,aiming at the characteristics of multi-inputs,non-linearity and large delay of WFGD system,the predictive control strategy,based on DMC,was proposed.And the group intelligence algorithm combined with the mechanism model was used to optimize the parameters of the transfer function.The control strategy based on DMC were established.By comparing with PID control,the control quality of DMC is verified.Simulation results showed that,compared with PID control,the distribution of the low-concentration side（less than 10 mg/m³）and the high-concentration side（more than 31 mg/m³）under the control of DMC is reduced.Compared with PID control,DMC has better control effect,,which provides theoretical support for engineering application research.Finally,based on the established DMC,taking the desulfurization system of a50MW cogeneration unit as the object,the industrial application verification of the WFGD control system was studied.Furthermore,the design and operation database of key equipment for wet desulfurization was constructed,and then the long-term stable operation effect of DMC in actual desulfurization system was studied,compared with the effect of PID.The results show that,the total current under variable-frequency control is 50-110 A lower than that under the conventional manual control,which has a good energy saving effect.In addition,compared with PID control,the outlet SO₂concentration fluctuation under DMC control is smaller and the distribution is more concentrated.The distribution on the low concentration side（outlet SO₂ concentration is less than 10mg/Nm³）is 16%less than that on PID control,and the distribution on high concentration side（outlet SO₂ concentration is greater than 35mg/Nm³）decreased by 6%.Compared with PID control,DMC has better control quality.