Numerical Simulation And System Optimization Of Sorbent Injection Into Flue Gas For Mercury Capture In Coal-fired Power Plant

Mercury is a toxic environmental pollutant with direct impact on human health.Mercury pollution from coal combustion accounts for the largest proportion of all anthropogenic mercury emission sources.The control and optimization of mercury removal system by injecting sorbent into coal-fired flue duct has become a hot research issue.With the help of CFD simulation software and the development of mercury removal model,the prediction and device optimization research on the flue gas injection mercury removal system of coal-fired power plants can provide technical and design guidance.In this paper,the numerical simulation method was used to predict the demercuration efficiency and study the influence of operation parameters on sorbent injection demercuration system of a 75t/h circulating fluidized bed（CFB）boiler and a 300 MW coal-fired unit,respectively.The optimization of operational parameters was also studied.NH₄Br modified rice husk coke sorbent（NH₄Br-RHC）was prepared and characterized.The adsorption experiments were carried out on fixed bed under different equilibrium mercury concentration and flue gas temperature.Based on the experimental data,the adsorption kinetic characteristics and adsorption equilibrium were analyzed.A mathematical model based on Freundlich isotherm for mercury adsorption was established according to the adsorption equilibrium parameters.The results show that NH₄Br-RHC improves the specific surface area and micropore volume ratio,and has rich pore structure.The equilibrium mercury concentration and flue gas temperature have an important influence on the adsorption kinetic parameters.The best temperature for NH₄Br-RHC mercury adsorption 120℃.External mass transfer and surface chemical adsorption are the main control processes.Freundlich and Langmuir equation can well describe the surface adsorption equilibrium process.The Freundlich adsorption constant K_F at different flue gas temperatures is obtained by fitting.A 75t/h CFB boiler sorbent injection demercuration system was designed and manufactured.The onsite experiment of mercury removal by injecting sorbent was carried out in this device,the results were compared with the numerical prediction results under the same conditions,the error between the prediction and experiment is 2.50%-7.66%,which verified the mathematical model.The results show that the Hg^p in flue gas share before injection is more than 90%under different injection rate conditions.The sorbent injection can improve the mercury removal performance of ACI+FF and the whole system.After injection,the Hg^Tremoval rate of WFGD decreases,as a result of part of Hg²⁺is converted into Hg⁰.the comparison between the prediction and the field test result,shows that this model can better predict the mercury removal rate of sorbent injection demercuration in flue duct of coal-fired power plant.On the basis of the above research,the CFD numerical simulation and prediction of a350MW coal-fired unit with sorbent injection demercuration device installed were studied,the influences of operation parameters（injection velocity,injection amount,sorbent particle size,flue gas mercury concentration,nozzle arrangement,nozzle injection angle,et al.）on mercury removal rate were obtained.On this basis,the optimized technical plans and technical parameters of sorbent injection demercuration system for coal-fired units were obtained.The results show that the uniformity of the flue gas flow field distribution is more affected by the geometry structure of the flue gas.It can be considered to improve the uniformity of the sorbent particle distribution by optimizing the arrangement of the sorbent nozzles and setting the flue gas guide plate.With the increase of the injection velocity,the uniformity of the sorbent distribution,the coverage ratio and the mercury removal rate increase,but it has little effect on the residence time.With the increase of the injection rate,the mercury removal rate increases.With the decrease of the sorbent particle size,the residence time decreases to some extent,while the distribution uniformity,coverage ratio and the mercury removal rate increase.With the increase of the mercury concentration at the inlet,the mercury removal rate increases.With the increase of the number of injection tubes or the number of nozzles on a single injection tube,the distribution uniformity,coverage ratio and the mercury removal rate increase.The influence of the above parameters on the system pressure drop can be ignored.The optimization results of sorbent distribution uniformity and mercury removal rate are:vertical flue gas injection>counterflow injection>downstream injection,that of sorbent residence time:counterflow injection>vertical flue gas injection>downstream injection,that of injection demercuration efficiency:vertical flue gas injection>counterflow injection>downstream injection.The sorbent injection rate and sorbent particle size are the main factors affecting the injection mercury removal rate.According to the results,an optimized injection scheme of a 350MW coal-fired unit sorbent injection demercuration system is proposed,which can realize the high uniformity of the cross-section sorbent distribution and the maximization of the coverage of sorbent.The predicted mercury removal efficiency of NH₄Br-RHC injection is 81.60%.