Numerical Simulation Of Combustion And In-furnace Denitration In A 220 T/h CFB Boiler

Circulating fluidized bed(CFB)boiler has been widely used in energy,chemical engineering and environment process due to its high combustion efficiency,fuel flexibility and low NOX emissions.However,with the implementation of ultra-low emission requirements for coal-fired boilers in China,the low NOx emission operation of CFB boilers faces great challenges.Compared with conventional flue gas denitrification technologies such as SCR,SNCR and low-temperature oxidation,in-situ catalytic reduction of NOx by adding denitrification catalyst shows great potential.In-depth study of flow,combustion and NOx behavior as well as in-situ reduction of NOx by denitration catalysts is of great significance.Based on MP-PIC method,a CPFD(Computational Particle Fluid Dynamic)model of a CFB boiler is established,model parameters are evaluated,and the model is then validated.As the grid number is more than 289000 and the cloud resolution is less than 1.71×107,the simulation results are independent.The Wenyu-Ergun model can better predict the volume fraction and the circulation ratio of particles in the furnace.The realtive deviation between simulation results and industrial results are generally less than 5%.There exist bottom dense phase zone,middle transition zone and upper dilute phase zone in the boiler furnace.Gas velocity is upward in the furnace core whilst downward in the near-wall zone,showing a typical core-annular flow structure.The furnace temperature is highest in the bottom dense zone(about 950℃);along the furnace height,the temperature gradually decreases;at furnace outlet,the temperature decreases down to about 850℃.Lean oxygen combustion results in a high CO concentration and strong reducing atmosphere in the bottom dense phase zone,inhibiting the generation of NOx.In the upper of the furnace,the NOx concentration gradually decreases due to reduction by CO species.Increasing primary air increases NOx concentration at the furnace outlet.The ratio of the upper to lower secondary air has little effect on the formation of NOx in the dense phase zone.In addition,small coal particles will increase the CO concentration and enhance the reducing atmosphere in the dense phase zone,which is beneficial to NOx reductions and low NOx emission.By adding denitration catalyst as the second particle phase,in-site NO-CO catalytic reduction reaction is investigated in the CFB boiler.For various particle property and filling way,the flow characteristics of catalyst are clarified.Simulation results show that with the increase of particle density/size,the volume fraction of catalyst particles in the dense phase zone gradually increases.For suspension-type and deposition-type catalysts,a high furnace inventory could be achieved.FillIng the denitration catalyst from the coal inlet is optimal.The NOx removal efficiencies for the fluidization circulation,suspension and deposition catalysts are 48.37%,57.47%and 54.21%,respectively.