| Biomass,as a renewable carbon-neutral energy,is an excellent substitute for fossil energy.The rational utilization of biomass energy is an important way for China to achieve the "carbon peaking and carbon neutrality" goal.Among the various biomass utilization technologies,cofiring biomass in coal-fired boilers can not only reduce the consumption of fossil fuels and the emissions of pollutants such as NOx and SO2,but also reduce the risk of the corrosion of the boiler tail heating surface caused by biomass burning,which is an effective measure to improve the efficiency of energy utilization.Therefore,in this paper,the 660 MW pulverized coal boiler is taken as the research object,and the numerical simulation software Fluent is used to study the co-combustion process of biomass and pulverized coal,and the influences of biomass injection location,biomass particle size and boiler working conditions on combustion process and NOx emission are investigated.(1)ICEM software is used to carry out physical modeling and partition meshing,and the main combustion area is locally refined.After mesh independence verification,the number of meshes is finally determined to be 2.35 million.In the research process,the realizable turbulence model is used to describe the gas-phase turbulence,and the discrete-phase model is performed to simulate the gas-solid two-phase flow.Meanwhile,the eddy dissipation model is used to calculate the combustion of volatiles,and the two-step competitive reaction model is used to simulate the precipitation of volatiles.In addition,the kinetic/diffusion control reaction rate model is used for coke combustion,and the P-1 radiation model is used for radiation heat transfer.On this basis,the temperature field,composition field and NOx generation the original working condition of the boiler are calculated,and the reliability of the selected model is verified.(2)The influences of biomass injection position and biomass particle size on the temperature field,component concentration field and pollutant emission in the pulverized coal boiler during the co-combustion of biomass are calculated.The calculation results show that the addition of biomass can promote the combustion of pulverized coal to a certain extent,whereas the NOx emission is reduced.When biomass is injected from the primary air vent at the bottom layer,it can release the reducing components such as volatile matter earlier,so that the NOx emission reduction effect is better.Although the change of biomass particle size has little effect on the flue gas temperature,O2 and CO volume fraction in the furnace,the excessive particle size will lead to the reduction of biomass particle burnout rate,thus reducing the required oxygen.As a result,the pulverized coal is easier to burn completely,and the NOx mass concentration in the outlet flue gas increases.(3)Numerical simulation of optimal air distribution and low load combustion of biomass mixing boiler of 660 MW subcritical unit is carried out to explore the influence of excess air coefficient(α),exhaust air ratio and boiler load on the velocity field,temperature field,group fraction field and NOx generation.The change of the burnout air rate has little effect on the temperature field in the furnace,but with the increase of the burnout air rate,the increase of the CO concentration increases the reducing atmosphere,thereby reducing the NOx concentration.However,low burnout air rate can cause problems such as corrosion and coking.The calculation results show that when the burnout air rate is 24%and a is 1.2,NOx emission reduction can be better achieved,and the safety of boiler operation can be ensured.When the boiler load decreases,the amount of air entering the furnace decreases,and the aerodynamic field becomes worse,so that the entrainment effect of flue gas is weakened,the temperature in the furnace gradually decreases,and the fuel burnout rate decreases,which is not conducive to the safe and stable operation of the boiler.To ensure the safety of boiler operation and reduce NOx emissions,it is necessary to optimize the air distribution adjustment when the boiler is operating at low load. |
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