| W-flame boiler is the main type of boiler in China that burns anthracite and low-quality lean coal.In order to ensure the ignition and burnout of low-volatile coal,it is necessary to arrange a fire protection belt in the furnace.The combustion temperature in the furnace can reach 1600-1800℃,and leads to a significant increase of thermal NOx.Generally,the original NOx concentration of a W-flame boiler can reach 1000-1800 mg/Nm3.In order to meet the environmental protection requirements of ultra-low emission,the couple denitration of low-NOx combustion,SNCR,SCR was used in a 600 MW supercritical W-flame boiler.However,it was found that after SNCR was put into operation,SCR ammonia escape was intensified and air preheater was seriously blocked,so SNCR denitration had to be stopped.In order to research the influence of low-NOx combustion and SNCR denitrification on SCR flow field and realize the deep coupling of different denitrification technology,the whole denitrification process including low-NOx combustion,SNCR,SCR was carried out based on CFD simulation.The study found that the vertical space air staged combustion technology could effectively reduce NOx emissions,and achieved a denitration efficiency of 41.3%-46.5%in the range of 50%-100%boiler load.The uneven arrangement of separate over fire air caused obvious temperature deviation at the SCR inlet section.The high temperature flue gas and low temperature flue gas cannot be fully mixed in the SCR device,and the temperature deviation at the inlet continues to the catalyst layer.The SNCR spray gun installed on the front wall of the boiler can only reduce the NOx near the front wall area.Part of the NOx in the flue gas was removed,which caused a serious uneven distribution of NOx concentration at the SCR inlet section.The deviation of the NOx concentration of the SCR inlet section increased with the increase of the SNCR denitration efficiency.The relative deviations of NOx concentration were 34.9%,40.2%,and 53%according to 100%,75%,and 50%boiler load.SNCR denitration had little effect on the velocity field and temperature field distribution in the SCR device.The internal flow of the SCR denitration tower under different boiler loads was calculated according to the non-uniform inlet conditions.The distribution of flue gas velocity and jet angle of the upstream section of the first catalyst layer can meet the technology requirements,but the NH3/NO molar ratios have large deviations.The values were 9.84%,11.63%,and 14.76%,which was the main reason for the increase of SCR ammonia escape.A vortex mixers were designed and installed upstream and downstream of the ammonia injection grid,respectively.The vortex mixer could effectively improve the uniformity of the flue gas temperature distribution and strengthen the mixing of NH3 and NO,but the NH3/NO deviation was still large.The relative standard deviation of NH3/NO upstream of the catalyst was significantly reduced by using vortex mixer and subarea ammonia injection.The values were 4.32%,4.53%and 4.89%respectively under three boiler loads,thus the effective coupling of low-NOx combustion,SNCR,SCR can be reached in the W-flame boiler. |
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