Numerical Simulation Investigations Into The Effects Of Blending With Bituminous And Brown Coal On The Combustion Characteristics In A 350MW Down-fired Boiler

In this paper,a 350MW supercritical down-fired boiler with multi-injection multi-stage combustion technology was taken as the research object.Numerical simulations were used to study the characteristics of ignition,burnout and NOx emissions of anthracite blending with bituminous coal and brown coal respectively,which can provide theoretical analysis and technical reference for the blending method of boiler coal grinding and coal blending in the furnace.The ignition distance measured by the anthracite industry test under the load of 230 MW of the boiler was compared with the ignition distance of the concentrated pulverized coal gas flow obtained by numerical simulation.The results showed that the difference between the simulated value and the experimental value was small,which verifies the rationality of the selected model.The combustion and NOx generation characteristics of anthracite and bituminous coal under different mixing ratios and grinding methods were studied.By comparing the in-furnace combustion characteristics and furnace outlet parameters of different blending methods under the same anthracite and bituminous coal ratio,the optimal bituminous coal grinding method was selected.When anthracite and bituminous coal were blended in a 3:1 ratio,the bituminous coal burner was arranged on the outside.When anthracite was blended with bituminous coal in a 1:1 ratio,the bituminous coal burner was arranged with an outer hedge arrangement.When anthracite and bituminous coal were blended in a 1:3 ratio,the anthracite burner was arranged on the outside.Through the comparison of the best blending methods under different bituminous coal ratios,when the proportion of bituminous coal blending increased from 0 to 75%,the NOx emission at the furnace outlet were gradually reduced from 681.3 mg/m3(O2=6%)to 477.2 mg/m3(O2=6%),the carbon in fly ash was gradually reduced from 5.49%to 1.45%.In different mixing ratios,the ignition distance of the hard-to-burn anthracite pulverized coal gas stream fluctuated between 1.28m and 1.64m,while the igniting distance of the easy-to-burn bituminous pulverized coal gas stream was between 0.68m and 0,72m.The combustion ratio and NOx generation characteristics of anthracite and brown coal were studied under different ratios and grinding methods.By comparing the in-furnace combustion characteristics and furnace outlet parameters of different blending methods under the same anthracite and brown coal ratio,the optimal brown coal grinding method was selected.When anthracite was blended with brown coal in a 3:1 ratio,the brown coal burner was placed on the outside.When anthracite was blended with brown coal in a 1:1 ratio,the brown coal burners were staggered.When anthracite was blended with brown coal in a 1:3 ratio,the anthracite burner was placed on the inside.Through the comparison of the best blending methods under different brown coal ratios,when the proportion of brown coal blending increased from 0 to 75%,the NOxemissions at the furnace outlet was gradually reduced from 681.3mg/m3(O2=60)to 440.7 mg/m3(O2=6%).However,the carbon in fly ash showed a trend of decreasing first and then increasing.When the proportion of brown coal blending was 25o,the minimum carbon in fly ash of 3.14%could be achieved.When the proportion of brown coal blending exceeds 50%,excessive moisture in the furnace would affect the burning effect of the pulverized coal,and the carbon in fly ash would be significantly increased,which seriously affected the combustion efficiency of the boiler,Considering comprehensively,when the in-furnace blending test of anthracite and bituminous coal was carried out on the boiler unit,the blending test of the bituminous coal ratio of 25%to 75%can be carried out When the in-furnace blending test of anthracite and brown coal was carried out,the blending ratio of brown coal should be controlled within 25%.