Effect Of Air Volume Ratio Flow And Combustion Characteristics Of A 350MW W-shaped Boiler

This work concentrated on the world's first 350 MW supercritical down-fired boiler with multi-injection multi-stage combustion technology.By means of single-phase small-scale cold test and numerical simulation,the effects of air volume ratios on the in-furnace flow,combustion and NOx emission characteristics were investigated,which provides theoretical guidance for air ratio design of combustion system.The flow and combustion characteristics under different inner and outer secondary-air ratios showed that,at inner and outer secondary-air ratio of 0:10,the asymmetrical flow field was formed in the furnace,and the downward airflow had great velocity decay and large penetration depth,which caused the obvious phenomenon of downward airflow washing the hopper declined wall.On increasing inner and outer secondary-air ratio from 0:10 to 5:5,the flow field gradually tended to be symmetrical,the airflow downward depth reduced,the flame center in the lower furnace moved upwards,the oxygen concentration in the hopper region rose,and corresponding NOx emissions reduced from 1375 to 699 mg/m3(O2=6%).On increasing inner and outer secondary-air ratio from 5:5 to 10:0,the strengthened carrying effect of the inner secondary air significantly reduced the diffusion of the fuel-rich flow.On one side,the ignition distance of pulverized coal particles was prolonged,which made a gradually shorter pulverized-coal burnout distance and made the burnout rate reduce.On the other side,the unburnt pulverized coal continued to burn under the mixing of OFA,which made the NOx emissions increase from 1375 to 699 mg/m3(O2=6%).At the inner and outer secondary-air ratios of10:0,the concentration of O2 in the area between the downward pulverized-coal flow and the front or rear wall was small,which was easy to cause slagging and high temperature corrosion of water wall.Ultimately the optimal inner and outer secondary-air ratio is determined to be 5:5.The flow and combustion characteristics under different fuel-rich and fuel-lean flow ratios showed that,the aerodynamic fields of the three conditions that fuel-rich and fuel-lean flow ratios are 3:7,4:6 and 5:5 were similar,and the flow fields in the furnace were basically symmetrical with respect to the furnace center line.At the fuel-rich and fuel-lean flow ratio of 0:10,the downward airflow had large penetration depth and kept a great rigidity when it met the tertiary air,so that the tertiary air was compressed with a declined angle.A large share of pulverized coal particles fell into the hopper zone and ignited,the temperature of hopper area washigh and the the concentration of O2 was low.With the fuel-rich and fuel-lean flow ratio increaseing,the airflow downward depth reduced,the flame center in the lower furnace moved upwards,and then the flue gas temperature at the furnace outlet increased.At the fuel-rich and fuel-lean flow ratio of 5:5,the weakened carrying effect of the inner secondary air significantly increased the diffusion of the fuel-rich flow,and then the pulverized coal stream diffused and burned at the center of the furnace not far from the nozzle,which made the residence time of the generated NOx in the reduction zone short.Meanwhile,the small airflow downward depth reduced the combustion stroke of the pulverized coal particles in the furnace,which was unfavorable to pulverized coal burnout.Ultimately the reasonable fuel-rich and lean flow ratio is determined to be 4:6.