Research About Effect Of Air Volume Ratio On A 600MW Supercritical Down-fired Boiler Combustion Characteristic

In recent years, the use of anthracite attracts more and more attention around the world, down-fired boiler has unique advantage in burning anthracite, multi-injection multi-stage combustion technology as a domestic intellectual property rights owned down-fired boiler combustion technology, having huge potential in research and application. This paper took a domestic power plant 600 MW supercritical down-fired boiler with this technology as the research object. On the basis of centralized arrangement of the burner on arch, Fluent14.5 software was used to research the effect of different air volume ratios and inclined angles of arch burners for combustion and the NOx emission characteristics in furnace.Firstly, the flow and NOx emission characteristics in furnace were studied when the inner and outer secondary air volume ratios were 0:10, 4:6, 5:5, 6:4 and 10:0. When the inner and outer secondary air volume ratio changed from 0:10 to 10:0, the symmetry of flow field in furnace significantly changed, the stroke of pulverized coal in furnace shorten at first, then gradually changed longer; when the inner and outer secondary air volume ratio were 4:6 and 10:0, the flow field in the furnace appeared skew. With the inner and outer secondary air volume ratio increased from 0:10 to 10:0, the flue gas temperature of furnace exit decreased, oxygen concentration gradually increased, NOx concentration significantly increased. Oxygen concentration in the middle and lower regions of cold ash hopper, when the inner and outer secondary air volume ratio was 5:5, oxygen concentration of the middle and lower regions was highest. Therefore, 5:5 was ultimately determined as the optimal inner and outer secondary air volume ratio.Secondly, the effect on combustion and NOx emission characteristics in furnace of different secondary air and tertiary air ratio were studied. Assumed that secondary air and tertiary air were keep unchanged at the ratio of 63.75% in research, changed the secondary air at a ratio of 19.16%, 27.38%, 33.64%, 42.36% and 51.30%. When the secondary air volume ratio was 19.16%, the flow field in furnace became skewed. When the secondary air volume ratio increased from 19.16% to 51.30%, the symmetry of flow field would gradually change for the better, the punch depth of pulverized coal in furnace gradually increased, the center of the furnace flame downward shifted, the flue gas temperature of furnace exit decreased, oxygen content gradually decreased, NOx concentration gradually decreased from 1290.4mg/m3 to 1040.6mg/m3(equivalent to 6% oxygen,same below). However, the secondary air ratio was too much as 51.30%, the center of the furnace flame was too low, which could easily cause burning loss cold ash hopper and thermal fatigue. In summary, 42.36% was selected as the best secondary air ratio.After that, the effect on flow and combustion characteristics in furnace of inclined angle of arch burners was researched. With the inclined angle of arch burners gradually increased, the symmetry of flow field was well, the symmetry of temperature field would gradually deteriorated, the scour action of primary air and secondary air against the front and rear walls gradually enhanced. When inclined angle of arch burners gradually increased from 0° to 20°, the punch depth of pulverized coal in furnace significantly shorten, the airflow inflection point height from the dense phase under arch decreased from 17 m to 7.5m, which to some extent eased over-temperature of cold ash hopper, but at the same time the center of the flame moved upward led to the flue gas temperature of furnace exit higher. When the inclined angle of arch burners increased, oxygen concentration in upper furnace gradually decreased, NOx concentration gradually increased from 889.1mg/m3 to 1220.6mg/m3. Taken together, 5° was recommended as the best inclined angle of arch burners, suggested not more than 10°.Finally, under the optimized parameters of inner and outer secondary air volume ratio, secondary air ratio and inclined angle of arch burners, pulverized coal could punch to the middle and upper region of cold ash hopper, the symmetry of flow filed, temperature field and flue gas component concentration field in furnace were well. High temperature region appeared in the upper furnace and the middle and upper of cold ash hopper, as for the front and rear walls, lower temperature and higher oxygen content, which could reduce the risks of high temperature corrosion and slagging of water cooled walls. Under the optimized parameters, the flue gas temperature of furnace exit was 1615.1K, oxygen concentration was 1.41% and NOx concentration was 918.4 mg/m3. Considering the above factors, the flow, combustion and NOx emission characteristics in furnace of the optimized parameters were ideal.