Simulation Of Combustion Characteristics In A 300 MW_e Down-fired Boiler With Multiple-injection And Multiple-staging Combustion Technology

China is one of the countries with rich reserves of lean coal and anthracite which are hard to burn coal in the world,and Down-fired boiler has been widely developed as the main furnace type for burning this kind of coal.Multiple-injection and multiple-staging combustion technology developed by Harbin Institute of Technology has been gradually applied due to its strong advantages in stable combustion,reduction of NOx and combustible matter in fly ash,etc.In order to more reasonably design and optimize the combustion technology and operating parameters of down-fired boiler and reduce NOx and combustible matter in fly ash,this article takes a second term 300 MW subcritical down-fired boiler which currently has high NOx emissions(896~1226 mg/m3(6%O2))and high ash content(>8%)in a power plant as the research object,and the boiler needs to carry out technical improvement of low NOx combustion.Numerical simulation is performed to study the combustion and NOx generation characteristics of the boiler under different combustion technologies firstly.And then the influence of tertiary air inclination angle and Secondary and tertiary air ratio on combustion and NOx generation characteristics in the boiler with multiple-injection and multiple-staging combustion technology is studied,and then the optimized parameters of boiler with multiple-injection and multiple-staging combustion technology is studied.Comparing the results of the numerical simulation with the industrial test under the MBEL combustion technology,the results show that the small diffence between the simulated value and the experimental value,which verifies the rationality of the numerical simulation model and the grid division.The results of different combustion technologies show that,under the MBEL combustion technology,the flow field and temperature field in the furnace are slightly skewed,and the particle clearly causes a certain degree of sour of the front water wall,and the NOx emission and combustible matter in fly ash at the furnace exit are relatively high,959.8mg/m3(6%O2)and 9.47%,respectively.In a company's combustion technology,the flow field and temperature field in the furnace are relatively symmetrical,but the airflow downward depth over the arch is shallower,and the NOx emission and combustible matter in fly ash at the furnace exit are 793.3 mg/m3(6% O2)and6.69%,respectively.The results of the research on the different tertiary air inclination angle with multiple-injection and multiple-staging combustion technology show that,the corresponding flow field and temperature field are skewed when the tertiary air inclination angle is 15°,and the rest of the working conditions form symmetricalflow field and temperature field in the lower furnace.With the increase of the tertiary air inclination angle,the airflow downward depth increases,the ignition distance of the pulverized coal decreases,and the NOx emission increases firstly and then decreases,finally rebounding.With the increase of the tertiary air inclination angle from 15° to 35°,the combustible matter in the fly ash at the furnace exit reduce from 6.02% to 4.19%,When the tertiary air inclination angle is40°,the value is 4.23%.When the tertiary air inclination angle is 35°,the NOx emission and combustible matter in fly ash at the furnace exit are lowest.Based on the research that the tertiary air inclination angle is 35°,the results of study different secondary and tertiary air ratios express that,symmetrical flow field and temperature field are formed in the furnace under all working conditions,and the combustible matter in the fly ash at the furnace exit was maintained at a relatively low level,with an average value of 4.289%.With the secondary air ratio increases from 30.54% to 42.54%,the airflow downward depth decreases,the ignition distance of pulverized coal decreases,and the NOx emission at the furnace exit decreases first and then increases.When the secondary air ratio is 36.54%,the combustible matter in the fly ash at the furnace exit is the lowest,and the NOx emission is relatively low,which are 675.92 mg/m3(6%O2)and 4.19%,respectively.By comprehensive consideration,it is recommended that the tertiary air inclination angle is 35°,and the secondary air ratio is 36.54%,and the corresponding tertiary air ratio is 21.99% with multiple-injection and multiple-staging combustion technology.Under this set of parameters,the NOx emissions and combustible matter in fly ash decreas by 29.58% and 55.76%,respectively,compared with the original boiler.