Experimental Study And Numerical Simulation On Co-combustion Characteristics Of Semi-char And High-volatile Coal

As a new type of fuel with high calorific value,semi-char has some problems,such as difficult ignition,low burnout rate and so on.At present,there are two research routes for semi-char clean and efficient combustion technology.The first is pure combustion semi-char technology,including preheating combustion technology,pressurized circulating fluidized bed technology and oxygen enriched combustion technology.Although the above technologies can realize semi-char combustion as a single fuel for power generation,they are still in the basic experimental research stage.The second is the semi-char co-combustion technology,that is,the semi coke and high-volatile coal are blended first,and the resulting blended fuel is then put into the coal-fired boiler in service for combustion and power generation,so as to realize the efficient and clean combustion technology of large proportion blending,stable combustion,burnout and low NOx emission.In this paper,a 350 k W pulverized coal combustion system was simulated by CFD and hot state experiments.The high-volatile coal was mixed with Shenmu semi-char to investigate the effects of semi-char blending ratio,primary air bias concentration ratio and blended coal quality on the ignition,burnout and NOx emission characteristics of mixed jet.The results show that,with the increase of the proportion of semi-char,the ignition distance and flame temperature of the mixing jet increase,the stable combustion position of coke gradually moves to the downstream of the furnace,the appearance of continuous flame zone is delayed,the precipitation of volatile matter and combustion reaction of semi-char particles become slower,the emission of NO decreases,and the burnout rate of blended coal gradually increases,which obviously deviates from the linear calculation value.The ignition distance obtained by"EGA gas release method"is different from that obtained by the second derivative method of axial temperature in numerical value,but it does not change the ignition sequence qualitatively.When the blending ratio is less than 50%,the temperature rise rate of the mixing jet does not decrease obviously.The primary air bias concentration ratio has two effects on the ignition for co-combustion of semi-char and bituminous coal.Too high or too low bias concentration ratio will lead to ignition delay.When the bias concentration ratio is 1,the radial temperature distribution is symmetrical.When the bias concentration ratio is greater than 1,the overall temperature level of the concentrated jet is higher than that of the light side,and O₂consumption and smoke generation are mainly concentrated on the concentrated side.With the increase of bias concentration ratio,the stable combustion position of coke gradually moves to the upstream of the furnace,the continuous flame zone becomes more concentrated,and the flame stability is enhanced.At the same time,the local high temperature zone is far away from the center of the furnace,and the NO generation concentration and fuel nitrogen conversion rate are gradually reduced.The highest burnout rate is 86.15%when the bias concentration ratio is 3.8.Considering the ignition,combustion stability,burnout rate and NO emission concentration at the exit of the main combustion zone,the bias concentration ratio of the mixed jet is recommended to be3.8～5.Changing the quality of blended coal,the ignition performance becomes worse with the decrease of volatile content of blended fuel,and the distance between burner outlet and stable combustion zone increases with the decrease of volatile content of blended fuel,but the delay effect of bituminous coal/semi-char mixed jet is not obvious,and the subsequent combustion is more intense.For lignite/semi-char,bituminous/semi-char and sub bituminous/semi-char jets,the NO generation concentrations are 473 mg/m³,486 mg/m³and 532 mg/m³respectively,and the burnout rates are 86.30%,81.99%and 73.64%respectively.It is recommended that the V_dafcontent suitable for mixed fuel combustion should not be less than 16%.