Study On Ignition And Fuel-N Conversion Characteristics During Blended Combustion Of Large Proportion Of Semi-Coke And Coal

With the rapid development of coal chemical industry in China,there is a serious overcapacity of by-product semi-char,which has become a bottleneck restricting the development of coal chemical industry.At present,large amounts of such semi-char need to be burned and utilized urgently.As a fuel,semi-char has the problems of difficult ignition and unstable combustion,low burnout ratio and high NO_x emission.Therefore,it is of great significance to explore how to realize the clean and efficient combustion of semi-char.Burning a large proportion of semi coke with coal for power generation will make use of the excess semi coke,which has a wide application prospect.In recent years,a lot of research has been done on the blended combustion of semi-char and coal in China,but most of the experiments are carried out in small devices,which is very different from the ignition environment of fuel jet in actual industrial boilers.In the 500 k W pilot-scale combustion pulverized coal combustion system,by analyzing the changes of flue gas temperature,flue gas composition,char composition and flame luminous intensity in the furnace,this paper studies the ignition and combustion characteristics of semi-char blended with coal in reducing atmosphere,which can well reflect the ignition process of fuel jet in actual boiler.Firstly,the effects of the blending ratio of semi-coke on the ignition and fuel nitrogen conversion characteristics of semi-coke and bituminous coal blends and semi-coke and lignite blends were investigated.The results show that with the increase of the blending ratio of semi-char,the ignition distance of the two blend fuels increases monotonously.When the blending ratio of semi-char in bituminous coal does not exceed 30%,the ignition characteristics are close to those of pure bituminous coal.When the blending ratio of semi-char is 30%,there is a positive synergy between bituminous coal and semi-char to reduce the conversion of fuel nitrogen to NO.Under this blending ratio,the final emission concentration of NO is 47 mg/m³ lower than that of pure bituminous coal.The NO emission concentration of lignite and semi-char blends is higher than that of bituminous coal and semi-char blends during combustion process.Considering that it is more difficult to reduce NO emission in actual boiler,bituminous coal is more suitable for blended combustion of large proportion of semi coke（≥50%）than lignite.Based on the effects of blending ratio on ignition and fuel nitrogen conversion characteristics for blending combustion of semi-coke and coal,the influence of primary air on the ignition and fuel nitrogen conversion characteristics of large proportion of semi-char（50%）blended with bituminous coal were investigated.The results show that when the primary air velocity is 18 m/s,the ignition distance of the blend fuel stream is the shortest,the NO emission concentration is the lowest and the burnout ratio is the highest.When the primary air velocity exceeds 18 m/s,the promotion effect of bituminous coal on semi-char ignition decreases.With the increase of primary air temperature from 100 ^oC to 280 ^oC and the blending ratio of semi-char is 30%,the ignition fraction of semi-char in the blend fuel decreases from 16%to 13%;When the blending ratio of semi-char is 50%,the fraction increases from 23%to 36%.When the primary air temperature is 280 ^oC and the blending ratio is 30%,the NO emission concentration at the outlet of the primary combustion zone is 813 mg/m³;When the blending ratio is 50%,the NO concentration is 689 mg/m³,and the burnout ratio under this blending ratio is 1.23%higher than that when the blending ratio is 30%.Therefore,high primary air temperature is more favorable for the ignition of 50%semi-char blended in coal.Based on the effects of primary air parameter on ignition and fuel nitrogen conversion characteristics for blending combustion of semi-coke and coal,the effects of fuel particle size and bias concentration ratio（BCR）on the ignition and fuel nitrogen conversion characteristics of large proportion of semi-coke（50%）blended with bituminous coal were investigated.The results show that as the fuel particle size increases from R₉₀=20%to R₉₀=25%the ignition fraction of semi-char decreases from35%to 13%,which is very unfavorable to the ignition of semi-char in the blend fuel,and the conversion ratio of fuel nitrogen to NO increases.Therefore,the particle size should be controlled below R₉₀=20%.In bias combustion,the ignition characteristics of blend fuel stream with BCR between 2.0 and 3.6 are significantly improved,and the ignition distance is significantly shorted.The volatile concentration in fuel is an important factor affecting the ignition of bias fuel streams;There is a positive correlation between the conversion rate of fuel nitrogen to NO and the area of strong reduction zone.Increasing the proportion of the area of strong reduction zone in the primary combustion zone is an effective measure to reduce the conversion of fuel nitrogen to NO for burning a large proportion of semi-char and bituminous coal under the bias combustion style.Based on the effects of conventional operation parameters and fuel properties on ignition and fuel nitrogen conversion characteristics for blending combustion of semi-coke and coal,the influence of steam concentration on the ignition and fuel nitrogen conversion characteristics of large proportion of semi-char（50%）and bituminous coal blends were investigated.The results show that when the added steam concentration is5%,the ignition characteristics of the blend fuel are enhanced and the corresponding ignition distance is the shortest.The main reason for the advanced ignition is that more highly reactive small aromatic ring structures are formed on the char surface.The value of I_{（Gr+VL+Vr）}/I_D in the char Raman structure directly shows a good linear relationship with the ignition distance.The ignition distance under different steam concentration can be well predicted by analyzing this ratio.The addition of a small amount of steam（5%）is conducive to the formation of a new and more reactive active site C（O）on the char surface,so as to promote the reduction of NO.However,in the environment with steam concentration higher than 10%,the contact between NO and C（O）is hindered,resulting in the increase of the conversion of fuel nitrogen to NO.