Study Of Combustion Characteristics Of Pulverized Coal And Char In O₂/CO₂Atmosphere

Oxy-coal combustion technology has aroused extensive concern since itshows significant advantages and great potential for emissions control ofgreenhouse gas CO₂and pollutants （SO₂,NOx） from coal combustion. Greatdifferences between oxy-coal combustion and traditionnal air combustionexist in respect of ignition, burning, reactivity of char, formation andemission of pollutants. Moreover, it can reproduce thermal conditions moresimilar to those of practical boilers in drop tube furnaces than in thermalgravimetric analyzers. So it’s considerably meaningful and valuable to studycombustion characteristics of pulverized coal and char in O₂/CO₂atmospherein drop tube furnaces.Experimental study and modeling prediction of ignition temperature foran anthracite in O₂/CO₂atmosphere have demonstrated that the ignitiontemperature of Yangquan anthracite in O₂/CO₂atmosphere is significantlyhigher than that in O₂/N2atmosphere at the same O₂concentration, itsdifference is140℃.The ignition temperature decreases exponentially with theincrease of O₂concentration in O₂/CO₂atmosphere. These can be mainlyexplained by the higher molar specific heat of CO₂than that of N2（approximately1.6times）, the lower mass diffusivity of O₂in CO₂than in N2（approximately0.76times） and the effect of O₂concentration on reactivity oflocal mixtures. The ignition temperatures predicted by the improvedheterogeneous thermal explosion model are in good agreement with those experimental ones in all atmospheres and the particle temperatures can bepredicted as well.Experimental study of combustion characteristics of a bituminous inO₂/CO₂atmosphere has demonstrated that O₂/CO₂mixtures cause the delayof pulverized coal combustion and thus a lower burnout rate than that inO₂/N₂atmosphere at the same O₂concentration. The combustion process canbe improved by increasing O₂concentration. The peak value of burning ratealong the drop tube furnace exists in each atmosphere and the location ofpeak in O₂/CO₂atmosphere is lower than that in O₂/N₂atmosphere at thesame O₂concentration, which means the delay of combustion process. Thelocation of peak moves upwards when increasing O₂concentration, however,coal char can be easily gasified by high concentration CO₂and makes thelocation of peak even upper. The NOxconcentration in O₂/CO₂atmosphere ismuch lower than that in O₂/N₂atmosphere at the same O₂concentration.Experimental study of kinetics for pulverized char surface oxidation inO₂/CO₂atmosphere has demonstrated that the activation energy E（95.4KJ/mol） of pulverized char obtained from Shenhua bituminous anddetermined in O₂/CO₂atmosphere is higher than that （87.9KJ/mol）determined in O₂/N₂atmosphere. How pre-exponential factor A varies withatmosphere （O₂/CO₂and O₂/N₂） is similar to that of activation energy E. Thespecific char oxidation rate coefficient Ksdetermined by activation energyand pre-exponential factor can be used to assess the char oxidation rate.