Correction To Mass Transfer Coefficient And Its Application In Oxy-fuel Combustion Of A Char Particle

As one of the most attractive carbon capture technologies, oxy-fuelcombustion for pulverized coal has won more and more attention. Incontrast with the conventional coal combustion, there have higher O2andCO2contents in the oxy-fuel combustion. At temperatures of pulverizedcoal flame, Stefan flow occurs at the char surface and has significanteffects on the char combustion, the heat transfer and the mass transfer. Inthis work, through detailed theoretical analyses and experimentalinvestigates, a method for predicting the effects of Stefan flow wasestablished when the CO gas-phase reaction occurs in the boundary layerof char particle. Also, a practical combustion model of char was developedfor the oxy-fuel combustion and the effects of Stefan flow and COoxidation on the combustion behavior of char particle were profoundlystudied. This work would lay a solid foundation for the industrialapplication of oxy-coal combustion technology.Considering the main surface reactions, boundary layer reaction andmulti-component diffusion, the corrections to O2and CO2mass transfercoefficients that describe the effects of Stefan flow and CO gas-phasereaction were presented based on the Maxwell-Stefan equation, and usedfor the analysis of the impacts of Stefan flow and CO homogeneousreaction on the mass transfer in O2/CO2atmosphere. The analysis showedthat the closer the CO flame sheet is to the char surface, the smaller themass transfer coefficient of O2and the larger that of CO2. If onlyconsidering the oxidation reaction at the particle’s surface, the minimum correction factor of O2is0.5which is32.5%lower than that in the absenceof CO gas-phase reaction. The flame sheet has a minimum location andcannot attach or approach to the surface under conditions of onlyconsidering the surface gasification and infinitely fast gas-phase reaction.If both surface oxidation and reduction were considered, the increase ofsurface reactions would decrease the mass transfer for both O2and CO2,and have greater influence on CO2transfer than on O2.The char sphere diffusion-controlled combustion experiments inO2/CO2atmosphere were conducted in a thermobalance and a tube furnace,respectively. The measurement method for mass transfer coefficient inoxy-fuel combustion was presented. The experiments verified the necessityto correct the mass transfer coefficient, and showed that the derivedcorrection factor can be used to describe the effects of Stefan flow and gasphase reaction.The char combustion model, moving flame front model (MFF model),was modified by coupling with the corrected mass transfer coefficients.The new MFF model, which takes the Stefan flow into account and is stillalgebraic expressed, is validated to being suitable for the comprehensivesimulation of pulverized coal flame and engineering application in theoxy-fuel combustion.