Temperature and Conversion of Char during Oxy-Fuel Combustion in Fluidized Bed: Influence of CO2 Concentration at High O2 Concentration

It is not yet well understood how the change of the atmosphere from O2/N2 to O2/CO2 at high O2concentration affects the conversion of fuel in a fluidized bed (FB). Most experimental work has beencarried out with O2 concentration lower than 25−35%v/v and some of the techniques used to measurethe temperature of the fuel particles, such as thermocouples, could interfere with the conversion. Theexisting theoretical studies assume that the coal particle always is in the emulsion phase, althoughobservations confirm that the particle is also in the bubbles and splash zone. In the present work,combustion of single beech-wood char particles are carried out in a 2D FB reactor in O2/CO2 andO2/N2, with the O2 concentration varying from 11 to 100%v/v. The reactor is equipped with a quartzwindow allowing measurement of the char temperature (by pyrometry with a digital camera) andconversion (by following the change of particle size). It is shown that in both atmospheres the chartemperature oscillates tens of degrees in just hundredths of a second, as a result of the particlemovement through the bed. The results also indicate that the transfer of O2 to the surface of the fuelparticle controls the overall rate of char conversion in both O2/CO2 and O2/N2, with a significantcontribution to carbon consumption by gasification superimposed on oxidation. Moreover, while thechar stays in the emulsion phase, its temperature seems not to be affected by the differentatmospheres, but the differences became significant when the particle are in the bubbles or in thesplash zone (the temperature becomes lower in the O2/CO2 than in O2/N2). Therefore, the differencesin the measured char conversion seems to be dominated (at least greatly influenced) by the timeduring which the char is out of the emulsion phase. These results underline the importance ofprecisely considering the char movement through the bed to understand the oxy-fuel behavior in FB