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

Paper i proceeding, 2018

It is not yet well understood how the change of the atmosphere from O2/N2 to O2/CO2 at high O2
concentration affects the conversion of fuel in a fluidized bed (FB). Most experimental work has been
carried out with O2 concentration lower than 25−35%v/v and some of the techniques used to measure
the temperature of the fuel particles, such as thermocouples, could interfere with the conversion. The
existing theoretical studies assume that the coal particle always is in the emulsion phase, although
observations 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 and
O2/N2, with the O2 concentration varying from 11 to 100%v/v. The reactor is equipped with a quartz
window allowing measurement of the char temperature (by pyrometry with a digital camera) and
conversion (by following the change of particle size). It is shown that in both atmospheres the char
temperature oscillates tens of degrees in just hundredths of a second, as a result of the particle
movement through the bed. The results also indicate that the transfer of O2 to the surface of the fuel
particle controls the overall rate of char conversion in both O2/CO2 and O2/N2, with a significant
contribution to carbon consumption by gasification superimposed on oxidation. Moreover, while the
char stays in the emulsion phase, its temperature seems not to be affected by the different
atmospheres, but the differences became significant when the particle are in the bubbles or in the
splash zone (the temperature becomes lower in the O2/CO2 than in O2/N2). Therefore, the differences
in the measured char conversion seems to be dominated (at least greatly influenced) by the time
during which the char is out of the emulsion phase. These results underline the importance of
precisely considering the char movement through the bed to understand the oxy-fuel behavior in FB.