Comparison of emission formation during pyrolysis and char combustion in air and oxyfuel conditions in fluidized bed

Oxyfuel combustion with CO2 capture is a considerable option for reduction of greenhouse gas emissions. Circulating fluidized bed (CFB) technology appears to be well suited to oxyfuel combustion. In order to study the differences in combustion phenomena and emission formation in air and oxyfuel combustion, the fuel pyrolysis and char combustion tests were performed with VTT’s bench scale bubbling fluidized bed reactor. In total seven fuels (Spanish anthracite, petroleum coke, Polish bituminous coal, Spanish lignite, South-African bituminous coal, wood chips, and straw pellet) were selected for these experiments. Three fuel batches were fed one at the time into reactor and pyrolyzed in sand bed. This was done for each fuel both in N2 and CO2 atmosphere. After the complete pyrolysis, gas was switched to 10% of O2 and the remaining char was combusted. The gas evolution was measured with FTIR and on-line analysers. Char converted mostly to CO2 in N2 environment. In the CO2 environment a high amount of CO formed compared to N2 environment. The total SO2 conversion increased in oxyfuel conditions compared to air combustion conditions. The effect of combustion gas on nitrogen emission formation appeared to be highly dependent on the fuel volatile and nitrogen content. More N2O and less NO were detected during the pyrolysis in CO2 atmosphere compared to N2 atmosphere. From the experimental data, conversion of different species during the pyrolysis and char combustion can be obtained. The conversion parameters can be used in furnace sub-models to design optimal boiler configuration. The overall results can be applied in the development of the flexible air/oxy CFB concept.