Pollutant emissions in a bubbling fluidized bed combustor working in oxy-fuel operating conditions: Effect of flue gas recirculation

In this work, pollutant emissions in a continuous bubbling fluidized bed combustor (∼3kWth) at oxy-firing conditions were measured. An anthracite coal was used as fuel and a limestone was added for sulfur retention. Flue gas recirculation was simulated by mixing different gases (CO2, SO2, steam, and NO) and the effect of varying the gas composition of the recycled flow on the pollutant emissions was analyzed. It was observed that the most important effect of CO2 recirculation was the increase of the optimum temperature for SO2 retention from ∼850°C (conventional air combustion) to 900–925°C. SO2 recirculation increased the Ca-based sorbent utilization and did not affect the N2O emissions at any temperature. In addition, at 850°C the CO emission increased and the NO emission decreased, however, at 925°C the CO variation was negligible and the NO reduction was low. About 60–70% of the recycled NO was reduced to N2 and N2O, being the NO converted to N2O lower than 5%. Steam recirculation mainly led to a sharp decrease in NO emission. A synergetic effect among the different recycled gases was not found in any case.