NO and N2O emissions in oxy-fuel combustion of coal in a bubbling fluidized bed combustor

• NO and N2O emissions follow the same trend with temperature under oxy-fuel and air-firing combustion.
• Higher rank coals generate higher N-coal conversion to NO and N2O.
• An increase in the coal moisture content decreases NO emissions and hardly affects N2O emissions.
• The presence of Ca-based sorbent affects the NO and N2O chemistry.
• The high levels of CO2 seem not to affect the catalytic activity of the sorbent.

Emissions from coal oxy-fuel combustion are receiving significant attention during the last years. This paper is focused on the analysis of fuel-N emissions in fluidized bed combustion systems. Experiments were carried out in a bubbling fluidized bed unit in the temperature interval 850–950 °C. Different coals (anthracite, bituminous and lignite) were used as fuels and different sorbents were employed for in-bed sulfur retention. In the experiments, NO, N2O and NO2 were measured. NO2 was not detected in any of the operating conditions. The influence of temperature on NO and N2O emissions was the same as in conventional combustion: NO emission increased as temperature increased while N2O emission decreased. Nevertheless, the total fuel-N conversion to nitrogenous species seemed to be lower than in combustion with air. As in air-firing combustion, the highest levels of NO and N2O were registered with the highest rank coals. The Ca-sorbent was found to have a key role on NO reduction via catalytic reaction and this catalytic effect seemed not to be affected by the high CO2 levels present in oxy-fuel combustion. Also, the moisture content in the coal affected the NO formation, which decreased with an increase in the coal moisture content. A similar effect was observed by increasing the oxygen concentration fed to the combustor.