Nitrogen transformation during pressurized oxy-biomass combustion process

Pressurized oxy-coal combustion in conjunction with carbon capture and sequestration is a promising technique for reducing carbon emissions. Biomass is a renewable energy source of zero carbon emissions. Therefore, negative carbon emissions can be achieved with pressurized oxy-biomass combustion, which is an appealing method for carbon peaking and carbon neutralization. NOx is an acidic gas produced from combustion that has the potential to corrode pipelines, demanding strict control. However, the evolution of NOx precursors and NOx formation in pressurized oxy-biomass combustion is still unclear. In this study, the NOx emissions in pressurized oxy-combustion and nitrogen migration characteristics of biomass in a pressurized CO₂ atmosphere were studied in a pressurized fixed bed reactor. The combustion results indicate that as the pressure increases, NO emission decreases while N₂O formation slightly increases. The pressurized pyrolysis/gasification results show that the release of NH₃ and HCN in CO₂ atmosphere significantly reduces with increasing pressure, while the other-N species are reduced to N₂ by char and CO at high pressure. NH₃ formation peaks at 700 °C while HCN emission increases significantly with increasing temperature. NH₃ yield is considerably higher than HCN yield when the temperature is below 800 °C. The correlation analysis shows that NO formation increases with the increase of the total amount of NH₃ and HCN released in the pyrolysis/gasification process. This study provides new insight into the evidence that the suppression of volatile-N under elevated pressure is one important reason for the decrease of NOx in the pressurized oxy-biomass combustion.