Abstract
Oxy-H2O combustion is a novel oxy-fuel combustion technology, different from conventional and oxy-CO2 combustion. NO and N2O emissions during devolatilization and char combustion of a single biomass particle in O2/N2 and O2/H2O were investigated in a tube reactor under different temperatures (700°C, 800°C, and 900°C) and oxygen concentrations (21%, 30%, 40%, and 50% by volume). Single biomass pellets of poplar wood, rice husk, and corn stalk were selected. The method of isothermal thermo-gravimetric analysis was applied to distinguish the two stages of biomass combustion based on its proximate analysis. The presence of H2O vapor can improve the free radicals, H2 and CO concentrations, promoting NO and N2O reduction. The total conversions of fuel-N to NO and N2O in O2/N2 are over 2 times higher than those in O2/H2O. In O2/N2, NO is formed as the sample heated up, while N2O is mainly formed during char combustion stage. However, NO is formed after N2O in O2/H2O. No matter in O2/N2 or in O2/H2O, a higher fuel-N content in biomass results in higher NO and N2O yields during the two stages, whilst the conversions of fuel-N to NO and N2O during the two stages decrease with increasing fuel-N/ash content. In O2/N2, the total conversion of fuel-N to NO reaches the maximum value (14.35%) at T=800°C, whilst the total conversion of fuel-N to NO increases with increasing temperature in O2/H2O. The total conversions of fuel-N to N2O decrease with increasing temperature in O2/N2 and O2/H2O. The influence of oxygen concentration on NO and N2O is similar to that of temperature. In O2/N2, there is a maximum value of the total conversion of fuel-N to NO (14.35%) at 〈O2〉=30%, whilst there is no obvious change of the total conversion of fuel-N to NO with increasing oxygen concentration in O2/H2O. The total conversions of fuel-N to N2O decrease with increasing oxygen concentration in O2/N2 and O2/H2O. The results are beneficial to understand the NO and N2O emission mechanisms and favorable for the NO and N2O control in O2/H2O.
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