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 O-2/N-2 and O-2/H2O were investigated in a tube reactor under different temperatures (700 degrees C, 800 degrees C, and 900 degrees 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, H-2 and CO concentrations, promoting NO and N2O reduction. The total conversions of fuel-N to NO and N2O in O-2/N-2 are over 2 times higher than those in O-2/H2O. In O-2/N-2, NO is formed as the sample heated up, while N2O is mainly formed during char combustion stage. However, NO is formed after N2O in O-2/H2O. No matter in O-2/N-2 or in O-2/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 degrees C, whilst the total conversion of fuel-N to NO increases with increasing temperature in O-2/H2O. The total conversions of fuel-N to N2O decrease with increasing temperature in O-2/N-2 and O-2/H2O. The influence of oxygen concentration on NO and N2O is similar to that of temperature. In O-2/N-2, there is a maximum value of the total conversion of fuel-N to NO (14.35%) at < O-2 > = 30%, whilst there is no obvious change of the total conversion of fuel-N to NO with increasing oxygen concentration in O-2/H2O. The total conversions of fuel-N to N2O decrease with increasing oxygen concentration in O-2/N-2 and O-2/H2O. The results are beneficial to understand the NO and N2O emission mechanisms and favorable for the NO and N2O control in O-2/H2O. (C) 2017 Elsevier Ltd. All rights reserved.