Abstract: The co-firing of biomass with coal is one of the technologies to reduce the carbon emission from the coal-fired power plant. The position of biomass co-firing has a significant impact on NO_x emission during the co-firing process. Therefore, in order to study the effect of the biomass co-firing position and the temperature on the NO_x emission, a two-stage drop-tube furnace was used to study the NO_x emission of biomass co-firing with coal from the primary combustion zone and the burnout zone as well as the migration of fuel N. The results show that NO_x emission behaviors are significantly different when biomass is mixed from the primary combustion zone and the burnout zone. When biomass is mixed from the primary combustion zone, NO_x emission at studied temperature shows a decreasing trend with the increase of the biomass co-firing ratio from 0 to 40%; with the increase of the over-fire air ratio, the lowest NO_x emission occurs when the ratio is 0.33. When biomass is mixed from the burnout zone, the NO_x emission decreases continuously with the increase of the biomass co-firing ratio from 0 to 40% at the burnout temperature of 1 000 ℃. At the burnout temperatures of 1200 and 1400 ℃, the NO_x emission is lowest when the biomass co-firing ratio is 10%. There is a significant difference in the conversion of fuel N to intermediate products of HCN and NH₃ during biomass co-firing in the burnout zone. At the studied burnout temperature, when biomass is mixed in the burnout zone, the conversion rate of fuel N to HCN always increases with the increase of biomass co-firing ratio. The conversion of fuel N to NH₃ increases with the increase of biomass co-firing ratio at the burnout temperature of 1 000 ℃; when the burnout temperatures is 1200 and 1 400 ℃, the conversion rate of NH₃ is the highest at biomass co-firing ratio of 10%. When the primary zone temperature is 1 200 ℃ and the burnout temperature is 1 400 ℃, about 94% of the fuel N is converted to N₂ and ash N, about 5% is converted to NO_x, and less than 1% is converted to HCN and NH₃. The co-firing of biomass can reduce the conversion of fuel N to NO_x compared to pure coal combustion. However, as the biomass co-firing ratio increases, the conversion rate of fuel N to NO_x and ash N increases, and the conversion rate of fuel N to N₂ decreases.