Co-firing of biomass residues with coal is continuously increasing in it's application in coal-fired boilers for electricity production. In this study, co-firing experiments were performed using a Russian coal with a range of biomasses, shea meal (SM), cotton stalk (CS), sugarcane bagasse (SB(T)), sugarcane bagasse (SB(R)) and wood chips (WC) as biomasses in 5%, 10% and 15% thermal fractions to evaluate their potential as substitute fuel and an agent for NO(x) control. It was found that the addition of biomass increased NO reduction under both un-staged and air-staged conditions. However, NO reductions obtained under optimum conditions of primary zone stoichiometry (SR(1) = 0.9) and over-fire air (OFA) injection port location 3, were found to be significantly higher than un-staged co-firing for the same biomass thermal share in the fuel blend. It was found that the addition of biomass has a positive effect on carbon burnout under the optimum conditions that were determined in the study. A 10% biomass blending ratio (BBR) was found to be optimum for air-staging conditions. When co-fired under optimum air-staged conditions, a 10% BBR of sugarcane bagasse (SB(R)), shea meal (SM), wood chips (WC), cotton stalk (CS) and sugarcane bagasse (SB(T)) in coal gave NO reduction of 49%, 51%, 53%, 60% and 72%, respectively. (C) 2010 Elsevier Ltd. All rights reserved.