Abstract: Cofiring ammonia (NH₃) in coal-fired boilers is a promising approach to reduce CO₂ emissions from thermal power plants. However, high NO_x emission is going to be the major challenge faced by of NH₃-coal cofiring. To explore the potential NO_x control strategies, the effects of NH₃ cofiring ratio (R_NH3), overfire air (OFA) rates and furnace temperature on the NO_x formation characteristics of NH₃/coal cofiring are studied in a test furnace designed to allow for flexible control of combustion environment of NH₃. By means of monitoring the distributions of key species, such as O₂ and NO_x, along the furnace, the NO_x formation process of NH₃ cofiring in the furnace is revealed. The results show that when NH₃ is injected with coal stream, the NO_x emissions tend to increase and then decrease with the increase of R_NH3, exhibiting peak values at approximately 15% R_NH3. The NO_x emissions decrease with the increase of OFA rates. It is found that the O₂ distribution in the furnace plays a key role in the formation of NO_x. The net formation of NO_x is governed by the initial NO formation under O₂-rich environment during the early stage of combustion and the subsequent reduction of NO by the residue NH₃ during the later stage of combustion. Increase of furnace temperature tends to promote the O₂ consumption by coal combustion so that the combustion of NH₃ takes place in a relatively O₂-deficient environment, leading to reduction of NO_x emissions. The findings of these paper are instrumental in the design and operation of NH₃ cofiring system in full scale coal-fired boilers.