Reaction Characteristics of Coal/NH₃ Co-combustion Affected by the Highly Preheated Temperature under MILD Combustion Mode

Co-combustion of coal blending with NH₃ is a promising technology to realize the large-scale CO₂ reduction in coal-fired plant. To offset the poor flammability of coal/NH₃ blend, an advanced moderate or intense low-oxygen dilution (MILD) combustion technology is implemented in coal/NH₃ co-combustion. The effects of the six different preheated temperatures from 1 173 K to 1 923 K on coal/NH₃ co-combustion under MILD mode are clarified based on the computional fluid dynamic software, i.e., ANSYS-FLUENT. Results show that, as the preheated temperature increases, the ignition of coal/NH₃ co-combustion is in advance. When the preheated temperature increases from 1 173 K to 1 923 K, the peak value of the furnace temperatres are increased from 1 823 K to 1 930 K, and the ignition times are in advance by 21.9%, and the ignition temperature and burnout time are reduced by 26.1% and 22.2%, respectively. These findings indicate that the high temperature can improve the ignition and burnout of coal/NH₃ co-combustion. In addition, the NO_x conversion ratio is increased from 2.78% to 3.25%, which is attributed to the decrease in the proportion of NO_x reduction (the decreases in the proportion of NH₃+NO pathway is from 38% to 30%). Although the highly preheated temperature is favorable to the ignition of coal/NH₃ co-combustion, it leads to an increase in fuel nitrogen conversion. Therefore, the regulation of the ammonia combustion reaction becomes the key to stable combustion and NO_x reduction. In the high temperature environment, reducing the local oxygen concentration in the reaction zone (e.g., air-staged combustion), promoting the reduction of NH₃ to NO, and inhibiting the conversion of NH₃ to NO will be expected to achieve stable combustion with low NO_x emission during coal/NH₃ co-combustion affected by the highly preheated temperature.