Reaction Characteristics of Coal/NH3 Co-combustion Affected by the Highly Preheated Temperature under MILD Combustion Mode
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Abstract
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 NOx conversion ratio is increased from 2.78% to 3.25%, which is attributed to the decrease in the proportion of NOx 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 NOx 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 NOx emission during coal/NH3 co-combustion affected by the highly preheated temperature.
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