Probing into Volatile Combustion Flame and Particulate Formation Behavior during the Coal and Ammonia Cofiring Process: Further Study on the Chemical Structure and Oxidation Reactivity of Soot Particles

Cofiring ammonia (NH3) fuel in coal-fired power plants is an important technical path for mitigating CO2 emissions. Combustion experiments of coal and ammonia were carried out on a McKenna flat flame system, and cumulative samplings of soot particles in the volatile flame were conducted. Detailed information on soot oxidation behavior, elemental composition, and chemical structure was obtained. The thermogravimetry results showed that the activation energy of the soot formed during coal and ammonia cofiring is higher than that without cofiring, and the comprehensive combustion characteristic index is lower, which means that the oxidation reactions of soot formed in cofiring conditions are more difficult to occur. The elemental analysis showed that, after ammonia cofiring, the C elements in soot increase, the S and H elements decrease, while N elements decrease in high oxygen conditions and increase in low oxygen conditions. In high oxygen fuel-lean conditions, ammonia cofiring increases the graphitization degree of the carbon structure of soot particles, making it more difficult to react. In low oxygen fuel-rich conditions, although ammonia cofiring does not significantly increase the graphitization degree of the carbon structure, it reduces the amount of reactive oxygen and allows N elements to enter the soot generation process in the form of N-5 and N-6, which bind to the edges of the aromatic structure, finally making the soot more difficult to react by increasing the N-containing structure in it.