The Influence of NH3 on Coal Combustion Kinetics during Coal/NH3 Cocombustion: Experiments and ReaxFF MD Simulations

To reduce CO2 emissions from coal-fired plants, coal/NH3 cocombustion has received widespread attention. In this work, the influence of NH3 on coal combustion kinetics during coal/NH3 cocombustion was studied by constant temperature thermogravimetric experiment and reactive molecular dynamics (ReaxFF MD). The experimental results show that coal combustion is inhibited during coal/NH3 cocombustion. The weight loss of coal during coal/NH3 cocombustion was always slower than that during pure coal combustion at all temperatures, and the burn out time of coal increases by 20–30s. The higher the NH3 cofiring ratio, the slower the weight loss rate of coal. When the NH3 cofiring ratio increases from 0 to 60%, the burnout time of coal increases by about 25%. The higher the carbon content in coal, the more obvious is the inhibition effect of NH3 on coal combustion. The inhibitory mechanism of NH3 on coal was then studied by ReaxFF MD. It is found that the combustion of coal during coal/NH3 cocombustion is inhibited mainly because NH3 will compete with coal for O2. Compared to pure coal combustion and pure NH3 combustion, the transfer of O to nitrogen oxides increases by about 1.5 times while the transfer of O to carbon oxides decreases by about 10% during coal/NH3 cocombustion. This proves that NH3 dominates in competition for O2. Compared with pure coal combustion, the combustion of char, tar, and carbonaceous gas from coal during coal/NH3 cocombustion is restrained. The decrease rate of H/C ratio of char will slow down. The attachment of O molecules on char will also decrease. These further confirm that NH3 inhibits coal combustion by competing with coal for O2. In this work, the competition mechanism during coal/NH3 cocombustion is studied, which is helpful for the development of stable coal combustion and unburned carbon emission reduction technology.