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

Abstract

To reduce CO 2 emissions from coal-fired plants, coal/NH 3 cocombustion has received widespread attention. In this work, the influence of NH 3 on coal combustion kinetics during coal/NH 3 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/NH 3 cocombustion. The weight loss of coal during coal/NH 3 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 NH 3 cofiring ratio, the slower the weight loss rate of coal. When the NH 3 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 NH 3 on coal combustion. The inhibitory mechanism of NH 3 on coal was then studied by ReaxFF MD. It is found that the combustion of coal during coal/NH 3 cocombustion is inhibited mainly because NH 3 will compete with coal for O 2 . Compared to pure coal combustion and pure NH 3 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/NH 3 cocombustion. This proves that NH 3 dominates in competition for O 2 . Compared with pure coal combustion, the combustion of char, tar, and carbonaceous gas from coal during coal/NH 3 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 NH 3 inhibits coal combustion by competing with coal for O 2 . In this work, the competition mechanism during coal/ NH 3 cocombustion is studied, which is helpful for the development of stable coal combustion and unburned carbon emission reduction technology.