Insight into the Competitive and Synergistic Effects during Coal/NH3 Cofiring via Reactive Molecular Dynamics Simulations

Abstract

In this work, reactive force field molecular dynamics (ReaxFF MD) simulations are performed to study competitive and synergistic effects and their impact on CO, CO2, unburned carbon (UC), and nitrogen oxide (NO) emissions in coal/NH3 cofiring at different conditions. Simulation results show that cofiring results in higher emissions of UC and NO. Increasing the temperature is helpful in reducing UC emissions but has little effect on NO emissions. Although decreasing the O2 equivalent ratio can help in reducing NO emissions, UC emissions increase significantly. The conclusion is that the best O2 equivalent ratio in cofiring is 0.6. Increasing the NH3 cofiring ratio will reduce NO emissions but seriously increase UC emissions. The competition between coal and NH3 for O2 will decrease the coal combustion rate and increase the NH3 combustion rate. The competition effect is further proved by tracing the OH distribution. The synergistic mechanism is revealed. The free radicals and intermediates generated from coal combustion, such as HO2, H2O2, H, and OH, will promote NH3 decomposition and oxidation. The kinetics analysis shows that the competition between NH3 combustion and coal combustion only influences the combustion rate of coal and NH3, while the synergistic effect of coal decreases the activation energy of NH3 combustion. The research results are helpful to further study the mechanism of coal/NH3 cofiring and guide the technology of NH3 blending in coal-fired power plants.