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.

The CaO sulfation is the main reaction to capture SO2 by limestone in circulating fluidized boilers. The mechanism and effects of H2O on CaO sulfation were investigated by density functional theory and experiments. The reaction paths and energy barriers for SO4 2– forming on CaO and CaSO4 surfaces as well as the outward diffusion of Ca2+/O2– in CaO and CaSO4 crystals were examined. Under conditions without H2O, the outward diffusion of Ca2+ on the CaSO4 surface is the rate-determining step for CaO sulfation because it has the largest energy barrier (3.63 eV) of all steps in the sulfation reaction. Under conditions with H2O, X-ray diffraction analysis indicates that H2O cannot directly accelerate the diffusion of Ca2+ in the CaSO4 layer. However, with the assistance of H2O, the interactions between Ca2+ and SO4 2– on the CaSO4 surface become weaker, the energy barrier for the diffusion of Ca2+ decreases from 3.63 to 1.62 eV, and thus the outward diffusion of Ca2+ on the CaSO4 surface is enhanced. The rate-determining step of the sulfation reaction under conditions with H2O became O2– diffusion in the CaSO4 layer with an energy barrier of 3.42 eV, which means that H2O decreases the largest energy barrier for sulfation by 0.21 eV. In addition, H2O can also enhance the adsorption of SO2 on a CaSO4 surface.