Effect mechanism of calcium on NO reduction in high temperature oxygen-lean zone of ammonia-coal co-firing

The co-combustion of ammonia and coal can effectively reduce CO₂ emissions in thermal power generation, but ammonia will increase NO_x emissions when it is co-fired with coal as an N source. Exploring the NO reduction mechanism in the co-combustion process of ammonia and coal is very necessary in achieving a low nitrogen emission. In this paper, the density functional theory was used to explore the mechanism of NO reduction in the high temperature oxygen depleted zone of ammonia and coal co-combustion, and the effect of calcium in coal, an important mineral, on the reduction of NO by NH₃ coke in coal was further analyzed. The theoretical calculation results show that the amino/coal coke in the high temperature oxygen depleted zone can reduce NO to N₂ by forming important transition intermediates such as NNH and N₂O. The formation of NNH in the reduction of NO by NH coke needs to overcome the energy barrier of 438.49 kJ/mol, which becomes the decisive step of the system reaction. The mineral calcium was not conducive to the adsorption of NH and NO on the surface of coal coke, and the adsorption energy of the two on the surface of coal coke was reduced by about 187.09 kJ/mol. In the presence of Ca at the top of coal coke surface, the NH reduction NO can be achieved by generating two paths: intermediate products NNH (path 1) and N₂O (path 2), and the deciding step energy barrier of path 1 was 636.41 kJ/mol, which was 197.92 kJ/mol higher than that of the NH/coal coke/NO system. With the participation of Ca, the formation of N₂O radicals in path 2 required 455.74 kJ/mol, which was 17.25 kJ/mol higher than that of NH/coal coke/NO system, and both paths showed that the metal mineral calcium inhibited the reduction of NH/coal coke to NO. The presence of Ca enhanced the binding energy between the NNH group and the surface of coal coke, making the inhibition of path 1 stronger than path 2 under the catalysis of calcium at the top. The kinetic parameters of the NH/coal coke/NO system before and after Ca participation were calculated by transition state theory. And the results showed that the rate of NH synergistic coal coke reduction NO under the participation of top calcium was lower than that of NO in the calcium-free participation system. The reduction rate of NH synergistic coal coke in path 1 was lower than that in path 2, and it was determined that path 1 had stronger inhibition than path 2 under the top calcium catalysis, and the kinetic results were consistent with the thermodynamic results.