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The catalytic effects of alkali metal ions (Na+ and K+) on NOx precursor formation during coal pyrolysis were investigated using the N-containing compound pyridine as a model compound. Density functional theory calculations at the B3LYP/6-31G (d, p) level of theory were conducted to elucidate the mechanism of pyridine pyrolysis and the pathways for HCN formation. The calculation results indicate that Na+ and K+ have distinct influences on different pyrolysis reactions; these alkali metal ions facilitate the initial hydrogen transfer from C1 to N and C2, whereas they hinder the other hydrogen migration reactions. Both Na+ and K+ significantly reduce the activation energies for C–C bond breakage and triple-bond formation, whereas they increase the activation energies for the isomerization reactions. The different effects essentially result from the distinct charge distributions induced by the two ions. Due to the distinct influences on the different reactions, the rate-determining steps are modulated, affecting the competitiveness of the different possible pathways of HCN formation. The formation of HCN from pyridine is promoted in the presence of Na+ and K+ because all the overall activation energies are decreased for different pathways. The calculation results agree well with previous experimental studies. Thus, the findings offer a new and promising approach to reveal the formation mechanism of NOx and facilitate the control of NOx for coal utilization.

Calcium-based compounds are major inorganic components in coal and also widely used as additives in thermal conversion of coal and, thus, have important effects on the decomposition of nitrogen-containing compounds to form NO x . In this work, the influence of Ca 2+ on the pyrolysis of pyrrole and benzopyrrole (indole) to form HCN, a precursor of NO x , was investigated via a density functional theory (DFT) calculation with B3LYP/6-31+G(d,p) basis set. The results suggest that Ca 2+ has strong interactions with pyrrole and indole by altering the original electron density distribution of the pyrrole ring and the configurations of pyrrole derivatives, respectively. Ca 2+ affects the energy barriers of the elementary pyrolytic reactions (i.e., internal hydrogen transfer, isomerization, and concerted decomposition) and particularly reduces the energy barriers of the ratedetermining steps for HCN formation. In comparison to alkali metal ion Na + , alkali earth metal ion Ca 2+ has a stronger influence on the pyrrole and indole pyrolysis to form HCN.

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