Effect of calcium on the secondary reactions of tar from Zhundong coal pyrolysis: A molecular dynamics simulation using ReaxFF

Abstract

In this paper, molecular dynamics method based on reactive force field is used to investigate the effect of calcium on the secondary reactions of nascent tar from Zhundong coal pyrolysis. The coal with and without Ca species are pyrolyzed at various temperatures. The results show that Ca rarely affects the primary pyrolysis of coal, but significantly promotes the secondary reactions of tar, hence resulting in the increase of gas and char yields. By extracting the tar radical fragments from coal primary pyrolysis products, the nascent tar systems with and without Ca species are constructed respectively. Simulations on these two tar systems are carried out at various temperatures ranging from 1800 K to 3000 K. The results show that very little amounts of gas-Ca and atomic Ca are observed at low temperatures. Ca is mainly involved in a repeated bond-breaking and bond-forming process between tar and coke. Ca species only promotes the polymerization of tar at the low temperatures. While at high temperatures, a large amount of Ca is released in the form of atom, which will recombine with tar radicals and thus promoting the polymerization of tar. In the meanwhile, the atomic Ca will also attack the tar fragments and enhance the cracking of tar. The activation energies of tar polymerization and cracking reactions are calculated as 26.6 and 20.3 kcal/mol in the absence of Ca, compared to 19.7 and 20.1 kcal/mol in the presence of Ca. This means that the role of Ca in reducing the activation energy for tar polymerization is much more significant than that for tar cracking reactions.