Atomic-level analysis of migration and transformation of organic sodium in high-alkali coal pyrolysis using reactive molecular dynamics simulations

Abstract

Monitoring the migration and transformation of free radicals and alkali metals (such as tar- and organic gas-bonded sodium) in high-alkali coal pyrolysis through experimental methods alone is a challenging task. To address this challenge, in this work we employed reactive force field (ReaxFF) molecular dynamics simulations to model the transformation behavior of sodium at the atomic level. The different forms of sodium [sodium atoms (Na(g)), sodium hydroxide (NaOH(g)), tar-bonded sodium, and organic gas-bonded sodium] were carefully analyzed to gain insights into their migration during the pyrolysis process. The results show that inherent organic sodium forms binary or multiple coordination structures with oxygen atoms in the coal matrix during pyrolysis. During pyrolysis, inherent organic sodium was transformed into three main sodium species: Na atoms, NaOH, and Na·H2O. The repeated reactions between these sodium-containing intermediates and the coal matrix strengthen the three-dimensional network structure of the coal matrix and hinder its graphitization. The preponderance of Na(g) predominantly stems from organic sodium C1–40+Na, while NaOH and Na·H2O contribute to a lesser extent. The formation of NaOH and Na·H2O can be primarily attributed to the presence of Na(g), with merely a limited portion arising from organic sodium (C1–40+Na). The results also show that organic sodium inhibits char and tar formation at high temperatures, whereas at low temperatures it promotes char formation and inhibits tar production.