Dynamic migration mechanism of organic oxygen in Fugu coal pyrolysis by large-scale ReaxFF molecular dynamics

Organic oxygen accounts for essential constituents of the chemical coal structure and makes significant contributions to the reactivity of coal in pyrolysis. The dynamic migration mechanism of organic oxygen during coal pyrolysis was explored by heat-up ReaxFF molecular dynamics simulations. A multicomponent molecular model for Fugu subbituminous coal was simulated in the temperature range of 300-2500 K with a heating rate of 1 K/ps using the GMD-Reax software. The difference between the pyrolysis products yields and their oxygen content indicates that organic oxygen in Fugu coal migrates preferentially into gas-phase oxygen during pyrolysis. Molecules of CO2, H2O, CH2O, and CO are identified as major O-containing gas products and their generation sequence shows consistency with reported experimental observations. CH2O molecules are produced mainly from cleavage of alkyl-alkyl ethers and their secondary cracking reactions will lead to their decrease trend and contribute partially to the rapid production of CHO radicals at high temperatures. The phenolics and carbonyl groups are found as the dominant functionalities in asphaltene and tar. The cleavage of alkyl-aryl and aryl-aryl ethers contributes to the production of phenolics that transform into carbonyl groups at higher temperatures through isomerization of aromatic rings. The decrease of phenolics and increase of carbonyl groups in the fast pyrolysis stage should be attributed mainly to the structure transformation reactions rather than migration among different pyrolyzate lumps of gas, tar, and asphaltene. This work suggests that ReaxFF MD simulation is useful for exploring the comprehensive dynamic migration mechanism of organic oxygen in coal pyrolysis.