Dynamic evolution and regulation of active hydrogen/oxygen migration during steam atmosphere pyrolysis of tar-rich coal: Insights from ReaxFF MD simulations and experiments

Steam regulates active hydrogen/oxygen migration and radical reactions during tar-rich coal pyrolysis, but its molecular role remains unclear. Here, ReaxFF molecular dynamics simulations combined with fixed-bed experiments were used to elucidate steam-assisted tar and gas formation. Atom labeling and trajectory tracking showed that steam promoted C O bond cleavage and aromatic side-chain dissociation, facilitating light tar and medium-molecular-weight tar precursor formation. Steam-derived ·H and ·OH stabilized hydrogen-deficient radicals through hydrogen transfer and radical termination, suppressing excessive cracking, crosslinking, and condensation. At 2400 K, the late-stage tar mass fraction stabilized at approximately 35%, with 8.03% of steam-derived hydrogen migrating into tar. Steam also promoted coal-derived hydrogen redistribution, with approximately 67% migrating into tar at 2600 K. Light tar and heavy tar mass fractions peaked at 21.99% and 43.20% at 2800 K and 2600 K, respectively, 3.67% and 7.48% higher than under an inert atmosphere. At 2800 K, steam supplied 32.59% of hydrogen and 18.92% of oxygen in pyrolysis gas, promoting H 2 , CO, and CO 2 formation. Fixed-bed experiments showed consistent product-yield and tar-composition trends, supporting the simulation-derived mechanisms and providing guidance for targeted tar and gas upgrading.