Evolutions of functional groups and polycyclic aromatic hydrocarbons during low temperature pyrolysis of a perhydrous bituminous coal

Understanding the molecular structural evolution mechanism during coal pyrolysis can help better regulate coal conversions for developing clean coal technology. This study investigated changes in functional groups and polycyclic aromatic hydrocarbons (PAHs) during pyrolysis of a perhydrous bituminous coal at temperature of 75–600 °C. Results indicated that aliphatic, C–O, C=O, OH, aromatic C=C, and aromatic C–H groups declined in relative abundance below 300 °C. These declines were attributed to cracking of hydrogen bonds and devolatilization. Meanwhile, the alkyl side chains of alkylated PAHs, i.e., alkyl-naphthalenes and alkyl-phenanthrenes, were continuously detached. Pyrolysis at 300–450 °C was characterized by elevated aromaticity, reflected by increments in the relative abundances of aromatic C–H, aromatic carbon, pyridine nitrogen, and protonated carbon. PAH evolutions at this stage exhibited relative increases of 4-–6-ring PAHs with relative decreases of 2-–3-ring PAHs. Pyrolysis at 450–600 °C was characterized by an increment of aromatic C=C and significant increases in relative abundances of 4-–6-ring PAHs in emitted particulate and gaseous phases. The elevation of pyrolysis temperature transformed coal macromolecules into more compact structure. The molecular structural units of chars were formulated as C245H227N3O14S, C239H193N3O8S, C228H152N2O3S, and C221H113NO3S for coal and chars produced at 300, 500, and 600 °C, respectively. Changes in three-dimensional molecular structural units reflected step-by-step intense deformation and condensed process.