Study on the mechanism of synergistic effect in co-pyrolysis of Huangling coal and enzymatic hydrolysis lignin by hydrothermal pretreatment

Co-pyrolysis of oil-rich coal and biomass is an effective approach to maximize the oil-gas properties of oil-rich coal. This study focuses on the challenge of unclear mechanisms underlying the enhancement of synergistic effects during the co-pyrolysis of oil-rich coal and biomass. Taking Huangling coal (H) and enzymatic hydrolysis lignin (E) as research samples, we systematically analyzed the structural evolution of raw materials before and after hydrothermal pretreatment (HTP) using techniques such as proximate analysis, ultimate analysis, scanning electron microscopy (SEM), inductively coupled plasma optical emission spectroscopy (ICP-OES), and nuclear magnetic resonance (13C-NMR). Combined with fixed-bed pyrolysis experiments, the enhancement mechanism of hydrothermal pretreatment on the co-pyrolysis synergistic effect of Huangling coal and enzymatic hydrolysis lignin blends was investigated. The results show that after hydrothermal pretreatment, the oxygen content of the raw materials decreases, the pore structure becomes more developed, and the concentration of inorganic metal ions such as calcium, potassium, and iron in the aqueous phase increases. The structures of Huangling coal (H) and enzymatic hydrolysis lignin (E) undergo significant changes: the differences in the relative contents of aliphatic carbon, aromatic carbon, and carbonyl/carboxyl carbon between the two are reduced, and the bridge carbon ratio increases. This improves the thermal stability of enzymatic hydrolysis lignin and makes its pyrolysis temperature range closer to that of Huangling coal. For the blends of Huangling coal and enzymatic hydrolysis lignin, the co-pyrolysis tar yield generally increases after hydrothermal pretreatment. When the H/E ratio is 8:2 and the hydrothermal pretreatment time is 24 hours, the co-pyrolysis tar yield increases by 80.52%, with significant increases in the contents of aliphatic compounds and monocyclic aromatics in the tar. Meanwhile, the yields of H2, CO, and CH4 in the gas phase increase by 5.47%, 10.98%, and 9.27% respectively, while the generation of CO2 and pyrolysis water is inhibited, and the pore structure of semi-coke is enriched. These phenomena are mainly attributed to the multiple mechanisms of "component interaction-structural modification-catalytic cracking" during hydrothermal pretreatment, which enhances the co-pyrolysis synergistic effect of H/E blends. The research results provide theoretical support for the development of technologies to strengthen the co-pyrolysis synergistic effect between oil-rich coal and biomass, and expand new pathways for low-carbon and high-quality utilization of oil-rich coal.