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

Abstract

Co-pyrolysis of oil-rich coal and biomass is an effective approach to maximize the oil and gas production from oil-rich coal. To address the challenge of unclear mechanisms for the enhancement of synergistic effects during the co-pyrolysis of oil-rich coal and biomass, a Huangling coal (H) and an enzymatic hydrolysis lignin (E) were used as the research samples and the structural evolution of raw materials and those by hydrothermal pretreatment (HTP) was systematically investigated using a suite of techniques such as proximate analysis, ultimate analysis, scanning electron microscopy (SEM), inductively coupled plasma optical emission spectrometry (ICP-OES) and nuclear magnetic resonance (¹³C-NMR). Combined with fixed-bed pyrolysis experiments, the mechanism by which hydrothermal pretreatment strengthens the co-pyrolysis synergistic effect of the Huangling coal and enzymatic hydrolysis lignin blend was explored. The results indicate that after hydrothermal pretreatment, the oxygen content of the raw materials decreases, the pore structure becomes more developed, and the concentrations of inorganic metal ions (e.g., calcium, potassium, and iron) in the aqueous phase increase. The structure of Huangling coal and enzymatically hydrolyzed lignin undergoes significant modification, with the contents of aliphatic carbon, aromatic carbon and carbonyl/carboxyl carbon in enzymatically hydrolyzed lignin approaching those in Huangling coal, and with the bridge carbon ratio increasing, resulting in improved thermal stability for the enzymatically hydrolyzed lignin and a pyrolysis temperature approaching that of Huangling coal. By the hydrothermal pretreatment, the co-pyrolysis of Huangling coal and enzymatic hydrolysis lignin generally exhibit an increase in tar yields. When the H/E ratio is 8:2 and hydrothermal pretreatment is conducted for 24 h, the co-pyrolysis tar yield increases by 80.52%, with significant increases in the contents of aliphatic compounds and monocyclic aromatic hydrocarbons in the tar. Meanwhile, the yields of H₂, CO and CH₄ in the gas increase by 5.47%, 10.98% and 9.27%, respectively, the generation of CO₂ and pyrolysis water is inhibited, and the pore structure of semi-coke is enriched. This is primarily attributed to the multiple mechanisms of “component interaction-structural modification-catalytic cracking” during hydrothermal pretreatment, which enhance the co-pyrolysis synergistic effect of the H/E blend. The findings provide theoretical support for developing the technologies to enhance the co-pyrolysis synergistic effect between oil-rich coal and biomass, and expand new avenues for the low-carbon, high-quality utilization of oil-rich coal.

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