Key Takeaways & Executive Findings
- •• • Hydrothermal pretreatment (24 h) of Huangling coal and enzymatic hydrolysis lignin reduces O/C ratio and increases fixed carbon, while developing pore structures and surface cracks, as confirmed by SEM and ultimate analysis. • • Co-pyrolysis of H/E blend (8:2) after 24 h HTP achieves an 80.52% increase in tar yield, with significant enrichment of aliphatic compounds and monocyclic aromatic hydrocarbons, indicating enhanced oil quality. • • Gas yields of H2, CO, and CH4 increase by 5.47%, 10.98%, and 9.27%, respectively, while CO2 generation is inhibited; pyrolysis water yield drops by 93.98%, improving gas calorific value and process efficiency. • • The synergistic mechanism involves 'component interaction-structural modification-catalytic cracking', where inorganic ions (Ca, K, Fe) leached during HTP act as catalysts, and structural convergence of H and E promotes thermal compatibility, leading to superior co-pyrolysis performance.
Abstract
Co-pyrolysis of oil-rich coal and biomass is a promising route to enhance oil and gas production, yet the underlying synergistic mechanisms remain poorly understood. This study investigates the effect of hydrothermal pretreatment (HTP) on the co-pyrolysis of Huangling coal (H) and enzymatic hydrolysis lignin (E). Raw and pretreated samples were characterized via proximate/ultimate analysis, SEM, ICP-OES, and 13C-NMR. Fixed-bed pyrolysis experiments were conducted to evaluate synergistic performance. Results show that HTP reduces oxygen content, develops pore structure, and increases concentrations of inorganic metal ions (Ca, K, Fe) in the aqueous phase. Structural modifications bring the carbon skeleton of E closer to that of H, with increased bridge carbon ratio and improved thermal stability, aligning pyrolysis temperature ranges. For the H/E blend (8:2) after 24 h HTP, tar yield increases by 80.52% compared to untreated blend, with significant rises in aliphatic compounds and monocyclic aromatic hydrocarbons. Gas yields of H2, CO, and CH4 increase by 5.47%, 10.98%, and 9.27%, respectively, while CO2 and pyrolysis water generation are inhibited (water yield decreases by 93.98%). Semi-coke pore structure becomes more developed. The enhanced synergistic effect is attributed to a multi-fold mechanism of 'component interaction-structural modification-catalytic cracking'. These findings provide theoretical support for developing technologies to improve co-pyrolysis of oil-rich coal and biomass, advancing low-carbon, high-quality utilization.
1. Introduction
Direct pyrolysis of oil-rich coal suffers from low tar yield, poor tar quality, and unfavorable economics, limiting its industrial viability. Co-pyrolysis with biomass, such as enzymatic hydrolysis lignin, offers a potential route to enhance volatile production and improve product distribution. However, the inherent differences in thermal behavior and chemical structure between coal and biomass often lead to weak synergistic effects, and the underlying mechanisms remain unclear. Conventional physical mixing fails to achieve intimate contact or favorable interactions, resulting in suboptimal yields and poor selectivity.
This study introduces hydrothermal pretreatment (HTP) as a strategic intervention to modify both feedstocks prior to co-pyrolysis. HTP is shown to reduce oxygen content, develop pore structures, and leach inorganic metal ions that can catalyze cracking reactions. Critically, HTP narrows the structural disparity between Huangling coal and enzymatic hydrolysis lignin, aligning their pyrolysis temperature ranges and enhancing thermal compatibility. This pretreatment enables a multi-fold enhancement mechanism—'component interaction-structural modification-catalytic cracking'—that significantly boosts tar yield and quality while optimizing gas composition. The findings offer a practical pathway to overcome the bottlenecks of direct co-pyrolysis, advancing low-carbon, high-value utilization of oil-rich coal.
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BAI Zhuangwei, ZHOU Anning, ZHANG Huaiqing, ZHANG Zhi, XI Dong, CHEN Fuxin, HE Xinfu (2026). Mechanistic Study on the Enhanced Synergistic Effect in Co-pyrolysis of Huangling Coal and Enzymatic Hydrolysis Lignin via Hydrothermal Pretreatment. Journal of Fuel Chemistry and Technology. https://doi.org/10.1016/S1872-5813(25)60615-3
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Frequently Asked Questions
What is the optimal hydrothermal pretreatment duration for maximizing tar yield in co-pyrolysis of Huangling coal and enzymatic hydrolysis lignin?
The study demonstrates that 24 hours of hydrothermal pretreatment yields the most significant enhancement, increasing tar yield by 80.52% compared to untreated blends. This duration optimally modifies the feedstock structure and leaches catalytic metal ions, leading to improved synergistic effects.
How does hydrothermal pretreatment affect the gas composition during co-pyrolysis?
After 24 h HTP, gas yields of H2, CO, and CH4 increase by 5.47%, 10.98%, and 9.27%, respectively, while CO2 generation is inhibited. This shift towards higher-value gases enhances the calorific value of the product gas, making it more suitable for industrial energy recovery.
What is the mechanism behind the enhanced synergistic effect after hydrothermal pretreatment?
The enhancement is attributed to a multi-fold mechanism: (1) component interaction—HTP reduces oxygen content and brings the carbon skeleton structures of coal and lignin closer, improving thermal compatibility; (2) structural modification—pore development and increased surface area facilitate better contact and mass transfer; (3) catalytic cracking—inorganic ions (Ca, K, Fe) leached into the aqueous phase act as catalysts, promoting cracking and reducing pyrolysis water formation.
Does hydrothermal pretreatment affect the quality of the tar produced?
Yes, the tar quality is significantly improved. After 24 h HTP, the contents of aliphatic compounds and monocyclic aromatic hydrocarbons increase substantially, indicating a higher proportion of valuable light fractions. This is beneficial for downstream refining and chemical production.
What are the implications of reduced pyrolysis water yield (93.98% decrease) for process economics?
The dramatic reduction in pyrolysis water yield indicates that HTP suppresses secondary reactions that produce water, thereby increasing the selectivity towards tar and gas. This improves overall carbon efficiency and reduces the energy penalty associated with water separation and disposal, enhancing the economic viability of the co-pyrolysis process.
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