Key Takeaways & Executive Findings
- •• • Acid washing (HCl and HCl-HF) reduced ash content but also decreased organic element contents; H/C atomic ratios for YL-De, YL-HCl, YL-De-Ca, and YL-HCl-Ca were 0.61, 0.64, 0.58, and 0.59, respectively, indicating structural changes that affect pyrolysis reactivity. • • Loading ion-exchangeable Ca2+ increased thermal weight loss rate in the 500–550 °C range, shifting the DTG peak temperature from 530 °C to 514 °C, demonstrating enhanced cracking ability and thermal reactivity. • • At 600 °C pyrolysis, kaolinite dehydroxylation to metakaolin occurred; the presence of acidic minerals promoted hydrogen transfer to volatiles, and the coexistence with Ca2+ increased aliphatic hydrocarbon content in tar from 13.60% (YL-De-Ca) to 21.98% (YL-HCl-Ca), a 61.6% relative increase. • • Acidic mineral components inhibit the adverse effect of ion-exchangeable Ca2+ on tar lightening, as evidenced by lower small aromatic ring content (<6 rings) in char from Ca-loaded coal, which is critical for improving tar quality and downstream processing.
Abstract
Inherent minerals significantly influence the thermal conversion of coal, yet the interaction mechanisms among minerals affecting tar generation during pyrolysis remain unclear. This study investigates the effect of acidic mineral components on the behavior of ion-exchangeable Ca2+ during coal pyrolysis. Coal samples were prepared via HCl and HCl-HF acid washing followed by Ca2+ ion exchange. Pyrolysis was conducted in a fixed-bed reactor. Acid washing effectively reduced ash content but also decreased organic element contents (carbon, hydrogen). Loading ion-exchangeable calcium enhanced the thermal weight loss rate in the 500–550 °C range, shifting the peak temperature from 530 °C to 514 °C. At a final pyrolysis temperature of 600 °C with slow heating, kaolinite in acidic minerals underwent dehydroxylation to form metakaolin. The content of small aromatic rings (<6 rings) in char from Ca-loaded coal was lower than that from acid-washed coal without Ca. Coexistence of acidic minerals with ion-exchangeable Ca increased aliphatic hydrocarbon content in tar: YL-HCl-Ca reached 21.98% versus 13.60% for YL-De-Ca. Acidic mineral components inhibit the adverse effect of ion-exchangeable Ca2+ on tar lightening. These findings provide insights into mineral interactions during pyrolysis, aiding in optimizing coal conversion processes for improved tar quality.
1. Introduction
Coal pyrolysis is a pivotal technology for low-rank coal utilization, yet its economic viability is hampered by low tar yields and high heavy component content. Inherent minerals, particularly calcium-based species, are known to catalyze secondary reactions that degrade tar quality, but the specific influence of acidic minerals—such as kaolinite—on these interactions remains poorly understood. Existing studies have focused on individual mineral effects, often overlooking the synergistic or antagonistic interactions that occur in real coal matrices. This lack of mechanistic insight has stalled the development of targeted mineral management strategies to optimize pyrolysis conditions for higher-value tar production.
This study addresses this bottleneck by systematically isolating the roles of acidic mineral components and ion-exchangeable calcium through controlled acid washing and ion exchange procedures. By comparing coal samples with and without acidic minerals and calcium loading, the research delineates how kaolinite dehydroxylation at 600 °C alters hydrogen transfer and aromatic ring condensation, ultimately influencing tar composition. The findings reveal that acidic minerals can counteract the detrimental effects of calcium on tar lightening, offering a potential pathway to engineer coal feeds or process conditions for improved product selectivity.
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LI Hongsheng, HAO Pan, YAN Lunjing, KONG Jiao, WANG Meijun, BAO Weiren, CHANG Liping (2026). Influence Mechanism of Acidic Mineral Components on the Reaction Behavior of Ion-Exchangeable Calcium during Coal Pyrolysis. Journal of Fuel Chemistry and Technology. https://doi.org/10.1016/S1872-5813(26)60642-1
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Frequently Asked Questions
What is the specific role of kaolinite dehydroxylation in influencing hydrogen transfer during coal pyrolysis at 600 °C?
At 600 °C, kaolinite undergoes dehydroxylation to form metakaolin, which is an active amorphous phase. This transformation promotes the transfer of hydrogen from the coal matrix to volatile matter, as evidenced by the increased aliphatic hydrocarbon content in tar when acidic minerals are present. Specifically, the coexistence of acidic minerals with ion-exchangeable Ca2+ increased aliphatic content from 13.60% (YL-De-Ca) to 21.98% (YL-HCl-Ca), indicating enhanced hydrogen donation and stabilization of aliphatic fragments.
How does the presence of acidic minerals affect the thermal decomposition profile of coal loaded with ion-exchangeable calcium?
The presence of acidic minerals does not significantly alter the peak temperature shift caused by calcium loading; the DTG peak shifted from 530 °C (without Ca) to 514 °C (with Ca) regardless of acidic mineral presence. However, acidic minerals influence the product distribution by promoting hydrogen transfer, which leads to higher aliphatic content in tar and reduced small aromatic ring formation in char, as observed in the lower content of <6 ring aromatics in Ca-loaded samples.
What is the industrial significance of the increased aliphatic hydrocarbon content in tar from 13.60% to 21.98%?
Aliphatic hydrocarbons are desirable components in coal tar as they are lighter and more valuable for producing fuels and chemicals. A relative increase of 61.6% in aliphatic content indicates that the presence of acidic minerals can significantly improve tar quality, making it easier to upgrade and increasing the economic value of the pyrolysis process. This could reduce downstream processing costs and enhance the feasibility of low-rank coal utilization.
How does acid washing affect the physical structure of coal and its subsequent pyrolysis behavior?
Acid washing, particularly the two-step HCl-HF procedure, increases pore volume and specific surface area compared to HCl washing alone. This structural modification enhances accessibility to active sites and may influence pyrolysis kinetics. However, acid washing also removes organic small molecules, altering the H/C ratio (e.g., YL-De: 0.61, YL-HCl: 0.64), which can affect the overall reactivity and product distribution. The study shows that these physical changes are coupled with chemical changes, and the combined effects must be considered when interpreting pyrolysis results.
What are the limitations of this study in terms of extrapolating to industrial-scale coal pyrolysis?
This study was conducted in a fixed-bed reactor at a slow heating rate to a final temperature of 600 °C, which may not fully replicate industrial conditions such as rapid heating, higher temperatures, or different reactor geometries. Additionally, the coal samples were artificially treated with acid washing and ion exchange, which may not represent the natural mineral distribution in run-of-mine coals. Therefore, while the mechanistic insights are valuable, direct scale-up requires validation under industrially relevant conditions, including the effects of particle size, pressure, and residence time.
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