Researchers have developed a simple visual method for tracking how volatile products move inside a fixed-bed coal pyrolysis reactor. Their findings show that carefully designed internal structures can redirect these vapors, limit undesirable secondary reactions, and improve the production of valuable tar and fuel gas.
Coal pyrolysis heats coal without oxygen to produce solid char, liquid tar, and combustible gases. It is considered a promising route for converting low-rank coal into more useful products. However, vapors released during pyrolysis may remain in high-temperature regions for too long. This exposure can cause valuable tar molecules to crack into non-condensable gases or combine into heavier compounds, reducing tar yield and quality.
In the new study, researchers from Chongqing University and Monash University used quartz particles as inert tracers to reveal the dominant movement of coal-derived vapors inside the reactor. Quartz placed in different regions changed color when exposed to tar-rich vapors because of carbon deposition and tar coking on its surface.
"The color patterns on the quartz gave us a direct and intuitive picture of where the volatile products traveled inside the reactor," said corresponding author Erfeng Hu of Chongqing University. "This helped us connect reactor flow behavior with changes in tar yield, tar composition, and gas quality."
The team compared conventional fixed-bed reactors with reactors containing four heating plates and a central gas-gathering tube. These internal components altered heat and mass transfer, directing volatile products away from the hotter reactor wall and toward the cooler central region.
Without the internal structures, many vapors passed through high-temperature char, where they experienced strong cracking and condensation reactions. In contrast, the modified reactor allowed heavy tar vapors to enter the cooler core, where they condensed temporarily and were later released or mildly cracked as the central region warmed.
This controlled vapor pathway increased total tar yield by approximately 8.5 percent and raised the light tar fraction to as much as 74.42 percent. Light tar is generally more desirable because it contains a greater proportion of lower-boiling compounds and is easier to process than heavy tar.
The redesigned flow field also changed the composition of the pyrolysis gas. Reactors with internal components produced less hydrogen and preserved more methane, indicating that severe secondary cracking had been suppressed. Under one tested arrangement, the higher heating value of the gas increased from 20.60 to 21.69 megajoules per normal cubic meter.
Thermal analysis showed that the main release of coal volatiles occurred near 500 degrees Celsius. The apparent activation energies ranged from about 67 to 306 kilojoules per mole, reflecting the complex sequence of chemical bond breaking and product-forming reactions involved in coal pyrolysis.
The researchers also tested a larger reactor with a diameter of 200 millimeters. Differences in the volatile and ash contents of char collected from different locations supported the vapor pathways observed using quartz particles.
"Our results show that reactor geometry is not simply a structural consideration. It can actively control where vapors travel and which products are ultimately formed," Hu said. "The approach provides a practical basis for designing more efficient fixed-bed systems for low-rank coal conversion."
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Journal reference: Zeng Y, Li M, Hu E, Ma Y, Xu F, et al. 2026. Inert particle tracking of flow fields for selective tar enhancement in a fixed-bed reactor with internals. Sustainable Carbon Materials 2: e024 doi: 10.48130/scm-0026-0019
https://www.maxapress.com/article/doi/10.48130/scm-0026-0019
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Sustainable Carbon Materials (e-ISSN 3070-3557) is a multidisciplinary platform for communicating advances in fundamental and applied research on carbon-based materials. It is dedicated to serving as an innovative, efficient and professional platform for researchers in the field of carbon materials around the world to deliver findings from this rapidly expanding field of science. It is a peer-reviewed, open-access journal that publishes review, original research, invited review, rapid report, perspective, commentary and correspondence papers.
