A research team has developed an inert-particle tracking method that makes volatile transport inside a fixed-bed coal pyrolysis reactor directly visible. By adding internal heating plates and a central gas-gathering tube, the researchers redirected volatile compounds from the reactor's hot outer wall toward its cooler core. This restructuring of the flow field suppressed excessive secondary cracking, increased total tar yield by approximately 8.5%, and raised the light-tar fraction to 74.42%. It also improved the fuel quality of the resulting gas.
Pyrolysis converts coal into tar, gas, and solid char and is considered a promising route for the cleaner utilization of low-rank coal. Researchers have investigated how coal rank, particle size, mineral composition, temperature, heating rate, pressure, and reactor configuration influence pyrolysis products. Indirectly heated fixed-bed reactors can produce relatively high tar yields, but their limited heat-transfer efficiency and strong secondary reactions may reduce tar quality. Internal components can modify heat and mass transfer, yet most previous studies have focused on overall product yields rather than directly determining how these structures alter volatile flow paths. A clearer understanding of this transport mechanism is therefore needed to guide reactor optimization.
A study (DOI: 10.48130/scm-0026-0019 ) published in Sustainable Carbon Materials on 27 May 2026 by Erfeng Hu's team, Chongqing University, shows that redirecting coal volatiles toward a cooler central zone can limit unwanted secondary reactions while increasing tar yield, light-tar content, and pyrolysis-gas heating value.
The researchers compared fixed-bed reactors with and without internal structures consisting of four heating plates and a central gas-gathering tube. Long-flame coal and chemically inert quartz particles were sieved to comparable particle-size distributions below 3 mm and loaded in different spatial arrangements. After nitrogen purging, the reactors were heated under identical conditions, and pyrolysis was stopped when the central bed temperature reached 500 °C. As tar-rich vapors passed through the quartz bed, condensation and coking darkened the quartz surface, enabling the dominant transport pathways to be visually traced. The team also measured tar, water, gas, and char yields and analyzed tar fractions and gas composition. Thermogravimetric analysis coupled with Fourier-transform infrared spectroscopy and gas chromatography-mass spectrometry was used to characterize volatile release, while Kissinger–Akahira–Sunose and Flynn–Wall–Ozawa models were applied to calculate apparent activation energies. Additional experiments in a 200-mm-diameter reactor examined whether the observed flow-field effects persisted at a larger scale. The thermal analysis identified three major pyrolysis stages and showed that the principal release of carbon-containing volatiles occurred between 300 and 600 °C, with particularly strong evolution near 500 °C. Apparent activation energies increased from approximately 67–73 kJ/mol at low conversion to 294–306 kJ/mol at high conversion, reflecting the progressive decomposition of more stable coal structures. Quartz discoloration revealed that the internals redirected vapors away from the high-temperature wall region and toward the cooler reactor core, producing a distinctive L-shaped deposition pattern. This altered pathway promoted the condensation and controlled recracking of heavy tar compounds while reducing their prolonged exposure to hot char. Consequently, the reactor equipped with internals produced more tar and less gas and water. Depending on the feeding arrangement, tar yield increased from 4.93 to 5.76 wt.% and from 5.62 to 6.10 wt.%. The light-tar fraction reached 74.42%. Hydrogen content fell while methane content increased, raising the gas higher heating value from 20.60 to 21.69 MJ/Nm³ in the corresponding configuration. Scale-up tests produced consistent changes in char properties, further supporting the proposed transport mechanism.
Overall, the study shows that reactor geometry can direct volatile movement and control coal pyrolysis reactions. Quartz tracing intuitively links internal flow patterns with tar, gas, and char properties, although it currently provides qualitative rather than quantitative evidence. Combining this method with grayscale imaging or surface-carbon measurements could improve assessments of volatile deposition. Together with product and kinetic analyses, the approach offers a scalable strategy for increasing tar yield, producing lighter fractions, and improving fuel-gas quality from low-rank coal conversion.
