Coal pyrolysis may have found a surprisingly simple way to become more selective. Researchers from Chongqing University and Monash University have used ordinary quartz particles to visualize how volatile products travel through a fixed-bed reactor, revealing that the internal geometry of the reactor can determine whether coal vapors become valuable liquid tar or degrade into lower-value gases and heavy residues.
The study addresses a long-standing challenge in the conversion of low-rank coal. When coal is heated in the absence of oxygen, its complex organic structure breaks down and produces three main product streams: solid char, liquid tar, and combustible gases. Tar is especially valuable because it contains chemical compounds that can be refined into fuels and industrial feedstocks. Yet the vapors carrying these compounds can undergo unwanted reactions before they leave the reactor. If they remain too long in hot regions, valuable molecules may crack into hydrogen and other non-condensable gases, or recombine into heavier compounds that are difficult to process.
To observe this hidden process, the research team placed inert quartz particles at different positions inside the reactor. The particles did not participate chemically in pyrolysis, but their surfaces became darkened when exposed to tar-rich vapors. Carbon deposition and tar coking produced visible color patterns, effectively turning the quartz into a passive flow-tracing system. Instead of relying only on complex simulations or indirect measurements, the researchers could inspect where tar vapors had traveled and compare those patterns with the products collected from the reactor.
“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. The visual evidence allowed the team to connect the movement of vapors with changes in tar yield, tar composition, and gas quality. In this way, a simple material provided information about the reactor’s internal flow field, including regions where vapors were likely to be trapped, redirected, condensed, or exposed to secondary reactions.
The researchers compared a conventional fixed-bed reactor with modified designs containing four heating plates and a central gas-gathering tube. These internal components changed the distribution of heat and mass transfer inside the reactor. Rather than allowing vapors to move primarily toward the hotter outer wall, the structures redirected them toward a cooler central region. This altered pathway reduced the time that heavy tar vapors spent in the most severe thermal environment, where cracking and condensation reactions could rapidly change their chemical composition.
In the conventional arrangement, many coal-derived vapors passed through hot layers of char. Char can act as a reactive surface, encouraging secondary cracking, reforming, and condensation. These reactions may break large tar molecules into smaller gases or cause them to form heavier aromatic compounds and coke. In the redesigned reactor, however, heavy vapors entered the cooler core more quickly. There, they could condense temporarily and later be released or undergo milder cracking as the central region warmed. The result was a more controlled sequence of vapor transport and chemical transformation.
This change in flow increased the total tar yield by approximately 8.5 percent, while the proportion of light tar reached as high as 74.42 percent. Light tar generally contains a greater share of lower-boiling compounds, making it easier to recover, transport, and process than heavy tar. The finding suggests that improving reactor performance does not necessarily require changing the coal feedstock or adding a chemical catalyst. In some cases, carefully engineered internal structures may be enough to steer the products toward the desired outcome.
The gas stream also reflected the altered chemistry. Reactors equipped with internal components produced less hydrogen while preserving more methane, a pattern consistent with reduced secondary cracking of hydrocarbons. Under one tested configuration, the higher heating value of the gas rose from 20.60 to 21.69 megajoules per normal cubic meter. That increase indicates that the gas retained more energy-rich hydrocarbons instead of being converted extensively into lighter species. For systems designed to produce both liquid fuels and combustible gas, this balance could be important for improving overall energy efficiency.
Thermal analysis showed that the main release of coal volatiles occurred near 500 degrees Celsius. The researchers calculated apparent activation energies ranging from approximately 67 to 306 kilojoules per mole, reflecting the multiple overlapping reactions involved in coal pyrolysis. These values encompass the breaking of chemical bonds in the coal matrix, the formation and transport of primary vapors, and the secondary reactions that occur after the vapors leave the solid fuel. Tests in a larger reactor with a diameter of 200 millimeters provided additional support: differences in the volatile and ash contents of char collected from separate locations matched the vapor pathways indicated by the quartz tracers.
The results highlight a broader principle in thermal conversion technology: reactor geometry is an active part of the chemistry. “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. By making vapor movement visible and linking it to product quality, the quartz-tracing approach could help engineers design more efficient fixed-bed reactors for low-rank coal and other solid fuels. The study, published in Sustainable Carbon Materials, offers a practical route toward producing more light tar and higher-quality fuel gas while limiting the secondary reactions that reduce the value of coal-derived products.
Subject of Research: Coal pyrolysis, vapor flow control, fixed-bed reactor design, tar enhancement, and fuel gas quality
Article Title: Inert particle tracking of flow fields for selective tar enhancement in a fixed-bed reactor with internals
News Publication Date: 27-May-2026
Web References: https://doi.org/10.48130/scm-0026-0019; Sustainable Carbon Materials
References: 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
Image Credits: Yongfu Zeng, Moshan Li, Erfeng Hu, Youcai Ma, Fan Xu & Run Dong
Keywords
Coal pyrolysis, fixed-bed reactor, tar production, light tar, volatile products, quartz tracers, reactor internals, vapor flow, fuel gas, methane, hydrogen, thermal conversion, low-rank coal, carbon materials
