In a review article published in the journal BiocharBiochar is a carbon-rich material created from biomass decomposition in low-oxygen conditions. It has important applications in environmental remediation, soil improvement, agriculture, carbon sequestration, energy storage, and sustainable materials, promoting efficiency and reducing waste in various contexts while addressing climate change challenges. More, a team of authors, including Yuxuan Sun, Jixiu Jia, Lili Huo, Xinyi Zhang, Lixin Zhao, Ziyun Liu, Yanan Zhao, and Zonglu Yao, investigated the preparation of bio-carbon from bio-tar polymerization. The study addresses a significant challenge in renewable energy: the formation of bio-tar as a by-product of biomassBiomass is a complex biological organic or non-organic solid product derived from living or recently living organism and available naturally. Various types of wastes such as animal manure, waste paper, sludge and many industrial wastes are also treated as biomass because like natural biomass these More pyrolysisPyrolysis is a thermochemical process that converts waste biomass into bio-char, bio-oil, and pyro-gas. It offers significant advantages in waste valorization, turning low-value materials into economically valuable resources. Its versatility allows for tailored products based on operational conditions, presenting itself as a cost-effective and efficient More, which often clogs pipelines and poses environmental risks due to its complex composition and toxicity. By converting this bio-tar into a valuable product, researchers aim to overcome this limitation and advance biomass energy utilization. The review summarizes the current understanding of the mechanisms, processes, and applications of this innovative technology, providing a framework for both bio-tar treatment and the development of advanced carbon materials.

Bio-tar is a complex organic mixture, primarily composed of aromatic compounds, with a total carbon content of 60–70% and less than 1% ashAsh is the non-combustible inorganic residue that remains after organic matter, like wood or biomass, is completely burned. It consists mainly of minerals and is different from biochar, which is produced through incomplete combustion.   Ash Ash is the residue that remains after the complete More. This makes it a carbon-rich material suitable for polymerization into bio-carbon. The polymerization process is heavily influenced by the composition of the bio-tar itself, which changes with pyrolysis conditionsThe conditions under which pyrolysis takes place, such as temperature, heating rate, and residence time, can significantly affect the properties of the biochar produced.   More. For instance, at temperatures between 200°C and 500°C, biomass components like cellulose and hemicellulose are converted into intermediates that form a “primary tar” consisting of oxygen-containing compounds such as aldehydes, phenols, and furans. As the temperature increases to 500-700°C, these compounds lose oxygen and become “secondary tar,” which has a higher molecular weight and better thermal stability. Above 700°C, this tar converts into “tertiary tar” with a more stable structure, eventually polymerizing into bio-carbon.

The polymerization is driven by specific components within the bio-tar. Oxygenated compounds with active functional groups like carbonyls and furan rings are crucial for promoting the reaction. For example, compounds like hydroxyaldehydes and hydroxyacetone, with their hydroxyl and carbonyl groups, facilitate electrophilic substitution and aldol condensation reactions. Furfural, a common furan, is a particularly effective polymerization agent, as its active carbonyl group and electron-rich ring activate the polymerization of bio-tar to form a cross-linked structure with biochar. In contrast, organic acids like formic acid and acetic acid do not participate directly but act as catalysts for the polymerization reactions. Macromolecular hydrocarbons, on the other hand, form the carbon skeleton framework through condensation and aromatic stacking mechanisms.

Polymerization process can be controlled by several factors, including reaction conditions and additives. The review highlights that reaction temperature, residence timeResidence time refers to the duration that the biomass is heated during the pyrolysis process. The residence time can influence the properties of the biochar produced.   More, and heating rate significantly impact the yield and properties of the final bio-carbon. While higher temperatures increase the carbon content and graphitization of bio-carbon, they also cause volatile components of bio-tar to escape, reducing the overall yield. For instance, one study found that increasing the temperature from 200°C to 500°C decreased the bio-carbon yield by more than 49%. Conversely, a longer residence timeThis refers to the amount of time that the biomass is heated during the pyrolysis process. The residence time can influence the characteristics of the biochar, such as its porosity and surface area.   More and a lower heating rate promote polymerization, leading to a higher yield. The addition of chemical activators, such as KOH and NaOH, and heteroatom dopants like urea can also improve the pore structure and functional group characteristics of bio-carbon, which are essential for high-value applications.

Bio-carbon has potential applications in several high-value areas, including adsorbents, electrode materials, catalysts, and fuels. Its porous structure and functional groups make it an effective adsorbent for heavy metals and organic pollutants. For instance, N/S dual-doped porous bio-carbon prepared from bio-tar showed a specific surface area of 957.44 m²g⁻¹ and achieved a CO₂ adsorption performance of 2.21 mmol/g at 0°C. As an electrode material for supercapacitors, bio-carbon’s low ash content and unique polymerization mechanism help it form a highly-developed pore structure, which is vital for efficient ion transport. One study showed a bio-carbon electrode with a specific capacitance of 351 Fg⁻¹ at a current density of 0.5 Ag⁻¹. As a fuel, bio-carbon has a high mass energy density, reaching 28.2 MJ/kg, comparable to commercial coal, and its low nitrogen and sulfur content reduce harmful emissions. Despite these promising applications, challenges remain in scaling up production and further optimizing the material’s properties for specific uses.

Source: Sun, Y., Jia, J., Huo, L., Zhang, X., Zhao, L., Liu, Z., Zhao, Y., & Yao, Z. (2025). Preparation of bio-carbon by polymerization of bio-tar: a critical review on mechanisms, processes, and applications. Biochar, 7(90), 1-19.