The study published in the journal Energies by Justyna Kujawska, Wojciech Cel, Barbara Charmas, and Dorota Szala explores how the temperature used to create biochar from digestate—the leftover material from biogas production—changes its chemical and thermal properties. As the global biogas sector expands, the amount of digestate waste is projected to rise from 31 million tons in 2022 to 177 million tons by 2050. Converting this massive volume of waste into biochar offers a sustainable solution that aligns with circular economy principles. The researchers tested temperatures ranging from 400 to 800 degrees Celsius to determine how heat influences the material’s ability to store carbon and resist breaking down in the environment.

The researchers discovered that as the processing temperature increased, the biochar underwent a dramatic transformation in its chemical structure. The hydrogen-to-carbon ratio, a key measure of how much a material has turned into stable “aromatic” structures, dropped from 0.565 at the lowest temperature to a mere 0.086 at the highest. This indicates that high heat effectively strips away unstable organic parts, leaving behind a resilient carbon skeleton. Furthermore, the oxygen-to-carbon ratio fell below 0.2 for the samples heated to 800 degrees, which scientists categorize as a threshold for extreme stability. Biochar with these specific chemical markers is predicted to remain in the soil for more than 1,000 years without decomposing.

Thermal stability tests further confirmed these results, showing that higher temperatures lead to a much higher percentage of solid residue. While the biochar produced at 400 degrees left behind about 61 percent of its mass after being reheated, the 800-degree version retained over 72 percent. This increase in solid residue reflects a material that is better at resisting heat and environmental decay. The study also used infrared technology to monitor the gases released during heating, finding that the high-temperature biochar emitted significantly fewer volatile compounds. This suggests that the material is “cleaner” and more chemically mature, making it safer for use in sensitive environmental applications.

The findings suggest that the intended use of the biochar should dictate the temperature at which it is made. Biochar produced at lower temperatures around 400 degrees contains more oxygen-based chemical groups, which makes it a better choice for stimulating soil health and aiding the fermentation process in biogas tanks. In contrast, the high-temperature biochar produced at 800 degrees is ideally suited for trapping pollutants in contaminated soil and for permanent carbon sequestration. By matching the production temperature to the specific environmental need, we can turn a difficult-to-manage waste product into a powerful tool for fighting climate change and restoring soil health.

Source: Kujawska, J., Cel, W., Charmas, B., & Szala, D. (2026). Effect of 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 temperature on chemical structure and thermal stability of digestate-based biochar. Energies, 19(1043).