The gas surrounding biomass during heating may be just as important as temperature in determining the quality of the biochar produced, according to a new review published in Biochar.
Biochar is a carbon rich material created when biomass, such as wood, crop residues, or organic waste, is heated with little or no oxygen. It can store carbon for long periods and may also improve soils, retain nutrients, remove contaminants, and support catalytic processes. Most studies have traditionally produced biochar under nitrogen, which provides a relatively stable and nonreactive environment.
The new review shows that alternative gases, including carbon dioxide, steam, oxygen, methane, ammonia, and recycled industrial gases, can actively shape the production process and the resulting materials.
"The atmosphere inside a pyrolysis reactor should not be viewed simply as a background condition," said lead author Professor Ondřej Mašek of the University of Edinburgh. "It can act as a powerful design tool, allowing us to control biochar yield, pore structure, surface chemistry, and the production of gases and liquids."
The researchers examined how different atmospheres influence the three main products of biomass pyrolysis: solid biochar, liquid bio-oil, and combustible gases.
Nitrogen and argon generally preserve more of the original biomass carbon in the solid product. This makes inert atmospheres a reliable option when the main goal is to maximize biochar production and retain carbon for long term storage.
Carbon dioxide and steam, however, can partially react with the developing biochar, opening pores and increasing its surface area. These changes may improve the material's ability to retain nutrients or capture heavy metals and organic pollutants. Carbon dioxide can also promote the breakdown of tar and shift more carbon into carbon monoxide rich gas, which may be recovered for heat or energy production.
Steam can create highly porous biochar and add oxygen containing surface groups that support adsorption. It may also increase the production or improve the composition of bio-oil. These benefits often come with a tradeoff, since steam can reduce the amount of solid biochar produced.
Small amounts of oxygen can provide heat directly inside the reactor, potentially lowering external energy requirements. Carefully controlled oxidative pyrolysis may also increase porosity and add acidic surface groups that improve ion exchange. Too much oxygen, however, can burn away valuable carbon and sharply reduce biochar yield.
Ammonia offers another route to specialized materials. At relatively low temperatures, it can introduce nitrogen containing groups into biochar, improving properties such as cation exchange capacity, adsorption, and catalytic activity. This one step approach may avoid the additional chemicals and processing normally required to modify biochar after production.
The review also highlights the potential use of flue gas, pyrolysis gas, and other recycled industrial gas streams. Using gases already available at industrial facilities could reduce reliance on purified gases, recover waste heat, and improve the overall environmental and economic performance of biochar production.
No single atmosphere is best for every purpose. An atmosphere that maximizes carbon retention may not produce the most porous material, while conditions that improve adsorption performance may lower solid yield.
"The key question is not which gas is universally superior," Mašek said. "It is which atmosphere best matches the intended outcome, whether that is carbon storage, contaminant removal, nutrient retention, fuel production, or process integration."
The authors call for more systematic experiments, pilot scale testing, real time monitoring, and environmental assessments. Future research should examine combinations of gas composition, temperature, feedstock type, residence time, and reactor design.
By treating pyrolysis atmosphere as a controllable engineering variable, researchers and manufacturers may be able to produce more effective biochars in fewer processing steps while making better use of energy and industrial gas streams.
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Journal Reference: Mašek, O., Buss, W., Wang, L. et al. Biochar production under different atmospheres: an overview. Biochar 8, 129 (2026). https://doi.org/10.1007/s42773-026-00626-8
https://doi.org/10.1007/s42773-026-00626-8
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About Biochar
Biochar (e-ISSN: 2524-7867) is the first journal dedicated exclusively to biochar research, spanning agronomy, environmental science, and materials science. It publishes original studies on biochar production, processing, and applications—such as bioenergy, environmental remediation, soil enhancement, climate mitigation, water treatment, and sustainability analysis. The journal serves as an innovative and professional platform for global researchers to share advances in this rapidly expanding field.
