Biochar, the carbon-rich material made by heating wood, crop residues, and organic waste, may have a hidden control knob that scientists have underestimated for decades: the gas surrounding it during production. A new review in Biochar argues that the atmosphere inside a pyrolysis reactor can be just as influential as temperature, determining whether the final material is optimized for long-term carbon storage, pollution removal, soil improvement, or energy production. Rather than treating the surrounding gas as an inert background, researchers say it should be viewed as an active engineering tool capable of reshaping biochar at the molecular and structural levels.
Biochar is produced through pyrolysis, a thermal process in which biomass is heated with little or no oxygen. Under these conditions, the feedstock separates into three principal products: a solid carbon-rich material, liquid bio-oil, and combustible gases. The process is already attracting global interest because biochar can lock carbon into a relatively stable form for long periods while improving soil structure, retaining nutrients, removing contaminants, and supporting chemical reactions. Yet most laboratory and industrial studies have traditionally used nitrogen, a relatively unreactive gas, to create what researchers assumed was a controlled environment.
The review, led by Professor Ondřej Mašek of the University of Edinburgh, shows that this assumption may be limiting the technology’s potential. The authors examined how nitrogen, argon, carbon dioxide, steam, oxygen, methane, ammonia, flue gas, and recycled pyrolysis gases influence the chemistry and physical structure of biochar. Each atmosphere can alter heat transfer, reaction pathways, carbon conversion, pore formation, surface chemistry, and the balance between solid, liquid, and gaseous products. The result is that two biochars made from the same biomass at the same nominal temperature may perform very differently if they are produced under different gases.
Inert atmospheres such as nitrogen and argon generally preserve more of the original biomass carbon in the solid fraction. This makes them attractive when the central objective is to maximize biochar yield and retain carbon for storage. However, their chemical neutrality can also limit the development of specialized surface properties. By contrast, reactive gases can interact directly with the evolving char. Carbon dioxide and steam, for example, may trigger gasification reactions in which carbon atoms are removed from the solid matrix, creating additional pores and increasing surface area.
That transformation could make biochar more effective as an environmental material. A larger internal surface area provides more locations where nutrients, heavy metals, and organic pollutants can attach through adsorption. Steam can also introduce oxygen-containing functional groups onto the biochar surface, changing its polarity and chemical reactivity. These groups may improve interactions with dissolved contaminants or soil nutrients. In some processing conditions, steam may increase bio-oil production or alter its composition, although the benefits can come at a cost: stronger reactions with steam may consume more solid carbon and lower the final biochar yield.
Carbon dioxide offers a different set of possibilities. As it reacts with hot carbon, it can enlarge pores and encourage the breakdown of tar compounds that might otherwise condense in the reactor or contaminate downstream products. The process can shift more carbon into carbon monoxide-rich gas, which may be recovered and burned to provide heat or generate energy. This creates the possibility of a more integrated system in which carbon dioxide is not merely emitted but is circulated through the reactor to influence the product while supporting energy recovery.
Oxygen is even more powerful—and more difficult to control. Small, carefully managed amounts can generate heat directly inside the reactor through partial oxidation, potentially reducing the external energy required to maintain pyrolysis temperatures. Controlled oxidative conditions may also increase porosity and create acidic surface groups that improve ion exchange, a property important for nutrient retention and some catalytic applications. But excessive oxygen can rapidly oxidize the char itself, burning away valuable carbon and sharply reducing the solid product. The boundary between useful process intensification and destructive combustion may therefore be narrow.
Ammonia could enable another form of customization by adding nitrogen to the material during production. At comparatively low temperatures, ammonia can react with the developing biochar and introduce nitrogen-containing functional groups. These groups may increase cation exchange capacity, improve adsorption, or enhance catalytic performance. In conventional manufacturing, similar properties might require post-production treatment with additional chemicals. Ammonia-assisted pyrolysis could combine production and modification in a single step, potentially simplifying the process, although safety, emissions, cost, and life-cycle impacts would need careful evaluation before large-scale adoption.
The review also points toward an industrial future in which biochar reactors use gases that facilities already produce. Flue gas, recycled pyrolysis gas, and other industrial streams could replace some purified nitrogen, reducing gas costs and lowering the energy associated with gas separation and compression. Such integration could also recover waste heat and connect biochar production with existing biomass, waste-management, or energy infrastructure. The authors emphasize that no atmosphere is universally superior: the best choice depends on whether a facility prioritizes carbon retention, contaminant capture, nutrient management, fuel generation, or overall process efficiency.
The researchers are calling for systematic experiments that vary gas composition alongside temperature, feedstock type, residence time, and reactor design. Pilot-scale trials will be essential because gas behavior in a small laboratory reactor may not translate directly to an industrial system. Real-time monitoring of gases, vapors, temperature, and char chemistry could help operators control the process as it unfolds rather than relying only on fixed settings. Environmental assessments must also account for emissions, energy use, chemical inputs, and the long-term fate of the resulting biochar. If these challenges are addressed, changing the atmosphere around biomass could transform pyrolysis from a one-size-fits-all heating process into a precision manufacturing platform for climate, agricultural, and environmental technologies.
Subject of Research: Biochar production and pyrolysis under different gas atmospheres
Article Title: Biochar production under different atmospheres: an overview
News Publication Date: 29-Jul-2026
Web References: https://doi.org/10.1007/s42773-026-00626-8; https://link.springer.com/journal/42773
References: Mašek, O., Buss, W., Wang, L. et al. “Biochar production under different atmospheres: an overview.” Biochar 8, 129 (2026). DOI: 10.1007/s42773-026-00626-8
Image Credits: Ondřej Mašek, Wolfram Buss, Liang Wang, Jiacheng Sun, Xutong Wang, Yue Wang and Øyvind Skreiberg
Keywords: Biochar, biomass pyrolysis, carbon storage, carbon dioxide, steam, ammonia, oxygen, gasification, soil improvement, pollutant removal, bio-oil, renewable energy
