In a comprehensive review published in Biochar, authors Ondrej Masek, Wolfram Buss, Liang Wang, Jiacheng Sun, Xutong Wang, Yue Wang, and Oyvind Skreiberg demonstrated that replacing standard nitrogen gas with reactive gas atmospheres fundamentally transforms 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. Traditional thermochemical conversion relies heavily on inert nitrogen purge gas to prevent combustion during heating. However, inert environments offer minimal control over the physical and chemical qualities of the resulting solid carbon. Introducing reactive carrier atmospheres such as carbon dioxide, steam, ammonia, methane, oxygen, or recycled industrial flue gas enables targeted physical and chemical modification during primary thermal conversion. This single-step engineering approach eliminates the need for expensive post-production chemical washing or high-temperature activation steps, offering a direct pathway to produce customized functional carbon materials.

The selection of process gas directly dictates the structural properties and surface area of biochar. When processing organic matter under a carbon dioxide atmosphere, the gas reacts with volatile compounds released during heating, promoting thermal cracking and reducing undesirable tar formation by up to seventy percent. This secondary reaction converts heavy organic vapors into energy-rich carbon monoxide gas while simultaneously etching micropores into the biochar matrix. Similarly, introducing steam into the primary reactor promotes gasificationGasification is a high-temperature, thermochemical process that converts carbon-based materials into a gaseous fuel called syngas and solid by-products. It takes place in an oxygen-deficient environment at temperatures typically above 750°C. Unlike combustion, which fully burns material to produce heat and carbon dioxide (CO2​), gasification More reactions that strip volatile matterVolatile matter refers to the organic compounds that are released as gases during the pyrolysis process. These compounds can include methane, hydrogen, and carbon monoxide, which can be captured and used as fuel or further processed into other valuable products.   More from the carbon skeleton. Steam treatment produces highly porous solid materials with expanded surface areas while simultaneously increasing overall liquid biofuel recovery. These structural enhancements make steam-treated and carbon dioxide-treated chars significantly more effective for water filtration and soil remediation than standard chars produced under pure nitrogen.

Introducing active chemical elements like nitrogen or oxygen through the carrier gas creates specialized chemical functionalities on the biochar surface. Thermal conversion carried out in an ammonia environment at temperatures below four hundred degrees Celsius incorporates basic nitrogen groups directly into the carbon structure. This chemical modification triples internal surface area to five hundred twenty-one square meters per gram and enhances the material’s ability to bind organic contaminants. Tests show that ammonia-modified biochar achieves phenol removal rates of one hundred seventy milligrams per gram, compared to one hundred five milligrams per gram for untreated char. Conversely, introducing low levels of oxygen creates exothermic partial combustion reactions. While this internal heating improves reactor thermal efficiency, excessive oxidation rapidly consumes solid carbon, cutting solid biochar yields by up to fifty percent at temperatures above seven hundred degrees Celsius.

Beyond single-gas systems, utilizing recycled process flue gases or methane offers practical operational benefits for commercial biochar facilities. Flue gas, which naturally contains a mixture of carbon dioxide, water vapor, and nitrogen, provides a low-cost reactive atmosphere that enhances surface porosityPorosity of biochar is a key factor in its effectiveness as a soil amendment and its ability to retain water and nutrients. Biochar’s porosity is influenced by feedstock type and pyrolysis temperature, and it plays a crucial role in microbial activity and overall soil health. Biochar More without requiring pure gas purchases. Reusing plant exhaust gases improves overall thermal efficiency and reduces facility operational costs. Meanwhile, methane atmospheres promote carbon deposition onto the solid surface, increasing overall carbon retention and char yield. Matching the carrier gas atmosphere to specific operational goals allows manufacturers to balance solid yield, pore network structure, and surface chemistry. This shift from passive inert processing to active gas engineering represents a major advance toward scalable, energy-efficient biochar production tailored for agricultural, environmental, and industrial applications.

Source: Masek, O., Buss, W., Wang, L., Sun, J., Wang, X., Wang, Y., & Skreiberg, O. (2026). Biochar production under different atmospheres: an overview. Biochar, 8(1), 129.