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 biomass 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. While traditional thermal conversion relies heavily on inert nitrogen to prevent combustion, introducing reactive gases such as carbon dioxide, steam, ammonia, oxygen, methane, or recycled industrial flue gas enables single-step physical and chemical engineering of biochar. This approach eliminates the need for expensive secondary chemical washing or physical activation procedures, allowing operators to directly control biochar yields, pore structure, and chemical functionalities during primary thermal processing.

The choice of processing atmosphere exerts a profound quantitative impact on product distribution and solid char properties. Processing organic materials under a carbon dioxide atmosphere actively promotes the thermal cracking of volatile organic compounds, reducing undesirable tar formation by up to seventy percent depending on the feedstockFeedstock refers to the raw organic material used to produce biochar. This can include a wide range of materials, such as wood chips, agricultural residues, and animal manure. More. This chemical interaction converts condensable organic vapors into carbon monoxide rich fuel gases that can be captured and burned for energy generation. At temperatures exceeding six hundred degrees Celsius, carbon dioxide reacts with the solid carbon framework through the Boudouard reaction, significantly expanding microporosity and raising the specific surface area of the resulting biochar.

Introducing steam into the thermal reactor similarly alters reaction pathways by favoring the formation of liquid biocrude and noncondensable gases while simultaneously generating porous carbon structures. Water molecules undergo 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 with nascent char, forming accessible mesopores and micropores throughout the material. However, this enhanced pore development comes at the expense of solid carbon yield. The structural changes driven by steam treatment increase oxygen-containing surface groups, rendering the material effective for moisture retention and agricultural applications, though the reduction in retained solid carbon requires careful balancing when long-term carbon storage is the primary objective.

Ammonia gas offers a uniquely powerful mechanism for chemical modification by embedding basic nitrogen functionalities directly into the carbon lattice. Conducting thermal conversion under an ammonia atmosphere at temperatures below four hundred degrees Celsius incorporates pyridinic, pyrrolic, and quaternary nitrogen groups without diminishing overall solid biochar yield. This nitrogenation process increases the internal surface area of the biochar from 158 to 521 square meters per gram and expands total pore volume from 0.075 to 0.248 cubic centimeters per gram. When applied to wastewater treatment, ammonia-modified biochar achieves phenol contaminant removal rates of 170 milligrams per gram compared to approximately 105 milligrams per gram for untreated char, demonstrating superior adsorption capabilities driven by strong electron interactions and structural pore expansion.

Oxidative atmospheres containing low concentrations of oxygen or ambient air introduce exothermic partial combustion reactions that improve reactor thermal efficiency. However, active oxidation rapidly consumes solid carbon, resulting in severe yield reductions. At high temperatures around seven hundred degrees Celsius, introducing oxygen reduces biochar yield by up to fifty percent compared to nitrogen baselines. While oxidative etching creates acidic carboxyl and hydroxyl surface groups that increase cation exchange capacity, the substantial carbon loss makes oxygen atmospheres less suitable for carbon removal initiatives.

Ultimately, selecting a pyrolysis atmosphere requires balancing trade-offs between solid carbon retention and specialized material functionality. Inert atmospheres remain the most direct route for maximizing stable carbon storage, whereas reactive atmospheres like carbon dioxide, steam, and ammonia provide tailored pathways for environmental remediation, water filtering, and renewable energy integration. Utilizing recycled industrial flue gas or captured combustion emissions as purge gases presents a practical, cost-effective strategy to scale high-value engineered biochar production while lowering operational carbon footprints across the bioeconomy.

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), Article 129.