Biochar production under different atmospheres: an overview
The composition of biomass feedstock and pyrolysis parameters, such as temperature, heating rate, and residence time, are key factors that affect the distribution and properties of pyrolysis products and are among the most extensively studied. However, the pyrolysis atmosphere also plays a crucial role in determining process efficiency and product quality. This review examines the effects of various pyrolysis atmospheres on biochar yields and properties, and also considers impacts on pyrolysis gases and liquids. While nitrogen is commonly used in research, the demand for biochar with specific properties like increased porosity and functionality has led to exploring alternative atmospheres such as steam, oxidative, pyrolysis gas, carbon dioxide, methane and ammonia. These alternatives can produce biochar with the desired characteristics in a single step, bypassing the need for multiple modifications. This review provides an overview of these pyrolysis atmosphere options, their applications, advantages, and potential challenges. Future research directions are also identified and highlighted, offering a roadmap for advancing biochar production technology. Reactive gases (CO2, H2O, O2, NH3) enable one‐step control of biochar yields, porosity and surface chemistry. CO2 and steam raise surface area, microporosity, and pyrolysis gas/bio‐oil yields (CO2 limits tars; steam adds oxygen functionalities). NH3 (< 400 °C) nitrogenates char to improve cation exchange capacity and catalytic performance. Reactive gases (CO2, H2O, O2, NH3) enable one‐step control of biochar yields, porosity and surface chemistry. CO2 and steam raise surface area, microporosity, and pyrolysis gas/bio‐oil yields (CO2 limits tars; steam adds oxygen functionalities). NH3 (< 400 °C) nitrogenates char to improve cation exchange capacity and catalytic performance.