Abstract
The decarbonization of metallurgical processes requires alternative carbonaceous materials that can partially or fully replace fossil-derived carbons such as coke and coal. Biochar is a potential candidate for carbothermal reduction processes; however, its suitability for metallurgical applications depends on its chemical and physical properties. One important aspect is the CO₂ gasification (Boudouard reaction) behavior of biochar, which controls carbon consumption and CO generation. Although CO₂ gasification of biochar has been widely studied, uncertainty remains regarding which reaction mechanisms dominate and how intrinsic material properties influence the reaction kinetics.
This study examines the CO₂ gasification kinetics of biochars produced from lignocellulosic biomass from various sources at low and high pyrolysis temperatures, which resulted in different properties. Isothermal CO₂ gasification experiments were carried out in a Thermogravimetric Analyzer over the temperature range 700 to 850 °C. Conversion data were analyzed using several solid-state kinetic models to determine which best represent the gasification behavior.
The results show that contracting geometric models provide the most consistent description of gasification behavior over the main conversion range. Accounting for pore evolution using the Random Pore Model improves the description of conversion-dependent reactivity. The temperature dependence of the reaction rates is consistent with chemically controlled gasification, indicating that intrinsic surface reaction kinetics dominate under the studied conditions. Higher reactivity observed in low pyrolysis temperature biochars is consistent with higher volatile matter content, which contribute to a less ordered carbon structure. In addition, the relatively high ash contents of the biochars may further enhance reactivity through catalytic mineral effects, increasing apparent reaction rates without changing the dominant reaction mechanism.
Overall, the results show that CO₂ gasification is controlled by the combined effects of pore structure, volatile matter, and inorganic constituents (ash). Understanding how each pillar contributes to gasification behavior provides a consistent basis for comparing biochars produced from different feedstocks and processing conditions. Such insight is essential for the informed deployment of biochar in metallurgical reduction processes and represents an important step toward defining criteria for potential use of biochar as a renewable metallurgical reductant.