Carbon-Negative Characteristics of Biochar and Gas Emissions during Biomass Pyrolysis: A Mini Review

Biomass pyrolysis has attracted increasing attention as a sustainable technology for simultaneous biochar production, renewable energy recovery, and carbon sequestration. This mini review discusses the carbon-negative characteristics of biochar, gas emissions during biomass pyrolysis, and the influence of pyrolysis conditions on syngas composition and carbon structural evolution. During pyrolysis, biomass undergoes devolatilization, deoxygenation, aromatization, and graphitization, producing gaseous products such as H2, CO, CO2, CH4, and light hydrocarbons together with stable carbonaceous residues. Elevated pyrolysis temperatures generally enhance the formation of combustible gases, particularly H2 and CO, while reducing oxygenated gases such as CO2 due to intensified secondary cracking and deoxygenation reactions. Simultaneously, carbon structures progressively transform from disordered precursor molecules into aromatic and graphitic domains with improved physicochemical stability. Biomass species, reactor configuration, carrier gas atmosphere, and thermal conditions significantly affect gas evolution behavior and carbon conversion efficiency. Overall, biomass pyrolysis demonstrates strong potential for carbon-neutral energy production, greenhouse gas mitigation, and development of sustainable carbon materials within circular bioeconomy systems.