Enhanced conversion of CO2 into O2-free fuel gas via the Boudouard reaction with biochar in an atmospheric plasmatron

•Energy efficiency can reach 36.4 % in the plasmatron CO2 decomposition process.•Max. CO2 conversion is largely enhanced from 6.9%–21.3% with biochar addition.•Boudouard reaction contributes >96.0 % of CO2 conversion in the presence of biochar.•O2-free fuel gas with high CO content can be produced in the presence of biochar.