Microwave-assisted gasification of rice straw pyrolytic biochar promoted by alkali and alkaline earth metals

•Microwave reduces 200 °C compared with conventional heating to obtain same results.•Syngas with 60 vol.% H2 and H2 + CO > 90% are obtained under Ca(OH)2 catalyst at 550 °C.•AAEM catalytic effects on gasification is related to forming of intermediate C(O).

A novel microwave assisted gasification technique was developed whereby pyrolytic biochar obtained from rice straw was used as the feedstock. The conversion efficiencies and gas composition obtained at 800 °C by conventional heating method were similar to that obtained at 550 °C by microwave-assisted gasification. At 550 °C using microwave heating, the CO + H2 yields were over 90 vol.% and H2 content was 60 vol.%. The effects of alkali and alkaline earth metals (AAEM) on microwave-assisted gasification activity of biochar were also investigated. The results indicated that 5% K2CO3 and 5% KOH increased the carbon conversion efficiency from 74.39% to 79.95% and 85.04%, respectively, but CO2 content increased remarkably. Ca(OH)2 was a suitable gasification catalyst and excellent CO2 absorber. 10% Ca(OH)2 reduced CO2 percentage from 21.92 vol.% to 10.83 vol.% and shortened reaction time from 90 to 60 min. Based on the AAEM catalytic mechanisms and microwave heating function, the intermediate C(O) and dipole rotation of materials molecules played a crucial role in the microwave-assisted gasification of biochar. This study proves that microwave heating coupled with the addition of AAEM result in low-temperature microwave-assisted gasification of biochar.