Effect of carbon structure on hydrogen release derived from different biomass pyrolysis

Abstract

Producing hydrogen from biomass via pyrolysis is a promising solution to meet the increasing demand for fossil fuel and CO2 release reduction. The correlation between hydrogen release and biomass structure during pyrolysis is investigated in this study. Pyrolysis gases including hydrogen, CH4, CO, and CO2 from three representative biomass, namely, pine, cottonwood and rice straw, are collected from fixed bed reactor and analyzed by gas chromatography (GC). The carbon structure is quantitatively determined by C-13 Nuclear Magnetic Resonance (13C NMR) using curve-fitting methods. Pyrolysis tar composition is monitored using gas chromatography coupled with a mass spectrometer (GC–MS). The results show that carbon structure in biomass has a significant effect on hydrogen release behavior. Hydrogen generation mostly caused by conversion of aromatics, steam reforming of alkanes and water gas reaction, which is related to lignin structures, the content of relative weak bond, and H/C ratio. The carbon structure in rice straw has more weak bonds, such as carbonyl group (2.08%), amorphous cellulose (11.19%) and ethers in lignins (9.22%), the H/C ration in rice straw is also 1.40, which is producing more tar and CH4 such that favoring the reforming reaction for hydrogen production. Its hydrogen release rate reaches to a maximum of 4.11 ml/min·g at 650 °C, higher than 2.86 ml/min·g of cottonwood and 4.05 ml/min·g of pine at 750 °C. The total hydrogen yield between 400 and 900 °C is in the order of rice straw (117.76 ml/g) > pine (98.95 ml/g) > cottonwood (76.99 ml/g).