Insights into chemical evolution mechanism of different biomass during torrefaction based on the properties of torrefied solid, liquid, and gaseous products

Understanding the complex structural changes and multitudinous chemical reactions during biomass torrefaction has a leading role for establishing the integral torrefaction mechanism. This study provides a systematic insight into the evolution characteristics of torrefied solid, liquid, and gaseous products of corn straw (CS) and walnut shell (WS) during torrefaction. The results show that the weight loss of CS during torrefaction is more significant than that of WS. With torrefaction severity, the relative amount of C=C gradually increases and the relative amounts of -OH and -COO decrease due to the aromatization reaction. Additionally, the relative amount of C=O in CS initially declines and then increases, while that in WS increases due to the chemical composition differences between CS and WS. After torrefaction, CS performs better in the aromaticity and orderliness than WS. GC/MS analysis confirms that phenols dominate in the torrefied liquid products of WS, while furans and ketones dominate in those of CS. Non-condensable gases mainly consist of CO and CO2. These observations reveal that the molecular transformation route of biomass during torrefaction is relevant to the behavior of three major ingredients. Such data supply fundamental theoretical knowledge to the advancement of torrefaction technique and the investigation of authentic kinetics.