Enhanced pyrolysis of lignocellulosic biomass by room-temperature dilute sulfuric acid pretreatment

In the present work, the effects of dilute sulfuric acid (0.2 wt%) pretreatment at room temperature (30 °C) on the pyrolytic behaviors of pubescens, rice straw, willow, and cypress were studied. For herbaceous biomass, the degradation temperatures (Ta) of cellulose increased by around 20 °C, and the yields of bio-char in slow pyrolysis decreased by around 4 % after the pretreatment. For the pretreated softwood biomass, a decreased Ta and an increased yield of bio-char were found for willow, while little variation for cypress was detected. Regarding the typical small molecular products in the bio-oil after the pretreatment, levoglucosan (LGA) was only found in those of the pretreated pubescens (14.3 %, 400 °C) and cypress (4.5 %, 400 °C; 9.3 %, 500 °C), whereas levoglucosenone (LGO) was only found in that of the pretreated willow (9.7 %, 400 °C; 10.5 %, 500 °C). Increased yield of the small lignin-derived compounds was found for rice straw (from 7.9 % to 14.5 %), while decreased yields were found for willow and cypress. For the oligomeric part in bio-oil, significantly promoted production of holocellulose-derived oligomers was found with slightly increased proportions of heavy oligomers (>1300 Da). Meanwhile, the production of lignin-derived oligomers was apparently promoted as well, especially for rice straw. In the 500 °C pyrolysis of the two woody biomass samples, in addition to more complete degradation (higher yields of bio-oil and small-molecular compounds, and lower yield of bio-char), it was also found that the proportion of the oligomers in 256–500 Da increased significantly, and more holocellulose-derived oligomers were found. It was, therefore, speculated that a higher pyrolysis temperature would mainly promote the pyrolysis of cellulose and hemicellulose.