Effects of different pretreatment methods on millet straw and millet bran for fuel properties and pyrolysis bio-oil

Effects of acid washing, torrefaction, and a combination of acid washing and torrefaction pretreatment millet straw (MS) and millet bran (MB) were investigated. The physicochemical properties of MS and MB before and after acid washing pretreatment were analyzed using industrial analysis, elemental analysis, component analysis, and Fourier infrared spectroscopy. Pyrolysis experiments using tube furnace and GC–MS analysis to investigate the composition and properties of bio-oil by comparing MS and MB under different temperatures. The results indicated that both acid washing and torrefaction pretreatment had a significant effect on fuel characteristics of MS and MB, while their effects on the molar ratios of C/O and C/H were opposed. The combination of acid washing and torrefaction pretreatment was beneficial for improving the fuel characteristics and increasing the yield of bio-oil. Compared to torrefaction pretreatment, the bio-oil yield of acid washing MS and acid washing MB all increased to 17% at 230 ℃. In addition, the relative content of sugars derived from MS and MB increased, but the relative content of oxygenates including acids, ketones, and furans, etc., decreased. These findings indicated that the combination of acid washing and torrefaction pretreatment was a promising method to improve the higher heating value and quality of bio-oil.