Effects of pretreatment sequence of torrefaction deoxygenation and acid washing on biomass pyrolysis polygeneration

Pyrolysis polygeneration converts raw biomass materials into solid (biochar), liquid (bio-oil), and gas (non-condensable gases) and is a promising way of biomass utilization. However, raw biomass material has high oxygen content, certain ash content, and low energy density, which greatly hampers the thermal conversion and utilization of biomass. In this study, the effects of single torrefaction pretreatment for deoxygenation, single washing pretreatment for deashing, and combination of torrefaction and washing pretreatment on pyrolysis polygeneration of rice husk, pine wood sawdust, and cotton straw were systematically studied. Results showed that the combination of pretreatment processes combined the advantages of single torrefaction and single washing pretreatments. Compared with single torrefaction pretreatment, the combined pretreatment increased the yield and higher heating value (HHV) of bio-oil, yields of gaseous products, and the volume fractions of CO and CH 4 . Compared with single washing pretreatment, the combined pretreatment increased the yield and the carbon content of biochar. The order of torrefaction and washing pretreatments also had an important influence on biomass pyrolysis. Compared with torrefaction-washing pretreatment, the washing-torrefaction pretreatment resulted in higher removal rate of metallic species, slightly higher relative sugars content in bio-oil, lower ash content of biochar, and further increase in HHV of gas. This study evaluated and summarized the optimal pretreatment methods based on the target pyrolysis products and provided a database for biomass pyrolysis polygeneration technology. • Pyrolysis polygeneration of biomass with torrefaction and acid washing is studied. • Pretreatment sequence of torrefaction and washing greatly affects biomass pyrolysis. • The combined pretreatment increases the yield and higher heating value of bio-oil. • The washing-torrefaction pretreatment results in higher removal rate of AAEMs. • Optimal pretreatments are summarized for target product of bio-oil, biochar, and gas.