With the increasing consumption of fossil fuels and the growing concerns about climate change, biomass is drawing increasing attention as a renewable energy source due to its advantages of renewal and abundance. Biomass can be converted into energy using bio-chemical and thermo-chemical processes, but the thermo-chemical conversion technology finds its dominance because of high efficient conversion to gas, liquid and solid products under thermal conditions. Biomass pretreatment can alter the physical features and chemical composition/structure of lignocellulosic materials. The pretreatment step has a significant influence on the quality and yield of products obtained from thermo-chemical conversion biomass. In this review, we discuss the applications of various pretreatment methods in the biomass thermo-chemical conversion, including torrefaction and gasification, pretreatment and biomass pyrolysis, pretreatment and biomass liquefaction. Torrefaction improves the hydrophobicity and grindability characteristics of biomass materials. Water or acid washing pretreatment can remove metal ions from biomass and the change in products distribution during the biomass pyrolysis is more obvious. Biomass pretreatment and liquefaction can increase the bio-oil yield and decrease the optimum reaction temperature compared to the untreated biomass liquefaction experiments.

Contents
1 Introduction
2 Main methods of biomass pretreatment
3 Pretreatment and thermo-chemical conversion biomass
3.1 Torrefaction and biomass gasification
3.2 Pretreatment and biomass pyrolysis
3.3 Pretreatment and biomass liquefaction
4 Conclusion and outlook