【文章內(nèi)容簡介】 w biomass as a fuel cost of biomass is relatively high。 due to its undesirable characteristics such as high moisture content and biological attack[14]. Technology advancement in areas such as power generation system and process selection will play a major role in assisting global nations in delivering their mitments and activities in renewable energy. In recent decade, a pretreatment technique known as torrefaction proposed for thermal conversion process has showed positive result. This review aims to pare and pile the various works on torrefaction presented by different studies.. Biomass to energy conversion processesBiofuel can be classified into three main types namely wood fuels, agro fuels and municipal byproducts which is based on the source of the biomass used. Fig. 1 summarizes the technological options to convert raw biomass into convenient energy carriers such as biogas, liquid fuel or processed solids. The technologies can be classified into three main categories: biochemical, mechanical and thermochemical conversion. Biofuel synthesized can be grouped as three main types namely the wood fuels, a gro fuel and municipal byproducts [17].Biochemical conversion utilizes biological organism and biological catalyst to convert biomass into convenient fuel such as bioethanol, biogas and bio diesel. Centuriesold technology of mechanical extraction is another option to obtain plant oil by physical rolling and crushing of seeds, kernel and fruits.Thermochemical processing relies on heat and chemical catalyst to synthesize useful secondary energy. This is an attractive option for conversion of biomass to energy due to its higher efficiencies,greater versatility as well as wider range of fuel feedstock. Thermochemical conversion of biomass pared to biological conversion is a faster process. Gasification technology offers advantages such as reduced emissions, improved thermal efficiency, and the ability to generate hydrogen and other highvalue fuels [18]. Gasification of biomass and coal are relatively identical as thermal deposition yields similar gaseous products. This factor offers considerable flexibility in cost, operation and range of secondary energy. However,the disadvantages related to the nature of biomass feedstock such as the position of more reactive ashes in biomass than in coal are among the undesirable characteristics that need to be resolved [19].In view of the problems associated with the undesirable characteristics of raw biomass, pretreatment offers a promising solution to enhance process efficiency prior to the main energy conversion step [20]. Torrefaction, a pretreatment technology that requires lower treatment temperature is reported to be highly efficient for thermochemical processing and will be the main pretreatment method discussed in this paper.Torrefaction is a thermolysis process that subjects the feedstock to thermal treatment at relatively low temperatures of 200–300 ?C in the absence of oxygen. Definition for torrefaction is monly associated with roasting, mild pyrolysis, slow pyrolysis, and thermal pretreatment, according to its utilization. Early research work on torrefaction was mainly on wood based material such as woodchipsand sawdust. In the recent years, more studies incorporate agricultural crops and a groforestry residue. Table 1 summarize the fuel properties of different biomass. Although various sources of biomass material were investigated, similar product properties can be attained through torrefaction process such as improved energetic value, enhanced hydrophobicity and friability。 which is a favored trend for thermochemical processing.The physical and chemical properties of biomass before and after torrefaction are analyzed for the following (a) yield, (b) energy content,(c) elemental position, (d) change in major ponents,(e) hydrophobicity, and (f) ease of minution.. Mass yield and energy yieldRaw biomass is deliberately subjected to limited conversion in the torrefaction process. The valuable intermediates synthesized in the process are used for energy recovery at a later stage [41].In the torrefaction temperature range of 200–300 ?C, mass loss is dominated by dehydration and de volatization in the reaction regime of hemicelluloses ponent [26].Mass spectrometry analysis indicates that weight loss is acpanied by reduction in the hemicelluloses and primary lignin sections [42]. The significant mass loss at the preliminary stage of torrefaction shows reduction in the moisture of the treated biomass [43].Mass yield and energy yield of different biomass subjected to torrefaction process is illustrated in Table 2. The mass yield of torrefied biomass can vary from 24% to 95% of its original rate of agricultural residues is paratively higher than woody biomass due to its higher hemicelluloses content, thus resulting in lower mass yield [33,39,44]. Studies show that the polymeric structure of the feedstock will affect the reactivity of torrefaction reaction [44,45]. Higher content of xylan, the main fraction in hemicelluloses fraction, will increase the rate of reaction [44]. Increasing the torrefaction temperature and residence time will improve the higher heating value (HHV) of biomass. HHV increment of torrefied biomass was in the range of 1–58% for the various biomasses as displayed in Table 2. Net calorific values of woody and nonwoody biomass are in the range of 18–26 MJ/kg and 12–25 MJ/kg, respectively.Energy yield based upon the mass yield and calorific value and can be viewed as an indicator of the amount of energy lost during torrefaction. Energy yield for woody biomass subjected to torrefaction temperatures below 250 ?C is above 95% except for Lucerne wood (88%). As torref