View Artide Online Chem Soc Rev Review Article Table 1 Typical elementary composition of bio-oil and crude oil. bio-oil,which is considered to be a primary issue among the (Adapted with permission from Dickerson et al,Energies,2013,6, 514-538.20 Copyright 2013 MDPL) differences between bio-oils and hydrocarbon fuels.18.24 The low heating value and flame temperature,greater ignition Composition Bio-oil Crude oil delay,and lower combustion rate of bio-oil are largely due to Water (wt%) 15-30 0.1 the high water content (15-30 wt%),although water could PH 2.8-3.8 reduce the viscosity and enhance the fluidity.25 The low pH Density (kg L-1) 1.05-1.25 0.86-0.94 of 2-3 of bio-oil is due to the significant amount of carboxylic Viscosity 50 C(cP) 40-100 180 HHV (MJ kg) 16-19 44 acids,mainly formic and acetic acids(Fig.1),and leads to its C (wt%) 55-65 83.86 corrosiveness to common construction materials such as carbon O(wt%) 28-40 1 steel and aluminum as well as some sealing materials.'The high H (wt%) 5-7 11-14 S(wt%) <0.05 <4 content of acidic components also makes the bio-oils extremely N(wt%) <0.4 <1 unstable.The physicochemical properties of bio-oils change as a Ash (wt%) <0.2 0.1 function of time under ordinary storage conditions.26 The visco- H/C 0.9-1.5 1.5-2.0 o/c 0.3-0.5 0 sity of bio-oil increases due to secondary condensation and polymerization of the high concentration of reactive compo- nents like aldehydes,ketones,and phenols.2 In distillation, 0 35-50 wt%of the primary bio-oil is left as a residue due to the Hemicellulose and Cellulose ☑Low wt% High wt% polymerization of reactive components and the substantial Acids Lignin amounts of nonvolatile sugars and oligomeric phenols.Highly Mise Oxy: 30 Acetic Glycolaldehyde oxygenated bio-oils are immiscible with hydrocarbon fuels, Propanoic Acctol DiOH-benzene which hinder their use as fuel additives. Dimeth-pnenol It is desirable and necessary to improve the quality of bio-oil Alcohols ydroglucose nol 20 toward properties similar to those of hydrocarbon fuel by certain Methanol Ethanol Furans Methyl guaiacol Ethylene Glycol upgrading techniques.Oxygen must be removed before the bio-oil can be used as a replacement for diesel and gasoline.10.23 Methyl formate Butyrolactone Ketor Bio-oil can be upgraded either off-line or during the fast pyrolysis Angelicalactone assisted by a catalyst,the so-called catalytic fast pyrolysis(CFP) process.Both cases require catalysts to efficiently remove oxygen. To date,catalytic deoxygenation has been extensively investigated for more than three decades and generally includes two approaches:catalytic cracking and hydrotreating.Catalytic Fig.1 Chemical composition of bio-oil from wood biomass and the most cracking creates products of lower oxygen content than the feed abundant molecules of each of the components.(Adapted with permission from Huber et al.Chem.Rev..2006.106.4044-4098.23 Copyright 2006 by solid acid catalysts,such as zeolites at atmospheric pressure American Chemical Society.) without the requirement of hydrogen.However,the process produces low grade products (benzene,toluene,and small chain alkanes),which require further refining,and has a low groups with relative abundance is shown in Fig.1.The main carbon yield because of significant coke formation,which components include three major families of compounds: results in a very short catalyst lifetime.34 Hydrotreating of bio- (i)small carbonyl compounds such as acetic acid,acetaldehyde, oil adopts the conventional fuel hydrotreating technologies and acetone,hydroxyaldehydes,hydroxyketones,and carboxylic acids;gives desired products by removing oxygen by hydrodeoxygena- (ii)sugar-derived compounds such as furfural,levoglucosan,tion and breaking the larger molecules in the presence of a anhydrosugars,furan/pyran ring-containing compounds;and pressurized hydrogen atmosphere and a catalyst such as sup- (iii)lignin-derived compounds,which are mainly phenols and ported molybdenum sulfide.3 Bio-oil hydrotreating has been guaiacols;oligomers of a molecular weight ranging from 900 to well developed and produces high grade products.There are 2500 are also found in significant amounts.The distribution excellent reviews that have summarized the historical develop- of these compounds mostly depends on the type of biomass ments,35 recent advances,36 and new focus on hydrogeoxygena- used and the process severity.4-2 Such a distribution also tion of lignin-derived bio-oils.However,because of bio-oil influences the physical properties of bio-oil. instability and the high oxygen content,hydrotreating suffers Some properties of bio-oil from fast pyrolysis of ligno-from high operating cost associated with significant catalyst cellulosic biomass significantly limit its direct utilization as deactivation,expensive catalysts used,and substantial hydro- transportation fuel in current systems.Generally,bio-oils are gen consumption.39 characterized by low vapor pressure,low heating value,high An alternative way is to use a catalyst to directly upgrade the acidity,high viscosity,and high reactivity.1819,23 Bio-oils show a pyrolysis vapors prior to quenching to produce bio-oil with wide range of boiling temperatures due to their complex com- improved quality,a process that is called catalytic fast pyrolysis positions.These adverse characteristics,particularly the instability (CFP).By instantly treating the hot pyrolysis vapor with a of bio-oil,are associated with the high oxygen content in the suitable catalyst,the pyrolysis intermediates are simultaneously 7596|Chem.Soc.Rev,2014.43.7594-7623 This joumal is The Royal Society of Chemistry 20147596 | Chem. Soc. Rev., 2014, 43, 7594--7623 This journal is © The Royal Society of Chemistry 2014 groups with relative abundance is shown in Fig. 1. The main components include three major families of compounds: (i) small carbonyl compounds such as acetic acid, acetaldehyde, acetone, hydroxyaldehydes, hydroxyketones, and carboxylic acids; (ii) sugar-derived compounds such as furfural, levoglucosan, anhydrosugars, furan/pyran ring-containing compounds; and (iii) lignin-derived compounds, which are mainly phenols and guaiacols; oligomers of a molecular weight ranging from 900 to 2500 are also found in significant amounts.16–18 The distribution of these compounds mostly depends on the type of biomass used and the process severity.14,18–22 Such a distribution also influences the physical properties of bio-oil. Some properties of bio-oil from fast pyrolysis of ligno￾cellulosic biomass significantly limit its direct utilization as transportation fuel in current systems. Generally, bio-oils are characterized by low vapor pressure, low heating value, high acidity, high viscosity, and high reactivity.18,19,23 Bio-oils show a wide range of boiling temperatures due to their complex com￾positions. These adverse characteristics, particularly the instability of bio-oil, are associated with the high oxygen content in the bio-oil, which is considered to be a primary issue among the differences between bio-oils and hydrocarbon fuels.18,24 The low heating value and flame temperature, greater ignition delay, and lower combustion rate of bio-oil are largely due to the high water content (15–30 wt%), although water could reduce the viscosity and enhance the fluidity.18,25 The low pH of 2–3 of bio-oil is due to the significant amount of carboxylic acids, mainly formic and acetic acids (Fig. 1), and leads to its corrosiveness to common construction materials such as carbon steel and aluminum as well as some sealing materials.18 The high content of acidic components also makes the bio-oils extremely unstable. The physicochemical properties of bio-oils change as a function of time under ordinary storage conditions.26 The visco￾sity of bio-oil increases due to secondary condensation and polymerization of the high concentration of reactive compo￾nents like aldehydes, ketones, and phenols.27 In distillation, 35–50 wt% of the primary bio-oil is left as a residue due to the polymerization of reactive components and the substantial amounts of nonvolatile sugars and oligomeric phenols. Highly oxygenated bio-oils are immiscible with hydrocarbon fuels, which hinder their use as fuel additives. It is desirable and necessary to improve the quality of bio-oil toward properties similar to those of hydrocarbon fuel by certain upgrading techniques.28–30 Oxygen must be removed before the bio-oil can be used as a replacement for diesel and gasoline.10,23 Bio-oil can be upgraded either off-line or during the fast pyrolysis assisted by a catalyst, the so-called catalytic fast pyrolysis (CFP) process. Both cases require catalysts to efficiently remove oxygen. To date, catalytic deoxygenation has been extensively investigated for more than three decades and generally includes two approaches: catalytic cracking and hydrotreating.31–33 Catalytic cracking creates products of lower oxygen content than the feed by solid acid catalysts, such as zeolites at atmospheric pressure without the requirement of hydrogen. However, the process produces low grade products (benzene, toluene, and small chain alkanes), which require further refining, and has a low carbon yield because of significant coke formation, which results in a very short catalyst lifetime.34 Hydrotreating of bio￾oil adopts the conventional fuel hydrotreating technologies and gives desired products by removing oxygen by hydrodeoxygena￾tion and breaking the larger molecules in the presence of a pressurized hydrogen atmosphere and a catalyst such as sup￾ported molybdenum sulfide.35,36 Bio-oil hydrotreating has been well developed and produces high grade products. There are excellent reviews that have summarized the historical develop￾ments,35 recent advances,36 and new focus on hydrogeoxygena￾tion of lignin-derived bio-oils.37,38 However, because of bio-oil instability and the high oxygen content, hydrotreating suffers from high operating cost associated with significant catalyst deactivation, expensive catalysts used, and substantial hydro￾gen consumption.39 An alternative way is to use a catalyst to directly upgrade the pyrolysis vapors prior to quenching to produce bio-oil with improved quality, a process that is called catalytic fast pyrolysis (CFP). By instantly treating the hot pyrolysis vapor with a suitable catalyst, the pyrolysis intermediates are simultaneously Table 1 Typical elementary composition of bio-oil and crude oil. (Adapted with permission from Dickerson et al., Energies, 2013, 6, 514–538.20 Copyright 2013 MDPI.) Composition Bio-oil Crude oil Water (wt%) 15–30 0.1 pH 2.8–3.8 — Density (kg L1 ) 1.05–1.25 0.86–0.94 Viscosity 50 1C (cP) 40–100 180 HHV (MJ kg1 ) 16–19 44 C (wt%) 55–65 83.86 O (wt%) 28–40 o1 H (wt%) 5–7 11–14 S (wt%) o0.05 o4 N (wt%) o0.4 o1 Ash (wt%) o0.2 0.1 H/C 0.9–1.5 1.5–2.0 O/C 0.3–0.5 B0 Fig. 1 Chemical composition of bio-oil from wood biomass and the most abundant molecules of each of the components. (Adapted with permission from Huber et al., Chem. Rev., 2006, 106, 4044–4098.23 Copyright 2006 American Chemical Society.) Chem Soc Rev Review Article Published on 07 May 2014. Downloaded by Shanghai Jiaotong University on 18/02/2016 07:32:58. View Article Online