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Author's personal copy Renewable and Sustainable Energy Reviews 24(2013)66-72 Contents lists available at SciVerse ScienceDirect Renewable and Sustainable Energy Reviews ELSEVIER journal homepage:www.elsevier.com/locate/rser Upgrading of bio-oil from biomass fast pyrolysis in China:A review Le Zhang,Ronghou Liu*,Renzhan Yin,Yuanfei Mei Biomass Energy Engineering Research Center,School of Agriculture and Biology.Shanghai Jiao Tong University.800 Dongchuan Road,Shanghai 200240.PR China ARTICLE INFO ABSTRACT Article history: Bio-oil is a brown liquid product from biomass fast pyrolysis.The upgrading of bio-oil has been a hotspot Received 16 December 2012 due to its contribution to the application of bio-oil.The properties of bio-oil,research progress, Received in revised form advantages and disadvantages of upgrading techniques of bio-oil from biomass fast pyrolysis in China 8 March 2013 are summarized,with the hope of promoting the development of upgrading and application of bio-oil in Accepted 15 March 2013 China.The upgrading techniques include hydrogenation,hydrodeoxygenation.catalytic pyrolysis. catalytic cracking.steam reforming.molecular distillation,supercritical fluids.esterification and Keywords: emulsification.Also,the current problems are summarized and several future development directions Bio-oil of bio-oil upgrading are pointed out. Upgrading 2013 Elsevier Ltd.All rights reserved. Fast pyrolysis Biomass China Contents 1. ntroduction.........。. 2. Properties ofbio-oib.........................................................6 3. Upgrading of bic0-oil……………… 67 3.1 Hydrogenation......+....+.+..... 67 3.2. Hydrodeoxygenation.......................................... 68 3.3. Catalytic pyrolysis…………… 68 3.4. Catalytic cracking............................................. 68 3.5. Steam reforming.....…* 69 3.6. Molecular distillation.................. 3.7. Supercritical fluids(SCFs) 9 38. Esterification ............. 69 3.9. Emulsification.,...,。 7 3.10. Industrial application of bio-oil.................................. 70 4.Conclusions and recommendations for future work......................... 7 4.1. 70 4.2. Recommendations for future work...................................................................................... 7 Acknowledgments.…………………………………………… 71 Reference6..............................................................................................................7I 1.Introduction resource of global fuel production.Compared with fossil fuels, biomass energy,for example bio-oil,has great potential to be an With the diminishing supply of fossil fuels and increasing alternative source of energy due to its advantages on reproducibility. environmental concerns,biomass is considered to be a promising resources universality [1]and environmental protection.Currently, producing biofuels,such as bio-oil,fuel gas and bio-char,through biomass fast pyrolysis has been a hotspot both at home and abroad. Corresponding author.Tel.:+86 21 34205744. However,as a promising alternate energy source,direct application E-mail addresses:liurhou@sjtu.edu.cn,zhangle2015@gmail.com(R.Liu). of bio-oil is limited due to its high viscosity,high water and ash 1364-0321/S-see front matter 2013 Elsevier Ltd.All rights reserved. http://dx.doi.org/10.1016/j.rser.2013.03.027
Author's personal copy Upgrading of bio-oil from biomass fast pyrolysis in China: A review Le Zhang, Ronghou Liu n , Renzhan Yin, Yuanfei Mei Biomass Energy Engineering Research Center, School of Agriculture and Biology, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, PR China article info Article history: Received 16 December 2012 Received in revised form 8 March 2013 Accepted 15 March 2013 Keywords: Bio-oil Upgrading Fast pyrolysis Biomass China abstract Bio-oil is a brown liquid product from biomass fast pyrolysis. The upgrading of bio-oil has been a hotspot due to its contribution to the application of bio-oil. The properties of bio-oil, research progress, advantages and disadvantages of upgrading techniques of bio-oil from biomass fast pyrolysis in China are summarized, with the hope of promoting the development of upgrading and application of bio-oil in China. The upgrading techniques include hydrogenation, hydrodeoxygenation, catalytic pyrolysis, catalytic cracking, steam reforming, molecular distillation, supercritical fluids, esterification and emulsification. Also, the current problems are summarized and several future development directions of bio-oil upgrading are pointed out. & 2013 Elsevier Ltd. All rights reserved. Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 2. Properties of bio-oil. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 3. Upgrading of bio-oil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 3.1. Hydrogenation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 3.2. Hydrodeoxygenation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 3.3. Catalytic pyrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 3.4. Catalytic cracking. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 3.5. Steam reforming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 3.6. Molecular distillation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 3.7. Supercritical fluids (SCFs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 3.8. Esterification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 3.9. Emulsification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 3.10. Industrial application of bio-oil. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 4. Conclusions and recommendations for future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 4.1. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 4.2. Recommendations for future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 1. Introduction With the diminishing supply of fossil fuels and increasing environmental concerns, biomass is considered to be a promising resource of global fuel production. Compared with fossil fuels, biomass energy, for example bio-oil, has great potential to be an alternative source of energy due to its advantages on reproducibility, resources universality [1] and environmental protection. Currently, producing biofuels, such as bio-oil, fuel gas and bio-char, through biomass fast pyrolysis has been a hotspot both at home and abroad. However, as a promising alternate energy source, direct application of bio-oil is limited due to its high viscosity, high water and ash Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/rser Renewable and Sustainable Energy Reviews 1364-0321/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.rser.2013.03.027 n Corresponding author. Tel.: þ86 21 34205744. E-mail addresses: liurhou@sjtu.edu.cn, zhangle2015@gmail.com (R. Liu). Renewable and Sustainable Energy Reviews 24 (2013) 66–72
Author's personal copy L.Zhang et al.Renewable and Sustainable Energy Reviews 24(2013)66-72 父 Table 1 kg).The properties of heavy petroleum fuel oil are significantly Comparison of selected properties of bio-oils derived from pyrolysis of rice husk different from bio-oils derived from the biomass pyrolysis and bio-oils derived from pyrolysis of wood and heavy petroleum fuel oil. processes. Properties Bio-oils derived Bio-oils derived Heavy petroleum According to Qiang et al.[12].it is known that the poor from pyrol小ysis of from pyrolysis of fuel oil [11] properties of bio-oil usually include high contents of water rice husk [9] wood [10] oxygen,ash and solids,low pH values,high viscosity.chemical and thermal instability.low heating value,and poor ignition and Water content 25.2 15-30 0.1 (wt%) combustion properties.For example,high oxygen content leads to pH 2.8 2.5 thermal instability,which hinders the storage stability of bio-oil, Elemental C41.7 54-58 85 and high acidity can result in corrosion of experimental facilities composition H77 55-7.0 11 Meanwhile,aldehyde and phenol in bio-oil are unstable,unsatu- (wt%) 050.3 35-40 1.0 N0.3 0-02 03 rated,and easily form macromolecules through polymerization, Ash 0-02 0.1 especially in the acidic condition,which will also increase the HHV (MJ/kg) 17.42 16-19 40 viscosity of bio-oil and reduce liquidity.The application of bio-oil Viscosity (at 128 40-100 180 is so far limited by these undesired properties.During the 50*C)(mPa s) production process of bio-oil,different biomass and reaction Solids (wt) 02-1 1 Distillation Upto 50 conditions can lead to bio-oils with different yield and quality. residue (wt) Despite these shortcomings of fuel properties,bio-oils also have some promising properties,such as less toxicity,good lubricity and greater biodegradation than petroleum fuels.Upgrading of bio-oil contents,low heating value,instability and high corrosiveness [2] is therefore necessary to improve its properties for its practical which leads to a series of problems in application of bio-oil. application as liquid fuel. So,in order to improve physicochemical properties of bio-oil for its practical application,upgrading of bio-oil is necessary.However. the process of upgrading of bio-oil is very difficult because of the 3.Upgrading of bio-oil complexity of the bio-oil contents [3].Although biomass fast pyrolysis for bio-oil production has aroused extensive attention Although fast pyrolysis can produce considerable amount of and interests both at home and abroad in recent years [4].there are bio-oils,for example a yield up to 56.8%was reported in domestic also lots of unknown mechanisms to be clarified before bio-oil can research [13].their direct applications as fuels are limited by the be used easily [3]. problems of high viscosity,high oxygen content and corrosion,as This paper reviews the properties and present situation of well as their thermal instability.Therefore,bio-oils should be upgrading technologies of bio-oil from biomass fast pyrolysis in upgraded using proper methods before they can be used in diesel China.The current problems of bio-oil upgrading in China are also or gasoline engines summarized.Besides,some recommendations on the develop- ment directions are put forward based on the current status of 3.1.Hydrogenation upgrading of bio-oil,with the hope of promoting the improvement of upgrading and application of bio-oil in China. The ultimate aim of hydrogenation is to improve stability and fuel quality by decreasing the contents of organic acids and aldehydes as well as other reactive compounds,because they not 2.Properties of bio-oil only lead to high corrosiveness and acidity,but also set up many obstacles to applications [14. Usually,bio-oil is a dark brown,free-flowing liquid with a Recently in China,many researchers have achieved consider- distinctive smoky smell.Many publications [5-8]have reported able progress in upgrading pyrolysis bio-oils using hydrogenation the physical properties of bio-oils.Compared with petroleum- technology.Traditionally researchers generally upgraded bio-oil derived oils,the different chemical composition of oils results in by single hydrogenation technology.Traditional hydrogenation is different physical properties of bio-oils.Bio-oil is a complex the treatment of pyrolysis bio-oil under specific conditions,such mixture,which consists of several hundreds of organic com- as high pressure(10-20 MPa).certain temperature and hydrogen pounds,mainly including alcohols,acids,aldehydes,esters, flow rate as well as proper catalyst.The bio-oil can be obtained by ketones,phenols as well as lignin-derived oligomers [2].Thorough various kinds of pyrolysis using several catalysts,such as Al2O3- understanding of bio-oil is a necessary precondition for research- based catalysts [15.16]and Ru/SBA-15 catalysts [17].etc.In the ers to clarify mechanisms of properties and upgrading of bio-oil. upgrading experiments,the following results [15,18]generally Table 1 shows comparison of selected properties of bio-oils could be observed:the pH value,the water content as well as derived from pyrolysis of rice husk and bio-oils derived from the H element content all increased in varying degrees while the pyrolysis of wood and heavy petroleum fuel oil. dynamic viscosity decreased to some extent.These experiments As shown in Table 1,the water content of bio-oil derived from also simultaneously indicated that the properties of the pyrolysis pyrolysis of rice husk is 25.2 wt%,while the values of bio-oils bio-oil were improved by hydrotreating and esterifying [16] derived from pyrolysis of wood and heavy petroleum fuel oil are carboxyl groups over these catalysts.At present in China,the 15-30 wt%and 0.1 wt%,respectively.The oxygen contents in largest scale of hydrogenation reactor is a cylindrical reactor with bio-oils derived from pyrolysis of rice husk and wood are a depth of 120 mm and an inner diameter of 32 mm [16]. 50.3 wt%and 35-40 wt%,respectively.while that in heavy petro- Recently a novel upgrading method named one-step hydro- leum fuel oil is 1.0 wt%.So,a conclusion can be drawn that genation-esterification(OHE)was established to convert acids and pyrolysis bio-oils have much higher oxygen and water contents aldehydes to stable and combustible components [19].The cata- than heavy petroleum fuel oil,which leads to lower heating values lysts for OHE reaction were bifunctional,such as Al-SBA-15 in bio-oils than in heavy petroleum fuel oil.The corresponding supported palladium bifunctional catalysts [20]and bifunctiona HHV (MJ/kg)of bio-oils from pyrolysis of rice husk and wood is Pd catalysts [21].which means they have properties of hydro- 16-19 MJ/kg.which is about 50%of that of heavy fuel oil (40 MJ/ genation and esterification [19,21].This is the advantage over the
Author's personal copy contents, low heating value, instability and high corrosiveness [2], which leads to a series of problems in application of bio-oil. So, in order to improve physicochemical properties of bio-oil for its practical application, upgrading of bio-oil is necessary. However, the process of upgrading of bio-oil is very difficult because of the complexity of the bio-oil contents [3]. Although biomass fast pyrolysis for bio-oil production has aroused extensive attention and interests both at home and abroad in recent years [4], there are also lots of unknown mechanisms to be clarified before bio-oil can be used easily [3]. This paper reviews the properties and present situation of upgrading technologies of bio-oil from biomass fast pyrolysis in China. The current problems of bio-oil upgrading in China are also summarized. Besides, some recommendations on the development directions are put forward based on the current status of upgrading of bio-oil, with the hope of promoting the improvement of upgrading and application of bio-oil in China. 2. Properties of bio-oil Usually, bio-oil is a dark brown, free-flowing liquid with a distinctive smoky smell. Many publications [5–8] have reported the physical properties of bio-oils. Compared with petroleumderived oils, the different chemical composition of oils results in different physical properties of bio-oils. Bio-oil is a complex mixture, which consists of several hundreds of organic compounds, mainly including alcohols, acids, aldehydes, esters, ketones, phenols as well as lignin-derived oligomers [2]. Thorough understanding of bio-oil is a necessary precondition for researchers to clarify mechanisms of properties and upgrading of bio-oil. Table 1 shows comparison of selected properties of bio-oils derived from pyrolysis of rice husk and bio-oils derived from pyrolysis of wood and heavy petroleum fuel oil. As shown in Table 1, the water content of bio-oil derived from pyrolysis of rice husk is 25.2 wt%, while the values of bio-oils derived from pyrolysis of wood and heavy petroleum fuel oil are 15–30 wt% and 0.1 wt%, respectively. The oxygen contents in bio-oils derived from pyrolysis of rice husk and wood are 50.3 wt% and 35–40 wt%, respectively, while that in heavy petroleum fuel oil is 1.0 wt%. So, a conclusion can be drawn that pyrolysis bio-oils have much higher oxygen and water contents than heavy petroleum fuel oil, which leads to lower heating values in bio-oils than in heavy petroleum fuel oil. The corresponding HHV (MJ/kg) of bio-oils from pyrolysis of rice husk and wood is 16–19 MJ/kg, which is about 50% of that of heavy fuel oil (40 MJ/ kg). The properties of heavy petroleum fuel oil are significantly different from bio-oils derived from the biomass pyrolysis processes. According to Qiang et al. [12], it is known that the poor properties of bio-oil usually include high contents of water, oxygen, ash and solids, low pH values, high viscosity, chemical and thermal instability, low heating value, and poor ignition and combustion properties. For example, high oxygen content leads to thermal instability, which hinders the storage stability of bio-oil, and high acidity can result in corrosion of experimental facilities. Meanwhile, aldehyde and phenol in bio-oil are unstable, unsaturated, and easily form macromolecules through polymerization, especially in the acidic condition, which will also increase the viscosity of bio-oil and reduce liquidity. The application of bio-oil is so far limited by these undesired properties. During the production process of bio-oil, different biomass and reaction conditions can lead to bio-oils with different yield and quality. Despite these shortcomings of fuel properties, bio-oils also have some promising properties, such as less toxicity, good lubricity and greater biodegradation than petroleum fuels. Upgrading of bio-oil is therefore necessary to improve its properties for its practical application as liquid fuel. 3. Upgrading of bio-oil Although fast pyrolysis can produce considerable amount of bio-oils, for example a yield up to 56.8% was reported in domestic research [13], their direct applications as fuels are limited by the problems of high viscosity, high oxygen content and corrosion, as well as their thermal instability. Therefore, bio-oils should be upgraded using proper methods before they can be used in diesel or gasoline engines. 3.1. Hydrogenation The ultimate aim of hydrogenation is to improve stability and fuel quality by decreasing the contents of organic acids and aldehydes as well as other reactive compounds, because they not only lead to high corrosiveness and acidity, but also set up many obstacles to applications [14]. Recently in China, many researchers have achieved considerable progress in upgrading pyrolysis bio-oils using hydrogenation technology. Traditionally researchers generally upgraded bio-oil by single hydrogenation technology. Traditional hydrogenation is the treatment of pyrolysis bio-oil under specific conditions, such as high pressure (10–20 MPa), certain temperature and hydrogen flow rate as well as proper catalyst. The bio-oil can be obtained by various kinds of pyrolysis using several catalysts, such as Al2O3- based catalysts [15,16] and Ru/SBA-15 catalysts [17], etc. In the upgrading experiments, the following results [15,18] generally could be observed: the pH value, the water content as well as the H element content all increased in varying degrees while the dynamic viscosity decreased to some extent. These experiments also simultaneously indicated that the properties of the pyrolysis bio-oil were improved by hydrotreating and esterifying [16] carboxyl groups over these catalysts. At present in China, the largest scale of hydrogenation reactor is a cylindrical reactor with a depth of 120 mm and an inner diameter of 32 mm [16]. Recently a novel upgrading method named one-step hydrogenation–esterification (OHE) was established to convert acids and aldehydes to stable and combustible components [19]. The catalysts for OHE reaction were bifunctional, such as Al-SBA-15 supported palladium bifunctional catalysts [20] and bifunctional Pd catalysts [21], which means they have properties of hydrogenation and esterification [19,21]. This is the advantage over the Table 1 Comparison of selected properties of bio-oils derived from pyrolysis of rice husk and bio-oils derived from pyrolysis of wood and heavy petroleum fuel oil. Properties Bio-oils derived from pyrolysis of rice husk [9] Bio-oils derived from pyrolysis of wood [10] Heavy petroleum fuel oil [11] Water content (wt%) 25.2 15–30 0.1 pH 2.8 2.5 – Elemental composition (wt%) C 41.7 54–58 85 H 7.7 5.5–7.0 11 O 50.3 35–40 1.0 N 0.3 0–0.2 0.3 Ash – 0–0.2 0.1 HHV (MJ/kg) 17.42 16–19 40 Viscosity (at 50 1C) (mPa s) 128 40–100 180 Solids (wt%) – 0.2–1 1 Distillation residue (wt%) – Upto 50 1 L. Zhang et al. / Renewable and Sustainable Energy Reviews 24 (2013) 66–72 67
Author's personal copy L Zhang et al.Renewable and Sustainable Energy Reviews 24 (2013)66-72 traditional hydrogenation process for OHE.Yu et al.[21]screened characteristic analysis of obtained bio-oils using elemental out 5%Pd/Al2(SiO3)3 with the best catalytic performance among GC-MS and FTIR technologies [31].Both in a fixed bed reactor tested bifunctional catalysts,and demonstrated that it is viable to [31,32]and in fluidized bed [13],the investigation indicated that convert these unstable constituents of bio-oil to esters and catalytic pyrolysis lowered the oxygen content of the bio-oils and alcohols through this simple and effective OHE reaction.Besides, aggrandized the calorific values compared to the direct pyrolysis for the OHE reaction,tests by Tang et al.[22]demonstrated the without catalysts.This conclusion can be drawn in many experi- effectiveness of the bifunctional catalyst system for combined ments with different biomass,including green microalga [31]. hydrogenation/esterification and a synergistic effect between corncob [13].herb residue [32]and waste woody biomass [33]. metal sites and acid sites over respective catalysts.Moreover. Besides,catalytic pyrolysis can lead to higher content of aromatic some measures were taken to improve the catalytic performance hydrocarbons in bio-oils with HZSM-5 [31],alumina [32]or HZSM- of the bifunctional catalyst [23].Obviously,the new hydrogenation 5/y-Al2O3 [35]as catalyst while direct pyrolysis promoted the method is much better than the traditional method due to the use increase of carbon chain compounds [31].However,Zhang et al of bifunctional catalysts. [13]reported that the addition of HZSM-5 zeolite catalyst in the experiments caused a significant decrease of heavy oil fraction and 3.2.Hydrodeoxygenation an increase of the coke,water and non-condensable gas yields This was because the HZSM-5 zeolite catalyst could promote the Hydrodeoxygenation(HDO).a variant of catalytic pyrolysis,is a conversion of oxygen element in heavy oil into CO,CO2 and H2O. bio-oil upgrading process which removes the oxygen under high So.choosing proper catalysts is crucial to catalytic pyrolysis. pressure of hydrogen with a catalyst.It can reduce oxygen content Besides,some new reports on the catalytic pyrolysis were of many kinds of oxygenated chemical groups,such as acids, reported recently in China.During biomass pyrolysis,Cao addition aldehydes,esters,ketones and phenols,etc.Hydrodeoxygenation could catalyze dehydration reactions [36].CaCl2 was reported to has been considered to be one of the most promising methods for have an apparent catalytic effect on elephant grass pyrolysis [37] bio-oil upgrading [24].Recently in China,a 500 ml autoclave and polluting heavy metal Cu showed effective catalytic activity in reactor [24]with diameter of 10 mm and length of 420 mm is the thermo-decomposition of biomass [33].This research can the largest experimental facility for hydrodeoxygenation.Addi- contribute to the development of catalysts applied on the more tionally,the largest dosage of the catalyst for this kind of research efficient catalytic pyrolysis process. is1.5g[241 Therefore,catalytic pyrolysis can promote the production and Most of the previous researches on hydrodeoxygenation of bio-oil quality of bio-oils through using the appropriate catalysts.But,it focused on industrial NiMo or CoMo sulfide/supported hydrotreating also encounters some problems,such as catalyst deactivation, catalysts.For instance,Wang et al.[24]demonstrated Pt supported reactor clogging,coke production and high water content in bio- on mesoporous ZSM-5 showed better performance than Pt/ZSM-5 oils,etc. and Pt/Al2O3 in dibenzofuran hydrodeoxygenation.However,these catalysts have several inherent shortcomings in hydrodeoxygenation, 3.4.Catalytic cracking such as product contamination and catalyst deactivation.The noble metal catalyst exhibits high catalytic activity in the HDO reactions Recently in China,catalytic cracking for upgrading pyrolysis [24-26].but with high cost.So,novel and economical catalysts that bio-oil can be divided into two patterns,the traditional catalytic can be used for hydrodeoxygenation of bio-oil with high oxygen cracking and the combination of catalytic pyrolysis and catalytic content should be developed. cracking.Traditionally,the catalytic cracking referred to a thermal Due to excellent hydrodeoxygenation activity and selectivity in conversion process of bio-oil under certain conditions,including the catalytic reactions [27,281,amorphous catalysts have aroused a hydrogen flow,proper catalysts (e.g.,HZSM-5 [38])and a specific great attention in China recently.Wang et al.[27,28]prepared and temperature higher than 350C as well as rather high pressure. tested hydrodeoxygenation activity of lots of amorphous catalysts, Hydrogenation with simultaneous cracking occurred during the and demonstrated that Co-W-B had higher thermal stability than catalytic cracking process.The products of the catalytic cracking Ni-Co-W-B and Ni-W-B catalyst.And,the catalyst activity could process consist of solid,liquid and gases.The solid is called coke, be increased with the Co/Mo ratio increase of surface composition, and the liquid can be divided into two phases:aqueous phase and which means the catalyst activity could be further improved at organic phase.The gas is combustible.In China,the traditional proper conditions.Therefore,this new kind of amorphous catalyst catalytic cracking had been carried out in a tubular fixed bed will be a potential candidate for the HDO process due to its many reactor [38]and micro-fixed bed reactor [391.The advantage of advantages,such as simple preparation,high thermal stability and this technique was the probability of obtaining a good deal of light high HDO activity as well as low cost [28,29]. product,but catalyst coke deposition was a bottleneck for sustain- able application of catalysts(e.g.,HZSM-5 [39]). 3.3.Catalytic pyrolysis Compared to the traditional catalytic cracking.scholars in China made the combination of catalytic pyrolysis and catalytic cracking to Recently,catalytic pyrolysis has aroused a great interest for the upgrade pyrolysis bio-oil.It adopted a sequential biomass pyrolysis advantages of operating at atmospheric pressure and the lack of reactor which consisted of a traditional pyrolysis reactor followed by need for hydrogen [30],which has been demonstrated by many the subsequent apparatus that supported decomposition of gaseous researches.The experiments on catalytic pyrolysis of biomass were intermediate [40].The largest reactor for this kind of investigation generally carried out in a fixed bed reactor [31]or fluidized bed was made of 316 stainless tube,with a length of 1000 mm and an [13]under nitrogen flow with some catalysts,such as HZSM-5 inner diameter of 20 mm [38].For example,Xiwei et al.[40]applied [13,311.ZSM-5 [32].Al-SBA-15 [321.alumina [32]and Cu [33].etc this method and found that biomass could be fully converted into So far in China,the largest scale of catalytic pyrolysis reactor is a gaseous products,such as H2,CH4 and CO,etc.Catalyst(Fe/y-Al2O3) tubular reactor with a length of 250 mm and an inner diameter of activities were affected by several factors,including calcination 38 mm.Many aspects of catalytic pyrolysis have been studied. temperature,temperature of catalytic pyrolysis and Fe/Al mass ratio. including screening of feasible catalysts with high deoxygenating Hong-yu et al.[41]demonstrated that when the cracking tempera- activities [32,34]or preferred selectivities [35].influence of tem ture was 500C,with a Weight Hourly Space Velocity of 3 h-,the perature and catalyst-to-material ratio on product yields [13]. liquid yield reached the maximum and the oxygenic compounds also
Author's personal copy traditional hydrogenation process for OHE. Yu et al. [21] screened out 5% Pd/Al2(SiO3)3 with the best catalytic performance among tested bifunctional catalysts, and demonstrated that it is viable to convert these unstable constituents of bio-oil to esters and alcohols through this simple and effective OHE reaction. Besides, for the OHE reaction, tests by Tang et al. [22] demonstrated the effectiveness of the bifunctional catalyst system for combined hydrogenation/esterification and a synergistic effect between metal sites and acid sites over respective catalysts. Moreover, some measures were taken to improve the catalytic performance of the bifunctional catalyst [23]. Obviously, the new hydrogenation method is much better than the traditional method due to the use of bifunctional catalysts. 3.2. Hydrodeoxygenation Hydrodeoxygenation (HDO), a variant of catalytic pyrolysis, is a bio-oil upgrading process which removes the oxygen under high pressure of hydrogen with a catalyst. It can reduce oxygen content of many kinds of oxygenated chemical groups, such as acids, aldehydes, esters, ketones and phenols, etc. Hydrodeoxygenation has been considered to be one of the most promising methods for bio-oil upgrading [24]. Recently in China, a 500 ml autoclave reactor [24] with diameter of 10 mm and length of 420 mm is the largest experimental facility for hydrodeoxygenation. Additionally, the largest dosage of the catalyst for this kind of research is 1.5 g [24]. Most of the previous researches on hydrodeoxygenation of bio-oil focused on industrial NiMo or CoMo sulfide/supported hydrotreating catalysts. For instance, Wang et al. [24] demonstrated Pt supported on mesoporous ZSM-5 showed better performance than Pt/ZSM-5 and Pt/Al2O3 in dibenzofuran hydrodeoxygenation. However, these catalysts have several inherent shortcomings in hydrodeoxygenation, such as product contamination and catalyst deactivation. The noble metal catalyst exhibits high catalytic activity in the HDO reactions [24–26], but with high cost. So, novel and economical catalysts that can be used for hydrodeoxygenation of bio-oil with high oxygen content should be developed. Due to excellent hydrodeoxygenation activity and selectivity in the catalytic reactions [27,28], amorphous catalysts have aroused a great attention in China recently. Wang et al. [27,28] prepared and tested hydrodeoxygenation activity of lots of amorphous catalysts, and demonstrated that Co–W–B had higher thermal stability than Ni–Co–W–B and Ni–W–B catalyst. And, the catalyst activity could be increased with the Co/Mo ratio increase of surface composition, which means the catalyst activity could be further improved at proper conditions. Therefore, this new kind of amorphous catalyst will be a potential candidate for the HDO process due to its many advantages, such as simple preparation, high thermal stability and high HDO activity as well as low cost [28,29]. 3.3. Catalytic pyrolysis Recently, catalytic pyrolysis has aroused a great interest for the advantages of operating at atmospheric pressure and the lack of need for hydrogen [30], which has been demonstrated by many researches. The experiments on catalytic pyrolysis of biomass were generally carried out in a fixed bed reactor [31] or fluidized bed [13] under nitrogen flow with some catalysts, such as HZSM-5 [13,31], ZSM-5 [32], Al-SBA-15 [32], alumina [32] and Cu [33], etc. So far in China, the largest scale of catalytic pyrolysis reactor is a tubular reactor with a length of 250 mm and an inner diameter of 38 mm. Many aspects of catalytic pyrolysis have been studied, including screening of feasible catalysts with high deoxygenating activities [32,34] or preferred selectivities [35], influence of temperature and catalyst-to-material ratio on product yields [13], characteristic analysis of obtained bio-oils using elemental, GC–MS and FTIR technologies [31]. Both in a fixed bed reactor [31,32] and in fluidized bed [13], the investigation indicated that catalytic pyrolysis lowered the oxygen content of the bio-oils and aggrandized the calorific values compared to the direct pyrolysis without catalysts. This conclusion can be drawn in many experiments with different biomass, including green microalga [31], corncob [13], herb residue [32] and waste woody biomass [33]. Besides, catalytic pyrolysis can lead to higher content of aromatic hydrocarbons in bio-oils with HZSM-5 [31], alumina [32] or HZSM- 5/γ-Al2O3 [35] as catalyst while direct pyrolysis promoted the increase of carbon chain compounds [31]. However, Zhang et al. [13] reported that the addition of HZSM-5 zeolite catalyst in the experiments caused a significant decrease of heavy oil fraction and an increase of the coke, water and non-condensable gas yields. This was because the HZSM-5 zeolite catalyst could promote the conversion of oxygen element in heavy oil into CO, CO2 and H2O. So, choosing proper catalysts is crucial to catalytic pyrolysis. Besides, some new reports on the catalytic pyrolysis were reported recently in China. During biomass pyrolysis, CaO addition could catalyze dehydration reactions [36]. CaCl2 was reported to have an apparent catalytic effect on elephant grass pyrolysis [37] and polluting heavy metal Cu showed effective catalytic activity in the thermo-decomposition of biomass [33]. This research can contribute to the development of catalysts applied on the more efficient catalytic pyrolysis process. Therefore, catalytic pyrolysis can promote the production and quality of bio-oils through using the appropriate catalysts. But, it also encounters some problems, such as catalyst deactivation, reactor clogging, coke production and high water content in biooils, etc. 3.4. Catalytic cracking Recently in China, catalytic cracking for upgrading pyrolysis bio-oil can be divided into two patterns, the traditional catalytic cracking and the combination of catalytic pyrolysis and catalytic cracking. Traditionally, the catalytic cracking referred to a thermal conversion process of bio-oil under certain conditions, including hydrogen flow, proper catalysts (e.g., HZSM-5 [38]) and a specific temperature higher than 350 1C as well as rather high pressure. Hydrogenation with simultaneous cracking occurred during the catalytic cracking process. The products of the catalytic cracking process consist of solid, liquid and gases. The solid is called coke, and the liquid can be divided into two phases: aqueous phase and organic phase. The gas is combustible. In China, the traditional catalytic cracking had been carried out in a tubular fixed bed reactor [38] and micro-fixed bed reactor [39]. The advantage of this technique was the probability of obtaining a good deal of light product, but catalyst coke deposition was a bottleneck for sustainable application of catalysts (e.g., HZSM-5 [39]). Compared to the traditional catalytic cracking, scholars in China made the combination of catalytic pyrolysis and catalytic cracking to upgrade pyrolysis bio-oil. It adopted a sequential biomass pyrolysis reactor which consisted of a traditional pyrolysis reactor followed by the subsequent apparatus that supported decomposition of gaseous intermediate [40]. The largest reactor for this kind of investigation was made of 316 stainless tube, with a length of 1000 mm and an inner diameter of 20 mm [38]. For example, Xiwei et al. [40] applied this method and found that biomass could be fully converted into gaseous products, such as H2, CH4 and CO, etc. Catalyst (Fe/γ-Al2O3) activities were affected by several factors, including calcination temperature, temperature of catalytic pyrolysis and Fe/Al mass ratio. Hong-yu et al. [41] demonstrated that when the cracking temperature was 500 1C, with a Weight Hourly Space Velocity of 3 h−1 , the liquid yield reached the maximum and the oxygenic compounds also 68 L. Zhang et al. / Renewable and Sustainable Energy Reviews 24 (2013) 66–72
Author's personal copy L.Zhang et al.Renewable and Sustainable Energy Reviews 24 (2013)66-72 69 decreased obviously.To summarize,researches demonstrated that distillation and flash distillation [51.Besides,molecular distillation the combined process had the superiority of promoting the liquid is currently suitable for the separation of heat-sensitive and high yield and improving the fuel quality over the separate processes. value-added substance,which limits the application of molecular distillation.To speak of,the molecular distillation apparatus is 3.5.Steam reforming urgently needed on account of the fact that most of experimental facilities for relative investigation in China were directly imported Steam reforming was also an effective method to upgrade from the foreign countries such as Germany. pyrolysis bio-oil.It could simultaneously produce renewable and clear gaseous hydrogen along with bio-oil upgrading,which was a 3.7.Supercritical fluids (SCFs) big advantage for steam reforming among various upgrading tech- nologies.Steam reforming generally used a fluidized bed reactor Recently,a new method for upgrading bio-oil from fast pyr system [42]or a fixed bed reactor system [43].The largest reactor olysis using supercritical fluids(SCFs)has drawn a great attention system up till now in China is a two-stage fixed bed reactor system at home and abroad.This method takes full advantage of the with the height of 800 mm and the inner diameter of 20 mm [42 unique and superior properties of supercritical reaction media In the steam reforming process,high temperature (800-900 C)and such as liquid-like density,faster rates of mass and heat transfer. proper catalysts [44-48]were generally necessary. dissolving power and gas-like diffusivity and viscosity [4].SCFs can However,coke formation caused catalyst deactivation,which be not only used as a reaction condition to produce bio-oils,but was a big problem in steam reforming of the bio-oil for sustainable also can be used as a superior medium to upgrade bio-oils,and hydrogen production.Chen et al.[48]and Wu et al.[49]investi- have shown great potential for producing bio-oils with much gated carbon deposition behavior in the steam reforming process lower viscosity and higher caloric values [2].In order to enhance of bio-oil for hydrogen production and demonstrated that for the the oil yields and qualities,some organic solvents,such as ethanol competition of carbon deposition and carbon elimination,a peak [55-59].methanol [60-62].water [63]and CO2 [64].etc.,were value of coking formation rate was obtained in a broad range of adopted in many relative researches. temperature (575-900 C).while high ratio of steam to carbon Usually.the upgrading method using SCFs performed effec- contributed to the carbon elimination.Also,regenerated catalyst tively in improving the quality and yield with the help of some showed slight drops in activity due to Fe contamination and Ni catalysts,such as aluminum silicate [65].HZSM-5 [66],bifunc- redispersion [50].Above all,upgrading bio-oil by steam reforming tional catalysts [67,68].etc.The upgrading experiments were was feasible in China but more appropriate catalysts and depend- mainly performed in the autoclave reactor,with a volume of able,steady and fully developed reactor systems still need to be 100 ml or 150 ml.After upgrading,the components of the bio-oil developed in the future. were optimized significantly and the properties of the bio-oil were improved greatly.The catalysts in supercritical media can facilitate 3.6.Molecular distillation the conversion of most acids into various kinds of esters in the upgrading process.As a result,kinematic viscosity and the density Bio-oil from biomass pyrolysis is a complex mixture of many of upgraded bio-oil decreased compared to that of crude bio-oil, chemical compounds with a wide range of boiling points.Due to the while the heating value and pH value of upgraded bio-oil thermo-sensitive property of bio-oil,it is easy to undergo reactions increased to a certain degree [55,59,64.Dang et al.[3]reported such as polymerization,decomposition and oxygenation [51.But, that higher initial hydrogen pressure (2.0 MPa)could effectively molecular distillation cannot be limited by these poor properties inhibit formation of coke.Although increasing temperature was and be appropriate for the separation of bio-oil.So,molecular helpful to promotion of heating value of upgraded bio-oil,the distillation is one of the most economically feasible methods to amount of desired products decreased and the formation of coke purify bio-oil. would be much more serious. Chinese researchers have carried out considerable work in Although the process of upgrading bio-oil using SCFs is envir- upgrading bio-oil by molecular distillation.Wang et al.[51] onmentally friendly,and can be carried out at a relatively lower separated bio-oil using KDL5 molecular distillation apparatus, temperature,it is not economically feasible on a large scale due to demonstrated the feasibility of using molecular distillation to the high cost of the organic solvents [2].Therefore,researchers in isolate bio-oil and came up with a separation factor to signify China should input more effort into testing less expensive organic the ability of isolating the chemicals of bio-oil during the mole- solvents as a substitute for SCFs. cular distillation process.The complexity of bio-oil was confirmed by studying the chemical composition of the three fractions 3.8.Esterification separated by molecular distillation using gas chromatography combined with mass spectrometry (GC-MS)[52].The results Due to the drawbacks of pyrolysis bio-oil,such as low heating showed that the light fraction consisted of CO2.water,hydrocar- value,high viscosity,high corrosiveness and poor stability,upgrad- bons and alcohols which evaporated fastest,while the heavy ing of bio-oil before practical application is necessary to acquire fraction had the highest char residue yield and the slowest rate high grade fuel.Organic acids in bio-oils can be converted into of decomposition due to the existence of saccharides,phenols and their corresponding esters by catalytic esterification and this the pyrolysis products,such as CO2.alcohols and phenols. greatly improves the quality of bio-oils [65]. The middle fraction was similar to the heavy fraction except for Upgrading the bio-oil through catalytic esterification has been the existence of water and formic acid.Using molecular distillation carried out widely in China.During the etherification process,the to refine biomass pyrolysis oil could upgrade the physical proper- experiment was generally conducted in a 250 ml or 300 ml ties of the refined bio-oil [53,54].such as carboxylic acids content, autoclave,and the catalysts included ion exchange resins [65]. water content and heating value. MoNi/y-Al2O3[69].etc.The results showed that the upgraded bio- In conclusion,molecular distillation is appropriate for the separa- oil had lower acid numbers,water contents,and viscosities. tion of bio-oil and is not restricted by its poor properties.However. Meanwhile,stability and corrosion properties of bio-oil were also due to the necessity of high vacuum condition,the energy consump- promoted [66].Junming et al.[67]reported their observations on tion in the process is usually larger than conventional distillation, ozone oxidation of bio-oil,and production of upgraded bio-oil such as vacuum distillation.steam distillation, atmospheric using subsequent esterification
Author's personal copy decreased obviously. To summarize, researches demonstrated that the combined process had the superiority of promoting the liquid yield and improving the fuel quality over the separate processes. 3.5. Steam reforming Steam reforming was also an effective method to upgrade pyrolysis bio-oil. It could simultaneously produce renewable and clear gaseous hydrogen along with bio-oil upgrading, which was a big advantage for steam reforming among various upgrading technologies. Steam reforming generally used a fluidized bed reactor system [42] or a fixed bed reactor system [43]. The largest reactor system up till now in China is a two-stage fixed bed reactor system with the height of 800 mm and the inner diameter of 20 mm [42]. In the steam reforming process, high temperature (800–900 1C) and proper catalysts [44–48] were generally necessary. However, coke formation caused catalyst deactivation, which was a big problem in steam reforming of the bio-oil for sustainable hydrogen production. Chen et al. [48] and Wu et al. [49] investigated carbon deposition behavior in the steam reforming process of bio-oil for hydrogen production and demonstrated that for the competition of carbon deposition and carbon elimination, a peak value of coking formation rate was obtained in a broad range of temperature (575–900 1C), while high ratio of steam to carbon contributed to the carbon elimination. Also, regenerated catalyst showed slight drops in activity due to Fe contamination and Ni redispersion [50]. Above all, upgrading bio-oil by steam reforming was feasible in China but more appropriate catalysts and dependable, steady and fully developed reactor systems still need to be developed in the future. 3.6. Molecular distillation Bio-oil from biomass pyrolysis is a complex mixture of many chemical compounds with a wide range of boiling points. Due to the thermo-sensitive property of bio-oil, it is easy to undergo reactions such as polymerization, decomposition and oxygenation [51]. But, molecular distillation cannot be limited by these poor properties and be appropriate for the separation of bio-oil. So, molecular distillation is one of the most economically feasible methods to purify bio-oil. Chinese researchers have carried out considerable work in upgrading bio-oil by molecular distillation. Wang et al. [51] separated bio-oil using KDL5 molecular distillation apparatus, demonstrated the feasibility of using molecular distillation to isolate bio-oil and came up with a separation factor to signify the ability of isolating the chemicals of bio-oil during the molecular distillation process. The complexity of bio-oil was confirmed by studying the chemical composition of the three fractions separated by molecular distillation using gas chromatography combined with mass spectrometry (GC–MS) [52]. The results showed that the light fraction consisted of CO2, water, hydrocarbons and alcohols which evaporated fastest, while the heavy fraction had the highest char residue yield and the slowest rate of decomposition due to the existence of saccharides, phenols and the pyrolysis products, such as CO2, alcohols and phenols. The middle fraction was similar to the heavy fraction except for the existence of water and formic acid. Using molecular distillation to refine biomass pyrolysis oil could upgrade the physical properties of the refined bio-oil [53,54], such as carboxylic acids content, water content and heating value. In conclusion, molecular distillation is appropriate for the separation of bio-oil and is not restricted by its poor properties. However, due to the necessity of high vacuum condition, the energy consumption in the process is usually larger than conventional distillation, such as vacuum distillation, steam distillation, atmospheric distillation and flash distillation [51]. Besides, molecular distillation is currently suitable for the separation of heat-sensitive and high value-added substance, which limits the application of molecular distillation. To speak of, the molecular distillation apparatus is urgently needed on account of the fact that most of experimental facilities for relative investigation in China were directly imported from the foreign countries such as Germany. 3.7. Supercritical fluids (SCFs) Recently, a new method for upgrading bio-oil from fast pyrolysis using supercritical fluids (SCFs) has drawn a great attention at home and abroad. This method takes full advantage of the unique and superior properties of supercritical reaction media, such as liquid-like density, faster rates of mass and heat transfer, dissolving power and gas-like diffusivity and viscosity [4]. SCFs can be not only used as a reaction condition to produce bio-oils, but also can be used as a superior medium to upgrade bio-oils, and have shown great potential for producing bio-oils with much lower viscosity and higher caloric values [2]. In order to enhance the oil yields and qualities, some organic solvents, such as ethanol [55–59], methanol [60–62], water [63] and CO2 [64], etc., were adopted in many relative researches. Usually, the upgrading method using SCFs performed effectively in improving the quality and yield with the help of some catalysts, such as aluminum silicate [65], HZSM-5 [66], bifunctional catalysts [67,68], etc. The upgrading experiments were mainly performed in the autoclave reactor, with a volume of 100 ml or 150 ml. After upgrading, the components of the bio-oil were optimized significantly and the properties of the bio-oil were improved greatly. The catalysts in supercritical media can facilitate the conversion of most acids into various kinds of esters in the upgrading process. As a result, kinematic viscosity and the density of upgraded bio-oil decreased compared to that of crude bio-oil, while the heating value and pH value of upgraded bio-oil increased to a certain degree [55,59,64]. Dang et al. [3] reported that higher initial hydrogen pressure (2.0 MPa) could effectively inhibit formation of coke. Although increasing temperature was helpful to promotion of heating value of upgraded bio-oil, the amount of desired products decreased and the formation of coke would be much more serious. Although the process of upgrading bio-oil using SCFs is environmentally friendly, and can be carried out at a relatively lower temperature, it is not economically feasible on a large scale due to the high cost of the organic solvents [2]. Therefore, researchers in China should input more effort into testing less expensive organic solvents as a substitute for SCFs. 3.8. Esterification Due to the drawbacks of pyrolysis bio-oil, such as low heating value, high viscosity, high corrosiveness and poor stability, upgrading of bio-oil before practical application is necessary to acquire high grade fuel. Organic acids in bio-oils can be converted into their corresponding esters by catalytic esterification and this greatly improves the quality of bio-oils [65]. Upgrading the bio-oil through catalytic esterification has been carried out widely in China. During the etherification process, the experiment was generally conducted in a 250 ml or 300 ml autoclave, and the catalysts included ion exchange resins [65], MoNi/γ-Al2O3 [69], etc. The results showed that the upgraded biooil had lower acid numbers, water contents, and viscosities. Meanwhile, stability and corrosion properties of bio-oil were also promoted [66]. Junming et al. [67] reported their observations on ozone oxidation of bio-oil, and production of upgraded bio-oil using subsequent esterification. L. Zhang et al. / Renewable and Sustainable Energy Reviews 24 (2013) 66–72 69
Author's personal copy 70 L Zhang et al.Renewable and Sustainable Energy Reviews 24 (2013)66-72 Yao et al.[68]and Zhou et al.[70]analyzed the component of emulsions.At present,the majority of bio-oil dosage in the esterified bio-oil using gas chromatography/mass spectroscopy(GC/ emulsification investigation is less than 400 ml.Moreover,design, MS)and Fourier transform infrared spectroscopy(FTIR).The former testing and production of injectors and fuel pumps with higher detected that 1,1-dimethoxypropan-2-one was the most abundant efficiency are urgently needed. species in the fraction of 60-80 C.Phenols were the most abundant species in the residue,followed by the ketones and hydrocarbons. The latter[70]demonstrated that upgrading significantly improved 3.10.Industrial application of bio-oil the dispersity of organic droplets in the bio-oil and completely removed char particles from the bio-oil.However,heavy species are Many substances,such as aromatics,olefins,resins,etc.,can be still the main components in the upgraded bio-oil. extracted from the pyrolysis bio-oil for practical industrial appli- In recent years,there were many investigations [71-74]into cation.Recently in China,Zhao et al.[84]extracted aromatics via upgrading bio-oil through catalytic esterification reaction using solid acid catalysts in China.The results consistently indicated that catalytic pyrolysis of pyrolytic lignins from bio-oil.The results indicated that without catalysts the main products were phenols the solid acid catalyst had a high catalytic activity to convert with a selectivity of above 90%at 600C,which demonstrated that organic acids,such as formic acid,propionic acid and acetic acid. it was an alternative way to produce useful chemicals and fuel into esters effectively.Meanwhile,properties of bio-oil,such as stability and fluidity,etc..could be improved. additives.Gong et al.[85]performed the selective production of light olefins from catalytic cracking of bio-oil using the La/HZSM-5 catalyst.Light olefin is a kind of basic building blocks for the 3.9.Emulsification petrochemical industry.Similarly,many other chemical products In order to promote the application of bio-oil as a combustion could be produced using pyrolysis bio-oil as feedstock.In sum- fuel,emulsification was performed as a feasible method to mary,it is crucial for Chinese researchers to develop more reliable upgrade the bio-oil.The research by Guoli et al.[75]demonstrated low cost refining and separation techniques before industrial that emulsion was a cheaper and convenient method for utiliza- production of chemicals from bio-oils is fully realized. tion of bio-oil. With the help of surfactants,pyrolysis oils can be emulsified with diesel.Zuogang et al.[76]produced bio-oil emulsion fuels 4.Conclusions and recommendations for future work using bio-oil and 0#diesel by power ultrasound.They studied the effects of treating time and ultrasound power on stability of the 4.1.Conclusions emulsion fuels.The results indicated that the emulsion fuels with a stable time as long as 35 h could be obtained under ultrasound Biomass pyrolysis is one of the most promising methods in power of 80 W with a treating time of 3 min. production of bio-oil,which has great development potential Wang [77]investigated the combustion characteristics of a around the world.Due to the advantages in economic problems blend fuel of bio-oil and diesel with different proportion of the and environmental protection,bio-oil as a new substitute of fossil two fuels using a numerical simulation method.The factors,such fuels,has acquired extensive recognition globally.Researches as combustion components distribution,ignition delay and tem- demonstrated that pyrolysis bio-oil could be upgraded using perature distribution in the combustor were studied. various approaches,such as hydrogenation,hydrodeoxygenation, Xu et al.[78]studied the lubricity of the bio-oil/diesel fuel catalytic pyrolysis,catalytic cracking,steam reforming,molecular using a High Frequency Reciprocating Test Rig(HFRR).They found distillation,supercritical fluids,esterification and emulsification, that the lubrication ability of the bio-oil/diesel fuel was better etc.Conducting such research is very important to offer basic compared with the conventional diesel fuel(number zero).Qiang information for the application of bio-oil from biomass fast et al.[79]conducted bio-oil emulsification with different percen- pyrolysis in the world.In addition,the commercialization of bio- tages of diesel oil,and evaluated the lubrication properties of oil oil upgrading technology needs to be developed further.In short, samples using a four-ball tester.They found that several proper- conclusions can be drawn as follows: ties,such as friction-reduction,anti-wear and extreme-pressure were better,which were not consistent with the conclusion of (1)the one-step hydrogenation-esterification (OHE)method is Xu et al.[78].The difference was likely to be caused by use of the much better than the traditional method due to the use of different device in their experiments.Meanwhile,increasing bifunctional catalysts. content of the bio-oil in the emulsions could promote the (2)Like amorphous catalysts,more novel and economical cata- lubrication ability of the emulsions.Moreover,the solid char lysts that can be used for hydrodeoxygenation of bio-oil with particles in the pyrolysis bio-oil could enhance its lubrication high oxygen content should be further developed performance. (3)Catalytic pyrolysis can promote the production and quality of Jiang et al.[80-83]reported their researches about upgrading bio-oils by using the appropriate catalysts while it encounters the storage properties and thermal stability of bio-oil through some critical problems,such as catalyst deactivation,reactor emulsification with biodiesel.During the aging research,a slight clogging,coke production and high water content in bio-oils,etc decrease in acid numbers and a slight increase in the molecular (4)The integrated upgrading process of catalytic pyrolysis and weight were observed and ES/biodiesel blends are stable under catalytic cracking has the superiority of increasing the liquid the conditions used in the research. yield and improving the fuel quality over the separate processes To conclude,using emulsification with diesel oil is a relatively (5)It is feasible to upgrade bio-oil by steam reforming in China simple and effective way of upgrading bio-oil.It is a short-term but needs appropriate catalysts. method for the application of pyrolysis bio-oil in diesel engines (6)Upgrading bio-oil using molecular distillation is appropriate and other devices.This method can improve some properties of for the separation of bio-oil and is not restricted by its poor bio-oil,such as ignition characteristics,but it is still difficult to properties,but is energy-consuming generally promote other fuel properties,such as heating value,corrosivity, (7)Using supercritical fluids(SCFs)is not economically feasible to cetane number and so on,to a satisfied degree.Besides,emulsi- upgrade bio-oil on a large scale due to the high cost of the fication required a large input of energy for high production of organic solvents
Author's personal copy Yao et al. [68] and Zhou et al. [70] analyzed the component of esterified bio-oil using gas chromatography/mass spectroscopy (GC/ MS) and Fourier transform infrared spectroscopy (FTIR). The former detected that 1,1-dimethoxypropan-2-one was the most abundant species in the fraction of 60–80 1C. Phenols were the most abundant species in the residue, followed by the ketones and hydrocarbons. The latter [70] demonstrated that upgrading significantly improved the dispersity of organic droplets in the bio-oil and completely removed char particles from the bio-oil. However, heavy species are still the main components in the upgraded bio-oil. In recent years, there were many investigations [71–74] into upgrading bio-oil through catalytic esterification reaction using solid acid catalysts in China. The results consistently indicated that the solid acid catalyst had a high catalytic activity to convert organic acids, such as formic acid, propionic acid and acetic acid, into esters effectively. Meanwhile, properties of bio-oil, such as stability and fluidity, etc., could be improved. 3.9. Emulsification In order to promote the application of bio-oil as a combustion fuel, emulsification was performed as a feasible method to upgrade the bio-oil. The research by Guoli et al. [75] demonstrated that emulsion was a cheaper and convenient method for utilization of bio-oil. With the help of surfactants, pyrolysis oils can be emulsified with diesel. Zuogang et al. [76] produced bio-oil emulsion fuels using bio-oil and 0# diesel by power ultrasound. They studied the effects of treating time and ultrasound power on stability of the emulsion fuels. The results indicated that the emulsion fuels with a stable time as long as 35 h could be obtained under ultrasound power of 80 W with a treating time of 3 min. Wang [77] investigated the combustion characteristics of a blend fuel of bio-oil and diesel with different proportion of the two fuels using a numerical simulation method. The factors, such as combustion components distribution, ignition delay and temperature distribution in the combustor were studied. Xu et al. [78] studied the lubricity of the bio-oil/diesel fuel using a High Frequency Reciprocating Test Rig (HFRR). They found that the lubrication ability of the bio-oil/diesel fuel was better compared with the conventional diesel fuel (number zero). Qiang et al. [79] conducted bio-oil emulsification with different percentages of diesel oil, and evaluated the lubrication properties of oil samples using a four-ball tester. They found that several properties, such as friction-reduction, anti-wear and extreme-pressure were better, which were not consistent with the conclusion of Xu et al. [78]. The difference was likely to be caused by use of the different device in their experiments. Meanwhile, increasing content of the bio-oil in the emulsions could promote the lubrication ability of the emulsions. Moreover, the solid char particles in the pyrolysis bio-oil could enhance its lubrication performance. Jiang et al. [80–83] reported their researches about upgrading the storage properties and thermal stability of bio-oil through emulsification with biodiesel. During the aging research, a slight decrease in acid numbers and a slight increase in the molecular weight were observed and ES/biodiesel blends are stable under the conditions used in the research. To conclude, using emulsification with diesel oil is a relatively simple and effective way of upgrading bio-oil. It is a short-term method for the application of pyrolysis bio-oil in diesel engines and other devices. This method can improve some properties of bio-oil, such as ignition characteristics, but it is still difficult to promote other fuel properties, such as heating value, corrosivity, cetane number and so on, to a satisfied degree. Besides, emulsi- fication required a large input of energy for high production of emulsions. At present, the majority of bio-oil dosage in the emulsification investigation is less than 400 ml. Moreover, design, testing and production of injectors and fuel pumps with higher efficiency are urgently needed. 3.10. Industrial application of bio-oil Many substances, such as aromatics, olefins, resins, etc., can be extracted from the pyrolysis bio-oil for practical industrial application. Recently in China, Zhao et al. [84] extracted aromatics via catalytic pyrolysis of pyrolytic lignins from bio-oil. The results indicated that without catalysts the main products were phenols with a selectivity of above 90% at 600 1C, which demonstrated that it was an alternative way to produce useful chemicals and fuel additives. Gong et al. [85] performed the selective production of light olefins from catalytic cracking of bio-oil using the La/HZSM-5 catalyst. Light olefin is a kind of basic building blocks for the petrochemical industry. Similarly, many other chemical products could be produced using pyrolysis bio-oil as feedstock. In summary, it is crucial for Chinese researchers to develop more reliable low cost refining and separation techniques before industrial production of chemicals from bio-oils is fully realized. 4. Conclusions and recommendations for future work 4.1. Conclusions Biomass pyrolysis is one of the most promising methods in production of bio-oil, which has great development potential around the world. Due to the advantages in economic problems and environmental protection, bio-oil as a new substitute of fossil fuels, has acquired extensive recognition globally. Researches demonstrated that pyrolysis bio-oil could be upgraded using various approaches, such as hydrogenation, hydrodeoxygenation, catalytic pyrolysis, catalytic cracking, steam reforming, molecular distillation, supercritical fluids, esterification and emulsification, etc. Conducting such research is very important to offer basic information for the application of bio-oil from biomass fast pyrolysis in the world. In addition, the commercialization of biooil upgrading technology needs to be developed further. In short, conclusions can be drawn as follows: (1) the one-step hydrogenation–esterification (OHE) method is much better than the traditional method due to the use of bifunctional catalysts. (2) Like amorphous catalysts, more novel and economical catalysts that can be used for hydrodeoxygenation of bio-oil with high oxygen content should be further developed. (3) Catalytic pyrolysis can promote the production and quality of bio-oils by using the appropriate catalysts while it encounters some critical problems, such as catalyst deactivation, reactor clogging, coke production and high water content in bio-oils, etc. (4) The integrated upgrading process of catalytic pyrolysis and catalytic cracking has the superiority of increasing the liquid yield and improving the fuel quality over the separate processes. (5) It is feasible to upgrade bio-oil by steam reforming in China but needs appropriate catalysts. (6) Upgrading bio-oil using molecular distillation is appropriate for the separation of bio-oil and is not restricted by its poor properties, but is energy-consuming generally. (7) Using supercritical fluids (SCFs) is not economically feasible to upgrade bio-oil on a large scale due to the high cost of the organic solvents. 70 L. Zhang et al. / Renewable and Sustainable Energy Reviews 24 (2013) 66–72
Author's personal copy L.Zhang et al.Renewable and Sustainable Energy Reviews 24 (2013)66-72 71 4.2.Recommendations for future work [7]Liu RH.Shen CJ.Wu H.Deng CJ.Liu SY.Characterisation of bio-oil from fast pyrolysis of rice husk in fluidised bed reactor.Journal of the Energy Institute 2011:842:73-9. The components of bio-oil are very complex.On one hand,the [Chen Tianju.Wu Ceng.Liu Ronghou.Fei Wenting.Liu Shiyu.Effect of hot vapor pyrolysis bio-oil has many advantages in properties,such as less filtration on the characterization of bio-oil from rice husks with fast pyrolysis toxic,good lubricity and stronger biodegradation and so on.On the in a fluidized-bed reactor.Bioresource Technology 2011:102:6178-85. other hand,it also has lots of disadvantages in characteristics,that [9]Zheng Ji-lu.Bio-oil from fast pyrolysis of rice husk:yields and related properties and improvem ent the pyrolysis system.Journal of Analytical is,high contents of water,oxygen,ash and solids,low pH value, and Applied Pyrolysis 2007:1(80):30-5. high viscosity,chemical and thermal instability,low heating value, [10]Zhang Q,Chang J.Wang T.Xu Y.Review of biomass pyrolysis oil properties and and poor ignition and combustion properties.Although the upgrading research.Energy Conversion and Management 2007:48:87-92. upgraded bio-oil could be used as alternative fuel of boiler and [11]Oasmaa A.Czernik S.Fuel oil quality of biomass pyrolysis oils-state of the art for the end users.Energy Fuels 1999:13(4):914-21. internal combustion engine,now it still cannot replace the fossil [12]Qiang Lu,Wen-Zhi Li.Xi-Feng Zhu.Overview of fuel properties of biomass fast fuels completely due to the restriction in technologies and costing. pyrolysis oils.Energy Com version and Management 2009:50(5):1376-83. [13]Zhang Huiyan,Xiao Rui.Huang He.Xiao Gang.Comparison of non-catalytic How to develop and use the upgrading technologies to realize the and catalytic fast pyrolysis of corncob in a fluidized bed reactor.Bioresource industrial application of bio-oil from biomass pyrolysis is a big Technology2009:100(3y1428-34. matter.In order to facilitate this issue,great efforts should be paid [14]Xu Ying.Wang Tiejun.Ma Longlong.Zhang Qi.Wang Lu.Upgrading of liquid fuel from the vacuum pyrolysis of biomass over the Mo-Ni/y-Al2O catalysts. in the following aspects in the future Biomass and Bioenerey 2009:3308):1030-6. [15]Ying Xu,Tiejun Wang.Longlong Ma.Guanyi Chen.Upgrading of fast pyrolysis (1)more appropriate catalysts (e.g.,bifunctional catalysts or liquid fuel from biomass over Ru/y-Al2O3 catalyst.Energy Conversion and multifunctional catalysts)and dependable,steady and fully Management 2012:55:172-7. [16]Xu Ying.Wang Tiejun,Ma Longlong.Zhang Qi,Wang Lu.Upgrading of liquid developed reactor systems need to be developed in the future. fuel from the vacuum pyrolysis of bior nas over the Mo-Ni/t-AlO catalysts (2)During the upgrading of bio-oil using various catalysts,the Biomass and Bioenergy 2009:33(8):1030-6. mechanism on catalyst deactivation needed to be further [17]Guo Jianhua,Ruan Renxiang.Zhang Ying.Hydrotreating of phenolic com- explained,and the catalysts with high durability.strong cndustral and E ch2012:51:6599-604 renewable ability and high efficiency need to be developed [18]Xiong Wan-Ming.Fu Yao,Zeng Fan-Xin.Guo Qing-Xiang.An in situ reduction urgently. approach for bio-oil hydroprocessing.Fuel Processing Technology 2011:92 (3)During the researches on emulsification,seeking for more 81599-605. [19]Yang Tang.Wanjin Yu,Liuye Mo.Hui Lou.Xiaoming Zheng.One-step economic and abundant surfactants as substitutes for the high- hydrogenation-esterification of aldehyde and acid to ester over bifunctional priced surfactants remains an interesting topic to investigate. Pt catalysts:a model reaction as novel route for catalytic upgrading of fast (4)The researches on how to combine pyrolysis reactors with ergy Fuels 008:22(53484-8. 1201 Yu Waniin.Tang Yang.Mo Liuve.Chen Ping.Lou Hui.Zheng xi reaction conditions organically and how to efficiently use Bifunctional Pd/Al-SBA-15 catalyzed one-step hydrogenation-esterification upgrading technologies of other oils for reference need to be of furfural and acetic acid:a model reaction for catalytic upgrading of bio- oil.Catalysis Communications 2011:13(1):35-9. done in the future so that new ideas would be found. [21]Yu Wanjin,Tang Yang.Mo Liuye,Chen Ping.Lou Hui,Zheng Xiaoming.One-step (5)In order to promote the industrialization of upgrading of bio- hydrogenation-esterification of furfural and acetic acid over bifunctional Pd oil,more efforts should be paid to develop more experimental catalysts for bio-oil upgrading.Bioresource Technology 2011:102(17):8241-6. facilities with larger scale and high efficiency. [22]Tang Yang.Miao Shaojun,Shanks Brent H.Zheng Xiaoming.Bifunctional mesoporous organic-inorganic hybrid silica for combined one-step hydro- genation/esterification.Applied Catalysis A 2010:375(2):310-7. 123 Tang Yang.Miao Shaojun,Pham Hien N.Datye Abhaya,Zheng Xiaoming. Shanks Brent H.Enhancement of Pt catalytic activity in the hydrogenation of aldehydes.Applied Catalysis A 2011:406(1-2):81-8. 124]Wang Yuxin,Fang Yunming.He Tao.Hu Haoquan,Wu Jinhu.Hydrodeoxy- Acknowledgments gena tion of dibenzofuran over noble metal supported on mesoporous zeolite Catalysis Communications 2011:12(13):1201-5. [25]Wang Yuxin,He Tao,Liu Kaituo.Wu Jinhu,Fang Yunming.From biomass to Financial support from National Natural Science Foundation of advanced bio-fuel by catalytic pyroly is/hydro-processing:hydrodeoxygena- China through contract(Grant no.51176121)and financial support tion of bio-oil derived from biomass catalytic pyrolysis.Bioresource Technol- from The National Science and Technology Supporting Plan 0gy2012:108:280-4. [26]Su-Ping Zhang.Yong-Jie Yan.Zher gwei Ren,Tingchen Li.Study of hydrodeox- through contract(Grant no.2011BAD22B07)are greatly acknowl- on of bio-oil from the fast pyrolysis of biomass.Energy Source edged.In addition,Daniel Lycett-Brown from the University of 2003:25(1):57-65. Southampton,UK is greatly acknowledged for his valuable sugges- [27]Wang Weiyan,Yang Yunquan.Luo Hean.Peng Huizuo,He Bing.Liu Wenying. tion and correction of the manuscript. Preparation of Ni(Co)-W-B amorphous catalysts for cyclopentanone hydro- deoxygenation.Catalysis Communications 2011:12(14):1275-9. [28]Wang Weiyan,Yang Yunquan,Luo Hean.Hu Tao,Liu Wenying.Amorphous Co-Mo-B catalvst with high activity for the hvdrodeoxvgenation of bio-oil References Catalysis Communications 2011:12(6):436-40. [29]Weiyan Wang.Yunquan Yang.Hean Luo.Tao Hu,Wenying Liu.Preparation and hydrodeoxygenation properties of Co-Mo-0-B amorphous catalyst. 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[32]Wang Pan,Zhan Sihui,Yu Hongbing.Xue Xufang.Hong Nan.The effects of 14]Zhang Qi.Chang Jie,Wang Tiejun.Xu Ying.Review of biomass pyrolysis oil temperature and catalysts on the pyrolysis of industrial wastes(herb residue) prop rties and upgrading research.Energy Conversion and Management Bioresource Technology 2010:101(9):3236-41. 2007:48(1):87-92. [33]Wu-Jun Liu.Ke Tian,Hong Jiang.Xue-Song Zhang.Hong-Sheng Ding.Han- [5]Yao Lu.Xian-Yong Wei.Jing-Pei Cao,Peng Li,Fang-Jing Liu.Yun-Peng Zhao. Qing Yu.Selectively roving the bio-oil quality by catalytic fast et al.Characterization of a bio-oil from pyrolysis of rice husk by detailed heavy-metal-pollu ed b omass: take copper Environ compositional analysi and structural investigation of lignin.Bioresource mental Science and Technology 2012:46(14):7849-56. 1 echnology2012.116114-9. [34]Huiyan Zhang.Rui Xiao,Denghui Wang.Zhaoping Zhong.Min Song.Qiwen [6]Rui Lu,Guo-Ping Sheng.Yan-Yun Hu,Ping Zheng.Hong Jiang.Yong Tang.et al. 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Author's personal copy 4.2. Recommendations for future work The components of bio-oil are very complex. On one hand, the pyrolysis bio-oil has many advantages in properties, such as less toxic, good lubricity and stronger biodegradation and so on. On the other hand, it also has lots of disadvantages in characteristics, that is, high contents of water, oxygen, ash and solids, low pH value, high viscosity, chemical and thermal instability, low heating value, and poor ignition and combustion properties. Although the upgraded bio-oil could be used as alternative fuel of boiler and internal combustion engine, now it still cannot replace the fossil fuels completely due to the restriction in technologies and costing. How to develop and use the upgrading technologies to realize the industrial application of bio-oil from biomass pyrolysis is a big matter. In order to facilitate this issue, great efforts should be paid in the following aspects in the future: (1) more appropriate catalysts (e.g., bifunctional catalysts or multifunctional catalysts) and dependable, steady and fully developed reactor systems need to be developed in the future. (2) During the upgrading of bio-oil using various catalysts, the mechanism on catalyst deactivation needed to be further explained, and the catalysts with high durability, strong renewable ability and high efficiency need to be developed urgently. (3) During the researches on emulsification, seeking for more economic and abundant surfactants as substitutes for the highpriced surfactants remains an interesting topic to investigate. (4) The researches on how to combine pyrolysis reactors with reaction conditions organically and how to efficiently use upgrading technologies of other oils for reference need to be done in the future so that new ideas would be found. (5) In order to promote the industrialization of upgrading of biooil, more efforts should be paid to develop more experimental facilities with larger scale and high efficiency. Acknowledgments Financial support from National Natural Science Foundation of China through contract (Grant no. 51176121) and financial support from The National Science and Technology Supporting Plan through contract (Grant no. 2011BAD22B07) are greatly acknowledged. In addition, Daniel Lycett-Brown from the University of Southampton, UK is greatly acknowledged for his valuable suggestion and correction of the manuscript. References [1] Xu Junming, Jiang Jianchun, Chen Jie, Sun Yunjuan. Biofuel production from catalytic cracking of woody oils. Bioresource Technology 2010;101(14):5586–91. [2] Xiu Shuangning, Shahbaz Abolghasem. Bio-oil production and upgrading research: a review. Renewable and Sustainable Energy Reviews 2012;16 (7):4406–14. [3] Dang Qi, Luo Zhongyang, Zhang Jixiang, Wang Jun, Chen Wen, Yang Yi. 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