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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