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 utiliza￾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 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 tem￾perature 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 percen￾tages of diesel oil, and evaluated the lubrication properties of oil samples using a four-ball tester. They found that several proper￾ties, 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 appli￾cation. 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 sum￾mary, 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 bio￾oil 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 cata￾lysts 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