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 tech￾nologies. 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] investi￾gated 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 depend￾able, 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 mole￾cular 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, hydrocar￾bons 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 proper￾ties 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 separa￾tion of bio-oil and is not restricted by its poor properties. However, due to the necessity of high vacuum condition, the energy consump￾tion 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 pyr￾olysis 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 effec￾tively in improving the quality and yield with the help of some catalysts, such as aluminum silicate [65], HZSM-5 [66], bifunc￾tional 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 envir￾onmentally 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, upgrad￾ing 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 bio￾oil 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