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Biofuels are liquid or gaseous fuels that are predominantly produced from biomass for transport sector applications. As biofuels arerenewable, sustainable, carbon neutral and environmentally benign, they have been proposed as promising alternative fuels for gasoline

Concerns about global climate change due to the emis-

energy consumption, i.e. industrial and city regions with

hydrogen, biofuels and batteries have been proposed assecondary energy carriers for the future transport sector.

are from fossil fuels, together with other signicant tech-nology and economic challenges in hydrogen storage,transportation and utilization. Electric and hybrid vehiclesare proposed as more viable alternatives to hydrogen vehi-

* Corresponding author. Tel.: +44 20 78823232; fax: +44 20 89831007.E-mail address: d.wen@qmul.ac.uk (D. Wen).

Available online at www.sciencedirect.com

Progress in Natural Science 19sion of greenhouse gases, and the projected decline inworld oil production has placed energy as the single mostimportant problem facing humanity in the next 50 years[1]. Securing clean, aordable energy for the long termbecomes one of the biggest challenges in modern societies.Increasing use of energy generated from renewableresources including biomass, wind energy, hydroelectricpower and solar energy will become viable, where geo-graphical and climatic prerequisites are favorable. Suchregions, however, seldom coincide with areas of high

To become replacement fuels for the future transportsector, the candidates would have to meet a number of cri-teria that include (1) abundance with enough resources thatcould replace petroleum-based fuels in the long term; (2)zero or low carbon emission with minimum detrimentaleect to the environment; (3) applicable to most runningvehicles based on existing infrastructures; and (4) econom-ically viable. Hydrogen fuel is dicult to become a realityin the short term as todays production is dependent oncrude oils or natural gas as raw material, or electricity thatand diesel engines. This paper reviews state-of-the-art application of the supercritical uid (SCF) technique in biofuels production thatincludes biodiesel from vegetable oils via the transesterication process, bio-hydrogen from the gasication and bio-oil from the lique-faction of biomass, with biodiesel production as the main focus. The global biofuel situation and biofuel economics are also reviewed.The SCF has been shown to be a promising technique for future large-scale biofuel production, especially for biodiesel production fromwaster oil and fat. Compared with conventional biofuel production methods, the SCF technology possesses a number of advantages thatincludes fast kinetics, high fuel production rate, ease of continuous operation and elimination of the necessity of catalysts. The harshoperation environment, i.e. the high temperature and high pressure, and its request on the materials and associated cost are the mainconcerns for its wide application. 2008 National Natural Science Foundation of China and Chinese Academy of Sciences. Published by Elsevier Limited and Science inChina Press. All rights reserved.

Keywords: Supercritical uids; Biomass; Transesterication; Biofuel; Hydrogen; Biodiesel; Gasication; Liquefaction

1. Introduction a high population density. Various sources includingRe

Supercritical uids technolog

Dongsheng Wen a,*, HaSchool of Engineering and Materials Science, QubState Key Laboratory of Heavy Oil Processing, C

Received 14 April 2008; received in revised form

Abstract1002-0071/$ - see front matter 2008 National Natural Science Foundation oand Science in China Press. All rights reserved.

doi:10.1016/j.pnsc.2008.09.001w

for clean biofuel production

Jiang a, Kai Zhang b

Mary University of London, London E1 4NS, UK

a University of Petroleum, Beijing 102249, China

September 2008; accepted 18 September 2008

www.elsevier.com/locate/pnsc

(2009) 273284f China and Chinese Academy of Sciences. Published by Elsevier Limited

ment, supercritical uids possess a number of unique

turacles for transportation, particularly in the short term [2,3];however they suer another serious problem: limitedresource of heavy metals needed for batteries. Biofuelsare liquid or gaseous fuels for the transport sector thatare predominantly produced from biomass. Biomass hasbeen recognized as a major world renewable energy sourceto supplement declining fossil fuel resources [4] and cur-rently supply 10% of our global energy needs withexpected fast growth in the near future, i.e. it wouldaccount for as much as 50% of the USs total energy con-sumption by 2050 [5]. Biofuels can be produced from avariety of bio-feedstocks, they are renewable, sustainable,biodegradable, carbon neutral for the whole life cycle andenvironmentally friendly so as to encourage green eldsand the agriculture industry, as well as applicable to run-ning vehicles with or without slight modications. Variousbio-origin fuels including bio-ethanol, biodiesel and bio-hydrogen appear to be attractive options for the futuretransport sector.

The decline of fossil fuel resources and the increasingprice of petroleum products have led to a major interestin expanding the use of biofuels. This has been reectedby the USs commitment of threefold increase in bioenergyin 10 years time, and the EUs new biofuel targets of reach-ing a minimum share of 5.75% of the transport fuel mar-

Fig. 1. Prediction of shares in the automobile market for three alternativefuels [7].

274 D. Wen et al. / Progress in Naket by the end of 2010 [6]. The shares of alternative fuels,biofuels, hydrogen and natural gas compared to the totalautomotive fuel consumption in the world are shown inFig. 1 as a futuristic view [7]. The production of biofuelsis expected to rise steadily in the next few decades.

Conventionally, biofuel production is based on tworoutes: either thermochemical conversion or biochemicalconversion, as illustrated in Fig. 2. The thermochemicalconversion route can be applied to wood, straw and refusethrough the gasication, liquefaction and pyrolysis pro-cesses to produce syn-gas, syn-oil and biochemicals. Bio-chemical conversion predominantly refers to bio-ethanoland biodiesel production through acid and enzyme hydro-lysis and/or fermentation from dierent sets of feedstocksthat include wood, wheat and sugar beet. In terms of abso-lute fuel costs, thermochemical conversion oers low-costproducts with some mature technologies. Biochemicaladvantages including increased species mixing, heat andmass transfer, fast reaction typically at a few minutes level,are environmentally benign, and have good scalability, aswell as being simple and easy for continuous production.The unique properties at supercritical conditions, i.e.strong dependence of the solubility of a material in a super-routes are more expensive. With strong competition fromthe global fuel market, there is a growing trend towardsemploying modern technologies for ecient biomass con-version. The supercritical uids (SCFs) technique is oneof the most promising ones.

2. Supercritical uids technique

In general, when a mixture of liquid and gas at equilib-rium is heated, thermal expansion causes the liquid tobecome less dense. At the same time, the gas becomes den-ser as pressure increases. At the critical point, the densitiesof the two phases become identical and the distinctionbetween them disappears. A supercritical status is denedas the uids temperature and pressure above its criticaltemperature, Tc, and critical pressure, Pc. In the gasliquidtransition regime, the SCF presents a combination of prop-erties of gases and liquids, which makes them very suitablefor the development of new processes that cannot be car-ried out with conventional liquid or gaseous uids. Thecritical parameters of some common uids are illustratedin Table 1.

Due to the creation of a homogeneous reaction environ-

Fig. 2. Conventional biomass conversion routes [8].

l Science 19 (2009) 273284critical uid to its density and good contact between oxi-dants and reactants, make SCFs ideal for separation andextraction of useful products and for oxidation of organicmaterials. However, these also have some limitationsrelated to the harsh operation environment and their eecton the materials. Corrosion and salt deposition are the twomain challenges for most of the industrial applications,especially for supercritical water (SCW) [1012]. SCW isfavorable for corrosion due to the presence of high pH val-ues, high concentrations of dissolved oxygen, ionic inor-ganic species and high temperaturepressure variations.Metal oxides can be formed due to the reduced salt solubil-ity, which could form stable solid particles that causeequipment fouling, plugging and erosion. A number of

Table 1Critical property of various solvents [9].

Solvent Molecular weight (g/mol) Critical temperature (K) Critical pressure (MPa) Density (kg/l)

Carbon dioxide 44.01 304.1 7.38 469Water 18.02 647.3 22.12 348Methane 16.04 190.4 4.60 162Ethane 30.07 305.3 4.87 203Propane 44.09 369.8 4.25 217Methanol 32.04 512.6 8.09 272Ethanol 46.07 513.9 6.14 276Acetone 58.08 508.1 4.70 278

D. Wen et al. / Progress in Natural Science 19 (2009) 273284 275plants could not meet their designed performance andsome have been closed for these reasons [13]. Besides these,the high energy intensity to reach supercritical status isanother big problem for the SCF technology, which couldbe solved with better heat recycling and improved systemdesign.

Despite these limitations, the SCF technique has beenproved to be an environmentally benign medium for anumber of chemical and related processes in the last fewdecades. Many new processes and products including thefractionation of products, dyeing of bres, treatment ofcontaminated solids, production of powders in micro/nanometer sizes and novel reactions [14,15] have also beendeveloped using the unique physical and chemical proper-ties of supercritical uids. For the energy industry, super-critical uids techniques have been used for coal-redpower plants [16], direct liquefaction or indirect liquefac-tion through gasication process for manufacturing syn-thesized gas, synthesized oil and chemical products, aswell as advanced nuclear systems. For a instace, supercrit-ical water reactors (SCWRs), have a high thermal eciencyof 45% in comparison with current light water reactorswhich have a thermal eciency of 33% [17]. Morerecently, there has been an emerging application of super-critical uids techniques for clean and high throughputbiofuel production. Compared with conventional thermo-chemical and biochemical methods, the SCF technologypossesses a number of advantages such as a high fuel con-version rate, quick reaction, clean production, easy andcontinuous operation, and elimination of the necessity ofcatalysts. This paper will review state-of-the-art biofuelproduction using the SCF technique, with the main focusTable 2Properties of the vegetable oils [18].

Vegetable oil Kinematics viscosity(mm2/s)

Cetanenumber

Cloud point(C)

Peanut 4.9 54 5Soya bean 4.5 45 1Babassu 3.6 63 4Palm 5.7 62 13Sunower 4.6 49 1Tallow 12Diesel 3.06 50 20% biodiesel

blend3.2 51 on biodiesel production through the transesterication pro-cess. Bio-hydrogen production through the gasicationprocess and bio-oil production from the liquefaction pro-cess of biomass will also be reviewed shortly.

3. SCFs for biodiesel production

3.1. Biodiesel production

Vegetable oil has been widely used for a long time. Eventhe rst diesel engine, named by the German scientist,Rudolph Diesel, was successfully run on peanut oil 100years ago. The thermo-physical properties of vegetableoil, mostly viscosity and volatility, however, limit its directapplication on diesel engines. A general list of properties ofvegetable oils from dierent sources is shown in Table 2.The viscosity value for most vegetable oils is at a rangeof 3560 cSt, which is much higher than that of standarddiesel fuels (4 cSt). This high viscosity can result in prob-lems in pumping and fuel spray processes such as the atom-ization and penetration eect. The low volatility ofvegetable oils can result in a high ash point, which willproduce a number of problems including injector choking,piston ring sticking, high carbon deposition, and lubrica-tion oil dilution and oil degradation [19]. The reactivityof unsaturated hydrocarbon chains can also bring otherproblems. The combination of all these factors makes thedirect application of vegetable oil unfeasible.

There has been, however, a renewed interest in vegetableoil for the transport sector recently due to the increasingprice of crude oil and environmental concerns. It could,

in the long run, substitute some fraction of petroleum dis-

Pour point(C)

Flash point(C)

Density(kg/l)

Lower heating value(MJ/kg)

176 0.883 33.67 178 0.885 33.5 127 0.875 31.8 164 0.880 33.5 183 0.860 33.59 96 16 76 0.855 43.816 128 0.859 43.2

tillates. However, economically it is not a competitive fuelat the moment due to the lack of practical on-farm process-ing technology and relatively high associated cost. Formeeting environmental and energy security concerns,acceptable alternative fuels for the transport sector have

studied and these processes have been commercialized[23], and some reviews on biodiesel production are alsoavailable [18]. The typical catalytic transesterication pro-cess includes the transesterication reaction, recovery ofun-reacted reactants, purication of the esters, separationof glycerol and the separation of the catalyst from the reac-tants and products, as shown in Fig. 3. Due to the need forvigorous stirring to mix the oil and alcohol and separate thecatalysts after the reaction, the catalytic processes have ahigh production cost and are energy intensive [24,25].

The supercritical uids technique can be used to synthe-size biodiesel through the transesterication of vegetable oilswithout using any catalysts. Compared to the conventional

276 D. Wen et al. / Progress in Natural Science 19 (2009) 273284to demonstrate that they do not sacrice the engines oper-ating performance. Vegetable oils have to be modied tobring their combustion-related properties closer to theirpetroleum-derived counterparts. The fuel modication forvegetable oils is mainly aimed at reducing their viscosityand increasing their volatility. Dilution, micro-emulsion,pyrolysis (thermal cracking) and transesterication to bio-diesel have been frequently used. Among all these tech-niques, the most successful one is to convert vegetableoils to biodiesel through the transesterication process [20].

Biodiesel is the methyl or ethyl ester of fatty acids madefrom virgin or used vegetable oils (both edible and non-edi-ble) and animal fat. Biodiesel has combustion-related prop-erties similar to those of petroleum diesel; it also operatesin compression ignition (diesel) engines and requires verylittle or no engine modications. Biodiesel can be blendedin any proportion with petroleum diesel to create a biodie-sel blend or can be used in its pure form. It can be storedjust like petroleum-derived diesel and hence does notrequire a separate infrastructure. The use of biodiesel inconventional diesel engines can result in substantial reduc-tion in emission of unburned hydrocarbons, carbon mon-oxide and particulate matters.

In chemical terms, transesterication is the process ofexchanging the alkoxy group of an ester compound byanother alcohol. The reactions are often catalyzed by anacid or a base. Transesterication is crucial for producingbiodiesel from biolipids. The transesterication process isthe reaction of a triglyceride (fat/oil) with a bioalcohol toform esters and glycerol [19,21,22]. The transestericationreaction can be initiated with or without a catalyst by usingprimary or secondary monohydric aliphatic alcohols hav-ing 18 carbon atoms, as shown below,

TriglyceridesMonohydric alcohol! GlycerinMono-alkyl

and a typical transesterication process is schematicallyshown in Fig. 3.

For biodiesel production, the transesterication can beconducted in either the presence or absence of a catalyst.The usual catalysts used are alkalis (NaOH, KOH), acids(sulfuric acid, HCl) and enzymes (lipases). The kinetics ofacid-catalyzed and alkali-catalyzed reactions has been wellFig. 3. Basic scheme for biodiesel production via transesterication.catalytic processes, the SCF technique possesses a numberof notable advantages such as easy separation, fast reactionand being environmentally friendly. This is primarilybecause alcohols and oil can co-exist in a single phase undersupercritical conditions. The increased solubility of organicmatters and the homogeneous environmentmake the transe-sterication process favorable. Compared with the catalytictransesterication process, relatively fewer investigationshave been explored through the supercritical uids route.The research on the topic was pioneered in Japan [18,2631], and recently it has enjoyed a sustained strong develop-ment in Europe [8,21,3234], China [3537] and India[24,38]. Most of these studies were conducted under labora-tory conditions, and there is still a lack of consensus on themechanisms of the reaction. Most of the transestericationmethods via the SCF techniques are based on the batch pro-ductionmethod [21,24,29,35]; very few are based on the con-tinuous production of biodiesel based on a ow loop [37],whose development is still at the beginning.

3.2. Biodiesel production from SCF transesterication

A number of parameters can aect the methyl ester yieldduring the transesterication reaction such as the reactiontemperature and pressure, alcoholic types, molar ratio ofalcohol to vegetable oil, residence time, water and free fattyacid content, solvents and catalysts, and operation modes.Examples of the inuence of these parameters on the bio-diesel production are reviewed below.Fig. 4. Biodiesel conversion from hazelnut kernel oil [32].

3.2.1. Temperature and residence time eect

It was observed that an increase in the reaction temper-ature, especially supercritical temperatures, had a favorableinuence on the ester conversion. Fig. 4 shows a typicalexample of the relationship between the biodiesel conver-sion and the reaction temperature for hazelnut kernel oilat a molar ratio of vegetable oil to methyl alcohol of 1:41[32]. Table 1 shows that the critical temperature of metha-nol is 512.6 K, there is a big jump in the conversion rate asthe temperature increases from sub-supercritical conditions(503 K) to the supercritical temperature. Nearly 100% con-

complete conversion.

which requires further extensive investigations.

oil, the conversion rate reached a plateau at a ratio of 40;

the allowable free fatty acids content [30,39,40]. As most ofthe waste vegetable oils and crude oils generally containwater and free fatty acids, these problems may reduce thebiodiesel production eciency [41].

For the supercritical methanol method, optimized oper-ation parameters have been found to be 350 C,43 MPa and residence time of 240 s with a molar ratioof 42 in methanol for transesterication of rapeseed oil tobiodiesel fuel [29]. Under supercritical conditions, free fattyacids in the oil could be simultaneously esteried. Thewater content eect on the yield of methyl esters by thesupercritical methanol treatment was studied by Kusdianaand Saka [30] and compared with those from alkaline- and

D. Wen et al. / Progress in Natura3.2.3. Molar ratio eect

The stoichiometric ratio for the transesterication reac-tion requires only 3 mole of alcohol and 1 mole of triglyc-eride to yield 3 mole of fatty acid ester and 1 mole of3.2.2. Alcohol eect

Vegetable oil can react with a number of alcohols. Fig. 5illustrates the role of dierent supercritical alcohols in thefatty acid alkyl ester conversion from triglycerides [28].The experimental results illustrated that alcohols withshorter alkyl chains gave better conversions under the samereaction time. Nearly 100% yield of alkyl esters wasobtained within 15 min treatment with methanol, while ittook 45 min by ethanol and 1-propanol methods. Undera similar condition, supercritical 1-butanol and 1-octanolproduced about 85% and 62% of alkyl esters, respectively,and the reaction reached a at conversion rate of 60%after 20 min for 1-octanol. As a consequence, the supercrit-ical methanol method has been widely investigated for bio-diesel production. Note that there is a big dierence in thereaction time to reach the equilibrium status for the meth-anol reaction between dierent research groups (Figs. 4and 5). This is common for all aecting parameters,although agreed qualitatively in general, quantitativeresults dier signicantly among dierent research groups,version is achieved in about 6 min. This is a signicantachievement compared with the conventional catalytictransesterication processes, which generally take a fewhours to reach equilibrium and are dicult to achieve aFig. 5. Alcohol eect on biodiesel conversion [28].further increase in the ratio did not help. Similar resultshave been obtained by other researchers [18,23,29]. Anoptimized excess of the alcohol of 40 is therefore gener-ally suggested in order to increase the yields of the alkylesters and to facilitate its phase separation from the glyc-erol formed.

3.2.4. Water and free fatty acids eect

For biodiesel production from the conventional cata-lytic transesterication reaction, the presence of watercan consume the catalyst, reduce catalyst eciency andcause soap formation and frothing, which increase the bio-diesel viscosity and make the glycerol separation dicultdue to the formation of gels and foams [8]. For catalyticreactions, the vegetable oils/fats used as a raw materialfor the transesterication should be water-free, or of extre-mely low concentration, i.e. below 0.06%, much lower thanglycerol. Various vegetable oils have been investigatedand it was found that they can be transesteried at widevegetable oil-alcohol molar ratios in supercritical alcoholconditions, ranging from 1:1 to 1:50 [18,29,32]. Examplesof the molar ratio eect are shown in Fig. 6 for batch bio-diesel production from cottonseed oil [32], and continuousbiodiesel production from soybean oil based on the super-critical methanol method under 300 C and 32 MPa condi-tions [37]. It is evident that higher molar ratios can result ina larger ester conversion rate in a shorter time. For soybean

Fig. 6. Eect of molar ratio on yield of methyl ester [32,37].

l Science 19 (2009) 273284 277acid-catalyzed methods. Examples of water content andfree fatty acid on the acid-, alkaline-catalyzed and super-

turacritical transesterication of vegetable oil are shown inFigs. 7 and 8. For acid catalytic reactions, as little as0.1% of water addition could lead to signicant reduction

Fig. 7. Yields of methyl esters as a function of water content [30].

Fig. 8. Yields of methyl ester as a function of fatty acids content [30].

278 D. Wen et al. / Progress in Naof the yield of methyl esters; the conversion was reducedto only 6% when 5% of water was added. A similartrend was also observed for the alkaline-catalyzed meth-ods. However, the amount of water added into the reactionsystem did not have any signicant eect on the conversionin the supercritical methanol method; and the presence ofwater positively aects the formation of methyl esters. Inaddition, compared with the alkaline-catalyzed method, ahigher yield could also be obtained from free fatty acids(Fig. 8).

The water-added supercritical methanol method hasanother feature of easier product separation, since glycerol,a co-product of transesterication, is more soluble in waterthan in methanol. It appears that the supercritical methodis specially good for converting a variety of resources withlarge contents of water and free fatty acid to biodiesel,which include crude vegetable oil, waste cooking oil andanimal fats.

3.2.5. Co-solvent eect

For most of the supercritical methods of biodiesel pro-duction, the reaction requires temperatures of 340400 Cand pressures of 2070 MPa, which is energy intensive.Such harsh operation conditions also lead to high produc-tion costs and material requirements. Various methodsincluding co-solvents and catalysts have been investigatedto reduce the reaction temperature and pressure whileachieving similar conversion rates.It is known that the solubility of methanol decreases atsupercritical conditions, being closer to that of vegetableoil at the appropriate temperature and pressure [42]. Somereports also show that the solubility of vegetable oils inmethanol increases at a rate of 23% per 10 C increase[39]. It would be of great interest from a practical point ofview to investigate the eect of a co-solvent. This could notonly increase the mutual solubility of methanol and vegeta-ble oil at low reaction temperatures, but also possiblydecrease the critical point of methanol, and allow the super-critical reaction to be carried out under milder conditions.

Using propane as the co-solvent, a study of the transest-erication of soybean oil in the supercritical methanol wasinvestigated [35]. Critical points for the binary system weredetermined by the content of propane in the binary system,which was found to decrease with increasing molar ratio ofpropane to methanol. The eect of propane on the conver-sion of soybean oil to methyl esters as biodiesel fuels isshown in Fig. 9. It is obvious that using propane as a co-sol-vent, the temperature can be reduced signicantly, i.e.330 C for methanol only and 280 C at propane-to-metha-nol molar ratio of 0.1, to reach a full conversion. As pro-pane is easy to add and separate, the reduction of reactiontemperature could make it viable for industrial applications.

3.2.6. Catalyst eect

For the conventional catalytic transesterication pro-

Fig. 9. Biodiesel conversions of propane and methanol under supercriticalconditions [35].

l Science 19 (2009) 273284cess, catalysts are classied as three types, alkali, acidand enzyme. Most of the reactions can be quickly precededwithout the need of a catalyst under supercritical methanoland ethanol conditions. However, a few catalysts have alsobeen introduced under such a condition in order to lowerthe reaction temperature and pressure, as outlined below.

Calcium oxide (CaO) has been known to catalyze reac-tions that require a base site. It is not dissolved in the reac-tion medium, and the transesterication reaction isheterogeneous. The roles of CaO in the supercritical transe-sterication of sunower seed oil to biodiesel were investi-gated by Demirbas [33]. It was found that the addition ofCaO could considerably improve the transestericationreaction. The experimental results are shown in Fig. 10 fora temperature of 525 K and a molar ratio of methanol tosunower oil: 41:1. It can be seen that the transesterication

low biodiesel conversions, further investigation of theenzymes eect on the total energy consumption and bene-t is still needed to assess this method.

3.2.7. Continuous production

Most of the biodiesel production via supercritical transe-sterication is based on the batch-type process. As thesupercritical methanol method requires a high temperatureof 350 C and a pressure of 45 MPa, and in addition, as alarge amount of methanol is necessary, it generally involves

Recently, He et al. [37] reported a continuous produc-

D. Wen et al. / Progress in Natural Science 19 (2009) 273284 279rate increases evidently with increasing CaO concentrations,and the reaction time of the yield reaching plateaus ofmethylester decreases with increasing catalyst concentrations.

Temperatures and molar ratios were also found to havegreat inuences on the catalytic supercritical transesterica-tion. Sunower oils could be fully converted to biodiesel in6 min under optimum conditions, i.e. at a temperature of525 K with 3 wt% CaO and 41:1 methanol/oil molar ratio.Of note is that the catalytic transesterication ability of CaOwas quite weak under ambient temperature, i.e. the yield ofmethyl ester was only about 5% in 3 h at 335K. CaO appearsto be a good catalyst under supercritical conditions.

Enzymatic reactions in supercritical carbon dioxide havebeen considered to be a practical way of achieving a betterbiofuel production rate. The requirement on power con-sumption and equipment is much lower for CO2 SCFs thanfor supercritical methanol and ethanol (Table 1). The sep-aration can also be easily achieved by the reduction of pres-sure, as the products and the enzyme do not dissolve incarbon dioxide at room conditions. Such an enzymaticreaction in supercritical carbon dioxide has been exploredand compared with non-catalytic supercritical methods[24]. One example of experimental results is shown inFig. 11 for the reaction at 45 C with 3 mg of enzyme.Enzyme reactions in supercritical carbon dioxide took amuch longer time and achieved only very low conversions(2730%), whilst high conversion rates (80100%) weretypically achieved under supercritical methanol and etha-nol conditions [29,32]. An improved reaction of supercriti-cal CO was developed for both edible and non-edible oils,

Fig. 10. Eect of CaO content on methyl ester yield [33].2

and a maximum conversion of less than 70% can beobtained after several hours of reaction [43]. Though with

Fig. 11. Biodiesel synthesis from supercritical CO2 [24].tion process for soybean oil conversion to biodiesel throughthe supercritical methanol method. The experiments wereoperated in a 75 ml tube reactor that supplied continuousow of soybean oil and methanol under molar ratios from6:1 to 80:1. After the reaction, the product was cooled toroom temperature, and then the crude methyl esters wereobtained in a separate vessel. Similar to the supercriticalbatch operation, it was observed that increasing the molarratio, reaction pressure and reaction temperature enhancedthe production yield eectively. However, there is also acritical value of residence time at high reaction tempera-ture, and the production yield will decrease if the residencetime surpasses this value. Some side reactions of unsatu-rated fatty acid methyl esters (FAMEs) also occurred whenthe reaction temperature was over 300 C, which led to abig loss of the material under a pressure of 32 MPa and amolar ratio of 40:1 as is shown in Fig. 12. Under the opti-mal reaction condition, only a maximum production yieldof 77% was observed, primarily due to the reactions ofunsaturated FAMEs at high temperature.

3.3. Reaction mechanism of transesterication

It was observed in many experiments that fatty acidspresent in the vegetable oil can be successfully convertedto methyl esters under supercritical methanol conditionshigh labor cost, unreliable production and relatively longertime. It would be very benecial to operate under continu-ous production conditions. A few continuous productionsystems have been developed for catalytic transestericationprocesses, which have resulted in increased production e-ciency and quality of biodiesel [4447].Fig. 12. Continuous synthesis of methyl esters from soybean oil [37].

tura[31]. Two types of reactions may exist in the supercriticalmethod for methyl esters formation: transesterication oftriglycerides and methyl esterication of fatty acids. It isexpected that a higher yield can be obtained than that pro-duced by the alkaline-catalyzed method [31].

Warabi et al. [28] studied the reactivity of transesterica-tion of triglycerides and alkyl esterication of fatty acids inthe supercritical alcohol process. In the experiments, thereaction temperature was set at 300 C, and methanol, eth-anol, 1-propanol, 1-butanol or 1-octanol was used as thereactant. It was shown that triglyceride was converted step-wise to diglyceride, monoglyceride and nally to glycerol asshown below.

Step I: triglyceride + methanol? diglyceride + methylester

Step II: diglyceride + methanol?monoglyceride + methylester

Step III: monoglyceride + methanol? glycerol + methylester

The formation of alkyl esters from monoglycerides is thecore step that determines the reaction rate, since monogly-cerides are the most stable intermediate compounds. Theresult also showed that transesterication of triglycerides(rapeseed oil) was slower in reaction rates than alkyl ester-ication of fatty acids, and the presence of saturated fattyacids such as palmitic and stearic acids had slightly loweredreactivity than that of the unsaturated fatty acids, oleic, lin-oleic and linolenic. Free fatty acids present in vegetable oilcould be completely converted to the alkyl esters under thesupercritical transesterication treatment.

3.4. Biodiesel economy

Although biodiesel has become more attractive recentlybecause of its abundance, carbon neutral eect and envi-ronmental benets, the economics of biodiesel is the mainobstacle for the commercialization of the product and forwide distribution in transport sectors.

A review of 12 economic feasibility studies shows thatfor biodiesel produced from conventional catalytic meth-ods, the projected cost from oilseed or animal fats fallswithin a range of US$0.300.69/l [48]. This includes themeal and glycerin credits and the assumption of reducedcapital investment costs by having the crushing and/oresterication facility added onto an existing grain or tallowfacility. Rough costs of biodiesel from vegetable oil andwaste grease are estimated to be US$0.540.62/l andUS$0.340.42/l, respectively. With pre-tax diesel priced atUS$0.18/l in the US and US$0.200.24/l in some Europeancountries, it is dicult for biodiesel to compete with petro-leum fuels without further economic and technologicaldevelopment breakthrough.

One reason for the non feasibility of biodiesel is the high

280 D. Wen et al. / Progress in Nacost of the feedstock as most of the biodiesel is currentlymade using soybean oil under alkaline catalyst conditions.The high value of soybean oil as a food product makes theproduction of a cost-eective fuel very challenging. How-ever, there are large amounts of low-cost oils and fats, suchas restaurant waste and animal fats, that could be con-verted to biodiesel. As reviewed earlier, these low-cost oilsand fats often contain large amounts of free fatty acids andwater that cannot be converted to biodiesel by the conven-tional catalytic method [30]. Only the supercritical process-ing method could solve the problems; it oers a greatadvantage to eliminate the pretreatment capital and oper-ating cost. It appears that using waste oil as a raw materialand employing a continuous transesterication processunder supercritical conditions, with recovery of high qual-ity glycerol as a biodiesel by-product, are primary optionsto lower the cost of biodiesel.

Very recently, such considerations have been incorpo-rated into an economic study that was conducted to focuson converting waste cooking oil via supercritical transeste-rication from methanol to methyl esters [49]. The eco-nomics of three plant capacities, 125,000, 80,000 and8000 tonnes biodiesel/year from waste cooking oil, werestudied for biodiesel production under continuous super-critical transesterication conditions. The results showedthat biodiesel produced by supercritical transestericationcan be scaled up with high purity of methyl esters(99.8%), and almost pure glycerol (96.4%) can be attainedas a by-product. The economic assessment of the biodieselplant shows that biodiesel can be sold at US$0.17/l(125,000 tonnes/year), US$0.24/l (80,000 tonnes/year)and US$0.52/l for the smallest capacity (8000 tonnes/year),which makes it a strong competitor for the catalyzedtransesterication process, and also in the near future asa promising replacement fuel for petroleum.

Such an economic analysis demonstrated that biodieselproduction from supercritical uid methods could becomeeconomically competitive even to the petroleum market.Further assessments of the sensitive key factors includingraw material price, plant capacity, glycerol price and capi-tal cost, as well as dierent supercritical techniques, are stillneeded to reach an impartial conclusion. In general,though, supercritical uids will be an interesting technicaland economic alternative for future biodiesel production.

4. Supercritical gasication of biomass

4.1. Conventional hydrogen production methods

The hydrogen economy is dependent on individual partsof a hydrogen energy system, which include production,delivery, storage, conversion, and end-use applications.The economic production of hydrogen and a highly e-cient conversion system, i.e. through fuel cell technologyto convert chemical energy to electricity and/or thermalenergy, are the two core elements. Currently over 70% ofhydrogen produced is from fossil fuels, mainly steam-meth-

l Science 19 (2009) 273284ane reforming (SMR). The process includes mainly threeparts: (i) pretreatment of the feedstock, (ii) steam reform-

turaCH4 H2O CO 3H2 1COH2O CO2 H2 2

The heat required for the rst reaction is generallyobtained by the combustion of fuel gas and purge/tail gasfrom the PSA system. Following the reforming step, thesynthesized gas is fed into the CO-conversion reactor toproduce additional hydrogen. Heat recovery for steam orfeedstock preheating takes place at dierent points withinthe process chain to optimize the energy eciency of thereformer system. In the third part, hydrogen puricationis achieved by means of PSA. The PSA unit consists of ves-sels lled with selected adsorbents, and could achievehydrogen purities higher than 99.999% by volume andCO impurities of less than 1 vppm (volumetric part per mil-lion) to meet the requirement of the fuel cells. Pure hydro-gen from the PSA unit is sent to the hydrogen compressor,while the PSA o-gas from recovering the adsorbents, thetailgas, is fed to the reformer burner. A recuperative burneris used with high eciency and low nitrogen oxide emis-sion. During a normal operation, the burner can be oper-ated solely on the tailgas stream.

Besides SMR, water electrolysis is also a major produc-tion process where electricity is used to split water intohydrogen and oxygen molecules. While the SMR processis heavily dependent on fossil fuel supply, which is limitedand causes environmental problems, the electrolysis pro-cess is very expensive and heavily depends on the supplyof electricity, which again is mostly from fossil fuels. Bio-mass is a large potential resource for economic productionof hydrogen. This interest is founded upon the expectationthat hydrogen will be produced at a competitive price withconventional fossil fuels.

4.2. Gasication of biomass for hydrogen production

A similar process with SMR can be used for hydrogenproduction from biomass, steaming reforming of biomass.

C6H10O5 7H2O 6CO2 12H2 3In this reaction, natural gas is replaced by cellulose that

is represented as C6H10O5. In the idealized, stoichiometricequation, cellulose reacts with water to produce hydrogenand carbon dioxide. The research on the gasication of bio-ing and watergas shift reaction and (iii) gas puricationthrough pressure swing adsorption (PSA). In the rst part,the hydrocarbon feedstock is desulphurised using activatedcarbon lters, pressurized and preheated and mixed withprocess steam. The fresh water is softened and de-mineral-ized by an ion-exchange water conditioning system. In thesecond part, methane and steam are converted within thecompact reformer furnace at approximately 900 C withthe addition of a nickel catalyst to a hydrogen-rich refor-mate steam according to the following reactions.

D. Wen et al. / Progress in Namass began a few decades ago, and much progress has beenmade. The major challenges facing biomass gasicationnow are to reduce and even eliminate the formation oftar and char so as to increase the conversion eciency,and to nd practical technologies to convert not only thecellulose, but also hemicellulose, lignin, protein, andextractive components of a biomass feedstock into a gasrich in hydrogen and carbon dioxide. Any production ofchar and tar represents an eective loss of gas.

For conventional biomass gasication under atmo-spheric pressure, biomass does not react directly with steamto produce the desired products. Instead, signicantamounts of tar and char are formed, and the gas containshigher hydrocarbons in addition to the desired light gases[50,51]. The formation of pyrolytic char and tar during gas-ication sets limits on the ecient production of hydrogenfrom biomass under atmospheric pressure. Both the tem-perature and pressure eects on reducing the char and tarproduction have been investigated. As the temperatureincreased to over 800 C, a nearly complete conversion oftar to gas could be realized [52], but the char by-productremained unconverted. For biomass gasication underhigh pressure, it was found that even cellulose, the moststable component of biomass, decomposes rapidly at atemperature below the critical temperature of water at apressure above waters supercritical pressure, 22.1 MPa[53]. The char formation can be fully suppressed as temper-ature is further increased [54]. However, tar gasicationbecomes the chief obstacle for a total steam reforming ofbiomass.

Based on previous experiments, it has therefore beenexpected that complete gasication could be achieved forsupercritical water under optimized operational conditions.

4.3. Biomass gasication in supercritical water (SCW)

Biomass gasication in supercritical water opens a doorto the realization of eective thermochemical gasicationof biomass, especially wet ones, as schematically shownin Fig. 13. In general, the supercritical gasication can becategorized into two areas, low temperature gasicationat 350600 C with the aid of some catalysts and high tem-perature gasication at 600800 C without any catalysts.For low temperature gasication, although catalysts aregenerally applied to enhance the reaction, complete gasi-cation of feedstock is still dicult. Due to the high depen-dence of the reactivity of biomass on temperature, acomplete conversion of biomass into combustible gas hasbecome possible at higher temperatures. However, the gas-ication eciency falls as the concentration of the organicfeedstock increases. There are a number of parameters thataect the thermochemical conversion eciency undersupercritical conditions, which include operating pressureand temperature, dierent catalysts and feedstock, interac-tions between dierent components and eect of partialoxidation. A recent review by Matsumura et al. can befound in Ref. [51].

l Science 19 (2009) 273284 281Throughout the development of the technology till todate, the possibility of biomass gasication in near- and

ized and clean medium caloric value gas with a high hydro-

catalysts. The techniques are still under development and

of

turagen content. Compared with conventional biomassgasication technologies, supercritical water gasicationpossesses a number of advantages that include high ther-mochemical conversion rate, suitable for wet feedstocksuch as water hyacinth and algae, high pressure productsthat are easy for future transportation and usage, opportu-nities for carbon capture, sequestration and storage, as wellas to further pure hydrogen production via a further steam-methane reforming process. However, some technicalbreakthroughs, especially on the complete gasication oftar and char, practical diculties of operation in harshtemperature and pressure conditions, material requirementand its associated high cost are the main barriers for thetechnology to become widely commercially available.

5. Supercritical liquefaction of biomass

Among the biomass energy conversion methods asshown in Fig. 1, thermochemical liquefaction is consideredsupercritical water (SCW) has been demonstrated, and acomplete conversion of biomass into combustible gas hasbeen achieved. SCW may become an important technologyfor converting wet biomass or organic waste to a pressur-

Fig. 13. Schematic of the application

282 D. Wen et al. / Progress in Nato be a promising method for converting biomass intohigher value fuels. Compared with the gasication tech-nique, the liquefaction process does not require a feedstockdrying process, which typically requires signicant heatingdue to the large latent heat of water vaporization. Thermo-chemical liquefaction can be an eective method for con-verting woody biomass into oil or other types of fuels.

Supercritical uid is a candidate for the chemical conver-sion of lignocellulosics due to its unique properties. Super-critical water treatment for cellulosic samples has beenextensively studied to obtain saccharides for subsequent fer-mentation to ethanol [55]. Co-liquefaction of cellulose andcoal in supercritical water has also been investigated aimingfor hydrogen production [56]. However, as the reaction isenergy intensive to reach the supercritical status of water, anumber of alcohols have been recently investigated, includ-ing methanol, ethanol and 1-propanol [27,5760]. Usinginterested readers may refer to the above reference fordetailed information.

6. Conclusion

This paper reviews state-of-the-art applications ofsupercritical uids (SCFs) technology for biofuels produc-tion, with the main focus on biodiesel production from veg-etable oil via the transesterication process. Bio-hydrogenfrom the gasication and bio-oil from the liquefaction ofbiomass from the SCF route are also briey reviewed. Itshows that SCF is a promising technique for future biofuelproduction. Compared with conventional biofuel produc-these low critical temperature andpressure alcohols, it is pos-sible to obtain liquid products as direct fuels. In addition,various alcohols can be produced from biomass, i.e. metha-nol from hydrogen and carbon monoxide gasied from bio-mass, and ethanol and butanol from fermentation ofbiomass saccharides. Various types of biofuels can beachieved by using dierent alcohols.

Compared with the conventional method, supercriticalbiomass liquefaction could oer a number of advantagessuch as high conversion rate, fast reaction and few or no

supercritical water gasication [51].

l Science 19 (2009) 273284tion methods, the SCF technology possesses a number ofadvantages that include fast conversion, high fuel produc-tion rate, ease of continuous operation and elimination ofthe necessity of catalysts. Some main conclusions can bedrawn:

1. Increasing temperature and molar ratios can enhancethe conversion rate and kinetics for supercritical biodie-sel production from vegetable oils.

2. Among all alcohols, methanol is the best for the transe-sterication process. Its conversion eect could be muchimproved under optimized conditions and in the pres-ence of either a co-solvent or a catalyst.

3. Water and free fatty acid have little or even positiveimpact on the biodiesel conversion under supercriticalconditions, which make SCF especially suitable for bio-diesel production from waster oil and animal fat.

tura4. The economic assessment shows that biodiesel producedfrom the SCF technology is comparable to conventionalcatalytic transesterication, and is competitive withpetroleum-derived fuels if waster oil is used as afeedstock.

5. Supercritical uid technology has favorable impact onbiogas production via the gasication, and bio-oil pro-duction via the liquefaction of biomass.

There are, however, a number of problems associatedwith the SCF technology. The mechanistic understandingof the process, the harsh operation environment such ashigh temperature and high pressure, and its request onthe materials and associated cost are the main concernsfor its wide application. Future research should focus onthe reduction of operating temperature and pressure whilemaintaining the high conversion rate.

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284 D. Wen et al. / Progress in Natural Science 19 (2009) 273284

Supercritical fluids technology for clean biofuel productionIntroductionSupercritical fluids techniqueSCFs for biodiesel productionBiodiesel productionBiodiesel production from SCF transesterificationTemperature and residence time effectAlcohol effectMolar ratio effectWater and free fatty acids effectCo-solvent effectCatalyst effectContinuous production

Reaction mechanism of transesterificationBiodiesel economy

Supercritical gasification of biomassConventional hydrogen production methodsGasification of biomass for hydrogen productionBiomass gasification in supercritical water (SCW)

Supercritical liquefaction of biomassConclusionReferences