-
vie
y
.
een
hin
16
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: [email protected] (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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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