Progress in Energy and Combustion Science 31 (2005) 466–487
www.elsevier.com/locate/pecs
Biodiesel production from vegetable oils via catalytic and
non-catalytic supercritical methanol transesterification methods
Ayhan Demirbas *
Department of Chemical Engineering, Selcuk University, Campus, 42031 Konya, Turkey
Received 18 April 2005; accepted 23 September 2005
Abstract
This paper reviews the production and characterization of biodiesel (BD or B) as well as the experimental work carried out by
many researchers in this field. BD fuel is a renewable substitute fuel for petroleum diesel or petrodiesel (PD) fuel made from
vegetable or animal fats. BD fuel can be used in any mixture with PD fuel as it has very similar characteristics but it has lower
exhaust emissions. BD fuel has better properties than that of PD fuel such as renewable, biodegradable, non-toxic, and essentially
free of sulfur and aromatics. There are more than 350 oil bearing crops identified, among which only sunflower, safflower, soybean,
cottonseed, rapeseed and peanut oils are considered as potential alternative fuels for diesel engines. The major problem associated
with the use of pure vegetable oils as fuels, for Diesel engines are caused by high fuel viscosity in compression ignition. Dilution,
micro-emulsification, pyrolysis and transesterification are the four techniques applied to solve the problems encountered with the
high fuel viscosity. Dilution of oils with solvents and microemulsions of vegetable oils lowers the viscosity, some engine
performance problems still exist. The viscosity values of vegetable oils vary between 27.2 and 53.6 mm2/s whereas those of
vegetable oil methyl esters between 3.59 and 4.63 mm2/s. The viscosity values of vegetable oil methyl esters highly decreases after
transesterification process. Compared to no. 2 diesel fuel, all of the vegetable oil methyl esters were slightly viscous. The flash
point values of vegetable oil methyl esters are highly lower than those of vegetable oils. An increase in density from 860 to
885 kg/m3 for vegetable oil methyl esters or biodiesels increases the viscosity from 3.59 to 4.63 mm2/s and the increases are highly
regular. The purpose of the transesterification process is to lower the viscosity of the oil. The transesterfication of triglycerides by
methanol, ethanol, propanol and butanol, has proved to be the most promising process. Methanol is the commonly used alcohol in
this process, due in part to its low cost. Methyl esters of vegetable oils have several outstanding advantages among other newrenewable and clean engine fuel alternatives. The most important variables affecting the methyl ester yield during the
transesterification reaction are molar ratio of alcohol to vegetable oil and reaction temperature. Biodiesel has become more
attractive recently because of its environmental benefits. Biodiesel is an environmentally friendly fuel that can be used in any diesel
engine without modification.
q 2005 Elsevier Ltd. All rights reserved.
Keywords: Alternative fuel; Vegetable oil; Biodiesel; Viscosity; Transesterification; Methanol
Contents
1.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.1. History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
* Tel.: C90 462 230 7831; fax: C90 462 248 8508.
E-mail address: [email protected]
0360-1285/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.pecs.2005.09.001
467
468
2.
3.
4.
5.
6.
7.
A. Demirbas / Progress in Energy and Combustion Science 31 (2005) 466–487
467
1.2. The use of vegetable oils and their derivatives as alternative diesel fuels . . . . . . . . . . . . . . . . . . . . . . . . .
1.3. Global vegetable oil resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Biodiesel (BD) as an alternative fuel for diesel engine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1. The importance of alcohols for diesel engines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.1. Methanol production methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.2. Ethanol production methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2. Hydrogen production methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Transesterification of vegetable oils and fats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1. Catalytic transesterification method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2. Supercritical methanol transesterification method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3. Recovery of glycerol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4. Reaction mechanism of transesterification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.1. Acid-catalyzed processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.2. Alkali-catalyzed processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.3. Enzyme-catalyzed processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.4. Non-catalytic supercritical alcohol transesterification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Fuel properties of vegetable oils and biodiesels (BDs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1. Emissions from biodiesel combustion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2. Comparison of fuel properties and combustion characteristics of methyl and ethyl alcohols and their esters
Engine performance tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
BD economy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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1. Introduction
The major part of all energy consumed worldwide
comes from fossil sources (petroleum, coal and natural
gas). However, these sources are limited, and will be
exhausted by the near future. Thus, looking for
alternative sources of new and renewable energy
such as hydro, biomass, wind, solar, geothermal,
hydrogen and nuclear is of vital importance. Alternative new and renewable fuels have the potential to
solve many of the current social problems and
concerns, from air pollution and global warming to
other environmental improvements and sustainability
issues [1].
Vegetable oil is one of the renewable fuels.
Vegetable oils have become more attractive recently
because of its environmental benefits and the fact
that it is made from renewable resources. Vegetable
oils are a renewable and potentially inexhaustible
source of energy with an energetic content close to
diesel fuel. The vegetable oil fuels were not
acceptable because they were more expensive than
petroleum fuels. However, with recent increases in
petroleum prices and uncertainties concerning petroleum availability, there is renewed interest in
vegetable oil fuels for diesel engines [2]. Diesel
boiling range material is of particular interest
because it has been shown to significantly reduce
particulate emissions relative to petroleum diesel
[3]. There are more than 350 oil-bearing crops
identified, among which only sunflower, safflower,
soybean, cottonseed, rapeseed, and peanut oils are
considered as potential alternative fuels for diesel
engines [4,5]. The major problem associated with
the use of pure vegetable oils as fuels, for diesel
engines are caused by high fuel viscosity in
compression ignition.
The use of vegetable oils as alternative renewable
fuel competing with petroleum was proposed in the
beginning of 1980s. The advantages of vegetable oils as
diesel fuel are [2]:
†
†
†
†
Liquid nature-portability
Ready availability
Renewability
Higher heat content (about 88% of no. 2 diesel
fuel)
† Lower sulfur content
† Lower aromatic content
† Biodegradability
The disadvantages of vegetable oils as diesel fuel
are:
† Higher viscosity
† Lower volatility
† The reactivity of unsaturated hydrocarbon chains
468
A. Demirbas / Progress in Energy and Combustion Science 31 (2005) 466–487
1.1. History
Transesterification of triglycerides are in oils is not a
new process. Scientists E. Duffy and J. Patrick
conducted it as early as 1853. Life for the diesel engine
began in 1893 when the famous German inventor
Rudolph Diesel published a paper entitled ‘The theory
and construction of a rational heat engine’. What the
paper described was a revolutionary engine in which air
would be compressed by a piston to a very high
pressure thereby causing a high temperature. Rudolph
Diesel designed the original diesel engine to run on
vegetable oil. Dr Rudolph Diesel used peanut oil to fuel
one of this his engines at the Paris Exposition of 1900
[6]. Because of the high temperatures created, the
engine was able to run a variety of vegetable oils
including hemp and peanut oil. At the 1911 World’s
Fair in Paris, Dr R. Diesel ran his engine on peanut oil
and declared ‘the diesel engine can be fed with
vegetable oils and will help considerably in the
development of the agriculture of the countries which
use it.’ One of the first uses of transesterified vegetable
oil was powering heavy-duty vehicles in South Africa
before world war II. The name ‘biodiesel’ has been
given to transesterified vegetable oil to describe its use
as a diesel fuel [7].
1.2. The use of vegetable oils and their derivatives as
alternative diesel fuels
The direct use of vegetable oils in fuel engines is
problematic. Due to their high viscosity (about 11–17
times higher than diesel fuel) and low volatility, they do
not burn completely and form deposits in the fuel
injector of diesel engines [2]. Different ways have been
considered to reduce the high viscosity of vegetable
oils:
(1) Dilution of 25 parts of vegetable oil with 75 parts of
diesel fuel,
(2) Microemulsions with short chain alcohols such as
ethanol or methanol,
(3) Thermal decomposition, which produces alkanes,
alkenes, carboxylic acids and aromatic compounds,
(4) Catalytic cracking, which produces alkanes,
cycloalkanes and alkylbenzenes, and
(5) Transesterification with ethanol or methanol.
Dilution of oils with solvents and microemulsions of
vegetable oils lowers the viscosity, some engine
performance problems, such as injector coking and
more carbon deposits. still exist Among all these
alternatives, the transesterification seems to be the best
choice, as the physical characteristics of fatty acid
esters (biodiesel) are very close to those of diesel fuel
and the process is relatively simple. Furthermore, the
methyl or ethyl esters of fatty acids can be burned
directly in unmodified diesel engines, with very low
deposit formation. Although short-term tests using neat
vegetable oil showed promising results, longer tests led
to injector coking, more engine deposits, ring sticking,
and thickening of the engine lubricant. These experiences led to the use of modified vegetable oil as a fuel.
Although there are many ways and procedures to
convert vegetable oil into a diesel-like fuel, transesterification process was found to be the most viable oil
modification process [8]. At present, the most common
way to produce biodiesel (BD or B) is to transesterify
triacylglycerols in vegetable oil or animal fats with an
alcohol in the presence of an alkali or acid catalyst.
BD is a renewable substitute fuel for petroleum
diesel (PD) made from vegetable or animal fats via
transesterification by alcohols. BD can be used in any
mixture with PD as it has very similar characteristics
but it has lower exhaust emissions. BD has better
properties than that of PD such as renewable,
biodegradable, non-toxic, and essentially free of sulfur
and aromatics.
Chemically, BD is referred to as the mono-alkylesters of long-chain-fatty acids derived from renewable
lipid sources. BD is the name for a variety of ester
based oxygenated fuel from renewable biological
sources. It can be used in compression ignition engines
with little or no modifications [9].
A number of methods exist to blend vegetable oil
with PD and create a low viscosity fuel oil with similar
properties to diesel. Benefits are substantially reduced
engine emissions, even with as small a blend as 20%
BD with 80% PD. Using BD results in large reductions
in overall carbon dioxide emissions and it is carbon
dioxide that is a major contributor to climate change.
Exploring new energy resources, such as BD fuel, is
of growing importance in recent years. BD is
recommended for use as a substitute for PD mainly
because BD is a renewable, domestic resource with an
environmentally friendly emission profile and is readily
available and biodegradable [10]. BD has become more
attractive recently because of its environmental benefits
[11,12]. This paper reviews the production and
characterization of BD from vegetable oils as well as
the experimental work carried out by many researchers
in this field.
Several types of vegetable oils can be used for the
preparation of BD. Soybean, rapeseed, sunflower
A. Demirbas / Progress in Energy and Combustion Science 31 (2005) 466–487
469
Table 1
Fatty acid compositions of vegetable oil samples
Sample
16:0
16:1
18:0
18:1
18:2
18:3
Others
Cottonseed
Poppyseed
Rapeseed
Safflowerseed
Sunflowerseed
Sesameseed
Linseed
Wheat graina
Palm
Corn marrow
Castorb
Tallow
Soybean
Bay laurel leafc
Peanut kerneld
Hazelnut kernel
Walnut kernel
Almond kernel
Olive kernel
Coconute
28.7
12.6
3.5
7.3
6.4
13.1
5.1
20.6
42.6
11.8
1.1
23.3
13.9
25.9
11.4
4.9
7.2
6.5
5.0
7.8
0
0.1
0
0
0.1
0
0.3
1.0
0.3
0
0
0.1
0.3
0.3
0
0.2
0.2
0.5
0.3
0.1
0.9
4.0
0.9
1.9
2.9
3.9
2.5
1.1
4.4
2.0
3.1
19.3
2.1
3.1
2.4
2.6
1.9
1.4
1.6
3.0
13.0
22.3
64.1
13.6
17.7
52.8
18.9
16.6
40.5
24.8
4.9
42.4
23.2
10.8
48.3
83.6
18.5
70.7
74.7
4.4
57.4
60.2
22.3
77.2
72.9
30.2
18.1
56.0
10.1
61.3
1.3
2.9
56.2
11.3
32.0
8.5
56.0
20.0
17.6
0.8
0
0.5
8.2
0
0
0
55.1
2.9
0.2
0
0
0.9
4.3
17.6
0.9
0.2
16.2
0
0
0
0
0
0
0
0
0
0
1.8
1.1
0.3
89.6
2.9
0
31.0
4.0
0
0
0.9
0.8
65.7
Legend: Source: Ref. [2].
a
Wheat grain oil contains 11.4% of 8:0 and 0.4% of 14:0 fatty acids.
b
Castor oil contains 89.6% ricinoloic acid.
c
Bay laurel oil contains 26.5% of 12:0 and 4.5% of 14:0 fatty acids.
d
Peanut kernel oil contains about 2.7% of 22:0 and 1.3% of 24:0 fatty acids.
e
Coconut oil contains about 8.9% of 8:0, 6.2% 10:0, 48.8% of 12:0 and 19.9% of 14:0 fatty acids.
and palm oils are the most studied. However, there are
no technical restrictions to the use of other types of
vegetable oils. The fatty acid compositions of vegetable
oil samples are given in Table 1. Considering the type
of the alcohol, the use of methanol is advantageous as it
allows the simultaneous separation of glycerol. The
same reaction using ethanol is more complicated as
it requires a water-free alcohol, as well as an oil with
a low water content, in order to obtain glycerol
separation [13].
Problems met in long-term engine tests according to
results obtained by earlier researchers may be classified
as follows: Injector coking and trumpet formation on
the injectors, more carbon deposits, oil ring sticking,
and thickening and gelling of the engine lubricant
oil [2].
The vegetable oils were all extremely viscous with
viscosities ranging 10–20 times greater than no. 2 diesel
fuel. Castor oil is in a class by itself with a viscosity
more than 100 times that of no. 2 diesel fuel [2].
Viscosity of oil can be lowered by blending with pure
ethanol. 25 parts of sunflower oil and 75 parts of PD
were blending as PD fuel. Dilution, micro-emulsification, pyrolysis and transesterification are the four
techniques applied to solve the problems encountered
with the high fuel viscosity. To reduce of the high
viscosity of vegetable oils, microemulsions with
immiscible liquids such as methanol and ethanol and
ionic or non-ionic amphiphiles have been studied
[12,14].
Vegetable oils have the potential to substitute a
fraction of petroleum distillates and petroleum based
petrochemicals in the near future. Vegetable oil fuels
are not petroleum-competitive fuels because they are
more expensive than petroleum fuels.
1.3. Global vegetable oil resources
Global vegetable oil production increased 56 million
tons in 1990 to 88 million tons in 2000, following a
below-normal increase. World vegetable and marine oil
consumption is tabulated in Table 2. Fig. 1 shows the
plots of percentages the world oil consumption by
years. Fig. 2 shows the total global production and
consumption of vegetable oil by years. Leading the
gains in vegetable oil production was a recovery in
world palm oil output, from 17.1 million tons in
1997/1998 to 19.3 million in 1998/1999.
The major exporters of vegetable oils are Malaysia,
Argentina, Indonesia, Philippines, and Brazil.
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A. Demirbas / Progress in Energy and Combustion Science 31 (2005) 466–487
Table 2
World vegetable and marine oil consumption (million metric ton)
1998
1999
2000
2001
2002
2003
Soybean
Palm
Rapeseed
Sunflowerseed
Peanut
Cottonseed
Coconut
Palm kernel
Olive
Fish
Total
23.5
18.5
12.5
9.2
4.5
3.7
3.2
2.3
2.2
1.2
80.7
24.5
21.2
13.3
9.5
4.3
3.7
3.2
2.6
2.4
1.2
85.7
26.0
23.5
13.1
8.6
4.2
3.6
3.3
2.7
2.5
1.2
88.4
26.6
24.8
12.8
8.4
4.7
4.0
3.5
3.1
2.6
1.3
91.8
27.2
26.3
12.5
8.2
5.3
4.4
3.7
3.5
2.7
1.3
95.1
27.9
27.8
12.1
8.0
5.8
4.9
3.9
3.7
2.8
1.4
98.3
Total production
Amount of vegetable oil, Million ton
Oil
90000
Total consumption
80000
70000
60000
Source: World statistics, 1998–2004 United Soybean Board.
The major importers of vegetable oils are China,
Pakistan, Italy and the United Kingdom. Few countries
such as Netherlands, Germany, United States and
Singapore are both large exporters as well as importers
of vegetable oils [8].
Global vegetable oil exports rose modestly from
29.8 million tons in 1997/1998 to 31.2 million in
1998/1999. A large portion of the gain went to India,
where even small price shifts can cause a substantial
change in consumption. Indian consumption of all
vegetable oils in 1998/1999 soared 26% from
1997/1998. Indian palm oil imports climbed to a record
2.5 million tons. Similarly, Pakistan, Iran, Egypt, and
Bangladesh sharply increased their vegetable oil
imports. In 1999, Pakistan reacted to falling vegetable
oil prices with a series of increases that doubled the
import duties on soybean oil and palm oil, while
eliminating duties on oilseeds. Pakistan also raised the
import duty on soybean meal from 10 to 35% to stem
the influx of Indian exports [8].
Annual oil consumption, wt%
Soybean
Palm
Rapeseed
Sunflowerseed
Peanut
Others
28
22
16
10
4
1998
1999
2000
2001
2002
2003
Year
Fig. 1. Plots of percentages the world oil consumption by years.
Source: World Statistics, 1998–2004 United Soybean Board.
50000
1990
1992
1994
1996
1998
2000
Years
Fig. 2. Total global production and consumption of vegetable oil by
years.
2. Biodiesel (BD) as an alternative fuel for diesel
engine
BD is a clear amber-yellow liquid with a viscosity
similar to PD. BD is non-flammable, and in contrast to
PD it is non-explosive, with a flash point of 423 K for
BD as compared to 337 K for PD. Unlike PD, BD is
biodegradable and non-toxic, and it significantly
reduces toxic and other emissions when burned as a
fuel. Currently, BD is more expensive to produce than
PD, which appears to be the primary factor keeping it
from being in more widespread use. Current worldwide
production of vegetable oil and animal fat is not enough
to replace liquid fossil fuel use (maximum replacement
percentage: w20–25%) [8].
Methyl esters of vegetable oils (BDs) have several
outstanding advantages among other new-renewable
and clean engine fuel alternatives. Methanol as
monoalcohol is generally used in the transesterification
reaction of triglycerides in the presence alkali as a
catalyst [15]. Methanol is a relatively inexpensive
alcohol. Several common vegetable oils such as
sunflower, palm, rapeseed, soybean, cottonseed and
corn oils and their fatty acids can be used as the sample
of vegetable oil. BD is easier to produce and cleaner
with equivalent amounts of processing when starting
with clean vegetable oil. The tallow, lard and yellow
grease BDs need additional processing at the end of
transesterification due to including high free fatty acid.
Diesel derived from rapeseed oil is the most common
A. Demirbas / Progress in Energy and Combustion Science 31 (2005) 466–487
BD available in Europe, while soybean BD is dominant
in the United States.
The emergence of the transesterification can be
dated back to as early as 1846 when Rochieder
described glycerol preparation through ethanolysis of
castor oil [16]. Since, that time alcoholysis has been
studied in many parts of the world. Others researchers
have also investigated the important reaction conditions
and parameters on the alcoholysis of triglycerides, such
as fish oils, tallow, soybean, rapeseed, cottonseed,
sunflower, safflower, peanut and linseed oils [17–26]. It
also prepared methyl esters from palm oil by
transesterification using methanol in the presence of a
alkali catalyst in a batch reactor [27]. Soybean oil was
transesterified into ethyl and methyl esters, and
compared the performances of the fuels with PD
[28,29].
Transesterification is the process of using an alcohol
(e.g. methanol, ethanol, propanol or butanol), in the
presence of a catalyst to chemically break the molecule
of the raw renewable oil into methyl or ethyl esters of
the renewable oil with glycerol as a by-product [2].
Methanol is the commonly used alcohol in this process,
due in part to its low cost. However, ethanol is a
preferred alcohol in the transesterification process
compared to methanol because it is derived from
agricultural products and is renewable and biologically
less objectionable in the environment. Alkali catalyzed
transesterification has been most frequently used
industrially, mainly due to its fast reaction rate.
Methyl, ethyl, 2-propyl and butyl esters were
prepared from canola and linseed oils through
transesterification using KOH and/or sodium alkoxides
as catalysts. In addition, methyl and ethyl esters were
prepared from rapeseed and sunflower oils using the
same catalysts [22].
2.1. The importance of alcohols for diesel engines
Practically, any of the organic molecules of the
alcohol family can be used as a fuel. The alcohols that
can be used for motor fuels are methanol (CH3OH),
ethanol (C2H5OH), propanol (C 3H7OH), butanol
(C4H9OH). However, only two of these alcohols
(methanol and ethanol) are technically and economically suitable as fuels for internal combustion engines
(ICEs). Main production facilities of methanol and
ethanol are tabulated in Table 3. Methanol is produced
by a variety of process, the most common are as
follows: Distillation of liquid products from wood and
coal, natural gas and petroleum gas. Ethanol is
produced mainly from biomass bioconversion. It can
471
Table 3
Main production facilities of methanol and ethanol
Product
Production process
Methanol
Distillation of liquid from wood pyrolysis
Gaseous products from biomass gasification
Distillation of liquid from coal pyrolysis
Synthetic gas from biomass and coal
Natural gas
Petroleum gas
Ethanol
Fermentation of sugars and starches
Bioconversion of cellulosic biomass
Hydration of alkanes
Synthesis from petroleum
Synthesis from coal
Enzymatic conversion of synthetic gas
also be produced by synthesis from petroleum or
mineral coal [30].
2.1.1. Methanol production methods
Methanol, also known as ‘wood alcohol’, is
commonly used in BD production for its reactivity.
Generally, it is easier to find than ethanol. Sustainable
methods of methanol production are currently not
economically viable.
The use of methanol as a motor fuel received
attention during the oil crises of the 1970s due to its
availability and low cost. Problems occurred early in
the development of gasoline–methanol blends. As a
result of its low price some gasoline marketers over
blended. Many tests have shown promising results
using 85–100% by volume methanol as a transportation
fuel in automobiles, trucks and buses [8].
Methanol can be used as one possible replacement
for conventional motor fuels. Methanol has been seen
as a possible large volume motor fuel substitute at
various times during gasoline shortages. It was often
used in the early part of the century to power
automobiles before inexpensive gasoline was widely
introduced. In the early 1920s, some viewed it as a
source of fuel before new techniques were developed to
discover and extract oil. Synthetically produced
methanol was widely used as a motor fuel in Germany
during the world war.
Before modern production technologies were
developed in the 1920s, methanol was obtained from
wood as a co-product of charcoal production and, for
this reason, was commonly known as wood alcohol.
Methanol is currently manufactured worldwide by
conversion or derived from syngas, natural gas, refinery
off-gas, coal or petroleum:
472
A. Demirbas / Progress in Energy and Combustion Science 31 (2005) 466–487
2H2 C CO/ CH3 OH
(1)
The chemical composition of syngas from coal and
then from natural gas can be identical with the same
H2/CO ratio. A variety of catalysts are capably of
causing the conversion, including reduced NiO-based
preparations, reduced Cu/ZnO shift preparations,
Cu/SiO2 and Pd/SiO2, and Pd/ZnO [31,32].
Biomass resources can be used to produce methanol.
The pyroligneous acid obtained from wood pyrolysis
consists of about 50% methanol, acetone, phenols and
water. [33,34]. As a renewable resource, biomass
represents a potentially inexhaustible supply of feedstock for methanol production. The product yield for
the conversion process is estimated to be 185 kg of
methanol per metric ton of solid waste [35,36].
Methanol is currently made from natural gas but can
be made using wood waste or garbage via partial
oxidation reaction into syngas, followed by catalytic
conversion into methanol called as biomethanol.
Adding sufficient hydrogen to the syngas to convert
all of the biomass carbon into methanol carbon would
more than double the methanol produced from the same
biomass base [37]. The composition of syngas from
biomass for producing methanol is presented in
Table 4. Current natural gas feedstocks are so
inexpensive that even with tax incentives renewable
methanol has not been able to compete economically.
Technologies are being developed that may eventually
result in commercial viability of renewable methanol.
Methanol from coal could be a very important
source of liquid fuel in the future. The coal is first
pulverized and cleaned, then fed to a gasifier bed where
it is reacted with oxygen and steam to produce the
syngas. Once these steps have been taken, the
production process is much the same as with the other
feedstocks with some variations in the catalyst used and
the design of the converter vessel in which the reaction
is carried out. Methanol made using synthesis gas
(syngas) with hydrogen and carbon monoxide in a 2–1
ratio (Table 4). The syngas was transformed to
methanol in a fixed catalyst bed reactor. Coal-derived
methanol has many preferable properties as free of
sulfur and other impurities, could replace petroleum in
transportation, or be used as a peaking fuel in
combustion turbines, or supply a source of hydrogen
Table 4
Analysis of syngas from typical coal
for fuel cells. The technology for making methanol
from natural gas is already in place and requires only
efficiency improvements and scale-up to make methanol an economically viable alternative transportation
fuel.
In recent years, a growing interest has been observed
in the application of methanol as an alternative liquid
fuel, which can be used directly for powering Otto
engines or fuel cells [38]. Biomass and coal can be
considered as a potential fuel for gasification and
further syngas production and methanol synthesis [32,
33]. The feasibility of achieving the conversion has
been demonstrated in a large scale system in which a
product gas is initially produced by pyrolysis and
gasification of a carbonaceous matter. Syngas from
biomass is altered by catalyst under high pressure and
temperature to form methanol. This method will
produce 100 gallons of methanol per ton of feed
material [38] Table 5.
2.1.2. Ethanol production methods
Ethanol, also known as ‘grain alcohol’, not
commonly used in making BD because of its low
reactivity than methanol. Ethanol is an alcohol-based
fuel produced by fermenting sugars from crop starches.
Currently, ethanol is generally produced from corn
kernels in USA. In this process, kernels are ground to a
fine powder, and all of it is cooked to liquefy it, without
removing the germ or fiber.
Ethanol has been used in Germany and France as
early as 1894 by the then incipient industry of ICEs.
Brazil has utilized ethanol as a fuel since 1925.
Currently, ethanol is produced from sugar beets and
from molasses in Brazil. A typical yield is 72.5 liter of
ethanol per ton of sugar cane. Modern crops yield
60 ton of sugar cane per hector of land. Production of
ethanol from biomass is one way to reduce both the
consumption of crude oil and environmental pollution
[39]. The use of gasohol (ethanol and gasoline mixture)
as an alternative motor fuel has been steadily increasing
in the world for a number of reasons. Domestic
production and use of ethanol for fuel can decrease
dependence on foreign oil, reduce trade deficits, create
Table 5
Composition of syngas from biomass for producing methanol (% by
volume)
Gases
CO
H2
CO2
CH4
N2
Ar
H2
CO
CH4
CO2
C2H4
H2 O
N2
Percentage
45.3
34.4
15.8
1.9
1.9
0.6
32–41
21–29
10–15
14–19
0.8–1.2
5.5–6.5
0.6–1.2
Source: Ref. [30].
Source: Ref. [7].
A. Demirbas / Progress in Energy and Combustion Science 31 (2005) 466–487
jobs in rural areas, reduce air pollution, and reduce
global climate change carbon dioxide buildup [40].
2.2. Hydrogen production methods
Hydrogen can be produced by several methods. The
predominant method for producing syngas is steam
reforming of natural gas, although other hydrocarbons
can be used as feedstocks. Approximately 95% of the
hydrogen is produced from fossil fuels conversion, such
as natural gas reforming.
Steam reforming of natural gas is an endothermic,
catalytic process carried out at about 1125 K and
around 2.5 MPa according to the following reactions:
CH4 C H2 O/ CO C 3H2
(2)
CO C H2 O/ CO2 C H2
(3)
Syngas or artificial water gas (COCH2) from coal
can be reformed to hydrogen. Hydrogen and oxygen
concentrations in coal increase as coal rank goes down.
The water vapor (steam) can be further shifted to
hydrogen by establishing conditions to drive the
reaction to produce additional hydrogen:
Coal C H2 O/ CO C H2
(4)
Near-term production of renewable hydrogen from
biomass requires a co-product strategy to compete with
conventional production of hydrogen from the steam
reforming of natural gas. The processing of pyrolysis
co-products from the production of activated carbon is
one possible path to demonstrate such a strategy.
Renewable hydrogen has the potential of being cost
effective and is environmentally friendly. The production of renewable hydrogen from biomass as a
renewable resource requires a co-product strategy to
compete with conventional production of hydrogen
from the steam reforming of natural gas. The process of
biomass to activated carbon is an alternative route to
hydrogen with a valuable co-product that is practiced
commercially [41]. Currently Czernik et al. [42] have
developed a method for producing hydrogen from
biomass and concluded that a co-products strategy
could compete with the cost of the commercial natural
gas-based technologies [43,44]. The yield of hydrogen
that can be produced from biomass is relatively low,
16–18% based on dry biomass weight [45]. Only the
carbohydrate-derived bio-oil fraction produced from
biomass undergoes reforming. The strategy is based on
producing hydrogen from biomass pyrolysis using a coproduct strategy to reduce the cost of hydrogen. The
process of biomass to activated carbon is an alternative
473
Table 6
List of some biomass material used for hydrogen production
Biomass species
Main conversion process
Bio-nut shell
Olive husk
Tea waste
Crop straw
Black liquor
Municipal solid waste
Crop grain residue
Pulp and paper waste
Petroleum basis plastic waste
Manure slurry
Steam gasification
Pyrolysis
Pyrolysis
Pyrolysis
Steam gasification
Supercritical water extraction
Supercritical fluid extraction
Microbiol fermentation
Supercritical fluid extraction
Microbiol fermentation
route to hydrogen with a valuable co-product that is
practiced commercially. The list of some biomass
material used for hydrogen production is given in
Table 6.
The first intermediate temperatures between 600 and
850 K pyrolysis carries out through the formation
mainly acetic acid, which partly forms H2, CO2 and CO
and a small amount of methane. The second intermediate temperatures between 900 and 1100 K pyrolysis
carries out through the formation mainly propionic
acid, which partly forms H2, CO2 and CO and a small
amount of ethylene, which partly reacts with hydrogen
to form ethane [46]. The high temperature pyrolysis
carries out via the formation of unstable free radicals
which react with water to form H2 and CO2 in
approximately a molar ratio of 2 mol of H2 per mol
of CO2 formed.
The gas products of H2, CO2, CO, CH4 and C2H6 are
formed by the secondary pyrolytic gasification
reactions, they will continue to react according to two
reversible tertiary reactions. The first is the water-gas
shift reaction [43]:
CO C H2 O$ CO2 C H2
(5)
and the second is the CH4 formation reaction:
CO C 3H2 O$ CH4 C H2 O
(6)
Hydrogen can be produced from biomass via two
thermochemical processes: (a) gasification followed by
reforming of the syngas, and (b) fast pyrolysis followed
by reforming of the carbohydrate fraction of the bio-oil.
In each process, water-gas shift is used to convert the
reformed gas into hydrogen, and pressure swing
adsorption is used to purify the product. Comparison
with other biomass thermochemical gasification such as
air gasification and/or steam gasification, the supercritical water gasification can directly deal with the wet
biomass without drying, and have high gasification
efficiency in lower temperature. The cost of hydrogen
474
A. Demirbas / Progress in Energy and Combustion Science 31 (2005) 466–487
Yield of hydrogen, vol.%
Supercritical fluid extraction
50
45
40
35
30
25
20
15
10
600
700
800
Conventional pyrolysis
900
1000
Temperature, K
1100
Steam gasification
1200
Fig. 3. Plots for yield of hydrogen from supercritical fluid (water)
extraction, pyrolysis and steam gasification [(W/S)Z2] of beech wood
at different temperatures. Source: Ref. [48].
production from supercritical water gasification of wet
biomass was several times higher than the current price
of hydrogen from steam methane reforming. Biomass
was gasified in supercritical water at a series of
temperature and pressure during different resident
times to form a product gas composed of H2, CO2,
CO, CH4, and a small amount of C2H4 and C2H6 [47].
Fig. 3 shows the curves for yield of hydrogen from
supercritical fluid extraction (SFE), pyrolysis and steam
gasification [(W/S)Z2] of beech wood at different
temperatures. Distilled water was used in the SFE (the
critical temperature of pure water is 647.7 K). As seen
from Fig. 3, the yield of hydrogen from SFE was
considerably high (49%) at lower temperatures. The
pyrolysis was carried out at the moderate temperatures
and steam gasification at the highest temperatures [48].
The oily liquid fraction from pyrolysis consisted of
two phases: an aqueous phase containing a wide variety
of organo-oxygen compounds of low molecular weight
and a non-aqueous tarry phase containing insoluble
organics of high molecular weight. Tar a viscous black
fluid that is a byproduct of the pyrolysis of woody
biomass. The chief constituents of tar are pyrocatechol,
phenol, guaiacol, cresol, creosol, methyl-creosol,
phlorol, toluene, xylene, naphthalene, and other
hydrocarbons.
3. Transesterification of vegetable oils and fats
The transesterification reaction proceeds with catalyst or without any catalyst by using primary or
secondary monohydric aliphatic alcohols having 1–8
carbon atoms as follows:
Triglycerides C Monohydric alcohol% Glycerin
C Mono- alkyl esters
(7)
Transesterification means taking a triglyceride
molecule or a complex fatty acid, neutralizing the
free fatty acids, removing the glycerin, and creating an
alcohol ester. The reaction is shown in Eq. (7).
Theoretically, transesterification reaction is an equilibrium reaction. In this reaction, however, more amount
of methanol was used to shift the reaction equilibrium
to the right side and produce more methyl esters as the
proposed product. A catalyst is usually used to improve
the reaction rate and yield.
Alcohols are primary or secondary monohydric
aliphatic alcohols having 1–8 carbon atoms Amongs
the alcohols that can be used in the transesterification
reaction are methanol, ethanol, propanol, butanol and
amyl alcohol. Methanol and ethanol are used most
frequently, ethanol is a preferred alcohol in the
transesterification process compared to methanol
because it is derived from agricultural products and is
renewable and biologically less objectionable in the
environment. However methanol is preferable because
of its low cost and its physical and chemical advantages
(polar and shortest chain alcohol). The transesterification reaction can be catalyzed by alkalis [10,49], acids
[50], or enzymes [51–55].
Several alcoholysis catalysts, known to be effective
for reactions between simple alcohols and soybean oil,
were evaluated and found to be ineffective toward
alcoholysis of ethylene glycol with soybean oil under
traditional reaction conditions. An initial survey of
alternative catalysts revealed that organometallic tin
complexes were effective but unsatisfactory due to
toxicity and difficulty in recovering the catalyst.
Satisfactory performance for several alcoholysis
reactions was achieved with calcium carbonate catalysts even though at higher temperatures, typically
greater than 475 K [56].
The physical properties of the primary chemical
products of transesterification are given in Tables 7 and
8 [11,57,58].
In the conventional transesterification of animal fats
and vegetable oils for biodiesel production, free fatty
acids and water always produce negative effects, since
the presence of free fatty acids and water causes soap
formation, consumes catalyst and reduces catalyst
effectiveness, all of which resulting in a low conversion
[59].
3.1. Catalytic transesterification method
The catalyst is dissolved into methanol by vigorous
stirring in a small reactor. The oil is transferred into the
BD reactor and then the catalyst/alcohol mixture is
A. Demirbas / Progress in Energy and Combustion Science 31 (2005) 466–487
475
Table 7
Physical properties of chemicals related to transesterification
Table 9
Critical temperatures and critical pressures of various alcohols
Name
Specific
gravity
(g/ml)
Melting
point
(K)
Boiling
point
(K)
Solubility
(!10%)
Alcohol
Critical temperature
(K)
Critical pressure
(MPa)
Methyl
myristate
Methyl
palmitate
Methyl
stearate
Methyl
oleate
Methanol
0.875
291.0
–
–
0.825
303.8
469.2
512.2
516.2
537.2
560.2
8.1
6.4
5.1
4.9
0.850
311.2
488.2
0.875
253.4
463.2
Benzene,
EtOH, Et2O
Et2O,
chloroform
EtOH, Et2O
Methanol
Ethanol
1-Propanol
1-Butanol
0.792
176.2
337.9
Ethanol
Glycerol
0.789
1.260
161.2
255.3
351.6
563.2
H2O, ether,
EtOH
H2O, ether
H2O, ether
Source: Refs. [7,57].
pumped into the oil. The final mixture is stirred
vigorously for 2 h at 340 K in ambient pressure. A
successful transesterification reaction produces two
liquid phases: ester and crude glycerol. Crude glycerol,
the heavier liquid, is collected at the bottom after
several hours of settling. Phase separation can be
observed within 10 min and can be complete within 2 h
of settling. Complete settling can take as long as 20 h.
After settling is complete, water is added at the rate of
5.5% by volume of methyl ester of oil and then stirred
for 5 min and the glycerin is allowed to settle again.
Washing the ester is a two-step process, which is
carried out with extreme care. A water wash solution at
the rate of 28% by volume of oil and g of tannic
acid/liter of water is added to the ester and gently
agitated. Air is carefully introduced into the aqueous
layer while simultaneously stirring very gently. This
process is continued until the ester layer becomes clear.
After settling, the aqueous solution is drained and water
alone is added at 28% by volume of oil for the final
washing [2,7,11].
3.2. Supercritical methanol transesterification method
[7]
The transesterfication of triglycerides by supercritical methanol (SCM), ethanol, propanol and butanol, has
proved to be the most promising process. Table 9 shows
critical temperatures and critical pressures of various
alcohols. A non-catalytic BD production route with
supercritical methanol has been developed that allows a
simple process and high yield because of simultaneous
transesterification of triglycerides and methyl esterification of fatty acids [9]. Because of having similar
properties to PD, BD, a transesterified product of
vegetable oil, is considered as the most promising one
for diesel fuel substitute. A reaction mechanism of
vegetable oil in SCM was proposed based on the
mechanism developed by Krammer and Vogel [60] for
the hydrolysis of esters in sub/supercritical water. The
basic idea of supercritical treatment is a relationship
between pressure and temperature upon thermophysical
properties of the solvent such as dielectric constant,
viscosity, specific weight, and polarity [61]. The
transesterification of sunflower oil was investigated in
SCM and supercritical ethanol at various temperatures
(475–675 K) [62]. Fig. 4 shows a SCM transesterification system.
The most important variables affecting the methyl
ester yield during transesterification reaction are molar
Table 8
Melting points of fatty acids, methyl esters and mono-, di-, and
triglycerides (K)
Name
Fatty
acid
Methyl
ester
1-Monoglyceride
1,3-Diglyceride
Triglyceride
Myristic
Palmitic
Stearic
Oleic
Linoleic
327.6
292.0
343.7
340.0
330.2
336.1
303.8
350.2
349.5
336.7
342.8
312.3
254.7
352.6
346.3
289.5
253.4
308.4
294.7
278.7
266.7
238.2
285.5
270.6
260.1
Source: Refs. [7,58].
Fig. 4. Supercritical methanol transesterification system. (1)
Autoclave; (2) electrical furnace; (3) temperature control monitor;
(4) pressure control monitor; (5) product exit valve; (6) condenser; (7)
product collecting vessel. Source: Ref. [2].
476
A. Demirbas / Progress in Energy and Combustion Science 31 (2005) 466–487
100
Yield of methyl ester, wt%
Yield of methyl ester, wt%
100
80
60
450 K
40
493 K
503 K
20
513 K
90
80
1.0:1.0
70
1.0:3.0
60
1.0:9.0
50
1.0:20
40
1.0:41
30
20
10
523 K
0
0
0
50
100
150
200
250
300
350
Reaction time (sec)
Fig. 5. Changes in yield percentage of methyl esters as treated with
subcritical and supercritical methanol at different temperatures as a
function of reaction time. Molar ratio of vegetable oil to methyl
alcohol: 1:41. Sample: hazelnut kernel oil. Source: Ref. [7].
ratio of alcohol to vegetable oil and reaction temperature. Viscosities of the methyl esters from the vegetable
oils were slightly higher than that of no. 2 diesel fuel.
Fig. 5 shows a typical example of the relationship
between the reaction time and the temperature [7].
The variables affecting the ester yield during
transesterification reaction are molar ratio of alcohol
to vegetable oil, reaction temperature, reaction time,
water content and catalyst. It was observed that
increasing the reaction temperature, especially to
supercritical temperatures, had a favorable influence
on ester conversion [7].
In the transesterification process, the vegetable oil
should have an acid value less than one and all
materials should be substantially anhydrous. If the acid
value is greater than one, more NaOH or KOH is
injected to neutralize the free fatty acids. Water can
cause soap formation and frothing. The resulting soaps
can induce an increase in viscosity, formation of gels
and foams, and made the separation of glycerol difficult
[11,63].
The stoichiometric ratio for transesterification
reaction requires 3 mol of alcohol and 1 mol of
triglyceride to yield 3 mol of fatty acid ester and
1 mol of glycerol. Higher molar ratios result in greater
ester production in a shorter time. The vegetable
oils are transesterified 1:6–1:40 vegetable oil-alcohol
molar ratios in catalytic and supercritical alcohol
conditions [7].
Fig. 6 shows the effect of the molar ratio of
vegetable oil to methanol on the yield of methyl ester.
–50
50
150
250
350
Reaction time (sec)
Fig. 6. Effect of molar ratio of vegetable oil to methanol on yield
of methyl ester. Temperature: 513 K, sample: methylester from
cottonseed oil. Source: Ref. [7].
As seen in Fig. 6, the cottonseed oil can be
transesterified at 1:1, 1:3, 1:9, 1:20 and 1:40 vegetable
oil-methanol molar ratios in subcritical and SCM
conditions [7].
In the supercritical alcohol transesterification
method, the yield of conversion raises 50–95% for
the first 10 min. Fig. 7 shows the plots for changes
in fatty acids alkyl esters conversion from triglycerides as treated in supercritical alcohols at 575 K
[64].
Water content is an important factor in the
conventional catalytic transesterification of vegetable
oil. In the conventional transesterification of fats and
vegetable oils for BD production, free fatty acids and
water always produce negative effects since the
presence of free fatty acids and water causes soap
formation, consumes catalyst and reduces catalyst
effectiveness. In catalyzed methods, the presence of
water has negative effects on the yields of methyl
esters. However, the presence of water affected
positively the formation of methyl esters in our SCM
method. Fig. 7 shows the plots for yields of methyl
esters as a function of free fatty acid content in BD
production [64].
Transesterification reaction of rapeseed oil in
SCM has been investigated without using any
catalyst. In addition, it was found that this new
SCM process requires the shorter reaction time and
simpler purification procedure because of the unused
catalyst [2,7]. Transesterification can occur at
different temperatures and the temperature influence
the reaction rate and yield of esters, depending on
the oil used. It was observed that increasing reaction
A. Demirbas / Progress in Energy and Combustion Science 31 (2005) 466–487
Fatty acid alkyl ester, %
100
80
60
Methanol
Ethanol
40
1-Propanol
1-Butanol
20
0
1-Octanol
0
10
20
30
Reaction time, min
40
50
Fig. 7. Plots for changes in fatty acids alkyl esters conversion from
triglycerides as treated in supercritical alcohol at 575 K. Source: Ref.
[64].
temperature, especially supercritical temperatures had
a favorable influence on ester conversion [7].
3.3. Recovery of glycerol
The standards make sure that the following
important factors in the BD fuel production process
by transesterification are satisfied: (a) complete
transesterification reaction, (b) removal of catalyst,
(c) removal of alcohol, (d) removal of glycerol, and
(e) complete esterification of free fatty acids. The
following transesterification procedure is for the
methyl ester production. The catalyst is dissolved
into the alcohol by vigerous stirring in a small
reactor. The oil is transferred into the BD reactor
and then the catalyst/methanol mixture is pumped
into the oil and final mixture stirred vigorously for
2 h. A successful reaction produces two liquid
phases: ester and crude glycerol. The entire mixture
then settles and glycerol is left on the bottom and
methyl esters (BD) is left on top. Crude glycerol, the
heavier liquid will collect at the bottom after several
hours of settling. Phase separation can be observed
within 10 min and can be complete within 2 h after
stirring has stopped. Complete settling can be taken
as long as 18 h. After settling is complete, water was
added at the rate of 5.0% by volume of the oil and
then stirred for 5 min and the glycerol allowed
settling again. After settling is complete the glycerol
is drained and the ester layer remains [8].
The recovery of high quality glycerol as a BD byproduct is primary options to be considered to lower
the cost of BD. With neutralizing the free fatty
acids, removing the glycerol, and creating an alcohol
477
ester transesterification occurs. This is accomplished
by mixing methanol with sodium hydroxide to make
sodium methoxide. This dangerous liquid is then
mixed into vegetable oil. Washing the methyl ester
is a two step process which is carried out with
extreme care.. This procedure is continued until the
methyl ester layer becomes clear. After settling, the
aqueous solution is drained and water alone is added
at 28% by volume of oil for the final washing. The
resulting BD fuel when used directly in a diesel
engine will burn up to 75% cleaner than no. 2 PD
fuel [8].
3.4. Reaction mechanism of transesterification
Transesterification consists of a number of
consecutive, reversible reactions [65,66]. The triglyceride is converted stepwise to diglyceride, monoglyceride and finally glycerol (Eqs. 8–11). The
formation of alkyl esters from monoglycerides is
believed as a step which determines the reaction
rate, since monoglycerides are the most stable
intermediate compound [11].
Fatty acid ðR1 COOHÞ C Alcohol ðROHÞ
$ Ester ðR1 COORÞ C Water ðH2 OÞ
Triglyceride C ROH$ Diglyceride C RCOOR1
(8)
(9)
Diglyceride C ROH$ Monoglyceride
C RCOOR2
(10)
Monoglyceride C ROH$ Glycerol
C RCOOR3
(11)
Several aspects, including the type of catalyst
(alkaline, acid or enzyme), alcohol/vegetable oil
molar ratio, temperature, water content and free
fatty acid content have an influence on the course of
the transesterification. In the transesterification of
vegetable oils and fats for biodiesel production, free
fatty acids and water always produce negative
effects, since the presence of free fatty acids and
water causes soap formation, consumes catalyst and
reduces catalyst effectiveness, all of which result in a
low conversion [67]. When the original ester is
reacted with an alcohol, the transesterification
process is called alcoholysis [68]. The transesterification is an equilibrium reaction and the transformation occurs essentially by mixing the reactants.
478
A. Demirbas / Progress in Energy and Combustion Science 31 (2005) 466–487
In the transesterification of vegetable oils, a
triglyceride reacts with an alcohol in the presence
of a strong acid or base, producing a mixture of fatty
acids alkyl esters and glycerol. The stoichiometric
reaction requires 1 mol of a triglyceride and 3 mol of
the alcohol. However, an excess of the alcohol is
used to increase the yields of the alkyl esters and to
allow its phase separation from the glycerol formed
[8].
vegetable oils were carried out in several studies [50,
69]. Solid superacid catalysts of sulfated tin and
zirconium oxides and tungstated zirconia were used
in the transesterification of soybean oil with methanol
at 475–575 K and the esterification of n-octanoic acid
with methanol at 450–475 K. Tungstated zirconia–
alumina is a promising catalyst for the production of
biodiesel fuels because of its activity for the transesterification as well as the esterification [69].
3.4.1. Acid-catalyzed processes
The transesterification process is catalyzed by acids,
preferably by sulfonic and sulfuric acids. These
catalysts give very high yields in alkyl esters, but the
reactions are slow. The alcohol/vegetable oil molar
ratio is one of the main factors that influence the
transesterification. An excess of alcohol favors the
formation of the products. On the other hand, an
excessive amount of alcohol makes the recovery of the
glycerol difficult, so that the ideal alcohol/oil ratio has
to be established empirically, considering each individual process. The protonation of the carbonyl group
of the ester leads to the carbocation which, after a
nucleophilic attack of the alcohol, produces the
tetrahedral intermediate, which eliminates glycerol to
form the new ester, and to regenerate the catalyst HC.
According to this mechanism, carboxylic acids can be
formed by reaction of the carbocation with water
present in the reaction mixture and acid-catalyzed
transesterification should be carried out in the absence
of water [8,68]. Acid catalytic transesterification of
3.4.2. Alkali-catalyzed processes
The reaction mechanism for alkali-catalyzed transesterification was formulated as three steps [70,71]. The
alkali-catalyzed transesterification of vegetable oils
proceeds faster than the acid-catalyzed reaction. The
mechanism of the base-catalyzed transesterification of
vegetable oils is shown in Fig. 8. The first step is the
reaction of the base with the alcohol, producing an
alkoxide and the protonated catalyst. The nucleophilic
attack of the alkoxide at the carbonyl group of the
triglyceride generates a tetrahedral intermediate, from
which the alkyl ester and the corresponding anion of the
diglyceride are formed. The latter deprotonates the
catalyst can react with a second molecule of alcohol
and starts another catalytic cycle. Diglycerides and
monoglycerides are converted by the same mechanism
to a mixture of alkyl esters and glycerol. Alkaline metal
alkoxides (CH3ONa) are the most active catalysts, since
they give very high yields (O98%) in short reaction
times (30 min) even if they are applied at low molar
concentrations (0.5 mol%). The presence of water gives
Fig. 8. Mechanism of the alkali-catalyzed transesterification of vegetable oils (B: base).
A. Demirbas / Progress in Energy and Combustion Science 31 (2005) 466–487
Supercritical methanol
Alkaline catalyst
Acid catalyst
100
Methyl ester, %
rise to hydrolysis of some of the produced ester, with
consequent soap formation. Potassium carbonate, used
in a concentration of 2 or 3 mol% gives high yields of
fatty acid alkyl esters and reduces the soap formation.
This can be explained by the formation of bicarbonate
instead of water (Fig. 8), which does not hydrolyse the
esters [8,68].
479
80
60
40
20
3.4.4. Non-catalytic supercritical alcohol
transesterification
BD, an alternative diesel fuel, is made from
renewable biological sources such as vegetable oils
and animal fats by non-catalytic supercritical alcohol
transesterification methods [2]. A non-catalytic BD
production route with supercritical methanol has been
developed that allows a simple process and high yield
because of simultaneous transesterification of triglycerides and methyl esterification of fatty acids [7].
The parameters affecting the methyl esters formation
are reaction temperature, pressure, molar ratio, water
content and free fatty acid content. It is evident that at
subcritical state of alcohol, reaction rate is very low and
gradually increases as either pressure or temperature
rises. It was observed that increasing the reaction
temperature, especially to supercritical conditions, had
a favorable influence on the yield of ester conversion
[2,7]. The yield of alkyl ester increased with increasing
the molar ratio of oil to alcohol [6]. In the supercritical
alcohol transesterification method, the yield of conversion raises 50–95% for the first 10 min.
Water content is an important factor in the
conventional catalytic transesterification of vegetable
oil. In the conventional transesterification of fats and
vegetable oils for biodiesel production, free fatty acids
and water always produce negative effects since the
presence of free fatty acids and water causes soap
formation consumes catalyst and reduces catalyst
effectiveness. In catalyzed methods, the presence of
water has negative effects on the yields of methyl
esters. However, the presence of water affected
0
0
1
2
3
4
5
Water content, %
Fig. 9. Plots for yields of methyl esters as a function of water content
in transesterification of triglycerides.
positively the formation of methyl esters in our
supercritical methanol method. Fig. 9 shows the plots
for yields of methyl esters as a function of water content
in transesterification of triglycerides. Fig. 10 shows the
plots for yields of methyl esters as a function of free
fatty acid content in BD production [67]. Comparisons
between catalytic commercial methanol process and
supercritical methanol (SCM) method for BD from
vegetable oils by transesterification are given in
Table 10.
4. Fuel properties of vegetable oils and biodiesels
(BDs)
Vegetable oils can be used as fuel for combustion
engines, but its viscosity is much higher than usual
diesel fuel and requires modifications of the engines.
The major problem associated with the use of pure
vegetable oils as fuels, for diesel engines are caused by
high fuel viscosity in compression ignition. Therefore,
vegetable oils are converted into their methyl esters
(BDs) by transesterification.
Viscosity is a measure of the internal friction or
resistance of an oil to flow. As the temperature of oil is
Supercritical methanol
alkaline catalyst
Acid catalyst
100
Methyl ester, %
3.4.3. Enzyme-catalyzed processes
Although the enzyme-catalyzed transesterification
processes are not yet commercially developed, new
results have been reported in a recent article [68]. The
common aspects of these studies consist in optimizing
the reaction conditions (solvent, temperature, pH, type
of microorganism which generates the enzyme, etc) in
order to establish suitable characteristics for an
industrial application. However, the reaction yields as
well as the reaction times are still unfavorable
compared to the base-catalyzed reaction systems [8].
80
60
40
20
0
0
10
20
Free fatty acid content, %
30
Fig. 10. Plots for yields of methyl esters as a function of free fatty acid
content.
480
A. Demirbas / Progress in Energy and Combustion Science 31 (2005) 466–487
Table 10
Comparisons between catalytic methanol (MeOH) process and
supercritical methanol (SCM) method for biodiesel from vegetable
oils by transesterification
Methylating agent
Catalyst
Reaction
temperature (K)
Reaction pressure
(MPa)
Reaction time (min)
Methyl ester yield
(wt%)
Removal for
purification
Free fatty acids
Catalytic MeOH
process
SCM method
Methanol
Alkali
303–338
Methanol
None
523–573
0.1
10–25
60–360
96
7–15
98
Methanol, catalyst,
glycerol, soaps
Saponified products
Methanol
Methyl esters, water
increased, its viscosity decreases and it is therefore able
to flow more readily. Viscosity is measured on several
different scales, including Redwood no. 1 at 100F,
Engler Degrees, Saybolt Seconds, etc. Viscosity is the
most important property of biodiesel since it affects the
operation of fuel injection equipment, particularly at
low temperatures when the increase in viscosity affects
the fluidity of the fuel. BD has viscosity close to diesel
fuels. High viscosity leads to poorer atomization of the
fuel spray and less accurate operation of the fuel
injectors. A novel process of BD fuel production has
been developed by a non-catalytic supercritical methanol method.
Viscosity, density and flash point measurements of
eight oil methyl esters are given in Table 11. Compared
to no. 2 diesel fuel, all of the vegetable oils were much
more viscous. Viscosity, density and flash point
measurements of ten vegetable oils given by Goering
et al. [4] are shown in Table 11. The density values of
vegetable oils are between 902.6 and 923.6 kg/m3 while
Table 11
Viscosity, density and flash point measurements of eight oil methyl
esters
Methyl ester
Viscosity
(mm2/s
(at 313 K))
Density
(kg/m3
(at 288 K))
Flash point
(K)
Cottonseed oil
Hazelnut kernel oil
Mustard oil
Palm oil
Rapeseed oil
Safflower oil
Soybean oil
Sunflower oil
3.69
3.59
4.10
3.70
4.63
4.03
4.08
4.22
880
860
881
870
885
880
885
880
437
401
446
443
428
453
447
443
Source: Ref. [72].
Table 12
Viscosity, density and flash point measurements of 10 vegetable oils
Oil source
Viscosity
(mm2/s (at
311 K))
Density
(kg/m3)
Flash point
(K)
Corn
Cottonseed
Crambe
Linseed
Peanut
Rapeseed
Safflower
Sesame
Soybean
Sunflower
34.9
33.5
53.6
27.2
39.6
37.0
31.3
35.5
32.6
33.9
909.5
914.8
904.4
923.6
902.6
911.5
914.4
913.3
913.8
916.1
550
509
447
514
544
519
533
533
527
447
The biodiesel (BD) was characterized by determining its density,
viscosity, high heating value, cetane number, cloud and pour points,
characteristics of distillation, and flash and combustion points
according to ISO norms. Source: Ref. [4].
those of vegetable oil methyl esters are between 860
and 885 kg/m3 (Table 12). The density values of
vegetable oil methyl esters considerably decreases via
transesterification process. The viscosity values of
vegetable oils are between 27.2 and 53.6 mm2/s
whereas those of vegetable oil methyl esters are
between 3.59 and 4.63 mm2/s. The viscosity values of
vegetable oil methyl esters highly decreases after
transesterification process. Compared to no. 2 PD
fuel, all of the vegetable oil methyl esters were slightly
viscous. The flash point values of vegetable oil methyl
esters are highly lower than those of vegetable oils
(Tables 11 and 12).
Density is another important property of BD. It is the
weight of a unit volume of fluid. Specific gravity is the
ratio of the density of a liquid to the density of water.
Specific gravity of BD fuels ranges between 0.87 and
0.89 kg/m3 (Table 11). Fuel injection equipment
operates on a volume metering system, hence a higher
density for BD results in the delivery of a slightly
greater mass of fuel.
Cetane number (CN) is a measure of ignition quality
of diesel fuel. The higher the CN, the easier the fuel
ignites when it is injected into the engine. The higher
the CN is the more fuel-efficient the fuel. BD has a
higher CN than petrol diesel because of its higher
oxygen content. This means that engines run smoother
and create less noise when running on BD. The CN is
based on two compounds, namely hexadecane with a
CN of 100 and heptamethylnonane with a CN of 15.
The CN scale also shows that straight-chain, saturated
hydrocarbons have higher CN compared to branchedchain or aromatic compounds of similar molecular
weight and number of carbon atoms. The CN of
A. Demirbas / Progress in Energy and Combustion Science 31 (2005) 466–487
Table 13
Comparisons of some fuel properties of vegetable oils and their esters
with diesel fuel
Fuel type
Calorific
value
(MJ/kg)
Density
(kg/m3)
No. 2 diesel
fuel
Sunflower oil
Sunflower
methyl ester
Cottonseed oil
Cottonseed
methyl ester
Soybean oil
Soybean
methyl ester
Corn oil
Opium poppy
oil
Rapeseed oil
43.4
815
4.3
47.0
39.5
40.6
918
878
58.5
10.3
37.1
45.5
39.6
40.6
912
874
50.1
11.1
48.1
45.5
39.6
39.8
914
872
65.4
11.1
38.0
37.0
37.8
38.9
915
921
46.3
56.1
37.6
–
37.6
914
39.2
37.6
Viscosity
at 300 K
(mm2 /s)
Cetane
numbera
Table 14
Fuel properties of methyl ester biodiesels
Source
Viscosity
(g/mL at
288.7 K)
Density
(cSt at 313.
2 K)
Cetane
number
Reference
no.
Sunflower
Soybean
Palm
Peanut
Babassu
Tallow
4.6
4.1
5.7
4.9
3.6
4.1
0.880
0.884
0.880
0.876
–
0.877
49
46
62
54
63
58
[73]
[73]
[74]
[75]
[75]
[76]
The BD was characterized by determining its
density, viscosity, high heating value, cetane number,
cloud and pour points, characteristics of distillation,
and flash and combustion points according to ISO
norms [8]. The higher heating values of the BD fuels,
on a mass basis, are 9–13% lower than no. 2 diesel fuel.
The cloud and pour points of no. 2 diesel fuel are
significantly lower than the BD fuels. The BD fuels
produced slightly lower power and torque and higher
fuel consumption than no. 2 diesel fuel. The properties
of BD are close to no. 2 diesel fuels. Some fuel
properties of methyl ester BDs are presented in
Table 13.
Two important parameters for low temperature
applications of a fuel are Cloud Point (CP) and Pour
Point (PP). The CP is the temperature at which wax first
becomes visible when the fuel is cooled. The PP is
the temperature at which the amount of wax out of
solution is sufficient to gel the fuel, thus it is the
lowest temperature at which the fuel can flow. BD
has higher CP and PP compared to conventional diesel
[77].
55
50
Viscosity, cSt
biodiesel is generally higher than conventional PD. The
CN is one of the prime indicators of the quality of diesel
fuel. It relates to the ignition delay time of a fuel upon
injection into the combustion chamber. The CN is a
measure of ignition quality of diesel fuels and high CN
implies short ignition delay. The longer the fatty acid
carbon chains and the more saturated the molecules, the
higher the CN. The CN of BD from animal fats is higher
than those of vegetable oils. Comparisons of some fuel
properties of vegetable oils and their esters with diesel
fuel are given in Table 13. Table 14 shows some fuel
properties of six methyl ester BDs given in literature.
Relationships between density and viscosity of
vegetable oils are depicted in Fig. 11. These figures
were plotted using the values in Table 12 given by
Goering et al. [4]. As seen Fig. 11, an increase in
density from 902.6 to 923.6 kg/m3 for vegetable oils
decreases the viscosity from 53.6 to 27.2 mm2/s and the
decreases are considerably regular (coefficient of
regression (r) is 0.7942).
Relationships between density and viscosity of
vegetable oil methyl esters are depicted in Fig. 12.
The Fig. 12 was plotted using the measured values [72].
As seen Fig. 12, an increase in density from 860 to
885 kg/m3 for vegetable oil methyl esters or BDs
increases the viscosity from 3.59 to 4.63 mm2/s. There
is high regression between density and viscosity values
vegetable oil methyl esters. The relationships between
viscosity and flash point for vegetable oil methyl esters
are irregular.
481
45
40
35
30
Source: Ref. [8].
a
Cetane number (CN) is a measure of ignition quality of diesel fuel.
25
900
905
910
915
920
925
Density, g/L
Fig. 11. Relationships between density and viscosity for vegetable
oils.
482
A. Demirbas / Progress in Energy and Combustion Science 31 (2005) 466–487
4.2. Comparison of fuel properties and combustion
characteristics of methyl and ethyl alcohols
and their esters
4.7
Viscosity, cSt
4.5
4.3
4.1
3.9
3.7
3.5
850
860
870
Density, g/L
880
890
Fig. 12. Relationships between density and viscosity for vegetable oil
methyl esters.
Previous studies on the effects of BDs on PD fuel
lubricity have shown an increase in lubricity associated
with the addition of BDs [78,79].
4.1. Emissions from biodiesel combustion
BDs have generally been found to be nontoxic and
are biodegradable, which may promote their use in
applications where biodegradability is desired. Neat
BD and BD blends reduce particulate matter (PM),
hydrocarbons (HC) and carbon monoxide (CO)
emissions and increase nitrogen oxides (NOx) emissions compared with diesel fuel used in an unmodified
diesel engine [80]. The emission impacts of 20 vol.%
BD for soybean-based BD added to an average base PD
is given in Table 15.
Results indicate that the transformities of biofuels
are greater than those of fossil fuels, thus showing that a
larger amount of resources is required to get the
environmental friendly product. This can be explained
by the fact that natural processes are more efficient than
industrial ones. On the other hand, the time involved in
the formation of the fossil fuels is considerably
different from that required for the production of the
biomass [81]. Coconut BD can yield reductions of
80.8–109.3% in net CO2 emissions relative to PD [82].
Table 15
Emission impacts of 20 vol.% BD for soybean-based BD added to an
average base PD
Percent change in emissions
NOx (nitrogen oxides)
PM (particular matter)
HC (hydrocarbons)
CO (carbon monoxide)
Source: Ref. [80].
C2.0
K10.1
K21.1
K11.0
Ethanol is an environmentally benign fuel. The
systematic effect of ethyl alcohol differs from that of
methyl alcohol. Ethyl alcohol is rapidly oxidized in the
body to carbon dioxide and water, and in contrast to
methyl alcohol no cumulative effect occurs. Ethanol is
also a preferred alcohol in the transesterification process
compared to methanol because it is derived from
agricultural products and is renewable and biologically
less objectionable in the environment. Methanol has a
higher octane rating than gasoline. Methanol has high
heat of vaporization that results in lower peak flame
temperatures than gasoline and lower nitrogen oxide
emissions. Its greater tolerance to lean combustion
higher air-to-fuel equivalence ratio results in generally
lower overall emissions and higher energy efficiency.
However, several disadvantages must be studied and
overcome before neat methanol is considered a viable
alternative to gasoline. The energy density of methanol
is about half that of gasoline, reducing the range a
vehicle can travel on an equivalent tank of fuel [8].
In general, the physical and chemical properties and the
performance of ethyl esters are comparable to those of the
methyl esters. Methyl and ethyl esters have almost the
same heat content. The viscosities of the ethyl esters are
slightly higher and the cloud and pour points are slightly
lower than those of the methyl esters. Engine tests
demonstrated that methyl esters produced slightly higher
power and torque than ethyl esters [23]. Some desirable
attributes of the ethyl esters over methyl esters are:
significantly lower smoke opacity, lower exhaust temperatures, and lower pour point. The ethyl esters tended to
have more injector coking than the methyl esters.
There are some important differences in the
combustion characteristics of alcohols and hydrocarbons. Alcohols have higher flame speeds and extended
flammability limits. Pure methanol is very flammable
and its flame is colorless when ignited. The alcohols
mix in all proportions with water due to the polar nature
of OH group. Low volatility is indicated by high boiling
point and high flash point. Combustion of alcohol in
presence of air can be initiated by an intensive source of
localized energy, such as a flame or a spark and also, the
mixture can be ignited by application of energy by
means of heat and pressure, as it happens in the
compression stroke of a piston engine. The high latent
heat of vaporization of alcohols cools the air entering
the combustion chamber of the engine, thereby
increasing the air density and mass flow. This leads to
A. Demirbas / Progress in Energy and Combustion Science 31 (2005) 466–487
increased volumetric efficiency and reduced compression temperatures [8].
Methanol is not miscible with hydrocarbons and
separation ensues readily in the presence of small
quantities of water, particularly with reduction in
temperature. Since alcohols, especially methanol, can
be readily ignited by hot surfaces, pre-ignition can
occur. It must be emphasized here that pre-ignition and
knocking in alcohol engine is a much more dangerous
condition than gasoline engines. Other properties,
however, are favorable to the increase of power and
reduction of fuel consumption.
When diesel engines are converted to alcohols, some
properties of gasoline, diesel and alcohol should be
concerned. Table 16 shows the characteristic properties
of the fuels. Because alcohols have limited solubility in
diesel fuel, stable emulsion must be formed that will
allow it to be injected before separation occurs. Hydroshear emulsification unit can be used to produce
emulsions of diesel-alcohol.
5. Engine performance tests
The methyl ester of vegetable oil was evaluated as a
fuel in compression ignition engines (CIE) by
researchers. They concluded that the performance of
the esters of vegetable oil did not differ greatly from PD
fuel [61,83]. The brake power was nearly the same as
with PD fuel, while specific fuel consumption was
higher than PD fuel. Based on crankcase oil analysis,
engine wear rates were low but some oil dilution did
occur. Carbon deposits inside the engine were normal
with the exception of intake valve deposits.
Fumigation is a process of introducing alcohol into
the diesel engine (up to 50%) by means of a carburetor
in the inlet manifold. At the same time, the diesel pump
operates at a reduced flow. In this process, BD fuel is
used for generating a pilot flame, and alcohol is used as
a fumigated fuel.
A visual inspection of the injector types would
indicate no difference between the BD fuels when
tested on no. 2 PD fuel. The overall injector coking is
considerably low. Linear regression is used to compare
483
injector coking, viscosity, percent of BD, total glycerol,
and heat of combustion data with the others.
Peak torque is less for the BD fuels than diesel fuel
but occurs at lower engine speed and generally the
torque curves are flatter. Test includes the power and
torque of the methyl esters and PD fuel and ethyl esters
versus PD fuel. The BD fuels on the average decrease
power by 5% compared to that of PD at rated load.
6. BD economy
The cost of BD fuels varies depending on the base
stock, geographic area, variability in crop production
from season to season, the price of the crude petroleum
and other factors. BD has over double the price of PD.
The high price of BD is in large part due to the high
price of the feedstock. However, BD can be made from
other feedstocks, including beef tallow, pork lard, and
yellow grease
Fatty acid methyl ester could be produced from tall oil,
a by-product in the manufacture of pulp by the Kraft
process. Tall oil consists of free C18 unsaturated fatty
acids, resin acids and relatively small amounts of
unsaponifiables [84,85]. The fatty acid fraction of tall oil
contains mainly oleic acid, linoleic acid and its isomers.
Tall oil fatty acids are easily converted into their methyl
esters by reaction with methanol, whereas the resin acids
are virtually unesterified due to hindered effect [86].
BD has become more attractive recently because of
its environmental benefits. The cost of BD, however, is
the main obstacle to commercialization of the product.
With cooking oils used as raw material, the viability of
a continuous transesterification process and recovery of
high quality glycerol as a BD by-product are primary
options to be considered to lower the cost of BD [10,11].
Vegetable oils are a renewable and potentially
inexhaustible source of energy with an energetic content
close to PD fuel. The vegetable oil fuels were not
acceptable because they were more expensive than
petroleum fuels. With recent increases in petroleum
prices and uncertainties concerning petroleum availability, there is renewed interest in vegetable oil fuels for
diesel engines.
Table 16
Comparison of characteristic properties of fuels
Fuel property
Gasoline
No. 2 diesel
Iso-octane
Methanol
Ethanol
Cetane number
Octane number
Auto-ignition temperature (K)
Latent heat of vaporization (MJ/Kg)
Lower heating value (MJ/Kg)
–
96
644
0.35
44.0
50
–
588
0.22
42.6
–
100
530
0.26
45.0
5
112
737
1.18
19.9
8
107
606
0.91
26.7
484
A. Demirbas / Progress in Energy and Combustion Science 31 (2005) 466–487
Most of the BD that is currently made uses
soybean oil, methanol, and an alkaline catalyst. The
high value of soybean oil as a food product makes
production of a cost-effective fuel very challenging.
However there are large amounts of low-cost oils
and fats such as restaurant waste and animal fats that
could be converted to BD. The problem with
processing these low cost oils and fats is that they
often contain large amounts of free fatty acids (FFA)
that cannot be converted to BD using an alkaline
catalyst [2,87].
A review of 12 economic feasibility studies shows
that the projected costs for BD from oilseed or
animal fats have a range US$0.30-0.69/l, including
meal and glycerin credits and the assumption of
reduced capital investment costs by having the
crushing and/or esterification facility added onto an
existing grain or tallow facility. Rough projections of
the cost of BD from vegetable oil and waste grease
are, respectively, US$0.54-0.62/l and US$0.34-0.42/l.
With pre-tax diesel priced at US$0.18/l in the US
and US$0.20-0.24/l in some European countries, BD
is thus currently not economically feasible, and more
research and technological development will be
needed [2,88].
7. Conclusion
With exception of hydropower and nuclear energy,
the major part of all energy consumed worldwide
comes from petroleum, charcoal and natural gas.
However, these sources are limited, and will be
exhausted on the near future. Thus, looking for
alternative sources of energy is of vital importance.
Vegetable oils are a renewable and potentially
inexhaustible source of energy with an energetic
content close to diesel fuel. The vegetable oil fuels
were not acceptable because they were more expensive
than petroleum fuels. With recent increases in
petroleum prices and uncertainties concerning petroleum availability, there is renewed interest in
vegetable oil fuels for diesel engines.
The purpose of the transesterification of vegetable
oils to their methyl esters (biodiesels) process is to
lower the viscosity of the oil. The main factors
affecting transesterification are molar ratio of
glycerides to alcohol, catalyst, reaction temperature
and pressure, reaction time and the contents of free
fatty acids and water in oils. The commonly
accepted molar ratios of alcohol to glycerides are
6:1–30:1.
Viscosity is the most important property of
biodiesel (BD) since it affects the operation of fuel
injection equipment, particularly at low temperatures
when the increase in viscosity affects the fluidity of
the fuel. BD has viscosity close to diesel fuels. High
viscosity leads to poorer atomization of the fuel
spray and less accurate operation of the fuel
injectors.
The parameters affecting on the methyl esters
formation are reaction temperature, pressure, molar
ratio, water content and free fatty acid content. It is
evident that at subcritical state of alcohol, reaction
rate is so low and gradually increased as either
pressure or temperature rises. It was observed that
increasing the reaction temperature, especially to
supercritical conditions, had a favorable influence on
the yield of ester conversion. The yield of alkyl ester
increased with increasing the molar ratio of oil to
alcohol. In the supercritical alcohol transesterification
method, the yield of conversion raises 50–95% for
the first 10 min. The BDs have high boiling points,
flash points, and extremely low vapor pressure, as
well as an inability to smoke under the smoke point
test.
BD is considered to be an attractive transportation
fuel for use in environmentally sensitive applications
due to its biodegradable nature, and essentially no
sulfur and aromatic contents, offers promise to reduce
particulate and toxic emissions. BDs have several
outstanding advantages among other new-renewable
and clean engine fuel alternatives. Fuel characterization
data show some similarities and differences between
BD and PD [2]:
† Sulfur content for BD is 20–50% that of D2 fuel
† Specific weight is higher for BD, heat of combustion is
lower, viscosities are 1.3–2.1 times that of No. 2 PD
fuel
† Pour points for BD fuels vary from 274 to 298 K higher
for BD fuels depending on the feedstock
† The BDs all have higher levels of injector coking than
no. 2 PD.
A novel process of BD fuel production has been
developed by a non-catalytic supercritical methanol
method. Supercritical methanol has a high potential for
both transesterification of triglycerides and methyl
esterification of free fatty acids to methyl esters for
diesel fuel substitute. In the supercritical methanol
transesterification method, the yield of conversion
raises 96% for 10 min.
A. Demirbas / Progress in Energy and Combustion Science 31 (2005) 466–487
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