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 new-

renewable 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 467

1.1. History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 468

Progress in Energy and Combustion Science 31 (2005) 466–487

www.elsevier.com/locate/pecs

0360-1285/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.pecs.2005.09.001

* Tel.: C90 462 230 7831; fax: C90 462 248 8508.

E-mail address: 8508.ayhandemirbas@hotmail.com

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 . . . . . . . . . . . . . . . . . . . . . . . . . 468

1.3. Global vegetable oil resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 469

2. Biodiesel (BD) as an alternative fuel for diesel engine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 470

2.1. The importance of alcohols for diesel engines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 471

2.1.1. Methanol production methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 471

2.1.2. Ethanol production methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 472

2.2. Hydrogen production methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473

3. Transesterification of vegetable oils and fats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 474

3.1. Catalytic transesterification method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 474

3.2. Supercritical methanol transesterification method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 475

3.3. Recovery of glycerol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 477

3.4. Reaction mechanism of transesterification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 477

3.4.1. Acid-catalyzed processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 478

3.4.2. Alkali-catalyzed processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 478

3.4.3. Enzyme-catalyzed processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 479

3.4.4. Non-catalytic supercritical alcohol transesterification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 479

4. Fuel properties of vegetable oils and biodiesels (BDs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 479

4.1. Emissions from biodiesel combustion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 482

4.2. Comparison of fuel properties and combustion characteristics of methyl and ethyl alcohols and their esters 482

5. Engine performance tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 483

6. BD economy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 483

7. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 484

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 484

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. Alterna-

tive 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 pet-

roleum 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

A. Demirbas / Progress in Energy and Combustion Science 31 (2005) 466–487468

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 experi-

ences 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, transester-

ification 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-alkyl-

esters 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

Table 1

Fatty acid compositions of vegetable oil samples

Sample 16:0 16:1 18:0 18:1 18:2 18:3 Others

Cottonseed 28.7 0 0.9 13.0 57.4 0 0

Poppyseed 12.6 0.1 4.0 22.3 60.2 0.5 0

Rapeseed 3.5 0 0.9 64.1 22.3 8.2 0

Safflowerseed 7.3 0 1.9 13.6 77.2 0 0

Sunflowerseed 6.4 0.1 2.9 17.7 72.9 0 0

Sesameseed 13.1 0 3.9 52.8 30.2 0 0

Linseed 5.1 0.3 2.5 18.9 18.1 55.1 0

Wheat graina 20.6 1.0 1.1 16.6 56.0 2.9 1.8

Palm 42.6 0.3 4.4 40.5 10.1 0.2 1.1

Corn marrow 11.8 0 2.0 24.8 61.3 0 0.3

Castorb 1.1 0 3.1 4.9 1.3 0 89.6

Tallow 23.3 0.1 19.3 42.4 2.9 0.9 2.9

Soybean 13.9 0.3 2.1 23.2 56.2 4.3 0

Bay laurel leafc 25.9 0.3 3.1 10.8 11.3 17.6 31.0

Peanut kerneld 11.4 0 2.4 48.3 32.0 0.9 4.0

Hazelnut kernel 4.9 0.2 2.6 83.6 8.5 0.2 0

Walnut kernel 7.2 0.2 1.9 18.5 56.0 16.2 0

Almond kernel 6.5 0.5 1.4 70.7 20.0 0 0.9

Olive kernel 5.0 0.3 1.6 74.7 17.6 0 0.8

Coconute 7.8 0.1 3.0 4.4 0.8 0 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.

A. Demirbas / Progress in Energy and Combustion Science 31 (2005) 466–487 469

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-emulsifica-

tion, 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.

Table 2

World vegetable and marine oil consumption (million metric ton)

Oil 1998 1999 2000 2001 2002 2003

Soybean 23.5 24.5 26.0 26.6 27.2 27.9

Palm 18.5 21.2 23.5 24.8 26.3 27.8

Rapeseed 12.5 13.3 13.1 12.8 12.5 12.1

Sunflowerseed 9.2 9.5 8.6 8.4 8.2 8.0

Peanut 4.5 4.3 4.2 4.7 5.3 5.8

Cottonseed 3.7 3.7 3.6 4.0 4.4 4.9

Coconut 3.2 3.2 3.3 3.5 3.7 3.9

Palm kernel 2.3 2.6 2.7 3.1 3.5 3.7

Olive 2.2 2.4 2.5 2.6 2.7 2.8

Fish 1.2 1.2 1.2 1.3 1.3 1.4

Total 80.7 85.7 88.4 91.8 95.1 98.3

Source: World statistics, 1998–2004 United Soybean Board.

50000

60000

70000

80000

90000

1990 1992 1994 1996 1998 2000

Years

Am

ount

of

veg

etab

le o

il, M

illio

n to

n

Total production Total consumption

Fig. 2. Total global production and consumption of vegetable oil by

years.

A. Demirbas / Progress in Energy and Combustion Science 31 (2005) 466–487470

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].

4

10

16

22

28

1998 1999 2000 2001 2002 2003

Year

Ann

ual

oil

cons

umpt

ion,

wt%

Soybean Palm Rapeseed Sunflowerseed Peanut Others

Fig. 1. Plots of percentages the world oil consumption by years.

Source: World Statistics, 1998–2004 United Soybean Board.

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

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

A. Demirbas / Progress in Energy and Combustion Science 31 (2005) 466–487 471

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 (C3H7OH), butanol

(C4H9OH). However, only two of these alcohols

(methanol and ethanol) are technically and economi-

cally 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

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:

A. Demirbas / Progress in Energy and Combustion Science 31 (2005) 466–487472

2H2 CCO/CH3OH (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 feed-

stock 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

Gases CO H2 CO2 CH4 N2 Ar

Percentage 45.3 34.4 15.8 1.9 1.9 0.6

Source: Ref. [30].

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 metha-

nol 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)

H2 CO CH4 CO2 C2H4 H2O N2

32–41 21–29 10–15 14–19 0.8–1.2 5.5–6.5 0.6–1.2

Source: Ref. [7].

Table 6

List of some biomass material used for hydrogen production

Biomass species Main conversion process

Bio-nut shell Steam gasification

Olive husk Pyrolysis

Tea waste Pyrolysis

Crop straw Pyrolysis

Black liquor Steam gasification

Municipal solid waste Supercritical water extraction

Crop grain residue Supercritical fluid extraction

Pulp and paper waste Microbiol fermentation

Petroleum basis plastic waste Supercritical fluid extraction

Manure slurry Microbiol fermentation

A. Demirbas / Progress in Energy and Combustion Science 31 (2005) 466–487 473

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 CH2O/CO C3H2 (2)

CO CH2O/CO2 CH2 (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 CH2O/CO CH2 (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 pro-

duction 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 co-

product strategy to reduce the cost of hydrogen. The

process of biomass to activated carbon is an alternative

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 intermedi-

ate 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 CH2O$ CO2 CH2 (5)

and the second is the CH4 formation reaction:

CO C3H2O$CH4 CH2O (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 super-

critical water gasification can directly deal with the wet

biomass without drying, and have high gasification

efficiency in lower temperature. The cost of hydrogen

10

15

2025

30

3540

4550

600 700 800 900 1000 1100 1200

Temperature, K

Yie

ld o

f h

ydro

gen,

vol

.%

Supercritical fluid extraction Conventional pyrolysis Steam gasification

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].

A. Demirbas / Progress in Energy and Combustion Science 31 (2005) 466–487474

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 cata-

lyst or without any catalyst by using primary or

secondary monohydric aliphatic alcohols having 1–8

carbon atoms as follows:

Triglycerides CMonohydric alcohol%Glycerin

CMono-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 equili-

brium 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 transesterifica-

tion 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 cata-

lysts 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

Table 7

Physical properties of chemicals related to transesterification

Name Specific

gravity

(g/ml)

Melting

point

(K)

Boiling

point

(K)

Solubility

(!10%)

Methyl

myristate

0.875 291.0 – –

Methyl

palmitate

0.825 303.8 469.2 Benzene,

EtOH, Et2O

Methyl

stearate

0.850 311.2 488.2 Et2O,

chloroform

Methyl

oleate

0.875 253.4 463.2 EtOH, Et2O

Methanol 0.792 176.2 337.9 H2O, ether,

EtOH

Ethanol 0.789 161.2 351.6 H2O, ether

Glycerol 1.260 255.3 563.2 H2O, ether

Source: Refs. [7,57].

Table 9

Critical temperatures and critical pressures of various alcohols

Alcohol Critical temperature

(K)

Critical pressure

(MPa)

Methanol 512.2 8.1

Ethanol 516.2 6.4

1-Propanol 537.2 5.1

1-Butanol 560.2 4.9

A. Demirbas / Progress in Energy and Combustion Science 31 (2005) 466–487 475

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].

Table 8

Melting points of fatty acids, methyl esters and mono-, di-, and

triglycerides (K)

Name Fatty

acid

Methyl

ester

1-Mono-

glyceride

1,3-Digly-

ceride

Trigly-

ceride

Myristic 327.6 336.1 342.8 289.5 266.7

Palmitic 292.0 303.8 312.3 253.4 238.2

Stearic 343.7 350.2 254.7 308.4 285.5

Oleic 340.0 349.5 352.6 294.7 270.6

Linoleic 330.2 336.7 346.3 278.7 260.1

Source: Refs. [7,58].

3.2. Supercritical methanol transesterification method

[7]

The transesterfication of triglycerides by supercriti-

cal 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 esterifi-

cation 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 transesterifica-

tion system.

The most important variables affecting the methyl

ester yield during transesterification reaction are molar

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].

0

20

40

60

80

100

0 50 100 150 200 250 300 350

Reaction time (sec)

Yie

ld o

f met

hyl

este

r, w

t%

450 K

493 K

503 K

513 K

523 K

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].

0

10

20

30

40

50

60

70

80

90

100

–50 50 150 250 350Reaction time (sec)

Yie

ld o

f m

ethy

l est

er, w

t%

1.0:1.0

1.0:3.0

1.0:9.0

1.0:20

1.0:41

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].

A. Demirbas / Progress in Energy and Combustion Science 31 (2005) 466–487476

ratio of alcohol to vegetable oil and reaction tempera-

ture. 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.

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 triglycer-

ides 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

0

20

40

60

80

100

0 10 20 30 40 50Reaction time, min

Fatty

aci

d a

lkyl

est

er,

%

Methanol

Ethanol

1-Propanol

1-Butanol

1-Octanol

Fig. 7. Plots for changes in fatty acids alkyl esters conversion from

triglycerides as treated in supercritical alcohol at 575 K. Source: Ref.

[64].

A. Demirbas / Progress in Energy and Combustion Science 31 (2005) 466–487 477

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 by-

product 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

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 trigly-

ceride is converted stepwise to diglyceride, mono-

glyceride 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 ðR1COOHÞCAlcohol ðROHÞ

$Ester ðR1COORÞCWater ðH2OÞ (8)

Triglyceride CROH$Diglyceride CRCOOR1 (9)

Diglyceride CROH$Monoglyceride

CRCOOR2 (10)

Monoglyceride CROH$Glycerol

CRCOOR3 (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 transester-

ification is an equilibrium reaction and the trans-

formation occurs essentially by mixing the reactants.

A. Demirbas / Progress in Energy and Combustion Science 31 (2005) 466–487478

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].

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 indi-

vidual 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

Fig. 8. Mechanism of the alkali-catalyzed trans

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 trans-

esterification as well as the esterification [69].

3.4.2. Alkali-catalyzed processes

The reaction mechanism for alkali-catalyzed trans-

esterification 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

esterification of vegetable oils (B: base).

0

20

40

60

80

100

0 1 2 3 4 5

Water content, %

Met

hyl

este

r, %

Supercritical methanol Alkaline catalyst Acid catalyst

Fig. 9. Plots for yields of methyl esters as a function of water content

in transesterification of triglycerides.

0

20

40

60

80

100

0 10 20 30Free fatty acid content, %

Met

hyl

este

r, %

Supercritical methanol alkaline catalyst Acid catalyst

Fig. 10. Plots for yields of methyl esters as a function of free fatty acid

content.

A. Demirbas / Progress in Energy and Combustion Science 31 (2005) 466–487 479

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].

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].

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 triglycer-

ides 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 conver-

sion 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

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

Table 10

Comparisons between catalytic methanol (MeOH) process and

supercritical methanol (SCM) method for biodiesel from vegetable

oils by transesterification

Catalytic MeOH

process

SCM method

Methylating agent Methanol Methanol

Catalyst Alkali None

Reaction

temperature (K)

303–338 523–573

Reaction pressure

(MPa)

0.1 10–25

Reaction time (min) 60–360 7–15

Methyl ester yield

(wt%)

96 98

Removal for

purification

Methanol, catalyst,

glycerol, soaps

Methanol

Free fatty acids Saponified products Methyl esters, water

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 34.9 909.5 550

Cottonseed 33.5 914.8 509

Crambe 53.6 904.4 447

Linseed 27.2 923.6 514

Peanut 39.6 902.6 544

Rapeseed 37.0 911.5 519

Safflower 31.3 914.4 533

Sesame 35.5 913.3 533

Soybean 32.6 913.8 527

Sunflower 33.9 916.1 447

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].

A. Demirbas / Progress in Energy and Combustion Science 31 (2005) 466–487480

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 metha-

nol 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 3.69 880 437

Hazelnut kernel oil 3.59 860 401

Mustard oil 4.10 881 446

Palm oil 3.70 870 443

Rapeseed oil 4.63 885 428

Safflower oil 4.03 880 453

Soybean oil 4.08 885 447

Sunflower oil 4.22 880 443

Source: Ref. [72].

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 branched-

chain or aromatic compounds of similar molecular

weight and number of carbon atoms. The CN of

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 4.6 0.880 49 [73]

Soybean 4.1 0.884 46 [73]

Palm 5.7 0.880 62 [74]

Peanut 4.9 0.876 54 [75]

Babassu 3.6 – 63 [75]

Tallow 4.1 0.877 58 [76]

A. Demirbas / Progress in Energy and Combustion Science 31 (2005) 466–487 481

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.

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)

Viscosity

at 300 K

(mm2 /s)

Cetane

numbera

No. 2 diesel

fuel

43.4 815 4.3 47.0

Sunflower oil 39.5 918 58.5 37.1

Sunflower

methyl ester

40.6 878 10.3 45.5

Cottonseed oil 39.6 912 50.1 48.1

Cottonseed

methyl ester

40.6 874 11.1 45.5

Soybean oil 39.6 914 65.4 38.0

Soybean

methyl ester

39.8 872 11.1 37.0

Corn oil 37.8 915 46.3 37.6

Opium poppy

oil

38.9 921 56.1 –

Rapeseed oil 37.6 914 39.2 37.6

Source: Ref. [8].a Cetane number (CN) is a measure of ignition quality of diesel fuel.

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].

25

30

35

40

45

50

55

900 905 910 915 920 925

Density, g/L

Vis

cosi

ty,

cSt

Fig. 11. Relationships between density and viscosity for vegetable

oils.

3.5

3.7

3.9

4.1

4.3

4.5

4.7

850 860 870 880 890

Density, g/L

Vis

cosi

ty,

cSt

Fig. 12. Relationships between density and viscosity for vegetable oil

methyl esters.

A. Demirbas / Progress in Energy and Combustion Science 31 (2005) 466–487482

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) emis-

sions 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) C2.0

PM (particular matter) K10.1

HC (hydrocarbons) K21.1

CO (carbon monoxide) K11.0

Source: Ref. [80].

4.2. Comparison of fuel properties and combustion

characteristics of methyl and ethyl alcohols

and their esters

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 tem-

peratures, 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 hydrocar-

bons. 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) 466–487 483

increased volumetric efficiency and reduced com-

pression 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. Hydro-

shear 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

Table 16

Comparison of characteristic properties of fuels

Fuel property Gasoline No. 2 diesel

Cetane number – 50

Octane number 96 –

Auto-ignition temperature (K) 644 588

Latent heat of vaporization (MJ/Kg) 0.35 0.22

Lower heating value (MJ/Kg) 44.0 42.6

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 avail-

ability, there is renewed interest in vegetable oil fuels for

diesel engines.

Iso-octane Methanol Ethanol

– 5 8

100 112 107

530 737 606

0.26 1.18 0.91

45.0 19.9 26.7

A. Demirbas / Progress in Energy and Combustion Science 31 (2005) 466–487484

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 pet-

roleum 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) 466–487 485

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