Biodiesel-production-from-vegetable-oils-via-catalytic-and-non-catalytic-supercritical-methanol-tran

Biodiesel production from vegetable oils via catalytic and non-catalytic supercritical methanol transesteri?cation methods

Ayhan Demirbas*

Department of Chemical Engineering,Selcuk University,Campus,42031Konya,Turkey

Received18April2005;accepted23September2005

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?eld.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 than350oil bearing crops identi?ed,among which only sun?ower,saf?ower,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-emulsi?cation,pyrolysis and transesteri?cation 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 between27.2and53.6mm2/s whereas those of vegetable oil methyl esters between3.59and4.63mm2/s.The viscosity values of vegetable oil methyl esters highly decreases after transesteri?cation https://www.360docs.net/doc/ae18728049.html,pared to no.2diesel fuel,all of the vegetable oil methyl esters were slightly viscous.The?ash point values of vegetable oil methyl esters are highly lower than those of vegetable oils.An increase in density from860to 885kg/m3for vegetable oil methyl esters or biodiesels increases the viscosity from3.59to4.63mm2/s and the increases are highly regular.The purpose of the transesteri?cation process is to lower the viscosity of the oil.The transester?cation 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 transesteri?cation reaction are molar ratio of alcohol to vegetable oil and reaction temperature.Biodiesel has become more attractive recently because of its environmental bene?ts.Biodiesel is an environmentally friendly fuel that can be used in any diesel engine without modi?cation.

Keywords:Alternative fuel;Vegetable oil;Biodiesel;Viscosity;Transesteri?cation;Methanol

Contents

1.Introduction (467)

1.1.History (468)

*Tel.:C904622307831;fax:C904622488508.

E-mail address:8508.ayhandemirbas@https://www.360docs.net/doc/ae18728049.html,

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.Transesteri?cation of vegetable oils and fats (474)

3.1.Catalytic transesteri?cation method (474)

3.2.Supercritical methanol transesteri?cation method (475)

3.3.Recovery of glycerol (477)

3.4.Reaction mechanism of transesteri?cation (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 transesteri?cation (479)

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

4.1.Emissions from biodiesel combustion (482)

https://www.360docs.net/doc/ae18728049.html,parison of fuel properties and combustion characteristics of methyl and ethyl alcohols and their esters482

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 bene?ts 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 signi?cantly reduce particulate emissions relative to petroleum diesel [3].There are more than350oil-bearing crops identi?ed,among which only sun?ower,saf?ower, 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 of1980s.The advantages of vegetable oils as diesel fuel are[2]:

?Liquid nature-portability

?Ready availability

?Renewability

?Higher heat content(about88%of no.2diesel 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 Science31(2005)466–487467

1.1.History

Transesteri?cation of triglycerides are in oils is not a new process.Scientists E.Duffy and J.Patrick conducted it as early as1853.Life for the diesel engine began in1893when 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 of1900 [6].Because of the high temperatures created,the engine was able to run a variety of vegetable oils including hemp and peanut oil.At the1911World’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?rst uses of transesteri?ed vegetable oil was powering heavy-duty vehicles in South Africa before world war II.The name‘biodiesel’has been given to transesteri?ed 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(about11–17 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 of25parts of vegetable oil with75parts 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)Transesteri?cation 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 transesteri?cation 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 unmodi?ed 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 modi?ed vegetable oil as a fuel. Although there are many ways and procedures to convert vegetable oil into a diesel-like fuel,transester-i?cation process was found to be the most viable oil modi?cation 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 transesteri?cation 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 modi?cations[9].

A number of methods exist to blend vegetable oil with PD and create a low viscosity fuel oil with similar properties to diesel.Bene?ts are substantially reduced engine emissions,even with as small a blend as20% BD with80%https://www.360docs.net/doc/ae18728049.html,ing 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 pro?le and is readily available and biodegradable[10].BD has become more attractive recently because of its environmental bene?ts [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?eld.

Several types of vegetable oils can be used for the preparation of BD.Soybean,rapeseed,sun?ower

A.Demirbas/Progress in Energy and Combustion Science31(2005)466–487 468

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 Table1.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 classi?ed 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 ranging10–20times greater than no.2diesel fuel.Castor oil is in a class by itself with a viscosity more than100times that of no.2diesel fuel[2]. Viscosity of oil can be lowered by blending with pure ethanol.25parts of sun?ower oil and75parts of PD were blending as PD fuel.Dilution,micro-emulsi?ca-tion,pyrolysis and transesteri?cation 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 increased56million tons in1990to88million tons in2000,following a below-normal increase.World vegetable and marine oil consumption is tabulated in Table2.Fig.1shows the plots of percentages the world oil consumption by years.Fig.2shows 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,from17.1million tons in 1997/1998to19.3million in1998/1999.

The major exporters of vegetable oils are Malaysia, Argentina,Indonesia,Philippines,and Brazil.

Table1

Fatty acid compositions of vegetable oil samples

Sample16:016:118:018:118:218:3Others Cottonseed28.700.913.057.400 Poppyseed12.60.1 4.022.360.20.50 Rapeseed 3.500.964.122.38.20

Saf?owerseed7.30 1.913.677.200

Sun?owerseed 6.40.1 2.917.772.900 Sesameseed13.10 3.952.830.200

Linseed 5.10.3 2.518.918.155.10

Wheat grain a20.6 1.0 1.116.656.0 2.9 1.8

Palm42.60.3 4.440.510.10.2 1.1

Corn marrow11.80 2.024.861.300.3

Castor b 1.10 3.1 4.9 1.3089.6

Tallow23.30.119.342.4 2.90.9 2.9 Soybean13.90.3 2.123.256.2 4.30

Bay laurel leaf c25.90.3 3.110.811.317.631.0

Peanut kernel d11.40 2.448.332.00.9 4.0 Hazelnut kernel 4.90.2 2.683.68.50.20

Walnut kernel7.20.2 1.918.556.016.20

Almond kernel 6.50.5 1.470.720.000.9

Olive kernel 5.00.3 1.674.717.600.8 Coconut e7.80.1 3.0 4.40.8065.7 Legend:Source:Ref.[2].

a Wheat grain oil contains11.4%of8:0and0.4%of14:0fatty acids.

b Castor oil contains89.6%ricinoloi

c acid.

c Bay laurel oil contains26.5%of12:0and4.5%of14:0fatty acids.

d Peanut kernel oil contains about2.7%of22:0and1.3%of24:0fatty acids.

e Coconut oil contains about8.9%of8:0,6.2%10:0,48.8%of12:0and19.9%of14:0fatty acids.

A.Demirbas/Progress in Energy and Combustion Science31(2005)466–487469

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 29.8million tons in 1997/1998to 31.2million 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 1998/1999soared 26%from 1997/1998.Indian palm oil imports climbed to a record 2.5million 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 10to 35%to stem the in?ux of Indian exports [8].

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-?ammable,and in contrast to PD it is non-explosive,with a ?ash point of 423K for BD as compared to 337K for PD.Unlike PD,BD is biodegradable and non-toxic,and it signi?cantly 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:w 20–25%)[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 transesteri?cation reaction of triglycerides in the presence alkali as a catalyst [15].Methanol is a relatively inexpensive alcohol.Several common vegetable oils such as sun?ower,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 transesteri?cation due to including high free fatty acid.Diesel derived from rapeseed oil is the most common

Table 2

World vegetable and marine oil consumption (million metric ton)

Oil

199819992000200120022003Soybean 23.524.526.026.627.227.9Palm 18.521.223.524.826.327.8Rapeseed

12.513.313.112.812.512.1Sun?owerseed 9.29.58.68.48.28.0Peanut 4.5 4.3 4.2 4.7 5.3 5.8Cottonseed 3.7 3.7 3.6 4.0 4.4 4.9Coconut 3.2 3.2 3.3 3.5 3.7 3.9Palm kernel 2.3 2.6 2.7 3.1 3.5 3.7Olive 2.2 2.4 2.5 2.6 2.7 2.8Fish 1.2 1.2 1.2 1.3 1.3 1.4Total

80.7

85.7

88.4

91.8

95.1

98.3

Source:World statistics,1998–2004United Soybean

Board.

410162228Year

A n n u a l o i l c o n s u m p t i o n , w t %

Fig.1.Plots of percentages the world oil consumption by years.Source:World Statistics,1998–2004United Soybean Board.

50000

60000

70000

80000

90000

Years

A m o u n t o f v e g e t a b l e o i l , M i l l i o n t o n

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

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

470

BD available in Europe,while soybean BD is dominant in the United States.

The emergence of the transesteri?cation can be dated back to as early as1846when 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?sh oils,tallow,soybean,rapeseed,cottonseed, sun?ower,saf?ower,peanut and linseed oils[17–26].It also prepared methyl esters from palm oil by transesteri?cation using methanol in the presence of a alkali catalyst in a batch reactor[27].Soybean oil was transesteri?ed into ethyl and methyl esters,and compared the performances of the fuels with PD

[28,29].

Transesteri?cation 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 transesteri?cation process compared to methanol because it is derived from agricultural products and is renewable and biologically less objectionable in the environment.Alkali catalyzed transesteri?cation 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 transesteri?cation using KOH and/or sodium alkoxides as catalysts.In addition,methyl and ethyl esters were prepared from rapeseed and sun?ower 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 Table3.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?nd 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 the1970s 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 using85–100%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 early1920s,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 the1920s,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,re?nery off-gas,coal or petroleum:

Table3

Main production facilities of methanol and ethanol

Product Production process

Methanol

Distillation of liquid from wood pyrolysis

Gaseous products from biomass gasi?cation

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 Science31(2005)466–487471

2H2C CO/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/SiO2and Pd/SiO2,and Pd/ZnO[31,32].

Biomass resources can be used to produce methanol. The pyroligneous acid obtained from wood pyrolysis consists of about50%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 be185kg 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 suf?cient 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 Table4.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?rst pulverized and cleaned,then fed to a gasi?er 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 a2–1 ratio(Table4).The syngas was transformed to methanol in a?xed 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 for fuel cells.The technology for making methanol from natural gas is already in place and requires only ef?ciency 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 gasi?cation 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 gasi?cation of a carbonaceous matter.Syngas from biomass is altered by catalyst under high pressure and temperature to form methanol.This method will produce100gallons of methanol per ton of feed material[38]Table5.

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 ?ne powder,and all of it is cooked to liquefy it,without removing the germ or?ber.

Ethanol has been used in Germany and France as early as1894by the then incipient industry of ICEs. Brazil has utilized ethanol as a fuel since1925. Currently,ethanol is produced from sugar beets and from molasses in Brazil.A typical yield is72.5liter of ethanol per ton of sugar cane.Modern crops yield 60ton 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 de?cits,create

Table4

Analysis of syngas from typical coal

Gases CO H2CO2CH4N2Ar Percentage45.334.415.8 1.9 1.90.6 Source:Ref.[30].Table5

Composition of syngas from biomass for producing methanol(%by volume)

H2CO CH4CO2C2H4H2O N2

32–4121–2910–1514–190.8–1.2 5.5–6.50.6–1.2 Source:Ref.[7].

A.Demirbas/Progress in Energy and Combustion Science31(2005)466–487 472

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.Approximately95%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 about1125K and around2.5MPa according to the following reactions: CH4C H2O/CO C3H2(2)

CO C H2O/CO2C H2(3) Syngas or arti?cial water gas(CO C H2)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 H2O/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 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 Table6.

The?rst intermediate temperatures between600and 850K pyrolysis carries out through the formation mainly acetic acid,which partly forms H2,CO2and CO and a small amount of methane.The second intermedi-ate temperatures between900and1100K pyrolysis carries out through the formation mainly propionic acid,which partly forms H2,CO2and 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 H2and CO2in approximately a molar ratio of2mol of H2per mol of CO2formed.

The gas products of H2,CO2,CO,CH4and C2H6are formed by the secondary pyrolytic gasi?cation reactions,they will continue to react according to two reversible tertiary reactions.The?rst is the water-gas shift reaction[43]:

CO C H2O$CO2C H2(5) and the second is the CH4formation reaction:

CO C3H2O$CH4C H2O(6) Hydrogen can be produced from biomass via two thermochemical processes:(a)gasi?cation 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 https://www.360docs.net/doc/ae18728049.html,parison with other biomass thermochemical gasi?cation such as air gasi?cation and/or steam gasi?cation,the super-critical water gasi?cation can directly deal with the wet biomass without drying,and have high gasi?cation ef?ciency in lower temperature.The cost of hydrogen Table6

List of some biomass material used for hydrogen production Biomass species Main conversion process

Bio-nut shell Steam gasi?cation

Olive husk Pyrolysis

Tea waste Pyrolysis

Crop straw Pyrolysis

Black liquor Steam gasi?cation

Municipal solid waste Supercritical water extraction Crop grain residue Supercritical?uid extraction Pulp and paper waste Microbiol fermentation Petroleum basis plastic waste Supercritical?uid extraction Manure slurry Microbiol fermentation

A.Demirbas/Progress in Energy and Combustion Science31(2005)466–487473

production from supercritical water gasi?cation of wet biomass was several times higher than the current price of hydrogen from steam methane reforming.Biomass was gasi?ed in supercritical water at a series of temperature and pressure during different resident times to form a product gas composed of H 2,CO 2,CO,CH 4,and a small amount of C 2H 4and C 2H 6[47].Fig.3shows the curves for yield of hydrogen from supercritical ?uid extraction (SFE),pyrolysis and steam gasi?cation [(W /S )Z 2]of beech wood at different temperatures.Distilled water was used in the SFE (the critical temperature of pure water is 647.7K).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 gasi?cation 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 ?uid 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.Transesteri?cation of vegetable oils and fats The transesteri?cation reaction proceeds with cata-lyst or without any catalyst by using primary or secondary monohydric aliphatic alcohols having 1–8carbon atoms as follows:

Triglycerides C Monohydric alcohol %Glycerin C Mono -alkyl esters

(7)

Transesteri?cation 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,transesteri?cation 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 1–8carbon atoms Amongs the alcohols that can be used in the transesteri?cation reaction are methanol,ethanol,propanol,butanol and amyl alcohol.Methanol and ethanol are used most frequently,ethanol is a preferred alcohol in the transesteri?cation 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 transesteri?ca-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 dif?culty 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 475K [56].

The physical properties of the primary chemical products of transesteri?cation are given in Tables 7and 8[11,57,58].

In the conventional transesteri?cation 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 transesteri?cation 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

101520253035404550Temperature, K

Y i e l d o f h y d r o g e n , v o l .%

Fig.3.Plots for yield of hydrogen from supercritical ?uid (water)extraction,pyrolysis and steam gasi?cation [(W /S )Z 2]of beech wood at different temperatures.Source:Ref.[48].

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

474

pumped into the oil.The?nal mixture is stirred vigorously for2h at340K in ambient pressure.A successful transesteri?cation 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 within10min and can be complete within2h of https://www.360docs.net/doc/ae18728049.html,plete settling can take as long as20h. After settling is complete,water is added at the rate of 5.5%by volume of methyl ester of oil and then stirred for5min 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 of28%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 at28%by volume of oil for the?nal washing[2,7,11].3.2.Supercritical methanol transesteri?cation method [7]

The transester?cation of triglycerides by supercriti-cal methanol(SCM),ethanol,propanol and butanol,has proved to be the most promising process.Table9shows 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 transesteri?cation of triglycerides and methyl esteri?-cation of fatty acids[9].Because of having similar properties to PD,BD,a transesteri?ed 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,speci?c weight,and polarity[61].The transesteri?cation of sun?ower oil was investigated in SCM and supercritical ethanol at various temperatures (475–675K)[62].Fig.4shows a SCM transesteri?ca-tion system.

The most important variables affecting the methyl ester yield during transesteri?cation reaction are

molar Fig. 4.Supercritical methanol transesteri?cation 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].

Table7

Physical properties of chemicals related to transesteri?cation

Name Speci?c

gravity

(g/ml)Melting

point

(K)

Boiling

point

(K)

Solubility

(!10%)

Methyl

myristate

0.875291.0––

Methyl palmitate 0.825303.8469.2Benzene,

EtOH,Et2O

Methyl stearate 0.850311.2488.2Et2O,

chloroform

Methyl

oleate

0.875253.4463.2EtOH,Et2O Methanol0.792176.2337.9H2O,ether,

EtOH Ethanol0.789161.2351.6H2O,ether Glycerol 1.260255.3563.2H2O,ether Source:Refs.[7,57].

Table8

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

Myristic327.6336.1342.8289.5266.7 Palmitic292.0303.8312.3253.4238.2 Stearic343.7350.2254.7308.4285.5 Oleic340.0349.5352.6294.7270.6 Linoleic330.2336.7346.3278.7260.1 Source:Refs.[7,58].Table9

Critical temperatures and critical pressures of various alcohols Alcohol Critical temperature

(K)

Critical pressure

(MPa) Methanol512.28.1

Ethanol516.2 6.4

1-Propanol537.2 5.1

1-Butanol560.2 4.9

A.Demirbas/Progress in Energy and Combustion Science31(2005)466–487475

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.2diesel fuel.Fig.5shows a typical example of the relationship between the reaction time and the temperature [7].The variables affecting the ester yield during transesteri?cation 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 in?uence on ester conversion [7].

In the transesteri?cation 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 dif?cult [11,63].

The stoichiometric ratio for transesteri?cation reaction requires 3mol of alcohol and 1mol of triglyceride to yield 3mol of fatty acid ester and 1mol of glycerol.Higher molar ratios result in greater ester production in a shorter time.The vegetable oils are transesteri?ed 1:6–1:40vegetable oil-alcohol molar ratios in catalytic and supercritical alcohol conditions [7].

Fig.6shows 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 transesteri?ed at 1:1,1:3,1:9,1:20and 1:40vegetable oil-methanol molar ratios in subcritical and SCM conditions [7].

In the supercritical alcohol transesteri?cation method,the yield of conversion raises 50–95%for the ?rst 10min.Fig.7shows the plots for changes in fatty acids alkyl esters conversion from triglycer-ides as treated in supercritical alcohols at 575K [64].

Transesteri?cation 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 puri?cation procedure because of the unused catalyst [2,7].Transesteri?cation can occur at different temperatures and the temperature in?uence the reaction rate and yield of esters,depending on the oil used.It was observed that increasing reaction

100

150

200

250

300

350

Reaction time (sec)

Y i e l d o f m e t h y l e s t e r , w t %

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]

–5050150250350

Reaction time (sec)

Fig.6.Effect of molar ratio of vegetable oil to methanol on yield of methyl ester.Temperature:513K,sample:methylester from cottonseed oil.Source:Ref.[7].

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

476

temperature,especially supercritical temperatures had a favorable in?uence 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 transesteri?cation are satis?ed:(a)complete transesteri?cation reaction,(b)removal of catalyst,(c)removal of alcohol,(d)removal of glycerol,and (e)complete esteri?cation of free fatty acids.The following transesteri?cation 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 ?nal mixture stirred vigorously for 2h.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 10min and can be complete within 2h after stirring has https://www.360docs.net/doc/ae18728049.html,plete settling can be taken as long as 18h.After settling is complete,water was added at the rate of 5.0%by volume of the oil and then stirred for 5min 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 transesteri?cation 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 ?nal washing.The resulting BD fuel when used directly in a diesel engine will burn up to 75%cleaner than no.2PD fuel [8].

3.4.Reaction mechanism of transesteri?cation Transesteri?cation consists of a number of consecutive,reversible reactions [65,66].The trigly-ceride is converted stepwise to diglyceride,mono-glyceride and ?nally 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 eR 1COOH TC Alcohol eROH T$Ester eR 1COOR TC Water eH 2O T

(8)Triglyceride C ROH $Diglyceride C RCOOR 1(9)

Diglyceride C ROH $Monoglyceride C RCOOR 2

(10)Monoglyceride C ROH $Glycerol C RCOOR 3

(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 in?uence on the course of the transesteri?cation.In the transesteri?cation 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 transesteri?cation process is called alcoholysis [68].The transester-i?cation is an equilibrium reaction and the trans-formation occurs essentially by mixing the reactants.

Reaction time, min

F a t t y a c i d a l k y l e s t e r , %

Fig.7.Plots for changes in fatty acids alkyl esters conversion from triglycerides as treated in supercritical alcohol at 575K.Source:Ref.[64].

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

477

In the transesteri?cation 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 requires1mol of a triglyceride and3mol 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 transesteri?cation 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 in?uence the transesteri?cation.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 dif?cult,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 H C. According to this mechanism,carboxylic acids can be formed by reaction of the carbocation with water present in the reaction mixture and acid-catalyzed transesteri?cation should be carried out in the absence of water[8,68].Acid catalytic transesteri?cation of 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 transesteri?cation of soybean oil with methanol at475–575K and the esteri?cation of n-octanoic acid with methanol at450–475K.Tungstated zirconia–alumina is a promising catalyst for the production of biodiesel fuels because of its activity for the trans-esteri?cation as well as the esteri?cation[69].

3.4.2.Alkali-catalyzed processes

The reaction mechanism for alkali-catalyzed trans-esteri?cation was formulated as three steps[70,71].The alkali-catalyzed transesteri?cation of vegetable oils proceeds faster than the acid-catalyzed reaction.The mechanism of the base-catalyzed transesteri?cation of vegetable oils is shown in Fig.8.The?rst 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(30min)even if they are applied at low molar concentrations(0.5mol%).The presence of water

gives Fig.8.Mechanism of the alkali-catalyzed transesteri?cation of vegetable oils(B:base).

A.Demirbas/Progress in Energy and Combustion Science31(2005)466–487

478

rise to hydrolysis of some of the produced ester,with consequent soap formation.Potassium carbonate,used in a concentration of2or3mol%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 transesteri?cation 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 transesteri?cation

BD,an alternative diesel fuel,is made from renewable biological sources such as vegetable oils and animal fats by non-catalytic supercritical alcohol transesteri?cation 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 transesteri?cation of triglycer-ides and methyl esteri?cation 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 in?uence 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 transesteri?cation method,the yield of conver-sion raises50–95%for the?rst10min.

Water content is an important factor in the conventional catalytic transesteri?cation of vegetable oil.In the conventional transesteri?cation 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.9shows the plots for yields of methyl esters as a function of water content in transesteri?cation of triglycerides.Fig.10shows 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 transesteri?cation are given in Table10.

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 modi?cations 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 transesteri?cation.

Viscosity is a measure of the internal friction or resistance of an oil to?ow.As the temperature of oil

Water content, %

Fig.9.Plots for yields of methyl esters as a function of water content in transesteri?cation of

triglycerides.

100

Free fatty acid content, %

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

A.Demirbas/Progress in Energy and Combustion Science31(2005)466–487479

increased,its viscosity decreases and it is therefore able to?ow more readily.Viscosity is measured on several different scales,including Redwood no.1at100F, 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?uidity 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?ash point measurements of eight oil methyl esters are given in https://www.360docs.net/doc/ae18728049.html,pared to no.2diesel fuel,all of the vegetable oils were much more viscous.Viscosity,density and?ash point measurements of ten vegetable oils given by Goering et al.[4]are shown in Table11.The density values of vegetable oils are between902.6and923.6kg/m3while those of vegetable oil methyl esters are between860 and885kg/m3(Table12).The density values of vegetable oil methyl esters considerably decreases via transesteri?cation process.The viscosity values of vegetable oils are between27.2and53.6mm2/s whereas those of vegetable oil methyl esters are between3.59and4.63mm2/s.The viscosity values of vegetable oil methyl esters highly decreases after transesteri?cation https://www.360docs.net/doc/ae18728049.html,pared to no.2PD fuel,all of the vegetable oil methyl esters were slightly viscous.The?ash point values of vegetable oil methyl esters are highly lower than those of vegetable oils (Tables11and12).

Density is another important property of BD.It is the weight of a unit volume of?uid.Speci?c gravity is the ratio of the density of a liquid to the density of water. Speci?c gravity of BD fuels ranges between0.87and 0.89kg/m3(Table11).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-ef?cient 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 of100and heptamethylnonane with a CN of15. 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

Table10

Comparisons between catalytic methanol(MeOH)process and supercritical methanol(SCM)method for biodiesel from vegetable oils by transesteri?cation

Catalytic MeOH

process

SCM method

Methylating agent Methanol Methanol Catalyst Alkali None

Reaction

temperature(K)

303–338523–573 Reaction pressure

(MPa)

0.110–25

Reaction time(min)60–3607–15

Methyl ester yield

(wt%)

9698

Removal for puri?cation Methanol,catalyst,

glycerol,soaps

Methanol

Free fatty acids Saponi?ed products Methyl esters,water Table11

Viscosity,density and?ash point measurements of eight oil methyl esters

Methyl ester Viscosity

(mm2/s

(at313K))Density

(kg/m3

(at288K))

Flash point

(K)

Cottonseed oil 3.69880437 Hazelnut kernel oil 3.59860401 Mustard oil 4.10881446 Palm oil 3.70870443 Rapeseed oil 4.63885428 Saf?ower oil 4.03880453 Soybean oil 4.08885447 Sun?ower oil 4.22880443 Source:Ref.[72].Table12

Viscosity,density and?ash point measurements of10vegetable oils Oil source Viscosity

(mm2/s(at

311K))

Density

(kg/m3)

Flash point

(K)

Corn34.9909.5550 Cottonseed33.5914.8509

Crambe53.6904.4447

Linseed27.2923.6514

Peanut39.6902.6544 Rapeseed37.0911.5519

Saf?ower31.3914.4533 Sesame35.5913.3533 Soybean32.6913.8527

Sun?ower33.9916.1447

The biodiesel(BD)was characterized by determining its density, viscosity,high heating value,cetane number,cloud and pour points, characteristics of distillation,and?ash and combustion points according to ISO norms.Source:Ref.[4].

A.Demirbas/Progress in Energy and Combustion Science31(2005)466–487 480

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 https://www.360docs.net/doc/ae18728049.html,parisons of some fuel properties of vegetable oils and their esters with diesel fuel are given in Table13.Table14shows 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?gures were plotted using the values in Table12given by Goering et al.[4].As seen Fig.11,an increase in density from902.6to923.6kg/m3for vegetable oils decreases the viscosity from53.6to27.2mm2/s and the decreases are considerably regular(coef?cient of regression(r)is0.7942).

Relationships between density and viscosity of vegetable oil methyl esters are depicted in Fig.12. The Fig.12was plotted using the measured values[72]. As seen Fig.12,an increase in density from860to 885kg/m3for vegetable oil methyl esters or BDs increases the viscosity from3.59to4.63mm2/s.There is high regression between density and viscosity values vegetable oil methyl esters.The relationships between viscosity and?ash point for vegetable oil methyl esters are irregular.

The BD was characterized by determining its density,viscosity,high heating value,cetane number, cloud and pour points,characteristics of distillation, and?ash and combustion points according to ISO norms[8].The higher heating values of the BD fuels, on a mass basis,are9–13%lower than no.2diesel fuel. The cloud and pour points of no.2diesel fuel are signi?cantly lower than the BD fuels.The BD fuels produced slightly lower power and torque and higher fuel consumption than no.2diesel fuel.The properties of BD are close to no.2diesel fuels.Some fuel properties of methyl ester BDs are presented in Table13.

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?rst becomes visible when the fuel is cooled.The PP is the temperature at which the amount of wax out of solution is suf?cient to gel the fuel,thus it is the lowest temperature at which the fuel can?ow.BD has higher CP and PP compared to conventional diesel [77].

Table13

Comparisons of some fuel properties of vegetable oils and their esters with diesel fuel

Fuel type Calori?c

value

(MJ/kg)Density

(kg/m3)

Viscosity

at300K

(mm2/s)

Cetane

number a

No.2diesel

fuel

43.4815 4.347.0

Sun?ower oil39.591858.537.1

Sun?ower

methyl ester

40.687810.345.5 Cottonseed oil39.691250.148.1 Cottonseed

methyl ester

40.687411.145.5 Soybean oil39.691465.438.0 Soybean

methyl ester

39.887211.137.0 Corn oil37.891546.337.6 Opium poppy

oil

38.992156.1–Rapeseed oil37.691439.237.6

Source:Ref.[8].

a Cetane number(CN)is a measure of ignition quality of diesel fuel.Table14

Fuel properties of methyl ester biodiesels

Source Viscosity

(g/mL at

288.7K)

Density

(cSt at313.

2K)

Cetane

number

Reference

no.

Sun?ower 4.60.88049[73] Soybean 4.10.88446[73]

Palm 5.70.88062[74] Peanut 4.90.87654[75] Babassu 3.6–63[75] Tallow 4.10.87758

[76]

Density, g/L

Fig.11.Relationships between density and viscosity for vegetable oils.

A.Demirbas/Progress in Energy and Combustion Science31(2005)466–487481

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 (NO x )emis-sions compared with diesel fuel used in an unmodi?ed diesel engine [80].The emission impacts of 20vol.%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 ef?cient 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 CO 2emissions relative to PD [82].

https://www.360docs.net/doc/ae18728049.html,parison 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 transesteri?cation 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 ?ame 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 ef?ciency.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:signi?cantly 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 ?ame speeds and extended ?ammability limits.Pure methanol is very ?ammable and its ?ame 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 ?ash https://www.360docs.net/doc/ae18728049.html,bustion of alcohol in presence of air can be initiated by an intensive source of localized energy,such as a ?ame 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 ?ow.This leads

3.53.73.9

4.14.34.54.7Density, g/L

V i s c o s i t y , c S t

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

Table 15

Emission impacts of 20vol.%BD for soybean-based BD added to an average base PD

Percent change in emissions

NO x (nitrogen oxides)C 2.0PM (particular matter)K 10.1HC (hydrocarbons)K 21.1CO (carbon monoxide)K 11.0

Source:Ref.[80].

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

482

increased volumetric ef?ciency 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.Table16shows 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 emulsi?cation 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 speci?c 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 to50%)by means of a carburetor in the inlet manifold.At the same time,the diesel pump operates at a reduced?ow.In this process,BD fuel is used for generating a pilot?ame,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.2PD fuel.The overall injector coking is considerably low.Linear regression is used to compare 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?atter.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 by5%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 C18unsaturated fatty acids,resin acids and relatively small amounts of unsaponi?ables[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 unesteri?ed due to hindered effect[86].

BD has become more attractive recently because of its environmental bene?ts.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 transesteri?cation 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.

Table16

Comparison of characteristic properties of fuels

Fuel property Gasoline No.2diesel Iso-octane Methanol Ethanol

Cetane number–50–58

Octane number96–100112107

Auto-ignition temperature(K)644588530737606

Latent heat of vaporization(MJ/Kg)0.350.220.26 1.180.91

Lower heating value(MJ/Kg)44.042.645.019.926.7

A.Demirbas/Progress in Energy and Combustion Science31(2005)466–487483

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 of12economic 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 esteri?cation 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 transesteri?cation of vegetable oils to their methyl esters(biodiesels)process is to lower the viscosity of the oil.The main factors affecting transesteri?cation 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?uidity 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 in?uence 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 transesteri?cation method,the yield of conversion raises50–95%for the?rst10min.The BDs have high boiling points,?ash 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 is20–50%that of D2fuel ?Speci?c weight is higher for BD,heat of combustion is lower,viscosities are1.3–2.1times that of No.2PD fuel

?Pour points for BD fuels vary from274to298K higher for BD fuels depending on the feedstock

?The BDs all have higher levels of injector coking than no.2PD.

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 transesteri?cation of triglycerides and methyl esteri?cation of free fatty acids to methyl esters for diesel fuel substitute.In the supercritical methanol transesteri?cation method,the yield of conversion raises96%for10min.

A.Demirbas/Progress in Energy and Combustion Science31(2005)466–487 484

References

[1]MacLeana HL,Laveb LB.Evaluating automobile fuel/propul-

sion system technologies.Prog Energy Combust Sci2003;29: 1–69.

[2]Demirbas A.Biodiesel fuels from vegetable oils via catalytic

and non-catalytic supercritical alcohol transesteri?cations and other methods:a survey.Energy Convers Manage2003;44: 2093–109.

[3]Giannelos PN,Zannikos F,Stournas S,Lois E,Anastopoulos G.

Tobacco seed oil as an alternative diesel fuel:physical and chemical properties.Ind Crop Prod2002;16:1–9.

[4]Goering E,Schwab W,Daugherty J,Pryde H,Heakin J.Fuel

properties of eleven vegetable oils.Trans ASAE1982;25: 1472–83.

[5]Pryor RW,Hanna MA,Schinstock JL,Bashford LL.Soybean oil

fuel in a small diesel engine.Trans ASAE1982;26:333–8. [6]Nitschke WR,Wilson CM.Rudolph diesel,pionier of the age

of power.Norman,OK:The University of Oklahoma Press;

1965.

[7]Demirbas A.Biodiesel from vegetable oils via transesteri?ca-

tion in supercritical methanol.Energy Convers Manage2002;

43:2349–56.

[8]Bala BK.Studies on biodiesels from transformation of vegetable

oils for diesel engines.Energy Edu Sci Technol2005;15:1–43.

[9]Demirbas A.Diesel fuel from vegetable oil via transesteri?ca-

tion and soap pyrolysis.Energy Sources2002;24:835–41. [10]Zhang Y,Dub MA,McLean DD,Kates M.Biodiesel production

from waste cooking oil:2.Economic assessment and sensitivity analysis.Bioresour Technol2003;90:229–40.

[11]Ma F,Hanna MA.Biodiesel production:a review.Bioresour

Technol1999;70:1–15.

[12]Ramadhas AS,Jayaraj S,Muraleedharan https://www.360docs.net/doc/ae18728049.html,e ofvegetable oils

as I.C.engine fuels—a review.Renew Energy2004;29:727–42.

[13]Schuchardt U,Ricardo Sercheli R,Vargas RM.Transesteri?ca-

tion of vegetable oils:a review.J Braz Chem Soc1998;9: 199–210.

[14]Mittelbach M,Gangl S.Long storage stability of biodiesel made

from rapeseed and used frying oil.J Am Oil Chem Soc2001;78: 573–7.

[15]Clark SJ,Wagner L,Schrock MD,Pinnaar PG.Methyl and ethyl

esters as renewable fuels for diesel engines.J Am Oil Chem Soc 1984;61:1632–8.

[16]Balat M.Bio-diesel from vegetable oils via transesteri?cation in

supercritical ethanol.Energy Edu Sci Technol2005;16:45–52.

[17]Goodrum JW.Volatility and boiling points of biodiesel from

from vegetable oils and tallow.Biomass Bioenergy2002;22: 205–11.

[18]Isigigur A,Karaosmonoglu F,Aksoy HA.Methyl ester from

saf?ower seed oil of Turkish origin as a biofuel for diesel engines.Appl Biochem Biotechnol1994;45/46:103–12. [19]Freedman B,Pryde EH.Fatty esters from vegetable oils for use

as a diesel fuel.Proceedings of the international conference on plant and vegetable oils as fuels1982;17–122.

[20]Fuls J,Hugo FJC.On farm preparation of sun?ower oil esters for

fuel Third international conference on energy use management 1981p.1595–602.

[21]Lang X,Dalai AK,Bakhshi NN,Reaney MJ,Hertz PB.

Preparation and characterization of bio-diesels from various bio-oils.Bioresour Technol2001;80:53–63.[22]Mittelbach M,Gangl S.Long storage stability of biodiesel made

from rapeseed and used frying oil.J Am Oil Chem Soc2001;78: 573–7.

[23]Encinar JM,Gonzalez JF,Rodriguez JJ,Tejedor A.Biodiesel

fuels from vegetable oils:transesteri?cation of Cynara cardunculus L.oils with ethanol.Energy Fuels2002;16:443–50.

[24]Canakci M,Van Gerpen J.Biodiesel production from oils and

fats with high free fatty acids.Trans ASAE2001;44:1429–36.

[25]Komers K,Stloukal R,Machek J,Skopal F.Biodiesel from

rapeseed oil,methanol and KOH3.Analysis of composition of actual reaction mixture.Eur J Lipid Sci Technol2001;103: 363–71.

[26]Acaroglu M,Oguz H,Ogut H.An investigation of the use of

rapeseed oil in agricultural tractors as engine oil.Energy Sources2001;23:823–30.

[27]Darnoko D,Cheryan M.Kinetics of palm oil transesteri?cation

in a batch reactor.J Am Oil Chem Soc2000;77:1263–7. [28]Diasakou M,Louloudi A,Papayannakos N.Kinetics of the non-

catalytic transesteri?cation of soybean oil.Fuel1998;77: 1297–302.

[29]Adams C,Peters JF,Rand MC,Schorer BJ,Ziemke MC.

Investigation of soybean oil as a diesel fuel extender:endurance tests.J Am Oil Chem Soc1983;60:1574–9.

[30]Chmielniak T,Sciazko M.Co-gasi?cation of biomass and coal

for methanol synthesis.Appl Energy2003;74:393–403. [31]Iwasa N,Kudo S,Takahashi H,Masuda S,Takezawa N.Highly

selective supported Pd catalysts for steam reforming of methanol.Catal Lett1993;19:211–6.

[32]Takezawa N,Shimokawabe M,Hiramatsu H,Sugiura H,

Asakawa T,Kobayashi H.Steam reforming of methanol over Cu/ZrO2.Role of ZrO2support.React Kinet Catal Lett1987;33: 191–6.

[33]Demirbas A,Gullu D.Acetic acid,methanol and acetone from

lignocellulosics by pyrolysis.Energy Edu Sci Technol1998;1: 111–5.

[34]Gullu D,Demirbas A.Biomass to methanol via pyrolysis

process.Energy Convers Manage2001;42:1349–56.

[35]Brown HP,Panshin AJ,Forsaith CC.Textbook of wood

technology.vol.II.New York:McGraw-Hill;1952.

[36]Sorensen HA.Energy conversion systems.New York:Wiley;

1983.

[37]Phillips VD,Kinoshita CM,Neill DR,Takahasi PK.Thermo-

chemical production of methanol from biomass in Hawaii.Appl Energy1990;35:167–75.

[38]Rowell RM,Hokanson AE.Methanol from wood:a

critical assessment.In:Sarkanen KV,Tillman DA,editors.

Progress in biomass conversion,vol.1.New York:Acedemic Press;1979.

[39]Lang X,Macdonald DG,Hill GA.Recycle bioreactor for

bioethanol production from wheat starch II.Fermentation and economics.Energy Sources2001;23:427–36.

[40]Demirbas A.Bioethanol from cellulosic materials:a renewable

motor fuel from biomass.Energy Sources2005;27:327–37. [41]Miranda R.Hydrogen from lignocellulosic biomass via

thermochemical processes.Energy Edu Sci Technol2004;13: 21–30.

[42]Czernik S,French R,Feik C,Chornet E.Chornet E.Production

of hydrogen from biomass by pyrolysis/steam reforming.In: Gregoire-Padro′C,Lau F,editors.Advances in hydrogen energy.

Dordrecht:Kluwer Academic/Plenum Publishers;2000.p.87–

91.

A.Demirbas/Progress in Energy and Combustion Science31(2005)466–487485