Liquid fuel from biomass: An overview - NOPR

Journal of Scientific & Industrial Research Vol. 64, November 2005, pp. 822-831

Liquid fuel from biomass: An overview Padma Vasudevan*, Satyawati Sharma and Ashwani Kumar Centre for Rural Development & Technology, Indian Institute of Technology, Hauz Khas, New Delhi 110 016 With depleting oil resources and negative environmental impacts associated with the use of petro fuels, there is a renewed interest in biomass based fuels, which can still form the base for sustainable development in terms of technoeconomics, environmental as well as socio-cultural considerations. As it is a locally available resource, energy equity can also be achieved at global levels and developing countries would stand to gain. However, to exploit the potential of biomass, more work is needed for converting it efficiently into modern energy carriers at competitive prices, supported by relevant policies. Currently, bioethanol and biodiesel have already reached commercial markets, especially as blends with petro fuels. This paper gives an overview on liquid biofuels covering the current and futuristic trends with respect to production and utilization of alcohols, vegetable oil based biodiesel and biocrude, emphasizing on the benefits to rural economy. Keywords: Biodiesel, Bioethanol, Biomass, Liquid fuel, Vegetable oil IPC Code: C10L1/02

Introduction Energy content of the biomass annually produced globally exceeds today’s world energy consumption by several factors1. Biomass can be converted into solid, liquid or gaseous fuels through thermo-chemical and biological routes. Liquid fuels, which are biofuels in the liquid form, are of special interest as petrol and diesel substitutes, in running internal combustion engines, especially for transportation. The rising cost of petroleum-based liquid fuels due to the depletion of oil sources, has brought biofuels back into focus. Biofuel is a non-polluting, locally available, accessible and reliable fuel obtained from renewable sources. For propagating biofuels, in addition to developing relevant technologies, sustainable production and availability of raw materials as well as marketing of final products have to be ensured. Raw material, technology to be adopted and the policy issues would vary from country to country2. Hall3 analyzed the potential of biomass energy in the context of “Industrialized Countries”. Country papers such as the “Potential of Liquid Biofuel in France” are also available4. A detailed report on Biofuel, by the Planning Commission, India5 and reports of various other ministries of Government of India have specifically emphasised the direct and indirect benefits of using biofuels. ________________ *Author for correspondence E-mail: [email protected], [email protected]

Liquid Biofuels

The cultivation, processing and use of liquid biofuels emit less climate-relevant CO2 than that of fuels from fossil sources. Biofuels are inherently more biodegradable than fossil fuels, and therefore represent a lower threat for inland and coastal waters. This, and the fact that biofuels are mostly consumed where they are produced, means that the risk of danger resulting from transportation is greatly minimized. Processing of biofuels and raw materials can pave way for multi-functional farming, which would lead to a new source of income and jobs in the area. For example, if European Union (EU) had a sustained demand for 2 million tons of biofuel, an estimated 2,000 jobs could be created in plant cultivation itself and 7,000 jobs would be generated in processing1. Given the labour surplus in developing countries with lower level of mechanization, employment potential is much higher5. Use of green energy sources, integrating biomass production with agriculture, forestry and wasteland regeneration would directly benefit rural economy through employment generation and increase of land productivity, at the same time reducing the rate of CO2 emission6. Types of Liquid Biofuels and Raw Material Sources

Liquid biofuels being considered world over fall into the following categories: i) Alcohols; ii) Plant seed oils; and iii) Biocrude and synthetic oils. Globally these are obtained from the following four broad categories of biomass sources: 1) Plantations specially raised for producing energy or energy and food such as energy

VASUDEVAN et al: LIQUID FUEL FROM BIOMASS: AN OVERVIEW

Table 1Current and projected gasoline and diesel consumption (billion litres)6 Region

Gasoline 2000 2020

Diesel 2000 2020

Africa Asea India Other Asia Brazil Other South America North and Central America Oceania Europe (including Russia) World

30 30 8 186 24 30 561 22 242 1132

34 60 43 253 3 34 242 16 333 1050

65 63 22 397 50 56 778 32 386 1829

65 111 100 469 61 56 293 21 439 1614 5

Table 2Properties of conventional and alcohol fuels Characteristics

Diesel

Energy content, MJ/kg 42.5 Kin Viscosity, mm2/s 4.01 Boiling point, oC 140-360 Flash point, oC 55-65 Auto ignition temperature, oC 230 Flammability limits, % gas in air 0.0-5.6 Motor Octane Number Cetane Number 45-55

Gasoline

Ethanol

44.0 0.6 37-205 -40 300 1.4-7.6 80-90-5

26.9 1.5 79 13 366 3.3-19.0 89 5

plantations, petro crops, agro-forestry etc.; 2) Agricultural residues and wastes including manure, straw, bagasse, and forest wastes; 3) Uncultivated biomass such as weeds; and 4) Organic urban or industrial wastes. Alcohols

Bioethanol is produced by fermentation of sugar and starchy crops. Cellulosic biomass is also being experimented for the production of bioethanol as this technology will help in using biomass residues from agricultural crops and forestry. Bio-methanol can be obtained by thermo-chemical degradation of lignocellulosic material. Biodiesel from Vegetable Seed Oils

Seed oils are combustible and have great potential to be used as biofuels. In principle, any vegetable or seed oil which essentially comprises triglycerides of long chain saturated and unsaturated fatty acids, can be burnt in a diesel engine. It is interesting to note that Rudolph Diesel in the preface of his patent of 1912 wrote “use of vegetable oil for engine fuel may seem insignificant today but such oil may become in the course of time, as important as petroleum”. Biodiesel is vegetable oils modified by transesterification to replace the glycerol molecules by methyl or ethyl groups. Biocrude

They are low molecular weight non-polar consti-

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tuents of plant, which can be directly extracted from biomass. These are generally a complex mixture of lipids, triglycerides, waxes, terpenoids, polysterol and other modified isoprenoids that can be catalytically upgraded for use as liquid fuels. The possibility of producing liquid fuel from biocrude have been considered using different kinds of raw materials, such as wood7,8 and laticiferous plant species9 such as Euphorbiaceae, Asclepiadaceae, Convolvulaceae and Moraceae. Some of the methods used are pyrolysis, hydropyrolysis and catalytic cracking. Applications of Liquid Biofuels

The liquid fuels are used for: a) Pure heat production; b) Electricity generation and combined heat production (CHP); and c) Vehicular transport. The first two come under stationary application in diesel pumps for irrigation and electricity generation. In this case, weight factor, which is always relevant to mobile applications, can be disregarded. In the latter, as in vehicular fuels for light and heavy vehicles, weight factor has to be considered. Automobile engines may be divided into two groups: (a) Constant volume cycle, spark ignition enginesFuel for this kind of engines is the gasoline fraction of crude oil used for light vehicles (car, two wheeler, three wheelers); and (b) Constant pressure cycle engine, alternately called compression ignition engine - Fuel for this kind of engine is diesel, used for heavy vehicles, in railway transport, tractors etc. Biodiesel is best suited to replace petro diesel, whose consumption is likely to go up substantially (Table 1). Bioethanol as a Substitute to Gasoline5

Petroleum reserves are finite. Emissions from engines using gasoline have NOx, SO2, CO2 and particulate matter (PM), which cause pollution. Gasoline has a knocking tendency. Tetra ethyl lead (TEL), as an additive, improves the anti-knocking rating of the fuel dramatically. However, due to harmful effects of lead, it has been banned. Benzene or cyclic compounds also increase the octane rating. Benzene is, however, a known carcinogenic material. Addition of oxygenated compounds helps in anti knocking. Ethanol as automotive fuel is advantageous as it contains oxygen (35%) based on biomass which is a renewable material. It reduces vehicular emissions of hydrocarbons (HC) and carbon monoxide (CO) and eliminates the use of lead, benzene, butadiene etc. The calorific value of ethanol is lower (by 40%) than that of gasoline, but increased efficiency in its use partly compensates for this (Table 2).

J SCI IND RES VOL. 64, NOVEMBER 2005

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Blends ( 20%) pose certain difficulties: (i) Higher aldehyde emissions; (ii) Corrosiveness, affecting metallic parts; (iii) Higher latent heat of vaporisation causing startability problem; (iv) Higher evaporation losses due to higher vapour pressure; and (v) Requiring large fuel tank due to lower calorific value. Ethanol is corrosive, absorbs moisture readily and can affect metallic parts (ferrous/ Table 3Cane ethanol blending: Supply and demand in 2020 (billion litres)6 Region

Africa Asean India Other Asia Brazil Other South America North and Central America Oceania Europe (including Russia) World

Demand 10% gasoline +3% diesel

Supply (E4 scenario)

Balance

9 10 6 56 7 8 88 4 52 239

22 29 49 23 62 17 31 7 0 239

13 19 43 -33 55 9 -57 3 -52 0

Table 4Properties of different methyl esters compared to diesel fuel5 Fuel Property

Formula Specific gravity Pour point, °C Viscosity mPa at 20°C Lower Heating Value, kJ/l Flash Point, °C Cetane Number

Rapeseed methyl ester C18-C19 0.88 -15 3.6 37 179 62

Soybean methyl ester C18-C19 0.87 -3 3.6 32 -52

No. 2 Diesel fuel C8-C25 0.81 -23 2.6-4.1 35-37 74 40-55

non-ferrous). Standards for ethanol use as fuel blending have been prescribed world over, Govt of India has initiated 5% ethanol-blended petrol with effect from 1st Jan 2003 and 10% ethanol blended petrol is also being envisaged10. Ethanol-diesel (15:85) emulsions can also give beneficial results in terms of emission reduction in diesel engines11,12 (41% PM, 5% NOx). The supply and demand in 2020 of cane ethanol blending has projected Brazil on top followed by India (Table 3). Biodiesel as Substitute to Petro Diesel5

Biodiesel has higher flash point temperature (>100°C), higher cetane number, lower sulphur content and lower aromatics than that of conventional diesel fuel. It could also be expected to reduce exhaust emissions due to fuel containing oxygen13. Presently, it is well established that significant reductions in emissions can be achieved by use of biodiesel14,15. Like other fossil fuels, High Speed Diesel (HSD) also makes net addition of carbon to the atmosphere. Further, petro diesel emits PM, especially below micron 2.5, which gets lodged in lungs causing reduction in its capacity. PM may carry un-burnt oil, which is carcinogenic. Also CO, HC, S and polycyclic aromatic hydrocarbon (PAH) emissions could be high. A 15% ethanol-diesel blend reduces PM emission. However, ethanol diesel blending requires emulsifier and there are certain storage and technical problems. Biodiesel is fatty acid ethyl or methyl ester and has properties similar to petroleum diesel (Table 4). Studies conducted with biodiesel on engines (Table 5) have shown reduction in PM (25-50%), HC, CO and PAH. However, a marginal increase in NOx (1-6%) is reported. This can be taken care of either by optimization of engine parts or by using a catalyst5,16. In conventional diesel fuels, additives for lubricity of fuel injection pump (FIP) are needed. Biodiesel is

Table 5Emission results of biodiesel and blends tests on IDI diesel engine5 Specifications

PM g/km

BS-II limit 0.17 Base line 0.129 With 10 % blend 0.093 With 15 % blend 0.080 % Improvement with respect to the base line 10 % blend 28 15 % blend 38

CO g/km

HC g/km

NOX g/km

HC+NOX g/km

1.5 0.77 0.65 0.62

0.37 0.22 0.16

0.79 0.83 0.89

1.2 1.16 1.04 1.05

15 20

41 50

-4 -12

10 10

Test Cycle: EEC+EUDC 90 kmph Cold Start (Mahindra & Mahindra)


reported to have superior lubricity. Its blending with petro diesel to increase flash point of the blend is useful, particularly in India, where petro diesel flash point is 35oC (well below the world average of 55oC). This is important for safety. The viscosity of biodiesel is higher and may lead to gum formation in injector, cylinder liner etc. Biodiesel can be blended in any ratio with petroleum diesel. The existing engines can use 20% biodiesel blend without any modification and reduction in torque output. Biodiesel is already in the commercial markets. USA uses B20 (20% blend) and B100 biodiesel. France uses B5 (5% blend) as mandatory in all diesel fuel. In EU, 5-15% blends are in use. The medium scale production of biodiesel on commercial basis has been developed in France, Germany, Italy, Austria and the USA17. They produced 1.5 million tons of biodiesel. In Philippines, diesel is mixed with coconut oil and the blend is used in tractors, buses and trucks, but it is not feasible in cooler countries where viscosity of oil increases and can cause damage to fuel pumps. Biomethanol

It is produced from synthetic gas or biogas and evaluated as a fuel for internal combustion engines18. Methanol can be used in blend with conventional fuels (without engine modification) or pure as a fuel. It can be used in traditional combustion engines and in direct methanol fuel cells, but it can also be used as a base product for making biodiesel from vegetable oils. The production of methanol is a cost intensive chemical process. Therefore, in current conditions, only waste biomass such as old wood or biowaste is used to produce methanol. Liquid Fuel Production Processes 1. Bioethanol Production5

Ethanol can be produced from any biological material that has sugar, starch or cellulose. The following are basic steps in converting biomass to bioethanol: 1) Converting biomass to a useable fermentation feedstock (typically some form of sugar) can be achieved using a variety of different process technologies; 2) Fermenting the biomass intermediates using biocatalysts (microorganisms including yeast and bacteria) to produce ethanol; and 3) Processing the fermentation product to yield fuel-grade ethanol and byproducts that can be used to produce other fuels, chemicals, heat and/or electricity.

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A) Sugar to Ethanol Production

In producing ethanol from sugar rich crops, sugar is fermented to alcohol using yeasts and other microbes. The fermentation process consists of breaking of starchy and sugary material into individual sugar molecules and then fermenting the sugar into ethanol and CO2. The fermentation of glucose/sugar to ethanol is energy efficient (93% of the feed energy is converted as ethanol and only a small amount is taken by yeast). [C6H10O5] n + nH2O → nC6H12O6 C6H12O6 → 2C2H5OH + 2CO2 A final step purifies ethanol to desired concentration and usually removes all water to produce ‘anhydrous ethanol’ that can be blended with gasoline. By fermentation alone, not more than 10 percent ethanol can be achieved. Distilling fermented liquor can provide a pure (95%) ethanol. Water in ethanol is undesirable in its use in gasoline blend and anhydrous alcohol (>99%) is required. Ethanol (95.6%) forms constant boiling mixture with water that does not allow simple distillation to meet the purpose. As a solution, azeotrophic distillation through solvent benzene or cyclohexane is used. Azeotrophic distillation, however, increases production cost of ethanol considerably. A cost effective solution is to use molecular sieve to eliminate water [Pressure Swing Adsorption-Molecular Sieve Dehydration Technology (MSDH)]. A synthetic adsorbent is used to dehydrate alcohol and to a high level of dryness with low energy inputs. Use of vapour phase adsorption has resulted into further energy saving in the process5. Today, bioethanol is the most widely used liquid fuel; the world ethanol production (60%) is from sugar crops. USA and Brazil are large-scale producers (65%)10. India is the largest producer of sugar in the world and has high potential for ethanol production19 (Table 6). SugarcaneThe major source of ethanol production in Brazil, India and other sugarcane-raising countries is sugar-molasses route. This provides better economy by sale of sugar; molasses becomes the byproduct of sugar. SugarbeetIn EU, sugarbeet is preferred. Sugarbeet has certain advantages5 over sugarcane (Table 7) as follows: i) Lower cycle (5-6 months) of crop production; ii) Higher yield (35-40 tons/acre);


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Table 6Ethanol availability based on production from molasses and its uses in India19 Year

Molasses production Ml

Production of alcohol Gl

Industrial use Ml

Potable use Ml

Other uses Ml

Surplus availability of alcohol Ml

1998-99 1999-00 2000-01 2001-02 2002-03 2003-04 2004-05 2005-06 2006-07

7.00 8.02 8.33 8.77 9.23 9.73 10.24 10.79 11.36

1.41 1.65 1.69 1.77 1.87 1.97 2.07 2.19 2.30

534.4 518.9 529.3 539.8 550.5 578.0 606.9 619.0 631.4

584.0 622.7 635.1 647.8 660.7 693.7 728.3 746.5 765.2

55.2 57.6 58.8 59.9 61.0 70.0 73.5 77.2 81.0

238.2 454.8 462.7 527.7 597.5 627.5 665.8 744.3 822.8

Table 7Comparison of cane and sugarbeet5 Properties

Cane

Sugarbeet

Yield per acre, tons Cycle of crop, months Sugar content on weight, % Sugar yield, tons/acre/y Ethanol yield (100%), l/acre/y

25-30 10-11 12-16 3.0-4.8 1,700-2,700

35-40 5-6 14-18 4.9-7.2 2,800-4,100 (with one cycle/y)

iii) High tolerance to wide range of climatic variation; and iv) Low water and fertilizer requirement (compared to sugarcane, sugarbeet requires 35-40% water and fertilizers). Ethanol yield from sugarcane is ~1,700-2,000 l/acre/y while for sugarbeet 2,800-4,100 l/acre/y per unit of land even taking only one crop (no credit for other crops). Harvesting of sugarbeet is also easier and it requires lower energy for juice extraction. Sweet sorghumSweet sorghum can be cultivated in temperate and tropical regions, increasing its potential benefits. Other crops that can yield oligosaccharides (potatoes, cereals, grapes, etc.) are generally not much utilized for bioethanol production (with the exception of corn in the USA). However, particular varieties of sweet sorghum recently developed in China, the USA, and the EU have very attractive and economically promising characteristics. India is also producing sweet sorghum. The Nimbalkar Agriculture Institute, India has claimed to have developed a variety of sweet sorghum with potential to produce 2-4kl/ha/y of ethanol20,21. Sweet sorghum produces a very high yield in terms of grains, sugar and lignocellulosic biomass (average, 30 dry tons/ha/y). B) Cellulose Biomass to Ethanol Production5

In this, cellulose is converted to simple sugars

(monosaccharides), which are enzymatically hydrolyzed to yield ethanol under following processes: Dilute Acid HydrolysisHydrolysis occurs in two stages to maximize sugar yields from the hemicellulose and cellulose fractions of biomass. The first stage is operated under milder conditions to hydrolyze hemicellulose, while the second stage is optimized to hydrolyze the more resistant cellulose fraction. Liquid hydrolyzates are recovered from each stage, neutralized, and fermented to ethanol. There is quite a bit of industrial experience with the dilute acid process. Germany, Japan, and Russia have operated dilute acid hydrolysis percolation plants off and on over the past 50 years. However, these percolation designs would not survive in a competitive market situation. Today, companies are beginning to look at commercial opportunities for this technology, which combine recent improvements and niche opportunities to solve environmental problems. Dilute acid hydrolysis can be used to recover sugar from sugarcane bagasse. Concentrated Acid HydrolysisThis process is based on concentrated acid decrystallization of cellulose followed by dilute acid hydrolysis to sugars. Separation of acid from sugars, acid recovery, and acid re-concentration are critical unit operations. Attempts are being made to commercially convert rice straw into ethanol and lignocellulosic components of municipal solid waste to ethanol.


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Table 8Properties of biodiesel from different oils26-29 Vegetable oil methyl esters (biodiesel) Peanut Soyabean Babassu Palm Sunflower Tallow Diesel 20% biodiesel blend

Kinematic viscosity mm2/s

Cetane no. °C

Lower heating value MJ/kg

Cloud point °C

Pour point °C

Flash point °C

Density kg/l

4.9 4.5 3.6 5.7 4.6 3.06 3.2

54 45 63 62 49 50 51

33.6 33.5 31.8 33.5 33.5 43.8 43.2

5 1 4 13 1 12 -

−7 9 −16 −16

176 178 127 164 183 96 76 128

0.883 0.885 0.875 0.880 0.860 0.855 0.859

Table 9Properties of vegetable oils30 Vegetable oil

Corn Cottonseed Linseed Rapeseed Safflower Sesame Soyabean Sunflower Palm Babassu Diesel

Kinematic viscosity at 38°C, mm2/s

Cetane no. °C

Heating value MJ/kg

Cloud point °C

Pour point °C

Flash point °C

Density kg/l

34.9 33.5 27.2 37.0 31.3 35.5 32.6 33.9 39.6 30.3 3.06

37.6 41.8 34.6 37.6 41.3 40.2 37.9 37.1 42.0 38.0 50

39.5 39.5 39.3 39.7 39.5 39.3 39.6 39.6 43.8

−1.1 1.7 1.7 −3.9 18.3 −3.9 −3.9 7.2 31.0 20.0 -

−40.0 −15.0 −15.0 −31.7 −6.7 −9.4 −12.2 −15.0 −16

277 234 241 246 260 260 254 274 267 150 76

0.9095 0.9148 0.9236 0.9115 0.9144 0.9133 0.9138 0.9161 0.9180 0.9460 0.855

Enzyme HydrolysisThe goal is to reduce cost of cellulase enzymes in bioethanol process by employing cutting-edge and efficient biochemical technologies. Therefore, research is being focused on development of biological enzymes that can breakdown cellulose and hemicellulose. An important process modification is the introduction of simultaneous saccharification and fermentation (SSF), which is another novel process for converting cellulose to ethanol22. This has recently been improved to include the co-fermentation of multiple sugar substrates. In SSF process, cellulose, enzymes and fermenting microbes are combined, reducing the number of vessels and improving efficiency. As sugars are produced, fermentive organisms convert them to ethanol. Thus, all required enzymes are produced within the reactor vessel, using same microbial community to produce both the enzymes that help break down cellulose to sugars and to ethanol23. Recent studies have employed genetically engineered Gram-negative bacteria to produce ethanol from sugars with high efficiency for example, Zymomonas mobilis genes alcohol dehydrogenase

(adhB) and pyruvate decarboxylase (pdc)24,25 were encoded in bacteria. 2. Biodiesel Production

Biodiesel generally refers to fatty acid methyl esters made by transesterification. This is a chemical process in which feedstock oil or fat reacts with methanol and KOH or other type of catalysts used. The feedstock can be vegetable oil, such as that derived from oil seed crops (soyabean, sunflower, rapeseed etc.), waste vegetable oil or animal fat. For biodiesel, oil extracted from crushed oil seeds is used, either directly or as heating oil. In seeds, oil energy content is around 40 Gj/ton, which is similar to that of diesel at 38-45 GJ/ton18. The most commonly used oils for the production of biodiesel26-30 (Tables 8 & 9) are soybean31,32, sunflower33,34, palm35, rapeseed36, canola37, cotton seed38, Jatropha39 and pongamia40. In India, the high cost of edible oils prevents their use in biodiesel preparation but nonedible oils are affordable for biodiesel production. India has more than 100 million ha of waste-


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lands, which can be utilized for the production of Jatropha (ratanjyot). However, in utilizing Jatropha seeds on a large scale, due care must be taken with regard to some toxic components41-50 present in the seeds as well as the oil cake. Other tree based oils with significant production potentials are sal (Shorea robusta), neem (Azadirachta indica), mahua (Mahua indica)51, besides karanj (Pongamia pinnata)52 and ratanjyot (Jatropha curcas). These oils have great potential to make biodiesel for supplementing other conventional sources. Waste vegetable oils are virtually inexhaustible source of energy, which might also prove an additional line of production. These oils contain some degradation products of vegetable oils and foreign material. However, analyses of used vegetable oils indicate that differences between used and unused fats are not much and in most cases simple heating and removal by filtration of solid particles suffices for subsequent transesterification5. TransesterificationThis process is the displacement of alcohol from an ester by another alcohol in a process similar to hydrolysis and has been widely used to reduce the viscosity of triglycerides. Most natural triglyceride oils are a mixture of 2-10 fatty acids. The chemical make up of fatty acid moiety in the triglyceride molecule depends on which oil is used as the original feedstock and what type of fatty acid it contains. The transesterification reaction is represented by the general equation: RCOOR’ + R” →RCOOR” + R’OH If methanol is used in the above reaction, it is termed methanolysis. The reaction of triglyceride with methanol is represented by the general equation: CH 2 − O − COR1  Heat CH − O − COR2 + 3CH 3 OH Catalyst  CH 2 − O − COR3 Triglyceride Methanol

→

R1 − COOCH 3 CH 2 OH +  R2 − COOCH 3 + CHOH +  R3 − COOCH 3 CH 2 OH Fatty acid methyl ester Glycerol

where, R1, R2 and R3 are specific fatty acids depending on the triglyceride used. Product recovery is done into

two phases18. In the first phase, there is easy removal of glycerol, a valuable industrial by-product. In the second phase, remaining alcohol/ester mixture is separated and excess alcohol is recycled. Then the esters are sent to the purification process, which consists of water washing, vacuum drying and filtration. A catalyst is usually used to improve the reaction rate and yield. Transesterification works well when the starting oil is of high quality. However, quite often low quality oils are used as raw materials for bio-diesel preparation. In cases, where free fatty acids (FFA) content of the oil is above 1 percent, difficulties arise due to soap formation, which promotes emulsification during the water washing stage and at an FFA content above 2 percent, process becomes unworkable. The process variables that influence transesterification reaction time and conversion are reaction temperature, oil temperature, and ratio of alcohol to oil, mixing intensity, purity of reactants, catalyst type and concentration. Technical details on these have been the subject of some recent reviews53. Catalysts are classified as alkali, acid, or enzyme. Alkali-catalyzed transesterification is faster than acidcatalyzed54. Alkalis include sodium hydroxide, sodium methoxide, potassium hydroxide, potassium methoxide, sodium amide, sodium hydride, potassium amide and potassium hydride55. If glycerides have higher FFA and more water, acid-catalyzed transesterification is suitable. The acids could be H2SO4, H3PO4, HCl or organic sulfonic acid. Besides chemical catalysts, enzyme catalysts and supercritical alcohol treatment are of interest, given the increasing environmental concerns. The use of lipase producing microbial cells immobilized within porous biomass support particles (BSPs) in whole cell biocatalysis is reported to be effective in improving cost efficiency since the immobilization can be achieved spontaneously during batch cultivation and no purification of lipase is necessary. For example, immobilized Rhizopus delemar and Rhizomucor miehei lipases efficiently catalyzed alcoholysis with long-chain fatty alcohols even in the presence of 20 percent water56-58. Lipases catalyze not only hydrolysis but also esterification and transesterification in nonaqueous medium. Methanolysis of triacylglycerols (TAGs) with a lipase thus is considered one of the effective reactions for production of biodiesel fuel from waste edible oil59. An immobilized lipase60 was employed to catalyze the


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methanolysis of corn oil in flowing supercritical CO2 with high ester conversion (>98%). Use of immobilizer Rhizopus oryzae for methanolysis of soyabean oil has been reported61.

experimental phase. Its use has only been tested in a few diesel vehicles. If diesel vehicles were designed and produced to run on DME, they would become inherently very low pollutant emitting vehicles.

3. Biomass Gasification62

Bio-oil

Another approach to convert biomass into liquid or gaseous fuels is direct gasification, which is based on the partial combustion of material in a restricted supply of air or oxygen to give producer gas18, which consists mainly of CO, H2 and CH4. Other possible target fuels include methanol, synthetic diesel and gasoline (latter two produced using the “FischerTropsch” process to build carbon chain molecules), dimethyl ether (DME, a potential alternative fuel for diesel engines with good combustion properties and low emissions), and gaseous fuels such as CH4 and H2. DME and the gaseous fuels are not compatible with today’s gasoline or diesel vehicles and would need both new types of vehicles (compressed natural gas or hydrogen fuel cell vehicles) and new refueling infrastructure.

When organic materials are subjected to fast pyrolysis by rapidly heating (450-600 °C) in absence of air, organic vapour, pyrolysis gases and charcoal are produced. Vapours are condensing to liquid fuel termed as bio-oil; typically 70-75 percent of feedstock can be converted into bio-oil depending upon condition64. Bio-oil can be used as liquid fuel in boilers, kilns, turbines and diesel engines.

Fischer-Tropsch (F-T) Fuels

F-T process converts “syngas” (CO and H2) into diesel fuel and naphtha (basic gasoline) by building polymer chains out of these basic building blocks. Typically, a variety of co-products (various chemicals) are also produced. F-T process is quite expensive if only gasoline and diesel products are considered63. Methanol

Syngas can also be converted into methanol through dehydration or other techniques and in fact methanol is an intermediate product of F-T process (and is therefore cheaper to produce than F-T gasoline and diesel). Methanol is somewhat out of favour as a transportation fuel due to its relatively low energy content and high toxicity, but might be a preferred fuel if fuel cell vehicles are developed with on board reforming of hydrogen (since methanol is an excellent hydrogen carrier and relatively easily reformed to remove the hydrogen). Dimethyl Ether (DME)

DME also can be produced from syngas, in a manner similar to methanol. It is a promising fuel for diesel engines, due to its good combustion and emissions properties. However, like LPG, it requires special fuel handling and storage equipment and some modifications of diesel engines, and is still at an

Conclusions Liquid fuels from biomass have already entered commercial markets in many countries especially as blends (up to 20-25 %) with gasoline and diesel. Bioethanol production is now well established and the issue is of making cheap alcohol through the use of waste feed stock such as cellulosic materials by enzyme fermentation techniques. Use of vegetable oils directly and as micro emulsions and processing through pyrolysis etc. for generating diesel substitute have been tried65. However, direct use of vegetable oils still faces many technical problems and transesterification has become the well-established route. Here again use of enzymes and immobilized whole cells are being tried for reducing cost of the product. Engine modifications for higher level of biodiesel and other products and new ways of getting power, such as the use of fuel cell technology are in consideration. Raw materials range from edible to nonedible oils must be selected carefully considering the toxicity and other aspects. Depending on the availability in a country, one or the other raw material seems economically viable for production of biomethanol and biodiesel. Every country has scope for developing biofuels for their energy security, with con-commitment economic and environmental benefits. However, more R&D is needed on improving production process with better catalysts and increasing yields. With suitable policy support, infrastructure has to be established ultimately aiming at using pure (100%) biofuels. Both the developed and developing countries which have the potential to grow biomass, can stand to gain by immediately adopting currently available technologies and participating in R&D for further development with regard to biofuels best suited to their conditions.


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