Energy Conversion and Management

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Energy Conversion and Management
journal homepage: www.elsevier.com/locate/enconman

Biofuels securing the planet’s future energy needs
Ayhan Demirbas *
Sila Science, University Mah, Mekan Sok No: 24, Trabzon, Turkey

a r t i c l e

i n f o

a b s t r a c t
The biofuels include bioethanol, biobutanol, biodiesel, vegetable oils, biomethanol, pyrolysis oils, biogas, and biohydrogen. There are two global biomass based liquid transportation fuels that might replace gasoline and diesel fuel. These are bioethanol and biodiesel. World production of biofuel was about 68 billion L in 2007. The primary feedstocks of bioethanol are sugarcane and corn. Bioethanol is a gasoline additive/ substitute. Bioethanol is by far the most widely used biofuel for transportation worldwide. About 60% of global bioethanol production comes from sugarcane and 40% from other crops. Biodiesel refers to a diesel-equivalent mono alkyl ester based oxygenated fuel. Biodiesel production using inedible vegetable oil, waste oil and grease has become more attractive recently. The economic performance of a biodiesel plant can be determined once certain factors are identified, such as plant capacity, process technology, raw material cost and chemical costs. The central policy of biofuel concerns job creation, greater efficiency in the general business environment, and protection of the environment. Ó 2009 Elsevier Ltd. All rights reserved.

Article history: Received 24 September2008 Accepted 16 May 2009 Available online 16 June 2009 Keywords: Biofuel Bioethanol Biodiesel Economic and environmental impacts

1. Introduction Liquid biofuels such as bioethanol and biodiesel may offer a promising alternative [1–3]. Because of increase in petroleum prices especially after petrol crisis in 1973 and then gulf war in 1991, geographically reduced availability of petroleum and more stringent governmental regulations on exhaust emissions, researchers have studied on alternative fuels and alternative solution methods [4–6]. The use of biofuels decreases the external energy dependence, promotion of regional engineering, increased R&D, decrease in impact of electricity production and transformation; increases the level of services for the rural population, creation of employment, etc. [7–9]. The term biofuel or biorenewable fuel (refuel) is referred to as solid, liquid or gaseous fuels that are predominantly produced from biomass [10–17]. Liquid biofuels being considered world over fall into the following categories: (a) bioalcohols [18–21], (b) vegetable oils [22–24] and biodiesels [25–27]; and (c) biocrude and synthetic oils [28–37]. Biofuels are important because they replace petroleum fuels. It is expected that the demand for biofuels will rise in the future. Biofuels are substitute fuel sources to petroleum; however, some still include a small amount of petroleum in the mixture [38,39]. Biofuels are generally considered as offering many priorities, including sustainability, reduction of greenhouse gas emissions, regional development,social structure and agriculture, security of supply [40–43].

Today bioethanol is the most used non-fossil alternative engine fuel in the world. The choice of raw material depends on local conditions. Bioethanol is good alternate fuel that is produced almost entirely from food crops. The primary feedstock of this fuel is corn. An important advantage of crop-based ethanol is its greenhouse benefits [18 ]. Due to the increasing concern on environmental protection, numerous researches on the usage of biodiesel are carried out in recent years. Biodiesel has become more attractive recently because of its environmental benefits [44–56]. The biggest difference between biofuels and petroleum feedstocks is oxygen content [57]. Biofuels are non-polluting, locally available, accessible, sustainable and reliable fuel obtained from renewable sources [58]. Sustainability of renewable energy systems must support both human and ecosystem health over the long term, goals on tolerable emissions should look well into the future [54 ]. Electricity generation from biofuels has been found to be a promising method in the nearest future [29 ]. The future of biomass electricity generation lies in biomass integrated gasification/gas turbine technology, which offers high-energy conversion efficiencies [8]. Liquid biofuels for transportation have recently attracted hugeattention in different countries all over the world because of its renewability, sustainability, common availability, regional development, rural manufacturing jobs, reduction of greenhouse gas emissions, and its biodegradability [39 ]. Biofuels offer significant benefits for energy security. Table 1 shows the availability of modern transportation fuels. Policy drivers for biorenewable liquid biofuels have attracted in rural development and economic opportunities for developing countries [62]. The European Union is on the third rank of biofuel production world wide, behind Brazil


2240 Table 1 Availability of modern transportation fuels. Fuel type

A. Demirbas / Energy Conversion and Management 50 (2009) 2239–2249

Availability Current Future Moderate–poor Excellent Excellent Moderate Excellent

Gasoline Bioethanol Biodiesel Compressed natural gas (CNG) Hydrogen for fuel cells

Excellent Moderate Moderate Excellent Poor

and the United States. In Europe, Germany is the largest, and France the second largest producer of biofuels [63]. 2. Bioenergy from biomass Modern bioenergy is commercial energy production from biomass for industry, power generation, or transport fuels. Biomass is the most common form of carbonaceous materials, widely used in the third world. Bioenergy is an inclusive term for all forms of biomass and biofuels. Green energy is an alternate term for renewable energy that the energy generated from sources which are considered environmentally friendly [39]. Green power refers to electricity supplied from more readily renewableenergy sources than traditional electrical power sources. Green power products have become widespread in many electricity markets worldwide which can be derived from renewable energy sources [64]. Using of green energy sources like hydro, biomass, geothermal, and wind energy in electricity production reduces CO2, SO2 and NOx emissions. Market research indicates that there is a large potential market for green energy in Europe in general. Green power marketing has emerged in more than a dozen countries around the world [65]. Biomass gasification technologies provide the opportunity to convert renewable biomass materials into clean fuel gases or synthesis gases. These gaseous products can be burned to generate heat or electricity, or they can potentially be used in the synthesis of liquid transportation fuels, hydrogen, or chemicals. Gasification offers a combination of flexibility, efficiency, and environmental acceptability that is essential in meeting future energy requirements. Biomass fuelled renewable distributed generation technologies in rural electrification provide no-regret options with significant CO2 emission mitigation potential when operated under net-metering scheme [61]. The future of biomass electricity generation lies in biomass integrated gasification/gas turbine technology, which offers high-energy conversion efficiencies [66]. Cogeneration, in and of itself, is an example of pollution prevention. Cogenerators, by using excess heat, may enhance the efficiency of total energy use by up to 80% or more from the typical 33–38% efficiency ofelectricity-only generation. A typical cogeneration system consists of an engine, steam turbine, or combustion turbine that drives an electrical generator. Fig. 1 shows a gas turbine topping cycle cogeneration system. Fig. 1 is a diagram of a
Exhaust STEAM Condensate Return

gas turbine of the type used for cogeneration power plants with sizes about 500 kW to 50 MW. These turbines are similar to jet engines used in aircraft. In this size range, gas turbines are more efficient than any other type of fossil fuel fired power plants, reaching electrical efficiencies of up to 45% and with the addition of waste heat recovery (cogeneration), efficiencies of over 80%. When used for cogeneration, the hot exhaust gases from the gas turbine are passed through a heat exchanger where the heat is transferred to water in pipes producing steam. A waste heat exchanger recovers waste heat from the engine and/or exhaust gas to produce hot water or steam. Cogeneration produces a given amount of electric power and process heat with 15–35% less fuel than it takes to produce the electricity and process heat separately [67]. Fig. 2 illustrates comparison between energy inputs to separate and cogenerative generation systems. Traditional coal, oil or natural gas fired thermal generating stations convert only about onethird of the initial energy contained within the fuel into useful electricity. The remainder of the energy is discarded as heat without serving any useful purpose. From 10% to 35% of primary energy use is wasted as conversion losses in power plants. Cogeneration can increase theefficiency of a fossil fuel from an average of 40% to over 80%. Fig. 2 illustrates the increase in efficiency. This increase in efficiency can translate into lower costs and fewer emissions of pollutants than the conventional alternative of generating electricity and heat separately [67]. 2.1. Biomass conversion processes Biomass, mainly in the form of wood, is the oldest form of energy used by humans. Wood has been used in direct combustions as an important energy source in developing countries [68]. Biomass is used to meet a variety of energy needs, including generating electricity, heating homes, fueling vehicles, and providing process heat for industrial facilities [69]. If briquettes from lignocellulosic materials are to be used efficiently and rationally as fuel, they must be characterized to determine such technological parameters as their reactivity, moisture content, density, ashes, volatile matter, and heat value, along with their major component elements [70]. Biomass can be used as a solid fuel, or converted into liquid or gaseous forms for the production of electric power, heat, chemicals, or gaseous and liquid fuels. Thermochemical conversion processes include three sub-categories: pyrolysis, gasification, and liquefaction. Fig. 3 shows the biomass thermal conversion processes. A variety of biomass resources can be used to convert to liquid, solid and gaseous fuels with the help of some physical, thermochemical, biochemical and biological conversion processes. Main biomass conversion processes are direct liquefaction, indirect liquefaction,physical extraction, thermochemical conversion, biochemical conversion, and electrochemical conversion. In liquefaction, pyrolysis and gasification processes, high temperatures are used to break down the wastes containing mostly hydrocarbons with no (in liquefaction and pyrolysis) or less oxygen than incineration (in gasification). 2.1.1. Mechanisms of thermochemical biomass conversion processes Thermal degradation of cellulose proceeds through two types of reaction: a gradual degradation, decomposition, and charring on heating at lower temperatures; and a rapid volatilization accompanied by the formation of levoglucosan on pyrolysis at higher temperatures. The hemicelluloses reacted more readily than cellulose during heating. Dehydration reactions around 473 K are primarily responsible for thermal degradation of lignin. Between 423 K and 573 K, cleavage of a- and b-aryl–alkyl-ether linkages occurs. Around 573 K, aliphatic side chains start splitting off from the aromatic ring [71 ].


Pyrolysis and Hydrothermal liquefaction

The thermal liquefaction process by using glycerol occurs in two steps, the faster first step in which the lignin is made available by breaking of the lignin–carbohydrate bonds. The released lignin is then dissolved in the organic phase. The slower second step may be indicative of a much more complex process. Glycerol reduces the surface tension of the solvent at high temperature, thus promoting the penetration of the alkali into the particles and the diffusion of the breakdown products of lignin from the wood into the solvent, assuring a uniform distribution of the reagents within the wood [31]. 2.2. Pyrolysis of biomass

The gasification of biomass is a thermal treatment which results in a high proportion of gaseous products and small quantities of char (solid product) and ash. Complete gasification of biomass involves several sequential and parallel reactions. Most of these reactions are endothermic and must be balanced by partial combustion of gas or an external heat source [73]. Liquefaction is a low-temperature, high-pressure thermochemical process using acatalyst. In the liquefaction process, the micellar-like broken-down fragments produced by hydrolysis are degraded to smaller compounds by dehydration, dehydrogenation, deoxygenation, and decarboxylation. These compounds once produced, rearrange through condensation, cyclization, and polymerization, leading to new compounds. Thermal depolymerization and decomposition of biomass, cellulose, hemicelluloses, and products were formed as well as a solid residue of charcoal [74]. Hydrogen liquefaction of sawdust in tetralin was performed in an autoclave at below conditions: temperature range from 473 K to 623 K; initial cool hydrogen pressure range from 4 to 10 MPa; reaction time range from 10 to 100 min. The effect of variables on the process of sawdust liquefaction was examined. The existence of H2 or tetralin improves both the conversion of sawdust and the oil yield. The maximum oil yield from the liquefaction was 67.1%. Temperature has a remarkable effect than initial cool hydrogen pressure and reaction time on the process of sawdust liquefaction [75].

Pyrolysis is the thermal decomposition of organic matter occurring in the absence of oxygen or when significantly less oxygen is present than required for complete combustion. Pyrolysis is the basic thermochemical process for converting biomass to a more useful fuel. A study of pyrolysis of olive cake at the temperature range from 673 K to 973 K has been carried out for production of bio-oil. As the pyrolysis temperature was increased, the percentage mass of char decreased whilst gas product increased[33]. Apricot stone (Prunus armeniaca L.) was pyrolyzed in a directly heated fixed-bed reactor under nitrogen atmosphere. Pyrolysis runs were performed using reactor temperatures between 673 K and 973 K with heating rate of about 300 K/min. As the pyrolysis temperature was increased, the percentage mass of char decreased while gas product increased. The bio-oil obtained at 825 K, at which the liquid product yield was maximum, was analyzed. Chemical fractionation of bio-oil showed that only low quantities of hydrocarbons were present, while oxygenated and polar fractions dominated [76]. A comparative study of the thermochemical behavior of cottonseed cake in static, nitrogen and steam atmospheres has been carried out. Pyrolysis under water vapor gave a rise in the yield as opposed to pyrolysis under static and nitrogen atmospheres [77]. Oxidative pyrolysis of Cuban pine sawdust was investigated using an autothermal fluidized bed reactor. The results indicated that the liquid and char products obtained may be a potentially valuable source of chemical feedstocks [78]. A continuous bench




fluidized bed pyrolysis has been designed and is currently under testing. The model and experimental results indicated that two zones exist inside of the fluidization column. The dense bed where the exothermic and endothermic reactions are active, and the freeboard zone where the temperature of the pyrolysis product decreases continuously; the bed temperature increases with an increase in the airfactor [79]. Bio-oils derived from biomass have been increasingly attracting attention as alternative sources of fuels and chemicals. Fixed-bed slow pyrolysis experiments have been conducted on a sample of hazelnut bagasse to determine particularly the effects of pyrolysis temperature, heating rate, particle size and sweep gas flow rate on the pyrolysis product yields. Under the various pyrolysis conditions applied in the experimental studies, the obtained char, liquid, and gas yields ranged between 26 and 35 wt , 23 and 34.40 wt.%, and 25 and 32 wt.%, respectively. The maximum bio-oil yield of 34.40% was obtained at the final pyrolysis temperature of 773 K, with a heating rate of 10 K/min, particle size range of 0.425– 0.600 mm and a 150 cm3/min of sweep gas flow rate [80]. The empirical formula of bio-oil that has a heating value of 34.57 MJ/ kg was established as CH1.45O0.33N0.127 [81]. The bio-oils were composed of a range of cyclopentanone, methoxyphenol, acetic acid, methanol, acetone, furfural, phenol, formic acid, levoglucosan, guaiocol and their alkylated phenol derivatives. The structural components of the biomass samples mainly affect pyrolytic degradation products [82].

3. Chemicals and fuels from biomass Biomass provides a potential source of added value chemicals, such as reducing sugars, furfural, ethanol and other products, by using biochemical or chemical and thermochemical processes. The gas phase of pyrolitic degradation products contains mostly carbon monoxide and carbon dioxide, and minor proportions of hydrogen, methane, ethane, andpropane. The liquid fraction consists mainly of water, with small proportions of acetaldehyde, propion aldehyde, butyraldehyde, acrolein, croton-aldehyde, furan, acetone, butanedione, and methanol. The sugars from the hemicelluloses are also fermented to bioethanol [32]. More recently, many attempts have been made to utilize biomass, such as wood, hazelnut shell, agricultural waste residues, waste paper and wheat straw, tea waste, and olive husk [5 –85]. The main biomass sources in use for energy production range from forest residues, agricultural residues, pulp and paper operation residues, animal waste, and landfill gas to energy crops [14,86–90]. Olive cake is a very promising material for the production of bio-oil. The highest bio-oil yields from the olive cakes were 31.0% at 700 K, 36.0% at 700 K and 41.0% at 700 K obtained from 10 K/s, 20 K/s and 40 K/s heating rate runs, respectively [34,22]. Olive cake is a by-product of olive oil production and is a solid material consisting of seed particles and the fleshy parts of olive. The Mediterranean region represents 98% of the world’s olive tree population [91]. Various types of agricultural residues such as straw, plant stems, leaves, pruning residues of fruit trees, oil seeds, stover, stalk, stone, peel, seed, kernel shell, and husk were can be used as biomass energy sources [40,92–95]. Ethanol from agricultural and forestry residues, energy crops, and other forms of lignocellulosic biomass could address these issues and result in net CO2 reductions [19]. Hazelnut shell was subjected tothermogravimetric analysis to establish burning profiles in a dynamic dry air atmosphere. A strong relation was determined between the heating rate and the intensity of the peaks on the burning profiles [96]. The fuel properties of mosses and algae, and the effect of pyrolysis temperature on the yield of bio-oil from moss and alga samples were investigated. The yield of bio-oil from pyrolysis of the samples increased with temperature [93 ]. Pyrolysis process of agricultural residues are the most common and convenient methods for conversion into bio-oil and bio-char [5]. Charcoal yield ranged from 24.23% to 37.89 wt and calorific value varied from 17.29 to 33.47 MJ/kg. Conversion of charcoal fines to solid fuel improved combustion quality [98]. Seven indigenous tree species of northeast India were pyrolyzed at temperatures ranging from 573 K to 1073 K with two different heating rates, 3 K/min and 20 K/min, and the effect of heating temperature and heating rate on the products yield and char quality were analyzed and discussed [13]. The bagasse char samples were prepared by carbonizing bagasse in a fixed-bed reactor at temperatures between 773 K and 1073 K. It was observed that raising the carbonization temperature resulted in a significant decrease in reactivity of bagasse char [99]. The chars from pyrolysis of the demineralized biomasses can be used in adsorption applications as activated carbons [35]. The properties of the char arising from pyrolysis of sugarcane bagasse at 873 K and 1073 K were determined to evaluate potentialities for specific end uses.The chars were found fairly adequate as solid biofuels [83]. The integrated organic waste-anaerobic digester-energy crop production system as a eco-agricultural system and to use anaerobically digested cattle slurry as fertilizer for safflower production was investigated [100]. Recycling of organic wastes by this system can decrease input of chemical fertilizer and use of fossil fuels [100]. Municipal wastewater treatment plants generate sludge as a by-product of the physical, chemical and biological processes used in the treatment of wastewater. Biogas can be produced from wastewater sludge by using anaerobic digestion [11]. Most of the total waste is organic, which could be utilized through a process of anaerobic digestion and already has been in use for decades in industrialized nations to produce clean burning methane gas, electricity, fuel, and fertilizers [101]. Reduction in the particle size of coir pith improved methane yield about 1.5 times compared to natural sample. Whereas acid treatment suppressed methane formation, alkali treatment has resulted in a slight improvement [102]. Greenhouse gases emissions from the widely accepted conventional biogas plants were investigated [103]. Biogas can be used instead of compressed natural gas to power gas vehicles, offering excellent air quality benefits as well as carbon savings. Bioethanol is a gasoline additive/substitute. Sugar cane, molasses and corn are good sources of bioethanol. Bioethanol is derived from alcoholic fermentation of sucrose or simple sugars, which are produced from biomass byhydrolysis process [12]. It is possible that corn stover may be economically converted to bioethanol [28]. Future technologies may allow bioethanol to be produced from a variety of source materials including wood, grass, straw and green waste. Methanol can be produced from hydrogen–carbon oxide mixtures by means of the catalytic reaction of carbon monoxide and some carbon dioxide with hydrogen. Biosynthesis gas (bio-syngas) is a gas rich in CO and H2 obtained by gasification of biomass. Mixture of gases from organic waste materials is converted to methanol in a conventional steam-reforming/water–gas shift reaction followed by high-pressure catalytic methanol synthesis [20]. Biodiesel is known as monoalkyl, such as methyl and ethyl, esters of fatty acids. Biodiesel is produced from triglycerides by transesterification process [45]. Biodiesel is the best candidate for diesel fuels in diesel engines. Green biodiesel can be produced by using natural biomethanol obtained from biosyngas [10]. Biodiesel can be produced from a number of sources, including recycled waste vegetable oil, oil crops and algae oil. Biodiesels play an important role in meeting future fuel requirements in view of their nature (less toxic), and have an edge over conventional diesel as they are obtained from renewable sources [104]. In general, the physical and chemical properties and the performance of the cotton seed oil methyl ester were comparable to diesel fuel [42]. The effects of cotton seed oil methyl esterand diesel fuel on a direct-injected, four-stroke, single-cylinder, air-cooled diesel engine performance and exhaust emissions were investigated. The results show that engine performance using cottonseed oil methyl ester fuel differed little from engine performance and torque with diesel fuel. As to the emissions, there was an approximate 30% reduction in CO and approximate 25% reduction in NOx [42]. The emission-forming gasses, such as carbon dioxide and carbon monoxide from combustion of biodiesel, generally are less than diesel fuel. Sulfur emissions are essentially eliminated with pure biodiesel [4]. Physical and chemical properties of methyl ester of waste cooking oil were determined in the laboratory. Obtained results were compared with No. 2 diesel fuel [56]. The specific fuel consumption for biodiesel fuels tended to be higher than that for normal diesel fuel, the exhaust smokiness values of biodiesels were considerably lower than that for petroleum diesel. On the other hand, there were no significant differences observed for torque, power and exhaust smokiness [105]. A new lipase immobilization method, textile cloth immobilization, was developed for conversion of soybean oil to biodiesel. The test results indicate that the maximum yield of biodiesel of 92% was obtained at the conditions of hexane being the solvent, water content being 20 wt , and reaction time being 24 h [51]. The dynamic transesterification reaction of peanut oil in supercritical methanol media was investigated. The reaction temperature and pressure were in the range of 523–583K and 10.0– 16.0 MPa, respectively. The molar ratio of peanut oil to methanol was 1:30. It was found that the yield of methyl esters was higher than 90% under the supercritical methanol [14]. Problems to be studied include fuel storage stability, fuel solubility, and oxidative stability of recycled soybean-derived biodiesel were investigated. Unlike newly manufactured soy oils, it was found that this recycled soy oil was not stable in fuels [53]. The oxidative and thermal degradation occurs on the double bonds of unsaturated aliphatic carbons chains in biolipids. Oxidation of biodiesel results in the formation of hydroperoxides [49]. Biohydrogen is gaining increasing attention as an encouraging future energy. Biomass can be thermally processed through gasification or pyrolysis to produce hydrogen. The combustion of hydrogen does not produce CO2, CO, SO2, VOC and particles, but entails emission of vapor and NOx [106]. The yields of hydrogen from the pyrolysis and the steam gasification increase with increasing of temperature. The highest yields were obtained from the pyrolysis (46%) and steam gasification (55%) of wheat straw while the lowest yields from olive waste [107]. Hydrothermal gasification of biomass wastes has been identified as a possible system for producing hydrogen. Supercritical and subcritical water has attracted much attention as an environmentally benign reaction medium and reactant [108]. Biohydrogen has the potential to solve two major energy problems: reducing dependence on petroleum and reducing pollution and greenhouse gas emissions[109]. Future technologies may allow biohydrogen to be produced economically from biorenewable feedstocks. Biohydrogen as the most promising candidates for tomorrow’s non-carbon transport fuels. The liquefaction of hydrogen gas on platinum (Pt)-supported carbon nano-layers was investigated. In the experiments, H2PtCl6 was used as Pt precursor and to prepare the Pt catalytic reaction nano-layer, the required amounts of H2PtCl6 were mixed with 5 wt carbon prepared by burning naphthalene in air. The yield of

liquefied hydrogen was 7.4% weight of Pt-catalyzed carbon for 30 min [110]. The mixture of H2 + CO is called as synthesis gas or syngas. Biosyngas is a gas rich in CO and H2 obtained by gasification of biomass. The aim of Fischer–Tropsch Synthesis (FTS) is synthesis of long-chain hydrocarbons from CO and H2 gas mixture. The products from FTS are mainly aliphatic straight-chain hydrocarbons (CxHy). Steam reforming of hydrocarbons, partial oxidation of heavy oil residues, selected steam reforming of aromatic compounds, and gasification of coals and solid wastes to yield a syngas, followed by water–gas shift conversion to produce H2 and CO2, are well-established processes [107,111]. 4. Economic and environmental impacts of biofuels Biofuels offer a number of technical and environmental benefits over conventional fossil fuels, which make them attractive as alternatives for the transport sector. The benefits include greenhouse gas reductions including reduced carbon dioxide emissions, which will contribute to domestic and international targets, thediversification of the fuel sector, biodegradability, sustainability, and an additional market for agricultural products. Biofuels help to protect and create jobs. Table 2 shows the major benefits of biofuels. Major benefits of biodiesel are given in Table 3. According to a study by the European Commission, European production of biofuels equivalent to 1% of EU automotive fuel consumption would help to protect and/or create between 45,000 and 75,000 jobs. Biofuels can be used as an alternative fuel for transport, as can other alternatives such as liquid natural gas (LNG), compressed natural gas (CNG) and liquefied petroleum gas (LPG). In the longer term, significant use of biofuels could offer large carbon savings. Policy drivers for renewable liquid biofuels have attracted particularly high levels of assistance in some countries given their promise of benefits in several areas of interest to governments, including agricultural production, greenhouse gas emissions, energy security, trade balances, rural development and economic opportunities for developing countries [1]. Bioethanol can be used directly in cars designed to run on pure ethanol or blended with gasoline to make ‘‘gasohol”. Anhydrous ethanol is required for blending with gasoline. No engine modification is typically needed to use the blend. Ethanol can be used as an octane-boosting, pollution-reducing additive in unleaded gasoline. World production of ethanol from sugar cane, maize and sugar

Table 2 Major benefits of biofuels. Economic impacts Sustainability Fuel diversity Increased number of ruralmanufacturing jobs Increased income taxes Increased investments in plant and equipment Agricultural development International competitiveness Reducing the dependency on imported petroleum Greenhouse gas reductions Reducing of air pollution Biodegradability Higher combustion efficiency Improved land and water use Carbon sequestration Domestic targets Supply reliability Reducing use of fossil fuels Ready availability Domestic distribution Renewability



A. Demirbas / Energy Conversion and Management 50 (2009) 2239–2249 Table 5 Fuel economy impacts of biodiesel use. Sustainability Fuel diversity Increased number of rural manufacturing jobs Increased income taxes Increased investments in plant and equipment Agricultural development International competitiveness Reducing the dependency on imported petroleum Inherent lubricity Higher cetan number Greenhouse gas reductions Reducing of air pollution Biodegradability Higher combustion efficiency Improved land and water use Carbon sequestration Lower sulfur content Lower aromatic content Less toxicity Domestic targets Supply reliability Higher flash point Reducing use of fossil fuels Ready availability Domestic distribution Renewability Percent of biodiesel in diesel fuel 20 100 % Reduction in miles/gallon 0.9–2.1 4.6–10.6

This represents around 3% of global gasoline use. Production is forecasted to almost double again by 2010 [112]. Biodiesel is a synthetic diesel-like fuel produced from vegetable oils, animal fats or waste cooking oil. It can be used directly as fuel, which requires some engine modifications, or blended with petroleum diesel and used in diesel engines with few or no modifications. At present, biodiesel accounts for less than 0.2% of the diesel consumed for transport [112]. Biodiesel has become more attractive recently because of its environmental benefits. The cost of biodiesel, however, is the main obstacle to commercialization of the product. With cooking oils used as raw material, the viability of a continuous transesterification process and recovery of high quality glycerol as a biodiesel by-product are primary options to be considered to lower the cost of biodiesel [113,114]. Table 4 shows biodiesel production capacity of European Union in 2003. The possible impact of biodiesel on fuel economy is positive as given in Table 5 [115]. Renewable alcohols are at present more expensive of synthesisethanol from ethylene and of methanol from natural gas. The simultaneous production ofbiomethanol (from sugar juice) in parallel to the production of bioethanol appears economically attractive in locations where hydro-electricity is available at very low cost. The EU production of biofuels amounted to around 2.9 billion L in 2004, with bioethanol totalling 620 million liters and biodiesel the remaining 2.3 billion L. The feed stocks used for ethanol production are cereals and sugar beet, while biodiesel is manufactured mainly from rapeseeds. In 2004, EU biodiesel production used 27% of EU rapeseed crop. In the same year, bioethanol production used 0.4% of EU cereals production and 0.8% of EU sugar beet production. The EU is by far the world’s biggest producer of biodiesel with Germany producing over half of the EU’s biodiesel. France and Italy are also important biodiesel producers, while Spain is the EU’s leading bioethanol producer [112]. Between 1991 and 2001, world ethanol production rose from around 16 billion L a year to 18.5 billion L. Brazil was the world’s leading ethanol producer until 2005 when US production roughly equalled Brazil’s. The United States become the world’s leading ethanol producer in 2006. China holds a distant but important third place in world rankings, followed by India, France, Germany


A. Demirbas / Energy Conversion and Management 50 (2009) 2239–2249 Table 6 Average international prices for common biocrude, fat, crops and oils used as feedstock for biofuel production in2007 (US$/ton). Biocrude Crude palm oil Maize Rapeseed oil Soybean oil Sugar Wheat Yellow grease 167 703 179 824 771 223 215 412
ance and taxes, represent about one-third of total cost per liter, of which the energy needed to run the conversion facility is an important (and in some cases quite variable) component. Capital cost recovery represents about one-sixth of total cost per liter. It has been showed that plant size has a major effect on cost [119]. The plant size can reduce operating costs by 15–20%, saving another $0.02–$0.03 per liter. Thus, a large plant with production costs of $0.29 per liter may be saving $0.05–$0.06 per liter over a smaller plant [119]. 4.2. Environmental impact of biofuels
and Spain. Fig. 4 shows the top five bioethanol producers in 2006 [116]. Fig. 5 shows world production of ethanol and biodiesel, 1980–2007. 4.1. Current costs, prices and economic impact of the biofuels Biofuels production costs can vary widely by feedstock, conversion process, scale of production and region. On an energy basis, ethanol is currently more expensive to produce than gasoline in all regions considered. Only ethanol produced in Brazil comes close to competing with gasoline. Ethanol produced from corn in the US is considerably more expensive than from sugar cane in Brazil, and ethanol from grain and sugar beet in Europe is even more expensive. These differences reflect many factors, such as scale, process efficiency, feedstock costs, capital and labor costs, co-product accounting, and the nature of the estimates. The cost oflarge-scale production of bio-based products is currently high in developed countries. For example, the production cost of biofuels may be three times higher than that of petroleum fuels, without, however, considering the non-market benefits. Conversely, in developing countries, the costs of producing biofuels are much lower than in the OECD countries and very near to the world market price of petroleum fuel [112]. Average international prices for common biocrude, fat, crops and oils used as feedstock for biofuel production in 2007 are given in Table 6 [1]. Agriculture ethanol is at present more expensive of synthesisethanol from ethylene. The simultaneous production of biomethanol (from sugar juice) in parallel to the production of bioethanol, appears economically attractive in locations where hydro-electricity is available at very low cost ($0.01 $ Kwh) [117]. Currently there is no global market for ethanol. The crop types, agricultural practices, land and labor costs, plant sizes, processing technologies and government policies in different regions considerably vary ethanol production costs and prices by region. Ethanol from sugar cane, produced mainly in developing countries with warm climates, is generally much cheaper to produce than ethanol from grain or sugar beet in IEA countries. For this reason, in countries like Brazil and India, where sugar cane is produced in substantial volumes, sugar cane-based ethanol is becoming an increasingly cost-effective alternative to petroleum fuels. Estimates show that bioethanol in the EU becomes competitive when theoil price reaches US$70 a barrel while in the United States it becomes competitive at US$50–60 a barrel. For Brazil the threshold is much lower – between US$25 and US$30 a barrel. Other efficient sugar producing countries such as Pakistan, Swaziland and Zimbabwe have production costs similar to Brazil’s [118]. Anhydrous ethanol, blendable with gasoline, is still somewhat more expensive. Prices in India have declined and are approaching the price of gasoline. For biofuels, the cost of feedstock (crops) is a major component of overall costs. In particular, the cost of producing oil-seed-derived biodiesel is dominated by the cost of the oil and by competition from high-value uses like cooking. The largest ethanol cost component is the plant feedstock. Operating costs, such as feedstock cost, co-product credit, chemicals, labor, maintenance, insur-Subsidies and incentives are provided independently from the environmental impact that ethanol may have during its entire life cycle, therefore, supporting biofuel production in the United States. In 2001, the European Commission launched a policy to promote the use of biofuels for transport in order to reduce greenhouse gas emissions and the environmental impact of transport, as well as to increase security of supply, technological innovation and agricultural diversification [112]. Biodiesel has become more attractive recently because of its environmental benefits. Biodiesel is superior to conventional diesel in terms of its sulfur content, aromatic content and flash point. It is essentially sulfur free andnon-aromatic while conventional diesel can contain up to 500 ppm SO2 and 20–40 wt aromatic compounds. These advantages could be a key solution to reducing the problem of urban pollution since gas emissions from the transportation sector contribute a significant amount to the total gas emissions. Diesel, in particular, is dominant for black smoke particulate together with SO2 emissions and contributes to a one-third of the total transport generated greenhouse gas emissions [55]. There was an average of decreasing of 14% for CO2, 17.1% for CO and 22.5% for smoke density when using biodiesel [56]. The biofuels include bioethanol, biobutanol, biodiesel, vegetable oils, biomethanol, pyrolysis oils, fuels from Fischer–Tropsch synthesis, biogas, and biohydrogen. There are several reasons for biofuels to be considered as relevant technologies by both developing and industrialized countries. They include energy security reasons, environmental concerns, foreign exchange savings, and socioeconomic issues related to the rural sector. Due to its environmental merits, the share of biofuel in the automotive fuel market will grow fast in the next decade [41,120]. 4.2.1. Reduction of exhaust emissions by using biodiesel in CIEs Biofuels such as bioethanol, biomethanol, biohydrogen and biodiesel generally results lower emissions than those of fossil based engine fuels. Many studies on the performances and emissions of compression ignition engines, fuelled with pure biodiesel and blends with diesel oil, have been performed and are reported in the literature [121,122]. Vegetableoils have become more attractive recently because of their environmental benefits and the fact that it is made from renewable resources. Dorado et al. [123] describe experiments on the exhaust emissions of biodiesel from olive oil methyl ester as alternative Diesel fuel fueled in a Diesel direct injection Perkins engine. The methyl ester of vegetable oil was evaluated as a fuel in CIE by researchers [124]. They concluded that the performance of the esters of vegetable oil did not differ greatly from that of Diesel fuel. The brake power was nearly the same as with Diesel fuel, while the specific fuel consumption was higher than that of Diesel 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. The results showed the transesterification treatment decreased the injector coking to a level significantly lower than that observed with D2 [125].


Although most researchers agree that vegetable oil ester fuels are suitable for use in CIE, a few contrary results have also been obtained. The results of these studies point out that most vegetable oil esters are suitable as Diesel substitutes but that more long term studies are necessary for commercial utilization to become practical. Fuel characterization data show some similarities and differences between biodiesel fuels and Diesel [125]:  Specific weight is higher for biodiesel, heat of combustion islower and viscosities are 1.3–1.6 times that of D2 fuel.  Pour points for biodiesel fuels vary from 274 to 298 K higher for biodiesel fuels depending on the feedstock.  Sulfur content for biodiesel fuel is 20–50% that of D2 fuel.  The esters all have higher levels of injector coking than D2 fuel. Several municipalities are considering mandating the use of low levels of biodiesel in diesel fuel on the basis of several studies which have found hydrocarbon (HC) and particulate matter (PM) benefits from the use of biodiesel. The use of biodiesel to reduce N2O is attractive for several reasons. First, biodiesel contains little nitrogen, as compared with Diesel fuel which is also used as a re-burning fuel. The N2O reduction was strongly dependent on initial N2O concentration and only slightly dependent upon temperature, where increased temperature increased N2O reduction. This results in lower N2O production from fuel nitrogen species for biodiesel. In addition, biodiesel contains virtually trace amount of sulfur, so SO2 emissions are reduced in direct proportion to the Diesel fuel replacement. Neat biodiesel and biodiesel blends reduce particulate matter (PM), hydrocarbons (HC) and carbon monoxide (CO) emissions and increase nitrogen oxides (NOx) emissions compared with diesel fuel used in an unmodified diesel engine [115]. The emission impacts of 20 vol soybean-based biodiesel added to an average base petrodiesel is given in Table 7 [115].
Table 7 Emission impacts of 20 vol soybean-based biodiesel added to an average base petrodiesel.

The total netemission of carbon dioxide (CO2) is considerably less than that of diesel oil and the amount of energy required for the production of biodiesel is less than that obtained with the final product. In addition, the emission of pollutants is somewhat less. CO2, one of the primary greenhouse gasses, is a trans-boundary gas, which means that, after it is emitted by a source, it is quickly dispersed in our atmosphere by natural processes. Biodiesel reduces CO2 emissions. Table 8 shows the average biodiesel emissions compared to conventional diesel, according to EPA [115]. Table 9 shows the average changes in mass emissions from diesel engines using the biodiesel mixtures relative to the standard diesel fuel [126]. Results indicate that the transformities of biofuels are greater than those of fossil fuels, thus showing that a larger amount of resources is required to get the environmental friendly product. This can be explained by the fact that natural processes are more efficient than industrial ones. On the other hand, the time involved in the formation of the fossil fuels is considerably different from that required for the production of the biomass [127]. Coconut BD can yield reductions of 80.8–109.3% in net CO2 emissions relative to PD [128]. Different scenarios for the use of agricultural residues as fuel for heat or power generation are analyzed. Reductions in net CO2 emissions are estimated at 77–104 g/MJ of diesel displaced by biodiesel. The predicted reductions in CO2 emissions are much greater than values reported in recent studies on biodieselderived from other vegetable oils, due both to the large amount of potential fuel in the residual biomass and to the low-energy inputs in traditional coconut farming techniques [128]. Unburned hydrocarbon emissions from biodiesel fuel combustion decrease compared to regular petroleum diesel. The use of blends of biodiesel and diesel oil are preferred in engines, in order to avoid some problems related to the decrease of power and torque and to the increase of NOx emissions with increasing content of pure biodiesel in the blend [129]. Emissions of all pollutants except NOx appear to decrease when biodiesel is used. The fact that NOx emissions increase with increasing biodiesel concentration could be a detriment in areas that are out of attainment for ozone. Fig. 6 shows the average emission impacts of vegetable oil-based biodiesel for Compression Ignition Engine (CIE). Fig. 7 shows average emission impacts of animalbased biodiesel for CIE [130]. One of the most impressive characteristics of biodiesel is that when burned it has fewer particulates than a petroleum-based diesel. 5. Conclusions This paper examines the potential roles of biofuels in the 21st century with a global energy model treating the entire fuel supply chain in detail. The biofuels as gasoline additive/substitute (bioethanol) and as diesel-equivalent (biodiesel) become major alternative of petroleum based transportation fuels. There are several reasons for biofuels to be considered as relevant technologies by both developing and industrialized countries. They include energy security reasons, environmental concerns, foreign exchange savings, and socioeconomic issues related to the rural sector. Biofuels are of rapidly growing interest for reasons of energy security, diversity, and sustainability benefits. Biofuels offer significant benefitsfor energy security. Biofuels also offer the promise of numerous benefits related to energy security, economics, and the environment. Bioethanol is a petrol additive/substitute fuel. It is possible that wood, straw and even household food wastes may be economically converted to bioethanol. Bioethanol is derived from alcoholic fermentation of sucrose or simple sugars, which are produced from biomass by hydrolysis process. Currently crops generating starch, sugar or oil are the basis for transport fuel production. There has been renewed interest in the use of vegetable oils for making biodiesel due to its less polluting and renewable nature as against the conventional petroleum diesel fuel. Biodiesel is a renewable replacement to petroleum-based diesel. Brief summaries of the basic concepts involved in the thermochemical conversions of biomass fuels are presented. Biofuels such as bioethanol, biomethanol, biohydrogen and biodiesel generally results lower emissions than those of fossil based engine fuels. References
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