Importance of rural bioenergy for developing countries

Energy Conversion and Management 48 (2007) 2386–2398 www.elsevier.com/locate/enconman

Importance of rural bioenergy for developing countries Ayse Hilal Demirbas *, Imren Demirbas Sila Science, University Mah, Mekan Sok No: 24, Trabzon, Turkey Received 20 January 2006; accepted 6 March 2007 Available online 24 April 2007

Abstract Energy resources will play an important role in the world’s future. Rural bioenergy is still the predominant form of energy used by people in the less developed countries, and bioenergy from biomass accounts for about 15% of the world’s primary energy consumption and about 38% of the primary energy consumption in developing countries. Furthermore, bioenergy often accounts for more than 90% of the total rural energy supplies in some developing countries. Earth life in rural areas of the world has changed dramatically over time. Industrial development in developing countries, coming at a time of low cost plentiful oil supplies, has resulted in greater reliance on the source of rural bioenergy than is true in the developed countries. In developed countries, there is a growing trend towards employing modern technologies and efficient bioenergy conversion using a range of biofuels, which are becoming cost wise competitive with fossil fuels. Currently, much attention has been a major focus on renewable alternatives in the developing countries. Renewable energy can be particularly appropriate for developing countries. In rural areas, particularly in remote locations, transmission and distribution of energy generated from fossil fuels can be difficult and expensive. Producing renewable energy locally can offer a viable alternative. Renewable energy can facilitate economic and social development in communities but only if the projects are intelligently designed and carefully planned with local input and cooperation. Particularly in poor rural areas, the costs of renewable energy projects will absorb a significant part of participants’ small incomes. Bio-fuels are important because they replace petroleum fuels. Biomass and biofuels can be used as a substitute for fossil fuels to generate heat, power and/or chemicals. Generally speaking, biofuels are generally considered as offering many benefits, including sustainability, reduction of greenhouse gas emissions, regional development, social structure and agriculture and security of supply.  2007 Elsevier Ltd. All rights reserved. Keywords: Rural energy; Developing countries; Bioethanol; Biomethanol; Hydrogen; Biodiesel

1. Introduction More than half of the world’s population living in rural areas still has no access to modern forms of energy. Energy is central to economic development, and there is a clear correlation between energy consumption and living standards. Energy resources will play an important role in the world’s future. The energy sources have been split into three categories: fossil fuels, renewable sources and nuclear sources. Table 1 shows the energy reserves of the world, and the world energy consumption is given in Table 2

*

Corresponding author. Tel.: +90 462 230 7831; fax: +90 462 248 8508. E-mail address: [email protected] (A.H. Demirbas).

0196-8904/$ - see front matter  2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2007.03.005

[1,2]. Worldwide energy consumption has increased 22.6fold in the last century, and emissions of CO2, SO2 and NOx from fossil fuel combustion are primary causes of atmospheric pollution [3]. The majority of the world’s energy needs are supplied through petrochemical sources, coal and natural gases. With the exception of hydroelectricity and nuclear fusion energy, all of these sources are finite, and at current usage rates, will be consumed shortly [4]. The rapid decrease in resources of fossil energy and the accumulation of carbon dioxide and other greenhouse gases in the atmosphere is thought to be at the origin of changes in climate, which are suspected to have dramatic consequences on humans and other living organisms. These changes have led to the development of renewable energy sources, sustainable development and eco-friendly

A.H. Demirbas, I. Demirbas / Energy Conversion and Management 48 (2007) 2386–2398 Table 1 Energy reserves of the world Deuterium

Uranium

Coal

Shale oil

Crude oil

Natural gas

Tar sands

7.5 · 109

1.2 · 105

320.0

79.0

37.0

19.6

6.1

15

Each unit = 1 · 10 Source: Ref. [1].

11

MJ = 1.67 · 10

Bbl crude oil.

Table 2 World energy consumption Years

1875

1900

1925

1950

1960

1970

1980

1990

2000

Cons. (Gtoe)

0.27

0.43

1.08

1.84

2.97

4.92

6.24

7.76

9.73

Source: Ref. [1,2].

concepts. Crude oils are limited reserves concentrated in certain regions of the world. Known crude oil reserves are estimated to be depleted in less than 50 years at the present rate of consumption [5]. Therefore, countries lacking such resources are facing foreign exchange crisis, mainly due to the import of oil [6]. Countries dependence on imported oil, environmental issues and employment in rural areas are reasons for replacement of fossil fuels by bio-fuels [7]. Renewable alternatives have been considered as sources of different energies and have led to the development of various research programs. The main renewable energy sources are biomass, hydropower, geothermal, wind and solar energies. Fig. 1 shows the percentage share of each renewable energy source in 1995 [8]. Investigations in this direction have been based on the following concepts: (1) renewable energy sources can be replenished in a short period of time; (2) renewable energy is a clean or inexhaustible energy like hydrogen energy; (3) renewable energy sources occur naturally in the environment and, therefore, should never run out; and (4) they also produce lower or negligible

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levels of greenhouse gases and other pollutants when compared with the conventional energy sources they replace. The petroleum crisis in the 1970s and rapidly increasing prices of petroleum leads scientists to work on new and renewable alternative energy sources, so vegetable oil studies become current among various investigations. There are mainly three objectives of energy policy, namely security of supply, competitiveness of the energy industry and environmental protection. Energy produced through centralized thermal, hydroelectric and nuclear power stations rarely flows to rural areas but has seen considerable progress in the most developed countries. Earth life in rural areas of the world has changed dramatically over time. Industrial development in developing countries, coming at a time of low cost plentiful oil supplies, has resulted in greater reliance on the source of rural bioenergy than is true in the developed countries. In developed countries, there is a growing trend towards employing modern technologies and efficient bioenergy conversion using a range of biofuels, which are becoming cost wise competitive with fossil fuels [9]. Interest in renewable energies has increased recently due to environmental concerns about global warming and air quality, a decline in the cost of the technologies for renewable energy and improved efficiency and reliability. Energy is central to current concerns, which is not an end in itself, but rather the means to achieve the goal of sustainable human development [10]. Every nation in the world has access to some form of renewable energy. Interest in harnessing this resource has been sparked by the central role of energy in development, which was highlighted during a decade of rapid escalation of world oil prices [11]. The diversification of kinds and sources of primary fuel is becoming vital energy issues in developing countries. In this regard, biomass energy like biodiesel and bio-oil fuels is, thus, becoming attractive due to environmental and energy policies for promoting sustainable development and environmental pollution mitigation in developing countries [12].

Fig. 1. Percentage share of each renewable energy source in 1995. Source: Ref. [7].

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The paper aims to present in a coherent way the energy status and life cycle assessment in the rural energy of developing countries.

tion in developing countries [15]. Furthermore, biomass often accounts for more than 90% of the total rural energy supplies in developing countries.

2. Importance of renewable energy for developing countries

3. Importance of biomass and bioenergy

Types of energy that are readily renewed are called renewable energy. Renewable energies have been the primary energy source throughout the history of the human race. Examples of renewable energy sources include biomass, hydropower, solar, wind and geothermal energies. Renewable energy sources are derived from those natural, mechanical, thermal and growth processes that repeat themselves within our lifetime and may be relied upon to produce predictable quantities of energy when required. In theory, renewable energy sources are infinitely available. As long as the earth continues to revolve around the sun, the sun will continue to produce harvestable energy. Heat from the sun additionally creates atmospheric conditions conducive to wind and water production, although not in even quantities throughout the world or with scientific precision in any one particular location. Finally, the sun produces the light necessary for growing the plants and trees that constitute the biomass category. Bioenergy can always be considered as a form of renewable energy. Currently, much attention has been a major focus on renewable alternatives in developing countries. Renewable energy can be particularly appropriate for developing countries. In rural areas, particularly in remote locations, transmission and distribution of energy generated from fossil fuels can be difficult and expensive. Producing renewable energy locally can offer a viable alternative. Renewable energy can facilitate economic and social development in communities but only if the projects are intelligently designed and carefully planned with local input and cooperation. Particularly in poor rural areas, the costs of renewable energy projects will absorb a significant part of participants’ small incomes. Energy policy must contribute to stable conditions for commercial activities related to the new renewable energy sources so as to stimulate rejuvenation of future research and development in this industry. Governments promoting renewable energy projects should require local needs assessment and community participation in project designs [13]. Biomass is an organic material that has stored sunlight in the form of chemical energy. Biomass fuels include wood, wood waste, straw, manure, sugar cane and many other byproducts from a variety of agricultural processes [14]. All biomass is produced by green plants converting sunlight into plant material through photosynthesis. Since the industrial revolution, the majority of the developed world’s energy requirements have been met by combustion of fossil fuels such as coal, oil and natural gas. Biomass, however, is still the predominant form of energy used by people in the less developed countries, and biomass energy accounts for about 15% of the world’s primary energy consumption and about 38% of the primary energy consump-

Bioenergy, the energy from biomass, has been used for thousands of years, ever since people started burning wood to cook food or to keep warm, and today, wood is still our largest biomass resource for bioenergy. According to the theory of the historical origin of biomass, around 600 million years ago, the first multi-cellular organisms appeared. Approximately 325 million years ago, amphibians, reptiles, insects and ferns evolved and most certainly resided in the earth life. Angiosperms, flowering plants that had first appeared about 140 million years ago, dominated the landscape, along with cycads, ginkgos, conifers and, in higher elevations, maples, birch, walnut and willow trees [15]. For thousands of years, since prehistoric humans started their first intentional fire, wood was the world’s most common source of energy for cooking, heating and manufacturing. Many countries in the developing world still use wood as their primary fuel. Half of the energy used in the continent of Africa is in the form of fuelwood. In most industrialized countries, the use of wood for fuel was minimal and declining until the oil crisis of 1973. As oil prices rose in the following years, the use of wood increased. In industrialized countries, wood is used as fuel in two distinct applications. Wood wastes from the forest products industry are burned to provide heat and/or electricity for the industry, and many homeowners use wood as a primary or secondary source of home heating. The average majority of biomass energy is produced from wood and wood wastes (64%), followed by municipal solid waste (24%), agricultural waste (5%) and landfill gases (5%) [16]. The world production of biomass is estimated at 146 billion metric tons a year, mostly wild plant growth. Some farm crops and trees can produce up to 20 metric tons per acre of biomass a year. Types of algae and grasses may produce 50 metric tons per year [17]. Worldwide, biomass ranks fourth as an energy resource, providing approximately 14% of the world’s energy needs; biomass is the most important source of energy in developing nations, providing; 35% of their energy [18,19]. Bioenergy crops include fast growing trees such as hybrid poplar, black locust, willow and silver maple in addition to annual crops such as corn, sweet sorghum and perennial grasses such as switchgrass [20]. Biomass combustion provides the basic energy requirements for cooking and heating of rural households and for process heat in a variety of traditional industries in developing countries. In general, biomass energy use in such cases is characterized by low efficiency, so the biomass fuels used could potentially provide a much more extensive energy service than at present if they were used efficiently. For example, new stove designs can improve the efficiency of biomass use for cooking by a factor of 2–3 [21,22].

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Biomass appears to be an attractive feedstock for three main reasons. First, it is a renewable resource that could be sustainably developed in the future. Second, it appears to have formidably positive environmental properties, resulting in no net releases of carbon dioxide and very low sulfur content. Third, it appears to have significant economic potential provided that fossil fuel prices increase in the future [23]. Lignocellulosic bio-methanol has such low emissions because the carbon content of the alcohol is primarily derived from carbon that was sequestered in the growing of the bio-feedstock and is only being rereleased into the atmosphere [24]. Reducing emissions of CO2 and other greenhouse gases are important strategies of mitigating the greenhouse effect. Thus, the need for developing carbon neutral and renewable sources of energy is more than ever before. The use of crop residues for bioenergy production must be critically assessed because of its positive impact on soil carbon sequestration, soil quality maintenance and ecosystem functions [25]. Biomass combustion is carbon or carbon dioxide neutral compared to fossil fuel combustion because the biomass combustion is simply releasing the carbon or carbon dioxide that was sequestered by growing the biomass in the beginning is certainly true. However, such thinking completely ignores the fact that fossil fuel combustion is also carbon or carbon dioxide neutral for exactly the same reason. The obvious difference lies in the elapsed time between the sequestration from the atmosphere and the return of the carbon or carbon dioxide to the atmosphere. 4. Current biomass conversion technologies Biomass is any photosynthetically derived material of biological origin. Most of the standing biomass is in the form of woody forest materials. Most land biomass is composed primarily of cellulose, hemicellulose and lignin [26]. Several types of biomass are used as an energy conversion feedstock including agricultural and forest product residues, municipal solid waste and industrial waste. Direct combustion is the old way of using biomass. Biomass thermochemical conversion technologies such as pyrolysis and gasification are certainly not the most important options at present; combustion is responsible for over 97% of the world’s bioenergy production. Some processes such as pyrolysis, gasification, anaerobic digestion and alcohol production have been widely applied to biomass in order to obtain its energy content. Biomass can be directly fired in dedicated boilers. However, cofiring biomass and coal has technical, economical, and environmental advantages over the other options. Cofiring biomass with coal, in comparison with single coal firing, helps reduce the total emissions per unit energy produced. The oldest of all fuels, wood (or biomass), and the old original fuel of the industrial revolution, coal, are key to this move to a new mission. Technical issues that can lead to doubt about biomass cofiring with coal are being resolved through testing and experience [27].

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The main current biomass technologies are [20,28]: 1. Destructive carbonization of woody biomass to charcoal [29] 2. Gasification of biomass to gaseous products [30,31] 3. Pyrolysis of biomass and solid wastes to liquid, solid and gaseous products [32,33] 4. Supercritical fluid extractions of biomass to liquid fuels [34] 5. Liquefaction of biomass to liquid products [35–37] 6. Hydrolysis of biomass to sugars and ethanol 7. Anaerobic digestion of biomass to gaseous products [38] 8. Biomass power for generating electricity by direct combustion or gasification and pyrolysis [39,40] 9. Cofiring of biomass with coal [41–50] 10. Biological conversion of biomass and waste (biogas production, wastewater treatment) [38] 11. Biomass densification (briquetting, pelleting) [51,52] 12. Domestic cookstoves and heating appliances of fuelwood 13. Biomass energy conservation in households and industry 14. Solar photovoltaic and biomass based rural electrification 15. Conversion of biomass to a pyrolytic oil (biofuel) for vehicle fuel [53–55] 16. Conversion of biomass to biomethanol and bioethanol for internal combustion engines [56] 17. Biodiesel is produced on a smaller scale from oil seed crops such as soy, sunflower or rapeseed for Diesel engines [57–61,6] 18. Hydrogen is produced from biomass by pyrolysis and steam gasification [62–69] 19. Sythesis gas and liquid fuels via Fisher–Tropsh senthesis [70] 20. Gasoline and Diesel fuel range transporting fuels from vegetable oils via pyrolysis [9,71–77].

5. Biofuels from lignocellulosic biomass materials Biofuel refers to liquid or gaseous fuels for the transport sector that are predominantly or exclusively produced from biomass. Biofuel has been a source of energy that human beings have used since ancient times [78]. Biofuels are important because they replace petroleum fuels. Bio-oil can be used as a substitute for fossil fuels to generate heat, power and/or chemicals. Upgrading bio-oil to a transportation fuel is technically feasible but needs further development. Generally speaking, biofuels are generally considered as offering many benefits including sustainability, reduction of greenhouse gas emissions, regional development, social structure and agriculture and security of supply. Biofuels are being investigated as potential substitutes for the current high pollutant fuels obtained from conventional sources [79].

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Diesel fuel can also be replaced by biodiesel made from vegetable oils. This fuel is now mainly being produced from soybean oil. However, any vegetable oil—corn, cottonseed, peanut, sunflower or canola—could be used to produce biodiesel. Researchers are also developing algae that produce oils, which can be converted to biodiesel [87–89].

Increasing use of biofuels for energy generation purposes is of particular interest nowadays because it allows mitigation of greenhouse gases, provides means of energy independence and may even offer new employment possibilities [80]. The opportunities for expanded biofuel production are bright, considering the high demand for petroleum products in both industrialized and developed countries of the world. Future raw material availability for worldwide biofuel production is significant [81]. Liquid biofuels presently are available as two forms, bioalchols and biodiesel. The first form is mostly used in combination with gasoline, and the second corresponds to a vast form of fatty acid esters or biodiesels for using in Diesel engines. Because biomass can be converted directly into a liquid fuel, it could someday supply much of our transportation fuel needs for cars, trucks, buses, airplanes and trains. This is very important because nearly one third of our nation’s energy is now used for transportation. The most commonly used biofuel is ethanol, which is produced from sugarcane, corn and other grains. A blend of gasoline and ethanol is already used in cities with high air pollution. However, ethanol made from biomass is currently more expensive than gasoline on a per gallon basis. So, it is very important for scientists to find less expensive ways to produce ethanol from other biomass crops. Today, researchers have found new ways to produce ethanol from grasses, trees, bark, sawdust, paper and farming wastes [82–86].

5.1. Bioethanol production from cellulosic materials Ethanol or ethyl alcohol (CH3CH2OH) is the most widely used liquid biofuel. It is an alcohol and is fermented from sugars, starches or from cellulosic biomass. Bioethanol represents an important, renewable liquid fuel for motor vehicles. Production of bioethanol from biomass is one way to reduce both the consumption of crude oil and environmental pollution. If bioethanol from biomass is used to drive a light duty vehicle, the net CO2 emission is less than 7% of that from the same car using reformulated gasoline [90]. In order to produce bioethanol from cellulosic biomass, a pretreatment process is used to reduce the sample size, break down the hemicelluloses to sugars and open up the structure of the cellulose component. The cellulose portion is hydrolyzed by acids or enzymes into glucose sugar that is fermented to bioethanol. The sugars from the hemicelluloses are also fermented to bioethanol. Fig. 2 shows the flow chart for the production of bioethanol from biomass. Primary consideration involves the production of ethyl alcohol from renewable resources and determination of

BIOMASS

Hemicellulose hydrolysis

Separation

Solid cellulose + Lignin

Xylose sugar

Cellulose hydrolysis Xylose fermentation Glucose sugar + Solid lignin

Glucose fermentation

Distillation for recover bioethanol

BIOETHANOL

Solid lignin for boiler

Fig. 2. Flow chart for the production of bioethanol from biomass.

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the economic and technical feasibility of using alcohol as an automotive fuel blended with gasoline [91]. Ethanol represents an important, renewable liquid fuel for motor vehicles [92]. The use of gasohol (an ethanol and gasoline mixture) as an alternative motor fuel has been steadily increasing around the world for a number of reasons. Domestic production and use of ethanol for fuel can decrease dependence on foreign oil, reduce trade deficits, create jobs in rural areas, reduce air pollution and reduce global climate change carbon dioxide buildup. Ethyl alcohol is not only the oldest synthetic organic chemical used by man, but it is also one of the most important. In an earlier study [93], physiological effects of inhibitors on ethanol from lignocellulosic materials and fermentation strategies were comprehensively investigated. The estimated world ethanol production in 1998 was 33.3 billion liters [94]. Approximately 9% of the ethanol is produced synthetically, and consequently, fermentation is responsible for 91% of global ethanol production [95]. Brazil is the dominant producer of alcohol with a production of 16.1 billion liters in 1998. The production of bioethanol in different continents is shown in Table 3. The use of ethanol as a motor fuel has as long a history as the car itself. It began with the use of ethanol in the internal combustion engine invented by Nikolas Otto in 1897 [96]. Alcohols have been used as fuels since the inception of the automobile. The term alcohol often has been used to denote either ethanol or methanol as a fuel. With the oil crises of the 1970s, ethanol became established as an alternative fuel. Many countries started programs to study and develop fuels in an economic way from available raw materials [97]. There are three principal methods to get the simple alcohols that are the backbone of aliphatic organic synthesis [98]. These are: (a) by hydration of alkanes obtained from the cracking of petroleum; (b) by the hydrolysis of cellulosic materials and (c) by fermentation of carbohydrates. Fermentation using genetically engineered yeast or bacteria will utilize all five of the major biomass sugars: glucose, xylose, mannose, galactose and arabinose. Bioethanol may be produced by direct fermentation of sugars, or from other carbohydrates that can be converted to sugar, such as starch and cellulose. Fermentation of sugars by yeast, the oldest synthetic chemical process used by man, is still of enormous importance for preparation of ethyl alcohol. Some sugars can be converted directly to bioethanol, whereas starch and cellulose must first be hydrolyzed to sugar before conversion to bioethanol. Most of the polymeric raw materials are available at prices lower than Table 3 Bioethanol production in different continents (billion liters/year) Continents

America

Asia

Europe

Africa

Oceania

Production

22.3

5.7

4.6

0.5

0.2

Source: Ref. [96].

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refined sugars. However, transportation costs of the raw materials make it necessary to use locally available raw material [93]. 5.1.1. Bioethanol production by acidic hydrolysis of biomass Pretreatment methods refer to the solubilization and separation of one or more of the four major components of biomass (hemicellulose, cellulose, lignin and extractives) to make the remaining solid biomass more accessible to further chemical or biological treatment. Hydrolysis (saccharification) breaks down the hydrogen bonds in the hemicellulose and cellulose fractions into their sugar components: pentoses and hexoses. These sugars can then be fermented into bioethanol. After the pretreatment process, there are two types of processes to hydrolyze the cellulosic biomass for fermentation into bioethanol. The most commonly applied methods can be classified in two groups: chemical hydrolysis (dilute and concentrated acid hydrolysis) and enzymatic hydrolysis. In addition, there are some other hydrolysis methods in which no chemicals or enzymes are applied. The dilute acid process is conducted under high temperature and pressure and has a reaction time in the range of seconds or minutes, which facilitates continuous processing. As an example, using a dilute acid process with 1% sulfuric acid in a continuous flow reactor at a residence time of 0.22 min and a temperature of 510 K with pure cellulose provided a yield over 50% sugars. In this case, 1000 kg of dry wood would yield about 164 kg of pure ethanol. The combination of acid and high temperature and pressure dictate special reactor materials, which can make the reactor expensive. The biggest advantage of dilute acid processes is their fast rate of reaction, which facilitates continuous processing [82]. Hydrolysis of cellulosic materials by concentrated sulfuric or hydrochloric acids is a relatively old process. The concentrated acid process uses relatively mild temperatures, and the only pressures involved are those created by pumping materials from vessel to vessel. Reaction times are typically much longer than for dilute acid. This method generally uses concentrated sulfuric acid followed by a dilution with water to dissolve and hydrolyze or convert the substrate into sugar. The primary advantage of the concentrated acid process is the potential for high sugar recovery efficiency [82]. The acid and sugar are separated via ion exchange, and then, the acid is re-concentrated via multiple effect evaporators. The low temperatures and pressures employed allow the use of relatively low cost materials such as fiberglass tanks and piping. The low temperatures and pressures also minimize the degradation of the sugars. 5.1.2. Bioethanol production by enzymatic hydrolysis of biomass Enzymes are naturally occurring plant proteins that cause certain chemical reactions to occur. There are two technological developments: enzymatic and direct microbial conversion methods. Chemical pretreatment of the

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cellulosic biomass is necessary before enzymatic hydrolysis. The first application of enzymatic hydrolysis was used in separate hydrolysis and fermentation steps. Enzymatic hydrolysis is accomplished by cellulolytic enzymes. Different kinds of ‘‘cellulases’’ may be used to cleave the cellulose and hemicelluloses. A mixture of endoglucanases, exoglucanases, b-glucosidases and cellobiohydrolases is commonly used [99,100]. The endoglucanases randomly attack cellulose chains to produce polysaccharides of shorter length, whereas exoglucanases attach to the nonreducing ends of these shorter chains and remove cellobiose moieties. b-glucosidases hydrolyze cellobiose and other oligosaccharides to glucose [86]. 5.2. Biomethanol production from organic wastes Methanol is mainly manufactured from natural gas. 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. Biomass sources are more preferable for biomethanol than for bioethanol because bioethanol is a high cost, low yield product. The production of methanol is a cost intensive chemical process. Therefore, in current conditions, only waste biomass such as old wood or bio-waste is used to produce methanol [53]. This is a promising alternative with a diversity of fuel applications with proven environmental, economic and consumer benefits. A 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. 5.2.1. Methanol production processes Before modern production technologies were developed in the 1920s, methanol was obtained from wood as a coproduct of charcoal production and, for this reason, was commonly known as wood alcohol. Methanol is currently manufactured worldwide by conversion or derived from syngas, natural gas, refinery off gas, coal or petroleum: 2H2 þ CO ! CH3 OH

ð1Þ

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 capable of causing the conversion, including reduced NiO based preparations, reduced Cu/ZnO shift preparations, Cu/SiO2 and Pd/ SiO2 and Pd/ZnO [101,102]. Methanol is currently made from natural gas but can also be made using biomass via partial oxidation reactions [103]. Biomass and coal can be considered as a potential fuel for gasification and further syngas production and methanol synthesis [102]. Adding sufficient hydrogen to the synthesis gas to convert all of the biomass into methanol can more than double the methanol produced from the same biomass base [104]. Waste material can be partially

converted to methanol, for which the product yield for the conversion process is estimated to be 185 kg of methanol per metric ton of solid waste [105,106]. Biomass resources can be used to produce methanol. The pyroligneous acid obtained from wood pyrolysis consists of about 50% methanol, acetone, phenols and water [103,107]. As a renewable resource, biomass represents a potentially inexhaustible supply of feedstock for methanol production. The composition of syngas from biomass for producing methanol is presented in Table 4. Current natural gas feedstocks are so inexpensive that even with tax incentives, renewable methanol has not been able to compete economically. Technologies are being developed that may eventually result in commercial viability of renewable methanol. In recent years, a growing interest has been observed in the application of methanol as an alternative liquid fuel that can be used directly for powering Otto engines or fuel cells [108]. The feasibility of achieving the conversion has been demonstrated in a large scale system in which a product gas is initially produced by pyrolysis and gasification of a carbonaceous matter. Syngas from biomass is altered by the catalyst under high pressure and temperature to form methanol. This method will produce 100 gallons of methanol per ton of feed material [56]. The gases produced can be steam reformed to produce hydrogen and followed by the water–gas shift reaction to enhance hydrogen production further. When the moisture content of biomass is higher than 35%, it can be gasified in a supercritical water condition [109]. Supercritical water gasification is a promising process to gasify biomass with high moisture contents due to the high gasification ratio (100% achievable) and high hydrogen volumetric ratio (50% achievable) [110–112]. Hydrogen produced by biomass gasification was reported to be comparable to that by natural gas reforming [113]. The process is more advantageous than fossil fuel reforming in consideration of environmental benefits. It is expected that biomass thermochemical conversion will be one of the most economical large scale renewable hydrogen technologies. The gas is converted to methanol in a conventional steam reforming/water gas shift reaction followed by high pressure catalytic methanol synthesis: CH4 þ H2 O ! CO þ 3H2 CO þ H2 O ! CO2 þ H2

ð2Þ ð3Þ

Eqs. (2) and (3) are called gasification/shift reactions. CO þ 2H2 ! CH3 OH

ð4Þ

or Table 4 Composition of syngas from biomass for producing methanol (% by volume) H2

CO

CH4

CO2

C2H4

H2O

N2

25–35

23–31

9–12

18–23

0.8–1.2

5.5–6.5

0.6–1.2

A.H. Demirbas, I. Demirbas / Energy Conversion and Management 48 (2007) 2386–2398

CO2 þ 3H2 ! CH3 OH þ H2 O

ð5Þ

Eqs. (4) or (5) are methanol synthesis reactions. Fig. 3 shows the production of biomethanol from carbohydrates by gasification and partial oxidation with O2 and H2O. The energy value of residues generated worldwide in agriculture and the forest products industry amounts to more than one third of the total commercial primary energy use at present [114]. Bioenergy supplies can be divided into two broad categories: (a) organic municipal waste and residues from the food and materials sectors; and (b) dedicated energy crops plantations. Bioenergy from biomass, both residues and energy crops, can be converted into modern energy carriers such as hydrogen, methanol, ethanol or electricity [115]. 5.3. Hydrogen generation from organic wastes Hydrogen, generated by passing an electrical current through water, can be used to store solar energy and regenerate it when needed for night time energy requirements. It can be burned to produce heat or passed through a fuel cell to produce electricity. Hydrogen can be produced by pyrolysis from biomass [65]. The pyrolysis based technology, in particular because it has co-product opportunities, has the most favorable economics. However, the gasification processes also produce hydrogen for less than many other renewable technologies. With scientific and engineering advancements, biomass can be viewed as a key economically viable component to a renewably based hydrogen economy [116]. Hydrogen produced from water, renewable organic wastes or biomass, either biologically (biophotolysis and

CARBOHYDRATES (CH2O)n Drying and Crushing into Powder

Gasification and Partial Oxidation with O2 and H2O Temperature: 1250 K

Mixture of gases: H2, CO, CO2, H2O

Catalytic synthesis: CO + 2H2 → CH3OH Pressure: 4-8 MPa

BIOMETHANOL Fig. 3. Biomethanol from carbohydrates by gasification and partial oxidation with O2 and H2O.

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fermentation) or photobiologically (photodecomposition), is termed ‘‘biohydrogen’’. Biohydrogen technology will play a major role in the future because it can utilize renewable sources of energy. Hydrogen is currently more expensive than conventional energy sources. There are different technologies presently being applied to produce hydrogen economically from biomass [117]. As a sustainable energy source, hydrogen is a promising alternative to fossil fuels. It is a clean and environmentally friendly fuel [118]. Hydrogen is the fuel of the future, mainly due to its high conversion efficiency, recyclability and nonpolluting nature. Hydrogen can be generated from water by electrolysis, photolysis, direct thermal decomposition or thermolysis and biological processes [119,120]. Many studies have reported on biohydrogen production by photocatalytic [121,122] or enzymatic [118,123] processes. Hydrogen can be produced from biomass by pyrolysis [63], gassification [124], steam gasification [64], steam reforming of bio-oils and enzymatic decomposition of sugars. Hydrogen is produced from pyroligneous oils produced from the pyrolysis of lignocellulosic biomass [125]. The yield of hydrogen that can be produced from biomass is relatively low, 16–18% based on dry biomass weight [63]. In the pyrolysis and gasification processes, the water– gas shift is used to convert the reformed gas into hydrogen, and pressure swing adsorption is used to purify the product. The cost of hydrogen production from supercritical water gasification of wet biomass was several times higher than the current price of hydrogen from steam methane reforming [126,127]. Biological generation of hydrogen (biohydrogen) technologies provide a wide range of approaches to generate hydrogen, including direct biophotolysis, indirect biophotolysis, photo-fermentations, and dark-fermentation [128]. Biological hydrogen production processes are found to be more environmentally friendly and less energy intensive as compared to thermochemical and electrochemical processes [119]. Researchers have started to investigate hydrogen production with anaerobic bacteria since the 1980s [129,130]. There are three types of microorganisms of hydrogen generation: cyano-bacteria, anaerobic bacteria and fermentative bacteria. The cyano-bacteria directly decompose water to hydrogen and oxygen in the presence of light energy by photosynthesis. Photosynthetic bacteria use organic substrates like organic acids. Anaerobic bacteria use organic substances as the sole source of electrons and energy, converting them into hydrogen. Biohydrogen can be generated using bacteria such as Clostridia by temperature, pH control, reactor hydraulic retention time (HRT) and other factors of the treatment system. Biological hydrogen can be generated from plants by biophotolysis of water using micro-algae (green algae and cyano-bacteria), fermentation of organic compounds and photodecomposition of organic compounds by photosynthetic bacteria. To produce hydrogen by fermentation of

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biomass, a continuous process using a non-sterile substrate with a readily available mixed microflora is desirable [131]. A successful biological conversion of biomass to hydrogen depends strongly on the processing of raw materials to produce feedstock that can be fermented by the microorganisms [123]. Hydrogen production from the bacterial fermentation of sugars has been examined in a variety of reactor systems. Hexose concentration has a greater effect on H2 yields than the HRT. Flocculation also was an important factor in the performance of the reactor [132]. Hydrogen gas is a product of the mixed acid fermentation of Escherichia coli, the butylene glycol fermentation of Aerobacter and the butyric acid fermentations of Clostridium spp [133]. The work was conducted to improve hydrogen fermentation of food waste in a leaching bed reactor by heat shocked anaerobic sludge and also to investigate the e1ect of dilution rate on the production of hydrogen and metabolites in hydrogen fermentation [118]. 5.4. Biodiesel production from vegetable oils Biodiesel has been defined as the monoalkyl esters of long chain fatty acids derived from renewable feedstocks, such as vegetable oils or animal fats, for use in compression ignition (Diesel) engines [134]. In recent times, biodiesel has become more attractive because of its environmental benefits and the fact that it is made from renewable resources [135]. Different ways of modifying vegetable oils and fats to use them as Diesel fuel, such as direct use, pyrolysis, dilution with hydrocarbons and emulsification, have been considered. Direct use of vegetable oils and the use of blends of oils have several problems [59]. Pyrolysis, defined as the cleavage to smaller molecules by thermal energy, of vegetable oils over petroleum catalysts has been investigated [136]. Emulsification with alcohols has been prepared to overcome the problem of high viscosity of vegetable oils [69]. The transesterfication of triglycerides by methanol, ethanol, propanol and butanol has proved to be the most promising process [137]. Several common vegetable oils such as sunflower, palm, rapeseed, soybean, cottonseed and corn oils and their fatty acids can be used as the sample of vegetable oil. Biodiesel is easier to produce and cleaner with equivalent amounts of processing when starting with clean vegetable oil. Biodiesel is generally made of methyl esters of fatty acids produced by the transesterification reaction of triglycerides with methanol with the help of a catalyst [138]. Methanol is a relatively inexpensive alcohol, and it has a small molecular mass. A reaction mechanism of vegetable oil in supercritical methanol was proposed based on the mechanism developed by Krammer and Vogel [139] for the hydrolysis of esters in sub/supercritical water. The basic idea of supercritical treatment is a relationship between pressure and temperature upon the thermophysical properties of the solvent such as dielectric constant, viscosity, specific weight and polarity [140].

In the conventional transesterification of animal fats and vegetable oils for biodiesel production, free fatty acids and water always produce negative effects, since the presence of free fatty acids and water causes soap formation, consumes catalyst and reduces catalyst effectiveness, all of which result in a low conversion [141]. Transesterification consists of a number of consecutive, reversible reactions [142,143]. The triglyceride is converted stepwise to diglyceride, monoglyceride and finally glycerol, in which 1 mol of alkyl esters is removed in each step. The reaction mechanism for alkali catalyzed transesterification was formulated as three steps [144,145]. The formation of alkyl esters from monoglycerides is believed to be the step that determines the reaction rate, since monoglycerides are the most stable intermediate compound [135]. Fatty acid ðR1 COOHÞ þ Alcohol ðROHÞ ¡ Ester ðR1 COORÞ þ Water ðH2 OÞ Triglyceride þ ROH ¡ Diglyceride þ RCOOR1

ð6Þ ð7Þ

Diglyceride þ ROH ¡ Monoglyceride þ RCOOR2 Monoglyceride þ ROH ¡ Glycerol þ RCOOR3

ð8Þ ð9Þ

The properties of biodiesel are close to those of Diesel fuels. The biodiesel was characterized by determining its viscosity, density, cetane number, cloud and pour points, characteristics of distillation, flash and combustion points and higher heating value (HHV) according to ISO norms [146]. Some fuel properties of methyl ester biodiesls are presented in Table 5. Viscosity is the most important property of biodiesel since it affects the operation of the fuel injection equipment, particularly at low temperatures when the increase in viscosity affects the fluidity of the fuel. Biodiesel has a viscosity close to that of diesel fuels. High viscosity leads to poorer atomization of the fuel spray and less accurate operation of the fuel injectors. A novel process of biodiesel fuel production has been developed by a non-catalytic supercritical methanol method. The supercritical methanol process is non-catalytic, simpler purification, lower reaction time and lower energy use. Therefore, the supercritical methanol method would be more effective and efficient than the common commercial process [57,75,140,147]. Biodiesel fuels have generally been found to be nontoxic and are biodegradable, which may promote their use in applications where biodegradability is desired. Neat biodiesel and biodiesel blends reduce particulate matter (PM), Table 5 Fuel properties of methyl ester biodiesels Source

Viscosity g/mL at 288.7 K

Density cSt at 313.2 K

Cetane Number

Reference

Sunflower Soybean Palm Peanut Babassu Tallow

4.6 4.1 5.7 4.9 3.6 4.1

0.880 0.884 0.880 0.876 – 0.877

49 46 62 54 63 58

[150] [142] [150] [151] [151] [152]

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hydrocarbons (HC) and carbon monoxide (CO) emissions and increase nitrogen oxides (NOx) emissions compared with petroleum based Diesel fuel used in an unmodified Diesel engine [148,149]. 6. Conclusion The energy sources have been split into three categories: fossil fuels, renewable sources, and nuclear sources. Rural bioenergy is available from biomass, which is a renewable form. Energy is central to economic development, and there is a clear correlation between energy consumption and living standards. Rural bioenergy is still the predominant form of energy used by people in the less developed countries. Currently, much attention has been a major focus on renewable alternatives in the developing countries. Renewable energy can be particularly appropriate for developing countries. Furthermore, bioenergy often accounts for more than 90% of the total rural energy supplies in some developing countries. Earth life in rural areas of the world has changed dramatically over time. Industrial development in developing countries, coming at a time of low cost plentiful oil supplies, has resulted in greater reliance on the source of rural bioenergy than is true in the developed countries. In rural areas, particularly in remote locations, transmission and distribution of energy generated from fossil fuels can be difficult and expensive. Producing renewable energy locally can offer a viable alternative. Renewable energy can facilitate economic and social development in communities but only if the projects are intelligently designed and carefully planned with local input and cooperation. Particularly in poor rural areas, the costs of renewable energy projects will absorb a significant part of participants’ small incomes. Biofuels are important because they replace petroleum fuels. Biofuels (mainly bioethanol, biomethanol, hydrogen and biodiesel) are obtained from biomass and can be used as a substitute for transportation fuels and to generate heat, power and/or chemicals. Generally speaking, biofuels are generally considered as offering many benefits, including sustainability, reduction of greenhouse gas emissions, regional development, social structure and agriculture and security of supply. Liquid biofuels presently are available in two forms, bioalchols and biodiesel. The first form is mostly used in combination with gasoline, and the second corresponds to a vast form of fatty acid esters or biodiesels for use in Diesel engines. Because biomass can be converted directly into a liquid fuel, it could someday supply much of our transportation fuel needs for cars, trucks, buses, airplanes and trains. This is very important because nearly one third of our nation’s energy is now used for transportation. References [1] Demirbas A. Carbonization and characterization of Turkish oil shales. Energy Sources 2000;22:675–82.

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