Reinforced concrete is obtained by adequately mixing specific proportions of aggregates (gravels and sand), cement, and water (Bartali, 1999). The water:cement ratio is 0.53 L kg-1 and the cement: sand: gravel mass ratio is 1:2.2:3.7 for floors, driveways, structural beams, and columns (Lindley & Whitaker, 1996). Cylindrical cast-in-place concrete tanks are commonly used in biogas plants for storing liquid manure during long periods. A serviceable tank should be watertight to prevent groundwater pollution and corrosion of the reinforcing rods. Therefore, these tanks should be designed to withstand different design loads, i. e. the loads of the soil outside the digester which is buried underground level and loads of the liquid stored inside the digester. Liquid manure is often stored in large cylindrical concrete tanks, which are partially underground. The dimensions of these tanks vary from 18 to 33 m in diameter with heights from 2.4 to 4.9 m and a uniform wall thickness varying from 150 to 200 mm (Ghafoori & Flynn, 2007; Godbout et al., 2003).

The internal volume of the tank can be calculated by multiplying the volume of substrates that should be stored in the tank by 1.10 in order to consider 10% as headspace. The cement mass (kg), gravels volume (m3), and sand volume (m3) required to build the tank can be calculated by multiplying the concrete volume of the tank by the constants C, G, and S, respectively, where C represents the mass of cement required to make 1 m3 of concrete (325 kg m-3), G is the volume of gravel required for 1 m3 concrete (0.8 m3 of gravel per m-3 of concrete), and S is the volume of sand required for 1 m3 concrete (0.4 m3 of sand per m-3 of concrete). The type of iron rods should be selected. The different types (N0D m-1, where N is the number of iron rods per meter length, and D is the diameter of the iron rod) are 606 m-1 (0.666 kg m-1) and 608 m-1 (0.888 kg m-1). In the case of constructing a tank without a concrete top, both types can be used. On the other side, in the case of building a tank with a concrete top, the type 608 m-1 must be used with two iron grids (Samer, 2008, 2010, 2011; Samer et al., 2008). The thickness of digester wall should be 35 cm and is built using reinforced concrete to bear the loads of the materials stored in the digester. Tables 1 through 3 show the typical digester specifications for a commercial biogas plant, the required quantities of construction materials to build the digester, and the quantities of the substrates.

1Consider a duration of 3 days for mixing and pumping, daily manure deposition of 6 m3 day-1, 1.8 m3 cow-1 month-1, and 100 cows

2Consider a storage duration of 7 days and liquid organic matter deposition of 3 m3 day-1 3Consider 40 days of storage duration and liquid substrate deposition of 2 m3 day-1 4Consider digester load of 4 kg m3 day-1 and density of 1.2 kg m-3

5Total quantity of substrates (10750 m3) that should be stored in a digester having a capacity of 11820 m3

Table 3. Quantities of the substrates

9.1 Global production of Jatropha

Jatropha is a shrub, belonging to the Euphorbiaceae family, thriving in various environments and across a wide range of ecosystems. It is a plant that can survive several months with minimal water and can actually live up to 40 years or more. It is not edible to human beings or animals. The jatropha industry is in its very early stages, covering a global area estimated at some 900,000 ha. More than 85 percent of jatropha plantings are in Asia, chiefly Myanmar, India, China and Indonesia. Africa accounts for around 12 percent or approximately 120,000 ha, mostly in Madagascar and Zambia, but also in Tanzania and Mozambique. The West African nations of Mali, Ghana and Senegal have also established lofty production targets for Jatropha notably; to cultivate 320,000 ha of Jatropha curcas in Senegal by 2012 and 1 million ha in Ghana in the medium term (OECD, 2008). Latin America has approximately 20,000 ha of jatropha, mostly in Brazil. The area planted with jatropha was projected to grow to 4.72 million ha by 2010 and 12.8 million ha by 2015. By then, Indonesia is expected to be the largest producer in Asia with 5.2 million ha, Ghana and Madagascar together will have the largest area in Africa with 1.1 million ha, and Brazil is projected to be the largest producer in Latin America with 1.3 million ha. Total biogas generation potential from Jatropha curcas cakes in India has been estimated as 2,550 million m3 from 10.2 lakh metric ton of J. curcas oil seed cakes.

Jatropha curcas contains about 30% oil leaving behind presscake (75% including about 5% losses of oil in extraction process in the mechanical expeller) with residual oil. The oil is used for preparing bio-diesel (Achten et al., 2008) and in soap preparation. The press cake is rich in organic matter (Abreu, 2009). It can be used as manure, as feedstock for biogas production, animal feed and so forth. (Agarwal, 2007). Also, Jatropha oil cake is used for enriching the soil (Reyadh, 1997). Envis (2004) observed that Jatropha oil cake is an organic fertilizer that is superior to cattle manure and it is in great demand by farmers.

There is a great challenge for the management of waste, especially in generating clean energy that will decrease the burden of environmental pollution, with the fields of both science and technology working in unison to develop new ways of utilizing and extending it’s shelf-life by developing alternative uses. Until now, there have been a lot of publications dealing with solid waste management, but there are still very few documents that can provide information regarding the use of this waste as a raw material. In the last few years, research has been focused on the transformation of waste into a useful product has made considerable progress. In developing countries, due to imbalance of demand and supply of energy, mainly in rural areas, choosing a source that fulfills the requirements has become essential, and they can use waste as other raw materials. Biogas, which is mainly generated from organic waste, is useful for them. In this context, a book on "Biogas", in which the emphasis is made on the chemistry of each step involved in biogas generation along with engineering principles and practices, is introduced. Each chapter of the book carries valuable and updated information from basics to apex, helping readers to understand more precisely. Different concepts have been covered to expand the views of the readers about the subject.

This publication will be very helpful to academics, researchers, NGOs and others working in the field.

Dr. Sunil Kumar

National Environmental Engineering Research Institute (NEERI), Kolkata Zonal Laboratory I-8, Kolkata,

India

The amount of gas produced depends on the slurry in the digester volume (Fulford, 1988). The digester volume is also related to the retention time measured in days and the loading rate, in terms of manure solids per unit liquid volume (San Thy et al., 2003). According to experiences in China, 97% of the total yield of gas from fermenting cattle manure will be produced in a period of 50 days at 350C. The hydraulic retention time (HTR) in anaerobic digesters is determined by calculating the number of days required for displacement of the fluid volume of the culture. At a given organic loading rate, the HTR is lower when using high water — content feeds than when using those containing less water (Fannin and Biljetina, 1987). The detention time is dependent on all the factors discussed above. Generally a retention time of between 30 and 45 days and in some cases 60 days is enough for substantial gas production (Clanton et al., 1985; Carcelon and Clark, 2002). A study by Hill (1982) found that detention times for digesters designed to produce maximum daily methane volume varied from 7.9 days for dairy waste to 14.8 days for poultry, and similar wide variations in loading rates existed between the two wastes.

Babajide A. Adelekan

Federal College of Agriculture Ibadan, Institute of Agricultural Research & Training, Ibadan,

Nigeria

1. Introduction

The chapter presents comprehensive and up-to-date knowledge on the themes of biogas, bioethanol, biodiesel as obtained from cassava, cocoyam, jatropha, grasses and manure. The author’s research findings as well as those reported by other researchers are used for the discussion. Recommendations as regards how to benefit much more from these biofuels derived from selected tropical crops are presented. It is anticipated that these recommendations will be of immense help to academics and industry specialists working in such areas.

None of the biological activities of anaerobic microorganisms, including their development, breeding and metabolism, requires oxygen. In fact, they are very sensitive to the presence of oxygen. The breakdown of organic materials in the presence of oxygen will produce carbon dioxide; in airless conditions, it produces methane (Buren, 1983; Voermans, 1985). Ferguson and Mah (1987) pointed out that methane-producing bacteria carry out the terminal step in the formation of biogas from the anaerobic decomposition of biomass. Methane is the final product of mineralizing the organic material in digesters and most anaerobic freshwater habitats. Most of the chemical energy in the starting materials (substrates) actually ends up in the methane released by these anaerobic bacteria. Ferguson and Mah (1987) noted further that in direct contrast, aerobic bacterial metabolism releases most of the chemical energy in the starting substrates by oxidizing them to carbon dioxide and water. Buren (1983) noted that if the digester is not sealed to ensure the absence of air. The action of the microorganisms and the production of biogas will be inhibited and some will escape. It is therefore crucial that the biogas digester be airtight and watertight.

In contemporary times, a great deal of interest has been generated worldwide regarding the use of biofuels namely biogas, bioethanol and biodiesel for energy supply. The most ambitious goal thus far in respect of the development and exploitation of renewable energy sources appear to be that articulated by the European Renewable Energy Council. According to European Renewable Energy Council EREC (2010) in March 2007, the Heads of States and Governments of the 27 EU Member States adopted a binding target of 20% renewable energy in final energy consumption by 2020 and 100% by 2050. Combined with the commitment to improve energy efficiency by 20% until 2020 and to reduce greenhouse gas emissions by 20% (or respectively 30% in case of a new international climate agreement) against the 1990 level, Europe’s political leaders paved the way for a more sustainable energy future for the European Union and for the next generations. In order to reach the binding overall target of at least 20% renewable energy by 2020, the development of all existing renewable energy sources as well as a balanced deployment in the heating and cooling, electricity and transport sectors is needed. According to estimates of the European renewable energy industry around 40% of electricity demand will be generated with renewable energy sources by 2020 (EREC, 2010). Furthermore, the new Renewable Energy Directive (RED) will undoubtedly stimulate the renewable energy heating and cooling market, and according to EREC’s projections, up to 25% of heating and cooling consumption can come from renewable energy by 2020. Similar kind of awareness is evident in other

regions of the world and cogent efforts are being made to increase the renewable energy share of the energy profile and reduce overdependence on fossil fuels.

For about 3 decades, Brazil has been in the forefront of using renewable energy in the form of bioethanol derived mainly from sugarcane to power fuel-flex vehicles or as oxygenate to gasoline and has made a remarkable success of it. Likewise, the USA has also to some extent used bioethanol to power vehicles. Bioethanol is the biofuel most widely used for transportation worldwide. The global annual production of fuel ethanol is around 40 to 50 billion litres, of which 90 percent is produced by the USA and Brazil from maize and sugarcane respectively (World Bank, 2008). Global ethanol production has seen steady growth since the search for alternatives to petroleum was prompted by the oil crisis of 1973/1974. The USA is now the largest consumer of bioethanol, followed by Brazil. Together they consume 30 billion litres, or three quarters of global production (Licht, 2005). The Economist (2005) reported that as at that time Germany was raising its output of biodiesel by 50% per year; USA was boosting its ethanol production by 30% per year; France aimed to triple its output of biodiesel and ethanol by 2007; China had just built the largest ethanol plant in the world; and also that Brazil was producing around 4 billion litres of ethanol per year, and hoped to export 8 billion litres per year by 2010. China’s Ministry of Science and Technology plans that the country would attain 12 million tonnes of biodiesel production by the year 2020 (GTZ, 2006).

According to OECD (2008), the global ethanol and biodiesel production in 2007 is given in Table 1. Certainly, successes recorded as regards exploitation and use of other biomass for energy supply, will further enhance global energy security. Some of the themes involved in this are discussed in this chapter.

Source: OECD (2008)

Table 1. Global Ethanol and Biodiesel Production for 2007 (in million litres)

There must be suitable moisture content of the feedstock as the microorganisms’ excretive and other metabolic processes require water. The moisture content should normally be around 90% of the mass of the total contents (Buren 1983). Both too much and too little water are harmful, with too much water the rate of production per unit volume in the digester will fall, preventing optimum use of the digester. If the moisture content is too low, acetic acids will accumulate inhibiting the digestion process and hence production. Furthermore, a rather thick scum will form on the surface of the substrate. This scum may prevent effective mixing of the charge in the digester.

Sparse evidence suggests that biogas was known to the Assyrians and Persians centuries before Jesus Christ was born. Further evidence is traceable to count Alessandro Volta who in 1776 concluded that there was a direct link between the amount of decaying organic matter and the amount of flammable gas produced. Sir Humphrey Davy determined in 1808 that methane was present in the gasses produced during the anaerobic decomposition of cattle manure. Helmont recorded the emanation of an inflammable gas from decaying organic matter in the 17th century (Brakel, 1980). It was not until towards the end of the 19th century that methanogenesis was found to be connected to microbial activity. In 1868, Bechamp named the organism responsible for methane production from ethanol. This organism could more accurately be described as a mixed population. Bechamp was able to show that, depending on the substrate different fermentation products were formed. Zehnder et al (1982) stated that it was in 1876 when Herter reported that acetate in sewage was converted to equal amounts of methane and carbon dioxide. Meynell (1976) noted that the first anaerobic digestion plant was built in Bombay, India in 1859. The first notable use of biogas in England occurred in 1859 when gas derived from a sewage treatment facility was used to fuel street lamps in Exeter (McCabe and Eckenfelder, 1957). Then in 1904, Travis put into operation a new two-stage process in which the suspended material was separated from the wastewater and allowed to pass into a separate ‘hydrolyzing’ chamber (Carcelon and Clark, 2002). Buswell and Hatfield (1936) and some other researchers in the 1930s identified anaerobic bacteria and the conditions that promote the production of methane. Their works also explained such issues as the fate of nitrogen in the anaerobic digestion process, stoichiometry of the reactions, as well as the production of energy from farm and industrial wastes through the anaerobic digestion process. Regarding anaerobic technology, farm — based facilities are the most common. In contemporary times low-technology biogas digesters have been most extensively used in China and India. Bui-Xuan (2004) pointed out that low cost biogas technology has been well received by small holder farms in many developing countries for producing a clean fuel to replace firewood, within the recent ten years. Stating that more than twenty thousand digesters have been installed in Vietnam, mainly paid for by the farmers; however biodigesters are still not fully integrated into the farming system as there is only limited use of by-products (effluent) as fertilizer for vegetables, fruit trees, fish pond and water plants. The paper further stated that the use of effluent from digester can be studied as a resource for small scale farmers. Interest in the technology is increasing in several other parts of the world.

The carbon:nitrogen (C/N) ratio expresses the relationship between the quantity of carbon and nitrogen present in organic materials. Materials with different C/N ratios differ widely in their yield of biogas. The ideal C/N ratio for anaerobic biodigesiton is between 20:1 and 30:1 (Marchaim, 1992). If C/N ratio is higher than that range, biogas production will be low. This is because the nitrogen will be consumed rapidly by methanogenic bacteria for meeting their protein requirements and will no longer react on the left over carbon remaining in the material. In such case of high C/N ratio, the gas production can be improved by adding nitrogen in farm cattle urine or by fitting latrine to the plant (Fulford, 1988). Materials with high C/N ratio typically are residues of agricultural plants. Conversely if C/N ratio is very low, that is outside the ideal range stated above, nitrogen will be liberated and it will accumulate in the form of ammonia. Ammonia will raise the pH value of the slurry in the digester. A pH value which is higher than 8.5, will be toxic to the methanogenic bacteria in the slurry. The cumulative effect of this is also reduced biogas production. Materials having low C/N ratio could be mixed with those having high C/N ratios so as to bring the average C/N ratio of the mixture to a desirable level. Human excreta, duck dung, chicken dung, and goat dung are some of the materials which typically have low C/N ratios.

According to Karki and Dixit (1984), typical C/N ratios of common organic materials are as shown in Table 3.