1. Introduction
Energy is the driving force for economic development and industrialization around the world. Fossil fuels are currently the main resources that are used for fuel, as such they are an important consideration in terms of finding solutions to the world׳s increasing demand for energy [1]. However fossil fuels are limited, and burning fossil fuels has a strongly negative impact on the environment [2]. Ever since the first oil crisis, renewable energy has attracted much by its renewability, sustainability and as a result of increased environmental awareness [3]. So, the role of renewable energy resources in electricity production is improving and it is currently receiving much research attention for its potential to replace with fossil fuel consumption in power plants. This has been brought about by increased public awareness of the negative environmental impact of conventional methods of generating power, especially from coal and petroleum power plants [4].
The main purpose of investigating potential sources of renewable energy is to identify localities that have a suitable supply of renewable energy resources. In the past, biomass energy has been a major source of fuel and it is now an important part of research in the field of renewable energy. According to historical accounts, people in the past relied mostly on dried leaves and wood for fuel [5]. Biomass, as a derivative of biological materials is a renewable resource. Any biological material including organisms or the remains of organisms such as wood and any other organic matter such as forest debris from trees and material from pruning plants and fragments of wood can be used as a source of biomass. Biomass resources include long chain organic compounds that are converted into simpler molecules by the process of digestion [6]. The first step in the digestion process is hydrolysis of polymers that converts polymers to monomers such as glucose and amino acids. In the next step the materials generated by microorganisms are converted to volatile fatty acids, hydrogen and acetic acid. By continuation of this process, active fatty acids (butyric acid and propionic acid) are converted to hydrogen and carbon dioxide by acidogenic bacteria and in the final step hydrogen, carbon dioxide and acetate are converted to carbon dioxide and methane by methanogenic bacteria. The biogas produced in this process consists of two components, methane and carbon dioxide with a small amount of other gases. Biogas usually contains about 55–65% methane, 30–35% carbon dioxide, and some hydrogen, nitrogen and other impurities. Its heating value is around 5300 kcal m−3 (600 BTU per cubic foot) and natural gas contains about 80% methane, with a heating value of around 9800 kcal m−3 (1000 BTU per cubic foot) [7].
In terms of an overall classification, the final products of biomass are biogas, biodiesel and bioethanol and the analysis in this current study relates to biogas and the potential of biogas production from livestock manure in Iran.
The first report on biomass by the Russian Pillini was on the production of biomass from landfill waste, and incomplete combustion was reported [1]. Von Helmond in 1630 was the first scientist to identify and formally report biogass flammability [3]. There is a wide range of material suitable for application as a biomass resource for bioenergy generation and these are categorized to 6 classes as listed:
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Forest
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agricultural and crop waste
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food industry waste
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municipal solid waste (MSW)
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livestock waste
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municipal sewage and industrial organic waste
The Food and Agricultural Organization (FAO) reported that 3,339,000 ha of forest exist in the region of the Middle East, of which 1,658,000 ha are located to Iran. It was also reported that each hectare has the potential to produce 80 m3 of biomass [2]. Thus, the output of Iranian forests has the potential of 133 million cubic meters or 331 million tons of biomass, annually. Efficient use of this resource could generate 218 TWh electricity [1]. As mentioned earlier, livestock waste is one of the main resources available for biogas production in Iran. It is obvious that varied climate conditions are an important consideration for keeping domestic livestock and Iran has suitable climate conditions, therefore good potential for significant amounts of biomass production. Therefore planning for production of biogas from livestock manure is an attractive option that needs to be highlighted in planning and policy development agendas for renewable energies.
The contribution of different biomass resources for biogas production in the world is shown in Table 1 which demonstrated that the agricultural solid waste and livestock manure most important ones with higher values. Preliminary investigations revealed that the Iran׳s biomass potential is equal to 15 million tons or 140 million barrels of crude oil equivalent, and that is equal to 13% of Iran׳s total crude oil production in the year 2010[1]. According to variety and diversity of biomass resources, different methods for resource assessment (calculations of available and accessible resources) are applicable. These methods can be applied for analysis and assessment of biogas production from Livestock manure in Iran.
Table 1.
The share of the biogas production resources in the world [1].
# Resource Share (%)
1 Municipal sewage and industrial organic wastes 2
2 Municipal organic wastes 11
3 Livestock wastes 28
4 Agricultural wastes 59
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Firstly, livestock resources in Iran were analyzed and amounts of animal manure produced in each province were estimated using different methods. Secondly, calculations were made using volume of biogas produced and fractions of methane gas from computed livestock resources. According to annual volumes of natural gas consumption and calculated volumes of biogas production from livestock waste in each province, evaluations were made for the portion of natural gas consumption that could potentially be replaced by biogas from livestock waste.
Application and utilization of biogas in Iran has a long history. Sheikh Bahai (1530–1622) was the first to use biogas in a bathhouse in Esfahan. In recent years, the first biogas production digester was built in 1975, Niazabad village, Lorestan western Iran. This digester has a volume of 5 m3 that used the livestock waste of the village to produce biogas for providing hot water for bath [1]. The livestock waste resources are under the attention of industrialized countries to produce biogas. The produced biogas can be used as an energy carrier after performing some refinement processes matching with global and environmental standards such as ISO: TC 255. Using biogas in industry can make a big saving in consumption of valuable fossil resources (Oil and Natural gas) [2].
This gas can be directly applied to a wide range of industries and existing heating systems, which can save natural gas and oil consumption as an exportable and valuable fossil fuel resource which reduces the environmental air pollutions. In most countries, general use of this gas is for cooking and lighting [3]. There are different types of reactors for the production of biogas and two types are most commonly used in Iran, Chinese and Indian models without any major modification in structures and digestion method. Comparisons of these two conventional biogas reactors are presented in Table 2[4] and also shown in Fig. 1.
Table 2.
Two types of most common biogas reactors in Iran [5].
Title Chinese model Indian model
Type of production Manure–gas Gas–manure
Building Building is integrated and creating it is possible in every where Materials and buildings method is simple
Inlet material Generally mixture of plants waste, animal waste, human waste Generally cow׳s manure
Material outlet method In many case, the pump or bucket Material after digestion will floating and will exit from bucket
Average size (m3) 18 14
Collecting the gas Dose not need cover and amount of gas will determined by gas pressure and exit material from outlet bucket In floating caps, height of gas tank show the amount of gas
Gas pressure Pressure is high (max: 1000 mmHg) Pressure is low (70–150 mmHg)
Cost Low cost High cost due to use of metal components
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Fig. 1.
Schematic diagram of biogas digester in Iran (upper) with floating gas holder (Indian model) and (lower) Fixed dome biogas digester (Chinese model).
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There are several parameters that affect the process of anaerobic digestion to achieve the ideal amount of methane production. The optimum conditions for production of biogas are:
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Temperature: the most suitable working range is 35–60 °C.
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pH: optimum pH for anaerobic digesters is in the range 6.8–7.2.
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Nutrients: anaerobic bacteria are needed for metabolism and cell survival and repair, and essential nutrients which are nitrogen, phosphorus, sulfur, carbon, magnesium, sodium, manganese, cobalt, iron and zinc. The amounts and percentages of these materials is very important in controlling and interactions of microorganisms.
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Toxic substance: if the concentration of required food for a microorganism is above its range of activity than it will become toxic and inhibit biological growth.
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Mixing of contents: mixing of contents and making a uniform solution can increase gas production and efficiency of anaerobic digesters [5] and [6].