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: