**1. Introduction**

Biomass can be regarded as any organic material that originated from plants or animals. Thus, the United Nations Framework Convention on Climate Change UNFCCC in 2005 [1], defined it as follows;

"A non-fossilized and biodegradable organic material originating from plants, animals and micro-organisms. This shall also include products, by-products, residues and waste from agriculture, forestry and related industries as well as the non-fossilized and biodegradable organic fractions of industrial and municipal wastes".

The world's energy demand has been steadily increasing in the last several decades. This is due to rapid increase in industrialization, population and the quest for improvement of the living standards for societies. On the other hand, this has caused an irreversible damage to the environment which leads to global warming, and climate change. These issues have been the topic of discussion among scientist and policy makers at national and international levels on how to mitigate the problem. The modern society is emphasizing on shift from non-renewable to renewable energy (such as wind, solar, tide, geothermal and biomass) in their search for energy source. Before the discovery of fossil fuels such as petroleum products, coal, and natural

gas, biomass was the main source of energy for heating and cooking [2]. Biomass is the term used to describe all materials that contain carbon in an organic form. This organic form of carbon can be transformed into inorganic through photosynthesis by forming bonds with other elements such as hydrogen, and oxygen using solar energy. The demolishing of these bonds (molecules) through physical or biological means, causes a closure in the cycle and making CO2 to be regenerated. During the regeneration process, energy is released which can be converted into other forms of energy. Therefore, as long as these equilibrium is maintained between use and regeneration, biomass is a renewable or inexhaustible source of energy [3]. Biomass is expected to be the leading form of energy with a significant global energy load of about 10–15%. However, biomass has a share of about 90% of total energy requirements for remote and rural areas of the developing countries. Therefore, it is likely to remain the future leading source of energy feedstock for the developing countries since about 90% of the world population is expected to live in the developing world by 2050 [4–6].

Biomass accumulates chemical energy in form of carbohydrates through combination of solar power and carbon dioxide during the process of photosynthesis. This has made it to be a potential energy source since the carbon dioxide captured during photosynthesis could be released when it burns. It is cheap and available in all forms such as forest and agricultural residues, wood, by-products of biological materials, organic components of municipal and sludge wastes, etc. There are several ways to convert biomass into useful products which largely depends on biomass characteristics and the end product [7]. The technologies applied in the conversion of biomass are mainly categorized under thermochemical or biological methods.
