Biomass as a raw material for energy production

In this paper, the meaning of biomass is defined and it is explained why it is a potential source of energy. The utilization of biomass as an energy source is based on heat energy production during its combustion. The solar energy captured and stored by plants is released in the form of heat energy during the biomass combustion. The variables that affect the energy value (calorific value) of forest biomass involve the chemical composition, percentage of extractives, moisture content, ash content and density. Softwoods generally contain more energy than hardwoods on a dry weight basis, due to higher lignin content plus the presence of more resinous extractives. Lastly, the advantages and disadvantages of biomass as an energy source are analyzed: biomass is renewable and eco-friendly, but its efficiency is low.


Introduction
The term biomass refers to the living materials in the biosphere and their refuse and waste products.It is defined as any organic matter that is available on a renewable basis, including agricultural crops and agricultural wastes and residues, wood and wood wastes and residues, animal wastes, municipal wastes and aquatic plants (ASTM, 1981).Biomass is the result of photosynthesis.Photosynthesis is the process by which chlorophyllcontaining cells in green plants convert incident light to chemical energy and synthesize organic compounds from inorganic compounds, especially carbohydrates from carbon dioxide and water with the simultaneous release of oxygen (ASTM, 1981).The total balance of photosynthesis is expressed by the reaction: Solar energy is transformed by plants with a course of procedures.The basic raw materials for this are water and carbon dioxide that abound in nature.Energy comes from the visible spectrum of solar radiation.The fundamental reactions take place in chlorophyll which captures the photons and activates the photosynthesis process (Apostolakis et al., 1987;Sooter, 1992).The process of photosynthesis embodies the two most important reactions in life.The first one is the water-splitting reaction which evolves oxygen as a by-product; all life depends on this reaction.Secondly is the fixation of carbon dioxide to organic compounds; all our food and fuel is derived from this CO 2 fixation from the atmosphere (Hall, 1983).These compounds contribute to the formation and growth of various plant organs and lead to biomass production.Also, inorganic elements as well as the proper temperature conditions for each plant species must coexist.
Consequently, it could be said that plants constitute a solar energy conversion system, since this is what photosynthesis is all about.Thus, everything which has been derived from the process of photosynthesis (biomass) is a potential source of energy.

Utilization of biomass as an energy source
The utilization of biomass as an energy source is based on heat energy production during its combustion (the solar energy captured and stored by plants is released in the form of heat energy during the biomass combustion).Combustion is the rapid chemical combination of oxygen with the elements of fuel that will burn with the release of heat energy.The maximum amount of heat energy which is produced when 1 g or 1 kg of dry mass is completely combusted is called calorific value or heating value and it is usually expressed in kcal/kg of absolutely dry mass (Tsoumis and Philippou, 1982).
The major combustible elements of lignocellulosic materials are carbon and hydrogen.Complete oxidation of these elements gives CO 2 , H 2 O and heat energy as follows (Philippou, 1981;Tsoumis and Philippou, 1982):

Energy value of forest biomass and variables which affect it
The value of biomass as a source of energy is based on its chemical composition.Also, its energy value is affected by the existence of extractives, as for instance the pine resin (Tillman, 1978;Tsoumis and Philippou, 1982).Various biomass types (wood, bark, foliage and so on) vary in chemical composition and concomitantly in energy value.The different chemical compositions are responsible for varying heat contents among biomass fuels (Tillman, 1978).Forest biomass is composed of holocellulose (cellulose and hemicellulose), lignin and various extractives (Table 1).Source: Philippou (1986).
The calorific value of holocellulose ranges from 4,100 to 4,350 kcal/kg of lignin from 6,100 to 6,500 kcal/kg, and of extractives from 4,100 to 9,150 kcal/kg.The percentage of the above chemical components differs among various forest species as well as among various parts (wood, bark, foliage, branches, tops) of the same tree (Philippou, 1982).Thus, the calorific value of various forest biomass types was found to range from 3,960 to 6,289 kcal/kg (Philippou, 1981(Philippou, , 1982(Philippou, , 1986)).
Concerning ash percentage it could be said that it influences proportionately the heating value.Lastly, heating value of forest biomass (on the basis of biomass volume that is burned) increases as its density increases (Tsoumis and Philippou, 1982;Philippou, 1982).

Advantages and disadvantages of biomass as an energy source
Biomass as a source of energy has advantages and disadvantages.The main advantages from biomass use are the following (Tsoumis and Philippou, 1982;Hall, 1983;Tsoumis, 1986;Philippou, 1986;Apostolakis et al., 1987;Sooter, 1992): it is renewable; b.
its utilisation can be attained by various conversion methods and by relatively simple technologies; c. its production and conversion do not create ecological and environmental problems (it does not produce oxides of S, N, Pd etc.); d.
it exists throughout the world; e.
biomass use for energy purposes creates employment.
The drawbacks which are related to biomass use are the following (Tsoumis and Philippou, 1982;Hall, 1983;Tsoumis, 1986;Philippou, 1986;Apostolakis et al., 1987;Sooter, 1992) By reason of these disadvantages, biomass cost remains, comparatively with oil, high.However, this problem is gradually disappearing because of upward trends in oil prices and the atmospheric pollution which is caused by its combustion (Hall, 1983;Apostolakis et al., 1987;Sooter, 1992).

Energy production methods from biomass
The potential offered by biomass for solving some of the world's energy problems is widely recognised.The energy in biomass may be realised either by direct use as in combustion or by upgrading into a more valuable and usable fuel.This upgrading may be by biological, chemical or thermal methods to give a solid, liquid or gaseous fuel (Beenackers and Bridgwater, 1989).In other words, the methods of energy biomass conversion are distinguished into biochemical (aerobic fermentation, anaerobic fermentation, alcoholic fermentation) (Apostolakis et al., 1987;Sooter, 1992) and thermochemical ones (direct combustion, pyrolysis, liquefaction, gasification) (Ward, 1983;Soltes, 1983;Beenackers and Bridgwater, 1989).

Conclusion
It could be said that plants constitute a solar energy conversion system, since this is what photosynthesis is all about.Everything which derives from the process of photosynthesis (biomass) is a potential source of energy.The utilisation of biomass as an energy source is based on heat energy production during its combustion; the solar energy captured and stored by plants is released in the form of heat energy during the biomass combustion.The energy value (calorific value) of forest biomass depends on its chemical composition, existence of extractives, moisture content, ash content and density.Lastly, the upward trends in oil prices and atmospheric pollution which is caused by its combustion, increasingly encourage a wider use of biomass as an energy source.
: HVw = HVd -(0.0114HV x Mw) HVd = HV -0.15 M Where: HVw : heating value of wet biomass (kcal/kg of wet biomass); HVd : heating value of dry biomass (kcal/kg of dry biomass); HV : heating value of absolutely dry biomass (kcal/kg of dry biomass); Mw : percentage of moisture on the basis of wet biomass weight (%); M : percentage of moisture on the basis of dry biomass weight (%).

Table 1 .
Chemical composition and calorific value of forest biomass.

Table 2 .
Characteristics of various fuels.