Biomass is organic material that has stored sunlight in the form of chemical en­ergy. It is a general term for material derived from growing plants or from animal manure. Biomass includes wood and logging residues, agricultural crops and their waste byproducts, the organic portion of municipal solid waste, animal wastes, mu­nicipal biosolids (sewage), waste from food processing, and aquatic plants and algae (Hall et al. 1993). All biomass is produced by green plants converting sunlight into plant material through photosynthesis. Photosynthesis is a carbon fixation reaction achieved by the reduction of carbon dioxide. The fixation or reduction of carbon dioxide is a light-independent process. Although some of the steps in photosynthe­sis are still not completely understood, the overall photosynthetic equation has been known since the 1800s.

Biomass appears to be an attractive feedstock for three main reasons. First, it is a renewable resource that could be sustainably developed in the future. Second, it appears to have considerable environmentally friendly properties, resulting in no net releases of carbon dioxide and very low sulfur content. Third, it appears to have significant economic potential provided that fossil fuel prices increase in the future.

Agricultural residues such as straws, nut shells, fruit shells, fruit seeds, plant stalks and stovers, green leaves and molasses are potential renewable energy sources.

Current disposal methods for these agricultural residues have caused widespread en­vironmental concerns. For example, disposal of rice and wheat straw by open-field burning causes air pollution (Zhang and Zhang 1999).

Agricultural energy or green energy production is the principal contributor to the economic development of a developing country, whose economic development is based on agricultural production, and most people live in rural areas. Implemen­tation of integrated community development programs is therefore very necessary. It is believed that integrated community development promotes the socioeconomic advancement of developing countries.

The biomass produced in Brazil results largely from an ethanol fuel production program started in 1975 from sugar cane crops grown specifically for fuel use, presently occupying 2.7 million hectares of land and employing about 350 distil­leries. Ethanol currently provides over 40% of the fuel consumed by cars and light trucks. It is estimated to have saved Brazil over $40 billion in oil imports, exclud­ing the costs of the program. In 1999, carbon emissions were reduced by almost 13 megatons as a result of the program, and local emissions of lead, sulfur, and carbon monoxide have been greatly reduced. In addition, according to the United Nations Development Programme, ethanol production supports about 700,000 rural jobs (UNDP 2000).

Biomass has historically been a dispersed, labor-intensive, and land-intensive source of energy. Therefore, as industrial activity has increased in countries, more concentrated and convenient sources of energy have been substituted for biomass. Selection of a particular biomass for energy requirements is influenced by its avail­ability, source and transportation costs, competing uses, and prevalent fossil fuel prices.

Biomass provides a clean, renewable energy source that could dramatically im­prove our environment, economy, and energy security. Biomass energy generates far less air emissions than fossil fuels, reduces the amount of waste sent to landfills, and decreases our reliance on foreign oil. Biomass energy also creates thousands of jobs and helps revitalize rural communities.

Energy can be obtained from direct combustion of biomass by burning of dry organic matter, such as woody scraps, grasses, and agricultural residues. Biomass can be thermochemically converted into liquid fuel and into gases, such as methane, carbon monoxide, or hydrogen, by pyrolysis. Bioethanol can be obtained from cel — lulosic biomass by fermenting and distilling sugar solutions. Vegetable oils such as soybean and canola oils can be chemically converted into liquid fuel known as biodiesel. These fuels can be used as diesel fuel and gasoline in conventional en­gines with little modification to the system. Certain organic compounds, specifically municipal biosolids (sewage) and animal wastes (manures), can be biochemically converted into methane by anaerobic digestion.

Certain transportation biofuels such as bioethanol, biodiesel, methyltetrahydro — furan, and dimethyl ether can be sustainably obtained from biomass. Biofuels can be used in pure form or blended with gasoline.

In industrialized countries, the main biomass processes utilized in the future are expected to be the direct combustion of residues and wastes for electricity gener­ation, bioethanol and biodiesel as liquid fuels, and combined heat and power pro­duction from energy crops. The electricity is produced by the direct combustion of biomass and by advanced gasification and pyrolysis technologies that are almost ready for commercial-scale use.

Bioenergy, the energy from biomass, has been used for thousands of years, ever since people started burning wood to cook food or to keep warm, and today wood is still our largest biomass resource for bioenergy. Many countries in the developing world still use wood as their primary fuel (Demirbas and Demirbas 2007). In the future, biomass has the potential to provide a cost-effective and sustainable supply of energy, while at the same time aiding countries in meeting their greenhouse-gas — reduction targets. In the short to medium term, biomass is expected to dominate energy supply. For the generation of electricity and heat, with the use of advanced combustion technology, organic wastes can be used as biomass. Also a number of crops and crop residues may fit modern bioenergy chains (Pimentel et al. 1981; Haberl and Geissler 2000; Hoogwijk et al. 2003). Biomass is the most common form of carbonaceous materials, which are widely used in the third world.

Biomass gasification technologies make it possible to convert renewable biomass materials into clean fuel gases or synthetic gases. These gaseous products can be burned to generate heat or electricity, or they can potentially be used in the synthe­sis of liquid transportation fuels, hydrogen, or chemicals. Gasification offers a com­bination of flexibility, efficiency, and environmental acceptability that is essential in meeting future energy requirements. Biomass-fueled renewable distributed gen­eration technologies in rural electrification provide no-regret options with signifi­cant CO2 emission mitigation potential when operated under a net-metering scheme (Karki et al. 2008).

Cogeneration, in and of itself, is an example of pollution prevention. Cogenera­tors, by using excess heat, may enhance the efficiency of total energy use by up to 80% or more from the typical 33 to 38% efficiency of electricity-only generation. A typical cogeneration system consists of an engine, steam turbine, or combustion turbine that drives an electrical generator. Figure 2.1 shows a gas turbine topping cycle cogeneration system. The figure is a diagram of a gas turbine of the type used in cogeneration power plants ranging in size from about 500 kW to 50 MW. These turbines are similar to jet engines used in aircraft. In this size range, gas turbines are more efficient than any other type of fossil-fuel-fired power plants, reaching elec­trical efficiencies of up to 45% or even, with the addition of waste heat recovery (cogeneration), efficiencies of over 80%. When used for cogeneration, the hot ex­haust gases from the gas turbine are passed through a heat exchanger where the heat is transferred to water in pipes producing steam. A waste heat exchanger recovers waste heat from the engine or exhaust gas to produce hot water or steam. Cogenera­tion produces a given amount of electric power and process heat with 15 to 35% less fuel than it takes to produce the electricity and process heat separately (Demirbas 2006c).

Figure 2.2 illustrates a comparison between energy inputs to separate and cogen­erative generation systems. Traditional coal-, oil-, or natural-gas-fired thermal gen­erating stations convert only about one-third of the initial energy contained within

the fuel into useful electricity. The remainder of the energy is discarded as heat with­out serving any useful purpose. From 10 to 35% of primary energy use is wasted as conversion losses in power plants. Cogeneration can increase the efficiency of

a fossil fuel from an average of 40 to over 80%. Figure 2.2 illustrates the increase in efficiency. This increase in efficiency can translate into lower costs and fewer emissions of pollutants than the conventional alternative of generating electricity and heat separately (Demirbas 2006c).