Biomass is mainly composed of the following constituents :
Biomass n \ Pre-treatment Gasification D—^ Gas clean up
Gas clean up
Power (Fuel cell)
CH3OH, DimethylEther, Fischer-Tropsch Diesel
FIGURE 9.1 Scheme of the possibilities involved in biomass gasification.
Cellulose, a linear polymer of anhydroglucose C6 units with a degree of polymerization of up to 10,000. The long strains form fibers, which give biomass its mechanical strength but also a strongly anisotropic character.
Hemicellulose, a linear polymer of C6 and C5 units with a degree of polymerization of less than 200.
Lignin, which is a random three-dimensional structure of phenolic compounds.
Traditionally, biomass (mainly in the form of wood) has been utilized by humans through direct combustion, and this process is still widely used in many parts of the world. Biomass is a dispersed, labor-intensive, and land-intensive source of energy. Therefore, as industrial activity has increased, more concentrated and convenient sources of energy have been substituted for biomass. Biomass currently represents only 3% of primary energy consumption in industrialized countries . However, much of the rural population in developing countries, which represents about 50% of the world's population, is reliant on biomass for fuel. Biomass accounts for 35% of primary energy consumption in developing countries, raising the world total to 14% of primary energy consumption. The Earth's natural biomass replacement represents an energy supply of around 3000 EJ (3 x 1021 J) per year, of which just under 2% is currently used as fuel. However, it is not possible to use all of the annual production of biomass in a sustainable manner. The main potential benefit in growing biomass especially for fuel is that, provided the right crops are chosen, it is possible to use poor-quality land that is unsuitable for growing food. Burning biomass produces some pollutants, including dust and the acid rain gases sulfur dioxide (SO2) and nitrogen oxides (NOJ, but it produces less sulfur than burning coal. Carbon dioxide, the greenhouse gas, is also released. However, as this originates from harvested or processed plants, which have absorbed it from the atmosphere in the first place, no additional amounts are involved. A wide variety of biomass resources can be used as feedstock (Figure 9.2, ), divided into three general categories :
Wastes. Large quantities of agricultural plant residues are produced annually worldwide and are vastly underutilized. The most common agricultural residue is the rice husk, which makes up 25% of rice by mass. Other plant residues include sugar cane fiber (bagasse), coconut husks and shells, groundnut (peanut) shell, and straw. Included in agricultural residue is waste, such as animal manure (e.g., from cattle, chicken, and pigs). Refuse-derived fuel (RDF) is the combustible material in domestic or industrial refuse. It consists mainly of plant material but may also include some plastics. RDF may be used raw and unprocessed, partially processed, or highly processed in the form of pellets . These latter burn more efficiently and with lower emissions.
Standing forests. Wood fuels are derived from natural forests, natural woodlands, and forestry plantations (fuelwood and charcoal). These fuels include sawdust and other residues from forestry and wood-processing activities. Fuelwood is the principal source for small-scale industrial energy in rural areas of developing countries. However, large reforestation programs will be required to meet future energy demands as the world population grows. In industrialized countries, the predominant wood fuels used in the industrial sector are from wood-processing industries. The utilization of this residue for energy production at or near its source has the advantage of avoiding expensive transport costs. Domestic wood fuels are sourced principally from land clearing and logging residues.
Energy crops. Energy crops are those grown specially for the purpose of producing energy. These include short-rotation plantations (or energy plantations), such as eucalyptus, willows, and poplars; herbaceous crops, such as sorghum, sugar cane, and artichokes; and vegetable-oil-bearing plants, such as soya beans, sunflowers, cotton, and rapeseed. Plant oils are important, as they have a high energy density. The oil-extraction technology and the agricultural techniques are simple, and the crops are very hardy.
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