The various feedstocks that can be used for the production of biofuels may be classified into three groups: cellulosic biomass, sugar and starchy crops, and oil-containing or oil-producing plants.
The first group of feedstock is cellulosic biomass. It is the type of feedstock that is increasingly becoming important for the production of biofuels. Its attractiveness as a feedstock results from the fact that it is made up of very complex sugar polymers that are not usually used as a source of human food. Cellulosic biomass includes a wide range of heterogeneous solid materials including, but not limited to, the following: (a) agricultural residues comprising leftover material from crops such as rice straw, or coconut coir, or the stalks, leaves and husks of corn plants; (b) forestry wastes such as chips and sawdust from lumber mills, dead trees, and tree branches; (c) municipal solid wastes such as paper products; (d) food processing and other industrial wastes such as slops from alcohol distilleries and black liquor from pulp and paper manufacturing; and (e) energy crops grown for fuel purposes such as fast growing trees and grasses.
The main components of cellulosic biomass are cellulose, hemicelluloses, and lignin. Depending on the source of biomass, cellulose may account for 40% to 60% by weight of the biomass and is thus the most common form of carbon in the biomass. It is a complex sugar polymer, or a polysaccharide, and is made from the six-carbon sugar called glucose. Because of its crystalline structure, it is resistant to hydrolysis, the chemical reaction that enables the production of simple, fermentable sugars from a polysaccharide. Hemicellulose is also a major source of carbon in biomass and may account for 20% to 40% by weight of the biomass. It is also a complex polysaccharide that is made from a variety of fivecarbon and six-carbon sugars. Although it is relatively easier to hydrolyze into simple sugars compared to cellulose, the sugars that are produced, however, are not easily fermented to ethanol. Lignin comprises from 10% to 24% by weight of biomass and provides structural integrity and strength in plants. It remains as the residual material after the sugars in the biomass have been converted to ethanol. As a highly complex carbon-containing polymer, it contains a lot of energy and can be burned to produce steam and electricity for use in the biomass-to-ethanol manufacturing process.
The second group of feedstock is sugar and starchy crops. These are plants such as sugar cane and sugar beets that can store through photosynthesis the energy from the sun by converting it into simple sugars. In a similar fashion, there are plants such as corn, cassava and sweet potato that store the energy as complex sugars or starches. Although the name sugar is most often used to refer to sucrose or table sugar, in general, sugars are water-soluble carbohydrates that have relatively low molecular weight and usually characterized with having a sweet taste. Carbohydrates, on the other hand, are a group of organic compounds that include sugars, starches, celluloses and gums. They provide a major source of energy in the diet of humans and animals. These compounds are produced through photosynthesis by plants and contain only molecules of carbon, hydrogen and oxygen, usually in the ratio 1:2:1. Simple sugars are called monosaccharides. More complex sugars comprise between two and ten monosaccharides that are linked together. Thus dissacharides are those that contain two monosaccharides, trisaccharides are those that contain three and so on. These biomass products are mainly used as human or animal food. However, because of the need to find alternative sources of energy other than fossil fuels, these products are increasingly being used for the production of biofuels, particularly ethanol as gasoline substitute or blend.
The third group of feedstock is composed of oil-containing or oil-producing plants. There are a large number of plants that produce oils, in particular fixed oils, which can be processed to produce biofuels that can be used as diesel substitute or blend. The common characteristic of these oils is that they may be decomposed into glycerin and one or more acids of the type called fatty acids. The physical properties commonly found in these plant oils are the following: (a) they float on water but are not soluble in it; (b) they are greasy to the touch and have lubricating properties; (c) they are not readily volatile; and (d) they may be burned without leaving any significant residue. Most of these oils such as soybean oil, coconut oil and palm oil have been used mainly for human or animal food or the production of various types of cosmetics and pharmaceuticals but increasing amounts are now being processed for the production of biodiesel.
The recent interest in the production of energy from biomass has focused primarily on technologies and applications that produce liquid fuels for the use of the transportation industry. Since ethanol in its anhydrous form can be blended with gasoline, it has become one of the most widely used biofuel. In the same manner, since biodiesel (the methyl ester of plant oil) can be easily blended with or used as a substitute for diesel fuel, its production and use have also become widespread. Their use has the additional advantage of reducing emission of air pollutants, buildup of greenhouse gas and dependence on imported oil. At the same time, the cultivation and harvesting of the feedstocks have positive impacts on the agricultural and rural economies.