Trees are seed-bearing plants that are subdivided into gymnosperms and angiosperms. Coniferous woods (softwoods) belong to the first category and hardwoods to the second group, and altogether 30,000 hardwoods and 520 softwoods are known (Sjostrom, 1993). Wood is mainly composed of cellulose, hemicelluloses and lignin. Simply put, cellulose forms a skeleton that is embedded in a matrix of the two other main components. In addition, minor amounts of extractives are also present in wood. Cellulose is themost abundant renewable biopolymer in the world, and is a linear homopolymer
composed of anhydroglucose units linked together by (1→ 4)-glycosidic bonds (Sixta, 2006). Cellulose has a strong tendency to form intra-and inter-molecular hydrogen bonds, resulting in a strong semi-crystalline fibrous structure that is insoluble in most solvents. Hemicelluloses are branched polymers consisting of both pentose (C5) and hexose (C6) sugars. Due to their branched, amorphous structure, hemicelluloses are usually water soluble and, unlike cellulose, can easily be hydrolyzed by acids to their corresponding monomeric components. Lignin is the world’s second most abundant biopolymer and is composed of randomly polymerized phenylpropane units. It has a heterogeneous, highly branched and relatively hydrophobic amorphous structure, resulting in poor water solubility (Sjostrom, 1993). Compounds extractable with organic solvents include terpenes, fats, waxes and low molecular weight phenols. The elementary composition of wood is approximately 49% carbon, 44% oxygen and 6% hydrogen (Sixta, 2006). Wood is a versatile material. When leaving the forest or plantation, wood can be divided
according to its use in fuel, sawn products, pulp wood and other industrial wood (Table 8.1). Sawn wood and veneer can be used as a construction material in buildings and pieces of furniture. Pulpwood is processed into particles and fibers and used in particle boards, fiber boards and paper. Wood can also be used as a fuel and finds its applications in many areas, from small bonfires to large CHP plants. Since wood can be employed in many areas, high quality wood may also be an expensive raw material for energy production. Low quality wood, such as scrap wood (used demolition wood) and sawdust, is better suited for bioenergy production. Not only the wood itself but also the wood components are of different value. Differentiating
the use of wood components is both sustainable and economically sound. By gentle chemical processing of wood a fraction of the lignin and hemicelluloses can be removed, resulting in cellulose fibers with relatively high hemicellulose content. These “crude” fibers are commonly used to make paper and board. Lignin and hemicelluloses can be nearly completely removed through more severe chemical treatments to obtain pure cellulose. These pure cellulose fibers are used as raw material for manmade fibers (e.g. viscose) and may also be converted to cellulose derivatives. Today, the sulfate (or Kraft) process, in which a solution of sodium sulfide and sodium hydrox-
ide is used for dissolving lignin from the wood fibers, is by far the most commonly used chemical pulping process (Sixta, 2006).While the fibers find their use in many types of paper products, the
Table 8.1. The 2009 world production of round wood. Data from FAOSTAT (FAO Statistics, 2011).