Environmental Engineering Reference
In-Depth Information
CHAPTER 8
Biorefineries using wood for production of specialty cellulose fibers,
lignosulfonates, vanillin, bioethanol and biogas - the Borregaard
Sarpsborg example
Stefan Backa, Martin Andresen & Trond Rojahn
8.1 INTRODUCTION
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 the most 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
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