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What is Lignin?
Occurrence, Chemical Structure, Function
Lignin is a constituent of the cell walls of almost all dry land plant cell walls. It is the second most abundant natural polymer in the world, surpassed only by cellulose. Of the polymers found in plant cell walls, lignin is the only one that is not composed of carbohydrate (sugar) monomers.
Lignin is unique in that it is the only large-scale biomass source of an aromatic functionality. It is composed of up to three different phenyl propane monomers, shown in Figure 1, depending on the species. Coniferyl alcohol occurs in all species and is the dominant monomer in conifers (softwoods). Deciduous (hardwood) species contain up to 40% syringyl alcohol units while grasses and agricultural crops may also contain coumaryl alcohol units.
An additional complexity of lignin is that there are many possible bonding patterns between individual units. Thus our knowledge of lignin chemical structure is less precise than our knowledge of other natural and synthetic polymers. Figure 2 shows a representative lignin fragment containing the most important bonding patterns.
Lignin and cellulose work together to provide a structural function in plants analogous to that of epoxy resin and glass fibres in a fiberglass boat. The fibrous components, cellulose or glass fibres, are the primary load-bearing elements while the matrix, lignin or epoxy resin, provides stiffness and rigidity. Thus trees (lignin content between 20% and 30% of dry weight) grow much taller than grasses (lignin content below 20%) before they bend under their own weight.
Beyond the structural function, lignin plays several other important biological roles in plants. Because it is much less hydrophilic than cellulose and hemicellulose, it prevents the absorption of water by these polysaccharides in plant cell walls and allows the efficient transport of water in the vascular tissues. Lignin also forms an effective barrier against attack by insects and fungi.
Commercial Sources of Lignin
As a source of papermaking grade bleached pulp, the sulfite process has been largely displaced by kraft (alkaline) pulping. Nonetheless, lignosulfonates isolated from spent sulfite pulping liquors are the most important commercial source of lignin today with global production being about 1 million metric tons per year. Lignosulfonates contains sulfonate (-SO3-) groups bonded to the polymer and are therefore soluble in water at a wide range of pH. The common applications of lignosulfonates are as dispersants, binders, complexing agents and emulsifying agents.
Kraft pulping is the dominant chemical pulping process in the world. It uses strong alkali with a sodium sulfide catalyst to separate lignin from the cellulose fibres. After pulping, the cellulose fibres go through several bleaching stages to remove residual lignin and produce a strong, white, stable papermaking pulp. The lignin and hemicellulose that is dissolved in the pulping stage is known as “black liquor” and is sent to a recovery system where it is burned. This recovery stage is crucial to the functioning of a kraft mill – it supplies much of the energy needed to operate the mill and regenerates the inorganic pulping chemicals.
Kraft lignins have different chemical properties than lignosulfonates. There are no sulfonate groups present, so kraft lignin is only soluble in alkaline solution (pH above 10). Kraft lignin can thus be precipitated from black liquor by lowering the pH to 10 with a suitable acid. However, only one company, Mead-Westvaco, isolates kraft lignin for sale as an industrial product. These lignins are used in niche applications such as dispersants for dyes and pesticides.
An interesting opportunity for lignin-based products exists in the Canadian kraft industry. In many mills, cellulose production is limited by the thermal capacity of the recovery furnace. Removing a portion of the lignin from the black liquor is currently the most cost-effective way to increase the production of cellulose without massive capital spending. The isolated lignin then provides an additional value-added revenue stream for the mill.
Tremendous effort is currently being expended, especially in the United States and Europe, to find feasible pathways to produce fuel ethanol from the cellulose contained in agricultural residues and waste wood. If a cellulosic ethanol industry is established, large amounts of lignin will be produced as a by-product. Most schemes propose to use the separated lignin as a fuel to run the ethanol plant. However, a process that converts all of the input biomass to fuel is unlikely to be economically feasible. To improve the economic feasibility, a portion of the lignin needs to be converted to higher-values chemicals or materials.