Lignin is the second most abundant natural polymer in the world and is a basic component of the cell walls of plant cell wall. It is composed of both aromatic and aliphatic functionality.
Plant lignins can be broadly divided into three classes: softwood (gymnosperm), hardwood (angiosperm) and grass or annual plant (graminaceous) lignin . Three different phenylpropane units, or monolignols, are responsible for lignin biosynthesis . Guaiacyl lignin is composed principally of coniferyl alcohol units, while guaiacyl- syringyl lignin contains monomeric units from coniferyl and sinapyl alcohol. In general, guaiacyl lignin is found in softwoods while guaiacyl-syringyl lignin is present in hardwoods. Graminaceous lignin is composed mainly of p -coumaryl alcohol units. These three lignin precursors are shown in Figure 1.
Sources of Lignin:
1. Pulp & Paper Mills:
2. Ligno-cellulosic Ethanol Industries: