1.3 Lignin

The structure of lignin (see Figure 4) is complex and changes with the type of biomass source. Lignin is composed of phenylpropenyl (C9) randomly branched units. The phenylpropenyl building blocks, like guaiacols and syringols, are connected through carbon-carbon and carbon-oxygen (ether) bonds. Trifunctionally linked units provide numerous branching sites and alternate ring units (Holladay et al., 2007). Lignin offers a signif­icant opportunity for enhancing the operation of a lignocellulosic biorefinery. Today, lignin is used as a source of heat and power for the processing plant (e. g., pulp and paper industry), but this approach seems to be shortsighted: lignin’s native structure suggests that it could play a central role as a new chemical feedstock, particularly in the formation of supramolecular materials and aromatic chemicals. All current commercial nonenergy uses of lignin, except combustion and production of synthetic vanillin and dimethylsulfoxide (DMSO), take

advantage of lignin’s polymer and polyelectrolyte properties. These are primarily applications targeted at dispersants, emulsifiers, binders, and sequestrants. Generally, lignin is used in these applications with little or no modification other than sulfonation or thio hydroxymethylation. These uses mainly represent relatively low value and limited volume growth applications. An economic study shows that when lignin is used for purposes other than power, the overall revenue improvement of a biorefinery concept is between $12 and $35 billion (Holladay et al., 2007). However, as will be shown hereinafter, significant technology developments are required to capture the lignin value benefit.

Besides the immediate opportunities for heat and power production, the specific types of products which can be produced from lignin can be grouped in two main categories:

1. Syngas-derived chemicals (near-term opportunity)

2. Aromatics (medium/long-term opportunity)