Because of the difficulties associated with separating it from lignocellulosic feedstocks, “lignin has typically been burned by papermakers to produce process heat or power. This is a tragic waste of a good source of carbon,” says Joseph J. Bozell, associate professor, Biomass Chemistry, Forest Products Center, University of Tennessee (Knoxville).
Though challenging, the use of lignin to produce higher-value products presents significant economic opportunities. For instance, analysis carried out by Bozell and others in 2007 for the U.S. Dept. of Energy (DOE; Washington, D.C.; doe.gov) suggests that ability to convert the 225 million tons/yr of available lignin in the U.S. for purposes other than power would create overall revenue-improvement opportunities of $12 to $35 billion compared to just burning the excess lignin for power or heat.
“When it comes to renewable feedstocks, carbohydrates can be separated from the parent biomass either intact (for later hydrolysis to monomeric carbohydrates) or directly as sugars. Plant oils are typically separated to produce triglycerides or free fatty acids. In either case, processes for production of carbohydrate-derived chemical products via catalysis or fermentation deal with a fairly consistent, uniform feedstock, largely independent of the recovery process,” says Bozell. “By comparison, the structural heterogeneity of lignin creates a primary difficulty in using it as a chemical feedstock. We don’t yet have the selective technology needed to convert it into discrete, low-molecular-weight aromatics.”
Nevertheless, Moens of NREL notes that a variety of chemicals have been produced in good yields from lignocellulosic feedstocks under a variety of process conditions. These include levulinic acid, methyltetrahydrofuran (MTHF; an industrial solvent, potential gasoline extender and oxygenate), delta-aminolevulinic acid (DALA; an environmentally benign herbicide), Diphenolic acid (DPA; a building block for plastics and coatings). “These molecules have distinctly different chemical properties, and they underscore the rich chemistry that one can develop from cellulosic and hemicellulosic fractions,” he adds.
All lignocellulosic materials contain lignin (for instance, 25% of wood is lignin, and lignin comprises 15–20% of grasses). The extraction of sucrose from sugar cane generates large volumes of bagasse (which is composed of 30-40% cellulose, 20–30% pentosans (hemicellulose), and about 20% lignin). Long considered a low-value lignocellulosic byproduct, the secondary use of bagasse as a chemical feedstock has long been hampered by the challenges associated with fractionating it into its useful cellulosic, hemicellulosic and lignin components, says Moens of NREL.
This means that two potentially valuable co-products are relegated to very low value applications such as fuel or animal feed, which constitutes a poor overall effectiveness in the use of this potentially valuable biomass resource. And, unlike cellulose that is produced for paper manufacture, cellulose produced for chemical applications (i.e., for the production of cellulose esters) typically has much higher purity requirements (on the order of 97%), says Moens of NREL.
* This article is an online sidebar to Renewable feedstocks: Trading barrels for bushels