(329h) A New Process for Recovering High Quality Lignins from Agricultural Black Liquors

Utilization of all the components of lignocellulosic biomass is essential for biorefineries to compete economically with their petroleum counterparts and establish themselves as a cornerstone of the green economy. Agricultural residues left over from annually harvested food crops, along with fast-growing energy crops, represent a vast resource that can be tapped for cellulosic bioethanol or bioproducts, but progress has been limited by the profitability of these biorefineries. The lignin byproduct from these biorefineries is key to resolving this issue, as it is the second most abundant component in the biomass and offers aromatic character that the carbohydrates lack.

The separation and subsequent extraction of the carbohydrates and lignin in the biomass is usually achieved through either dilute acid pretreatment or alkaline pretreatment, with the latter being favored here for its tendency to leave the cellulose and the lignin less degraded. However, little work has been done to develop economically and environmentally friendly techniques to recover lignin from the lignin-containing byproduct stream (known as black liquor). Instead, most techniques today for recovering lignin from agricultural black liquors involve energy-intensive centrifugation steps and/or extensive water washing. Considering lignin is intended to be part of the solution to the sustainability problem, using expensive or polluting techniques on what is supposed to be a low-value green material defeats the purpose lignin was originally intended to serve.

In this work, we have developed a process to recover lignins directly from the alkaline black liquors of multiple agricultural feedstocks (e.g., corn stover and wheat straw). The process requires no washing or centrifugation and only one filtration step. This new recovery process, when used in conjunction with the Aqueous Lignin Purification using Hot Agents (or ALPHA) process, provides the missing link to directly convert agricultural black liquors into high quality lignins of <0.25% sugar and <0.5% ash with molecular weights differing by an order of magnitude. The ALPHA process uses only simple green solvent systems like ethanol/water and acetone/water at low solvent-to-feed ratios (<6:1 S:F) to accomplish simultaneous fractionation and purification. In summary, this new recovery technique offers an economical and sustainable alternative to traditional recovery and purification/fractionation techniques currently being applied to the alkaline liquors of agricultural residue streams.