With global agricultural residue production estimated at 2.8 to 3.8 billion tons annually, there is an abundance of agricultural (ag) residue that can be leveraged. For context, corn stover, the most abundant residue, could in principle yield up to 200 million tons of lignin. However, today there is limited commercial accessibility to ag lignin, as the existing methods for lignin recovery (e.g., filtration and centrifugation) were essentially developed for laboratory application and so are relatively expensive and do not readily lend themselves to commercial scale-up. For example, filtration, the most widely used method, is limited by the fine-powdered nature of the extracted lignin, which tends to clog filters.
The recently developed RUSH (Recovery of Lignin using Separation by Heat) process is uniquely suited for isolating lignins from dilute (i.e. unevaporated) black liquors derived from agricultural residues. By using a combination of carefully calibrated temperature increase and mixing during acidification of the black liquor, we have created a surprisingly simple process that can be operated batchwise or continuously. It is important to note that RUSH eliminates both the filtration and/or evaporation steps typically used in lignin recovery, which can account for 50-70% of the total production cost of the lignin.
Using a continuous version of RUSH, we have achieved >80% yield of the lignin at a purity of >85%. Although the continuous setup is currently lab-scale, the method is readily scalable.