“Lignin” May Be the Magic Word
Cellulosic material from non-food crops—substances like grass, leaves and wood—is an attractive source for environmentally friendly, domestically produced biofuels. If the biorefineries that produce those fuels could also get a second high-value product from the same feedstock, the economics of such plants would be even more attractive. A researcher at the Savannah River National Laboratory has developed a process that may make that kind of two-for-one production both technologically and economically feasible. Steven Sherman’s work has resulted in a process that separates and purifies lignin, a polymer material with a large number of commercial uses, from biofuels production.
Cellulosic ethanol can be made from a wide variety of plants, including switchgrass, sorghum, pine and many others, some of which can be grown on land that would be considered too poor for other profitable crops. The cell walls of these and almost all other land-based plants contain large quantities of lignin, which, conducts water through the plant stem and adds stiffness or hardness to the plant’s structure.
A purified form of lignin extracted from these plants would have excellent potential for high-value commercial uses. It could provide a source of aromatics for liquid fuel additives and industrial bulk chemicals, or a drop-in replacement for phenol in phenolic resins. It has also been studied for use in coatings and as a source material for carbon fiber manufacturing.
Previous methods for isolating lignin from biofuels production have not resulted in a product that would be suitable for these high-value uses. Most result in a product with a lot of impurities that is only suitable for burning to generate steam for heat. For that basic use, lignin is not competitive with lower-cost natural gas.
“Our process results in a much purer, solid lignin product that could have a lot of commercial uses,” says Sherman, who invented the process. “It’s a valuable material, but with a lot of other processes, you wind up throwing it away. We’re trying to generate an additional revenue stream for a biorefinery, using something that otherwise would be a waste stream.”
Because the process is so efficient at removing lignin, it greatly reduces the amount of biological material being disposed in the resulting waste stream. Other processes for isolating a pure form of lignin do exist. The wood pulping industry, for example, uses a pretreatment method that produces pure lignin. Their processes, however, use harsh chemicals and require expensive capital equipment, so they have not been seriously considered for the biofuels industry.
The biorefineries that make ethanol and other fuels from plants typically use an ammonia-based pretreatment process to separate the lignin from the cellulosic material that is converted to fuel. The vegetation is ground up, then mixed with a solution of ammonium hydroxide and heated to release the lignin and prepare the remaining cellulose for conversion to sugars.
Savannah River’s process treats the remaining pretreatment solution with a series of steps that use no strong chemicals, no high pressure and only moderately high heat. First, the solution is heated to evaporate most, but not all of the ammonia. A small amount is retained to keep an alkaline pH, which prevents premature condensation of the lignin. Next, acid is added to the hot solution, which causes the lignin to begin to coalesce. The solution is boiled until the volume reduces by about half, and the lignin begins to coagulate into large particles. Once the solution is cooled, the lignin particles are either allowed to settle out of the solution, or they are removed by filtration. If desired, the lignin is rinsed with a dilute acid solution to help remove remaining salts and ash.
The laboratory has conducted successful bench-scale demonstrations of this process using several of the plants that have good potential as sources of cellulosic ethanol—switchgrass, sorghum, sweetgum and loblolly pine. Lignin was successfully recovered from all of them, recovering 66 percent to 100 percent of the lignin in the pretreatment solution. The lab also conducted studies to evaluate the cost-effectiveness of this approach.
Researchers looked at the cost to add the lignin purification process to an existing biorefinery operation, including chemical, equipment and utility costs. Then they compared this cost to the anticipated revenue that could be expected from the purified lignin. This analysis showed that an industrial-scale facility processing 350,000 metric tons of switchgrass per year could use this process to produce lignin at a cost that could make it feasible to sell it as a feedstock for commodity chemicals.
The analysis also showed that some of the components of the resulting waste stream, such as ammonium sulfate and protein, might also have potential to be exploited for additional revenue.
Sherman, a chemical engineer, is working on several different projects related to different forms of energy, including improvements to fossil energy and new approaches to nuclear energy, but it is bioenergy that gets him really excited. “These processes are not being developed just because they are interesting, but because the United States needs to diversify its energy supplies. Bioenergy is one way of doing that,” he says. “The United States is rich in land and agricultural resources, and we have the know-how to meet a large percentage of our fuel needs domestically using materials grown locally. We are not there yet, and costs are still too high. All of our work is directed towards lowering processing costs through gains in energy efficiency, better chemistry, process intensification and any other method that may lower costs.”
Angeline French is a writer at Oak Ridge National Laboratory.