Cellulosic Ethanol: What to Do with the Lignin

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Lignocellulosic ethanol production has been a goal of the U.S. DOE for some time, and yet it remains at least a few years off as a demonstrated viable technology.

Lignocellulosic biomass can be converted to ethanol through either a biochemical or thermocatalytic process. The biochemical process utilizes several pretreatment and hydrolysis steps to rupture the lignin walls surrounding the cellulose and hemicellulose fibers. This makes these fibers available for fermentation to ethanol. This process always leaves about 15 percent to 30 percent of the input biomass mass as unconverted lignin, a complex polymer found in most plant material. Lignin poses either a potential disposal liability or a byproduct opportunity.

The University of North Dakota Energy & Environmental Research Center sees the lignin as a byproduct opportunity and is pursuing thermocatalytic conversion of the lignin residue to synthetic fuels using gasification and catalysis. The EERC believes this is a great approach as opposed to some of the more popular alternatives to utilizing the lignin, such as simply burning it for energy. Gasifying the lignin and recombining the gas constituents into alcohol result in a higher overall conversion efficiency of lignocellulosic biomass to ethanol.

One challenge to the lignin gasification approach is that biomass gasifiers are still in the developmental stage when it comes to producing a very clean synthetic gas from lignin.

Coal gasification experience is helping to meet the challenge, since lignin should perform similar to a low-rank, lignite coal in a gasification environment. When analyzed for their components, lignin and lignite show a similar composition.

To best convert this byproduct lignin to synthetic fuels and chemicals bench- and/or pilot-scale demonstrations are needed to adequately assess the physical characteristics, feeding properties and gasification performance for specific lignin types in given gasification systems. Each ethanol process will produce a specific lignin material with its own unique characteristics. Selection of gasifier type should be coordinated with the type of gas-to-liquids catalyst used and the lignin gasification characteristics.

The proper choice of gasifier type is critical for efficient conversion of the lignin to synthetic gases (syngas) that can be recombined into ethanol. Gasifiers can be pressurized or remain at atmospheric pressure. The type of gasifier to employ requires an economic analysis of the trade-offs associated with each type of biorefinery plant. High-pressure gasification minimizes the need for expensive postgasification compression and is better overall for gas cleaning. Atmospheric gasifiers may require several staged compression steps to bring the syngas to the pressure needed for the gas-to-liquid plant. Syngas scrubbing and conditioning systems are also more complex at higher pressures.

While lignin gasification to ethanol holds great promise, researchers still need to work with biorefineries to prove the technology. Several U.S. federally funded cellulosic ethanol projects are using biochemical processes, which might pave the way for further research and development of thermochemical approaches to utilizing the lignin byproduct.

Bruce Folkedahl is a senior research manager at the EERC. Reach him at [email protected] or (701) 777-5243.