ARS | Publication request: Response of thermochemical and biochemical conversion processes to lignin concentration in alfalfa stems

Submitted to: Energy and Fuels
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 2, 2008
Publication Date: June 24, 2008
Citation: Boateng, A.A., Weimer, P.J., Jung, H.G., Lamb, J.F. 2008. Response of thermochemical and biochemical conversion processes to lignin concentration in alfalfa stems. Energy and Fuels. 22:2810-2815.

Interpretive Summary: One variable in the composition of biomass that is thought to affect its conversion to energy is lignin, the complex polymer that provides structure to plant cells. There is evidence that lignin inhibits the process of breaking down biomass to sugars for fermentation, one method of producing fuel alcohols from biomass. However, lignin is thought to enhance thermal conversion, an alternative method to fermentation in biofuel production, although the extent of these effects are not well known. We studied the response of these two processes to lignin concentration in alfalfa stems to determine the relative effects that the biological range in lignin concentration might have on the yields of the product gases. We found that whereas gas produced by fermentation is strongly and negatively impacted by increasing lignin concentration, the effects were minimal for thermal degradation. The results are useful to agronomists, farmers and geneticists who plan to use measures such as genetic modification and harvest practices to produce biomass for use as energy feedstocks.

Technical Abstract: The technologies currently in place to convert lignocellulosic biomass to energy are either biochemical or thermochemical, the efficiencies of which may vary depending on the composition of the feedstock. One variable that conversion technologists have wrestled with, particularly in the simultaneous saccharification and fermentation process, is biomass lignin content. While lignin is considered a recalcitrant to biochemical conversion, it can be a good source of combustion fuel, but the true effect of composition on thermochemical conversion has not been well quantified. In this study we examined the effect of lignin content of alfalfa stems on two biofuel conversion methodologies: (i) biochemical conversion using in-vitro ruminal fermentation as a surrogate for fermentability to ethanol and (ii) thermochemical conversion using pyrolysis. Lignin was found to account for little of the variation in pyrolysis product yield compared to biochemical conversion. Linear regression of lignin concentration on pyrolysis product yields resulted in few significant relationships whereas in-vitro gas production exhibited a strong negative response to lignin content. For alfalfa stems, lignin was much more important to biological conversion potential than to thermochemical conversion potential. The results suggest that genetic modification or agronomic management of lignocellulosic biomass for bioenergy feedstock composition should be based on the intended energy conversion platform.