Can xylose be fermented to biofuel butanol in continuous long-term reactors: If not, what options are there?

Authors: Qureshi, Nasib; LIN, XIAOQING - Guangdong University; TAO, SHUNHUI - Guangdong University; Liu, Siqing; HUANG, HAIBO - Virginia Polytechnic Institution & State University; Nichols, Nancy

Submitted to: Energies
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/21/2023
Publication Date: 6/26/2023
Citation: Qureshi, N., Lin, X., Tao, S., Liu, S., Huang, H., Nichols, N.N. 2023. Can xylose be fermented to biofuel butanol in continuous long-term reactors: If not, what options are there? Energies. 16(13). Article 4945. https://doi.org/10.3390/en16134945.
DOI: https://doi.org/10.3390/en16134945

Interpretive Summary: There is avid interest in developing biological source of fuel. Butanol produced by fermentation is one possible source. Butanol already commands a 350 million gallons per year market and the potential market for commercial biofuel is huge at 143 billion gallons per year. Fermentation of cellulosic feedstocks are necessary to produce large volumes of advanced biofuels. Traditionally, butanol has been produced using grain and sugar crops by fermenting the sugars like glucose, fructose, and sucrose. However, cellulosic feedstocks contain large amounts of a different sugar, xylose. It was found that the industrial bacterium used here for the fermentation produced as much product from xylose as from glucose. However, when the xylose concentration was increased to a level relevant for commercial fermentations, much of the xylose went unused. It was hypothesized that the fermentation stopped once the butanol concentration reached a toxic level for the bacterium. We tested this by adding a system to allow for removing and collecting butanol (and the other fermentation products) while the fermentation was ongoing. Using this system and with a few other minor modifications, up to a 15% xylose (150 gL-1) solution could be completely fermented to butanol. This research directly relates to the growing field of biofuel and by agricultural processors and farmers looking to expand their markets.

Technical Abstract: This study applied concentrated xylose (60–250 g/L) medium to produce butanol (acetone butanol ethanol, or ABE). A control batch fermentation of 61 g/L initial glucose using Clostridium beijerinckii P260 resulted in a productivity and yield of 0.33 g/L·h and 0.43 g/g, respectively. Use of 60 g/L xylose in a batch system resulted in productivity and yield of 0.26 g/L·h, and 0.40 g/g, respectively. In these two experiments, the culture fermented 89.3% glucose and 83.6% of xylose, respectively. When ABE recovery was coupled with fermentation for continuous solvent removal, the culture fermented all the added xylose (60 g/L). This system resulted in a productivity and yield of 0.66 g/L·h and 0.44 g/g, respectively. When the sugar concentration was further increased above 100 g/L, only a small fraction of the sugar was fermented in batch cultures without product removal. However, with simultaneous product removal, all the xylose (150 g/L) was fermented provided the culture was fed with nutrients intermittently. In this system, 66.32 g/L ABE was produced from 150 g/L xylose with a productivity of 0.44 g/L·h and yield of 0.44 g/g. Using the integrated culture system allowed sugar consumption to be increased by 300% (150 g/L). The continuous system using xylose as a feed did not sustain and after 36 days (864 h) of fermentation, it produced only 2–3 g/L ABE. Rather, the culture became acidogenic and produced 4–5 g/L acids (acetic and butyric). This study suggested that xylose be fermented in batch reactors coupled with simultaneous product recovery rather than in continuous reactors.