Uncovering Challenges for Complete Carbohydrate to Bioethanol Utilization in a Reductive Catalytic Fractionation Biorefinery
Thomas Nicolaï, Wouter Arts, Sergio Calderon‐Ardila, Ruben Smets, Mik Van Der Borght, Johan M. Thevelein, Bert F. Sels
Abstract
The reductive catalytic fractionation (RCF) biorefinery, developed for high-quality lignin oil extraction from lignocellulose biomass, concurrently produces a (hemi)cellulose pulp suitable for bioethanol production. Depending on the RCF severity and delignification, 5 to 25% of the biomass’ carbohydrates are solubilized, ending up in a distinct water fraction. With the premise of full biomass exploitation, while minimizing waste and the processing costs, this work studies the integration of both pulp and water fractions from varied RCF processing parameters into separate hydrolysis and fermentation (SHF) using birch wood and a xylose-utilizing Saccharomyces cerevisiae strain. Besides advancing water fraction characterization revealing certain (reduced) sugars (oligomers), methyl sugars, and shorter polyols, efficient saccharification of the RCF pulps, up to 98% sugar yield, is observed. With high delignification and carbohydrate solubilization in the RCF, a significant 6.2% ethanol titer increase can be achieved by addition of the water fraction to the SHF. We identify both carbohydrate polymers and α/β-methyl carbohydrates in the water fraction as a carbon source for additional bioethanol production. Introducing water fractions from alternative RCF conditions results in similar or lower ethanol yields, influencing both saccharification and yeast’s xylose utilization. From the perspective of the entire, raw biomass, the maximum carbohydrate to bioethanol utilization is achieved with specific RCF parameters that focus on sufficient delignification and high carbohydrate retention, concurrently obtaining water fractions with low carbohydrate content that is better not used for bioethanol production, overall yielding 79% sugar and 59% bioethanol, of their theoretical maximum, comparable to the reported yields for established second-generation carbohydrate-centered biorefineries using hardwood.