High yield co-production of isobutanol and ethanol from switchgrass: experiments, and process synthesis and analysis

Biofuels from sustainable feedstocks are a promising option for carbon-neutral bioenergy, where isobutanol has been receiving attention due to its advantageous physical and chemical properties. In this work, the production of isobutanol from carbohydrates in ammonia fiber expansion-pretreated switchgrass hydrolysate is investigated. We engineer a yeast strain by hybridizing an industrial starch isobutanologen with a strain that can tolerate the stresses of lignocellulosic hydrolysates. This strategy increases isobutanol production through ethanol co-production, which enables improved yeast growth and higher metabolic flux under these stressful conditions, likely due to the presence of at least some pyruvate decarboxylase. Furthermore, we develop a process for the recovery of isobutanol and ethanol from the broth and perform technoeconomic analysis of the switchgrass-to-alcohol biorefinery based on experiments. The yeast consumes all available glucose, but no xylose, available in the hydrolysate and co-produces isobutanol and ethanol at 23.7% and 61.8% theoretical yields, respectively. An estimated baseline minimum selling price of $11.41 per GGE for isobutanol and ethanol is determined and sensitivity analysis identified the key parameters affecting the economic feasibility of the process. Specifically, hydrolysis enzyme loading, the sugar concentration in hydrolysate, and potential fermentation technological advances, such as xylose conversion to alcohols, were shown to have the greatest economic impact.