Fermentation Cost Improvement Through Propagation Advances for a Commercially Relevant Biomass-to-Ethanol Process

Compared to their starch-to-ethanol counterparts, pentose-utilizing organisms for biomass-to-ethanol processes are less robust while having to ferment mixed sugars in a significantly more inhibitory substrate. This inhibition can generally be overcome with a high yeast pitch to maximize xylose uptake rates in fermentation. The cost driven trade-off of a high yeast pitch during fermentation is the necessity of dedicated onsite propagation, as opposed to purchase of all yeast.  Novozymes’ Advance Biofuel cost model suggest the majority of the fermentation cost when using high yeast pitches can be attributed to the propagation variable costs, most significantly from the carbohydrate source. Therefore, maximizing the cost performance of C5 yeast propagation is important to ensure an economically feasible biomass-to-ethanol fermentation step. 

Novozymes has identified two approaches to reduce the fermentation cost: first, increase propagation cell yields from sugars to reduce the realized organism cost, preferably on cheap carbohydrate sources, or second, produce a more robust organism from propagation with a higher realized xylose uptake rate in fermentation which would enable fermentation at a lower yeast pitch in a reasonable processing time. This presentation will discuss cost model results in conjunction with reactor data to pinpoint potential savings and opportunities to lower overall fermentation cost for second generation ethanol. Sensitivities on cost from propagation and fermentation process variables targets will be presented. Propagation optimization with respect to higher realized xylose uptake rate in fermentation will be addressed as well. Lastly, the holistic minimum ethanol selling price implications for process optimizations will be given.