Evaluation of sodium sulfite detoxification of pretreated softwood slurries for ethanol production

During acid pretreatment of lignocellulosic feedstocks partial degradation of hemicelluloses and lignin leads to formation of inhibitory compounds that constrain enzymatic hydrolysis and ethanolic fermentation. Detoxification by different chemical, biological and physical means is one of the strategies for minimising the inhibition problems. In this work, the effect of in situ detoxification with sodium sulfite was applied to steam-pretreated spruce slurries, and its effect on simultaneous saccharification and fermentation (SSF) for ethanol production was evaluated. The detoxification significantly enhanced the fermentability of the slurries, and as a result the enzyme and yeast requirements were considerably reduced, and the SSF time was shortened. The detoxification method was first evaluated at lab scale, and it was further validated at demonstration scale. A techno-economic evaluation based on the experimental results was performed. The evaluation revealed the required reductions of enzyme and yeast loads in order to compensate the cost of the sodium sulfite addition, and it showed that the achieved shortening of SSF time allows a decrease of the bioreactor volume that results in investment savings, which are considerably higher than the detoxification expenditures. It was thus shown that the cost of sodium sulfite can be offset by the benefits resulting from its addition. These results demonstrate the high potential of sodium sulfite detoxification as a part of a softwood-to-ethanol technology.