Separate versus Simultaneous Saccharification and Fermentation of Two‐Step Steam Pretreated Softwood for Ethanol Production

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Abstract
In two previous studies, optimal conditions were identified for two‐step steam pretreatment of SO2‐ and H2SO4‐impregnated softwood. In the present study the yield of sugar and ethanol was determined in a process development unit where pretreatment was performed in a 10‐L reactor and simultaneous saccharification and fermentation (SSF) or enzymatic hydrolysis (EH) were performed in 30‐L reactors. The study showed that a steam pretreatment reactor should be larger than 2 L to yield acceptable results. Two pretreatment combinations were studied. In the H2SO4 case, the first pretreatment step was at 180°C for 10 min with 0.5% H2SO4 and the second step at 210°C for 2 min with 1% H2SO4. In the SO2 case, first step was at 190°C for 2 min followed by a second step at 210°C for 5 min. The concentration of SO2 was 3% in both steps. EH and SSF were performed on the whole slurry after the second pretreatment step to determine the yield of sugars and ethanol. The liquid after the first pretreatment step was also analyzed and fermented. When SSF and EH were performed at the same dry matter content and enzymatic activity, the ethanol yield in SSF exceeded the yield obtained with EH in both pretreatment cases, even when 100% yield in the fermentation step was assumed. Thus SSF is a better process if yield is the main priority. Comparison of the yields with the two acid catalysts showed higher yields with SO2 in both SSF and EH. The overall ethanol yield following SSF of SO2‐impregnated and pretreated wood reached 81% of the theoretical, that is, 357 liters per metric ton of dry raw material.