Process Considerations and Economic Evaluation of Two-Step Steam Pretreatment for Production of Fuel Ethanol from Softwood

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Abstract
To increase the overall ethanol yield from softwood, the steam pretreatment stage can be carried out in two steps. The two‐step pretreatment process was evaluated from a techno‐economic standpoint and compared with the one‐step pretreatment process. The production plants considered were designed to utilize spruce as raw material and have a capacity of 200 000 tons/year. The two‐step process resulted in a higher ethanol yield and a lower requirement for enzymes. However, the two‐step process is more capital‐intensive and has a higher energy requirement. The estimated ethanol production cost was the same, 4.13 SEK/L (55.1 ¢/L) for both alternatives. For the two‐step process different energy‐saving options were considered, such as a higher concentration of water‐insoluble solids in the filter cake before the second step, and the possibility of excluding the pressure reduction between the steps. The most optimistic configuration, with 50% water‐insoluble solids in the filter cake in the feed to the second pretreatment step, no pressure reduction between the pretreatment steps, and 77% overall ethanol yield (0.25 kg EtOH/kg dry wood), resulted in a production cost of 3.90 SEK/L (52.0 ¢/L). This shows the potential for the two‐step pretreatment process, which, however, remains to be verified in pilot trials.

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