Process considerations and economic evaluation of two-step steam pretreatment for production of fuel ethanol from softwood

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 cent/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 cent/L). This shows the potential for the two-step pretreatment process, which, however, remains to be verified in pilot trials.