Improved one-step steam pretreatment of SO2-impregnated softwood with time-dependent temperature profile for ethanol production.

In the production of ethanol from lignocellulosic material, pretreatment of the raw material before enzymatic hydrolysis and fermentation is essential to obtain high overall yields of sugar and ethanol. Two-step steam pretreatment results in higher ethanol yields from softwood than the standard one-step pretreatment process. However, the difficulty with separation and washing of the material at high pressure between the two pretreatment steps is a major drawback. In this study, a new one-step pretreatment procedure was investigated, in which the time-temperature profile was varied during pretreatment. The efficiency of pretreatment was assessed by performing simultaneous saccharification and fermentation on the pretreated slurries. Pretreatment of SO(2)-impregnated softwood performed by varying the temperature (190-226 degrees C), the residence time (5-10 min), and the mode of temperature increase (linear or stepwise), resulted in recovery of about 90% of the mannose and glucose present in the raw material. The highest ethanol yield, 75% of theoretical based on the glucan and mannan content of the raw material, was obtained at pretreatment conditions of 190 degrees C for 12 min. Similar ethanol yields were achieved when running the pretreatment as one-step (190-200 degrees C), two levels of temperature, at shorter residence time (7 min), which results in lower capital costs for the process.