2-10: Two-stage pretreatment to enable a cleaner process – insights from lab scale testing and experiences in pilot scale-up

As a leading pretreatment method, dilute acid steam explosion relies on sufficient hemicellulose removal and lignin modifications to attain high cellulose digestibility. To make the process cleaner, it is desirable to use lower or even no acid in the pretreatment. However, as the acid use drops below a certain level, one has to compromise between substrate digestibility and sugar recovery. In order to enable a cleaner low-acid or no-acid process with high sugar yields, we explored the technical feasibility of two-stage pretreatment.

Two-stage pretreatment of corn stover using different combinations of acid and hot water was studied in the lab. The first stage (e.g. 170^oC, 40~50 min) was optimized to achieve moderate xylan removal (~50%) and minimal xylose decomposition (<5%). After solid-liquid separation, the second stage was controlled to remove the more recalcitrant xylan and achieve high digestibility of cellulose. Some of the best conditions identified were further scaled up in an advanced 2-stage pilot plant which had a build-in solid-liquid separation apparatus operated above the glass transition temperature of lignin. Our laboratory insights and pilot scale up experience will be shared in this talk.

As the pretreatment liquors contain substantial sugar oligomers (50~85%), hemicellulases are needed to break down the oligomers to fermentable sugar monomers. Also, the inhibitors in the liquor streams must be removed to improve the downstream fermentation. Several different process configurations were examined to handle those liquid streams besides the pretreated solids. The enzymes of appropriate compositions to hydrolyze those substrates will be discussed.