8-01: Trend of reducing acid use in dilute acid pretreatment for biomass to ethanol

Dilute acid pretreatment is a leading pretreatment technology for biomass to ethanol conversion due to the comparatively low chemical cost, high hemicellulose solubilization efficiency, and short processing time. However, it also places significant constraints on the materials of construction of the pretreatment reactor and other accessory equipment. In addition, the process requires large quantities of alkali for pH adjustment and generates significant amount of sulfate salts as byproducts. Reducing acid use to the lowest level possible would be of great interest to the emerging industry.

Our recent work on dilute acid pretreatment optimization indicates that at least a 50% reduction in acid use can be achieved without compromising pretreatment efficiency when the pretreatment time was extended from 1-5 min to 8-15 min. Further acid reduction is possible if the pretreatment time is extended beyond 15 min. To avoid undesired sugar degradation and inhibitor generation, temperatures should be controlled below 170^oC. Another strategy we examined to reduce acid use was to increase the solid content in the pretreatment reactor. This strategy was based on the assumption that the solution strength of the acid, rather than the acid loading on biomass solids, is critical for pretreatment. However, the experimental results showed the opposite. Therefore, it is unlikely that acid use can be lowered simply by increasing the solid content in pretreatment reactors.

Moving to lower acid concentrations and lower temperatures also changed the requirement of enzymes for both cellulose and xylan conversions. These new findings will be discussed in detail.