10-15: Maximizing potential ethanol yield by increasing biomass moisture content prior to pretreatment

In order to meet current and future demands for cellulosic bioethanol, biorefineries utilizing lignocellulosic biomass to produce ethanol must be run as efficiently as possible. These future biorefineries will likely utilize biomass from multiple sources. While characteristics like particle size can be specified and controlled, storage and shipping may cause non-uniformity in the chip moisture content. Maintaining uniform biomass moisture content is essential for consistency and to maximize the efficiency of downstream processes.

The objective of this research was (1) to determine what effect the biomass moisture content (MC) has on the overall ethanol yield after bioconversion and (2) to ascertain whether the MC can be artificially increased in dried chips to achieve the same yields as never dried chips. It has previously been shown that the MC of herbaceous biomass prior to pretreatment is a determinant of the efficacy of added SO₂. In this study, never-dried hybrid poplar chips were air dried to three different moisture contents, then re-wetted with water and steam. Chips were then impregnated with 3% SO₂, steam pretreated at 195 °C for 5 minutes, and the solid fraction enzymatically hydrolyzed. Chips with less than 25% moisture at the time of pretreatment were pretreated unevenly and exhibited significant amounts of poorly cooked rejects. Hydrolysis of rejects showed only 57% conversion compared to 80% conversion of the accept fraction. Hydrolysis of all solids from chips greater than 25% moisture showed 75-80% conversion, indicating that moderate air-drying of chips does not have a significant effect on hydrolysis yields.