3-10: Does steam pretreatment of softwoods produce more durable pellets in which the carbohydrates can still be readily utilized as a feedstock for a biomass-to-ethanol process?

One of the challenges with any future biomass-to-fuels/chemicals process will be the ability to access large amounts of biomass. These large volumes of biomass (greater than two-to-five thousand tons dry wt./day) required if the benefits from economies of scale are to be achieved. One way to improve the economics of transporting over distances is to densify the material, as demonstrated by the ever increasing global trade in wood pellets. The durability and bulk density of pellets are important characteristics for their handling and storage. Improved bulk density reduces transportation costs while improved durability reduces dust production that is often associated with health problems and dust explosion. In this study, Douglas-Fir woodchips were treated with high pressure steam at 200 °C and 4% SO₂ for 5 min. Steam pretreatment resulted in a material which could be directly pelletized, without any further size reduction. Pellets were made using a force load of 4000 N and a typical piston-die setup at 70 °C. Pellets made from Steam pretreated softwoods were found to have a higher density and almost two times higher mechanical strength compared to non-pretreated pellets. Although the dried, denser pellets were expected to enhance thermochemical conversion of the biomass we were apprehensive that these characteristics would be problematic for a bioconversion approach.

We will show that steam pretreatment and pelletization slightly reduced the overall carbohydrate content of the biomass. However, most of this loss could be attributed to partial hemicellulose degradation while the cellulosic component still could be readily enzymatically hydrolyzed.