Outdoor storage performance of chopped switchgrass for bioenergy use

Bulk outdoor storage piles are a potential cost saving alternative to enclosed storage for on-site queuing prior to conversion. Storage performance of switchgrass in this format is unknown. This study baselines storage performance—moisture content and distribution, dry matter loss (DML), and compositional change—using field-scale and laboratory reactors. Three switchgrass piles were made on-site by Genera Energy, Vonore, TN. Conditions included field chopped, tub-ground, and tub-ground covered with a tarp. Piles were monitored for evidence of biological activity throughout six months of storage. Laboratory storage experiments were performed using switchgrass at high and low moisture contents. Results showed that dry matter losses in the reactors remained <5% at 25% moisture content (MC) and <12% at 50% MC and that losses occurred within the first weeks of storage. Piles were sampled for moisture and dry matter loss using in-situ DML samples recovered after storage. Losses from each pile were calculated to be 5.0%, 4.5%, and 3.6% for the chopped, ground, and tarped piles respectively. Relative to uncovered bales, these piles protected feedstock quality as well as the inner most bales, which incurred 10% DML over 18 months of storage. In practice, this storage method is intended for use during harvest season, when field chopped switchgrass is available and for queuing at a conversion facility. This short-term storage option performs sufficiently well to warrant further study, to include cost estimates for material handling and grinding trials to determine the impact of the outer layer of materials, which contain as much as 75% moisture.