Agricultural residues are only available for a limited time frame each year, but processing facilities must be run around the clock for virtually the entire year to have any hope of real­izing reasonable returns on the huge investments required. This requirement is particularly critical for production of commodity products as margins are thin. Thus, a vital consideration is how to ensure a year-round supply, and only two options are apparent: (1) store enough material during the harvest season to last the entire year or (2) have access to a number of feedstocks that can be harvested year round. The latter may be a viable strategy but is very regional in nature. Thus, our focus will be to discuss considerations in storing agricultural residues.

Given the low margins and high capital costs, it is highly desirable to store agricultural residues in the simplest and least costly manner possible, and simply piling up biomass is about as simple a way as there is. For example, sugarcane bagasse has been stored in piles that are properly maintained to prevent moisture accumulation. Tractor trailer rigs drove on to large piles of bagasse at a furfural plant in Belle Glade, Florida over a period of about 30 years to dump their loads, and bulldozers then spread the material around so the surface slopped downward. Driving large equipment on the piles compacted the bagasse, thereby making it nearly impenetrable to water and also difficult for the pile to catch fire. Slopping the piles not only allowed trucks and bulldozers to travel on the pile but ensured that water would run off, thereby keeping the moisture levels reasonably constant at about 50%. The result was little degradation of the hemicellulose or cellulose in bagasse.

In a systematic study, samples taken from various depths in a pile of sugarcane bagasse over a period of 3-26 weeks were analyzed by analytical pyrolysis (Agblevor et al. 1994). It was found that samples taken from the center of the piles after 3.25, 6.5, 13, and 26 weeks of storage had very small changes in the distribution of hemicellulose, cellulose, and lignin, while hemicellulose and cellulose contents dropped while lignin increased for samples taken from the regions near the outer surface layer. They also found that pentosans were degraded more rapidly than hexosans in the outer regions in the pile where microbial activity was high, consistent with experience that hemicellulose is more readily decomposed than cellulose which is in turn more susceptible to attack than lignin. Thus, these results demonstrate that deterioration only occurred in a very shallow layer near the surface of the piles but not in the bulk of the material, consistent with industrial experience with large piles. If such an approach would work as well for corn stover and other agricultural residues, it would greatly simplify storage.

It has been shown that wrapped bales of corn stover can be stored for extended times with little degradation, consistent with experience with sugarcane bagasse. All bales were stored in the open, with high moisture bales and uncovered square bales processed first while the normal moisture round bales, protected from the weather by their plastic wrap, could be processed later. Open-field storage of wrapped round bales is common practice for straw, hay, and similar materials, with open fields of such bales a common sight in Vermont and New Hampshire, as well as Nebraska, Iowa, and other parts of the Midwest. Corncobs have been compacted for long-term storage for furfural production over more than a year, similar to the approach taken for bagasse described above. However, it is unclear if the lower bulk packing density of corn stover and other agricultural residues will allow this approach.

Another consideration is whether to store biomass near the source or near the conversion facility. The former simplifies gathering operations and avoids moving material that could degrade during storage. However, it is simpler to manage a few piles of biomass than to try to ensure numerous piles are properly cared for to minimize deterioration.