By Yebo Li, an assistant professor in the Ohio State University Department of Food, Agricultural and Biological Engineering
Silos for storing lignocellulosic biomass
Farmers are familiar with storing high moisture forage crops as silage. Tall silos, horizontal or bunker silos, and more recently “shrink-wrapped” round bales are common examples of storing crops “wet” instead of “dry.”
Today, scientists are looking at “silage” techniques as a way of preserving lignocellulosic biomass. The most common “lignocellulosic biomass” on Ohio farms is corn stover. Storing the crop for use year ‘round is essential to the future success of a bio-refinery. Wet storage has been used since the 1800s for preserving green crops for livestock. Now it is being considered as a storage method for a new industry: bio-refining.
Dry vs. wet storage
For dry storage, lignocellulosic biomass is typically harvested dry at 20% to 25% moisture. For instance, after corn harvest, stover is chopped with a flail shredder, field dried, raked and finally baled with a large round baler. The stover can be windrowed as it is shredded, but this can slow down the drying process. The common practice is to shred and lay the stover as wide as possible to facilitate drying. The time required to reach baling moisture is several days to a few weeks because of the low ambient temperatures and frequency of rain during the fall. Besides the difficulties in drying, the dry matter losses for round bales stored outdoors can range from 10% to 25%.
For wet storage, the biomass is harvested wet (greater than 45% moisture) and directly preserved by ensiling upon harvest. For example, when corn stover is harvested as a wet product, a shredder typically follows right behind the combine. Corn stover is shredded and windrowed in a single pass. A forage harvester gathers and grinds the stover prior to ensiling. Wet harvest and storage avoids field drying of lignocellulosic biomass thus greatly improving harvest efficiency and timeliness. Furthermore, the dry matter losses during wet storage can be reduced to less than 5%, and the product is more digestible than dry-stored lignocellulosic biomass.
Wet storage (as silage) is a process of preserving green feed under wet and airtight conditions. During wet storage, fermentative microorganisms convert sugars to acids. The acidic and anaerobic environment prevents further microbial growth and facilitates long-term storage. Wet storage can be in a horizontal silo (an airtight pit), or in plastic wrapping.
Before starting the anaerobic stage there is an aerobic phase in which the trapped oxygen is used. The remaining oxygen after filling a silo enables some respiration and some dry matter loss, but this stage ends quickly. After oxygen depletion, the anaerobic phase starts. Silage undergoes anaerobic fermentation, which starts about 48 hours after the silo is filled and lasts a few weeks. Finally, pH drops to about 3 or 4, and the microbial activity is inhibited.
Ensilage fermentation has generally been conducted by natural occurring microorganisms, but inoculation with specific microorganisms also has been adopted to speed up the fermentation process and/or improve the resulting silage quality. With inoculation, the fermentation process is essentially complete in about two weeks. Silage inoculants contain one or more strains of lactic acid bacteria, and the most common is Lactobacillus plantarum. Other bacteria used in inoculants include Lactobacillus buchneri, Enterococcus faecium and Pediococcus species. During wet storage, fermentative bacteria produce volatile fatty acids, such as acetate, propionate, lactate and butyrate, which preserve the forage.
The advantages of wet storage compared to dry storage include: 1) lower harvesting costs, 2) lower dry matter losses during storage, 3) increased product uniformity, 4) improved feedstock susceptibility to enzymatic hydrolysis, 5) reduced risk of fire, and 6) value added to the feedstock by integrating a chemical or biological pretreatment. However, there are potential drawbacks that need further investigation. First, wet storage sometimes releases liquid (leachate) containing nitric acid (HNO3), which is corrosive. And plastic sheeting used for sealing a pit or wrapping bales needs proper disposal or recycling.
Wet storage is a promising technique for preserving the carbohydrates in lignocellulosic biomass. However, further development of wet storage and the integration of this technology with lignocellulosic biomass harvesting, transportation, storage and preprocessing are needed to match the requirements for a year-round secure supply for a biorefinery.