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 and engineers are looking at "silage" techniques as a way of preserving lignocellulosic biomass for use as a feedstock for biobased energy and products. Lignocellulosic biomass is an abundant, widely available resource and includes agricultural crop residues, such as corn stover and wheat straw; energy crops, such as switchgrass; and municipal waste. The most common lignocellulosic biomass on Ohio farms is corn stover. The ability to store biomass feedstocks 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 feed; now it is being considered as an alternative storage method for a new industry: biorefining.
Dry vs. Wet Storage
Typically, farmers use either dry or wet storage methods to preserve lignocellulosic biomass, such as corn stover. These methods differ in the way the biomass is harvested, processed, and stored, and each method has both advantages and disadvantages. These differences are discussed below based on corn stover, which is collected after the corn is harvested, as an example.
For dry storage, the stover is typically harvested and baled at 20% to 25% moisture. The most common practice is to shred and lay the stover as wide as possible to facilitate drying after which it is raked and baled. The time required to reach baling moisture is several days to a few weeks depending on ambient temperature and rainfall. In addition to difficulties in drying the stover, dry matter loss for round bales stored outdoors can range from 10% to 25% (Shinners et al., 2007).
For wet storage, the corn stover is harvested at moisture greater than 45% and preserved by ensiling it immediately after harvest. Typically a shredder follows directly behind the combine and the 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 loss during wet storage can be reduced to less than 5%, and the product is more digestible than dry-stored lignocellulosic biomass (Shinners et al., 2007).
The advantages of wet storage compared to dry storage include: (1) lower harvesting cost, (2) lower dry matter loss 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 (Ren et al., 2006).
|Figure 1. Wet (top) and dry (bottom) biomass storage configurations for lignocellulosic grasses.|
However, there are potential drawbacks of wet storage that need to be considered. First, wet storage sometimes releases liquid (leachate) containing nitric acid (HNO₃), which is corrosive. Also, the plastic sheeting used for sealing a pit or wrapping bales needs proper disposal or recycling. Because dry storage relies on low water activity to prevent microbial growth, it is most effective in arid regions. Wet storage, or ensilage, under anaerobic conditions encourages a natural acid fermentation that lowers pH and reduces microbial activity. Wet storage can keep dry matter loss below 5% for a full year, even in humid regions (Richard, 2010).
Customizing the Wet Storage Process
The advantages of generating a uniform product that is more susceptible to enzymatic hydrolysis makes wet storage the better alternative for storing biomass for biorefining. Currently, researchers are evaluating the wet storage process and methods for enhancing its use for storing a variety of lignocellulosic biomass feedstocks. Wet storage (as silage) takes place under airtight conditions, during which 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, enabling some respiration and dry matter loss. After about 48 hours, the oxygen is depleted and anaerobic fermentation begins and lasts a few weeks. During fermentation, the pH drops and, when it reaches about 3 or 4, the microbial activity is inhibited.
Ensilage fermentation has generally relied on naturally 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.
Organic acids generated during wet storage can adversely affect fermentative microbes used to produce biofuels. Therefore, chemical stabilizers have been introduced to retard ensiling and thus reduce the production of organic acids during wet storage. As a result, the enzymatic conversion of plant carbohydrates into fermentable sugars at the biorefinery is enhanced. Microbial growth will decrease at both low and high pH values. Adding 1–5% sulfuric acid (H₂SO₄) to lower the pH to less than 3 can block bacterial growth. Likewise, adding an alkali to raise the pH to at least 10 will also stop microbial activity. Typical alkaline agents include sodium hydroxide (NaOH), ammonia (NH₃) and calcium hydroxide (Ca(OH)₂). NaOH is a popular choice because it reacts quickly (in a few hours) with lignocellulosic biomass. However, although Ca(OH)₂ may take weeks to react, it has advantages compared to NaOH, including lower cost, easy handling, and minimal residual salts (Digman, et al., 2010).
Wet storage is a promising technique for preserving the carbohydrates in lignocellulosic biomass for up to a year. Further development of wet storage methods for lignocellulosic biomass and the integration of this technology with harvesting, transportation, storage, and preprocessing are needed to provide a year-round supply of biomass as required for a biorefinery.
Reviewed by Mary Wicks, Dr. Harold Keener, and Lindsay Kilpatrick.