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The major barrier to wide application of anaerobic fermentation technology in the agricultural sector is economics. Research and development efforts in anaerobic fermentation technology have shown that methane can be produced from livestock manures but that economies-of-scale have a significant impact on the economic feasibility.
Farmer-constructed and -operated systems were estimated to be economically feasible for beef feedlots between 1,000 to 2,000 head, and commercial "turn-key" systems were feasible for feedlots larger than 8,000 head. However, since the average U.S. beef feedlot capacity is about 150 head, and less than three percent have capacities greater than 1,000 head, this means that only large feedlots would benefit from this technology. This is true for other species as well; that is, methane production is economically feasible only for larger-than-average sized livestock enterprises.
By combining crop residues with manure, smaller livestock enterprises may be able to produce methane at a lower unit cost because of the larger plant size. Another advantage of combining crop residues with manures is the large amount of crop residue in close proximity to livestock enterprises. In the U.S., about 200 million tons (dry weight basis) of collectable corn stalk and wheat straw could be available for fermentation, as opposed to about 30 million tons of collectable manure. Thus, there is at least seven times more crop residue than manure for fermentation. A third advantage of mixing crop residue with manure is that, nutritionally, the highly nitrogenous manure complements the highly carbonaceous but nitrogen-deficient crop residue.
There are, however, several problems associated with fermenting crop residue. The major problems are the relatively low biodegradability of untreated residue, the cost and possible adverse side-reactions of pretreating crop residues, the increased materials-handling problems associated with mixing and transporting manure-crop residue mixtures, and the long-term agronomic consequences of removing large amounts of crop residue from productive crop land.
The "dry fermentation" system proposed by researchers at Cornell University has several advantages for fermenting crop residue. In essence, the system is a batch fermentation of crop residue at moisture contents between 75 to 85 percent. Advantages of this system are: simple "hole-in-the-ground" design; no need for size reduction or mechanical mixing of the residue; and the residue remaining after fermentation can be applied back on the land as mulch. Disadvantages of the system are: the high buffer requirement to maintain a neutral pH; the large volume of "seed" required to innoculate the fermentor; and the slow reaction rate in the fermentor.
This report describes a two-stage fermentation system that allows rapid conversion of easily degraded compounds to methane and long-term fermentation for more slowly degraded compounds. The advantages of this system are: thermo-chemical pretreatment or size reduction of the straw are not required; the straw is handled only at the beginning and end of fermentation (Le., materials handling problems associated with mixing and pumping straw slurries are minimized); and the system will selectively ferment the easily and less degradable compounds. This report also describes studies evaluating whether anhydrous ammonia treatment can increase the methane yield of straw.