1a. Objectives (from AD-416):
Our overall goal is to develop a productive, efficient, and sustainable biomass feedstock supply system using perennial grasses and legumes as a primary feedstock. This project addresses critical needs for feedstock development using perennial grasses and legumes by developing innovative ways to fractionally harvest and store these feedstocks. Specific objectives are: 1) design and fabricate new harvesting mechanisms to separate the high-protein and high-fiber fractions from these crops at harvest; 2) quantify the machine's field performance using a controlled set of operating variables; 3) use this information to improve the mechanisms through re-design during the off-season; and 4) collaborate to develop on-farm storage and pretreatment systems to preserve and add value to both the high-protein and high-fiber fractions. Additional objectives in the extended project include: 5) determine storage characteristics of switchgrass & reed canarygrass stored under anaerobic conditions in bunker & bag silos at different moisture contents; 6) quantify packing density, porosity & temperature profile of biomass materials during storage; 7) determine aerobic stability of stored feedstocks; 8) assess composition & bioconversion potential of feedstocks before & after storage under various conditions; & 9) estimate storage costs under different conditions, taking into account storage losses & changes in quality, as well as bioconversion potential of the stored material.
1b. Approach (from AD-416):
Design and fabricate equipment for field-fractionation of bioenergy crops during harvest. Test equipment on established fields of alfalfa, switchgrass, and reed canarygrass. Determine yield and quality of fractions obtained, along with power requirements and operating costs. Improve design of equipment to improve performance, reliability, and operating costs. Store harvested materials under different conditions, and determine dry matter losses and quality changes resulting from storage under these methods. We will also compare the yield and quality of switchgrass and reed canarygrass when stored at different moisture levels in either bag silos (a widely adopted storage technology) or in bunker silos (a potentially lower-cost method that does not generate plastic waste). Conventional measures of silage preparation (packing density, porosity, temperature profiles) will be combined with measures of microbial conversion in the silo (fermentation acid production), and of subsequent bioconversion potential of feedstock after storage to fuels and fuel precursors. Because exposure of anaerobically stored feedstocks to air can cause undesirable spoilage by microorganisms, we will conduct aerobic stability tests to determine if storage method affects the rate and extent of feedstock spoilage, and if different microbial agents are responsible for spoilage of different feedstocks stored under different conditions. We will estimate costs of storage of each feedstock that will take into account the dry matter losses and quality change of each feedstock during storage, and the bioconversion potential after storage.
3. Progress Report:
This research is related to Objective 2 of the parent project: Develop new commercially viable technologies for harvest, storage and/or on-farm pretreatment and biorefining of perennial bioenergy crops, and use modeling to assess the economic and environmental impacts of integrating these new technologies into sustainable farming system. Research was conducted by the cooperator on single-pass fractional harvest of alfalfa, switchgrass, and reed canarygrass. An experimental harvest fractionation attachment for a self-propelled forage harvester was designed and fabricated. Leaves of biomass plants were stripped by a multi-tine rotor similar to that used to strip snap beans. The stripped leaves and upper plant parts were captured in an auger, processed by the forage harvester cutterhead, and collected in a trailing wagon. A cutterbar cut the stems and formed them into a windrow underneath the harvester. Stripped fraction yield depended on rotor:ground speed ratio and rotor:plant height ratio. The latter ratio had a much greater impact on the yield of the stripped fraction than did the former ratio. Fractionation was more easily accomplished for alfalfa than for reed canarygrass or switchgrass. Varying plant height and lodging caused difficulties in reed canarygrass. Switchgrass was especially difficult to fractionally harvest because the brittle stems of the plant were easily broken by the stripping rotor, causing the stems to entangle in the rotor and eventually rendering the mechanism inoperable. Crude protein (CP) content of the stripped fraction averaged 23.1%, 8.9%, and 7.1% for alfalfa, reed canarygrass and switchgrass, respectively. CP content of the cut stem fraction averaged 12.0%, 4.0%, and 3.7% for alfalfa, reed canarygrass and switchgrass, respectively. In alfalfa, the stripping rotor removed 40% more material than the estimated leaf mass (estimated by hand sampling prior to harvest). The rotor removed not only the leaves, but also the top portion of the stem, which accounts for the recorded mass difference. The rotor removed only 1% more mass than the estimated leaf mass of reed canarygrass, indicating that most of the stripped mass was leaves. In switchgrass, the rotor removed 100% more material than the estimated leaf mass. Moisture content of the stripped fraction for the two perennial grasses averaged 63.5% [wet basis (w.b.)], which should produce successful direct ensiling. Alfalfa leaves averaged 77.1% (w.b.) moisture, so direct ensiling would only be possible with appropriate amendments or additives. Stripped alfalfa material was used by the ARS scientist to determine storage characteristics of direct-ensiled leaves. The cut fraction (consisting mainly of stems) dried readily to either chopping or baling moisture because the stripping rotor removed some of the waxy outer layer of the stem, facilitating evaporation of water from the stems. The cooperator also helped collect data during field evaluation of an experimental leaf stripper designed/fabricated by ARS scientists, in order to characterize the optimum performance configuration of the leaf stripping rotor. The cooperator also evaluated a new wide-body leaf stripper on alfalfa that was designed/fabricated by ARS scientists. The harvested leaves will be used for a feeding trial in late 2013. Research was also conducted on anaerobic storage and aerobic stability of moist perennial grasses. These crops are typically harvested in the late fall when drying conditions are difficult. Delay of harvest until spring can insure a dry-standing crop, but winter losses often exceed 15% of crop dry yield. Biorefineries require a consistent feedstock, but bales stored outdoors can have highly variable moisture and quality. Indoor storage overcomes this problem, but adds significant cost. Reed canarygrass and switchgrass were harvested at two moisture contents (~54 and 43% w.b.) and stored in pilot-scale silo bags until late spring. Loss of dry matter (DM) during storage ranged from 2.3% to 3.4%, with an average across all treatments of 2.3%. There was no significant difference in storage losses between high- and low-moisture treatments. Both grasses were removed from storage with uniform moisture through the cross section and length of the bag. Moisture content at removal only varied 1 to 2 percentage units from the harvest moisture. Aerobic stability was determined during 2- and 7-day aerobic exposure after removal from storage whereby losses of 1.6% and 2.3% of DM, respectively, occurred. Lactobacillus buchneri was applied as a bacterial inoculant at the time of harvest to improve aerobic stability of the grasses. The inoculant was successful in reducing the heating degree days and temperature rise above ambient in both grasses during aerobic exposure. L. buchneri also produced significantly lower counts of undesirable yeast and mold than did the control. Inoculation did not have a significant effect on DM loss during storage or aerobic exposure. This system offers a wider harvest window, produces a size-reduced material at time of harvest, and well-conserved feedstock DM aerobic stability, especially when inoculated with L. buchneri.