Environmental Engineering Reference
In-Depth Information
material into a precompression chamber. A cutting rotor forces material through a series of station-
ary knifes to reduce the length of material. The cutting mechanism can be engaged or disengaged.
Round balers densify biomass by packing and rolling the material. Square balers densify “flakes”
of biomass and then compress the flakes into a bale. Large square balers have the highest field
efficiency among these three alternatives in terms of mass throughput. Prewitt et al. (2007) baled
corn stover and achieved densities up to 176 kg/m
3
, whereas Cundiff and Marsh (1996) found the
density of switchgrass bales to be 143 and 200 kg/m
3
for round and large square bales, respectively.
Round bales generally work well for in-field storage in more humid climates because their shape
can shed water. Round balers require less energy to run and also require a lower initial investment
cost. Sokhansanj and Turhollow (2004) also found that round balers reduced cost as compared with
large square balers when harvesting corn stover. Square bales are inherently easier to transport
and store compactly because of the ease in which they can be stacked. However, square bales have
the disadvantage of large mass loss because of deterioration if stored in a field without adequate
covering and protection from the ground surface. Baling also depends on local weather, conditions
of biomass harvesting, and the end requirements of bioenergy production. If the biomass cannot be
dried because of weather restrictions, the harvesting and handling systems must be able to process
wet materials. For high-volume harvest operations that cannot risk wet weather delays, ensiled stor-
age appears to have significant logistical advantages, with the caveat that drying would be required
later for thermochemical processing downstream.
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hopping
Pull-type and self-propelled forage harvesters are currently used on farms to harvest corn and
hay silage. A forage harvester picks up, chops, and transfers the mowed windrow into a truck or
wagon after the harvester. These machines are designed to harvest materials at a range of moisture
contents. Because the material is loaded and leaves the field immediately as it is harvested, several
forage trucks are needed to drive side-by-side along with a forage harvester to collect and transport
harvested materials. When these pieces of equipment are used to harvest biomass or dry crops,
the forage harvester may experience large mass loss, friction, and combustion problems. A forage
harvester was evaluated in Europe for harvesting miscanthus (
Miscanthus x giganteus
) at 11- and
44-mm chop length and resulted in a bulk density of 95 and 70 kg DM/m
3
, respectively (Venturi
et al. 1998).
Another option for harvesting high moisture biomass materials is a self-loading forage wagon.
This system combines a chopping mechanism with a transport wagon, eliminating the separate
forage truck required for a typical forage harvester system. Self-loading wagons rely on tractor
power to harvest and transport the biomass and thus do not have an independent power unit. Self-
loading wagons have been shown to reduce labor and fuel costs compared with forage harvesters
(Savoie et al. 1992; Brownell et al. 2009). Despite these benefits of self-loading wagons, the self-
propelled forage harvester remains the most commonly used machine for silage harvest at most
farms in the United States. This is due to its low cost, high field efficiency, and low risk of forage
quality problems and nutrient loss.
Corn and other grain combines can be modified to harvest grain and chopped crop residue in a
single-pass operation. This operation can be implemented with single-stream or split-stream collection
of the grain and residue. With single-pass, single-stream collection, grain and residue are collected and
transported together to a centralized processing location where grain may be separated in a stationary
combine, or the combined stream can be made into energy and fuels (Kadam et al. 2000). With single-
pass, split-stream collection, the combine separates the grain in the field, and the stover/crop residue is
immediately chopped and blown into a receiving truck or trailer. Single-pass, split-stream collection
reduces field traffic, thus minimizing compaction, and it also reduces the soil contamination that
inevitably occurs when biomass is dried in the field (Shinners et al. 2007a).
