Harvesting System Design and Performance - Engineering and Science of Biomass Feedstock Production and Provision - page 82

research and development efforts worldwide. For bioenergy crops, the current
approach is to use existing harvesting equipment, with or without modifications,
rather than design a whole new machine. Field studies have demonstrated that exist-
ing harvesting machines can be used when the biomass crop characteristics are a
close match with a crop already in cultivation. For example, hay and forage machin-
ery can be used for switchgrass. However, high yield of switchgrass poses chal-
lenges to achieve a high throughput rate [ 2 ].
The development of special purpose machines to account for the unique crop
characteristics and increased productivity and efficiency is also being explored [ 3 ].
For example, harvesting energy grasses such as Miscanthus does not require field
drying so a single-pass machine would make more economic sense [ 4 ]. It could also
reduce the intake of impurities such as soil and decayed litter while harvesting.
Designing harvesting equipment requires careful consideration of the functional
requirements, biomass quantity to be harvested, and desired biomass quality. Key
processes such as cutting, conveying, conditioning, chopping, and densification are
affected by the properties and condition of the biomass. The form in which the bio-
mass is prepared for transportation impacts on transport efficiency and logistics. For
example, to meet weight limits of semitrucks, the bale density should be around
225 kg m −3 comparedtocurrentbaledensityof150-180kgm −3 [ 5 ].
The goal of this chapter is to review past literature on the harvesting of dedicated
energy crops, which includes the discussion of the field experiment data as well as
conclusions drawn from those field studies. This chapter is accordingly arranged as
follows. The next section describes crop harvesting characteristics. The third sec-
tion explains the functional processes, and the fourth section focuses on harvesting
machinery systems for the four main categories of bioenergy crops: energy grasses,
short rotation woody crops, green energy crops, and agricultural residue. The last
section describes future challenges.

5.2 Crop Harvesting Characteristics

5.2.1 Biomass Properties

Morphological properties of bioenergy crops influence the material flow and energy
consumption. The properties of interest include distribution of vascular bundles in
the stem, degree of lignification of vascular bundles, and geometric size and shape
of the stem. Tall and sturdy Miscanthus stems can cause inconsistent crop flow in a
mower-conditioner and plugging of the pickup unit in a baler if not conditioned
enough [ 4 ]. Similarly, the stem moisture content affects cutting. The moisture con-
tent varies with harvest time, and higher moisture content requires more cutting
energy because it provides viscous damping effect during cutting [ 6 ]. The knowl-
edge of expected crop moisture content at harvest is necessary for designing effec-
tive and efficient harvesting machinery. The desired moisture content at harvest is
often defined by the needs of the subsequent processes. For example, baling corn
stoverismoreeffectiveatabout15-25%moisturecontent,whereaschoppingcan