Harvesting System Design and Performance - Engineering and Science of Biomass Feedstock Production and Provision - page 88

5.3 Harvesting Subsystems

A typical biomass harvesting machine consists of one or more of following subsys-
tems: cutting, conveying, conditioning, collecting, baling, and chopping. If two
machines are involved in harvesting a crop, then it is called a two-pass system, such
as mowing and baling. If only one machine is involved, then it is called a single-pass
system, such as a sugar cane chopper-harvester. A single-pass system is preferred
when the crop needs to be harvested green such as sugar cane or there is no need for
field drying such as corn silage. A two-pass system is preferred when the crop needs
to be field dried to the desired moisture content before baling such as sorghum.
A bioenergy crop harvesting system consists of cutting plants and transforming the
cut plants to a transportable form. The common transportable forms are bale, chopped
biomass, wood chips, and billets. The main functional processes in forming a bale are
cutting, material conveying, conditioning, windrowing, picking up windrows, com-
paction, knotting, and bale release. The functional processes in chopped biomass or
wood chips are cutting, material conveying, chopping, and blowing chopped biomass
into a wagon. Similarly, the functional processes in forming sugar cane billets are
base cutting, cutting stems into billets, cleaning trash, and conveying billets into a
wagon. The following sections describe each of the main processes in detail.

5.3.1 Mowing

Cuttingdevicesareclassiiedbasedonthecuttingmodeforwhichtheyaredesigned.
Impact cutting and shear cutting are the commonly used cutting modes in bioenergy
harvesting machinery.

5.3.1.1 Reciprocating Sickle Bar Mowers

Sickle bar mowers cut the crop by slicing it between a moving knife section and a
stationary ledge plate (Fig. 5.4a ). The construction of a typical cutter bar section is
shown in Fig. 5.4b . Knife section edges can be smooth or serrated and can be
re-sharpened or replaced. Ledger plate edges are usually serrated on the underside
and are not re-sharpened. The correct clearance between a knife section and ledger
plate is maintained by a knife clip. The guards protect the knives from being dam-
aged by rocks and also help to deflect stems at the end of a sickle stroke.
The cutter bar mowers are of two types: (1) single oscillating element with a
fixed finger bar or (2) dual oscillating elements. The single oscillating element
mowers consist of a fixed part (bar with guards, fingers, or teeth) and a moving part
(the cutter blade which is composed of many knives) (Fig. 5.5a ). The ground speed
for the single oscillating element (see Fig. 5.5a )isabout5-7 kmh −1 (Table 5.3 ),
whereas the dual oscillating elements (see Fig. 5.5b ) have relatively higher ground