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While bioenergy feedstock is a less demanding product than pulp wood chip in terms of quality of
chip, many of the chip quality issues arising from chipper design and maintenance also impact
directly upon chipper efficiency, and operating the chipper accounts for a significant proportion of
the diesel used throughout the biomass supply chain (Wu et al, 2008).
The quality of cut by the chipper also impacts upon bulk density. At the extreme in terms of poor
cutting, shredding of whole trees is commonly observed to produce a very low density product with a
high proportion of long pieces, which also makes material handling more difficult. Within chippers,
disc chippers do not process shrubby material and small whole trees as effectively as drum chippers
because of the way in which the knives approach the biomass (whereas disc chipping is the preferred
method in log chipping). Chipper knife-to-anvil clearance is another variable that affects both chip
quality and material handling properties.
2.3.2 Pour Rate
Pour rate is an important performance measure in any harvesting system.
Sugar System
For sugarcane harvesting there are four different pour rate definitions commonly used. These include:
Instantaneous pour rate: Instantaneous pour rate is defined as how fast cane flows through the
machine and is measured in tonnes per hour. Instantaneous pour rate is taken as the product of crop
size and harvester forward speed, equating to the average instantaneous processing rate of the
harvester. Instantaneous pour rate is difficult to measure and only used in research trials. The
maximum instantaneous pour rate of a cane harvester is around 400 tonne/hr.
Elevator pour rate: Elevator pour rate is the tonnes per hour delivered off the end of the elevator
while the machine is continuously harvesting. The elevator pour rate in the Australian sugar industry
is around 100 to 150 tonne/hr in green cane.
Delivery rate: Delivery rate is the tonnes delivered to the mill delivery point per harvesting hour. A
clear distinction is made between elevator pour rate and delivery rate. Elevator pour rate is the
instantaneous rate at which cane is leaving the elevator while the harvester is continuously cutting
and is not the average delivery rate of the machine. For example, a harvester travelling at 7 km/hr in
a 100 tonne/ha crop has an elevator pour rate of 105 tonne per harvesting hour. After turning,
waiting and other delay time is accounted for, this operation might deliver cane to the receival point
at 60 tonne per harvesting hour. This is the difference between elevator pour rate and delivery rate.
The typical delivery rate in the Australian sugar industry is around 60-80 tonne/hr in green cane.
Field efficiency: Field efficiency is the time spent harvesting divided by the total time spent on
harvest activities. In the example above, with 105 tonne/hr elevator pour rate and a 60 tonne/hr
delivery rate, field efficiency would be 57%
Engine hour pour rate: Engine hour pour rate is defined as the tonnes processed per harvester
engine operating hour and is not to be confused with instantaneous, elevator or delivery pour rate.
This method of measurement does not account for servicing and repair time and other harvest
activities occurring when the harvester is not operating. It is an often used comparison as engine
hours are easily measured.
A very high degree of variability can be expected in the field. Identical harvesters can have
dramatically different harvesting rates under identical field conditions, depending on the operators
priorities, time pressures etc. Figure 2.7 illustrates the delivery rate of a sugarcane harvester for
various crop sizes. Even with the same operator and machine, harvesting rate is not solely dependent
on crop size, as a 100 tonne/ha badly lodged cane can have a dramatically lower maximum harvesting
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