Almost all woody biomass needs to be chipped
or ground for use. The Forest Operations
Research Unit has several chipping-related publications dating from 1985
through 2002.
One of the unit’s first energywood-related
publications (Sirois
and Stokes 1985) was presented in
1985. This paper reviewed the types of
wood available for energy use (forest residue, smallwood and prunings,
wastewood and mill residue) and included general discussions of processing
methods. The raw material costs of
forest residue are minimal, but the cost of harvesting, or gathering, processing,
and transporting can make it more expensive than coal or oil. Short rotation woody crops, thinnings from
plantations, or small understory trees, require highly efficient handling
systems to be economical. Prunings from
large commercial orchards are typically located in urban areas and buried in
landfills, the cost of processing and short transporting distances could offset
this landfill expense. Sources for
wastewood include demolition of structures, broken pallets, mill trimmings,
railroad dunnage and more. Mill residue
may consist of bark, sawdust, shavings, bolt ends, knot rings or veneer
clippings.
The most common processing method at the
logging/harvest site is whole-tree chipping, but shredding, grinding and
chunking may also be used depending on the raw material (Sirois
and Stokes 1985). Moisture content is important in many
combustion methods because the water content must be evaporated before energy
can be released. Five methods of
moisture reduction are described.
The size of the chipped material needs to be
considered when the type of boiler to be used requires a uniform particle size
(Sirois
and Stokes 1985). Small particle sizes allow for faster
combustion, will evaporate moisture quickly, and can be easily suspended in a
suspension type burner.
Other chipping-related research focused on
comparisons of in-woods chipping to systems that haul tree-length stems to be
chipped at a mill. In a cooperative
study with Auburn University and Mead Coated Board Corporation (Shrestha
and Lanford 2002), tree-length
(TL) and in-woods chipping (IWC) operations were compared for utilization and
revenue. Both operations removed
sawtimber along with the pulpwood. The
IWC operation realized more wood recovery, but the TL operation merchandised
the value better. The IWC operation sent
small sawlogs to the chipper which reduced the value. The TL operation topped the pulpwood at 2.5
inches which resulted in less pulpwood utilization.
Recovery of products from in-woods flail and
chipping (IWC) was compared to tree-length (TL) and whole tree (WT) systems (Stokes
and Watson 1991). Flail delimbing and debarking recovered the
highest percentage of clean chips. The
WT operation hauled the whole tree, including the tops and limbs, to the mill
for processing. In terms of biomass, the
WT system recovered the most. The IWC
system produced the most forest residue.
The quality of in-woods pulp quality chips
was compared to woodyard chips (Watson
and others 1991) in a cooperative
study with
A smaller chipper was incorporated into a
traditional cut-to-length harvest operation to remove traditional wood products
and energywood (Bolding
2002 and Bolding
and Lanford 2001). Auburn University, a cooperator in this
study, chose a smaller chipper to keep the ownership and operating costs lower
and to allow operations to stay small and efficient. Energywood was not processed by the
harvester. It was felled and forwarded
in full tree form to the chipper. This
concept study determined that the addition of the small chipper increases the
utilization of the non-merchantable portion of merchantable stems. This system can also reduce fire
hazards. The delivered cost of
energywood was higher than the market rate for energywood, but site preparation
savings could make this an economically viable system.
A study with Weyerhaeuser Company and
Mississippi State University incorporated a tub grinder (Barkbuster 1100) into
an in-woods chipping operation to process flail delimber residue into
energywood (Baughman
and others 1990). Production rates were acceptable to keep pace
with the equipment mix. This machine
could follow the flail/chipping operation as a separate function. Fuel yield accounted for approximately 26.5%
of the total volume of chips and fuel.
After drying, the heat content of the flail debris compared favorably to
whole tree pines and hardwood.
Some publications are short technical
releases. Ashmore
and Stokes (1987b) documented the
development of a technique to use a metal sign to help position chip vans in
the field. The use of the signs assisted
in quick and safe placement of chip vans for in-woods loading.
A cooperative study with Mississippi State
University examined the productivity (Watson
and others 1986a) and power
requirements (Stokes
and others 1987c) of Morbark
chippers (Models 20 and 27).
Researchers examined the chipping productivity differences between
different species and d.b.h. classes (pine, hard hardwood, and soft hardwood),
and the effect of moisture content on chipper productivity. Moisture content did not impact the
productivity of the 650 hp Morbark 27.
Generally, as d.b.h. increased, productivity increased for all species
tested. The soft hardwood productivity
was at a maximum in the 9 inch d.b.h. class.
Maximum productivity for the hard hardwoods increased beyond the tested
15 inch d.b.h. class. Moisture content
impacted productivity of the 350 hp Morbark 20.
Productivity decreased as moisture content increased. As with the Morbark 27, the productivity
increased with increasing d.b.h.
Power requirements of two chippers were
analyzed as a function of the number of stems being chipped and d.b.h. (Stokes
and others 1987c). The smaller Morbark 20 reached maximum engine
power when five 8-inch stems were fed into the chipper. The larger chipper, the Morbark 27, reached
maximum engine power when eight of these stems were fed into the chipper. The authors determined that both of these
chippers had the necessary power for the opening size of the chipper.
Some paper companies have tested growing
hybrid poplar as a short rotation woody crop species to meet their fiber
needs. Whole trees can be chipped into
pulp quality chips using chain flail delimber/debarker/chippers (DDC), but the
production rates of these multiple use machines are impacted by the time
required to remove limbs and bark. Hartsough
and others (2000a)
and Hartsough
and others (2002) explored delimbing this small hybrid poplar
prior to processing with a flail/chipper to increase the productivity of the
chipping process. Pre-delimbing
increased the productivity of the DDC by 10%.
The reduced cost of flail/chipping would not cover the additional cost
of delimbing with the machine mix tested.
In a 1988 study with the University of
California, the Wood Research Institute, and Boise Cascade, hybrid poplar was
chipped using a Peterson Pacific DDC 5000 chipper. Chip recovery based on a percentage of the
total tree weight, bark separation and wood losses were tested and documented
in Hartsough
and others 2000b. For all of the test trees, 95% of the
potentially available wood was hauled in the chip vans. More chips were rejected as tree size
increased because of the amount of stringy material produced from the surface
area of the tree. Bark discharge was
higher for smaller trees due to breakage of the small diameter section of the
bole.
Stokes
and Sirois (1989) summarized
processing alternatives (including field chipping equipment, chunking,
crushing, baling and grinding) and harvesting methods (post-harvesting,
pre-harvesting, and integrated) used in the Southern United States.