CHIPPING SYSTEMS

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 unitís 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 Mississippi State University.Bark content and chip size are of concern to mills.The use of chain flails has reduced the bark content to less than 1% from pine stems and 3% for soft hardwood species.The effectiveness of removing bark in the woods with chain flails is comparable to woodyard drum debarking for pine stems.The flails are not as effective as woodyard drums in debarking hardwoods.Flails are not as efficient in delimbing when several small stems are fed at once.IWC can produce fewer pins and fines than woodyard processes, but they also produced a higher percentage of overs.

 

Mississippi State University and Boise Cascade Corporation cooperated with the unit to compare the utilization, production, and cost of a TL system to an IWC system (Watson and Stokes 1994).The IWC used a Peterson 5000 flail-chipper unit.Cost to the digester was about the same for in-woods chips as compared to woodyard chips, given the costs of the woodyard used in the study.The IWC operation cost about 32% more per ton delivered than the roundwood operation, but the mill processing costs of the roundwood are higher than for the in-woods chips.The IWC system recovered more acceptable chips than the TL system.

 

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.

 

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