Large chipper loading biomass
into a chip van at the landing

Compared to other available woody biomass sources in the forest, recovering unused residues from conventional sawlog and pulpwood logging operations has the greatest potential for providing biomass for energy production (Perlack and others 2005). Of the total forest biomass available nationally, over 40 million tons per year are logging residues. These logging residues can be augmented by removing wood from energy plantations and by harvesting trees that are otherwise non-merchantable due to size, species, or form, as well as dead and damaged trees from any number of agents like insects, disease and natural catastrophes. More information about this is in the Biological and Environmental Sustainability, Bioenergy Production from Southern Forests, and Supply of Forest Biomass sections. However, with the prices available for biomass products circa 2006, it is a challenge to make a harvesting operation profitable - see the section on Feedstock Production.

This section of the Encyclopedia of Southern Bioenergy will synthesize what we know about forest harvesting and transportation practices with primary focus on bioenergy products. In many cases, bioenergy products will be the least valuable products that can be commercially produced in our southern forests. Also, this section deals specifically with the cost factors associated with harvesting operations. Issues dealing with other aspects of the economics of bioenergy production and utilization can be found elsewhere in the Economics and Utilization sections. The Biological and Environmental Sustainability section deals specifically with the many important ecological considerations of producing forest products, including bioenergy. Finally, links are provided in this section to other encyclopedia pages that discuss conventional harvesting systems.

This harvesting and transportation section assumes that a market for bioenergy products exists and deals primarily with the question of where, when and how to harvest biomass that can then be transported to utilization centers some distance from the harvest site. Woody biomass products can take on several different forms which each have their own storage and cost considerations. Many harvesting systems are available to recover biomass. These systems will be reviewed in terms of cost and productivity factors associated with harvesting, pre-processing (chipping or bundling), drying, transportation, and storage.

This section of the Encyclopedia of Southern Bioenergy will comprehensively discuss all operations along the value chain from in-woods harvesting through pre-processing and transport to storage in the following sections.

When designing appropriate systems, it is fundamental to remember that woody biomass or feedstock can be processed, stored and delivered in three different forms. These forms include unconsolidated slash, chips and other comminuted (reduced size) materials, or bundled materials. Different parts of the tree, including stumps, bark, leaves and needles, and the wood itself, may be included.

Horizontal grinder being used to comminute
biomass at a land clearing site

However, it is possible to separate different parts of the tree through storage procedures and mechanical means. These operations are explained in the Pre-Processing and Drying section. All forms can be stored in-woods or at an end-use location.

Each of the pre-processing, sorting and storage alternatives has major implications for feedstock quality and positive or negative implications for biomass use in combustion technology or bio-based product production equipment. These topics will be discussed in the following sections.

• Unconsolidated Materials
• Comminuted Materials
• Composite Residue Logs

Wood Quality is an especially important issue regardless of the form of the feedstock and is treated in the Storage section.

Logging slash normally left in the woods

Unconsolidated slash consists of stumps, tree tops, limbs and branches, and unmerchantable trees of all sizes and species. In most Southern harvesting operations involving tree-length or cut-to-length systems, this unmerchantable biomass typically has been cut and left in place on the logging site or concentrated at the log landing.

While not commonly practiced, this slash can be transported to a biomass-using facility by specialized containers on trailers. There have been efforts made to transport logs with limbs and tops intact and also efforts made to compress this material allowing for heavier, higher density loads for transport. But, these have not proven to be feasible operationally due to costs. The major feasibility issue centers around the fact that this slash is a low density, bulky material that is too costly to transport due to low weight and volume considerations.

Historically, when this material is delivered to a wood manufacturing facility it has been used as hog fuel to generate power. Even in this case, there remain specific drying issues to consider with unconsolidated slash, primarily related to pile sizes, how long to let the piles dry, and the season of the year that piles are produced. These are treated in the Drying section. Innovations about the transport of this unconsolidated feedstock is in the Transportation and Delivery section under Container Trailers.

Clean wood chips of fairly
uniform size

Chips and other comminuted (mechanically reduced in size) materials are generated from in-woods chippers, tub grinders and/or horizontal grinders. Chips and other comminuted materials are desirable because they have a higher bulk density than unconsolidated materials, making transport more economically feasible. Some end users have tight specifications on chip size and the amount of allowable foreign matter. Depending on their requirements, clean chips can be produced by removing bark, needles, and leaves.

This can be done before or during the chipping operation. Chips tend to be uniform in size with each one being fewer than three inches in any dimension. For a review of this process go to chipping in the Basic Steps of Timber Harvesting section under Processing.

"Dirty" chips are those that require less care in handling since they can include some debris like leaves, bark and soil removed from clean chips. Dirty chips are not used for for paper, and thus generally are used for hog fuel to produce steam and heat at wood manufacturing plants. Dirty chips could be used as feedstock for pellet production and other products depending on the sensitivity of the conversion technology. Material produced from tub grinders and shredders is not uniform in size and also includes debris and thus tends to be used for hogfuel.

Wood residue being comminuted by horizontal grinder after logging

The newest material concept that has been introduced involves compaction of logging residues into cylindrical bales or bundles known as composite residue logs (CRLs). Typically, CRLs have a diameter of approximately 2 to 2.5 ft and 10 ft long. Each weighs between 880 and 1320 lb fresh weight (Andersson and others 2002).

This technology is attractive because CRLs can be handled as roundwood suitable for conventional harvesting, transportation, and storage at the wood-using plant. Past technology issues have been addressed by machinery manufacturers. CRLs can be flimsy as a direct result of operators not feeding the bundler correctly thus causing a possible lose of material during transport. In addition, CRLs require an integrated two-pass harvesting operation as well as chipping in the woods or at the mill with a large industrial chipper. This option is explored in more detail in the Timber Harvesting for Biomass section. Technology issues of

In-woods pile of composite residue logs (CRLs)

CRLs being unloaded at roadside for later transport by truck

Timber harvesting technology is always changing and adapting to new product opportunities and challenges, including recovering woody biomass for feedstock.  Even with change, harvesting systems designed to process and deliver woody biomass involve several different operations.  The order of the operations can change from system to system and some of the operations can be combined or even by-passed.  Generally speaking the operations include the four major steps: harvesting or felling, processing and drying, transportation and delivery, and storage.  If the biomass feedstock is not comminuted (reduced in size) in the woods then the processing and drying component will be completely different than for a system that involves in-woods chipping, grinding or shredding.  Storage of the biomass in the woods or at a concentration yard is not always feasible, so storage may not even occur in some systems. 

The technology to be employed is determined by the conditions under which the fuel is harvested and by the scale of operations. The nature of the harvesting system used is determined by forest site, forestry traditions, infrastructure, and the desired level of integration into conventional logging systems.  All these factors influence the choice of technology and methods used (Harstela 1993).

A key issue for a successful forest fuel recovery sytem is the degree of integration with other harvesting operations (Bjorheden 1989).  Higher levels of integration incorporate methods and technology associated with parallel and coordinated one-pass harvesting operations.  In highly integrated systems, wood fuel recovery is an integral part of operational planning; decisions on assortment range are based on their net contribution, and technology and methods are adapted to the task of integrated harvesting. 

In one-pass, or fully-integrated harvesting systems, all products are harvested in one operation.   One-pass operations may be based on processing into different products at the stump.  One-pass systems are used to produce forest fuel commercially in addition to conventional roundwood (Richardson 1986).  One-pass harvesting systems take a number of forms and utilize equipment in a variety of combinations.  Various forms of processing, such as comminution and measures to facilitate handling of the material, as well as storage, can take place at different points in the supply chain (Bjorheden and Eriksson 1989).

Two-pass harvesting systems do exist and tend to include a low-level of integration between the two harvesting operations.  Typically, logging residues are removed in a separate operation after final harvest of traditional wood products.  The final harvest can be adapted to facilitate removal of residues through accumulation of branches and tops into large piles.  The piles are positioned on site to remain undisturbed until collected after harvest (Andersson 2002).  To work effectively in biomass recovery, two-pass systems require a high level of integration between the two harvesting operations.  The productivity of the residual operation is directly correlated to how the traditional wood products are harvested.  A variety of harvesting methods can be used depending on equipment configuration and type of stand being harvested. 

Harvesting biomass from thinnings represents a unique challenge due to the felling, delimbing and handling of small-diameter trees.  Because tops and branches constitute a high proportion of biomass in small trees, cost-effective harvest requires specialized equipment.  Accumulating felling heads and multi-tree delimbing devices, which are standard in the South, must be used.  If limbs are to remain intact then a tree-section method of harvest can be used.  Trees can be cut then bucked without delimbing then hauled to a landing for processing with a bunch-delimber.  Another approach would be to haul tree-sections to the end-user in trailers with special metal-sided covers.  Following delivery to a processing yard, wood is separated and processed into pulpwood and biomass.  This tree section method of utilization from thinnings is not used currently in the South.

This section of the Encyclopedia of Southern Bioenergy will address harvesting system design in the following ways:

Drawing of mechanized wheel-based
timber harvesting systems

There are several phases commonly considered in designing harvesting operations. However, some phases can be combined or even eliminated depending on the product or in this case the woody feedstock being generated. To reduce confusion that might be caused by unfamiliar terminology in the following sections on Timber Harvesting for Biomass and Practical Biomass Harvesting Systems, it is worthwhile to review all these operations in Basic Steps in Timber Harvesting in The Encyclopedia of Southern Appalachian Forest Ecosystems.

The graphic below shows some of the options for mechanical ground-based harvesting systems. The phases shown from upper left to right include felling, delimbing or processing and then lower left to right the processes involve extraction, loading and transport.

Biomass harvesting systems focus on removal of the largest percentage of total biomass present by weight regardless of conventional ?merchantibility? specifications. For this reason, equipment design and operation may be somewhat different than those found in conventional tree-length or cut-to-length harvesting systems. Work in biomass harvesting in the 1980s focused on adapting existing conventional harvesting systems to harvest both conventional products and unmerchantable stems plus residues from conventional logging operations (Stokes and others 1984; Stokes and Sirois 1989). Conventional harvesting equipment was coupled with whole tree chippers, flail delimbers and grinders that allowed the collection and particle size reduction of non-merchantable material. Stokes (1992) reported at that time that several fuel harvesting systems were in operation, and that most utilized feller-bunchers (generally rubber-tired), grapple skidders and large chippers operating at the landing.

While chipping technology has not changed greatly since the 1980s, chipping operations are still the most widely used methods for generating biomass feedstock. Systems or the configuration of machinery using a tub grinder or shredder are similar and are included in the discussion of chipping. As an alternative, composite residue logs (CRL) systems have been examined to determine feasibility. Both are discussed in the sections below.

• Comminution Systems
• Composite Residue Log System

Biomass being processed into chips
by a chipper with in-feed system

Bundler machine creating and
stacking composite residue logs

Hartsough and others (1995) analyzed five different systems of conventional equipment for harvesting Western U.S. stands for health improvement, four of which processed the unmerchantible portion as biomass fuel. The five harvesting systems are described below:

1. Feller/Buncher-Skidder-Flail/Chipper: In this system, one combination flail delimber/debarker/chipper converted round wood into pulp-grade chips. There are two feller/bunchers used; the type of feller/buncher used was dependent upon slopes. The steeper slopes required a more expensive, tracked swing-to-tree machine and the more level terrain allowed the use of less expensive rubber-tired, drive-to-tree feller/bunchers. Three rubber-tired skidders were used in this system. This system was used where there was a strong local market for pulp chips.
2. Feller/Buncher-Skidder-Processor-Loader-Chipper: All material was felled with two tracked feller/bunchers that separated merchantable sawlog trees and biomass. Three rubber-tired skidders transported the sawlogs to one processor that decked the logs and piled the tops for later chipping. Most limbs were returned to the woods by the skidders. One log loader was used to load the sawlogs. One chipper was brought to the site after the sawlogs were loaded and removed, and the chipper processed the tops for biomass fuel.
3. Harvester-Forwarder-Loader-Chipper: This system utilized one harvester that processed (felled, delimbed, bucked and piled) both biomass and sawlog trees in separate piles. One forwarder loaded both types of logs and carried them to a landing. Sawlogs were loaded with one conventional log loader and the biomass was chipped with one chipper. This system leaves the residues (tops and branches) from both types of trees in the woods.
   

   Grapple skidder bringing biomass
   to chipper at the landing
4. Feller/Buncher-Harvester-Skidder-Loader-Chipper: Here the merchantable trees were processed with one harvester, and one feller/buncher felled the biomass trees. Three skidders pulled both types of piles to a landing, where one conventional log loader loaded the sawlogs and one chipper handled the biomass.
5. Harvester-Forwarder: In this system merchantable stems only were harvested. One harvester (or possibly two if stem sizes were small) processed the trees, and one forwarder transported the trees to the landing to be loaded by the forwarder onto trucks. The non-merchantable biomass was left in the stand.  A scarifying device can be attached to the forwarder between the front axles and the track.  This provides partial site preparation each time the forwarder is driven in to the harvest area thereby increasing the productivity of the machine even when empty.
   

The observed pros and cons to the various components of these systems are discussed throughout the harvesting section. Other possible systems mentioned but not studied were horse logging, chainsaw-prebunch-skidder-flail, feller/buncher-skidder-flail, harvester-skidder, and harvester-cable-yarder. An important conclusion from this study was that harvest system choice depends on a number of factors and conditions, and even when the range of conditions is limited to relatively flat terrain with small trees in overstocked stands, there is no one "optimal" system. In addition, it was concluded that the range of available harvesting system options makes it likely that an operations planner with substantial experience will find the best option.

Forwarder loading logging residues
for transport to landing

Bolding and Lanford (2001) studied cut-to-length (CTL) systems combined with a small chipper to harvest energy wood in Alabama. In this system, non-merchantable trees are felled and piled with limbs and tops from merchantable trees during harvest. A forwarder transports the non-merchantable material to a chipper and the merchantable logs to log trailers. The chipper is smaller than traditional whole-tree chippers used in conventional harvesting systems, and more closely matched to the capacity of one harvester and one forwarder. For regenerated stands the economic advantage of the system includes reduced site preparation costs and the value of the energy.

Conventional harvesting systems have become larger, more capital intensive and designed for large tracts of land. Smaller, labor-intensive operations such as the shortwood truck and the manual chain saw feller are now rare in commercial pulpwood harvesting operations. Small-scale operations using manual chainsaw felling and either cable skidders or animals for skidding are still used, however very little, for sawtimber harvests, especially in situations where volume is low or aesthetics are more of a concern. Urban logging in the southern region often uses shortwood equipment and manual felling. Smaller and multiple function equipment has been examined for use in biomass harvesting for several years. In 1986 Stokes and Sirois evaluated a chipper-forwarder; this machine chips in the woods into an inboard container, and transports the chips to a landing for transport when the container is full. One of the setbacks in any multiple function machine is that when it is performing one function, it is unavailable for the other.

Chipper-forwarders are not in common use in the US today, but are used with some variation in other parts of the world where higher fuel prices result in greater biomass utilization.  New technologies have been developed in Scandinavian counties with a bioenergy havester.  This one machine performs CTL operations with an accumulating duel function processing head utilizing both a chainsaw and shear head, an integrated front mounted chipper, and a 10-ton chip bin on the rear of the machine.  This enables one operator to harvest and sort veneer, sawlogs, pulp, and chips from one operation and can be used in thinning operations.  The unit self-unloads chips utilizing chip augers and can load forwarders fitted with a chip bin or drive to a landing area to live load chip vans.  A log carrying forwarder would be needed to haul out sawlogs.

The most common operation is comminution by chipper, tub grinder or horizontal grinder. Hakkila (1989) notes that comminution has a central role in the overall design of the harvesting system layout. According to Richardson (2002) "... (t)he choice of comminution device and the location of comminution in the forest fuel supply chain are among the most important components of a supply system for fuel wood. Factors affecting choices include: customer requirements for the raw material, total volume of fuelwood in the system, stand characteristics and nature of the road network, conditions at the end-user reception facility, and feasibility of creating terminals for efficient handling and storage without incurring excessive additional haul distances."

Continuing to quote from Richardson (2002) the following observations are significant to note.

"Sometimes the whole-tree method is used and the trees are transported to the road, landing or central processing plant for separation and processing of components. In whole-tree harvesting systems for fuelwood, either the whole-tree or the branches and tops are used for fuel, the rest of the tree being utilized as industrial wood. In Sweden and Finland, the harvesting of fuelwood in the form of whole-trees or tree-sections is less common than the removal of logging residue from final fellings. In Denmark, fuel chips are produced mainly from early thinnings. Whole-tree and tree-section methods can also be employed for clearcutting with under-sized trees handled as tree-sections in diameter-graded harvesting. Chipping or delimbing can be carried out efficiently through bunch processing at the landing, mill or heating plant. Specially-equipped haul vehicles are used for carrying tree-sections."

"Chipping at the landing and direct-loading of custom chip vehicles is a hot (concurrently operating machines linking material flow) or highly sensitive system dependent on the smooth functioning of all components of the productivity chain. If a delay occurs at chipper, truck, or reception facility, downtime will be costly. This problem is greatest when a container system or custom chip vehicle is being used."

Finland has addressed the issue of a delay in productivity due to the components required in the flow of biomass by eliminating the 'hot concept' and utilizing 'cold' chipping/loading at the landing.  There are dedicated on-road chipping trucks that can fill vans on the landing using previously gathered and piled material.  This allows for chipping/loading and hauling a drier material made available when the demand requires.

JEnergy Wood Harvester bundling
compositee residue logs

Finding an effective method of densifying residues would be a key development to reduce the costs of transporting low-density biomass feedstock (Rummer and others 2004). The use of conventional round balers for collecting residues was examined by several researchers (Woodfin and Stokes 1987; Curtin 1987; Stokes and others 1987; Johansson and others 2005). However, residue baling or bundling has not been widely adapted to date. One machine that can offer a technical solution is the Energy Wood Harvester shown below. It can be more closely examined at http://www.deere.com/en_US/cfd/forestry/deere_forestry/harvesters/wheel/1490d_general.html. This machine bundles forest residues into composite residue logs (CRLs). Pinox, Valmet, and Timberjack also have commercially available bundling equipment. Most current technology does not lend itself to integration with logging operations, although there is a possibility of using a conventional forwarder for CRL transportation. Later in the chain CRLs may offer high integration opportunities since hauling can be carried out by conventional roundwood rigs.  This needs to be integrated with logging operations to achieve productivity.  If a cut-to-length system is used in a clearcut operation, it is important for the operators to windrow slash to be bundled.  Also, nutrient cycling could easily be achieved if the slash were allowed to remain on the ground before bundling.

For anyone, including landowners, interested in or already in the logging business to consider harvesting biomass, there may be several potential options available. However, transportation of the feedstock can be a problem especially when delivering unconsolidated materials. An agreement has to be made in advance with the wood-using facility about accepting the feedstock. Transporting biomass will be difficult without special trucks designed for the work. Also, consideration about off-loading the feedstock at the wood-using facility has to be made.

A recent study conducted by Westbrook and others (2006) in southern Georgia looked at the addition of a small chipper (Conehead 565) to a mechanized, tree-length system to harvest tops, limbs, and understory (dbh 1-4 inch) biomass in addition to roundwood. The site contained an estimated 7.7 tons per acre of understory biomass with an average dbh of 2 inches. Several interesting findings came out of this study. First, harvest of understory stems did not significantly increase fuel consumption per ton when roundwood was harvested along with the chipping of limbs, tops, and understory. Second, harvesting and chipping understory stems did not reduce roundwood output. In addition, cost projections suggest that this method of producing chips can be competitive with open market energy chips if no more than 10 loads of roundwood are harvested to produce a load of chips or at least one load of chips is produced daily. Finally, site prep costs were reduced by approximately US$20, and the operation yielded a net energy ratio of 44:1.

Small in-woods chipper processing
chips and loading chip van

Recognizing that scaled-down operations may be an option, many manufacturers are designing and manufacturing smaller scale harvesting equipment. In a recent trade publication six manufacturers advertised harvesting attachments and carrier modifications for farm tractor or skid steer machines, and nine manufacturers offered smaller-scale chippers for timber harvesting (Anon 2005).

Other efforts have been made to investigate new, smaller harvesting technology systems, much of it through the modification of agricultural or industrial machines. Two primary objectives cited by Willhoit and Rummer (1999) describe the following important characteristics of an optimum small-scale harvesting system:

• Four-wheel drive farm tractors, small rubber-tired skid-steer machines, and larger rubber-tracked skid-steer machines are the three main types of base machines to be considered for small-scale mechanized harvesting systems;
• Hydraulic shears for felling;
• Processing by harvester head; and
• Loading with front loader attachment or small knuckleboom attachment.

While small scale systems may optimize at lower levels of productivity, it is important to recognize that the use of small scale systems to harvest lower value material on less productive cuts (i.e., biomass yielding harvests--usually as a result of uneven-aged silviculture, timber strand improvement, thinning, etc.) is rarely economically feasible (Jensen and Visser 2004). In addition, the evidence is mixed at best reqarding the ecological effects of small-scale equipment (Marui and others 1995; Updegraff and Blinn 2001).

ATV with self-loading log trailer

Furthermore, small scale-systems tend to require more operator skill in an effort to maximize productivity, are more labor intensive, and have problems meeting OSHA logging safety requirements (Updegraff and Blinn 2001). Safety standards currently lacking in many small-scale systems include: OSHA-approved roll bars, radiator guards, valve stem protection, engine guards, cab protection, and hydraulic tanks (Updegraff and Blinn 2001; Shaffer 1992).  Retrofitting small industrial and/or farm equipment can present problems with stability and safety for the operator.  While retrofits can be made, most industrial equipment is engineered to be utilized on level surfaces, not on slopes or uneven ground with stumps and slash to maneuver over. 

Horse drawn log skidding

The use of livestock for timber harvesting systems (animal logging by horse, mule, and oxen) has nearly disappeared from commercial forestry in the US, but may be considered by landowners having small, specialized objectives (Visser and others 2006). In countries where labor is relatively cheap, the cost of animal skidding for small tree harvesting, particularly in thinning operations, is lower than machine skidding; however, this is not true in countries like the United States with high labor costs. In addition, animal harvesting systems are more sensitive to severe weather conditions than machine skidding, and distances greater than 500 feet are not conducive to the use of animals because of low ground speed (Wang 1999) . Visser and others (2006) report the existence of approximately 50 or so animal powered logging operations in the Southeast.

Animal logging requires specialized support equipment and special sets of operator skills and experience (Rummer 1996). It may be applied to the some areas but will depend upon locally available contractors and a landowners willingness to pay for such services.

Unconsolidated slash left after a
logging operation

In woods chipper with chip van
and knuckleboom loader

Pre-processing and drying feedstocks are important from the standpoint of transport costs as well as combustion efficiency. The drier the wood, the less water transported. Assuming total transportation vehicle payload capacities stay the same, there will more wood (and less water) delivered per load. This will reduce the transport cost per unit of wood, and reduce the amount of water being handled, transported, and evaporated through combustion.

To facilitate efficient transport, material handling, and utilization at the conversion facility, biomass is further pre-processed during or after harvesting. The optimum treatment of the material depends upon the characteristics of the material, the end-use, and the site management requirements. Pre-processing should address two critical characteristics of biomass: particle size and moisture content.

The desired particle size is defined by the end user of the biomass. A rule of thumb is that the size must be small enough to be conveyed without material binding up or bridging. Several biomass power plants in the US have material specifications that provide for nominal 3- inch material; this means that nearly all of the material does not exceed 3 inches in any dimension. This can be accomplished with either chipping or grinding.

• Pre-Processing
• Drying

For nearly all end uses, ultimate material handling is optimized for the end-user by having uniform particle size. Large-scale conversion facilities usually have additional communition and classifying equipment on site, but initial communition often takes place prior to transport (exceptions are bundling residues or transporting whole logs). Particle sizing is often integral to biomass harvesting. Because of the diversity of biomass material taken from Southern forests, some consideration of particle-size management technology is helpful. Forestry residues are usually processed to reduce the material for more economical removal, transport and handling, and size reduction can assist in reducing transport costs by increasing density of shipped material and decreasing the air space in transported loads. Stokes (1998) pointed to a study that showed where fractioning (chipping) of logging residues resulted in a 25 percent decrease in hauling costs.

There are still only a few commonly-used methods to convert large pieces of woody material into small pieces, and all require a relatively large amount of diesel fuel to operate, approximately 40-50 gallons per hour. The following general categories of biomass size reduction technology can be used for comparison.

1. Chippers: machines that rely on cutting material with a slicing action.
2. Tub Grinders: machines that reduce particles in size by repeatedly pounding them into smaller pieces through a combination of tensile, shear and compressive forces.
3. Horizontal Grinders: machines that generally tear particles apart by shearing versus smashing (Goldstein and Diaz 2005).

In-woods chipper preparing to
process woody biomass

Chippers are the most commonly used size-reduction equipment in forestry. They are characterized by high output high-speed cutting knives, and most have integrated capacity to throw chips into transport trailers for hauling. They are reliant upon sharp knives, and are susceptible to knife wear from high soil content, metal contamination, or rocks and stones. Chippers are well integrated into existing harvesting systems.

Tub grinder comminuting woody
biomass

Grinders are types or derivatives of hammer mills, and include horizontal feed grinders (material fed to hammers horizontally and vertical feed grinders such as tub grinders. Grinders are more forgiving of contamination and accept a wider range of piece size. Tub grinders are designed to take short, non-oriented pieces including stumps, tops, brush, and large forked branches. Grinders rely on hitting a piece of wood often enough to finally break it into the piece size desired, usually with high speed rotating hammers. They require more energy than chippers per ton of output, and excessive soil can increase internal wear.

Horiztonal grinder comminuting trees

Horizontal grinders are generally slow-speed rip/shear devices, used widely in tire shredding. Material is pinched between rotating devices, and ripped apart or sheared. Horizontal grinders are used where signifcant contamination is evident, because the internal parts are slow-moving, and frequent damage due to metal, rock or concrete is avoided. They are not in wide use in biomass operations except as a first-stage size reduction, as their capital cost is high and the particle size is larger and less uniform in size. Material from horizontal grinders usually requires further size reduction.

Stokes and others (1987) reviewed a 1982 study addressing ways to increase bulk density of roundwood loads from thinnings. Two experimental trailers were built with specialized load compression fixtures. In most cases the load compression increased the total load biomass over the uncompacted trailers by 50 to 100 percent for softwoods and somewhat less for hardwoods. Results were better when lower moisture content material was used. However, costs for compressed loads tended to be higher because equipment costs were higher and the trailers had smaller load capacities.

In 2004 Rummer and others, evaluated the efficiency of Scandinavian biomass collection systems, which were biomass bundlers coupled with those countries conventional harvesting configurations. Biomass bundlers collect, compress, and bind forest residues into cylindrical bundles approximately 2 ft in diameter and 10 ft long. This simplifies handling residues by compacting loose slash into a form that resembles a log. They reported that in Swedish studies, biomass bundling was cost-competitive with their alternative treatment of roadside chipping residues. This process enables residues to be economically shipped to conversion centers where more efficient size reduction can take place. The study reported that the residue material had to be properly staged to allow efficient bundling, and that there is no single residue treatment that will meet the needs of all situations.

Biomass feedstock moisture content is dependent on post-harvest handling and storage conditions.  Moisture may be lost by transpiration drying, through foliage, or from open wood surfaces.  The rate of drying depends on many factors including ambient temperature, relative humidity, wind speed, season, rainfall pattern, tree species, and tree size.  The best season for drying usually is dry summer in the South. 

Transpirational drying, also known as ?leaf seasoning?, ?biological drying?, and ?delayed bucking? occurs when felled trees are left for several weeks with the tops, branches, and leaves intact (Stokes and others 1993).  Such drying can be successfully used with most species.  Moisture contents as low as 30 percent may be reached under favorable conditions.

Research studies have been conducted on transpirational drying in the US, and a review of several studies is reported by McMinn (1986) and Stokes and others (1993).  They report several earlier findings related to transpirational drying, including the following.

• There is little or no difference in moisture content of red oak trees felled with tops intact and dried for three weeks compared to similar trees felled, bucked and stacked for the same period.  In the same study, paper birch showed significant moisture loss.
• Loblolly pine, white oak, and sweetgum logging residues were compared for moisture loss.  Results showed higher moisture losses in pine, less moisture loss for sweetgum, and the least for oak.  It was recommended allowing residues to remain in the woods for 10 to 12 weeks in the winter to reduce moisture content.
• In Virginia, it was shown that after four months of field drying, minimum moisture contents were approximately 29 percent.  The main drying variables identified were season of felling, species, and diameter.  The net energy value increased up to 50 percent (pine heartwood).
• Results for eucalyptus and slash pine in Florida showed that the season of felling significantly influenced the drying of pines, and that at least four weeks of drying may be needed for optimal drying.

Large logging slash pile suitable for
biomass recovery

Stokes (1987) provides a basis for predicting transpirational drying of species groups in the southern US based on these variables.  It was noted that pine stem weights began to stabilize after about 50 days, soft hardwoods after about 30 days, and hard hardwoods after 40 days.  Moisture contents after that period were about 37 percent for pine, 33 percent for soft or low-density hardwoods, and 32 percent for hard or high-density hardwoods (wet weight basis).  This pretreatment is important from the standpoint of transport costs as well as combustion efficiency.  Each transport load of drier wood will carry less water.  Assuming total payload capacities stay the same, there will be more wood (and less water) delivered per load.  This will reduce the transport cost per unit of wood, and reduce the amount of water being handled, transported, and evaporated through combustion.

Additional research was performed on another approach to field drying. Sirois and others (1991) reviewed tests performed on a prototype roller crusher.  This machine was designed to crush round, smaller diameter stems and facilitate drying by opening the wood to transpirational drying effect.  The study found that crushing facilitated drying during periods when rain was absent, and any drying benefit was attained during the first five weeks of drying.  The study concluded that there is no guaranteed benefit from crushing trees to increase the rate of moisture loss over long drying periods or periods of heavy rainfall.

Excessive precipitation or low temperatures may hinder the efficiency of transpiration drying (Lehtikangas and Jirjis 1993a).  If on-site storage is extended to late fall or even until winter in temperate countries, the advantage gained by transpiration drying will be lost due to absorption of moisture directly from the air and from precipitation (Lehtikangas 1991).   One benefit of on-site storage in small heaps is that leaves may be left in the forest, resulting in reduced nutrient loss from the site and better fuel quality.

Truck and chip van being loaded
at a sawmill

Transport of the feedstock after harvesting, pre-processing and storage is a key element to the successful and profitable use of biomass. The way transport is organized and distance to the end-user can have implications for the production system as a whole. When the commodity is forest fuel, the transported product is actually energy. The goal should be to transport energy as efficiently as possible. This is not necessarily the same as optimizing transport of a physical load.

A basic problem of forest energy transportation is that slash, un-delimbed small trees and tree-sections, are the typical forest bioenergy products (Hakkila 1989). "Its low bulk density increases the cost of transportation. Water is a major constituent of the transported mass and the complex texture of the material makes handling technically difficult. Bulk density may be increased by compaction or by chipping. Processing into chips will decrease durability during storage. Green chips are highly vulnerable to microbiological, physical and chemical degradation which can cause health hazards, loss of substance, loss of energy content and add to the risk of self-ignition" (Bjorheden and Eriksson 1990; Kofman 1994). Chipping can only be recommended if it takes place shortly before consumption." (Richardson 2002).

Selection of transport systems is affected by the quality and structure of the forest road network and by conditions at the landings. To become the exclusive transport flow, comminution at the heating plant must generally be 30-45 percent less expensive than chipping at the landing, in order to compensate for the high cost of slash transport. For more information, please see the Storage section.

Water-based transport of feedstock is possible. This can be applied in two different situations. First, feedstock can be transported on barges along rivers in the southern United States. Barges have long been used to transport wood raw material especially hardwoods within the Mississippie river corridor. Secondly, the biomass can be exported to other countries in bulk transport ships. Chipped biomass is currently being texported via ships from several ports in the South. Aruna (1997) says that sea transport is the most efficient means of transporting bulky biofuels from Sweden to other countries.

• Trucks
• Trailers and Vans

Most forestry products and harvesting material is transported by truck. About 80 percent of the pulpwood delivered to US mills in 1996 arrived by truck (McDonald and others 2001). Truck transport is usually the least expensive, but in the past railroads played a larger role. A study in 1985 reported that rail rates for transport of fuelwood then were about 35 percent lower than trucks for haul lengths averaging 80 miles (Stokes and others 1993). Rail transportation has been used for providing material to large facilities such as pulp mills or power plants, but because of the discontinuous and fragmented nature of many Southern timber stands, rail transport will probably not be used.

Trucks for transporting commodities can be generally placed in one of two categories. Most goods in the South are currently transported in 80,000 lbs gross vehicle weight (GVW) road tractor-trailer combinations. These utilize standard highway road tractors, six to ten wheel tandem axle highway trucks with either a conventional (engine in front of compartment) or cab-over-engine design. Typical road tractors weigh about 12,000 to 20,000 lbs, and can include sleeping compartments and provisions for hydraulic power for trailer functions such as operating self-unloading floors.

Fixed trucks are vehicles with a cargo area integral to the operator cab and chassis. Fixed trucks are for the most part less than 40 ft in length, and the payload capacity is less than road tractors. Road tractors are designed to pull cargo trailers. These trucks are designed for greater capacity and offer the versatility of changing the type, size, and configuration of cargo space. Fixed trucks are usually shorter than road tractor-trailer combinations, and allow more maneuverability in tighter areas. Rummer and Klepac (2003) explored the use of fixed trucks with removable roll-off trucks trailers with pallet racks to supply small-scale users. Rawlings and others (2004) examined the use of fixed trucks and roll-off containers for hauling logging slash from harvesting sites to mills in Montana. Because of the lower payload capacity, applications of fixed trucks are best when hauling distances are shorter.

Logging truck and trailer loaded with CRL bundles

Truck with duel chip bin trailers loaded with chips

Woody biomass feedstock can be delivered to wood-using facilities by truck or rail.  Rail can be used to transport manufacturing residue feedstock (chips) to a facilities using that material for energy; however, this does not happen often.  Trucks are used exclusively for transporting feedstock from the woods.  The configuration can take several forms that vary from the type of truck used and the type of truck-trailer combination used.  Of course how the biomass is pre-processed at the logging site determines the transportation configuration used. 

• Log Trailers
• Container Trailers
• Vans

The most common trailers used for timber harvesting come in several forms.  They can be either pole trailers or frame trailers all with dual-axles.  The log frame type of trailer is designed to haul trees, poles, or shortwood in racks.  They are relatively lightweight and because of this have high payload capacities.  Most require unloading equipment at the receiving facility, although some are modified to drop one side of the log restraints allowing a front loader to push the load off one side of the trailer.  This latter type is no longer very common. The residue bundles researched by Rummer and others (2004) can also be transported in these types of trailers.

Container trailers are designed to hold bulk material and the container is designed to be handled full. Because of this, they are built with sturdy walls and supports and their total capacity in cubic volume is less than bulk vans or log trailers. However, they can be left on a site and filled as desired, and then removed and replaced with an empty container at the same time. They can also be used as storage at the end user?s site. In addition, container trailers may be more suitable for collecting yards where road access is limited, or where smaller volumes are present.

Container trailers handle most of the international trade that is moved by truck from ship ports, and a large portion of the collected solid waste in the US. These consist of a trailer chassis with a removable cargo container or box. The containers can be varied in size, construction and volume, and the chassis can have the capacity to load and unload containers. The roll-off trucks and containers commonly used for collecting and hauling solid waste, and the container trailers used for distributing goods from ships, are two common varieties.

Container or bin loaded with logging slash ready to be loaded
onto a trailer

Small chip van and truck loaded with woody biomass

Vans are enclosed box trailers generally 8 to 8.5 ft in width and designed to be less than 12.50 ft height when pulled by a road tractor.  Vans and flatbed trailers are used to transport most highway cargo throughout the US.  The difference between the box trailers seen on most highways and vans hauling harvesting products (bulk vans) is that most vans are built for containerized cargo (commodities in boxes or on pallets).  Transported in water-tight and sometimes climate controlled conditions, this cargo does not shift or settle, and the cargo containers help to support the goods inside. Bulk vans haul bulk commodities that can shift, settle, freeze, and often contain moisture or soil.  The vans require additional support, especially on the side walls and floors.  Many timber harvesters have attempted to use conventional box vans with little modification, and some of these vans have structurally failed while full and in transit.

Bulk vans for forestry are also known as chip vans.  Bulk vans have either an open end or an open top.  Open-top bulk vans are usually loaded with front wheel loaders from the side, or from overhead bins.  These must have removable tarps to comply with most state regulations.  Open-end bulk vans are generally used for chippers, where material is blown into the trailer, and must have tailgates that allow both loading and reduce or eliminate flying material while in transit.

Top loading chip van being lloaded
by a chipper

Bulk vans used in the southern region are mostly for maximum capacity of 80,000 lbs although in many western states legal weight limits are higher and bulk vans are designed for higher capacities.  Depending upon the weight of the road tractor and the trailer itself, this means that they carry a legal payload of about 42,000 to 52,000 lbs.  The cubic yard capacity of the trailer is matched to the material being hauled. In order to achieve the maximum weight capacity for lighter material, the bulk van must have more volume capacity.  Most bulk vans carry between 97 and 131 cubic yards, although specialty chip vans for extremely light material (such as planer shavings) can hold 150 cubic yards or more.  Bulk vans are used for hauling garbage and debris, although bulk vans for this purpose have stronger walls and floors, and usually lower cubic volume capacity.

Bulk vans can be unloaded by placing them on a tipping platform, raising the front of the trailer and unloading the contents from the rear.  In areas without major biomass facilities, bulk vans increasingly have integral hydraulically-operated self-unloading floors (or live floors) that move the contents from inside the trailer to the rear and out of the tailgate.  The advantages of these trailers are that they 1) do not require a truck unloading platform at the destination; 2) can place material within any area of the storage yard; 3) can unload unacceptable loads at a different location; and 4) can backhaul other material on the return trip to places without truck unloading platforms, increasing the revenue per trip and therefore potentially lowering the cost to the producer. However, live floor bulk vans are heavier than regular bulk vans, and this reduces the potential legal weight capacity of the payload.

Bulk vans are generally considered to be the most cost-efficient mode of transporting pre-processed biomass provided the access roads are suitable for these over-the-highway carriers.  In less accessible areas, other options such as container trailers should be considered as discussed by Rawlings and others (2004).

Mills and other wood-using facilities keep chip or sawdust piles on-site or at nearby facilities for periods when supply is low. The ideal storage period is determined by each facility’s woody supply situation, but typically varies between two and six weeks (Fuller, 1985). Woody biomass is reduced in size in the forest and then transported for storage, or it is transported, reduced in size at the mill, and then stored. The resulting material, usually chips, is stored outside in large piles and under cover in large silos or bins. Chips stored in bins are typically to be used within several hours or days while silos are used for longer-term storage needs. Silos and bins protect against contamination while at the same time allowing for uniform feeding and metering of the material. While storing comminuted biomass makes handling and transport relatively easy, if not managed carefully, the biomass will succumb to dry-matter loss and in some cases self-ignitioniv. High temperatures and acetic acid odor are signs that a chip pile is in danger of dry-matter loss and self-igniting (Fuller, 1985). Additionally, chip piles with excessive mold and fungi growth can lead to health risks for humans.

Woody biomass can be bundled and stored under cover to gain the advantages that come along with storing chipped material, ease of transport and handling, while at the same time protecting the material from the disadvantages that come along with chipped material, dry matter loss, moisture retention, heat generation, and health hazards (Richardson et al., 2002).  Logging residues should be allowed to dry during the summer months before being bundled and stored (Richardson et.al., 2002).

Associated topics concerning bundling are covered in the Composite Residue Log section.

Dry-matter loss, which is the degradation of lignin, cellulose, and hemicellulose, occurs when wet woody biomass, in any form, is not utilized immediately. The degree to which dry-matter loss occurs, is largely dependent on the materials moisture content. Woody biomass having higher moisture content is more susceptible to colonization by fungi and mold and at a faster rate (Richardson et. al., 2002). These microorganisms, via metabolic activity, generate heat which in turn accelerates oxidation, moisture adsorption, hydrolysis, pyrolysis, and other chemical processes resulting in dry-matter loss. Dry-matter loss results in a reduction of overall energy content as well as leads to an increase in ash content (Richardson et. al., 2002).

Several studies (Thornqvist and Jirjis, 1990; Fredholm and Jirjis, 1998) have observed dry matter loss in stored woody biomass. Green chips stored in a large pile for seven months lost approximately 12 percent of their dry matter and bark stored in a large pile for six months lost approximately 26% of their dry matter. The dry-matter loss in the bark pile resulted in a 20 percent decrease in energy content

Dry-matter loss is particularly a problem in chipped material. This is because 1) chipping increases the area of exposed surfaces on which microbial activity can occur, 2) the small particle size gained by chipping restricts air flow and prevents heat dissipation, and 3) chipping releases the soluble contents of plant cells providing microbes with nutrients (Richardson et.al., 2002). Increases in ash content due to dry-matter loss are also higher with chipped material; although, the reasons for this remain unclear (Richardson et. al., 2002).

Piles of wood chips, sawdust and bark are more likely to self-heat with risk of self-ignition at a faster rate because the material has low thermal conductivity (Richardson et. al., 2002). Heat is generated in several different ways. First, parenchyma cells often survive after harvest, continuing respiration which in turn generates heat (Richardson et al., 2002). In addition, microbial metabolic activity generates heat. The heat then becomes trapped because of small air passages. This heat encourages more microbial activity which then generates more heat at higher temperatures.  The self-heating process is slow with only moderate heat generation in piles where particle size and air passages are larger (Richardson et. al., 2002).  The actual temperature of self-ignition varies and is determined by pile size, season, moisture content, and oxygen concentration. Concentration of oxygen in the pile is one of the most important factors influencing self-ignition. At reduced oxygen concentrations the minimum temperature required for self-ignition of wood is increased.  However, self-­heating may occur at oxygen concentrations as low as 4 percent (Feist and others 1971).  Heat generated from microbial activity cannot alone lead to self-ignition. It is suspected that the presence of metals such as copper, manganese, and iron, may be catalysts to such occurrences (Wolfaardt and Rabie, 2003).

Fungi and bacteria begin colonization almost immediately after a pile of woody biomass, whether comminuted or not, is created.  The rate at which this occurs and the types of fungi and bacteria that exist are dependent on moisture content, wood composition, particle size, size and form of pile, as well as storage duration (Richardson et. al., 2002).  Large piles of chipped material tend to have higher growth rates as well as varieties than other material piles do not have.

Molds and actinomycetes (bacteria having a growth patter similar to fungi) produce a large number of microspores during their relatively short life cycle. These microspores along with hyphal fragments are released during handling of the wood chips. The amount of microspores and hyphal fragments released depends on disturbance amount, location, and weather (Richardson et. al., 2002). Exposure to large amounts of airborne microspores and fragments poses a health hazard in humans. Organic dust toxic syndrome (ODTS) and allergic alveolitis are the two most common diseases resulting from microspore and fragment inhalation. Allergic alveolitis is a an immunolgical lung disease and ODTS is a transient febrile condition. The use of a ventilated helmet fitted with a filter effective against particles smaller than 5 µm in diameter when handling stored woody biomass material, especially chips, will prevent exposure (Richardson et.al., 2002).

Forest processing industries generally display strong economies of scale, but with increased size, wood acquisition costs will also rise due to longer hauling distances.  The recent trend towards decreasing transportation costs is pushing the optimum size towards larger and larger units.  It is impossible to optimize the system by optimizing the individual parts separately.  Optimization must be addressed at the system level.

As the biomass harvesting systems change the associated harvesting, processing, delivery, storage costs change too.  These costs become a function of tract size, tree species, volume of wood, distance to the wood-using facility, terrain and other considerations.  Even within the South there is a tremendous variation in all of these factors and harvesting systems must be designed to meet the constraints on a local level.

The two major cost considerations will be examined in the following sections.

• Factors Affecting Harvesting Costs
• Factors Affecting Transportation and Delivery Costs

Transportation of wood fiber accounts for about 25 to 50 percent of the total delivered costs and this is likely to increase as fuel prices escalate (McDonald and others 2001).  This is also true for biomass projects.  Demeter and others (2003) report that for biomass power plants delivery of biomass from the harvesting site to a conversion facility is a significant portion of the materials delivered cost.  Issues discussed in previous sections such as density and moisture content contribute to the delivery costs. Transportation of products generally contributes a significant portion of overall harvest cost variability.  Stokes and others (1993) cited a study that found costs per mile decrease with increasing distance up to about 100 miles, after which they level off.  The decrease was about 40 percent for an increase in distance of 20 to 100 miles.  Beyond a certain distance, transport becomes limiting and its costs become directly proportional to haul lengths.

Bulk vans are generally considered to be the most cost-efficient mode of transporting preprocessed biomass provided the access roads are suitable for these over-the-highway carriers. In less accessible areas, other options such as container trailers should be considered as discussed by Rawlings and others (2004).

Because transport is a high portion of the overall costs, studies have been conducted to increase transport efficiency. Locating, moving, loading, hauling, unloading, and returning transport vehicles are all logistical challenges faced with every load of material removed from the forest.  Coordination is important.  One study in 2001 looked at methods to dispatch trucks based upon different input data, and the associated difference in costs (McDonald and others 2001).  Rummer and Klepac (2003) explored the use of fixed trucks with removable roll-off trucks trailers with pallet racks to supply small-scale users.  Rawlings and others (2004) examined the use of roll-off containers in hauling logging slash from harvesting sites to mills in Montana.

The Forest Residues Transportation Model (FoRTS) is a spreadsheet calculator designed to help users compare alternative methods of moving biomass from the forest to a wood-using facility. The program is available at http://www.srs.fs.usda.gov/forestops/biomass.htm.  It will:

• estimate loading and hauling costs for different combinations of equipment
• evaluate the best mix (numbers and types) of equipment
• compare different hauling routes
• examine reloading, or two-stage hauling opportunities

The Texas Forest Service (2006) developed a case study for biomass logging in East Texas.  This study is focused on a logging business for whole-tree chipping of small diameter trees - between 6-12 years old.  Thinning of these small diameter trees for stand improvement would normally be done at a financial loss, since there is no commercial value for the thinned material.  The need for woody biomass may provide an opportunity to make pre-commercial thinning financially feasible.

Basic assumptions of this case study included a five year length of project, new machinery, 25 percent residual value of equipment, a ten percent interest rate.  To start a biomass logging business from scratch, approximately $755,000 of initial capital investment is necessary.  The machines needed for this project include a feller-buncher, grapple skidder, whole tree chipper, bulldozer, and two service trucks.  The machines are capable of producing 20 green tons of biomass per hour on average.  The total annual production of biomass would be about 48,000 green tons.

A crew of four was needed.  Other operating costs include fuel, lube, repair and maintenance, major parts, insurance, and others.  The annual total operating cost was calculated to be $506,905.  About $250,000 operating capital was considered to be sufficient to cover the project operation.  This meant that the owner of the business may need to obtain approximately $1 million in the beginning of the project to cover the need of capital investment and operating funds.

The annual total cost, obtained by adding annual capital cost and annual operating cost was $669,481, making the per ton logging cost $13.95 and the average hourly cost was $278.95.  Transportation cost was also included by using a prevailing cost of biomass hauling of $0.15 per ton per mile.  For a truck and chip van hauling 25 tons of biomass for 50 miles to the mill, it would cost $187.50, or $7.50 per ton.

For a biomass logging business, the break-even price would be $21.45 per ton delivered.  This price did not include any payment to the landowner.  If payment to the landowner is required, the delivered biomass chip price could be $25/ton or higher.  One limiting factor could be landowner willingness or availability for pre-commercial thinning operations.

In a recent study (Langholtz 2006) about the economic assessment of biomass resources the researchers took into account that costs vary with biomass type, distance, and transportation infrastructure.  When transportation costs are taken into account, more costly resources in close proximity were shown to be economically competitive with cheaper resources farther away, and vice versa.

The study methods included calculation of transportation costs and haul times, determination of physical availability and geographic distribution of biomass, and creation of biomass resource supply curves.  The delivered cost of woody biomass were defined as the sum wood procurement, harvest, and transportation costs.  The authors assumed different harvest costs for different types of biomass resources and calculated transportation cost as a function of haul time.  A geographic information system (GIS) was used for evaluating woodshed procurement areas and transportation costs.

Assessing transportation cost based on haul time rather than distance allowed accounting for site-specific road infrastructure and geographical constraints within a woodshed.  The procurement, harvest, and transportation costs were summed to calculate the total delivered cost of each woody biomass resource within a given haul time.

With the information on quantities, distribution, procurement, harvest, processing, and transport costs for each woody biomass resource, supply curves were constructed.  Typical demand was estimated in the range of 2 to 4 trillion British thermal units (BTUs) to produce approximately 20 to 40 megawatts (MW) of electricity.  For the study area the average cost curve, quantities in this range cost $1.57 to $1.91 per gigajoule (GJ-1) or $1.66 to $2.01 per thousand thousand BTUs (MMBTU-1) which is competitive with current coal energy costs. These supply curves illustrated the local economic availability of woody biomass resources and prices that might be paid as a function of demand.

Harvesting systems used for short rotation woody crops like eucalyptus, willow, and hybrid poplar are very sensitive because fuel prices must be high enough to cover the cost of comminution and transport.  In most cases, harvesting of short rotation woody crops is accomplished with harvesting systems similar to those used in the harvesting of more traditional forest products.  When a viable fuel market exists, system components include feller/bunchers, skidders, delimbers, debarkers, chippers, and chip vans.  Problems occur when a viable fuel market does not exist.  The following studies look at the productivity and cost effectivness of alternative systems.

Hartsough and Cooper (1998) looked at the productivity and cost effectivness of a cut-to-length system comprised of a harvester, forwarder, and chipper.  At costs of $30-35/BDT for short rotation eucalyptus that averaged 6"DBH, it was found that the cut-to-length system could be cost competitive with a more traditional whole tree system which in a previous study (Hartsough et al 1992) was found to cost about $33/BDT for short rotation poplar that averaged 6" DBH.

Spinelli and Hartsough (1999) compared the forwarding of bunched whole trees in a eucalyptus plantation from stump to landing with a front-end loader versus skidding with a grapple skidder.  Results of this study are very promising.  The study showed that front-end loaders as forwarders in SRWCs, because they can pick up more trees in a given pass than skidders, were 40-60% more productive than the grapple skidder.  In addition because front-end loaders can handle both extraction of trees and handling of trees at the landing, only one machine is need for these two activities.  In the skidder system, two machines were needed, one skidder for extraction and one for handling at the landing.  The drawbacks of front-end loaders are that 1) they have higher capital investment requirements; 2) they are heavier; 3) they are not sensitive to soil conditions and 4) they require gentle slopes, even terrain, and small, uniform trees.