By R. Schroeder, B. Jackson, and S. Ashton
A major challenge associated with transporting woody biomass for energy and bio-based products is that woody biomass in its raw form -- slash, un-delimbed small trees and tree sections -- has a low-bulk density. Low-bulk density increases the cost of transportation because air is a major component of the transported volume. In addition, the complex texture of the material makes handling technically difficult. Bulk density may be increased and the problems associated with the material’s texture reduced by compaction or by comminution via chipping, grinding or shredding. Processing feedstock into small pieces may however introduce new problems by decreasing durability and longevity during storage. Transportation of woody biomass feedstock is a key element of forest- derived product activities. The way they are organized can have implications for the production system as a whole.
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 facility 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 preprocessed at the logging site determines the transportation configuration used.
Most forestry products and harvesting material are transported by truck. About 80 percent of the pulpwood delivered to U.S. mills in 1996 arrived by truck (McDonald et. al., 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 et al, 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-pound gross vehicle weight (GVW) road tractor-trailer combinations. These utilize standard highway road tractors, six- to 10-wheel tandem axle highway trucks with either a conventional engine in front of the compartment or a cab-over-engine design. Typical road tractors weigh about 12,000 to 20,000 pounds 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 feet in length, and the payload capacity is less than that ofroad 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 truck 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.
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 that of 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 United States. 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.
Vans are enclosed box trailers generally 8 to 8.5 feet in width and designed to be less than 12.5 feet in height when pulled by a road tractor. Vans and flatbed trailers are used to transport most highway cargo throughout the United States. The difference between the box trailers seen on most highways and bulk vans hauling harvesting products 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 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 reducing or eliminating flying material while in transit.
Bulk vans used in the South are mostly for maximum capacity of 80,000 pounds, 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 pounds. 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. However,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 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).
Woody biomass feedstock can be delivered to wood-using facilities by truck or rail. Rail can be used to transport manufacturing residue feedstock to a facility 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 preprocessed at the logging site determines the transportation configuration used.