A Preliminary Assessment of the State of Harvest and Collection Technology for Forest Residues

To meet the 'Twenty in Ten Initiative' goals set in the 2007 State of the Union address, forest resources will be needed as feedstocks for lignocellulosic ethanol production. It has been estimated that 368 million dry tons can be produced annually in the U.S. from logging residues and fuel treatment thinnings. Currently, very little of this woody biomass is used for energy production due to the costs and difficulty in collecting and transporting this material. However, minimizing biomass costs (including harvest, handling, transport, storage, and processing costs) delivered to the refinery is necessary to develop a sustainable cellulosic ethanol industry. Achieving this goal requires a fresh look at conventional timber harvesting operations to identify ways of efficiently integrating energy wood collection and developing cost-effective technologies to harvest small-diameter trees. In conventional whole-tree logging operations, entire trees are felled and skidded from the stump to the landing. The residues (also called slash), consisting of tops and limbs, accumulate at the landing when trees are delimbed. This slash can be ground at the landing with a mobile grinder or transported to another central location with a stationary grinder. The ground material is transported via chip vans, or possibly large roll on/off containers,more » to the user facility. Cut-to-length harvesting systems are gaining popularity in some locations. In these operations, specialized harvesters that can fall, delimb, and cut logs to length are used. The small diameter tops and limbs accumulate along the machine's track. It can be left in the forest to dry or removed soon after harvest while logs are extracted. Removing slash during the same operation as the wood has been shown to be more efficient. However, leaving residue in the forest to dry reduces moisture content, which improves grinder performance, reduces dry matter loss during storage, and inhibits colonization of fungi that produce harmful spores. In recent years, new machines that are specially designed for collection of small diameter wood have been developed in the U.S. and Europe. Residue bundlers and balers improve transportation and handling efficiency by densifying the material and packaging it so that it can be handled with conventional equipment. An experimental integrated harvester/grinder can fall small diameter trees and feed them into a grinder. The ground material is collected in a bin that can be dumped into a chip van. The harvester head is also capable of delimbing and bucking (cut into sections) small timber to be used for pulp and posts. Limitations of these new technologies are their large capital costs and complexity, leading to high maintenance costs and the need for highly trained operators. To ensure that quality feedstock materials consistently enter the mouth of the refinery, the uniform format supply system concept proposes that feedstock diversity be managed at harvest, much like the current grain supply system. This allows for standardization of key infrastructure components and facilitation of a biomass commodity system. Challenges in achieving a uniform woody biomass supply include, but are not limited to, developing machines for efficient harvest of small-diameter trees in a range of topographies and conditions, developing machines and operating plans for grinding biomass as near to the stump as possible, developing cost-effective drying strategies to reduce losses and mold growth during wood chip storage, and quantifying environmental impacts of slash removal and fuel thinnings to aid landowner decisions and policy development. « less