Review of Harvesting Systems

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: