In most cases, woody biomass derived from the forest for energy applications today comes from either roundwood timber or forest residues recovered in conjunction with conventional harvesting activities. Certain bioenergy applications, including energy pellets, require or prefer clean fiber feedstock with very low bark content and soil contamination, which results in low ash content of the final product. Also, certain biofuel conversion technologies, specifically certain biochemical platforms, are best adapted to narrowly specified clean fiber feedstocks, often of a single species or species group. When clean fiber is required, conventional harvesting and debarking systems for pulpwood and other small diameter timber are commonly employed. These could include conventional longwood systems for delivering tree-length material to the conversion facility or in-woods chipping operations. In the former case, the timber would typically be debarked and chipped at the conversion facility. In the latter case, debarking would occur in the forest, usually by means of a flail debarking system, close-coupled to the chipper. In this case, clean chips are normally blown directly from the chipper outfeed into a chip van for delivery to the plant.

Logistics associated with utilizing woody biomass from slash, tops, and unmerchantable stem portions produced as a by-product of logging operations depend on the type of harvesting method used. The majority of logging in North America uses ground-based harvesting systems, with a variety of skidder or forwarder types. However, on steep slopes (>40%), cable logging is required. Industrial forest ownerships in the western United States and Canada most commonly require a mix of ground-based and cable logging. The difference in systems has important implications for the cost of extracting woody biomass. In general, cable logging operations are both more expensive and less productive than ground-based logging operations. Landing sizes tend to be smaller due to the steep terrain, and logging roads are more difficult to navigate with conventional chip trailers. In particular, curve radii engineered for conventional log trucks in the western United States may not be suitable for possum-belly chip trailers. A variety of emerging options to productively transport biomass on low volume forest road networks designed for roundwood transport are described in Section 14.7. In this environment, it is rarely cost effective to handle logging residues using cable systems.

14.1.1 Whole Tree Versus Cut-to-Length

As mentioned in the previous section, the distinction between cable and ground-based logging affects the production rate and cost of woody biomass utilization from logging residues. Within ground-based systems, feasibility of biomass extraction, production rates, and costs are further affected by the type of harvest and processing system in use. Whole tree harvesting that involves felling of stems with a feller-buncher, followed by grapple skidding or shovel logging to forward whole trees (including branches and tops) to roadside or a centralized landing, is, by design, paired with a processing method that accumulates loose woody biomass at the roadside. Processing with a grinder or chipper step at a landing or a concentration yard is then required, prior to subsequent transport. By contrast, in cut-to-length harvesting systems, stems are bucked into sawlogs in the woods by a feller — processor that delimbs and tops trees immediately after felling, at the location of the stump. Piled sawlogs are loaded by a log forwarder, which advances them to the landing. This process leaves the majority of logging residue in the woods following the initial harvesting and processing step (Figure 14.2), and thus requires an additional, separate slash bundler, slash forwarder, chipper-forwarder, or other equipment option to collect and move slash to the roadside. If slash is forwarded without processing, or is bundled and compressed for forwarding, it must then be ground or chipped at the roadside, a landing, or a concentration yard before transport. Figure 14.3 shows a small number of the many possible systems and equipment configurations available for moving logging slash from the woods to a conversion facility in whole tree and cut-to-length harvesting operations. From the figure, it is evident that there are various points at which comminution may occur, and the number of pieces of equipment that handle materials along the supply chain can range from very few to very many (Figure 14.4).

When in-woods residues are collected by a self-feeding chipper-forwarder, then, depend­ing on the system, they may be off-loaded directly from the chipper-forwarder to a chip van for subsequent transport. Open top ‘roll-off’ and hook lift containers are another useful option for advancing loose logging residues, either as a forwarder to advance residues to the roadside in cut-to-length operations, or to advance residues from the roadside to cen­tralized concentration depots in whole-tree harvesting. Following a grinding or chipping step at the roadside or concentration depot, chips or hog fuel may be conveyed directly onto a chip trailer for transport to a processing facility (as part of the step, e. g., via equipment outfeed), or it may be piled and loaded at a later time. For example, a large chip trucking contractor in Idaho has developed specialized, large capacity wheel loader buckets for load­ing hog fuel onto chip trucks with higher production rates than could be achieved with a conventional loader.