New Paradigms in Harvesting Woody Biomass for Energy

Image:Bundler_170px.jpgeXtension’s team of wood energy experts discuss new paradigms in harvesting woody biomass for energy. Check it out?

By Charlie Blinn

Because woody biomass as a product is low-value, low density, and irregularly shaped, machinery used to harvest it is expensive to own and operate and transportation is a high percentage of the overall delivered cost. Anyone who is considering harvesting this material should consider the following points:

In-woods Considerations

  • Ensure that your market(s) will demand enough volume throughout the year and for a number of years to make it economical to purchase the necessary equipment.
  • As the end user defines the particle size, shape and quality of the woody biomass, understand those specifications so that any processing equipment meets those requirements. As market specifications will likely change over time, purchase equipment which gives you the flexibility to meet varying requirements.
  • Match the size (diameter), shape (straightness) and amount of soil contamination of your feedstock to the type and size of comminution (chipping or size reduction) equipment and the in-feed on the machinery.
  • Regardless of the cost of the communition equipment, ensure that it is fully-utilized and used efficiently and that the cost of the feedstock is low to achieve a reasonable cost per ton of production.
  • A one-pass system is generally preferred over a two-pass system. Here the biomass is gathered along with the solid wood products like logs and pulpwood.
  • Recovering logging residue in a two-pass system may result in increased contamination of the residue (more soil) when the material is piled and increased entanglement of the materials. It can also result in increased breakage due to the additional handling. This situation may favor use of a tub grinder instead of a chipper (Visser et al., 2009).
  • If a large proportion of the feedstock to be comminuted is comprised of larger diameter sections that are short (e.g., butt cuts), a grinder is preferred. Material could also be bundled instead of comminuted, as long as there enough branches and tops to build tight bundles (Visser et al., 2009).
  • While processing biomass simultaneous to a commercial roundwood harvest may make landing operations more efficient and reduces handling, possible reasons to delay processing of biomass which has been concentrated at one or landings include: chipping becomes more efficient, inconsistent markets don’t require consistent production, some markets pay more for material with reduced moisture content, and limitations of landing space (Robertson et al., 2008).
  • Commercially available biomass round balers are available. They can convert logging slash into bales that can be transported and comminuted by the end-user. A biomass square baler has been developed and studied in the western US. It will offer the same advantages as the round balers.

Transportation Considerations

  • Ensure that one-way haul distances to the wood-using facility are fewer than 100 miles. Shorter distances also minimize the need for a larger fleet of trucks to haul material.
  • Transporting loose residue may be the least expensive alternative when the one-way transportation distance is less than 25 miles (Spinelli et al. 2007).
  • To the extent possible, make sure that transport trailers are as full as legally possible when they leave the in-woods harvest site.
  • Roll-on-roll-off containers are finding their way into biomass transportation systems. Containers can be loaded on forwarders and taken to roadside for loading on trucks for further processing. They allow for gathering of small debris that would otherwise be left within the harvested site.

General Considerations

  • Do not expect to collect lots of logging residues per acre. Following the 80-20 rule-of-thumb, the first 80 percent of residues may be relatively easy and cheap to collect, process, and transport. The more you go after the little pieces, the higher the average cost per ton.
  • Reduce unnecessary operations and limit the handling of materials. Integrate biomass recovery into the merchantable product harvesting operation to gain operational efficiencies. If you try to collect residues from a conventional system after harvest, they will be more costly to recover than residues from a harvest that anticipated residue collection. While it may not take any more time to put all the residues in one place compared to scattered, it makes a huge difference in the cost to further recover biomass.
  • If woody biomass is to be removed using a two-pass system, pile unconsolidated biomass as close as possible to the conventional landing for later processing and transport.
  • While the transportation of biomass which has been air dried to reduce its weight is advantageous because it reduces transportation costs, dry biomass may be more difficult to comminute (chip, grind or shred).
  • If the woody biomass will not be chipped, ground, or shredded in-woods, handling (e.g., loading, subsequent processing) will be more efficient it the material is compressed, bound, and then bundled of stored in a road transport bin (Rummer et al. 2004).
  • Identify the piece of machinery that is most expensive to own and maintain a high level of production for that machine. Make sure that the machinery is designed to handle the volume it is expected to process.
  • If biomass processing is a secondary product of your operation, make sure that it does not interfere with production of the primary product as it could lower overall profitability and productivity.
  • Composite residue logs (CRLs) require new equipment (i.e., a bundler that is either mounted on a forwarder or trailer) but allow transport on conventional log truck and trailer rigs. The receiving facility must have comminution capability.
  • In-woods portable pyrolysis units that convert biomass into bio-oil in the woods are being considered. They represent a real cost-effective opportunity as they result in fuel, and not wood, being transported.