Residues to Riches: Tools of the Trade

There are three basic methods for handling forest residues: transportation of loose residues, compaction, or comminution.  For the most part, transporting residues in a loose form is not practical or economical.  The payload is very low, restricting their economical transport to very short haul distances.  Loading, handling, and transporting loose debris is also problematic, requiring special trucks and unloading facilities.  A number of companies are starting to ship full trees or roundwood as an energy source.  In this case, conventional equipment is used, but full trees can raise safety concerns because of branches sticking out beyond the trailer.  In trials, the wood was sent to a central landing to be chipped, so all transport was off-highway.  Should highway travel be necessary, a walled trailer should be used.

Apart from bundling equipment, the art of compacting forest residues is not well developed.  Bundlers were originally developed in Sweden, but their application has been more widespread in Finland.  They are used to produce round bales, called compacted energy logs, from limbs and tops.  At one time, there were a number of manufacturers, but today there are only the John Deere Energy Wood Harvester and the ABAB Rogbico.  Forwarder-mounted bundlers were developed for recovering residues from cut-to-length harvesting systems; however, the Rogbico is truck mounted, designed for producing large energy logs from residues placed at the roadside.  Bundling improves the payload, but a comminution stage is still required at a central terminal or energy plant.  There are no bundling systems in use in Canada at this time, and the place for bundlers in Canadian supply systems will be discussed in a future Residues to Riches article.

Comminution, which is just a fancy word for particle-size reduction, is the critical stage in the design of an efficient supply chain for harvest residues.  It is the chipping, grinding, hogging, or shredding phase done in the woods, with the purpose of increasing the payload in the van and facilitating the handling of material further down the chain.

As I stated in my previous column, the supply chain begins with the customer, in this case, an energy plant.  The comminution method plays an important role in meeting the feedstock quality demands of the conversion facility, and in many cases, it will determine the final product quality, as there may be no further re-hogging capacity in the system.  Smaller- to medium-sized heat plants are more susceptible to variations in feedstock particle size, resulting in a narrow allowance for delivered particle size.  Even at large combined heat and power facilities, there may be other physical limitations to the feedstock because of the types of feeding devices that are used, such as augers.

A few years ago, the choices in comminution equipment were somewhat limited, but recent development and refinement of horizontal grinders and the emergence of more drum chippers in North America is opening up more avenues for tailoring the feedstock quality to the needs of the energy plant.  With greater choice, however, comes a greater risk of making the wrong choice.

Comminution equipment includes high- and low-speed shredders, tub and horizontal grinders, and disc and drum chippers.  All have a place for treating woody residues, but some are more suitable to forest conditions and for harvest residues than others.
High-torque, slow-rotation shredders are common, especially in larger communities, where they are used to process municipal debris.  They can treat highly contaminated debris, but they also produce large oversized pieces not readily suitable for burning.  Tub grinders are also used in many applications, but again, they are more suitable for municipal debris and for producing mulch from bark.  However, they are not suitable for handling long pieces and tops.  High-speed shredders are not common, and their use is limited to specialized applications such as producing mulch.  This leaves three main equipment types: disc chippers, drum chippers, and horizontal grinders.

The choice of these tools and their specific configuration will be determined by the customers’ needs, the forest conditions, the nature of the material to be treated, and how well the residues were handled before the comminution stage, i.e., the level of contamination.

This stage is one of the most critical in designing an efficient supply system.  I will examine these equipment choices in more detail in my next column, called “To chip or to grind?” •

Mark Ryans is with FPInnovations–Feric Division and can be reached at mark.ryans@fpinnovations.ca.