Residues to Riches: To Chip or To Grind?

Should one purchase a chipper or a grinder?  This is a very relevant question in the design of an efficient supply chain for forest-origin biomass for two reasons.  The comminution phase determines the product quality, and the equipment is the second most costly component in the system after transportation.

As I mentioned in my previous column, Tools of the Trade, the most suitable equipment for comminuting harvest residues are disc and drum chippers and horizontal grinders.  But why are some suitable to certain applications and not to others?  Why do we see extensive use and so many configurations of drum chippers in Europe, but less so in North America?  Why is whole-tree chipping more common below the border, yet almost nonexistent in Canada?

The main reason is customer need.  Small boilers are common in Europe, but are more finicky in their feedstock needs, and particle size is an important criterion.  Small heating plants are not very common (yet) in Canada, and those in existence have used sawmill waste streams.  With sawmill closures and curtailments, and with fierce demand for the residues from those still in operation, there will be more need for feedstock coming directly from the forest.  Getting back to the comminution stage, a chipper is the most energy efficient machine for producing a finer and more uniform material.  Chippers are also available in different sizes to suit operating chances and volume requirements, so they are the machine of choice for smaller heating plants and lower volume contracts.

Because of the lack of an infeed table and their sensitivity to contaminants, disc chippers are used only for full (whole) trees and larger hardwood tops that have been handled with care.  In these suitable conditions, they produce an even chip quality that is desirable to any heat plant.

Drum chippers are more multi-use.  With an infeed table (the larger the better), large horizontal feed roller, and drum configuration, drum chippers can treat a larger variety of feedstocks, including softwood harvest residues and shorter pieces that disc chippers cannot handle.  They can also produce an even product quality, depending on the incoming material and, in some cases, the use of a screen.  Most chippers also have a blower, which makes them more suitable for on-road operation.  Blowing the material can also increase the payload in the chip van.

The main drawback with chippers is contamination.  Drum chippers are less sensitive than discs, but they are not suitable to medium or high levels of contamination, including sand and mud, and of course, rocks and metal.  Horizontal grinders are therefore the choice for contractors that have no control over how the feedstock has been handled before the comminution stage.  Horizontal grinders can handle a variety of materials from full trees and harvest residues to mill, sort yard, and woody municipal wastes.  Thus, they can be used in the woods and to clean up mill yards during spring breakup or wet fall periods.

When the customer is a larger heat plant with a boiler that can handle larger and less uniform particle sizes, a grinder is a suitable comminution machine.  With variable and lower quality (contaminated) material as the feedstock, larger (hog fuel) screens permit high throughput, leading to lower overall costs.  Screens can be changed to make smaller particles, but it is not an efficient machine to produce small particles just for a heating market.  Horizontal grinders, by their size, configuration, and discharge conveyor, require more room to operate and are more difficult to transport from one site to another.  These factors and their need to operate perpendicular to the trailer mean that they are not suitable for small operating chances and difficult terrain conditions that limit off-road mobility.

In summary, there is no one-machine-fits-all solution.  The choice is determined by a variety of factors: customer needs, form of material, level of feedstock contamination, operating conditions, size of contract, available capital, and regrinding (hogging) capabilities at the plant.

Is there a new hybrid solution?  With the scarcity of sawmill residues and new opportunities for pellets, there is a need for a smaller chip size produced from roundwood in the bush.  Two years ago, one grinder manufacturer produced a conversion kit to change a horizontal grinder into a large drum chipper.  The solution was a replaceable drum with a two- or four-pocket knife configuration.  Over the last month, at least two other grinder manufacturers have announced their solutions and conversion kits to do the same thing.  This is good news for a user that has a variety of customers with different feedstock needs and changing operating conditions. •

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