Forest residuals (tree limbs, branches, small trees and unusable trunk bits) come in all different sizes. Before they can be efficiently transported to and processed at a conversion facility for biofuels and co-products, they need to be ground up into small pieces.
To break up the wood material, a diesel-powered grinder is typically used. This machinery forces the forest residual material past bits mounted on a cutting rotor. The bits hammer or cut the wood material into smaller pieces, which are then passed through screens that separate the wood grindings based on size. The choice of bits and screens used can affect the bulk density and particle size distribution of the reduced product. A higher bulk density translates into more forest residual material per truckload, and the particle size distribution can be an important quality characteristic depending on the user.
To help wood processors understand how grinder bit and screen configurations impact forest residual processing and costs, structured tests were performed and funded by the USDA-NIFA (through NARA), and the results were recently published in the Biomass and Bioenergy Journal.
This is the first peer-review study to isolate the effects of grinder bit types and screen sizes on particle distribution, bulk density, bark content and fuel cost when processing forest residuals. The authors evaluated the impact of two bit types (carbide-hammer and knife-edge) and three screen sizes on particle size distribution, bulk density and fuel consumption from three feedstock size classes: branches-and-tops, pulpwood, and butt-log-chunks. These feedstock size classes had a moisture content of 15.3%, 24.8%, and 26.3 % respectively. The screen size combinations tested were small (2-3 inches), medium (3-4 inches) and large (4-5 inches). The tests consisted of six treatments ⎯ knife–edge bits plus a small, medium, or large screen and carbide–hammer bits plus small, medium, or large screen. Each of these treatments were applied to the three feedstock size classes. The study used approximately 180 tonnes of Douglas-fir forest residuals generated four months after timber harvest in western Oregon.
When grinding branches-and-tops, the screen size and bit type selected had little impact on bulk density for the final product. Bit type and screen size did, however, impact the bulk density of grindings derived from pulpwood and butt-log-chunk size classes. In this case, using knife-edge bits and smaller screen sizes provided grindings with the highest bulk density for these two feedstock classes. Knife-edge bits produced grindings that were 16% denser that those produced using carbide-hammer bits. The use of small screens with carbide-hammer or knife-edge bits increase bulk density by 16% and 9% respectively compared to using large screens.
The bulk density of grindings from the three size classes differed with the branches-and-tops grindings reporting the highest bulk density levels.
Particle size distribution
Screen size and bit type impacted particle size distribution for the branches-and-tops plus pulpwood size classes but not the butt-log-chunk size class. Knife-edge bits and smaller screen sizes produced a higher percentage of fine particles. The authors speculate that the increased screen-residual contact provided by smaller screens contributed to producing finer particles. Large screens produced a greater amount of large particles.
Grinder fuel consumption increased as forest residual feedstock size increased; for instance, grinding branches-and-tops sized feedstock consumed 2.1 L t-1 less fuel than for grinding pulpwood and 3.3 L t-1 less fuel that grinding butt-log chunks. Smaller screen size use resulted in higher fuel consumption. For pulpwood and butt-log-chunks size classes, the use of knife-edge bits reduced fuel consumption compared to carbide-hammer bits.
Bark and non-wood substances
The branches-and-tops size class contained 7.5% and 7.9% more bark and other substances than the pulpwood and butt-log-chunks size classes respectfully. The authors point out that the higher surface-volume ratio afforded by the branches-and-tops would partially account for the increased bark and non-wood substance content. The choice of bit type or screen size did not significantly affect bark or non-wood substance content in the grindings.
The results show that the efficiencies attributed to the choice of screens and bits are dependent on the forest residual size class. The results indicate that using knife-edge bits over carbide-hammer bits to reduce forest residuals can result in increased bulk density and lower fuel cost depending on the forest residual size class. The authors point out that knife-edge bits wear more quickly that carbide-hammer bits and that bit replacement costs would need to be considered.
The information provided in the study is being used to develop a techno-economic analysis and life cycle assessment for the conversion of forest residuals into biojet fuel and co-products. The information is also intended to provide wood processors science-based information to help increase efficiencies and improve the economics of processing forest residuals for transport.