Transporting a low value product like forest residuals from a timber harvest site to a processing facility represents a significant economic and logistical challenge for a wood to biofuel supply chain. One way to ensure that transport costs are as low as possible is to maximize the amount of wood residuals carried per trailer load.
Placing ground wood residuals (chips) into a trailer sounds pretty simple — just fill up the trailer to legal weight and go. However, trailers carrying dry wood chips that contain less than 35% moisture content generally do not exceed weight restrictions when filled, which makes the load volume limited. In this case, there is opportunity to carry additional wood residuals per load if the load density can be increased.
In a recent paper published in the Forest Products Journal and funded by NARA, authors Rene Zamora-Cristales, John Sessions, David Smith and Gevan Marrs analyze methods to increase the density of wood chips per trailer load.
Two grades of wood chips considered
Two feedstock size classes were used for this evaluation: Douglas-fir branches-and-tops and larger, thicker pieces commonly called pulpwood. Both feedstocks are commonly found mixed in timber harvest slash piles. Depending on market conditions, pulpwood is selectively removed or excluded from slash piles and used to produce higher value products. The average moisture content for the branches-and-tops was 17.3%, and the bark content was 15.7%. The average moisture content for the pulpwood was 20.12% and the bark content was 4.4%. Both feedstock classes were reduced in size using a horizontal grinder equipped with 7.61 cm screens followed by 10.16 cm screens.
Loading method and bit type
The study evaluated two factors that contribute to wood chip bulk density: loading method and bit type. For loading method, the study blew residual grindings vertically into the trailer at high speed (~120 mph) and compared these results to a more standardized method of loading where grindings are dropped into the trailer from a conveyor belt.
What they found was that the high speed blowing increased the load’s bulk density by a range of 24% to 35% over conventional loading. The variation in density improvement depended on the feedstock and on the bit types used. To put this into economic perspective, the authors calculated that the increased bulk density could cut delivery costs by $8.3 per ton for branches-and-tops and $14.7 for pulpwood on a 200 km trip.
As mentioned, the type of bits used with the grinder could impact load density. Ether carbide hammer bits, knife-edge bits, or a combination of the two were installed on the grinder. According to the authors, “Carbide-coated hammer bits have relatively blunt edges that are highly abrasive and tend to hammer shred the material. Knife-edge bits tend to cut shred the residue using the sharp edge of the bit. Carbide hammer bits tend to produce less dense material compared with knife-edge bits, but they are less susceptible to contaminants.” Turns out that knife-edge bits did improve the bulk density of pulpwood over carbide-coated bits, however, bit selection did not influence the bulk density of branches-and-tops.
As stated in the paper, “Although extensive research was found in the horizontal blowing of chips, few studies were found that applied directly to grinding operations, and no research was found in relation to high speed blowing of grindings in a vertical orientation during loading.”
They conclude their investigation by pointing out that even though the use of high speed blowing can increase load density significantly, other considerations such as ease of unloading and blower costs will need to be considered to evaluate overall cost savings.