Common Causes of Off-Spec Product: Part 2

Producers who manufacture wood pellets for domestic heating markets, export heating markets or overseas power companies likely intend to achieve certain quality criteria.
By Chris Wiberg | March 25, 2020

Producers who manufacture wood pellets for domestic heating markets, export heating markets or overseas power companies likely intend to achieve certain quality criteria. If all goes as planned, final product verification testing confirms that product is in-spec and life is good. But what happens if intended specifications are not met? In a prior column, I described issues related to moisture, ash and chlorine. Now, I will discuss bulk density, durability, pellet length and heavy metals.

Several factors contribute to the bulk density of the final product. First and most obvious is the die selected for use at a particular production site. Once a die configuration is in place, the bulk density of the finished product should be fairly consistent, as long as other parameters such as material type, moisture content and operating conditions remain consistent. When bulk density begins to drift, it is often the result of one of the other parameters changing, such as an increase in moisture content, which causes bulk density to drop, or that the pellets are becoming too short, which will increase the bulk density. In my experience, it has often been that bulk density issues are blamed on the die (e.g., the effective length needs to be increased or decreased). This may be the case if the die type is not well matched to the application, but if close attention is paid to other easy-to-monitor parameters, simple adjustments may bring bulk density back in line.

Durability is similar to bulk density in that high moisture content will also cause a drop in durability. If both bulk density and durability have decreased, then it is very likely a moisture control issue. Beyond moisture, an observed drop in durability can be related to an upset in process temperature. One example is during pellet mill startup. The first several minutes of production may need to be discarded, as the process temperature is increasing and durability is stabilizing.While this generally results in only a small amount of impacted production, it is common for producers to collect a sample at the beginning of a day or shift. The collected sample shows low durability but may not be representative of production once all has stabilized. If adjustments are made based on this early data, then it could result in overcorrection as the process stabilizes.

Another example of a cause for reduced durability, is a drop in process temperature related to weather. If the pellet presses are in an unheated area, on cold mornings, standard operating conditions may not generate enough heat to overcome the ambient temperatures, thus resulting in insufficient heat for the pellets to sufficiently bind. It is common to have one or two production sites per year experience durability failures due to this type of weather impact.

While length seems the most trivial of all off-spec product issues, it can become a serious compliance issue for wood pellets intended for residential heating markets. Both the Pellet Fuels Institute and the ENplus wood pellet quality certification schemes have strict requirements when it comes to pellet length. The PFI limit is no more than 1% by weight can exceed 1.5 inches in length, and the ENplus limit is no more than 1% by weight can exceed 40 millimeters (mm) in length, but not even a single pellet can exceed 45 mm. Issues with overlength pellets are typically caused by pellet breakers falling out of adjustment, a simple fix. Over time, however, producers can become complacent with pellet length and may not notice that the percent of overlength pellets is gradually creeping up. Once over the limit, it can be days before the problem is noticed, creating a potentially serious compliance issue.

Heavy metals is the final parameter I will address. For export power markets, metals are tested on every shipment. For residential markets, metals are tested annually through the PFI and ENplus wood pellet quality certification schemes. Based on years of monitoring for heavy metals, if the fiber being used consists of virgin wood sources, then there is very little chance of ever seeing metals test above any of the specified limits. Heavy metals are generally the result of chemical treatments or wood that has become contaminated through contact with other materials. To date, when metals are found to be out of spec, it generally isn’t a situation where a producer has knowingly accepted a material stream with chemicals or contaminates, but rather, a fiber supplier has begun delivering these types of materials without the knowledge of the producer. Keeping tight control over suppliers is key to assuring there are no issues with heavy metals.  Let suppliers know what you accept, and even more importantly, what you do not accept. Be strict with your incoming feedstock inspections.

As is apparent with my two-part column, the topic of common causes of off-spec product is quite deep, and I have only relayed the basics. Hopefully, this sheds some light on some of the issues producers may be facing in their facilities.

Author: Chris Wiberg
Manager, Biomass Energy Laboratory
[email protected]