Pellet Specification & Method Comparisons
Recently, I was asked to compare the methods and specifications referenced in the PFI standards with the seemingly similar ISO 17225-2 standard.
Bear in mind that the PFI standards were developed for the North American wood pellet industry, while in most cases, the newly published ISO standards closely resemble former EN standards, which were written for the European markets. ENplus and CANplus now reference the specifications for quality classes A1, A2 and B, as outlined in ISO 17225-2, but producers primarily manufacture “A1 grade.”
Also, while the PFI standards provide criteria for premium, standard and utility grades, the vast majority of producers manufacture premium grade. This exercise compares the requirements of PFI’s premium grade with ISO 17225-2 A1 grade.
PFI specifications allow a bulk density range of 40 to 48 pounds per cubic foot, while ISO 17225-2 references a range of 600 to 750 kilograms (kg) per cubic meter. (37.5 to 46.8 pounds per cubic foot). The test methods are different in that they use different-sized containers, different methods of compaction and different pour heights. In addition to these differences, both methods inherently have a large degree of variability as a result of the test being dependent on individual technique. Despite all of these differences and the inherent variability, the two methods do seem to generate similar results.
PFI’s diameter range is 0.230 to 0.285 inches (5.84 to 7.24 millimeters (mm). This is with the understanding that U.S. producers predominantly use a one-quarter-inch die and some slightly larger die sizes. ISO 17225-2 requires that producers declare 6 or 8 mm, each with a tolerance plus or minus 1 mm, allowing for a potential range of 5 to 9 mm (0.197 to 0.354 inches). Given that the 6 mm diameter most closely resembles the customary one-quarter-inch (6.35 mm) die size, it would be expected that producers would declare 6 mm. It is uncertain as to how the 8 mm diameter product would affect stove performance. Both test methods use calipers to measure the diameter where the mean value is reported.
For durability, the PFI method follows the tumbler method, where the chamber dimensions are 12 inches by 12 inches by 5.5 inches (305 mm by 305 mm by 140 mm). The ISO method uses a similar tumbler that is just slightly smaller (300 mm by 300 mm by 120 mm). I have not found the differences in the box dimensions to cause a significant difference in test results, but in theory, the slightly larger box could suggest a slightly more aggressive test for the PFI method.
PFI defines fines as material passing through a one-eighth-inch wire mesh screen (3.175-mm square hole). For ISO 17225-2, fines are defined as material passing through a 3.15-mm round hole screen. Even though the screen dimensions 3.175 and 3.15 seem similar, because the PFI screen has square holes and the ISO screen has round holes, the difference in aperture size is about 30 percent. As such, the PFI test classifies a larger portion of the material as fines making it harder to pass the PFI fines test, despite having a comparable fines requirement for ISO (both reference a fines limit of 0.5 percent for bagged material). In addition, this causes the durability test result to be approximately 0.7 lower when tested via the PFI method.
For ash content, both PFI and ISO use fairly similar temperatures for ashing, 580 to 600 degrees Celsius for PFI, and 550 C for ISO. I have not seen a significant difference between these temperatures, and I consider these two methods to deliver comparable results. The PFI limit for ash is 1 percent, and the ISO 17225-2 limit for ash is 0.7 percent.
Regarding length, PFI does not allow more than 1 percent to be longer than 1.5 inches (38.1 mm), while ISO does not allow more than 1 percent to be longer than 40 mm (1.57 inches) and no pellets longer than 45 mm. When comparing 38.1 mm 40 mm, the PFI test is more rigorous, however, the ISO specification that no pellet can be longer than 45 mm can make the ISO specifications more rigorous. For the test method, the PFI test is more thorough, in that the test is performed on a minimum sample size of 2.5 pounds (1,134 grams) while the ISO test is performed on 30 to 40 grams.
PFI and ISO use calorimeter methods for determining the heating value, and both referenced tests yield comparable results direct from the instrument. For ISO 17225-2, however, the specified limit for energy content is expressed as the net calorific value, also referred to as lower heating value. For PFI, the heating value is expressed as the gross calorific value, or higher heating value (HHV). These parameters are not directly comparable. ISO provides a limit that the A1 pellets need to be greater than or equal to 4.6 kilowatt-hour per kg (equivalent to 7119 Btu per pound). The PFI Standard requires the producer to disclose the minimum HHV as-received.
The ISO method for chlorine references ion chromatography as the primary method, but has language for allowing several direct analysis techniques. PFI lists several accepted methods. All differ in their detection limits and instrumentation required. PFI’s limit for chlorine is 300 milligrams (mg), per kilogram (kg) and the ISO requirement is 200 mg per kg.
PFI does not currently have metals listed in its standard, and no test method is specified. ISO has limits for eight metals, and references an ISO test method for analyzing metals. ISO 17225-2 also lists requirements for several additional parameters not included in the PFI standards, including deformation temperature, nitrogen and sulfur.
While the PFI and ISO standards seem very similar in many ways, it is important to note the often subtle differences in the specifications and the referenced test methods, as PFI and ISO are not always comparable.
Author: Chris Wiberg
Manager, Biomass Energy Laboratory