Metals Testing Questions Answered

In November 2017, the PFI Standards Program was updated to include the testing of the same metals that are referenced in the international pellet specifications published under ISO 17225-2, and referenced in wood pellet quality certification schemes.
By Chris Wiberg | March 24, 2018

In November 2017, the PFI Standards Program was updated to include the testing of metals. The minimum requirements are to conduct a metals test at least annually, using a recently published ISO test method (ISO 16968), and to test for eight specific metals: arsenic, cadmium, chromium, copper, lead, mercury, nickel and zinc. These are the same metals that are referenced in the international pellet specifications published under ISO 17225-2, and referenced in wood pellet quality certification schemes ENplus, CANplus and DINplus. The new metals testing provisions are being implemented in 2018, and have resulted in many questions, including: Why test for these specific metals? What are the baseline levels for these metals in wood? What type of instances could lead to a failure of the metals testing, and why are we adopting ISO test methods? In this column, I will try to answer these questions.

The original list of metals and the test method itself was developed by a European initiative to develop standards for solid biofuels, in the early 2000s. The European Committee for Standardization (CEN) created Technical Committee 335 to spearhead this work, and as part of the initiative, two research projects were funded (BioNorm I and BioNorm II), to answer technical questions about biomass and the appropriateness of various testing methodologies available at that time. The result was information that allowed technical experts to develop specifications and test methods specific to solid biofuels. These were originally published as European National (EN) standards between 2000 and 2008, but subsequently moved to ISO to allow for their development into ISO standards, and to facilitate global adoption. It was this research that led to the adoption of the current testing method, and the focus on the eight metals listed above.

Regarding the development of a specific list of metals, during the BioNorm I and BioNorm II projects, studies were performed to assess metals levels in wood, including baseline levels present in virgin wood, metals levels commonly found in various nonvirgin streams of recycled wood, as well as wood recovered from construction and demolition activities. Additionally, wood treatments were assessed for metal components. As a result, the list of metals provided in ISO 17225-2 were determined to be key indicators of the use of contaminated wood and/or wood that contains chemicals commonly found in wood treatments. 

Regarding the specific limits provided for each metal, the intent of the ISO standard is to provide a limit that is above the highest amount observed in virgin wood. In theory, the metals limits can’t be exceeded unless the feedstock materials used for wood pellet production contain some form of recycled wood that has been contaminated, construction and/or demolition waste, or chemically treated materials. It is noteworthy that the primary research behind the creation of the ISO limits was based on studies of European wood. A review of North American-based studies of metals in wood does provide a basis for comparison. While metals research data on the North American wood basket is not as extensive as the information available in Europe, I will note that the North American data supported the European findings, hence the adoption of the ISO-listed metals and their associated limits.

But that still does not answer the question as to what is normal for virgin wood in the North American wood basket. For this knowledge, I can only speak based on our own experience. Our lab in Conyers, Georgia, regularly tests for metals in wood pellet samples from across North America. Test data is provided to numerous clients, but specific test data is confidential, and can’t be shared in a public format such as this, so I will generalize. From our experience, metals such as chromium, copper, mercury and nickel are rarely found in virgin wood at concentrations above our detectable limits for these metals. Baseline levels of arsenic are typically below our detection limit of 0.04 milligrams (mg) per kilogram (kg), but can be found up to about 0.16 mg/kg. Cadmium typically ranges between 0.04 and 0.12 mg/kg.  In some instances, we have seen cadmium approach 0.3 mg/kg, but not above the limit of 0.5 mg/kg. Lead commonly ranges between 0.05 and 0.7 mg/kg. Zinc ranges between 6 and 13 mg/kg, but we have seen instances where it can be as high as 30 mg/kg, which is still well below the zinc limit of 100 mg/kg. In our experience, these ranges are typical of both hardwoods and softwoods from diverse areas across North America.

The final question is, what could trigger a failure? Essentially, the answer is easy—don’t use contaminated or chemically treated wood. Copper, chrome, arsenic and zinc are readily found in pressure treated lumber (especially copper), so steer clear of residuals that may have pressure-treated wood residuals as part of the mix. Cadmium, lead, nickel, mercury and zinc are largely the result of making use of recycled wood that has been contaminated.

I realize that metals testing is complex, and creates a sense of uneasiness. I can only say that you should have no problem passing these new requirements if you are running a clean operation, and do a good job of screening your suppliers to be sure nothing that can risk failing a metals test is being introduced into the raw material stream.

Author: Chris Wiberg
Manager, Biomass Energy Laboratory