The Weight of Biomass Analysis

Biomass quality testing involves a series of meticulous procedures that assure industry confidence.
By Chris Hanson | May 06, 2013

The three grades of pellet classifications issued by the Virginia-based Pellet Fuels Institute––premium, standard and utility––serve to ensure residential and commercial consumers they are getting consistent products. Furthermore, using advanced laboratory testing and quality control of the product could bring more stability and credibility to the biomass industry. “There is a long history of people being able to put whatever they want on the bag, and you’ll see premium as pretty much the predominant grade classification,” says Chris Wiberg, biomass lab manager for Timber Products Inspection Inc. Some producers have even placed super premium and ultrapremium as quality identifiers on their bags, he says, and historically, there has not been a process to verify their claims.

The Pellet Fuel Institute has had standards and listed characteristics of the fuel for many years, but they were never fully enforced, Wiberg says. It is very common for producers to not comply with the grade requirements, and others do not even know they are supposed to test their product. “I had a producer in 2006 …he came up to me after my presentation and said, ‘so what you’re telling me is I should test my product?’” Wiberg recalls. When the producer brought in the bag label, it was stamped as a premium grade. “The bag supplier was just so used to what goes on the bag, and wood is wood is wood,” he says.

Why Test?

The main purpose for biomass testing is to assure the product quality is consistent among producers, Wiberg says. Failing to test biomass could hold potential repercussions for consumers, as well as producers. When producers fail to follow a quality program, he adds, they are doing the industry a disservice because it raises concern about the use of inappropriate materials, and consumers might not have the experience the industry would like them to have. One example he cites is a producer suspected of manufacturing pellets containing blue tarpaulin fragments. 

Biomass testing also aims at building confidence in the industry. When a producer is putting questionable material in a premium pellet, it could result in lower confidence in the system on the part of regulatory bodies. The PFI program is intended to be referenced with the U.S. EPA’s new source performance standard for residential wood heaters, Wiberg says. By following standards built in conjunction with the biomass industry and the EPA, product quality and market confidence can become more established, leading to better consumer experiences, resulting in more product recommendations and consumer support for the pellet industry.

Testing Procedures

To claim a premium grade, the PFI tests biomass materials for bulk density, diameter, pellet durability and other properties, using procedures mostly taken or adapted from ASTM International, formerly the American Society for Testing and Materials. For instance, PFI adapted a protected ASTM test for bulk density of wood pellets by utilizing a minimum, 12-pound sample in a 0.25-cubic-foot container that is tapped 25 times from a distance of 1 inch.

The percentage of fines is determined using a 2.5-pound sample weighed with an analytical balance and recorded to the nearest 0.1 gram. A receiving pan is weighed and recorded and a one-eighth-inch screen is attached. The pellet sample is moved to the screen and tilted 10 times to each side to sift the entire sample. Last, the screen is removed and the weight of the base pan with the fines is recorded. To figure the percentage, the recorded weight of the base pan is subtracted from the combined weight of the base pan with the fines, then divided by the initial sample weight and multiplied by 100.

The National Renewable Energy Lab tests biomass ash content through the use of a muffle furnace. First, specialized bowls, called crucibles, are placed in a muffle furnace for a minimum of four hours at 550 to 600 degrees Celsius. The crucibles are then removed from the furnace, cooled, weighed and placed back in the muffle furnace to determine a constant weight. Next, 0.5 to 2 grams of the test sample is placed in the crucible and placed over an ashing burner until smoke appears. The smoke is ignited and burned until no more smoke or flames appear. The crucible is placed back in the muffle furnace for 18 to 30 hours, protected from drafts to avoid sample loss. The crucible is then placed in a vacuum-sealed enclosure, called a desiccator, cooled for a specific amount of time and weighed with the ash to the nearest milligram to determine the remaining ash.

The gross calorific value of a prepared sample of solid forms of refuse-derived fuel is determined by a bomb calorimeter according to ASTM International. In this process, described by the German-based IKA Group, which manufactures bomb calorimeters, 1 gram of a solid is weighed and placed in a stainless steel container filled with roughly 435 pounds per square inch (PSI) of oxygen. The sample is then ignited using cotton thread connected to an ignition wire inside the decomposition vessel and burned. During the test, the core temperature can reach 1,800 degrees Fahrenheit and pressures of 2,900 PSI for a couple milliseconds. The generated heat is then transferred into an inner vessel filled with water where it can be measured.

The pellet durability index (PDI) recommended by PFI was outlined by Kansas State University. In the PDI test, a 500-gram sample is run through a pellet durability tester—a 1-foot-tall box with a 9-inch baffle that rotates to agitate the pellets. Before being placed in the tester, fines are removed with a one-eighth-inch wire sieve. The sample is tumbled through the durability tester at roughly 50 rotations per minute for a total of 500 rotations. The tumbled sample is rescreened on the one-eighth-inch sieve and the weight is recorded. The PDI is calculated by dividing the whole pellet sample weight by the initial sample weight, and multiplied by 100.

Upon completion of the tests, PFI then classifies the biomass based on the results. To qualify for premium grade for residential and commercial use, pellets need to have a bulk density of 40 to 60 pounds per cubic foot, a diameter of 0.23 to 0.28 inches, length of 1.5 inches with less than 1 percent variation, a PDI greater than or equal to 96.5, fines of 0.5 percent or less, an ash content of 1 percent or less, moisture content of 8 percent or less and chloride content of 300 parts per million or less.

In order to enforce the standards, PFI requires random samples be pulled and tested and says producers must continually test their products. On-site testing may be done, but only if testing will also be completed by an accredited laboratory, at least twice a year. PFI’s quality mark can be used only for products that have met the standards. PFI asks that if producers have other, untested product lines, they not imply the quality mark is all encompassing. This lowers the risk of subpar products reaching the customer and establishes credibility for the product.

Domestic vs. Export

The criteria for pellets for residential and commercial use are the same. While commercial boilers require the same quality standards as residential units, the industrial uses for biomass are much more varied than residential, Wiberg says. Industrial fuel quality specifications are often negotiated between a biomass supplier and a buyer, reflecting the specific requirements for the power plant or other industrial use and incorporating logistics concerns. 

Work is being done in Europe to create new standards based on contract agreements currently in use among industrial users, power plants and pellet producers, according to Wiberg. The establishment of European standards could demand further quality studies of biomass on the industrial level. Timber Product Inspection is currently the only lab in the U.S. to achieve ISO 17025 accreditation by the International Organization for Standardization for the European test method, Wiberg adds. He hopes PFI might adapt ISO testing methods to help create a more international standard.

Author: Chris Hanson
Staff Writer, Biomass Magazine