Common Causes of Off-Spec Product

If all goes as planned, final product verification testing confirms that your product is in spec and life is good. But what happens if your product doesn’t hit the intended specifications?
By Chris Wiberg | September 30, 2019

Whether you’re producing wood pellets for domestic heating markets, export heating markets or overseas power companies, you’re likely manufacturing with the intent of hitting certain quality criteria. Monitored parameters often include moisture, ash and calorific value, as well as potentially numerous chemical and physical properties. If all goes as planned, final product verification testing confirms that your product is in spec and life is good. But what happens if your product doesn’t hit the intended specifications? Failure to hit the intended quality criteria can result in a variety of issues ranging from unhappy customers to product rejection. When test parameters fall out of spec, it usually results in a significant investigation to identify the source of the problem and bring the product back to spec. Over the years, I have been involved in seemingly countless off-spec product investigations and thought a good topic for this column would be to share what we have found to be common causes of off-spec product.

I’ll start with moisture content (MC). For most certification schemes and contracts, the requirement for moisture is high enough that wood pellet producers rarely fail the actual MC criteria. However, MC affects several other parameters—most notably, calorific value, bulk density and durability. Most problems occur when moisture gets too high, but other problems can occur if moisture is too low. The best approach is to identify a MC that works for your production situation, and then try to stay as close to that MC as possible (plus or minus 1 percent from your target generally means you have a good level of control). It is also important to identify a not-to-exceed upper limit for your MC. This is often driven by a minimum calorific value requirement or possibly a minimum bulk density requirement. If you have a minimum calorific value limit and some historical data on your product, you can calculate the upper limit of moisture that would put you below the calorific value limit.

Ash content is probably the next biggest driver of off-spec product. Ash limits are often very low, so if the process and materials aren’t kept clean, it is very likely you will fail your ash limit directly. Even if you have a little breathing room with your ash limit, you may find that small increases in ash can result in other issues such as clinkers in a pellet stove or slag in a boiler. When all is clean, most woody biomass materials will perform well. For softwoods, the baseline ash content is around 0.2–0.3%, and for hardwoods, it is around 0.4–0.5%. As your ash content increases above these values, it indicates you are getting dirt or contaminants into your system, which can cause a problem. If you see ash content above what is expected, then it is advisable to investigate the reason, even if you are still within your ash limit, as it could result in other downstream problems. One specific situation last winter was that fiber was harder to come by for many wood pellet producers. As a result, it became tempting for producers to accept higher-ash materials that would not typically be used. We saw several ash exceedances directly related to a tough fiber supply.

Next is chlorine. Pretty much all the certification schemes and export contracts have limits on chlorine content due to its corrosive nature in combustion systems. Chlorine limits are generally around 200 or 300 ppm (0.02–0.03%). The baseline level of chlorine in most wood fiber is around 50 ppm (0.005%), with little chance of exceeding any limits. Imagine the surprise of a wood pellet producer that receives a test report showing high chlorine when they have made no changes to the fiber supply. As for some examples of how chlorine can find a way your fiber, the biggest problem we come across is salt used to melt ice on roads in the winter.  Plows and other snow removal machinery spread various mixtures of sand and salt on roadways to melt snow and ice. As it melts, the cars (and logging trucks) driving on it cause it to become an airborne spray that covers your windshield, as well as your load of logs (or any other uncovered wood materials you may be carrying). Even for closed trucks, the snow that accumulates in the wheel wells and truck body can fall off when on a dump pad, causing a large slug of chlorine contamination into the fiber.

Another situation involved truck drivers that were applying salt to the bed of their truck before filling it with fiber, based on the theory that salt will keep the wet chips from freezing to the bed of the truck. Other sources of chlorine have included sea sprays in coastal regions, using residues from sea water-soaked logs, and in some regions, there are material types that simply contain higher levels of chlorine.

There are many more stories related to numerous other properties, but due to space constraints, I will revisit this  topic in a future column.

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