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?
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.