Canadian Biomass • Summer 2023 • Equipment spotlight

Making wood pellets is not easy. You need the correct process, consistent material, and reliable equipment among several other factors that are difficult to control. There is nothing you can do to control the weather, but there are a few things that you can control to maintain the lowest operating cost, and highest reliability in your pellet mill. Like anything else, the more care and attention you put into your pellet mills, the healthier they will be.

Pelleting is a constant balancing act to achieve peak performance. There are three keys to squeezing the most life and value out of your equipment:

When you know your equipment inside and out, you know when things are going wrong. Catching problems as soon as possible is critical to prevent larger issues.

Daily inspections allow you to know your pellet mill and address any issues immediately. Walk around the pellet mill each shift, check oil levels, temperatures, and gauges. Listen for anything unusual, and look for anything unusual. Make sure to inspect your dies any time you open the pellet mill. Check the inside of the die for tramp metal; look for impact craters or embedded metal in the holes. It requires less labour to get tramp metal out when it first occurs rather than after it becomes embedded deeper in the holes. Don’t forget to identify and correct the source of the tramp metal as well.

Do you see uneven wear on the die face? Adjust the deflector or wipers to within proper recommended clearance. The longer you allow uneven wear to continue, the shorter the wear life of the die and rollers will be since you eventually will not be able to adjust the rolls properly. A change in material can occasionally create the need to change the die specification in order to solve uneven wear. Also remember that an old die is a history lesson, make sure to check them as well.

Routine maintenance is necessary in our industrial environment. Adjusting rolls is one of the most common things you will do. CPM recommends a skip-touch adjustment of the rollers to the die, where the roller barely skips across the die as it’s rotating to maximize die life. While a tight adjustment of the rollers to the die may be less prone to plug, it does cause accelerated wear and fatigue on the die. Rolls have a harder surface than the die, so the metal-on-metal contact will quickly wear the die, and cause die rollover. Die rollover is when the holes become partially blinded over by metal, which reduces capacity, causes plugging. Enough increased face pressure can lead to die failure.

Die rollover can also be a sign of die movement. As die movement occurs, rolls will contact the die face causing rollover. Changing dies is a great opportunity to do some preventative maintenance on die movement. Clean die clamps and the quill flange, and measure wear on these parts with a wear gauge. Check the die, wear ring, clamps, and die pilot for any shiny areas as this is a warning that something is worn or loose. Check the key and die keyway for signs of hammering, wear, or movement. Appropriate parts should be replaced immediately to prevent continued movement of the die. Coat mating surfaces with anti-seize compound, and use the recommended torque on the bolts or clamps. Make sure to recheck after one shift, and do not overtighten.

Wipers and deflectors are there to protect your equipment, and maintain even wear, extending the life of your parts. Wipers take feed from the rear of the die and spread it back onto the die surface, and help keep material out of the mainshaft seal. A close adjustment will prevent build up and slugs of material coming loose. Slugs of material should be avoided as it can cause plugging and premature parts failure.

Deflectors take material off the feed cone and direct it across the die face. Ideally each roll gets equal feed, resulting in even wear of rolls and smooth operation of pellet mill and pellet mill amperage. When the recommended adjustment can no longer be made to the wipers or deflectors they should be replaced. Wipers and deflector adjustment can often be overlooked, but are critical to maintain consistent process operating conditions. A strong preventative maintenance program will lead to the lowest total cost of ownership.

Regular testing of material before and after pelleting is key to optimize your process and equipment. Check incoming moisture, it should typically be 10 per cent when wood pelleting, and you’ll want to keep it as consistent and in as tight of a range as possible. Feed must be sized properly to meet capacity and quality targets. As the hammers and screens wear on your hammermills they will discharge a larger product, which will negatively affect pellet mill performance.

Regularly check pellet temperature, PDI (Pellet Durability Index), and moisture leaving the cooler. Is something off? The sooner you catch these variances, the sooner you can resolve them. Oftentimes a reduction in capacity or PDI will be the first signs that it’s time to change your die. A new die can quickly pay for itself in the lost capacity that it regains.

You can use your lost capacity to quickly estimate how many hours it will take to pay for the new die.

(New Die Price) / (Pellet Price per Ton x Lost TPH) = # Hours to Pay for New Die

For example, if your normal capacity is 5 TPH, and you are losing five per cent, or 0.25 TPH due to wear, and a new die costs $5,000, while the pellets sell for $200 per ton:

$5,000 / ($200 x 0.25 TPH) = 100 Hours This formula only takes into account lost production and does not consider that labour and electricity costs are still the same at the lower throughput. The new die will pay for itself quickly.

It has been proven time and again that following your manufacturer’s recommended maintenance plans will lead to the most profitable pelleting operation. There will be plenty of factors that cannot be controlled in a pellet mill, but you can control the effective practice of daily inspections, routine maintenance, and material testing. We wish you the best in your operations.

Kyle Hoffman is a biomass sales and applications manager for CPM. Reach him at [email protected], 765-730-6651