When Life Hands You Green Chips, Make Pellets

Tackling a new pellet mill of any size is a massive undertaking, and the devil is in the details. Even if this isn’t your first rodeo, the considerations are daunting, to say the least. Each improvement over your last build is considered and weighed against cost and timing. It has to produce, but it has to be profitable. It has to fit in the space you procured, and it has to be up and running by third quarter. Steel is up, but your favorite vendors are at least six months out. Thee traffic study is done—you have how many trucks per day? And what about the wood yard?

When the phone isn’t ringing off the hook, you’re making calls and setting up meetings. You turn to the usual suspects and begin to prepare to take quotes on truck dumps, hoppers, drag chains, belt conveyors, stacker reclaimers, silos, green hammer mills, dry hammer mills, dryers, cyclones, surge bins, and of course, the pellet mills themselves. The thought lingers in the back of your mind, “Has anything changed in the past few years?”

“This kind of research can be fruitful,” says Chris Duffy, sales manager at Bruks Siwertell. Acknowledged as a leader in bulk handling, Duffy points out that in certain applications, air-cushion conveying and enclosed conveyors are a functional shift for the better. He also points to their Tubulator, a fully enclosed system eliminating the environmental impact of fugitive dust. Many clients turn to engineering solutions to remediate fugitive dust on older systems. From truck dumps to conveyors to the various systems in the mill, this is a constant battle.

Bob Decker, Advanced Conveyor Technologies, says another emphasis is to avoid drag conveyors in favor of belt conveyors to move the final product. One advantage, he points out, is the ability to forgo dust explosion vents that would be necessary with drag chains. Drag chains are entirely appropriate with the delivered chips, but the industry has moved away from them for pellets. Additionally, with belt conveyors, operators can avoid the pellet degradation issues inherent to drag chains moving finished product. “Ideally, after the cooler, you would use only belt conveyors,” Decker says.

Once operators are confidently moving material toward and through their processes, they should shift gears and focus on sizing, screening and drying. Andy Turner, BM&M Screening Solutions, describes the virtues of a gyratory screen offering full contact for greater efficiency and proper sizing. When sizing the machine, he likes to stay twice as long as wide with the screen. Fines are a significant consideration, meaning the higher fines content of the feedstock, the greater the need for more square footage of screen area, so this must be carefully considered by the project owner and their consultants. “Higher capacity is the hallmark of the gyratory screen,” Turner says. “The gyratory screen’s full contact configuration allows you to make use of the whole screen.”

The chips may be right where they’re wanted, but they’re still the size they were when they came in—and that’s not going to work. Enter Mark Lyman. Lyman is president of West Salem Machinery, and an all-around sizing expert. The company’s Super Shredder was developed to handle high volumes of green wood fiber from diverse biomass feedstocks.  Lyman says the company has come up with a unique concept for high-volume green milling. “Our machine has stood out because it handles high volumes and diverse feedstocks,” he says. “When you’re trying to feed a plant with a million tons of fiber, it increases the ability to handle sustainable biomass materials, including forestry and sawmill residuals, and properly prepares the furnish so downstream equipment can operate to its highest level of performance. It can be a huge advantage, allowing the project owner to go after lower-priced feedstocks. As the industry moves toward those sustainable inputs, it’s important for the machinery to adapt to the broader range of feedstocks.”

Raise your hand if you hate damp chips. I see you there, and so does your pelletizer, so let’s tackle the daunting task of drying those out. “So many considerations go into selecting a dryer, from heating technology to passes to material classification,” says Trevor Frazier of TSI Inc. Frazier’s high-level pass at explaining their offering included a few bits of advice project owners can take to heart. TSI’s dryers feature bolt-together setup so they can be deployed quickly. Additionally, the dryer is single pass. The goal of a single-pass dryer is to create a “blizzard” effect inside the drum with a homogenous shower of chips, across the full width and along the full length of the drum. The gas velocity inside the drum is one-third that of an equivalent triple pass. The chips are nudged forward by the hot gas as they shower, and there is a “classification” effect, with small chips traveling faster than larger or wetter chips. This enables the drum to dry everything to the same end moisture content. Typical moisture tolerance is less than +/- 1% (compared to +/- 3% to 5% in a triple pass)—certainly something to consider when making a choice.

Naturally, every individual mill has a number of unique factors to weigh, so no solution is one-size-fits-all—and there is innovation in space. A good place to start when determining how to outfit your pellet mill is by choosing a good engineering partner—that is a relationship sure to bear fruit.


Author: Aaron Edewards
Business Development Director
Evergreen Engineering Inc.