Pellet Production: What Type of Conveyors Should You Use?

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North American production of wood pellets has soared over the past decade due to demand for renewable fuels in Europe. Barring the unforeseen, that demand is unlikely to abate anytime soon. Biomass-powered thermal energy plants in Asia are now drawing demand across the Pacific, and some power plants in the United States are cogenerating energy from coal and pellets. The consumer market has likewise bolstered pellet manufacturing as grill enthusiasts have embraced wood pellets as a cooking fuel. Provisions in the 2021 Build Back Better Act[1] (if passed in its current form) may likewise drive forward the pellet industry, as the bill extends tax credits for clean residential-based heating units and other improvements related to energy efficiency through 2032.[2] Biomass heating systems with efficiency ratings of 75 percent or higher are included in these improvements.[3]

Backing these trends is the fact that the BBBA sets aside $10,000,000,000 (yes, that’s 10 billion dollars) to prevent wildfires via thinning. While the funds will certainly be spent inefficiently, $10 billion in thinning efforts will create a lot of waste biomass. One can see how at least some of this material may end up being utilized for pellets.[4]

As the recent failure of Active Energy Group exemplifies, though, there’s still much to be learned in the pellet industry, especially that manufacturers can’t get by with ultra-cheap infrastructure. Biomass is harsh, abrasive, and often acidic. It doesn’t flow well, and is combustible. Handling it effectively therefore requires robust equipment designed for the material, not repurposed grain equipment, not low-bid machinery, and not screws or blowers.

AEG is only the latest casualty of poor choices in facility design. It isn’t alone in having chosen shoddy equipment. It happens all the time because the production of pellets at the scale of today’s industrial plants didn’t occur until recently, and new players regularly enter the market who have no prior experience handling pellets, let alone its feedstock. Thus, engineers and company owners equip these plants with equipment unsuited for the application—something we hope you may avoid.

There’s much to be said about the equipment that goes into a pellet plant, but one article can’t cover everything, of course. So, because we’ve written much about handling biomass (pellet feedstock) in general, we’ll focus this piece on handling the end product itself, pellets.

Pellet Characteristics

Of primary concern in handling pellets is their fragility, a trait they display due to their low moisture content and the “glue” that holds them together: lignin, a rigid polymer that reinforces cell walls in plants. Lignin is all that holds the pellets together (the biomass fibers have been broken down at this point), so pellets are brittle as soon as they leave the pellet mill, especially in a hot state where the lignin has not totally solidified.

The fragile nature of pellets means manufacturers must take care to protect them from shattering or being crushed. To this end, manufacturers can avoid shattering pellets by reducing the height from which they drop the pellets and decreasing the number of transfer points through which the pellets flow. In regard to height, the farther a pellet falls, the more likely it is to break when it hits a hard surface. Silos obviously come to mind when thinking about this issue—after all, pellets have a long way to fall from the infeed chute to the floor. Indeed, pellets are likely to break as they enter storage. Because of this, engineers have at times designed elaborate systems to slow pellets’ descent. However, these concerns are somewhat misplaced, as breakage, though it does occur, isn’t as much of an issue in storage as one might suppose. Once the silo has begun filling, pellets don’t have far to fall, and they strike an angled mound when they fall, which partially deflects the pellets, thereby minimizing damage.

Reducing the number of transfer points similarly affects pellet quality, as each transfer point presents an opportunity for pellets to break as they fall from one machine into another.

Pellet Conveyors

Pneumatic Conveyance

The type of transfer machinery also plays a large role in preserving pellets. Pneumatic conveyance systems are the worst in this regard, as they move the pellets at great speed and slam them against the duct walls at every turn. Then, they eject the pellets at high velocity from the system, and they again slam into something—either a plate or a wall or a floor. At such speeds, striking a pile of pellets causes damage as well.

Bucket Elevators

Centrifugal bucket elevators similarly damage pellets as they toss them against the hood at discharge. If you wish to opt for a bucket elevator, you should choose a model with continuous discharge, which relies on gravity to empty the buckets. You should avoid both centrifugal bucket elevators and pneumatic conveyance systems for pellet transfer.

Screw Conveyors

Screw conveyors are a third system manufacturers should avoid for transporting pellets. Screws are problematic because they crush pellets underneath the flights as they turn. This issue isn’t unique to pellets—it occurs with any material screws carry—but the abrasive characteristics of biomass, pelleted forms included, make this action worse. Not only do the screws break the pellets, but they grind the fines against the trough walls, which forms holes and encourages the formation of rust. Due to these issues, screws should be used to move pellets short distances only.

Drag-Chain Conveyors

Drag-chain conveyors can likewise prove troublesome for pellets because they, too, can grind them up, though the mechanism is different. A conveyor’s design drastically influences this tendency, however. It isn’t a universal flaw, though it does accompany the most common chain conveyor design: a single chain that runs freely down the trough. Because nothing holds down the chains, they and the paddles are able to arch over pellets that lodge beneath them. As they do so, they grind them up. The number of pellets that get damaged when this occurs isn’t trivial—chain conveyors have been known to significantly increase the volume of fines.

Despite this tendency, single-chain drag conveyors are commonly employed to transport pellets. The primary reason for this is that basic models can be made cheap. (These are what you call “run-of-the-mill” conveyors.) The secondary reason is that engineers and owners who select them are usually ignorant of the damage they’ll cause. Other reasons are practical, like the fact that they can elevate pellets and other materials at steeper angles than belt conveyors.

On the other hand, a well-designed drag-chain conveyor can prove a good, if not the best, medium for transporting wood pellets. Again, the specific design of the conveyor is what makes the difference. SMART Conveyors™ from Biomass Engineering & Equipment, for example, are designed with chains that run through channels outside the material path. The channels prevent the chains and paddles from riding over the pellets, the action that damages pellets. Instead, they sweep the pellets through the trough because the chains and paddles are prevented from vertical movement.

Belt Conveyors

As for belt conveyors, they’re an obvious choice for handling pellets gently. And they’re the only practical solution for jobs like loading a sea carrier or transporting pellets long distances (miles or thousands of feet). Their maintenance costs are lower than other types of conveyors, as well, and they are generally reliable. They certainly aren’t the worst option for moving pellets.

But they do come with downsides, which is why they aren’t always selected for handling pellets. Primary among these downsides is that they do a poor job containing dust, which is not an insignificant problem. The National Fire Protection Agency’s standards require dust be contained in applications where it could potentially fuel an explosion. Outdoors, where explosions aren’t a concern, dust poses risks of fires and flash fires. It’s also a general problem in terms of the mess it creates—at a time when labor is in short supply, hours can be spent cleaning up dust.

Also important is the fact that belt conveyors cannot elevate material at steep angles without a sidewall belt, which is an expensive option. Thus, in order to raise material any significant height, belt conveyors must either be very long or zig-zag to the desired elevation. Both solutions are suboptimal and serve to increase construction costs, maintenance costs, and the amount of dust that escapes them.

SMART Conveyors™

Paul Kalil of PK Consulting, who represents BE&E in Canada, is convinced SMART Conveyors™ perform better than belt conveyors in applications involving pellets. His conviction is based on experience. At Pacific BioEnergy, where he formerly worked, they replaced a problematic belt conveyor with a SMART Conveyor™ from BE&E. Since installing BE&E’s conveyor, “They’d not noticed any deterioration or any additional fines being created through their process,” Kalil said. “So right now, we’re confident you can use paddle conveyors to move pellets and not generate any more fines than you would using belts.

“Certainly, the BE&E conveyors are far better from a site, fugitive-dust point of view because these belt conveyors leave fines all over the place,” Kalil said. “It’s difficult to seal up [belt conveyors], and they have carryback. Often, they have to open up the conveyors and let the material fall back, they’re getting so much carryback,” he said.

Other parts of the SMART Conveyor™ design impresses Kalil, too, such as the optional finishes to withstand hot materials and the performance of the plastic paddles. “BE&E’s conveyor have an advantage at the presses, too,” he said. “The pellets coming out of a pellet press are extremely hot if they do not go into a cooler. Even if they do come out of a cooler, they’re 40-50°C. That’s a hot, steamy environment. The ability to go with a galvanized or stainless-steel conveyor is a plus in a highly corrosive environment. The paddles themselves can handle those temperatures easily.”

Of course, we are biased in favor of our conveyor. So don’t take our word for it. Do your research. Talk to pellet manufacturers to learn about their experiences. Talk to our customers to learn theirs. We challenge you to find a better drag-chain conveyor than our SMART Conveyors™. (We’re confident you won’t.) But whatever you end up choosing to handle your pellets, don’t settle for run-of-the-mill conveyors that will eat your profits. If you wish to take full advantage of the market, you need a robust material handling system. To ensure that’s what your plant is supplied with, contact us today.

[1] As of Nov. 23, 2021 the Build Back Better act has passed the U.S. House of Representatives. The Senate is now reviewing the bill. President Biden is expected to sign the bill upon approval and any further changes.

[2] H.R.5376 SEC. 136302 and 136303.

[3] U.S. tax code § 25D(d)(6)

[4] While we don’t put it beyond the government’s genius to transport green wood a hundred miles or more to a pellet plant, in-woods production of pellets, biochar, or other products makes the most sense for this application.