Biomass energy encompasses a wide range of installations, from universities to saw mills to large-scale power plants. Unfortunately, with countless varying site specifics and fuel characteristics, there is no one-size-fits-all solution when it comes to moving and delivering fuel into a boiler. Screw, drag and belt conveyors are well-known designs and commonly applied throughout the industry.
Cost-effective and ideal for handling a wide range of materials from wood chips to sludge, screw conveyors provide enclosed transfers while moving materials horizontally, vertically or at an incline, and can deliver materials from one or more inlet points to one or more discharge points. Most economical for transferring fuel short distances and typically requiring more power than other conveyor types, a screw conveyor operates with rotating shafts attached to a longitudinal axis. “As the shafts turn, the spiral flight is positioned at an angle such that material is forced down the length of the conveyor,” explains Cliff Moss, vice president and director of material handling at ProcessBarron.
A good choice for transporting unscreened or dusty biomass, or where space is limited, drag chain conveyors consist of chains that pull along the length of the conveyor by sprockets. “Attached to the chain or chains are drag flights, that push, or in some cases enhance or support, the material to carry it along,” Moss says, adding that there are some different conveying principles with drag conveyors, one of which uses flights to push wood along. Besides “bulldozing,” drag conveyors can be designed such that flights are placed every five or six feet to push material along, or possess many smaller flights that are placed closer together. “The material sits atop them [flights], and because of the cohesion between them, you can pile the material really high—a foot—and it is carried along at the same speed as the chain. That’s called en masse conveying.”
And nearly as simple as its title, belt conveyors are economical and reliable when needing to transfer biomass over long distances at high capacities. Today’s designs vary from simple, open solutions that are recognized as the conventional belt design, to closed systems that use pressurized air to move fuel.
For Bruks, its nontraditional belt conveyor, the tubulator, has been a great fit at biomass operations requiring long-distance conveying. Pellet plant Green Circle Bio Energy in Florida, Ameresco’s biomass cogeneration plant in South Carolina, We Energies’ biomass power facility in Wisconsin and Mt. Poso Cogeneration in California are all utilizing the technology. “Conventional belt conveyors run atop idlers or rollers, but in this case, the belt is actually supported by air,” says Daniel Pace, sales engineer at Bruks. “The air is blown by fans that are located roughly every 250 feet, and they elevate the belt. It’s still driven by a head and tail pulley like a conventional conveyor, but it’s a lower-horsepower way of doing it.”
A benefit of this design is that it is fully enclosed, alleviating dusting issues. “These are really great when you need to span long distances,” Pace adds. “We’ve had installations go over buildings, roads and bodies of water, and we have cable towers that support these conveyors and can reach as far as 250 feet per cable tower. This is great, opposed to having to have bents every 40 feet with standard conveyors.”
Pace says that the tubulator design is competitive to traditional belt designs, but better priced if dealing with longer distances.
And yet another conveying innovation is being brought to the industry by Mayfran International, manufacturer of a shuffle drive conveyor that was originally designed for the metals industry. Paul Tamlin says he developed the design in 1988 for stamping plants that were all facing similar problems moving scrap metal, and began with a small system that moved a tray back and forth. “This eliminated a lot of parts that existing systems had, so it increased reliability to customers,” he says.
The shuffle drive may be mounted on or located away from the system. Via a connecting rod, it moves the torsion element-supported trays, allowing for a gentle rocking motion that moves material forward.Though the scrap metal industry has kept the company busy, it’s making a push into the biomass industry. “For example, in Canada, a customer with a pelletizing plant—one that takes garbage from Toronto suburbs, sorts out recyclables and puts everything left into a pellet—was considering a system to convey pellets throughout the plant because of the gentle motion of our system. It doesn’t cause the pellets to break down—there’s no degradation, and there’s no exposure to a belt.”
A fit for pellet plants, what about a biomass heating facilities? “We feel it is [a good fit], in a feeder-type situation,” Tamlin says. “It can handle extremely high loads, where a belt conveyor cannot. We have advantages that way, and there is no place for raw material to get within the conveying mechanism itself, depending on the product type. With straw or cane fiber, those all have the opportunities to get caught within belts and pulleys and cause bearing problems and shut down. We describe it as a beltless conveying system.”
The largest unit size Mayfran offers today is 350 feet long—the largest installation has been over 1,000 feet—and maximum system speed is 40 feet per minute. “For biomass that might be lower than what they’re used to, but we can increase the tray size to increase the flow rate,” he says. Mayfran has also designed a vertical conveyor to move product straight up or down in a spiral motion. “Sometimes in biomass or pellet handling, they want to dry the product,” Tamlin says, “so it can also be used as a vertical cooling tower.”
The downside of such a system is that the upfront cost is more than other conveyor designs, he notes. “It might be tougher getting into this industry because of that, but in the long run, there’s no maintenance.”
Besides up-front cost, with the Mayfran shuffle conveyor and so many other options on the market, much has to be taken into consideration when determining the best set up for a respective operation.
The material being handled is an obvious and essential project specific. “The size, type, volumes, and distance fuel will be traveling should be evaluated,” Moss says, as well as the importance of protection of the environment. “Lots of plants don’t like anything exposed to the atmosphere—say, dust, for plants near neighborhoods. When it’s windy, dust can blow off the bark, and belt conveyors are dusty by nature anyway, so one might want to use a fully enclosed system in that case, which may be a drag conveyor. It’s a lot more money, but you’ll have a dust-free environment.”
That applies largely to universities, which are more sensitive to environmental causes. “They won’t want to use any conveyors that aren’t completely enclosed, so traditional, standard belt conveyors don’t work very well because dust can build up and cause a fire. There, using drag chain, enclosed conveyors or screw conveyors would work best; you’ll want to keep any transport ports exposed under a vacuum-type atmosphere where dust is collected at exposure points and run into a bag house or filter.”
University and similar-sized systems are also very compact, so certain conveyors will not work at the high inclines required. “It’s difficult to go from the ground to 60 feet up in the air, 10 feet away, so you get into issues with selecting a system based on elevation,” Moss says.
Pace reiterates that notion. “You definitely need to consider elevations you have to achieve,” he says. “If you’re talking belt vs. chain, you can achieve elevations a lot easier with chain conveyors. When dealing with just biomass, if you have the real estate for a belt conveyor, you might want to go with that. A tubulator would work well there, but it likely comes down to pricing, as it’s more competitive over longer distances.”
On the other hand, power plants require higher fuel volumes and woodyards are far away from the power house. “Boilers are also bigger, so conveyors are longer by consequence of that,” Moss says. “They are less sensitive to environmental issues as well. Long conveyors are much more expensive, and you won’t have the dust-tight, sexy drag conveyors that can be done at a university. Rather, they’ll have belt conveyors that run great distances.”
If a facility is converting to a different fuel—from coal to biomass—there will be big differences with volumetric feed rates. “It takes a lot more wood than coal,” Moss says. “Many times these conveyors can be reused, but overhauled to run at faster speeds to handle more volume.”
Coal is much denser than biomass, so belt width may need to be increased, Pace offers. “You’re definitely going to need more volume.”
While there’s a good possibility a facility can use some of the same equipment, some will need modifications or replacement. “Coal flows out of hoppers and silos very easily, whereas you can’t just pile biomass in a coal bunker—it would not flow out the bottom,” Moss says. “That’s where the size and type of material come into play. You might be able to cut the bottom off of the bunker and put in a drag or screw reclaimer to force the material out, and place the conveyor below the bunker. A belt conveyor or drag could be reused, but overhauled and sped up. You can definitely reuse or retrofit existing equipment without using a whole new system, and we’ve done that over and over with a lot of coal to biomass conversions.”
For other potential scenarios, fuel size is yet another element of the conveyor puzzle—large pieces of fuel coming into an operation prior to being processed, or construction and demolition waste, won’t work on a belt conveyor or standard screw. “If it is big biomass that hasn’t been ground up yet, you can’t just plop it on a belt or screw conveyor, it’ll jam it up,” Moss says. “If it’s really fine, you can’t use a belt conveyor because it’ll blow everywhere. If it’s hog fuel, your options are wide open.”
Finally, climate can play a surprisingly influential role in moving material. “If you’re burning wood pellets, you can’t have those rained on, so the environment can play a big factor,” Moss says.
Intertwine moisture with harsh temperatures, and some conveyors may not be adequate for the job. “Certain [conveyors] have limits of their degree of incline. If you get a frozen belt, it becomes like an ice hockey table, especially when conveying biomass into the boiler. If you’re getting that material from a woodyard, you’ll have frozen material and need conveyors that can handle it. It does change your conveying design criteria if it’s in a frozen climate.”
In such a climate, with a belt design, an appropriate incline would max out at 15 degrees. On a drag, freezing isn’t a problem, so 18 degrees would work, according to Moss. And inclines affect cost. “If we have to get to 80 feet, we can get there a lot quicker on a belt of an incline on 18 degrees and save a lot of money,” he says. “For 14 degrees, a conveyor reaches further back, so that might increase the cost by 20 or 25 percent.”
No matter the project scenario, today’s modern conveying technology suppliers are ready and eager to develop custom, innovative systems based on facility needs. “Give us your worst case conveying situation in your plant,” Tamlin adds. “We’ll focus on that to solve it.” Author: Anna Simet Managing Editor, Biomass Magazine 701-738-4961 [email protected]