Flexible Biomass Conveyance
The biomass gasifier at Chippewa Valley Ethanol Co. is fuel-flexible by design. Thisrequires a handling system engineered to move feedstocks of varying volumes and densities. Biomass Magazine speaks with Rapat Corp., the engineer of the bulk conveyance system, and equipment vendor Robert White Industries Inc., about the project.
"It throws it on the vendor to size it, dry it, do whatever needs to be done before it comes to the ethanol plant," says Bob White, owner of Robert White Industries Inc.-the integrator for CVEC's handling system from receiving to storage. "Having said that, the ethanol plant doesn't always get what they bargained for. They might say we want all our material to be smaller than 3 inches in length, but sometimes vendors will put material in the middle or bottom of the truck that is 10 or 12 inches long. That can cause problems downstream." White says the unfortunate occurrence of receiving out-of-spec wood on occasion is why his company provided disc screens on the front end of CVEC's biomass handling system, to prevent improperly sized material from reaching the storage silo.
Industrial conveyance specialists Rapat contracted with one company, Robert White Industries, for the receiving system on the front end, and another, Pessco, for the handling system from the storage silo to the plant. Koenig says two different companies were used because each one specialized in different areas. Robert White Industries specializes in mechanical handling systems while Pessco is experienced in pneumatic, or air-driven, material movement.
The rate of unloading in the wood receiving area at CVEC is much faster than the rate at which the woody biomass is fed from the silo to the plant, which is why mechanical means were employed on the front end and pneumatic conveyance in the backend. To truckers hauling in 20 to 25 tons of material in a load, time is money, and therefore haste is necessary in receiving. "Most of the wood comes in on live floor trailers, which are expensive, so the people who own them think that the time spent loading and unloading them is costing them money-they see it as downtime," White tells Biomass Magazine. "The trailers have the ability to unload themselves in 10 minutes, so if we want to unload a trailer in 15 minutes we have to move the material at the rate of about 100 tons an hour." Moving material pneumatically at that rate is simply not practical, which is why CVEC and Rapat chose mechanical conveyers to move the material from receiving to storage.
Receiving and Storage
In addition to wood, CVEC anticipates using corn stover, cobs and other ag residues for gasifier fuel, and as one can imagine there are great material differences between corncobs and stover. Because the system was engineered to gasify various materials, which is facilitated by multi-fuel burners, the biomass handling system was also required to be designed with built-in flexibility. The ability to remain versatile with feedstocks will be important as time goes on, as supplies of one variety or another become scarcer and fluctuate in price. "We designed the capacity for low-density material because there's a lot of volume but not a lot of weight, but designed the horsepower and mechanical capability for high-density material because that's going to demand more power," White says. "So we tried to give them the best of both worlds-heavy enough equipment and horsepower to do the high-density materials but a large enough system to handle the higher volume materials." White says the mechanical system his company designed can move up to 100 tons of
biomass an hour, however, Bill Lee, general manager of CVEC, says the company hasn't had to unload material at quite that high a rate yet.
An alternative to unloading the incoming material straight from the trucks in the receiving area to storage would be to employ what's called a surge bin to receive most or all of a whole semi load, after which the biomass could be metered out slowly over time. "But the price of that would be almost as much, or maybe more, than to just unload and convey straight to the storage silo," White says.
Storage silo capacity on-site is 375 tons, and when that much biomass is being stored there is always the risk for spontaneous combustion, explosion and fire. In fact, not too far from Benson, Minn., where CVEC is located, Central MN Ethanol Co-op experienced an unfortunate incident with its wood storage facility in Little Falls, Minn. When massive amounts of biomass are stored the moisture levels must be kept below or above a certain percentage in order to prevent the conditions conducive for spontaneous combustion. "The primary way we're managing that risk with our feedstock is through moisture control-we have a spec of 20 percent moisture max," Lee says. "Above and beyond that, we monitor the temperature in the silo and we have provisions to re-circulate the silo to kind of turn the pile, if you will. We've even installed some fire suppression in the silo so we've got a pretty good handle on that."
The safety measures engineered into the storage and handling design to help prevent or contain explosion include explosion-proof electrical devices, explosion panels on enclosed conveying, and explosion-suppression systems such as chemical-charged canisters. "If there's an explosion, that sets off the canisters, which dampens the explosion or fire," Koenig says. The silos are equipped with "burst panels" designed to burst and help contain fire when pressure inside the silo exceeds a set limit.
The handling capacity from the silo to the gasifier is considerably less than that needed in receiving. The mechanical receiving system leading to the silo is designed to handle up to 100 tons an hour whereas the pneumatic or air-driven system moving the material from the silo to the plant only needs to move a little more than that in an entire day-approximately 115 tons per day currently, according to Lee. By press time Biomass Magazine was unable to reach anyone from Pessco, the company that provided the handling equipment for taking material from the silo to the plant, but Koenig describes some of the unique and important features Rapat incorporates in its engineering design. "Rapat was one of the first companies to develop enclosed conveyers," he says, which reduces fugitive dust emissions inside the plant. "What that relates to is a safer work environment and less housekeeping issues. There's still a requirement for dust collection on an enclosed conveyer, but the difference is on open conveyers there's lots more fugitive dust in the atmosphere and it accumulates on equipment, which would have to be dealt with in maintenance procedures."
Enclosed conveyers consist of a conveyer belt trapped inside a four-sided enclosure. "The fugitive dust is dealt with using a reloading and self-cleaning system on the bottom where the belt rides on the bottom cover," Koenig says. "The bottom cover has an anti-static liner, and that liner blows the belt with a special rubber wiper that's attached to the leading edge of the splice allowing the belt to ride back on that anti-static bottom. It pulls the material back toward the inlet end of the conveyer and with special devices in the tail section it reloads the material centrifugally to the topside of the belt, so any of the fines or dust will be dealt with by re-circulating them at the tail section of the process."
Lee says the biomass gasification system, including the gasifier, handling and storage equipment, is passed the commissioning period and in routine operation. "Everything is going good," Lee says. "If I had the chance to do it all over again, I can't think of anything I would do differently."
Ron Kotrba is a Biomass Magazine senior writer. Reach him at firstname.lastname@example.org or (701) 738-4942.