“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.
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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.”
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