LOGISTICS¦ fuel have different fuel handling systems and somewhat different boilers so it can sit out in the snow and rain in those situations, but when you’re dealing with a processed biomass fuel, it has to be stored.” Adding a new biomass storage system to an existing coal plant can be costly. “We’ve seen fuel handling yards as cheap as $12 million, to all the way up to $50 million,” Browers says. “It’s a very expensive part of burning biomass; using biomass as you get it doesn’t work everywhere, and there isn’t always an active forest products residuals industry. The whole volume of storage becomes a critical driver. It’s a huge issue.” Since the biomass is used so quickly at Minnesota Power, potential storage issues such as molding aren’t relevant. At other large-scale cofiring operations, which are common in the U.K., molding and caking are potential problems when the biomass and coal are mixed and stored together.
THE POWER AND
PERFORMANCE THAT ONLY THE
BEAST
®
CAN PROVIDE... ON SITE...
Minimizing Mold and Dust “When you have a mix of biomass and coal, the water transferred from the coal to the biomass often causes it to go moldy,” says Mike Bradley, director and professor of bulk and particulate technology at the Wolfson Centre for Bulk Solids Handling Technology on the Medway campus of the University of Greenwich in the U.K. The mold growth gives strength to the material, which is referred to as caking. “You tend to get hard cakes in nonmoving areas of bunkers and silos, and this prevents flow,” Bradley says. Likewise, water ingress into the storage area can cause mold growth and caking. “The best way to prevent this is to ensure that the bunker or other storage vessel discharges in a ‘first-in, firstout’ pattern, so the oldest material is used first and any discharge of material disturbs everything above it,” Bradley says. “We call this mass flow, which means good stock rotation. It prevents areas of long-term static residence.” Many biomass materials that have high moisture content will mold and ferment by themselves in storage, so they must be used in strict rotation to prevent self-ignition, much like a wet haystack can do, Bradley adds. The same is true for coal. “With direct cofiring the coal is pretty hands off,” Polzin says. “The only problem with coal is that if we get wet coal and leave it in the storage bins for too long, it’ll self-combust. We have to make sure that if we’re having an outage we don’t put too much in the storage silos. Managing your coal inventory is critical.” Usually, mold growth that leads to caking takes about two days to become significant, but it depends on the temperature, according to Bradley. “Warm, moist conditions accelerate it; cold conditions retard it. It also depends very much on the particular biomass—some do not cake readily, others cake very easily and quickly.” In contrast to moist conditions creating mold, dust is also a potential issue at cofiring operations. “Dust is an issue because biomass has a much greater capacity to suck up water than coal,” Bradley explains. “When you mix
...OR IN THE FIELD!
BIOSAWDUST: BIOSAWDUST:
PERFECT FOR CO-FIRING WITH COAL
ONLY FROM BEAST® RECYCLERS
Experience for yourself the economic benefits that the Beast® can bring. Our equipment can accurately size material to move most efficiently through whatever biomass process your operation uses! When designing your facility, it would be a mistake not to put the Beast® to work on-site sizing your material. Eliminate frayed edges on your material to ensure smooth passage through your plant, thanks to the cutting action of the Beast®. Whether chipping in the field or stationary grinding on-site, the Beast® produces a uniform, dimensional end-product in a single pass. When it comes to the sizing your material, leave it to the Beast®! Call today to learn more.
6750 Millbrook Road • Remus, MI 49340 • USA FIND US ONLINE
1-800-952-0178 SEPTEMBER 2011 | BIOMASS POWER & THERMAL 37