EMISSIONS
Trail consumes approximately 100,000 tons of coal a year and produces 11,000 tons of ash. At Red Trail, fly ash created in the coal boiler is taken through an ash collection system, which works off a set of blowers. “The ash collection system is going to take ash from the boiler itself and from our fly-ash baghouse on this facility,” Thomas says. “The ash is pneumatically conveyed then to a holding silo which has a baghouse on it to clean the air that we are essentially blowing in there. From there, there’s an unloading system which shares the same baghouse as the pneumatic conveying system, so that when we load ash out we put a little bit of negative pressure on the dump spout and control dust through that.” Environmental regulations differ from state to state so some agencies might require fugitive dust testing on the baghouse for the ash silo itself—but not in the largely rural North Dakota area where Red Trail is located. “At this facility, all we are required to do is watch the pressure drop,” he says. “We don’t have a physical test that we have to do.” Not only would this vary from state to state but also within a region where a major emitter may have to meet different regulations than a minor emitter. Graham notes the most common problem attributed to using baghouses as ash control devices: “The high temperatures and periodic cinders from the plant boiler can cause fire problems with baghouses.” Ron Renko, regional sales manager for the Geoenergy Division of AH Lundberg Associates Inc., expresses the same safety concerns to EPM that Graham writes about. “One big concern is fire,” he says. Geoenergy is an emissions controls vendor for biomass-powered plants. Renko says baghouses reside somewhere at the bottom of his fly-ash tech list. Geoenergy Manager Steve Jaasund tells EPM that baghouses just
won’t work for fly-ash control in a plant like the Florida pellet mill. His company supplied the monster wood pellet mill with its wet ESP equipment. “A baghouse is not feasible,” Jaasund says. “The reason is three-fold: First, the gas stream exits the dryer much too close to the water vapor dew point so water condensation would be a big worry. Second, the condensible hydrocarbons (from the drying of wood biomass) at high temperatures will tend to plug the filter media. And finally, the great majority of the particulate— large fiber and condensible hydrocarbons—are combustible.” On safety and the potential for baghouse fires, Thomas says Red Trail has not encountered any problems with its baghouse system. Graham, Renko and Jaasund all mention the possibility of moisture, tars and plugging of the fabric filter media. Thomas says Red Trail has experienced minor interruptions in production due to media clogging, but nothing debilitating. “We’ve had a little bit of that, but what we’ve done is work with the baghouse manufacturer and through some of the controls we’ve been able to work what’s in the PLC [programmable logic controller] and fine tune the cycles of when the bag is cleaned,” he says. “Our baghouse in this facility has six cells and we can run with five of them while one is off-line. If one was off-line, obviously you’re reducing your air capacity and you could see some plants go down because of that, but to date it hasn’t been limiting for us. A lot of that comes into play with the type of fuel being used and its ash content, the different additives used and how those materials are combusted.” Thomas mentions, however, some unforeseen factors which could affect the performance and longevity of baghouses. “The temperature of that flue gas going into it can degrade your bags over time,” he says. Graham agrees. “Periodic bag replacement is a definite operating cost consideration,” he writes. Thus, while