Biomass and Coal: A Powerful Combination
When Minnesota Power began cofiring coal and biomass at its power plants in Duluth and Grand Rapids, Minn., more than 20 years ago, they started out combusting 75-25 coal to waste wood ratios.
Back then, biomass was plentiful and cheap and it was common to get it for free, says Mike Polzin, renewable fuels coordinator for Minnesota Power. “We started burning wood in Duluth only because a new paper mill had bark they had to get rid of and they didn’t know what to do with it,” he says.
That has changed significantly as the years have gone by, as biomass has become a hot commodity. Competition for material has become stiff and in many cases can be pricey. Still, certain incentives that have come into play during that time—such as state renewable energy credits—make it enticing for power utilities such as Minnesota Power to use more biomass.
And, that’s precisely what the utility has done and continues to do. Today, the facility in Grand Rapids produces about 25 megawatts (MW) of electricity from a 15-85 coal to biomass ratio, and the Duluth facility typically generates about 50 MW from a 10-90 coal to biomass ratio. Using that much biomass at facilities that were meant for coal hasn’t come without its challenges, Polzin says, the biggest of which are related to materials handling and storage.
While it won’t be the case at every facility, so far, there have been ways around these challenges.
Biomass can be bulky and requires a lot of space, but storing it isn’t as simple as just finding more space—it must be kept dry, hence requiring some type of covered storage system. “The biggest challenge has been on-site storage,” Polzin says. “We just don’t have enough of it—we’re at 90 percent biomass at Duluth, but site storage is much too small.”
That necessitates deliveries of biomass seven days a week, a process which Polzin is charged with facilitating. “We have about 900 tons of on-site storage, but we burn over 1,000 tons of biomass each day,” he explains. “So it’s there only for a number of hours. We dump it into a large building then pull it back out to be used; all the wood that comes in and gets stored is gone in 24 hours or less.”
Getting the appropriate amount of deliveries each day is where the challenge lies, as the weather can create havoc on timber harvesting. “Significant snow or rain handicaps loggers from getting into the woods and bringing materials to us, and spring road restrictions can also create problems,” Polzin says. When there isn’t enough stored fuel, it could mean a shortage.
To mitigate that problem, Minnesota Power has 50 possible suppliers, and requires only two to three truckloads per day from a given supplier. “They have the potential to put out five or six [truckloads], but if we can get two or three from several suppliers, we make sure we can always get enough to satisfy our appetite for wood,” Polzin says. “We suggest that our suppliers have more than one market to deliver to, because inevitably we will have to go off-line due to plant outages, equipment failures and things like that. That way, they can reroute deliveries to another market. If those other markets have outages, they can deliver more to us.”
However, if a situation does arise where they can't secure enough wood—and it does happen—it’s just a matter of calling for more coal. “[Using more coal] doesn’t create any problems with our infrastructure,” Polzin says. “There are large coal suppliers close to both facilities, so we can get more coal within two hours, to keep us online.”
Bruce Browers, senior engineer at Barr Engineering, agrees that storage is one of the biggest challenges in cofiring. Last summer, Barr completed biomass cube test burns at Wyandotte Municipal Services in Wyandotte, Mich., and says if the company were to cofire biomass for the long-term, the current storage system would have to be significantly modified. “[Biomass] doesn’t stand up to moisture,” says Browers, who, coincidentally, was a project manager at Minnesota Power when its plants were built in the mid-1980s. “Pulp and paper guys who burn waste biomass 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.
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, first-out’ 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 muchon 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 coal—typically wet on the surface at 6 to 10 percent moisture—with biomass, which is often 10 percent water, the biomass sucks in all the water and that leaves the coal dry and dusty. Hence, you get much more coal dust emissions when you cohandle.”
In addition, biomass has its own dust content, so even materials that have low dust tend to break down in handling and produce dust, Bradley says. “Both coal and biomass dusts are a serious explosion hazard. With the dust on cohandled coal and biomass being more mobile than on coal alone, it can easily create [hazardous] conditions in a plant.”
These types of explosions occur when a mix of combustible dust and air are ignited by a hot surface or spark. “Usually this happens locally at first, typically due to an overheating bearing, a static discharge or someone doing some grinding or welding,” Bradley explains. “The draught from that small initial explosion whips accumulated dust up off the floor and other flat surfaces, creating an explosive mix of dust and air—then a fireball sweeps through the whole building in seconds, devastating all before it.”
Polzin agrees that dust can create a hazardous situation. At Minnesota Power’s Duluth facility, all of the conveyors are covered with metal, horseshoe-shaped hoods so dust cannot be blown off the conveyors. “The storage facility is completely closed, so we don’t get a lot of fugitive dust blowing around,” he says. “We have a large truck dump where, when a surge of wood is dumped from the truck into the bin or receiving hopper, it creates a dust plume, so we have some vacuums to help mitigate or minimize that.”
Additionally, the company’s operating permit requires dust minimization in the entire area of the plant, so parking lots and roads are paved, and a street sweeper is used to sweep up loose dust and wood chips. The situation is similar at the Grand Rapids plant. “We’re sensitive to dust blowing around; we don’t like a lot of fines,” Polzin says. “The reason we have a hog fuel grinder at each facility is because we’d rather have material come to us at a larger size initially, so it doesn’t blow around.”
Once biomass is ready to enter the plant, another slew of potential problems can surface, including fuel feed system issues.
Other Potential Issues
Biomass won’t always flow smoothly through a feeding system meant for coal for a number of reasons, one of which is winter weather conditions. “On the biomass side, [frozen] wood going through our chutes and storage system is a big problem because it tends to bridge and plug up the system because in the winter it often comes in frozen chunks,” Polzin says.
Plant attendants usually realize that not enough wood is getting into the boiler when the heat rate can’t be sustained, or bin indicators sound an alarm. “We simply have to go out there and ram the wood through,” he says. “We have customized some hatch doors that we open, and then we poke and prod the wood with long rods until it is muscled through.”
At Wyandotte in Michigan, which has a 25-MW circulating fluidized bed (CFB) system designed to burn coal and tire-derived fuel (TDF), Browers says the plan going into the test burns was to do 30 and 60 percent biomass ratios. Due to the limestone feeder capabilities, however, the tests had to be modified to 15 percent. “[Running the biomass through the limestone feeder] wasn’t the best way we could do the test burns, but it was about the only way,” Browers says. Since there was no way to get the desired blend out in the coal yard, the only way get it was to let the fuel feeders take the coal and TDF, and put the biomass fuel in the limestone feeders. “The measuring device coming out of the feeders allows us to control the blend.”
A 60 percent blend would have worked fine in the boiler, he adds, but it wasn’t feasible because of the delivery issues with the limestone system.
Another issue Browers says that needs attention are potential sparklers. “Sparklers are what happen when you burn biomass in a CFB or stoker,” he explains. “There are a lot of particles to it, and they don’t always combust where they are supposed to. They can carry over from the bed into the convection pass, and you don’t want combustion in there.”
Attendants must physically open the boiler door to check for sparklers. When they do occur, it indicates a couple of problems—inefficient combustion, and carry-over of carbon particles. “Then you have to worry about fires,” Browers says. “There isn’t enough to explode, but as it falls out in your duct work or gets removed in your particulate removal device, you’ve got carbon particles in there and they’re susceptible to fires.”
Overall, Browers says that cofiring isn’t all that simple. “People who think it’s easy to chip raw biomass and run it through their coal system may want to rethink that assumption,” he says.
In fact, it can take a long time to work out the kinks. Even at Minnesota Power, which has been cofiring for decades, the company is working to optimize its cofiring operations, according to Polzin. In an upcoming project that will take place over the next couple of years, the company will put in new feeders, more robust material handling systems and more storage, though biomass deliveries will have to be more coordinated than ever with the near doubling of biomass consumed at each plant.
Unlike the U.K., cofiring biomass with coal in the U.S. is still largely in the testing phases, and many utilities that have been interested in cofiring in the past have slowed down plans due to regulatory uncertainties surrounding biomass. “More incentives are needed,” Browers adds. “There is a lot of talk here about cofiring, but if you look at who has actually done it, there aren’t many.”
Author: Anna Austin
Associate Editor, Biomass Power & Thermal