The Art of Biomass Pelletizing
Pelletizing biomass can be challenging. The lack of a one-size-fits-all process means that it can be more art than science.
The variable and often high moisture content in biomass and its natural decay can lower its value. Fortunately, biomass can be condensed to produce a uniform, competitive fuel product.
One of the methods for condensing biomass is pelletizing. Corn stover, for example, can be made 10 times denser if it is first ground to a five-thirty-seconds-inch particle before pelletizing, according to Alan Doering, associate scientist of coproducts at the Agricultural Utilization Research Institute office in Waseca, Minn.
The most common biomass pelletized for fuel is wood, mainly from sawdust, wood chips and shavings. Eighty wood pellet mills across North America produce 1.1 million tons of pellets annually and 23 fireplace manufacturers make pellet stoves and fireplace inserts for burning pellets, according to the Pellet Fuels Institute, a nonprofit in Arlington, Va., that serves the pellet industry. The institute says 800,000 homes in the U.S. use wood pellets for heat.
Wood pellets are also used on a commercial scale. The institute cites a stage theater, prison and hydroponic tomato farm as examples.
The wood pellet market is larger in Europe where 29 countries consumed 6 million metric tons of pellets in 2007, according to Force Technology, an industrial design company in Brøndby, Denmark. The company says Sweden was the largest consumer, followed by The Netherlands, Belgium, Germany, Austria and France. Sweden was the largest producer of pellets, followed by Germany and Austria. The Netherlands and Belgium were large net importers, while Germany and Austria were significant exporters. Some countries import wood waste for pelletizing.
Because wood pellets compete with fiberboard, particleboard and oriented strand board for raw materials, there have been recent reports of wood pellet shortages in the U.S.
To satisfy demand for pellet fuels, agricultural residues and industrial food byproducts are being pelletized for fuel, although on a much smaller scale. According to Robert Hubener, sales manager for pelletizing equipment supplier Freedom Equipment LLC of Rockford, Ill., more customers are pelletizing products for fuel. "An interesting one is manure mixed in with wood pellets, basically [used] animal bedding," Hubener says. "It's a product that a lot of people [want] to get rid of."
At Colorado Mill Equipment, a Cañon City, Colo., supplier of pelletizing equipment, Marcel Madar, operations manager, says pelletizing operations used to be limited "pretty much to feed," he says, "but now fuel-it's very, very popular." Madar says his company receives two to three samples weekly from companies interested in pelletizing. "It's amazing what kind of materials we get in," he says. "Everybody is trying to make a pellet out of whatever they can get their hands on. We are basically testing them to see how suitable they are for fuels." Madar says his company assisted the U.S. military with pelletizing cafeteria trash-paper, plastics and Styrofoam-to make liquid fuels through pyrolysis.
Agrecol Corp., a Madison, Wis., grower of native plants and seeds, began producing biomass pellets four years ago. According to Mark Doudlah, president of Agrecol, the company began making pellets to deal with the high quantities of MOG (material other than grain) byproduct produced during seed-cleaning. "It didn't belong in a landfill," he says, "and composting is quite messy. Land-spreading for us wasn't a good option, either. We decided, let's densify and burn it." Doudlah says the company modified equipment from a retired feed plant to pelletize MOG, and later biomass from the fields. Agrecol uses the pellets to heat its facility and sells the rest.
Pelletizing new forms of biomass is challenging. "We don't have much hair left, that's for sure," Doudlah says. "It's much more of an art than it is a science."
In a report for the European Biomass Industry Association, French agronomist Olivier Pastré says nonwoody biomass generally has more hemicellulose and less cellulose and lignin than wood, giving it less tensile and compressive strength. In Europe, the Danish Technological Institute has been testing which combinations of biomass are best suited for pellet production and combustion, he says. Doering says AURI has been doing some of the same for its clients.
AURI tests how biomass must be pretreated and milled to produce a high-quality pellet. "We like to see a pellet with a pellet durability index of 92 percent or better," Doering says, which is determined by tumbling pellets for a period of time to find the volume of fines produced. "You can make pellets that are 99 percent durable," he says, "but then you typically sacrifice throughput in terms of tons per hour."
"It's about having that high-gloss sheen on the side of the pellet," Doudlah says, "[so that] when it gets to the end user, it doesn't contain a lot of dust."
Hubener says pellet quality needs can vary. "We have a long list of questions that we ask all of our customers," he says. "Some customers plan to burn all of the fuel they make, in which case a low-quality pellet is perfect for them. [But] if they are planning on shipping it on a barge or shipping it overseas, they are going to have to produce a very hard pellet."
To make pellets, the biomass must first be cleaned to remove contaminants. The clean biomass is then ground in a hammer mill or chipped to a uniform size, which must be less than the thickness of the pellet that will be produced. Grinding down biomass helps to reduce the horsepower the pellet mill must produce. If the biomass is high in moisture, it must be dried to approximately 10 percent moisture content.
While the lignin content in wood is generally enough to bind pellets, other forms of biomass require special conditioning to strengthen them. Sometimes binders such as starch, sugars, paraffin oils, or lignin must be added to make the biomass malleable.
Before pelletizing, the mixture must be conditioned using water of varying temperatures or steam. "Corn stover has a lower glass-transitioning temperature than switchgrass," Doering says. "You can get a very durable corn stover pellet at 165 degrees Fahrenheit, whereas to get an equivalent pellet with switchgrass, you have to obtain temperatures greater than 200 degrees, oftentimes 210 or 220 degrees."
Once conditioned, the biomass is fed to a pellet mill. Inside the mill, rollers extrude the mix through a perforated flat or ring die, which effectively condenses the product into pellet form. The hot pellets must then be cooled to harden. They are then screened to separate residual fines, which can be reused.
Changing the thickness of the pellet mill die and also milling speed, temperature, and pressure are keys to optimizing pelletizing efficiency and pellet quality.
"The dies are all different," Doudlah says. "The rollers that extrude [the biomass] through the die are different, too, and that's important to watch."
Doering agrees. "All feedstocks pellet differently," he says. "They require different-sized pellet dies or die thicknesses. One die doesn't fit all."
Doudlah says because some biomass can be quite abrasive, additional products might be added to increase the service life of a pellet mill. "In any given seed cleaning day or week there might be six to eight different prairie species in that batch," Doudlah says. "We don't regularly pellet things twice the same way. If you can get 2,000 to 2,500 hours out of a die, you're probably doing pretty well." Doudlah says between pelletizing runs, Agrecol flushes the die by pelletizing grains, such as corn. "Those things also lubricate well because of the oils and things," he says.
As more U.S. states and Canadian provinces adopt or increase renewable portfolio standards for electrical utilities, more power companies are looking at burning wood pellets in coal-fired boilers. Ontario Power Generation Inc., based in Toronto, Ontario, has been testing cofiring wood pellets with low-sulfur lignite coal at its Atikokan Generating Station in Atikokan, Ontario, with considerable success.
But utilities are also looking beyond pelletized wood. Vattenfall AB, a European heat and electrical utility based in Stockholm, Sweden, has been renovating its 438-megawatt Amager Power Station in Copenhagen, Denmark, to cofire straw pellets with coal. The plant, which supplies both heat and electricity, burns 70,000 metric tons of pellets with 700,000 metric tons of coal annually. The company expects to increase the pellet portion to 150,000 metric tons this year.
In the U.S., AURI has seen a continual growth in interest from industries for pelletizing nonwoody biomass over the past six years, spurred in part by sporadic increases in wood prices, Doering says. "[They are] looking at pellet fuels to displace natural gas," he says, "or looking at densified solid fuels to cofire with coal. We're working with some utilities, investigating that potential."
"The big demand, as we see it right now, is going to be the electrical utility plants, the coal-burning plants," Doudlah says. "We have been involved in some test burns [with] some power plants and they are trying to see what the emissions are going to be. So far they have come back quite promising."
Pastré notes that the fluidized bed combustion technology used at power plants is inherently flexible and can burn fuels with a wide range of calorific values, ash and moisture content and they have successfully been used to cofire wood, biomass and waste materials, in addition to coal.
Before power companies can use biomass pellets, however, they must address emissions issues. Pastré notes that compared with wood, agricultural residues typically have higher nitrogen, sulfur, chlorine and potassium content due to increased use of fertilizers, pesticides and herbicides in agriculture. He says agro-pellets should primarily be used in large-scale combustion plants equipped with sophisticated combustion control and flue gas cleaning systems. In a report for Pellets Atlas, dubbed pellets@las, an Intelligent Energy Europe funded project for the European Union, Martin Junginger, a researcher at Utrecht University in The Netherlands, notes that unknown emissions from biomass pellets is one of the major factors preventing the development of a larger, nonwoody biomass pellet market.
Doudlah says having a power utility as a client is critical for setting up large-scale pelletizing operations. "I think to get one of these large plants going-to spend between $5 million and $15 million-you need a substantial anchor," he says.
But like many renewable energy pioneers, Doudlah says economics aren't always the main driver. "Our goal at Agrecol is not to see yet another monoculture across millions of acres," he says, "but an ecologically sound and sustainable feed mix that, hopefully, includes enough lignin so that we can wean ourselves away from the amount of nitrogen used in this country."
Ryan C. Christiansen is a Biomass Magazine staff writer. Reach him at firstname.lastname@example.org or (701) 373-8042.