An Unconventional Pellet Feedstock
Cattails are extremely fast-growing and competitive plants. In lakes everywhere, they are displacing native plant populations that support wildlife habitat and prevent erosion, in addition to hindering swimming and boating activities in recreational areas.
Though generally considered a nuisance, cattails do provide a benefit to the health of a watershed by filtering out excessive toxins and nutrients, such as phosphorous and nitrogen, before they reach a lake.
That’s the case in Lake Winnipeg, the sixth-largest freshwater lake in Canada. The Red River, which flows north into the lake through the Netley-Libau Marsh, is nutrient rich and is the main source of the high phosphorous and nitrogen levels in the lake. Too much phosphorous promotes the uncontrolled growth of pesky plants such as algae, which then grows too quickly and thickly and absorbs much of the oxygen and sunlight needed by other plants and fish.
While cattails help mitigate nutrient loads, they can only absorb so much phosphorus until they are saturated, says Richard Grosshans, a researcher at the University of Manitoba, Canada. “Some nutrients such as nitrogen naturally break down, but phosphorous is stored in the sediments, so over time it can actually accumulate to the point of complete saturation,” he says.
Several years ago, Grosshans and his team began a project to evaluate how much phosphorous could be permanently removed from the lake by harvesting marsh grasses. They hoped to understand the effects it would have on the health of Netley Marsh, a coastal wetland on the south end of the lake, as well as the health of Lake Winnipeg. Since the 1970s, the marsh has seen a significant loss of habitat.
“We ended up mostly focusing on cattails, as it is pretty effective as a wastewater treatment plant,” says Grosshans, who also works for the International Institute for Sustainable Development, a project co-partner. “Our cattails were loaded with phosphorous, which is one of the things we’re interested in taking out of the marsh, and cattails have a really high energy value. It’s bound in the biomass, and we found it to be valuable as a biomass fuel.”
Unwanted nutrient removal, coupled with the benefit of bioenergy production, creates the greatest economic feasibility of harvesting the cattails.
A Match for Wood Pellets
When the research project began four years ago, the idea was to harvest and bale the cattails. “We were working with a company that has a gasifier with an automatic bale feeder, and they were going through about six bales a day,” Grosshans says. “About halfway through the project we were exploring different funding sources, and Manitoba Hydro expressed interest in funding the concept of densified fuel.”
Starting out with 1-inch by 1-inch cubes, the team first determined that no binder was needed. “Just heat,” Grosshans says. “There’s enough lignin in the cattails so it naturally binds itself. We did a trial with a binder and then with no binder, and found that they had the exact same durability—about 97 percent.”
Moving on to other pellet sizes, the researchers found that the cattail material compresses and binds efficiently in standard pelletizing equipment. And, energy-wise, the cattail pellets are comparable to standard wood pellets. “We worked with the Alberta Research Council, and they did pellet trials and all of the tests and comparisons for us,” Grosshans says. “They found that the calorific value [of the cattail pellets] was anywhere from 16 to 20 megajoules (MJ) per kilogram (kg), about same value of the standard wood pellets that they were comparing, which ranged from 17 to 18 MJ per kg. So they actually have the same energy value, if not more.”
The only superior characteristic of wood pellets was its low ash content, compared to the cattail pellets. “Cattail has about 6 percent ash content, compared to wood pellet standards which are less than one-half of a percent to 3 percent,” Grosshans says. “So we determined that the best option would be to produce mixed pellets. We're working with a company right now that produces mixed-fuel pellets from forest and ag waste, to integrate the cattails in on a commercial scale.”
Feedstock availability and sustainability are not an issue with cattail pellets, Grosshans says. “When we harvest, we leave about a foot of stubble above the water level, only harvesting in about 1 to 6 inches of water.”
Cattails grow fast, and are extremely competitive and resilient, he adds. Properly harvesting them facilitates their health. “We found that after the first year of harvest—because initially there was so much dead stuff out there—the following year, the plants were coming up two weeks earlier because the sunlight could get to them easier.”
Nearby areas not harvested were negatively impacted. “Growth of cattail per square meter was about half that of our harvested site,” he says. “Leaving the stubble allows the plants to breathe and come up again next year, and we’re getting about 14 to 19 tons per hectare (2.47 acres), with 90 days to maturity. It does depend on weather and the area because there are different varieties, but we have a hybrid variety up here that produces an amazing amount of biomass—we had cattails upwards of 12 feet tall in some areas.”
The real challenge to the entire scheme is getting the cattails out of the water, and harvesting when they have the lowest moisture content.
Prepping for Pelletization
“There’s a lot of moisture in cattails,” Grosshans says. “We’ve harvested at different times of the year, and found that a winter harvest is next to impossible.”
Summer is the best time to harvest relative to nutrient removal because that’s when cattails have the highest amount of phosphorus in them, but that’s also when nutrient reserves in the ground are the lowest. “If you keep doing that over time—and it would take a long time—it could negatively impact your community,” Grosshans says. “In the fall, cattails send these nutrient reserves back into the ground, into the roots and rhizomes, so they can survive until next year.”
Without enough stored nutrients, the cattails wouldn't be able to survive the long winter.
Harvesting in the spring is the best option if it is being done specifically for bioenergy purposes, because the cattails are dry and easy to harvest right after the snow is gone and the ground is still frozen. “But it has very little nutrient value in it still; it’s lost about all but 5 percent of its phosphorus remaining,” Grosshans says. Therefore, from a nutrient management perspective, harvesting in the spring is completely useless.
“We found that a good compromise is to harvest in the fall, because the plant has put some of its nutrients back into the soil to survive, and they’ve lost about 30 percent of moisture naturally,” Grosshans says. “You also avoid wildlife effects because most of the ducks and geese have gone south by that time.” Most importantly, the cattails have a significant amount of phosphorous in them to make it worthwhile.
To harvest the cattails, the team initially built a custom-designed small, wetland harvester, but it was designed when the focus was specifically to make bales. “It cuts it and leaves it in a swath, and then we moved the swathes to allow them to dry naturally in the sun,” Grosshans says. “Later, we had a farmer come along and bale it.”
In the next phase of the project, which the researchers are just beginning, some significant funding from the Manitoba government will be used to develop a commercial cattail harvester.
“As the project has developed, we have decided just to make pellets rather than bales and we’re working with companies to develop something similar to a corn forage harvester,” Grosshans says. “It can drive in there, cut the cattails, shred them and store them in a hopper all in one shot. Then we’ll transport the shredded material to wherever we’re going to pelletize it.”
Besides completing the design of the new harvester, the next phase of the project will take the research to a pilot-scale harvest of 300 hectares of cattails, looking at how much phosphorous is removed as a result, how much bioenergy could be created, as well as the carbon offsets that could be generated. Canada has a new carbon offset system, where the government will issue credits for greenhouse gas emissions reductions and removals.
“We’re interested in nutrient management and the bioenergy part of it, but somebody came to us with a proposal to buy carbon credits,” Grosshans says. “So a big driver of the next phase of research is on the end of the process—selling the carbon offsets from burning the cattail pellets.”
Overall, the real driver of the project was to try to understand the value of net removal of marsh to the health of Lake Winnipeg, while working in multiple cobenefits such as bioenergy production, Grosshans says. He adds that it’s not economically feasible to harvest cattail only for bioenergy purposes. “You can do it though, especially if you have areas where you are already growing them such as at storm water treatment plants or wastewater treatment plants, and you can manage the water levels,” he says. “If you combine bioenergy with nutrient management and carbon credits, then it is a lot more economically viable. The great thing about cattail harvesting is that there are no inputs, and it grows everywhere, and it absorbs stuff that causes problems in the environment.”
Author: Anna Austin
Associate Editor, Pellet Mill Magazine