On Board with Energy Crops
Near the Columbia River just outside of eastern Oregon’s Boardman sits the state’s last operating coal plant, a 600-MW facility built in the late 1970s. Though the plant has plenty of years left in it, the state's decision to phase out coal left Portland General Electric exploring its options.
In 2010, PGE was approved to continue to burn coal at Boardman until 2020, with some temporary emissions controls upgrades. After that, $500 million in additional pollution controls would be required to comply with federal and state sulfur, nitrogen and mercury rules, thus enabling the plant to continue operations until at least 2040.
Ultimately, PGE faced three possibilities—closing by 2020, making costly upgrades, or switching to another fuel source. If closed, it would make history as the youngest coal plant in the U.S. to shut down as a result of air quality regulations, but doing so and building a new plant elsewhere makes more economic sense than keeping it open for upgrades.
With the upgrade option ruled out, the fate of the plant rests on the feasibility of using torrefied energy crops as fuel, and PGE has spent the past several years conducting in-depth research and rigorous testing to determine what the possibilities are.
Initially, PGE looked into repowering with natural gas, but rendered that option unfeasible. “We did a study on natural gas and found the area didn’t have a gas line, but that wasn’t the real issue,” says Jaisen Mody, PGE projects manager. “The issue was that the Boardman boiler was designed for coal combustion, and using gas in the existing boiler made it highly inefficient. The cost wasn’t conducive to running the plant long-term, as we would have to change out the boiler. We decided that converting an old Rankine cycle coal boiler wasn’t the way to go because of the capital expenditure.”
Basically, it boiled down to the notion of using gas meant building a new gas plant, adds Steve Corson, PGE spokesman.
When PGE began evaluating biomass back in 2010, wood pellets were tested but gummed up the plant’s pulverizers. Crop research began at that point, and arundo donax was chosen as a fuel of interest due to its great growth potential. It’s been found to produce upwards of 35 dry tons per acre per year, compared to switchgrass, which will yield 4 to 13 dry tons per acre per year.
PGE has been growing arundo test plots around the Boardman area for the past couple of years—about 92 acres—and has harvested it a few times, storing the crop for test burns, Mody says. He adds that while the initial emphasis was mostly on arundo, that’s changed a bit.
On one hand, a single energy crop is attractive because it’s dedicated to producing feedstock volumes needed, but reliance on a single fuel source is risky for a number of reasons, including harsh weather, natural disasters or pests. “So we’re also investigating other biomass sources, including sorghum and ag waste,” says Mody.
One thing that’s certain is that if energy crops and biomass are used at Boardman, they will be torrefied first. “Torrefaction is the right way to repower Boardman with biomass, because we’re anticipating no changes to plant equipment,” Mody says.
Corson adds that torrefaction would allow the plant to pulverize the fuel just as it is doing with coal, but green biomass would require a lot of changes. Additionally, researchers have found that torrefied biomass is more hydrophobic than Powder River Basin coal, which is currently used at Boardman.
Later this year, PGE is installing a torrefier at Boardman, and will then begin its test burns, according to Mody. “These test burns are critical for us,” he says. “We think running this test will prove to us that we can run torrefied biomass through the plant, and we’ll also collect emissions data. Then we’ll sit down and figure out what it’ll take to run the plant for air permitting and the economics of that.”
Mody notes that each feedstock tested—arundo or sorghum—could have a different effect on the boiler, slagging or fouling it, so close attention will be paid as to what source is torrefied and how.
According to a study done in 2012 by researchers at the University of Washington, Washington State and Oregon State University, operating at 300 MW and producing power under optimal economic conditions, about 1.25 million tons of torrefied arundo would be used by Boardman, based on the Btu content of torrefied arundo (10,400 Btu per pound). About 94 dry tons of arundo would produce 52.7 tons of torrefied chips, the researchers found, so a total of 67.6 thousand acres of arundo would be required to produce 1.25 million tons of torrefied chips and support torrefaction, assuming 33 dry tons per acre per year.
Of course, while multiple sources would be used, Mody admits obtaining necessary quantities remains PGE’s biggest challenge in the quest to repower with biomass.
“It's [repowering] always been one issue—the source of biomass,” says Mody. “How can we procure and move enough in an economic manner that would sustain a large plant? The production of biomass, whether we’re growing or buying it, remains our biggest challenge. That’s why we’re looking at diversity now—one species isn’t the answer. It’s about what we can grow at a reasonable price, and what’s available out there.”
If the torrefaction test burns are successful, more work has to be done to calculate the economics and emissions profiles of a full-scale torrefier. Once that data is complete, PGE will bring it to its integrated resource planning process, which is a comprehensive plan presented to the public utility commission that lays out its generating portfolio resource requirements.
At that time, the next step for Boardman will be decided, Corson adds. “At this point, what we’ll really be saying is, okay, we know we can do this, is it better than the other options?”
Author: Anna Simet
Managing Editor, Biomass Magazine