Coal Plants Get Second Lives
AGING, INEFFICIENT AND EXPENSIVE to upgrade, the Boardman Plant could have joined dozens of similar coal-fired generating stations consigned to the scrap heap of history as its owner prepared to permanently mothball its 600 megawatts in 2020.
Instead, Portland General Electric is in the middle of a grand experiment to see if the plant in eastern Oregon can be successfully converted into the world's largest biomass power plant.
The plant dates from the 1970s and the utility already filed plans with state regulators to permanently shut it down because of the cost associated with bringing it into compliance with early 21st-century environmental mandates currently on the books and with any future carbon regulations.
"The mandate allows a small opening for repowering, so we looked at biomass and natural gas, but its large Rankine cycle boiler is very inefficient for gas," Jaisen Mody, PGE's general manager of generation projects, said.
"We started looking at biomass and looked at a technology called torrefaction, and the more we looked at it, the more it intrigued us."
But biomass power plants may fail because they don't receive a consistent supply of fuel, which makes securing a long-term power purchase agreement difficult.
There are other obstacles, mainly due to the plant's size.
"On paper, it looks like it will work, but no one has demonstrated that on a boiler like ours with 100 percent fuel switching," Mody added.
The plan is to complete engineering studies and reconstruction of the boiler, so that a test burn could be performed over a few days in 2014. Emissions data would be collected and operational issues would be assessed. The Electric Power Research Institute is the research partner with PGE in the project.
If the tests prove successful and the emissions data falls within certain parameters, the permitting would be pursued and the project would be completed over several years.
But as PGE emphasizes, that's a very big "if."
The other challenge is fuel sourcing. A baseload plant of this size will require 4 million tons of green biomass, so the plan is to run the plant at 100 percent power for six months of the year when it's needed the most.
This is where local sourcing of green biomass and the torrefaction process come in.
Torrefaction is a charring process of agricultural material that essentially removes moisture - roasting it in the absence of oxygen - that creates a brick-like material that could be described as green coal. It could be thought of as a half-step below turning wood into charcoal.
PGE believes it has found a grass that will be better than coal, which burns at 8,400 BTUs per pound. PGE is investigating many potential plant sources, but it believes it has found one that burns at 10,000 BTUs per pound.
Wayne Lei, director of research and development, said the utility found a nearby university that had been experimenting with a plant that can replace hardwood fiber in pulp production.
"It came to our attention that University of Washington was testing a plant called arundo donax, a perennial grass that grows faster, like a switchgrass, but it's the most productive in its class," Lei said.
The plant is prolific: An acre can yield 25 to 30 tons of the grass with a low moisture content. And with arundo donax having similar requirements for water and a growing season like alfalfa and corn, crops already grown in the Columbia River Basin, the local sourcing issue may be solved.
Coal-to-biomass conversions under way or that have been successfully completed are well below the scope of the Boardman Plant.
"Successful biomass plants tend to be under 100 megawatts," said Bob Cleaves, executive director of the Biomass power Association. "Developers are challenged by making sure that the coal plant can be appropriately sized as a biomass opportunity as the two tend to be different. Biomass tends to be local in its supply and frankly few places exist that could accommodate a few hundred megawatts in biomass capacity."
DTE Energy has been one of the more active companies in the conversions of old coal plants during the past eight years, with the Detroit-based holding company owning, operating or currently converting plants in three states.
"We looked at the growing trend of coal plant shutdowns and decided that if biomass made sense, we were zeroing in on plants that could generate 40 megawatts to 50 megawatts," said Steve Sorrentino, vice president, power and renewables. "If you're looking at the 15-to-25-megawatt plant, that's a tough economy-of-scale play in capital costs as well as in fuel acquisition."
DTE Energy has completed biomass conversions in Cassville, Wis., and Bakersfield, Calif. It has operating biomass power plants in Woodland., Calif., and Mobile, Ala.
One of the first was the Wisconsin project of a 1950s plant that the Dairyland Cooperative formerly owned and that operated sporadically in recent years.
After feasibility studies and a power contract with Dairyland, which was seeking renewable electricity, permitting came in a relatively quick 15 months. Operation started in late 2010.
"If you can do this in the range of $1,000 per kilowatt, that's one of the other drivers," Sorrentino said. "When you build a new biomass plant, the number is $4,000 to $5,000 per kilowatt."
Not all conversions come that cheaply, and perhaps many are closer to $2,000 per kilowatt, but they're worthwhile when there is a ready customer, as in the case of the ongoing Stockton conversion.
The 45-megawatt plant's output will be sold to Pacific Gas and Electric under a 25-year power purchase agreement. "PG&E is very aggressive with renewables," Sorrentino said. And the plant qualifies under California's renewable portfolio standard of 33 percent by 2020.
The plant is on track for a late 2013 opening and will be supplied by tree trimmings and the like, as well as wood pallets and other so-called "urban waste."
So what is the future of coal-to-biomass conversions?
"It's going to continue, but I don't think at the same pace as before natural gas became so prevalent," Cleaves said. "We think there will be a greater trend toward co-firing as opposed to complete conversion. In smaller projects it makes lots of sense as small coal plants struggle to meet EPA mandates."
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