Aug. 19, 2007

By Russell Stigall/Frontiersman

MAT-SU - To date, the only thing Matanuska Electric Association's plans to build a coal-fired generation plant has generated is passionate debate - at times aggressively so. While MEA and environmental concerns wrangle over the electric cooperatives proposal for a circulating fluidized bed coal plant, others are left to wonder what the heck is a CFB generator and why does MEA want one?

Matanuska Electric Association plans to use new technology to address local concerns over coal plant emissions. It wants to spend about $350 million to build 200 megawatts of new generation in the Mat-Su Valley by 2015. Half of this new generation would come from a circulating fluidized bed coal combustor and half from a natural gas-fired plant. An additional 5 megawatts would come from a small renewable energy source like landfill methane or in-stream hydroelectric.

Steve Denton, vice president of business development for Usibelli Coal, said Alaska has no CFB combustors, but thousands exist around the world.

“Typically, [CFB plants are] used for small industrial process, working alongside manufacturing plants,” Denton said. He also explained the process of how CFB combustion works.

A circulating fluidized bed combustor suspends its solid fuel on jets of air, like sand on an air hockey table. The tumbling, roiling action of the suspended fuel gives the combustor its name. The action of the combustor allows for limestone to be mixed into the fuel during combustion, which traps sulfur and lowers the plant's sulfur dioxide emission. The process converts limestone into a gypsum material, like wall board.

CFB combustors burn cooler than traditional pulverized coal burners, about 1,400 degrees to 1,700 degrees Fahrenheit. This is below the temperature nitrous oxide is formed. Sulfur dioxide and nitrous oxide are acidic, and can kill plants and fish and cause smog in concentration.

Several tons of coal, ash and limestone flakes mix in the combustion chamber. This huge mass of hot material quickly raises the temperature of added fuel. The combustor's boiler, where water is converted to steam used to spin a turbine, has walls full of water in pipes. Air jets blow hot ash up through the boiler to heat a brick-like material. Some ash goes into a bag house, some is re-injected into the fluidized bed. And the process repeats. Lime flakes are carried up through the fly ash through the boiler. The fuel for a circulating fluidized bed combustor can be made of larger particles than pulverized coal, reducing handling cost.

Because much of the process' particulate matter is suspended in the fluid rather than expelled into the atmosphere, it's much cleaner that the traditional method of simply burning pulverized coal, Denton said. However, a circulating fluidized bed combustor loses about a 10 percent efficiency compared to a pulverized coal burner.

This inefficiency combined with Usibelli Coal's low-grade product means Matanuska Electric Association's CFB plant would likely emit more of the greenhouse gas carbon dioxide than a pulverized coal plant, since 1 ton of coal produces about 3 tons of carbon dioxide, according to Jeff Goodell, author of “Big Coal.”

Matanuska Electric Associations Integrated Resource Plan, a study commissioned to investigate power generation options and their impacts, shows that the cooperative would keep low the potential offset taxes associated with CO2 emissions by carbon capture and sequestration.

MEA Assistant General Manager Tuckerman Babcock said the co-op would wait to hear from its engineers and consultants whether carbon sequestration would be used in its proposed coal plant.

“They would let us know what is available and what the alternatives are and what they cost,” Babcock said.

A lot of money is being dedicated to the issue of carbon capture and sequestration, Denton said. “But we are quite [some way] from being able to do that effectively, both capture and sequestration.”

In sequestration, carbon dioxide typically is captured and injected into spent oil wells, like Nikiski's Agrium plant with the Cook Inlet oil wells near by, Denton said.

That would be “quite a bit less so in the Mat-Su Valley,” Denton said.

Although there is now not a tax on carbon emissions, Denton believes some type of tax on carbon emissions is on the way.

“It is a real case of politics and ignorance over science,” he said.

While new scrubbing technology developed for traditional pulverized coal burners can produce an emissions profile similar to a fluidized bed coal burner, the fluidized bed technology would allow Matanuska Electric Association the option to burn biomass like trees, tires, wood chips and trash.

The main attraction of a circulating fluidized bed combustor is its ability to burn biomass along with coal, Babcock said. Matanuska Electric Association estimates that it could burn up to 30 percent biomass in its circulating fluidized bed power plant.

While pulverized coal plants can match a circulating fluidized bed plants on emissions levels, the scrubber systems to clean them add complexity to the overall system, Babcock said.

“It's one less piece of equipment that could go wrong, in that way [CFB] is a simpler design,” he said.

Babcock said both pulverized coal and CFB were investigated by CH2M Hill, the engineering consultant hired to complete the co-op's Integrated Resource Plan for future generation.

“What tipped the scales between a modern CFB plant and a modern pulverized coal plant is the ability of the CFB to burn biomass,” Babcock said, adding the plant would also have limitations when built as to which kinds of fuel can be efficiently handled.

“The CFB plant is well-suited to Usibelli's low grade sub-bituminous coal,” MEA's Integrated Resource Plan says. “It is also suitable for co-firing coal with low grade fuels, including some waste materials. However, the advantage of fuel flexibility often mentioned in connection with CFB units can be misleading; the combustion portion of the process is inherently more flexible than PC [pulverized coal], but material handling systems must be designed to handle larger quantities associated with lower quality fuels. Once the unit is built, it will operate most efficiently with whatever design fuel is specified.”

Contact Russell Stigall at 352-2267 or russell.stigall@frontiersman.com.