Burning questions about biomass

Of cane, coal and carbon dioxide

  • Torrefied cane.

    Portland General Electric
 

At first glance, it looks like a great way to counter climate change -- convert a greenhouse-gas-spewing coal power plant to a clean, modern facility fueled by plant matter. But the devil, as always, is in the details.

The Boardman power plant -- Oregon's only coal-fired power plant -- sits amid farmland in the eastern part of the state, near the scenic Columbia River Gorge. Since 1980, it's provided juice for 250,000 consumers. More recently, it's been charged with violating the Clean Air Act, and is about to get slapped with yet more stringent state and federal pollution rules. Bringing the aging plant into full compliance would be prohibitively expensive, so Portland General Electric is considering either installing minimal pollution controls and then closing it in five to 10 years, or converting it to renewable power.

One such option is biomass. PGE officials and scientists at Oregon and Washington state universities are testing whether a fast-growing Asian cane, Arundo donax, could be used to fuel the Boardman plant. The perennial -- best known as a source for clarinet and saxophone reeds -- becomes black and dense like a charcoal briquette when roasted in a process called "torrefaction."  If converted, Boardman would be the first power plant in the world to run completely on torrefied biomass.

But it'll take years of study to determine if that would work, plus at least $600 million to convert the plant, install additional pollution controls, and build a torrefaction facility. To reduce transport costs, PGE also would need to grow as much as 100,000 acres' worth of cane on arable land within 50 miles of the plant. "It's a giant experiment," says Jaisen Mody, PGE's general manager of generation projects, "with a lot of challenges."

Logistical and financial challenges have long plagued biomass power plants, regardless of fuel source. The industry peaked in the mid-'90s; since then, it's been hampered by supply difficulties, deregulation and the relatively low cost of fossil fuels. This summer, plans for a $500 million, 107-megawatt hybrid solar thermal and biomass plant in Fresno County, Calif., were scrapped; developers blamed project economics and an inadequate local biomass supply. An $8 million plant built in 2007 to power a Nevada prison with forest waste was shut down in September, after running into wood-supply problems and local clean-air standards more stringent than the federal government's. Still, "if the world puts a price on carbon," says Lee Rybeck Lynd, biomass expert and professor of engineering at Dartmouth College, "we'll see better economics and greater activity in biomass."

That raises a more fundamental question: Just how green is biomass? Burning trees or plants to generate electricity still releases particulate matter, nitrogen oxides and sulfur dioxide, albeit at lower levels than coal. It also releases a lot of carbon dioxide.

Burning  coal and other fossil fuels increases the net amount of CO2 in the atmosphere. But because trees and plants absorb CO2 as they grow, then release it when they decay or burn, they can be a carbon-neutral energy source -- theoretically.

Whether any given form of biomass energy is indeed carbon-neutral depends on a lot of factors, including the fuel source, whether it's used to produce electricity, heat or some combination, what type of fossil fuel is replaced, and what would happen to the trees or plants if they weren't used for fuel. Newer technologies to produce energy from biomass -- converting it into liquid fuel or combustible gas, for example -- are cleaner and more efficient than burning it. Changes in indirect land use also matter: If a soybean farmer switched to a biomass crop like cane, another farmer might be tempted to clear forested land for soybeans, thus releasing additional carbon.

Accounting for all of this is a huge analytical challenge, notes Daniel Kammen, the World Bank's chief technical specialist for renewable energy and energy efficiency. Hence the disagreement about how, whether and how long it might take for biomass combustion to actually decrease the total amount of carbon dioxide in the atmosphere. A recent report from the Massachusetts-based nonprofit Manomet Center for Conservation Sciences indicates that burning wood from managed forests releases carbon dioxide that may linger in the atmosphere for decades before it's soaked up by new forest growth.

According to Manomet's report, when woody biomass replaces coal in power plants, it generally releases more carbon per unit of energy production than coal does. It takes 10 to 20 years for the excess carbon emissions to be taken up by new tree growth. When woody biomass replaces natural gas, absorption time is more like 50 to 90 years. The American Forest & Paper Association disputes Manomet's analysis, saying that the report considers biomass and forest growth at an unrealistically small scale, and that it doesn't account for the carbon removed from the atmosphere during the combusted plants' lifetimes.

The Environmental Protection Agency itself seems unsure whether biomass plants result in a net increase in carbon dioxide. The agency has referred to biomass energy as carbon-neutral in several reports, but new rules proposed this spring could put biomass on the same level as coal. One specifies which industrial sources will need permits for greenhouse gas emissions under the Clean Air Act, and does not exempt biomass-fired plants, although the EPA has yet to determine the exact permitting requirements for the various types of fuel. Even so, proponents say the permit requirements may cripple the still-nascent industry. "It'll all come to a head in national policy," says Mody of PGE. "In our minds (torrefied biomass) is carbon-neutral. But all of that is debatable."

Despite the obstacles, some small biomass projects are under way in the West. Lincoln County, Nev., is considering a 10 MW plant that would burn pinon-juniper from forest thinning, and the University of Montana is planning a $16 million boiler fueled by beetle-killed trees and forest waste. Such projects may prove to be the most feasible form of biomass power. "These ‘forest off-takes' are an excellent source of energy," says Kammen, "because they do not demand new land or crops, and just work off of waste."

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