Repowering with Biomass


Waste fuels struggle despite coal’s decline.

Waste fuels struggle despite coal’s decline.

Fortnightly Magazine - December 2010

Shale gas has changed the game for power generation fuels in America. With gas prices projected to remain low for years or even decades to come, coal-fired power generation in particular faces a dim outlook. Most new coal projects have already been sidelined, and now, cost pressures and changing environmental policies are forcing utilities to re-evaluate their plans for existing coal plants—especially those that are nearing the end of their useful lives.

Many such plants are being mothballed or decommissioned. In late 2009, for example, Exelon announced it would permanently shut down four Pennsylvania coal-fired generation plants with a total capacity of 933 MW. Around the same time, Progress Energy announced the closure of 11 North Carolina coal-fired plants with a total of 1,500 MW. And in mid-2010, TVA said it would idle nine coal-fired power units, with the goal of replacing them with low carbon or carbon-free generation.

Natural gas seems destined to become King Coal’s successor, and the dash to gas is causing resource planners to consider ways to ensure ongoing fuel diversity. Solar and wind have a role to play in the country’s generation future, and energy policy trends favor some level of nuclear power expansion. However, often lost in the shuffle is another fuel—one that doesn’t have the glamor and glitz of solar, wind, and nuclear. This fuel is biomass.

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According to a report published by consulting firm Norbridge Inc., biomass accounts for about 1.4 percent of U.S. power generation and about 15 percent of renewables generation. While not 100 percent environmentally friendly, biomass is considered carbon neutral, because plant matter absorbs about the same amount of CO 2 during its growth that it later emits when burned. Of course, harvesting and transportation activities generate greenhouse gases (GHG), but such emissions are inconsequential compared to GHGs produced during coal combustion.

Some utilities and independent power companies are building biomass plants from scratch. Others are considering fuel switching options, toward burning 100 percent biomass or co-firing with coal or other fuels.

There are three ways to co-fire. With direct co-firing, biomass and coal are mixed together. With indirect co-firing, the biomass goes through gasification and pyrolysis—a thermochemical decomposition of the biomass at elevated temperatures—and the resulting gaseous fuel is injected into the furnace. With parallel co-firing, a completely separate biomass-fired boiler is set up next to a coal-fired boiler, and the two are cross-connected into a common header. [Editor’s note: Researchers also are working to commercialize biocoal, a direct substitute for coal made from crop wastes. See “Biocoal Options,” p.31 ].

Repowering a coal-fired plant to burn biomass, however, is far from easy. Three projects, sponsored by Xcel Energy, Georgia Power and FirstEnergy, exemplify both the promise of biomass repowering and the challenges such projects encounter. In some cases, these challenges prove to be insurmountable.

Wood in Wisconsin

Xcel’s Bay Front plant, a three-unit generating station in Ashland, Wisc., is a pioneer in biomass fueling, having begun burning waste wood and