King Coal Eats Its Vegetables

Blending in biomass makes coal-fired generators seem a little cleaner

PHOTO: Nigel Roddis/Reuters

Capturing carbon-dioxide emissions and sequestering them deep underground is often presented as the sole means of significantly cutting the carbon emissions from a coal-fired power plant. In fact, this as-yet-unproven scheme is being beaten to the punch by a comparatively simple alternative: blending biomass into the plant’s coal feed.

Adding agricultural leftovers, wood chips, or even dried sewage replaces some of the power plant’s fossil carbon with renewable carbon. Such ”cofiring” of coal and biomass is beginning to boom, thanks to preferential pricing and tax incentives supporting renewable energy, standards mandating utilities to obtain more of their power from renewable resources, and carbon caps that make utilities pay for CO2 emissions. Europe, which already employs all three of these policies, is leading the way.

In the United Kingdom, cofiring has become one of the fastest-growing sources of power, following the introduction of the country’s renewable portfolio standard in 2002. The UK’s first cofiring operation started up the same year, and by 2006 cofiring was generating more than 2.5 terawatt-hours of electricity annually and displacing 2.6 percent of the power sector’s coal use. More cofiring is coming, as Drax Power, operator of the UK’s largest generator (4000 megawatts), in North Yorkshire, boosts the proportion of power the utility generates through cofiring from 2.5 percent to 12.5 percent. Drax boasts that from next year its renewable generation will rival that of 600 wind turbines, slashing annual CO2 emissions by over 2.5 million metric tons.

Cofiring is attractive due to its low installation price relative to most alternative power technologies. Coal plants accommodate up to 20 percent biomass with a modest addition of equipment to store and handle the renewable fuels. The upgrade costs for cofiring range from US $50 to $300 per kilowatt. That’s less than a third of the price tag for an onshore wind farm and one order of magnitude cheaper than building a standalone biomass generator.

Biomass burned with coal also delivers more energy per ton of fuel than biomass burned alone. The typically smaller biomass-only generators convert just 25 percent of their fuel energy into electricity. Conventional coal plants average 36 percent thermodynamic efficiency, and the efficiency lost when blending in modest proportions of biomass is small.

If carbon capture and storage does ultimately prove economic and safe, what then for cofiring? Cofired plants that include CO2 capture provide a net extraction of carbon from the atmosphere. ”If you combine the two, you have a sort of CO2 vacuum,” says Marc Londo, a biofuels expert at the Energy Research Centre of the Netherlands, in Amsterdam.

On the flip side, says Londo, cofiring may look less beneficial if carbon capture fails to pan out. That’s because cofiring extends the economic viability of coal-fired power while diluting support for alternatives such as solar and wave power. For this reason, the UK has capped cofiring’s contribution to its renewables mandate. Londo predicts that Europe may similarly limit cofiring’s role in its mandates. As Londo puts it, ”It’s a bad idea to keep an old and relatively inefficient coal plant alive just via biomass.”

About the Author

Peter Fairley, a contributing editor, writes about energy for Spectrum. When he started research for ”Germany’s Green-Energy Gap”, he anticipated that the nation’s efforts to replace coal- and nuclear-fueled electricity with power from offshore wind turbines might provide a road map for other countries. He discovered, though, that Germany’s green-energy push has stalled. His article reveals why.

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