Study: Are Wood Pellets A Green Fuel?

James Watt’s steam engine vaulted coal to its major role as a fuel for the Industrial Revolution. Today, about 40% of the world’s electricity is generated in coal-fired power plants, consuming more than 80% of the coal mined each year. Because combustion of coal produces carbon dioxide (CO2) and other air pollutants, efforts to combat […]

James Watt’s steam engine vaulted coal to its major role as a fuel for the Industrial Revolution. Today, about 40% of the world’s electricity is generated in coal-fired power plants, consuming more than 80% of the coal mined each year. Because combustion of coal produces carbon dioxide (CO2) and other air pollutants, efforts to combat climate change have now turned to seeking alternatives to coal. Natural gas is cleaner and less expensive but, like coal, returns fossil carbon to the atmosphere. Recently, attention has focused on woody biomass—a return to firewood—to generate electricity. Trees remove CO2 from the atmosphere, and burning wood returns it. But recent evidence shows that the use of wood as fuel is likely to result in net CO2 emissions and may endanger forest biodiversity.

In recent years, ∼7 million metric tons of wood pellets per year have been shipped from the United States to the European Union (EU), where biomass fuels have been declared carbon neutral and are thus considered to count toward fulfilling the commitments of the Paris Agreement. The EU aims to generate 20% of its electricity by 2020 using renewable sources, including burning woody biomass. In part to revive a languishing forest products industry, the U.S. Congress may also declare wood a carbon-neutral fuel. Despite its withdrawal from the Paris Agreement, the United States may see a few utilities switch from coal to wood, which costs roughly the same as natural gas. The switch could be further incentivized with a carbon tax on fossil carbon.

Cutting trees for fuel is antithetical to the important role that forests play as a sink for CO2 that might otherwise accumulate in the atmosphere. Each year, an estimated 31% of the CO2 emitted from human activities is stored in forests. However, managed forests store less carbon than their native counterparts, and harvesting of native forests will therefore be a source of, not a sink for, atmospheric CO2. Furthermore, wood contains less energy than coal, and wood burning thus generates higher CO2 emissions per kilowatt of electricity. The CO2 emissions from burning wood offset CO2 that might otherwise be emitted from fossil-fuel combustion, but full carbon accounting must also consider how long it takes to restore the carbon pool of forested land that has been converted to atmospheric CO2.

The large-scale abandonment of agricultural activities during the Great Depression (1929 to 1939) led to a rapid expansion of mostly natural forests across the southeastern United States. Later, these natural stands were replaced by plantations of loblolly and slash pine, which were the favorites of the forest products industry because they grow well in the warm, wet climate of the Southeast. Loblolly pine plantations achieve a maximum biomass of 125 metric tons per hectare in about 40 years. But because this species achieves its maximum rate of biomass accrual in about 20 years, rotations are usually kept short to maintain the fastest carbon uptake possible. Thirty-four operating and proposed wood-pellet plants dot the landscape of the Southeast, each anticipated to receive logs from the region within an 80-km radius. Maine and the Canadian Maritime provinces also eye the potential for wood pellets to revitalize their forest products industries; most of this wood would derive from natural forests and not plantations.

Read more on this from Science Magazine at http://science.sciencemag.org/content/359/6382/1328.full.

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