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Cleaner Than Coal? Wood Power Makes a Comeback

Converting from power plants from coal- to wood-fired may not deliver environmental benefits as advertised
atikokan-wood-silos



© Invest in Ontario

In the midst of black spruce and jack pine stands in northwestern Ontario's Crown forests, a global trend has come home to roost. Atikokan Generating Station ceased burning coal last year to prepare for its new fuel: locally sourced wood pellets.

Canada already sends wood pellets abroad for power generation, but it is now leveraging the resource on a large scale in its own backyard. Atikokan will be the largest commercial power plant in North America to convert from coal to biomass, a trend that has caught on worldwide, especially in Europe.

The retrofit is part of Ontario's plan to be the first jurisdiction in North America to shut down its coal fleet. In Europe the drive to retrofit coal-fired power plants to biomass comes from the European Union Renewable Energy Directive, which calls for 20 percent of energy to come from renewables, including biomass, by 2020. Much of Europe's wood pellets are being imported from private forests in the southeastern U.S. as well other parts of North America.

The move away from dirty coal to a renewable resource is seen as a win in terms of carbon emissions by some governments, but not everyone agrees. The definition of sustainably harvested wood pellets varies by power plant and country, and some environmentalists fear that the growing need for biomass will outstrip any benefits from sidelining coal. "If we're going to have this global trade in wood energy," says Brian Kittler, project director at the Pinchot Institute for Conservation, "we have to ask, how well do the European and U.S. systems match up?’"

Studies have found that burning wood over fossil fuels produces about 65 to 95 percent less greenhouse gas emissions. Those studies, however, do not take into account the change in forest carbon stocks, according to Thomas Buchholz, a senior scientist at Spatial Informatics Group who studies greenhouse gas emissions related to bioenergy systems. When researchers start adding in the change to the forest carbon stock, the picture changes, which worries environmentalists. Although trees can be replanted, whereas coal is a finite resource in the short term, there is debate as to how much carbon smaller trees take up compared with mature trees. Based on a study Buchholz did for the Southern Environmental Law Center, if Europe imports southeastern U.S. wood instead of burning coal, atmospheric greenhouse gases could up to a 300 percent in the first 50 years, although they would drop below fossil fuel levels eventually.

In the U.K. Drax Power Station is spending $1.1 billion to convert half of its 4,000-megawatt coal-fired facility to wood pellets, much of which will be imported from North America. Although Drax has drafted sustainability guidelines, environmentalists question whether the world's largest biomass power station will be able to sustain itself on wood pellets made from just logging residue. The European Union has delayed its final decision on wood pellet sourcing criteria several times.

Drax will need seven million metric tons annually for the three boilers that are undergoing conversion. "We're in the forefront of developing sustainability biomass criteria," asserted Melanie Wedgbury, a spokesperson for Drax. "We fully buy into the fact that biomass is carbon-neutral." Drax already ships in coal from across the globe, some as far away as Australia. Drax's estimate is that the plant will cut 10 million metric tons of carbon dioxide emissions annually, and that they will be able to get enough wood that meets their sustainability requirements.

If some pellets for Drax are not coming from residues, but from cutting whole trees as National Resources Defense Council (NRDC) asserts is already happening, the carbon payback can look worse compared with burning coal because more mature trees absorb more carbon than smaller trees do. If hardwood is harvested only for pellets, the carbon payback will take at least decades and possibly more than 100 years, according to some studies. "How we do carbon accounting is at the heart of the issue," says Nathanael Greene, director of renewable energy policy at NRDC. "We need our policies to drive our investments that are truly long-term sustainable so we don't waste time and taxpayer dollars that we are going to slowly realize aren't good for the environment or climate."

Atikokan, which will spend $170 million to upgrade the plant, does not face quite same wood sourcing challenge as Drax. It's a far smaller facility, a 200-megawatt plant that only runs at about 10 to 12 percent capacity. All of the wood will come from within 200 kilometers of the power plant, from the province’s Crown forests, greatly reducing shipping costs and emissions. Atikokan's analysis found that burning wood pellets would have a 90 percent greenhouse gas emissions benefits over the lignite coal it previously consumed, most of which came from western Canada. It estimates 85 percent of the 90,000 metric tons of pellets will come from existing logging residues.

"We think that we have a good, robust system," says Robert Lyng, director for environmental governance for Ontario Power Generation, which owns Atikokan.

There are benefits for other emissions, including lower sulfur oxides (SOx), nitrogen oxides (NOx) and mercury, when coal is completely replaced by wood pellets. Biomass does not have lower SOx and NOx emissions, however, when compared with new natural gas combined-cycle plants, which have gas turbines and steam turbines that run off waste heat. The study also found it is cheaper to install emissions controls on coal plants than to convert the facilities to biomass if done just for the SOx and NOx benefits.

Environmentalists argue that "co-firing" retrofit projects such as Drax, in which some boilers burn wood, others coal, only prolongs the life of outdated coal plants. They point out it would be better to use wood residues in more efficient energy production, such as combined power and heat systems that provide electricity and then capture and use the waste heat. Power plants that are switching to pellets counter that higher-value lumber is being harvested anyway. Wood residues, which go into pellets, have a far lower value than lumber so the plants argue that because they are only buying the low-value product, their market power is limited and it is the higher-value products that drive decisions on how forests are logged.

Although replacing coal with wood is hotly debated, the extent to which older coal plants will be converted partially or completely to biomass is limited by various factors, such as energy efficiency and the price of natural gas. Cheap natural gas has meant that many utilities have decided to build gas plants or make more use of existing ones, instead of upgrading old coal plants. In 2010 FirstEnergy Corp. canceled plans to convert two units of a plant in Ohio to biomass because of low energy prices, which were partially driven by the price of natural gas. Tilbury power station in the U.K. decided earlier this year to shut down completely after partially converting to biomass because it could not secure the subsidies needed to move fully to biomass. In Ontario most of the coal capacity is being replaced by solar, wind and natural gas, the latter of which can respond faster to balance intermittent renewables on the grid than coal-fired power plants.

"You can't just take biomass and count it as carbon-free," says Greene. "The science is clear—it's just a mistake."

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