WASHINGTON — Even as the Environmental Protection Agency considers requiring coal-fired power plants to cut their carbon dioxide output, some utilities have started to use a decidedly low-tech additive that accomplishes that goal: wood.
Ranging in size from sawdust to chunks as big as soup cans, waste wood from paper mills, furniture factories, and logging operations has been used with varying levels of success.
Minnesota Power, which once generated almost all of its power from coal and is now trying to convert to one-third renewables and one-third natural gas, found that co-firing with wood was a quick way to move an old plant partly to the renewable category.
“We’re finding an emissions improvement benefit and an economic benefit,” because the wood is cheaper than coal, said Allan S. Rudeck Jr., Minnesota Power’s vice president for strategy and planning. One boiler at the company’s Rapids Energy Center, near Grand Rapids, Minn., has run at up to 90 percent wood.
For companies like Minnesota Power, co-firing will be one of the leading options if the EPA, which recently proposed limits on carbon emissions for new plants, follows through on its plan to develop limits for old ones. Using modest amounts of wood at a large number of coal plants could be a relatively quick way to phase in renewable energy.
And unlike wind or solar power electricity from a boiler, burning wood is easy to schedule and integrate into the grid.
The EPA is in the midst of “listening sessions” in 11 cities to gather ideas from the public about putting carbon limits on existing plants. Last week it held an eight-hour session in Denver.
Wood does release carbon when burned, as nearly all fuels do. But taking woody material from forests or farms leaves space for new growth, which will absorb carbon dioxide from the atmosphere as it grows. Although some opponents of using wood say that disrupting forests means added carbon dioxide in the atmosphere for generations, regulators usually count its use as zero carbon.
Coal plants are finely engineered, designed to burn one particular kind of coal, and adding wood can be tricky. But their carbon output, like their overall efficiency — that is, the amount of coal burned compared to the amount of electricity generated — has grown worse in many cases in recent years. Earlier EPA rules that cover emissions of soot, nitrogen oxides, and other pollutants have required plants to install pollution control equipment that itself consumes a lot of energy. In the end, some older plants, less efficient to begin with, are more likely to be retired and replaced with natural gas ones, industry executives say, but younger ones judged to have decades of life left in them are good candidates for co-firing.
Co-firing has its drawbacks. In some cases, particularly for bigger companies, there is simply not enough wood. American Electric Power, trying to meet Ohio’s renewable energy standard, tried wood twice at its Picway plant south of Columbus, which was built in 1955. In 2003, it used sawdust from a cabinet manufacturer, and in 2010, waste wood chunks. But it dropped the test.
“The material is difficult to get in any quantity and any predictable form,” said Mark C. McCullough, American Electric Power’s executive vice president for generation. Wood was “difficult to introduce in our combustion systems, if we don’t know what to expect,” he said. The feeder system had trouble handling the bigger chunks, he said.
The larger mass of wood compared to coal is an issue, too. A pound of wood can produce only about two-thirds as much heat as a pound of coal, and it is a lot bigger. To produce the same amount of energy, companies must enlarge fuel-handling systems. And coal-fired plants are not used to handling fuel that can rot.
Small amounts of wood can be mixed with coal and added to existing equipment that pulverizes coal into powder, which is then burned, but that limits co-firing to about 5 percent of fuel, and some companies say that their pulverizing equipment cannot handle the wood. Other companies have cut holes in the boiler and blown in wood, chopped into confetti-size pieces. That requires expensive modifications, but it allows wood to substitute for 15 percent of the coal, or more. European utilities have experimented with heating the wood in a chamber outside the plant, producing a fuel gas of carbon monoxide and hydrogen, and pumping that into the boiler, but that is even more expensive.
Nearly 200 plants have conducted test burns worldwide, said David L. Nicholls, a forest products technologist at the US Forest Service.
Long term, experts say, to reach carbon goals power companies will have to capture the carbon from all the coal they use, and probably most of the natural gas, too. But in the meantime, Nicholls said, “You could look at co-firing as a bridge strategy.”