Wood is a renewable energy resource. It’s local: so local that, for many of us, it’s available just a few steps out the back door. It’s nearly carbon neutral: trees absorb enough carbon while they grow to almost balance out the carbon released when those trees are burned as firewood. And wood is cheap: if you cut it yourself from your own woodlot, it’s more or less free.
People in Vermont and New Hampshire clearly appreciate wood’s benefits as a heating fuel. The New Hampshire Office of Energy and Planning estimates that up to 25 percent of New Hampshire homes have woodstoves. The Vermont Department of Public Service found, in a 1997 study, that 31 percent of Vermonters burned wood for at least some of their heating needs.
There is at least one problem, though. “It’s renewable, but dirty,” says Philip Etter of the State of Vermont’s Air Pollution Control Division. Even the cleanest woodstove, he says, still creates more air pollution than burning oil in a heating furnace.
“People say, ‘It’s just natural wood. What’s wrong with that?’ There is a lot wrong with that,” Etter says, explaining that wood smoke contains polyaromatic hydrocarbons, or PAHs, some of which are carcinogens.
Then there are the tiny particles. “PM 2.5 means particulate matter less than 2.5 microns in diameter,” says Etter. A human hair is 70 microns wide. “These particles tend to get deep into your lungs,” where they can cause bronchitis and lung damage.
In some parts of the country, where population densities are high and air is trapped in valleys on cold winter days, pollution from burning wood can make up a significant slice of the total air pollution. This tends not to be a big problem in Vermont and New Hampshire, but it does happen.
Still, a renewable, low-net-carbon fuel is a terrible thing to waste. It makes sense to burn wood cleanly.
The best way to do that is with an EPA (Environmental Protection Agency)-certified woodstove, says Kathy Brockett, with the State of New Hampshire’s Air Resources Division. Since 1988, all woodstoves for sale have been required to be EPA-certified, she says. “But there are a lot of woodstoves around that predate that law, and they are more polluting than the certified ones.”
The old, uncertified stoves on average release about 40 to 60 grams of particulate matter into the air per hour. The current regulations require that stoves release no more than 4.4 grams per hour for catalytic stoves, and no more than 7.5 grams per hour for non-catalytic stoves. But most certified woodstoves do much better than that.
If you know something about woodstoves and air pollution, you might be expecting to read something about catalytic woodstoves here, and you would be right. If you know something about catalytic woodstoves, however, you may be already shaking your head.
“We haven’t made a catalytic woodstove in 10 years,” says David Kuhfahl, president of HearthStone, the soapstone woodstove maker in Morrisville, Vermont. “That technology worked fine in the lab, but in the real world, those stoves didn’t work so well. They were high maintenance.”
Today, HearthStone’s non-catalytic stoves emit on average about 4 grams per hour of particulates, Kuhfahl says. That’s less than the regulations specify for a catalytic stove.
Kuhfahl explains the technology that lets the new stoves burn cleanly, without catalysts: a baffle made of ceramic material, similar to the stuff on the belly of the space shuttle, lengthens the path of the rising smoke. Just below the baffle, two to four air pipes bring in oxygen-rich air.
When the fire is turned down low, the new technology becomes obvious: the smoke reignites as it reaches the baffle and mixes with the fresh oxygen. The heat from this re-ignition both makes the stove more efficient and also helps keep the wood burning.
It looks like a gas stove, Kuhfahl notes. And in some ways it is, since it is the smoke that is burning, as well as the wood.
This technology, similar in all the certified, non-catalytic woodstoves, sends less pollution into the air and has other benefits as well. Cleaner smoke means a cleaner chimney. It also burns wood so efficiently that some people find they use one-third less wood to get the same amount of heat.
In 2008, a grant from the EPA will bring rebates on these new, cleaner-burning woodstoves to low-income residents of Rutland, Vermont, in a woodstove swap-out program, says Marci Young with the Vermont Air Pollution Control Division. Because Rutland is a city in a valley, it is one of the places in Vermont where air pollution tends to be trapped.
Kathy Brockett says that if you already have an EPA-certified stove, hot fires and dry wood can help your stove burn even cleaner.