Wood Stoves Clean Up Their Act
To avoid woodsmoke pollution, manufacturers develop technologies to burn hot and fast
REINHART ROSS speaks in glowing terms about his new wood stove.
"I fire it up once in the morning, then I don't have to do it again," he says.
Mr. Ross lives in a hilltop A-frame house he and his wife, Dorothy, built in Bellingham, Wash., in 1986. The house was designed to be heated efficiently by their wood stove, which sits in the corner of the cathedral-ceiling living room that faces a forest. He keeps the house at about 68 degrees "because any warmer than that, and people start getting tired," he says.
The clean-burning stove is a big improvement over his previous one, which "was state of the art for its time," he says. "The glass window in the door never gets dirty."
In addition to burning less wood and producing less smoke, new wood stoves accumulate less tarry, black creosote. Many stoves create airflow across their glass doors to prevent any buildup there.
"I'm convinced it makes a visible difference in chimney emissions," Ross says of his $1,200 Pacific Energy Super Series stove. "I can see [the difference] from the living room just by looking up out the windows."
The trees Ross cleared to build the house provided enough wood to heat the house for his first three years there, he says.
Like Ross, hundreds of thousands of homeowners in the United States bought wood stoves as the cost of energy rose through the 1970s and '80s. Wood was a popular way to provide heat while cutting back on oil, electricity, and natural gas consumption. But the resurgence of organic heating fuel brought days when burning wood was banned.
The bans have been so frequent in some areas, says John Kowalczyk, manager of air planning for the Oregon Department of Environmental Quality, that "they have actually pushed people away from wood heat" despite its low cost in rural areas.
Traditional wisdom says that the most efficient way to extract heat from wood is to let it smolder for hours, never letting too much air get to the fire so the wood won't burn too fast. But this kind of fire is the most polluting, says Will Peck of the Bow and Arrow Stove and Fireplace shop in Cambridge, Mass.
By comparison, today's clean-burning wood stoves are designed to burn fires hot and fast and will not permit an operator to cut off the air supply.
They must be fed smaller amounts of wood continually, rather than using one big log, which turns into coals that will last all night, Mr. Peck says.
Ross, however, says a moderately full load of logs fed into his stove at 9 p.m. will leave coals hot enough at 6 the next morning to start a fire without kindling.
The key to cutting smoke emissions, says Chris Stoneman, who is in charge of implementing wood-stove standards for the Environmental Protection Agency (EPA) in Washington, D.C., is to burn less wood.
New stove technology focuses on lean-burning fires that consume lots of air and relatively little fuel and burn wood completely. As an added benefit, the fires deposit less of the black, tarry creosote that often coats the walls of older model wood stoves.
"Any wood stove is only as clean as the person operating it," says Mr. Peck.
To ensure complete consumption of fuel, manufacturers rely on three primary devices not present in older technology stoves (generally those sold before 1988).
* A secondary air system automatically allows sufficient air into the stove to burn fuel thoroughly, even when the operator has closed down the stove's primary air-intake system.
* A ceramic blanket near the top of the stove traps enormous amounts of heat to promote secondary burning of smoke particles on their way toward the exhaust flume.
* A catalytic combustor in the chimney of many stoves uses platinum and palladium to catalyze further reduction of the particulate emissions.
THE goal of all three devices is to increase what Mr. Stoneman refers to as the "three T's": burn time, temperature, and turbulence. The longer it takes each molecule to pass through the stove, the more completely it will burn, and the more heat it will give off in the stove. The hotter the temperature of the fire, the more smoke particles will burn. And the more turbulence generated, the more air will mingle with each fuel particle to consume it.
Before 1988, wood stoves used none of this technology.
Geoff Wurzel, speaking for the Hearth Products Association of Washington, D.C., cites two types of energy released in wood fires: 50 percent of it from the wood itself and 50 percent from gases stored in the wood. The previously cited technologies were developed to make most efficient use of both sources, he says.
Besides wood stoves, pellet stoves that burn recycled cardboard, sawdust, and corncobs have become commonplace. Unlike split logs, the pellets are of a uniform composition and have fewer polluting, unburnable molecules, says Kenny Taylor of Bow and Arrow. The pellet stoves do not fall under EPA regulations and do not use catalytic combustors, but they do pollute less than even other new-technology wood-burning stoves, Mr. Taylor says, though some people find them unattractive.
The push for cleaner wood stoves came after a sales boom in the late 1970s and early 1980s, when woodsmoke accumulation began to violate the federal Clean Air Act in areas susceptible to atmospheric inversions, in which temperatures increase with height, putting a lid on convection and trapping pollutants near the ground.
Today, wood heat ranks as the third most common source of heat in American homes, says Mr. Wurzel.
To head off a barrage of various state regulations in the mid 1980s, the Hearth Products Association, then called the Wood Heating Alliance, lobbied the EPA to devise nationwide standards, and cooperated in setting those standards.
The laws adopted by the EPA in 1988 restricted the emissions of new wood stoves sold to not more than 5.5 grams per hour (gph) of particulate matter larger than 10 microns for stoves with catalytic combustors and not more than 8.5 gph for non-catalyst stoves. The regulations were tightened in 1992 to 4.1 gph for catalyst stoves and 7.5 gph for "non-cats."
The regulations did not affect existing stoves, and burn bans continue in many areas of the US.
Wood-stove manufacturers support the clean-burning regulations, says Wurzel, because they believe heavy woodsmoke pollution has given wood burning a bad reputation.
In various cities the association is sponsoring programs to encourage owners of older stoves to trade them in for cleaner-burning ones. (See related story at left.) When Ross replaced his old stove, which had cracked, the EPA paid him $100 to trade it in.
Still, the atmosphere inside the house is a main reason the Rosses enjoy their wood stove. "When you are looking through the glass and watch the flames dancing inside, it sure is a cozy way of living," he says. "I wouldn't trade it for anything."