Issues

Promoting Clean Technology

Technology for heating with biomass has developed rapidly over recent decades.  The advent of pellet stoves and the development of ultra-clean furnaces and wood chip boilers are helping to push the frontiers of how biomass can heat America.

Technological Advances

The advances in reducing emissions from biomass stoves and furnaces in America have been largely the result of engineering innovation in the private sector with virtually no government support for research and development. The Department of Energy has funded extensive research on renewables, but for wood biomass, ethanol has been almost exclusively the favored fuel.

The EPA jump-started cleaner residential stoves in 1988, and has not revisited its emissions standards since then. However, in 1995, Washington State sent another strong message to the industry, setting stricter standards for wood stoves sold in the state. Instead of the EPA maximum of 7.5 grams per hour for wood stoves, and 4.5 for pellet stoves, Washington uses 4.5 for wood stoves and 2.5 per hour for pellet stoves.

The bottom line is that wood stoves used to emit 20 to 40 grams per hour, and those that are still in use give the technology a bad name. Wood stoves tend to last a long time and are replaced less frequently than any major appliance. In both America and Europe, the future of emissions from biomass use is partially tied to replacing older stoves, and to removing loopholes so that as sales of stoves increase due to higher fossil fuel prices, virtually all new stoves will be ultra clean.

Catalytic systems and then non-catalytic alternatives were created in the 1990s to help stove manufacturers alter their product lines to meet the EPA regulations. More recently, pellet stoves have come onto the market as an alternative to burning traditional cordwood. These stoves are exempt from EPA regulation because their design allows for low emissions.

The potential for biomass stoves to help curb global warming, reduce emissions further and reduce heating bills in both America and Europe lies in developing markets for furnaces. This is already well underway in Europe, but has yet to take off in America.

EPA-regulated wood fired furnaces and boilers are even cleaner burning than wood stoves. These furnaces and boilers can work with forced-air or radiator systems to heat homes with closed floor plans and existing ductwork.

Can we burn cleaner in the future?

Yes. The Alliance for Green Heat supports stricter and more comprehensive national emissions standards. The EPA has not revisited their standards since they were first developed in the late 1980s. Meanwhile, technological advances have all but made EPA standards obsolete.

At the residential level, the 4.5 gram per hour standard introduced by Washington State is gaining traction and has been adopted in some fashion in multiple states. There are many issues surrounding how well the third party emissions tests hold up in the real world, especially over time.

For produces that may sell only 10,000 units, costs of those tests may pose serious issues. Unlike the “white” appliances (refrigerators, dishwashers, dryers, etc.) that sell hundreds of thousands of units, wood and pellet stove makers have a harder time absorbing the regulatory costs. Alleviating this with federal or state funding, may spur a wave of new technological advances, making stoves even cleaner. If each stove and each variation of each stove continues to require funding for tests from the manufacturer alone, experimentation may be suppressed.

Technological advances made in Europe with wood and pellet furnaces have clearly outstripped American companies, partly because that there is a much greater demand for cleaner burning furnaces in Europe. These are being imported to America and will help American producers create similar models.

Research, development and innovation in the thermal biomass arena are happening, and stoves will become cleaner and more efficient every year. But the advances are not as fast or as complex as those facing the solar PV industry, for example. The United States government and the Department of Energy in particular, has put few research and development resources in this arena, and this should change. Not only can the American market compete internationally in producing the cleanest and most efficient models, but the prices of those models need to come down to make them more available to low and middle income families.

Why older stoves pollute

Traditional fireplaces and old-style wood stoves and inserts tend to smoke excessively because their burn methods do not produce complete combustion.  The smoke associated with these appliances is essentially unburned fuel - potential heat energy sent out of a chimney instead of being put to use.  In order for wood to burn completely, the proper mix of fuel, oxygen and heat must be managed.  New hearth products developed over the last few decades include technology to create this optimum combustion environment, producing enough heat to burn away almost all the components of smoke.

Catalytic and Non-Catalytic Systems

The descriptions of combustion options below have been adapted from those developed by The Wood Heat Organization, a Canadian non-profit that promotes the use of wood for heating. Gabriel Daher of August West Chimney Co. in Pembroke, MA explains the differences between catalytic and non-catalytic stoves.

Catalytic Stoves

In catalytic combustion, smoky exhaust is passed through a coated ceramic honeycomb inside the stove where the smoke gases and particles ignite and burn. This honeycomb structure is coated with a noble metal like palladium, which interacts with unburned volatile organic compounds, particulate matter and carbon monoxide in the exhaust gases and lowers their combustion temperature. The catalyst structure also retains heat, reaching temperatures of nearly 1,800 degrees farenheight.

All catalytic stoves have a lever-operated catalyst bypass damper, which is opened for starting and reloading, then closed when the fire has become hot enough for the catalyst to work. The damper is then closed to direct all exhaust through the catalyst structure. The catalytic honeycomb degrades over time and must be replaced, but its durability is largely in the hands of the stove user. The catalyst can last more than six seasons if the stove is used properly; but if the stove is over-fired, garbage is burned and regular cleaning and maintenance are not done, the catalyst may break down in as little as two years. (EPA note: Garbage should never be burned in a wood stove or fireplace.) Catalytic stoves are capable of producing long, even heat output.

Non-Catalytic Stoves

Non-catalytic stoves do not use a catalyst, but have three other internal characteristics that create a good environment for complete combustion. These are firebox insulation, a large baffle (to produce a longer, hotter gas flow path), and pre-heated combustion air introduced through small holes above the fuel in the firebox.

Non-catalytic stoves cannot match the even heat output of catalytic stoves, but their owners love watching the beautiful fire created by the combustion air introduction. Like the catalyst in catalytic stoves, the baffle and some other internal parts of a non-catalytic stove will need replacement from time to time as they deteriorate with the high heat of efficient combustion.

Although most stoves on the market are non-catalytic due to their ease of use, some of the most popular high-end stoves use catalytic combustion. Because they are slightly more complicated to operate, and the best of them do perform exceptionally, catalytic stoves are suited to people who like technology and are prepared to operate the stove properly, so it continues to perform at peak efficiency. Both options have their pros and cons, and buyers should consider their needs and abilities when choosing a model.

A Partial Solution for Emissions

A commercially viable pellet stove emerged in Washington State in the 1980s, potentially changing the future of wood burning in America. While wood stoves will always remain popular with many, the cleaner burning pellet stove industry is growing much faster because of its convenience. Pellet furnaces are now beginning to approximate the same convenience of a fossil fuel furnace.

Pellet stoves, fueled by pellets made out of sawdust, wood products or other biomass materials, are assigned a default efficiency rating of 78 percent and tend to have very good emissions. The biomass fuel is dried and pressed into small pellets resembling rabbit food in factories, then bagged or shipped in bulk for home or commercial use. Wood pellets have a very low ash content (typically in the region of 0.2 percent to 0.5 percent) compared to other solid fuels (coal 8 to 12 percent). The resultant wood pellet ash (potash) can be used as a fertilizer, reducing disposal costs. Other biomass fuels like corn kernels and cherry pits can often be burned in these stoves if the user has an affordable source.

Pellet stoves use active airflow systems and a unique grate design to accommodate this type of fuel, typically a turning auger to automatically transfer pellets from a storage hopper to the stove’s combustion chamber. Automation of many of the time-consuming steps required for wood stoves make pellet stoves an attractive option for users accustomed to hands-off fossil fuel heaters. Unlike most cordwood stove models, pellet stoves usually require some electricity input to run the various automated systems. Air is provided for the combustion by an electric blower. The ignition is automatic, using a stream of air that is heated by an electrical element. The rotation speed of the feed auger and the fan speeds can be varied to modulate the heat output. All of these elements combine to make a controlled combustion chamber, ensuring such low emissions that they can be vented through a wall in the house just like a clothes dryer.

Whole-house systems

Modern wood-fired furnaces and boilers, generally installed in a home’s basement or utility room are much more likely to produce the hot fires required to create maximum usable heat and minimal emissions in the exhaust air. They typically incorporate a smaller firebox than outdoor models, allowing more regulation of air and temperature conditions for a cleaner burn. The wood and pellet furnaces generally function more or less like a traditional oil or gas-fired furnace or boiler system, except that the wood versions need to be manually fed by the homeowner once or twice a day. Furnaces and boiler are usually controlled by a standard thermostat. When the home is warm enough, the thermostat causes the damper to close, and the fire burns at a reduced level. When more heat is called for, the damper automatically opens again, and the fire comes back to a higher temperature in the increased oxygen flow.