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 did not revise its emissions standards until 2015. However, in 1995, Washington State set stricter standards for wood stoves sold in the state. Instead of the EPA maximum of 7.5 grams per hour for non-catalytic wood stoves and 4.1 g/hr for catalytic wood stoves, Washington used 4.5 for non-catalytic wood stoves and 2.5 per hour for catalytic wood stoves. But as of May 15, 2015, all wood and pellet stoves in the United States must emit no more than 4.5 grams per hour.

Wood stoves used to emit 20 to 40 grams per hour, and millions of them are still in use and help form public opinion about the viability wood heat as a clean, renewable energy technology. Wood stoves tend to last a long time and are replaced less frequently than other major appliances. In both America and Europe the future of emissions from biomass use depends upon the replacement of older stoves, and the removal of loopholes that allow unregulated new stoves to stay on the market.

Catalytic systems and then non-catalytic alternatives built in the 1990s helped stove manufacturers produce large firebox stoves that still met 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 partially because they were viewed as sufficiently clean already.

The potential for biomass stoves to help curb global warming, reduce emissions further (see the emissions section at the end of the page) and reduce heating bills in both America and Europe lies in developing markets for cleaner and more affordable furnaces, boilers and stoves. This is already well underway in Europe, but has yet to take off in America.

Can we burn cleaner in the future?

Yes. The Alliance for Green Heat supports stricter and more comprehensive national emissions standards. The EPA is now revisiting their standards for the first time since they were 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.

For producers that may sell only 10,000 units or less in a year, costs of testing is a significant part of getting a product to market. 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.

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 in the solar PV industry, for example. Unfortunately, the United States government and the Department of Energy in particular has put few research and development resources in this arena. In order for the American market to compete internationally in producing the cleanest and most efficient models, the prices of those models needs 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

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.

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 Fahrenheit.

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. Some pellet furnaces are now approaching the same level of convenience and emissions as fossil fuel furnaces.

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 low 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 has 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 boilers 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.

Comparing Emissions of Residential Heating Fuels

The most common fuels used for residential heating, are (in order of popularity) natural gas, fuel oil, wood, electricity, liquefied petroleum gas (LPG), coal and kerosene. Emissions from these fuels can be grouped into three categories: greenhouse gases, particulate matter and acidifying pollutants.

Greenhouse gases that come from home heating sources are carbon dioxide (CO2), nitrous oxide and methane. These compounds trap heat in the atmosphere, changing the planetary energy balance and contributing to global warming.

Fine particulate matter, also known as PM2.5, is directly emitted or formed as a secondary pollutant in the atmosphere, and consists mostly of sulfate, nitrate, chloride and ammonium compounds, and organic and elemental carbon. Particulate matter negatively impacts respiratory and cardiovascular health and reduces visibility.

The acidifying pollutants, sulfur dioxide, nitrogen oxides and ammonia aggravate/cause respiratory system problems and interact with water in the atmosphere, creating acid rain.

It is notable that wood heat has the lowest emissions of greenhouse gases among the home heating fuels (see Figure 1 below). The greenhouse gas emissions from the study of Air Emissions from Residential Heating are conservative in the estimate of the benefits of wood burning for CO2 reduction. Rather than assuming a nearly carbon-neutral cycle (which is theoretically possible), the study reduced the CO2 emissions from wood by a reasonable 40% based on current standard wood harvesting practices (including the fossil fuels used to transport and process the wood). This reduction in net CO2 emissions reflects the fact that trees consume CO2 while growing, and the release of CO2 is part of the natural carbon cycle. This is in contrast to the burning of fossil fuels, which transfers carbon from storage in the earth's crust into the atmosphere as CO2, where it remains for 100 years on average. Note: the results in all of these graphs reflect the energy trajectory of the fuels rather than just end point emissions.

Wood also has the lowest acid equivalence (see Figure 3 below) because of its low sulfur content compared to fossil fuels, which results in less sulfur dioxide formation upon burning and less sulfuric acid formed in the atmosphere to contribute to acid rain. Acid rain damages crops, ecosystems and even statues and buildings.

Electricity and coal have the highest overall emissions across categories. Natural gas ranks well in terms of emissions, but recently it has come to light that pollution caused by natural gas extraction, in a process known as fracking http://www.gaslandthemovie.com/whats-fracking has been underestimated.

Unfortunately, residential wood-burning appliances have the highest emissions of particulate matter (see Figure 2 below). This is because of the prevalence of old dirty stoves still in use, and highlights the need to replace these relics with new cleaner-burning models. New wood-burning technology stoves that are EPA-certified, labeled "new wood" in the graph, have significantly lower emissions of particulate matter than old wood stoves, and also lower emissions than coal and electric residential heating systems.

Source: Air Emissions from Residential Heating: The Wood Heating Option Put into Environmental Perspective. The Proceedings of a U.S. EPA and Air Waste Management Association Conference: Emission Inventory: Living in a Global Environment, v. 1, pp 373-384, 1998. Available at http://basineducation.uwex.edu/centralwis/pdfs/hpawma.pdf.