Unsteady combustion phases of pellet stoves are associated with high particulate, CO and PAH emissions that are not reflected in testing values.

Problems with Pellets

Pellet stoves are promoted as a cleaner burning option. However, while it is true they emit fewer particulates than a log-burning stove, they are still significantly more polluting than a non-wood-burning form of heat. A pellet stove emits 15 times more PM2.5 particulate pollution than oil heating, and up to 1800 times that of gas.

 

Some Emissions Not Counted Officially

Photo of pellets burning.As one study noted, emissions tests for pellet stoves generally omit the start-up and stop phases of operation. Yet, it is during these “transient operations” that pellet stoves produce “significantly high emissions such as carbon monoxide and hydrocarbon from incomplete combustion.” The researchers noted that when these higher-emitting phases of the burn cycle are included, the pellet stoves exceed their “eco” certification limits.

 

Another study similarly concluded that the emissions from pellet stoves “measured according to technical standards does not provide representative data with respect to a real domestic utilization,” due to differences in operation and combustion conditions as well as the exclusion of the start-up phase from testing. This study also noted that settings on pellet stoves are often not optimized for the type of pellet being burned, further increasing emissions of various compounds.

 

It has also been demonstrated that carcinogenic and mutagenic polycyclic aromatic hydrocarbons (PAHs) emitted from pellet stoves during start-up “have a higher toxicological burden” than during steady-state burning and that the ignition phase contributes substantially to the overall pollution output from pellet stoves. The researchers of this study concluded, “… in order to evaluate the real impact of pellet stoves on the environment, transient conditions should be taken into account. The ignition phase, even though it lasts only 20 minutes, can significantly contribute to pollutant emission.”

 

Pellet Stove Emissions

A study by the Northeast States for Coordinated Air Use Management (NESCAUM) noted that the combustion of wood pellets “produces atmospheric emissions of particulate matter, carbon monoxide, nitrogen oxides, volatile organic compounds, mineral residues, and to a lesser extent sulfur oxides.”

 

Pellets, however, are assumed to be cleaner than other kinds of wood combustion, largely on the assumption that pellets are relatively uniform in content and low in moisture, and their combustion in automated pellet stoves is more controlled than that of log-burning stoves. However, emissions can still vary significantly depending upon what kind of pellet stove is used, how it is operated and what kind of pellets are used.

 

Pellet Quality Impurities

The NESCAUM study analyzed 23 wood chip samples and 132 wood pellet samples manufactured in the U.S. and Canada and available for sale in eastern parts of the United States. They found that most of the pellets tested would meet U.S. voluntary standards, but would not be likely to meet the higher standards for residential use in Europe.

 

Some pellet samples in their study had “unusually high concentrations of several heavy metals, including arsenic, copper and chromium.” It was assumed that this was due to the use of recycled preservative-treated and painted waste wood. Their analysis found a wide range of results, with many samples “higher than the ‘normal’ benchmark value.” Some samples had levels of heavy metals 4 to 7 times higher than the next highest sample.

 

Photo of bag of pellets displaying CA Prop 65 toxicity warning.A legally mandated warning label on a bag of wood pellets in California.They also found that some pellets, in spite of claiming to be made from debarked wood, were “distinctly elevated for many metals,” while others were “relatively clean.” They found “no clear pattern relative to barked or debarked wood and elevated levels of metals as might be expected.” Even different samples from the same brands often had differing levels of contaminants.

 

Some pellets that were darker, suggesting the inclusion of bark, had high cadmium concentrations. The study noted that high cadmium concentrations have been found in the bark of coniferous trees, debarked pine and willow. Trees generally store the metals that they absorb through the roots in their bark, which tends to have higher cadmium levels than in core wood.

 

Others researchers have also noted the heavy metal content in both pellet and log-burning stove ashes.

 

Toxic PAHs in Pellet Ashes

It has also been noted that ash from pellet stoves contains highly toxic polycyclic aromatic hydrocarbons (PAHs). One study notes that ash is often used as a soil amendment, and recommends that ash from pellet stoves not be used on plants destined for human and animal food due to toxicity concerns.

 

Pellet Emissions: Small Particle Size

Other research has shown that the particles from pellet stoves, although fewer in number than from a log-burning stove, tend to be in the small PM1 size range that are particularly damaging to health (please see our Particulate Pollution page for more information about fine particulates).

 

For more information about common household wood-burning devices, please visit the following links: