Pellet stoves

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.

An analysis conducted in 2009 found that particle emissions from wood pellet stoves were “approximately 15 times greater than those of oil-fired (PDF) units or approximately 1800 times greater than gas fired units.”

A 2022 Italian study that measured the ultrafine particle emissions from different types of “automatically-fed small-scale heating systems” found that pellet stoves had, by far, the highest emissions.

Start-up emissions aren’t counted

Emissions tests for pellet stoves omit the start-up phase of operation. In the US, the testing protocol for EPA approval allows the pellet stove to be run for an hour before emissions are officially counted.

A review of the US EPA’s wood stove certification program conducted by the Northeast States for Coordinated Air Use Management (NESCAUM) found that, “on average, first-hour emissions were 175 percent higher than the [pellet stoves’] certification value. The first-hour values ranged from 14 percent to 576 percent higher (PDF).”

A European study found that when higher-emitting phases of the burn cycle were included, pellet stoves exceeded their 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.

An Italian study found that the 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.”

More emissions testing issues

The NESCAUM review found that issues with pellet stove certification testing and test reports are widespread. “Of the 86 test reports reviewed, all had at least one element that triggered revocation criteria.”

Pellet quality impurities

A previous NESCAUM study 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 in 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.

NESCAUM analyzed 23 wood chip samples and 132 wood pellet samples manufactured in the US and Canada and available for sale in eastern parts of the United States. They found that most of the pellets tested would meet US 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.

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.

Toxic PAHs in pellet ashes

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