EThere is a dispute about pellet heating systems. Initially praised as climate-friendly, they are now being pilloried as forest destroyers and emission slingshots. Beat Müller from the Swiss Federal Office for the Environment in Bern explains which pollutants are emitted, why carbon monoxide is not a problem – and what particle filters and separators do.
Mr. Müller, your authority has researched the exhaust gases from various heating systems. How much fine dust does a classic stove release?
A stove emits an average of around 100 milligrams of fine dust per megajoule.
Is that much?
That’s quite a lot. Oil and gas furnaces have values of around 0.1 to 0.2 milligrams of fine dust per megajoule.
Wood-burning stoves therefore blow a thousand times more fine dust into the air than gas or oil heating.
Yes, on average. When hundreds of thousands of these stoves smoke, the air pollution increases enormously. If you live in a village where many people heat with wood, there is quickly a high level of fine dust pollution in winter. I see that here in the village where I live. There are two or three bad wood furnaces. When they start, you can smell it immediately.
Pellet stoves are considered by many in Germany to be the heating of the future. If you look at the emissions, however, they also blow enormous amounts of fine dust into the air, about half of the emissions from wood-burning stoves.
Pellets are often a bit better than firewood, that’s true. Our results show mean values, the scatter is large. Not necessarily with oil and gas, but above all with wood in general and with individual room furnaces, the fine dust values can also be ten times higher, but also lower. But it doesn’t have to stay that way. You can make heating with wood cleaner.
How come?
The most important measure for reducing air pollutant emissions from wood firing is the use of clean and natural wood and combustion that is as complete as possible. This requires care when operating the system and regular checks by the chimney sweep or furnace inspector. The prerequisite is a system that allows complete combustion.
Why are the emissions from wood so enormously high compared to oil and gas?
When burning wood, the wood outgasses, and in the end wood gas burns. The way to this wood gas is long, starting with the solid, which first has to be gasified, sometimes with a lack of oxygen, which results in soot and other pollutants. The combustion conditions for wood are generally more demanding. Oil, on the other hand, is finely atomized and burns with a lower level of fine dust pollution.
In terms of energy density, wood releases more carbon dioxide and methane than oil and gas, and burning wood can also produce dioxins.
Dioxins are not the primary problem when burning wood. They come about when the wood that you burn is not natural. Dioxin can be emitted under unfavorable conditions, but this is rather an exception.
In terms of all pollutants, pellets perform slightly better overall than split logs. Is that because pellets are standardized and therefore contain hardly any water?
The combustion of pellets can be better controlled, they also have a large surface area and therefore burn cleaner.
What other pollutants are emitted when burning wood?
Particulate matter is the biggest problem. The volatile organic compounds, the NMVOC, are also harmful to the environment. These are small molecules that are emitted as gas. However, there are also larger organic molecules that are emitted in gaseous form at high exhaust gas temperatures and condense into particulate matter in the ambient air. Such secondary organic aerosols have a toxic effect, just like fine dust in the form of soot. Even larger molecules are referred to as tar, which blocks the chimney and can lead to chimney fires.
There is also the risk of carbon monoxide poisoning.
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