Two technologies make wood-burning stoves environmentally friendly

A study shows that a combination of two technologies is the best way to reduce pollutant emissions from wood-burning stoves. The authors recommend making this mandatory. Currently, around 3.5 million stoves in Germany need to be retrofitted if they are not to be replaced or shut down.

Many German citizens urgently need to consider how they can reduce the pollutant emissions from their wood-burning stoves or tiled stoves so that they comply with the Ordinance on Small and Medium-Sized Combustion Installations. This is because it stipulates that all single-room firing systems built between 1995 and March 21, 2010 must be shut down, retrofitted or replaced with a new low-emission stove by the end of this year if the existing one does not comply with the applicable limit values. According to the annual survey by the chimney sweep trade, around 3.5 million systems are affected, the majority of which are wood-burning stoves.

The measure makes sense. This is because wood-burning stoves not only produce large quantities of fine dust, but also polycyclic aromatic hydrocarbons (PAHs) and formaldehyde. According to the Federal Environment Agency (UBA), these include some carcinogenic, mutagenic and/or reprotoxic pollutants. The amount of harmful emissions emitted by wood-burning stoves even exceeds that of car and truck engines. The older an installation is, the greater its emissions of particulate matter and PAHs usually are.

Catalytic converters and electric separators

If a new oven is not to be purchased, there are basically two technologies to reduce pollutant emissions to a level that is acceptable for people and the environment: electrostatic dust collectors and catalytic converters. An electrostatic precipitator charges the particles in the flue gas electrostatically, causing them to be separated at the chimney pipe. The catalytic converter causes toxic gaseous substances to react to form non-toxic substances. For example, carbon monoxide and hydrocarbons are transformed into carbon dioxide and water.

However, very little research has been carried out into exactly how effective the two techniques really are. Scientists from Goethe University Frankfurt, RWTH Aachen University and the University Hospitals of Aachen and Freiburg have now made up for this with the joint project"TeToxBeScheit". They have investigated the extent to which catalytic converters and e-separators individually and in combination reduce pollutant emissions and how well this protects people and the environment.

Completely new findings

"This is the first study of pollutants in wood-burning stoves with an integrated approach that goes far beyond the chemical analysis of individual substances," says Henner Hollert from Goethe University Frankfurt. "Together with the other partners, we took a comprehensive look at the flue gas, the particles emitted and the effect of the reduction measures."

Not only the chemical-physical side, but also the effect of pollutants and combinations of pollutants on humans and ecosystems was researched. "This effect-based investigation can also prove the detrimental effect of previously unknown pollutants and pollutant mixtures and has not been carried out in similar studies before," emphasizes Hollert.

Experiments with water fleas and algae

In nature, pollutants also enter bodies of water when they are washed out of the air by rain, for example. Therefore, in addition to chemical-physical measurements, the researchers looked at the reaction of water fleas, algae and fish embryos to pollutants in water.

The toxicity was clearly evident in water with untreated flue gas: the organs of the fish embryos were damaged, the water fleas died and the algae grew more slowly. With an upstream catalyst, on the other hand, there were no more toxic effects and the pollution of aquatic systems was greatly reduced.

The electrostatic precipitator proved to be less effective directly at the fire. The toxicity only decreased when the device was installed further away. The reason: the temperature influences the efficiency of the separator. Certain substances only bind to particles in the cooled exhaust gas and can thus be separated.

State-of-the-art lung model

In order to investigate the consequences of people breathing in pollutants emitted by stoves, a cell-based lung model was used, which is currently considered the most advanced method. In terms of human toxicology, the catalytic converter also initially performed better than the e-separator. This was again due to the fact that, although e-separators significantly reduce fine dust pollution, they only neutralize gaseous pollutants from the exhaust gas to a limited extent.

Sabrina Schiwy from Goethe University therefore sees catalytic converters as the "winner" of the study. They are universally effective and can reduce highly reactive substances that enter human lungs in gaseous form or even as fine particles, she says.

Catalysts are the winners

An additional disadvantage of e-separators is that their use is currently counterproductive in some cases. This is because it has led to increased levels of unhealthy volatile and highly volatile organic compounds (VOC/VVOC) in the tests. The separator also promoted the formation of soot deposits. According to the researchers, solutions still need to be found for this, for example additional downstream catalysts are conceivable.

The good news for stove owners: the catalytic converters are not too expensive, they can be retrofitted for around 400 euros. However, the authors of the study also consider the use of electric separators to be indispensable. Their immediate effect is initially less noticeable, they say. But they could reduce dangerous particulate matter emissions by up to 95 percent and therefore have an effect in an area that catalytic converters do not cover, they argue.

Combination allows stricter limits

The researchers therefore recommend the combined use of both technologies in the medium term. The e-separator should be installed upstream of the catalytic converter so that it separates the particles first. The catalytic converter then takes care of the gaseous substances.

According to their findings, stricter limit values should apply in future, the scientists write in the summary of their work. In the first instance, incentives should be created for the use of catalytic converters, followed by e-separators in a second step. They also advocate the use of electronic furnace controls to prevent operating errors and sub-optimal furnace operation.