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No. 779: Contribution to air pollution from residential wood combustion. Results from the Wooduse project

Olesen, H.R., Wåhlin, P. & Illerup, J.B. 2010: Contribution to air pollution from residential wood combustion. Results from the Wooduse project. National Environmental Research Institute, Aarhus University. 71 pp. - NERI Technical Report no. 779

Summary

This report is based on results from the research project WOODUSE. Furthermore, it summarises some of the available knowledge on air pollution from residential wood combustion in Denmark. One objective of the report is to clarify issues that may cause misunderstandings in the public debate on smoke from wood stoves. However, the report does not pretend to cover all aspects of air pollution from wood combustion in Denmark.

WOODUSE is a comprehensive research project with the title Residential wood combustion and the interaction between technology, user and environment. The project is interdisciplinary and covers a wide range of issues related to residential wood combustion. It includes investigations of emissions, air pollution level in outdoor and indoor air, health effects and social aspects. The project has had participation by four Danish institutions (National Environmental Research Institute at Aarhus University, National Research Centre for the Working Environment, Department of Public Health, University of Copenhagen, Department of Civil Engineering, Technical University of Denmark). The project has received funding from the Strategic Research Council (grant ENMI-2104-05-0010, period 2006-2009).

The central new work in the present report concerns results from a measurement campaign in the village of Slagslunde and derived results that have been obtained by combining the measurements with air quality model computations. Furthermore, the report presents results from other Danish and international studies, compiled here in order to provide an overview of central issues related to residential wood combustion.

One can distinguish between primary particles and secondary particles. Primary particles exist as particles immediately after they have left the source. Secondary particles were not 'born' as particles, but are created from gases, which undergo chemical and physical transformation during transport – a process that continues for several hours or days after the pollution has left the source. Secondary particles can be further characterised as secondary inorganic particles or as secondary organic particles.

Emission inventories of particle pollution address only primary particles. Domestic wood combustion is a dominating source to Danish emissions of primary particles.

On the other hand, concentration measurements in the atmosphere comprise both primary and secondary particles. Considered on a yearly basis, the contribution from wood combustion does not constitute a dominant fraction of the total amount of particle pollution in the air.

However, this does not mean that the contribution from wood combustion is unimportant. The contribution is considerable, and many studies in literature have established a clear statistical correlation between particle pollution and adverse health effects. There is no indication that particles from wood combustion should be less dangerous than other particles.

Furthermore, on a short time basis one may experience episodes with pronounced wood smoke pollution at various times and locations. Complaints from neighbours are often caused by this type of episodes.

The report presents estimates for the contribution to particle concentrations (measured as PM2.5) in outdoor air for three locations, where studies addressing pollution from residential wood combustion have been conducted. When generalised to a yearly basis, estimates for the contribution from wood combustion range from 0.5 to 2.3 microgram/m3 for specific locations and years.

In the case of Slagslunde, it has been possible to substantiate the estimate with quite detailed model calculations of the wood stove contribution to concentrations. The calculations provide information on the geographical distribution of the contribution and its dependence on meteorology – e.g., low wind speeds lead to relatively high concentrations.

There are two locations with a contribution of around 2 microgram/m3 on a yearly basis – Gundsømagle and Slagslunde. These must both be regarded areas with a high level of wood combustion activity. A third area – Vindinge – with an estimated contribution around 0.5 microgram/m3 on a yearly basis is characterized by moderate combustion activity. These estimates provide an indication of what can be expected in other Danish residential areas with moderate to high combustion activity.

It is important to note that the firing habits of wood stove users are extremely important to emissions. There is a large potential for reduction of pollution if the extent of bad firing habits can be reduced.

Anyhow, even a modern wood stove pollutes much more than a central heating plant per kilo combusted wood. For instance, the emission from a stove that just complies with the Danish wood stove regulations (Brændeovnsbekendtgørelsen), is several hundred times larger than for a large wood-fired combustion plant.

It is a great challenge to determine representative emission factors for wood stoves, because the frequency of bad and good firing habits is all-important.

In the Slagslunde study it has been possible to investigate how well the official emission factors fit with concentrations measured, based on information on actual firewood consumption etc.

In order to achieve consistency, in Slagslunde it is pertinent to use a considerably smaller average emission factor than the one resulting from the set of official emission factors. The results suggest that the emission factors for stoves categorised as 'old stoves' and 'newer stoves' should be decreased to around half of the official values which were used in 2005, in order to fit with Slagslunde data. Consistency can be acheived by using an aggregated average emission factor for Slagslunde of around 7 g PM2.5/kg firewood (440 g/GJ). However, it should be stressed that such adjustment of emission factors is subject to considerable uncertainty, and that it cannot readily be assumed to be valid for Denmark in general. It should further be noted that the official emission factors are updated annually, resulting in a decrease in the official aggregated emission factor from 672 g/GJ in 2005 to 533 g/GJ in 2008.

In a chapter on emission factors, a potential pitfall is pointed out: There is a dramatic difference between emission factors based on measurements which include condensable particles (such as the Norwegian standard NS 3058-2), and emission factors based on measurements which neglect condensable particles (such as the German standard VDI 2066). The difference is modest if a stove is operated under completely ideal conditions, whereas the difference is a factor 2-10 for a stove in realistic operating conditions. Emission factors accounting for condensable particles – as those commonly used in Denmark – are the most relevant as an indicator of outdoor air pollution.

Finally, interesting results have emerged from a limited study of indoor pollution, where focus was on soot in indoor air. In a house with a wood stove, the stove can contribute considerably to indoor pollution. In a case study, bursts of pollution were observed to be emitted each time a cold stove was lit. Poor draft had the result that pollution was dispersed into the living area instead of being emitted through the stack.

Full report in pdf (1,54 MB)