Big emissions from small chimneys

Domestic wood burning is a major source of air pollutant emissions – a new eco-labelled wood stove is allowed to emit 25 times more health-damaging particles than a ten-year old diesel truck.

Wood burning is perceived by many as being natural and therefore environmentally benign, but it is in fact a significant source of several harmful air pollutants, including fine particulate matter (PM_2.5), black carbon (soot), dioxins, polycyclic aromatic hydrocarbons (PAHs) and volatile organic compounds (VOCs).

These emissions contribute significantly to premature mortality and morbidity and the soot particles also contribute to Arctic warming and global climate change. Moreover, the share of emissions from residential wood burning in the EU is expected to increase as other key emission sources gradually become more efficiently regulated and because increasing costs of conventional home heating will continue to stimulate cheap wood burning.

In European emission inventories, small-scale combustion of fuels for domestic heating is reported under a sector called “commercial, institutional and household”. In the EU28, fuel combustion in this sector is the major emission source of primary PM_2.5, contributing 56 per cent of total emissions, as well as PM10 (40%), black carbon (46%) and the carcinogenic PAH compound bens(a)pyrene (71%).

Expressed as grams of particles per unit of energy, the emission levels from residential wood burning are so high that they totally overshadow those from other heat sources (see figure). In addition, detailed measurements from chimneys show that emissions of PM_2.5 may increase up to 30 times if a stove is not operated properly and up to 250 times if it is misused, which underlines the fact that emission levels can be much higher under real-life conditions.

Detailed measurements in residential areas in Denmark show that wood burning can increase local concentrations of PM_2.5 and PAH to levels similar to those found in rush hour traffic in central Copenhagen.

Other measurements have shown that wood stoves can give rise to very high levels of indoor air pollution, a significant problem especially in the winter period, when people spend most time indoors and ventilation is limited.

Biomass fuel is usually regarded as carbon neutral, because trees take up carbon dioxide (CO₂) from the atmosphere as they grow, and burning wood simply releases the same amount of CO₂ back into the air. This, however, assumes that the wood fuel comes only from forests that are managed and harvested in a sustainable manner, and it ignores the carbon footprint of transporting and drying the wood.

In addition, wood burning generates soot particles (black carbon) that contribute to global warming, especially in the shorter term. So even if wood burning may be largely carbon neutral, it is definitely not climate neutral.

Key challenges related to residential wood burning are the high pollution levels combined with the  long lifetime of each unit (usually more than 30 years) and the intensive use of wood burning because it is a cheap or sometimes even free method for heating in many parts of Europe.

So how can the emissions from domestic wood burning be reduced?

A first measure is to minimise the heat consumption of houses through better insulation. That way you will effectively reduce the amount of wood burned, independent of whether wood is a supplementary heat source (stove) or the primary heat source (boiler).

Then you look for cleaner alternative – primarily renewable – heating sources, such as solar heat, biogas, and geothermal energy, including heat pumps (driven by green electricity). District heating, especially from combined heat and power plants that are fuelled by renewables and from industrial surplus heat, is another option.

New stoves and boilers, especially those that are eco-labelled, are generally more efficient and cleaner than old ones, even though they will still generate higher emissions than the alternative heating sources. Replacing wood logs with pellets is usually beneficial, as it reduces the influence of bad burning behaviour and inferior fuel quality.

Improving stove operation can significantly reduce emissions – but this is a complex issue, and even well operated wood stoves can still be high emitters.

In 2015, the EU’s Ecodesign directive was extended to include energy efficiency and emission standards that will apply to new solid fuel boilers as from 2020 and to new local space heaters as from 2022. While these new standards will help to keep the worst products from entering the market, they are much less stringent than what can actually be achieved by the best already existing appliances. Consequently there is an urgent need to revise and strengthen these standards.

Because the Ecodesign standards are harmonized at EU level, member states are not allowed to introduce stricter national requirements. But there are a number of other measures that can be taken at national and/or local level, including:

• Incentivising energy renovation and home insulation, as well as small-scale solar and geothermal energy installations;
• Banning or restricting the installation and use of domestic solid fuel appliances – especially in highly polluted areas or during pollution episodes;
• Incentivising quicker replacement or shutdown of old appliances;
• Introducing emission-related taxes/fees on domestic solid fuel burning;
• Introducing ambitious eco-labels for stoves and boilers to promote front runners in the market;
• Information campaigns to make citizens in general, and wood fuel users in particular, more aware and to improve stove operation;

For example, replacing old wood stoves with Swan-labelled stoves and replacing old wood boilers with new boilers could halve the pollution from private wood burning. And if all wood stoves and wood boilers were replaced with wood pellet equivalents, the pollution from residential wood burning could be cut by more than 90 per cent.

Economic incentives, such as emission taxes or charges, combined with strict emission limit values, are efficient instruments for speeding up the replacement (phase-out) of heavily polluting stoves and boilers.

The Danish Ecological Council has designed a tax proposal for stoves and boilers in Denmark, which is based on the type of installation, varies with emission levels and is differentiated for urban and rural areas. The tax would be paid per hour of pollution (i.e. when the stove/boiler is used), and the hours of pollution are measured by a small temperature sensor placed in the chimney that counts the number of hours of use. The Danish Economic Councils, as well as the Danish tax authorities (Ministry of Taxation), have both calculated in 2016 that this is the most cost-efficient way to regulate wood burning. The polluter pays principle is then introduced by making the tax equal the pollution costs depending on the emission (type of device) and the health costs (urban/rural area).

The proposal would result in a fairer taxation of heat sources by increasing the fee on the most polluting types. Leading Danish health experts and economists estimated in 2016 that this tax could save about 350 annual premature death in Denmark and save Danish society health costs worth more than €450 million per year. Furthermore, incentives to insulate houses would be enhanced, as well as the promotion, sales and development of better stoves and cleaner heat sources.

Kåre Press-Kristensen and Christer Ågren

Sources: Information from the EU LIFE project Clean Heat (www.clean-heat.eu), especially the brochure “Residential wood burning – Environmental impact and sustainable solutions” and the booklet “Pollution from residential burning – Danish experience in an international perspective.”

Figure: Emissions of fine particles from different sources, expressed as grams of PM_2.5 per gigajoule of energy.