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How to reduce smoke emissions from domestic wood-burning appliances

Posted by Sandra Hayes on 13 July 2017 at 2:30 pm

Burning wood in a domestic stove or boiler can generally be regarded as a more sustainable approach to heating than traditional methods such as coal or gas heating. This is because the carbon dioxide (CO²) emissions produced as a result of burning wood fuel remain part of the carbon cycle, being taken up by new plants elsewhere via photosynthesis.

In common with conventional combustion systems, burning wood in a wood burning stove or boiler can emit a number of pollutants including nitrogen dioxide (NO2), particulate matter (PM), carbon monoxide (CO) and volatile organic compounds (VOCs) and sulphur dioxide (SO2).  The mix and amounts of pollution produced will depend on the size and design of the stove or boiler, the quality of the fuel used and the presence of any emissions abatement (cleaning) equipment (such as a secondary flow or a catalytic convertor -  in very large wood fuel boilers).  Generally a well maintained stove or boiler will produce more pollution than a similar gas system, but less than an equivalent coal or oil fired stove or boiler.

It is very important when using a wood burning stove or boiler to ensure that combustion is complete as incomplete combustion increases the level of pollutants and smoke production.  This can happen when wet wood is burned or if the fire is damped down to keep it going overnight.  Burning wood contaminated with paint or other finishes will also cause pollution and is likely to damage the stove or boiler.  Appliances also need to be maintained in compliance with the manufacturer’s instructions in order to keep potential pollutants to a minimum.

Stoves and boilers with a secondary air system will generally burn cleaner due to the increased levels of oxygen they provide.  Fuels such as wood pellets will generally burn the cleanest as they are small (meaning they have a greater surface area for their size) with a consistent, low moisture content. 

Wood can also be manufactured into compressed briquettes (also referred to as heat logs), which are usually between 60 mm and 150 mm in length and can be used as an alternative to logs in a log burning stove. Briquettes can offer a cleaner alternative to logs due to their higher energy density, low moisture content and steady combustion, although it is always worth checking whether they contain any additives before you buy.

Smaller chips and pellets can offer an even cleaner burn, as the large surface area to volume ratio means combustion is achieved in a very efficient manner. This makes a pellet-fuelled boiler or stove a suitable choice for those living within smoke-controlled areas (where only exempt appliances can be used) and other areas were background levels of pollutants are already an issue, usually in towns or cities.

Using an energy efficient stove or boiler is also a way of reducing potential pollution levels. As of 2022, any new stoves produced must meet lower energy-usage requirements set out by the European Union.  Further information on this initiative can be found at: http://www.which.co.uk/news/2017/03/new-wood-burning-stoves-to-produce-fewer-emissions/ and http://www.hetas.co.uk/ecodesign-ready-scheme/

Further useful information on this topic can be found at http://www.environmental-protection.org.uk/policy-areas/air-quality/air-pollution-law-and-policy/using-wood-and-coal-for-home-heating/

Image credit: Steve Douglas.
 

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