Off the back burner

Rick Gould looks at the effects of wood-burning stoves on air quality

Last September, London’s Mayor, Sadiq Khan, called for strong restrictions on wood-fuelled stoves, following reports that burning wood makes a big contribution to particulate air pollution. His proposals included a ban on solid fuels in low emissions zones. A report published in 2017 jointly by King’s College London (KCL) and the National Physical Laboratory (NPL) and released in early 2018 confirmed the link between wood-burning and poor air quality.

But this research also showed how complex the situation is; for example, the team found a downward trend in emissions, while the levels of particulate emitted strongly depends on the type of wood-burning appliance.

The rise of the stove

According to the Stove Industry Alliance (SIA), the sector’s trade body, UK suppliers sell about 175,000 stoves annually, with around a million stoves sold over the past decade and a greater proportion in London. This growth has been catalysed by schemes such as the Renewable Heat Incentive, the Merton Rule, and the perception that wood is a cleaner, greener, low-carbon source of energy.

Indeed, the SIA adds that “modern wood-burning stoves are virtually carbon neutral when using current burn technology”.

However, wood-burning can also emit considerable amounts of respirable particulate matter, PM10, as well as its more harmful, smaller component, PM2.5. Owing to the rising popularity of wood-burning, UK researchers and their overseas counterparts have examined how this affects air quality.

Aethalometers and levoglucosan

In 2014, a team from KCL, NPL and the Norwegian Institute for Air Research published the results of a three-year investigation of wood smoke in London.

“Our wood-burning investigations began in 2009. We were concerned about increased particulate concentrations in UK cities from schemes that were being introduced to decarbonise heating,’ explains Dr Gary Fuller of KCL. “We measured particulate along a 35km transect across London in 2010, as a baseline to assess any changes. I had many sleepless nights worrying if our measurements would be sensitive enough to detect wood-burning in London,” he describes. However, his concerns were allayed. “Wood-burning was already well established and adding about 1.1 µg m3 to the annual mean particulate matter in London,” he says.

Cities overseas, such as Paris and Berlin, had seen similar results. Putting these figures in context, the emissions from wood-burning greatly outweighed the reductions in PM10 from traffic that resulted from the first two phases of the London Low Emission Zone.

When determining airborne particulate, the team analysed the ambient concentrations of levoglucosan, a by-product of wood combustion, as well as measuring particulate matter with an optical instrument known as an aethalometer. This instrument can show whether the particulate matter originates from motor vehicles or burning wood.

The team didn’t distinguish between domestic, municipal and commercial wood-burning, but the data itself was informative. ‘In 2010, the diurnal and seven-day variations pointed strongly to domestic sources and not offices or schools, for example,’ explains Fuller.

Based on these findings, KCL and NPL continued this research, adding data from Birmingham and extending the investigation into 2016. They then determined that wood burning could contribute between 23% and 31% of airborne PM2.5.

When considering the headline data at face value, it would be simplistic to conclude that wood-burning stoves are solely responsible. But the team emphasises that the emissions from wood-burning vary widely depending on the appliance used, while The Department for Environment, Food and Rural Affairs (Defra) and the SIA point out that the type of wood and its moisture content also influence the emissions. So how do these factors affect air quality?

From open fires to eco-stoves

In the report released this year, the authors point out that there is a wide spectrum in emissions from open fires to modern, wood-burning stoves and boilers that have higher combustion efficiencies.

For example, the team cites emission tests performed in Portugal; this research found that particulate emissions from fireplaces were three times greater than those from a traditional stove, 12 times greater than those from a modern, eco-labelled stove, and fifteen times greater than a similar stove fuelled with wood pellets.

Moreover, the SIA points to estimates that in domestic properties in London, up to 70% of wood is burned in open fires. Additionally, a survey of national, domestic wood-use carried out in 2015 by the Department for Business, Energy & Industrial Strategy revealed that the proportion is 40% nationally. This suggests that burning wood in open fires is the biggest source of poor air quality. Notably, the Clean Air Act prohibits wood-burning in open fires in much of London.

Most stoves in the UK are traditional ones that can be used in smokeless zones where they have an exemption. Such appliances have to be tested and meet specified limit values for emissions of particulate matter. From 2022, all new stoves in the UK must meet the EC’s eco-design criteria, such as much tighter emission limits for particulate matter.

In simple terms, modern stoves typically pollute less. For example, a report in the 2012 Journal of Occupational and Environmental Medicine described a programme in the US town of Libby, Montana, where 1,100 older stoves were replaced with modern, cleaner, more efficient types between 2005 and 2007. Following this, PM2.5 concentrations dropped by about 28%, whilst the incidence of respiratory illnesses also fell.

Similarly, when considering the downward trend in emissions from wood-burning, KCL and NPL suggest that “one possible explanation is the replacement of high-emission fireplaces with newer, lower-emission wood-stoves, balancing an increase in total wood heating”.

Let’s not forget the seasoning…

Numerous studies have also found that the type of wood fuel significantly affects emissions. Kiln-dried or seasoned wood, for example, burns much more efficiently than freshly cut wood, with the latter having emissions that can be several times higher. Waste wood has even higher emissions and can result in releases of heavy metals.

Prompted by the research results, Defra has produced guidance on wood-burning, providing users with advice to keep emissions low. The SIA is also promoting eco-design-ready stoves before the 2022 deadline, as there are several brands already approved. Both the SIA and Defra recommend using certified Ready to Burn wood with a moisture content below 20%.

In February, Defra consulted on wood-burning and will use the responses to determine its options for reducing pollution. It has emphasised that it does not intend to ban wood-burning, but will act to reduce emissions and improve air quality. This is likely to involve stronger measures to enforce the Clean Air Act and stop wood-burning in open fires and a faster transition to eco-compliant stoves.

Ultimately, however, perhaps we should consider just using less wood fuel or none at all if air quality is at risk. “I think we should really question if home wood-burning has a place in the city of the 21st century,” concludes Fuller.

Rick Gould MIEMA is writing in a personal capacity as a freelance journalist