Is biomass heating causing urban air pollution?


In the UK, more energy is used to provide heating and hot water in our buildings and industries than for any other purpose. As a result, a third of the UK’s carbon emissions originate from heating. Heat in the UK is currently supplied predominantly by fossil fuels – with less than 6.5% [2015] from renewable sources.

At the same time, urban centres are facing severe air quality issues with air pollution limits frequently breached[1]. National and local policies aim to reduce emissions in urban areas, where the concentration of emissions are highest. Overall, the majority of air quality emissions originate from road transport, however, some have also been identified as originating from wood burning. As figures 1 and 2 below from the Greater London Authority show, around 4% of PM10 emissions and 3% of NOx emissions originate from commercial and domestic other fuels, which include emissions from wood burning. However, the GLA data does not go further into details of the “other fuel” category. This white paper further looks at the evidence on how biomass boilers are regulated, urban wood burning emissions, and origin of the emissions.

Figure 1: Origin of NOx emissions in Greater London (2013, GLA LAEI2013 update)


Figure 2: Origin of PM10 emissions in Greater London (2013, GLA LAEI2013 update)


Urban Wood Burning Emissions

The research and measurement programme of Fuller et al. (2014)[2] suggest that emissions related to wood burning mainly arise from “a decorative or secondary heating source”, from garden waste burning and from patio wood burners. This would indicate that the relatively minor emissions from urban wood burning are not from the types of biomass boilers as supported under the RHI, but instead from open fires and older domestic stoves.

Open fireplaces and older stoves differ from biomass boilers in a number of ways. Biomass boilers produce hot water for central heating systems and water heating, and even process heat at commercial scale. They vary in size from 5kWth for low energy houses up to 5,000kWth or more for large building complexes or industrial process heat, and are many times more fuel efficient than open fires. Pellet boilers operate with automatic feeding systems that bring in fuel from a storage room and can replace oil heating systems. Open fires and wood burning stoves which are the sole source of heating are unusual in urban areas, but more common in rural areas, where wood or other solid fuels are sometimes used provide direct space heating and central heating and hot water in homes.

The Government’s Domestic Wood Survey[3] shows that 68% (2014) of wood burning appliances in London were open fires. Together with the results from Fuller et al (2014), this would suggest that the main issue of emissions from wood burning in urban centres is related to open fires and, in part, older stoves.  Poor enforcement of Clean Air Act requirements, which are primarily intended to regulate pollution from domestic solid fuel, is undoubtedly a major contributing factor to urban air pollution in towns and cities.

The new EU Ecodesign Directive includes new emission standards for new wood stoves, and UK regulations were planned to be implemented prior to 2022. Many Ecodesign compliant stoves are available on the market now as “Ecodesign Ready” which is supported by Defra. Newer wood stoves are therefore expected to have significantly lower measured emissions.

Biomass boiler emission regulations

Biomass heating systems burn fuel such as wood pellets, chips or logs to provide central heating and hot water in homes, hospitals, schools, the public sector, industrial sites, not-for-profit organisations, small and large businesses, and care homes. They can also generate high-temperature heat for industrial processes.

Since September 2013, any biomass boilers installed and commissioned under the Renewable Heat Incentive (RHI) scheme have been required to meet strict air quality levels. The air quality requirements set limits on the emissions a product can produce. Products must operate within these limits to be eligible for the Domestic and Non-domestic RHI scheme. Under the scheme, PM emissions must not exceed 30 grams per gigajoule net heat output, and NOx emissions must not exceed 150 grams per gigajoule net heat output. No biomass boiler installed since September 2013 should receive any government support without meeting the requirements.

The analysis below assesses the RHI Emission Certificates for biomass boilers installed under the Renewable Heat Incentive in Great Britain.


Through a Freedom of Information Request sent to the Department for Business, Energy and Industrial Strategy, the Wood Heat Association has accessed data on the biomass boiler emission test scores. The data reflects the 1,205 biomass boilers that have RHI Emissions Certificates, with information on measured emissions of PM and NOx in g/GJ net heat input. As several boilers had different measured emissions depending on the type of fuel (usually wood chip and wood pellets), the average value was selected, since there was no further description to describe which value was most appropriate.

Analysis of biomass boiler emissions

The analysis of the emissions test results confirms that the measured emissions for all boilers are within the required maximum. The average grams of PM emissions per gigajoule net heat output are 11.92g PM/GJ, well below the limit of 30g PM/GJ. As demonstrated by figure 3, the vast majority of boilers have particulate matter emissions that are considerably below the legal limit. Figure 3 shows how many tested boilers have measured PM emissions within different emission ranges, which shows that 77.1%, or 929 boilers, have tested measured PM emissions at less than half the legal limit, and 48.2%, or 581 boilers, emitted less than a third of the legal threshold.

 Figure 3: Number of registered biomass boilers per particulate matter (PM) emissions


The average grams of NOx emissions per gigajoule are 83.45g NOx/GJ, also notably below the legal limit of 150g NOx/GJ. Figure 4 demonstrates that the vast majority of biomass boilers had measured emissions in the range of 51-100g NOx/GJ. Specifically, 37.5% had measured NOx emissions at less than half of the legal limit, and 84% emitted less than two-thirds of the legal maximum (i.e. 100g NOx/GJ or less).

Figure 4: Number of registered biomass boilers per nitrogen oxides (NOx) emissions



PM10 and NOx emission analysis for London shows that the majority of emissions originate from road transport and other forms of transport. In urban centres like London, only 3-4% of air quality emissions stem from “other fuels”, and it is unclear how much of the “other fuels” related emissions can be attributed to wood heating. Measurement programmes show that the emissions related to wood burning mainly originate from garden waste burning, patio wood burners, and “decorative or secondary heating source[s]”; and government data show that most of the decorative/secondary heating sources in London are open fires.

Unlike open fires and old (non-Ecodesign) stoves, the majority of RHI-registered biomass boilers are strictly regulated by Ofgem in terms of their PM and NOx emissions, and analysis of official emissions certificates shows that the vast majority of RHI supported biomass boilers emit less than half of the legal limit, and most significantly less if operated in line with manufacturers’ instructions.

This, and the fact that most biomass boilers are installed off the gas grid, makes it unlikely that biomass boilers contribute significantly to urban air quality issues, as the main causes are transport, and the few emissions originating from wood heating stem from open fires. It is, therefore, wrong to say that biomass boilers contribute to air quality problems in cities.

For further information, please contact Frank Aaskov, Policy Analyst at the Wood Heat Association at or 020 7925 3570.


[1] Carrington, D. (2017, Jan., 6), London breaches annual air pollution limit for 2017 in just five days, The Guardian, Retrieved from

[2] Fuller, Gary W. (2014), Anja H. Tremper, Timothy D. Baker, Karl Espen Yttri, David Butterfield, Contribution of wood burning to PM10 in London, Atmospheric Environment 87 (2014) 87-94, Retrieved from

[3] DECC (2016), Summary results of the domestic wood use survey, Renewables statistics, 31 March 2016, Retrieved from