Does household use of solid biomass-based heating affect air quality?

Solid biomass combustion can emit various air pollutants that negatively affect human health, and air pollution from biomass combustion in the residential sector has recently come under the spotlight. In the best cases, these emissions can be maintained within acceptable limits, but emissions levels vary significantly according to type of heating system, how the system is operated, and the characteristics of the fuel used.

The most relevant pollutants from solid biomass combustion that affect air quality and pose health risks are products from incomplete combustion (PIC), such as volatile organic compounds (VOC) and solid PM (biomass char, soot and tars). Incomplete combustion also emits carbon monoxide (CO).

In addition, solid biomass combustion results in additional gaseous pollutants such as NO_X and SO_X. Waste wood contaminated with heavy metals and organic chlorine compounds can emit even more air pollutants, but for large-scale combustion of such fuels, appropriately designed flue gas cleaning equipment can reduce emissions to acceptable levels. In small-scale heating applications, uncontaminated woody biomass is predominantly used.

When solid biomass combustion contributes to the PM already in the ambient air from other sources, adverse health effects can result. Inhalable PM smaller than 10 micrometres (PM₁₀) is a key indicator of the health impacts of air pollution. There is strong evidence that exposure to PM₁₀ and PM_2.5 can cause cardiopul­monary and cardiovascular diseases, and consequently lead to higher rates of mortality (Nussbaumer, 2017). Combustion particles in the ambient air can also act as carriers of toxic and carcinogenic substances, and CO is a toxic gas that can cause death at high concentrations.

Air pollutant emissions from residential biomass-based heating vary considerably depending on the type of heating system, how the system is operated, and the characteristics of the fuel used.

Open fireplaces and simple log stoves generally create high air pollutant emissions and should be discouraged in urban areas for air-quality reasons. Conversely, modern biomass boilers and stoves can comply with stringent emissions limits when operated appropriately, as outlined in Table 5.3, which shows typical organic carbon (OC) fractions of PM from different biomass heating technologies, indicating the potential health impact of emissions.

While only biomass combustion technologies are shown above, fossil heating fuels, particularly coal, also result in high air pollutant emissions. For example, coal combustion for residential heating can lead to PM emissions of 20 g/GJ to 430 g/GJ (Butcher and Ellenbecker, 2012) as well as generally higher sulphur dioxide (SO₂) emissions than from biomass fuels. Conversely, PM emissions from natural gas are lower than from biomass fuels.

The type and level of emissions from biomass heating is linked to whether complete or incomplete combustion occurs in the heating system, as reflected in the wide range of typical PM emissions for some biomass heating devices listed in Table 5.3. Complete combustion minimises harmful air pollutants from PIC, but an ideal mixture of combustible gases and combustion air, air-fuel ratio, residence time in the combustion chamber and flame temperature are required to achieve complete combustion.

If the conditions necessary for complete combustion are not met, secondary measures such as catalytic converters and electrostatic precipitators are available to reduce pollutant emissions from wood stoves and biomass boilers. However, secondary measures are not commonly used in residential-scale systems because they can be costly. Furthermore, the effect of secondary measures can be limited by inappropriate combustion conditions, so they only partly replace the need for near-complete combustion.

How the heating system is operated strongly influences air pollutant emissions, especially for manually operated boilers and stoves. For manual systems, the manner of fuel ignition, quantity of fuel introduced to the combustion chamber and level of combustion air supplied must be carefully administered according to best practices to minimise pollutant emissions. Managing these conditions manually to ensure near-complete combustion is complicated, so actual emissions can significantly exceed those achieved for a given system under test conditions. This highlights the importance of considering real-world operational conditions when developing emissions standards.

More sophisticated automated heating devices can maintain the conditions necessary to achieve near-complete biomass combustion, and therefore generate very low PIC emissions when operated appropriately. For automated systems, design guidelines and best-practice system specifications are preconditions for ideal operation, and these systems have higher investment costs than simpler, manually operated boilers and stoves.

Ensuring the appropriate fuel is used in the combustion device is also of key importance. To maintain low air pollutant emissions factors such as fuel moisture content, size and ash content must be appropriate. The application of biomass fuel standards, e.g. ENplus for wood pellets, and quality standards for combustion systems and emissions measurement, can further minimise the impact of residential biomass-based heating on air quality.

In 2017 the European Commission announced it was pursuing infringement actions against 16 member states for excessive PM₁₀ levels. Consequently, air pollutant emission regulations, monitoring and enforcement is likely to become more stringent for residential solid biomass heating systems. Italy, which has the largest residential wood pellet heating market in Europe, recently passed new legislation to classify biomass stoves according to level of pollutant emissions.

Although solid biomass-based heating offers a renewable, lower-carbon alternative to fossil fuel-based systems, to ensure there is no trade-off between decarbonisation and air quality, policy support may increasingly require that biomass heating systems limit air pollutant emissions to acceptable levels.

Minimising air pollution emissions through sophisticated technologies, appropriate fuel selection and best-practice operation is likely to be key to future market prospects for residential biomass heating. This is already evident, as subsidy eligibility for biomass boilers in Germany and the Netherlands is linked to minimum emissions criteria. Such criteria also apply to wider bioenergy applications, for example in France, where biomass co-generation tenders include air quality in the selection criteria.