Current Understanding

Environmental impacts of residential, solid fuel combustion:

Although solid fuels contribute a relatively small fraction of the energy required for domestic space heating in urban areas, the relatively low combustion efficiencies of the appliances – especially open fireplaces – in which they are burned ensures that their combustion results in a significant contribution to particulate matter (PM) concentrations in ambient air.  Studies in Cork, for instance, have found that 44% of the total organic aerosol mass (PM₁), and 28% and non-refractory total PM₁, derive from residential combustion of wood, coal and peat (Dall’Osto et al, 2013).  Subsequent analysis indicates that, despite regulations for restricting the use of smoky fuels, solid fuel burning is the major source (46–50%) of PM_2.5 in wintertime in Cork, and also likely in other areas of Ireland (Dall’Osto et al., 2014).  The study further noted that, although wood combustion was strongly associated with both organic Carbon (OC) and elemental Carbon (EC), peat and coal combustion were linked mainly with OC, and that the aerosol from peat and coal combustion appeared to be more volatile than that produced by wood combustion.  Overall, these studies conclude that particulate emissions from residential combustion of wood, coal and peat have a major impact on the air quality of Cork (Dall’Osto et al, 2013).

More recent studies in Dublin, Galway, Enniscorthy, Killarney and Birr, under the AEROSOURCE and SAPPHIRE projects, support these findings and show that residential combustion of solid fuels contribute over 50% of organic aerosol mass from small towns to the capital city (Lin et al., 2018;Lin et al., 2019;Lin et al., 2020;Lin et al., 2022;Wenger et al., 2020;Ovadnevaite et al., 2021).

Moreover, source apportionment studies indicate that all observed extreme pollution events were driven by solid fuel (mostly peat) combustion, and that the amount of PM generated is disproportional to the number of homes using it as a fuel source (Lin et al., 2018; Ovadnevaite et al., 2021).  The traffic contribution to these events is usually low, especially in residential areas (Lin et al., 2019; Lin et al., 2018; Lin et al., 2017).  Even at the sites closer to the main congested streets, which are important for pedestrian health, solid fuel burning contributes significantly to PM during winter (Lin et al., 2020), while traffic results in higher black carbon (BC) contributions surpassing that of OM (Lin et al., 2020) during non-heating periods.

Organic pollutants are particularly associated with residential-scale combustion of solid fuels, where the quality of the fuel, of the appliance, and of operator behaviour are completely uncontrolled.  As noted in the EMEP/EEA Guidebook 2019, the emissions due to incomplete combustion of solid fuels are generally many times greater in small appliances than in bigger plants.  This is particularly valid for manually-fed appliances and poorly controlled automatic installations (GB19, p22).

Moreover, particles emitted from incomplete combustion in manually-operated stoves and fireplaces exhibit a high cytotoxicity, whilst particles from correctly operated, automated, biomass boilers and furnaces are mainly inorganic (derived from ash constituents in the biomass), and exhibit significantly lower or even undetectable cytotoxicity (Nussbaumer, 2017).  The AEROSOURCE project revealed that solid fuel burning contributes significantly to PM toxicity, pointing to an important role for solid biomass and peat burning in determining health effects, alongside coal or oil combustion (Ovadnevaite et al., 2021).

Quantifying PM emissions:

Quantifying and characterising the emissions of PM from solid-fuel combustion is inherently challenging, and the values obtained depend on both the details of the combustion process, and the measurement technique employed – as noted in (Smith and Quinn, 2020).  “PM” is used as a catch-all term for solid, semi-solid and liquid aerosols that span a wide range of chemical compositions and physical features.  Unlike gaseous pollutants such as CO or NOx, PM undergoes substantial physical and chemical changes as it emerges from the combustion zone, passes through the flue, and mixes with ambient air.  Figure 1 (Nussbaumer, 2017) provides an indication of the myriad processes and factors that govern the ultimate physical and chemical characteristics of the combustion-derived PM found in ambient air.

It can be seen from Figure 1 that the reactions leading to PM formation can be associated with four distinct, physical locations:

1. in the fuel bed itself;
2. within the combustion chamber of the stove;
3. in the flue or chimney;
4. in the atmosphere, following emission from the stove.

The changes experienced by an individual particle depend on the physical conditions – such as temperature and oxygen concentration – within those zones.  Conditions within the first three zones depend, in turn, on a varying number of factors including:

• the size, shape, arrangement, moisture content and chemical composition of the fuel elements;
• the availability of combustion air;
• the design of the stove;
• the quality of the flue and the flue installation.

_{Figure 1   An outline of the principal features of PM formation, and subsequent secondary aerosol formation in the atmosphere, in biomass combustion. u, gas velocity; τ, residence time (< denoting short residence time, > denoting long residence time); 1, solid particle path; 2, solid vapour particle path. COC, condensable organic compounds; EC/BC, ratio of elemental carbon to black carbon; PIA, primary inorganic aerosol; PM10, particulate matter with a characteristic diameter of 10 μm or less; POA, primary organic aerosol; SIA, secondary inorganic aerosol; SOA, secondary organic aerosol. Source: Nussbaumer (2017)}

Hence, the combustion of low-grade fuel, and/or inappropriate control of the inlet air supply, can lead to substantial quantities of PM still being emitted.  Moreover, the conditions in each of the first three zones change as the combustion proceeds from ignition, through light off and steady combustion, and on to smouldering combustion and extinction. 

The apparent quantity and characteristics of PM emitted from solid fuel appliances are therefore sensitive to both the timing, and the location, of the measurement.  In general, a substantial fraction of total PM emissions occurs during the ignition and light off phases, when temperatures within the appliance are low.  In contrast, standard methods for testing the performance of fuels and stoves measure PM only during the stable combustion phase.  PM emissions measurement during the early phases is further complicated, however, by the presence of firelighters or some other ignition agent in the stove.  Conventional, kerosene-based firelighters – widely used in Ireland – have been shown to make a disproportionately high contribution to PM emissions, with emission factors up to ten times higher than dry wood (Smith and Quinn, 2020).

From a measurement perspective, even greater significance is attached to the fact that particulate emissions continue to evolve after they have exited the flue and entered the atmosphere; PM emissions differ completely from gaseous emissions such as NOx in this respect.  As the exhaust gases are cooled and diluted in the atmosphere, volatile organic compounds (VOCs) and condensable organic compounds (COCs) may either:

• form liquid droplets,
• adsorb onto the surface of solid carbonaceous particles
• and/or react with CO, H₂O and other compounds.

Hence, the chemical composition and physical characteristics of PM observed in ambient air may differ substantially from those observed in the exhaust flue, and the total mass of PM formed following dilution and cooling in the atmosphere always exceeds that present within the flue.  Estimates in the literature put the ratio of PM mass obtained using these two methods at between 2 and 10 for biomass fuels, and that ratio can be substantially influenced by the moisture content of the raw fuel (Smith and Quinn, 2020). 

Further evidence of the importance of PM measurement technique is provided in (Trubetskaya, 2020), where it was concluded that the PM emission factor (EF) determined for a solid fuel, when burned in a residential stove, depends strongly on the measurement method employed and on user behaviour, and less strongly on secondary air supply and stove type.  That study found that organic aerosol (OA) dominated PM composition for all fuels tested (bituminous coal, low-smoke manufactured fuels, peat and wood), constituting 50−65% of PM from bituminous and low-smoke ovoids, and 85-95% from torrefied olive stone (TOS) briquettes, sod peat, and wood.

The magnitude of the organic (i.e. condensable) component in PM is significant, because published data on PM emissions (and EFs) are sometimes derived from measurements in the hot, raw flue gas, and sometimes from measurements following dilution.  However, EF obtained using these two methods are not directly comparable, since much OC remains in the vapour phase in hot flue gases.

The Irish fuel and appliance mix is unusual

In Ireland, official data indicates that the mix of solid fuels for domestic heating is dominated by peat, followed by bituminous coal and manufactured “smokeless” fuels, and then biomass (Dineen, 2021).  This is very different to the fuel mix in the UK or in most of continental Europe, where dry wood is by far the dominant fuel.   Although the amount of nontraded wood and sod peat used in the residential sector is highly uncertain, a more detailed analysis of the nontraded sector suggests that, in a worst-case scenario, wood might account for 75 ktoe (~15%, on an energy basis) of solid-fuel consumption in the residential sector (Trubetskaya, 2020).  Ireland also diverges from most of mainland Europe, in our widespread use of open fireplaces (rather than stoves) as the principal appliance for combustion of solid fuels in the Residential sector.

The unique mix of fuels and appliances presents a significant challenge to the modelling community, as regional air quality and climate model outputs depend greatly on the accuracy of carbonaceous aerosol emissions, particularly those from residential fuel combustion (Gao et al., 2018;Hu et al., 2019), which currently contribute to the highest model uncertainties (Ots et al., 2018;Li et al., 2017).  The standard emission inventory data from EMEP, TNO or NAEI has resulted in a tenfold underestimation of emissions, and consequent underestimation of ambient PM concentrations, in Dublin city (Hu et al., 2019).   Estimates are expected to be even worse elsewhere in Ireland, where solid fuels contribute a higher proportion of residential heating and open fireplaces are more widely used.  Ireland-specific emission factors for these fuel-appliance combinations are therefore needed, to provide accurate and representative inputs to regional air-quality models, and to generate national emission inventories that can support informed policy development.