Concept

Although consumption of solid fuels in the Residential sector decreased by about 60% in the decade from 1990-2000, it has decreased by only 30% in the two decades since then (Dineen, Holland and Howley, 2021).  Recent figures suggest a gentle but steady increase in solid fuel consumption over the past five years – a trend that may well accelerate, given the soaring prices of oil and gas resulting from the war in Ukraine. 

Irish emissions from this source are currently estimated using emission factors (EF) from the Air Pollutant Emission Inventory Guidebook 2019, produced by the European Monitoring and Evaluation Programme (EMEP) and the European Environment Agency (EEA).  Based on these estimates, Residential solid fuel heating is the largest single source of fine particulate matter (PM) emissions in Ireland, accounting for over 50% of all emissions of PM2.5.  Very significantly, the European Environment Agency (EEA) estimates that PM2.5 is responsible for over 90% of all mortality attributable to air pollution in Ireland.   

Solid fuels in the Irish residential sector

However, many of the solid fuels used in residential combustion in Ireland, including peat-based fuels and manufactured “smokeless” coals, are poorly represented in the EMEP/EEA Guidebook.  Data on emissions from open fireplaces, widely used for residential heating in Ireland, are also sparse and of variable quality.  Moreover, as discussed in Section A3, EF related to particulate matter (PM) are highly sensitive to the nature of the measurement approach employed, and to the timing of that measurement within the complete cycle incorporating ignition—light off—main combustion—smouldering—extinction.  The default EF suggested in the EMEP/EEA Guidebook, therefore, may not accurately represent emissions from the types of solid fuel and appliance combinations widely used in Ireland.

This weakness has been partly addressed in a previous EPA project (EFDOSOF, 2011-CCRP-MS-4.5).  However, that project produced EF for only one combustion device – a relatively large and simple stove.  PM emissions were determined from the undiluted flue gas, using a “hot filter” method.  Although the results provided many valuable insights, the project also identified a need to capture condensable components in the exhaust stream.  The concentration and composition of these condensable compounds varies between fuels and appliances, but they have a significant impact on both the physical and chemical characteristics of the PM as it is diluted and cooled in ambient air.

Addressing the knowledge gap

This proposal seeks to address that knowledge gap by developing a comprehensive suite of emission factors for combinations of solid fuel and combustion appliance widely used in Ireland.  Both wood (seasoned and green/wet) and peat (sod and briquettes) will be tested.  Each fuel type will be burned in an open fireplace, and in a residential stove typical of those commonly used in Ireland.

Measurements will be performed in both raw and diluted flue gas, as described below in the “Measurement approach”, and discussed in more detail in Section C.  This approach ensures that detailed information will be available for both the “hot-filterable” and condensable emissions from each fuel.  In particular, the incorporation of dilution will allow the project team to assess the impact of solid fuel combustion on the quantity and characteristics of the PM added to the ambient air, rather than in the flue of the device.

Detailed chemical speciation of the diluted PM will be an important output from the project, and represents a significant step in capability relative to EFDOSOF.  The output will be further extended by the determination of PM-bound emission factors for all metals listed in the EMEP/EEA guidebook, from each fuel.  Research-grade gas analysers will measure the concentration of gaseous pollutants in the diluted gas.

In summary, this proposal extends the existing knowledge and capabilities developed during the EFDOSOF project, by:

• obtaining measurements from an open fireplace – although open fireplaces are widely used in Ireland, data on their emissions characteristics is sparse, and of variable quality;
• measuring PM emissions in both the raw and the diluted flue gas, thereby enabling both “hot filterable” and condensable components to be quantified;
• delivering highly detailed, time-resolved chemical characterisation of PM in the diluted state, i.e. as it will appear in ambient air;
• extending the range of pollutants examined, to include PM-bound metals in emissions to air

The experimental concept

EFs will be determined over the complete combustion cycle beginning with a cold appliance, and will incorporate emissions associated with all phases of combustion, from ignition through to extinction.  Multiple repeats of each test will be performed, to establish the degree of variability of emissions between nominally identical combustion experiments, and to provide confidence intervals for the EF derived from the raw measurements.

The detailed test protocol – how much fuel to consume per test, whether to refuel the stove during a test, how to manage the ignition and light-off phases, how to determine the end of the test, etc. – will be developed in consultation with stakeholders from the relevant industries, from the EPA, and with other technical experts.  A Stakeholder Workshop is scheduled in the first few months of the project for this purpose.  Further workshops will be held each year throughout the project, to promote dialogue, and to maintain and encourage stakeholder engagement.

Measurement approach

As mentioned above, we plan to measure PM and gaseous emissions in both the raw, and the diluted, exhaust streams.  Measurements in the raw gas involve much higher concentrations of pollutant, facilitating the detection of trace compounds such as metals, and provide valuable information on the properties of the PM prior to being diluted and cooled in the atmosphere.  The diluted gas measurements, on the other hand, provide a realistic representation of the PM as it will appear in the ambient air, and therefore offer greater insight into its potential health and environmental impacts.   Moreover, the advanced measurement systems that will be applied to the diluted exhaust stream can provide detailed, time-resolved, physico-chemical characterisation of that PM, and can accurately quantify the mass of condensable compounds that contribute to it.

Data synthesis

Approximately seventy combustion tests are planned.  Each test will generate tens of thousands of time-resolved data points, and multiple integrated samples such as filter papers.  Auditing, synchronising, and interpreting these voluminous and diverse data sets will require substantial time, effort, and expertise.  The outputs from that work, however, will include not only comprehensive EF for ten solid fuels used in Ireland, but a greatly improved understanding of the factors determining the relative emissions from each fuel, and of the overall health impacts of emissions to air and soil associated with residential, solid-fuel combustion in Ireland.