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No. 442: Updating the emission model for residential wood combustion

Nielsen, O.-K., Nielsen, M. & Plejdrup, M.S., 2021. Updating the emission model for residential wood combustion. Aarhus University, DCE – Danish Centre for Environment and Energy, 86 pp. Scientific Report No. 442. http://dce2.au.dk/pub/SR442.pdf 

Summary

Residential wood combustion is an important source of air pollution in Denmark and therefore attracts great attention. The share of residential wood combustion to several pollutants such as carbon monoxide, volatile organic compounds, fine particulate matter (PM2.5) and polycyclic aromatic hydrocarbons (PAHs) are very high, and especially for the pollutants most associated with adverse impacts on human health such as particulate matter.

In 2019 and 2020, work was done to update and document the model used to estimate emissions from residential wood combustion. The results of this work is presented in this report.

The model has been described in Chapter 2 of this report including a description of the most important model parameters.

There is however, large uncertainties associated with the emission inventory, as there are many parameters in the emission calculation model, all of which have varying levels of uncertainty. The parameters include the number of appliances per technology, the age distribution, the wood consumption and the emission factors. To estimate the age distribution it is necessary to make assumptions on the expected lifetime and replacement rates of stoves and boilers.

The assumptions behind the total number of wood burning appliances and the replacement rates of various stove technologies were revisited and changes were made using the best available knowledge. The model improvement is based on a review of available literature and analysis of data from surveys, relevant projects and from the chimneysweeper association.

Time series for the number of wood burning appliances have been updated. The methodology has been updated so that time series is being based on replacement rates and sales numbers.

Unit consumptions per technology is used for bottom-up estimation of wood consumption. In accordance with the international guidelines for emission inventories submitted to the UN and the EU, the emissions from residential wood combustion must be based on the fuel consumption as given in the national energy statistics published by the Danish Energy Agency. To comply with the guidelines, the bottom-up estimated wood consumption is scaled to the fuel consumption as given in the national energy statistics.

For the emission factors, recent literature was analysed to update emission factors. The update focussed on the pollutants with the highest impact on national emissions, and categories (technologies) where the existing emission factors had been identified as being in need of updating. Revised emission factors were established for particulate matter (PM), black carbon (BC), polycyclic aromatic hydrocarbons (PAH), polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F), nitrogen oxides (NOx), carbon monoxide (CO) and sulphur dioxide SO2.

The updated model caused significant changes in emissions of several pollutants. The largest changes were related to particulate matter, carbon monoxide, black carbon, dioxins (PCDD/F) and PAHs. For the other pollutants, changes were smaller.

While the updated model and data are improved compared to the old model, there are still significant uncertainties associated with the estimation of emissions from residential wood burning. There are uncertainties associated with all parameters included in the model and while uncertainties can be lowered, emissions from residential wood combustion will continue to have a high uncertainty.

Future improvements to the model and input data will primarily depend on new and improved data becoming available. Emission measurement programmes producing emission factors are periodically carried out and the results of these measurements will continue to be of interest and may cause updates to the emission factors.

New data on the use of wood burning appliances are produced as part of the survey conducted for the energy statistics. While data historically have been very fluctuating making it difficult to draw firm conclusions, the results of these surveys will continue to be monitored.

For future improvement to the model and emission estimation, data on the number, type and age of the appliances could be an important element to update. The chimneysweepers inspect all appliances frequently at least once a year and it would be beneficial if further details was included in the registration and made available. This includes information about appliance age, further details about technology, and indication of the extent to which the appliances are used, e.g. primary heating, supplementary heating or occasionally. Such information can contribute to update the total appliance numbers, to improve the age and technology distribution and in the longer term to verify or improve the replacement rates.