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No. 426: Emission scenarios and air quality modelling for residential wood combustion

Plejdrup, M.S., Nielsen, O.-K., Christensen, J.H. 2021. Emission scenarios and air quality modelling for residential wood combustion. Impact analysis of measures for small wood burning appliances in Denmark and effect on transport of black carbon to the Arctic. Aarhus University, DCE – Danish Centre for Environment and Energy, 54 p. – Scientific report no. 426. http://dce2.au.dk/pub/SR426.pdf 

Summary

In this project the emission impacts of three scenarios for residential wood combustion are estimated and the impacts of the concentrations of black carbon BC are modelled over Denmark and the Arctic using the Danish Eulerian Hemispheric Model (DEHM). Additionally, the modelled concentrations are compared to measurement results. The overall climate effect of residential wood burning in Denmark is estimated for pollutants where the IPCC Fifth Assessment Report provides global warming potentials.

Three reduction scenarios for residential wood burning

Three emission scenarios have been defined and the emission consequences for selected pollutants have been compared to a baseline scenario based on the latest official emission projections. The three scenarios are:

  1. Ban on stoves that do not meet the requirements according to the Nordic Ecolabel in district heating areas
  2. Particle filters on 20 % of the wood stoves and wood boilers that do not meet the requirements according to the Nordic Ecolabel
  3. Phasing out of older stoves (2003 and before) in connection with transfer of house ownership. 

Impacts on particle emissions

The emission consequences for PM2.5 (particles less than 2.5 micro meters in diameter) and black carbon were estimated showing that the effect in 2030 for scenario 1, 2 and 3 is a reduction of 382 tonnes, 337 tonnes and to 446 tonnes of PM2.5, respectively, compared to the basic scenario emission in 2030. This corresponds to reductions of 8 %, 7 % and 10 %, respectively compared to 2030 emissions based on the latest emission projection (baseline scenario) from 2020 (2018 base year). For BC, the effect are an increase of 8 tonnes for scenario 1, corresponding 2 %, a decrease of 15 tonnes (3 %) for scenario 2 and an increase of 26 tonnes (6 %) for scenario 3. The increased BC emission observed for two of the three scenarios are caused by newer technologies having higher black BC emission factors.

Impacts on particle concentrations in Denmark and in the Arctic

The transport and transformation of Elemental Carbon (EC) and other air pollutants of the emissions scenarios have been determined by using the Danish Eulerian Hemispheric Model (DEHM). While BC is the term used in the emission inventory due to the definitions in the reporting requirements, the term EC is used in air quality measurements and modelling. In many cases, BC and EC is used interchangeably and EC is considered to be a good approximation for BC in most cases. The model runs made in this project are single year model runs, and the Danish emissions for year 2018 is used for the basic background model run with DEHM, which calculates the atmospheric concentrations and depositions of pollutants. The Danish emissions for year 2018 are the latest available. Four model runs have been made for 2030 for the scenarios basic, 1, 2 and 3. The results of the model runs show a decrease of the mean EC from 2018 to 2030 of 11.7 %. The largest decrease of the mean EC concentration in Denmark in 2030 compared to the basic scenario in 2030 is found for scenario 2 (0.16 percentage point), while scenario 1 and 3 results in minor increases compared to the basic scenario (0.08 percentage point and 0.28 percentage point, respectively). Further, the mean concentration and deposition for the Arctic area has been modelled showing only minor changes for the three scenarios.

Comparison of model results and measurements in Denmark

The concentrations of EC modelled with DEHM has been compared to air quality measurements at three different sites; one rural, one suburban and one urban background site in Denmark. The comparison generally shows a good agreement between observed and modelled concentrations, with the best result for the rural measurement site. This is expected since the DEHM model will perform better for a rural area, where emissions from a larger area contributes to the concentrations, compared to the two other sites, where more local sources contribute significantly, e.g. residential wood burning and road transport. The model overestimates the concentration by about 11 % at the rural measurement site and captures the same trend as can be observed in the measurement.

Climate effects of residential wood burning in Denmark

Based on global warming potentials, the total climate effect of residential wood burning in Denmark has been estimated in CO2 equivalents. In international reporting of national emission inventories, CO2 emissions and removals due to the harvesting, combustion and growth of biomass are included in the carbon stock changes of the relevant land use category of the Land Use – Land Use Change and Forestry (LULUCF) sector for the country where the biomass originates. Therefore, CO2 emissions from biomass burning is not included the Energy sector. However, that does not mean that CO2 is not emitted when combusting wood in residential plants. For the purposes of this report, we have also included information on the CO2 emissions associated with residential wood combustion to provide a more complete view of the overall climate impact. In fact, the scenario calculations show that by far the largest contribution to the total greenhouse gas emission from residential wood combustion expressed in CO2 equivalents is from CO2, which constitutes approximately 66 % of the total greenhouse gas emissions expressed in CO2 equivalents. This emphasizes the importance of sustainable forestry. By using sustainably grown wood, CO2 emissions are compensated by CO2 uptake and thereby do not contribute to raising the CO2 content in the atmosphere. The second most important pollutant contribution to the total greenhouse gas effect in the scenarios is black carbon (approximately 32 %) whereas the remaining pollutants considered (methane (CH4), nitrous oxide (N2O), non-methane volatile organic carbons (NMVOCs) and nitrogen oxides (NOx)) contributes very little to the total greenhouse gas effect in the scenarios (2 %). There is very little difference between the baseline scenario and the three scenarios. In fact, the difference is at most 2 percentage point.