Specific chimney emissions and biofuel characteristics of softwood pellets for residential heating in Sweden

Softwood pellets are mainly produced from sawdust and to some extent from wood shavings. The moisture content is typically less than half of that in firewood. The present annual residential use in Sweden for pellet burners in boilers and for pellet stoves amounts to more than 100 000 tonnes and is increasing rapidly. The total annual production capacity at the more than 20 units exceeds one million tonnes, and permits continued rapid replacement of firewood and petroleum oil for residential heating. With the purpose of characterising emissions to air, chimney smoke was sampled, and specific compounds were assessed by gas chromatography and mass spectrometry. Benzene was the predominant aromatic compound in emissions from pellet burners. The smoke from stoves contained methoxyphenols with antioxidant properties and lower proportions of aromatic hydrocarbons. Observed differences in emissions from specific burning appliances are of interest with regard to both health hazards and regulation of emissions. Consideration of ecological aspects and pollution hazards indicates that wood pellets should be used primarily for residential heating, whereas controlled large-scale combustion is preferable for most other types of biomass waste.