Residential biomass combustion - emissions of organic compounds to air from wood pellets and other new alternatives

Doctoral thesis, 2006

It is important to increase the use of biofuels for residential heating in order to decrease the use of fossil fuels and their impact on global warming. The smoke from the combustion of biomass contains a large number of compounds, which to varying degrees can affect the environment and human health. In this study, specific organic compounds were therefore analysed in the smoke from the selected fuels and combustion appliances using various gas chromatographic and mass spectrometric techniques. Softwood pellets are increasingly common as a residential fuel in Sweden and annually almost 500 000 tonnes are used. Measurements showed that the emissions from different stages of incomplete laboratory burning of softwood pellets differ greatly in amount and composition. Initial smouldering and flaming burning emitted methoxyphenols with an antioxidant effect in high concentrations, whereas glowing burning emitted the carcinogenic benzene and polycyclic aromatic hydrocarbons in low concentrations. With the increasing demand for wood pellets, alternative fuels, such as oats, wheat straw and peat, are of great interest. Results from incomplete combustion of these fuels on a laboratory scale indicate that they give rise to relatively low emissions of organic compounds, almost as low as those from softwood pellets. High concentrations of furan-related compounds and anhydrosugar were emitted during the initial smouldering of oats, while high concentrations of methoxyphenols were emitted from wheat straw and peat/wood pellets. The emissions from the combustion of softwood pellets in residential appliances were generally low and wood pellets are environmentally well suited to replace traditional firewood and oil boilers. The combustion of softwood pellets in two stoves and one boiler gave rise to emissions of methoxyphenols together with aromatic hydrocarbons. The combustion in pellet burners was more complete and emitted benzene and other aromatic hydrocarbons in low concentrations. The large variations observed in emissions from different residential pellet burning appliances make it important to choose the best available appliance and to install and maintain it correctly. The studied ecolabelled wood boiler showed high combustion efficiency. The emissions of compounds hazardous to health and the environment were low and the boiler is therefore recommended as an environmentally sound option for residential firewood combustion.