Comparison of emissions and toxicological properties of fine particles from wood and oil boilers in small (20-25 kW) and medium (5-10 MW) scale

The aim of this study was to compare four alternatives for providing decentralized energy production in small communities in terms of their flue gas emissions and toxicological properties of the emissions. In this study, two different size classes of boilers were examined and the use of fossil fuel oils was compared against wood fuels. The lowest PM₁ emission, 0.1 mg MJ⁻¹, was observed from small-scale light fuel oil combustion. In medium-scale wood combustion, PM₁ emission values from a grate fired wood combustion boiler (10 MW) without particulate filtration were the highest (264 mg MJ⁻¹) but were substantially reduced down to 0.6 mg MJ⁻¹ due to the usage of an electrostatic precipitator (ESP). The wood combustion particles were mainly formed of potassium salts. In light fuel oil combustion, one of the main components in the particles was sulphate whereas in heavy fuel oil combustion also significant amounts of V and Ni were emitted. Pellet combustion produced the lowest PAH emissions. Overall, oil combustion produced higher amount of PAHs than wood combustion. This was indicated also as a higher cytotoxicity of the oil combustion samples when compared to those from wood combustion in the corresponding scale of boilers. However, when calculated on an equal mass basis, the particles collected after ESP were even more cytotoxic which can be explained by the altered chemical characteristics of the emissions in the ESP. Due to the variation in the emissions and in the toxicity of the emissions, we propose that in the long term, not only the emission levels but also the toxicity of the emissions should be taken into account in the regulations of the emission limits of the combustion plants.