Field studies of combustion aerosols

In particular, the behaviour of fine and ultra fine particles, i.e. particles with diameter <2.5 mum, has been investigated. Field measurements are made simultaneously before and after the electrostatic precipitator and in the stack. These data thus show the formation of particles in the boiler, their removal in the electrostatic precipitator and the particle transformations in the desulfurization plant. Several measuring methods are used, including total dust measurements and fine particle size distributions by low-pressure cascade impactors and scanning mobility particle sizers. Size fractions collected in the impactor are chemically analysed by EDX (Energy Dispersive X-ray spectroscopy) and NAA (Neutron Activation Analysis). The investigation has contributed with a large volume of data characterizing the particle variation from boiler to stack for the boilers and fuels studied. The data are documented in separate reports for the individual plants and in special reports in which the particle dynamics in the boiler, ESP and desulphurization unit for the different plants are compared. Many observations are new and surprising, because this type of measurement has not been made previously. In particular for the fine particles, which mainly are generated by the condensation of volatile ash components, theories are proposed for the formation mechanisms. The fraction of fine particles formed in the boiler depends on the volatility of the ash and varies from less than 1 weight percent for coal firing to approximately 50 percent for straw and Orimulsion firing. For coal, the fraction of submicron particles increases from 1% in the boiler to 38% in the stack, due mainly to the larger penetration of fine particles through the electrostatic precipitater. The dust emissions from the four plants are well below the emission limits allowed in Denmark, but almost all emitted particles are below 10 mum. Calculations show that their contribution to both the short and the longe range particle pollution of the environment are small