ANALYSIS OF SIZE-FRACTIONATED COAL COMBUSTION AEROSOLS BY PIXE AND OTHER ANALYTICAL TECHNIQUES

Particle-induced X-ray emission (PIXE) analysis, instrumental neutron activation analysis (INAA) and inductively coupled plasma mass spectro metry (ICP-MS) were used to study the chemical composition of size-fra ctionated in-stack fly-ash particles emitted during coal combustion. T he samples were collected before the electrostatic precipitator at a g as temperature of 120-degrees-C during the combustion of Venezuelan co al in a 81 MW capacity circulating fluidized bed boiler. The sampling device consisted of a Berner low pressure impactor, which was operated with a cyclone precutter. The Nuclepore polycarbonate foils, which we re used as collection surfaces in the low pressure impactor, were anal yzed by the three techniques, and the results of common elements were critically compared. The PIXE results were systematically lower than t he INAA data, and the percentage difference appeared to be stage-depen dent, but virtually independent upon the element. The discrepancies ar e most likely due to bounce-off effects, particle reentrainment and ot her sampling artifacts, which may make that a fraction of the aerosol particles is deposited on the impaction foils outside the section anal yzed by PIXE. However, by resorting to a ''mixed internal standard'' a pproach, accurate PIXE data are obtained. Also in the comparison betwe en the ICP-MS and the INAA data significant discrepancies were observe d. These are most likely due to incomplete dissolution of the particul ate material, and in particular of the alumino-silicate fly-ash matrix , during the acid digestion sample preparation step for ICP-MS. It is suggested that a comparison between ICP-MS data of acid digested sampl es and INAA can advantageously be used to provide speciation informati on on the various elements. Selected examples of size distributions ar e presented and briefly discussed.