| Popis: |
The temporal evolution of the size-segregated composition of multiple, heavy metal, combustion-generated aerosols was investigated. The purpose was to gain insight into the competition between metal condensation and aerosol coagulation processes that affect the partitioning of toxic, heavy metals during combustion and their impact on the environment. Experiments were conducted on a2-to 16-kW downflow combustor, which supported a natural gas flame through which aqueous solutions of metal salts were sprayed. The behavior of both single metals and binary mixtures of lead and cadmium (which are both considered semivolatile) and of cadmium and nickel (the latter of which is refractory) was investigated. Particulate samples were withdrawn isokinetically through a rapid-dilution sampling probe from which they were quickly size segregated in a Berner low pressure impactor, allowing physical and chemical resolution in the submicron particle size range. Experimental data agreed with theoretical predictions using an existing literature model. This model solved the general dynamic equation numerically but was modified here to include the effects of film condensation in a simple manner. For samples withdrawn above the semivolatile metal dew points, theory showed that nucleation of the semivolatile metal in the probe, followed by coagulation, could explain completely, the measured size-segregated composition. With similar dew points and number concentrations, cadmium and lead particles interacted primarily through coagulation. However, cadmium and nickel particles did not interact by coagulation in the probe because of disparties in particle size and number concentrations. Further down the furnace, both model and data showed that compositions of particles were determined by condensation on existing nickel particles, rather than by nucleation of the semivolatile component (cadmium), followed by coagulation. These results are consistent with a literature hypothesis, on the effect of temperature quench rate on pertinent aerosol mechanisms. |
Full Text from ScienceDirect