Tammaro, Marco (2010) Heterogeneous condensation for submicronic particles abatement. [Tesi di dottorato] (Unpublished)

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It is now well established that the emission of sub-micrometric particulate matter entrained in flue gases of industry and vehicles exhausts, is one of the most critical treats for human health because of the toxicological effects of ultrafine particles on the respiratory system and their ability to cross alveoli’s membranes reaching the circulatory system too. Albeit this scenario, the traditional particle abatement devices are mainly designed and optimised to treat particles larger than 1 μm, but they are far less effective towards the sub-micrometric particles. In particular, the existing technologies are far less efficient in collecting sub-micrometric particles, especially in the range 0.1 - 1 μm, called Greenfield gap. The aim of this work is the study of the heterogeneous condensation, a promising method that can be used to achieve very high particles removal efficiency with reduced costs: particles undergoing heterogeneous condensation increase their diameter by deposition of a liquid film on the particles surface, generating a liquid-solid aerosol with desired diameter. Therefore, this process can be used as a preconditioning technique to enlarge the particles diameter at micrometric size, allowing the use of conventional separators for gas cleaning. Although largely studied for its effect on atmospheric phenomena and for the application to particle measurement techniques (the CPC units), heterogeneous condensation has never been used at industrial scale as a method for gas cleaning. In this study, theoretical and experimental analyses on the heterogeneous condensation have been carried out. For this purpose instrumented lab scale equipment has been designed, constructed and tested. Experimental protocols and measurement techniques have been optimized. The core of the equipment is the growth tube, that consists in a glass tube where the particle laden gas flows comes into contact with a supersaturated water vapour environment, generated by a liquid film flowing on the tube internal walls. Preliminary experimental tests have been carried out to define adequate experimental protocols and to find out the most reliable techniques to measure the size and concentration of the liquid-solid aerosol in the water saturated gas that exists the growth tube. Afterwards, an experimental campaign has been carried out to study the heterogeneous condensation of soot particles produced by a ethylene-air premixed flame at different working conditions. Eventually, the experiments have been interpreted through a simplified theoretical model that allows a good description of observed results. Experimental and modelling results eventually provide preliminary guidelines for the design of a pilot scale unit to be tested for future experimentation.

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