Cozzolino, Gennaro (2013) Water condensation for submicronic particles abatement. [Tesi di dottorato]

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Control of particulate matter (PM) emissions from industrial processes is important for protection of human health and the environment. A promising technique for submicron PM abatement is by condensing water vapor onto the particles, which enhances particle growth and improves the performance of traditional particle collection devices. This thesis analyzes the flue gases cleaning process by means of activation heterogeneous water nucleation mechanism on particles surface as a function of working condition, inlet temperature and vapor concentration, with particular emphasis on the effect of their dimension, number concentration, morphology and chemical nature on nucleation activation process. The temporal evolution of particles growths process has been studied with a proper experimental apparatus. Submicronic particles with a different wettability (C, Fe and Ni) have been produced by means of an aerosol spark generator. Vapor condensation on submicronic particles has been followed along the axis of the laminar flow chamber in dependence of operative conditions and particles characteristics by measuring the intensity of vertical and horizontal component of elastically scattered light and then evaluating the polarization ratio. Furthermore a series of off-line measurements have been performed to characterize solid particles. The results obtained represent a first database for kinetic data of condensation process. They show that particle growth mechanism involves different physical processes. By means a specific numerical modeling the polarization ratio of composite sphere made by a water shell hosting a solid particles was been calculated. In such a way the final size of droplet and so the particle growth factor have been evaluated. The main result obtained is that particle covering process related to its morphology and chemical properties is active at under-saturation conditions even if their size variation is relatively low. While for supersaturation conditions the particles coverage is enhanced by mass transport of vapor toward particles surface. In such a case the final size of the composite droplet is of the order of micron. It is worthwhile to note that both mechanisms are particularly effective for particles capture showing an unitary activation efficiency in the operative conditions considered. It is worth noting that extreme working conditions may be useful for the particle coverage. In addition, this work demonstrated that the theoretical prediction related to classical nucleation theory could be used to determine the process effectiveness at higher vapor concentration and lower inlet temperature, even though it fails to predict the induction time. At low values of vapor concentration, this approach was not valid in predicting the occurrence of the vapor condensation on particles. For this purpose the theory of capillary condensation seems to be an useful tool to predict the vapor deposition on particles and the induction time of growth process in so severe condition. In this PhD thesis, it was assessed that the characteristic induction and growth times are compatible with practical applications. The identification and estimation of this information can be useful in the design and dimensioning of a real abatement unit.

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