Ricchiari, Domenico (2025) Design and Optimisation of Exhaust Gas Cleaning Systems with Chlorite-based Wet Oxidative Scrubber (WOS). [Tesi di dottorato]

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Abstract

This thesis investigates the design and optimisation of a chlorite-based wet oxidative scrubber (WOS) for the simultaneous removal of sulphur dioxide (SO₂) and nitrogen oxides (NOₓ) from industrial flue gases. The work addresses three main gaps hindering industrial deployment: the lack of rigorous thermodynamic and kinetic models, the need for reliable mass-transfer correlations under reactive conditions, and the quantification and mitigation of chlorine-species emissions. A multiscale experimental campaign was carried out using a lab-scale fed-batch bubble column to determine SO₂ and NO solubility and acidification dynamics in aqueous NaClO₂ solutions, a falling-film column to characterise liquid-film hydrodynamics and to derive Enhancement Factors and intrinsic rate expressions for SO₂–NaClO₂ and NO–NaClO₂ systems, and a semi-pilot packed-bed column to evaluate De-SOx/De-NOx performance and validate a rigorous design model under conditions representative of real flue gases. The results show that chlorite concentration strongly affects gas–liquid equilibria and that all solutions evolve towards acidic conditions, activating the acid oxidation pathway, while the initial pH mainly controls the time needed to reach equilibrium. New kinetic correlations, combined with Danckwerts’ theory, reproduce the measured Enhancement Factors with high accuracy (R² ≈ 0.97 for SO₂ and 0.90 for NO) and enable reliable prediction of column performance. Semi-pilot tests confirm SO₂ removal efficiencies close to 100% and NOₓ removal above 95% at optimal pH and oxidant dosage, with model predictions within ±30% for SO₂ and ±20% for NOₓ outlet concentrations. Dedicated campaigns on euchlorine formation demonstrate that Cl₂ and ClO₂ slip can be minimised by appropriate operating conditions and effectively reduced (>99%) by a secondary polishing stage for both gas and liquid streams. Overall, the thesis provides a complete thermodynamic, kinetic, and design framework, demonstrating that the NaClO₂-based WOS is a technically mature and environmentally compatible option for integrated SO₂/NOₓ abatement and ready for industrial scale-up in collaboration with Boldrocchi S.p.A.

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