Real-time application and modelling of the NOx-sorption reaction on a particulate calcium carbonate surface-flow filter exposed to combustion exhaust

Abstract

Of major interest, especially in city environments, and increasingly inside vehicles or industrial plants, is the drive to reduce human exposure to nitrogen oxides (NO_x). This trend has drawn increasing attention to filtration, which has developed remarkably owing to the capabilities of recently developed mathematical models and novel filter concepts. This paper reports on the study of the kinetic modelling of adsorption of nitrogen dioxide (NO₂), collected from the tailpipe of a diesel engine, reacting to calcium nitrate salt (Ca(NO₃)₂) on a surface flow filter consisting of a coating of fine ground limestone or marble (CaCO₃) in combination with micro-nanofibrillated cellulose (MNFC) acting as binder and humectant applied onto a multiply recycled newsprint substrate. The coating and substrate are both porous, but on different pore size scales, with the coating having significantly lower permeability. To maximise gas-coating contact, therefore, the coating deposition is pixelated, achieved by pin coating. An axially dispersed gaseous plug flow model (dispersion model) was used to simulate the transport within the coating pore network structure, following earlier flow modelling studies, and a kinetic reaction model was used to examine NO₂ to NO₃^- conversion in correlation with experimental results. Modelling results indicate a 60.38% conversion of exposed NO₂ gas to Ca(NO₃)₂ under the specific conditions applied, with an absolute relative error between the predicted and experimentally estimated value being 0.81%. The model additionally enabled a prediction of effects of changing parameters over a limited perturbation range, thus assisting in predicting filter element consumption, with attention given to the active component CaCO₃ surface as a function of particle size in relation to the gas contact exchange, promoting the reaction over time. It is intended that the Ca(NO₃)₂ formed from the reaction can go on to be used as a value-added fertiliser, thus contributing to circular economy.

Keywords: Clean technologies; Emissions control; Gaseous reaction modelling; NOx mitigation; Surface flow filtration.