Numerical and experimental study of heterogeneous reactions involving carbonaceous compounds in clay brick firing

Abstract

Clay bricks have been used as a construction material by humans since thousands of years. Modern brick making however is challenged by the still energy-intensive production, coming together with considerable amounts of greenhouse gas emissions and consequently climate impact. Therefore, the physicochemical fundamentals of the brick firing process should be studied, in order to provide insight to possible technological changes. The present study deals with the heterogeneous chemical reactions, which occur in the thermo-chemical conversion of a clay brick mixture rich in carbonates and blended with organic additives. An integrated solution of the actual mass and heat transfer phenomena resulting from the reactions of carbon-related compounds inside a porous brick body under heat exposure is provided. The partial and global mass balances as well as the heat balance are represented by the relevant differential equations, which were solved using a finite differences method. The chemical kinetics of the carbonates’ calcination and the oxidation of the added organic substance (wood particles) were obtained by thermogravimetric analysis and the results deployed for parameterizing the global reaction model. An accurate description of the effective diffusion coefficients by means of the mean transport-pore model was implemented to the global reaction model. Combustion experiments with cylinder-shaped clay specimens conducted in a fluidized bed lab reactor were used for evaluation of the model. After simple adaptations of the parameterization, the model predictions showed good agreement with the experimental data. Parameter sensitivity analyses and various model tests demonstrate the practical suitability of the presented reaction model for the thermo-chemical process of brick firing.