CFD Modelling and Experimental Testing of Thermal Calcination of Kaolinite Rich Clay Particles - An Effort towards Green Concrete

Cement industry is one of the major industrial emitters of greenhouse gases, generating 5-7% of the total anthropogenic CO2 emissions. Consequently, use of supplementary cementitious materials (SCM) to replace part of the CO2-intensive cement clinker is an attractive way to mitigate CO2 emissions from cement industry. SCMs based on industrial byproducts like fly ashes and slags are subject to availability problems. Yet clays are the most ubiquitous material on earth's crust. Thus, properly calcined clays are a very promising candidate for SCMs to produce green cements. Calcination at inappropriately high temperatures or long retention time will not only waste energy but also decrease the reactivity of the calcines due to possible recrystallization of the reactive phase into a stable crystalline phase. Therefore, it is very crucial to achieve an in-depth understanding of the calcination processes in a calciner and develop a useful tool that can aid in design of a smart clay calcination technology, which makes the major objective of this study.

In this thesis, a numerical approach is mainly used to investigate the flash calcination of clay particles. A transient one-dimensional particle model which fully addresses not only the particle-ambient flow interaction but also the intra-particle processes has been successfully developed in a C++ program to examine calcination of clay particles suspended in a hot gas. The calcination process is also numerically studied using gPROMS (a general PROcess Modeling System) software, which is suspended during the project due to the adjustment made by the project consortium. The model results from both C++ and gPROMS software show good similarity. Various experiments have been performed to derive key kinetic data, to collect data from a gas suspension calciner (GSC), and to characterize the calcines obtained under different calcination conditions, which are either provided to the numerical model as inputs or as database for model validation.

The model is able to reliably predict the temperature and residence time at which a given clay material attains optimum composition of the required material, metakaolinite. For kaolinite rich clay particles with mean particle size of 13.74 μm in diameter, moderate calcination temperatures (1173-1200 K) tend to display optimum amount of metakaolinite in a fraction of seconds with less risk of further phase transformation. High calcination temperatures (>1300 K), however, deplete the amount of metakaolinite and promote further recrystallization of metakaolinite into undesired mullite phase that influences pozzolanic property of calcines negatively. Different indicators have been used to spot the optimum pozzolanic property of the calcined clay material, among which is the density of calcines. By using the variation in density of calcines, an optimum residence time has been marked. At this time the calcines display a minimum density that corresponds to the most dehydroxylated calcines. The behavior of flash calcined kaolinite rich clays has also been examined experimentally. The composition and property of calcines observed experimentally supports model prediction. The agreement between model and experimental results confirms the validity of the model.

The optimum calcination parameters predicted in this study are crucial not only to maximize the yield of the desired product but also minimize the energy consumption during operation. Thus, the experimentally validated calcination model and simulation results can aid in an improved understanding of clay calcination process and also new conceptual design and optimization of clay calciners.