Abstract
The manufacturing process of ceramic building materials is exceptionally energy-intensive, involving several critical stages such as raw material extraction, mixing, shaping, drying, and firing. Each of these stages requires strict energy management to ensure efficiency and quality. Drying and firing, in particular, demand substantial energy inputs. The drying process necessitates a controlled environment to remove moisture from the wet materials, which typically involves the use of heated air, often generated by burning fossil fuels such as natural gas. This step is crucial to prevent defects during firing but is also energy demanding. Firing, the next vital stage, involves heating the dried materials to extremely high temperatures to induce sintering, a process that hardens the materials and imparts the necessary durability and strength. This phase is especially energy-demanding, as kilns must maintain consistent high temperatures, exceeding 900°C, for extended periods. The energy re ...
The manufacturing process of ceramic building materials is exceptionally energy-intensive, involving several critical stages such as raw material extraction, mixing, shaping, drying, and firing. Each of these stages requires strict energy management to ensure efficiency and quality. Drying and firing, in particular, demand substantial energy inputs. The drying process necessitates a controlled environment to remove moisture from the wet materials, which typically involves the use of heated air, often generated by burning fossil fuels such as natural gas. This step is crucial to prevent defects during firing but is also energy demanding. Firing, the next vital stage, involves heating the dried materials to extremely high temperatures to induce sintering, a process that hardens the materials and imparts the necessary durability and strength. This phase is especially energy-demanding, as kilns must maintain consistent high temperatures, exceeding 900°C, for extended periods. The energy required for this is typically sourced from the combustion of natural gas. This stage accounts for the majority of the thermal energy consumption in ceramic production, significantly impacting the overall energy footprint of the manufacturing process. Therefore, optimizing energy use in drying and firing is essential for improving the efficiency and sustainability of ceramic building material production. This thesis considers the development of numerical models based on first principle equations to capture the physical heat and mass transfer phenomena during the drying and firing processes of ceramic materials. The proposed modeling methodologies also elaborate on physical aspects of the studied processes that have not received enough attention in the open literature. Specifically, the thesis focuses on the shrinkage occurring ought to moisture migration during the drying stage and the solid-state sintering during the firing process. These phenomena are crucial for understanding the behavior of ceramic materials under thermal treatment. The models aim to provide a more comprehensive understanding of these processes while quantifying the overall process performance and the final product quality. By conducting simulations of heat and mass transfer while considering the structural changes that occur within the material, the research seeks to improve predictive accuracy and provide insights into optimizing these energy-intensive stages without compromising product quality. This work not only advances the theoretical framework but also has practical implications for enhancing the efficiency and sustainability of ceramic manufacturing processes. More specifically, the drying of a ceramic slab is initially examined, taking into account shrinkage, which is the movement of the clay matrix towards the interior of the body as moisture migrates towards the drying surface. Shrinkage is included as a two-way coupled phenomenon with respect to heat and mass transfer. The numerical model is capable of predicting changes in local body porosity and solid phase concentrations imposed by drying-induced shrinkage, providing significant insights into the drying process of ceramic materials. The proposed mathematical framework is used to conduct a parametric analysis that examines the effects of different shrinkage scenarios on heat and mass transfer rates, as well as on the structure and physical properties of the material, demonstrating the impact of shrinkage phenomenon on drying dynamics. A realistic shrinkage scenario case study is also examined to demonstrate the full range of the model's predictive capabilities. Next, the drying process is modeled on an industrial scale by developing a numerical framework for an existing ceramic tunnel dryer. The proposed model captures the complex heat and mass transfer phenomena inside the dryer, predicting the gas phase composition, temperature, and relative humidity distributions, as well as the tile temperature and moisture content distributions along the dryer. This model is used to systematically derive optimal operating policies for the tunnel dryer, focusing primarily on controlling the shrinkage stage, during which the risk of product fracture is highest, while minimizing energy costs. First, energy consumption during the steady-state dryer operation is minimized, while ensuring that product quality is assured. Then, the transient state operation is optimized, investigating two case studies. The first case examines an increase in dryer productivity whilst the second case explores the coexistence of two different raw clay-based ceramics in the dryer. The optimal operating scheme ensures that body shrinkage is well regulated throughout the drying process with minimal fuel consumption. The third thematic section of this thesis focuses on the firing process of ceramic tiles. The analysis involves developing a numerical model to capture the heat transfer phenomena between the gas and solid phases inside a tunnel kiln. The impact of kiln operating conditions on ceramic product quality is quantified using a novel sintering model based on experimental results. This mathematical framework is employed in an optimization study aimed at minimizing natural gas consumption while maximizing kiln production rates and ensuring a minimum sintering threshold for the fired ceramics. To resolve the described multi-objective optimization problem, the ε-Constraint method is used to attain a set of non-dominated optimal solutions, each of equal physical importance. Finally, the attained non-dominated solutions are compared in a distinct optimization study focused on minimizing specific energy consumption, which is the ratio of the objectives in the bi-objective optimization problem. The final part of this thesis presents a model-based optimization approach to derive optimal temperature set-points for the firing zone of an industrial ceramic tunnel kiln under feedback control, accounting for different production loads. For this analysis, a detailed mathematical model is developed, assuming that the tunnel kiln is divided into a series of control zones, each equipped with a natural gas burner regulated by a PID controller. The optimization framework incorporates both the physical model of the process and constraints related to kiln operation and end-product quality, aiming to minimize total fuel consumption in the burners. In this context, optimal set-points are derived for various kiln production rates, ensuring that product quality is maintained with minimal energy consumption. A case study is conducted to demonstrate the benefits of adjusting temperature set-points to accommodate changes in production rates.
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