As an emerging heat storage technology in recent years, molten salt phase change heat storage technology has the advantages of high heat storage density, long cycle stability, energy conservation, environmental protection, safety and reliability, etc. It can be used to well realize the recovery, storage and time-space utilization of high temperature flue gas waste heat in the intermittent ceramic kiln, so as to alter the current situation of high energy consumption, high pollution and high carbon emission of intermittent kiln. However, the heat conduction performance of molten salt heat storage materials is still too low, while the heat storage efficiency of molten salt is not fully utilized in the existing industries. It is still necessary to to enhance the heat storage performance of molten salt. The most common method is to add fins, while most of existing researches on strengthen phase change heat storage with fins focus on low temperature application scenarios, such as electric vehicle thermal management, chip temperature control and heat dissipation, etc. Although high temperature molten salt heat storage has been widely used in recent years, it is mainly used for heat energy storage from the new energy surplus power electric or the photo-thermal. No matter the application scenario, heat source form or heat transfer mode, the heat storage of high temperature flue gas is quite different from the existing ones, while the heat transfer characteristics and the influence law of fins structural parameters of the heat storage process are different.

In this study, a heat transfer enhancement scheme with fins was proposed for molten salt heat transfer pipe to achieve efficient recovery of the waste heat from the high temperature flue gas, while a mathematical physics model was constructed to simulate the high temperature flue gas molten salt heat storage process. More attentions were paid on the influences of the fins number, length and tilt angle on the liquid phase change, flow field distribution and heat storage performance. In this way, the influence laws of the fin structural parameters on the heat transfer characteristics and heat storage performance are reached.

Although adding fins will play a certain inhibitory role in the flow of molten salt near the wall in the direction of buoyancy to some extent in the initial stage, it can accelerate the melting rate of molten salt on the near wall side, prompting the molten salt system to enter a thermal storage mode dominated by convective heat transfer earlier and hence improving the overall thermal storage performance. There exists an optimal set of fin structural parameters that enable to achieve desired heat storage performance, i.e., the fins length L=52.5 mm, fins number n=4 and fins angle θ=45°. In this case, the liquid phase mass fraction f and the molten salt average temperature T both achieve the maximum value during the heat storage process. However, any one of structural parameter of the fins exceeds its optimal value, the flow resistance of the molten salt thermal fluid near the wall along buoyancy direction increases and the flow path increases, whereas the local vortices between the fins are strengthened evidently, thus suppressing the global heat transfer and deteriorating the thermal storage performance.

In this study, the fundamental link between the structural parameters of heat transfer tube among the heat transfer characteristics and the heat storage performance is established for the melting salt heat storage scenario of high temperature flue gas. The results have important guiding value for the efficient recovery, storage, conversion and utilization of the medium-high grade industrial waste heat. On this basis, the subsequent research on structural innovation and multi-parameter collaborative optimization can be further expanded.

As a high energy consuming industry, the ceramic industry generally faces the problem of excess waste heat from the flue gases at medium and low temperatures in the production process. If the waste heat cannot be effectively utilized, it is not only energy waste, but also seriously hinders the low-carbon, energy-saving production and sustainable development of the ceramic industry. It is imperative to explore effective technical solutions for utilizing the kiln waste heat, in order to achieve energy conservation and emission reduction in ceramic production. Ammonium chloride is an important raw material for ceramic production, while the drying process usually requires huge amount of heat energy. Therefore, utilizing kiln waste heat to dry ammonium chloride not only improves energy efficiency but also reduces the production costs, thus having important practical significance. The aim of this study is to construct a kinetic model for the drying of ammonium chloride, focusing on drying characteristics and moisture migration laws, thus providing theoretical and technical supports for optimizing the waste heat utilization in ceramic production processes.

To achieve the above goals, a technical solution based on kiln waste heat to dry ammonium chloride was proposed, by constructing a kinetic model with user-defined functions (UDF). The water migration law and drying characteristics during the drying process were analyzed through numerical simulation methods. The contents include three aspects. Firstly, through drying experiments, five thin-layer drying models were established, while the most suitable model to describe the moisture change law during the ammonium chloride drying process will be identified. Accordingly, a mathematical model for the drying process of ammonium chloride was constructed. Secondly, the spatial and temporal variations of water migration rate (WMR) during the drying process of ammonium chloride will be analyzed, especially the water migration characteristics in the x- and z-directions. Thirdly, by simulating and analyzing the water migration behavior during the early, middle, and late stages of drying, the phenomenon of water accumulation and the impact on drying efficiency will be revealed.

The two-term model can be used to accurately predict the drying process of ammonium chloride, which can be divided into accelerated drying stage and deceleration drying stage. In the initial stage of drying, the water migration rate (WMR) in ammonium chloride products was rapidly increased from the edge to the center, and then slowly decreased, with slight water accumulation in the central area. As the drying process was progressed, the WMR in the central area was gradually increased, while the overall WMR showed a deline trend. In the x-direction, WMR maintained an upward pattern from the edge to the center. In the z-direction, the variation trend in WMR was similar to that in the x-direction, but its peak shifted from the central region to a slightly higher location. In addition, it was also found that the spatial distribution of water migration rate was closely related to the drying time, so that the drying efficiency could be significantly improved by optimizing the drying process parameters.

In order to reveal the water migration law during the drying process of ammonium chloride, a thin-layer drying kinetics model was proposed, while the user-defined functions (UDF) were combined to numerically simulate the drying process driven by kiln waste heat. The two-term model can be used to accurately predict the drying process of ammonium chloride, with a correlation coefficient R²=0.99891, root mean square error RMSE of 0.006454, and squared error of 0.001532936, indicating high reliability of the model. The drying of ammonium chloride was divided into two stages: acceleration and deceleration. The maximum drying rate was 1.28833×10^–5 kg·kg^–1·s^–1, while it took 14400 seconds to achieve a moisture content of 0.1%. During the drying process, the moisture content was gradually decreased, whereas the moisture content in the edge area was lower than that on the inner side. In the initial stage, the water migration rate (WMR) was increased and then decreased from the edge to the center, whereas water accumulation occurred in the central region. In the later stage, WMR was increased from the edge to the center, while the overall WMR showed a decline trend. In the z-direction, the variation pattern of WMR was similar to that in the x-direction, but the peak shifted from the central region to a higher location and the overall WMR was gradually decreased with drying time.

Ceramic kiln production process is often accompanied by a large number of high-temperature flue gas waste heat emissions, resulting in increased production costs and carbon emissions per unit of output value. However, the current ceramic kiln in the high temperature flue gas waste heat recovery and utilization technology is relatively scarce, especially for some of the intermittent ceramic kilns, waste heat recovery is more difficult. Therefore, intermittent production mode of flue gas waste heat staggered utilization of high temperature heat storage technology is an urgent need to solve the bottleneck problem.

High-temperature molten salt as a commonly used high-temperature heat storage medium has a wide working temperature (150-1300 ℃), low saturation vapor pressure, high security, large heat storage capacity, high chemical stability, cheap raw materials and so on, which has been widely used in many high-temperature heat storage industries. Obviously, high-temperature molten salt waste heat recovery technology is fully applicable to intermittent ceramic high-temperature flue gas waste heat recovery and storage. Based on this, the molten salt thermal storage technology is introduced to realize the recovery and storage of high-temperature flue gas waste heat and staggered time utilization in intermittent ceramic kilns, while a mathematical-physical model of the high-temperature flue gas molten salt thermal storage process is established, numerical simulation methods are used to reveal the characteristics of the high-temperature flue gas molten salt phase-change thermal storage and the heat transfer law, and the influence of the number, spacing and shape of the heat exchanger tubes on the process of the molten salt phase-change thermal storage process in the heat storage tank is systematically analyzed.

The number of heat exchanger tubes was varied from 1 to 6 and the effective heat exchange area increased linearly, but the gain effect of enhanced molten salt melting became smaller and leveled off. The maximum heat transfer area between models increased by 245%, while the corresponding two-stage melting time decreased by 62% and 59%, respectively. In contrast, the heat transfer area between the five and six heat transfer tubes is increased by 22%, and the two-stage melting time is reduced by only 4%. For the two fixed tube number models, the effect of different heat exchanger tube spacing on the melting time of the two stages of molten salt is small, the maximum reduction of the melting time is 7% in the two stages and the reduction of the time in the stage of T=600 ℃ is even smaller compared with that in the stage of complete melting. When the isotropic tube spacing becomes larger, the melting time of the two stages decreases and then increases. When the tube spacing is 100 mm, the melting time of the molten salt domain reaches the extreme value, whereas the overall average temperature of the molten salt is effectively enhanced. When the area ratio above and below the heat exchanger tube varies from 1/1 isotropic to 1/11, the heat storage effect of the molten salt is increasing in the complete melting stage, the slope of the melting time variation decreases and the melting time is shortened by a maximum of 10.6%, whereas the change in the melting time in the stage of T=600 °C is small. When the area ratio between the upper and lower surface is 1/11, it can not only ensure the heat transfer efficiency at the stage of T=600 ℃, but also significantly increase the melting speed and shorten the melting time from room temperature to complete melting stage of the molten salt.

As the heat exchanger tube volume remains unchanged and the amount of flue gas per unit of time is under the same premise, the tube diameter will be shortened, the heat transfer area is greatly enlarged, the density of the heat flow received per unit volume of molten salt is improved, so that the efficiency of heat transfer is enhanced, which will speed up the melting process. Although increasing the number of heat exchanger tubes can significantly improve heat transfer efficiency, this is not a linear effect. At the same time, the variation of the pipe spacing has a significant impact on the melting process of the molten salt.100 mm pipe spacing achieves a good heat balance between the inside and the wall of the tank, avoiding large areas of non-melting in the center area and hence demonstrating a significant melting advantage. Different top and bottom area ratio on the molten salt in the heat storage tank mainly lies in the upper and lower part of the heat transfer tube heat transfer area ratio. Smaller top and bottom area ratio means larger lower part of the heat transfer tube heat transfer area. The bottom of the heat transfer tube diameter becomes larger, equivalent to squeezing the bottom of the heat storage tank molten salt to the upper part of the tank area. Therefore, the amount of the molten salt at bottom is reduced and the relative area of the heat transfer is increased, so that the bottom of the tank under the dual role of the molten salt melts more quickly.