ZHANG Sigen, TANG Yongzhi, LIU Qi, LU Lin, FENG Qing

(Department of Energy Science and Engineering, School of Materials Science and Engineering, Jingdezhen Ceramic University, Jingdezhen 333403, Jiangxi, China)

Extended abstract:[Background and purposes] 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.[Methods] 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.[Results] 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.[Conclusions] 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.

Key words: molten salt phase change; heat transfer enhancement; waste heat utilization; numerical simulation; heat transfer optimization