MENG Dezheng, XIONG Zhao, TANG Yongzhi, ZHENG Hao, LU Lin, HUANG Yichen, DING Zhonggen

(School of Energy and Power Engineering, Jingdezhen Ceramic University, Jingdezhen 333403, Jiangxi, China)

Extended abstract:

[Background and purposes] China ceramic industry boasts a vast industrial chain, yet as a high-energy-consuming, high-polluting and high-carbon-emission sector, it runs counter to the principles of green and sustainable development. The high-temperature firing stage consumes immense energy, while raw material processing involves substantial water and electricity usage with difficult-to-treat wastewater. Additionally, ceramic production bases demand significant cooling, while existing refrigeration equipment further exacerbates energy consumption through additional electricity use. The production process also consumes large volume of water and generates high levels of wastewater. Given the difficulty of revolutionary changes in production processes, advancing the low-carbon transformation of the ceramic industry hinges on the conversion and utilization of waste heat. Currently, the application of ceramic industry flue gas waste heat to drive refrigeration and wastewater desalination is neglected. Applying seawater desalination technology to purify wastewater offers a pathway for efficiently converting and utilizing inexpensive and readily available low-grade industrial waste heat. Recent studies on coupling thermal-driven refrigeration with seawater desalination systems have primarily focused on solar-driven and thermal power plant waste heat-driven approaches. This study is aimed to not only establish a hybrid system integrating ejector refrigeration with humidification-dehumidification wastewater desalination but also propos three optimization strategies, including enhancing refrigerant condensation heat release, implementing deep cascade utilization of end-of-pipe flue gas waste heat and reusing waste heat from industrial concentrated wastewater. These approaches further enable simultaneous production of freshwater and refrigeration capacity, while advancing the low-carbon and energy-efficient transformation of the ceramic industry.

[Methods] This paper is aimed to establish a hybrid system of ejector refrigeration and humidification-dehumidification wastewater desalination driven by the flue gas waste heat from the ceramic industry, as well as its corresponding thermodynamic theoretical model. The Engineering Equation Solver (EES) software is employed for programming and calculation. The reliability of the model is verified by comparing theoretical results with experimental data. Three performance optimization schemes for the hybrid system are proposed, while comprehensive thermodynamic process analysis is conducted and the working performances of different optimized coupled systems are compared.

[Results] Through the optimization scheme for heat recovery from refrigerant condensation, the outlet temperature of wastewater from the ejector refrigeration (ER) subsystem can be increased by 3–5 ℃, while the energy utilization factor (EUF) can be raised by at least 14.8%, as compared with the initial scheme. In the optimization scheme for deep utilization of flue gas waste heat at the tail end, when the flue gas temperature is lower than the dew point, not only the latent heat and sensible heat released by the flue gas can be recovered, but also a certain amount of condensed freshwater can be produced. With this scheme, the flue gas waste heat utilization rate can be increased by 8.6–12.4%, the EUF is raised by at least 42.6%, the freshwater yield in increased by more than 72.3% and the flue gas discharge temperature is decreased to about 48 ℃. The optimization scheme for reutilization of industrial concentrated wastewater can be used to raise the spray temperature of terminal wastewater up to 74.8 ℃, with the freshwater yield to be increased to 392.2 L·h⁻¹, representing a 74.9% rise as compared with the initial scheme and over three times that of the standalone humidification-dehumidification (HDH) system. The EUF reaches as high as 0.88, and the flue gas waste heat utilization rate is not less than 70.48%. Among the three optimization schemes, the one for deep utilization of flue gas waste heat at the tail end exhibites the highest flue gas waste heat utilization rate, while the scheme for reutilization of industrial concentrated wastewater delivers the maximum energy efficiency and freshwater yield.

[Conclusions] An integrated ejector refrigeration and humidification-dehumidification wastewater desalination (ER-HDH) system driven by waste heat from the ceramic industry was developed. Three progressive optimization schemes were proposed to enhance the preheating effect of front-end wastewater in the condenser and achieve deep recovery of residual heat from end-stage flue gas and industrial concentrated wastewater. These findings hold significant implications for the efficient recovery of waste heat in the ceramic industry.

Key words: ceramic industry flue gas waste heat; ejector refrigeration; humidification-dehumidification; performance optimization; energy utilization factor