Fundamentals and Technologies of Waste Heat Recovery from High Temperature Solid Granules

Zhigang Guo, Xing Tian, Zhihong Wu, Jian Yang, Qiuwang Wang*

Key Laboratory of Thermal-Fluid Science and Engineering, Ministry of Education,

School of Energy and Power Engineering, Xi’an Jiaotong University,

Xi’an, Shaanxi, 710049, China

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In the field of metallurgy and building materials, there are a huge waste heat contained in the high temperature solid granules (HTSG). In China, the annual production of HTSG can achieve 4.5 billion tons/year, of which the residual heat exceeds 100 million tons of standard coal. Nowadays, the coke dry quenching, annular cooler machine and grate cooler machine have been the mature technologies for the lumps. The multistage cyclone exchanger, fluidized beds and drum cooler have been also widely applied for the powders. However, the industrial materials such as the slag, usually contains both lumps and powders, and there are the wide ranges of diameter, temperature and yield. Therefore, it is quite difficult to develop the efficient heat recovery for these granular mixtures, of which the proportion reaches more than 60% of the total HTSG. Above all, it is still worthy of investigating the mass-and-heat-transfer mechanism for particles in the wide size, and the related efficient technology of waste heat recovery remains open. A project funded by the National Key R&D Program of China (No. 2017YFB0603500, 2017-2020) has been conducted to overcome the limitations above. Based on the optimized separation devices and moving bed heat exchangers, the indirect heat recovery is applied for powders with the major particle size less than 3 mm. By comparison, the direct heat recovery is adopted for the other lumps which may still contain the fine powders. As a result, a demonstration project of graded recovery and utilization system has been built in the Chengde Branch of HBIS Group Co., LTD. for the HTSG. The particle size located between 0.045-45 mm, the efficiency of waste heat recovery was larger than 70%, the testing running time achieved more than 200 h, and the yield could exceed 600 t/d.

In the project, the major investigations are shown as follows. Firstly, the fundamental of heat transfer in the HTSG were focused on. For the direct heat recovery between the air and the lumps, the factor of irregular shape was considered to modify the heat transfer correlation. Moreover, the influence of swimming powders between large particles was explained in the modifications. As for the indirect heat recovery, the mechanism of wall effect was clarified in the granular flow around the circular tube, and the corresponding suppression method was proposed. The porosity distributions were presented for the spherical particles in uniform diameter size and multi-particle size. In addition, the contact thermal resistance and the penetration resistance were built to predict the indirect heat transfer of dense gravity-driven granular flow. The heat transfer was concerned with the particle contact near the wall and the inside flow structures. Secondly, the different heat recovery technologies for the HTSG were proposed. To uniformly separate and distribute the gravity-driven granular flow with the wide diameter size, the multiple technologies combined with fast-falling surfaces were developed. Correspondingly, the design methodology of the spiral discharge devices was also established for the granular mixtures. Besides, to improve the heat recovery efficiency, both the directional control of heat flow transportation on the granular side and the synergistic heat transfer enhancement on the fluid side play remarkable roles. The methods of granular flow structures adjustment and the optimized inserted element on the fluid side were therefore proposed for the improvement. Moreover, the combined storage method including the heat-and-mass self-equilibrium, and the efficient graded-recovery design were proposed for the waste heat. Finally, the anti-clogging, anti-wear and anti-corrosion technologies were all developed in the moving bed heat exchangers. It demonstrated the design criteria of tube materials, tube diameter, the distances between tube banks and the coating on tube out-wall surfaces in the complex granular flows.

Keywords: High temperature solid granule; Mass-and-heat-transfer mechanism; Technology of waste heat recovery; Moving bed heat exchangers