Thermal Shock Resistance Evaluation Of Coal Water Slurry Nozzle

In actual use of coal-water slurry (CWS) nozzle, there is a very high thermal stress in the nozzle due to the high temperature in coal boiler and extremely cold by CWS. CWS ceramic nozzle concentrates thermal stress cracks and chipping damage phenomenon, which has seriously affected the life of the coal-water slurry nozzle. How to evaluation CWS nozzle’s thermal shock resistance reasonably is particularly important. In this paper, the thermal shock resistance of the Al2O3/TiC ceramic composite nozzle and YG8cemented carbide nozzle is studied by the finite element calculation and water quenching test.According to the theory of heat transfer and thermal mechanical theory, coal-water slurry nozzle temperature field and stress field were analyzed in the steady-state and transient state. The mathematical model is established and the reasons for the coal-water slurry nozzle thermal shock damage are analyzed. The effect of materials and structure on temperature filed and stress field of the nozzles was studied. The steady-state heat transfer analysis results show that:the maximum thermal stress occurs at the nozzle outlet, which is consistent with the actual use. The maximum thermal stress of the nozzles is as follows:flat-ceramic>flat-carbide>cone mouth ceramics>mouth cone carbide. The nozzle structure has important effect on the temperature field and stress field, the thermal stress of nozzle with cone angle is significantly lower than that of nozzle with through-hole structure. The reason is the cone outlet structure decreases the contact area between the nozzle and the high temperature environment, and increases the contact area with the cryogenic CWS, which could take away more heat.Materials significantly affect the nozzle temperature and stress field, the highest temperature and thermal stress of Al2O3/TiC ceramic material are higher than that of the YG8carbide material, the reason is that cemented carbide has higher thermal conductivity and lower thermal expansion coefficient than ceramic material, and makes the heat transfer more quickly and evenly, as a results, the temperature gradient and the deformation resistance are reduced. Transient heat transfer simulation results show that:heat distribution of the nozzle is uneven because of the forced convection cooling, namely the surface has higher tensile thermal stress than internal area, the thermal stress of Al2O3/TiC ceramics is higher than that of YG8cemented carbide. Thermal stress increases along with the increase of temperature. In the cooling process, the thermal stress increases with cooling time to a maximum value quickly and then decreases slowly, the maximum value appears at the time of2-3s.The thermal shock resistance of Al2O3/TiC ceramic and YG8carbide are studied by water quenching test and indentation-quenching test. The results show:after water quenching, the hardness of Al2O3/TiC ceramic and YG8cemented carbide decreases, the decreasing degree of hardness of YG8carbide is more obvious. Al2O3/TiC ceramic and YG8carbide show different failure forms in water quenching test. The main failure mode of the ceramic nozzle in the water quench test is whole cracking, fissure first formation on the surface, and then extended to the depth of the entire nozzle. The form of major damage of carbide nozzle is massive fall off, concentrated in the inner edge of the hole and the edge of the outer cylindrical surface after water quenching test. The thermal shock resistance ability of carbide nozzle is better compared with that of the ceramic. The two materials nozzles have some degree of oxidation, the oxidation of YG8carbide is more serious. In the pre-crack indentation-quenching experiments, the crack growth rate of ceramic nozzle is higher than cemented carbide, which proved the resistance to crack propagation of cemented carbide is better than ceramic.