Synthesizing Cordierite From Calcined Bauxite And Its Application In The Thermal Energy Storage Materials

Solar thermal power generation is one of the most promising technologies of converting solar energy into electricity. While thermal storage is the key to realize efficient operation of the solar thermal power plant. Researches and developments of high-performance thermal storage materials are increasingly raising people’s attention. Among them, Cordierite is considered to be an excellent candidature due to its low thermal expansion, favorite thermal shock resistance and high-temperature resistant. This thesis, therefore, mainly focuses on the in-situ synthesis of cordierite using calcined bauxite and talc, and it is applied in the field of high temperature solar thermal storage material.Particularly, the mircostructure, characterization and sintering properties of calcined bauxite were studied in details. In this study, we designed and prepared a series of silicon-rich cordierite, magnesium-rich cordierite, aluminum-rich cordierite and conventional cordierite by using in-situ synthesis. The relationship between composition, preparation techniques, micro structures and performances are respectively investigated by using modern testing technologies including XRF, XRD, SEM, EPMA, TEM, Raman spectrum, Infrared spectrum, Nuclear magnetic resonance (NMR) and etc. The influence of different compositions on the in-situ synthesized cordierite is studied. The structure and synthesis mechanism of cordierite are investigated. Moreover, high-temperature thermal storage materials with improved thermal shock resistance and increased thermal storage capacity are prepared by doping in-situ synthesized cordierite with silicon carbide, zirconium oxide and mullite. The mechanism of performances enhancement is also discussed. In addition, in order to increase the specific surface area of heat storage materials and enhance the efficiency of convective heat transfer, pore structures including its shape and size of the high-temperature thermal storage materials are particularly modeled and studied using the laws of thermodynamic. Finally, selected phase change materials (PCMs) are encapsulated in the ceramic matrix to further increase the thermal storage density of the heat storage materials. The encapsulating mechanism and corrosion behaviors between the encapsulation agent and ceramic matrix materials are investigated. Importantly, for the evaluation of the performance of thermal storage systems using the developed materials, we also developed a thermal storage system to assess its heat transfer performance and thermal storage behavior (e.g. endothermic and exothermic processes). The main conclusions of the above researches are listed as follows:(1) Calcined bauxite, as an excellent raw material for producing ceramics, is suitable for preparing industrial ceramic products with a high strength and high-temperature resistance. Calcined bauxite has a favorable high-temperature resistance with a melting point of higher than1650℃). Its bending strength, bulk density, specific heat, and thermal conductivity are improved with an increasing sintering temperature. The main crystal phases of calcined bauxite are corundum and mullite. The corundum is metastable and maintained a diaspore flake and granular appearance, which broadens the contact surface of reaction and increases the reactivity of corundum grains. Because of orientations and column shape of the mullite grains, mullite is used as nucleation agent in the process of synthesis cordierite in order to promote the growth of hexagonal cordierite. The impurities contained in the calcined bauxite will enter the lattice of cordierite at high temperatures in the forms of Fe2+, Fe3+, Ti4+, and K+etc., which increases the lattice defects and reduces the grain formation temperature. All of the above-mentioned phenomenon reduce the synthesis temperature, broaden the synthesis temperature range, and improve the high-temperature resistance of cordierite.(2) The calcined bauxite synthesized cordierite with the theoretical chemical composition has a low synthesis temperature (1160℃), a high purity (95.63%), a low thermal expansion coefficient (2.22×10-6/℃), a high-temperature resistance (1500℃), and a wide synthesis temperature range (1160℃～1430℃). In contrast, the cordierites with the magnesium-rich composition has a reduced synthesis temperature and thermal expansion coefficient, which is unfavorable for improving its bending strength, purity, high-temperature resistance, and thermal shock resistance; Silicon-rich composition is favorable for improving the synthesis temperature and thermal expansion coefficient, and reducing the high temperature resistance, thermal shock resistance, purity and bending strength of cordierite; Aluminum-rich composition is favorable for improving bending strength, thermal shock resistance and high temperature resistance, while improving the thermal expansion coefficient and synthesis temperature of cordierite. The comprehensive performance of sample F2is the optimal (Calcined bauxite39.80%, Gguangxi talc41.64%, Guangdong quartz18.56%) sintered at1420℃, the water absorption (Wa), porosity (Pa), bulk density (D), linear shrinkage after sintering, bending strength, thermal expansion coefficient (RT～850℃), thermal conductivity at room temperature, heat capacity at room temperature, and heat storage density at temperatures from0to800℃are12.96%,24.56%,1.97g·cm-3,0.21%,53.92MPa,2.22×10-6/℃,2.20W/(m·K),0.60kJ/(kg·K), and869kJ/kg, respectively. XRD and SEM analysis shows that the phase composition is indialite (high temperature cordierite) in the form of hexagonal columnar and granular.(3) The results of the TEM, Raman spectroscopy, infrared spectroscopy, and nuclear magnetic resonance (NMR) analyses are consistent with the results from the XRD analysis, confirming that (i) indialite is generated in the process of synthesizing cordierite using calcined bauxite, and (ii) the cordierite has not gone through mutual transformation between the high temperature cordierite and low temperature cordierite in the whole process. It is feasible to measure the thermal expansion coefficient of cordierite using the degree of order of the Si/Al alignment, namely the thermal expansion coefficient is decreased with an reducing degree of order of the Si/Al alignment TEM analysis shows that a lot of microcrystalline (about5nm) exist in the glass phases, which helps to increase the thermal conductivity and thermal shock resistance while reducing the thermal expansion coefficient of the sample.(4) The addition of mullite and andalusite in the sample is not effective in improving the bulk density and thermal conductivity of the sample. In contrast, adding zirconium oxide in the sample can significantly improve the bulk density, while adding silicon carbide can improve both the bulk density and thermal conductivity of the sample. In comparison with the above additives, in-situ synthesis of mullite is the best approach of improving the bulk density and thermal conductivity. Overall, the sample E1prepared using the in-situ synthesis of cordierite-mullite and sintered at1450℃have the best performances. For example, its bending strength is improved by28.11%after a thermal shock resistance test of30times. Moreover, the thermal conductivity (room temperature), specific heat capacity (room temperature), heat storage capacity (0～800℃), Wa, Pa, D and bending strength are3.71W/(m·K),0.87kJ/(kg·K),1416kJ/kg (increased by62.95%),1.33%,3.26%,2.52g·cm-3and76.95MPa, respectively. The mechanism of the improved thermal shock resistance can be attributed to the fact that the in-situ synthesized columnar mullite and in-situ generated columnar cordierite are intertwined with each other in the micro scale. Moreover, the low expansion coefficient of cordierite helps to ease the contraction/expansion due to volumetric changes of the sample. This is added on top of the fact that few glass phase and more closed porosity are presented in the sample. All of the above-mentioned effects attributes to an elevation of the energy barrier of crack propagation and a prolonged path for the developmental crack extension, which all lead to a high bending strength of the sample. It should be also mentioned that when the mullite phase is encompassed by cordierite phase which has a smaller thermal expansion coefficient, according to the principle of Thermal Expansion Mismatch, the residual compressive stress area is produced around the mullite resulting in a small stress value. Moreover, the stress can cause crack bifurcation and deflection in the process of thermal shock resistance test. The above mechanism leads to an improved thermal shock resistance of the sample. Therefore, it is concluded that the sample El with a good thermal shock resistance, high-temperature resistant, high strength can meet the requirement of heat storage material for solar thermal power generation.(5) Thermodynamic simulations on the geometry of heat storage materials were performed. The results show that the resistance of heat accumulator using honeycomb ceramic is less than that of the ceramic tube and ceramic ball. Moreover, the honeycomb ceramic is shown as the favorable material for the design and selection of the induced draft fan, leading to a low temperature environment for the long-term operation of the draft fan. The results further shows that the heat transfer coefficient of the honeycomb ceramic with square holes is larger than that of the honeycomb ceramics with round and hexagonal holes for a same heat transfer duty. Meanwhile, using the honeycomb ceramic the size of the heat storage device can be largely reduced, which is beneficial in terms of the optimization of the design and application of a thermal store device or system.(6) The combination of encapsulating agent and ceramic matrix are affected by the addition of the high-temperature flux in the encapsulating agent. When the high-temperature flux content is more than70%, the difference of thermal expansion coefficient between the encapsulation agent and ceramic matrix becomes significant leading to a poor combination. Moreover, the poor strength of the encapsulating agent also results in a bad combination. It is concluded that the optimum content of high-temperature flux is about65%. The PCM with composite substrate material was examined after200times’thermal cycle tests, which shows that the generated permeable zone (45microns in width) is formed between K2SO4and the ceramic substrate impeding the further infiltration of molten salt. Therefore, the combination of the PCM and composite substrate material shows favorable compatibility between the two materials and is suitable for the preparation of the encapsulated PCM materials. It should be also noted that the adaptability of different kinds of PCM and cordierite ceramic is different, and appropriate PCMs should be carefully selected to prevent any possible reactions with the cordierite ceramic. In this way, the heat storage system with an improved heat capacity can be achieved.(7) Air flow rate directly affects the overall heat transfer coefficient and heat transfer resistance. With an increased air flow, both the convective heat transfer coefficient and the heat transfer resistance of the heat storage system increase. Therefore, careful consideration should be given to the increased heat transfer resistance as a result of the increased air flow, since an increased heat transfer resistance affects greatly the stable operation of the system. Moreover, the honeycomb ceramic thermal storage materials, when filled with0.5m3encapsulated PCM, can store up to924.86MJ, which is equivalent to about280kWh. The volumetric heat storage density of this heat storage system is about1849.72MJ/m3. Our tests and analyses all show that the thermal storage device (i.e. the cordierite honeycomb ceramic encapsulated with phase change materials)is a good candidate to be used for thermal storage in a solar thermal power plant.