| The absorber material is the core components of the receiver in solar thermal power plant (STPP). Due to the energy flux density inhomogeneity and the instability which can easy lead to local hot spot in the absorber, and result to a series of proplems such as thermal stress damage, low air movement stability and durability. Therefore, it is urgent to develop new absorber material, which should have good oxidation resistance, good thermal shock resistance, with three or two dimensional connected structure, high specific surface area and high heat conductivity. Based on the requirement of the absorber material and the characteristic of the receiver, this paper have studied the batch formula composition which would use as absorber material in STPP system.In the experiment, Si3N4and SiC were used as substrate material, andalusite,α-Al2O3, Y2O3, Cr2O3and etc. were choosed as additives to synthesise high-temperature resistance binder phase, than form the ceramic composite absorber material which meet the demand of absorber material in STPP system.Modern testing techniques were taken to study the influence regularity of composition and preparation process on the microstructure and properties.The mechanism and methods to improve the oxidation resistance and thermal shock resistance of the ceramic absorber material were also discussed.The foam ceramic used as absorber which suitable for STPP was developed. A high efficient receiver was designed to fit the prepared absorber material by using the air as heat transfer medium.What’s more, simulation analysis of the receiver and absorber material on temperature field, pressure field and velocity vector field was catched out by Ansys workbench. The main results of the research are as follows:(1) At first, the composite ceramic of mullite bonded SiC and Si3N4was conduct-ed by pressureless firing with SiC and Si3N4as the main materials, andalusite and α-Al2O3as additives. The batch formula of series A have designed, the thermal shock resistance, oxidation resistance, phase composition, microstructure and performances have been tested and analyzed. Results showed that while fired in the air, the composite ceramic of Si3N4-SiC ceramic contains a large number of quartz phase, which result the decrease of the thermal shock resistance.(2) Based on the experiments of series A, ways for improving thermal shock performance of the composite materials are researched. Through additional bauxite source of aluminum in the experiments, which enabled more free SiO2can react with Al2O3to generate mullite in high temperature reaction process, reducing glass phase in the samples and improving thermal shock resistance. Batch formula of series B is designed.The properties effects of the fired samples by pressureless firing in air and burying powder firing (using Qingdao graphite powder to cover sample in firing process) have been studied respectively. Study on the buried fired samples showed that B3is the optimal formula after firing, the addition of bauxite is15%, the optimum firing temperature is1480℃, the bending strength is53.20MPa, while the bending strength changed to87.86MP after thermal shocks from1100℃to room temperature for30times, which increased by65.15%. Phase compositions analysis showed that the phases are a-SiC, Si3N4and mullite before thermal shock, while it is α-SiC, Si3N4, quartz, and mullite after thermal shock. A small amount of quartz phase is generated after thermal shock, filling in the pores, covering on the crystal surface, thus improving the thermal shock resistance of samples. Using burying powder firing method can reduce the content of quartz phase, and improve the thermal shock resistance of fired samples.(3) On the basis of batch formula of B3, methods for improving the density of the absorber ceramic have been researched. By adding different additives in the experiments, the batch formula of series F is designed. Results showed that adding Y2O3in batch formula of B3with the burying firing process, can greatly improve the density of the fired samples, it has changed from2.06g.cm-3up to2.57g.cm-3.F3is the optimal formula, that is plusing9%of Y2O3in the B3formula to fire, and the best firing temperature is1500℃, the bending strength is100.02MPa. Phase compositions analysis showed that the main phases of the samples are α-SiC, Si3N4, O’-Sialon and mullite.The microstructure of the samples by using SEM showed a compact structure, meanwhile, massive O’-Sialon solid solution can be observed in the grain interface which is useful for improving of the density.(4) On the basis of burying firing of series F, methods for improving oxidation resistance performance of the absorber ceramic material are studied. Based on the formula F1and F3, batch formula of series G is designed, and results showed that adding Cr2O3and Y2O3is beneficial to improve the oxidation resistance of the composite ceramics. The optimum sample is formula Gl fired at1500℃, the bending strength is157.04MPa. The oxidation reaction rate constant is1.7389mg2·cm-4·h-1after oxidation for100h at1300℃, and oxidation resistance is better than the sample of batch formula series A, B and F. The main phases are a-SiC, Si3N4, mullite and O’-Sialon. The compound addition of Y2O3and Cr2O3can form a protective layer which could slow down the spread velocity of O, thus will mprove the oxidation resistance of the sample.(5) Methods to improve the thermal shock resistance performance of composite ceramics have been studied. By adding ZrO2in the samples, which can create small crack with temperature changing to toughening composites, thus improving the thermal shock resistance of composite ceramic materials. Batch formula of series H is designed in the experiment, and results showed that samples exposed good thermal shock resistance by adding8%ZrO2,7.26%Y2O3on the basis of Gl formula. The formula of H3fired at1480℃is the optimum, which have a bending strength of100.26MPa. The bending strength of samples increased after thermal shock for30times from1100℃to room temperature, the increase rate is10.34%. The phases of the fired samples are α-SiC, Si3N4, mullite and O’-Sialon; also a small amount of quartz (SiO2) existed. The microstructure and EPMA results indicated that adding Y2O3, Cr2O3and ZrO2had obvious effects on formatting nucleus, than generate flowers and reticular structure solid solution, which are useful for improving thermal shock resistance.(6) The foam ceramic used as the absorber for the STPP have been prepared and studied. The absorbers of foam ceramic have been prepared from polymeric sponge replication method by using the optimum formula of G1and H3as slurry matrix, rheological agent as solvent, and polyurethane foam as impregnation. Results showed that the foam ceramics that prepared by G1formula slurry had the best performance. The optimal firing temperature is1500℃, the porosity rate is93.7%, the compressive strength is0.27MPa, the compressive strength is0.30MPa after thermal shock for30times.The main phases of the optimum foam ceramic after firing are a-SiC, S13N4, mullite and O’-Sialon.The macrostructure and microstructure analysis of foam ceramics prepared by Gl formula showed an uniform aperture, ranging from1to3mm, and the sample frame is relatively sturdy.Microscopic image of the sample showed that the sample’s skeleton is relatively dense. It has an expectation to use the foam ceramic as absorber in STPP system, which could solve the defects of high temperature oxidation and thermal shock performance to some degree.(7) Efficient volumetric receiver structure by using air as heat transfer medium and the prepared foam ceramic as absorber have been designed. The heat absorption temperature field, pressure field and velocity vector field have been simulated and analysed by using Ansys Workbench. In the design of the paper, the opening angle of the receiver is50°, inlet diameter is180cm, the outlet diameter of the receiver is50cm, and the total length of the absorber is110cm. Simulation results on heat absorption temperature field with different porosity of Si3N4-SiC foam ceramic showed that with the increasing of the porosity,the outlet air temperature increase as well.The pressure distribution analysis with different porosity showed that the higher of the foam ceramic porosity, the smaller of pressure drop in the receiver is, which is more conducive to strengthen heat transfer in the receiver, and benefit to the improvement of the thermal efficiency. At the same time, the inlet air velocity and receiver pressure distribution have simulated by adding the porosity of0.95, and the results showed that when the inlet air velocity is5-8m/s, it is advantageous to keep the receiver working steadily. |
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