Temperature variations in the oxygen carrier particles during their reduction and oxidation in a chemical-looping combustion system
Temperature variations in the oxygen carrier particles during their reduction and oxidation in a chemical-looping combustion system
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DOI：
10.1016/j.ces.2004.09.049
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发表时间：
2005-02
影响因子：
4.7
通讯作者：
F. García-Labiano;L. F. D. Diego;J. Adánez;A. Abad;P. Gayán
中科院分区：
文献类型：
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作者：
F. García-Labiano;L. F. D. Diego;J. Adánez;A. Abad;P. Gayán
建立了一个包含质量和传热的颗粒反应模型，以了解化学环燃烧（CLC）系统中氧载体颗粒在循环还原和氧化反应过程中产生的温度变化。考察了不同氧载体Cu、Co、Fe、Mn、Ni在燃料气体（CH4、Co、H2）和氧化（O2）还原过程中的反应。在这些系统中，氧化反应总是放热的，随后放热；然而，根据金属氧化物和燃料气体的不同，还原反应可以是放热的也可以是吸热的。在放热反应过程中，氧载体内部产生的热量使颗粒温度升高，当温度升高接近活性物质的熔点时，会影响颗粒结构。分析了影响反应速率和热传递过程的几个变量，了解了它们对颗粒内部温度的影响。对于给定的氧载体和反应，颗粒的最高温度主要取决于颗粒大小、反应速率和外传热阻力，而受氧载体孔隙率、惰性材料类型和金属氧化物含量的影响较小。氧化反应的温度变化最大，最大对应于1mm颗粒的Ni和Co氧载体，温度为~ +90K。在还原反应中观察到的最高温度升高对应于CuO与CO的反应，1mm颗粒的温度升高为~ +40K。对于其余的反应和金属，对于粒径小于1mm的颗粒，颗粒温度的变化在10K以下。在CLC系统中存在的典型操作条件下，当颗粒尺寸小于0.3mm，金属氧化物含量为40wt%，总转化时间小于30s时，对于任何氧载体的任何反应，相对于体积条件的温度升高都小于15K。此外，在大多数实际条件下，颗粒内部的温度分布接近平坦，没有发现局部高温点。因此，在典型的CLC系统温度（1000-1300K）下，流化床反应器氧化过程中由于烧结而导致载体固体多孔结构的变化预计不会发生。
A particle reaction model including mass and heat transfer has been developed to know the temperature variations produced inside the oxygen carrier particles during the cyclic reduction and oxidation reactions taking place in a chemical-looping combustion (CLC) system. The reactions of the different oxygen carriers based on Cu, Co, Fe, Mn, and Ni during the reduction with fuel gas (CH4, CO, and H2) and oxidation (O2) have been considered. In these systems, the oxidation reaction is always exothermic with subsequent heat release; however, the reduction reaction can be exothermic or endothermic depending on the metal oxide and the fuel gas. The heat generated inside the oxygen carriers during the exothermic reactions increases the particle temperature, and could affect the particle structure if the temperature increase is near to the melting point of the active materials. Several variables that affect the reaction rate and the heat transport process have been analyzed to know their effect on the internal particle temperature. For a given oxygen carrier and reaction, the maximum temperature of the particles depended mainly on the particle size, the reaction rate, and the external heat transfer resistance, being lower than the effect of the oxygen carrier porosity, type of inert material, and metal oxide content. The highest temperature variations were reached for the oxidation reactions, with the maximum corresponding to the Ni and Co oxygen carriers with values of ∼+90K for 1mm particles. The highest temperature increase observed during the reduction reactions corresponded to the reaction of CuO with CO, with values of ∼+40K for 1mm particles. For the rest of the reactions and metals, the variations in the particle temperature were below 10K for particle sizes below 1mm. Under the typical operating conditions that exist in a CLC system, with particle sizes lower than 0.3mm, 40wt% of metal oxide content, and overall conversion times lower than 30s, the increases of temperature with respect to the bulk conditions were lower than 15K for any reaction of any oxygen carrier. Moreover, the temperature profiles inside the particles were near flat in most of the practical conditions, and no local points with high temperatures were found. Thus, changes in the solid porous structure of the carrier due to sintering during oxidation in fluidized bed reactors are not expected working at typical temperatures of CLC systems (1000–1300K).