Experimental And Kinetics Investigation Of Iron/Copper/Manganese Based Oxygen Carriers For Chemical Looping Combustion Of Coal

Chemical Looping Combustion (CLC) is a novel technology for combustion, which is different to conventional combustion technology due to its inherent CO2 capture characteristic. In recent years, more and more investigations were focused on using coal for CLC due to its higher storage than the other fossil fuels. Depending on the different mechanisms for combustion, two approaches are always involved in CLC for coal combustion, i.e. in-situ Gasification Chemical Looping Combustion (iG-CLC) and Chemical Looping with Oxygen Uncoupling (CLOU). For both iG-CLC and CLOU, the development of suitable oxygen carriers is a cornstone, which is essential for the development of CLC.In this context, this work proposed two novel Fe2O3/Al2O3 and CuO/CuAl2O4 oxygen carriers with high reactivity, based on sol-gel synthetic technique. Also a further development of manganese minerals was accomplished for their use in CLC technology.For the synthetic Fe-based material, the preparation process was optimized to control the physical and chemical structures of products, which further derived the Fe2O3/Al2O3 oxygen carrier with high reactivity, high crushing strength and good resistance to agglomeration. Subsequently, this oxygen carrier was used for the tests with gaseous fuels and solid fuels, for its application to CLC and iG-CLC. During calcinations, the temperature for Fe2O3/Al2O3 should be higher than 1100? to obtain particles with sufficient crushing strength. During the reactivity tests, Fe2O3/Al2O3 oxygen carrier possesses sufficient reactivity with the gases CH4, CO and H2 mainly involved in CLC. With the attention to the tests with solid fuels, the oxygen carrier was subjected to the simulations of iG-CLC in batch fluidized reactors. Steam concentration, oxygen carrier to fuel ratio and cycles were focused for their influence on the performance of the oxygen carrier. In the end, the developed Fe2O3/Al2O3 oxygen carrier was compared to the Fe-based oxygen carriers in the literatures, which suggested that the sol-gel Fe2O3/Al2O3 can be considered as a highly reactive oxygen carrier for iG-CLC of coal. That is high combustion efficiency can be achieved with lower amount of oxygen carrier inventory.With regard to the CuO/CuAl2O4 material, the only available component for CLOU is CuO, where the CuAl2O4 has an extremely slow rate of oxygen release. The first series of test with this oxygen carrier was conducted with gaseous fuels due to its potential to improve the performance of gas combustion. After that, the CuO/CuAl2O4 was applied to the CLOU experiments with coal combustion, where the temperature, volatiles content, coal rank and cycles were evaluated for their effects on the combustion performance. The results show that high-rank coal can lead to higher CO2 capture efficiency but lower rate of conversion. The opposite occurs for the low-rank coal. During the oxidation of gaseous fuels with CuO/CuAl2O4, CLC and CLOU mechanisms exist. Identification indicates that the CLOU mechanism dominates for the combustion of gases with low concentration, whereas CLC is relevant for the case of gases with high concentration.Additionlly, the use of manganese minerals as oxygen carriers was further developed through the present work. Four manganese minerals were considered for their utilizations in CLC. In the tests of CLOU feasibility of these manganese minerals, it was observed that the Mn2O3 in the minerals can only release gaseous O2 during the first cycle, which, however, cannot be regenerated in the followed oxidation process. Therefore, the four studied manganese minerals have no CLOU effect. In this scenario, the following experiments were carried out for their use in CLC for gaseous fuels and iG-CLC for solid fuels. The reactivity of all the manganese minerals decreased during the first 10 cycles and then became stable. The enhancement of the investigated manganese minerals on char gasification is better than the previously used ilmenite. According to the experiments, MnGBMPB and MnBR exhibited high attrition rates and low crushing strength after cycles. After 60 cyclic tests, MnSA and MnGBHNE were suggested as the most promising oxygen carriers for CLC. Thus, MnSA and MnGBHNE were especially concerned during the subsequent iG-CLC tests with solid fuels in fluidized bed reactor. During the experiments with char, the alkali metal elements contained in the manganese minerals catalysis the gasification of char which showed unexpected high rates of char gasification.Finally, using temperature programmed reaction experiments this work carried out the kinetic analysis of Fe2O3/Al2O3 and CuO/CuAl2O4 for the reactions involved in their use in iG-CLC and CLOU. The involved method was for the first time used in the kinetics determination for CLC process. For the Fe2O3/Al2O3 oxygen carrier, the reductions by H2 and CO can be described with nucleation and nuclei growth model, however, different mechanism functions were involved depending on gaseous fuels. The mechanism functions were G(X)=[-ln(1-X)]1/4 and [-ln(1-X]1/3 respectively for the oxidation of H2 and CO. The corresponding activation energy is 19.5 and 170.5 kJ/mol, respectively. In the case of CuO/CuAl2O4 decomposition, nucleation and nuclei growth model can also be applied to the description of the oxygen release process from this oxygen carrier. The mechanism function for the decomposition of CuO to Cu2O is G(X)=[-ln(1-X)]2/3, and the corresponding activation energy is 343.7 kJ/mol. With regard to the application of this model to the CLOU process with coal combustion, modification to the model developed in N2 atmosphere should be carried out with the partial pressure of O2 in the reaction atmosphere.