Prior to operating a Chemical Looping Combustion (CLC) process, suitable oxygen carrier particles should be developed for the effective CLC process. The CLC process does not need to separate $CO_2$ and the nitric oxides $NO_x$ production is rather low since it operates at lower temperatures. This system consists of oxidation and reduction reactors where metal oxides particles are circulating through these two reactors. The metal particles are oxidized with air in an air reactor and the oxidized metalparticles are reduced by fuel in a fuel reactor. Therefore, a circulating fluidized bed (CFB) combustor is considered as an oxidizer or combustor of the chemical looping combustion system since a CFB has excellent gas-solid mixing characteristics.
The reactivity of various oxygen carrier particles has been studied to find higher conversion of metal oxides. However, the effective carrier particles are rather expensive chemical agents so that cheaper oxygen carrier materials are needed for large-scale power plants more than 10 MW.
In the present study, NiO and $Fe_2O_3$ as the carrier particles and bentonite, $TiO_2$, and $Al_2O_3$ as the supporters were selected. These particles were prepared by the direct mixing of fine metal oxides whose size is less than 10 ㎛. The ratio of carrier/supporter is 6/4, and the paste was made by adding diluted distilled water to the mixed powder. The paste was dried at 110℃ for 24 hours, and calcined at 1,000℃ for 6 hours. Then it was crushed in a ball mill and the sample particles were taken in the size range of 106-150 ㎛ by sieving. The reactivity of the oxygen carrier particles was evaluated in a thermobalance reactor (0.050 m-I.D. 0.60 m-height) at the alternating atmosphere of oxygen reduction by $10%-CH_4$ and oxidizing the metal by $10%-O_2$ as in case of a chemical looping combustion system at 850℃. Also, the dual metal oxides of NiO-$Fe_2O_3/bentonite$ particles were prepared with different loading ratios of $NiO/Fe_2O_3$. The reduction and oxidation characteristics of these particles were analyzed at 650 ~ 950℃.
The shapes, mapping of each component, crystalline phases, compositions of metal oxides and pore area and pore volume of the particles were measured by SEM, EDS, XRD, ICP and $N_2-BET$, respectively.
The NiO exhibits higher reactivity than $Fe_2O_$3 and the particles supported on bentonite or $Al_2O_3$, produce higher reactivity than those on $TiO_2$. Therefore, the metal oxides on bentonite support were selected since they have reasonable reactivity and bentonite is a rather cheap supporter. The reactivity increases with reaction temperature and content of NiO. The obtained kinetic data is analyzed in terms of the gas-solid reaction. From the Arrhenius plot, the activation energies and pre-exponential factors are found to be 1.766 ~ 4.280 kJ/mol and $0.00880 ~ 0.02002 s^{-1}atm^{-1}$ for the reduction of each particle, respectively and 2.435 ~ 6.041 kJ/mol and $0.00806 ~ 0.1243 s^{-1}atm^{-1}$ for the oxidation reaction, respectively.
The carbon deposition characteristics on the metal oxide particles have been determined in this study. Carbon and some carbide are formed after the reduction reaction and the crystalline phases were determined by XRD analysis. The carbon deposition is started faster with more highly reactive particles. The metal oxide particles supported on bentonite do not exhibit high reactivity, but carbon deposition began rather slowly at the conversion above 0.9 than the other metal oxide particles.
The attritions of each particle were determined in the attrition tester of ASTM D5757-95 (0.035 m-I.D. 0.71 m-height) at gas velocity up to 17 cm/s based on the ASTM standard. The obtained data was analyzed in terms of the attrition index (AI) and the corrected attrition index (CAI). The values of AI and CAI are found to be 0 ~ 62.50 and 0 ~ 23.61, respectively. The particles supported on $TiO_2$ and $Al_2O_3$ has good resistance against attrition, but those on bentonite do not have good resistance. The optimum condition for good resistance against attrition for $NiO-Fe_2O_3/bentonite$ particles is found to be around $NiO:Fe_2O_3$ = 3:1.
From the economic analysis of the oxygen carrier particles based on the reactivity and price of raw materials, $Fe_2O_3$ supported on bentonite is a best candidate as an oxygen carrier particle for the CLC process.
