(16b) Attrition and Reactivity Analysis of NiO/NiAl2O4 Catalyst Particles for Chemical Looping Combustion

Chemical Looping Combustion (CLC) is one of the promising methods for inherent CO₂ separation with minimal energy penalty. This process is done in two interconnected reactors where metal oxide transfers required oxygen from air reactor to fuel reactor.  Selection of a metal oxide is crucial for the continuous operation of CLC and must ensure high attrition resistivity, high reactivity, and be environmentally benign. Attrition of particles changes the particle size distribution affecting the hydrodynamics of the system. Low reactivity of particles decreases the conversion of fuel and the regeneration of oxides. Finding an oxygen carrier to fulfill the required strength and reactivity is a challenge. During the last decade, many studies have been reported on the development of various oxygen carriers for CLC process [1-3]. Among them, NiO based oxygen carrier particles have shown to be excellent candidates [1,2,4-11].

Catalyst attrition tests for CLC have been reported mostly for testing in fluidized beds at low temperatures [12-14]. In this study, the air jet attrition unit is operated at elevated temperatures (in the range of 550°C and higher) to assess the attrition behaviour of catalysts at temperatures relevant to the CLC operation. NiO/NiAl₂O₄ catalysts used in this study are prepared by VITO, Flemish Institute for Technological Research,
Belgium, using the freeze granulation technique and for the size range of 106-180 μm. As shown in Figure 1, N₂ gas (line1) with flow rate of 10 L/min and with a back-pressure of 170-200 kPa is mixed with steam (line 2) and injected to the bottom of attrition tube. N₂ is used instead of air to avoid any reaction during the attrition test. Furthermore, the steam is mixed with N₂ to prevent carbon deposition on catalyst particles in the attrition tester. As a result, gas jet with a velocity of 450 m/s is formed through small orifices of the attrition tube distributor. Ceramic heaters are used for steam generation and maintaining the temperature of the attrition tube at higher than 550 °C. Under these conditions only fine particles with diameters of less than 20 μm are transferred to the fine collector where particles are collected for attrition index determination.

The effect of particle size distribution on the attrition is tested using the high temperature attrition tester and compared with those tests conducted at room temperature. Particle size analyser, Malvern Mastersizer 2000, is used before and after each test. In addition, the reaction kinetics of the oxygen carrier is investigated under oxidizing and reducing conditions. Thermogravimetric analyzer, TA instrument SDT Q600, is used to test the reactivity of metal oxides before and after the attrition tests. The effects of particle size, gas concentration, and temperature in the range of 773-1373 K on the reactivity are examined. Figure 2 compares the typical changes in catalyst weight fraction at 600 and 900 °C. Initially the temperature is ramped from the ambient temperature with the rate of 100 °C/min to the desired temperature in which no significant reaction happens during this period except for 900 °C which after 700 °C reduction is shown by the weight loss. During the isothermal condition, rapid reduction of metal oxide with methane occurs while enough oxygen is supplied from the surface of the particles. Further reaction leads to carbon deposition on the surface inhibiting the oxidation reaction. Comparing the trends for 600 and 900 °C, the reduction reactivity is increased with temperature. Further experiments are conducted at elevated temperatures to find an appropriate temperature in which lowest carbon deposition and highest reaction rate are achieved. Moreover, the results of the experiments are used for investigating the reaction kinetics for oxidation and reduction by considering different reaction models.

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