Defluidization Conditions for a Fluidized Bed of Iron Oxide-, Nickel Oxide-, and Manganese Oxide-Containing Oxygen Carriers for Chemical-Looping Combustion

Abstract

For combustion with CO2 capture, chemical-looping combustion with inherent separation of CO2 is a promising technology. Chemical-looping combustion uses oxygen carriers that are composed of metal oxide to transfer oxygen from the combustion air to the fuel. The defluidization of oxygen-carrier particles was investigated to improve the understanding of when particle agglomeration may occur. The study was made in a laboratory fluidized-bed reactor at 950 °C, simulating a chemical-looping combustion system by exposing the sample to reducing and oxidizing conditions in an alternating manner. The oxygen-carrier particles used were based on oxides of iron, nickel, and manganese and produced by freeze granulation. For iron oxide particles, there was no defluidization of the bed when the content of available oxygen in the particle was high. The defluidization occurred during the oxidation period after long reduction periods, in which a significant reduction of the magnetite to wustite occurred. This is an important observation, because the reduction to wustite is not expected in chemical-looping combustion with high fuel conversion. Thus, laboratory experiments with iron oxide performed with long reduction times may give an unduly exaggerated impression of the risks of agglomeration. For nickel oxide, the defluidization was dependent on the sintering temperature with no defluidization in experiments conducted with particles sintered at 1300 and 1400 °C. The nickel oxide particles that were sintered at 1500 °C only defluidized once in a total of 49 cycles, whereas the particles that were sintered at 1600 °C defluidized already in the first cycle. For the nickel oxide particles, it was not possible to see any effect of the length of the reducing period on the defluidization. There was no defluidization of the manganese oxide particles. The defluidization of the bed leads to agglomeration for the iron oxide particles, but not for the particles of nickel oxide, where the bed was still loosely packed. Carbon was formed on the particles based on nickel oxide and manganese oxide.

Testing of a highly reactive impregnated Fe2O3/Al2O3 oxygen carrier for a SR–CLC system in a continuous CLC unit

“…As it was pointed out, some authors have found agglomeration problems working with Fe-bases OCs [17][18]. However, although a high degree of conversion was reached in the batch fluidized reactor the carrier did not show any agglomeration behaviour during operation.…”

“…Other chemical characteristics are advantageous for the use of Fe-based oxygen carriers: low tendency to carbon deposition [12] and no risk of sulphide or sulphate formation at any concentration or operating temperature when sulphur containing fuels are used [13]. [17][18] To develop a suitable oxygen carrier, synthetic materials, low-cost minerals, ores and waste products have been evaluated. There are several studies working with synthetic Fe-based oxygen carriers using different supports, (sepiolite, SiO 2 , TiO 2 ZrO 2 and Al 2 O 3 ), different preparation methods, and metal contents [14][15][16]19].…”

Testing of minerals and industrial by-products as oxygen carriers for chemical-looping combustion in a circulating fluidized-bed 300W laboratory reactor

“…First, because complete fuel conversion to CO 2 and H 2 O can thermodynamically not be reached anymore [12,13]. Second, because those phases are known to have a high tendency to form agglomerates [14][15][16].…”

“…until steam was added to the fuel flow, carbon deposition in the fuel line occurred. The reaction that is believed to be responsible is known as the Boudouard reaction, see equation (14).…”

Chemical-looping combustion and chemical-looping with oxygen uncoupling of kerosene with Mn- and Cu-based oxygen carriers in a circulating fluidized-bed 300W laboratory reactor

“…Figures 6 and 9, might be an indicator for coke formation on the oxygen carrier. Carbon formation on manganesebased oxygen carrier with methane as fuel has been mentioned in earlier studies [16,28,29], whereas oxygen release properties of C2Z-1050 oxygen carrier might have inhibited coke formation on the particles to a large extent.…”