High Reactivity and Mechanical Durability of NiO/NiAl₂O₄ and NiO/NiAl₂O₄/MgAl₂O₄ Oxygen Carrier Particles Used for more than 1000 h in a 10 kW CLC Reactor

Abstract

Chemical-looping combustion (CLC) is a promising technology for CO 2 capture in the process of combustion of gaseous fossil fuels. Oxygen carrier materials based on Ni/NiO on NiAl 2 O 4 and on NiAl 2 O 4 /MgAl 2 O 4 support have previously shown high initial activity combined with high conversion of methane and low concentration of outgoing CO. The feasibility of the Ni/NiO system for CLC depends largely on the lifetime of the oxygen carrier particles in the reactor due to the high price of the material. Avoiding chemical and physical degradation of oxygen carriers is essential for long-term industrial-scale chemical-looping combustion operations, and the particles' activity and mechanical durability have to remain high during long operation times. In this study, a series of oxygen carrier samples were collected from a 10 kW CLC combustor operated for a total of 1016 h using oxygen carrier materials based on NiO/NiAl 2 O 4 (N-VITO) for 405 h and a mixture of used NiO/NiAl 2 O 4 with fresh NiO/NiAl 2 O 4 /MgAl 2 O 4 (N-VITOMg) for 611 h, respectively. These samples were collected after certain time intervals and analyzed in terms of reactivity with methane in a quartz batch reactor. Also, the structural and mechanical properties of these samples were investigated by means of powder XRD, BET surface area measurements, light and scanning electron microscopy, energy dispersive X-ray spectrometry, and crushing strength evaluation. It was shown that both N-VITO and N-VITO/N-VITOMg demonstrate high reactivity and mechanical durability after having been used for more than 1000 h in the CLC 10 kW reactor, which makes them excellent candidates for applications within the area of chemical-looping combustion.

Iron oxide looping for natural gas conversion in a countercurrent moving bed reactor

“…7a. As should be predicted based on Le Chatelier's principle, the simulation results indicate that at temperatures below 950°C, increasing temperature increases methane conversion, as Reactions (14)- (20) are mostly endothermic. For example, when U = 2.5, the methane conversion is 87.7% at 750°C, and it increases to its maximum 99.998% at 950°C.…”

“…For example, the lines of Reactions (7) and (10) intersect at $550°C. As a result, Reaction (20), i.e., methane decomposition, has an equilibrium constant of 1 at $550°C. The reaction becomes more thermodynamically favorable at higher temperatures because the slope of Reaction (7) is more negative than the slope of Reaction (10), thus producing a more negative Gibbs free energy as the temperature increases.…”

“…Further, the reducer should be operated above 750°C to facilitate Reactions (24)- (26) and to avoid carbon deposition from Reaction (20). The high temperature limit depends on the melting, sintering and sticking properties of the solid materials.…”

“…A small amount of H 2 was observed from the reducer gas outlet, which might have resulted from Reaction (20).…”

“…Nickel oxide based materials have been widely developed and applied for methane conversion because of its fast kinetics for methane conversion [17,18]. The thermodynamics of NiO/Ni, however, restrict the steam to H 2 conversion in Reaction (5) and CH 4 conversion in Reaction (6) [19,20]. In addition, the high cost of nickel-based materials inhibits its further scale up for commercial application.…”