Performance in a fixed-bed reactor of titania-supported nickel oxide as oxygen carriers for the chemical-looping combustion of methane in multicycle tests

Chemical-looping combustion has been proposed as an alternative process for the complete elimination of CO2 emissions to the atmosphere in the combustion of carbonaceous products, such as natural gas. In this case, the combustion is a two-stage process. In the first stage, the structural oxygen contained in a reducible inorganic oxide is used for the combustion of the natural gas. In the second stage, the reduced oxygen carrier is regenerated with pure air to reinitiate a new combustion cycle. In this paper, nickel oxide supported on porous rutile is used as an oxygen carrier for the chemical-looping combustion of methane, as the main component of natural gas. The performance is assessed in 20-cycle tests in a fixed-bed reactor at 900 degrees C, using either dilute (20 vol % in N-2) or pure methane for the reduction stage and pure air for the regeneration stage. The experimental results reveal that the reactions in the two involved processes are fast, as CO2, before breakthrough, is the only compound detected in the outlet gas of the reduction stage. However, in the reduction stage, the thermal decomposition of methane appears as a side reaction, already acting at the start of the test in clear competition for methane consumption with the main reaction of the chemical-looping combustion. In this case, carbon is mostly deposited as uniform coatings on Ni catalyst particles. Because this deposited carbon will evolve then as CO2 in the outlet gas of the next regeneration stage, its presence poses some limitations to the achievable maximum efficiency in CO2 capture in a chemical-looping process. Moreover, rutile does not behave as a completely inactive support, especially using pure methane. Conversely, through its partial reduction, it acts as an additional oxygen source for methane combustion that must be taken into account. A slight performance decay and significant porosity increase of the oxygen carriers with the number of cycles were observed in a 20-cycle test in a fixed-bed reactor, which should be assessed in further long-term tests in future work.