In a chemical-looping combustion (CLC) process, gas (natural gas, syngas, etc) is burnt in two reactors. In the first one, a metallic oxide that is used as oxygen source is reduced by the feeding gas to a lower oxidation state, being CO 2 and steam the reaction products. In the second reactor, the reduced solid is regenerated with air to the fresh oxide, and the process can be repeated for many successive cycles. CO 2 can be easily recovered from the outlet gas coming from the first reactor by simple steam condensation. Consequently, CLC is a clean process for the combustion of carbon containing fuels preventing the CO 2 emissions to atmosphere. The main drawback of the overall process is that the carriers are subjected to strong chemical and thermal stresses in every cycle and the performance and mechanical strength can decay down to unacceptable levels after enough number of cycles in use.In this paper the behaviour of CuO as an oxygen carrier for a CLC process has been analysed in a thermogravimetric analyser (TGA). The effects of carrier composition and preparation method used have been investigated to develop Cu-based carriers exhibiting high 2 reduction and oxidation rates without substantial changes in the chemical, structural and mechanical properties for a high number of oxidation-reduction cycles. It has been observed that the carriers prepared by mechanical mixing or by coprecipitation showed an excellent chemical stability in multicycle tests in thermobalance, however, the mechanical properties of these carriers were highly degraded to unacceptable levels. On the other hand, the carriers prepared by impregnation exhibited excellent chemical stability without substantial decay of the mechanical strength in multicycle testing. These results suggest that copper based carriers prepared by impregnation are good candidates for chemical-looping combustion process.

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Chemical looping combustion (CLC) of methane has been proposed in the past decade as an efficient method for CO 2 capture without important cost penalties. The combustion is carried out in a two-step process using, in the first one, the lattice oxygen of a reducible inorganic oxide for methane combustion and, in the second one, air for further carrier regeneration. An additional advantage of the CLC is the improbable generation of thermal NO x because the operating temperature used for carrier regeneration is relatively low. Copper-based oxygen carriers with different copper contents have been prepared by successive wet impregnations on porous titania, used as support, with an aqueous solution of copper nitrate. The prepared oxygen carriers have been subsequently studied in five-cycle reduction-regeneration tests in a fixed bed reactor at atmospheric pressure with the aim of carrier characterization, analysis of the components in the outlet gas, and the study of the effect of some of the main parameters influencing the problem, including copper content and operating temperature. A further 20-cycle performance test has also been carried out with the oxygen carrier with the highest copper loading. The study reveals that copper does not interact with titania as support, which remains unaltered as rutile along all the two steps involved in the process. However, it is redistributed on the support because the melting points of some of the involved copper phases are close to the operating temperature. Neither carbonaceous deposits on the carrier in the reduction step nor subsidiary chemical reactions, especially those involving CO formation, takes place. The copper-based oxygen carriers exhibited a good performance in 20-cycle tests in a fixed bed reactor showing high reactivity and no substantial decay in efficiency with the number of cycles.

Chemical-looping combustion of carbonaceous compounds is a proposed two-step process for complete CO2 capture and substantial reduction of NO x emissions. In the first stage, the reduction stage, the framework oxygen of a reducible inorganic oxide is used for the combustion of the carbonaceous material. In the second stage, the regeneration stage, the carrier in a reduced state is regenerated with air to recover the properties of the fresh carrier, ready to reinitiate a new cycle. This article provides results for the performance of a copper oxide silica-supported oxygen carrier in a 20-cycle test of chemical-looping of methane in a fixed-bed reactor at 800 °C and atmospheric pressure. The mesoporous nature of silica provided a good dispersion of the active phase imparting a high mechanical strength to the overall carrier. Additionally, silica is stable under highly reducing agents and inert in the two involved processes. The respective CH4, CO2, and CO breakthrough curves in the reduction stage show that the reduction reaction rate is fast and highly selective to CO2 formation. CO emissions are very low, only yielded at the end of the reduction stage, when the reduction stage should be stopped to initiate a regeneration stage. Characterization studies using different techniques, such as TPR, SEM-EDX, and powder XRD, reveal that CuO might decompose into Cu2O at the operating conditions used in the reduction stage, but fortunately, the decomposition rate is so low that it has no effect on the oxygen amount initially available for chemical-looping combustion. Copper does not promote the thermal decomposition of methane, and deposited carbon, consequently, could not be detected in the reduced carrier. In a 20-cycle test neither performance decay nor mechanical degradation of the oxygen carrier has been observed.

Recent investigations have shown that in the combustion of carbonaceous compounds CO2 and NOx emissions to the atmosphere can be substantially reduced by using a two stage chemical-looping process. In this process, the reduction stage is undertaken in a first reactor in which the framework oxygen of a reducible inorganic oxide is used, instead of the usual atmospheric oxygen, for the combustion of a carbonaceous compound, for instance, methane. The outlet gas from this reactor is mostly composed of CO2 and steam as reaction products and further separation of these two components can be carried out easily by simple condensation of steam. Then, the oxygen carrier found in a reduced state is transported to a second reactor in which carrier regeneration with air takes place at relatively low temperatures, consequently preventing the formation of thermal NOx. Afterward, the regenerated carrier is carried to the first reactor to reinitiate a new cycle and so on for a number of repetitive cycles, while the carrier is able to withstand the severe chemical and thermal stresses involved in every cycle. In this paper, the performance of titania-supported nickel oxides has been investigated in a fixed-bed reactor as oxygen carriers for chemical-looping combustion of methane. Samples with different nickel oxide contents were prepared by successive incipient wet impregnations, and their performance as oxygen carriers was investigated at 900 degrees C and atmospheric pressure in five-cycle fixed-bed reactor tests using pure methane and pure air for the respective reduction and regeneration stages. The evolution of the outlet gas composition in each stage was followed by gas chromatography, and the involved chemical, structural, and textural changes of the carrier in the reactor bed were studied by using different characterization techniques. From the study, it is deduced that the reactivity of these nickel-based oxygen carriers is in the two involved stages and almost independent of the nickel loading. However, in the reduction stage, carbon deposition, from the thermal decomposition of methane, and CO emissions, mainly derived from the partial reduction of titania as support acting as an additional oxygen source, may impose some constraints to the efficiency of the overall chemical-looping combustion process in CO2 capture.

Chemical-looping combustion has been proposed as an alternative process for the complete elimination of CO 2 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 °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 CO 2 , 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 CO 2 in the outlet gas of the next regeneration stage, its presence poses some limitations to the achievable maximum efficiency in CO 2 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 longterm tests in future work.

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