This work is a comprehensive review of the Chemical-Looping Combustion (CLC) and Chemical-Looping Reforming (CLR) processes reporting the main advances in these technologies up to 2010. These processes are based on the transfer of the oxygen from air to the fuel by means of a solid oxygen-carrier avoiding direct contact between fuel and air for different final purposes. CLC has arisen during last years as a very promising combustion technology for power plants and industrial applications with inherent CO 2 capture which avoids the energetic penalty present in other competing technologies.CLR uses the chemical looping cycles for H 2 production with additional advantages if CO 2 capture is also considered.The review compiles the main milestones reached during last years in the development of these technologies regarding the use of gaseous or solid fuels, the oxygen-carrier development, the continuous operation experience, and modelling at several scales. Up to 2010, more than 700 different materials based on Ni, Cu, Fe, Mn, Co, as well as other mixed oxides and low cost materials, have been compiled. Especial emphasis has been done in those oxygen-carriers tested under continuous operation in Chemical-Looping 2 prototypes. The total time of operational experience (≈ 3500 h) in different CLC units in the size range 0.3-120 kW th , has allowed to demonstrate the technology and to gain in maturity. To help in the design, optimization, and scale-up of the CLC process, modelling work is also reviewed. Different levels of modelling have been accomplished, including fundamentals of the gas-solid reactions in the oxygen-carriers, modelling of the air-and fuel-reactors, and integration of the CLC systems in the power plant. Considering the great advances reached up to date in a very short period of time, it can be said that CLC and CLR are very promising technologies within the framework of the CO 2 capture options.

Chemical-looping combustion (CLC) has been suggested as an energetically efficient method for capture of carbon dioxide from the combustion of fuel gas. This technique involves the use of an oxygen carrier that transfers oxygen from the air to the fuel, preventing direct contact between them. The oxygen carrier is composed of a metal oxide as an oxygen source, and an inert as a binder for increasing the mechanical strength of the carrier. In this work, 240 samples composed of 40-80% of Cu, Fe, Mn, or Ni oxides on Al 2 O 3 , sepiolite, SiO 2 , TiO 2 , or ZrO 2 were prepared by mechanical mixing as cylindrical extrudates. The samples were sintered at four temperatures between 950 and 1300 °C. The effects of the chemical nature and composition of the carrier and the sintering temperature were investigated by reactivity tests in a thermogravimetric analyzer using CH 4 as fuel, and the mechanical strength of the solids. On the basis of these properties, the most promising carriers to be used in a CLC system were selected. The best Cu-based oxygen carriers were those prepared using SiO 2 or TiO 2 as inert, and sintered at 950 °C. Among the Fe-based oxygen carriers, those prepared with Al 2 O 3 and ZrO 2 as inerts showed the best behavior. ZrO 2 was the best inert for those Mn-based oxygen carriers. Finally, TiO 2 was the best inert for those Ni-based oxygen carriers.

Ilmenite, a natural mineral composed of FeTiO 3 , is a low-cost and promising oxygen carrier (OC) for solid fuels combustion in a chemical-looping combustion (CLC) system. The aim of this study is to analyze the behavior of ilmenite as an OC in CLC and the changes in its properties through redox cycles. Experiments consisting of reduction-oxidation cycles in a thermogravimetric analyzer were carried out using the main products of coal pyrolysis and gasification, that is, CH 4 , H 2 , and CO, as reducing gases. Characterizations of ilmenite through scanning electron microscopy-energy-dispersive X-ray (SEM-EDX), X-ray diffraction (XRD), Hg porosimetry, N 2 fisisorption, He pycnometry, and hardness measures have been performed. Both fresh and previously calcined at 1223 K ilmenite have been used as initial OCs. Fresh ilmenite reacts slowly; nevertheless, there is a gain in reactivity in reduction as well as in oxidation with the number of cycles. This activation occurs for all tested fuel gases and is faster if ilmenite has been previously calcined. The initial oxygen transport capacity was measured to be 4%, and it decreases with the number of cycles up to 2.1% after 100 redox cycles. Nevertheless, ilmenite shows adequate values of reactivity and oxygen transport capacity for its use in CLC technology with solid fuels. The trade-off between the increase in reactivity and decrease in oxygen transport capacity on ilmenite performance in a CLC system has been evaluated through the estimation of the solids inventory needed in the fuel reactor. If fresh or calcined ilmenite is fed into the CLC system, the activation process could happen in the CLC itself. Also, a previous step for activation can be designed.

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.