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) is a two-step combustion process that produces a pure CO 2 stream, ready for compression and sequestration. A CLC system is composed by two reactors, an air and a fuel reactor, and an oxygen carrier (OC) circulating between the reactors, which transfers the oxygen necessary for the fuel combustion from the air to the fuel. This system can be designed similar to a circulating fluidised bed, but with the addition of a bubbling fluidised bed on the return side. A mapping of the range of operational conditions, design values, and OC characteristics is presented for the most usual metal oxides (CuO, Fe 2 O 3 , and NiO) and different fuel gases (CH 4 , H 2 , and CO). The pressure operation of a CLC system is also considered. Moreover, a comparison of the possible use of three high reactive OCs (Cu10Al-I, Fe45Al-FG, Ni40Al-FG) previously characterised is carried out. It was found that the circulation rates and the solids inventories are linked, and the possible operating conditions are closely dependent on the reactivity of the OCs. The operational limits of the solids circulation rates, given by the mass and heat balances in the system, were defined for the different type of OCs. Moreover, a plot to calculate the solids inventories in a CLC system, valid for any type of OC and fuel gas, is proposed. The minimum solids inventories depended on the fuel gas used, and followed the order CH 4 > CO > H 2 . Values of minimum solids inventories in a range from 40 to 133 kg/MW f were found for the OCs used in this work, excepting for the reaction of Fe45Al-FG with CH 4 , which needs a higher amount of solids because of its low reactivity. From the economic analysis carried out it was found the cost of the OC particles does not represent any limitation to the development of the CLC technology. ᭧

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.

The kinetics of reduction with CH4, H2, and CO and oxidation with O2 of a Cu-based oxygen carrier prepared by impregnation on alumina to be used in a chemical-looping combustion (CLC) system have been determined in a thermogravimetric analyzer. The oxygen carrier exhibited high reactivity in both reduction and oxidation with times for complete conversion lower than 40 s at 1073 K and 5−70 vol % of the fuel gas and 5−21 vol % of O2. The analysis of the sample carried out by scanning electron microscopy using energy-dispersive X-ray and chemisorption showed that the CuO was well dispersed in the porous surface of the alumina matrix and a uniform thin layer on the porous surface was considered. The shrinking-core model for platelike geometry of the reacting surface was used for the kinetic determination, in which the chemical reaction controlled the global reaction rate. No effect of the gas products (H2O and CO2) on the reaction rate was detected. The reaction order depended on the fuel gas, and values of 0.4, 0.6, and 0.8 were found for CH4, H2, and CO, respectively. The order of the oxidation reaction was 1. The activation energies for the reduction and oxidation reactions varied between 14 and 60 kJ mol-1. The reactivity data together with the operating variables were used to calculate some design parameters for a CLC system. It was found that the total solid inventory and the recirculation rate are linked. To optimize both parameters, conversion variations of the oxygen carrier in the fuel and air reactors, ΔX s, should be about 0.2−0.4. For a typical CLC operating condition, the total solids inventory for this Cu-based oxygen carrier (10 wt % CuO) was 133 kg/MWf if the fuel gas was CH4, 86 kg/MWf if the fuel gas was H2, and 104 kg/MWf if the fuel gas was CO, with recirculation rates of about 12 kg s-1 per MWf. The high reactivity of the material, in both reduction and oxidation, demonstrated the feasibility of this oxygen carrier to be used in a CLC system.

The objective of this study was to establish the kinetic of both reduction and oxidation reactions taking place in the Chemical-Looping Combustion (CLC) process using ilmenite as oxygen carrier. Because of the beneficial of the use of pre-oxidized ilmenite and the activation of the ilmenite during the redox cycles, both the reactivity of pre

Chemical Looping Combustion (CLC) is an attractive technology to decrease greenhouse gas emissions affecting global warming, because it is a combustion process with inherent CO 2 separation and therefore without needing extra equipment for CO 2 separation and low penalty in energy demand. The CLC concept is based on the split of a conventional combustion of gas fuel into separate reduction and oxidation reactions. The oxygen transfer from air to fuel is accomplished by means of an oxygen carrier in the form of a metal oxide circulating between two interconnected reactors. A Cu-based material (Cu14Al) prepared by impregnation of -Al 2 O 3 as support with two different particle sizes (0.1-0.3 mm, 0.2-0.5 mm) was used as an oxygen carrier for a chemical looping combustion of methane. A 10 kWth CLC prototype composed of two interconnected bubbling fluidized bed reactors has been designed, built in and operated at 800 o C during 100 h for each particle size. In the reduction stage full conversion of CH 4 to CO 2 and H 2 O was achieved using oxygen carrier-to-fuel ratios above 1.5. Some CuO losses as the active phase of the CLC process were detected during the first 50 h of operation, mainly due to the erosion of the CuO present in external surface of the alumina particles. The high reactivity of the oxygen carrier maintained during the whole test, the low attrition rate detected after 100 hours of operation, and the absence of any agglomeration problem revealed a good performance of these CuO-based materials as oxygen carriers in a chemical-looping combustion process.