The kinetics of reduction with CH 4 , H 2 and CO and oxidation with O 2 of two NiO-based oxygen carriers prepared by impregnation, NiO18-Al and NiO21-Al, for chemicallooping combustion (CLC) and chemical-looping reforming (CLR) was investigated in this work. The experimental tests were carried out in a thermogravimetric analyzer using different reacting gas concentrations (5-20 vol. % for CH 4 , 5-50 vol. % for H 2 and CO and 5-21 vol. % for O 2 ) and temperatures (1073-1223 K). The reduction stage using both materials proceeded in two steps: a first period of high reactivity attributed to the presence of free NiO and then a low reactivity period that corresponded to NiAl 2 O 4 reduction. Therefore, the kinetic parameters for NiO and NiAl 2 O 4 reduction were determined separately for each oxygen carrier and each reacting gas. An empirical linear model was developed to describe NiO reduction. The solid conversion was a function of the fuel concentration and NiO content in the solid. The effect of temperature was low (E a ~ 5 kJ mol -1 ) although a chemical reaction rate controlled by diffusional processes was dismissed. The shrinking core model for spherical grain 2 geometry was used to obtain the kinetic parameters of the NiAl 2 O 4 reduction working with both NiO-based oxygen carriers. Chemical reaction control was assumed for NiO18-Al whereas diffusion through product layer was also considered when NiO21-Al was used as oxygen carrier. 82-472 kJ mol -1 activation energies were obtained with NiO18-Al particles. The activation energies were 237 and 373 kJ mol -1 for the kinetic constant and 200-281 kJ mol -1 for the diffusion coefficient, respectively, in the case of NiO21-Al oxygen carrier. Reduction reaction of NiO21-Al with CO was extremely slow and the kinetic parameters were obtained for comparison purposes with chemical control of the reaction rate. In this case, the reaction order was 1 and the activation energy 89 kJ mol -1 . The combined model for consecutive reduction of NiO and NiAl 2 O 4 in the NiO18-Al particles with the kinetic parameters obtained in this work predicted adequately the experimental results. The oxidation step of both oxygen carriers was fast and complete until reaching the initial condition. An empiric linear model, which assumes a linear relation between time and conversion, was developed to determine the kinetics of Ni oxidation. The reaction order of oxidation was about 0.8 for NiO18-Al and NiO21-Al and the activation energies were low, 22 kJ mol -1 in both cases. 

Different Ni-based oxygen carriers were prepared by dry impregnation using -Al 2 O 3 as support. The reactivity, selectivity during methane combustion, attrition rate and agglomeration behaviour of the oxygen carriers were measured and analyzed in a termogravimetric analyzer and in a batch fluidized bed during multicycle reductionoxidation tests. showed very high reactivity and high methane combustion selectivity to CO 2 and H 2 O because the interaction between the NiO and the support was decreased. In addition, these oxygen carriers had very low attrition rates and did not show any agglomeration problems during operation in fluidized beds, and so, they seem to be suitable for the chemicallooping combustion process.

ABSTRACT. Chemical-looping combustion (CLC) is a promising method for the combustion of fuel 11 gas with CO 2 capture and sequestration (CCS). This paper presents the methate combustion results 12 obtained in a continuous CLC prototype using an oxygen carrier containing 18 wt% NiO impregnated 13 on alumina. The design of the CLC prototype was a circulating fluidized bed reactor, consisting of two 14 interconnected fluidized bed reactors, the fuel reactor (FR) and the air reactor (AR). The main operating 15 conditions affecting combustion, as fuel gas flow, solids circulation rate, and FR temperature, were 16 analyzed. The CLC operation was carried out using methane as fuel gas in the FR with a thermal power 17 between 500 and 850 W th . The prototype was successfully operated during 100 h, of which 70 h were at 18 2 combustion conditions. No methane was detected at the FR exit, being CO and H 2 the unconverted 1 gases. Increasing the temperature in the FR or the solids circulation rates increased the combustion 2 efficiency, reaching efficiencies values as high as 99% at temperatures in the range 1073-1153 K, and a 3 solid inventory in the FR of 600 kg per MW th . The effect of operating conditions on the performance of 4 the oxygen carrier in the CLC prototype was analyzed. During operation of the CLC prototype, no signs 5 of agglomeration or carbon formation were detected and the main properties of particles did not vary. 6The two different phases in the oxygen carrier, NiO and NiAl 2 O 4 , were active to transfer oxygen to the 7 fuel gas. The NiO/NiAl 2 O 4 ratio increased with a decrease in the solids circulation rate, which affected 8 to the reactivity of the oxygen carrier. 9INTRODUCTION. 10

15The reactivity of a Ni-based oxygen carried prepared by hot incipient wetness 16 impregnation (HIWI) on -Al 2 O 3 with a NiO content of 18 wt% was studied in this 17 work. Pulse experiments with the reduction period divided into 4-second pulses were 18 performed in a fluidized bed reactor at 1223 K using CH 4 as fuel. The number of pulses 19 was between 2 and 12. Information about the gaseous product distribution and 20 secondary reactions during the reduction was obtained. In addition to the direct reaction 21 of the combustible gas with the oxygen carrier, CH 4 steam reforming also had a 22 significant role in the process, forming H 2 and CO. to the design of a CLC system were investigated. When formation of NiAl 2 O 4 occurred, 5 the average reactivity in the fuel reactor decreased. Therefore, the presence of both NiO 6 and NiAl 2 O 4 phases must be considered for the design of a CLC facility. 7 8

A synthetic Fe-based oxygen carrier prepared by impregnation using -Al 2 O 3 as support, successfully tested for gas CLC combustion, has been now evaluated with respect to gas combustion in a 500 W th CLC continuous unit when the fuel, CH 4 , contained variable amounts of H 2 S.Full gas combustion was reached at oxygen carrier-to-fuel ratio values higher than 1.5. The presence of sulfur in the fuel gas hardly affected the reactivity and combustion efficiency of the oxygen carrier, independently of the amount of sulfur in the gas stream. All the sulfur introduced as H 2 S in the fuel reactor (FR) was released in the same reactor as SO 2 . Furthermore, oxygen carrier particles extracted from the CLC unit after the experimental tests with H 2 S addition were 3 characterized by means of different techniques and no sulfur was detected in any case. The impregnated Fe-based oxygen carrier was highly reactive and sulfur resistant, and can be considered as a suitable material for CLC with gaseous fuels that contain sulfur.

Ni-based oxygen carriers (OC) with different NiO content were prepared by incipient wet impregnation, at ambient (AI), and hot conditions (HI) and by deposition-precipitation (DP) methods using -Al 2 O 3 and -Al 2 O 3 as supports. The OC were characterized by BET, Hg porosimetry, mechanical strength, TPR, XRD and SEM/EDX techniques. Reactivity of the OC was measured in a termogravimetric analyzer and methane combustion selectivity towards CO 2 and H 2 O, attrition rate, and agglomeration behaviour were analyzed in a batch fluidized bed reactor during multicycle reduction-oxidation tests.XRD and TPR analysis showed the presence of both free NiO and NiAl 2 O 4 phases in most of the OC. The interaction of the NiO with the alumina during OC preparation formed NiAl 2 O 4 that affected negatively to the OC reactivity and methane combustion selectivity towards CO 2 and H 2 O during the reduction reaction. The NiO-alumina interaction was more affected by the support type than by the preparation method used. The NiO-alumina interaction was stronger in the OC prepared on -Al 2 O 3 . 2The OC were evaluated in the fluidized bed reactor with respect to the agglomeration process.OC prepared by the AI and HI methods with NiO contents up to 25 wt%, OC prepared by the DP method on -Al 2 O 3 with NiO content lower than 30 wt%, and OC prepared by the DP method on -Al 2 O 3 with a NiO content lower than 26 wt% did not agglomerated. OC that agglomerated showed an external layer of NiO over the particles. It seems that the most important factor affecting to the formation of the external NiO layer on the OC, and so to the agglomeration process, was the metal content of the OC. The attrition rates of the OC prepared using -Al 2 O 3 as support were higher than the ones prepared using -Al 2 O 3 as support, and in general the attrition rates of all the OC were low.The OC prepared by AI, HI or DP methods on -Al 2 O 3 as support had appropriated characteristics to be used in the chemical-looping combustion process.

Chemical-looping combustion (CLC) has been suggested among the best alternatives to reduce the economic cost of CO 2 capture using fuel gas because CO 2 is inherently separated in the process. For gaseous fuels, natural gas, refinery gas, or syngas from coal gasification can be used. These fuels may contain different amounts of sulfur compounds, such as H 2 S and COS. An experimental investigation of the fate of sulfur during CH 4 combustion in a 500 W th CLC prototype using a Ni-based oxygen carrier has been carried out. The effect on the oxygen carrier behavior and combustion efficiency of several operating conditions such as temperature and H 2 S concentration has been analyzed. Nickel sulfide, Ni 3 S 2 , was formed at all operating conditions in the fuel reactor, which produced an oxygen carrier deactivation and lower combustion efficiencies. However, the oxygen carrier recovered their initial reactivity after certain time without sulfur addition. The sulfides were transported to the air reactor where SO 2 was produced as final gas product. Agglomeration problems derived from the sulfides formation were never detected during continuous operation. Considering both operational and environmental aspects, fuels with sulfur contents below 100 vppm H 2 S seem to be adequate to be used in an industrial CLC plant.

Greenhouse gas emissions, especially CO 2 , formed by combustion of fossil fuels, highly contribute to the global warming problem. Chemical-Looping Combustion (CLC) has emerged as a promising option for CO 2 capture because this gas is inherently separated from the other flue gas components and thus no energy is expended for the separation.