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.