Reduction and oxidation kinetics of nickel-based oxygen-carriers for chemical-looping combustion and chemical-looping reforming

Abstract

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. 

Reduction kinetics of lanthanum ferrite perovskite for the production of synthesis gas by chemical-looping methane reforming

“…High gas to solid molar ratio is necessary to increase the oxygen carrier conversion at short reduction time, which has been observed in our previous work (Dai et al, 2012). The reduction rate of LaFeO 3 oxygen carrier is accelerated with the reaction time, and the trend is more dominant at high gas to solid molar ratio, which is clearly different from the two-stage reduction process over Nibased oxygen carriers, characterized by the fast NiO reduction and low nickel aluminate reactivity (Dueso et al, 2012). The oxygen carrier conversion (α) for the various temperatures under gas to solid molar ratio of 26.3 is presented in Fig.…”

“…Applying the chemical looping methane reforming (CLMR) concept to POM enables an opportunity to resolve the two issues by spatially separating CH 4 conversion into two half-steps over the fuel reactor and air reactor. The CLMR process eliminates the expensive air separation and mixing unit, and fulfills the heat balance between the endothermic reduction reaction in fuel reactor and exothermic oxidation reaction in air reactor by the circulating oxygen carrier (de Diego et al, 2008;Dueso et al, 2012). The reduction of oxygen carrier by methane shows the potential for a new green technology of syngas production without using any catalyst and its deactivation problem.…”

“…Notably, the reactivity and selectivity in CLMR process can be improved by combining different materials to bring synergetic effect or form solid solution (spinel, perovskite, et al) (Cabello et al, 2014a;Neal et al, 2014Neal et al, , 2015Bhavsar and Veser, 2013;Zhu et al, 2014;Chen et al, 2014). As mentioned above, although many work to increase the reactivity and syngas selectivity have been carried out for a long time, the reduction kinetics of oxygen carriers using methane as fuel at representative conditions of CLMR system has been seldom investigated (Dueso et al, 2012;Rashidi et al, 2013aRashidi et al, , 2013b). Many models have been used to describe the conversiontime profiles of noncatalytic gas-solid reactions in the literature.…”

Syngas/H2 production from bioethanol in a continuous chemical-looping reforming prototype

“…The reduction and oxidation kinetics of these oxygen carriers with regards to the main gaseous products existing in CLC and CLR processes, and their catalytic activity have been determined in previous works [31,32]. Table 1 The gases were fed by means of specific mass-flow controllers.…”

Kinetic determination of a highly reactive impregnated Fe2O3/Al2O3 oxygen carrier for use in gas-fueled Chemical Looping Combustion

“…with the Fe20Al oxygen carrier. The reacting model for reduction kinetics assumes a first step with the reaction rate controlled by chemical reaction in the grain surface [53][54]. During this first step, alumina and oxygen must diffuse towards the reaction interphase.…”