Methane Steam Reforming Kinetics on a Ni/Mg/K/Al2O3 Catalyst

Abstract

The kinetics of methane steam reforming were studied on a Ni/Mg/K/Al2O3 catalyst that was developed for conditioning of biomass-derived syngas. Reactions were conducted in a packed-bed reactor while the concentrations of reactants (methane and steam) and products (hydrogen, carbon monoxide, and carbon dioxide) were varied at atmospheric pressure, with the effects of temperature (525–700 °C) and residence time also being investigated. A power law rate model was developed using nonlinear regression to provide a predictive capability for the rate of methane conversion over this catalyst, to be used for reactor design and technoeconomic analysis of process designs. In order to provide some mechanistic insight, and to compare this catalyst to other non-promoted Ni/Al2O3 catalysts reported in the literature, a reaction mechanism consisting of five elementary steps, using a Langmuir–Hinshelwood type approach, was also considered. These five steps included: (i) CH4 adsorption, (ii) H2O adsorption, (iii) surface reaction of adsorbed CH4 and H2O to form CO and H2, (iv) CO desorption, and (v) H2 desorption. Nonlinear regression was then used to fit each of the rate laws to the experimental data. From these results, the model that assumed CH4 adsorption to be the rate determining step provided the best fit of the experimental data. This finding is consistent with literature studies on non-promoted Ni/Al2O3 catalysts, in which methane adsorption has been proposed to be the rate determining step during catalytic methane steam reforming. Both the power rate laws and the rate law assuming CH4 adsorption to be the rate determining step can be used as predictive tools for determining methane conversion for a given set of process conditions. Additionally, a rate expression that assumed the rate was only a function of methane partial pressure was considered, namely, \(rate = k*P_{{CH_{4} }}\), where \(k = k_{0} *e^{{^{{ - {\text{Ea}}/{\text{RT}}}} }}\), with PCH4 in units of Torr. This first-order-methane rate expression fit the data well, yielding an apparent activation energy over this catalyst of Ea = 93 kJ/mol and the pre-exponential rate constant of k0 = 7.67 × 105 mol/(g-cat s Torr CH4).