Modeling the CO₂ capture and in situ conversion to CH₄ on dual function Ru-Na₂CO₃/Al₂O₃ catalyst

Highlights

• A kinetic model is developed to describe the CO₂ capture and conversion to CH₄.

• The adsorption site may be occupied by CO₂ or H₂O, or both can share a single site.

• The model predicts the evolution of CO₂, CH₄ and H₂O concentration with time.

• Species concentrations and covering factors are predicted along the reactor length.

• The model is validated in a wide range of temperatures and reagents concentration.

Abstract

A dynamic kinetic model is proposed to describe the CO₂ adsorption and in situ hydrogenation to CH₄ on a 4%Ru-10%Na₂CO₃/Al₂O₃ catalyst. One dimensional isothermal heterogeneous plug flow reactor model with axial dispersion is considered. The reaction network and kinetic equations are deduced from experimental data analysis. The molecular modeling includes competitive adsorption of CO₂ and H₂O on same adsorption site. During CO₂ storage period, CO₂ is adsorbed onto Na₂O species to form Na₂CO₃. Alternatively, CO₂ can be also adsorbed onto NaOH species releasing H₂O to the gas phase, which can further be adsorbed onto free adsorption sites located downstream. It was needed to consider formation of unstable bicarbonates to fit the experimental data. During CO₂ hydrogenation step, adsorbed carbonates are decomposed, promoted by the presence of H₂. CH₄ is then formed through the Sabatier’s reaction and the as-formed H₂O interacts with regenerated adsorption sites. Temporal evolution of CO₂, CH₄ and H₂O during CO₂ storage and hydrogenation cycles is accurately predicted by the model, for a wide range of reactant concentrations and temperature (250−400 °C), gaining understanding on mechanisms and dynamics of the process on dual function materials.

Introduction

The power to gas (PtG) technology produces methane or synthetic natural gas (SNG) through the Sabatier’s reaction (Eq. (1)) [1]. This reaction was first presented in 1902 by the French chemist Paul Sabatier and provides the theoretical basis for the CO₂ methanation [2]. The PtG concept allows the storage of energy using SNG as energy carrier. First, H₂ is produced by electrolysis using the surplus electricity obtained from renewable resources. Then, H₂ is catalytically reacted with pre-concentrated CO₂ to produce methane. In this way, SNG can be fed directly to the existing extensive transport and distribution network. Besides, the utilization of the as-produced SNG as a fuel represents a carbon neutral cycle provided that H₂ is exclusively obtained from renewable resources.

CO₂ + 4H₂ ⇆ CH₄ + 2H₂O

The PtG technology can store and balance surplus energy caused by the intermittency of renewable energies and replace fossil fuels in applications historically difficult to access for renewables, such as energy transport [3]. In recent years, its interest has increased and the number of publications in the field is growing exponentially [4]. The CO₂ hydrogenation reaction is highly exothermic and thermodynamic equilibrium limits the progress of the reaction at high temperatures. Thus, to avoid hot spots, an efficient management of the heat generated by the reaction is particularly relevant.

Modeling the CO₂ hydrogenation has gained interest as a useful tool for designing efficient reactors to maximize CO₂ conversion. Schlereth et al. [5] modeled an externally cooled fixed bed reactor with a pure stoichiometric feed. They showed that reaction rates and exothermicity prevent a fixed bed reactor of technical dimensions to be operated at high conversions without runaway of the reactor. The study was completed by modeling a fixed-bed membrane reactor as an example of structured reactor that offers improved temperature control through separate and controlled feeding of hydrogen from carbon dioxide. Kiewidt et al. [6] addressed the problem of exothermicity by applying a method based on Semenov's number optimization to calculate optimal axial temperature profiles in single stage fixed bed reactors that take into account kinetic and thermodynamic limitations simultaneously. They improve the methane production compared to isothermal and adiabatic operation. Chein et al. [7] used non-isothermal aximometric governing equations for the flow of gas in a fixed bed reactor, using the reagent inlet temperature as the main parameter. They showed that the elimination of heat characterized by the heat transfer coefficient plays an important role in the conversion of CO₂. Kreitz et al. [8] presented a heterogeneous dynamic 1D model for a microstructured fixed bed reactor at industrially relevant operating conditions with periodic oscillations of the input feed composition. The simulation showed that the hot spot temperature changes significantly during the periodic operation, both in position and in magnitude.

Recently, the CO₂ hydrogenation in consecutive adsorption and hydrogenation cycles has been proposed [[9], [10], [11], [12], [13]]. This alternative can be directly applied to CO₂ diluted streams, without the need of previous sequestration and purification steps, and present a better management of the heat generated. This strategy requires the utilization of dual function materials (DFM), packed in two parallel reactors that operate in alternate cycles of CO₂ capture and methanation. The DFM contains an alkaline or alkaline earth element that acts as CO₂ adsorbent and a noble metal that assists the methanation reaction. First, CO₂ is adsorbed in the DFM until saturation. Then, when H₂ is injected, a spillover phenomenon occurs that leads the chemisorbed CO₂ to the noble metal where the methanation takes place. Both the CO₂ capture process and the hydrogenation to CH₄ can operate at temperatures in the range of 250−400 °C. Those temperatures are usually found in the effluent gas of a combustion process and therefore there is no need for external heat input.

The modeling of the CO₂ hydrogenation in consecutive adsorption and hydrogenation cycles can also be a suitable tool to define optimal operation conditions, such as, the duration of adsorption and hydrogenation cycles, temperature or H₂ dose during the hydrogenation. There are several studies regarding the modeling of CO₂ adsorption and desorption processes [14,15]. With respect to those models, we are including a reaction step in which the adsorbed carbonates react with hydrogen to produce CH₄ and H₂O. Novel specific kinetic equations to describe the regeneration of the adsorption sites and the formation of CH₄ and H₂O will be proposed. Besides, the interaction of the as-formed H₂O with the adsorption sites will be also considered. Very recently a kinetic model for the joint adsorption and desorption of CO₂ and H₂O on hydrotalcites has been proposed [16]. This model includes three adsorption sites, where two sites model the weaker chemisorption of H₂O and CO₂, whereas a third site accounts for stronger bond CO₂ and H₂O on the adsorbent. With respect to this model, we are proposing a model consisting on only one adsorption site where CO₂ and H₂O compete for the adsorption site. Besides, we also consider the joint adsorption of CO₂ and H₂O onto a single site forming a bicarbonate.

As far as our knowledge is concerned, the modeling of the CO₂ adsorption and hydrogenation to CH₄ in consecutive cycles using DFMs has not yet been reported in the literature. Thus, the main objective of this paper is to develop a kinetic model able to describe the evolution of CO₂, CH₄ and H₂O during the adsorption and hydrogenation periods. The model will be based on the complete reaction scheme proposed in our previous works on DFMs with formulation Me-Na₂CO₃/Al₂O₃ [12,13]. A dynamic one dimensional isothermal heterogeneous plug flow reactor model with axial dispersion will be considered. To solve the partial differential equation (PDE) system, the axial coordinate of the reactor will be discretized based on finite differences. The resulting ordinary differential equations system will be solved by a dedicated program developed in Matlab. The kinetic parameters will be estimated based on the least squares method, using the temporal evolution of the gaseous species concentrations (CO₂, CH₄ and H₂O) as experimental responses.