Modeling Elementary Heterogeneous Chemistry and Electrochemistry in Solid-Oxide Fuel Cells

This paper presents a new computational framework for modeling chemically reacting flow in anode-supported solid-oxide fuel cells (SOFC). Depending on materials and operating conditions, SOFC anodes afford a possibility for internal reforming or catalytic partial oxidation of hydrocarbon fuels. An important new element of the model is the capability to represent elementary heterogeneous chemical kinetics in the form of multistep reaction mechanisms. Porous-media transport in the electrodes is represented with a dusty-gas model. Charge-transfer chemistry is represented in a modified Butler-Volmer setting that is derived from elementary reactions, but assuming a single rate-limiting step. The model is discussed in terms of systems with defined flow channels and planar membrane-electrode assemblies. However, the underlying theory is independent of the particular geometry. Examples are given to illustrate the model

Microkinetic Modeling of CO2 and H2O Electrolysis on Ni in a Solid Oxide Electrolysis Cell: A Critical Evaluation

Solid oxide cells (SOC) are ideal candidates for the electrochemical conversion of H2O and CO2 into H2 and CO using renewable sources. This work develops different electrochemical models for the reduction of H2O and CO2 based on elementary step kinetics and discriminates them based on their ability to predict experimentally measured cell performances. The thermo-catalytic chemistry is represented using a micro kinetic model, which is coupled to the electrochemical model through the surface coverage terms. A one dimensional representation of SOC resolving the cell across the thickness of the electrodes is used for simulations. The source terms for the species transport Eqs. are calculated using the micro kinetic model by applying mean field approximation. The discussion in the paper covers aspects related to parameter fitting, model development, solution methodology, model discrimination and identification of rate limiting step. © 2021 The Electrochemical Society ("ECS"). Published on behalf of ECS by IOP Publishing Limited

Mechanistic Kinetic Model for Biogas Dry Reforming

A thermodynamically consistent mechanistic kinetic model for dry reforming of biogas/CH4 (DRM) is developed. Since reverse water gas shift (RWGS) reaction always occur alongside DRM, the overall rate is expressed in terms of individual rates of DRM and RWGS reactions. In order to identify the rate-limiting steps for the derivation of mechanistic kinetics, a microkinetic model is initially developed and validated against experimental measurements. Reaction rate analysis and partial equilibrium analysis are then performed on the microkinetic model to identify the rate-limiting steps. The parameters of the mechanistic kinetic model are estimated using a genetic algorithm. t statistic is performed to establish the confidence level of the estimated parameters. A brute-force sensitivity analysis is performed to identify the most sensitive parameters in the microkinetic model. The models are validated over wide ranges of conditions

Detailed Chemical Kinetic Modeling of Pyrolysis of Ethylene, Acetylene, and Propylene at 1073-1373 K with a Plug-Flow Reactor Model

ABSTRACT: This study examines the predictive capability of our recently proposed reaction mechanism (Norinaga and Deutschmann, Ind Eng Chem Res 2007, 46, 3547) for hydrocarbon pyrolysis at varying temperature. The conventional flow reactor experiments were conducted at 8 kPa, over the temperature range 1073-1373 K, using ethylene, acetylene, and propylene as reactants to validate the mechanism. More than 40 compounds were identified and quantitatively analyzed by on-and off-line gas chromatography. The chemical reaction schemes consisting of 227 species and 827 reactions were coupled with a plug-flow reactor model that incorporated the experimentally measured axial temperature profile of the reactor. Comparisons between the computations and the experiments are presented for more than 30 products including hydrogen and hydrocarbons ranging from methane to coronene as a function of temperature. The model can predict the compositions of major products (mole fractions larger than 10 −2 ) in the pyrolysis of three hydrocarbons with satisfactory accuracies over the whole temperature range considered. Mole fraction profiles of minor compounds including polycyclic aromatic hydrocarbons (PAHs) up to three ring systems, such as phenanthrene, anthracene, and phenylnaphthalene, are also fairly modeled. At temperatures lower than 1273 K, larger PAHs were underpredicted and the deviation became larger with decreasing temperature and increasing molecular mass of PAHs, while better agreements were found at temperatures higher tha

Experimental studies of catalyst deactivation due to carbon and sulphur during CO 2 reforming of CH 4 over Ni washcoated monolith in the presence of H 2 S

This study presents the CO2 reforming of CH4 over Ni coated monolith catalyst at 800°C and 101.325 kPa. The high CH4 to CO2 ratio employed in this study is similar to the CH4:CO2 ratio of >1 found in biogas. Cordierite monolith samples (0.258 channels per m2) washcoated with alumina are used for the experimental purpose. The study considers the combined deactivation effect due to sulphur poisoning and fouling due to carbon deposition. Four different cases with respect to the introduction and removal of H2S are considered. The rate of deactivation due to simultaneous carbon deposition and sulphur poisoning is much faster than the individual poisoning processes. The catalyst shows almost stable operation for 6 h without the presence of (Formula presented.) in the feed stream. From the conversion studies, it appears that the pre-treatment of catalyst samples with H2S leads to negligible sulphur coverage. The sulphur poisoning effect is also found to be reversible. © 2021 Canadian Society for Chemical Engineering

Effect of Calcination Time on the Catalytic Activity of Ni/γ-Al2O3 Cordierite Monolith for Dry Reforming of Biogas

Ni/γ-Al2O3 wash coated cordierite monolith catalysts are calcined in air at 800 °C for 4, 10, and 20 h in order to study the effect of calcination time on the activity of the catalysts for dry reforming of model biogas. Catalytic activity studies are performed at 800 °C with three different CH4/CO2 ratios of 1.0, 1.5, and 2.0. The catalyst calcined for the longest time (C-20) displays higher stability and activity in terms of CH4 and CO2 conversion compared to those calcined for 4 h (C-4) and 10 h (C-10). XRD data and TPR analysis detect the maximum amount of NiAl2O4/MgAl2O4 phases and strongest metal-support interaction, respectively, for the C-20 sample. FESEM reveals the particle size of the calcined and reduced C-20 sample to be smaller than that of the C-4 and C-10 samples. Whereas, H2 pulse-chemisorption characterization demonstrates the highest metal surface area, metal dispersion, and smallest Ni particle size for the C-20 catalyst. While, no carbon deposition on any catalyst occurs for the CH4/CO2 ratio of one, lowest amount of carbon nanotubes is formed on the C-20 sample for the CH4/CO2 ratio of 1.5 and 2.0, as observe by DTA-TGA. EDX reveals concentration variation of Mg and Si from the cordierite monolith wall along the thickness of the coating for all the samples. In addition, the maximum amount of these elements is observed for the calcined C-20 catalyst coating. These implies that the diffusion of Mg and Si from the cordierite monolith to the catalyst coating during calcination contribute significantly in controlling the physicochemical properties of the catalysts. As a result, the higher stability and activity of the C-20 could be attributed to the formation of higher amount of the Ni– Mg- alumina spinel complex in the catalyst coating during longer calcination time, which leads to the improved metal-support interaction and higher nickel dispersion over monolith