1,721,113 research outputs found
Chemical and geometric effect of ceria and washcoat addition on catalytic partial oxidation of CH4 on Rh probed by spatially resolved measurement
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A multistep model for the kinetic analysis of the impedance spectra of a novel mixed ionic and electronic conducting cathode
No full textA one-dimensional, heterogeneous and dynamic model is applied to kinetically analyze impedance experiments performed on a novel NdBa0.9Co2O5.6 (NBC) MIEC cathode. The model simulates the spectra in the time domain by accounting for the gas diffusion inside the electrode pores, and for the solid state diffusion of oxygen vacancies inside the bulk of the cathodic material. A detailed kinetic scheme is applied to describe the oxygen reduction mechanism, which includes steps for adsorption and desorption, first and second electronation at the gas/electrode interface, and ion transfer at the electrode/electrolyte interface. The kinetic investigation is based on impedance spectra collected on symmetric NBC/GDC/NBC cells, at open circuit voltage, between 550 and 700°C, and 5–100% O2 molar fraction. The vacancies diffusion coefficient and the kinetic parameters of the reaction steps are fitted to describe the data. At the highest temperatures, a sensitivity analysis reveals that the rate determining step is the first electronation of the oxygen adatom, while the second electronation and the interfacial ion transport are kinetically irrelevant. Overall, the model allows to individuate the key parameters for capturing the kinetics of a MIEC cathode
Numerical analysis of NOx production within a hydrogen catalytic combustor
Full text linkHydrogen combustion reactions produce nitrogen oxides as a byproduct. They can be reduced by exploiting the catalytic combustion of hydrogen in a monolith. However, some nitrogen oxides can still be produced in the gas-phase of the catalytic combustor. The present work numerically investigates the production of nitrogen oxides as a by-product of the combustion of a lean air-hydrogen mixture in a catalytic monolith with an equivalence ratio lower than 0.3. The analysis is carried out with a 2D dynamic numerical model implemented in MATLAB. The model solves mass and energy balances in a domain describing a single channel of the monolith. The model involves a detailed reaction mechanism for the gas-phase combustion, including the subsets that model the production of nitrogen oxides. As a result, the model indicates that the catalytic combustor does not produce nitrogen oxides with an inlet hydrogen fraction lower than 9%vol. Furthermore, the maximum value of nitrogen oxide at the outlet of the channel is lower than 0.4 ppm, obtained with the highest hydrogen fraction simulated in this work (12%vol inlet hydrogen fraction)
Catalytic partial oxidation of methane over a 4% Rh/α-Al2O3 catalyst: Part I: Kinetic study in annular reactor
No full textAbstract
The catalytic partial oxidation (CPO) of CH4 to synthesis gas over a 4 wt% Rh/α-Al2O3 catalyst was investigated by means of a short contact
time annular reactor, specifically designed for testing very fast and exothermic reactions. Data were collected by feeding CH4/O2/inert gas
mixtures, at varying temperature (from 350 to 850 ◦C), GHSV (up to 4.5 × 106 N l/Kgcat/h), O2/CH4 ratio (from 0.56 to 1.3), reactant dilution
(1 to 27% CH4 v/v) and adding CO2 (1%) and H2O (1 and 2%) to the standard feed. Steam reforming, CO2 reforming, water gas shift (WGS),
reverse-WGS, H2 and CO combustion tests were also carried out to refine the study. A quantitative analysis of the experimental data was performed
by a 1D mathematical model of the reactor, wherein a molecular kinetic scheme of the process was incorporated. The scheme consists of CH4
total oxidation and reforming, the water gas shift and reverse water gas shift reactions, and H2 and CO post-combustion reactions. On the basis of
experimental data and numerical analysis, it was found that, under the CPO conditions: (1) the kinetic role of CO2 reforming is negligible, so that
steam reforming and CH4 total combustion alone can account for the consumption of CH4; (2) oxidation and steam reforming of methane have
comparable intrinsic kinetics under differential conditions, but surface coverages differently influence the reaction rates under integral conditions;
(3) the direct and the reverse water gas shift reactions (WGS and RWGS), when far from the chemical equilibrium, have independent kinetics;
(4) the process kinetics is significantly affected by the dilution of the reacting mixture: since the global reaction order is lower than 1, conversion
and selectivity decrease at decreasing dilution. Part I of the work deals with the development and the validation of the proposed kinetic scheme;
Part II deals with the analysis of CO2 reforming and RWGS experiments and supports the assumption that CO2 reforming can be excluded form
the CPO kinetic scheme
Optimization of the thermal behavior of an adiabtic reformer for the catalytic partial oxidation of CH4 at short contact time
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Catalytic partial oxidation of methane over a 4% Rh/α-Al2O3 catalyst Part II: Role of CO2 reforming
No full textA kinetic study of the CO2 reforming of CH4 over a 4 wt% Rh/α-Al2O3 catalyst was performed in a short contact time annular reactor.
Experiments were carried out under nearly isothermal conditions, at high space velocity (2×106 Nl/Kgcat/h), within the temperature range 300–
800 ◦C, at varying feed composition. CO2/CH4 tests with excess CO2 showed a strong similarity with previous H2O/CH4 tests. At CO2/CH4 = 1
(an experiment characterized by negligible amount of H2O in the product mixture), the measured conversion of methane was significantly lower.
Additional experiments with co-feed of O2 or H2 indicated that H2O had a limiting role on the conversion of CH4. A quantitative analysis of data
was performed by means of a 1D heterogeneous model of the reactor, by assuming that steam reforming and reverse water gas shift were uniquely
active and proceeded according to kinetics that were estimated on the basis of independent data. Though neglecting the rate of CO2 reforming, all
the observed trends could be well described as a cycle of H2O reforming and RWGS (initiated by a trace amount of H2O in the feed) wherein the
rate determining step (either SR or RWGS) depends on the gas-phase composition. Finally, experiments confirmed that the addition of CO to the
reaction mixture partly slowed down the kinetics of methane activation, which had been indirectly postulated in Part I of this work on the basis of
CH4 CPO data at varying reactant concentrations
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