Microkinetic analysis of CH4 CPO tests with CO2-diluted feed streams

Autothermal CH4 catalytic partial oxidation (CPO) tests were performed in an adiabatic lab-scale reformer over 2 wt% Rh/α-Al2O3 catalysts supported on 600 cpsi cordierite honeycombs. The effect of adding either CO2 or N2 to CH4/air mixtures was investigated at constant O2/CH4 ratio (0.56) and total flow rate (10 Nl/min). At increasing dilution the conversion of CH4 and the gas temperatures decreased. Also, at equal CH4 concentration, the addition of CO2 decreased the temperatures and the conversion of CH4 more importantly than N2. Thus, in line with previous studies from this and other laboratories, no evidence of a direct dry reforming route was found; the observed thermal behavior appeared largely controlled by the heat capacity of the feed streams. However an important chemical effect of CO2 addition, not previously appreciated in the literature, was shown by the evolution of the outlet H2/CO ratio; at increasing N2 dilution, the H2/CO ratio slightly increased from about 2 to 2.5, but at increasing CO2 dilution, the H2/CO ratio decreased from about 1.7 to 0.7. These trends are in line with the thermodynamics of the reverse water gas shift (RWGS) reaction; such a reaction was thus extremely fast in all the experiments with CO2, even at the highest values of dilution, while CH4 conversion did not reach the equilibrium.A C1 microkinetic model was used to analyze the results. A close match between calculated and measured temperatures, conversions and syngas composition was obtained under all conditions. Notably, the present microkinetic scheme, which incorporates steps for CO2 adsorption and reactivity, was able to account for the observed net consumption of CO2 and for the lowering of the H2/CO ratio in the experiments with CO2 co-feed. Surface coverages were analyzed for the various investigated conditions and the effects on the kinetics of methane steam reforming were evaluated in detail; the factors which control the onset of a more kinetically controlled regime at high degrees of dilution were highlighted.

The effect of CO2 dilution was investigated in autothermal CH4 catalytic partial oxidation tests at short contact time over Rh-based catalysts. A novel C1 microkinetic scheme and a detailed reactor model were used for the analysis. CO2 plays an important kinetic role by activating the reverse-WGS reaction, which strongly affects the outlet H2/CO ratio. Despite the influence on the surface coverages of CO* and H*, the role of CO2 on the kinetics of methane reforming is negligible, since the coverage of Rh free sites is practically independent from the nature of the diluting gas (CO2 or N2).Figure optionsDownload high-quality image (85 K)Download as PowerPoint slideResearch highlights▶ CO2 recycle in CH4 CPO activates the RWGS reaction and modifies the H2/CO ratio. ▶ The role of CO2 feed enrichment on the kinetics of methane reforming is negligible. ▶ The coverage of Rh free sites is nearly independent of the nature of the diluting gas (CO2 or N2).