Microkinetic analysis of CH 4 CPO tests with CO 2-diluted feed streams

Abstract

Autothermal CH 4 catalytic partial oxidation (CPO) tests were performed in an adiabatic lab-scale reformer over 2 wt% Rh/α-Al 2O 3 catalysts supported on 600 cpsi cordierite honeycombs. The effect of adding either CO 2 or N 2 to CH 4/air mixtures was investigated at constant O 2/CH 4 ratio (0.56) and total flow rate (10 Nl/min). At increasing dilution the conversion of CH 4 and the gas temperatures decreased. Also, at equal CH 4 concentration, the addition of CO 2 decreased the temperatures and the conversion of CH 4 more importantly than N 2. 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 CO 2 addition, not previously appreciated in the literature, was shown by the evolution of the outlet H 2/CO ratio; at increasing N 2 dilution, the H 2/CO ratio slightly increased from about 2 to 2.5, but at increasing CO 2 dilution, the H 2/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 CO 2, even at the highest values of dilution, while CH 4 conversion did not reach the equilibrium. A C 1 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 CO 2 adsorption and reactivity, was able to account for the observed net consumption of CO 2 and for the lowering of the H 2/CO ratio in the experiments with CO 2 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.