Hysteresis loops of methane catalytic partial oxidation for hydrogen production under the effects of varied Reynolds number and Damkohler number

摘要

Hysteresis loops of catalytic partial oxidation of methane (CPOM) for hydrogen production under the effects of varied Reynolds number and Damkoehler number are investigated numerically in this study. The physical phenomena are predicted using the indirect mechanism, which consists of the total oxidation (or combustion), steam reforming and CO_2 reforming of methane in a catalyst bed. Numerical results reveal that, when the Damkoehler number is relatively low, a hysteresis loop of CPOM from varying Reynolds number develops. Increasing the Damkoehler number leads to the loop shifting toward the regime of high Reynolds number. However, once the Damkoehler number is large to a certain extent, the chemical reactions are always exhibited for the Reynolds number less than 2000. A closed loop is thus not observed. Alternatively, for a given Reynolds number, an ignited Damkoehler number and an extinguished Damkoehler number can be obtained. Accordingly, three different regions in the plot of Damkoehler number versus Reynolds number are identified. Physically, when the role played by Damkoehler number on CPOM is much more important than by the Reynolds number (Region I), the thermal effect governs the chemical reactions. In contrast, if the Reynolds number plays a key role in determining the CPOM (Region Ⅲ), the chemically frozen flow prevails over the catalyst bed. When the residence times of the total oxidation and convection in the catalyst bed are in an equivalent state (Region Ⅱ), CPOM is characterized by a dual-solution, rendering the hysteresis loops. From the distributions of ignited and extinguished Damkoehler numbers, the catalytic reactor and operation of partial oxidation of methane and other fuels can be designed accordingly.