Effects of heterogeneous–homogeneous interaction on the homogeneous ignition in hydrogen-fueled catalytic microreactors

Abstract

The effects of heterogeneous–homogeneous interaction on the homogeneous ignition behavior in hydrogen-fueled catalytic microreactors in the submillimeter to millimeter range were investigated numerically. A two-dimensional CFD (computational fluid dynamics) model that includes detailed hetero-/homogeneous chemistry and transport was employed to study the effect of mechanistic interactions between heterogeneous and homogeneous reaction pathways on the homogeneous ignition behavior of hydrogen-air mixtures over platinum in parallel-plate microreactors. Parametric studies were carried out to identify the effect of various operating parameters (inlet temperature, reactor size, inlet pressure, and feed composition) on the homogeneous ignition behavior. Comparisons with purely homogeneous cases were made. Heterogeneous–homogeneous interaction diagrams delineating the regions of the different homogeneous ignition types were constructed in terms of microreactor dimension and inlet temperature. An inflection point temperature of homogeneous ignition mode transition was found. The catalyst promotes the homogeneous ignition at lower temperatures but inhibits the homogeneous ignition at higher temperatures. The homogeneous inhibition is mainly induced by heterogeneous reactions; decreasing the reactor size enhances this inhibiting effect and moves the inflection point toward higher temperatures. Microreactor dimension has a strong impact on the homogeneous ignition behavior at lower temperatures, but has little effect at higher temperatures. Three homogeneous ignition regions can be observed in heterogeneous–homogeneous interaction diagrams, and a transition from completely heterogeneously- to heterogeneously-, and eventually homogeneously-controlled ignition occurs with increasing reactor size and inlet temperature. The diverging ignition behavior occurs with increasing pressure. The homogeneous ignition location at higher temperatures is found to be nearly independent of feed compositions that has an important effect at lower temperatures.