This paper describes the use of a commercial CFD code, FLUENT, to model a reverse flow catalytic converter used for a lean burn natural gas engine. A three dimensional model is used to study methane ignition. The methodology for the construction of pseudo-homogeneous and heterogeneous continuum models in the context of FLUENT is described. For the heterogeneous model, a dual zone approach is used, where double cells or nodes are used to distinguish between fluid and solid temperatures. The heterogeneous model generally requires much more computational time than the pseudo-homogeneous model under conditions of uni-directional flow, and slightly more under conditions of reverse flow. The use of a heterogeneous model is justified when there are large changes in the inlet conditions. Overall, the use of 3D modelling is feasible, if time consuming, using FLUENT on a modern PC. From a reactor operational perspective, it is demonstrated that methane ignition can be achieved at a lower inlet gas temperature under conditions of reverse flow, compared to uni-directional flow. When the inlet temperature is sufficient to achieve ignition under conditions of uni-directional flow, then the use of reverse flow increases the time required to achieve ignition. Hence, the selection of flow mode must be selected depending on the inlet condition. (c) 2006 Elsevier Ltd. All rights reserved.