Auto-cyclic reactor: Design and evaluation for the removal of unburned methane from emissions of natural gas engines

Abstract

A non-adiabatic fixed bed auto-cyclic reactor (ACR) consisting of two counter-current concentric compartments was designed and built for removing low concentrations of methane from exhaust gases from natural gas engines. The length was based on simulations by a simple heterogeneous one dimensional model using literature parameters and kinetic data, while the diameter was selected to assure a linear fluid velocity between 0.5 and 2 m/s. Its innovative design consists of a judicious combination of 14 longitudinal fins welded to the outlet part of inner reactor compartment to maximize the heat transfer to the inlet section and highly active pellet type catalyst filling the space between fins to lower the ignition temperature. The experimental ACR pilot unit was loaded by a combination of highly active laboratory prepared catalysts: palladium/alumina pellets and palladium/alumina coated cordierite monoliths. The efficiency of methane removal from air and from synthetic exhaust gas containing 7 vol% CO 2 and 14 vol% H 2O was evaluated under a wide range of operating conditions: temperature from 290 to 500 °C, methane concentration between 500 and 3800 ppm. The reactor performance was monitored in terms of axial temperature profiles and methane conversion both in transient and steady state conditions. Reproducible performance of the ACR was observed even after 1200 h of cumulative operation and complete methane removal was obtained at relatively low temperatures. To simulate the obtained experimental data, a heterogeneous one-dimensional model was developed to suit the final reactor configuration using actual laboratory determined kinetic data. The model described adequately the experimental temperature profiles and methane conversion when heat transfer between the reactor compartments and heat loss were taken into account.