Characteristics of Oxy-fuel Combustion in an Oxygen Transport Reactor

This study aims at investigating the characteristics of oxy-fuel combustion in an oxygen transport reactor (OTR). The cylindrical reactor walls are made of dense, nonporous, mixed-conducting ceramic membranes that only allow for oxygen permeation from the outside air into the combustion chamber. The oxygen permeate mixes with a mixture of CO2 and CH4 (sweep gas) that enters the reactor, resulting in combustion products composed of H2O and CO2. The modeling of the flow process considers a numerical solution of the conservation equations of mass, momentum, energy, and species in the axi-symmetric flow domain. The oxygen permeation across the membrane depends upon the prevailing temperatures and the oxygen partial pressure at both sides of the membrane. The simulations are performed for different compositions of CH4/CO2 mixtures and different mass flow rates. First, the comparison between the reactive and separation-only OTR units showed that combining reaction and separation increases the O2 permeation rate significantly to about 2.5 times under the assumptions given herein. Second, a mass flow rate of 1.625 × 10–7 kg/s with a CH4/CO2 mass ratio ranging from 0.5:0.5 to 1.0:0 gives an almost uniform axial temperature of about 1250 K in most of the reactor length with a high CH4 conversion of 75 to 35%, respectively. In all of the simulations, the total O2 permeation flux is almost the same, except for 1.625 × 10–7 kg/s with a CH4/CO2 mass ratio less than 0.3:0.7. The results indicate that the heat of reaction is mostly transferred to the air side with a portion used to heat the O2-permeating flux. For higher mass flow rates, the OTR operates with a rich mixture, resulting in low CH4 conversion. The combustion process in such cases can be improved by splitting the OTR into a series of units, where the fuel is added at stages along the reactor network.