Application of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ membranes in an oxy-fuel combustion reactor

Oxygen separation from air through ceramic ion transport membranes is anticipated to be one of the technologies that can make fossil fuel power plants to operate without carbon dioxide (CO₂) emission. Consequently, the level of carbon emission in the atmosphere, which causes global warming, could be contained. The most promising material for such ion transport membranes is the Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF), which has been reported to have the highest oxygen permeation flux. However, lack of chemical stability of these membranes in CO₂ rich environment is recognized as a major problem. In this study, the performance and stability of BSCF membranes in an oxy-fuel combustor have been evaluated using methane gas. The flow rate of the methane (CH₄) gas has been optimized to the value of 0.65 ml/min at 920 °C. Under this flow rate, the BSCF stability has also been evaluated. The study has shown that the BSCF stability depends on the amount of excess-oxygen in the reactor. If all oxygen, generated by the membrane, is consumed during the combustion reaction then BSCF starts to deteriorate and its permeability decreases with time. On the other hand, when the combustion reaction does not consume all oxygen, and a minimum excess-oxygen is retained, the BSCF remains stable with constant oxygen flux. This fact has been established using two BSCF membranes with different thicknesses and same exposed area of 113 mm². The first membrane has 1.4 mm thickness and its initial oxygen permeability under combustion reaction is 0.83 µmol cm⁻² s⁻¹ with an excess of 7.5% oxygen. After 190 h of combustion, the oxygen permeability has dropped to 0.76 µmol cm⁻² s⁻¹ (7.9% reduction). The second BSCF membrane has a thickness of 1.0 mm produces 0.91 µmol cm⁻² s⁻¹ with a 15% excess oxygen during the combustion reaction. This membrane has shown excellent stability for more than 200 h of complete methane combustion. This is a remarkable result which shows that a carefully controlled fuel volume introduced in a BSCF membrane-reactor provides high oxygen output and excellent stability for a long period of time.