A review on combustion instabilities in energy generating devices utilizing oxyfuel combustion

This paper reviewed theoretical, numerical, and experimental studies on combustion instabilities in combustors for energy-producing devices such as gas turbines, boilers, and furnaces utilizing oxyfuel combustion technology. Oxyfuel combustion as a CO2 capture technology gives encouraging and promising solutions for controlling greenhouse gases emission from combustion devices. However, the major concerns with oxyfuel systems are the combustion instabilities mainly arising from the CO2 dilution required for controlling excessive temperatures. Higher dilution levels or significantly nonuniform distribution of CO2 in the mixture can potentially lead to static and/or dynamic instabilities associated with local flame extinction and unsteady heat release rate, respectively. As in the case of lean premixed air/fuel flames, combustion instabilities are among the most critical operational challenges encountered in CO2-diluted oxyfuel systems as well. Existing literature on flame instabilities has predominately focused on air-fuel flames, with very few research studies dedicated to the same phenomena in oxyfuel combustion. With an ever-increasing push toward carbon capture and reduction in NOx emissions, it has become important for the combustion community to accelerate research efforts in the direction of oxyfuel combustion systems. This review study on flame instabilities in oxyfuel combustors is one small step in that very direction. This paper starts with discussions on the theoretical mechanisms of combustion instabilities. Then a range of numerical and experimental studies, from premixed to non-premixed flames, are presented and discussed. The main objective is to summarize recent progress in understanding instabilities in oxyfuel combustion systems, focusing primarily on the effect of CO2 dilution on flame stability. A range of experimental and numerical studies by various groups have strongly associated both static and dynamic instabilities in oxyfuel systems with diluent concentration, suggesting higher concentrations potentially result in a stratified mixture field which leads to higher heat release fluctuations and pressure oscillations. As such, many combustion systems use hydrogen enrichment to enhance static stability while employing flow and/or fuel modulation to dampen dynamic instabilities in oxyfuel combustors. Finally, we highlight and summarize some of the key issues and unsolved questions that still challenge researchers and, therefore, need to be the focus of future research work as regards to oxyfuel combustion.