Membrane technology for challenging separations: Removal of CO2, SO2 and NOx from flue and waste gases

The major gaseous pollutants emitted during the combustion of fossil fuels, during energy-intensive production of materials, and during waste processing are CO2, SO2, and NOX. The combined efforts of industry and the research community aim to intensify their capture and decrease their emissions, with the final objective of meeting stricter emission limits, which is becoming more challenging with conventional technologies. Membrane separation technology is a particularly significant environmental protection application because it can be effectively combined with other separation technologies and generally uses much less energy. As membrane processes represent an energy-efficient alternative to traditional separation processes, in view of today's emissions policy, they have become economically competitive to purify flue gases emitted from small point sources, such as waste incineration units and steel or cement production facilities. Different membrane types with various specific functions, such as high solubility for certain gases, thermal and mechanical stability, and ease of manufacturing, are presented as perspective technologies for flue gas purification. Unique systems are also discussed, including facilitated or retarded transport of permeants in membranes and mixed-matrix membranes. We first describe the basic operating mechanism based on their structure and material properties to compare the advantages and disadvantages of suitable membranes. This review presents state-of-the-art membrane technologies in CO2, SO2, and NOX separation from flue gases and discusses the potential applications of the most promising membranes.