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The atmospheric concentration of anthropogenic carbon dioxide (CO 2 ) has been increasing since the start of industrialization in the mid 19th century, and the rate is increasing. It is highly unlikely that fossil fuel combustion, the main contributor to anthropogenic CO 2 , will be replaced in the foreseeable future. Therefore, CO 2 capture and storage offer a new set of options for reducing greenhouse gas emissions, in addition to the current strategies of improving energy efficiency and increasing the use of renewable energy resources. Carbon dioxide selective membranes provide a viable energy-saving alternative for CO 2 separation, since membranes do not require any phase transformation. This review examines various CO 2 selective membranes for the separation of CO 2 and N 2 , CO 2 and CH 4 , and CO 2 and H 2 from flue or fuel gas. This review attempts to summarize recent significant advances reported in the literature about various CO 2 selective membranes, their stability, the effect of different parameters on the performance of the membrane, the structure and permeation properties relationships, and the transport mechanism applied in different CO 2 selective membranes. Finally, the future direction for CO 2 selective membranes is proposed. Hybrid organic-inorganic membranes have become an expanding field of research, as the introduction of organic molecules can improve the characteristics of a matrix. Hydrotalcite-type materials, perovskite-type oxides, lithium zirconate, and lithium silicate are also suggested as candidate materials for high temperature CO 2 selective membranes.

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Chemical-looping technology is one of the promising CO2 capture technologies. It generates a CO2 enriched flue gas, which will greatly benefit CO2 capture, utilization or sequestration. Both chemical-looping combustion (CLC) and chemical-looping gasification (CLG) have the potential to be used to generate power, chemicals, and liquid fuels. Chemical-looping is an oxygen transporting process using oxygen carriers. Recently, attention has focused on solid fuels such as coal. Coal chemical-looping reactions are more complicated than gaseous fuels due to coal properties (like mineral matter) and the complex reaction pathways involving solid fuels. The mineral matter/ash and sulfur in coal may affect the activity of oxygen carriers. Oxygen carriers are the key issue in chemical-looping processes. Thermogravimetric analysis (TGA) has been widely used for the development of oxygen carriers (e.g., oxide reactivity). Two proposed processes for the CLC of solid fuels are in-situ Gasification Chemical-Looping Combustion (iG-CLC) and Chemical-Looping with Oxygen Uncoupling (CLOU). The objectives of this review are to discuss various chemical-looping processes with coal, summarize TGA applications in oxygen carrier development, and outline the major challenges associated with coal chemical-looping in iG-CLC and CLOU. OPEN ACCESSEnergies 2015, 8 10606

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