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.

Chemical-looping with oxygen uncoupling (CLOU) is considered a promising technology to burn solid fuels with improved CO₂ capture and has the potential to improve fuel conversion and reaction rates. Cu-based oxygen carriers (Cu-OC) are often used in solid fuel CLOU. This study focused on investigating Cu-OC derived from a natural mineral for solid fuel CLOU because of their potentially lower cost compared to synthetic OCs. Reactivity and recyclability of a natural ore-derived Cu-OC on coal char (Powder River Basin sub-bituminous coal) were studied at 900 °C in Ar and air using TGA-QMS and fixed-bed reactor-QMS for five cycles. Cu-OC was prepared by simply heating chalcopyrite in air. Chalcopyrite is one of the principle copper sulfide ores and one of the primary ores for copper. The prepared Cu-OC had primarily CuO and CuFe₂O₄ (CuOFe₂O₃) as active compounds based on XRD analysis and an oxygen capacity 3.3% from oxygen uncoupling. The carbon conversion efficiency Xc was 0.94 for reduction at a ratio of Cu-OC to char ϕ = 75 and the product gas was primarily CO₂ with trace O₂. The reactivities and the rates were similar for five redox cycles. These results indicate that the natural ore-derived material with low cost has potential as a competitive oxygen carrier in solid fuel CLOU based on its reactivity in this study.

FeAl_xO_y powders, fabricated by solution combustion synthesis (SCS), are promising catalysts for microwave-assisted pyrolysis of methane. However, the effects of SCS parameters on the pyrolysis are not well understood. In the present work, two fuels (citric acid and glycine), two Fe:Al molar ratios, and two heating modes (a hotplate and a muffle furnace) are tested. All catalysts exhibit CH₄ conversion of around 70% and H₂ composition of about 93%. The process is one to two orders of magnitude more efficient than conventional pyrolysis. Increasing the Fe:Al ratio from 1:1 to 2:1 and using a hotplate improve H₂ generation efficiency. Use of glycine decreases the CO₂:H₂ ratio, but citric acid yields more readily reducible products due to differences in phase evolution, detected by X-ray diffraction analysis. Scanning electron microscopy and energy-dispersive X-ray spectroscopy reveal carbon nanotubes and exsolution of Fe. FeAl_xO_y catalysts, prepared via incipient wetness impregnation, are ineffective.