Solvent extraction of coal is an effective method for removing coal ash, thereby generating the ultraclean fuel that can be directly combusted in gas turbine. Due to their organic affinity, a few inorganic elements can be extracted as well. Ti in coal extract, its elution from raw coal as well as the transformation of eluted Ti during coal extract combustion, have been investigated. Two coals of the U.S. and their acid-washed samples were extracted under 1 MPa N2 (cold) at 360C. The solvents used include nonpolar 1-methynaphthalene and its mixtures with polar indole. The results indicate that, Ti in coal extracts is mainly composed of nanoparticles including TiO2 (anatase) and Ti associated with quartz. These particles are insoluble in any acids, having a fine dispersion into coal matrix. Upon coal fragmentation at 360C, they could be librated, and, hence, traversed the filter for isolating coal extracts. The organo-Ti was preferentially extracted as well, which is most likely in a form of Ti porphyrin or Ti chelated with phenol-oxygen. These findings also have implications for revealing the modes of occurrence of Ti in the raw coals. Combustion of coal extract at 1,000C resulted in the formation of nanometric TiO2 polymorphs and much complex compound like FeZnTiO4. The former species could be mainly formed by the phase change of TiO2 (anatase) at high-temperatures, while formation of the latter one could involve the capture of metallic vapors like Zn on TiO2 polymorphs. The resultant nanoparticles may escape the conventional pollution control devices, causing environmental concern.
Zhang, L., Takanohashi, T., Saito, I., Wang, Q., & Ninomiya, Y. (2008). Elution of Ti during solvent extraction of coal and the transformation of eluted Ti upon combustion. AIChE Journal, 54(6), 1646 - 1655. https://doi.org/10.1002/aic.11474