Electron spin resonance studies of coals and coal conversion processes: A review

Electron spin resonance (ESR) is a spectroscopic method to detect paramagnetic species, which has been applied in coal and coal conversion research over 60 years. The major advancements made before 1990s were reviewed but the studies and findings reported since then have not been reviewed. The recent studies not only showed new phenomena on ESR radical behaviors associated with coal conversion processes, but also revealed some misunderstanding of earlier studies on the nature of ESR radicals and its relationship with the coal conversion processes, especially with the total radicals generated in coal conversion processes. This article provides a dedicated review of ESR studies in coal research from 1954 to now, with emphases on research advancements in this century. It also includes ESR studies on other heavy organic matters, such as biomass and oil shale, because their reactions are similar in principle with coal reactions. It starts with the basic knowledge on ESR analysis, including signal assignment, spectra analysis, and cautions needed to ensure the reliability of spectra, and then moves to the significance of ESR spectra for coals and coal conversions especially new phenomena and understandings reported in recent decade, and finally ends with the challenging researches needed in future studies.