Abstract
Electron spin resonance (ESR) spectroscopy is used to study materials with unpaired electrons, such as organic radicals and metal complexes. This method can also be used to follow radical reactions during pyrolysis of carbonaceous materials. However, the temperature dependence of ESR measurement should be considered. To enable reasonable comparisons, results measured at different temperatures must be converted. In this study, we investigated the behavior of free radicals in the process of coal pyrolysis using in situ and ex situ ESR. The ESR data were collected at both pyrolysis and room temperatures, and apparent differences were analyzed. The differences were diminished when our data were converted to the same measurement temperature level based on the Boltzmann distribution law. Furthermore, we investigated the effects of process conditions on the behavior of free radicals in the solid phase of coal. We found that temperature is the most important factor determining the formation and behavior of free radicals in the solid phase, followed by the residence time. Relatively active radicals were quenched by hydrogen-donor solvents to some degree, while stable radicals remained.
Highlights
The fundamental mechanisms and behaviors in the thermal decomposition of coal have attracted considerable attention in the field of coal conversion processes, such as coal coking, coal gasification, and direct coal liquefaction
During the in situ experiments, the coal sample was heated in a spectrometer-equipped heater at the designated pyrolysis temperature for 120 s and Electron spin resonance (ESR) measurements were performed at the pyrolysis temperature
We investigated the behavior of free radicals in coal pyrolysis processes using in situ and ex situ ESR
Summary
The fundamental mechanisms and behaviors in the thermal decomposition of coal have attracted considerable attention in the field of coal conversion processes, such as coal coking, coal gasification, and direct coal liquefaction. Coal pyrolysis is a primary reaction in the above-mentioned processes,[1,2,3,4] and the final products are determined by the formation and the subsequent reactions of free radicals. The overall mass-change is one characteristic, which is used to determine the solid and volatile changes in the solid and the distribution of volatiles, in tars.[5,6,7,8] Another common used approach is to correlate process variables during coal pyrolysis with the formation of designated products.[9,10,11,12,13,14] these studies have contributed to advances in our understanding of the behavior of coal pyrolysis, there have been few reports that have revealed the mechanism of coal pyrolysis from the perspective of free radicals in the reactions. An effective technique for measuring free radicals is desirable
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