Influences of particle size, ultraviolet irradiation and pyrolysis temperature on stable free radicals in coal

Free radicals play an important part in coal utilization, such as carbonization, gasification, liquefaction and pyrolysis processes. The diagenesis of organic sediment, pyrolytic reactions during metamorphosis and radiolysis are the three possible geneses of stable free radicals in coal. The influences of different origins on the nature of free radicals and their subsequent behaviors during the coal utilization are olgreat interest. In this paper, three experimental studies.of super-fine comminution, fixed-bed pyrolysis and in situ ultraviolet irradiation were adopted to investigate the behaviors of the paramagnetic centers in coal/char. The nature of different radical species in coal/char during the influential processes was focused through the deconvolution study of the electron paramagnetic resonance (EPR) spectra. Final results indicate that the comminution, pyrolysis and ultraviolet (UV) irradiation are all effective ways to initiate the free radicals. The super-fine comminution has noticeable influence on the aromatic hydrocarbon radicals. The UV irradiation can promote the formation of the a-type oxygen-containing radicals in coal. Furthermore, the simple aromatic clusters and the sigma-type oxygen-containing radicals are the most active types during thermal degradation of pulverized coal particles. (C) 2014 Elsevier B.V. All rights reserved.