Effect of cell geometry on the in situ measurement of e.s.r.-free radical concentrations during pyrolysis of a ZnCl 2-impregnated coal

Abstract

Changes induced by ZnCl 2-impregnation in the development of spin population profiles during the in situ pyrolysis of a bituminous coal were investigated as a function of cell geometry, carrier gas flow rate and heating rate. At a heating rate of 10 °C min −1, ZnCl 2 was observed to lower spin populations over the whole temperature range (up to 500 °C) for all cell geometries. At the higher heating rates (1.5–4 °C s −1), small increases in spin population were observed at slow carrier gas flow rates (<0.15 ms −1), whereas, at 2.5 m s −1, the role of ZnCl 2-impregnation could not be evaluated with accuracy, due to agglomeration of untreated coal samples in the flow cell. Direct comparison with previous work appears difficult, owing to the extreme sensitivity of spin populations to cell geometry and local hydrodynamic conditions. It is noted that the observed changes in free radical populations are not necessarily compatible with proposed Friedel-Crafts type reaction mechanisms, where reaction intermediates are assumed to be carbonium ions.