Correlation of tar yields from a fixed bed coal pyrolysis reactor with in-situ e.s.r.-spin population measurements

Abstract

Changes in pyrolysis tar yield with increasing sweep gas superficial velocity were measured in a fixed bed reactor operated at 10 K min −1 at temperatures up to 500 °C. Tar yields were observed to increase with gas velocity by about 5 wt% when the superficial velocity was changed over the range 0.15–2.5 m s −1. Examination of average chemical structures of the tar suggests that at the highest flow rate used (2.5 m s −1), entrainment of tar volatiles from coal particle surfaces may be taking place, contributing to an increased yield of volatiles from pyrolysing coal particles. A parallel set of in situ e.s.r.-spin population measurements, using an identical time-temperature ramp were performed in a fixed bed quartz e.s.r.-cell, under vacuum and as a function of increasing carrier gas flow rate. Char free radical populations were observed to increase systematically with increasing gas velocity, suggesting that evolving pyrolysis volatiles quench char free radicals if allowed to remain in contact with the pyrolysing solid. Qualitatively, similar temperature dependences of the e.s.r. signal were obtained from chars produced during the two different sets of experiments.