Pyrolytic toluene conversion to benzene and coke over activated carbon in a fixed-bed reactor

The catalytic decomposition of tar compounds is an important issue in the cleaning and upgrading of gasification product gases. This work focuses on toluene pyrolysis in a quartz tube reactor with a fixed bed of activated carbon. A toluene to benzene and coke conversion mechanism is also proposed. Liquid decomposition products were analysed with GC-FID. The ash content in the spent carbon was measured, allowing for the determination of coke deposit growth on the carbon surface. Pyrolysis runs were performed at temperatures ranging from 650 to 800 °C and with varying toluene feeding times. It was determined that toluene conversion is maintained above 90% for the first 20 min of pyrolysis at 800 °C. At this initial time, pronounced coking on the carbon surface was observed. Subsequently, toluene conversion decreased, coke deposition was hindered and benzene, a decomposition side product, yield was greatly enhanced. At lower temperatures coking is inhibited and toluene conversions were relatively low. For short, 10 min feeding times and at low temperatures, the benzene share of overall converted toluene is significant. The toluene to benzene conversion model, necessarily based on both experimental results and the literature, shows that coke and benzene creation are competing reactions.