Determination of pyrolysis behavior of brown coal in a pressurized drop tube reactor

Abstract

Pyrolysis of a pulverized German brown coal was investigated using a new high pressurized drop tube reactor. Influence of temperature (600, 700, 800 °C), pressure (25, 60 bar (g)) as well as residence time (variation of heated reactor length: 500, 1500, 2500 mm) on pyrolysis product yields as well as char and gas composition were evaluated. Particle heating and residence times of gas and particle in the heated reactor section were calculated based on the obtained data. To trace the pyrolysis progress during particle drop in the reactor tube gas sampling over reactor height was done. A good reactor performance and reliable results for the pyrolysis experiments could be obtained for the new reactor equipment. Only a slight pressure influence on product yields, char composition, and gas composition was found, which was contrary at the different temperatures. This indicates competing reactions taking place at the given conditions. Gas species evolution over the reactor height is mainly promoted by rising temperatures. The long gas residence times in the reactor and the obtained CH4, H2, and C2 and C3 hydrocarbon yields are a result of secondary cracking of tar and gaseous hydrocarbons at temperatures above 600 °C. Furthermore, it was found, that heterogeneous gasification reactions take place at higher temperatures as well. Comparison between thermogravimetric experiments in “drop-in” mode and the DTR results show significant differences attributed to heat transfer limitations in the particle bulk (TGA) and to the long gas residence times (78–368 s) in the drop tube reactor.