Behaviors of coking and kinetics of volatiles’ reaction during coal pyrolysis in a two-stage reactor

Abstract

The reaction of volatiles, especially the coking of volatiles, is the core reaction in the thermal conversion of low-rank coal. In this work, a two-stage reactor is designed and used to investigate the coking behavior of volatiles by adjusting the temperature and residence time of volatiles in second-stage reactor. The variation of tar properties and coke structure is analyzed by Fourier transform infrared spectroscopy (FTIR), electron spin resonance (ESR), and Gel permeation chromatography (GPC). The results show that the temperature of tar condensation (540 °C) is 60 °C lower than that of coke formation (600 °C). The oxygen-containing functional groups in tar can inhibit the coking of volatiles, although oxygen content is higher in the initial coking. The coking behavior of volatiles can be expressed by first-order + autocatalytic kinetics with activation energy lower than 40 kJ/mol when the high temperature volatiles produced by the first-stage reactor directly enter the second-stage reactor to generate coke. The coke is categorized into two types, one suspended in the tar (coke-tar) and another deposited on the wall of reactor (coke-wall). The conversion of coke-tar to coke-wall lead to a big error in the first order + autocatalytic kinetics fitting.