A comparative study on thermal behavior of functional groups in coals with different ranks during low temperature pyrolysis

Abstract

Pyrolysis has widely found applications in the processes of coal conversion and utilization. Good understanding of its reaction mechanism contributes to the optimization of coal conversion process and the controlling of the emission of pollutants during coal utilization. The evolution of the six main functional groups in coal with different ranks during pyrolysis was compared based on the data from in-situ transmission FTIR. As the increase of the coal rank, the initial temperature for the primary decomposition of aliphatic group rises and the remaining proportion at the final temperature gets bigger due to the more stable aliphatic structure in the high rank coal. The evolution of methyl, methylene and methine presents more detailed information in different rank coal. For the decomposition of aromatic CH in the three relatively low rank coals, there are two stages: the decreasing stage and the increasing stage with temperature rising. The temperature range of the decreasing stage depends on the interaction of coal molecules, while the growth extent of aromatic CH in the increasing stage can indicate the degree of the substitutions at the aromatic rings. Both aromatic CC and CO in the two high rank coals decrease slower in the high temperature stage than in the low temperature stage, while the situation is opposite for the two low rank coals. The number of CO type decrease with the rising of coal rank. Among the six functional groups, OH is the most insensitive to the coal rank. For kinetics analysis, the pattern of activation energy for the loss of the four functional groups in different rank coals is presented. The activation energy for the decomposition of aliphatic groups and OH in the high temperature stage is much higher than that (<50 kJmol−1) in the low temperature stage. The decomposition of CO and CO shows relatively low activation energies in all stages. The kinetic models are mostly one way diffusion and nth order chemical reaction and the pre-exponential factor appears obvious compensation effect with the activated energy.