Tar evolution from coal and model polymers: 2. The effects of aromatic ring sizes and donatable hydrogens

Abstract

Several ethylee bidged polymes cntaining benzene, naphthalene, anthracene, and hydroaromatic rings have been synthesized. Pyrolysis data for these polymers have led to an improved understanding of coal pyrolysis and have been used to validate and extend the tar formation model recently proposed by Solomon and King 1. It has been found that the molecular weight ranges of tars are controlled to first approximation by the temperature where tar is evolved and that tar evolution temperatures are determined by the stability of the bridging bonds between the aromatic rings in a polymer chain. For ethylene bridged polymers, tar formation appears to be rate-limited by homolytic cleavage of the central carbon-carbon bond in the bridges. Ethylene bridges adjacent to large aromatic rings tend to decompose at lower temperatures during pyrolysis of both coals and polymers. The rate constants for cleavage of these bonds are in good agreement with those calculated using thermochemical kinetics. Char and tar yields from these polymers seem to be determined by the availability of donatable hydrogens to ‘cap’ the free radicals formed during bond homolysis. Polymers which break a lot of bonds during tar formation and also have limited supplies of donatable hydrogens tend to produce more char than polymers with abundant donatable hydrogens. An improved tar formation model has been developed using Monte Carlo solution techniques. This model predicts char and tar yields and molecular weight distributions directly from polymer structures and the chemical rate laws for cleavage of the bridging bonds and no longer uses an adjustable parameter to simulate the consumption of donatable hydrogens. Preliminary results with this model indicate that ipso substitution, radical recombination, and high temperature crosslinking reactions play important roles in the formation of tar by ethylene bridged polymers.