Structural characterization of tar from a coal gasification plant: Comparison with a coke oven tar and a crude oil flash-column residue

Abstract

Residual tar from a coal gasification plant was characterized using a wide array of analytical techniques. Structural comparisons were made with a coke oven tar and a crude oil flash-column residue. The gasification tar residue was found to have a carbon aromaticity >94%, with extensively dealkylated aromatic structures and only small amounts of oxygenates, compared with the 56% carbon aromaticity and significant occurrence of alkyl substitution in the coke oven tar. The proportion of material boiling > 450°C as well as the elemental carbon and ‘fixed carbon’ contents of the gasification tar residue were also higher than those of the other samples. As expected, the petroleum flash-column residue was mainly aliphatic ( f a = 0.12), with small aromatic ring systems attached to long alkyl chains. For all three samples, MALDI mass spectra showed a signal at molecular masses > 20 000 u, with the gasification tar residue showing the narrowest range of molecular masses. Taken together, structural differences between the gasification tar and coke oven tar reflect the more severe thermal treatment and chemical history of the gasification tar, leading to the survival of low amounts (∼ 1 wt% of coal feed) of an apparently, chemically, very stable residue.