The Pathways for Thermal Decomposition of Coals with High Content of Sulphur and Oxygen

Ludmila Butuzova, Vladimir Safin, Stefan Marinov, Natalia Yaneva, Oksana Turchanina, Genadij Butuzov

Donetsk National Technical University

Bulgarian Academy of Sciences, Institute of Organic Chemistry

L. M. Litvinenko Institute of Physical Organic and Coal Chemistry, National Academy of Sciences of Ukraine

Reference: GeoLines 22, 2009 - s. 15-19.

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Abstract

This study was undertaken to obtain more definite information about the peculiarity of chemical reactions under the pyrolysis of coals with different cjontent of heteroatoms using the pairs of low- and high-sulphur samples of the same rank (76-88% Cdaf) in parallel experiments. Products were characterized by the standard chemical methods, FT-IR spectroscopy, extraction, liquid and gas chromatography. The results suggest that the sensitivity of pyrolysis products yield to changes of coal genetic type as well a coal rank is competing rates of volatile release and resolidification processes. The high content of active oxygen- and sulphur-containing groups appears to shift this competition in favor of greater volatile products release and slower resolidification. Phenolic OH (and SH) groups take part in ether bonds or C-C bonds creation by a dehydration reaction in a temperature range of 300-500°C. This agrees with the high contents of H2O, CO,CO2 in gas and asphaltenes in liquid and many -O-, -S- type bridges in solid semi-coking products. In contrast, pyrolysis of the good coking coals results in predominant formation of aromatic structures of semi-coke with potymethylene bridges and produces the gaseous products with high portion of H2 and CH4.

Experimental

Four pairs of Middle Carboniferous Donets coals of the same rank level but different genetic types by reductivity were investigated. According the Ukrainian classification they are low- and medium rank coals (DSTY 3472-96) which can be subdivided on D (Rr=0,49-0,71) and G (Rr=0.96-1,18) grades. Their rank parameters are in correlation with bituminous coals of the International Classification (ECE-UN Geneva 1998).

Petrographic composition of samples is uniform enough: 80-90% vitrinite. The samples of reduced (RC) and low reduced coals (LRS) were collected from the close coal seams (the distance between stratigraphic columns is less then 100m). The limestone of a layers roof and finely crystalline pyrite and data about the content of microlithotypes (carbopyrite) with a fine pyrite were used for selecting and testing of a type by reductivity. The coals were classed to a low-reduced or reduced type if the content of carbopyrite was less than 20% or more than 40% according to Butuzova at al. (2005a).

To solve this problem the samples of investigated coals were examined by thermal, physico-chemical analyses. The thermal behavior of coals was studied by classical Fisher method and pyrolysis in the water vapour stream.

The composition of solid pyrolysis products was measured by elemental, technical, petrographic analyses and DRIFT-spectroscopy. The petrography, proximate and ultimate analyses of the samples, including the total (St), organic (So), pyritic (Sp) and sulphates (Ss) sulphur were determined by standard methods. IR-spectra of solid samples were recorded on a FT-IR «BioRad» FTS-7 spectrometr with DRIFT technique.

Extraction, column chromatography and gas chromatography – mass spectrometry (GC-MS) of the saturated and aromatic hydrocarbon fractions were used to the separation and identification of liquid pyrolysis products (Safin et al. 2005). The determination of semi-coking gas composition was performed on apparatus VTI.

Conclusions

There have been shown the differences in the origin coals, semi-cokes and cokes structure and microstructure for low-reduced and reduced samples and the possibility to change them by the chemical pretreatment.

RC and their semi-cokes are distinguished for the greater ratio number of the -S- and -O- and Car-H groups. Their semi-cokes and cokes are a weak-coked material with less compactness, a higher volume of pores in comparison to semi-cokes of LRC type coals.

The results of this paper prove that the dependence of the coal structure and reactivity on genetic type is fairly strong. The reactions of -S- and -O- containing groups affect the course of thermodestruction processes permitting directed variations in the yield and composition of liquid, solid and gaseous products, the degree of COM conversation.

The results presented in this paper prove that during thermodestruction of coking coals the processes related with the cleavage of ether groups and with changes in the package method of the aromatic rings via the polymethylene bridges are of considerable importance.

This results evidence that existing coals classification system can be improved if the coal type by reductivity is taken in consideration.

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