Reductive pyrolysis of Miocene-aged lignite humic acids, Bulgaria

Abstract

Atmosphere Pressure-Temperature Programmed Reduction (AP-TPR) technique coupled "off-line" with thermal desorption gas chromatography-mass spectrometry (TD-GC/MS) can be effectively used to specify qualitatively/quantitatively organic functionalities in humic substances. The AP-TPR technique has been initially developed and successfully applied for sulphur speciation in coals and in their biodesul-phurized products. The aim of the present study is to analyze volatiles in flue gases of a reductive pyrolysis of Miocene-aged lignite humic acids by AP-TPR "off-line" TD-GC/MS and to assign identified components to the structural compositions of the samples studied. Humic acids from two coal-bearing provinces in Bulgaria, i.e. Thrace and Sofia, have been considered. The following compounds were tracked and quantified in the flue gases released during the AP-TPR experiments: aliphatics, i.e. nC(6)-nC(14), mono unsaturated/saturated pairs, alkylated benzenes, styrenes, naphthalenes, biphenyls, fluorene, acenaphthylene; oxygen containing compounds, i.e. phenols, methoxy phenols, furans, benzofurans; sulphur containing compounds, i.e. sulphides, thiophenes, benzothiophenes; nitrogen containing compounds, i.e. pyrroles, indoles, quinolines, benzonitriles, etc. Total content of organics in pyrolysates amounts to 55.9 mg/g Corg for Thrace humic acids (Maritza-East deposit) and 55.0 mg/g Corg for Sofia humic acids (Stanjanci deposit). Alkyl benzenes (C-6-C-10) highly dominate in the programs up to half of the recorded components (46-58 rel.%). The low sample maturity is reflected in high abundance of phenols, C-6-C-8 (20 rel.%), methoxy phenols (2.7 rel.%) and degradation products from carbohydrates, i. e. furaldehydes, 2-cyclopenten-1-ones, etc. Plots of four group composites, i. e. aliphatics, aromatics, O-, S-and N-containing compounds, in "spider web" diagram seem promising for bulk interpretation of pyrolysates data. According to the reductive pyrolysis data, structural units of Miocene-aged humic acids are composed by 1-2 aromatic rings, condensed or coupled, including heteroatoms arranged in linear and cyclic forms. (C) 2015 Elsevier Ltd. All rights reserved.