Self-heating-generated compounds release to water phase simulated by hydrous pyrolysis

            Spontaneous heating of coal waste rocks stored within the dumps is the worldwide phenomenon. It occurs in oxygen-deficient conditions that can be well simulated by hydrous pyrolysis. The process leads to production of new, relatively well water soluble compounds. They should be considered a hazard to the aquatic systems, both to the surface and groundwater since many older coal waste dumps are not isolated from below. However, the amounts of water soluble compounds produced and their fingerprint are not well recognized. In this project we aimed to identify types of compounds produced using hydrous pyrolysis as laboratory simulation of self-heating carried out in controlled conditions. This will allow for identification of distribution patterns of self-heating-produced compounds also in natural waters.

            Four mudstones from two coal mines, the Janina (subbituminous) and Marcel (bituminous) (Upper Silesia Coal Basin, Poland) were selected for hydrous pyrolysis. The experiments were conducted in 1-liter reactors (Parr Co.) in temperatures 250, 360, and 400^oC during 72 h. the procedure details are presented by Lewan et al. (2008). Amount of water added ranged from 200-380 mL. Dissolved organic compounds were isolated using solid phase extraction on C18 PolarPlus columns (BAKERBOND, 3g). Compounds were eluted with dichloromethane (HPLC grade). The compositions of SPE extracts was investigated with an Agilent 6890 gas chromatograph coupled with an Agilent Technology 5973 mass spectrometer.

            Hydrous pyrolysis released compounds such as phenols, carboxylic acids, aldehydes, and ketones, including numerous aromatic ketones and quinones, and S-heterocyclic compounds such as dibenzothiophenes. Phenolic derivatives, dominating in pyrolytic water phase (up to 60% of the total extract composition), comprised compounds from phenol (C₀) to C₄ phenols. The minimal temperature of phenol release, caused by the macromolecule cracking, was 360^oC. Water phase from 250^oC pyrolysis contained phenols in minor amounts only, and vitrinite, the main source of them, was not changed. The general composition of organic phase at this temperature corresponds to water leachates of Upper Silesia coal.

            Thus the major hazard to the aquatic environment is sites in coal waste dumps with self-heating temperature exceeding 250^oC and compounds indicating this pollution origin are phenols with cresols and xylenols domination in the distribution.

 

Acknowledgements

The financial support of the National Science Centre, grant No 2017/27/B/ST10/00680 is gratefully acknowledged.

Lewan, M.D., Kotarba, M.J., Więcław, D., Piestrzyński, A., 2008. Evaluating transition-metal catalysis in gas generation from the Permian Kupferschiefer by hydrous pyrolysis. Geochim. Cosmochim. Acta 72, 4069-4093.