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Groundwater pollution is considered the most serious potential environmental risk related to the underground coal gasification technology (UCG). A variety of hazardous water-born contaminants have been identified during different UCG operations conducted so far, and in some locations long-term groundwater contaminations were observed. Characteristic organic UCG-related pollutions are mostly the phenols, benzene with its derivatives, polycyclic aromatic hydrocarbons, and heterocycles. In the inorganic array, ammonia, cyanides, sulphates, and heavy metals are usually identified. Although the fact that the coal rank affects the contaminants’ formation and release processes during the UCG is known, the information is still scarce. The main goal of the study presented in the paper is the comparison of the processes of formation and release of water contaminants produced during UCG simulations on hard coal and lignite. Significant differences in the qualitative and quantitative description of the contamination profiles were identified for both types of coal. From the analysis it follows that the formation of contaminants is the function of coal rank, the elemental composition of coal, and the gasification temperature. In case of hard coal gasification, the total load of inorganic and organic pollutants in the process water is substantially higher in comparison to lignite. It has been identified that the reaction pH is the parameter affecting concentrations of heavy metals in process waters. The macromolecular structure of coal and temperature were recognized as the main factors governing the distribution of organic compounds. The highly aromatised structure of hard coal becomes the source of substantial quantities of aromatic compounds; still the contribution of smaller species (one or two rings) is high. For lignite, the relative contributions of higher aromatics are greater as compared to hard coal.