Pollutants release from the residues remaining after underground gasification (UCG) of ortho- and meta-lignites

Abstract

One of the most significant environmental risks related to underground coal gasification (UCG) is groundwater contamination, which could potentially occur after process shutdown. This work comprises the results of laboratory tests on characteristics of UCG solid residues which were produced in the ex situ underground gasification of ortho- and meta-lignites and which may be a potential source of pollutants release to the surrounding area. Two multi-day trials were carried out in a large scale UCG ex situ reactors. The gasification tests were conducted in an artificial coal seam in the atmospheric pressure system. Two different types of lignite were gasified, i.e. meta-lignite from Velenje Coal Mine (Slovenia) and ortho-lignite from Oltenia Coal Mine (Romania). The aim of this study was to examine whether the type of lignite has an influence on the composition of the post-gasification residues, including contaminants that can be leached to the surrounding aquatic environment from the cavity formed as a results of UCG. The solid post-process residue samples obtained from different sections of the reactor were characterized and the leaching behaviour of some organic and inorganic parameters present in post-process residue was investigated. Additionally, the partitioning of metals and metalloids during underground gasification of two types of lignites (ortho- and meta-lignite) was discussed. The effect on lignite properties on the behaviour of metals and metalloids in the process of gasification and leaching to the water phase was also confirmed in the study. Leaching tests have been performed in terms of evaluation the capability for inorganic and general organic compounds elution from post-process residues after UCG of lignites. The study shows that samples originating from various parts of the UCG reactor differ strongly and may release contaminants to the vicinity of the georeactor. However, no dangerous trace elements such as Cr, Cd, Hg and Pb occurred in the aqueous extracts, which may indicate that in the post-operational phase, the UCG of lignite is a process safe for the aquatic environment.