Abstract

Investigations at the Cospuden–Zwenkau dump, in the southwest of Liepzig in Saxony, Germany, show that as sources for CO2 in the lignite open-pit mining dumps, both lignite particles as well as carbonate dissolution come into question. The lower dump material was laid down as the waste material after lignite mining and was set down in the form of ribs parallel to the exhumation direction; it makes up about two-thirds of the dump mass. The surface of this first component was allowed to sit fallow for many years and so had an intensive air admittance before the upper cover material, made up of local materials, was superposed. The residual large quarries, caused by coal excavation, were flooded and converted into lakes. The change in the lake levels with time and the horizontal influx of lake waters into the dump mean that carbon dioxide is altered from free-phase gas to gas in water solution with concomitant changes in dissolution and outgassing capabilities.

As a consequence of redumping the mining spoils, the original formations were destroyed, as were the various water transport pathways through the geologically deposited rocks, and the redeposition was a mix of unconsolidated spoil material. Because of aeration (groundwater withdrawal), naturally occurring reducing anaerobic regions were changed to aerobic oxidizing conditions. Near the formations impeding gas exchange at the dump surface, a considerable amount of carbon dioxide would seem to be dissolved in water. Measurements of the carbon dioxide flow at the dump surface show no increase in outgassing rate relative to a control area outside the dump region.

The determination of the isotopic composition of the carbon dioxide taken from many probes through the dump is inconsistent with the origin of the gas from various sources. The negative δ13C values (between −24.5 and −26.5‰) imply a dominant organic origin instead of one from carbonate dissolution.

Outgassing measurements show that, despite the higher carbon dioxide content of the lignite open-pit dump in comparison to ordinary earth, no enhanced emission rate is seen. For the organic and carbonate contents in the dump materials, one can calculate the maximum possible carbon dioxide development. From 1 t of lignite (water content 52% and coal material content 48%), about 582 m3 (20,553 ft3) CO2 result from burning (corresponding to about 1144 kg). Considering the fact that the dump is composed of about two-thirds of lower dump materials and about one-third of upper dump material, there is about 29 kg of coal material per tonne of mixed dump material. From this figure, one can obtain about 106 kg (54 m3) of carbon dioxide. Because the extraction of each tonne of lignite required 3 t of mining spoils, the carbon dioxide amount that could be produced by direct burning of lignite in the atmosphere can be further increased by about 30% as a consequence of oxidation and carbonate dissolution processes in the dump itself.

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