State-owned power utility Eskom reports that a design study is currently under way for an underground coal gasification (UCG) demonstration plant, which will comprise a 250 000 Nm3/h gas production plant and a 100 MW to 140 MW gas turbine plant.
Research and development into UCG is increasing as more unmineable coal deposits are being found and owing to dwindling oil and gas reserves. The UCG Association describes the process as a method of converting unworked coal, while still underground, into a combustible gas, which can be used for industrial heating, power generation or to manufacture hydrogen, synthetic natural gas or diesel fuel.
Eskom’s Majuba project, located in the Amersfoort magisterial district, in Mpumalanga, was chosen for the project as its coalfield was found to be unmineable by conventional means, owing to dolorite intrusions, heat-affected coal and seam displacements.
The coal seam lies between 280 m and 300 m deep. Eskom’s studies show that the gas could be cofired with coal to power the 4 100 MW Majuba power station, providing direct use for the gas. Further studies show that gas could be used as a fuel for a modern, high-efficiency integrated gasification combined-cycle (IGCC) power station.
Meanwhile, with the increased pressure on mining industries to reduce pollution emissions, UCG is seen as advanced clean coal technology. Eskom reports that no UCG contamination is evident in water samples, compared with baseline water sampling performed in aquifers, including testing the coal seam. Water from surrounding aquifers is also monitored from shallow and deep boreholes placed in concentric rings around the process location.
Eskom reports that UCG is also being extensively monitored with piezometers for measuring aquifer pressure and temperature. The use of UCG-IGCC power stations show a decrease in greenhouse-gas emissions, compared with conventional power stations. Eskom undertook baseline air quality tests before carrying out UCG, while a permanent meteorological station on the UCG site monitors wind speed, direction, ambient temperature and barometric pressure.
Eskom’s continuous ambient emission monitoring stations adjacent to the UCG site monitor ambient gas species and meteorological parameters. A further comprehensive audit has been completed and a permanent monitoring site will be established on the UCG site.
Meanwhile, Eskom has done extensive studies on the land impact of UCG and is currently in the process of undertaking regular audits. A baseline fauna and flora survey and soil tests were undertaken before starting UCG. The Majuba site has shown that natu- ral vegetation re-establishes itself rapidly following drilling disruption and the surface impact is minimal postgasification.
Subsidence is also being monitored at Majuba, using devices for conventional underground monitoring, while the potential for strata collapse is also being modelled. There has been no surface subsidence as yet for the four-and-a-half years of the pilot UCG operations.