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Engineering: Underground Coal Gasification for Sasol and Eskom

 





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South Africa is endowed with about 192-billion tons of coal reserves of which 32-billion tons are viewed as economically extractable. Until quite recently, there was little prospect of exploiting this pent-up energy potential
Bohlweki-SSI Environmental was appointed by Sasol as well as Eskom to undertake Environmental Impact Assessments (EIAs) for these companies who are particularly interested in the 160-billion tons in resource the country has no current plans to exploit.

The technology that could potentially unlock this potential is known as Underground Coal Gasification, or otherwise known as “UCG” which is now been tested locally.
What is UCG?

Underground Coal Gasification is a method of converting unworked coal, deep underground, into a combustible gas, which can be used for industrial heating, power generation or the manufacture of hydrogen, synthetic natural gas or other chemicals. The gas can be processed to remove the CO2 before it is passed on to end users, thereby providing a source of clean energy with minimal green house gas emissions.

The concept of gasifying coal underground and bringing the energy to the surface as a gas for subsequent use in heating or power generation has considerable attraction.
UCG is achieved by drilling two boreholes from the surface, one to supply oxygen and water/steam, the other to bring the product gas to the surface. After the coal has been ignited, oxygen and water/steam are pumped into the injection well to create a controlled burn.

Development of UCG to date

UCG is conceptually very simple but the development of a working system has proven to be more difficult in practice. The main problems are drilling the boreholes, controlling the reaction within the seam and producing a gas of a consistent and high quality.

The gasification of coal seams in situ was first developed in the former Soviet Union during the 1930’s and commercial-scale schemes have been operating since the Second World War; one project in Uzbekistan is still operating today. A pilot plant has also been successfully operated since 1999 in Chinchilla, Australia.
UCG is of growing interest in the largest coal producing countries, such as China and Australia, where new trials and supporting studies are underway. The prospect of international collaboration is growing, and a successful demonstration of this technology in countries such as South Africa could offer interesting export opportunities.

How clean is UCG

Although the EIAs to be undertaken for the Sasol and Eskom UCG projects will provide confirmation on the possible negative environmental impacts associated with UCG [i.e. noise levels, air pollution, the effects on soil (i.e. subsidence) and water, loss of captured Co2 through fissures in the earth surface etc], the following were identified and assumed as environmental advantages associated with this coal recovery method:
No above-ground scarring of the earth;

It extends the lifespan of the complex by using coal that is difficult to mine;
The UCG technology proved a 95% coal extraction and far exceeds that of conventional mining (the current extraction percentage is 37%);

It is a commercially competitive energy supply;

No handling and storage of large volumes of ash and slag in ash dumps is required, that are exposed to ash leaching, and have the potential to cause soil contamination;
Decrease in environmental footprint (i.e. no large tracks of land are not buried under overburden rock and tailings dumps and their would be no acid mine drainage caused by reactions of the overburden rock with atmospheric water and air);
There are numerous usages of the gas (e.g. gas turbines for electricity generation, fuel gas, raw gas etc; and
There is reduced coal wastage / increased efficiency

The climate imperative

The other big assumption is that UCG offers a coal based solution that is climate friendly. At present, more than 70% of South African energy comes from coal, and Bohlweki-SSI is of the view that this technology may become very important in future when calculating the cost associated with carbon-dioxide emissions under the Kyoto Protocol.

UCG could be an attractive choice for companies to obtain carbon credits. It further provides the opportunity to extract coal that would otherwise be un-minable as improved drilling techniques now allow for access to larger coal volumes at reduced cost. At this stage, it is perceived to have a lower environmental impact than other mining or coal utilisation processes. This will however be confirmed by the environmental assessments to be undertaken by Bohlweki-SSI and its team of specialists.

 
 
 
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