The use of UCG gas as a fuel for advanced clean coal technology power generation, has the potential to address a number of strategic drivers for Eskom and South Africa :
UCG enables an independent, long term fuel source by being able to potentially extract the substantial unminable coal resources in South Africa. This has obvious primary energy inventory advantages, and less obvious advantages due to the broader geographic spread of such unminable coal. The Eskom pilot plant has proven this for the Majuba coalfield, and this now needs to be extrapolated for other coalfields in South Africa.
UCG enables significant particulate, sulphur and nitrous oxide emission reductions compared to conventional power generation. Eskom’s research has however highlighted that its carbon emissions are determined by the geological conditions and quality of the coal, thus requiring very careful site selection.
UCG effectively shortens the coal value chain, by reducing the number of steps between the mining of the energy resource and the generation of electricity. This has obvious advantages for the cost of electricity, as well as additional less obvious advantages for safety and the environment (due to the absence of handling and transportation of solids).
The significantly improved mining efficiency of UCG compared to other conventional underground coal mining technologies also has obvious primary energy inventory advantages, and less obvious safety advantages due to the absence of people underground.
UCG has the potential to supply a baseload fuel source from the Majuba coalfield in Mpumalanga. Conceptually, the combination of several different users, each having synergistic fuel requirements, could enable peaking, mid-merit or baseload power generating options. This will require further research.
Several of the above drivers are expanded upon below.
a) Potential application in South Africa
UCG technology typically has the following coal resource requirements :
Coal seam thickness from 0.5 m to 30 m.
Dip from 0o to 70o.
Depth from 30 m to 800 m
Calorific value (Lower Heating Value -LHV) from 8.0 to 30.0 Megajoules per kilogram (MJ/kg)
Coal rank from low-quality lignite to bituminous
The technology can also tolerate geological faulting or displacement by dykes, by virtue of modularity, mobility, low surface site establishment costs, and re-usability of some components.
The above requirements are generally the opposite of the requirements for conventional underground mining, and hence offer an opportunity for expanding South Africa’s mineable coal reserve base by extracting coal previously disregarded as being unminable. To quantify this, almost three quarters of the country’s coal resources are presently regarded as conventionally un-minable, but could be extracted using UCG technology.
Figure 3 South African Coal Resource Map, indicating potential UCG sites (circled)
UCG technology effectively extends South Africa’s coal reserves and broadens the geographic availability of sites for new power plant (illustrated in part by Figure 3), compared to the availability of presently mineable coal reserves. This enables far more strategic positioning of new power generating plant, to support demand side needs and stabilise the transmission network.
Many of the areas indicated in Figure 3 show definite potential for UCG application. Apart from its Majuba UCG site, Eskom has already completed a detailed conceptual study of a second site with significant potential.
a) Environmental Performance
The outcome of a highly idealised life cycle assessment of current and future fossil-fuelled power generating options is depicted in the figure below. Eskom has concluded from its research on the Majuba site that the specific CO2 emissions are primarily affected by the auxiliary power consumption, which is dictated by the underground operational pressure. This in turn is limited by the need to maintain operational pressure below hydrostatic pressure to safeguard the environment (discussed below). The hydrostatic pressure is determined by the depth of the coal seam below the water table. Therefore the geological setting plays a crucial role on determining the CO2 emissions from a UCG plant. Eskom’s research is now focussing on optimising the fuel specification from its Majuba pilot plant, to determine the minimum CO2 emissions with this particular site. Other sites with more favourable geological settings are also being considered.
IEA CCC Report CCC/32, Simon Walker,June 2000
Figure 4 Emissions performance of fossil-fuelled power stations
UCG consumes water in the gasification process, to produce hydrogen. This does entail consumption of the water in the coal seam, and in the immediate surrounding strata. The underground aquifers are therefore closely monitored to ensure no impact on aquifers closer to the surface that may be in use for domestic or agricultural purposes, or may evacuate into surface streams. Apart from the usage aspect, there is also a risk of contamination of aquifers and water bodies with UCG products. This risk is mitigated by maintaining a negative hydraulic gradient into the underground cavity (refer to the figure below), thereby forcing removal of UCG products with the resulting water influx. The UCG products are removed via the steel-lined wells to surface process plant, designed to separate and clean the products. The steel lining of the wells ensures separation from surrounding aquifers and strata.
Eskom is applying this technology approach (called eUCG, which is proprietary to Ergo Exergy Technologies Inc. of Canada) in the Majuba pilot plant. The Ergo Exergy technology is based on expertise gained over several decades in the former Soviet Union commercial UCG plant.
ACARP Project C9058, “Coal in a Sustainable Society”, Case study B20, BHP Billiton, Australia, July 2002
MS Blinderman, SR Fidler, Proc.Int.Conf. Water in Mining 2003, Brisbane, Australia, October 2003
Figure 5 A simplistic view of the UCG process, showing how it is controlled with a negative gradient into the cavity to reduce contamination risks
The UCG process is analogous to conventional longwall underground mining, and surface subsidence is therefore expected unless pillars are deliberately left behind to support the overlying strata. In the case of coalfields that are geologically disrupted by dolorite intrusions (dykes), these structures would naturally provide similar support to pillars. The extent of pillars or structures is inversely proportional to mining efficiency.
Subsidence monitoring equipment is typically installed around the UCG process, to measure rates and extents of subsidence. The undermining of surface infrastructure and natural features (such as rivers) can be avoided, as UCG direction and extent can be accurately controlled underground by the orientation of air injection. This can be explained by bearing in mind that the coal seam is far under the water table, and the air injected into the wells is the only source of air in the coal seam. One of the basic principles of gasification/combustion then comes into play, being that the UCG process will naturally proceed in the direction of the air source.
The UCG process leaves behind the ash associated with the coal, as there is insufficient velocity to convey the bulk of this ash to the surface. The ash remaining inside the UCG cavity therefore partially fills the mining void, and reduces the extent of subsequent goafing and subsidence by acting as a buffer. This contrasts with conventional mining, where the ash is removed with the coal to the surface, hence leaving no buffer.
a) Cost of Generation
UCG effectively shortens the conventional mine-to-power station coal value chain, as illustrated in the following figure, by reducing the number of steps between the mining of the energy resource and the generation of electricity. The shorter value chain translates to lower cost, which has been confirmed in Eskom’s concept-level studies and international costing comparisons.
The UCG-IGCC technology also enables modular expansion, which has further cost and lead-time benefits.
Schematic courtesy of Ergo Exergy Technologies Inc.
Figure 6 Conventional Mining vs. UCG – the Coal to Power Journey
a) Coal Resource Recovery
UCG offers a higher coal mining efficiency than conventional underground mining does. With the usage of fire as a mining method, the Eskom Majuba UCG pilot plant achieved a mining efficiency of 83%. The reason for this is that the entire coal seam is consumed in the process, including any methane gas and combustible roof, floor and partings. In traditional underground mining, resources are intentionally left behind due to access, safety and quality reasons, resulting in a maximum mining efficiency of 75%.
b) Power Generation Options
UCG should be considered as a baseload fuel source, which can then be applied in various ways. There are however various factors to consider :
· Due to its dilute nature, it is undesirable to store UCG gas other than in the inherent storage capacity of the UCG cavity(s), the gas cleaning plant and the pipelines conveying the gas.
· UCG gas can be made less dilute if oxygen, or oxygen-enriched air, is used for the UCG process.
· If a rapid start-up gas turbine or reciprocating engine can use the UCG gas during peak power demand periods, then it could achieve mid-merit or peak power production which has significant advantages for South Africa. However, a use for the gas would be required during off-peak periods, as UCG production is baseload. This brings into consideration the potential for off-peak chemicals production, or co-firing the gas into a conventional coal-fired power plant and stockpiling the coal.
Eskom is initiating research into the combination of UCG with several users, each having synergistic fuel requirements, to enable peaking, mid-merit or baseload power generating options.