Underground energy - underground gasification of coal
With soaring costs, high emissions and problematic waste materials, it’s no wonder coal is under continuous environmental scrutiny. But a new technique is surfacing that could change all that – can coal really give renewables a run for their money? Michael Forrest reports.
The use of coal as a prime energy source has been on the increase over the past 10 years, in response to high demand for electrical energy that, despite high investment, has outstripped the ability of renewables to satisfy. According to the International Energy Agency, the use of coal in energy generation will increase by 55% by 2030. Coal burning results in high emissions, mining it is expensive, and the waste materials require environmental management. Although oﬀering relatively clean and, in some countries, cheap energy, other forms of hydrocarbons, particularly shale gas, have the unresolved antipathy of public opinion over fracking.
There is an alternative, however, in the underground gasiﬁcation of coal (UGC). The process was ﬁrst identiﬁed in 1868 by Sir William Siemens, whose objective was to recover gas from slack (ﬁnegrained coal) and waste. Russian chemist Dmitri Mendeleev continued research, but the most progress was made by the Skochinsky Institute of Mining in the 1930s.
The process relies on the controlled underground ignition and partial combustion of coal, which raises temperatures and causes chemical reactions that produce coal gas, leaving the incombustible portion of residue (ash) behind. This is the same technique that was used in the UK until the 1960s when coal gas was produced for home use and street lamps, except in this case it takes place underground. The technique involves drilling an injection borehole to enter the coal seam, and another to allow the product gases from gasiﬁcation to be piped safely to the surface.
Site selection is critical for successful implementation of this process. The coal horizon must be sealed by the under- and overlying lithology that must be impervious to water and gas, thereby providing an underground gas-tight chamber. By managing the amount of air or oxygen entering the system, the rate of gasiﬁ cation can be controlled, maintaining optimum ignition rate for gas generation. Cutting the oxygen supply stops the process. The gas generated, syngas, is a mixture of carbon monoxide, hydrogen, CO2 and methane. Syngas can be easily treated to remove CO2 and any particulates, rendering a cleaner supply of gas that can be piped to a new or existing power station. As conversion takes place underground, there is very little in the way of waste products at surface.
The ideal location for UGC requires a reasonably thick coal seam that is undisturbed by faulting and other geological discontinuities, and with an ash content of less than 45%. High-volatile bituminous coal is sought, but caloriﬁ c quality is not a determining factor and low-grade coal can also be used.
Trial and error
Underground coal gasiﬁcation began commercially in the 1930s in the Ukraine and Uzbekistan. In the former, gasiﬁ cation was trialed in thin, steeply dipping seams in Donbass, Kusbass and Moscow coalﬁ elds, with mixed success. However, in the Angren coalﬁeld in Uzbekistan, a gasiﬁ cation project was based upon lignite with 30% moisture content and 11% ash. The seams dipped at 5–50° in a seam measuring 4–24m thick, at a depth of 110–250m. The recovered gas fuelled a power station, but it was not until 1961 that the plant was reopened and 100MW of power from syngas was produced.
Trials were undertaken in Europe and the USA, with the Lawrence Livermore National Laboratory in California taking a lead role. Some trials were more successful than others, most notably the fourth series from 1984–1989, when 14,000t of coal in Arizona was gasiﬁed in 93 days.
Early methods of drilling used vertical holes for pumping air, water or oxygen down to the coal bed, relying on natural porosity for the generated gas to ﬂow to the exhaust vertical drilling. In some cases explosives were used to provide better gas ﬂow, while in others the injection borehole was moved forward down-strata to continue gasiﬁcation. However, it was not until the directional drilling technology developed in the oil and gas industry that gasiﬁcation came of age. This allowed in-seam drilling, providing a conduit for generation and recovery of syngas.
Hungry for energy
The choice of coal deposit is paramount. The seam should be suﬃciently deep and free from any faulting that would prevent the syngas from dissipating into the host geology. ‘This can be a challenge,’ says Dries du Plooy, Exploration Manager at Wildhorse Energy Ltd, an Australian-listed company with a gasiﬁcation project in Hungary. Wildhorse is developing a UGC project based on the Mecsek coalﬁeld in the south of the country. Aside from coal, Hungary is an energypoor country importing oil and gas from Russia, so oﬀers a ready market for syngas.
The Mecsek ﬁeld is a Mesozoic coal complex, which is a remnant of pre-Tertiary rocks within the Tertiaryaged Pannonian Basin. The area has seen signiﬁcant historic coal-production over a period that ended in 2004, yielding approximately 300Mt. However, not all areas of the ﬁeld were mined and some 1,000Mt of coal has been identiﬁed within the northern section.
The regional tectonic history is complex, with four major extensional and compressional deformation phases identiﬁed of late Permian to late Tertiary age. Coal-bearing horizons exist within three horizons formed around 200 million years ago, which contain between 10 and 42 seams greater than 0.5m thick. Of greatest interest for UGC is the middle unit, which measures 200–500m thick. ‘The northern part is of greatest interest, particularly the unmined sections,’ says du Plooy. Here Wildhorse has identiﬁed large deposits through historical drilling records. Conﬁrmation of its suitability for UGC required detailed 3D geophysics, as the structural complexity due to extensional and compressional tectonics of the Mecsek coalﬁeld is the principal risk to the project. Having a clear understanding of the distribution of the major faults is, therefore, essential. This geophysical exploration revealed that the Váralja area, on the northern margin of the coalﬁeld, has the basin’s largest known coal deposits.
The survey also revealed that within the Váralja section, some 20 seismotectonic blocks were identiﬁed within the coal formations that seem to be internally unaﬀected, although their boundaries were usually fault-controlled. ‘The quality of the seismic surveys was much better than expected considering the terrain,’ says du Plooy. Here coherent and continuous reﬂectors can be identiﬁed, which are in agreement with geological sections inferred from earlier exploration wells. To the south of exploration area the strata are heavily tectonised, again showing good spatial correlation with borehole data. This new tectonic model shows that the heavily tectonised nature of the Mecsek coalﬁeld in the southern Váralja area is of utmost importance for the future planning of the UGC project, which relies on the presence of continuous and undisturbed coal bodies.
‘Our prefeasibilty work has shown that good syngas yields can be obtained through directional drilling and the injection of oxygen,’ says du Plooy. ‘We have an agreement for oxygen supply and are in talks with energy companies with regards to oﬀtake agreements.’ The technical team at Wildhorse has experience with Sasol and Eskom, the South African coal-to-oil and power-generation companies that did much to commercialise coal-to-liquids technology. UGC has the ability to reach many uneconomic coal resources. The process leaves the ash, char and sulphur within the gasiﬁed coal horizons, and as the host rock is volumised by the process there is little threat of surface movement. The surface footprint of the process is small compared to mining, and the syngas can be easily processed to a cleaner fuel. It also takes out the major mining, handling and transport components of a conventional coal mine – not to mention their costs.
Dries du Plooy - tel +36 72 511 413