Underground Coal Gasification – A clean indigenous energy option
Dr Michael Green, Director, UCG Partnership
Introduction
Events around the world, in energy and clean coal have been important drivers in the advancement of underground coal gasification (UCG) over the past 12 months. UCG is a gasification technology that is well placed to meet the high environmental emission standards and to provide lower cost carbon capture from the pre-combustion, high pressure gas which reaches the surface from deep coal seams. The energy chain for UCG also scores well environmentally, because the energy for transportation is negligible and there are virtually no leaks of methane and other gases to atmosphere (unlike Russian and other gas infra-structures). A conference on UCG, organised by the UCG Partnership in February 2007 in London, demonstrated the extensive interest in UCG around the world.
UCG has been around since the original experiments by Sir William Ramsey in the Durham Coal field in 1912, and the concept has reached the commercial stage on several occasions over the past 95 years. The FSU still has large UCG operations in place supplying gas to power stations, and detailed designs for ammonia and SNG plant were produced at the end of the US development programme in the early 1990’s, but were curtailed because of cheap natural gas. Europe and Australia revived interest with UCG trials in Northern Spain and Queensland and UK DTI undertook a series of supporting feasibility studies. By 2005, key opinion-formers began to accept the benefits of UCG as a clean coal technology with global warming credentials.
India and China and more recently South Africa are among a growing list of countries that recognise the potential of UCG as a national energy option. Previous experience of UCG, in contrast has tended to be with developed countries like Australia, UK, and the US, the transfer of expertise is underway. A variety of studies on UCG has started again in most of these countries, but compared with similar energy related activities like IGCC and CCS, the expenditure is still small. The recent UCG conference in London reported that feasibility studies are underway or completed in the Gujurat Region of India, the Firth of Forth in Scotland, the Majuba Coal field in South Africa and the Surat Basin in Queensland, Australia. Several of these have reached the exploratory drilling stage, and one new gas flaring was reported in 2007. Another significant development is the stock exchange offering of A$22M by Linc Energy in 2006 to extend the development of its UCG to utilisation opportunities in gas turbines and GTL plant.
A number of large players are watching the current developments in UCG. Some of these, like Statoil, Tullow Oil and the ABN AMRO Bank, became initial or founder members of the UCG Partnership. Membership has now grown to 32 organisations with other large companies like E.ON now on board. The UCG Partnership is already the centre of excellence for UCG, and brings together those with an interest in developing and promoting UCG as a clean coal technology. The Partnership is largely web based www.ucgp.com, it organises conferences and training and holds the definitive data base on UCG for its members.
This article will explain why UCG has recently become a significant energy option for coal and where UCG is going over the next year or two.
Coal and Security of Supply issues
The first is the recognition that in spite of renewables, nuclear options and the presence of still large supplies of natural gas around the world, coal will remain a supplier of large scale energy to the world’s economies, until 2050’s or beyond. This is new thinking for countries like the UK, where until recently a mixture of “wind-plus-gas” , was going to meet its energy requirements for the foreseeable future. Coal is in plentiful supply; the European reserve alone is 130BT and the US equivalent, which is the largest in the world, is 240BT. The US is committed to exploiting coal as a security alternative to Middle East oil. Even Europe, alarmed at its growing dependence on Russian oil and gas, is prepared to look at alternatives like coal and nuclear in its energy and research policies.
Calculations of the likely additional coal reserve that UCG would provide based on some conservative assumptions for the discount factors of coal resources have suggested that the current declared world coal reserve of around 1,000 BT would increase by an additional 600BT with UCG. The principle explanation is that the extraction of energy from unmineable coal, in deep seams, near shore and in the large lignite deposits, becomes economic with UCG. Expressed as equivalent natural gas, see figure 1, the gas reserves of countries like India, Australia and Europe would benefit considerably by the greater conversion of coal to UCG syngas.
The security-of-supply advantages of coal are generally accepted, but differences remain about the conversion route for power generation. Both supercritical thermal plant and IGCC coal plant are being installed in similar numbers, and both are capable of similar plant efficiencies, but gasification appears to be edging in the lead when CO2 CCS or so-called capture ready plant is also being planned. The advantages of pre-combustion capture from gasification apply equally well to UCG; in fact, the high concentration of the CO2 in UCG product gas makes separation more favourable.
Another question, particularly for the UK and Europe is whether indigenous coal, in spite of the still vast resources that still exist in the Member State Countries, should ever be extracted again in large quantities, because of the perceived environmental disadvantages of coal and coal mining. Here the benefits of UCG come into their own, because no coal or ash handling is required at surface and the network of UCG process wells are temporary and much less conspicuous than a colliery. UCG also avoids completely the safety issues and associated equipment of men underground.
UCG with CO2 Capture and Storage
In a carbon constrained world, UCG, must demonstrate that carbon in the fuel can be prevented from entering the atmosphere as CO2. Most UCG processes are oxy-fuelled, which means only CO2 and water are produced after combustion, thereby making CO2 separation simpler and cheaper.
In addition, the production gases are also open to pre-combustion capture, and can benefit from the high concentration and pressure of the CO2 in the product gas. UCG is unique as a gasification process, see table 1, in producing CO2 and methane as well as the carbon monoxide and hydrogen found in surface gasification. This is because lower temperatures and higher temperatures in parts of the UCG cavity favour the formation of methane. Much of the CO2 from UCG can be captured more cheaply than in other applications, and UCG has the option of supplying either pure hydrogen or methane/hydrogen mixtures. The latter have favourable burning properties in turbines and the costs of transmission are lower than for hydrogen alone.
A key area of future research is the study of UCG with local CO2 capture and storage (CCS). UCG-CCS creates voids in the coal and a highly stressed area above it. Under the right conditions, these could be suitable for permanent CO2 storage, and possible storage receptors for CO2 are the deeper coal seams in the vicinity of the UCG process and the use of the abandoned cavity and surrounding stressed area. The UCG conference also heard about a new EU project lead by the Central Mining Institute, Poland, which aims to direct the underground reactions towards hydrogen by chemically fixing the carbon in the operating cavity. The project, which has partners from Poland, Czech Republic, UK, The Netherlands and Germany, is approved with plans to start in 2007.
Modern UCG Technology
Underground coal gasification works by constructing vertical wells into a coal seam to supply the injection gases O2 and H2O, and to discharge a mixture of production gases, CO, H2, CH4 and CO2 to surface. While the principle is deceptively simple, control of the gasification process has been at the heart of UCG development over many years.
Most coals can be gasified in-situ but opening up the coal seam between the two vertical boreholes is necessary, and techniques such as fraccing, reversed combustion and electrical discharge have all been employed. The method, adopted in the FSU and used subsequently in the recent Australian UCG trial, rely on closely spaced vertical wells to move the combustion front through the coal field.
The alternative is directional drilling, which allows wells to be constructed at a precise horizon in coal seams and links these accurately to other wells which connect to surface. Bottom hole drilling assemblies have sensors to detect seam boundaries and even look ahead of the drilling bit to identify, in advance, faults and areas of unacceptable structure. Directional drilling and moveable injection was first used in the final US trials (late 1980’s) and taken to greater coal-seam depth in the European trials (1988-1998). These latter trials have also shown that moving the injection point within the channel, figure 2, gives greater control of the process and leads to a wider gas cavity and more efficient process overall. It appears that the latest tests of the FSU method, namely the flaring test in the Majuba coal field (2007), are using some form of directional drilling to improve access to the coal field.
The latest technology of UCG, whether it uses vertical wells only, directional drilling or moveable injection are now well proven at the pilot scale (4-25MW thermal output). It was clear from the UCG conference in London that a number of entrepreneurial and established organisations that are making feasibility studies of commercial schemes, seeking suitable sites, and defining development programmes.
Environmental Risk Management and Regulatory Issues
The environmental impacts of a UCG process are visual, acoustic, and include air emissions and groundwater effects. The inherent environmental benefits of UCG are the simplicity of surface plant, the absence coal storage and transportation, and the ability to remove minor polluting constituents such as SOx, particulates and heavy metals from the production syngas.
Like any geological extraction process, the geological and hydrogeological risks of UCG have to be carefully managed. Control has advanced considerably since the early UCG trials and most UCG processes now have active control of the operational conditions in the cavity to ensure an inward flow of groundwater and to prevent gas seepage. Site selection is also very important to achieve the necessary separation from ground water pathways and the isolation of the cavity by means of naturally occurring impermeable geological strata.
The European Groundwater Directive although not specifically written with UCG in mind, is likely to require that the groundwater surrounding the process is declared permanently unsuitable for other purposes like irrigation or animal consumption, and that the hydrogeology surrounding the process is monitored and modelled. Strict controls are imposed on the by-products of combustion produced underground, and the models will need to address the close interdependence between combustion control and contaminant fate and transport. Contaminant risk and product gas quality need to be integrated in prediction models to assess the environmental and economic constraints of potential UCG sites. Work to date has established that UCG in deep coal seams, in so called “permanently unsuitable water” are likely to satisfy current and future European ground water regulations.
Authorisation to operate a UCG plant in Europe would be made, according to the UK Environment Agency at the UCG Conference, under a Pollution Prevention and Control Permit, issued by the Environmental Authorities of the Region. This is likely to cover the UCG reactor, the surface plant and the implications for ground water. The Coal Authority also described its pioneering work on the coal licensing of UCG in the UK.
The conference drew attention to the risk management techniques that will need to be applied and the wide range of tools from around the world that can be used to provide insights into the underground process for regulatory purposes. Examples include the packed bed cavity models from India, the risk analysis techniques from the US, and the feasibility studies, seismic exploration, micro-seismic and project evaluation from the UK, US and Australia.
Concluding Remarks
UCG, until recently, was seen as an unconventional coal exploitation technology to be treated as a long term prospect for clean energy. The world has realised that the technology, which is largely from the oil and gas industry, actually works, the underground risks are manageable, and there are very good reasons to make the leap from proven pilot trials to commercial schemes. Its credentials as a source of clean gas, its use of indigenous coal resources with security of supply benefits and its potentially low environmental impact are the main reasons. UCG also offers potential new routes to CCS, which are likely to be cheaper, easier to manage and possible less fraught with legal challenges. The recent UCG conference in London showed that first movers are already in place to take advantage of UCG as a profitable commercial clean coal opportunity.
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