Going underground for new clean coal opportunities5 July 2002
The time has come for underground coal gasification, a technology with a surprisingly long but so far largely experimental history, to be considered as a serious long-term solution for clean fossil-fuelled power production.
Underground coal gasification is the conversion of unworked coal, deep underground, into a combustible gas. Currently this is achieved by injecting of oxygen and steam under pressure. The product gas can be brought to the surface and used for industrial heating, power generation or the manufacture of hydrogen, SNG or other chemicals.
The UCG process has been the subject of development and field trials since the early Durham, UK, experiments in the 1920s. Different techniques have been used, with varying degrees of success, to access the coal, provide the injection gases and extract the gasification products. The most active period was between 1960-1980, when both the Soviet Union and the United States were attempting to commercialise the process. Although large-scale schemes were developed in the former Soviet Union, and one is apparently still operating in Uzbekistan, interest began to wane in both countries as a result of the introduction of low-cost natural gas, and the growing environmental concerns about operating UCG at shallow depth. Nevertheless, a substantial body of new technology and operating experience had been developed during this period.
European interest during the same perion had been in the exploitation of the thinner coal seams of northern Europe. The principal UCG activities were the early UK trial at Newman Spinney in Derbyshire (1950-1955), Thulin in Belgium (1971-1976) and various smaller trials in France and Morocco. A trial was also planned by British Coal in Nottinghamshire in the mid 1980s but never carried out.
The EU decided to concentrate on the gasification of deeper coal seams by supporting a proposal from three member states, Belgium, Spain and the UK, to undertake a major European trial under its framework research programme. The trial, which was carried out at 550m in the El Tremedal coal field of eastern Spain (1992-1998), clearly demonstrated that, using the latest drilling and injection control technology, UCG in deeper coal seams was technically feasible.
Events have moved on since the Spanish trial ended in 1998. The DTI has set up a programme on UCG in the UK, and other countries such as Australia and China are actively pursuing UCG trials. Furthermore, the whole field of coal gasification is now at the forefront of fossil-fuel technology for cleaner coal and CO2 capture.
The composition of the gas reaching the surface from a UCG process depends on the depth and pressure of the process, the composition of the injection gases and how the process is controlled. The main components of value to power generation are hydrogen, carbon monoxide, methane, and some higher hydrocarbons. In addition, the raw gas contains carbon dioxide, water vapour and nitrogen in substantial quantities, and contaminant concentrations of minor components such as ammonia, hydrogen sulphide, organic sulphur components and the various constituents of tar.
This syngas from UCG is not dissimilar to the raw product gas from surface gasification plant, and analogous processes of separation and gas cleanup will be required. Unlike the product of surface gasification, UCG gas can vary much more widely as the cavity develops and new CRIP (see later) manoeuvres are initiated: this may present difficulties for the gas turbine. Another issue is whether air or oxygen is used for gasification: clearly, the nitrogen content of the syngas will be much higher after gasification with air, and the calorific value significantly lower.
Gas turbine manufacturers like GE, who have examined the product gas composition from Chinchilla, have concluded that gas turbines such as the GE Frame 6B can operate satisfactorily on air blown UCG gas, and they have extensive test experience on low-cv gas to support this view. Similarly the coal fired gasification plants in Europe and the US, such as Puertollano in Spain and Buggenheim in The Netherlands have been operating on syngas from oxygen-fired gasifiers for some years.
Clean up technology for power generation from coal generated syngas is also well established. The process generally involves washing, acid gas removal, particulate filtering and possibly some balancing of gas composition prior to the gas turbine. These plants are well understood by the process and power industry, and reliable power production can be produced from UCG gas. A schematic of UCG in power production, with options for hydrogen production and CO2 sequestration, is shown in Figure 1 (figures currently not available).
Environmental considerations are the over-riding concern in any new energy process, and UCG is no exception. Recent studies by the EU (Green Paper Towards a European strategy for the Security of Energy Supply) and the UK government (UK Energy Review PIU Report March 2002) recognised the ongoing requirement for coal in the short to medium term, and accept the need both for improvements in generation efficiency, and the importance of CO2 capture and sequestration.
UCG has substantial environmental benefits. The process enables the energy content of the coal to be brought to the surface as a high-pressure gas, which can be readily treated to remove contaminants and capture the CO2. In addition, UCG has no ash disposal or coal handling requirements at the surface, and the spent cavity and adjacent coal might also be used for CO2 sequestration.
UCG is conceptually very simple and has potential advantages over virtually all other methods of coal utilisation. It does not require a gasification reactor at the surface and the product is a gas, which, once processed, can be used in the most efficient combined cycles for power generation.
The success of UCG is dependent on the geology of the coal deposition, with its natural faults, variable coal properties and site-specific hydrogeology. The areas of coal best for UCG are often difficult to find and past attempts to gasify in-seam have often run into problems related to coal permeability, access to the seam or the production of a variable poor quality gas.
These problems have largely been overcome as a result of major advances in exploration, surveying and drilling technniques used by the oil and gas industry in recent years.
Coal seams vary enormously in geological structure and compositional maturity. Reliance on the coal itself to form the interconnecting pathways for UCG may work in high permeability coals, but for the more mature coals found in America, Europe and parts of the Soviet Union, the permeability is generally too low. Attempts to improve the connection between vertical wells have been made by hydraulic fracturing, reverse combustion and electrical arcs: overall, these methods did not live up to expectations.
An obvious solution is to connect the injection and production wells by in-seam drilling (Figure 2). The earliest attempt was the Newman Spinney trial, where an in-seam borehole, drilled from a surface outcrop, was used to connect two vertical wells. Later, in the 1950s, the Soviets began to develop crude methods of steering an in-seam drill bit, and applied the technology to UCG.
It took a long time, considering the guidance systems available from aerospace technology, to develop fully steerable down-borehole motors for drilling, and only in the 1990s were these advanced systems, developed originally for oil and gas exploration, applied to coal. America and Australia were the first to use in-seam directional drilling in coal for exploration and degassing. The Spanish trial was the first to use production tools and down-shaft assemblies for constructing and inter-connecting the UCG process wells.
Today, specialist drilling suppliers are offering a growing range of logging tools for inclusion in the down-shaft assembly and smaller diameter equipment, ideal for UCG is becoming available. Discontinuities in the surrounding strata, the position of the floor and roof, and physical coal properties can all be sensed with the appropriate geophysical logging tools.
Furthermore, it is possible to steer towards homing devices emitting electromagnetic signals, thereby dramatically improving the reliability of intersections. These developments will assist greatly in the construction of process wells for UCG, and eliminate much of the risk previously associated with coal seam drilling.
Advanced drilling techniques, however, are generally expensive to contract for and an important question for the UCG engineering design is how much sensing technology to employ, and how much to rely on the skill of the drilling operatives to correct unacceptable faults in the trajectory. Much depends on the quality of the geological data.
A thorough understanding of the coal seam and associated geology is an essential step towards any successful UCG trial or commercial operation. It is possible to make product gas from most in-situ coals, but the process will only develop satisfactorily if the conditions are conducive to cavity development. The rank of the coal, its physical properties such as swelling, permeability, cleat structure, its sulphur, chlorine and ash content are all primary considerations.
The characteristics of the coal bed such as thickness, dipping angle, the presence of dirt bands, and depth of the seam are also important. Sites that have several coal seams in succession are clearly preferable, and the absence of significant fault structures, wash-outs and other undesirable geological features are additional constraints on site selection.
Beyond the coal seam itself, the ability of the adjacent strata to retain the gas and contaminants of the cavity will have an important bearing on the environmental acceptability of the proposed site for UCG. The presence of aquifers, and the hydrogeology of the site also need to be carefully evaluated.
There are few coal prospects where the detailed geology and hydrogeology evaluation is available. Any geological information on the prospect will probably need to be supplemented by borehole coring, 2D or 3D seismic surveys and hydrogeological testing and modelling in adjacent strata. This is expensive and time-consuming in the development of a UCG scheme but absolutely essential to its success.
CRIP, controlled retractable injection point, is a method of directing the injected stream of oxygen and water to the precise section of the coal seam required for gasification.
The gasification cavity will form around the injection point and develop a shape, possibly ellipsoidal, which points towards the production well and grows as the process develops (Figures 3, 4). The gasification reactions take place mainly at the outer surface of the cavity space. There is evidence to suggest that cavity gasification from a fixed injection point is a process of growth followed by decline as the volume becomes very large.
The CRIP manoeuvre has been developed to control the injection process in relation to the developing cavity, and expose new coal to the injection gases. It uses a retractable coiled tube inside the injection well to re-position the injection point when the section of cavity becomes exhausted. This technology, which was developed originally for oil and gas wells, allows the gasification process to advance along the in-seam well, while maintaining a good quality production gas. The width of the cavity is determined by its depth, and coal properties such as permeability and cleat structure.
The CRIP is just part of the overall control of the UCG process. Injection rates, gas composition and pressure control are equally important in maintaining the process and maximising production rates. The control envelope is now reasonably well understood but further data from future trials is required before a commercial sized UCG scheme built around CRIP could be confidently developed.
The CO2 capture and sequestration option is now a topic of major research interest around the world. The US DOE has embarked on a large programme to support its "Pathway to stabilisation" scenarios and the EU has included CO2 capture and sequestration in the 6th Framework proposals. UCG is a suitable target for research in this area. It has the same advantages as surface gasification for CO2 capture, and can access potential underground storage locations for CO2 through its association with drilling underground strata.
Environmental concerns about UCG have been raised especially when the gasification takes place in shallow seams. The tars produced in the cavity during gasification have the potential to disperse, and under adverse geological circumstances can leach into aquifers in the upper layers of the strata.
The technical response is to focus UCG on deeper coal seams, where the probability of contaminants reaching the surface is much reduced. It is also important that the strata between the coal seam and any surface aquifers have low permeability and that geological faults between them are self-sealing. A thorough understanding of the geology, good borehole data on the strata and modelling of the hydrogeology are essential parts of the environmental impact assessment of any UCG planning proposal.
Although contaminants like tars and phenols are a small part of the gasification process, and coal is known to absorb gases and liquid contaminants, the concern of underground contamination must be addressed. European ground water legislation and other legislation also probably need clarifying in respect to UCG and CO2 sequestration operations.
The aim of the Spanish UCG trial was to demonstrate the technical feasibility of gasifying European coals, which are typically deeper and thinner than those used in previous UCG trials in the USA and elsewhere. A site was chosen which had been extensively surveyed for a possible mining project, and the trial was based on the two key technologies of deviated in-seam drilling and CRIP injection.
The process well structure for the trial consisted of two principal wells, an in-seam injection well run along the dip of the coal seam and a vertical production well, which intersects the other well in-seam.
The completed wells included casing and concentric tubing to provide the necessary paths for production, injection, purge gas and cooling water flows. The gasification products passed to a surface production line for flow measurement and sampling of gas and condensate products. The product gases were flared, and the liquids from the well were collected for disposal and treatment. Maximum power produced at surface during the Spanish trial was 8 MWt.
The trial achieved its principal objectives of in-seam drilling, channel communication, CRIP manoeuvre and gasification of a significant quantity of coal. A post-gasification drilling study also based on directional drilling, identified the shape and extent of the cavity.
The operating and drilling experience provided a number of useful lessons for future trials in terms of the detailed engineering design of the underground components, the control of the in-seam drilling process and the geological selection of trial sites. The problems identified during the Spanish trial are relatively easy to solve, and a further trial of sustained channel gasification would lay the technical foundations for commercial operations, and provide a basis for a detailed economic assessment of the process of UCG.
The UK has large-scale coal deposits in deep seams, which extend far out into the North Sea. These could reduce the UK's future dependence on imported fuels when North Sea oil and gas supplies are exhausted. Largely as a result of the successful outcome of the Spanish trial, the UK Department of Trade and Industry in 1999 began a UCG study programme to critically assess the opportunities offered by UCG as a long-term energy prospect.
The overall objective of the current programme is to demonstrate the technical feasibility of UCG in typical UK coal seams and to establish the economic and commercial conditions under which it will be competitively viable. These objectives are being achieved by a combination of feasibility studies and field trials. The study is also reviewing the most recent developments in directional in-seam drilling, and will demonstrate the technology by means of a series of test drillings under UK coal conditions. These drilling tests will be undertaken at the same site planned for the UCG trial.
A study of advanced drilling is currently underway to critically review, on a worldwide basis, the techniques and equipment available for the construction of intersecting in-seam process wells for trial and commercial UCG. The knowledge gained from this study will be used in the design and construction of the borehole configurations for any future UK trial. The results of the drilling investigation may also have application to enhanced coal bed methane (ECBM) and CO2 sequestration in low permeability coals.
The principle activity of the UK programme to date has been to identify potential trial sites. The site should be able to accommodate two different trials, namely:
• A directional drilling trial to prove the accuracy and control of in-seam drilling along seam lengths of up to 400 m.
• A sustained single channel gasification trial to monitor cavity development in long in-seam channels.
A further objective is to identify a site, not necessarily the trial site, for a semi-commercial project.
The target is bituminous or sub-bituminous coal in seams of at least 2-3 m thickness and at depths between 600 and 1200 m. The site would require an overall coal reserve of about 400 000 tonnes contained within a surface area of 10 hectares, and should be some distance from abandoned mineral workings. The major UK coalfields are well documented and large and detailed geological and mining data sets on potential mining prospects are available. Contracts have already been placed with major consultants to undertake a UK-wide search for a suitable UCG trial site. A preliminary review has been completed of geological setting and coal requirements. The presence of water aquifers, planning, and surface land requirements have also been considered and a number of suitable coalfield prospects that meet the criteria have been identified throughout the UK.
Selected site areas within the coal prospects with good borehole data are now being subject to more detailed geological, hydrogeological and planning investigations. The aim is to identify by end 2002 potential sites for the trials. The process of site selection has once again highlighted the importance of the environmental issues and it is proposed that before site work is initiated that a thorough study of the environmental issues is undertaken.
UCG is of growing interest in Australia where new trials and supporting studies are underway.
A UCG trial at Chinchilla, 350 km West of Brisbane, by a private company supported by Australian power interests has been operating continuously since December 1999 in high-ash Queensland coals. It uses UCG know-how from the former Soviet Union to gasify a section of a 10 m thick seam at a depth of 140 m. Access to the seam is achieved by 8 vertical wells which act as both injection and production wells as the gasification of the seam proceeds from one to the other. Output is limited to 60 tonne/day of coal (5-8 MWt) and is in full compliance with Australian environment and health and safety regulations.
The use of vertical wells in shallow seams leads to low drilling costs, and projected costs of power from the Chinchilla UCG process scaled up to 70 MWe are as low as US $1.2 c/kWh compared with a typical figure of US$4.0 c/kWh for IGCC. (Figures from An IGCC project at Chinchilla, Australia based on UCG, a paper to the 2002 Gasification Technologies Conference, San Francisco, Oct 2001). A UCG power project at Chinchilla based on a GE Frame 6B gas turbine with an estimated combine cycle output of 67 MWe is now being planned.
CSIRO Exploration and Mining, Brisbane, is building its capabilities in UCG, and has recently published work on cavity modelling and process studies. The conversion of UCG syngas to hydrogen and liquid fuels is of particular interest in Australia.
Directional drilling by a number of companies has been used extensively for the degassing and exploration of coal. This experience could be very useful in future UCG trials and commercial operations.
China has probably the most active UCG programme in the world today. The UCG centre at the China University of Mining and Technology, Beijing, is testing UCG in abandoned coal mines.
The trial schemes use two parallel galleries in the coal as headers for the injection and production gases. These galleries connect to the surface by drilled vertical boreholes, and to each other by suitable passages. The coal is ignited in one of the cross passages, air and steam are the gasifying agents, and the product gas reaches the surface through one of the vertical boreholes.
An interesting variation by the Chinese is the two-stage process, in which the injection gases alternate between air and steam, thereby increasing the hydrogen content of the process during the steam phase. At least four trials have been undertaken in shallow coalmines, and larger schemes are planned.
A project due to start in Shanxi Province this year will use UCG gas for the production of ammonia and hydrogen. Small scale power production schemes using converted coal boilers or gas turbines are also under consideration. A technology transfer study between the UK and China on UCG is currently underway.
Most other coal producing countries keep a watching brief on UCG. South Africa, Canada, India and Ukraine are reported to have shown interest or undertaken feasibility studies in recent times. The power industry of New Zealand made a test burn in 1984.
Japan, which has substantial coal interests overseas has UCG in its future research plans for coal exploitation, and has been maintaining a low level programme for many years. Economic and technical studies have been produced, and there are reports that a Japanese sponsored trial, possibly overseas, will be undertaken in the near term.
Re-examination of UCG
The coal-producing countries of the world are re-examining UCG as an option for clean coal exploitation. The reasons are:
• The substantial technological improvements that have taken place in surveying, drilling and completion technology in recent years, much of which has direct application to UCG.
• The high efficiency of IGCC and the possibility of using proven technology for the processing and utilisation of UCG product gas.
• The results of the Spanish trial, which demonstrated the feasibility of UCG at depths greater than 550 m. The prospect of UCG at these depth, or greater, will eliminate much of the previous environmental concerns about ground water contamination.
• The ability to easily capture CO2 from the high-pressure product gas of UCG.
• The potential link between UCG and CO2 sequestration. Drilling and injection technologies are similar, suitable coal seams may be available nearby and the UCG underground cavity itself may be a possible CO2 storage medium.