Underground coal gasification (UCG) is the gasification of coal in-situ, which involves drilling boreholes into the coal and injecting water/air or water/oxygen mixtures. It combines an extraction process and a conversion process into one step, producing a high-quality, affordable synthetic gas, which can be used for power generation. Still in the early stage of commercialization, UCG is poised to become a future major contributor to the energy mix in countries around the world.
Coal gasification is a well-known chemical process that converts solid carbonaceous material into synthetic gas (syngas), which consists predominantly of methane (CH4), carbon monoxide (CO), carbon dioxide (CO2), hydrogen (H2), and water (H2O) steam. Gasification differs from combustion (or burning) because burning coal takes place in excess O2 and produces only CO2 and water steam.
In the underground coal gasification (UCG) application, air and/or oxygen is introduced to coal while it is still in the ground by pumping it down boreholes (called injection wells), which are drilled into the coal seam from the ground surface (Figure 1).
|1. Making coal cleaner. The underground gasification of steeply dipping coal seams was demonstrated in a pilot project near Rawlins, Wyo. In the underground coal gasification process, air and/or oxygen is introduced to the coal while it is still in the ground by pumping it down boreholes (called injection wells), which are drilled into the coal seam from the ground surface. Courtesy: Paul Ahner|
Rohan Courtney, chairman of the trustees of the UCG Association (UCGA, http://www.ucgp.com) and also chairman of Clean Coal Ltd., told POWER in May about the UCG process. He explained that once syngas is formed in the coal seam, the syngas then flows back to the surface under pressure via a second borehole (the production well), which is linked through the coal seam to the injection well (Figure 2). A linked injection well and production well is called a UCG “module,” which is the cornerstone of UCG.
|2. Surfacing. After syngas is formed in the coal seam, it then flows back to the surface under pressure via a second borehole (the production well), which is linked through the coal seam to the injection well. Courtesy: Marc Mostarde|
“In many ways, it is the relatively recent perfection of drilling and methods for linking the injection and production wells that has led to the huge resurgence of interest in UCG that we are now witnessing across the globe,” he said. (See sidebar, “Coal Gasification Pilot Projects.”)
Two UCG Methods
The two main methods used to carry out UCG are often referred to generically as the linked vertical well (LVW) method and the controlled retractable injection point (CRIP) method. Both of these methods rely on a module of at least two linked boreholes to inject the oxidant and remove the syngas.
The LVW method uses vertically drilled wells to access the coal seam and different techniques to link the boreholes. In contrast, the CRIP method relies on a combination of conventional drilling and directional drilling to access the coal seam and physically form the link between the injection and production wells.
Evidence from previous and current trials suggests that the two basic methods are generally suited to exploit different coal resources. LVW methods are more suited to shallow coals seams, and CRIP methods are more suitable for deeper coal seams.
There is a fairly equal distribution of methods employed on modern UCG projects around the world.
However, Courtney pointed out that “on balance, it’s fair to say that the LVW method is currently used more.” It is used in projects in Uzbekistan, South Africa, China, and New Zealand. On the other hand, the company Carbon Energy uses the parallel CRIP method in Australia; Swan Hills in Canada is using the linear CRIP method; and Linc Energy has experimented with most of the available techniques.
“As long as the techniques are deployed in properly selected sites (site selection is fundamentally important for UCG) and operated correctly, there are no major technical challenges to these techniques used for UCG,” he said. “Much of the work nowadays is in refining the techniques to increase efficiency and reduce costs, rather than in overcoming major technical challenges.”
UCG plants can produce syngas by exploiting coal resources located both onshore and offshore (Figures 3 and 4).
|3. Hitting pay dirt. This diagram illustrates the controlled retractable injection point (CRIP) method being used to access a coal seam onshore. The CRIP method relies on a combination of conventional drilling and directional drilling to access the coal seam and physically form the link between the injection and production wells. Source: UCG Association|
|4. Going to the ocean depths. UCG plants can also be constructed with injection and production wells that can access coal seams located offshore. Source: UCG Association|
The Linked Vertical Well Method
The LVW method uses reverse combustion (RC): The coal is ignited from one of the vertical wells and air/oxygen is introduced into the coal from the other well. The combustion front then moves toward the air/oxygen, forming linkages between the wells by progressively consuming small amounts of coal and forming tube-like channels as it goes. Once the linkage is established, forward combustion (where the combustion front moves in the same direction as the injected air/oxygen toward the production well) is used to gasify the coal.
“An advantage of the RC-LVW method is that it is relatively inexpensive, as no expensive directional drilling is required to link the wells,” Courtney said.
It is also possible with RC-LVW to use greater-diameter injection and production wells than with the CRIP methods because no deviated in-seam drilling (where large borehole diameters are a disadvantage) is required. This means that greater syngas flow rates could be achieved using RC-LVW than with CRIP at shallow depths (<300 meters or 984 feet).
A disadvantage of the RC-LVW method is that, because it relies on the natural permeability of coals, it is not particularly well suited to low-permeability coal, or deeper coal seams (>300 meters [m]), which tend to be under great pressure and consequently have reduced permeability, according to Courtney. The reliance on natural permeability may also force the linkage to take unpredictable paths, as the linkage will likely follow preexisting fractures or paths of low permeability. Furthermore, it is more difficult to ensure that the linkage in the coal seam is maintained as close as possible to the base of the seam using this method.
“This is important because early UCG trials using the RC-LVW method showed strong evidence that maintaining the injection point at a low position in the coal seam is essential for obtaining good syngas quality and high mining/gasification efficiency,” he said.
The Controlled Retractable Injection Point Method
The modern CRIP techniques use a combination of conventional and directional drilling techniques to drill and complete both the injection and production wells, Courtney pointed out. The vertical section of the CRIP module injection well is drilled to a predetermined depth, after which directional drilling is used to deviate the hole and drill along, and at bottom of, the coal seam.
“With the CRIP technique, the location of the injection point can be precisely controlled and retracted back along the bottom of the coal seam,” he noted. “This is of benefit because it allows for fresh coal to be accessed each time the syngas quality drops as a result of cavity maturation.”
The main difference between the linear and parallel CRIP methods is in the production well design:
- The linear CRIP concept uses a vertical production well located at least 500 m from the deviated injection well. The in-seam section of the injection well is drilled such that it intersects the production well.
- The parallel CRIP method uses a deviated in-seam production well drilled parallel to the injection well with an interwell spacing of around 30 m. The two wells are deviated at a predetermined in-seam length toward a third vertically drilled ignition well, which is used to initiate gasification.
Gasification efficiency drops as the UCG reactor grows because more and more of the barren roof rock is exposed, which conducts heat away from the reactor and impacts syngas quality, Courtney said. The CRIP method allows for the injection point to be retracted back within the coal seam when the efficiency drops.
“The large spacing between the injection and production wells also means that fewer boreholes are required to gasify a certain volume of coal, and so the CRIP methods have a smaller surface impact than LVW methods,” he said.
Furthermore, as the CRIP methods do not rely on natural coal permeability to create the linkage, this method can be used at great depths (1,400 m deep has been achieved at the Swan Hills project in Alberta, Canada), significantly increasing the resource base for UCG around the world.
Syngas Cost Comparison
The cost of producing syngas on a per unit energy basis is very closely linked to a number of key variables, and so it is not really possible to give one overall figure appropriate for all UCG projects, Courtney explained.
One of the most important variables is coal seam thickness. This is because the cost of producing syngas is linked closely to the cost of installing a module, and so the more coal a module can gasify, which is a function of coal seam thickness, the lower the costs to produce syngas.
“It is possible to give a rough idea of costs for specific projects, so the UCGA recently produced a price comparison with other energy-producing technologies in response to a UK government-sponsored report entitled ‘UK Electricity Costs Update, June 2010,’” he said.
The UCGA used the same methods to calculate the costs for UCG as those used in the report to ensure a fair comparison (Figure 5). These and other cost estimates, including work by Lawrence Livermore National Laboratory, consistently show UCG to be very competitive with existing mature coal- or gas-fired electricity generation technologies. It is worth pointing out that economics will only improve as UCG develops into a mature industry.
|5. A cost-effective option. This cost estimate shows UCG to be competitive with existing mature coal- or natural gas–fired electricity generation technologies. Note that 1.00 Euro = $1.4320 USD, May 2011. For UCG, no payment for the value of coal is included, and the cost of UCG includes the cost of 90-plus% carbon capture and storage. Source: UCG Association|
The Impact of Low Natural Gas Prices on UCG Deployment
“Historically, low natural gas price is one of the main contributing factors for delaying the commercialization of UCG,” Courtney said. “Nowadays, with the advances made in directional drilling and other technologies, the production of syngas on a calorific value per unit cost basis is much more competitive with natural gas, even relatively cheap natural gas.”
“Since the beginning, the UCGA has had a world map detailing all the interest in UCG around the globe,” he said. “We are currently finding that we cannot update it fast enough to keep pace with the projects being undertaken in different countries.”
There are many reasons why UCG is attractive to coal-bearing countries:
- Some countries, such as Poland, rely on coal to produce the vast majority of their power. As the amount of economically minable coal declines, UCG is becoming increasingly attractive.
- Another key driver for interest in UCG in Europe is energy security concerns, particularly in those countries that rely on Russia for their natural gas supplies.
- Other countries, such as China and India, simply require huge amounts of energy to fuel their economic expansion. As these countries contain vast quantities of coal, much of it unminable, UCG is being developed as part of the energy mix.
“In contrast, we are also seeing interest from energy-rich regions, such as Alberta in Canada. Alberta has huge oil sand reserves and is not short of energy,” he said. “Alberta also has very large quantities of very deep unminable coal, which is suitable for UCG, so many companies are looking to exploit this valuable resource.”
Obstacles to Deployment
There are few UCG-specific technical issues that need to be overcome because, in recent years, the technologies have matured to a stage where companies are now moving from pilot stage to a commercial stage, Courtney said. In his opinion, “what we need to see now is more projects moving into the commercial stage to give investors more confidence in the technology and fund more projects.”
“This is closely linked to another challenge we are seeing: There currently is only a handful of people with direct experience of UCG,” he said. “These people generally reached their professional peak in the last ‘phase’ of UCG development between the 1970s and late 1990s. Therefore, we now need a new generation of UCG experts to develop UCG in the 21st century. We are now seeing universities offer UCG modules in their undergraduate degrees as well as UCG-related PhD programs and post-doctorate research, but what we really need is more practical experience of UCG.”
The environmental challenges are well understood, as a result of the significant knowledge gained from previous UCG trials. These include groundwater contamination, subsidence, surface contamination, and gas emissions. They can be managed, however, by careful site selection, the correct operation of the UCG module, and by the use of appropriate engineering materials/surface plants, according to Courtney. Safety is also often mentioned as a challenge, although experience is showing that with good process control and operations, safety issues are no different than those in other process industries (see sidebar “UCG Safety Issues”).
Groundwater Contamination. Contaminants, such as benzene and other hydrocarbons, can be produced during UCG by a natural process called coal pyrolysis. Coal pyrolysis occurs during the breakdown of coal at temperatures less than those required for gasification. During UCG, this will likely happen within the coal at a distance less than 0.5 m from the coal face.
“As UCG takes place below the groundwater level, in a ‘groundwater bubble,’ there is a risk that some of the contaminants could leave the UCG reactor and impact groundwater resources,” Courtney said. “It is possible, however, to stop contaminants from entering the groundwater by ensuring that water only flows into the UCG cavity, because contaminants will not be transported against the direction of flow.”
Selecting the appropriate site for UCG is of fundamental importance, and a correctly sited UCG project would not be located anywhere near an aquifer used to extract water for use. The suitability of a site with respect to assessing risks to aquifers can be determined using a number of existing and well-tested methods, so it is relatively straightforward to avoid high-risk locations in the first place.
Subsidence. Ground subsidence is the propagation of the UCG cavity toward the surface following collapse of the cavity roof rocks. The distance that subsidence can propagate is strongly dependent on cavity size, the depth to the cavity, and the mechanical properties of the rocks overlying the cavity. Significant surface subsidence has not been observed in all UCG trials; in fact, it is very rare. Site selection is one of the most important factors in managing the risks from subsidence. Coal seam depth is critical, and seams >200 m deep minimize the risks to the surface. It is also necessary to target coal seams with strong, fully consolidated roof rocks that can resist the effects of subsidence.
The risks of surface subsidence from UCG are analogous to those from conventional mining, and so there are many standard, well-accepted techniques for assessing this risk. The fact that UCG is increasingly carried out at depths greater than conventional mining, however, lowers the risk of surface subsidence compared with traditional mining techniques.
Surface Contamination. Risks to the environment from UCG at the surface are essentially restricted to how contaminants are handled once they are condensed out with the water in the syngas. UCG “blackwater” is no different from the wastewater produced during conventional surface coal gasification. Therefore, technologies that can treat the water are well established and readily available.
Atmospheric Emissions. The emissions from UCG are essentially no different than from any other industrial process using coal, and so existing, tried and tested technologies can be employed to reduce atmospheric emissions. Emissions from a UCG plant are, however, considerably lower than from conventional coal mines, because no coal is brought to the surface, and methane emissions (often associated with coal mining) are minimized.
Permitting Issues for UCG Projects in the U.S.
It is well within the interests of UCG operators to demonstrate clearly that no impacts to groundwater resources on- or off-site will occur as a result of UCG, Courtney said. Impacts to aquifers from any contamination will have serious implications for a project under its environmental permitting regime. For example, a regulator could order a project to stop all UCG activities if it has breached its permitting conditions. Additionally, the UCG operator would likely have to pay the significant costs associated with aquifer remediation as well as expose itself to the risk of litigation from adjacent landowners.
All UCG operators are aware of these issues and, consequently, ensure that numerous monitoring wells are distributed throughout their sites. Maintaining groundwater levels is critical for the long-term sustainability of UCG projects. Therefore, many groundwater monitoring wells have to be on UCG sites irrespective of their use for groundwater quality monitoring. Typically, groundwater monitoring wells are positioned at the site periphery, as well as off-site, whenever feasible. This allows UCG operators to demonstrate that they are not impacting aquifers.
One of the current hindrances to widespread deployment of UCG around the world is the uncertain regulatory environment in some areas, Courtney said. Countries and regions with existing UCG regulations (or with other existing policies that can be easily adapted to deal with UCG projects) will have an advantage over others.
He pointed out that “it is clear that countries such as the UK, China, India, Turkey, U.S. (Wyoming), Australia, and Canada are moving forward with their regulations and stimulating interest in UCG projects.” Those countries with abundant, unminable coal reserves suitable for UCG and a strong need for affordable energy, such as China and India, will probably undertake widespread deployment of UCG first. Currently, five UCG projects are being carried out in Australia, China, Canada, and South Africa.
“We are at the early stage of commercialization of UCG. As more and more projects around the world prove that UCG produces affordable, clean, and efficient energy, the use of UCG in the U.S. will naturally grow both in the short and long term,” he said. “Other factors, such as energy security and carbon emissions, will likely have an impact on the deployment of UCG in the U.S, but probably in the longer term.”
Courtney also emphasized the impact that the successful implementation of carbon capture and sequestration (CCS) could have on UCG’s future development. “In the longer time frame, it is our firm belief that CCS will become commercial, and given UCG’s inherent benefits for carbon capture (CO2 can be captured relatively cheaply from oxygen-fired UCG syngas at high pressure), this trend will see UCG being increasingly exploited in regions such as Europe,” he said.
Rohan Courtney would like to acknowledge the contributions of the following people: Julie Lauder, CEO, UCG Association; Marc Mostade, technical director; Shaun Lavis, senior geoscientist; and Paul Ahner, chief UCG technician, Clean Coal Ltd.
— Angela Neville, JD, is POWER’s senior editor.