Gasification in Detail - Types of Gasifiers - Gasifiers for Special Applications
Underground Coal Gasification
Underground Coal Gasification (UCG) uses a similar process to that used in surface gasification. The main difference is that in UCG the underground coal seam itself becomes the reactor, so that the gasification of the coal takes place underground instead of in a manufactured gasification vessel at the surface.1
In the UCG process, injection wells are drilled into an unmined coal seam, and either air or oxygen is injected into the seam along with water. The coal face is ignited, and the high temperatures (about 1,200°C) from the combustion and limited oxygen causes nearby coal to partially oxidize into hydrogen, carbon monoxide (CO), carbon dioxide (CO2), and minimal amounts of methane (CH4) and hydrogen sulfide (H2S). These products flow to the surface through one or more production wells located ahead of the combustion zone. As the face is burned and an area depleted, the operation is moved to follow the seam. The graphic below illustrates the general process.
(source: Science & Technology Review)
UCG eliminates the need for mining, and the dangers to miners and environmental degradation that are associated with it. It also makes deep or difficult to access coal seams into usable energy assets, as only one-sixth to one-eighth of the world’s coal reserves are economically mineable. Scientists estimate that with UCG, the U.S. usable coal reserves could increase by 300% .2
In terms of the use of coal, UCG retains many of the advantages of other forms of gasification. UCG has been demonstrated on almost all types of coal, although coal with lower ash content is preferable. Compared to surface gasification, UCG requires much smaller gas cleanup equipment, because both the tar and ash content of UCG-based syngas is substantially lower than that obtained from a surface gasifier.3 Because the processing of the coal is kept underground, surface and air emissions of sulfur, nitrous oxides, and mercury are dramatically reduced.2
Challenges with underground coal gasification stem from the potential leaching of unwanted substances into groundwater. Subsidence, where the surface actually sinks as the deep seam is gasified, can also be an issue. Mitigation of these risks is being investigated. Subsidence can be reduced or eliminated through careful analysis of geology and selective gasification of seam areas, much like pillar and chamber underground mining practices. Controlling leaching also requires extensive geological study. One approach demonstrated in Australia involves optimizing operating conditions such that the pressure in the gasifier is lower than pressure in the coal seam and in the surrounding strata. As a result, there is no drive for groundwater flow from the gasifier chamber or loss of product or contaminants into the surroundings.3
Since some coal is burned in order to gasify surrounding coal, some of the heat content of the coal seam is lost. However, it is estimated that this energy loss is less than the energy that would be required to mine the seam.
Two different methods of UCG have evolved, and both are commercially available. The first, based on technology from the former Soviet Union, uses vertical wells and a method like reverse combustion to open up the internal pathways in the coal. The process has been used in several operating facilities and demonstration projects.
The second, tested in European and American coal seams, creates dedicated inseam boreholes, using drilling and completion technology adapted from oil and gas production. It has a moveable injection point known as controlled retraction injection point (CRIP) and generally uses oxygen or enriched air for gasification.6 The schematic below illustrates the CRIP method.
(source: Lawrence Livermore National Laboratory)
Site selection is paramount to a successful UCG project. The characteristics of the coal seam, the permeability and fault structure of the local strata and the geology and hydrogeology of the area which surrounds the target coal seam must be fully understood. This requires the drilling of pilot bore holes to coal seam depth for coring and seam characterization, and a good quality seismic survey (preferably 3D) of the whole area. Modeling of the hydrogeology will be required to meet most countries ground water requirements.4
The combustion of coal releases CO2, and combustion of coal in UCG is no exception. However, like other forms of gasification, UCG offers enhanced potential for carbon capture and storage (CCS). The syngas produced from UCG can be processed and the CO2 separated for sequestration or other use.
Long-term storage of CO2 in geological targets is being widely researched. The chief geological targets for carbon storage include deep saline aquifers, depleted gas fields, active oil fields, depleted oil or gas fields, and unmineable coal seams. All of these targets are frequently found near coal seams that are candidates for UCG. Therefore, underground CO2 storage options are generally expected to be available at UCG sites. Carbon capture economics and coincidence of storage targets make UCG-CCS an attractive carbon management package. In a UCG-CCS scenario, UCG-generated syngas would be taken from the ground and the by-products separated out. The CO2 would then be returned to nearby geological formations.
The potential for using the cavity in the coal seam created by UCG for CO2 storage has been suggested. Using the coal seam cavity has the advantages of pre-existing boreholes and large volumes, but there are potential hurdles as well: the integrity of the cavity can be compromised by cracking and collapsing caused by the UCG process. Secondly, CO2 will interact with water to form carbonic acid and may interact with the coal, char, and ash to form sulfuric acid. These acids could migrate out of the cavity, along with CO2. In these cases, the risk for leaching metals and other harmful chemicals into water may be substantial. In addition, volatile organic compounds (VOCs) like benzene may dissolve into the CO2 and be transported out of the reservoir and travel upwards through the crust with CO2. Such processes could conceivably increase the risk of groundwater contamination even for deep UCG projects. The use of UCG-created cavities for carbon storage requires further study.
History of UCG
UCG has been identified as a potential process for utilizing unmineable coal since the late nineteenth century. Vladimir Lenin was an early proponent of the technology’s ability to eliminate the need for miners to work in underground mines, and the former Soviet Union invested heavily in UCG research. By 1939 the Soviets had successfully begun operating a UCG plant in the Ukraine, which was later shut down by German occupation. Later (and to this day) the Skochinsky Institute of Mining in Moscow became a center for UCG expertise. The UCG technology developed by the Institute was implemented in three brown coal and two black coal power stations in the 1960s. One of these facilities, the power station at Angren, Uzbekistan, still operates, producing about a million standard cubic feet of syngas per hour. The others have been converted to gas fired stations due to the significant natural gas reserves in the former Soviet Union.1
In the late 1970s and 1980s, the U.S. government instituted several research projects and trials of UCG. The Rocky Mountain 1 trial demonstrated the gasification of about 10,000 tons of coal. Over 30 UCG pilot tests were run across the United States. At that time, the hydrogen by-product of UCG was viewed as a liability, reducing the perceived quality of the gas. In addition, groundwater-contamination problems resulted at two sites. When gas and oil prices dropped in the 1980s and 1990s, efforts to commercialize UCG came to a halt.2
Recent UCG Research and Demonstrations
With increasing demand for natural gas and chemical products and increasing concerns over mining practices, interest in UCG has revived around the world. The map below overlays significant UCG projects on identified potential CO2 geologic storage.
(source: Science & Technology Review)
UCG projects have been developed extensively in Australia. The Chinchilla project in Queensland is the most recent large demonstration project, operating from 1997 to 2003. The project developers claim that 35,000 tonnes of coal were gasified with no observed subsidence or contamination of groundwater. The project achieved 95% recovery of the coal resource, 75% recovery of the total energy, and a controlled shutdown. Australia is now considering wider commercial application of UCG.
In South Africa, a pilot scale UCG project at the Majuba Coal Field north of Johannesburg achieved ignition in January 2007. The coal seam supplies a 4,200 MW power plant but the field is severely faulted with volcanic intrusions, making mining difficult. The pilot scale UCG process produces only a small stream of syngas that is flared, but plans call for a 1,200-MW UCG plant and an IGCC plant constructed in parallel.
In the United Kingdom, the government undertook a five-year effort to review UCG and study the feasibility of using the technology for exploiting coal on land and offshore. A new UCG Partnership, launched in the United Kingdom in 2005, draws its membership from more than eight countries.
In India, interest in the potential of UCG is particularly high. India has vast coal resources, a shortage of natural gas, and much of the nation’s coal lies in steeply dipping deposits that are difficult to mine conventionally. At least three pilot projects are now in the planning stages.
Chinese trials of UCG at shallow depth have been underway since about 1985. In addition, a UCG field trial leading to commercial production has been proposed within the concession area of the Liaohe oil field in Liaoning Province.4
In the United States, R&D has remained mostly in the private sector only, although the state of Indiana is conducting some research into UCG.5 Recently, a Colorado-based company (Luca Technologies) announced research into UCG using microorganisms to break down coal into methane and other gases, rather than heat from combustion. This technology appears to be very early in its development.6