12 S&TR April 2007

Fire Underground coal gasification may provide a secure in the energy supply and reduce greenhouse gas emissions. Hole

Lawrence Livermore National Laboratory S&TR April 2007 Underground Coal Gasification 13

ORLDWIDE coal reserves are vast, In the early years of UCG, the produce secure and clean energy from its Wover 10 trillion metric tons, but technology gained an “ugly duckling” domestic coal reserves, coal-to-gas and unless cleaner and cheaper ways can be reputation in the U.S. The UCG process coal-to-liquid conversions may become found to convert coal to gas or liquid fuels, yielded gas of low heating quality with commonplace. coal is unlikely to become an acceptable too much hydrogen, and it was considered Applying improved UCG technology replacement for dwindling and uncertain an environmental risk. But as coal-rich to gasify deep, thin, and low-grade coal supplies of oil and natural gas. Mining countries now look to replace imported oil seams could vastly increase the amount coal is dangerous work, coal is dirty to with secure domestic energy sources, make of exploitable reserves. The coal could be burn, and much of the coal in the ground hydrogen fuels, and find ways to limit converted to gas for a variety of uses, and is too deep or too low in quality to be their greenhouse gas emissions, they are emissions of sulfur, nitrous oxides, and mined economically. Today, less than one- rediscovering the potential of UCG. mercury could be dramatically reduced. sixth of the world’s coal is economically In the U.S., coal supplies about “UCG could increase recoverable coal accessible. However, Livermore is helping 50 percent of this country’s electricity reserves in the U.S. by as much as 300 to to revive an old technology that offers because it is the least expensive energy 400 percent,” says Julio Friedmann, who promise to substantially increase usable source. Coal can be gasified or liquefied to leads Livermore’s Carbon Management coal reserves and make coal a clean and make transportation fuels, natural gas, or Program. Another benefit of UCG is economic alternative fuel. Known as chemical feedstocks. Today, the U.S. has that hydrogen accounts for half the total underground coal gasification (UCG), this only one operating coal gasification plant gas product. technology converts coal to a combustible and no commercial liquefaction operations. As with any hydrocarbon combustion gas underground. However, because of the nation’s goal to process, UCG generates carbon dioxide

Hoe Creek Centralia Angren

Highly prospective Prospective Nonprospective Prior test sites Chinchilla Sites of note Announced or planned

This map shows underground coal gasification (UCG) sites worldwide, including planned sites and prior pilot test sites. The sites of note are Centralia, Washington, and Hoe Creek, Wyoming, which are two Lawrence Livermore test sites; Chinchilla, Australia, where the longest pilot in the Western world was recently completed; and Angren, Uzbekistan, where a commercial UCG plant has operated for 50 years. The underlying gray shading shows potential areas for geologic carbon storage.

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(CO2), a greenhouse gas. Fortunately, in Centralia, Washington, and the other UCG Revives potential sites for UCG operations in Hoe Creek, Wyoming. Livermore Four years ago, former Laboratory correspond to locations where sites are researchers also patented a UCG process engineer Ray Smith, who led the Energy plentiful for sequestering CO2 in geologic called Controlled Retraction Ignition Point, Program in the Energy and Environment formations underground. UCG also which was used in pilot tests performed in Directorate, encouraged DOE to revisit enhances the storage capacity of the coal Europe during the 1990s. In the U.S., when UCG as a part of its program to develop seam itself to store injected CO2. The gas and oil prices dropped in the 1980s and hydrogen-from-coal technology. After generated gas, called syngas, would be 1990s, efforts to commercialize UCG came Smith’s retirement a year ago, his team taken from the ground and the by-products to a halt. of chemical engineers, geologists, and separated out. The CO2 would then be Today, high prices have returned for all environmental scientists pursued the returned downhole nearby. kinds of fuel, and uncertainties exist about revival of UCG through Friedmann’s political stability in the Middle East. A Carbon Management Program. In February Ups and Downs of UCG renewed U.S. interest in coal gasification 2006, DOE commissioned the team to The idea for coal gasification, either is not surprising. Furthermore, hydrogen prepare a document evaluating the current underground or in aboveground plants is now a welcome by-product because state of UCG technology. Best Practices using mined coal, has been around for of the current interest in alternatively in Underground Coal Gasification was more than 150 years. The technology was fueled vehicles. completed at the end of 2006 and is first widely used in the U.S. during the late awaiting official release by DOE. 1800s. Lamplighters made their rounds The document explores the UCG efforts in many of our largest cities lighting that have been undertaken worldwide. streetlights fueled by “town gas,” the Importantly, it also addresses the issues product of early and relatively crude forms that were problematic in previous of coal gasification. Once vast fields of UCG operations by evaluating the potential natural gas were discovered and pipelines application of technological advances built to transport the gas to consumers, the in areas such as environmental risk use of town gas disappeared. assessment, combustion-process modeling, From the 1930s through the 1990s, geologic subsurface characterization, and the former Soviet Union invested in geomechanics. developing UCG technology at numerous Over the last few years, the number sites and was successful at the commercial of activities throughout the world scale in several locations. China has been focusing on UCG has rapidly increased. developing the technology since the 1980s The Chinchilla project, operating from and currently has the largest operational 1997 to 2003 in Queensland, Australia, UCG program. Their approach uses demonstrated the first long-term UCG abandoned tunnels in conventional mines. pilot in the Western world. That project During the energy crisis of the 1970s, has now advanced to the stage of raising U.S. interest spiked in all forms of capital for a coal gas-to-liquids pilot that alternative energy, and the Department of will make ultraclean diesel and aviation Energy (DOE) invested billions of dollars fuel. In South Africa, the electricity to develop efficient coal-gasification supply company Eskom is developing technologies for power generation. Over UCG at the Majuba Coal Field and 30 UCG pilot tests were run across the achieved ignition in January 2007. U.S. At that time, the hydrogen by- In the United Kingdom, the government product of UCG was viewed as a liability, undertook a five-year effort to review reducing the perceived quality of the gas. The Majuba UCG Project in Mpumalanga, UCG and study the feasibility of using In addition, groundwater-contamination South Africa, is producing high-quality syngas the technology for exploiting coal on land problems resulted at two sites. for power generation. This photo shows the first and offshore. A new UCG partnership, The Laboratory, a pioneer in the study flare on January 20, 2007, when the UCG plant launched in the United Kingdom in 2005, of UCG, developed two test sites—one successfully started operations. draws its membership from more than

Lawrence Livermore National Laboratory S&TR April 2007 Underground Coal Gasification 15

eight countries. The partnership hosted Energy Dialogue Coal Working Group, The Powder River Basin is a massive its second international conference in with collaboration from governments coal and natural-gas deposit that spans the February 2007, and Livermore chemist within the Asian-Pacific Partnership on Montana–Wyoming border. It is the largest and environmental scientist Elizabeth Clean Development and Climate. Burton source of coal mined in the U.S. and is one Burton delivered the keynote address. represented the Laboratory and made of the largest deposits of coal in the world. In India, interest in the potential of technical presentations at the workshop. GasTech, Inc., and the Wyoming Business UCG is particularly high. India is the Industry is reengaging with the Council recently completed a feasibility world’s third largest coal producer Laboratory to pursue UCG. On March study showing UCG to be a better option (383 million tons of bituminous coal in 30, 2006, Lawrence Livermore signed with respect to cost, emissions, and 2005) and uses coal for about 60 percent a memorandum of understanding with environmental effects compared with of its own energy needs. Although India Ergo Exergy, which has licensed its conventional coal-fired stations and has vast coal resources, much of it is low proprietary Exergy UCG technology, integrated gasification combined-cycle grade, with high ash content. In addition, or EUCG, to clients in Wyoming, South plants. New UCG field pilots are planned the coal lies in steeply dipping deposits Africa, India, Australia, New Zealand, for the Powder River Basin. Ergo Exergy that are difficult to mine conventionally. Canada, and Italy. The two organizations Technologies, Inc., will be involved in this India also has a shortage of natural gas. have agreed to cooperate in conducting test operation. A November 2006 workshop on UCG, research on UCG technology, specifically hosted jointly by the Indian Ministry of in the areas of process simulation and The UCG Process Coal and DOE, was heavily attended by carbon sequestration, with an emphasis on In the UCG process, injection wells representatives from India’s coal industry. evaluating the environmental performance are drilled into an unmined coal seam, At least three pilot projects are now in the of a large-scale operation. In addition, and either air or oxygen is injected into planning stages. Livermore, through DOE, BP (formerly British Petroleum) recently the cavity. Water is also needed and may is working closely with India on its UCG executed an agreement with the Laboratory be pumped from the surface or may come development. Ravi Upadhye, a chemical to develop simulations for optimizing from the surrounding rock. The coal engineer who was involved in some of the UCG process as well as tools for face is ignited, and at high temperatures Livermore’s early UCG projects, played drilling, monitoring, and environmental (1,500 kelvins) and high pressures, this a major role in organizing the workshop management that are essential for field combustion generates hydrogen, carbon under the auspices of the U.S.–India deployment of UCG. monoxide, carbon dioxide, and minimal

Air in Gas out

Ground level Water table

Overburden

At a UCG production facility, air is injected into the cavity, water enters from surrounding rock, and partial combustion and gasification take place at the coal seam face after ignition. The resulting high-pressure syngas stream is returned to the surface, where the gas is separated and contaminants are Coal seam removed.

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amounts of methane and hydrogen sulfide. 1980s. The model quite accurately contaminants away from the cavity, which These products flow to the surface through predicts the hydrogen, methane, and introduced benzene, a carcinogen, in one or more production wells. As the water content of the gas. However, it potable groundwater. The contamination face is burned and an area depleted, the predicts twice the actual level of carbon has required an expensive and long-term operation is moved to follow the seam. monoxide and about two-thirds the actual cleanup effort at the site. Upadhye and chemical engineer level of CO2. “Verification of the model’s Since these problematic tests in the Henrik Wallman from the University accuracy can only be done with field 1970s, environmental scientists have of California at Berkeley are developing experiments,” notes Upadhye. “We cannot learned a great deal about the behavior improved combustion process models run laboratory experiments to verify the and types of contaminant compounds and a computational fluid dynamics models. UCG takes place many hundreds produced by UCG as well as about model. Their goal is to optimize the of feet underground, and its results can contaminant transport and environmental design, operation, and control of UCG be difficult to measure.” Upadhye hopes risk assessment. Several steps can be processes so that the composition of to test and improve the model using field taken to avoid groundwater pollution. the product gas can be predicted and, data from the UCG pilot tests that will One is balancing operating conditions to despite variable subsurface conditions, occur throughout the world over the next minimize the transport of contaminants constrained within acceptable limits. few years. from overpressurized burn zones. Another Gas composition affects the economic is to locate a UCG site where natural viability of the operation and must stay Meeting Environmental Challenges geologic seals isolate the burn zone from within the limits of the capabilities of the The new field pilots will also provide surrounding strata. Isolating the site from gas-processing plant at the surface. Thus key data for the environmental models current or future groundwater sources far, they have developed the essentials of being developed by a team of environmental and understanding how UCG affects the process model and have integrated it scientists led by Burton at Livermore. the local hydrogeology are essential. with Aspen Plus, a commercial software Although most of the previous UCG pilots This knowledge greatly benefited the package for simulating steady-state did not produce significant environmental Chinchilla project. “Chinchilla is an chemical processes. consequences, Livermore’s 1970s test site excellent example of how to plan a Upadhye and Wallman’s simplified at Hoe Creek, Wyoming, unfortunately site and operate a UCG plant,” says model may work for some variables but resulted in contaminated groundwater, Friedmann. “The operators maintained not for all, as shown in the table below, as did one pilot in Carbon County, negative pressure in the combustion which compares model results for UCG Wyoming. At Hoe Creek, operation of cavity so that contaminants could not flow gas component levels to measurements the burn cavity at pressures higher than beyond the cavity.” made during the U.S. field tests in the that in the surrounding rock strata pushed Burton’s team is creating the first detailed models of contaminant flow and transport specifically for UCG operations. Model results for UCG gas composition compared with field measurements made “The standard types of hydrologic models during the 1980s Rocky Mountain 1 Controlled Retraction Ignition Point test. used for environmental assessments do not consider the full effects of UCG operations,” she says. UCG requires Component UCG model predictions (percent) Field measurements (percent) integrated simulations that capture the complex geochemical, geomechanical, Hydrogen 27.2 27.3 and geohydrological processes occurring during a burn. Carbon monoxide 13.0 6.4 Initially, Livermore groundwater specialists Walt McNab and Souheil Ezzedine created and tested a modified Carbon dioxide 19.4 27.2 version of the groundwater-modeling tool Flex to generate simple models Methane 7.4 6.4 of contaminant transport from UCG combustion. The models included Water 33.0 33.0 thermal buoyancy effects on contaminant plume migration.

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burned coal seam. Ideally, remaining CO2 would be sequestered in deep geologic formations nearby. UCG processes cause thermal, geochemical, and geothermal changes to the surrounding rock reservoir, which may affect the reservoir’s ability or capacity for CO2 storage. Such changes include the effects of heating and quenching on fractures and rock properties. The reservoir may become more porous as acid leaches from ash, tars, char, coal, and rock minerals. Fluid densities in the reservoir may change because of high combustion temperatures. In addition, organic contaminants in CO2 and metals in acid groundwaters may become more soluble because of UCG. “At this point, CO2 storage in UCG zones comes with caveats,” says Friedmann. Additional research is essential to quantify and Groundwater modeling was used to simulate a hypothetical contaminant transport scenario in a characterize the effect of these processes subsurface homogeneous aquifer above a UCG coal seam. The model couples flow, heat, mass, before any CO2 can be pumped back and density to calculate changes in the temporal and spatial distribution of contaminants generated down near a UCG production facility. by the gasification process. In this simple case, the heat and contaminant sources are modeled for a If the CO2 is not sequestered in place, stationary ignition point, and groundwater flow rates are minimized to highlight the buoyancy effects it can be piped to oil fields. U.S. oil on the contaminant plume. The contaminant concentrations over time, from top left to bottom right, are companies can then inject it underground shown (purple is lowest, red–yellow is highest). to increase production from oil and natural gas wells, a process called enhanced oil recovery. The U.S. already leads the world These first simplistic models used a syngas usable. After cooling, the gas is in enhanced oil recovery technology, which homogeneous subsurface. The layering and filtered to remove ash and tar particles. represents an opportunity to sequester permeability contrasts that characterize Removal technologies are well established carbon at a lower cost compared with natural rock sequences associated with for hydrogen sulfide and ash products storing it in geologic repositories. Sales of coal seams were ignored. In this way, the such as arsenic, mercury, and lead. These the recovered oil and gas could generate researchers could isolate important thermal compounds are then disposed of safely. revenues to help offset the expenses of changes when predicting and assessing Hydrogen can be separated out for use sequestration. The only operating coal UCG environmental effects. alone, or it can be included as a component gasification plant in the U.S., at Beulah, Another environmental concern is that in the syngas, which is a mixture of North Dakota, has been piping its captured the void created by gasification may cause hydrogen and carbon monoxide. CO2 to oil fields in Canada for years. the land surface to subside. Subsidence is If the CO2 is to be captured at Livermore researchers have developed likely to be more of a problem if gasification the surface and sequestered, it must electromagnetic imaging and electrical occurs in a shallow coal seam, closer to the be separated from the syngas. The resistivity tomography to monitor the surface. This phenomenon also often occurs Laboratory is a leader in the field of CO2 injected underground and ascertain above long-wall underground coal mines but carbon management and is developing its location over time. Electromagnetic is less of a problem if the seam is deep. a number of separation or “capture” imaging was originally designed technologies. (See S&TR, May 2005, as an aid in oil recovery. Electrical Managing Greenhouse Gases pp. 12–19.) At a UCG production site, a resistivity tomography was designed At the surface, the various combustion significant percentage of the CO2 would for environmental research but has products are separated out to make the likely be sequestered in the void left by the since been extended for use in oil

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1 4 6 5

3 2

1 kilometer Geologic storage options for CO2 1 Depleted oil and gas reservoirs 2 Use of CO2 in enhanced oil recovery 3 Deep unused saline water–saturated reservoir rocks 4 Deep unminable coal seams 5 Use of CO2 in enhanced coal-bed methane recovery 6 Other suggested options (basalts, oil shales, cavities) Produced oil or gas

Injected CO2 Stored CO2 2 kilometers

Options for storing carbon dioxide (CO2) in deep geologic formations are represented here. (Courtesy of Intergovernmental Panel on Climate Change, 2005.)

fields. When existing well casings are been rediscovered globally, its future in all phases, from planning to operations used as electrodes, electrical resistivity maturation depends on the success of the to site closure. tomography is a nearly noninvasive and pilot tests that are just beginning. The U.S. —Katie Walter low-cost method for monitoring. Three- government has declared “clean coal” dimensional modeling at Livermore a critical goal for the near term, and the Key Words: carbon dioxide capture and storage, allows researchers to examine injection state of California and other government carbon sequestration, hydrogen production, scenarios in detail, including those entities have mandated the reduction of underground coal gasification (UCG). involving enhanced oil recovery, and CO2 emissions. “Current plans are for an to “test” monitoring tools in a virtual additional 120 coal-fired power plants environment before expensive prototypes around the world in the next decade,” For further information contact are built. says Friedmann. “UCG could be used to Julio Friedmann (925) 423-0585 ([email protected]). power many of them.” Success requires UCG for the Future that the right tools are available to Although the potential of UCG as a accurately assess the economic viability transformational technology for coal has and environmental consequences of UCG

Lawrence Livermore National Laboratory