Underground Coal Gasification

David W. Camp Joshua A. White Underground Coal Gasification: An Overview of Groundwater Contamination Hazards and Mitigation Strategies March 2015 Lawrence Livermore National Laboratory LLNL-TR-668633 Disclaimer This document was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor Lawrence Livermore Na tional Security, LLC, nor any of their employees makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or useful ness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or Lawrence Livermore National Security, LLC. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or Lawrence Livermore National Security, LLC, and shall not be used for advertising or product endorsement purposes. v Acknowledgements This work was funded by a 2012 Applied Science Grant from the Office of Surface Mining Reclamation and Enforcement. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE AC52-07NA27344. The authors wish to thank Duane Matt and other members of the OSM Underground Coal Gasification Working Group for their support and recommendations. We also thank the Clean Air Task Force (CATF) for earlier funding that allowed LLNL to begin investi gating groundwater contamination issues. Gas sampling recommendations benefitted from research funded by the DOE Basic Energy Sciences Program. vii Preface All energy production technologies produce environmental impacts, and create environ mental risks. It is always difficult to weigh the benefits of additional energy production against the risks of that activity. Further, if a venture is approved, it is difficult to balance the trade-offs between environmental protection and costs. The main purpose of this report is to provide information and understanding so that these difficult decisions can be made as well as possible. The hazard of groundwater contamination by underground coal gasification (UCG) op erations is real and must be addressed seriously by all projects. In the past, some UCG projects have unacceptably contaminated groundwater resources, while others have been carried out safely. The goal of this work is to provide an understanding of specific con tamination hazards and mitigation best practices. This report describes how UCG can work without contaminating groundwater. It also describes things that can go wrong. The under lying goal is to identity mitigation strategies and best practices to minimize groundwater risk. The audience for this report is regulators, project developers, and stakeholders with a wide range of expertise in UCG. The emphasis is education and information, not prescrip tion. The phenomena are too complex and the range of possible specifics are too wide for specific prescription. Each case will be unique and require detailed assessment and analy ses, using this document to guide appropriate questions and critical review. ix Summary Underground coal gasification is the in situ conversion of coal into an energy-rich product gas. It takes place deep underground, using chemical reactions to consume the coal and grow a cavity. Gas wells, drilled into the coal seam, inject reactant air, oxygen, and/or steam to sustain the reactions. Production wells then extract the product gas. Preventing groundwater contamination during a UCG operation requires care at many steps. Key requirements include: • Selecting a favorable site • Analyzing failure modes • Careful and conservative design, construction, and testing • Conservative operations • Quality-assurance and operational controls • Monitoring, by a combination of measurements and modeling • Early detection and correction of unwanted conditions or escaping gas • Proper closure procedures • Post-closure monitoring and management During normal UCG operation contaminants are continually generated, destroyed, and removed, leaving only small amounts confined locally. The gas within parts of the UCG cavity during operations will contain many organic contaminants, including those formed by pyrolysis of the coal. These species will include aliphatic and, especially, aromatic hydrocarbons—including benzene, polyaromatic hydrocarbons, and phenols. In proper op eration virtually all of this gas will be converted to a more benign composition, produced out of the ground within contained piping, and properly separated above ground. At termination of a properly operated module, a small inventory of contaminant species will be present in the cavity and its walls and rubble zone. Post-burn module cleaning operations—such as nitrogen sweep, steam sweep, and/or gradual steam-producing wa ter infiltration—can greatly reduce the local inventory of these contaminants. These post- burn cleaning operations—coined “Clean Cavern” by the Rocky Mountain 1 team—were demonstrated during the Rocky Mountain 1 test and are being evaluated in recent Australian UCG panels. Thus, a properly operated and cleaned UCG module will terminate with a small quantity of organic contaminants present in the immediate vicinity of the cavity. xi Preface Transport of contaminants outside the local confinement zone is abnormal, either during operations or after closure. It will occur if there is an outward pressure gradient and a pathway for flow. Whatever the amount and distribution of residual contaminants, their impact is dependent on the proximity and value of nearby groundwater resources, and the transport barriers (or hydrologic gradients) between the contaminants and protected waters. UCG should be avoided within some keep-out distance of a high-value ground water resource. Site-specific geomechanical and hydrologic studies, coupled to the UCG designs being considered, will be needed to set this keep-out distance as a function of various fac tors. Qualitatively, one wants to avoid having open pathways extend up into a valuable aquifer. Over the years, the UCG community has improved in its understanding and approach to mitigating contamination risks. Some of the general approaches include the following: • The operating pressure of the UCG cavity must be controlled below the surrounding water pressure at the highest gas-connected part of the cavity. • The UCG production wells and their completion must be designed and constructed prop erly to maintain their integrity despite thermal and mechanical stresses. • As with any industrial operation that produces waste-water, care must be taken to keep it from spilling at the surface. • Careful geotechnical analysis must be carried out on the geomechanical behavior of the open (or rubble filled) cavity, possible extent of roof collapse, and likely fracture extents. • In general, narrower cavities with larger pillars will increase subsidence protection, but at the expense of higher well-placement costs and reduced resource recovery. • Monitoring of the operation, including indications of vertical cavity growth, is important. • Rings of groundwater sampling wells can typically provide early detection of any prob lems close to the operation, while confirming far-field cleanliness in outer, regulatory- compliance rings. • A method for detecting gas leakage to the formation can be used, as it may detect contamination-spreading conditions earlier and further away. • A proper shutdown and follow-up procedure should be followed, along the lines of the Clean Cavern concept demonstrated at Rocky Mountain 1 and other subsequent field tests. Careful analysis and understanding of likely failure modes will help prevent and minimize impacts. This document provides a general description of the relevant processes, potential failure modes, and practical mitigation strategies. It can guide critical review of project design and operations. xii Contents 1 Introduction to UCG .................................................. 1 1.1 What is UCG? .................................................... 1 1.2 Major hazards .................................................... 3 1.3 Physical Processes ................................................ 4 1.4 Gasifier Designs .................................................. 8 1.5 Process waste and byproduct streams ................................. 11 2 Site Selection and Characterization ..................................... 15 2.1 Overview of Site Characteristics ..................................... 15 2.2 Regional Structural Geology ........................................ 16 2.3 Local Coal Seam Characteristics ..................................... 17 2.4 Hydrologic Properties .............................................. 19 2.5 Geomechanical Properties .......................................... 20 2.6 Coal Rank, Properties, and Chemical Attributes ........................ 22 3 Contaminant Behavior During Proper Operation ........................ 25 3.1 Generation of contaminants ......................................... 25 3.2 Destruction of organics during operation .............................. 31 3.3 Containment and removal of contaminants during operation . 33 3.4 Removal of more contaminants during

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