&EFK United States Environmental Protection Agency Industrial Environmental Research Laboratory Cincinnati OH 45268 Research and Development EPA-600/S7-81-035 May 1981 Project Summary Environmental Effects of In Situ Gasification of Texas Lignite T. F. Edgar, M. J. Humenick, W. R. Kaiser, and R. J. Charbeneau A general survey of environmental effects of in-situ gasification of Texas lignite has been undertaken. The sur- vey has emphasized the following subjects: (1) Identification of the location, quality, and quantity of lignites in Texas: An exploration model for the various lignite-bearing formations has now been devel- oped, thus allowing evaluation of the commercial potential of various regions of Texas where deep basin lignite can be found. In addition, hydrological and baseline environmental data have been obtained for several areas in Texas. (2) Assessment of in-situ gasifica- tion technologies: The effects of various geological conditions, coal characteristics, and operat- ing conditions have been re- viewed with reference to Texas lignite. Recent field test data for both air and oxygen injection have been analyzed; it appears that medium Btu gas (oxygen- blown system) will be the most attractive product from in-situ gasification. (3) Determination of possible ad- verse environmental impacts (air, land, and water) resulting from underground coal gasifica- tion (UCG): Air pollution for UCG is essentially the same as for surface gasification, and overall air emissions are lower compared to those from a con- ventional solid coal burning fa- cility. Subsidence is the major land impact, but it is difficult to predict the extent of subsidence at a given site. The major impact to be considered from UCG is subsurface water pollution; field test data from Texas and Wyom- ing have been reviewed, both for organic and inorganic compounds. Applicable regulations are dis- cussed. (4) Evaluation of the dispersion of pollutant species and the appli- cation of mathematical models to predict pollutant transport: The literature on dispersion mod- eling has been reviewed and specific problems posed by typi- cal groundwater flows in Texas are discussed. Hydrological data at UCG test sites in Texas are reviewed. (5) Assessment of existing water pollution control technology to counteract pollution caused by UCG: The philosophy of site restoration, given that some natural renovation occurs under- ground after a test is performed, is discussed, and strategies for site restoration have been identi- fied. This Project Summary was developed by EPA's Industrial Environmental Research Laboratory, Cincinnati, OH, to announce key findings of the re- search project that is fully documented ------- in a separate report of the same title (see Project Report ordering informa- tion at back). Introduction The future use of underground coal gasification (UCG) in the state of Texas to produce a synthetic fuel to replace oil and gas appears to be both technically and economically feasible. UCG has some environmental advantages over conventional extraction and conversion technologies, in that strip mining is avoided and sulfur emissions are more readily controlled. However, one disad- vantage of UCG is that subsurface water pollution does occur, although the short- and long-term water quality impacts are difficult to predict. The evaluation of UCG for Texas has required analysis of engineering and geological data available in the litera- ture. Operating data from UCG tests, both in the U.S. and U.S.S.R., and geological, hydrological, and environ- mental quality data have been accumu- lated. Relevant data have been analyzed in order to assess the environmental (land, air, water) impact of UCG in Texas. Due to an inadequate data base in the above three areas, engineering analysis must be employed to reach quantitative and in some cases qualitative conclu- sions. Therefore this report should be considered as a preliminary evaluation of UCG in Texas and serves to identify those areas of research which should be pursued in more detail. The major alternatives for use of UCG include on-site power production using low Btu gas and manufacture of chemi- cal products from medium Btu gas. Oxygen injection and subsequent processing of the medium Btu gas to produce methanol or gasoline is a promising near-term technological al- ternative for UCG in Texas. Resources: A detailed analysis of well logs in the state of Texas has led to the development of a regional exploration model for lignite. This model has allowed the estimation of deep basin lignite re- sources which might be recoverable via underground gasification. The minimum seam thickness for economic or techni- cal feasibility has been selected as five feet; approximately 35 billion short tons of lignite lie in five distinct regions under less than 2000 feet of cover. These blocks have also been evaluated in terms of their hydrology and baseline water quality, although the data are limited. Overall geological criteria for selecting gasifier sites have been devel- oped. Preferred hydrological factors have also been identified. Figure 1 and Table 1 summarize the deep basin resources of lignite. Environmental Impacts Water Pollution Of the possible environmental impacts of UCG, the effects on local ground- water quality may be the most significant concern in implementation of the process. Documentation exists in the literature and from current projects that pollutants are released to groundwaters when normal flow returns to a post gasification zone. Above-ground processing of product gas streams can also present water pollution control problems. Although there has been no demonstration of large-scale gas cleaning facilities in conjunction with UCG field tests, the wastewaters generated should be similar to those produced by above-ground gasification facilities. Available data on inorganic and organic water pollutants for UCG field tests in Texas and Wyoming have been accumulated and analyzed. The organic compounds include light hydrocarbons, phenols, oils, and tars. Heavier organics include some polynuclear aromatic hydrocarbons (PAH's) and hetero-cyclic compounds. Other gaseous components that condense or are absorbed in sur- rounding groundwater are ammonia, carbon dioxide, hydrogen sulfide, and methane. Qualitatively similar pollutant profiles are obtained. Field data have indicated that the pollutants tend to decrease both with time and distance from the burn cavities. There are natural means by which the groundwater concentrations can be reduced. Most interpretations of field data attribute the improved water quality to adsorption and ion exchange of properties of surrounding strata, precip- itation reactions, dilution and dispersion by groundwater flow, and biological conversion reactions. It should be noted that only the last process, biological conversion, is ultimately useful in the final destruction or conversion of harm- ful contaminants to nonharmful prod- ucts. Inorganic pollutants are not as susceptible to attenuation as are the organic pollutants. The attenuation of phenol is shown in Figure 2. After gasification is completed, an ash residue remains in the burn cavity which yields soluble inorganic compo- nents to reinvading groundwaters. The soluble ash components can greatly increase the total dissolved solids con- tent of the groundwater. These soluble materials include a wide array of ionic species, mainly calcium, sodium, sulfate, and bicarbonate. These components yield increases in non-specific param- eters such as conductivity and total dissolved solids. The exact quantities of these materials are a function of initial coal composition, combustion tempera- ture, water temperature, and ground- water composition. There are, however, many other inorganic materials leached into the groundwater which are of interest even though they are present in lesser quantities. These include alumi- num, arsenic, barium, boron, iron, zinc, cyanide, selenium, and hydroxide. Again the exact compositions depend on the variables cited above. The conceptual model of pollutant generation and transport proposed in the literature appears valid. Materials having little affinity for solid surfaces, such as many of the TDS components. Table 1. Deep-Basin Resources in Texas (millions of short tons) x 10s short tons Percent Data (E-logs) Wilcox east central 10499 30 626 Wilcox north east 0 0 506 Wilcox Sabine Uplift 8814 25 396 Wilcox south 4631 14 363 Wilcox subtotal 23944 69 1891 Jackson east 5600 16 587 Jackson south 5275 15 547 Jackson subtotal 10875 31 1134 Yegua east 0 0 595 Total 34819 100 3620 ------- 'exarkana .••"•.... Drainage divide 4 Resource block number '//? s/'ru gasification field test Deep-Basin Lignite i;;i|'-| lower Wilcox Group Co/vert Bluff Formation (Wilcox Group} undivided Wilcox Group lower Jackson Group Manning and Wellborn Formations (Jackson Group) 40 80 mi 0 40 80 km W.R. Kaiser 1979 Figure 1. Distribution of deep-basin lignite in Texas. ------- 600 100 10 o 0) 0.1 0.01 0.001 7 JO 100 Distance from burn boundary — ft Figure 2. Concentration of phenolic materials at Hoe Creek I as a function of distance from burn boundary and time after gasification had ceased. will not be retained by surrounding strata and will travel at the rate of groundwater flow with little diminuation of peak concentrations. Those materials with medium to large affinity for solid surfaces will travel much more slowly and have significant decreases in peak concentrations as time progresses. Of particular concern is the deposition of tars and oils in the surrounding strata. Some of the materials comprising the tars and oils are toxic chemicals. It appears that the solubility of these components are very low and they have high adsorption affinity for surrounding solid strata. Thus, aqueous concentra- tions of these components should be low in the region of the tar deposits, and their high affinity for solid surfaces should prevent rapid migration in the water phase. However, considering solubility and adsorption only, it should take a very long time before the source of pollution is dispersed, but little quantitative information is available. Dispersion of potential groundwater pollutants has been analyzed and found not to be very significant, due to low groundwater velocities (typically one meter/year or less). Disturbance of aquifer flow patterns and local hydrology may, however, result from roof collapse after gasification is completed. An important consideration in dispersion analysis is the ability of the subsurface strata to adsorb potential contaminants, thus gradually improving water quality after gasification. The general mathematical models used for prediction and simulation of flow and transport are deemed to be adequate. The science and art of numer- ical modeling is highly developed, and there are few physical problems which defy analysis. The limitation of numeri- cal models lies in the difficulty in finding the appropriate parameter values to use in the models. Fractures within the porous media present a number of problems for modeling. There is no generally accepted model of the hydrau- lics of flow in fractured media. Also, development of fractures in the over- burden presents an avenue for increased cross-formation flow between neigh- boring water bearing units. Air Pollution While the water pollution impact from underground coal gasification is localized in nature, the air quality considerations in UCG have much broader geographical ramifications. This is because in-situ gasification offers the possibility of actually reducing regional air quality impacts, when compared to the alterna- tive of strip mining and burning solid fuel in a conventional steam-electric plant. If, however, the power generation plants are located at the lignite deposits themselves and the power is transmitted to the consuming centers on the Gulf Coast, there may be shifts in the state- wide air quality patterns. While the consuming areas will experience little or no adverse environmental impact, the predominantly rural areas where lignite deposits exist may experience some problems. This of course will also be tied to medium/ and long/range transport of pollutants in the atmosphere. A large number of lignite-based power plants are indeed planned for the future (up to 1989) at various locations along the Texas lignite belt. These are shown in the full report along with the plants which will use western (non-Texas) coals. All the lignite-based plants are presently scheduled to use conventional pulverized coal boilers. Table 2 summa- rizes the projected effect of underground coal gasification on the emission rates. Subsidence from Underground Coal Gasification The controlled operation of any under- ground coal gasification plant requires the ability to predict thein-situ behavior of the relevant rock strata, which in turn presupposes a knowledge of the me- chanical and physical rock properties. Without this ability unforeseen opera- tional and environmental problems may arise. Of particular importance in this context is the ability to predict roof stability and subsidence during and after the productive life of the operation. Significant fracturing, deformation, or collapse of the rocks surrounding the cavity can affect the overall operation in many ways. As discussed in Section 3 of the full report, roof collapse can be beneficial by minimizing void space underground. Excessive void space encourages gas phase oxidation of CO to CO2, which lowers the heating value of the product gas significantly and causes excessive gas outlet tempera- tures. The ultimate sweep efficiency of the process depends upon the complete- ness of the roof collapse. Moreover, if the immediate cavity roof is coal, as it may well be early in the development, collapse of this roof will be advantageous in giving a "self-fueling" effect. On the other hand, extensive fractur- ing of the surrounding rocks, or collapse of the roof, could cause serious opera- tional and environmental problems. Roof collapse could lead to excessive gas losses and excessive water influx, possibly causing problems of pollution in overlying aquifers. Finally, any defor- mation or collapse of the roof rock will ultimately be reflected on the surface as subsidence. ------- Table 2. Comparative End Use Air Pollutant Emissions from Combustion of Medium Btu Gas (UCG) and Direct Lignite/Coal Combustion-Houston-Galveston Air Quality Region (1985) Incremental emission rate, tons/year' Relative increase. Air pollutant Paniculate matter Sulfur oxides Nitrogen oxides Direct combustion 20,000 33,6002 (224.00O3) 172,000 UCG5 1,800 100 57,000 Regional total (1973)4 224,000 195.000 299,000 Direct combustion 9.1% 17.2% (1 14.6%3) 57.6% UCG 0.8% 19.2% Reduction obtained by gasification 90.8% 99.7% 66.6% Notes: 'Based on additional 19X106 tons coal/year required in 1985 to meet energy demands. 235% S02 removal. 3Uncontrolled SO2 emissions. 'Regional emission totals for Region 7 courtesy of Texas Air Control Board. s Based on emissions from combustion of natural gas (EPA AP-42, "A Compilation of Air Pollutant Source Emission Factors,' EPA Office of Air Quality and Standards, Research Triangle Park, N.C., 1977) Conclusions An extensive subsurface study of lignite in Texas has led to the develop- ment of a fairly complete regional exploration model for lignite. New resource maps for the Wilcox and Jackson Groups in South Texas have been completed. This model has allowed the estimation of deep basin lignite resources which might be recoverable via underground gasification. The mini- mum searrt thickness for economic or technical feasibility has been selected as five feet (1.5 m), and the maximum seam depth is considered to be 2000 feet (600 m); approximately 35 billion short tons of lignite, found in five regions, meet these criteria. Resource blocks have also been evaluated in terms of their hydrology and baseline water quality, although the data are limited. Overall geological criteria for selecting gasifier sites have been developed. The technical factors which are con- ducive to application of UCG for Texas lignites have been identified, including coal properties, seam thickness, water influx, and roof collapse. Water influx appears to be a crucial factor in Texas both in linking and gasification and is expected to be more of a problem in the Wilcox Group than the Jackson Group. Field test data from Texas sites, all in the Wilcox Group, have shown very high water influxes; mathematical model calculations for gas composition indicate that the thermal efficiency of the process suffers greatly for large water influx rates. This suggests that dewatering of a site may have a beneficial effect on heating value and thermal efficiency. Calculations on design and operation of dewatering wells could provide some insight as to the economic trade-offs of such wells. The role of production pressure in UCG to reduce water influx is also poorly understood. Prior esti- mates of water influx for a given site are desirable but are difficult to calculate with present knowledge. Field data have indicated that the pollutants tend to decrease both with time and distance from the burn cavities. There are natural means by which the groundwater concentrations can be reduced. Most interpretations of field data attribute the improved waterquality to adsorption and ion exchange proper- ties of surrounding strata, precipitation reactions, dilution and dispersion by groundwater flow, and biological con- version reactions. It should be noted that only the last process, biological conversion, is ultimately useful in the final destruction or conversion of harm- ful contaminants to nonharmful products. The impact from air emissions from UCG has much broader geographical ramifications than the water quality problem, which is more site-specific in nature. Air emissions from a UCG facility plus conversion plant are ex- pected to be lower than those resulting from conventional coal burning steam- electric plant. The gas cleanup opera- tions for UCG should be the same as those needed for surface gasification plants. For a given region, the lower emissions would mean that the available PSD increments in pollutant concentra- tions in ambient air are consumed to a lesser extent by each new source or major modification. This could promote a greater industrial growth in the region while minimizing adverse environmental effects on air quality. A qualitative analysis of existing emission sources in those regions where UCG might be used reveals few sources which affect air quality. However, a more detailed study involving meteorological considerations should be performed. The prediction of roof collapse and subsequent surface subsidence is not possible given the state of knowledge for large-scale rock mechanics. The surface subsidence is site-specific, and available field data do not allow any specific conclusions regarding predicted subsidence at Texas sites for a commer- cial operation. Only one U.S. field test (Hoe Creek III) has experienced measur- able subsidence. The relatively thin seams of Texas lignite do appear to be advantageous for minimizing surface movement. ------- T. F. Edgar. M. J. Humenick, W. R. Kaiser, and R. J. Charbeneau are with the University of Texas, Austin, TX 78712. Robert Thurnau is.the EPA Project Officer (see below). The complete report, entitled "Environmental Effects of In Situ Gasification of Texas Lignite." (Order No. PBS 1-171 654; Cost: $14.00, subject to change) will be available only from: National Technical Information Service 5285 Port Royal Road Springfield. VA 22161 Telephone: 703-487-4650 The EPA Project Officer can be contacted at: Industrial Environmental Research Laboratory U.S. Environmental Protection Agency Cincinnati, OH 45268 > US. 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