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--> 9 Advanced Research Programs The present chapter provides a brief overview of the organization and budgets for the DOE's Office of FE advanced research programs relating to coal. The DOE and committee perspectives on the role of advanced research for coal-based technologies are then presented. The chapter concludes with a brief discussion of opportunities for advanced research in three areas: combustion and gasification, coal conversion and catalysis, and materials. It is not the intention of the committee to provide a comprehensive list of research opportunities for coal-based technologies but rather to highlight key areas. The specific research opportunities discussed were identified by the committee on the basis of its review and analysis of current DOE programs (chapters 5 through 7), and particular importance was accorded activities unique to coal technologies. In each case the proposed research is directed toward meeting, and ultimately exceeding, DOE's targets for advanced coal-based power systems and the production of clean fuels from coal. PROGRAM ORGANIZATION AND BUDGETS The advanced research programs within the DOE FE coal R&D program consist of a set of cross-cutting programs within the AR&TD (Advanced Research and Technology Development) budget category and a set of technology-specific programs falling under the general category of Advanced Research and Energy Technology (AR&ET), formerly known as Advanced Research. The AR&ET technology-specific programs fall within the Advanced Clean Fuels and Advanced Clean/Efficient Power Systems budget categories (see Table 2-1). This
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--> program organization responds to a directive from Congress stating that, for ease of budget presentation and receiving testimony, advanced research directly related to a specific coal area should be presented both in the budget and in testimony as part of the total program for that specific technology area, rather than as part of the AR&TD budget category. The AR&TD program includes both research and nonresearch portions. The Technology Crosscut activities within AR&TD (see Chapter 2) include all nonresearch areas, namely, Environmental Activities, Technical and Economic Analyses, International Program Support, and Coal Technology Export, as well as two advanced research areas, specifically, Instrumentation and Diagnosis and Bioprocessing of Coal. For the purposes of the present discussion, the nonresearch portion of the AR&TD program will not be considered, and corresponding budget data are not included in Tables 9-1 and 9-2. Table 9-1 presents the funding history of advanced research programs on coal since 1988. The FY 1995 budget request represents the DOE and administration proposal to Congress. When these budget numbers are expressed in constant dollars, it can be seen that there was a decrease of approximately 30 percent in the advanced research budget between FY 1988 and FY 1994, with an additional decrease of approximately 25 percent from the FY 1994 level proposed for FY 1995. A more detailed comparison between the FY 1994 enacted appropriation and the 1995 congressional request is shown in Table 9-2, which also shows in more detail the advanced research activities funded under AR&TD and AR&ET. Major budget reductions are proposed in FY 1995 for the programs in materials (25 percent), components (50 percent), and, most notably, coal liquefaction (85 percent). It is proposed that the FY 1994 budget of $5.2 million for coal liquefaction be reduced to $0.8 million in FY 1995. TABLE 9-1 Trends in Advanced Research Budgets (millions of current dollars) Area FY FY FY FY FY FY FY FY 1995 1988 1989 1990 1991 1992 1993 1994 (request) AR&TD (research) 21.1 20.2 21.2 22.9 23.0 20.4 22.6 19.8 AR&ET (fuels) 6.1 6.9 6.8 8.1 7.1 7.4 5.2 0.8 AR&ET (power systems) 8.1 7.4 7.1 8.2 7.3 2.9 2.1 1.8 Totala 35.3 34.5 35.1 39.2 37.4 30.7 29.9 22.4 (41.1) (38.5) (37.5) (40.3) (37.4) (29.9) (28.4) (20.7) a Figures in parentheses represent total budget in constant 1992 dollars. Source: Personal communication from David Beecy, U.S. DOE, to Jill Wilson, National Research Council, July 20, 1994.
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--> TABLE 9-2 Advanced Research Budgets for FY 1994 and FY 1995 (millions of current dollars) Area FY 1994 (enacted) FY 1995 (requested) AR&TD Coal Utilization Science 3.1 3.1 Materials 8.9 6.9 Components 1.7 0.9 Bioprocessing of Coal 1.9 1.9 University and National Laboratory Coal Research plus University Coal Research 5.0 5.0 HBCUs,a Education, and Training 1.0 1.0 Instrumentation and Diagnostics 1.0 1.0 Subtotal (AR&TD) 22.6 19.8 AR&ET Advanced Clean Fuels Research Coal Liquefaction 5.2 0.8 Advanced Clean/Efficient Power Systems Combustion Systems 0.5 0.4 Control Technology and Coal Preparation 1.1 1.0 Surface Coal Gasification 0.5 0.4 Subtotal (AR&ET) 7.3 2.6 TOTAL 29.9 22.4 a Historically black colleges and universities. Source: DOE ( 1994c). THE ROLE OF ADVANCED RESEARCH DOE Perspective A perspective on the mission, vision, and goals of the FE advanced research activities is provided in a recent document from DOE (1994b). The role of advanced research within the FE program is "to stimulate, nurture, and advance critical enabling science and technologies for fossil energy systems." A series of advanced research goals, strategies, and success indicators have been selected to support relevant DOE business lines and the core Office of FE business lines of clean fuel systems and clean/efficient power systems (see Chapter 2) and to directly reflect customer and stakeholder expectations. The goals are as follows: Provide the core competencies in the critical enabling science and technologies that enable the Office of FE business lines to succeed in their missions. Through feasibility testing, identify and nurture innovative concepts for advancing the technology and removing barriers to achieving the Office of FE's business line goals.
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--> Provide the fundamental data, information, materials, and tools required by the U.S. fossil energy industry to bring advanced fossil energy systems to commercial fruition. Improve the environmental performance of fossil energy systems by performing research that significantly increases system efficiencies, provides advanced environmental control systems, and shifts from waste management to pollution prevention/waste minimization. The Committee's Perspective Based on its analysis of likely future trends in coal use and ongoing DOE coal programs, the committee observed that the use of coal for power generation is confronting an increasingly demanding set of requirements. Following many years of gradual improvement of pulverized coal-steam turbine baseload power systems, with limited add-ons for emissions control, there is a need for greatly enhanced technology for emission control, for improved efficiency, and for improvements in the overall economics of power generation. Similarly, during the time periods considered in this study it is probable that liquid and gaseous fuels manufactured from coal will be needed. Improvements in the cost and efficiency of manufacturing processes will depend on further advances in the chemistry and engineering related to coal use. In light of the continuing needs for advances beyond the 2010 targets defined for the power systems and fuels programs (Chapter 2) and the goals defined in DOE's Strategic Plan (DOE, 1994a), the committee identified a critical role for DOE advanced research programs on coal. Such programs have the potential to exploit the extensive opportunities for improved coal technology while compensating for the decline in industrial and non-DOE government support for long-range research on coal. The optimum role for DOE differs from one advanced research area to another but is largely determined by technology needs and their degree of specificity to coal-based systems and by complementary research activities in industry and government organizations outside DOE. The following discussions of some major research areas address opportunities for DOE advanced research programs to contribute to the development of coal technologies. The research areas discussed are combustion and gasification, coal conversion and catalysis, and materials. COMBUSTION AND GASIFICATION Research on oxidation of fuels to provide useful energy with acceptable emissions is the subject of a large international activity. Much current work relates to gas-phase reactions and to soot formation and oxidation (see, for example, The Combustion Institute, in press). However, coal combustion research falls outside these areas because of the large amount of char formed by the
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--> pyrolysis step and because of the ash content of coal. Problems directly related to coal—such as emissions, waste products, and char oxidation efficiency—are receiving far less attention than problems relating to other fuels. Much of the recent advanced research on coal-related combustion issues, notably the interaction with coal ash and the final stages of oxidation, has been conducted in the United States, principally under DOE and, to a lesser extent, National Science Foundation sponsorship. The committee noted that, in the absence of research needs and funding from other sources, DOE support is important to achieve progress in quantitative understanding of coal-related combustion and gasification issues and to identify innovative concepts for further investigation. While still a promising area for research, gas-phase chemistry of NOx formation and destruction and of the oxidation of carbon monoxide (CO) and hydrocarbons, has advanced to the point where simplified gas kinetic models can be used in conjunction with primitive turbulence modeling as a semiquantitative design and development tool for low-emission furnaces and gas turbine combustors. However, in the case of coal, the early release of gas-phase hydrogen cyanide introduces NOx production pathways not yet quantitatively explored. Moreover, promising research opportunities still exist, including implementation of more sophisticated models. In contrast, the understanding and quantitative treatment of carbon kinetics, taking into account catalytic and physical interactions with ash and graphitization of carbon as the oxidation process proceeds, is at a relatively primitive stage. Since future innovations in coal gasifier and combustor design will depend, to a considerable extent, on quantitative understanding of the interaction between pyrolysis, carbon oxidation, and emissions, the committee noted that DOE's advanced research program for coal needs to address this issue. The final stage of carbon oxidation is of special interest because of the observed reduction of reactivity at high conversion rates (Davis et al., in press). The long reaction times and high temperatures required for high carbon conversion will increase thermal NOx formation in the presence of excess air. The interactions involved are complex, and improved quantitative understanding of the evolution of carbon reactivity and its interaction with the physical and catalytic properties of the coal ash is needed for choice of optimum levels of carbon oxidation. Two major advanced research opportunities were identified by the committee as a basis for improving high-performance gasification systems. In low-temperature and countercurrent fixed-bed gasification processes, escape of fuel nitrogen as ammonia can occur, resulting in the formation of additional NOx on combustion if not removed. Quantitative treatment of this problem is needed for improvement of these processes. For low-temperature gasification processes where high carbon conversion is needed, catalysis of carbon gasification by ash constituents, such as calcium, or by added catalysts remains a promising area related to future advances in gasification efficiency.
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--> COAL CONVERSION AND CATALYSIS The complexity of coal structure and chemistry has important implications for conversion technologies and catalysis. Coals are inhomogeneous on the macroscopic, microscopic, and molecular levels. They are insoluble, opaque, macromolecular systems composed of a mixture of organic and inorganic constituents. While knowledge of coal's physical and chemical structures remains rudimentary, knowledge and understanding of coal reactivity are even more limited. Most of the available tools for determining chemical structure are designed to work with systems of pure compounds and either do not work when applied to coals or become much more complex in their application. The efficacy of solid catalysts when used with solid coals decreases very significantly compared to their effectiveness in fluid systems. Opportunities exist to develop entirely new catalysts that will contact coals and effect desired reactions. The committee identified a role for DOE in supporting advanced research on coal conversion and catalysis to ensure the cleanest and most efficient utilization of coal, consistent with the goals of the advanced fuels and power systems programs, and to compensate for the absence of significant industrial research in this field. In reviewing current DOE coal advanced research programs, the committee particularly noted the decline in efforts devoted to coal liquefaction technology. Given the likely growth in importance of coal liquids in the mid and long-term, as described in the committee's strategic planning scenarios (see Chapter 4), the committee identified coal liquefaction as an important area for advanced research within the DOE coal program. Industrial transformations of fossil fuels are catalytic, and the creation of new and improved catalysts and better reactors to use those catalysts has been a central thrust of fuel chemistry for almost a century. The use of catalytic chemistry with coals presents unique and difficult problems. Since coal is a solid, it cannot move around into contact with a catalyst surface. Thus, the use of immobile solid catalysts typical of oil and gas processing is not possible with coal. It is necessary either to render the coal fluid, to use catalysts of extraordinarily high dispersion, or to use catalysts that are themselves mobile fluids. All three approaches have been used with some success, and there has been a fairly continuous improvement in catalysts used. Further enhancements can be anticipated based on a mix of applied and fundamental studies on topics such as highly dispersed catalysts, diffusion in coals and coal-catalyst contacting, and effective mobile catalysts. Both lower-temperature catalysts and more selective chemistries have the potential to reduce costs. Research opportunities can be conveniently divided into two major categories: improvements in current processing chemistry and technology and liquefaction processes based on new chemistries. Possible improvements in chemistry and technology (see also Chapter 6) include: low-pressure reaction at 2.17 MPa (300 psig) or less;
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--> use of low-cost subbituminous coal or lignite, especially deposits having high hydrogen-to-carbon ratios; removal of coal oxygen as carbon dioxide; complete conversion to liquids with boiling points below 540 °C (1000 °F); improved selectivity to minimize production of hydrogen, water, and hydrocarbon gases; coproduction of high-value chemical and other nonfuel products; and direct use of gas from IGCC systems equipped with hot gas cleanup for F-T synthesis and to produce hydrogen for direct liquefaction. For direct liquefaction, existing processes require cold gas cleanup, shifting to convert carbon monoxide and water to hydrogen and CO2, and scrubbing to remove CO2. There would be significant energy and capital cost savings if the hot gasifier gas could be used without cooling and further processing. Water/gas shift activity in the catalyst system used would be desirable; however, currently available catalysts are not sufficiently sulfur resistant. The use of hot gasifier product for F-T synthesis would require new catalysts capable of carrying out the reaction in the presence of the sulfur concentrations and traces of heavy metals remaining after hot gas cleanup. More active or selective sulfur-tolerant catalysts could markedly improve both direct liquefaction and the upgrading of coal liquids. Alternative process chemistries of potential interest include: coprocessing based on alkylation or transalkylation chemistry rather than hydrogenation, oxidative depolymerization to oxygenate fuels, and new depolymerization chemistry followed by fixed-bed catalytic upgrading. The DOE AR&TD budget for bioprocessing of coal was $1.9 million in FY 1994, and the same funding has been proposed for FY 1995. The main thrusts of the bioprocessing program in recent years have been to explore and apply recent advances in biotechnology to convert coal to liquid fuels and to improve the environmental acceptability of advanced power systems. Activities have included characterization of the metabolic features of bacteria found to remove organic sulfur, mineral matter, and metals from coal and investigation of mechanisms for bioconversion of coal. Most experts in the field now agree that biotechnology is best suited for the manufacture of high-value-added products and is least well suited for the production of very large amounts of low-value-added materials, as in the case of coal processing. Thus, current and proposed future DOE coal program efforts in biotechnology will focus on cleanup of sulfur- and nitrogen-containing compounds in combustion gases, rather than on coal desulfurization and demineralization. The committee notes that, although there are possible opportunities for biological cleanup of flue gas (NOx and SO2 removal), significant
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--> technological difficulties remain because of the relatively long processing times and large volumes of gas to be treated. MATERIALS General Comments R&D aimed at developing high-performance materials designed to operate in hostile environments is a very large and active area of endeavor worldwide. Given the limited resources of DOE's coal advanced research program in materials, the committee identified a need for this program to focus on key materials development issues for coal-based technologies while leveraging more generic materials developments from other programs. The preceding review of DOE's coal R&D programs given in chapters 5 through 7 has been used by the committee as a basis for identifying opportunities in materials research specific to coal-based technologies. Three areas have been selected for emphasis and are discussed below: advanced gas turbines; high-temperature, high-pressure heat exchangers; and inorganic membranes. The present discussion is not intended to provide an exhaustive list of materials research opportunities relevant to the coal program but rather to highlight key materials-based enabling technologies critical to the success of DOE programs in advanced clean fuels and advanced power systems. Advanced Gas Turbines Many of the advanced coal-based power generation technologies currently being developed incorporate gas turbines (e.g., IGCC, advanced PFBC, direct coal-fired gas turbines, and IFC [indirectly fired cycles]). Thus, gas turbines constitute a key component in advanced coal-based power generation technologies. The ATS (Advanced Turbine Systems) program, funded under the natural gas component of the FE R&D program budget, aims to develop advanced land-based turbines for natural gas systems but adaptable to coalor biomass-derived fuels. The systems efficiency target using natural gas is greater than 60 percent based on lower heating value (approximately 55 percent HHV equivalent). Many generic materials issues,1 such as increased temperature capability and extended operating lifetime, are being addressed in the ATS program by DOE and industry participants, and related developments for natural-gas-fired turbines should be broadly applicable to turbines using coal-derived fuels. The committee recommends that activities in the FE coal R&D program focus on materials issues specific to the use of coal-derived fuels in advanced turbines. 1 See NRC (1986) for an assessment of materials needs for large land-based gas turbines.
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--> All attempts to date to direct fire gas turbines with coal have resulted in significant ash deposition and corrosion of hot gas path components as a result of the aggressive chemical nature of the products of coal combustion (LaHaye and Bary, 1994). The development of turbine materials capable of surviving the hostile environment of direct coal-fired systems represents a major challenge. In the case of gasification-based systems, the environmental constraints imposed on the turbine materials are less demanding than in the case of direct coal firing but more severe than in a natural-gas-fired system. Coal gasification produces a raw syngas consisting mainly of carbon monoxide and hydrogen but with substantial quantities of CO2 and water; minor quantities of hydrogen sulfide, ammonia, and hydrogen chloride; and a few parts per million of alkali metals (NRC, 1986). While unprocessed natural gas can contain large amounts of hydrogen sulfide, pipeline natural gas contains no hydrogen sulfide and a sulfur weight fraction of only 0.000007 (DeLuchi, 1993). The major issue associated with the use of coal-derived gas in advanced turbines is the effect of contaminants, notably sulfur and alkali metals, on turbine performance (operating temperature and lifetime). Possible penalties in the overall efficiency of gasification-based systems can be anticipated based on the need to operate at lower temperatures to reduce the corrosive effects of contaminants in the coal-derived gas. Corrosion is also likely to severely reduce the lifetime of the turbine components. The ability of hot gas cleanup systems to reduce contaminants to levels acceptable for high-temperature advanced turbines has not yet been demonstrated. Reverting to cold gas cleanup would involve an efficiency penalty of one to three percentage points. From a materials perspective, the critical issue for coal gas-fired systems is the extent to which corrosion-resistant turbine blade materials and coatings can increase the environmental tolerance of advanced turbines, thereby reducing (or eliminating) the need for gas cleanup and possible associated efficiency penalties. Allowable levels of contaminants depend on engine design and turbine pressures and temperatures, but the corrosion problem is likely to be the most severe for the first-stage blades that are exposed to the highest temperatures and the full concentration of impurities in the gas stream (Bernstein and Allen, 1992). Given the increased likelihood of environmental attack, evaluation of candidate material systems for coal-fueled turbine systems is necessary, with an accompanying search for better materials. The superalloys currently used for turbine blades are generally protected from high-temperature oxidation and corrosion attack by a variety of coatings. Formation on the coating surface of reaction products—specifically, adherent alumina or chromia scales—retards subsequent reaction between the coating and the environment. A recent review of high-temperature coatings for combustion turbine blades (Bernstein and Allen, 1992) addresses coating requirements for protection from different types of environmental attack. Since fuel type is probably the most important variable influencing the choice
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--> of a coating, the use of gas derived from coal gasification is likely to have a significant impact on the choice of turbine blade coatings. The complex chemical reactions that occur at high temperatures, and the susceptibility of these reactions to small chemical changes in the coating and gaseous environment, suggest that significant effort will be necessary to develop and evaluate coatings for turbines used in coal gasification-based power systems. There are a large number of commercial coatings available, and a number of different application methods that influence the coating behavior, but there is no one coating that is resistant to all types of high-temperature attack. It has been suggested that in systems using coal-derived fuel, coatings on advanced superalloys and the alloys themselves will need to form chromia rather than alumina scales for increased corrosion resistance (Bannister et al., 1994). In the case of the substrate (blade) materials, this constraint may limit the availability of suitable high-strength alloys. Recently, the use of thermal barrier coatings (TBCs) has proven extremely useful in extending the temperature capabilities of existing superalloys. TBCs are ceramic coatings applied over metal substrates to insulate them from high temperatures. They consist of a layer of stabilized zirconium oxide that is 0.12 to 0.38 mm (0.005 to 0.015 inches) thick applied over a bond coat composed of an oxidation-resistant metal coating. Although TBCs themselves are expected to be only minimally corroded by the more aggressive environment in coal-fueled turbines, both the substrate and the bond coat may be adversely affected. The development of alternative turbine materials with higher-temperature capability than existing superalloys—notably monolithic ceramics and ceramic matrix composites—is being addressed in the ATS program. The potential improvements in high-temperature corrosion resistance of ceramic materials compared to state-of-the-art superalloys is of interest for turbines using coal-derived fuels. Heat Exchangers In terms of materials behavior, the critical requirements for the ceramic heat exchanger for EFCC power generation systems (see Chapter 7) are to maximize operating temperatures for the proposed duty cycle, notably combinations of high-temperature and pressure; to resist fouling and alkali corrosion, with emphasis on the latter for low-rank coals; and to avoid catastrophic failure. Although advanced ceramics offer excellent high-temperature properties, such as high strength, corrosion and erosion resistance, and refractoriness, they are subject to brittle fracture due to critical flaws. High-velocity fragments from a failed ceramic tube have the potential to initiate rapid sequential failure of the array of ceramic tubes in the heat exchanger. The current proprietary tube design permits ''graceful" rather than catastrophic failure.
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--> Advanced structural ceramics with increased temperature capability and improved toughness are under development in a number of government/industry programs, including the ATS program (see above), the Integrated High-Performance Turbine Energy Technology program, including the U.S. Air Force, Navy, Army, Advanced Research Projects Agency, and the National Aeronautics and Space Administration (NASA), and the NASA Enabling Propulsion Materials program. Materials developed in these and other programs for high-temperature gas turbine applications may offer the higher operating temperatures and improved brittle fracture characteristics required for the ceramic heat exchanger in EFCC power generation systems. Since the proposed ceramic heat exchanger involves no moving parts, it is significantly less susceptible to deterioration from ash deposition or corrosion than are rotating components in the gas path of a turbine (LaHaye and Bary, 1994). However, the ash deposition and corrosion problems encountered using pulverized coal and the high-pressure cycles encountered in EFCC applications are unlikely to be addressed in materials development programs that are not targeted at coal-based technologies. In the view of the committee, the DOE coal materials program should focus on such issues specific to coal-based systems. Current materials development and testing of the ceramic heat exchanger for EFCC systems is being conducted by Hague International (Orozco and Vandervort, 1993; Vandervort et al., 1993; Orozco, 1993; LaHaye and Bary, 1994). Activities are focusing on pressure and environmental testing. Over 2 million hours of successful operation of low-pressure ceramic heat exchanger units in corrosive high-temperature industrial environments has already been demonstrated. A series of tests is planned to demonstrate that a complete ceramic heat exchanger can contain pressures up to 1.21 Mpa (175 psia), endure at least 100 hours of operation under static and dynamic loadings, and meet thermal performance requirements. During these tests, the combustor will be fired with natural gas for operational simplicity. Subsequent testing with a coal-fired combustor will verify the ability of the slag screen to protect the ceramic heat exchanger from coal ash. Ceramic materials demonstrate superior corrosion resistance compared to conventional metals and superalloys but can be severely degraded by alkali metals in coal combustion products. In particular, nonoxide ceramics such as silicon carbide (SiC) corrode in an oxidizing environment. The corrosion process is affected by the material processing technique, grain size, and impurity content. Hague International has conducted a series of corrosion tests on 46-cm (18-inch) long, 2.5-cm (1-inch) diameter tubular coupons of candidate heat exchanger materials, notably, an alumina matrix composite, reaction-bonded SiC, mullite (orthorhombic aluminum silicate, Al6Si2O13), and monolithic alumina (Al2O3). Preliminary results indicate that mullite shows the highest temperature capability and good corrosion resistance. After 300 hours at 1090 °C (2000 °F) with brief excursions to 1480 °C (2700 °F), little corrosion was observed.
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--> Despite performance enhancements in advanced ceramics, a temperature limit of approximately 1090 °C (2000 °F) currently exists for ceramic heat exchanger materials. A report published in the late 1980s (OTA, 1988) noted that federal government support has been necessary to accelerate development of the ceramic materials and system technology for heat exchangers, despite projected economic and performance advantages. Material manufacturers and end users have considered the technical risks too high to invest their own funds in systems development and implementation. Membranes Membranes play a key role in the production of fossil-fuel-based products that meet composition standards for engine and combustor performance and provide environmental compliance through the removal of pollutant molecules (NRC, 1993). Possible applications of membranes to coal-based systems include the separation of hydrogen from coal gas streams and of impurities such as hydrogen sulfide (H2S), ammonia (NH3), SO2, NOx, and trace metal compounds from coal conversion (e.g., gasification) and combustion (flue gas) streams. Such separations can account for a major fraction of the investment and operating cost for coal-based systems. A particularly important application for advanced clean/ efficient power systems is the cleanup of coal gasification streams to drive advanced turbines. As discussed above, the ability of hot gas cleanup systems to reduce the contaminants to levels acceptable for high-temperature advanced turbines remains to be demonstrated. Another possible application of membranes is for the separation of methane from very dilute coalbed methane streams (see Chapter 5). Low-temperature polymer membrane technology is fairly well developed and is useful for liquid-liquid, liquid-gas, and gas-gas separations (DOE, 1992). However, polymer membranes are limited to relatively low temperatures (less than 250 °C [480 °F]) and are subject to chemical and abrasive attack, particularly in the aggressive environments encountered in coal-based systems. Inorganic (ceramic) membranes have the potential to operate at the high temperatures required for advanced power generation systems (e.g., 815 °C [1500 °F] for removal of hot gas particulates from advanced PFBC and IGCC systems) and to provide significantly enhanced corrosion and erosion resistance compared to polymer membranes. Other expected advantages of advanced inorganic membranes include high permeability (1,000 to 10,000 times organic membrane permeability) and high selectivity (DOE, 1993). In materials terms, refractory behavior and resistance to environmental attack depend on a suitable choice of ceramic material and associated fabrication process. Possible problems can be anticipated in coal-based systems due to reaction of candidate ceramic membrane materials—such as alumina, zirconia, and silica—with gas stream components, notably SO2 and alkali metals, at tempera-
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--> tures in the range of 540 °C to 1090 °C (1000 °F to 2000 °F). The presence of steam is likely to accelerate the degradation process. Requirements for high separation efficiency impose further materials constraints in terms of pore size distribution and mean pore size in the membrane. A high degree of control during membrane fabrication is necessary to achieve the desired microstructural features. Ceramic membranes consist of a porous support a few millimeters thick, a porous intermediate layer 10 to 100 microns thick with pore diameters in the range of 0.05 to 0.5 microns,2 and the separation layer with a thickness of 1 to 5 microns (Burggraaf et al., 1989). Generally, the separation layer must have pore diameters less than 10 nm for effective separation of gaseous components by diffusion (Krishnan et al., 1993); in some cases a mean pore size of 2.5 nm may be necessary. 3 Current commercially available membranes do not meet all performance requirements for cleanup of coal-gas and flue gas streams, although several manufacturers produce inorganic membranes for micro- and ultrafiltration applications, and some of these have pore diameters less than 10 nm and are capable of separating gaseous components. However, extensive membrane technology has been developed over the past 40 years for nuclear gaseous diffusion applications, and alumina and zirconia membranes have been used for the separation of uranium hexafluoride (UF6) isotopes for the nuclear industry since 1950 (Krishnan et al., 1993). Current DOE programs to develop ceramic membranes for coal-based applications are attempting to leverage this existing knowledge base. Investigators at the Oak Ridge Gaseous Diffusion Plant have produced alumina (ceramic) membranes with pore radii as small as 70 nm. Membrane separation tests have demonstrated a capability to separate hydrogen from gas mixtures (DOE, 1992). Membrane material research opportunities specific to coal-based systems involve primarily the development of inorganic membranes for separation of coal-derived products and impurities at elevated temperature and in corrosive environments. Improvements can be anticipated in the selectivity and separation efficiency based on enhanced understanding of the relationship between pore structure and the physical chemistry of molecular separations (NRC, 1993). Opportunities also exist for the development of membranes with improved resistance to the environments characteristic of coal-based systems, such that operating lifetimes can be extended. Given the likely increase in concerns over greenhouse gas emissions, the investigation and demonstration of cost-effective separation of methane from very dilute coalbed methane streams using membrane techniques also merit some attention. 2 One micron = 10-6 m. 3 One nm = 10-9 m.
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--> FINDINGS Future innovations in coal gasifier and combustor design will depend largely on an improved quantitative understanding of the interactions between coal ash, carbon oxidation, and emissions. The committee identified this topic as being of importance for DOE's coal-related advanced research activities, given its relevance to improved coal-based systems for power generation and fuel production. Advanced research on coal liquefaction has the potential to achieve significant cost savings, either through improvements in current processing chemistry and technology or through processes based on new chemistries. There is currently very little industrial research on coal liquefaction; most activities are funded by DOE. The operating environment in a coal-gas-fired turbine is more corrosive than that in a natural-gas-fired turbine, due primarily to the presence of sulfur and alkali metals. Evaluation of existing and emerging turbine material systems is needed to determine their suitability for advanced coal gasification-based power generation systems. This evaluation will require appropriate test rigs and methods for accelerated long-term testing in corrosive environments. Subsequent materials development will likely be necessary to optimize substrate and coating materials. The need for improved corrosion-resistant turbine materials is dependent on the ability of hot gas cleanup systems to reduce contaminant levels in coal-derived gas to acceptable levels for advanced gas turbines. The more successful the hot gas cleanup, the less demanding are the materials requirements, and vice versa. The performance of high-temperature, high-pressure heat exchangers for EFCC power generation systems is currently limited by the properties of available materials. In particular, the maximum operating temperature of approximately 1090 °C [2000 °F] would not provide efficiencies significantly higher than state-of-the-art pulverized coal systems. The corrosive environment resulting from coal combustion imposes additional severe demands on materials. The ability to reach operating temperatures of 1370 °C to 1425 °C (2500 °F to 2600 °F)—corresponding to the inlet temperatures of future advanced gas turbines—represents a major materials challenge and is far from the current state of the art. Inorganic membranes with high separation efficiencies and long-term resistance to high-temperature corrosive environments have the potential to improve the economics of power generation from coal, particularly for systems using advanced turbines. Materials development is required to improve the separation efficiency of ceramic membranes used for hot gas and flue gas cleanup. Improvements in durability at elevated temperatures in corrosive environments are also needed. Additional research opportunities exist to investigate membrane separation of methane from very dilute coalbed methane streams.
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--> REFERENCES Bannister, R.L., N.S. Cheruvu, D.A. Little, and C. McQuiggan. 1994. Turbines for the turn of the century. Mechanical Engineering 115(6): 68-75. Bernstein, H.L., and J.H. Allen. 1992. A review of high-temperature coatings for combustion turbine blades. Pp. 6.17-6.47 in Proceedings of the Steam and Combustion Turbine-Blading Conference and Workshop, January 29-31, Orlando, Florida. Palo Alto, California: Electric Power Research Institute. Burggraaf, A.J., K. Keiser, and B.A. van Heisel. 1989. Nanophase ceramics, membranes and ion implanted layers. Pp. 705-724 in Surface and Interfaces of Ceramics Materials. Dufour, L.C., Monty, and Petot-Evans, eds. Boston: Kluwer Academic Publishers. Davis, K.A., R.H. Hurt, N.Y.C. Yang, and T.H. Headley. In press. Evolution of char density crystallinity and ultra fine structure during pulverized coal combustion. Pittsburgh, Pennsylvania: The Combustion Institute. DeLuchi, M.A. 1993. Emissions of Greenhouse Gases from the Use of Transportation Fuels and Electricity. Argonne National Laboratory, ANL/ESD/TM-22. Washington, D.C.: U.S. Government Printing Office. DOE. 1992. Advanced Research Program Plan: Cross-cutting Fossil Fuels Science and Technology. U.S. Department of Energy, DOE/FE-0250T. Washington, D.C.: DOE. DOE. 1993. Assessment of the Potential for Refinery Applications of Inorganic Membrane Technology—An Identification and Screening Analysis. U.S. Department of Energy, DOE/FE-61680H3. Washington, D.C.: DOE. DOE. 1994a. Strategic Plan: Fueling a Competitive Economy, U.S. Department of Energy, DOE/S0108. Washington, D.C.: DOE. DOE. 1994b. Fossil Energy Advanced Research: Strategic Plan, Review Draft, July 15. Washington, D.C.: DOE. DOE. 1994c. FY 1995 Congressional Budget Request. U.S. Department of Energy, DOE/CR-0023, Vol. 4. Washington, D.C.: DOE. Krishnan, G.N., A. Sanjurjo, and B.J. Wood. 1993. Thermal/chemical degradation of inorganic membrane materials. Pp. 211-219 in Proceedings of the Coal-Fired Power Systems 93—Advances in IGCC and PFBC Review Meeting held June 28-30, 1993, Morgantown Energy Technology Center, Morgantown, West Virginia. U.S. Department of Energy, DOE/METC-93/6131. Washington, D.C.: DOE. LaHaye, P.G., and M.R. Bary. 1994. Externally Fired Combustion Cycle (EFCC): A DOE Clean Coal V Project—Effective Means of Rejuvenation for Older Coal-Fired Stations. Paper presented at the ASME Turbo Expo '94, The Hague, Netherlands, June 13-16. NRC. 1986. Materials for Large Land-Based Gas Turbines. National Materials Advisory Board, National Research Council. Washington, D.C.: National Academy Press. NRC. 1993. Advanced Exploratory Research Directions for Extraction and Processing of Oil and Gas. Board on Chemical Science and Technology, National Research Council. Washington, D.C.: National Academy Press. Orozco, N.J. 1993. High Pressure Ceramic Air Heater for Indirectly Fired Gas Turbine Applications . Paper presented at the Joint Contractors Review Meeting, FE/EE Advanced Turbine Systems Conference, FE Fuel Cells and Coal-Fired Heat Engines Conference, U.S. Department of Energy, Morgantown Energy Technology Center, Morgantown, West Virginia, August 3-5. Orozco, N.J., and C.L. Vandervort. 1993. Ceramic Air Heater for an Indirectly Fired Gas Turbine Using Low Rank Fuels. Paper presented at the Low Rank Fuels Symposium, St. Louis, Missouri, May 11-12. OTA. 1988. Advanced Materials by Design. U.S. Congress, Office of Technology Assessment, OTA-E-351. Washington, D.C.: U.S. Government Printing Office.
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--> The Combustion Institute. In press. Proceedings of the 25th International Symposium on Combustion, July 1994, in Irvine, California. Pittsburgh, Pennsylvania: The Combustion Institute. Vandervort, C.L., M.R. Bary, L.E. Stoddard, and S.T. Higgins. 1993. Externally Fired combined-cycle Repowering of Existing Steam Plants. Paper presented at the Meeting of the International Gas Turbine Institute, Cincinnati, Ohio, May 24-25.
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