Supporting Technologies and Systems Integration
In this chapter, the supporting technologies and systems necessary for the enabling technologies of Vision 21 to function in commercial applications are reviewed. The ability of a process or power plant to perform to its design capability depends in large part on the engineering integrity of its components and support systems. Because the committee concluded in the previous chapter that the Vision 21 Program should concentrate on coal gasification as its principal enabling technology, this chapter is focused on supporting technologies and systems integration associated with gasification. The activities and milestones often referenced by FY date in the following discussion of the program goals and milestones can be found in Appendix C. The committee's findings and recommendations for specific areas are stated in each section of this chapter.
ADVANCED COMPUTATIONAL MODELING AND SYSTEMS ANALYSIS
Program Goals and Schedules
In the Vision 21 Program Plan (DOE, 1999), computer-based modeling and simulation play a critical role in three general areas: (1) systems analysis; (2) advanced computational modeling and the development of virtual demonstration capability (a program element of enabling technologies); and (3) systems integration. As distinct from the development of ''hardware" (i.e., enabling and support component technologies for Vision 21 energy plants), the development of "software" involves using computer-based tools for designing and analyzing plant configurations. This capability is critical for identifying the most promising
configurations and reducing the development time and cost of building systems and components.
The committee strongly endorses the need for advanced modeling and systems analyses. Indeed, because the emphasis of the Vision 21 Program is on the development of hardware components, computer-based capabilities for system simulation, integration, and analysis will be essential to the development and deployment of Vision 21 facilities. Thus, modeling and simulation comprise critical components of the overall Vision 21 Program.
The Vision 21 Program Plan identifies the key developments in modeling and software for simulations of Vision 21 components and systems and sets milestones for accomplishing most of these objectives. The following discussion focuses primarily on the overall structure of the program plan relative to the long-term objectives of Vision 21. The findings and recommendations relate to four areas: modeling goals and objectives; modeling capabilities; modeling needs; and management.
But even with the most sophisticated modeling techniques available, Vision 21 will inevitably have to build or renovate facilities to test new components and systems. There appears to be an assumption by Vision 21 that this demand will somehow be met outside the program. The committee believes the Power Systems Development Facility, all of the Clean Coal Technology (CCT) Program projects, and the NETL facilities could be used for the sequential and concurrent development and testing of component technologies. Modifications and sidestream tests could be done at CCT plants, particularly IGCC plants, to test breakthroughs or evolve processes.
Modeling Goals and Objectives
The program plan identifies a number of specific technologies (e.g., fuel cells, gasifiers, turbines) for which simulation models are to be completed at specified times over the next ten years (Romanosky, 1999a, 1999b). But the exact definition of a completed model, the levels of detail and methods of validation, the intended users, the purposes and how and when revisions will be made in response to new data or process developments have not been determined. Until these and other questions have been answered, the adequacy of Vision 21 models cannot be assessed.
Unanswered questions also remain with regard to integrated systems models and virtual demonstration capability. What are the desired outputs of these models? For what purposes are they intended? For example, some committee members assumed that the virtual demonstrations were intended to provide a basis for the commercialization of a product or process without full-scale testing or physical demonstration. Although this does not appear to be DOE's intent, the current program plan and supporting documentation are ambiguous enough to raise
questions concerning the modeling goals and objectives. The draft program plan does not establish priorities for advanced modeling and systems analysis. Because the scope of the comprehensive modeling and analysis activities could be enormous, prioritization will be crucial.
Finding. The goals, objectives, and priorities of the modeling and systems analysis activities have not been adequately defined.
Recommendation. The U.S. Department of Energy should work closely with potential model users and model developers in industry, academia, government, and nongovernmental organizations to define the specific goals, objectives, and priorities of the component modeling and systems modeling for Vision 21. The goals, objectives, and priorities should reflect the integration of, and need for, experimental or field data to support computer models.
Expectations for Model Capabilities
The program plan emphasizes the development of "transient 3-D simulations that realistically account for all the physically relevant phenomena, such as fluid flow, heat transfer, chemistry, radiation, material stress, etc." (DOE, 1999, p. 22). Although this is a laudable long-term research goal, the development of mechanistic models in all of the areas of endeavor covered by Vision 21 would require a foundation of scientific knowledge and understanding that is simply unattainable in the next 15 years given the current state of knowledge and the historical pace of basic research. Although reasonably sophisticated mechanistic models may indeed become available for some Vision 21 components, the committee believes it very unlikely that the level of detail described in the plan will be possible for most of Vision 21's advanced technologies.
Furthermore, integrating component models of this complexity into an overall systems model of a Vision 21 plant is likely to be extremely cumbersome and time consuming at best, and completely unworkable at worst. Detailed component models of the type described by DOE are typically designed as stand-alone entities by different individuals or organizations, with little or no thought or capability for linking them with other models of similar complexity. Even if integration were feasible technically and allowing for substantial improvements in capabilities, the time and computational resources required would be extraordinary.
Finding. The technical level of the models described in the Vision 21 Program Plan is unrealistic and unlikely to be achieved in the Vision 21 time frame (i.e., the next 15 years).
Recommendation. The U.S. Department of Energy should review the current state of the art of science-based modeling capabilities at both the component and systems levels and use this review as a basis for refining its expectations for Vision 21 models. This assessment should clearly identify and distinguish among modeling capabilities at different levels of detail (e.g., mechanistic [molecular or microscale] modeling vs. more empirically based engineering models).
Hierarchy of Modeling Needs
Detailed mechanistic models are not necessary, or even appropriate, for many types of design and analysis. For example, computer-based modeling tools for preliminary design and analysis, including scoping studies of various types, can usually be done with much simpler models, often based on a combination of basic principles (e.g., mass and energy balances) and empirical, statistical, or "reduced-form" constructs rather than "molecular-level" mechanistic models. The program plan does not include the parallel development of a simpler, faster, and more economical modeling framework suitable for preliminary design or scoping studies, which require a minimum of detailed or site-specific information. Nor does it include the coupling of performance models and cost models or a characterization of the uncertainties in underlying model structures and parameters.
In addition to models of specific Vision 21 plant designs, the program will need modeling capabilities to assess the market potential for these technologies in the context of the overall energy picture. Tools such as the National Energy Modeling System (NEMS) developed by DOE's Energy Information Administration (and used for the DOE/Energy Information Agency annual energy outlook report) offer one possible way of providing such capabilities. However, the Vision 21 Program Plan is silent as to how the proposed periodic studies of Vision 21 market potential will be carried out.
Finding. No hierarchy of models at different levels of complexity suited for different purposes is reflected in the current Vision 21 Program plan. Instead, the plan emphasizes the development of a "virtual demonstration" capability for detailed final designs of specific projects.
Recommendation. The Vision 21 Program Plan should be modified to address the need for a hierarchy of models suitable for preliminary design and scoping studies, as well as for detailed final designs. This hierarchy should range from simple, transparent models showing basic mass and energy balances, costs for process units, and integrated systems to more complex and detailed models of components and systems. The hierarchy should also reflect the differing needs for dynamic simulations and steady-state models of components and systems. It
should also include the capability of coupling performance models and cost models, as well as analyzing the effects of uncertainty.
Management of Modeling Activities
The Vision 21 Program Plan is silent about how various component models and integrated systems models will be brought together, validated, and made accessible to interested users and how the substantial difficulties of model integration will be addressed. At the present time, computer-based models of energy technology components and systems are spread across a wide array of organizations, both public and private. Although some of these models are in the public domain, many others are proprietary. In the absence of a clear management plan and institutional structure for the development of advanced modeling capabilities, Vision 21 program objectives are not likely to be achieved. Issues of intellectual property could also be a barrier to the development or availability of some types of modeling capabilities, including virtual demonstration capabilities. The development of an effective management plan and institutional capability to deal with the issues outlined above will be critical to the success of the Vision 21 Program.
Finding. The Vision 21 Program Plan does not comment on the use of facilities that will inevitably have to be built or renovated to test new components and systems. There appears to be an assumption that this demand will somehow be met outside the program.
Recommendation. The U.S. Department of Energy (DOE) should make facilities at the Power Systems Development Facility, all of the Clean Coal Technology (CCT) Program projects, and the National Energy Technology Laboratory facilities available for sequential and concurrent development and testing of Vision 21 component technologies. DOE should fund modifications and sidestream tests at the CCT plants, particularly for integrated gasification/combined-cycle plants, to test breakthroughs or evolve processes.
Finding. Vision 21 lacks a clear management plan for the development of advanced computer-based modeling capabilities.
Recommendation. The U.S. Department of Energy should develop a management plan and an institutional capability to carry out computer-based modeling, systems analysis, and systems integration activities. The management plan should include mechanisms for verifying or qualifying the performance of models; assessing and characterizing their reliability; maintaining and updating all Vision 21 models and modeling capabilities; and ensuring that models and modeling
capabilities are openly developed, validated, and made available to interested parties.
MATERIALS AND HEAT EXCHANGERS
The commercial availability and long-term survivability of materials is considered a critical area for all advanced power-generation technologies, regardless of fuel source. Of course, resolving materials-related problems and issues is much more challenging for coal-based technologies. Numerous technology configurations are possible for Vision 21 systems, as well as for future electric power-generation options using coal as feedstock, and each option presents different materials problems. Therefore, the R&D materials projects, including commercial deployment, should be closely coordinated.
Currently, new materials (both alloys and ceramics) suitable for use at high temperatures in corrosive and erosive environments are being developed as part of DOE's Base Technology Program, as well as by the Vision 21 Program (Judkins, 1999). R&D is carried out mostly at Oak Ridge National Laboratory (ORNL) and is being monitored by one FETC associate for its applicability to the Vision 21 Program.
DOE's overall research program on advanced materials and heat exchangers includes the following areas:
new classes of materials for high-temperature alloys, ceramics, and intermetallics
testing and demonstration of materials for advanced, high-temperature heat exchangers
development of high-temperature filter materials
development and testing of high-temperature materials for refractories, sensors, and the production of advanced fuels
development of new gas-separation membranes
development of advanced composite materials
development of oxide-dispersion strengthening (ODS) materials and production technologies
development of catalytic materials for gas purification
Although the ATS is an integral part of the Vision 21 Program, work on the ATS is carried out in a separate DOE program for the development of advanced gas turbine materials that can withstand 1,500°C to 1,650°C firing temperatures. The goal of materials research for the ATS is to improve the performance and durability of thermal barrier coatings-substrate materials, as applied to turbine blades in advanced land-based gas turbines. The focus is on thermal barrier
coatings applied by air plasma spray and chemical-physical vapor deposition methods. Materials research projects in the ATS program fall into three areas: modeling and durability experiments for thermal barrier coatings, new coating techniques, and life prediction and nondestructive evaluation.
General Requirements for Vision 21 Materials
The success of the Vision 21 Program will depend largely on the timely development, demonstration, and commercialization of advanced, cost-competitive, reliable materials and high-temperature, corrosion-resistant coatings (such as weld overlays of high Cr-Fe based composition and iron-aluminide coatings) for critical components. However, rather than a pure materials program, Vision 21 has a component development program that places demands on materials. As the committee noted in the previous chapter, the 60-percent efficiency target (HHV) and carbon-sequestration technologies can only be met by coal-gasification technologies integrated with fuel cells and advanced gas turbines. DOE's entire materials R&D program related to Vision 21 should be driven by component design and reassessed in light of the aggressive goals of the Vision 21 Program.
The development of an economical gas-separation membrane is probably the most important materials R&D challenge related to Vision 21. Robust membranes for separating hydrogen, oxygen, and carbon dioxide will have significant implications for the technical and economic success of Vision 21 systems.
DOE, in partnership with industry, is aggressively developing the ion-transport membrane for separating oxygen from air. This membrane is based on a mixed-conducting ceramic that has both electronic and oxygen-ionic conductivity when operated at high temperatures (typically 815°C–870°C). Development is directed towards a low-cost membrane for oxygen production.
The separation of hydrogen from carbon dioxide is also critical because pure hydrogen is the preferred reactant for fuel cells. The focus of an R&D program on hydrogen separation would be to minimize the energy and capital costs of the separation process. The proposed Vision 21 Program focuses on porous inorganic membranes that are selective for the transport of hydrogen. These membranes must operate at high temperatures and require low energy of separation. Recently, metal-based (palladium) membranes have been shown to transfer hydrogen atoms. The current research program also includes an investigation of a ceramic proton-transfer membrane for the generation and separation of hydrogen from high-temperature steam.
Materials Requirements for Fuel Cells
Fuel cell components, including electrolyte, electrodes, cell supports, current collectors and interconnects, and ducts and manifolds for fuel cell stacks and/or arrays, require special materials. Some of the important materials characteristics for advanced fuel cell concepts include relatively low-temperature (540°C–815°C) electrolytes, catalytically active electrodes compatible with electrolytes, metallic interconnects, seals, and economical thin-film fabrication techniques. For present-generation concepts, advanced and lower cost fabrication techniques are critical for fuel cell economics.
High-temperature heat exchangers will also be required in high-efficiency fuel cell power systems for preheating fuel and air reactant streams and/or to make use of the hot combustion-product exhaust stream. Heat exchangers will have to be able to tolerate top temperatures of 760°C to 870°C in either oxidizing or reducing conditions and pressure differences of 2 to 4 atmospheres over the surface. Practical fuel cell power systems must also be compact and affordable.
High-Temperature Filter Materials
Reliable hot-gas filters have long been a major barrier to the deployment of coal-gasification technology. Significant R&D programs, in both the private and public sector, have been addressing this issue. Both advanced ceramics and metallic filters appear to have reasonably good chances of being used in future coal-based energy technologies.
Oxide-Dispersion Strengthened Alloys
ODS alloys could increase the operating temperature by 150°C, much higher than the temperature with current high-strength austenitic materials. The current program includes the development of ferritic ODS alloys for use at temperatures exceeding the capabilities of advanced steels and heat-resistant alloys. These alloys would have a broad range of uses in high-efficiency Vision 21 systems. Joining, the quantification of corrosion resistance in typical service environments, and mechanical and physical property measurements are issues that should be addressed.
Ultra High-Temperature Intermetallics
Current materials development programs suggest that some intermetallics with very high melting points might be used as alternatives to ceramics in very high-temperature heat exchangers.
DOE's ATS Program is responsible for developing information on advanced gas turbine materials that can withstand 1,500°C to 1,650°C firing temperatures. However, no reference is made in the Vision 21 Program Plan to testing these materials with coal-derived syngas at high temperatures. For Vision 21, these tests will be necessary to determine the life and survivability of these materials with coal-derived syngas, as well as hydrogen, in this temperature range.
Resolving materials-related problems and issues is much more challenging for coal-based technologies. Numerous technology configurations are possible for Vision 21 systems, as well as for future electric power-generation options using coal as feedstock, and each option presents different materials problems. Therefore, the R&D materials program, including commercial deployment, should be closely coordinated. The success of the Vision 21 Program will depend largely on the timely development, demonstration, and commercialization of advanced, cost-competitive, reliable materials and high-temperature corrosion-resistant coatings. The use of bottoming cycles could be the natural evolution for cycle improvement. The economics of particular bottoming cycles would have to be explored and consideration given to incorporating potentially attractive cycles into gasification/gas turbine/fuel cell cycle configurations, but special materials would have to be developed for this type of high-temperature steam cycle.
The Vision 21 goal of 60-percent net efficiency can only be achieved with a high-temperature fuel cell (probably a solid-oxide fuel cell). Therefore, the availability of low-cost materials for advanced solid-oxide fuel cells should be considered a critical element of the Vision 21 Program.
Findings and Recommendations
Finding. DOE's development of materials and heat exchangers is dispersed among several programs. The focus of the Vision 21 Program should be on high-temperature components, not high-temperature materials.
Recommendation. All materials-related development activities for the Vision 21 Program should be focused and coordinated with other federally funded materials research and development (R&D) programs. Vision 21 has no room for a pure materials program but should have a component development program that places demands on materials. All Vision 21 materials R&D should be driven by component design and reassessed in light of the aggressive program goals.
Finding. The development of improved ceramics and/or porous, inorganic membranes that allow hydrogen gas separation from syngas is one of the most important
materials R&D programs related to Vision 21. An ion-transport membrane for separating oxygen from air could also be very important. The development programs for functional materials (i.e., materials that process functions, such as gas-separation membranes), do not seem to be adequate in the Vision 21 Program Plan.
Recommendation. The U.S. Department of Energy should develop a systematic, well defined program for deploying high-temperature membranes for oxygen separation from air and hydrogen separation from syngas. Because separation is a critical step in the gasification-based Vision 21 plant, this research area should be given a high priority.
Finding. High-temperature, high-pressure, and more corrosion-resistant and durable materials (including high-temperature filter materials) will be necessary for various systems and subsystems of Vision 21 plants. Exposure and durability tests take time after candidate alloys are developed. The Vision 21 goal of evaluating and selecting materials suitable for high-temperature application by FY02 seems to be too optimistic. For all pressure/boundary materials (e.g., heat exchangers, structural materials, etc.), appropriate ASME Code Cases will be required, which could take two to three years. The Vision 21 timeline does not include time for this process.
Recommendation. The Vision 21 Program for developing high-strength materials that can withstand high temperatures and high pressures should be narrowed down and focused on materials that will be used in reducing conditions only. To ensure that a small number of candidate materials for particulate filters can be demonstrated, the U.S. Department of Energy should develop a five-year to seven-year program for the development and demonstration of materials to be used in reducing conditions.
Finding. Currently, only limited work is being done on oxygen-dispersion strengthened alloys and ultra high-temperature intermetallics that have the potential of operating at high temperatures under various operating conditions.
Recommendation. Research on the development of oxygen-dispersion strengthened (ODS) alloys should be substantially increased with a strong focus on obtaining fabrication and performance data. Recently developed ODS alloys should be evaluated and tested for high-strength requirements at or above 1,100°C because traditional ingot-processed alloys are not strong enough to perform at this temperature. ODS alloys could be used for high-temperature heat exchangers at potentially lower cost.
Finding. Vision 21's materials R&D program does not include the development of high-temperature coatings to protect against accelerated corrosion and erosion, especially for gas turbine blades and other structural components.
Recommendation. The U.S. Department of Energy should place a high priority on the development of high-temperature coatings and materials, which will be essential for all advanced-generation technologies. Coatings for corrosion protection, such as weld overlays of high Cr-Fe-based composition and iron-aluminide coatings, should also be evaluated.
Finding. Although some work on materials-related R&D is being performed by private industries (with partial funding from DOE), additional work will be necessary for materials development for next-generation solid-oxide fuel cells. Currently, no R&D is being carried out on metallic interconnections and seals or the development of low-cost, thin-film fabrication techniques. Advanced, low-cost fabrication techniques will be critical to the commercialization of fuel cells.
Recommendation. The committee strongly recommends that a formal research and development program in the Vision 21 Program be initiated to develop metallic interconnections and seals and to develop low-cost, thin-film fabrication techniques.
Finding. The Vision 21 Program plan does not take enough account of the need for high-temperature refractory materials and high-temperature sensors.
Recommendation. Vision 21 should put a high priority on the development of high-temperature refractory materials and high-temperature sensors.
ENVIRONMENTAL CONTROL TECHNOLOGIES
A five-part structure has been delineated in the program plan to address the overall goal of removing environmental barriers to the use of fossil fuels. The technologies included under the umbrella of environmental control are listed below:
advanced, low-NOx combustion
advanced, fine particulate matter (PM2.5) controls
management of by-products of coal combustion
revolutionary approaches to capturing and separating of carbon dioxide
energy systems with terrestrial sinks
Advanced emissions controls is a key technology area on the technology road map for electric power and fuels R&D. Vision 21 proposes reducing emissions of SOx, NOx, and primary particulate matter to address environmental concerns associated with PM2.5, visibility impairment, acidification, and eutrophication. Thus, Vision 21 focuses on ''eliminating environmental issues associated with the utilization of fossil fuels" (DOE, 1999, p. 1). Emissions of air pollutants, such as SOx, NOx, and mercury, would be reduced to essentially zero. "Emissions of carbon dioxide, a greenhouse gas, would be dramatically decreased because of higher efficiency" (DOE, 1999, p. 1). Vision 21 plant designs would also include the option of capturing and sequestering carbon. Vision 21 proposes that changes in the approach to emissions control be shifted gradually from an expensive afterthought imposed by legislation and regulation towards emissions management as an integral part of the design and operation of power-generating systems.
These are worthwhile and responsible goals that would indeed render the use of fossil fuels environmentally more benign and more viable for use in the twenty-first century. In the case of NOx, a reasonable goal would be to reduce emissions levels to those of natural gas-fueled power plants. However, for SOx, mercury, particulates, and trace metals, these levels would be unrealistic in the time frame of the Vision 21 Program. Mercury is mentioned separately from other trace metals because unlike most metals that end up in by-product ash or slag, a significant portion of the mercury in coal can be volatilized. The committee believes that defining environmental emissions goals as "near zero" is a dangerous oversimplification. The environmental goals should be quantified and based on an assessment of the total fuel-cycle emissions, including production of the fuel (coal). Emission levels for each of the pollutants under consideration should be defined.
Balance in the Portfolio
Environmental control techniques include reducing emissions of NOx, SOx, particulates, mercury, organics, and trace metals, as well as minimizing emissions of CO2 and solid waste. The timelines for implementing controls are not specified in detail. NOx and SOx solutions are currently being implemented, but both incremental and step-out improvements will be necessary to meet the near-zero emissions specifications. Improvements should be implemented as they become available, either as enhancements or replacements of selective catalytic reduction (SCR) technologies. The particulates problem will be addressed mainly by the control of NOx and SOx emissions. The cost-effective elimination of emissions of mercury and trace metals will require long-term research; current approaches based on carbon adsorbents are very expensive. Finally, the capture of carbon dioxide and the use of by-products will require an even longer timeline,
probably until the end of the Vision 21 timeline. The staggered implementation of these technologies will require detailed planning to ensure that resources are allocated in each area at the critical times.
In the committee's opinion, this area should have a low priority because changes will be evolutionary and will have a minimal impact in the gasification-based approaches that are likely to be the focus of the Vision 21 Program. The development of low-NOx combustion is being addressed and implemented by other ongoing research programs at DOE and elsewhere and should remain part of the baseline DOE program, in close collaboration with vendor-driven applications and activities.
Advanced Control Technology for Fine Particulates
R&D in this area is closely related to the reduction of emissions of NOx and SOx. The Vision 21 Program should focus on catalytic and non-SCR technologies and combined NOx/SOx catalytic removal. In the early stages, R&D should focus on the reduction or removal of NOx and SOx molecules; long-term R&D should continue on potential SCR technologies that do not require the use of ammonia (or that produce ammonia in situ) and exploit hydrocarbons or hydrogen as selective reductants, or NO decomposition catalysis.
The control of fine particulates, an area that has been widely studied and will play a critical role in the reduction of ozone levels and acid rain, will require a fresh and more fundamental approach than the one that can be inferred from the Vision 21 Program Plan. Based on the results of parallel programs in DOE currently addressing automotive-related emissions control, early Vision 21 research should focus on the fundamentals of catalytic reactions of NOx and SOx, which react and condense in the atmosphere to produce fine particulates. Improvements in current technologies should, of course, be encouraged, but Vision 21 should also be thinking in terms of new chemistry and engineering concepts that could lead to breakthroughs. In this area, continuity (i.e., long-term approaches to very difficult problems) and parallel research paths (i.e., multiple approaches to the solution of a given problem with a rigorous downselection process) should be specified, which will require coordination and communication among these programs. Vision 21 should consider its entire time frame instead of focusing exclusively on short-term solutions in the first few years.
Management of Combustion By-products
The objective of R&D in this area should be to turn all solid wastes into marketable, usable products that safely contain residual impurities. No materials
should leave a plant site unless they can be recycled into usable products. Plant designs should include the processing of periodic wastes, such as spent catalysts, saturated absorbents, contaminated solvents, and water treatment sludge. In a gasification system, for example, many of these wastes could be fed to the gasifier; the carbonaceous and hydrocarbon components could be converted into syngas; the mineral matter could be captured in the molten slag and ash agglomerates that could be used as a construction material.
Instead of the current emphasis on the conversion of coal combustion solid by-products into marketable materials, the Vision 21 Program Plan should include a general assessment of the disposition and containment of all solid, liquid, and gas streams. All material flows should be managed according to the highest standards of industrial ecology by following the principles of "recycle, sell, or bind." Standards should be based on health and environmental concerns and should incorporate relevant solutions from other countries dealing with similar issues. Successful international strategies for dealing with combustion by-products might be adaptable to local market drivers. For example, in the Netherlands, legislation requires that markets be found for IGCC plant waste solids.
Capture and Separation of Carbon Dioxide
The most recent environmental driver in energy systems is the requirement that new power-generation technologies consider the feasibility and cost of capturing carbon dioxide. In keeping with this trend, Vision 21 should concentrate its R&D on strategies that yield more concentrated carbon dioxide effluent streams and novel sequestration technologies. Vision 21 should clearly differentiate between improvements in current technologies for capturing carbon dioxide that are part of DOE's current baseline program and the breakthrough technologies that Vision 21 plants will require. Although Vision 21's highest priority should be on revolutionary systems, the program would be ill advised to disregard research to improve current technologies. The thermodynamic viability, energy requirements, and long-term environmental risk of potential revolutionary approaches should be assessed as early as possible.
Energy Systems Integrated with Terrestrial Sinks
The Vision 21 Program proposes attempting to locate sources of carbon dioxide (i.e., Vision 21 plants) near customers for captured carbon dioxide. Although the use of biomass in general may or may not be an advisable approach to carbon management, the location of plant sites is a political rather than technical issue that should not be included in the Vision 21 Program Plan. Site-specific decisions are not directly related to technology R&D issues and can best be made in the context of the exigencies of a particular facility.
The planned environmental control technologies do not explicitly establish priorities among cleanup requirements or indicate the level of difficulty or support required for each research area. The objective of near-zero emissions is not qualified by cost constraints, and the costs of many of the technical objectives may be prohibitive. The carbon dioxide containment problem is not clearly defined, either in terms of (1) the differences between evolutionary (in DOE's current baseline program) and revolutionary technologies (in the Vision 21 Program) or (2) the benefits associated with a concentrated carbon dioxide stream. In general, the level of detail of strategies for emissions controls is insufficient for the committee to judge the likelihood of success. The ultimate disposition of carbon dioxide can be better addressed by a science-based program very different in nature from the technology-based focus of Vision 21.
Hence, there is a logical division of labor between efforts to develop carbon dioxide separation and capture technology for an energy facility and other efforts to dispose of carbon dioxide in oceans or wells. Transport technology is usually assumed to be via pipeline and thus requires no new R&D. Overall, however, not enough emphasis has been placed on fundamental chemistry or models/simulations of chemical reactions, especially those occurring on catalytic surfaces. The required tools are likely to be developed in basic research currently coordinated through DOE's Office of Basic Energy Sciences and the National Science Foundation. Effective Vision 21 management will be necessary for these advanced new concepts to be incorporated into Vision 21 technologies. Advances in emissions control technologies are likely to require the application of newly emerging theoretical and experimental methods to a greater extent than for past emissions control challenges.
Vision 21 will have to take a more proactive approach in the areas of environmental control and gas-stream purification. The current program does not focus enough on new chemistries, new catalytic approaches, or innovative approaches at the boundaries between chemistry, physics, and engineering. Advances will require a new outlook and an aggressive approach to research, as well as the incorporation of contributions from fundamental research in related disciplines. Although the objectives of the emissions control component of Vision 21 are reasonable, they have not been prioritized to reflect their importance and potential impact.
Findings and Recommendations
Finding. Advanced low-NOx combustors will have a minimal impact on gasification-based approaches for Vision 21.
Recommendation. The program to develop a low-NOx combustor and integrate energy systems with terrestrial sinks should be removed from the Vision 21 Program and transferred to the U.S. Department of Energy's ongoing program. Vision 21 should focus on much larger and more fundamental issues, such as the removal of NOx, sulfur dioxide, metals, and other toxins from effluent streams, evolutionary and revolutionary technologies for capturing carbon dioxide, and other separation technologies.
Finding. As environmental emissions at Vision 21 plants approach the "near-zero" goal, upstream emissions from the mining, beneficiation, transportation of coal, and other fossil fuels will become increasingly troublesome. Trade-offs may be necessary between in-plant and upstream emissions to approach near-zero emissions overall.
Recommendation. The selection criteria and program planning framework for Vision 21 designs and concepts should include a full life-cycle analysis that incorporates the entire fuel cycle.
Finding. If the gasification-based approaches recommended by the committee are adopted, traditional effluent cleanup requirements for combustion-based processes will be replaced by new and different requirements.
Recommendation. Combustion effluent streams from gasification-based processes will contain significantly different concentrations of impurities than those in traditional combustion effluent streams. Thus, the capture and disposal of effluents must be given a high priority. Vision 21 should make early investments in new approaches based on solid scientific and engineering concepts and should adopt a longer time frame for the implementation of these novel approaches. Less revolutionary approaches should continue to be pursued in other U.S. Department of Energy programs. Molecular simulations, surface and catalytic science, and high-temperature catalytic materials should be incorporated more effectively into the proposed program, especially in the early stages of program definition and concept development.
Finding. Meeting the environmental objectives of Vision 21 will require intense research. However, the environmental control component of the program is not well integrated, and no strategic plan has been developed.
Recommendation. The U.S. Department of Energy (DOE) should conduct a critical-path analysis and rigorously prioritize activities in the area of advanced emissions control. Workshops, broad technical symposia, and other methods could be used to help delineate the state of the art, technical hurdles, and mechanisms for encouraging new approaches and revolutionary advances in this area.
Complex processes will have to be understood for removing or controlling multiple contaminants. DOE should strive to develop tools that will enable clear thinking and new approaches that will make progress possible for the Vision 21 Program.
Finding. The Vision 21 Program boundaries and responsibilities for capturing and sequestrating carbon dioxide are not clearly defined.
Recommendation. All activities related to the separation and capture of carbon dioxide should be included in the Vision 21 Program, as befits their application within the extended timeline of the program and their impact on the design and operation of future energy complexes. Issues related to the off-site transport and the ultimate disposition of carbon dioxide should not be included in the Vision 21 Program. All activities outside the plant boundary should be outside the Vision 21 Program. The carbon capture/separation and carbon sequestration programs will have to be coordinated to ensure that logical decisions are made during the design of Vision 21 plants.
The Vision 21 Program Plan recognizes the necessity of removing particulates, alkali, and sulfur from internal process streams to protect downstream equipment, catalysts, and components. Some contaminants must be removed for reasons of efficiency and some for environmental reasons. Those driven by improvements in efficiency must be evaluated against the potential losses in efficiency associated with the removal process. Removals for environmental reasons must be evaluated in the context of alternatives, such as cold-gas purification or the removal of impurities at the end of the process. Contaminants removed to protect catalysts and equipment, which are sensitive to the details of specific process schemes, must be evaluated in this context. Gas streams for the operation of water-gas shift, hydrogen membranes, and electrodes must have fewer impurities if the product will be used for fuel cells. If the focus of Vision 21 is on gasification as the committee recommends, gas-stream purification will require R&D on high-temperature particulate filters and contaminant-removal technologies.
High-Temperature Particulate Filters
Hot-gas filtration is currently a limiting operation for the removal of fine particulates. Nevertheless, hot-syngas cleanup methods will be necessary for
many of the Vision 21 technologies. The critical issues of costs and lifetimes have been widely recognized and studied in the last few years. A 1995 report, Coal: Energy for the Future, identified hot-gas cleanup as a critical need, but progress thus far has been slow (NRC, 1995). The recent start-up of facilities for testing at operating conditions is likely to accelerate progress, but more fundamental studies of basic processes and likely solutions to the mechanical breakdown and binding of materials will be necessary to meet the time targets for Vision 21. Thermal shock and alkali deposition processes must also be studied and addressed as longer term goals.
In the current plan, the high-temperature filter component of the program is scheduled to be completed in FY04, making it unlikely that the benefits of much longer range programs in materials can be incorporated into the design of high-temperature filters. In addition, specific temperatures are not included in the milestones, and the 840°C temperature in the program plan may not be high enough to meet the requirements of the process schemes in Vision 21 (DOE, 1999).
Meeting the Vision 21 requirements will require a longer term, more ambitious approach, as well as a fundamental understanding of materials and design and continued testing of available materials and concepts. Testing of materials in existing or emerging facilities in FY04 will be necessary to achieve stable operation at 815°C. Parallel work on fundamental materials science should also be initiated immediately and should continue beyond FY04 with the objective of providing designs for filters capable of operating at the high temperatures (approximately 930°C) necessary for the high-efficiency processes Vision 21 proposes for the FY10-FY12 time frame. Finally, DOE should keep in mind that hot-gas filtration may be significantly less important for gasification-based power generation than it has been in the traditional power-generation environment.
Contaminant removal (i.e., the removal of sulfur, ammonia, mercury, and trace metals) will be critical to the high-efficiency process concepts proposed in Vision 21. This long-standing problem will require revolutionary ideas and strategies that are not likely to result from modifications of existing adsorbents, from engineering concepts associated with the introduction/removal of the adsorbent, or from changes in the contacting patterns between the adsorbent and the effluent stream. Significant progress in this area will require a fundamental understanding of the chemistry of gas-solid reactions at high (and low) temperatures, the structural transformations caused by cyclic adsorption/desorption, and the formation of compounds. Devising new testing methods for materials may provide both serendipitous discoveries of interesting materials properties and add significant information about relationships between materials composition and structure, and the performance and durability of materials as adsorbents.
Finally, a clearer, more rigorous assessment of the requirements of downstream equipment and catalysts for fuel cell-based approaches will be necessary for defining the cleanup approaches required to meet purity specifications. The description in the program plan does not explain the balance between hot-gas and cold-gas cleanup or how the economic/efficiency benefits of the two approaches will be evaluated. R&D on cold-gas cleanup should be continued both as an alternative to hot-gas cleanup and as a way to satisfy the potentially more stringent constraints for systems with fuel cells as the power-generating system. The milestones in the current plan will have to be revised to include the temperatures of streams at which the intended separations must take place. The milestones should also include long-range research to ensure that improvements in this important area continue beyond FY06 as Vision 21 plants become better defined and designed and as new gas-stream specifications are established. Purity requirements and specifications for gas-streams in fuel cell applications may not be possible until significant design modifications are made, which may not happen until well beyond FY06.
The purification unit must be long lasting; it must be able to remove contaminants continuously under normal operating conditions. Gas-stream purification must be effective at high temperatures, probably under pressure and under strong reducing conditions. Satisfying these requirements will be a major challenge that will require a close cooperation between the materials research and development program and the gas-purification process technology program. The overall strategy for ensuring this cooperation has not been spelled out.
Findings and Recommendations
Finding. Some of the proposed research programs in the area of gas-stream purification are scheduled to end well before the enabling technologies of Vision 21 have been developed and tested.
Recommendation. The U.S. Department of Energy should extend the time frame of some of the research on contaminant removal to overlap more with the rest of the program. The extension would allow for more fundamental science and engineering research early in the program in preparation for a long-term, more aggressive phase to meet the purity requirements of gasification-based systems. Many of these requirements may not be well understood or defined by 2004–2006, the planned end-point for research on gas purification. The revised milestones should be closely coordinated with the rest of the Vision 21 plan.
Finding. The issue of testing pilot-scale filter systems has not been fully addressed. The development of commercially attractive contaminant-removal technology requires a very well planned testing protocol. Challenges to the
technology include demonstrating that high levels of contaminant removal can be accomplished at operating conditions and confirming that the system can continue to remove contaminants from the gas stream over time.
Recommendation. The Vision 21 Program should evaluate nonfuel operating costs associated with gas-stream purification technologies. The program plan should include requirements for assessing gas-stream purity requirements to protect downstream equipment and catalysts in fuel cell-based systems.
Finding. The most difficult challenge for the Vision 21 Program will be the development of materials that can meet the requirements for new power-generating systems. The development of a long-lasting purification unit will require close coordination between the materials research and development program and the gas-purification process technology program. A strategy to ensure this coordination has not been established.
Recommendation. The U.S. Department of Energy (DOE) should continue testing materials with near-term implementation at existing facilities until 2004. DOE should also develop a fundamental materials science program to improve the mechanical properties and chemical reactions of materials at high temperatures. The materials development program should be coordinated with the testing to ensure that the materials meet the requirements for the Vision 21 technologies.
Conventional technologies for separations of oxygen, hydrogen, and carbon dioxide have been optimized for decades. Gas-separation technology will also be a critical process step in gasification-based Vision 21 plants. Therefore, all of the separation concepts being investigated are novel and technologically challenging. Vision 21 should establish clear milestones to measure progress and should frequently assess the economic feasibility of the new technologies.
The current program plan addresses the following key technologies:
ceramic membranes for oxygen production
high-temperature membranes for hydrogen production
carbon dioxide-hydrate separation for hydrogen production
hydrogen production from water dissociation
Ceramic Membranes for Oxygen Production
In partnership with DOE, an industry-led team has been aggressively developing an ion-transport membrane for oxygen separation. Ion-transport membranes
are based on mixed-conducting ceramic membranes that have both electronic and oxygen ionic conductivity when operated at high-temperature, typically 800–900°C. The goal of the Vision 21 Program is to develop a significantly less expensive technology for oxygen production. Preliminary work indicates that a 30-percent reduction in oxygen costs is possible. Compared to current cryogenic oxygen production technology, ion-transport membrane technology can improve thermal efficiency for the integrated IGCC system by about 3 percent, decrease carbon dioxide and sulfur emissions, and reduce the cost of generated electric power by more than 6 percent (Foster, 1999; Van Eric and Richards, 1999).
The committee agrees with the major milestones and schedule in the Vision 21 Program Plan. Phase 1, which commenced under a DOE cooperative agreement in October 1998, is a three-year program focused on construction of a technology development unit for validation tests of process concepts at a capacity of 0.1 ton per day of oxygen. After at least one intermediate scale up, Phase 2 and Phase 3 activities will culminate with scale up to a 25 ton per day (or larger) precommercial demonstration unit fully integrated with a gas turbine. Phases 2 and 3 are planned for three years each.
Finding. Ceramic membrane technology has the potential to significantly lower the cost of oxygen in the time frame required for Vision 21 plants.
Recommendation. The U.S. Department of Energy should continue to support research on ceramic membranes for oxygen production to meet the milestones and schedule in the Vision 21 Program Plan.
Advanced Hydrogen Separation
The separation of hydrogen from carbon dioxide is a critical technology that should be pursued in Vision 21. A carbon dioxide-free hydrogen stream can be used in air-fed combustion turbines to produce electric power with zero emissions of carbon dioxide. Pure hydrogen is not only the preferred reactant for fuel cells, it can also be used to increase the ratio of hydrogen to carbon monoxide, thus broadening the range of fuel and chemical products that can be made from synthesis gas (carbon monoxide and hydrogen). The hydrogen membrane program, which is funded under the materials R&D program at Oak Ridge National Laboratory, is in the early materials development stage. Membrane materials less than one inch in diameter are being produced and tested.
The separation of carbon dioxide and hydrogen is done throughout the world to make high-purity hydrogen for the synthesis of ammonia and for other chemical and refining operations. The most widely used technology is based on the absorption of carbon dioxide in a liquid solvent. However, this process has significant energy penalties because the absorption takes place at relatively low
temperatures (less than 200°C), and the desorption of the carbon dioxide occurs at low pressures. Moreover, the desorption step requires heat to separate the carbon dioxide from the solvent. Although these commercial processes have been improved, the energy loss in separation and compression of the carbon dioxide is still as much as 10 percent of the power produced by the energy complex (Elsevier Science, Ltd., 1999). The technologies chosen by Vision 21 for carbon dioxide recovery should avoid low-temperature absorption and regeneration.
The focus of a Vision 21 R&D program on hydrogen separation should be on technologies that minimize the energy and capital costs of the separation process. Processes must be able to operate at high temperatures and pressures to minimize the energy required for the separation and to achieve high rates of separation per unit volume of processing equipment. The proposed Vision 21 program focuses on porous inorganic membranes that are selective for the transport of hydrogen. This technology, which was developed for the gas-diffusion membranes used for uranium enrichment, should operate at high-temperature and require low energy for the separation.
Other membrane concepts that should be considered in this program area include metal-based (palladium) membranes that dissociate and rapidly diffuse hydrogen through metal and proton-transfer membranes that, as their name implies, transfer hydrogen as protons. Because hydrogen-separation technology involving high-temperature membranes is still at an early stage of development, the Vision 21 Program should evaluate a wide range of membrane materials before an expensive scale up of the chosen technology is initiated.
Finding. The hydrogen-separation membrane program is in the early materials stage of development.
Recommendation. When bench-scale data are available on permeation rates, selectivity, and lifetimes, Vision 21 should conduct an independent economic assessment of advanced hydrogen-separation membrane technology, including associated cost uncertainties and a comparison with conventional hydrogen-recovery technology, before initiating a major development program.
Carbon Dioxide Hydrate-Based Technology
Numerous commercial processes for capturing carbon dioxide from gas mixtures are widely used in the refining industry (EPRI, 1991). The use of these processes in a power plant is not economically feasible, however, because of the high energy consumption and high equipment costs of the many large absorbers and regenerators. A process for removing carbon dioxide as solid carbon dioxide hydrate from a methane-rich stream was investigated by the California Institute
of Technology in 1993 (EPRI, 1993). The use of this process in a syngas stream was later patented (Spencer, 1999).
Subsequent process evaluations have indicated that the carbon dioxide hydrate removal process may be able to operate at very low temperature and remove more than 80 percent of the carbon dioxide from a syngas stream in an IGCC plant while consuming less energy and at lower capital cost than for a commercial amine process (Spencer and White, 1998). Although these preliminary results are encouraging, the findings have not been verified independently by a third party. Therefore, DOE has provided funding to a joint industry/national laboratory team to validate the data. If they are validated, additional bench-scale experiments will be carried out to determine scale up and design data for a commercial-scale, single-reactor-train pilot plant. Considering the amount of R&D in this area, capturing carbon dioxide in its hydrate precursor form might meet the Vision 21 goal of having a commercial design by 2015.
Finding. The hydrate-based technology for capturing carbon dioxide operates at very low temperature and includes a number of thermodynamic phase changes. Claims have been made that it is a low energy-consumption process with reasonable investment costs.
Recommendation. The hydrate-based technology for capturing carbon dioxide should be pursued in the Vision 21 Program, but energy and capital cost assumptions should be reviewed periodically to confirm the validity of the low energy consumption and costs claimed in preliminary process design studies.
Hydrogen Production from Water Dissociation
The Vision 21 program on hydrogen production from water dissociation will investigate using an inorganic proton-transfer membrane for generating and separating hydrogen from high-temperature steam. The technology program is in the early stages of material development, with work being done at Argonne National Laboratory and FETC. The water dissociation reaction is endothermic, which will add to the energy requirements of the process. Reducing the energy requirement for heating water and developing lasting membranes that can selectively transfer hydrogen ions at the high (greater than 1,000°C) temperatures required for water dissociation are among the major technological challenges.
Finding. Hydrogen production from water dissociation technology faces major fundamental challenges that will have to be addressed early in the R&D process to determine whether further R&D expenditures might result in a commercially viable technology.
Recommendation. The first milestone for hydrogen production from water dissociation should be an independent review to identify all of the engineering and economic hurdles that will have to be overcome. If the technology is judged to be capable of meeting Vision 21's performance and cost goals, then research and development should continue to the next stage. If performance and cost goals cannot be met, Vision 21 Program funding should be withdrawn.
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Elsevier Science, Ltd. 1999. Greenhouse Gas Control Technologies. Oxford, U.K.: Elsevier Science, Ltd.
EPRI (Electric Power Research Institute). 1991. Engineering and Economic Evaluation of Carbon Dioxide Removal from Fossil-Fuel-Fired Power Plants. Vol. 2. Coal Gasification-Combined Cycle Power Plants. EPRI IE-7365. Palo Alto, Calif.: Electric Power Research Institute.
EPRI. 1993. Investigations of CO2 Hydrate Formation and Discussion: Progress Report for October 1, 1991, to June 30, 1993. August 1993. Palo Alto, Calif.: Electric Power Research Institute.
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Judkins, R. 1999. Materials R&D Opportunities for the Advanced Fossil-Fueled Energy Roadmap. Presentation by R. Judkins, Oak Ridge National Laboratory, to the Committee on R&D Opportunities for Advanced Fossil-fueled Energy Complexes, National Research Council, Washington, D.C., June 30, 1999.
NRC (National Research Council). 1995. Coal: Energy for the Future. Board on Energy and Environmental Systems. Washington, D.C.: National Academy Press.
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Spencer, D. 1999. CO2 Hydrate Gas Separation. Presentation to the 7th Clean Coal Technology Conference, Knoxville, Tennessee, June 21-24, 1999.
Spencer, D., and J. S. White. 1998. Hydrate Process Design for Extracting and Sequestering Carbon Dioxide from Power Plant Flue Gases or Shifted Synthesis Gas. Final Report to the U.S. Department of Energy-Office of Fossil Energy. Unpublished.
Van Eric, E., and R. Richards. 1999. Development of Ceramic Ion Transport Membranes for Oxygen Production. Presentation to the 7th Clean Coal Technology Conference, Knoxville, Tennessee, June 21-24, 1999.