The Vision 21 Energy Plant: Clean Energy from Fossil Fuels
Oral Presentation

By Lawrence A. Ruth

Senior Management and Technical Advisor, National Energy Technology Laboratory, Pittsburgh, PA 15236

ABSTRACT

Fossil fuels now provide about 85 percent of the primary energy resources for the United States and the world, and forecasts indicate that this percentage will change little in the foreseeable future. Moreover, total energy use is expected to grow. In the United States, energy consumption is expected to increase by over 30 percent in the next 20 years; worldwide, the energy increase is estimated to be 60 percent over the same period. Coal plays a major role because of its wide distribution and low cost and accounts for about 22 percent of energy consumption both in the United States and worldwide. Although this percentage is expected to change little, the total amount of coal used will increase considerably because of the increase in total energy consumption.

Given this large increase in energy demand and the continuing need to protect our environment, the U.S. Department of Energy has begun an initiative called Vision 21. The goal of Vision 21 is to effectively remove, at affordable costs, environmental concerns associated with using fossil fuels to produce electricity and other valuable energy products. Achieving this ambitious goal will require an intensive, long-range research and development effort aimed at providing substantial advances in fossil energy-related technologies such as gasification, combustion, gas purification and separation, fuel cells, turbines, environmental controls, and advanced materials. This paper provides an overview of Vision 21, including examples of specific projects already underway and the impacts of coal-quality parameters on Vision 21 energy technologies.

Vision 21 is the U.S. Department of Energy's (DOE) new initiative for developing the technology needed for ultraclean 21st century energy plants. The goal of Vision 21 is to effectively remove, at affordable costs, environmental contaminants generated by using fossil fuels to produce electricity and other valuable energy products. To achieve the Vision 21 goal, DOE's National Energy Technology Laboratory (NETL) has implemented an intensive, long-range, 15- to 20-year research and development effort.

Industry and academia have been collaborating on Vision 21. One result of this effort is the Vision 21 Technology Roadmap (U.S. Department of Energy, 2001), which outlines a 15-year research and development strategy and includes objectives, barriers to achieving objectives, and strategies for overcoming barriers. The goal of Vision 21 is very aggressive and, if successful, will provide the United States, and the world, with new methods of fossil-based power generation that would have significant advantages over current methods.

Vision 21 is needed because fossil energy will remain a substantial part of the future energy mix. Fossil fuels now account for about 85 percent of our primary energy sources, both in the United States and in the world. Energy use is growing. In the United States, total energy consumption is projected to increase from 96 to 127 quads (1 quad = 1015 Btu) between 1999 and 2020, an average annual increase of 1.3 percent (Energy Information Administration, 2000a). Worldwide, energy growth is more dramatic; the projection for world energy consumption is for a 60 percent increase over the period from 1997 to 2020, or about 2.6 percent annually. In the developing countries, including Asia, the Middle East, Africa, and Central and South America, the increase is estimated to be even higher, about 6.5 percent annually (Energy Information Administration, 2000b).

Coal plays a major role because it is widely distributed and low in cost. The United States has the largest reserves of any country, about a 250-year supply at current use rates. Although coal's "market share" may decrease slightly over the next 20 years, the amount of coal used will increase because of the growth in total energy use. Although Vision 21 is a fossil fuel initiative rather than a coal-only initiative, the formidable environmental challenges associated with coal use require a disproportionate share of attention.

The Vision 21 initiative is timely because of the confluence of energy, environmental, and market drivers. These drivers include:

• Growing energy demand, particularly for electricity, both in the U.S. and worldwide
• Recognition that fossil energy needs to be part of the future energy mix
• The need to "solve" environmental concerns associated with fossil fuel use
• Restructuring of the energy industry with new players open to multiple feedstocks and products
• Underinvestment in research and technology development
• Recognition of the value of "future options," such as the hydrogen economy

A final point is that Vision 21 stresses innovation and revolutionary technologies rather than attempting to achieve evolutionary improvements in existing technologies. This emphasis is necessary if Vision 21 is to be successful at rapidly developing the technology basis for 21st century energy plants with unprecedented efficiency and environmental performance.

THE VISION 21 ENERGY PLANT

Table 1 shows the performance targets, including costs and timing, for Vision 21 energy plants. Targets are clearly very aggressive, especially because designs for commercial plants are to be available in only 15 years. However, benefits from Vision 21 could be realized much sooner. Spinoff technologies -- some becoming available before 2005 -- are likely to result from Vision 21 R&D. These spinoffs, which are expected to have applications beyond the electric power and energy industries, could include low-cost gas separation technology (for example, for H₂ from syngas and O₂ from air), better gas purification/cleaning processes, better catalysts for producing clean fuels and chemicals from low-valued raw materials, more efficient and lower cost environmental control technology, high-temperature heat exchangers, and improved materials for service under high-temperature conditions.

Table 1. Vision 21 energy plant performance targets

¹HHV, higher heating value; LHV, lower heating value.
²Environmental Protection Agency (1998).

Figure 1 shows an artist's rendition of a possible configuration of a Vision 21 energy plant. The gasifier produces a fuel gas that is cleaned and used to produce power with gas turbines and fuel cells as well as clean transportation fuels and chemicals. The plant features a modular design and makes a market-driven product slate. Coal and "opportunity" feedstocks are gasified by using oxygen produced with a low-cost air separation membrane. The fuel gas is cleaned, and a second membrane is used to separate hydrogen. Carbon monoxide in the fuel gas may be oxidized to CO₂ and the CO₂ sequestered if necessary. Electricity is generated with a fuel cell by using the hydrogen and a gas turbine utilizing the energy in the fuel-cell exhaust. Heat remaining in the gas turbine exhaust is used to generate steam for process heating. A portion of the fuel gas is diverted for the production of clean liquid fuels and high-value chemicals.

Figure 1. Example of a Vision 21 energy plant.

It needs to be emphasized that all Vision 21 plants will not produce a slate of products nor utilize multiple feedstocks. Indeed, it is likely that electricity will be the major or only product most of the time. Other products, such as clean transportation fuels, would be produced where it makes economic sense (that is, where it lowers the cost of producing electricity). The important point is that gasification provides the option of coproduction. Vision 21 plants also might be based on combustion, especially combustion with oxygen rather than with air. Just as DOE has no intention of imposing preselected technology on the market, DOE believes that choices concerning products and feedstocks should be based on market and economic forces.

Figure 2 shows a gasification/gas turbine/fuel cell cycle that was studied to determine how a coal-based system might be configured to achieve the Vision 21 efficiency target of 60 percent for electricity production. This system was designed for analysis purposes, and this paper does not intend to imply that it is a likely configuration for a future commercial Vision 21 plant. However, the heat and mass balances indicate that it is thermodynamically feasible to achieve 60-percent efficiency (on the basis of a higher heating value of fuel) using coal as the fuel. The gas turbine, fuel cell, and gasifier technology selected for the cycle represent the state-of-the-art in DOE's current development programs. Many of the subsystems and components shown in figure 2 have not been tested at the scales or operating conditions necessary for a large commercial plant. The challenges are to develop the required subsystems (for example, fuel cells and gas cleanup technology), integrate the subsystems, simplify the cycle, develop a control strategy and the means to implement it, and reduce cost.

Figure 2. Gasification/gas turbine/fuel cell cycle.

The design shown produces 560 MW (gross) or 520 MW (net) power. The fuel is Illinois No. 6 coal containing 2.5 percent sulfur. The coal is gasified in an entrained bed gasifier operating at 15 atm. pressure. A cold gas conversion efficiency of 84 percent is assumed. The fuel gas is cleaned, cooled, and desulfurized before entering a solid oxide fuel cell (SOFC) operating at 15 atm. and 1830°. A portion of the gasifier fuel gas is reduced in pressure through an expander/turbine before entering a second low-pressure SOFC operating at 3 atm. Ninety percent of the fuel constituents are converted within the cell chambers to produce electricity. The remaining fuel is combusted with the oxidant exhaust streams from the SOFC cathodes to boost the heat energy available for use in the two cascaded turboexpanders. Heat from the turbine exhausts and from the fuel gas cooler is used to generate steam for a reheat steam cycle operating at 1450 psi and 1000°. Of the 560 MW gross power, 33 percent is provided by the high-pressure SOFC, 21 percent by the low-pressure SOFC, 25 percent from the turboexpanders, and 21 percent from the steam turbine.

The differences between a traditional coal-fueled powerplant and a Vision 21 energy plant are summarized in figure 3. The environmental design of Vision 21 plants will be based on the philosophy of industrial ecology. Rather than emission controls being "added on," Vision 21 environmental controls will be a fundamental part of the plant from the outset of the design process. Vision 21 plants will produce no "wastes." All conversion products will be either recycled or converted to useful coproducts. "Smart design" refers to achieving reliability not by redundancy or overdesign but by using inherently more reliable components with predictable service lifetimes. Ideally, critical components would be replaced during routine plant servicing before they fail.

VISION 21 TECHNOLOGIES

Vision 21 focuses on developing the key, critical technologies that will be needed to design and build Vision 21 energy plants. Specific types of plants or plant configurations are not emphasized because it is unknown what kinds of plants, feedstocks, and products the market will favor 15 to 20 years into the future. DOE and industry worked together to identify the key technologies that are most likely to be needed in future Vision 21 plants, regardless of the specific plant configuration. These technologies include:

• Fuel-flexible, thermally efficient gasification to allow the use of low-cost feedstocks (for example, municipal waste, petcoke, and biomass, with coal)
• Gas separation (for example, membranes that can be used to separate oxygen from air, hydrogen from syngas, and CO₂ from combustion products)
• High-temperature heat exchangers (for example, alloy-tube exchangers capable of heating steam or air for use in advanced, high-efficiency cycles)
• Gas stream purification systems capable of operating at high temperatures for removing sulfur compounds and other constituents that may corrode or erode downstream components (for example, turbines, or poison downstream catalysts)
• High-performance combustion systems, both suspension fired and fluidized bed, including ultra-low-NOx combustion and systems that burn fuels in O₂/CO₂ mixtures and produce exhaust streams containing only CO₂ and water
• Fuel-flexible combustion turbines and engine systems, especially turbines and engines capable of operating on coal-derived gases or hydrogen; fuel cell/turbine-engine hybrids capable of 70- to 80-percent efficiency; advanced combustion turbines, including ceramic turbines and engines; advanced steam turbines
• Fuel cells (for example, high-efficiency, low-cost fuel cells; cascaded fuel cell systems capable of operating at multiple temperatures and pressures; fuel cells bottomed by fuel cells; fuel cell/turbine hybrids; new, low-cost, fuel cell concepts capable of approaching $100/kilowatt stack costs and, when incorporated into a system, 70- to 80-percent system efficiency)
• advanced fuels and chemicals development; systems and catalysts for fuels and chemicals production; hydrogen production and storage
• advanced materials for high-temperature applications in aggressive environments, e.g., boiler tubes for high-temperature steam bottoming cycles, and very high-temperature (>2000°F) heat exchangers for use in indirectly fired cycles and other applications, as well as functional materials needed for gas cleanup or separation
• advanced controls and sensors for highly integrated Vision 21 plants; new algorithms that utilize state-of-the-art hardware to assure reliable performance, optimum plant efficiency, and low emissions
• environmental control technology for low-NOx emissions, control of fine particulate matter, and management of byproducts from Vision 21 plants; improved concepts for CO₂ capture and separation
• computational modeling and virtual simulation

Vision 21 will need substantially improved design and simulation tools, including development of a virtual simulation capability. These tools will be used to aid in designing individual components and subsystems, to evaluate and compare the performance of different configurations of Vision 21 systems, and to simulate the performance of plant sections and complete Vision 21 plants. Availability of advanced simulation software will reduce the cost of developing Vision 21 systems. Development of this capability will be a key part of the Vision 21 program.

FEEDSTOCK REQUIREMENTS FOR VISION 21 PLANTS

Vision 21 plants will utilize fossil fuels, primarily coal and gas, along with "opportunity" feedstocks. Opportunity feedstocks include biomass, petroleum coke, and even municipal wastes. It is highly unlikely that a single Vision 21 plant would utilize a wide range of feedstocks; however, it would be feasible for a plant to be designed to accommodate alternative feeds, such as coal and natural gas or coal and biomass when this reduces the cost of making the desired products.

Coal parameters that are important in today's energy plants will continue to be important in future Vision 21 plants (table 2). These parameters relate to coal properties that can affect corrosion, slagging and deposition, coal reactivity, environmental control, and "poisoning" of downstream components or materials that may be used in fuel cells or chemical processing.

Table 2. Coal parameters important for future energy plants

However, Vision 21 plants are likely to differ from today's energy plants in ways that may affect feedstock requirements. The need to achieve higher thermal efficiency is a driving force for higher operating temperatures for components such as turbine blades, boiler tubes, refractory linings, heat exchangers, and gas cleaning systems. This means that feedstock constituents, such as alkali metals, halides, sulfur and iron, associated with high-temperature corrosion should become more important. Environmental corrosion is also a major factor in the design of specialized components that use functional materials, such as ceramic ion-transport membranes. Future energy plants are also more likely to be based on gasification than are current plants. Thus, coal properties, such as porosity and surface area, that can influence gasification rates may become more important.

High-temperature gas turbines, fuel cells, and catalysts for chemical synthesis are more likely to be used in future energy plants. Attention will need to be given to feedstock constituents that can affect these components. Table 3 gives composition (purity) targets for gases that will be used in high-temperature combustion turbines, fuel cells, and fuels and chemicals production.

Table 3. Gas purity targets for Vision 21 energy plants

[U.S. Department of Energy, 2001]

VISION 21 PROJECT PORTFOLIO

The Vision 21 project portfolio comprises ongoing activities in various NETL product areas, e.g., gasification, combustion, turbines, fuel cells, environmental technology, clean fuels, and advanced research, that relate to achieving Vision 21 objectives. In addition, DOE issued a Vision 21 solicitation in September 1999 to competitively seek additional cost-shared projects to develop technologies and analytical capabilities needed for Vision 21. Fifteen projects, out of over nearly 70 proposals received, were selected as of August 2001. Their combined value is approximately $32 million, with about $8 million being provided by the participants and $24 million by DOE.

Although the ongoing activities and new projects address a diverse set of technologies, they collectively share a singular theme and constitute the Vision 21 program. This common theme is the focus on developing the technology and know-how needed to design the subsystems and components that make up Vision 21 plants. Some of the recently selected Vision 21 projects are described below.

• Huntington Alloys, Huntington, WV, and its partners are developing stronger, heat- and corrosion-resistant Oxide Dispersion Strengthened (ODS) alloys for Vision 21 heat exchangers and other applications not possible with existing metallic materials. ODS alloys retain their strength at very high temperatures but their application is limited by poor weldability and relatively poor circumferential creep strength. This latter property is being improved by modifying the grain structure so that ODS tubing can be used at high-temperature (c. 2000°), high-pressure conditions that will often be encountered in Vision 21 plants. Improved joining methods are also being developed and high-temperature corrosion limits are being established.
• CFD Research, Huntsville, AL, along with team members and an industrial consortium comprising turbine and combustion equipment manufacturers, are applying an advanced computational tool called "large eddy simulation" to design low-emission combustion systems for gas turbines. Using a computer to simulate the effect on emissions caused by changes in fuel composition or combustor design can reduce the costs of developing new, more efficient, and cleaner combustor designs and lead to innovative concepts for reducing emissions.
• Clean Energy Systems, Sacramento, CA, is developing a "rocket engine" steam generator to power an advanced turbine, generating electricity and emitting only water and a stream of CO₂ ready for sequestration. Efficiencies exceeding 60% can be achieved with turbines operating at 3000 lb/in2 and 2600°. The size of the steam generator is dramatically smaller than boilers used today; a unit of several hundred megawatts capacity would be about the size of an office desk, opening up possibilities for transportability and greatly reduced cost.
• Fuel Cell Energy, Danbury, CT, is beginning the development of a "hybrid" power system that combines a fuel cell and gas turbine to generate electricity at efficiencies unprecedented even for gas-based systems, 65 percent (near-term) to 80 percent (by 2015). An existing 250 kW fuel cell will be combined with a commercially available microturbine and a conceptual design of a 40 MW Vision 21 plant will be prepared. The proposed system uses a novel approach in which the heat energy in the fuel cell exhaust is transferred indirectly to the turbine air with heat exchangers. A benefit of this approach is that the fuel cell can operate at ambient pressure, saving the costs of piping and pressure vessels, and allowing the turbine to operate at its optimum pressure ratio.
• Foster Wheeler Development Corporation, Livingston, NJ, is leading a team developing a pressurized circulating fluidized bed partial gasification module that offers the advantages of gasification while providing substantial process flexibility because it produces both solid and gaseous fuels that can be utilized in advanced steam and gas turbine cycles. The module also operates at much lower temperatures than existing gasifiers, potentially improving reliability and reducing operating costs.
• GE Energy and Environmental Research Corporation, Irvine, CA, is developing an innovative, fuel-flexible gasification-combustion concept to convert coal and opportunity feedstocks to hydrogen and a separate stream of sequestration-ready CO₂. The hydrogen could be used in fuel cells and combustion turbines to generate electricity with zero pollutant emissions. The three-year program includes lab-, bench-, and pilot-scale activities.
• Eltron Research, Boulder, CO, and ITN Energy Systems, Wheat Ridge, CO, are developing alternative approaches for using non-porous membranes to separate hydrogen from gas streams such as those from coal gasification. In this approach, the hydrogen-containing gas mixture is passed across the membrane surface where hydrogen is catalytically oxidized. The protons and electrons generated are incorporated into the membrane material lattice and conducted to the reduction surface on the other side of the membrane where the reverse reduction reaction occurs to produce pure hydrogen. The technical challenge is to create materials that have both satisfactory proton and electron conductivity and chemical stability at the conditions of use, and to develop thin ceramic structures that achieve hydrogen separation rates comparable to those used in industrial processes.
• Siemens-Westinghouse Power Corporation, Pittsburgh, PA, is teaming with Praxair, Tonawanda, NY, to develop a zero-emission power plant that integrates a solid oxide fuel cell (SOFC) with an oxygen ion-transport membrane (OTM). The approach will be to modify the design of a tubular SOFC by including a stack of OTMs. Oxygen transported through the membrane would be used to oxidize the SOFC's depleted fuel, converting it into CO₂ and steam. The CO₂ can be easily separated for eventual sequestration by condensing the steam. OTM technology provides a much more efficient method of supplying oxygen than conventional cryogenic air separation. In addition, the comparable operating temperatures of SOFCs and OTMs allow for an energy-efficient and cost-effective integration.

CONCLUSIONS

DOE has set ambitious goals for the Vision 21 energy plant of the future. Developing the needed technology will be a major challenge but, compared to current fossil energy based plants, success will result in near-zero environmental impact increased thermal efficiency feedstock and product flexibility affordable costs for energy well into the 21st century

REFERENCES

Energy Information Administration, 2000a, Annual energy outlook 2001: Washington, D.C., U.S. Department of Energy, 262 p.

_______________2000b, International energy outlook 2000: Washington, D.C., U.S. Department of Energy, 250 p.

Environmental Protection Agency, 1998, Study of hazardous air pollutant emissions from electric utility steam generation units : Washington, D.C., Report EPA-453/R-98-004b, v. 2, 295 p.

U.S. Department of Energy, 2001, Vision 21 technology roadmap: Washington, D.C.,, National Energy Technology Laboratory (available at http://www.netl.doe.gov/coalpower/vision21/pubs/v21rdmp.pdf).