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Novel Approaches to Carbon Management: Separation, Capture, Sequestration, and Conversion to Useful Products - Workshop Report 2 Advanced Separations Techniques The workshop sessions on separations techniques were aimed at identifying novel materials and process concepts for CO2 and hydrogen production. The streams from which these gases need to be separated include low-pressure flue gas from the combustion of fossil fuels, and high-pressure streams from the gasification of fossil fuels, in addition to the separation of CO2 from the atmosphere. Hydrogen was included because efficient and cost-effective purification methods, when coupled with low CO2 emission generation methods, could enable wide applications of H2 as fuel. Purification of oxygen was also on the agenda, but no notable ideas were presented. At the present time, technology exists for the separation of CO2 and hydrogen, but the capital and operating costs are very high, particularly when existing technology is considered for fossil fuel combustion or gasification streams. This limitation applies to all available separation schemes—to absorption by a liquid, adsorption by a solid, or separation by selective transport through a membrane. At the fundamental level, the high cost for all of these schemes results from two key factors: (1) the low mass fluxes in the separation units and (2) the high energy consumption during regeneration of the separation agent and/or the production of a high-pressure stream of CO2 or hydrogen. The cost of the separation agents, their operating life, their selectivity, and the complexity of the process are also important factors in making any process economically attractive. High mass flux (reduced capital cost) and low energy consumption are key features that will make any novel material or process concept attractive and worth investigating. During the workshop sessions, many potential areas of research in advanced separations techniques were discussed at length. The following eight were considered to have promise and are discussed in the sections below: CO2 absorbents; CO2 sorbents; Nanoscale materials as separation agents; High-temperature membranes; Electrochemical approaches for gas separations; Small-scale widely distributed CO2 recovery processes; Carbon management through carbon monoxide; and Novel hydrogen storage concepts. CO2 ABSORBENTS Current commercial processes for CO2 absorption utilize either the chemical interaction between CO2 and a compound (e.g., amine, alkali metal hydroxide) or the physical interaction with a solvent (e.g., alcohol, ether). In either case, CO2 is removed from the gas stream into the sorbent at a lower temperature or higher pressure and is later released at a higher temperature or lower pressure. Although absorption by chemical interaction is efficient, the process suffers from high energy consumption and degradation of the sorbent due to other contaminants (such as sulfur compounds and trace metals) in the flue gas and
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Novel Approaches to Carbon Management: Separation, Capture, Sequestration, and Conversion to Useful Products - Workshop Report decomposition in the case of amines. Physical solvent absorbents also degrade for similar reasons and suffer loss due to evaporation. There is need for new, low-cost sorbent materials that have enhanced stability, are less volatile and less viscous, have higher CO2 capacity per unit of mass, are more environmentally friendly, and require less energy consumption for operation. Potential candidate materials include but are not limited to the following: molten metal oxides, medium-temperature eutectics, ionic liquids, biphasic materials, and CO2 transfer agents that reversibly form compounds with CO2 (e.g., alkyl carbonate). Some of these materials offer the potential advantages of being stable above 300 °C, are nonvolatile, and have tunable properties. Hybrid materials that possess synergistic effects may offer additional advantages of being multifunctional. Recent developments in experimental methods and computational techniques, such as density-functional theory (DFT) and molecular dynamic methods, provide new tools for designing and synthesizing tailored molecules with unique properties. CO2 SORBENTS Sorbents are used to remove CO2 from a gas stream typically at higher temperatures than those used for absorbents, up to 700 °C or 800 °C in a combustion process. The common sorbents are metal oxides, such as calcium oxide (CaO). These materials chemically react with the carbon dioxide, in the case of CaO by forming carbonates. In most cases, the sorption capacity is limited to about 30 percent—that is, only about 30 percent of the CaO is converted to carbonate. The capacity can be improved by better engineering of the pore structure of the CaO in which case close to 100 percent capacity can be achieved. However, significant improvements in the operational characteristics of the sorbent would make this approach much more attractive. A desirable sorbent should have high CO2 capacity (up to 100 percent of theoretical capacity), function in the presence of water vapor in the gas stream, and have fast reaction and regeneration kinetics, high durability, and the ability to be regenerated with minimal energy consumption. Sorbents that can operate at high temperatures (600 °C to 700 °C) could eliminate the need to cool the gas. The ability to remove other pollutants also is desirable. High-temperature sorbents can also be applied to the production of hydrogen from fossil fuels. Natural gas or coal can be gasified to a mixture of carbon monoxide and hydrogen (CO/H2). Increased hydrogen production is traditionally achieved by employing the water-gas shift (WGS) reaction. However, the equilibrium of the WGS reaction requires a low reaction temperature in order to achieve high hydrogen concentration. Research is under way to separate hydrogen from high-temperature gas mixtures by means of high-temperature hydrogen separation membranes to shift the equilibrium toward hydrogen formation. Similar results can be achieved by removing CO2 from a high-temperature gas mixture by the reaction of CO2 with high-temperature sorbents, leading to the production of pure hydrogen. Metal oxides can also be effective for multifunctional pollution control. For example, calcium-based sorbents can react with sulfur oxides, hydrogen sulfide, and chlorine to a high extent as well, thus reducing their concentration in effluent streams to parts per million (ppm) levels. New oxide compositions (e.g., multicomponent oxides, supported oxides) and/or oxides of engineered porosity are candidate materials. Completely novel sorbents, such as a
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Novel Approaches to Carbon Management: Separation, Capture, Sequestration, and Conversion to Useful Products - Workshop Report solid sorbent that becomes a liquid, or a liquid that turns into a solid upon adsorption of CO2, may offer significant advantages. Using modern computational methods and experimental techniques developed for the design and synthesis of nanostructured materials, it may be possible to engineer materials with tailored pore structures and connectivity, particle size, and chemical environment of the binding sites. Research and development on such materials can lead to breakthrough discoveries in adsorption technology. NANOSCALE MATERIALS AS SEPARATION AGENTS Nanotechnology offers the potential to make materials with nanometer-sized dimensions. It also allows the predictable fabrication of structures with similar dimensionality. The appeal of these materials and structures is that they can have physical and chemical properties different from those of their larger-scale counterparts. The potential of these nanoscale materials in separations technology is just beginning to be demonstrated. Separations technology is based on the physical and chemical interactions between the separations agent and the molecules that need to be separated. Physical properties such as pore size dimensions, pore size distribution, and connectivity have a major impact on the effectiveness of the separations agent. The chemical interactions between the separations agent and the molecules to be separated are also of paramount importance. These can be dramatically different at the nanoscale. Functionalization at the nanoscale may also prove to be more effective in terms of selectivity toward small molecules as well as the ability to change the binding energy. Nanoscale porous structures in which molecular exclusion is the basis for many concepts in membrane separations offer the potential for greater selectivity. Nanoscale porous structures with minimum irregularities in the molecular path may significantly increase the molecular flux through the pores. This is a very important feature, since a major barrier to the use of membranes in large-scale separations is inadequate flux through membranes. Methods to design and construct such structures and their incorporation into defect-free membranes are research opportunities. HIGH-TEMPERATURE MEMBRANES There is a need for novel membranes that can perform the separation of CO2 and H2 at high temperature (700 °C to 800 °C) and pressures above 20 bars. These membranes must show very high permeation rates in order to be an option for the large-scale separation of hydrogen and CO2. Moderate selectivities are sufficient in the case of CO2. High selectivities are required for hydrogen. Membrane types under development include polymeric materials, microporous membranes, and liquid membranes, as well as solid membranes. For the needs described, solid membranes offer the greatest potential. Significant progress has been made in the development of ionic membranes for use in solid oxide fuel cells and for the high-temperature separation of oxygen from air. Novel concepts that build on these developments are needed for the effective separation of hydrogen and CO2 at high pressures and temperatures. Nanoscale membranes that are thermally and mechanically stable and that do not require a support or that can utilize a support that is not flux-limited will represent a major technology breakthrough.
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Novel Approaches to Carbon Management: Separation, Capture, Sequestration, and Conversion to Useful Products - Workshop Report Another promising area of research is in the design of high-temperature, stable porous structures that have the porous space filled with functionalized molten salts. The molten salts would perform the separation, taking advantage of facilitated transport mechanisms since the molten salts need to operate under minimum pressure differential. These structures will require stable, nonvolatile molten salts with high permeance and good selectivity. ELECTROCHEMICAL APPROACHES FOR GAS SEPARATIONS Most state-of-the-art gas separation technologies require significant expenditures of energy. For example, adsorption and desorption processes require energy to regenerate the separation agent. In other separations that are driven by pressure differentials, the separated product stream is at low pressure. Since in most CO2 sequestration schemes it is necessary to produce high-pressure CO2, many separation schemes require expensive and energy-intensive compression of the CO2. Electrochemical approaches that allow the recovery of CO2, hydrogen, and other gases from low-pressure streams deserve more attention. Conceptually, one can envision absorbing the low-pressure gas and pumping the gas-rich absorbent to high pressure. Pressurizing a liquid requires only a fraction of the electrical energy required to pressurize a gas. The challenge is to desorb the gas at this high pressure and regenerate the absorbent. Electrochemical methods could be a novel way to accomplish this desired step. It is known that electrolysis of water can result in high-pressure hydrogen and oxygen. Experiments have been reported in the literature indicating that CO2 can be absorbed in the liquid at pressures below atmospheric and desorbed at atmospheric pressure in an electrochemical redox system. Research that leads to absorbents that have a high capacity for low-pressure CO2 and whose CO2 solubility at high pressure can be controlled using electrochemistry offers significant promise. Systems that incorporate redox agents that have different binding strength for CO2 depending on the oxidation state are also promising. SMALL-SCALE WIDELY DISTRIBUTED CO2 RECOVERY PROCESSES Significant potential impact on the management of greenhouse carbon would result if economic processes were developed to recover CO2 from distributed sources, such as exhaust gases from transportation vehicles, or from dilute sources such as the atmosphere. Although the weight of the absorbent and the absorbed CO2 together is many times the weight of fuel burned, onboard absorption systems suitable and selective for extracting CO2 from exhaust gas should be considered. CO2-saturated absorbers might be exchanged for fresh ones at the time of refueling. An infrastructure could be developed to collect and centrally process the absorbers for sequestration and regeneration. Similarly, passive, home-scale, distributed absorption systems might be developed for CO2 removal from the atmosphere. Suitable distributed passive systems—based on bodies of water, absorbers, adsorbent materials, and biofiltration systems—deserve more attention. A centralized collection infrastructure capable of regenerating distributed absorbents may be required. New families of absorbents (or adsorbents or membranes) will need to be developed for use with systems to recover CO2 directly from the atmosphere. Such absorbents will need
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Novel Approaches to Carbon Management: Separation, Capture, Sequestration, and Conversion to Useful Products - Workshop Report to be more effective than today’s amines at the 400- to 500-ppm CO2 concentration range, but they will have to be less costly to regenerate than hydroxides. It may be necessary to extend or develop new structure-property relationships from existing or new experimental data to identify new molecular compositions and materials. Finally, it may be possible to develop distributed systems to recover CO2 from small domestic point sources or even from the atmosphere to produce materials such as fuel-grade methanol at rates significantly greater than cultivating biomass for use as a fuel feedstock. Such systems may be driven by photochemistry, electrochemistry, or diurnal temperature swings. (See the discussion in Chapter 5, “Novel Niches.”) In all cases, novel passive absorption materials and systems as well as low-cost (capital and energy) regeneration systems need to be developed. CARBON MANAGEMENT THROUGH CARBON MONOXIDE In the gasification of fossil fuels, most of the carbon is in the form of carbon monoxide. The conventional processing options for this gas are combustion to CO2 (for electric power or heat generation), reaction with water to produce hydrogen and CO2, or reaction with hydrogen to produce a broad range of chemical and fuel products. In a scenario in which carbon management (collection and sequestration) is desired, it is worthwhile to explore ways to achieve this through use of carbon monoxide. One starting venue would be the production of power by means of a partial combustion process designed to produce carbon monoxide as the principal product. The energy or power from the partial combustion will be less than it would be if the fuel were fully combusted to CO2. However, carbon monoxide may be easier to capture, separate, and “store” than CO2. Moreover, carbon monoxide can be more easily converted to useful products than CO2. It is important to keep in mind that all of these reaction schemes require hydrogen. If the hydrogen is derived from fossil fuels using conventional technology, this option is not attractive. Thus, it is essential that novel ways to produce this hydrogen be part of the equation. In the case of this carbon management option, the first step should be an engineering evaluation of novel process concepts that encompass the whole carbon cycle, starting from the fossil fuel and including the separation, transportation, and sequestration of the ultimate carbon species. Simple energy and material balances can help identify attractive options that can guide the selection of novel materials and process chemistry concepts. These schemes should include process concepts in which heat and/or electric power is generated while the carbon monoxide is converted to an easily “sequestrable” state. It should also include concepts in which carbon monoxide is converted to fuels and chemicals as an intermediate step leading to simpler recovery and sequestration of carbon. NOVEL HYDROGEN STORAGE CONCEPTS The production of hydrogen by means of gasification of coal (or other low-quality carbonaceous feedstocks) offers a low-cost route to fossil fuel decarbonization. With current technology, hydrogen (gas) can be produced from coal in large plants with near-zero emissions of carbon dioxide at costs in the range of $1 to $1.75 per kilogram (equivalent to
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Novel Approaches to Carbon Management: Separation, Capture, Sequestration, and Conversion to Useful Products - Workshop Report $1 to $1.75 per gallon of gasoline equivalent).1,2 But the real cost of hydrogen to automotive consumers can be 2 1/2 to 3 times as much. The high retail cost of hydrogen inhibits the development of a hydrogen economy.3 The large difference between the retail and wholesale costs of hydrogen arises in large part because the volumetric energy density of hydrogen is low (with gaseous hydrogen at 350 bar, one-tenth that of gasoline). The only commercially viable strategy for hydrogen storage in a vehicle at the present time is compressed gaseous storage (350 to 700 bar), which is costly not only for the storage canisters onboard vehicles but also for the compressors and electricity required for compression at refueling stations. Refueling time and safe hydrogen storage and handling facilities at retail stations are also factors that have a negative impact on the development of a hydrogen economy for transportation. A low-cost hydrogen storage technology offering high volumetric storage densities with modest pressurization requirements could lead to a major reduction of the retail-wholesale gap for hydrogen. A mid-sized car with a hydrogen fuel cell could have a fuel consumption of 1 kg of hydrogen per 80 miles (80 miles per gallon of gasoline equivalent). If the desired range is 350 miles, a storage system with 4 percent hydrogen by weight translates to less than 120 kg of total weight. This weight penalty can be offset by reductions in the weight of other auto body parts. Some present hydrogen storage sorbents have capacities below the target indicated, other sorbents suffer deterioration in use, and still others require complex systems to handle the energy requirements during sorption and desorption. However, the range of materials investigated until now has been limited (metal hydrides, carbon). The theoretical design of sorbents (liquids or solids) with unique compositions and structure coupled with favorable physical and chemical interactions with hydrogen molecules is a good starting point for further research. 1 Williams R.H. Decarbonized Fossil Energy Carriers and Their Technological Competitors. Prepared for the International Panel on Climate Change Workshop on Carbon Capture and Storage, Regina, Saskatchewan, Canada, 18–21 November 2002. 2 Simbeck D.R. and Chang E. Hydrogen Supply: Cost Estimate for Hydrogen Pathways—Scoping Analysis. NREL/SR-540–32525. July, 2002. Golden, CO: National Renewable Energy Lab. 3 The hydrogen economy has been envisioned as the large-scale use of hydrogen as an energy carrier generated from any of a variety of fuels or feedstocks, to be used in the transportation, industrial, and building sectors, and requiring an infrastructure for its transmission and delivery.
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