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Beyond the Molecular Frontier: Challenges for Chemistry and Chemical Engineering 10 Energy: Providing for the Future1 Some Challenges for Chemists and Chemical Engineers Develop more stable and less expensive materials and methods for the capture of solar energy and its conversion to energy or to useful products. Design inexpensive, high-energy-density, and quickly rechargeable storage batteries that make electric vehicles truly practical. Develop practical, less expensive, more stable fuel cells with improved membranes, catalysts, electrodes, and electrolytes. Develop materials, processes, and infrastructure for hydrogen generation, distribution, storage, and delivery of energy for vehicles. Develop photocatalytic systems with efficiencies great enough to use for chemical processing on a significant scale. Learn how to concentrate and securely deal with the radioactive waste products from nuclear energy plants. Develop practical superconducting materials for energy distribution over long distances. 1 As part of the overall project on Challenges for the Chemical Sciences in the 21st Century, a workshop on Energy and Transportation will lead to a separate report. The reader is urged to consult that report for further information.
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Beyond the Molecular Frontier: Challenges for Chemistry and Chemical Engineering Develop technologies and catalysts for the cleaner use of coal as a fuel and for the conversion of coal to other fuels. Develop methods to use biomass as a renewable fuel source. Develop technologies for the improved extraction of conventional fossil fuels, including unconventional sources such as oil shale, tar sands, and deep-sea methane hydrates. Develop practical and environmentally responsible methods of carbon dioxide capture and sequestration. Develop lower cost, lighter weight, more durable, more resilient, and recyclable materials for the construction of safer lighter-weight vehicles. Develop improved materials, processes, and practices that will allow reduced energy use per unit of gross domestic product. Chemistry and chemical engineering are intimately concerned with the generation and use of energy. We need energy for manufacturing, for transportation, for heating and cooling our homes, for lighting, and for cooking. Currently about 85% of the world’s energy is obtained by burning fossil fuels—petroleum, natural gas, and coal—but this must change soon. Affordable supplies will become scarcer, and burning fossil fuels produces carbon dioxide that contributes to the greenhouse effect by which solar energy is trapped within the atmosphere and warms the planet. Burning fossil fuels, at least with current technology, also produces oxides of nitrogen and sulfur and other pollutants that affect plants and animals. The problem of having enough clean energy is related to population, standard of living, and the efficiency with which energy is used to provide a unit of economic output. Humans will always need energy, and chemists and chemical engineers will continue to play a central role in learning how to produce and use it. GOALS Chemists and chemical engineers will need to join with experts in other disciplines to invent new ways to generate and transport energy for human use and provide for the needs and aspirations of a growing population in a sustainable manner. New ways will also be needed to minimize the energy used for human activities, including manufacturing.
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Beyond the Molecular Frontier: Challenges for Chemistry and Chemical Engineering PROGRESS TO DATE Generating Energy Once petroleum is removed from the earth it is refined. It is separated into its various components by distillation, producing not only gasoline but also higher boiling materials such as diesel oil, kerosene, lubricating oil, and asphalt. Then part of it is cracked, converting some of the larger less volatile molecules to smaller ones more useful in gasoline, and it is reformed, transforming some of the molecules into others with smoother burning properties. Some of the products are also used as chemical raw materials, in addition to fuels. Both cracking and reforming use special catalysts and processes. The overall process is highly efficient, so that essentially none of the components of petroleum are wasted. However, most agree that the supply of inexpensive petroleum will run out sometime, although there is disagreement as to exactly when. At current rates of consumption, we may still be using petroleum as a major source of energy 50 years from now. Natural gas may last 100 years, while coal reserves could last for perhaps four centuries. If population growth and standard of living improvement lead to increased energy consumption rates despite conservation efforts, the currently economically recoverable reserves will be depleted in a shorter period of time. In any event, development of more expensive fossil fuel sources—such as oil shale, tar sands, methane hydrates that are found at the high pressure of the deep sea, and coal, oil, and gas that current extraction technology leaves behind—will also be required. Chemists and chemical engineers have developed processes for the gasification of coal, converting it by chemical transformation into syngas, a cleaner, more convenient energy source. They have also devised ways to convert coal into a liquid that can be used in place of gasoline in combustion engines. However, whether coal is used as a solid or converted to a liquid or gas, when it is burned the result is still carbon dioxide, a greenhouse gas. Some chemical processes use energy directly to drive the transformation. For example, the conversion of iron ore, iron oxide, to iron metal requires chemical energy to remove the oxygen atoms. In early times the iron ore was heated with charcoal; in more recent times it is heated with refined coal (coke), but in both cases the result is conversion of coal or wood into carbon monoxide, which is toxic but can be burned to carbon dioxide to generate needed heat. There is now interest in devising processes that do not use carbon in this way, but use electrical energy to avoid the production of carbon oxides. There is also interest in finding feasible and harmless ways to capture and permanently sequester carbon dioxide underground or in the oceans, thereby preventing its accumulation in the atmosphere.2 Carbon dioxide concentration in the 2 Carbon Management: Implications for R&D in the Chemical Sciences, National Research Council, National Academy Press, Washington, D.C., 2001.
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Beyond the Molecular Frontier: Challenges for Chemistry and Chemical Engineering atmosphere has increased by about one-third since the beginning of the industrial revolution. Unless combustion of fossil fuels is halted, or unless carbon dioxide from fossil fuel burning is completely and permanently sequestered, carbon dioxide concentrations in the atmosphere will inevitably continue to rise with potentially significant consequences for global warming. Alternatives to Fossil Fuels Solar Energy Almost all of our energy has come from the sun. This is obvious when we burn wood, in which solar energy has been in a sense stored in a tree by photosynthesis. When whale oil was a significant fuel it was only a little more indirect— the whales ate plant material that was also produced by sunlight, and then converted that food energy into the energy in their oil. Petroleum, natural gas, and coal are also just stored forms of solar energy, from plants and animals that lived and were buried long ago, while hydroelectric and wind power are derived from more contemporary solar-driven oceanic evaporation and atmospheric pressure gradients. In photosynthesis, plants convert carbon dioxide into oxygen of the air and carbonaceous materials of the plant; burning the plants simply sends the carbon back to carbon dioxide again. Thus the cycle—carbon dioxide plus light to form plant material and oxygen, then burning the plant material to consume the oxygen and regenerate carbon dioxide and liberate energy—is one way to capture the energy of the sun. The critical need is to avoid overloading the cycle, producing carbon dioxide faster than it can be recycled into plant material—a significant concern for current levels of fossil fuel combustion. Although energy systems are currently dominated by fossil fuels, alternatives need to be developed. There is no obvious single solution, but a variety of approaches could make useful contributions. One plan would be to grow special grasses or other plants that are particularly efficient at converting sunlight into biomass (plant matter), then convert the biomass to electricity either by burning or by some version of a fuel cell. Genetic engineering may permit us to design new green plants that are particularly efficient at converting sunlight into useful fuels. Water and arable land would be needed for this scheme, but it is argued that we do not yet need all the arable land for the production of food and fiber. Another alternative would be to encourage the growth of phytoplankton in the ocean by intentional fertilization with limiting micronutrients such as iron. The process would serve as a mechanism for sequestering some carbon dioxide from the atmosphere; the resulting organic material would not be harvested as fuel but some instead would sink into the ocean depths before completing the cycle back into carbon dioxide. As with other proposals for carbon sequestration, there is no scientific consensus on its efficacy and safety.
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Beyond the Molecular Frontier: Challenges for Chemistry and Chemical Engineering Scientists, including chemists and chemical engineers, have also been pursuing the direct capture of solar energy, either for heating or for directly generating electricity.3 One plan would cover significant amounts of arid desert and other surfaces such as rooftops with photovoltaic cells that directly convert solar energy into electricity. At the present time, photovoltaics can convert as much as 30% of the incident sunlight to electricity. The technical challenge, however, is to devise materials and manufacturing processes for photocells that are cheap, long lasting, and efficient in the conversion of light to electricity; ways are also needed to collect, store, and distribute the energy when and where it is needed. These problems have not yet been completely solved. A related advance is the invention of electrically conductive polymers, which are not metals. Alan J. Heeger, Alan G. MacDiarmid, and Hideki Shirakawa received a Nobel Prize in 2000 for opening up this important new area of science. In an alternative way to take advantage of the sun’s energy, photocells are under development using sunlight to drive chemical transformations, perhaps even producing chemicals that in turn could be used to generate electricity. For example, the photochemical generation of hydrogen, by splitting water, could be combined with a hydrogen fuel cell in this way. This area strongly depends on basic research in photochemistry. Nuclear Energy An alternative to solar energy is provided by nuclear energy—currently the source of 7% of the world’s total energy and 20% of U.S. electrical energy. Chemists and chemical engineers have devised the processes for producing the nuclear fuels from crude uranium ores. In many countries nuclear power plants are major sources of electricity (as much as 75% in France), but one of the problems is nuclear waste. A typical nuclear energy plant produces 20 metric tons of radioactive waste each year. Chemists and chemical engineers are working to devise methods to separate the radioactive material from the inert material in which it is produced. If this is successful the volume of radioactive substances to be handled will be much less, and some of the purified radioactive materials may be available for other uses (including medical diagnostics and treatment). As another approach, converting the waste products to tough ceramics could make them stable for very long periods of time. The development of safe methods for dealing with radioactive waste—together with public acceptance of them—pose a challenge to which chemists and chemical engineers can respond.4 3 For further discussion of this topic, see ref. 1 and ref. 2. 4 See for example: Electrometallurgical Techniques for DOE Spent Fuel Treatment: Final Report, National Research Council, National Academy Press, Washington, D.C., 2000; Alternatives for High-Level Waste Salt Processing at the Savannah River Site, National Research Council, National Academy Press, Washington, D.C., 2000.
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Beyond the Molecular Frontier: Challenges for Chemistry and Chemical Engineering Nuclear energy offers many advantages if the waste problem can be solved. The fuel is inexpensive, the energy can be generated near where it is to be used, and there are no greenhouse or acid rain effects. Of course, a special problem with nuclear energy is the hazard if the plant is run carelessly, and it is possible that the operation of a nuclear power plant could be diverted to develop material for nuclear weapons or the radioactive by-products could be used in terrorist attacks. However, it is important to solve these problems so that the currently negative public perception of nuclear energy undergoes a change, and permits nuclear energy to make its full possible contribution to the world, particularly after we have stopped burning fossil fuels. Whether nuclear power generation increases as a contributor to our energy supply or merely continues on its present course, the aging population of nuclear chemists and engineers poses a significant concern. There has been a steady reduction in the number of university programs in nuclear chemistry, radiochemistry, and nuclear engineering—and in the number of graduates they produce.5 Unless a new pool of expertise is developed, it will become increasingly difficult to safely operate existing reactors, manage the radioactive waste that will be produced (along with that which already exists), and clean up radioactive contamination from earlier activities. Water and Wind Approximately 10% of U.S. electrical energy is produced by hydroelectric dams.6 Although there are few economic and environmentally acceptable dam sites remaining, in some places it is possible to use wind power, or perhaps even the ocean tides, to generate electricity. Here the opportunity for chemists and chemical engineers is the invention and production of modern materials that can make such approaches possible. Energy Efficiency, Conversion, Storage, and Distribution In recent years, much attention has been devoted to improving the efficiency with which energy is produced and used by society in general and also in chemical manufacturing. Higher fuel efficiency in automobiles, better insulation materials and construction practices for homes, and energy efficient lighting and ap- 5 Nuclear Education and Training: Cause for Concern? The Nuclear Energy Agency, Organisation for Economic Co-operation and Development (OECD), Paris, 2000 [http://www.nea.fr/html/ndd/reports/2000/nea2428-education.pdf]; Training Requirements for Chemists in Nuclear Medicine, Nuclear Industry, and Related Areas, Report of a Workshop, National Research Council, National Academy Press, Washington, D.C., 1988. 6 Renewable Power Pathways: A Review of The U.S. Department of Energy’s Renewable Energy Programs, National Research Council, National Academy Press, Washington, D.C., 2000, p. 116.
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Beyond the Molecular Frontier: Challenges for Chemistry and Chemical Engineering pliances are familiar examples. Internal combustion-electric hybrid vehicles are another area in which multidisciplinary science and engineering research is striving to produce extremely efficient, if perhaps not yet economically practical, transportation systems. For electrical power generation, complex systems such as coal-gasification combined-cycle and natural gas combined-cycle are being installed to reduce the amount of carbon dioxide produced per unit of electricity generated. A combined-cycle generator increases efficiency by capturing heat from the gas-turbine exhaust stream and produces additional power with a steam turbine. As a consequence of electricity deregulation, various heat and power cogeneration and distributed power generation schemes with micro turbines or fuel cells are also being explored. Again, contributions by chemists and chemical engineers will be critical, especially in the development of high-temperature and other advanced materials. Electrochemical Cells Instead of burning fuels such as petroleum or coal or natural gas to produce steam for electrical turbines, electrochemistry can be used to generate electricity directly from chemical reactions—thus avoiding the waste of energy and the pollution that comes from combustion. This approach is particularly attractive for use in portable energy sources and vehicles. A common example of such an electrochemical cell is an old-style flashlight battery that cannot be recharged. In such a battery, a metal such as zinc gives up electrons as it is oxidized to a zinc ion, while another material such as manganese dioxide is reduced by those electrons to form manganese ions. The battery is constructed with separators or membranes in such a way that the electrons from the zinc must travel outside the battery through an external circuit before returning to the battery to reduce the manganese dioxide. In this way electricity is used directly. The zinc metal and the manganese dioxide were both originally made in a factory by oxidation-reduction reactions of zinc salts and from manganese salts, so the battery represents a convenient way to store and deliver electricity. Batteries have been developed from many pairs of chemicals capable of being oxidized and reduced. Some systems are rechargeable; after the chemicals in the battery have been exhausted, the reactions can be reversed by the application of an external source of electricity. The lead-acid automobile battery is a familiar example. In many applications, such as cell phones and laptop computers, the weight of a portable electricity supply is critical. This has led to the development of batteries based on lightweight lithium chemistry, for which challenges still remain. Electrochemists have also learned how to make electrochemical cells in which one of the reactants is continuously supplied from outside the cell rather than contained within. One example is the zinc-air cell in which particles of zinc
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Beyond the Molecular Frontier: Challenges for Chemistry and Chemical Engineering metal contained in the cell react with the oxygen of the air brought in from outside the cell to make zinc oxide, again with the generation of electricity. One can imagine a scenario in which such a cell could replace the petroleum-fueled engine of an automobile. The vehicle would be driven by an electric motor with the electricity supplied from a zinc-air cell. When all the zinc had been oxidized, a “service station” would remove the part of the cell with the zinc oxide and replace it with a fresh supply of zinc metal. The station would return the zinc oxide to a factory where electricity would be used to convert it back to zinc metal. Fuel Cells Electrochemical cells in which both reactants are continuously supplied from outside the cell, particularly those in which the reactants are oxygen and an otherwise combustible fuel, are generally referred to as fuel cells. Chemists and chemical engineers have devised a large number of fuel cells that differ in the reactants used, as well as in the separators, electrolytes, catalysts, operating temperatures, and other construction details. One example uses hydrogen and air. The cell reaction converts hydrogen fuel and the oxygen from the air to water, just as if the hydrogen were being burned. But this process generates electricity directly with a greater energy efficiency than that obtained from a process such as burning the hydrogen in a combustion turbine generator. A hydrogen fuel cell also operates in a cycle. The reaction of hydrogen and oxygen in the fuel cell to produce water is one part of the cycle. Elsewhere, a source of electricity could be used to electrolyze water and generate hydrogen and oxygen. The net result is to move energy from an electrical generating plant to a remote fuel cell. A fuel cell avoids the production of nitrogen oxide pollutants that are generally formed in combustion processes. Nitrogen oxides formed during generation of the energy needed for production of hydrogen (or other fuel) can be removed by scrubbing the exhaust gases in the generating plant. Hydrogen fuel cells were first used in the space program (with pure oxygen rather than air), but they are being developed for a variety of terrestrial applications that range from portable electronics, to vehicles, to back-up emergency power systems. There are still many unsolved problems for hydrogen fuel cells. One is the rate of reaction of the oxygen at the electrodes, which is not yet as rapid as desired. Related to this is the high cost of current membrane and catalyst systems. Another unsolved problem is how to transport and store hydrogen safely for use with the fuel cell. For space applications, hydrogen has been stored as a cryogenic liquid. For terrestrial applications, one approach is to store the hydrogen as a very high-pressure gas in very strong lightweight cylinders made of advanced composite materials. Chemists and engineers are working on other approaches, such as adsorbing the hydrogen on carbon or spongy nickel, or as metal hydrides that can be stored safely at low pressure. But this and related problems have not yet
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Beyond the Molecular Frontier: Challenges for Chemistry and Chemical Engineering been solved. When they are, we could have what has been called the “hydrogen economy.” In another version of a fuel cell, a carbon compound such as methanol (from natural gas or from the reaction of coal with water) reacts directly in an electrochemical cell with the oxygen in air to form water and carbon dioxide. This approach has fewer problems with storage of the fuel, and it has higher efficiency than power production from combustion of the same fuel—but it still produces carbon dioxide with its contribution to global warming. The performance of current fuel-cell catalysts may be degraded by carbon monoxide, an intermediate in the reaction with carbon-based fuels. In yet another variant, a hydrocarbon is caused to react with water in a separate device called a reformer to produce hydrogen along with a mixture of carbon monoxide and carbon dioxide; after separation, the hydrogen can be fed to a conventional hydrogen fuel cell. This approach avoids carbon monoxide degradation of the fuel cell performance, but it still produces carbon dioxide. Although there is still much to do to make fuel cells widely practical, experimental automobiles have recently been exhibited that are powered by fuel cells. Thus there is every reason to expect fuel cells to play a major role in electricity production in the future. Storage Batteries In the above scenarios the energy to produce the reactants for a battery or fuel cell was supplied in a factory, but another possibility is a storage battery that can be easily and quickly recharged either at a service station or at home. The familiar lead/acid car battery is an example, but it is not good enough to replace combustion of gasoline as the main power source in typical transportation vehicles. The batteries are too heavy and take too long to recharge. What is needed is a high-capacity storage battery that is lightweight, inexpensive, long lasting, and rechargeable. The batteries now used in golf carts and in some recent electric automobiles do not meet all these criteria, but there is active research by chemists and chemical engineers to develop such batteries. When they exist, drivers will be able to stop at a service station and recharge their batteries in 10 to 15 minutes, then drive on for another 300 miles before a recharge is needed. In another scenario, the drivers might simply trade in their depleted batteries for others that have been recharged by a service station. When the problems are solved, we will be able to reduce our dependence on petroleum-fueled automotive engines. We also use small rechargeable batteries to power cell phones and portable computers. They are reasonably light and have the capacity to go for some hours before requiring recharging, but improvements are still needed. As chemists and chemical engineers develop better battery technology we can expect to be freed
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Beyond the Molecular Frontier: Challenges for Chemistry and Chemical Engineering from the need to recharge the batteries in our computers or cell phones quite so often. Energy Distribution Portable energy is important for vehicles and small electronic objects such as cell phones, but we also need to distribute energy from the generating plant to the place where it is to be used. Whether it is generated using photocells in the desert, by growing biomass in agricultural areas, or by operating nuclear power plants, it must go from there to the manufacturing plants and homes where it is needed. Currently this is done with power lines made of ordinary conducting materials such as copper or aluminum, but the electrical resistance of those metals results in considerable loss of energy. Thus there is great interest among chemical scientists in developing practical superconducting materials for power distribution. Superconductors pass electricity with no resistive loss, but so far they operate only at extremely low temperatures, and are impractical for power distribution. However, as the basic science of materials progresses it is hoped that eventually superconductors will become practical for operation closer to normal temperatures while carrying a large current flow. This is a particularly difficult challenge, but if it can be met, the rewards will be enormous. There is already progress—superconducting power cables that can operate when cooled by liquid nitrogen are being made for short-distance power distribution in some urban areas. The approach described above involves the idea that there is a central power plant from which electricity is distributed, but there is another choice. It might be possible to distribute the power generation itself, having small generators locally sited where the power is used. Already many large buildings have their own power generators (as do some private homes), although these are primarily for emergency use. This distributed approach offers the advantage that power is generated locally and only when needed, so energy losses from transmission would not cause problems. For this approach, fuel cells may have an important role if they become practical and can operate using locally available fuels. CHALLENGES AND OPPORTUNITIES FOR THE FUTURE A variety of opportunities and challenges have been described in the preceding section along with an indication of where current progress is inadequate. We must eventually learn how to operate in a world that is not energized by burning fossil fuels, and the opportunities and challenges are clear. In the meantime, while we are still burning coal and hydrocarbons, we need to learn how to deal with the carbon dioxide that is produced. This must be done to address the problem of global climate change and to eliminate the environmentally harmful side products of combustion. We need to devise better ways to use solar energy, for ex-
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Beyond the Molecular Frontier: Challenges for Chemistry and Chemical Engineering ample by creating practical cells for conversion of sunlight to electricity. We need to replace combustion by fuel cell technology, and we need to solve the problem of how to transport and store hydrogen. We need to invent rechargeable batteries that are practical for vehicles that have electric motors instead of gasoline engines. Advances in both basic and applied chemistry and chemical engineering are needed to achieve these goals. WHY ALL THIS IS IMPORTANT The challenges and opportunities in the field of energy are critical for a world in which inexpensive, readily available fossil fuels will eventually be exhausted. Unless we learn how to generate and store energy, not just burn up the fuels formed in earlier times, we will be unable to continue to advance the human condition or even maintain it at its current level. The problems are of central importance, but they can be solved—and the chemical sciences are a necessary part of the solution.
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