【徐遐生院士談地球新能源】 原文網路全文版 EARTH ENERGY: GIFTS FROM NATURE (Online English Full Version) Frank H. Shu1, Michael J. Cai2, Fen-Tair Luo3 1 Institute of Astronomy and Astrophysics, Academia SInica Research Corporation, University of Hawaii2 Institute of Chemistry, Academia Sinica3 Introduction The astronomical heritage of the Earth makes it rich with energy. In its oceans are water molecules that contain two isotopes of hydrogen that date back to the big bang. The light form is what powers thermonuclear fusion in the Sun; the heavy form underlies the hope behind thermonuclear fusion on Earth. Helium is the second most abundant element in the Universe after hydrogen, but none of the helium remaining on Earth came from the big bang; they all come from the alpha particles (helium nuclei) spit out from unstable isotopes of heavy elements that make up the rocks of Earth. These radioactive elements are relics from supernovae that made neutron stars and provide the heat that keeps the interior of the Earth hot. The most neutron-rich of these heavy elements, uranium, forms the basis of fission reactions that power most of today’s terrestrial nuclear reactors. The moderator that slows down fission neutrons is the same as the coolant that carries away the heat from the core of these reactors, water with the light form of hydrogen. This use gives these machines their name: light water reactors (LWRs). LWRs have no emissions of carbon dioxide, but they play a controversial role in Earth energy because of misconceptions that they lack 4 S’s: Solutions (for the nuclear waste problem) Safety (with respect to massive release of radioactivity to the environment) Security (with respect to weapons proliferation) Sustainability (of high-grade uranium ore) Radiation from the thermonuclear powered Sun is the natural energy source that sustains all life on Earth. Sunlight passes through an optically transparent atmosphere to warm the surface of the earth. If sunlight falls on the oceans, heating the water causes some of it to evaporate. The salt of the seawater is left behind, so when the water vapor precipitates, the rain is a source of fresh water. If it is cold, and snow instead falls on high mountain passes, when the snows melt, the streams of fresh water collect into mighty rivers. If the rivers are dammed, high reservoirs of water build up behind the dams. Released from these great heights, the falling water can rush past water turbines that turn powerful magnets inside coils of wire that hum with alternating current. On inhomogeneous terrain, and because of night and day variations, the heating by sunlight is uneven and gives differences in temperature and pressure that create wind, which can power turbines also generating electricity (at about 50% efficiency versus 90% for hydroelectricity). Because air is 800 times less dense than water, wind-electricity is considerably more expensive than hydroelectricity. 1/16
If sunlight strikes solar panels, the photovoltaic effect generates solar electricity (at efficiency up to 20%). Solar electricity ceases at night and is highly variable during cloudy days, so it requires backup from other sources of “base-load” power. If the sunlight falls on green plants, photosynthesis is able to take the energy in the photons to convert the carbon dioxide in the air and water in the ground into the organic compounds necessary for plant growth and reproduction (at about 1% efficiency). These organic compounds contain proportionally fewer O compared to C and H than present in CO2 and H2O, so free molecular oxygen O2 is released to the atmosphere as a byproduct of photosynthesis. Conversely, when plants die, the incompletely oxidized C and H in organic matter can combine with the O2 in air, releasing heat in the process, and reform CO2 and H2O, both of which are greenhouse gases (GHGs). If the reactions occur in a flame, we call the process “burning,” with the heat of combustion used perhaps to boil water that causes the steam to expand past a steam turbine that can again generate electricity (at about 20% efficiency if the biomass is burned directly). If the reactions occur more slowly in animals, we call the process digestion, with the animals (unicellular or multicellular) making use of the food energy (at a low efficiency dependent on the species) and exhaling or excreting the waste products, CO2 and H2O. Biomass that got buried in past eons deep into the Earth, where there is no oxygen but ample heat and pressure, produced the fossil fuels, coal, petroleum, and natural gas that powers the modern technological society. Coal burning is used mostly for electricity generation (at about 35 to 40% efficiency), with noxious emissions of volatile heavy metals (like mercury) because coal is dug out of the ground with small bits of stone in it that contain such heavy metals. Petroleum holds an almost unassailable position as the feedstock of choice for transportation fuel because it of its advantage in ETUDES: Extraction, with historical energy return on investment (EROI) ratios > 10 Transportation, worth doing because petroleum is an energy dense liquid Upgrading, refining to separate low and high molecular weight hydrocarbons and processing to produce a variety of chemical products (e.g., plastics) Distribution, extensive network of suppliers and outlets for products Establishment, with market penetration into all segments of society Storage, e.g., in gasoline tanks, available for usage when one wants In the public mind, natural gas is a clean burning cooking fuel with almost no noxious emissions and yields CO2 about a half that of coal with the same energy content. But natural gas can also be burned so that the expanding flue gas turns turbines to generate electricity at an efficiency that can reach 60% in so-called “combined cycle” power plants where the waste heat in the flue gas is used to help boil water in the steam boiler of a coal-fired power plant. Natural gas in the United States produced by the method of hydraulic fracturing of shale has unbelievably low production costs. Other nations, notably China, are joining the “rush to gas.” For these various reasons, natural gas is often touted as the “bridge fuel” to a carbon-free future, where human energy needs are entirely supplied by renewables like solar and wind. One can question how natural gas can serve this temporary role given that it is needed to take up the slack when the wind is not blowing or when the 2/16
Sun is not shining in the sky. Building more wind and solar makes humans more dependent of natural gas, not less. Mitigating Climate Change Human burning of fossil fuels has increased the atmospheric concentration of carbon dioxide from 280 ppm before the industrial revolution to 395 ppm at the time of the writing of this article (Fig. 1). Overwhelming scientific consensus holds that this increase is the main cause of modern climate change. Because of space limitations, we do not discuss the evidence that supports this conclusion. We hope that a future issue of the ASIAA Quarterly can focus on this important subject. From the perspective of mitigating the effects of climate change, we can divide the major terrestrial energy sources mentioned above into four categories: Category I, sources that produce copious emissions of carbon dioxide: coal oil natural gas Although always lumped together, the three fossil fuels Figure 1. The concentration of CO2 in the atmosphere in are not equal. Coal powered ppm as a function of time during the past ten thousand years the Industrial Revolution; for (up to 2005). Data source: IPCC the Age of Innovation, we need something better. But if we are to stop using coal, thought has to be given to how we salvage the investment made on all the new coal-fired power plants that are springing up in China, India, and Germany (which shut down its nuclear power plants because a tsunami disabled three nuclear reactors in Japan). For sound technical reasons, civilization uses oil as the transportation fuel of choice. Easy to extract, transport, upgrade, distribute, and store, it is priced per unit energy at ten times the value of coal and shale gas for its convenience of use. Natural gas is cheap in some parts of the United States because of the practice of hydraulic fracturing. In its low-density state as a gas; transporting it in pipelines is very expensive compared to doing the same for oil, because to carry the mass mass-flow the natural gas pipes have much larger diameters. Shipping natural gas overseas is economically feasible only if it is liquefied into a denser state. Liquefied natural gas (LNG) requires cryogenically low temperatures and high pressures, so by the time LNG reaches Taiwan from the United States the cost of natural gas has increased by a factor of six. As a result of these difficulties, shale gas is not today transported from where it is produced, with the result that local supply greatly exceeds
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the local demand, which explains why current prices for shale gas are so low. Moreover, if there are leaks during extraction, then methane, which makes up 90% of what is in natural gas, is, as a GHG, 72 times worse than the equivalent amount of CO2 for 20 years, and 25 times worse for 100 years. Methane is gradually destroyed by oxidization in the atmosphere, but its potential for harm in the environment if not used wisely does not bode well for it being a panacea for humanity’s problems with climate change. Group II. Sources that are renewable and reliable, that produce essentially zero emissions of carbon dioxide: hydroelectric power biofuels geothermal solar thermal Hydroelectricity is a wonderful twentieth century technology. It has little room for expansion in the twenty-first because almost all the large rivers of the world have already been dammed. Biofuel technologies are often judged on their ratio of energy return on energy invested (EROI). Economists argue whether corn ethanol is being produced in the United States with EROI > 1 or < 1. Brazil claims that its EROI for producing sugar-cane ethanol averages about 8.3; however, their calculation does not count as input the bagasse (the material left after the sugar has been pressed out of the cane) burnt in the fields to help power the plant. If this input is included, the Brazilian EROI is probably closer to 2. Corn ethanol is notorious for driving up worldwide food prices. Researchers hope to proceed to second-generation biofuels where the feedstock does not compete with food. To accomplish this aim requires using (a) non-food feedstocks, e.g., waste wood, wild grasses, etc; (b) marginal lands not suitable for the planting of food crops. The second requirement is at odds with having biomass yields per hectare high enough to sustain economic biofuel production. Almost by definition, marginal lands either lack water or lack the soil nutrients necessary for productive vegetative growth, or both. To supply this water and/or the chemical fertilizer (which is today produced by the petrochemical industry) requires large fossil-fuel inputs that may be self-defeating if the goal is to reduce our dependency on fossil fuels. This realization has spurred some to look at oil produced by algae, where the effort is in a state of relative infancy. When the source of Earth heat is close to the surface, as in Iceland, geothermal is a reliable, established technology, especially when used for space heating. In warm climes, like Taiwan, it makes more sense to look at using cold seawater at depth as a source for air chilling in the summertime. To drill ten km deep to tap geothermal heat where it is not available from the surface, as some have proposed, seems an unnecessary invasion of the environment, given the bad accidents that have occurred with deep drilling for oil. In solar thermal, the heat of the Sun is captured by parabolic east-west troughs and stored in molten salt for energy conversion at night. Solar thermal suffers from the dilute nature of sunlight and the inefficient use of its energy compared to 4/16
photovoltaics, which directly converts sunlight into electricity. Group III, sources that are renewable but unreliable, and produce essentially zero emissions of carbon dioxide: wind solar PV With hydroelectricity, we can control the release of water behind dams to satisfy the timing of human demands. The wind changes speed and direction according to the vagaries of a turbulent lower atmosphere of the Earth. During hot or cold spells, when one needs electricity the most, the wind can stop blowing for weeks on end. Wind is strongest at night, when the cold air is descending and everybody is sleeping with little need for electricity. Thus, wind behaves like a car with a mind of its own, starting when the traffic light is red, and stopping when it is green. Solar photovoltaics (PV) is intermittent because it ceases when the Sun sets, which is when we need to turn the lights on. It is not completely dependable even during the daytime because passing clouds can interfere with the efficient operations of solar panels. Nevertheless, because electricity demand is highest around noon, solar PV is well matched to “peak-load” power. Solar PV is the only energy generation technology that offers personalized action, i.e., each family and business can own and control their own system to reduce the electricity demand on the power grid. The main failing of solar PV is its heavily subsidized costs, including installation. As long as solar PV needs government subsidies, which can change in democracies with each election cycle, making the market for solar panels highly volatile, it cannot have an impact much greater than its current contribution of about 0.01% of total world energy usage. (Beware that articles about solar PV usually quote nameplate power. Nameplate power refers to electricity generation on a clear day at noon when the Sun is highest in the sky. The average contribution is typically only 20% of nameplate power.) Group IV, sources that are reliable, sustainable, and have essentially zero emissions of carbon dioxide: advanced fission nuclear reactors thermonuclear fusion Nuclear power based either on fission or fusion are not renewable because the fuel – uranium or thorium in the case of fission, deuterium in the case of terrestrial fusion – are irreversibly transformed into non-fissionable and non-fusionable substances. Nevertheless, the stock of deuterium in the oceans is so large that fusion could supply all the world energy needs until the Sun turns itself into a red giant. In that sense, fusion energy is not renewable, but it is sustainable. Unfortunately, fusion power is unlikely to become a commercial reality in time to help with climate change. Thus, it remains a terrestrial energy source for the future. In contrast, if U-235 continues to be the world’s sole source of fissile material, then the stocks of high-grade uranium ore are sufficient only to supply about six years of total projected world energy needs in 2050. We cannot even make it to 2050 at that
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rate. Fissioning U-235 for terrestrial power is neither renewable nor sustainable. Nuclear Breeder Reactors Molted salt breeder reactors (MSBRs) offer solutions for the nuclear waste problem, safety against the massive release of radioactivity into the environment, security against weapons proliferation, and sustainability of the nuclear fission option. Before we discuss MSBRs, however, we briefly review the subject of breeder reactors more generally. U-238 is more than 100 times as abundant as U-235, and adding a neutron to U-238 makes U-239, which becomes Pu-239 after two beta decays to turn two neutrons into two protons. Pu-239 is fissile. Such a program of “breeding” to turn a fertile (U-238) into a fissile (Pu-239) raises the high-grade uranium ore use (if all power came from fission reactors) to 600 years. . Uranium-bearing minerals are soluble in seawater, leading to Japanese proposals to use polymer filters to trawl for uranium from seawater. Experiments have been carried out showing that the technology is economically viable. The supply of uranium in the oceans suffices to power a “plutonium economy” for hundreds of thousands of years. Thus, U-238 breeder reactors are a sustainable energy resource for the Earth. Bill Gates has invested money in this technology. The potential for thorium breeder reactors is even better. Thorium has only one stable isotope, Th-232, which eliminates the need for expensive isotope separation. Moreover, while Th-232, an even-even nuclide with 90 protons and 142 neutrons, is only fertile, it can be made fissile by absorbing a neutron. This turns Th-232 into Th-233, which, after two beta decays that convert two neutrons into two protons, produces U-233. An even-odd nuclide with 92 protons and 141 neutrons, U-233 is fissile. When U-233 has a slow neutron added to it (one with a spin opposite to the unpaired neutron that must be in U-233 because it has an odd number of neutrons), the increase in the energy of the large nucleus is enough to cause the resulting nucleus to vibrate violently into two uneven pieces, called fission products. Fission products from the breakup of a neutron-rich parent are too neutron rich to remain in such states without spitting out an additional 2 or 3 neutrons. When a U-233 nucleus absorbs a slow neutron and fissions, an average of 2.49 (fast) fission Because this average output of neutrons per fission is greater than 2, apart from the 1 neutron needed to sustain the chain reaction, another is available to turn a neighboring Th-232 nucleus into Th-233, that then decays into a new fissile U-233. If the neutron economy is managed properly by building the reactor core out of materials that do not absorb fission neutrons parasitically while slowing them down to low speeds, the extra 0.49 neutrons on average per fission reaction can make more U-233 from Th-232 than we started out with. In principle, then, thorium breeder reactors could exponentially expand their numbers until we have enough to supply the total energy needs of the world. Thorium is 3 to 4 times as abundant as uranium in the crust of the Earth. What is a 600 year depletion time for high-grade uranium ore becomes something more like 2000 years for the depletion of high-grade thorium ores. As a chemical element, thorium behaves oppositely to uranium in one important respect: thorium minerals are 6/16
not soluble in seawater. Thus, they are not found in the oceans of the Earth, but are ample in beach sand of a variety black in color called monazite. Lots of monazite exits on Taiwan beaches. If you think it is not enough, just go out in the ocean and get some more from the ocean bottom. Because thorium has no other commercial applications, no one has surveyed how much thorium might exist in the world as potential nuclear fuel. The reserves are likely to last millions of years, if not billions if one were to go to lower grades of ore. Thus, thorium MSBRs are sustainable. Molten Salt Breeder Reactor Our discussion of MSBRs begins with the observation that it offers a solution to the nuclear waste problem that has accumulated from half a century of operating LWRs. Figure 2 schematically provides the solution. The high-level nuclear waste from the spent fuel rods of LWRs consists of three main components: Unreacted U-235, mixed with U-238 Pu-239 and higher actinides from collateral neutron irradiation of U-238 Fission products from the splitting of fissile nuclei
MSRs Can Rid LWR Waste & Safely Breed for U-233
Unreacted uranium can be Chain reaction, breeding, and processing in liquid salt safely separated from the Pu in • LWR spent fuel Th-232 Blanket Pu-239 and minor actinides core Enough in Lehmi Pass for – U-238, U-235 turns by the standard process of 1,000 yr of USA energy use – Pu/actinides Th-232 fluorination to produce a into – Fission prod’s gas UF6 that rises out of a U-233 Core • Th-232 molten salt system. Once U-233 Blanket processing: in core separated, the large amounts UF4 (liquid) + F2 (gas) gives of U-238 mixed in with the 300 yr ! UF6 (gas) Yucca IFR or breeder both U-233 & U-232 Mtn TWR Ground U-235 (converted from the UF6 form to more stable 2/15/13 Frank H. Shu Figure 2. Schematic diagram of how solving the nuclear 9waste oxide forms) makes this problem of LWRs provides a method to start up MSBRs. material unsuitable for ©ASIAA bomb making, and it can either go to a geological repository (like Yucca mountain or its replacement), or be given as fuel for proponents of reactor technology like the integral fast reactor (IFR) or traveling wave reactor (TWR). A process called “pyroprocessing” developed at the Idaho National Laboratory then safely separates the Pu-239 and minor actinides from the fission products. With a few unimportant exceptions, the fission products contain radioactive elements that have half-lives of order 30 years or shorter. Such material can be packed in dry casks and stored underground for 300 years, after which their radioactivity has dropped below background levels. The casks can be opened to retrieve rare substances that have great economic and medical value.
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The Pu-239 and minor actinides are chemically made into fluoride compounds, such as PuF3, and dissolved in eutectic NaF/BeF2 molten salt (our preferred choice of the carrier solvent salt). We pump enough of PuF3/NaF/BeF2 fuel salt into the core of a molten salt converter reactor (MSCR) to achieve a critical mass and to sustain a chain fission reaction. The excess neutrons above what is needed to sustain the chain reaction (against parasitic neutron captures by non-fissiles in the system) random walk their way out of the core to irradiate a blanket salt in a pool surrounding the reactor core that consists of ThF4 dissolved in moleten eutectic NaF/BeF2. The thorium is entirely in the form Th-232, and neutron captures by Th-232 result, after two beta decays, in U-233. When the Pu-239 and minor actinides are consumed, we have solved the nuclear waste problem of LWRs.
Figure 3.One design possibility for a two-fluid MSBR (patent pending). Four molten-salt pumps in the foreground, fuel salt circulates into the vertical channels in the black-colored core. Reaching a compact configuration with moderator graphite all around it, the fuel salt sustains a chain reaction. Pumps in the background pull blanket salt through the core in horizontal channels that alternate with the vertical channels, but separated from them by walls of graphitic material. Heat from fission reactions in the vertical channels conducts across the graphite into the blanket salt in the horizontal channels. The blanket salt then flows into a secondary heat exchanger in the background outside the pool. The secondary heat exchanger transfers the heat from the radioactive blanket salt to a non-radioactive working salt (e.g, the NaAc/KAc used for supertorrefaction of biomass). After the secondary heat transfer, the cooled blanket salt flows to rejoin the pool at the top. The cooler blanket salt lying above the hotter blanket salt induces a convection patter that keeps the blanket salt well mixed. In the interim the cooled fuel salt flows out of the core into the foreground pumps, where any fission gases in the salt are flushed out of the system by helium gas flowing through the white pipes. The fuel salt then circulates back into the core via the red pipes to begin the
The solution for LWR waste has two side benefits: It has eliminated the “dirty bomb” risk from the existence process anew. © ASIAA of LWR plutonium It offers a way to start up MSBRs when U-233 does not exist in nature
The manufactured U-233 in the blanket salt exists chemically as UF4 in the pool. To extract it, we continuously pump small amounts of the pool salt to a chamber where gaseous F2 bubbled through the molten salt combines with UF4 in solution to form a gas UF6 that bubbles out of the liquid. The UF6 then flows to another chamber where it attacks metallic Be to produce UF4 and BeF2. When we dissolve the 233UF4 in eutectic NaF/BeF2 molten salt and pump this fuel salt into the core of the reactor, the 8/16
replacement fissile has turned a MSCR (converter reactor) into a MSBR (breeder reactor). Electrolysis of the BeF2 can recover the Be and F2 needed to process the next batch of 233UF4. The chemical processing is straightforward and can be carried out remotely without endangering the operators. The energy needed for the chemical processing is minuscule (~ 10-5) compared to the nuclear energy benefit. Because the fuel salt in MSBRs circulates indefinitely until all fissiles are consumed, there are only fission products to deal with by underground storage for 300 years. Thus, MSBRs have no waste problem of their own without a good solution. What about security? Cannot U-233 be used to make bombs? No, when one has fast fission neutrons flying around, one cannot avoid reactions with one fast incoming neutron and two outgoing neutrons. Such reactions create U-232 that accompanies the U-233. In its decay chain, U-232 is a powerful gamma emitter, and U-232 is almost impossible to separate from U-233. Even if martyrs were willing to make a bomb using unseparated U-233/U-232, the presence of the U-232 would make the bomb easily detectable by Geiger counters if one tried to smuggle it into a city, say, in a port container. The gamma rays would also interfere with the sensitive electronic control mechanisms that must be part of any weapons assemblage. No nation or terrorist organization would attempt to make a bomb this way, when much simpler alternatives are possible. Thus, MSBRs are secure. Figure 3 shows a possible design for a two-fluid molten-salt breeder-reactor of a type described schematically in Figure 2. To slow the fission neutrons from the fast speeds at which they emerge from the fission reactions without absorbing them, we build the reactor core entirely out of carbon-based materials (except for metallic nuts and bolts). Graphite is impervious to chemical attack by hot NaF/BeF2 as long as there is no water in the salt. Doubled for safety of containment, the walls of the pool are made of metal (Hastelloy N resistant to attack by the salt). The random walking neutrons in the pool will be mostly absorbed by Th-232 (in the form of ThF4 dissolved in molten NaF/BeF2 in the pool) before they can strike the walls of the pool and activate the metal to become nuisance low-level waste. Nuclear Accidents All nuclear reactors are designed to shut themselves off automatically in the case of an emergency. The MSBR is no different, it just has larger safety margins. No reactor accident has ever occurred because of a runaway chain reaction (with the exception of the Chernobyl reactor, which had a horrible flaw in its design that could never pass the nuclear regulatory review outside of the former Soviet Union). Most nuclear accidents occur after the reactor has shut down safely. They arise because of problems in dissipating the decay heat from the fission products. For reactors with fixed solid fuel elements, the possible problems are exemplified by Fukushima. An emergency arises (a tsunami of historical proportions strikes the station). The reactors shut down safely, but the fuel rods continue to put out decay heat that is a few percent of reactor full power. Something knocks out the cooling systems normally used to cool the fuel rods (the whole electrical grid goes down because of the earthquake and tsunami). Emergency equipment has to cool the fuel rods while they remain in the same cramped space of the operational configuration. 9/16
The auxiliary power goes out (fuel for diesel generators swept away, batteries run down), and there is a loss of coolant fluid (because the water boils away). Now, the plants are in big trouble. Without active cooling of the fuel rods, the rods melt down. Steam interacts with the superhot fuel rods, generating hydrogen. The hydrogen escapes into the containment buildings and explodes. Not designed to be strong, the buildings blast apart. Containment is breached, and massive amounts of radioactivity escape into the environment. None of these events would have occurred in two-fluid molten-salt breeder-reactors of the design in Figure 3 because of the following safety features: MSBRs do not use water, so they do not need to be located near large bodies of water, like rivers or ocean sides, where people like to live. They can survive earthquakes and cannot be overwhelmed by tsunamis Molten salt reactors run themselves, without operator intervention needed Neutron absorber elements buoyant in the blanket salt automatically descend into the core if the pool loses coolant (the blanket salt of the pool) If the fuel salt overheats for any reason, a drain plug melts that dumps the fuel salt into an air-cooled tank absent of moderators and of a geometry where reaching accidental criticality is impossible In MSBRs, if reactions run too fast, the fuel salt heats up. The molten fuel salt will then expand partially out of the core, and the reactions will slow. Conversely, if we need extra power, we pump on the blanket salt harder. This cools the fuel salt, causing it to contract into the core more, thereby making the reactions run faster. These principles are exactly how the Sun, having a gaseous core that expands when heated and contracts when cooled, regulates its thermonuclear fusion reactions in the core to balance what is lost in radiation from the surface. We no more have to worry about a molten salt reactor overheating or overcooling than we have to worry that the Sun tomorrow won’t be the same as it is today. The idea of a drain plug originated at Oak Ridge National Laboratory, who invented the concept of reactors with liquid fuel elements. With solid fuel elements, as we have seen in the example of Fukushima, if something goes wrong with the primary cooling system, the problem needs fixing with the equipment in the same place where something broke. With liquid fuel, we can move it to another place (the dump tank) where we have prepared a separate emergency cooling system. We choose the coolant to be air, because although we can lose water, and we can lose molten salt, it is almost impossible to lose air. To be able to use air to cool nuclear power equipment, however, the decay heat cannot be overwhelming. This is where online cleaning of the fission products (needed to maintain the breeding ratio above unity) makes its contribution to reactor safety – it allows even reactors with fairly large full-power operations to have relatively little decay heat when one has reactor shutdown in an emergency. To be supersafe, we should avoid building reactors that are too big (because the amount of decay heat scales with operational full power). Nevertheless, it is conceivable that with complete station blackout (as happened with Fukushima), the power needed even to run fans won’t be available. Suppose the fuel salt then melts through the air-cooled dump tank. For this contingency, we’ve added a 10/16
steel salt catcher into which the molten salt will spread into a thin sheet, conducting its heat to inside the steel as it flows. The design is such that the salt freezes in less than 10 seconds to immobilize any fission products that the fuel salt might contain. Because solid salt has a very low vapor pressure, no radioactive gases will escape. There is no water in the system, so hydrogen will not be generated to cause an explosion. The salt is composed of elements on opposite sides of the periodic table, one being very electropositive and the other being very electronegative. No other element can get between them, so there are no chemical reactions that can threaten the system. In other words, salt cannot catch on fire. One extra precaution must be taken: a containment dome that can prevent intrusion by jet airplanes that try to crash into the reactor. We have to design the dome so that in case the unthinkable happens, and the operators have to abandon the site, the reactor is walk-away safe. This means that decay heat cannot be trapped inside the dome, but needs to be able to work its way out. A good design is exemplified by the Westinghouse AP 1000, which has a thin steel cap that traps gases inside but allows conduction of heat to the upper surface, which is cooled by convection in a protective concrete dome partially open to circulating outside air. Finally, MSBRs can be located in remote places where any accident would have a minimal impact on surrounding human populations. Thus, MSBRs are walk-away safe. Supertorrefaction of Biomass into Biofuel With oilâ&#x20AC;&#x2122;s advantages in ETUDES (which have made them rich and powerful), oil companies are tough to displace with technologies that depend on primitive micro-organisms performing fermentation reactions at room temperature, where all chemical reactions are slow. (If they were not slow, the organisms would char.) The strategy of our research group is to Figure 4. Torrefaction of woody plant material. Data source: fight fire with fire, or Bergman et al. 2005). more accurately, with supertorrefaction. Torrefaction is generally recognized as the most efficient way of harnessing biomass energy (Fig. 4). The traditional method involves burning a fuel and letting the flue gas heat biomass in a partially enclosed environment that has a limited intake of oxygen in air. The process drives out volatile organic compounds (VOCs), including water vapor, leaving behind a blackened solid residue, charcoal. The VOCs are usually burned to supplement the fuel, which can be natural gas or a portion of the biomass or its torrefaction products. 11/16
Supertorrefaction (patent pending) is an improved process conceived as part of a general program using molten salts to generate alternative energies by the first author and brought to maturity at Academia Sinica. Supertorrefaction uses molten salt as a medium to transfer heat to the biomass with which the salt is in direct contact. Immersion beneath the surface of the salt excludes oxygen and air. In contrast with traditional torrefaction, where many hours are required for the completion of the charring process, supertorrefaction requires typically only ten minutes because the heat capacity of molten salt per unit volume is about 2000 times larger than that of flue gas if both heat-transfer fluids Figure 5. The Crankberry machine for tabletop supertorrefaction. are at atmospheric ©ASIAA pressure and a given temperature. The second author of this article designed a tabletop machine (“crankberry”, Fig. 5) which automates the process of supertorrefaction on a laboratory scale. Using the crankberry, the third author and his group have supertorrefied a wide variety of biomass feedstocks, with uniformly good results 6. Examples of charcoal making by supertorrefaction with (Fig. 6). From data that Figure molten acetate salt (NaAc/KAc) from different biomass feedstocks. we have accumulated ©ASIAA from such experiments and using the same rules of calculation as Brazilian sugar cane ethanol, we estimate that the EROI ratio for a demonstration-scale supertorrefaction project is of order 40:1. If we include internal inputs of energy from renewable sources in the denominator, but not the crude glycerol that should be charged to biodiesel making, the EROI drops to about 9.6:1, still very good by Brazilian standards, and comparable to the record of established oil companies. With “peak oil,” our EROI will improve relative to that of the oil industry. Moreover, burning our products is a carbon-neutral activity.
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The molten salt we use for supertorrefaction is a eutectic mixture of sodium acetate, NaAc, and potassium acetate, KAc. (The same combination is used to flavor â&#x20AC;&#x153;salt-and-vinegarâ&#x20AC;? potato chips). This salt mixture melts at 235 oC and decomposes to sodium carbonate, Na2CO3, and potassium carbonate, K2CO3, plus acetone if the temperature exceeds 460 oC. If the temperature of the salt is 300 oC, a product ecocoal results that is a clean-burning, carbon-neutral, replacement for natural coal; whereas if the temperature is 500 oC, the product biochar is a fine carbon-negative soil amendment (Fig. 7). We note that burying bichar is a carbon-negative activity, beneficial not only to the host country, but to the whole world. Thus, in principle, biochar production and burial can become the basis of true carbon trading, where, for example, oil companies that extract a tonne of petroleum from anywhere in the world are required to pay someone else to bury a tonne of biochar on land in need of improvement in soil quality. The resulting flow of money from the rich to the poor in rural communities facing desertification is a win-win proposition, with everyone receiving the benefits of a cleaner environment. Because the VOCs driven from the biomass are recovered rather than burned, the economic return per unit weight of the biomass is higher than in traditional torrefaction. In particular, apart from water (which we recover and recycle for washing and recovering the salt in the finished biochar), acetic acid is the most abundant component of the VOC yield. As mentioned earlier, we are able to generate acetone and Na2CO3/K2CO3 if we take NaAc/KAc above 460 oC. By reacting the Na2CO3/K2CO3 with acetic acid, which is a fast acid-base reaction, we are able to recover the NaAc/KAc that we decomposed (plus CO2 and H2O).
Figure 7. Scanning electron microscope (SEM) images of (left) ecocoal made from leucaena supertorrefied at 300 oC for ten minutes, and (right) biochar made from leucaena supertorrefied at 500 oC for eleven minutes. The bar at the bottom left of the left image is 10 microns; of the right image, 20 microns. Supertorrefaction at 300 oC drives out VOCs from ecocoal, but leaves many microstructures within cell walls, whereas supertorrefaction at 500 oC decomposes some acetate salt into carbonate salt and leaves behind only cell walls. Below the image we give the Brunauer-Emmett-Teller (BET) measure of porosity (area per unit mass) in m2/g. ŠASIAA
Acetone is a high-value chemical, useful as an industrial solvent as well as a feedstock for general aviation fuel, so the technique not only creates a high-throughput solid biofuel to compete with natural coal, but also a liquid feedstock to lessen the dependence on petroleum for one segment of the transportation industry. We also get uncondensed gases combustible as a replacement for natural gas. Supertorrefaction allows a greatly reduced size of the equipment needed to produce a given throughput (tonne per day) for the biomass processing, even when the slight loss of the salt encased in the pores of the charcoal is taken into account. This 13/16
reduction lowers considerably the initial investment of capital equipment. Indeed, it is possible to have supertorrefaction throughputs that generate attractive economic returns with batch-process equipment compact enough to be transportable by truck to remote batch supertorrefaction sites where the biomass is harvested. These capabilities make commercialization of supertorrefaction possible in startup environments that hold many barriers for traditional torrefaction technologies. The next step may be to conduct a demonstration project in Penghu County to prove the economic feasibility of scaled-up, mobile, batch-process supertorrefaction. Our target biomass is a bush called leucaena that has over-grown 70% of Penghu County (Fig. 8). Introduced to Taiwan under the Japanese occupation, leucaena was originally cultivated for firewood. Figure 8. Leucaena fields occupy 70% of the land area of Penghu Leucaena is and threaten to invade the remaining 30%. In autumn and winter the plants are very dry, in optimum condition for harvest and nitrogen-fixing and supertorrefaction. (Photo taken Oct 19, 2012). requires no chemical fertilizer to grow in ŠASIAA poor soil. Now that everyone uses natural gas or propane for cooking, the leucaena, with its adaptive advantages, has become an invasive species that threatens the habitats of the native vegetation of Taiwan (and, indeed, much of Southeast Asia). Using this biowaste as a bioresource is consistent with the sustainable development goals of Penghu County. Taiwanâ&#x20AC;&#x2122;s Council of Agriculture (CoA) prefers to try to eradicate this invasive species. Eradication of established leucaena is impossible without digging up its deep roots, and killing all viable seeds dispersed on and in the soil. To harvest the leucaena, we would therefore clear-cut the branches, allowing the CoA to experiment with eradication schemes. If eradication efforts fail, as is likely from experience in other parts of the world, each topped bush will regenerate new growth in ensuing seasons, recovering fully in about three years. Another bad situation exists in Western North America, where winters that are too mild, combined with drought-like conditions in the summers, are blamed for an outbreak of pine bark beetle disease in mountain forests stretching from Southern California to British Columbia (Fig. 9). Hundreds of thousands of pine trees fall per day. We propose that the felled trees should be supertorrefied before they become ground tinder for wildfires, or rot and release greenhouse gases into the atmosphere, or have falling limbs that bring down power lines and cause expensive and dangerous outages. We would bury the resulting biochar in the same forests, not only sequestering for thousands of years the resulting carbon, but also encouraging new 14/16
growth that would lock up more carbon. The forest crisis affects more than just North America. A survey that appeared in Nature magazine in 2012 found that 70% of 226 forest species in 81 forests of the world are on the verge of dying from the stress placed on root systems when there is too little water in the soil. This existential threat deserves an adequate response.
Figure 9. Pine trees in Colorado dying or dead from bark beetle infestation (AP/Colorado Forest Service/Jen Chase).
Figure 10. Left (picture taken in July 2010): how an abandoned silver mine in Hope, Colorado looked for a century before the addition of biochar soil amendment made by torrefying diseased pine trees. Right (picture taken in August 2011): how the same mine tailings site looked a year later after the application of biochar soil amendment at a rate of about 100 tonne per hectare. (Photo credit: Troy Hooper).
Biochar is also useful for land reclamation. Experiments carried out at an abandoned silver mine in Hope, Colorado show that each hectare treated with 100 tonne of biochar will permanently require 17% less water to rejuvenate vegetative growth (Fig. 10). We propose to use charcoal fines, generated by supertorrefaction whenever one has bark mixed in with the woody stems, in experimental trials to see whether the use of charcoal fines as a soil amendment stimulates a similar dramatic improvement in soil productivity of Penghuâ&#x20AC;&#x2122;s infertile soil while decreasing the share of water that needs to be devoted to agricultural irrigation. With the data in hand, Penghu County can make better informed decisions whether it should (a) undertake a systematic effort to eradicate leucaena over the next decade, (b) passively harvest leucaena as a bioresource while controlling its spread, or (c) actively cultivate leucaena, but without the application of ammonium fertilizers that are based on petroleum feedstocks.
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The Grand Challenge Climate change is the grand challenge of the twenty-first century. The fate of human civilization may well depend on whether we rise in a rational and scientific manner to meet this challenge. The ultimate goal of our group is to marry the technologies of molten salt reactors and supertorrefaction. There are physical and economical reasons why it is hard to beat natural gas for turbine electricity generation, or to beat natural gas as the input heat for supertorrefaction. But we do not have to use the nuclear heat from a MSBR for turbine electricity generation (a difficult coupling). Instead, we can transfer the heat carried in the radioactive blanket salt (ThF4/NaF/BeF2) to a non-radioactive working salt (NaAc/KAc) via a secondary heat exchanger (an easy coupling depicted in the background of Fig. 3). Then we have a combination that can beat natural gas at both tasks. Although nuclear electricity is expensive, nuclear heat is cheap – much cheaper than natural gas. We can therefore use nuclear heat to produce from biomass, at very high throughputs, biochar, acetone, and syngas cheaper and cleaner than mined coal, drilled petroleum, and fracked shale gas. Baseload electric power can be generated from syngas; liquid transportation fuels can be made from acetone; and carbon-negative sequestration can be achieved with biochar. Coal, oil, and natural gas are valuable Earth resources, and they would not contribute to climate change if they were used to make durable goods, rather than burned. We do not need fossil-fuel companies to go out of business; we need them to go into a different business. Other researchers may have even better ideas for effecting a realistic transition from an economy based on fossil fuels. If so, they should get to work. Through nearly fourteen billion years of the evolution of the physical universe, nature has given us a bounty of Earth energy that can, in principle, replace fossil fuels. It is time for us to do our part. （ Authors/Frank Shu, Michael Cai, Fen-Tair Luo; Translator/Chun-Hui Yang; Reviewer/ Michael Cai）
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