Solar thermochemical and electrochemical research - how they can help reduce the carbon dioxide burden

Energy Vol. 21, No. l/g. pp. ‘739-745, 19%

Copyright0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved

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SOLAR THERMOCHEMICAL AND ELECTROCHEMICAL RESEARCH HOW THEY CAN HELP REDUCE THE CARBON DIOXIDE BURDEN

Edward A. FLETCHER University of Minnesota Department of Mechanical Engineering 111 Church Street, S.E., Minneapolis, MN 55455, U.S.A. (received 30 August, 1995)

Abstract- Any process which decreases the use of fossil fLels as a p.rime energy source will be used only if it is attractive to industry. To be attractive, an alternative energy source must be cost effective. The only alternative prime energy sources which appear likely to be cost effective in the foreseeable fiture are nuclear fission and the various manifestations of solar. Fission, no matter how well it is engineered on earth, can cause major disasters because of human error; its parent cost effectiveness is illusory. Thermonuclear fusion energy is no closer to fiuition than it was fifty years ago, when it was first proposed. Solar energy b thermonuclear fusion. The source is far removed from humans. We can’t manipulate the safety devices. The realization that one cannot divorce nuclear energy from the hazards of human error and malice is already a given in public policy. Being a 58OOK source, solar is most efficiently used when it is directly absorbed at the site of an endothermic reaction at the highest practicable temperature. In recognizing the special thermodynamic attributes of solar energy, for the past 20 years my students and I have explored various solar thermochemical and solar thermoelectrochemical processes. This paper presents a summary of some of our pertinent observations and suggests directions that I believe future research and development should take. Copyright o 1996 Elsevier Science Ltd.

INTRODUCTION

Any process which decreases the use of fossil fuels as a prime energy source will be used only if it is attractive to industry. To be attractive, an alternative energy source must be cost effective. The only alternative prime energy sources which M likely to be cost effective in the foreseeable fYbtureare nuclear fission and the various manifestations of solar. Fission, no matter how well it is engineered, can cause major disasters because of human error; its qparent cost effectiveness iis illusory. The realization that one cannot divorce nuclear energy from the hazards of human error and malice is 739

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already a given in public policy. Thermonuclear fusion energy is no closer to fruition than it was fifty years ago, when it was Qsfproposed. Solar energy & thermonuclear fusion. The source is far removed from humans. We can’t manipulate the hardware and safety devices. There is no question that solar energy can supplement and replace fossil fuels to reduce carbon dioxide emissions. The question is, “Are there solar processes that can be made cost-effective enough to attract industry?” Present industrial processes work well. They have survived competition. Industry has already committed vast sums of money to them. We face a difficult task. Being a 5800K source, solar is most efficiently used when it is directly absorbed at the site of an endothermic reaction at the highest practicable temperature.” For about twenty years my students and I have explored the use of solar energy as an alternative to fossii and nuclear Gels. The studies have been both analytical and experimental.‘*“”It has become evident that some applications are more likely to be attractive to industry than others. In this paper I discuss, with the objective of making them attractive to commerce and industry, the avenues that I believe future research and development should take. HYDROGEN SULFIDE SPLITTING

A recent reviewI strongly suggested that well-conceived technology applied to the sulfur contained in fossil fuels can have an powerful impact on global energy economy and, by implication, on carbon dioxide emissions. This excellent review summarizes the collected thoughts of the many presumed that the processes would be limited to authors cited. The pioneering studie?= temperatures of about 1OOOK.At Minnesota we focused on two concepts: high-temperature solar thermochemical at temperatures up to 1800KX’z’J*” and ambient temperature electrolysis.” In this paper I .shall concern myself mostly with solar-thermalstudies but electrolysis is also an attractive option, and I shall mention it briefly, as well. Solarthermochemical -- Our studies are predicated on the notion that solar energy has the thermodynamic quality of a very high temperature source. It should therefore be used at high temperatures. Most of the studies cited in Ref. 19 were limited to temperatures of about 1100-l ZOOK.

Fig. 1. Variation of the extent of dissociation of H$ with temperature at 1 atm. Lower pressures increase the extent of dissociation; higher temperatures decrease it.

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Figure 1 shows the variation of the extent of dissociation of H$ with temperature at 1 attn. Dissociation is greater at lower pressures. It is possible to get Eonversions greater than these at any particular temperature. If one of the products is continually removed from the reaction arena, I+$ continues to dissociate until the hydrogen and sulfur are virtually completely separated, This rationale has been used to just@ running the process at lower temperatures.” One way to do that is to continuously condense and separate liquid sulfur from the remaining gases. That procedure is not without problems. The normal boiling point of sulfur is. 718K. At that temperature the extent of dissociation at 1 atm is 0.0043. The equilibrium partial pressure of S, is 0.0014 atm. Condensation of sulfur would not occur under such conditions. We therefore chose to use the procedure of heating H,S to high temperatures and quenching the product. In our exploratory studies we werle able, in this way, to achieve very high conversions and quench fractions.‘s*‘7 Our studies provided us with a basis for an economic evaluation” of a solar process for thermosplitting I-JS and comparing it with other thermosplitting alternatives. The economics of the process and its comparison with alternatives is quite complicated; there are many caveats. In determining the payback time on the capital investment for such a plant, one should remember that it depends strongly on the market values of HZ and of sulfur, whose price varies widely. One should also note that I-@ actually has a negative value; the value added in any conversion process is greater than the value of the product. In our study we ascribed to H.$ a zero value; our payback times are thus pessimistic. In comparing our solar plant with plants using a more conventional source of process heat, electricity in our study, the comparison depends strongly on the current cost of electrical energy. Moreover, if nuclear energy is not used, the electric power augmented plants will, themselves, entail a substantial CO, burden. Another point is pertinent: if one applies appropriate environmental constraints on sulfur oxides emissions, the sale of the only useful product from a currently used Claus unit, sulfur, is unlikely to ever recover its capital cost. _ Figure 2 shows the variation of the payback period with the cost of electrical energy. Three options are compared: A plant which uses sunlight to provide all its process heat, a solar-electric plant, which uses sunlight to provide process heat when the sun is shining and supplements it with electric power when the sun is not shining, and a plant which uses electric power for all its energy needs. 12

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Fig. 2. Variation of the payback period of a unit for handling HJ with the cost of electrical energy for three kinds of units: an intermittently operating all-solar unit and two continuously operating units, one of which uses solar augmented with electrical energy when the sun is not shining and the other of which uses no solar at all. They all operate at the same H.$ conversion rate, but the solar unit operates only 20% of the time. (This figure was taken from Ref 13.)

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The solar plant, because of the intermittent nature of sunlight operates 20% of the time. The other two plants operate continuously, 100% of the time. The plants are comparable in that each of them splits H$ at the rate, 100 gmol/s. On an annual basis, the solar plant processes one-fifth as much sulfur as the other two. The payback period of the solar plant, operating 20% of the time, is 6.3 years when the cost of electric power is $O.OS/kWhr.The payback periods for the others, which operate continuously, are about a year. But the payback time for the solar plant is relatively insensitive to the cost of electricity, and the others are not. When the cost of electric power goes to %O.I3/kWhrthe electric plant can never recover its capital cost. When the cost of electric power goes to %0.16/kWhr the solar-electric plant can never recover its capital cost. It is worth noting that in the United States the cost of domestic electric power now ranges from about $0.05 to about $O.l5/kWhr. The daily production of the electrically augmented plants is about 300 tonnes. For perspective, we note that, the capital cost of a comparable Claus unit, which produces no hydrogen, is estimated to be about $45 million and the capital cost of an all solar unit with a comparable daily production would be about $90 million The capital costs of the continuously operating thermochemical units would be about $18 million. Electrolysis - If it were not for the environmental consequences of using it as such, H,S would be a good fuel. Since it is much less stable than H,O, it is thermodynamically possible to use a small fraction of the hydrogen in a hydrogen-air tie1 cell to provide energy to electrolyze the H,S, recover all the sulfin and pump the product hydrogen to pipeline pressure.” If this technique were used, all of the H,S could be converted to valuable products without producing any CO,. About 17% of the hydrogen would, ideally, be required to pump the remaining hydrogen to a 100 bar pipeline pressure. Electrolysis might well be economically viable now using commercial electrical energy, but, depending on the prime energy source, it might incur a substantial CO, burden. STEAM GASIFICATION OF CARBONACEOUS MATERIALS In accordance with Gibbs phase rule F=C+2-P, where F, C, and P are the numbers of degrees of freedom, components, and phases. Carbon-hydrogen-oxygen is a three-component system if we permit the elemental composition to vary. If we specify a particular overall elemental composition, it is a one component system. Thus, if we choose a particular overall composition, a particular mixture of carbon and water, for example, under conditions where there is only one phase, gas, for example, there are two degrees of freedom. When we have chosen two intensive variables, all of the remaining intensive variables have been specified. If we choose temperature and pressure, the concentrations of all the molecular and atomic species in the system are specified. Although there may be many of them, CO, q. CO,, H,O etc., their mole fractions and partial pressures will all depend on the temperature and pressure we have chosen. Since elemental carbon (graphite) can exist in this mixture of elements, there is a substantial range of conditions under which the system will be a two-phase system. Under these conditions, once we have chosen a temperature, the tigacities, and by implication, mole fractions of all the species in all of the phases the system are, and thus the pressure, (the “saturation” pressure of the condensed phase) are determined by nature. However, because of the idealizations we make in doing the thermodynamic calculation (We assume the gases are completely insoluble in the condensed phase.) we can assign a pressure and still calculate useful answers. The pertinent point is that the equilibrium compositions of materials containing the same relative amounts of C, H, and 0 are similar, regardless of where the elements came fi-om. This point is shown in Fig. 3. Each of the corners of this triangular composition diagram represents a pure element. The overall atomic composition of any compound or mixture is represented by a point in the diagram. Thus

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H,O is a point on the H-O axis, CH,, is a point on the C-H axis, and CO and CO, points are on the C-O axis. Cellulose and a typical bituminous coal with its ash, sulfur, and nitrogen removed are found in the body of the diagram. Any combination of two substances is found on the straight line connecting them. For the steam gasification of coke (carbon), for example, the elemental composition of the product falls on the line that connects the point C (carbon) with the point I$O. The location of the point depends on the mole fractions of the components according to the convention used in ordinary phase diagrams. In a similar way, the steam gasification of coal or wood, or the CO., reforming of hydrocarbons might be represented. The important point is that the equilibrium composition, at any given combination of temperature and pressure, is ‘unique at every point in the diagram, i.e. at any particular combination of the elements, regardless of what was the source of the elements. An equimolar mixture of C and KO has the overall composition CqO. The equilibrium product will be synthesis gas, (CO+HJ, over a wide range of high temperatures and pressures. Such a mixture might be made by using carbon and water, or cellulose and water. The gasification is endotthermic. If the product is cooled quickly to ambient temperature the high temperature composition will be retained, even though the low temperature equilibrium product is not syngas, but mO+carbon).

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Fig. 3. Product equilibrium composition diagram for the C-H-O system. The temperature is 1600K. The pressure is 1 atm. The equilibrium product compositions are given for selected feedstocks. Any point in the diagram or on its boundary can be selected by an appropriate selection of a mix of feedstocks. Common binary feedstocks, (coke-water), (coal-water), and (methane-CO,), are connected by tie-lines, Any binary combination of reactants can be represented by a point on the appropriate tie-line. All points which lie on the line that connects H to CO give compositions for which solid carbon is an incipient equilibrium product. All compositions which lie above that line have solid carbon as an equilibrium composition product. The formation of solid carbon may be avoided by bringing elemental compositions to points which lie below that line, by adding water or carbon dioxide, for example.

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The higher heating value of the reactant is 94.05 kcal per mole of contained carbon. The higher heating value of the product is 135.95 kcal per mole of contained carbon, 45% higher than that of the reactants; the lower heating value of the product is 125.43 kcal per mole of carbon, an increase of 33%. The heating value of the fuel has thus been increased by the storage of sunlight. It thus takes less carbon to produce a fuel having a given heating value; carbon dioxide emissions are lower by virtue of the substitution for fossil fuel combustion of stored solar energy. A cellulose, (C,H,,O,),-HO mixture of formula weight ratio one has the same atomic composition. It gives the same product at 1600K and 1 atm. The higher heating value of C,H,,O,, is about 679 kcal per formula weight. The heating value of the steam gasification product is 816 kcal, an increase of 20%. The lower heating value is also increased by 20%. The storage of solar energy has resulted in a reduction of the amount of CO, produced for a given heating value. Equimolar methane-water yields (3H+CO), a product rich in H. The higher heating value of a mole of methane is 212.79 kcal. The higher heating value of the product is 272.59 kcal, an enhancement of 28%. The enhancement of the lower heating value is comparable. Thus, solar steam gasification of all carbonaceous materials are just variations on the same theme. Coal gasification with cogeneration -- The energy for coal gasification is now provided by burning some of it.24 That wastes coal. It also produces dirty nitrogen which has to be cleaned up. Solar steam gasification can thus help achieve the worthy ,objective of improving coal economy and reducing CO, emissions. But it can also, ideally, do much more. If gasification is accompanied by electric power generation it can provide a quick quench that requires no heat exchange as it stores solar energy for later use. The process would thus produce electric power as it stores solar energy in the form of the augmented heating values of the fossil fuels. The thermodynamic details of solar steam gasification of biomass and coal appear elsewhere.*’ There, it was suggested that the endothermic reactions be effected at supercritical temperatures (1200-l 600K) and pressures (up to 400 atm) in a solar furnace using a reactor in which sunlight impinges directly on a reacting absorbing surface. Concentrated sunlight is eminently suited for such a process. It can transfer process heat to high temperature systems efficiently.‘” When those papers were written, a very important and as yet unsolved problem was the need for a windowed reactor which could operate at the high temperatures and pressures required to achieve it. Recently Kami et al have described a reactor which gives promise of achieving such a milestone. With the development of reactors of the sort he is now pioneering, it will be appropriate to consiaer electric power cogeneration with the concurrent solar beneficiation of carbonaceous fuels. Such processes give promise of substantially reducing the amount of carbonaceous fuel required to achieve many objectives. REFERENCES 1. E.A. Fletcher and R.L. Moen, Science 197. 1050 (1977). 2. R.B. Diver, “Reactor-Receiver Concepts for Thermochemical Transport of Solar Energy,” Proc. 2 1st Intersociety Energy Conversion Engineering Conf, pp. 96 l-966, (1986), San Diego, CA. 3. A.J. Hunt, J. Ayer, P. Hull, F. Miller, J.E. Noring, and D. Worth, “Solar Radiant Heating of Gas-Particle Mixtures.” Lawrence Berkeley Laboratory, University of California LBL-22743, (1986), Berkeley, CA 94720. 4. D. Fraenkel, R. Levitan, and M. Levy, Int. J. Hydrogen Energy 11,267 (1986). 5. E.A. Fletcher, Energy 4,61 (1979). 6. J.E. Noting, R.B. Diver, and E.A. Fletcher, Energy 6, 109 (1981). 7. J.E. Noring and E.A. Fletcher, Energy 7,651 (1982). 8. J.E. Noring and E.A. Fletcher, Energy 8,247 (1983). 9. R.B. Diver, D.E.E. Carlson, F.J. Macdonald, and E.A. Fletcher, J. Solar Energy Engineering 105, 288 (1983).

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10. E.A. Fletcher, Energy 8,835 (1983). 11. R.B. Diver, S. Pederson, T. Kappauf, and E.A. Fletcher, Energy 8,947 (1983). 12. J.E. Noring, J.P. Murray, and E.A. Fletcher, Int. J. Hydrogen Energy 9, 587 (1984). 13. R.B. Diver and E.A. Fletcher, Energy 10,831 (1985). 14. E.A. Fletcher, F.J. Macdonald, and D. Kunnerth, Energy 10, 1255 (1985). 15. T. Kappa&, J.P. Murray, R. Palumbo, R.B. Diver, and E.A Fletcher, Energy 10, 1119 (1985). 16. R.D. Palumbo and E.A Fletcher, Energy 13, 121 (1985). 17. T. Kappauf and E.A. Fletcher, Energy 14,443 (1989). 18. A. Steinfeld and E.A. Fletcher, Energy 16, 1011 (1991). 19. J. Zaman and A. Chakma, Fuel Processing Technology 41, 159 (1995). 20. M.E.D. Raymont, Hydrocarbon Process. 54, 139 (1975). 21. M. Dokiya, T. Kamayama, and K. Fukuda, Denki Kagaku Oyobi Kogyo Butsuri Kagaku 45, 70 1 (1977). 22. K. Fukuda, M. Dokiya, T. Kamayama, and Y. Kotera, Ind. Eng. Chem. Fundam. 17, ,243 (1978). 23. J.P. Murray and E.A. Fletcher, Energy 19, 1083 (1994). 24. Probstein, R.F. and R.E. Hicks (1982). Svnthetic Fuels, McGraw-Hill Book Company, New York, Chapter 4. 25. E.A. Fletcher, “Coal and Biomass Gasification in a Solar-thermal Process Which Uses a Gas Turbine to Cool and Quench the Product as it Generates Electric Power,” Proc. of the Biennial Congress of the International Solar Energy Society, Denver, CO, 19-23 August, pp. 790-795 (1991). 26. J. Karni, A. Kribus, R. Rubin, P. Doron, A. Fiterman, and D. Sagie, “The DIAPR: A High-Pressure, High-Temperature Solar Receiver,” Solar Engineering 1995, Proc. 1995 ASME/JSES International Solar Energy Conference 1, pp. 591-596, Maui, Hawaii, 1995, W.B. Stine, T. Tanaka, and D.E. Claridge, eds., American Sot. Mechanical Engineers, New York, NY.