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Hydrogen—the optimum chemical fuel
Applied Energy 47 (1994) 183-199
Hydrogen
The Optimum Chemical Fuel
'The idea of using hydrogen as the ultimate, ecologically clean synthetic fuel--a notion that spawned the concept of a hydrogen economy during the oil-shock round of the early 1970s--is slowly making headway again, especially overseas.'
Peter Hoffman,
Chemical Engineering, October 26, 1987, p. 26
ABSTRACT The preceding paper showed that despite the broad desirability of fusion as a base-load energy source it is necessary to implement a manufactured chemical fuel for energy transport, storage and as a transportation fuel In this paper it will be shown that hydrogen is certainly the optimum, and maybe the only, choice for the chemical fuel The fuel use properties of hydrogen will be reviewed along with its detailed chemical and physical properties.
POTENTIAL CHEMICAL FUELS Specific criteria must be satisfied by the chemical fuel selected to support the fusion energy system. To be useful the fuel must: (1) provide a large energy storage capability per unit mass to mini mize the amount of fuel material manufactured, handled, stored, and transported; (2) be a fluid for ease of transportation in pipe lines and transfer to and from storage vessels; (3) be non-toxic to plants and animals. (No matter how Carefully the fuel is handled accidents, mistakes and natural disasters will result in spills); (4) provide only gaseous products when burned in air to allow disposal of the reaction products directly to the atmosphere; (5) when burned with air, yield non-toxic products harmless to the environment. (The reaction products must be totally non-toxic or 183
Applied Energy 0306-2619/94/$07.00 © 1994 Elsevier Science Ltd, England. Printed in Great Britain
184
(6) (7)
(8) (9)
L. O. Williams
the shift from hydrocarbons to the new fuel will just change one set of toxins for another); be made of c o m m o n chemical elements to ensure an abundant supply; be made from elements available in most locations to reduce the a m o u n t of shipping required in the production and use of the fuel; be easily manufactured by a low cost, process to be economically viable; be easy to use in existing power generation equipment to aid transition with a m i n i m u m of equipment modification.
These criteria will be applied to possible substitutes to determine which have the potential to serve as the manufactured fuel for use with the fusion energy generation system. To search for substances to replace the chemicals found in fossil fuels, all chemical elements and compounds with potential as fuels were examined. The nine criteria were used to judge each element and comp o u n d to determine if it was a suitable candidate. There are 83 non-radioactive elements from which to choose. The elements from 1-18 and their c o m p o u n d s will be evaluated against the criteria to determine their suitability as fuels. The elements with an atomic number higher than 18 (i.e. argon) are metals or semi-metals. They release little energy when they react with the oxygen. On the basis of their failure to meet criterion 1 they can be easily eliminated from consideration. Table 1 shows how elements 1-18 fare when evaluated against the nine criteria. All data regarding physical and chemical properties, abundance, toxicity, and reaction energies of elements and compounds were derived from values taken from the H a n d b o o k of Physics and Chemistry. The first evaluation factor is the a m o u n t of energy produced on combustion. A measure of energy must be selected for comparison of the energy produced by various fuels. The technically proper measure of energy is the joule. A joule is a quantity of energy, as are British Thermal units (Btu) and calories. A watt is a measure of power, the potential for the delivery of energy. Since a watt is a measure of potential to provide energy, watts per unit time must be specified to provide a quantity of energy. The joule (a measure of quantity) is defined as 1.00 Ws. Thus a 1.0 W light bulb uses energy at a rate of 1.0 J s-l and a 100 W bulb uses 100 J S-1. When fuels burn they release a discrete quantities of energy depending of the mass of fuel reacted. The joule is a relatively small unit of energy.
Hydrogen--the optimum chemical fuel
TABLE The
Element
18 E l e m e n t s
185
1
Evaluated
as Potential
State
Fuels
Pass?
Criterion no. 1
2
3
4
5
6
7
9
+
+
1. H y d r o g e n
Gas
+
+
+
+
+
+
2.
Helium
Gas
-
+
+
+
+
.
3.
Lithium
Solid
+
.
4.
Beryllium
Solid
+
.
5.
Boron
Solid
+
-
+
.
6.
Carbon
Solid
+
-
+
+
-
+
+
+
+
7. N i t r o g e n
Gas
-
+
+
+
-
+
+
+
-
No
8. O x y g e n
Gas
-
+
+
+
+
+
+
+
-
No
9.
.
. .
.
.
.
. .
.
Gas
-
+
-
+
.
Neon
Gas
-
+
+
+
+
11.
Sodium
Solid
+
.
12.
Magnesium
Solid
+
-
+
-
13. A l u m i n u m
Solid
+
-
+
14.
Silicon
Solid
+
-
+
15.
Phosphorus
Solid
+
.
16.
Sulfur
Solid
-
-
+
+
-
+
17. C h l o r i n e
Gas
-
+
-
+
-
18.
Gas
-
+
+
+
+
Argon
+, Pass;
-,
.
.
.
-
. .
No No
.
.
Yes No
+
. .
Fluorine
.
-
.
10.
.
.
+
.
.
+
8
No
.
No
No
+
+
+
-
+
+
+
-
No
+
+
+
+
-
No
-
+
+
+
+
-
No
-
+
+
+
-
-
No
+
+
-
No
+
+
+
-
No
+
+
+
-
No
.
.
.
.
.
.
.
No
No
fail.
In this paper, units of 1000 J g - 1 m a s s , i.e. kJ g-l, will be used. If a fuel produces 10 kJ g-1 and the energy is converted to electricity at 100% efficiency, burning 1 g in 1 s would produce 10 kJ s-I or 10000 J s-l. This would light one hundred 100 W light bulbs for 1 s. Combustion of fuels in air is the primary method of energy production from common fuels. Complete combustion of methane with air or oxygen generates 57.8 kJ g-l. Gasoline or fuel oil are mixtures of several substances and have a variable heat of combustion ranging from 4 2 46 kJ g-l dependent on the quantities of the various ingredients. Coal is even more variable than oil and has a heat of combustion ranging from 16- 30 kJ g-l. Coal's energy production potential depends on its moisture and ash content and the ratio of hydrogen to carbon. These values provide the standard against which candidate fuels can be compared to determine if they meet the first criterion. When burned in air, several of the 18 pure elements release an amount of energy greater than coal. The largest amount of energy, 142 kJ g-i is released by the combustion of hydrogen. Others are: beryllium, 67.9 kJ g-i; boron, 58.2 kJ g-i; lithium, 43.6 kJ g-i; carbon, 32.7 kJ g-i (the major
186
L. O. Williams
element in coal); silicon, 31.4 kJ g-l; aluminum, 31.0 kJ g-l; magnesium, 26.0 kJ g-l; and phosphorus, 24.3 kJ g-l. These elements are all solids and fail the second criterion of being a fluid which would give ease of handling. On combustion, all but carbon produce solid oxides and thus fail the fourth criterion with reaction products that cannot be disposed of into the air. Carbon passes the fourth criterion, but fails the fifth because combustion results in highly toxic carbon monoxide when burned with insufficient oxygen, and carbon dioxide (the major greenhouse gas) when burned with excess oxygen. Two of the remaining elements, sodium and sulfur burn in air releasing 9.09 kJ g-1 and 9.30 kJ g-l respectively. These energy releases are less than coal making them unsuitable because of the failure to meet the first criterion. Both are solids and fail the second criterion. Sodium produces a solid oxide causing it to fail the fourth criterion and sulfur dioxide, the product of the combustion of sulfur, is a very toxic gas causing sulfur to fail the fifth criterion. This accounts for 11 of the first 18 elements. The remaining seven elements---chlorine, nitrogen, oxygen, fluorine, helium, neon, and argon---do not burn in air and fail the first criterion. Several of the chemical elements were rejected because they were solids. If methods of handling were developed that would allow the use of solid fuels in all applications, the second criterion could be ignored and the solid fuel adopted. Unfortunately, all of the energetic elements, save hydrogen and carbon, produce solid oxides. Use of a fuel that produces a solid oxide will require a solid oxide return system capable of handling two or more times the mass handled in the fuel supply system. More than tripling of the mass of material handled will be an extremely large burden in both cost and logistics. Some of the chemical elements that release large amounts of energy when burned in air, combine with hydrogen, carbon and sulfur to produce hydrides, carbides and sulfides respectively. These products will burn in air with an energy release and must be evaluated as potential fuels. The hydrides deserve special attention because our current fuels are chemical compounds made up of hydrogen and carbon: the hydrocarbons. Table 2 shows how each of the hydrides of the first 18 elements fare when examined against the nine criteria. The hydrides are excellent fuels on an energy basis, but all are solid except those of boron, carbon and nitrogen. The hydrides of boron have a high energy content, but are extremely toxic making them non candidates on the basis of the third criterion. Upon oxidation the metal hydrides yield the same solid metal oxides as the pure metals. The nitrogen hydrides, ammonia and hydrazine, are potential fuels releasing
Hydrogen--the optimum chemical fuel
187
TABLE 2 Hydrides Evaluated as Potential Fuels Element
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
Hydrogen Helium Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon Sodium Magnesium Aluminum Silicon Phosphorus Sulfur Chlorine Argon
State
Gas N Solid Solid S/L/G S/L/G Liquid Liquid Liquid N Solid Solid Solid Gas Gas Gas Gas N
Criterion no.
Pass?
I
2
3
4
5
6
7
8
+
+
+
+
+
+
+
+
Yes
+ + + + -
. . + + + + +
.
-
+
+ + +
+ + +
No No No No No No No
+ + + + + -
. + . + +
.
+ + + +
+ + + -
+ +
+ +
. .
.
+
. . + + + -
.
.
. + .
.
. + +
+ + + . -
. .
. + + +
.
.
.
. -
. .
+ + + +
+ + .
.
9
.
+ + + + .
+
m
. + +
m
m
No No No No No No No
+, Pass; - , fail; N, no hydride. 23.9 a n d 19.3 k J g-~, r e s p e c t i v e l y . A m m o n i a b o i l s a t - 3 3 ° a n d h y d r a z i n e at +11 I°C. These temperatures would allow either to be handled as f l u i d s . T h e y f a i l c r i t e r i o n 3 b e c a u s e a m m o n i a is t o x i c t o a n i m a l s ( m a x i m u m a l l o w a b l e c o n c e n t r a t i o n 50 p p m ) a n d t o p l a n t s ( a t c o n c e n t r a t i o n s a b o v e 1%). H y d r a z i n e is t o x i c ( m a x i m u m a l l o w a b l e c o n c e n t r a t i o n 1 p p m ) a n d is a s u s p e c t e d c a r c i n o g e n . T h e r e m a i n i n g h y d r i d e s , n a m e l y t h o s e o f carbon (commonly called hydrocarbons), are the very materials for which we are seeking a substitute. Metallic carbides and sulfides are solids, produce solid and gaseous c o m b u s t i o n p r o d u c t s a n d r e l e a s e less e n e r g y t h a n t h e p u r e m e t a l s . A s a result they fail to meet several of the criteria. Carbon f o r m s a n i t r i d e , c y a n o g e n , w h i c h is r e l a t e d t o h y d r o g e n cyanide. Both cyanogen and hydrogen cyanide are potential fuels, but both are extremely toxic and can be rejected on the basis of the third c r i t e r i o n . C a r b o n d i s u l f i d e is a r o o m t e m p e r a t u r e l i q u i d w i t h f u e l p r o p e r ties. I t is less t o x i c t h a n c y a n o g e n , b u t is s u f f i c i e n t l y t o x i c t o b e r e j e c t e d o n t h e b a s i s o f t h e t h i r d c r i t e r i o n . W h e n it b u r n s it p r o d u c e s l a r g e a m o u n t s o f t o x i c s u l f u r d i o x i d e . N i t r o g e n s u l f i d e is a n e x p l o s i v e s o l i d .
188
L. O. Williams
Boron forms a solid carbide. It can be burned to release a large amount of energy, but it is a refractory solid, and on combustion, solid boron oxide is formed. Boron nitride is a solid and releases so little energy on combustion that it cannot be considered a fuel.
HYDROGEN FUEL From an excursion through the chemical periodic table it is clear the only material fulfilling all the criteria is hydrogen. With regard to the first criterion, 'Provide a large energy storage capability per unit mass', hydrogen is the best possible choice. No other material produces more energy when burned in air. Hydrogen not only fulfills this criterion, but on a weight basis its energy content is 3.3 times higher than gasoline. As a result of this high energy release, on the basis of energy stored per unit mass, hydrogen is the best chemical fuel. For aircraft and rocket systems, for which fuel weight has great importance, the use of hydrogen dramatically improves the performance. For other systems the high energy per unit weight is of value but may not be a critical advantage. Hydrogen easily fills the first criterion to 'Provide a large energy storage capability per unit mass'. The second criterion is the fuel must 'Be a fluid'. Hydrogen is a fluid and meets the primary thrust of this criterion. Hydrogen gas can be transferred and shipped long distances by pipelines, pumped by available pumps and stored in the same types of containers currently used for the storage of other gases. At low temperatures (i.e. -253°C) hydrogen condenses into a liquid. Liquid hydrogen is a fluid and can be transferred like other liquids. In all circumstances hydrogen can be handled either as a ambient temperature gas or as a cryogenic liquid at about 20 K. The third criterion for the new fuel is that it must 'Be non-toxic to plants and animals'. All fuels are more toxic than hydrogen. The toxicity of hydrogen is so low it is on a par with nitrogen, the major constituent of the atmosphere. Long exposure to an atmosphere of pure hydrogen can smother plants and animals. The effect is not due to toxicity--pure hydrogen simply prevents the animal from receiving the oxygen it requires for respiration and prevents the plant from absorbing carbon dioxide. Pure nitrogen (80% of air), helium, and argon will have the same effect. Even oxygen is more toxic than hydrogen. At high concentrations oxygen can be harmful to animals by causing damage to their lungs. Animals can survive exposure to high hydrogen concentrations without harmful effects if they receive adequate oxygen. Plants can survive if
Hydrogen--the optimum chemicalfuel
189
there is adequate oxygen and carbon dioxide. The toxicity of hydrogen is so low that mixtures of hydrogen and oxygen have been evaluated as breathing gas for divers operating at great depths. In this application hydrogen appears less toxic than helium and may offer some advantages for divers at extreme depth or for long exposure periods. Because of its lack of effect on plants and animals, leakage of hydrogen presents no toxic hazards to the environment. 2 The fourth criterion is the new fuel must 'Provide only gaseous products when burned in air'. When hydrogen burns in air the product is water vapor. Direct disposal of the water vapor in the ambient air cannot cause harm. The fifth criterion requires that the new fuel must 'Yield only nontoxic products that do not harm the environment when reacted with ambient air'. Under all conditions water vapor is the only significant product of hydrogen combustion. Mixture ratios producing high temperatures (as a result of combustion) with a slight excess of oxygen can result in the production of nitrogen oxides. The energy from the hydrogen combustion forces the nitrogen to react with the oxygen to form nitric oxide. As the hot gas cools, the nitric oxide reacts with more oxygen to form nitrogen dioxide. Under the worst possible conditions, namely high temperatures with excess oxygen, the reaction product gas contains 400-600 ppm nitrogen oxides. The magnitude of this problem was shown in Fig. 1 in the area labeled 'Hydrogen lean'. By careful control of the hydrogen-air mixture ratio and, in some cases, the use of chemical reaction accelerators (i.e. catalysis) the output of nitrogen oxides can be reduced to low and safe levels. The reaction products from the fuel must be gaseous so that they can be directly vented to the air. This eliminates the requirement for hardware to collect, store and return the spent solid or liquid reaction products. The product of the reaction of hydrogen with oxygen from the air is water. There is no carbon so neither un-burned hydrocarbons nor toxic carbon monoxide is produced. All fossil fuels contain some amount of sulfur compounds. These are converted to sulfur dioxide when the fuel is burned. Most processes under consideration for the production of hydrogen are free from sulfur or any other harmful contaminants. Thus, unlike fossil fuel hydrocarbons, hydrogen combustion products will not be contaminated with sulfur compounds. In low temperature combustion reactions (i.e. flame temperature less than 2500 K) all of the toxic products observed in the combustion of fossil fuels are completely eliminated by the use of contaminant-free hydrogen as the fuel. The open air temperature of a hydrogen flame is 2318 K. Thus, low temperature reactions will occur in properly adjusted
L. O. Williams
190
Weight Ratio - Air : Hydrogen l"-Air : Hydrogen Equivalency Ratio (34.5 : I) by Weight Hydrogen Rich ~ Hydrogen Lean
80 Atm Premure 2180 1 Arm Pressm~ 1~
2442~87 94819801 879
2002
1789
1624
734
632
558
1491 501
Nitrogen Water Oxygen Argon
v
e~ o
Hydrogen f
-2 -3
Ammonia _--¢.
Parts Per Thousand
o c.)
-5 Parts Per Million
-6
-7
f-
-8
---..,
-9 E
Nitrogen Dioxide Parts Per Billion
~Nitric Oxide i ~ Hydroge~ 0 xygen --7 %,---Nitric Oxide and Nitrogen Dioxide
-ll -12
~X
Ammonia
Fig. 1. Equilibrium products of combustion of air with hydrogen ( at 80 atm. pressure expanded to 1 atm. pressure).
kitchen stoves, home and industrial furnaces, and in other low pressure open-air combustion. If the hydrogen combustion reaction is conducted under conditions that result in high temperatures (i.e. flame temperature above 2500 K) and an excess of air, the excess oxygen will react with the nitrogen of the air to produce small quantities of nitric oxide. Figure 1 is a plot of the calculated theoretical concentration of the products of combustion of hydrogen in air at various mixture ratios. This plot shows on the hydrogen lean side of the mixture ratio there is a small amount of nitrogen oxides (NO and NO2). In hydrogen rich mixtures excess hydrogen reacts with nitrogen to form trace amounts of ammonia. When the combustion mixture has a large excess of hydrogen, substantial amounts of ammonia are produced (see the far right side of Fig. 1). As the amount of hydrogen in the mixture is decreased (moving from left to right on Fig. 1) less ammonia is produced. At a concentration of one part hydrogen to 32.8 parts air the ammonia concentration drops below 1 ppm. As the concentration of hydrogen is further decreased to one part hydrogen to 34.6 parts air oxides of nitrogen increase to concentrations greater than I ppm. There is a range of mixtures from one part hydrogen
Hydrogen--the optimum chemicalfuel
191
to 32.8-34.6 parts air where both ammonia and oxides of nitrogen are present at concentrations less than 1 ppm. With precise mixture ratio control at exactly one part hydrogen to 34.45 parts air the concentration of both is less than 10 ppb. While it will be difficult to operate all combustion equipment with this degree of accuracy, the potential for exceedingly low emissions is available and can be approached with good engineering design and operational discipline. As a result of hydrogen's combustion behavior it should be feasible to operate high temperature hydrogen combustion processes without creating significant air pollution. In low temperature applications the products of hydrogen combustion are completely non-toxic. Figure 1 shows well controlled, high temperature hydrogen combustion produces only trace amounts of air pollutants. As a result of the capability of adjusting the mixture ratio to provide non-toxic combustion products, hydrogen meets the fifth criterion. Hydrogen is the only chemical fuel that can meet this criterion. In all respects, hydrogen offers great improvement in environmental safety when compared to any other fuel. When too little oxygen is present in the combustion reaction a trace of ammonia is produced, Fig. 1 shows that the mixture ratio ranges where ammonia is produced in the area labeled 'Hydrogen Rich'. Ammonia is not a desirable exhaust product. When animals are exposed to ammonia it is about a tenth of the toxicity of the oxides of nitrogen. The maximum safe exposure level for ammonia is 50 ppm; for nitrogen dioxide the maximum level is 5 ppm. Plants use ammonia as a source of nitrogen, so it is actually a nutrient and is only harmful at high concentrations. The direct reaction of hydrogen with air produces no toxic substances. Side reactions of hydrogen or oxygen with the nitrogen of the air can produce small quantities of toxic substances under certain circumstances. By control of the reaction conditions to suppress the side reactions hydrogen can be made to meet the fifth criterion. The sixth criterion requires the fuel, 'Be made of common chemical elements'. Water is the combination of hydrogen and oxygen with a chemical formula of H20. This formula shows two atoms of hydrogen are combined with one of oxygen. The atomic weight of hydrogen is 1 and oxygen is 16, so providing a molecular weight of 18, 2 units for the two hydrogen atoms and 16 units for the oxygen atom. As a result water is 11.1% (2/18) hydrogen. Hydrogen produced from water easily meets the sixth criterion. The seventh criterion requires the fuel 'Be made from elements available in most locations'. Hydrogen can be made from water. Fresh water can be easily purified to the level needed for the production of hydrogen. If the salt is removed, sea water can also be used for the production of
192
L. O. Williams
hydrogen. Water is available virtually everywhere on the face of the Earth~ thus, hydrogen can easily meet the seventh criterion. The eighth criterion is that the fuel must, 'Be easily manufactured by a simple, reasonable cost process'. Hydrogen can be produced from water by an extremely simple process. Two electrical conductors (electrodes) are placed in water. A direct electric current passes from the electrodes through the water. At a voltage above 1.3 V, the water will decompose into hydrogen and oxygen. As this simple experiment is performed in a manner allowing observation of the electrodes, when the current is turned on bubbles will be seen to form on the electrodes. More bubbles will form on the negative (hydrogen) electrode than on the positive (oxygen) electrode. As the bubbles grow they will break loose and float to the surface: hydrogen from the negative electrode and oxygen from the positive electrode. An inverted tube filled with water can be placed over each electrode to collect the gas produced. This process is called electrolysis and the device in which it is performed is an electrolyzer. Figure 2 shows a simplified schematic diagram of a water electrolyzer. Electrolysis is potentially highly efficient. In the laboratory, under carefully controlled conditions of slow production, electric energy can be converted to potential energy as hydrogen at essentially 100% efficiency. As the production rate is increased, to obtain better utilization of the electrolysis equipment, the efficiency is reduced. In designing electrolysis equipment the engineer must make a trade between a large expensive electrolyzer producing hydrogen at a low rate and high efficiency or a less costly smaller system producing more hydrogen at lower efficiency. The actual trade-off between these efficiencies is dependent on a large number of complex factors that must be analyzed for each specific hydrogen production facility. Industrial electrolyzers use electrodes and gas collection schemes optimized to produce the maximum amount of hydrogen from the minimum size equipment and minimum quantity of electric energy. Their basic principle of operation is an extension of the simple process described above. Equipment for the industrial scale production of hydrogen by electrolysis is available from a number of manufacturers. These include: Brown Bovary (Switzerland), General Electric (USA), Te.ledyne Energy Systems (USA), Norsk Hydro (Norway) and Stewart Electrolyzers (Canada). This equipment employs a wide variety of engineering solutions to handle the electrolysis process. The efficiency of the various processes ranges from 50% to about 75% depending on the type of equipment used and how hard the process is driven. Further research and development can undoubtedly improve the performance of electrolysis hardware, but it will not be necessary. Any of this equipment can serve
[
[
-
?ig. 2.
.e- + 02 (Gas) + 2H20
•O H
leaction
)xygen Electrode
)utput
Ixygen
Electrons in
~
H 2 (Gas)
;xarnple: Sodium Hydroxide
Water solution ol an ionizing acid, alkali or salt
Electrolyte
ton-conductive container
2e-
Simple schematic diagram of a water electrolysis device.
ectrons out
Leads
~eaction !H+ +
lydrogen Electrode
Output
Hydrogen
~J
?,
r~
!
194
L. O. Williams
as the basis for the production of hydrogen from electrical energy derived from any potential source. The current availability of the necessary equipment to liberate hydrogen from water allows hydrogen to meet the eighth criterion. The ninth criterion is that the fuel must 'Be easy to use in existing power generation equipment'. The use of hydrogen as a fuel has been demonstrated in virtually every type of fuel-using device in existence. The International Journal of Hydrogen Energy, published by Pergamon Press, has printed hundreds of articles describing equipment operating with hydrogen as the fuel. These conversions cover hardware as diverse as automobiles, boats, airplanes, home furnaces and stoves. 5 The Institute Of Gas Technology (IGT) in Chicago, has demonstrated the use of hydrogen in a whole spectrum of residential applications including space heating, cooking, and water heating. Hydrogen can be used in about the same manner as natural gas. IGT has demonstrated the feasibility of hydrogen as a substitute for natural gas in most current applications. In many cases, the necessary equipment for the use of hydrogen can be obtained by simple modifications of existing natural gas equipment. The Canadians are considering the conversion of railroads to the use of hydrogen. The Denver Research Institute, Los Alamos National Laboratory, The University of Southern California and Billings (Wyoming) Energy Research have demonstrated hydrogen-fueled automobiles. The Federal Republic of Germany has converted BMW automobiles to hydrogen and the Japanese have converted several small cars to hydrogen fuel. These research programs and hardware demonstrations have shown there are no insurmountable technological barriers to the adoption of hydrogen as a general-purpose fuel. 3 Hydrogen gas can be used in the same manner as natural gas is used. The mixture ratio of hydrogen with air is different from than that of natural gas. In all other respects--flame temperature, ignition requirements, flow control equipment, corrosion and flue requirements etc. hydrogen acts about the same as natural gas. Any piece of equipment fueled with natural gas can be fueled with hydrogen by adjusting the mixture ratio of the air to the fuel. Virtually every fuel-using device has models or examples routinely operated on natural gas. Automobiles, trucks, trains, boats, homes, power plants, and manufacturing plants all have current working examples of day-to-day operation with natural gas as the fuel. Of the major fuel users only aircrafts are not currently operated on natural gas. Most of these, including aircrafts, have been operated on hydrogen as research and development demonstrations. The similarity of the operation of natural
Hydrogen--the optimum chemicalfuel
195
gas and hydrogen coupled with the feasibility demonstrations of hydrogen as a fuel in all types of equipment provide the basis for accepting that hydrogen can easily meet the ninth criterion. The portable chemical fuel for the future energy system is pure hydrogen. No other chemical substance is available that will meet the nine criteria established. The chemical elements discussed above are all that are available, or ever will be available, for use as a chemical fuel. New elements may be produced by nuclear reactions, but they will be radioactive and will have such high atomic weights they will be of no value as fuels. Hydrogen meets the criteria established for the chemical fuel. In most cases it is the best possible candidate, but all its properties are not ideal. These areas of imperfect fit with the requirements will define areas for future research.
H Y D R O G E N PROPERTIES A N D C H A L L E N G E S 'Hydrogen, with its low transportation costs, as a gas or as a liquid, is ideally suited as an energy vector for very large nuclear or fusion primary energy generators.' C. Marchetti,
International Journal of Hydrogen Energy, 13 (12) (1988) 725.
The statement by Dr. Marchetti highlights the value of hydrogen as the method of handling and transporting energy produced by fusion energy. In many applications it is the best possible choice. For other applications, research and engineering development is required for its application. To understand which applications require the most research, hydrogen properties must be reviewed. Although hydrogen is the only substance meeting all the criteria established at the beginning of the paper it is not identical to any fuels in current use. The properties of hydrogen are provided in Table 3. The data from Table 3 will aid in understanding what and how great are the differences between hydrogen and current fuels. Some properties of hydrogen display a less than optimum fit to the requirements of a fuel. The existing knowledge of these properties will aid in early definition of where the most research will be required for the implementation of hydrogen as the general-purpose fuel. Table 3 contains far more detail than is necessary for this discussion. This level of detail is presented for reference and to demonstrate that hydrogen has been studied in an exhaustive manner and most of the necessary data required to identify the required engineering research and development are available.
L. O. Williams
196
TABLE 3 Properties of Hydrogen
Property Molecular weight Triple point pressure Triple point temperature Normal boiling point Critical pressure Critical temperature Density at critical point Denisty of liquid at triple point Density of solid at triple point Density of vapor at triple point Density of liquid at NBP Density of vapor at NBP Density of gas at NTP Density ratio: NBP liquid to NTP gas Heat of fusion Heat of vaporization Heat of sublimation Heat of combustion: to steam at 100°C Heat of combustion: to water at 0°C Specific heat (Cp) of NTP gas Cp of NPB liquid Cv/Cv of NTP gas Cr/Cv of NBP liquid Viscosity of NTP gas Viscosity of NBP liquid Thermal conductivity of NTP gas Thermal conductivity of NBP liquid Surface tension of NBP liquid Dielectric constant of NTP gas Dielectric constant of NBP liquid Index of refraction of NTP gas Index of refraction of NBP liquid Adiabatic sound velocity in NTP gas Adiabatic sound velocity in NBP liquid Compressibility factor (z) in NTP gas Compressibility factor (z) of NBPliquid Gas constant (R) Isothermal bulk modulus of NBP liquid Volume expansivity of NBP liquid Limits of flammability in air Limits of detonatability in air Stoichiometric composition in air Minimum energy for ignition in air Auto-ignition temperature in air Hot air jet ignition temperature Flame temperature in air Thermal energy radiated from flame Burning velocity in NTP air Detonation velocity in NTP air Diffusion coefficient in NTP air Diffusion velocity in NTP air Buoyant velocity in NTP air Maximum experimental safe gap in NTP air Quenching gap in NTP air Detonation induction distance (length/diameter) in NTP air Limiting oxygen index Vaporization rate of liquid pools Burning rate of spilled liquid pools
Value 2.015 9 0.069 5 13-803 20.268 12.795 32.976 0.032 4 0.077 0 0.086 5 125.60 0.070 8 134.0 83.746 845 58.23 445-59 507.39 119.93 141.86 14.89 9.69 1.383 1.688 8-75 × 10-5 1.33 x 10~ 1.897 1.00 1-93 x 10-3 1.000 26 1-233 1.000 12 1,110 1 294 1 093 1.0006 1,712 x 10-2 40.703 7 50.13 1,658 x 10-2 4-75 18.3-59 29.53 0,020 858 943 2 318 17-25 265-325 1.48-2.15 0.61 2 1.2-9 8 x 10-t 4.6 × 10-2 100 5,0 2.5-5 3,0-6.6
Units amu atm K K atm K g c m -3 g c m -a g c m -3 g m-~ g c m -3 g m -3 g c m -3 -j g-t j g-l j g-t j g-1 J g-t j g-i K-l j g-I K-J --g crn-1 s-l g c m -1 s -t mW em -I K -l mW cmq K -l N m -l
m s-t m s-~ crn3 atm g-i K-l MN m -3 Vol. % Vol. % Vol. % mJ K K K % cm s-I km s-~ cm2 s-I cm s-~ m s-t era cm -Vol. % cm rain-~ cm rain-1
amu, Atomic mass unit; Cp, specific heat at constant pressure; Cv, specific heat at constant volume: N, newton; NBP, normal boding point; NTP, normal temperature and pressure (1 atm at 288.15 K).
Hydrogen--the optimum chemicalfuel
197
There are two slightly different varieties of hydrogen molecule. In one the spin of the hydrogen nuclei are parallel (para) and in the other the spins are opposed (ortho). At room temperature hydrogen is an equal mixture of the two types. In liquid hydrogen orthohydrogen slowly changes to parahydrogen with the release of a small amount of heat. The physical properties of the two types are slightly different. After a long period of time or treatment with a catalyst the two types reach an equilibrium mixture. Properties given in the table are for equilibrium parahydrogen-orthohydrogen at the temperature at which the property was measured. Data adapted from Ref. 4. Energy per unit mass is of primary importance. The amount of energy stored per unit volume is important in systems that are volume constrained. When compared to other fuels hydrogen has a low energy content per unit volume. A specific volume of hydrogen gas, say 1 m3, weighs only 12.5% as much as the same volume of methane gas. While hydrogen releases 2.45 times more energy per unit weight than methane, when compared on a volumetric basis, its light weight results in an volumetric energy content only 30% of methane. This low energy content, on a volumetric basis, is not fatal to the use of hydrogen. It is however, the major problem associated with adoption of hydrogen as a general purpose fuel. The impact of the low volumetric energy density will be addressed in detail in the paper' Hydrogen in Transportation', and 'The Fusion Hydrogen Energy System'. At this point it is sufficient to observe that storage tanks, pipelines and other hydrogen fuel systems will be lighter in weight, but larger than those used with current fuels. Hydrogen must be cooled to 20 K before it condenses to a liquid. A gas stored in a liquefied form at low temperatures is termed a cryogen. As a cryogenic liquid, hydrogen can be stored and handled on a large scale using current technology handling equipment. In this form it is far more dense than it is in the gaseous state, but its low density characteristics, as compared to other fuels, are still present. A volume of liquid hydrogen weighs only 10% as much as the same volume of gasoline. For equal weights of hydrogen and gasoline, hydrogen has 290% more energy. For equal volumes of hydrogen and gasoline, hydrogen has only 29% as much energy. Its very low temperature (20-30 K) presents a handling problem not encountered with current common fuels. Current technology equipment is available for handling and storage of liquid hydrogen. Research to improve storage methods and technology will provide techniques that will ease the adoption of hydrogen as the general purpose fuel. This work must be directed at the development of improved solutions to the handling and storage problems created by liquid hydrogen's low temperature and density in both the gaseous and
198
L. O. Williams
liquid state. The low density of hydrogen and its low boiling point present the major research challenge in the development of the new energy system.
NOTE ON SYNTHETIC HYDROCARBONS Hydrocarbons can be synthesized by reacting hydrogen with carbon-containing materials. Farm waste and coal have been suggested as suitable carbon sources. Investigators have suggested large quantities of hydrogen should be produced for the synthesis of liquid hydrocarbon fuels. Chemical engineers currently know how to process coal into synthetic liquid fuels. The first step in the synthetic fuel manufacturing process is the production of hydrogen from water using energy derived from coal combustion or from non-fossil energy sources (hydropower, nuclear, etc.). The hydrogen is reacted with coal converting it to a liquid and/or gaseous hydrocarbon fuel. Today the technology is in limited use in places such as South Africa where coal is cheap and oil derived fuels are expensive. The end result of this synthesis process is a fuel with all the shortcomings of natural liquid fossil fuels and in addition, the shortcomings of coal use. These synthetic chemical fuels, made of hydrogen and carbon, have the same basic chemical composition as the fossil fuels we are trying to eliminate. Use of these compounds will simply perpetuate the problems described in the first paper of this issue, for which we are seeking a solution. If implemented on a wide scale, synthetic fuel production processes provide a method of continuing the undesirable addition of carbon dioxide and other pollutants to the atmosphere. In addition, they encourage increased production of coal, the most polluting of the fossil fuels. The fuel for the future must be hydrogen used alone, without carbon.
SUMMARY The potential chemical fuels that might be used to replace the fossil fuels have been examined. The data indicates that there is only one element or compound, namely hydrogen, with physical properties that fulfill all the criteria established for the future fuel. However, two properties were not optimum in this application: its low density and low boiling point. Because of the low density hydrogen handling and storage equipment will be large and bulky. The bulk will produce only minor problems for stationary handling and storage, but will provide challenges for the
Hydrogen--the optimum chemicalfuel
199
designers of mobile equipment. The low temperature of liquid hydrogen will necessitate the use of vacuum jacketed superinsulated storage vessels. These must be equipped with appropriate safety features and equipment to accommodate the slight continuous boil off of liquid hydrogen, resulting from the inability to manufacture a perfect insulator. In the following paper, hydrogen will be examined as the general purpose transportation fuel. In 'The Fusion Hydrogen Energy System', hydrogen's use will be woven with fusion energy to provide the tapestry depicting the environmentally sound energy system of the future.
REFERENCES 1. Weast, R. C., Handbook of physics and chemistry. CRC Press Inc. Boca Raton, F1. 33431, 68th edition, 1987-1988. 2. Williams, L. O., Hydrogen power. Pergamon Press Inc., New York, 1980. 3. Hord, J., National Bureau of Standards, Technical Note 690. U.S. Department of Commerce, National Bureau of Standards, Colorado, October 1976. 4. Edeskuty, F.J., et al. Hydrogen safety and environmental control assessment. Los Alamos National Laboratory Rep. LA-8225-PR, September 1979. 5. Anon, The International Journal Of Hydrogen Energy (13 volumes), The Proceedings of The seven World Hydrogen Energy Conferences and related materials. The National Hydrogen Association, Washington, D.C. Information on hydrogen technology is available from: The International Association for Hydrogen Energy (IAHE), PO Box 248266, Coral Gables, FL. 33124, USA.
Hydrogen
The Optimum Chemical Fuel
'The idea of using hydrogen as the ultimate, ecologically clean synthetic fuel--a notion that spawned the concept of a hydrogen economy during the oil-shock round of the early 1970s--is slowly making headway again, especially overseas.'
Peter Hoffman,
Chemical Engineering, October 26, 1987, p. 26
ABSTRACT The preceding paper showed that despite the broad desirability of fusion as a base-load energy source it is necessary to implement a manufactured chemical fuel for energy transport, storage and as a transportation fuel In this paper it will be shown that hydrogen is certainly the optimum, and maybe the only, choice for the chemical fuel The fuel use properties of hydrogen will be reviewed along with its detailed chemical and physical properties.
POTENTIAL CHEMICAL FUELS Specific criteria must be satisfied by the chemical fuel selected to support the fusion energy system. To be useful the fuel must: (1) provide a large energy storage capability per unit mass to mini mize the amount of fuel material manufactured, handled, stored, and transported; (2) be a fluid for ease of transportation in pipe lines and transfer to and from storage vessels; (3) be non-toxic to plants and animals. (No matter how Carefully the fuel is handled accidents, mistakes and natural disasters will result in spills); (4) provide only gaseous products when burned in air to allow disposal of the reaction products directly to the atmosphere; (5) when burned with air, yield non-toxic products harmless to the environment. (The reaction products must be totally non-toxic or 183
Applied Energy 0306-2619/94/$07.00 © 1994 Elsevier Science Ltd, England. Printed in Great Britain
184
(6) (7)
(8) (9)
L. O. Williams
the shift from hydrocarbons to the new fuel will just change one set of toxins for another); be made of c o m m o n chemical elements to ensure an abundant supply; be made from elements available in most locations to reduce the a m o u n t of shipping required in the production and use of the fuel; be easily manufactured by a low cost, process to be economically viable; be easy to use in existing power generation equipment to aid transition with a m i n i m u m of equipment modification.
These criteria will be applied to possible substitutes to determine which have the potential to serve as the manufactured fuel for use with the fusion energy generation system. To search for substances to replace the chemicals found in fossil fuels, all chemical elements and compounds with potential as fuels were examined. The nine criteria were used to judge each element and comp o u n d to determine if it was a suitable candidate. There are 83 non-radioactive elements from which to choose. The elements from 1-18 and their c o m p o u n d s will be evaluated against the criteria to determine their suitability as fuels. The elements with an atomic number higher than 18 (i.e. argon) are metals or semi-metals. They release little energy when they react with the oxygen. On the basis of their failure to meet criterion 1 they can be easily eliminated from consideration. Table 1 shows how elements 1-18 fare when evaluated against the nine criteria. All data regarding physical and chemical properties, abundance, toxicity, and reaction energies of elements and compounds were derived from values taken from the H a n d b o o k of Physics and Chemistry. The first evaluation factor is the a m o u n t of energy produced on combustion. A measure of energy must be selected for comparison of the energy produced by various fuels. The technically proper measure of energy is the joule. A joule is a quantity of energy, as are British Thermal units (Btu) and calories. A watt is a measure of power, the potential for the delivery of energy. Since a watt is a measure of potential to provide energy, watts per unit time must be specified to provide a quantity of energy. The joule (a measure of quantity) is defined as 1.00 Ws. Thus a 1.0 W light bulb uses energy at a rate of 1.0 J s-l and a 100 W bulb uses 100 J S-1. When fuels burn they release a discrete quantities of energy depending of the mass of fuel reacted. The joule is a relatively small unit of energy.
Hydrogen--the optimum chemical fuel
TABLE The
Element
18 E l e m e n t s
185
1
Evaluated
as Potential
State
Fuels
Pass?
Criterion no. 1
2
3
4
5
6
7
9
+
+
1. H y d r o g e n
Gas
+
+
+
+
+
+
2.
Helium
Gas
-
+
+
+
+
.
3.
Lithium
Solid
+
.
4.
Beryllium
Solid
+
.
5.
Boron
Solid
+
-
+
.
6.
Carbon
Solid
+
-
+
+
-
+
+
+
+
7. N i t r o g e n
Gas
-
+
+
+
-
+
+
+
-
No
8. O x y g e n
Gas
-
+
+
+
+
+
+
+
-
No
9.
.
. .
.
.
.
. .
.
Gas
-
+
-
+
.
Neon
Gas
-
+
+
+
+
11.
Sodium
Solid
+
.
12.
Magnesium
Solid
+
-
+
-
13. A l u m i n u m
Solid
+
-
+
14.
Silicon
Solid
+
-
+
15.
Phosphorus
Solid
+
.
16.
Sulfur
Solid
-
-
+
+
-
+
17. C h l o r i n e
Gas
-
+
-
+
-
18.
Gas
-
+
+
+
+
Argon
+, Pass;
-,
.
.
.
-
. .
No No
.
.
Yes No
+
. .
Fluorine
.
-
.
10.
.
.
+
.
.
+
8
No
.
No
No
+
+
+
-
+
+
+
-
No
+
+
+
+
-
No
-
+
+
+
+
-
No
-
+
+
+
-
-
No
+
+
-
No
+
+
+
-
No
+
+
+
-
No
.
.
.
.
.
.
.
No
No
fail.
In this paper, units of 1000 J g - 1 m a s s , i.e. kJ g-l, will be used. If a fuel produces 10 kJ g-1 and the energy is converted to electricity at 100% efficiency, burning 1 g in 1 s would produce 10 kJ s-I or 10000 J s-l. This would light one hundred 100 W light bulbs for 1 s. Combustion of fuels in air is the primary method of energy production from common fuels. Complete combustion of methane with air or oxygen generates 57.8 kJ g-l. Gasoline or fuel oil are mixtures of several substances and have a variable heat of combustion ranging from 4 2 46 kJ g-l dependent on the quantities of the various ingredients. Coal is even more variable than oil and has a heat of combustion ranging from 16- 30 kJ g-l. Coal's energy production potential depends on its moisture and ash content and the ratio of hydrogen to carbon. These values provide the standard against which candidate fuels can be compared to determine if they meet the first criterion. When burned in air, several of the 18 pure elements release an amount of energy greater than coal. The largest amount of energy, 142 kJ g-i is released by the combustion of hydrogen. Others are: beryllium, 67.9 kJ g-i; boron, 58.2 kJ g-i; lithium, 43.6 kJ g-i; carbon, 32.7 kJ g-i (the major
186
L. O. Williams
element in coal); silicon, 31.4 kJ g-l; aluminum, 31.0 kJ g-l; magnesium, 26.0 kJ g-l; and phosphorus, 24.3 kJ g-l. These elements are all solids and fail the second criterion of being a fluid which would give ease of handling. On combustion, all but carbon produce solid oxides and thus fail the fourth criterion with reaction products that cannot be disposed of into the air. Carbon passes the fourth criterion, but fails the fifth because combustion results in highly toxic carbon monoxide when burned with insufficient oxygen, and carbon dioxide (the major greenhouse gas) when burned with excess oxygen. Two of the remaining elements, sodium and sulfur burn in air releasing 9.09 kJ g-1 and 9.30 kJ g-l respectively. These energy releases are less than coal making them unsuitable because of the failure to meet the first criterion. Both are solids and fail the second criterion. Sodium produces a solid oxide causing it to fail the fourth criterion and sulfur dioxide, the product of the combustion of sulfur, is a very toxic gas causing sulfur to fail the fifth criterion. This accounts for 11 of the first 18 elements. The remaining seven elements---chlorine, nitrogen, oxygen, fluorine, helium, neon, and argon---do not burn in air and fail the first criterion. Several of the chemical elements were rejected because they were solids. If methods of handling were developed that would allow the use of solid fuels in all applications, the second criterion could be ignored and the solid fuel adopted. Unfortunately, all of the energetic elements, save hydrogen and carbon, produce solid oxides. Use of a fuel that produces a solid oxide will require a solid oxide return system capable of handling two or more times the mass handled in the fuel supply system. More than tripling of the mass of material handled will be an extremely large burden in both cost and logistics. Some of the chemical elements that release large amounts of energy when burned in air, combine with hydrogen, carbon and sulfur to produce hydrides, carbides and sulfides respectively. These products will burn in air with an energy release and must be evaluated as potential fuels. The hydrides deserve special attention because our current fuels are chemical compounds made up of hydrogen and carbon: the hydrocarbons. Table 2 shows how each of the hydrides of the first 18 elements fare when examined against the nine criteria. The hydrides are excellent fuels on an energy basis, but all are solid except those of boron, carbon and nitrogen. The hydrides of boron have a high energy content, but are extremely toxic making them non candidates on the basis of the third criterion. Upon oxidation the metal hydrides yield the same solid metal oxides as the pure metals. The nitrogen hydrides, ammonia and hydrazine, are potential fuels releasing
Hydrogen--the optimum chemical fuel
187
TABLE 2 Hydrides Evaluated as Potential Fuels Element
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
Hydrogen Helium Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon Sodium Magnesium Aluminum Silicon Phosphorus Sulfur Chlorine Argon
State
Gas N Solid Solid S/L/G S/L/G Liquid Liquid Liquid N Solid Solid Solid Gas Gas Gas Gas N
Criterion no.
Pass?
I
2
3
4
5
6
7
8
+
+
+
+
+
+
+
+
Yes
+ + + + -
. . + + + + +
.
-
+
+ + +
+ + +
No No No No No No No
+ + + + + -
. + . + +
.
+ + + +
+ + + -
+ +
+ +
. .
.
+
. . + + + -
.
.
. + .
.
. + +
+ + + . -
. .
. + + +
.
.
.
. -
. .
+ + + +
+ + .
.
9
.
+ + + + .
+
m
. + +
m
m
No No No No No No No
+, Pass; - , fail; N, no hydride. 23.9 a n d 19.3 k J g-~, r e s p e c t i v e l y . A m m o n i a b o i l s a t - 3 3 ° a n d h y d r a z i n e at +11 I°C. These temperatures would allow either to be handled as f l u i d s . T h e y f a i l c r i t e r i o n 3 b e c a u s e a m m o n i a is t o x i c t o a n i m a l s ( m a x i m u m a l l o w a b l e c o n c e n t r a t i o n 50 p p m ) a n d t o p l a n t s ( a t c o n c e n t r a t i o n s a b o v e 1%). H y d r a z i n e is t o x i c ( m a x i m u m a l l o w a b l e c o n c e n t r a t i o n 1 p p m ) a n d is a s u s p e c t e d c a r c i n o g e n . T h e r e m a i n i n g h y d r i d e s , n a m e l y t h o s e o f carbon (commonly called hydrocarbons), are the very materials for which we are seeking a substitute. Metallic carbides and sulfides are solids, produce solid and gaseous c o m b u s t i o n p r o d u c t s a n d r e l e a s e less e n e r g y t h a n t h e p u r e m e t a l s . A s a result they fail to meet several of the criteria. Carbon f o r m s a n i t r i d e , c y a n o g e n , w h i c h is r e l a t e d t o h y d r o g e n cyanide. Both cyanogen and hydrogen cyanide are potential fuels, but both are extremely toxic and can be rejected on the basis of the third c r i t e r i o n . C a r b o n d i s u l f i d e is a r o o m t e m p e r a t u r e l i q u i d w i t h f u e l p r o p e r ties. I t is less t o x i c t h a n c y a n o g e n , b u t is s u f f i c i e n t l y t o x i c t o b e r e j e c t e d o n t h e b a s i s o f t h e t h i r d c r i t e r i o n . W h e n it b u r n s it p r o d u c e s l a r g e a m o u n t s o f t o x i c s u l f u r d i o x i d e . N i t r o g e n s u l f i d e is a n e x p l o s i v e s o l i d .
188
L. O. Williams
Boron forms a solid carbide. It can be burned to release a large amount of energy, but it is a refractory solid, and on combustion, solid boron oxide is formed. Boron nitride is a solid and releases so little energy on combustion that it cannot be considered a fuel.
HYDROGEN FUEL From an excursion through the chemical periodic table it is clear the only material fulfilling all the criteria is hydrogen. With regard to the first criterion, 'Provide a large energy storage capability per unit mass', hydrogen is the best possible choice. No other material produces more energy when burned in air. Hydrogen not only fulfills this criterion, but on a weight basis its energy content is 3.3 times higher than gasoline. As a result of this high energy release, on the basis of energy stored per unit mass, hydrogen is the best chemical fuel. For aircraft and rocket systems, for which fuel weight has great importance, the use of hydrogen dramatically improves the performance. For other systems the high energy per unit weight is of value but may not be a critical advantage. Hydrogen easily fills the first criterion to 'Provide a large energy storage capability per unit mass'. The second criterion is the fuel must 'Be a fluid'. Hydrogen is a fluid and meets the primary thrust of this criterion. Hydrogen gas can be transferred and shipped long distances by pipelines, pumped by available pumps and stored in the same types of containers currently used for the storage of other gases. At low temperatures (i.e. -253°C) hydrogen condenses into a liquid. Liquid hydrogen is a fluid and can be transferred like other liquids. In all circumstances hydrogen can be handled either as a ambient temperature gas or as a cryogenic liquid at about 20 K. The third criterion for the new fuel is that it must 'Be non-toxic to plants and animals'. All fuels are more toxic than hydrogen. The toxicity of hydrogen is so low it is on a par with nitrogen, the major constituent of the atmosphere. Long exposure to an atmosphere of pure hydrogen can smother plants and animals. The effect is not due to toxicity--pure hydrogen simply prevents the animal from receiving the oxygen it requires for respiration and prevents the plant from absorbing carbon dioxide. Pure nitrogen (80% of air), helium, and argon will have the same effect. Even oxygen is more toxic than hydrogen. At high concentrations oxygen can be harmful to animals by causing damage to their lungs. Animals can survive exposure to high hydrogen concentrations without harmful effects if they receive adequate oxygen. Plants can survive if
Hydrogen--the optimum chemicalfuel
189
there is adequate oxygen and carbon dioxide. The toxicity of hydrogen is so low that mixtures of hydrogen and oxygen have been evaluated as breathing gas for divers operating at great depths. In this application hydrogen appears less toxic than helium and may offer some advantages for divers at extreme depth or for long exposure periods. Because of its lack of effect on plants and animals, leakage of hydrogen presents no toxic hazards to the environment. 2 The fourth criterion is the new fuel must 'Provide only gaseous products when burned in air'. When hydrogen burns in air the product is water vapor. Direct disposal of the water vapor in the ambient air cannot cause harm. The fifth criterion requires that the new fuel must 'Yield only nontoxic products that do not harm the environment when reacted with ambient air'. Under all conditions water vapor is the only significant product of hydrogen combustion. Mixture ratios producing high temperatures (as a result of combustion) with a slight excess of oxygen can result in the production of nitrogen oxides. The energy from the hydrogen combustion forces the nitrogen to react with the oxygen to form nitric oxide. As the hot gas cools, the nitric oxide reacts with more oxygen to form nitrogen dioxide. Under the worst possible conditions, namely high temperatures with excess oxygen, the reaction product gas contains 400-600 ppm nitrogen oxides. The magnitude of this problem was shown in Fig. 1 in the area labeled 'Hydrogen lean'. By careful control of the hydrogen-air mixture ratio and, in some cases, the use of chemical reaction accelerators (i.e. catalysis) the output of nitrogen oxides can be reduced to low and safe levels. The reaction products from the fuel must be gaseous so that they can be directly vented to the air. This eliminates the requirement for hardware to collect, store and return the spent solid or liquid reaction products. The product of the reaction of hydrogen with oxygen from the air is water. There is no carbon so neither un-burned hydrocarbons nor toxic carbon monoxide is produced. All fossil fuels contain some amount of sulfur compounds. These are converted to sulfur dioxide when the fuel is burned. Most processes under consideration for the production of hydrogen are free from sulfur or any other harmful contaminants. Thus, unlike fossil fuel hydrocarbons, hydrogen combustion products will not be contaminated with sulfur compounds. In low temperature combustion reactions (i.e. flame temperature less than 2500 K) all of the toxic products observed in the combustion of fossil fuels are completely eliminated by the use of contaminant-free hydrogen as the fuel. The open air temperature of a hydrogen flame is 2318 K. Thus, low temperature reactions will occur in properly adjusted
L. O. Williams
190
Weight Ratio - Air : Hydrogen l"-Air : Hydrogen Equivalency Ratio (34.5 : I) by Weight Hydrogen Rich ~ Hydrogen Lean
80 Atm Premure 2180 1 Arm Pressm~ 1~
2442~87 94819801 879
2002
1789
1624
734
632
558
1491 501
Nitrogen Water Oxygen Argon
v
e~ o
Hydrogen f
-2 -3
Ammonia _--¢.
Parts Per Thousand
o c.)
-5 Parts Per Million
-6
-7
f-
-8
---..,
-9 E
Nitrogen Dioxide Parts Per Billion
~Nitric Oxide i ~ Hydroge~ 0 xygen --7 %,---Nitric Oxide and Nitrogen Dioxide
-ll -12
~X
Ammonia
Fig. 1. Equilibrium products of combustion of air with hydrogen ( at 80 atm. pressure expanded to 1 atm. pressure).
kitchen stoves, home and industrial furnaces, and in other low pressure open-air combustion. If the hydrogen combustion reaction is conducted under conditions that result in high temperatures (i.e. flame temperature above 2500 K) and an excess of air, the excess oxygen will react with the nitrogen of the air to produce small quantities of nitric oxide. Figure 1 is a plot of the calculated theoretical concentration of the products of combustion of hydrogen in air at various mixture ratios. This plot shows on the hydrogen lean side of the mixture ratio there is a small amount of nitrogen oxides (NO and NO2). In hydrogen rich mixtures excess hydrogen reacts with nitrogen to form trace amounts of ammonia. When the combustion mixture has a large excess of hydrogen, substantial amounts of ammonia are produced (see the far right side of Fig. 1). As the amount of hydrogen in the mixture is decreased (moving from left to right on Fig. 1) less ammonia is produced. At a concentration of one part hydrogen to 32.8 parts air the ammonia concentration drops below 1 ppm. As the concentration of hydrogen is further decreased to one part hydrogen to 34.6 parts air oxides of nitrogen increase to concentrations greater than I ppm. There is a range of mixtures from one part hydrogen
Hydrogen--the optimum chemicalfuel
191
to 32.8-34.6 parts air where both ammonia and oxides of nitrogen are present at concentrations less than 1 ppm. With precise mixture ratio control at exactly one part hydrogen to 34.45 parts air the concentration of both is less than 10 ppb. While it will be difficult to operate all combustion equipment with this degree of accuracy, the potential for exceedingly low emissions is available and can be approached with good engineering design and operational discipline. As a result of hydrogen's combustion behavior it should be feasible to operate high temperature hydrogen combustion processes without creating significant air pollution. In low temperature applications the products of hydrogen combustion are completely non-toxic. Figure 1 shows well controlled, high temperature hydrogen combustion produces only trace amounts of air pollutants. As a result of the capability of adjusting the mixture ratio to provide non-toxic combustion products, hydrogen meets the fifth criterion. Hydrogen is the only chemical fuel that can meet this criterion. In all respects, hydrogen offers great improvement in environmental safety when compared to any other fuel. When too little oxygen is present in the combustion reaction a trace of ammonia is produced, Fig. 1 shows that the mixture ratio ranges where ammonia is produced in the area labeled 'Hydrogen Rich'. Ammonia is not a desirable exhaust product. When animals are exposed to ammonia it is about a tenth of the toxicity of the oxides of nitrogen. The maximum safe exposure level for ammonia is 50 ppm; for nitrogen dioxide the maximum level is 5 ppm. Plants use ammonia as a source of nitrogen, so it is actually a nutrient and is only harmful at high concentrations. The direct reaction of hydrogen with air produces no toxic substances. Side reactions of hydrogen or oxygen with the nitrogen of the air can produce small quantities of toxic substances under certain circumstances. By control of the reaction conditions to suppress the side reactions hydrogen can be made to meet the fifth criterion. The sixth criterion requires the fuel, 'Be made of common chemical elements'. Water is the combination of hydrogen and oxygen with a chemical formula of H20. This formula shows two atoms of hydrogen are combined with one of oxygen. The atomic weight of hydrogen is 1 and oxygen is 16, so providing a molecular weight of 18, 2 units for the two hydrogen atoms and 16 units for the oxygen atom. As a result water is 11.1% (2/18) hydrogen. Hydrogen produced from water easily meets the sixth criterion. The seventh criterion requires the fuel 'Be made from elements available in most locations'. Hydrogen can be made from water. Fresh water can be easily purified to the level needed for the production of hydrogen. If the salt is removed, sea water can also be used for the production of
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hydrogen. Water is available virtually everywhere on the face of the Earth~ thus, hydrogen can easily meet the seventh criterion. The eighth criterion is that the fuel must, 'Be easily manufactured by a simple, reasonable cost process'. Hydrogen can be produced from water by an extremely simple process. Two electrical conductors (electrodes) are placed in water. A direct electric current passes from the electrodes through the water. At a voltage above 1.3 V, the water will decompose into hydrogen and oxygen. As this simple experiment is performed in a manner allowing observation of the electrodes, when the current is turned on bubbles will be seen to form on the electrodes. More bubbles will form on the negative (hydrogen) electrode than on the positive (oxygen) electrode. As the bubbles grow they will break loose and float to the surface: hydrogen from the negative electrode and oxygen from the positive electrode. An inverted tube filled with water can be placed over each electrode to collect the gas produced. This process is called electrolysis and the device in which it is performed is an electrolyzer. Figure 2 shows a simplified schematic diagram of a water electrolyzer. Electrolysis is potentially highly efficient. In the laboratory, under carefully controlled conditions of slow production, electric energy can be converted to potential energy as hydrogen at essentially 100% efficiency. As the production rate is increased, to obtain better utilization of the electrolysis equipment, the efficiency is reduced. In designing electrolysis equipment the engineer must make a trade between a large expensive electrolyzer producing hydrogen at a low rate and high efficiency or a less costly smaller system producing more hydrogen at lower efficiency. The actual trade-off between these efficiencies is dependent on a large number of complex factors that must be analyzed for each specific hydrogen production facility. Industrial electrolyzers use electrodes and gas collection schemes optimized to produce the maximum amount of hydrogen from the minimum size equipment and minimum quantity of electric energy. Their basic principle of operation is an extension of the simple process described above. Equipment for the industrial scale production of hydrogen by electrolysis is available from a number of manufacturers. These include: Brown Bovary (Switzerland), General Electric (USA), Te.ledyne Energy Systems (USA), Norsk Hydro (Norway) and Stewart Electrolyzers (Canada). This equipment employs a wide variety of engineering solutions to handle the electrolysis process. The efficiency of the various processes ranges from 50% to about 75% depending on the type of equipment used and how hard the process is driven. Further research and development can undoubtedly improve the performance of electrolysis hardware, but it will not be necessary. Any of this equipment can serve
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L. O. Williams
as the basis for the production of hydrogen from electrical energy derived from any potential source. The current availability of the necessary equipment to liberate hydrogen from water allows hydrogen to meet the eighth criterion. The ninth criterion is that the fuel must 'Be easy to use in existing power generation equipment'. The use of hydrogen as a fuel has been demonstrated in virtually every type of fuel-using device in existence. The International Journal of Hydrogen Energy, published by Pergamon Press, has printed hundreds of articles describing equipment operating with hydrogen as the fuel. These conversions cover hardware as diverse as automobiles, boats, airplanes, home furnaces and stoves. 5 The Institute Of Gas Technology (IGT) in Chicago, has demonstrated the use of hydrogen in a whole spectrum of residential applications including space heating, cooking, and water heating. Hydrogen can be used in about the same manner as natural gas. IGT has demonstrated the feasibility of hydrogen as a substitute for natural gas in most current applications. In many cases, the necessary equipment for the use of hydrogen can be obtained by simple modifications of existing natural gas equipment. The Canadians are considering the conversion of railroads to the use of hydrogen. The Denver Research Institute, Los Alamos National Laboratory, The University of Southern California and Billings (Wyoming) Energy Research have demonstrated hydrogen-fueled automobiles. The Federal Republic of Germany has converted BMW automobiles to hydrogen and the Japanese have converted several small cars to hydrogen fuel. These research programs and hardware demonstrations have shown there are no insurmountable technological barriers to the adoption of hydrogen as a general-purpose fuel. 3 Hydrogen gas can be used in the same manner as natural gas is used. The mixture ratio of hydrogen with air is different from than that of natural gas. In all other respects--flame temperature, ignition requirements, flow control equipment, corrosion and flue requirements etc. hydrogen acts about the same as natural gas. Any piece of equipment fueled with natural gas can be fueled with hydrogen by adjusting the mixture ratio of the air to the fuel. Virtually every fuel-using device has models or examples routinely operated on natural gas. Automobiles, trucks, trains, boats, homes, power plants, and manufacturing plants all have current working examples of day-to-day operation with natural gas as the fuel. Of the major fuel users only aircrafts are not currently operated on natural gas. Most of these, including aircrafts, have been operated on hydrogen as research and development demonstrations. The similarity of the operation of natural
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gas and hydrogen coupled with the feasibility demonstrations of hydrogen as a fuel in all types of equipment provide the basis for accepting that hydrogen can easily meet the ninth criterion. The portable chemical fuel for the future energy system is pure hydrogen. No other chemical substance is available that will meet the nine criteria established. The chemical elements discussed above are all that are available, or ever will be available, for use as a chemical fuel. New elements may be produced by nuclear reactions, but they will be radioactive and will have such high atomic weights they will be of no value as fuels. Hydrogen meets the criteria established for the chemical fuel. In most cases it is the best possible candidate, but all its properties are not ideal. These areas of imperfect fit with the requirements will define areas for future research.
H Y D R O G E N PROPERTIES A N D C H A L L E N G E S 'Hydrogen, with its low transportation costs, as a gas or as a liquid, is ideally suited as an energy vector for very large nuclear or fusion primary energy generators.' C. Marchetti,
International Journal of Hydrogen Energy, 13 (12) (1988) 725.
The statement by Dr. Marchetti highlights the value of hydrogen as the method of handling and transporting energy produced by fusion energy. In many applications it is the best possible choice. For other applications, research and engineering development is required for its application. To understand which applications require the most research, hydrogen properties must be reviewed. Although hydrogen is the only substance meeting all the criteria established at the beginning of the paper it is not identical to any fuels in current use. The properties of hydrogen are provided in Table 3. The data from Table 3 will aid in understanding what and how great are the differences between hydrogen and current fuels. Some properties of hydrogen display a less than optimum fit to the requirements of a fuel. The existing knowledge of these properties will aid in early definition of where the most research will be required for the implementation of hydrogen as the general-purpose fuel. Table 3 contains far more detail than is necessary for this discussion. This level of detail is presented for reference and to demonstrate that hydrogen has been studied in an exhaustive manner and most of the necessary data required to identify the required engineering research and development are available.
L. O. Williams
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TABLE 3 Properties of Hydrogen
Property Molecular weight Triple point pressure Triple point temperature Normal boiling point Critical pressure Critical temperature Density at critical point Denisty of liquid at triple point Density of solid at triple point Density of vapor at triple point Density of liquid at NBP Density of vapor at NBP Density of gas at NTP Density ratio: NBP liquid to NTP gas Heat of fusion Heat of vaporization Heat of sublimation Heat of combustion: to steam at 100°C Heat of combustion: to water at 0°C Specific heat (Cp) of NTP gas Cp of NPB liquid Cv/Cv of NTP gas Cr/Cv of NBP liquid Viscosity of NTP gas Viscosity of NBP liquid Thermal conductivity of NTP gas Thermal conductivity of NBP liquid Surface tension of NBP liquid Dielectric constant of NTP gas Dielectric constant of NBP liquid Index of refraction of NTP gas Index of refraction of NBP liquid Adiabatic sound velocity in NTP gas Adiabatic sound velocity in NBP liquid Compressibility factor (z) in NTP gas Compressibility factor (z) of NBPliquid Gas constant (R) Isothermal bulk modulus of NBP liquid Volume expansivity of NBP liquid Limits of flammability in air Limits of detonatability in air Stoichiometric composition in air Minimum energy for ignition in air Auto-ignition temperature in air Hot air jet ignition temperature Flame temperature in air Thermal energy radiated from flame Burning velocity in NTP air Detonation velocity in NTP air Diffusion coefficient in NTP air Diffusion velocity in NTP air Buoyant velocity in NTP air Maximum experimental safe gap in NTP air Quenching gap in NTP air Detonation induction distance (length/diameter) in NTP air Limiting oxygen index Vaporization rate of liquid pools Burning rate of spilled liquid pools
Value 2.015 9 0.069 5 13-803 20.268 12.795 32.976 0.032 4 0.077 0 0.086 5 125.60 0.070 8 134.0 83.746 845 58.23 445-59 507.39 119.93 141.86 14.89 9.69 1.383 1.688 8-75 × 10-5 1.33 x 10~ 1.897 1.00 1-93 x 10-3 1.000 26 1-233 1.000 12 1,110 1 294 1 093 1.0006 1,712 x 10-2 40.703 7 50.13 1,658 x 10-2 4-75 18.3-59 29.53 0,020 858 943 2 318 17-25 265-325 1.48-2.15 0.61 2 1.2-9 8 x 10-t 4.6 × 10-2 100 5,0 2.5-5 3,0-6.6
Units amu atm K K atm K g c m -3 g c m -a g c m -3 g m-~ g c m -3 g m -3 g c m -3 -j g-t j g-l j g-t j g-1 J g-t j g-i K-l j g-I K-J --g crn-1 s-l g c m -1 s -t mW em -I K -l mW cmq K -l N m -l
m s-t m s-~ crn3 atm g-i K-l MN m -3 Vol. % Vol. % Vol. % mJ K K K % cm s-I km s-~ cm2 s-I cm s-~ m s-t era cm -Vol. % cm rain-~ cm rain-1
amu, Atomic mass unit; Cp, specific heat at constant pressure; Cv, specific heat at constant volume: N, newton; NBP, normal boding point; NTP, normal temperature and pressure (1 atm at 288.15 K).
Hydrogen--the optimum chemicalfuel
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There are two slightly different varieties of hydrogen molecule. In one the spin of the hydrogen nuclei are parallel (para) and in the other the spins are opposed (ortho). At room temperature hydrogen is an equal mixture of the two types. In liquid hydrogen orthohydrogen slowly changes to parahydrogen with the release of a small amount of heat. The physical properties of the two types are slightly different. After a long period of time or treatment with a catalyst the two types reach an equilibrium mixture. Properties given in the table are for equilibrium parahydrogen-orthohydrogen at the temperature at which the property was measured. Data adapted from Ref. 4. Energy per unit mass is of primary importance. The amount of energy stored per unit volume is important in systems that are volume constrained. When compared to other fuels hydrogen has a low energy content per unit volume. A specific volume of hydrogen gas, say 1 m3, weighs only 12.5% as much as the same volume of methane gas. While hydrogen releases 2.45 times more energy per unit weight than methane, when compared on a volumetric basis, its light weight results in an volumetric energy content only 30% of methane. This low energy content, on a volumetric basis, is not fatal to the use of hydrogen. It is however, the major problem associated with adoption of hydrogen as a general purpose fuel. The impact of the low volumetric energy density will be addressed in detail in the paper' Hydrogen in Transportation', and 'The Fusion Hydrogen Energy System'. At this point it is sufficient to observe that storage tanks, pipelines and other hydrogen fuel systems will be lighter in weight, but larger than those used with current fuels. Hydrogen must be cooled to 20 K before it condenses to a liquid. A gas stored in a liquefied form at low temperatures is termed a cryogen. As a cryogenic liquid, hydrogen can be stored and handled on a large scale using current technology handling equipment. In this form it is far more dense than it is in the gaseous state, but its low density characteristics, as compared to other fuels, are still present. A volume of liquid hydrogen weighs only 10% as much as the same volume of gasoline. For equal weights of hydrogen and gasoline, hydrogen has 290% more energy. For equal volumes of hydrogen and gasoline, hydrogen has only 29% as much energy. Its very low temperature (20-30 K) presents a handling problem not encountered with current common fuels. Current technology equipment is available for handling and storage of liquid hydrogen. Research to improve storage methods and technology will provide techniques that will ease the adoption of hydrogen as the general purpose fuel. This work must be directed at the development of improved solutions to the handling and storage problems created by liquid hydrogen's low temperature and density in both the gaseous and
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liquid state. The low density of hydrogen and its low boiling point present the major research challenge in the development of the new energy system.
NOTE ON SYNTHETIC HYDROCARBONS Hydrocarbons can be synthesized by reacting hydrogen with carbon-containing materials. Farm waste and coal have been suggested as suitable carbon sources. Investigators have suggested large quantities of hydrogen should be produced for the synthesis of liquid hydrocarbon fuels. Chemical engineers currently know how to process coal into synthetic liquid fuels. The first step in the synthetic fuel manufacturing process is the production of hydrogen from water using energy derived from coal combustion or from non-fossil energy sources (hydropower, nuclear, etc.). The hydrogen is reacted with coal converting it to a liquid and/or gaseous hydrocarbon fuel. Today the technology is in limited use in places such as South Africa where coal is cheap and oil derived fuels are expensive. The end result of this synthesis process is a fuel with all the shortcomings of natural liquid fossil fuels and in addition, the shortcomings of coal use. These synthetic chemical fuels, made of hydrogen and carbon, have the same basic chemical composition as the fossil fuels we are trying to eliminate. Use of these compounds will simply perpetuate the problems described in the first paper of this issue, for which we are seeking a solution. If implemented on a wide scale, synthetic fuel production processes provide a method of continuing the undesirable addition of carbon dioxide and other pollutants to the atmosphere. In addition, they encourage increased production of coal, the most polluting of the fossil fuels. The fuel for the future must be hydrogen used alone, without carbon.
SUMMARY The potential chemical fuels that might be used to replace the fossil fuels have been examined. The data indicates that there is only one element or compound, namely hydrogen, with physical properties that fulfill all the criteria established for the future fuel. However, two properties were not optimum in this application: its low density and low boiling point. Because of the low density hydrogen handling and storage equipment will be large and bulky. The bulk will produce only minor problems for stationary handling and storage, but will provide challenges for the
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designers of mobile equipment. The low temperature of liquid hydrogen will necessitate the use of vacuum jacketed superinsulated storage vessels. These must be equipped with appropriate safety features and equipment to accommodate the slight continuous boil off of liquid hydrogen, resulting from the inability to manufacture a perfect insulator. In the following paper, hydrogen will be examined as the general purpose transportation fuel. In 'The Fusion Hydrogen Energy System', hydrogen's use will be woven with fusion energy to provide the tapestry depicting the environmentally sound energy system of the future.
REFERENCES 1. Weast, R. C., Handbook of physics and chemistry. CRC Press Inc. Boca Raton, F1. 33431, 68th edition, 1987-1988. 2. Williams, L. O., Hydrogen power. Pergamon Press Inc., New York, 1980. 3. Hord, J., National Bureau of Standards, Technical Note 690. U.S. Department of Commerce, National Bureau of Standards, Colorado, October 1976. 4. Edeskuty, F.J., et al. Hydrogen safety and environmental control assessment. Los Alamos National Laboratory Rep. LA-8225-PR, September 1979. 5. Anon, The International Journal Of Hydrogen Energy (13 volumes), The Proceedings of The seven World Hydrogen Energy Conferences and related materials. The National Hydrogen Association, Washington, D.C. Information on hydrogen technology is available from: The International Association for Hydrogen Energy (IAHE), PO Box 248266, Coral Gables, FL. 33124, USA.