Hydrogen Fuel

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From the 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 these criteria, but on a weight basis, its energy content is 3.3 times higher than gasoline. Because of this high-energy release, based on 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. The Space Shuttle main engines burn hydrogen with oxygen. 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 gas. At low temperatures, 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 an ambient temperature gas or as a cryogenic liquid at about 20 Kelvin.

The third criterion for the new fuel is that it "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 damaging to their lungs. Animals can survive exposure to high hydrogen concentrations without harmful effects if they receive adequate oxygen. Plants can survive if there is adequate oxygen and carbon dioxide.

The toxicity of hydrogen is so low that mixtures of hydrogen and oxygen have been experimentally evaluated as breathing gas for divers operating at great depth. In this application, hydrogen appears less toxic than helium and may offer some advantages for divers at extreme depth or for long exposure periods. 170 Because of its lack of effect on plants and animals, leakage of hydrogen presents no toxic hazards to the environment and hydrogen easily meets the third criterion of no toxicity to plants and animals.171

The fourth criterion is the new fuel "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.

170 http://www.mtsinai.org/pulmonary/books/scuba/sectionl.htm

171 Edeskuty, F. J., et. el. "Hydrogen Safety and Environmental Control Assessment", Los Alamos National Laboratory Report Number LA-S225-PR, September 1979

The fifth criterion requires that the new fuel "Yield only non-toxic 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 with a slight excess of oxygen can result in the production of trace amounts 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, high temperatures with excess oxygen, the reaction product gas contains 400 to 600 parts per million nitrogen oxides. The magnitude of this problem was shown in Figure 5.1 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 (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 no un-burned hydrocarbons or 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.

When contaminant free hydrogen is used as the fuel in low temperature combustion reactions (flame temperature less than 2500 Kelvin) all of the toxic products observed in the combustion of fossil fuels are eliminated. The open-air temperature of a hydrogen flame is 2318 Kelvin. Thus, low temperature reactions will occur in properly adjusted 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 (flame temperature of 2488 Kelvin) and an excess of air, the excess oxygen will react with the nitrogen of the air to produce small quantities of nitric oxide. 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 ammonia is produced. As the amount of hydrogen in the mixture is decreased, less ammonia is produced. At a concentration of one part hydrogen to 32.8 parts air the ammonia concentration drops below one part per million. 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 one part per million.

Figure 5.1 shows that there is a range of mixtures from one part hydrogen to 32.8 to 34.6 parts air where both ammonia and oxides of nitrogen are present at concentrations less than one part per million. With precise mixture ratio control at exactly one part hydrogen to 34.45 parts air the concentration of both ammonia and the oxides of nitrogen are less than 10 parts per billion. Most combustion equipment can be operated with this degree of accuracy. The potential for exceedingly low emissions is available and can be approached with good engineering design and operational discipline. Because of hydrogen's combustion behavior, it is 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 and well-controlled, high temperature hydrogen combustion produces only trace amounts of air pollutants.


Combustion at 80 Atmospheres Pressure Expanded to One Atmosphere Pressure

Weight Ratio - Air : Hydrogen

— Air : Hydrogen Equivalency Ratio (34.5 : 1) by Weight

Hydrogen Rich -«--Hydrogen Lean

|- 20:1 |30:1 r <0:1 r 50:1 |- 60:1 p 70:1 p 80:1

80 Atm Pressure 2180 1 Atm Pressure

Hydrogen a Ammonia

Hydrogen Rich -«--Hydrogen Lean

|- 20:1 |30:1 r <0:1 r 50:1 |- 60:1 p 70:1 p 80:1

a Ammonia

2004 Ford Fuse Box Diagram

Nitric Oxide


Nitric Oxide and Nitrogen Dioxide Ammonia

Figure 5.1 Equilibrium Combustion products of hydrogen with air

Nitric Oxide


Nitric Oxide and Nitrogen Dioxide Ammonia

Figure 5.1 Equilibrium Combustion products of hydrogen with air

Because 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. Ammonia is not a desirable exhaust product. When animals are exposed to ammonia, it is about 0.1 as toxic as the oxides of nitrogen. The maximum safe exposure level for ammonia is 50 parts per million, for nitrogen dioxide the maximum level is 5 parts per million. Plants use ammonia as a source of nitrogen so it is actually a nutrient and is only harmful at high concentrations. Ammonia has a very distinctive odor what is easily recognized. This combination of characteristic can be employed to provide added environmental safety. Equipment can be designed in such a manner that should a mixture ratio shift happen due to equipment failure it will be biased to become hydrogen rich. This will reduce the probability of production of the more dangerous oxides of nitrogen and provide an odorant warning signal that the equipment needs service.

The direct reaction of hydrogen with air produces no toxic substances. Side chemical 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: "When burned with air, yield non-toxic products, harmless to the environment".

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 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 of being common.

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, seawater can also be used for the production of hydrogen. Water is available virtually everywhere on the face of the earth; thus, hydrogen can easily meet the seventh criterion.

The eighth criterion is the fuel, "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, 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 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.

Electrolysis is 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. To achieve this level of efficiency requires the apparatus to produce almost no hydrogen. 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 using 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. Some of these are:

Brown Bovery, Switzerland — General Electric, United States - Teledyne-Brown Energy Systems, United States — Norsk Hydro, Norway - Stuart 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 95% depending on which type of equipment is 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 the currently available equipment can serve as the basis for the production of hydrogen using 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 "Be easily manufactured by a simple, reasonable cost process". Electrolyzers manufactured by Stuart Energy Systems, in Canada, and Teledyne-Brown, in the United States are shown in Chapters 6 and 7.

The ninth criterion is the fuel "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.

The Institute Of Gas Technology (IGT can be reached at www.igt.org) 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 Energy Research (Wyoming) have demonstrated hydrogen-fueled automobiles. The German manufacturer, BMW, has converted automobiles to liquid hydrogen and the Japanese have converted several small cars to hydrogen fuel. Ford has demonstrated a hydrogen fuel cell automobile. In addition, there are hydrogen fuel cell buses in operation. Hydrogen fuel cell for transportation will be discussed at length in Chapter 7. These hardware demonstrations have shown there are no technological barriers to the adoption of hydrogen as a general-purpose transportation fuel.

Hydrogen gas can be used in the same manner as natural gas is used. The mixture ratio of hydrogen with air is different from that of natural gas. In all other respects, flame temperature, ignition requirements, flow control equipment, corrosion, 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 airplanes are not currently operated on natural gas. Most of these fuel using devices, including airplanes, have been operated on hydrogen as research and development demonstrations. The similarity of the operation of natural 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 criteria: "Be easy to use in existing energy generation equipment".

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 at the beginning of this chapter. 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 for fuels.

Hydrogen meets the criteria established for the chemical fuel. In most cases, it is the best possible candidate, but not all its properties are ideal. These areas of imperfect fit with the requirements will define areas for future research.

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