GB2165532A - Thermochemical hydrogen generator - Google Patents

Abstract

A lightweight thermochemical hydrogen generator comprises a metallic hydride e.g. magnesium hydride matrix and an ionic hydride e.g. lithium hydride matrix disposed coaxially within a first compartment of a container so as to form a passageway therebetween which communicates with a hydrogen-collecting plenum. Water, pressurized by inert gas, is disposed within another compartment of the container and is supplied to the ionic hydride matrix through a normally closed valve and a water distribution manifold pipe having a plurality of apertures uniformly distributed therein. Upon opening the valve, the resulting ionic hydride/water reaction liberates hydrogen and produces heat which decomposes the metallic hydride. The proportions of the hydrides and the water are selected to control the temperature within the container to ensure decomposition of the metallic hydride, to optimize hydrogen generation and heat dissipation, and to minimize weight and volume. The generator may be employed for the high altitude inflation of a balloon launched from a rocket, as a source of fuel for combustion engines, and as a safe, lightweight hydrogen transportation system. <IMAGE>

Description

SPECIFICATION Lightweight thermochemical hydrogen generator Background of the invention This invention relates generally to hydrogen generation systems and methods, and more particularly to hydrogen generation from the combined decomposition of endothermically and exothermically decomposable hydrides.

The thermal generation of hydrogen from metal hydrides or from reactions between metals or metal hydrides and water is well known, and such hydrogen generation systems have well-recognized advantages, principally related to safety, over gaseous or liquid hydrogen storage systems. However, most metallic hydrides are capable of storing only relatively small percentages of hydrogen per unit weight of hydride, which may impose unacceptable weight and, in some instances, volume penalties on hydride.

based systems. For example, double metal hydrides, as of titanium and iron, have been employed for the commercial storage of hydrogen. However, sixty pounds of hydride (TiFeH) are required for each pound of hydrogen released. Therefore, in order to generate ten Ibs. of hydrogen, the weight of the hydride alone would be 600 Ibs., and additional weight would be required for the hydride reaction initiator. For applications requiring low weight hydrogen generation, such as the high altitude inflation of balloons launched from rockets, such weights would be unacceptable.

Other lighter weight metal hydrides can reduce the weight of a hydride-based system. Magnesium hydride, for example, is capable of generating approximately 1 Ib. of hydrogen for each 14 Ibs. of hydride, thus reducing the total hydride weight of the system to 140 Ibs. for generating 10 Ibs. of hydrogen. An additional complication is added, however, due to the fact that it is necessary to supply a significant quantity of heat to magnesium hydride for decomposition. Approximately 3,900 kcal/lb. or 17.8 kcai/mole of hydrogen generated must be supplied at an operating temperature in the vicinity of 300"C to magnesium hydride for decomposition. This necessitates a heat source which will add additional weight to the system.If a chemical heat source is employed, it is expected that approximately 1 gram/kcal or 8.6 Ibs/lb of hydrogen generated would be added according to the following stoichiometry:

Where AH = 17.9 kcal.

Accordingly, to generate 10 Ibs. of hydrogen with a magnesium hydride/chemical heater system would require approximately 226 Ibs. (102 kg) of chemicals alone, and the estimated volume of the system would be approximately 90-100 liters, or about 3.2-3.5 cubic feet (cf).

Other sources of hydrogen include the ionic hydrides of calcium and lithium, and double hydrides such as LiAIH4 which react with water and produce heat and hydrogen. Hydrogen generators employing calcium hydride are based upon the reaction

The exothermicity of reaction (2) at 25"C is 0.7 kcal/g of reactants, and the hydrogen yield is 5.17 Ibs.

per 100 Ibs. of reactants. Accordingly, to generate 10 Ibs. of hydrogen, approximately 104.4 Ibs. of calcium hydride and 89.4 Ibs. of water, approximately 200 Ibs. total, would be required. Calcium hydride has a density of 1.9 g/cm3, a melting point greater than 1000 C, and a heat of formation of -45 kcal/mole.

The reaction of lithium hydride with water yields more hydrogen than calcium hydride, but also more heat according to the following equation:

where AH = 26 kcal.

The exothermicity of reaction (3) at 25"C is 1.0 kcal/g of reactants, and the hydrogen yield is 7.75 Ibs.

per 100 Ibs. of reactants. Hence, 40 Ibs. of lithium hydride and 90 Ibs. of water, a total chemical weight of 130 Ibs., are required to liberate 10 Ibs. of hydrogen. Lithium hydride has a density of 0.78 g/cm3, a melting point of 686"C, and a heat of formation of -21.7 kcal/gmol. Lithium hydride melts without decomposition at 4710C. The exothermicity of this reaction will raise the temperature of the products to approximately 900 C, which is much above the melting point of lithium hydroxide.

Because of the exothermicity of the calcium hydride and lithium hydride reactions, it is necessary to include provisions for discarding the heat generated, which may add significant weight and complexity to the system.

It is desirable to provide hydride-based hydrogen generation systems and methods in order to take advantage of the safety features and rather long storage life of hydride media, while avoiding the abovenoted disadvantages of known hydride-based systems and methods, and it is to this end that the present invention is directed.

Summary of the invention The invention provides highly advantageous hydrogen generation systems and methods which are noteworthy for their high volumetric capacity, low weight, small external size, and relative simplicity. As such, they are well adapted to such applications as inflation systems for the high altitude inflation of balloons, for generating fuel for combustion engines, and for affording safe, compact and lightweight hydrogen transportation systems.

The invention is based upon hydrogen generation by the combined action of an endothermically decomposable metallic hydride and an exothermically decomposable hydride, both of which liberate hydrogen, wherein the heat liberated by the exothermically decomposable hydride is employed for decomposing the endothermically decomposable hydride. The relative porportions of the hydrides are adjusted to control the temperature of the reaction, to optimize hydrogen generation and heat dissipation, and to minimize weight and volume.

Briefly stated, a hydrogen generator in accordance with the invention comprises a container enclosing a first matrix of a metallic hydride which decomposes endothermically to liberate hydrogen and a second matrix of an ionic hydride which reacts with water to decompose exothermically and liberate hydrogen, the first and second matrices being disposed adjacent to one another so that the heat produced by the decomposition of the inionic hydride is transferred to the metallic hydride. The porportions of the metallic hydride and the ionic hydride are adjusted so as to ensure decomposition of the metallic hydride and to control the temperature within the container.A source of pressurized water is connected by way of valve means to water distribution means for distributing water uniformly to the ionic hydride matrix, and valve actuating means is included for opening the valve means to initiate reaction. Means is further included for collecting the hydrogen liberated from the hydrides and for supplying the hydrogen to an outlet of the container.

Preferably, the metallic hydride is magnesium hydride and the ionic hydride is lithium hydride. The first and second matrices may be formed to have an annular cylindrical shape with the first matrix disposed coaxially about the second matrix within a first compartment of the container with a passageway between the matrices for liberated hydrogen, and with the second matrix being disposed coaxially about the water distribution means. The water distribution means may comprise a manifold having a plurality of apertures therein for uniformly distributing the water to the ionic hydride matrix. The water may be disposed within another compartment of the container and pressurized by an inert gas.

Brief description of the drawings Figure 1 is a longitudinal sectional view of a hydrogen generator in accordance with the invention; and Figure 2 is a cross sectional view taken approximately along the lines 2-2 of Figure 1.

Description of the preferred embodiments As previously indicated, the invention is particuarly well adapted to providing safe, lightweight high volumetric capacity hydrogen generation systems for the high altitude inflation of balloons launched from rockets and the like, and will be described in that context. However, as will be apparent from the description which follows, this is illustrative of only one utility of the invention.

As previously noted, the invention affords a lightweight high volumetric capacity hydrogen generator that is based upon the thermochemistry which optimizes a metallic hydride/ionic hydride and water system by combining the chemical advantages of the ionic hydride and water reaction in efficiently generating hydrogen and heat, and by subsequently employing the heat generated in this reaction for the decomposition of the metallic hydride to generate additional hydrogen, at a lower temperature, instead of discarding the heat to the environment. By employing the heat generated by the exothermic reaction of the ionic hydride and water to decompose the metallic hydride, the necessity of providing a heat dissipation mechanism for this heat is avoided, thereby enabling the system weight (and volume) to be reduced.The proportions of the three chemical compounds (ionic hydride, water and metallic hydride) will control the final temperature since the decomposition of the metallic hydride will act as a moderator.

A preferred endothermically decomposable metallic hydride is magnesium hydride, since among the metallic hydrides it has a rather high hydrogen storage capacity per unit weight, being capable of generating about 1 lb. of hydrogen for each 14 Ibs. of hydride. It will be understood, however, that any endothermically decomposable metallic hydride may be employed in the practice of the invention. A preferred ionic hydride is lithium hydride which, as previously pointed out, yields approximately 7.75 Ibs. of hydrogen per 100 Ibs. of reactants and has an exothermicity of reaction at 25"C of 1.0 kcal/g of reactants. It will be further understood that any ionic hydride which exothermically reacts with water to produce hydrogen may be employed in the practice of the invention. The chemical weight of the lithium hydride/water system is selected relative to that of the magnesium hydride such that the heat produced decomposes the magnesium hydride and provides a final operating temperature of the order of 300"C. Combining a lithium hydride/water system and a magnesium hydride system in this manner does not reduce the amount of hydrogen generated per unit chemical weight, and avoids the necessity for discarding the excess heat, thereby reducing the final system weight.

Table I lists the thermochemical properties of the lithium hyride/water and magnesium hydride system and the calculations leading to the following overall balanced equation for a 300"C reaction:

TABLE I Thermodynamic properties and calculations Heat of reaction

(#H) 26 kcal/mole - EXOTHERMIC 19.8 kcal/mole - ENDOTHERMIC cp UOH Y (cal/mole deg) 10 11.6 7 At 600"K pg/cc muMH LiH 1.8 0.78 Heat required to reach 300"C 300 C LiOH MgH2 H2 CpdT 3 kcal/mole 3.48 2.1 RT Net heat available a) Theoretical for MgH2 17.42 kcal/mole (LiH + H2) decomposition at 300"C b) Twelve percent loss (Assumed) 15.3 kcal/mole Mole ratio MgH2 (Decomposed) LiH (Reacted) 0.8 The following Table II shows the total calculated weight and volume of a system for generating 10 Ibs.

of hydrogen. As shown, the total chemical weight and volume are 130.9 Ibs. and 1.78 c.f., respectively.

TABLE II Chemical weight and volume Compound Weight Volume H2 Generated 4.53 kg (10 Ibs) Balloon (variable) LiH 10.17 kg (22.45 Ibs) 13.04 Liters H2O 22.88 kg (50.48 Ibs) 22.88 Liters MgH2 26.27 kg (57.99 Ibs) 14.59 Liters Total required 59.32 kg 50.51 Liters (130.9 Ibs) - (1.78 c.f.) Figures 1 and 2 illustrate a preferred form of a metallic hydride/ionic hydride and water hydrogen gen erator 10 in accordance with the invention for the high altitude inflation of a balloon launched from a rocket. As shown, the generator comprises a container 12, which may be cylindrical and formed of stainless steel, for example, since this configuration is readily adaptable to conform to limited space.The container is divided into a plurality of compartments 14, 16, 18 and 20, and has an outlet 22 which is adapted for connection to a balloon filler tube (not illustrated). The upper compartment 14 of the container serves as a plenum for coilecting the hydrogen liberated by the hydride reactions (as indicated by the arrows in the figure) in compartment 16, as will be described more fully shortly. The plenum may be connected to the outlet 22 through a pressure-actuated normally closed check valve 24 which is designed to open at a predetermined pressure.

The generator is preferably a magnesium hydride/lithium hydride and water system, for the reasons previously described. The lithium hydride and magnesium hydride are disposed as respective hydride matrices 26 and 28 in compartment 16. Each matrix preferably has an annular cylindrical shape, and the cylindrical matrices are preferably sized such that the magnesium hydride matrix 28 may be disposed coaxially about the lithium hydride matrix 26, as shown, and such that an annular passageway 30 is formed between the two matrices.The lithium hydride matrix 26 is preferably perforated or porous so as to have a predetermined void volume to afford gas liberation without creating excessive local pressure, and may be formed with a plurality of radially extending passageways 34 which communicate with annular passageway 30 and with a coaxial longitudinally extending cylindrical opening 36 within the lithium hydride matrix. The magnesium hydride may be either solid or granular. If granular, it may also include a binder to bond the granules together and to slow down the reaction, any may be disposed within an annular retainer screen (not illustrated) for support. The relative sizes of the hydride matrices are determined by equation (4) as previously described.

Disposed within coaxial opening 36 is a water distribution system which may comprise a longitudinally extending manifold pipe 40 having a plurality of openings 42 uniformly distributed longitudinally and circumferentially therein. The lower end (in Figure 1) of the manifold pipe may be connected to a source of pressurized water in compartment 20 through a shut-off valve 44. Valve 44, which is normally closed, may be operated by an actuating mechanism 46 comprising a pressure switch actuated electrically ignited explosive squib of known construction. Electrical power for the actuating mechanism may be supplied from a battery 48, and atmospheric pressure may be supplied to the pressure switch via a pressure sensor port 50 in the sidewall of container 12. A water fill port 52 communicating with compartment 20 may also be located in the sidewall adjacent to the pressure sensor port.

Compartment 20 may be divided into two portions by a movable piston 54. Water is supplied to the upper portion of the compartment via the water fill port, and pressurized inert gas may be supplied to the lower side of piston 54 via a gas fill valve 56.

To enable generator 10 to be stored safely, it is desirable that the water and inert gas portions of compartment 20 be left unfilled and that the explosive squib actuating mechanism for valve 44 not be inserted. Prior to deployment, the upper portion of compartment 20 may be filled with water via water fill port 52 and pressurized inert gas may be supplied to the lower portion of the compartment via gas fill valve 56. Lastly, the explosive squib actuating mechanism may be inserted. Upon launch, the pressure sensor of actuating mechanism 46 senses the atmoshperic pressure via pressure sensor port 50 and, upon a predetermined altitude being reached, actuates the electrically ignited explosive squib to open valve 44.Water under pressure from the inert gas behind the piston will be forced into manifold pipe 40 as the piston moves upwardly (in Figure 1) to exhaust the water from the compartment, the velocity of the piston being regulated by the speed of the hydride reaction in compartment 16. As the water comes into contact with the inner lithium hydride matrix, it converts lithium hydride to lithium hydroxide and produces heat and hydrogen gas, which may carry some steam with it initially. The heated gases pass through the perforated lithium hydride matrix and into passage 30, where they give up the steam and some of their thermal energy. Most of the thermal energy, however, will flow to the outer magnesium hydride matrix. The magnesium hydride will absorb the heat, thereby reducing the temperature within the compartment, and thermally decompose to generate additional hydrogen.The hydrogen will flow into the plenum (compartment 14) above the hydride compartment 16 and build up pressure. Upon the pressure in the plenum reaching a predetermined value, check valve 24 will open to enable the hydrogen to flow freely into the balloon filler tube until the process ceases or is terminated. By selecting the quantities of the hydrides and water in accordance with equation (4), the heat produced by the LiH/H2O reaction can be absorbed substantially completely in the MgH2 and maintain its temperature at the required temperature of 300"C for hydrogen dissociation.

The following Table Ill gives representative dimensions, volumes and weights for the chemical components of a generator of the type illustrated in Figures 1 and 2 that has a volumetric capacity that is twice that required to generate 10 Ibs. of hydrogen gas. Thus, the generator of Table Ill has a safety factor of 2.

Assuming all materials react to completion, the minimum generator parameters for generating 10 Ibs. of hydrogen would be one-half of those given in the Table.

TABLE Ill Generator dimensions OD (in.) ID (in.) LD (in.) VOL .liy.3) WGT. (Ibs.) MgH2 17 13 19 1790 116 LiH 12 3 19 2010* 43 H2O 17 -- 12-1/2 2840 103 * 25% VOID Assuming a maximum allowable balloon payload weight of 300 Ibs., a generator having the parameters given in Table Ill would leave 38 Ibs. for structure, mechanism and associated items.

As will be appreciated from the foregoing, the invention provides a compact, lightweight, highly efficient thermochemical hydrogen generator which is useful for a number of different applications. In addition to its use as an inflation system for balloons, a hydrogen generator in accordance with the invention is also useful as a source of fuel for combustion engines and as a safe, compact, lightweight hydrogen transportation system.

While preferred embodiments of the invention have been shown and described, it will be apparent to those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims.

Claims (22)

1. A hydrogen generation system comprising a container, the container enclosing a first matrix of a metallic hydride which decomposes endothermically to liberate hydrogen and a second matrix of an ionic hydride which reacts with water to decompose exothermically and liberate hydrogen, the first and second matrices being disposed adjacent to one another within the container such that the heat produced by the decomposition of the ionic hydride is transferred to the metallic hydride, and the proportions of the metallic hydride and the ionic hydride being adjusted so as to ensure decomposition of the metallic hydride and to control the temperature within the container; a source of pressurized water; means for distributing the water uniformly to the ionic hydride matrix; valve means connected between the water distribution means and the source of pressurized water; and means for actuating the valve means to admit pressurized water to the water distribution means.

2. The generator of claim 1, wherein said metallic hydride comprises magnesium hydride.

3. The generator of claim 1, wherein said ionic hydride is selected from the group consisting of lithium hydride and calcium hydride.

4. The generator of claim 1, wherein said ionic hydride comprises lithium hydride.

5. The generator of claim 1, wherein said first and second matrices each have an annular cylindrical shape, the first matrix being disposed coaxially about the second matrix within a first compartment of the container so as to form a passageway between the matrices for liberated hydrogen, and the second matrix being disposed coaxially about the water distribution means.

6. The generator of claim 5, wherein said second matrix has a plurality of radially extending passageways therein which communicate with said passageway between said matrices and with the interior of the second matrix.

7. The generator of claim 5, wherein said water distribution means comprises a manifold pipe disposed coaxially within the interior of the second matrix, the pipe having a plurality of apertures distributed longitudinally and circumferentially therein, and wherein said source of pressurized water comprises another compartment of the container which is divided into first and second parts by a movable piston, the first part containing water and the second part containing pressurized gas.

8. The generator of claim 7, wherein said valve means comprises a normally closed valve connected between the manifold pipe and the first part of said other compartment, and wherein said actuating means comprises an electrical actuator for opening the valve, and means responsive to a predetermined condition for operating the electrical actuator.

9. The generator of claim 8, wherein said predetermined condition comprises atmospheric pressure, and said operating means comprises an electrical source and a pressure sensitive switch for connecting the electrical source to the electrical actuator.

10. The generator of claim 7, wherein said container includes water fill means and gas fill means communicating with said first and second parts of said other compartment, respectively, to enable said first and second parts to be respectively filled with water and gas, and wherein said container is further formed to enable insertion of the actuating means prior to use.

11. The generator of claim 1, wherein said container is formed with a gas plenum communicating with said passageway between said matrices and with an outlet opening of the container, and wherein a normally closed pressure operated valve is disposed between said plenum and said outlet opening.

12. The hydrogen generator of claim 1, wherein said generator is formed to be disposed within a rocket for the high altitude inflation of a balloon launched from said rocket.

13. A method of generating hydrogen comprising disposing within a first compartment of a container a first matrix of magnesium hydride and a second matrix of an ionic hydride which reacts with water to decompose exothermically; disposing within a second compartment of the container pressurized water; supplying pressurized water uniformly to said second ionic hydride matrix so as to cause reaction between the ionic hydride and the water; transferring heat produced by said reaction to the magnesium hydride to endothermically decompose the magnesium hydride, the proportions of the magnesium hydride, the ionic hydride and water disposed within said container being selected so as to ensure control of the temperature within the container and the decomposition of the magnesium hydride; collecting hydrogen produced by the ionic hydride reaction with the water and the decomposition of the magnesium hydride; and supplying said hydrogen to an outlet of the container.

14. The method of claim 13, wherein said ionic hydride is selected from the group consisting of lithium hydride and calcium hydride.

15. The method of claim 13, wherein said ionic hydride comprises lithium hydride.

16. The method of claim 13 further comprising forming said first and second matrices as annular cylinders of magnesium hydride and ionic hydride, respectively, and wherein said hydride disposing step comprises disposing the first matrix coaxially about the second matrix so as to afford a passageway therebetween, and wherein said second matrix is formed with a plurality of radially extending passageways which communicate with said passageway between said matrices and with an interior opening of the second matrix.

17. The method of claim 16, wherein said second ionic hydride matrix is formed to have a predetermined volumetric percentage of voids therein so as to afford hydrogen liberation without the creation of excessive local pressure.

18. The method of claim 13, wherein said magnesium hydride is granular, and wherein said first matrix includes a binder to bond the granules of magnesium hydride together so as to control the rate of decomposition of the magnesium hydride.

19. The method of claim 13 further comprising sensing a predetermined atmospheric condition, and wherein said supplying comprises supplying water in response to said predetermined atmospheric condition.

20. The method of claim 19 further comprising employing the hydrogen generated to fill a balloon launched at a high altitude from a rocket.

21. A method of generating hydrogen substantially as hereinbefore described with reference to the drawing.

22. A hydrogen generation system substantially as hereinbefore described with reference to and as illustrated in the drawing.