Patent Application
20090104481
The present invention relates to the generation of hydrogen from a fuel that is stored in solid form and from which hydrogen is generated using a liquid reagent.
Hydrogen is the fuel of choice for fuel cells. Widespread use of fuel cells is dependent on finding a convenient hydrogen source due to the difficulties in storing hydrogen gas. Many hydrogen carriers, including hydrocarbons, metal hydrides, and chemical hydrides are being considered as hydrogen storage and supply systems. In each case, systems need to be developed to release the hydrogen from its carrier, either by reformation as in the case of hydrocarbons, desorption from metal hydrides, or hydrolysis of chemical hydrides.
Complex chemical hydrides, such as sodium borohydride and lithium borohydride, have been investigated as hydrogen storage media with high gravimetric hydrogen storage densities. Sodium borohydride has garnered particular interest, because it can be dissolved in alkaline water solutions with virtually no reaction—hydrogen is not generated until the solution contacts a catalyst to promote hydrolysis. In a typical heterogeneous catalyzed system, the stoichiometric reaction of borohydrides with water to produce hydrogen gas and a borate is illustrated by the following chemical reaction for alkali metal borohydride compounds:
MBH4+4H2O→MBO2.2H2O+4H2+heat (1)
To maintain the borohydride and borate solids in solution, water in excess of that required for the stoichiometric hydrolysis reaction is typically stored, since water generally reacts with the borate products to form hydrated borate compounds. Extra water may be added to the system to compensate for this loss, such as by using a dilute borohydride fuel solution, which limits the effective hydrogen storage density of such hydrogen generation systems.
It is desirable to have a hydrogen generator that maximizes the hydrogen stored within a given volume. Such generators offer the potential of compact and safe hydrogen storage that, when coupled with a fuel cell, can provide systems to meet the growing demand for portable power.
The present invention provides systems and methods for hydrogen generation by the hydrolysis of a solid fuel, and methods of operating a power module. The system includes a reaction chamber adapted to contain at least one solid fuel capable of generating hydrogen upon contact with a liquid reagent, and a liquid distributor for contacting the solid fuel with the liquid reagent in the reaction chamber to produce hydrogen gas and a substantially fluid nongaseous product.
A complete understanding of the present invention may be obtained by reference to the accompanying drawings when considered in conjunction with the following detailed description, in which:
The present invention provides methods and apparatus for contacting a chemical hydride fuel with an aqueous reagent to generate hydrogen gas and nongaseous products.
The invention provides systems and methods for hydrogen generation by the hydrolysis of a solid fuel. In a preferred embodiment, the system includes a liquid reagent storage region, a reaction chamber adapted to contain at least one solid fuel capable of generating hydrogen upon contact with the liquid reagent, and a liquid distributor disposed in the reaction chamber for contacting the solid fuel with the liquid reagent in the reaction chamber to produce hydrogen gas and a substantially fluid nongaseous product.
In another embodiment, apparatus are provided for hydrogen generation by the hydrolysis of a solid fuel including a storage area adapted to contain a liquid reagent, a reaction chamber adapted to contain a solid fuel capable of generating hydrogen upon contact with the liquid reagent, the storage area bounded by at least one moveable wall, and a liquid distributor disposed in the reaction chamber for contacting the solid fuel with the liquid reagent in the reaction chamber to produce hydrogen gas and a substantially fluid nongaseous product.
The present invention further provides methods of generating hydrogen gas by a hydrolysis reaction utilizing a solid fuel capable of generating hydrogen and a substantially fluid nongaseous product when contacted with a liquid reagent. A liquid reagent is provided, and the liquid reagent and the solid fuel are contacted in a reaction chamber in which a liquid distributor is disposed, wherein such contact generates hydrogen gas and a substantially fluid nongaseous product, and the liquid distributor is configured to move through the substantially fluid nongaseous product.
In one embodiment, the present invention provides methods of generating hydrogen gas by a hydrolysis reaction utilizing a solid fuel capable of generating hydrogen and a substantially fluid nongaseous product when contacted with a liquid reagent. A liquid reagent is provided, and the liquid reagent and the solid fuel are contacted in a reaction chamber bounded by at least one hydrogen separator and in which a liquid distributor is disposed, wherein such contact generates hydrogen gas and a substantially fluid nongaseous product and movement of the liquid distributor in the reaction chamber exposes additional hydrogen separators.
In another embodiment, the present invention provides methods of operating a power module, by providing a power module having a hydrogen gas inlet in communication with a hydrogen gas outlet of an associated hydrogen generator. A solid fuel capable of generating hydrogen and a substantially fluid nongaseous product when contacted with a liquid reagent is provided. A liquid reagent is provided, and the liquid reagent and the solid fuel are contacted in a reaction chamber in which a liquid distributor is disposed, wherein such contact generates hydrogen gas and a substantially fluid nongaseous product. In a preferred embodiment, water generated as a product in a fuel cell power module is transported back to the liquid reagent storage chamber of the hydrogen generator.
As used herein, the term “nongaseous” comprises solid and liquid forms; the nongaseous products may be a mixture of solid and liquid materials and may comprise a metal salt product.
The preferred chemical hydride fuel components for the present invention are chemical hydrides in solid form. These chemical hydrides may be utilized in mixtures, but are preferably utilized individually. The term chemical hydrides as used herein includes the alkali and alkaline earth metal hydrides and boron hydrides; these compounds react with water to produce hydrogen gas and a metal salt, the nature and composition of which depends on the nature of chemical hydride.
The term “solid form” encompasses any dry or substantially dry form, including powder, granules, monoliths, or pellets.
The alkali and alkaline earth metal hydrides have the general formula MHn wherein M is a cation selected from the group consisting of alkali metal cations such as sodium, potassium or lithium and alkaline earth metal cations such as calcium, and n is equal to the charge of the cation. Examples of suitable metal hydrides, without intended limitation, include NaH, LiH, MgH2, and the like. The alkali and alkaline earth metal hydrides typically produce metal hydroxide salts and hydrogen gas when hydrolyzed, for example, the reaction of sodium hydride with water produces hydrogen gas and sodium hydroxide, though the products are not limited to metal hydroxide salts.
The terms “boron hydride” or “boron hydrides” as used herein include boranes, polyhedral boranes, and anions of borohydrides or polyhedral boranes, such as those provided in co-pending U.S. patent application Ser. No. 10/741,199, entitled “Fuel Blends for Hydrogen Generators,” filed Dec. 19, 2003 (U.S. Pat. Publ. No. 2005/0132640), the entire disclosure of which is hereby incorporated herein. Suitable boron hydrides include, without intended limitation, the group of borohydride salts M(BH4)n, triborohydride salts M(B3H8)n, decahydrodecaborate salts M2(B10H10)n, tridecahydrodecaborate salts M(B10H13)n, dodecahydrododecaborate salts M2(B12H12)n, and octadecahydroicosaborate salts M2(B20H18)n, among others, where M is a cation selected from the group consisting of alkali metal cations, alkaline earth metal cations, aluminum cation, zinc cation, and ammonium cation, and n is equal to the charge of the cation; and neutral borane compounds, such as decaborane(14) (B10H14); ammonia borane compounds of formula NHxBHy, wherein x and y independently=1 to 4 and do not have to be the same, of formula NHxRBHy, wherein x and y independently=1 to 4 and do not have to be the same, and R is a methyl or ethyl group, and of formula NH3B3H7, and dimethylamine borane (NH(CH3)2BH3). For the above-mentioned boron hydrides, M is preferably sodium, potassium, lithium, or calcium. The boron hydrides typically produce a boron-oxygen salt and hydrogen gas when hydrolyzed. For example, the reaction of an alkali metal borohydride with water as shown in Equation (1) produces a hydrated alkali metal metaborate which may be represented by formula MBO2.nH2O, though other products may be produced. For sodium borohydride (NaBH4), n preferably is 2; however, n is variable and is determined by the temperature and the borohydride salt, among other factors.
The chemical hydride may be anhydrous or hydrated and preferably contains less than about 50 wt % water. The hydrated forms of certain borohydride salts, notably sodium borohydride, exist at low to moderate temperatures. For example, sodium borohydride dihydrate (NaBH4.2H2O, 51.2 wt % NaBH4 and 48.8 wt % H2O) is formed at temperatures below 36.4° C., potassium borohydride trihydrate exists at temperatures below 7.5° C., and potassium borohydride monohydrate exists at temperatures below 37.5° C.
A metal borohydride fuel component may be combined with a solid stabilizer agent, preferably one selected from the group consisting of metal hydroxides, anhydrous metal metaborates, hydrated metal metaborates, and mixtures thereof. Solid stabilized fuel compositions comprising about 20 to 99.7 wt-% borohydride and 0.3 to 80 wt-% hydroxide salts are disclosed in co-pending U.S. patent application Ser. No. 11/068,838 entitled “Borohydride Fuel Composition and Methods” filed on Mar. 1, 2005, the disclosure of which is incorporated by reference herein in its entirety.
The aqueous reagent may be water or may comprise a soluble reagent in an aqueous solution. The aqueous reagent may be an aqueous acidic solution, i.e., a reagent having a pH less than about 7. Suitable acidic solutions include, but are not limited to, inorganic acids such as the mineral acids hydrochloric acid (HCl), sulfuric acid (H2SO4), and phosphoric acid (H3PO4), and organic acids such as acetic acid (CH3COOH), formic acid (HCOOH), malic acid, maleic acid, citric acid, and tartaric acid, among others. The acidic reagents may also comprise a combination of organic and/or inorganic acids. Different acids have different characteristics such as solution density and viscosity so the choice of acid may be different for various applications. Preferably, the liquid reagent is a solution containing the acidic reagent in a range from about 0.1 to about 40 wt %. In some embodiments, the liquid reagent is an aqueous solution with a water concentration in the range of about 44 to about 52 molar (M) water, preferably about 46 to about 50 M water and most preferably about 48 M water (in comparison, pure water can be considered to have a water concentration of about 55 M water) and has a pH less than 7. Higher reagent concentrations may be stored through the use of water recaptured from hydrogen consumption devices, such as fuel cells. For instance, if about 50% of the water exhausted at the cathode of a fuel cell were recovered the concentration of acid might be increased by about 25%, depending on fuel cell efficiency and acid concentration.
The liquid reagent may be a transition metal solution, i.e., a solution containing a water soluble transition metal salt, for example, the chloride salts of cobalt (COCl2), nickel (NiCl2), or copper (CuCl2). In such cases, as the reagent solution contacts the borohydride, the metal ion is typically reduced by the borohydride and deposited as metal particles or metal boride compounds in the solid borohydride contained within the reaction chamber, and accumulate in the reaction chamber as the borohydride is consumed. Since these materials can also catalyze hydrolysis of borohydride, the increased concentration of metal catalyst with increased time of operation ensures that the borohydride fuel is completely converted to hydrogen. Alternatively, the hydrogen generator may operate by initially feeding a transition metal solution to the borohydride fuel to accumulate metal particles or metal boride compounds in the solid fuel, and then feeding only water or an acidic reagent to the borohydride fuel to react with the remaining fuel.
In hydrogen generation systems in accordance with embodiments of the present invention, hydrogen is produced by contacting a solid chemical hydride fuel with a liquid reagent to transform the chemical hydride fuel into hydrogen gas and an oxidized product which is typically a metal salt or oxide compound (“product” or “discharged fuel”). The rate of hydrogen generation can be regulated by controlling the contact between the liquid reagent and the solid chemical hydride. The hydrogen generation reaction can be stopped by preventing contact between the liquid reagent and the solid chemical hydride.
Preferred embodiments of the present invention provide hydrogen generation systems in which a liquid reagent distributor is disposed within a reaction chamber containing the chemical hydride compound in solid form. For the method of the present invention, it is preferred that the product produced by the contact of the liquid reagent and solid chemical hydride be substantially fluid. In this context, the term “substantially fluid” means a product or products in a liquid or slurry state at least temporarily such that the product or products produced can pass through channels in the liquid reagent distributor to a “trailing” edge, i.e., the side opposite or distal to unreacted solid fuel, and facilitate the movement of the liquid reagent distributor through the remaining unreacted fuel; the product does not need to remain fluid after passage through the channels, and may solidify over time. In this manner, the leading surface of the liquid reagent distributor is exposed to unreacted fuel.
Preferably, the hydrogen generation systems allow volume exchange such that the products can occupy the space originally occupied by the solid chemical hydride and/or liquid reagent.
The state of discharged fuel and distribution of products can be controlled by the selection of particular liquid reagents, reagent concentration, and the ratio of liquid reagent to solid chemical hydride, for example, as shown in Table 1 for the reaction of hydrochloric acid with a mixture comprising about 87 wt-% sodium borohydride and about 13 wt-% sodium hydroxide.
Additionally, controlling the temperature at the reaction site and of the byproduct will affect the hydration and physical state of the product. For example, the hydrolysis of sodium borohydride with water yields a mixture of sodium metaborate hydrates, including the tetrahydrate (4 waters per boron on a molar basis), the dihydrate (2 waters per boron on a molar basis), and the hemihydrate (0.5 waters per boron on a molar basis). At temperatures around about 53° C., the tetrahydrate will melt in its waters of hydration producing a fluid product.
Exemplary embodiments of liquid reagent distributors useful in embodiments of the present invention are illustrated in
The surface of the liquid distributor adjacent to outlets 155 may comprise a metal that promotes the hydrolysis of a chemical hydride. Suitable transition metal catalysts for the generation of hydrogen from a metal hydride solution include metals from Group IB to Group VIIIB of the Periodic Table, either utilized individually or in mixtures, or as compounds of these metals. Representative examples of these metals include, without intended limitation, transition metals represented by the copper group, zinc group, scandium group, titanium group, vanadium group, chromium group, manganese group, iron group, cobalt group and nickel group, among others. Examples of useful catalyst metals include, without intended limitation, ruthenium, iron, cobalt, nickel, copper, manganese, rhodium, rhenium, platinum, palladium, and chromium, for example, used individually or as mixtures. The preparation of supported catalysts is taught, for example, in U.S. Pat. No. 6,534,033 entitled “System for Hydrogen Generation,” wherein the catalyst metal is deposited on, or bound to, a support structure.
Referring now to
The liquid distributor 150 may further comprise a heating element that may be run continuously or intermittently to accelerate the rate of hydrolysis or maintain the products in a substantially liquid state, for example. The heating element may provide for increased efficiency in system startup.
Referring now to
Reaction chamber 110 comprises a liquid distributor 150 disposed axially therein, an actuator 170, and a hydrogen outlet 195. The hydrogen generator may be a single discrete unit or may be comprised of separable components; for instance, the reaction chamber 110 may be removably attached to the liquid reagent storage chamber 120, and thus one or both of these compartments are refillable or replaceable.
Liquid reagent regulator 140 may comprise, for example, a pump including, but not limited to, a peristaltic pump, a piezoelectric pump, a piston pump, a diaphragm pump, a centrifugal pump, or an axial flow pump; or a valve including, but not limited to, a solenoid valve, a ball valve, a pinch valve, or a diaphragm valve.
Hydrogen outlet line 195 connects reaction chamber 110 to a power module comprising a fuel cell or hydrogen-burning engine for conversion to energy, or to a hydrogen storage device, such as a balloon, a gas cylinder or a metal hydride. A hydrogen separator 190 is arranged such that the hydrogen generated in the reaction chamber passes through the separator 190 to separate the hydrogen gas from the solids and liquids within the reaction chamber 110 before the hydrogen gas is removed via the hydrogen outlet line 195. The separator is preferably in communication with hydrogen outlet line 195, and can be incorporated at the inlet. The separator may be a gas permeable membrane or filter that is preferably substantially impermeable to liquids and solids. “Substantially impermeable” in this context means preferentially allowing passage of gases relative to the passage of solids and/or liquids or, in preferred cases, allowing passage only of gases. Examples of suitable gas permeable membranes include materials that are more permeable to hydrogen than to a liquid such as water, for example, silicon rubber, polyethylene, polypropylene, polyurethane, fluoropolymers or any hydrogen-permeable metal membranes such as palladium-gold alloys. The gas permeable membrane is preferably microporous and hydrophobic.
The actuator 170 provides mechanical energy to move the liquid distributor 150. Actuators suitable for use in embodiments of the invention include mechanical springs, air springs, magnetic drives, and screw drives. Preferably, the actuator is a mechanical spring that moves the liquid distributor 150 by either being expanded or compressed beyond its relaxed, neutral state. Nonlimiting examples of useful springs include tension/extension springs, compression springs, helical springs, and coil springs. In some configurations, the spring will be extended during operation, and will “push” the liquid distributor through the reaction chamber 110 as provided in
The actuator may be used to provide unilateral movement such that only the liquid distributor 150 moves; in such cases it would be attached to a fixed, i.e., non-moveable, wall. In these cases, wall 180 (as in
Alternatively, the actuator 170 may exert a pressure on both the liquid distributor 150 and the wall 180, or separate actuators may be used for movement of liquid distributor 150 and the wall 180. Preferably, the liquid reagent contained within chamber 120 occupies a finite volume, that is, it is incompressible, and wall 180 cannot move until the regulator 140 operates, that is, a valve is opened or a pump is turned on. In such a design wherein both the liquid distributor 150 moves through the reaction chamber and the wall 180 moves to diminish the volume of the liquid storage chamber 120, the relative movements do not need to be symmetric. That is, one or the other of wall 180 and liquid distributor 150 may move a greater linear distance from its respective origin than the other.
An optional inlet 200 may be included to allow water generated by the hydrogen fuel cell or collected from a condenser or dehumidifier elsewhere within the power system to be passed into the liquid reagent reservoir 120 to be contained within either the reservoir 120 or an inner container. This allows the water to be collected as it is generated.
As illustrated in
Referring now to
The actuator 330 is preferably a twisted spring, or a rotary spring. The walls 310 and 320 may independently be flexible or rigid. One or both of first wall 310 and second wall 320 may comprise a rotary sliding wall, for example, with a sliding gasket seal against the outer wall.
In reference to
The liquid reagent regulator 140 may comprise a separable pump wherein a pump head resides in one of the fuel cartridge or fuel cell power module and a controller resides in the other of the fuel cartridge or fuel cell power module. The controller comprises a motor or an electrical contact. In general, peristaltic and piston pumps operate through the use of a pump head comprised of a series of fingers in a linear or circular configuration or at least one piston which can compress the fuel line; the fingers may be in a variety of configurations and alternatively referred to as rollers, shoes, or wipers. The compression of the fuel line by the fingers forces the liquid through the line; when the line is not compressed and open, fluid flows into the fuel line. A diaphragm pump configuration comprises a diaphragm in the wall of fuel line, check valves on the upstream and downstream sides of the diaphragm, and a pump head. In general, diaphragm pumps operate through the use of a pump head comprised of a series of cams in a linear or circular configuration or at least one piston which can compress the diaphragm; the compression of the membrane by the fingers forces the liquid through the line; when the membrane expands and is not compressed, fluid is drawn into the fuel line. The cams may be in a variety of configurations and alternatively referred to as rollers, shoes, or wipers. The check valves constrain and control the directional flow through the diaphragm and fuel line.
The liquid reagent stored within chamber 120 is compressed by a wall 180. In this embodiment, the wall 180 moves in response to movement of the liquid distributor 150 and actuator 170. This wall 180 cannot move without operating a liquid reagent regulator, e.g., a valve is opened or a pump is turned on. Suitable liquid reagent regulators include, but are not limited to, peristaltic pumps, piezoelectric pumps, piston pumps, diaphragm pumps, centrifugal pumps, axial flow pumps, solenoid valves, ball valves, pinch valves, and diaphragm valves.
The chamber 110, liquid distributor 150, actuator 170, and liquid reagent chamber 120 may be contained within an inner housing 420 and an outer housing 410. The region bounded by the inner housing 420 and the outer housing 410 forms a gas ballast region 430. During startup of the hydrogen generation system, the gas ballast region 430 can provide stored hydrogen to supply the demand of the power module or other hydrogen device prior to onset of hydrogen generation. This ballast hydrogen may comprise hydrogen generated from residual fuel components after the liquid reagent feed is stopped or hydrogen previously unconsumed by the hydrogen device. Hydrogen is fed to the hydrogen device from one or more hydrogen outlets 195.
At least one hydrogen separator 190 is present in the wall of inner housing 420, and may be located at any position along the length of the generator. Preferably, at least a portion of at least one hydrogen separator 190 is located on the distal side of the liquid distributor and at least a portion of at least one hydrogen separator 190 is located on the proximal side of the liquid distributor. In some configurations, a single separator 190 may be present; in other configurations, a plurality of separators 190 are present. The size of the generator, intended use, and available surface area of inner housing 420 will determine the appropriate number of separators.
Referring to
The liquid reagent is fed from storage chamber 120 though outlet 135a which is connected via conduit 135 (the external portion of this conduit is not illustrated in
An embodiment of the invention as shown in
While the present invention has been described with respect to particular disclosed embodiments, it should be understood that numerous other embodiments are within the scope of the present invention. Accordingly, it is not intended that the present invention be limited to the illustrated embodiments, but only by the appended claims.
