Patent Application
20090101520
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. However, its widespread use can be complicated by the difficulties in storing the 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. The gravimetric hydrogen storage density of sodium borohydride is 10.8% and lithium borohydride is 18%. 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 apparatus and methods for hydrogen generation by the hydrolysis of a solid fuel, and methods of operating a power module. The apparatus include a reaction chamber bounded by at least one moveable wall and adapted to contain at least one solid fuel capable of generating hydrogen upon contact with the solid fuel, and at least one liquid outlet for contacting the solid fuel with a liquid reagent in the reaction chamber to produce hydrogen gas and a product. The apparatus further includes a hydrogen outlet line in communication with the reaction chamber, and a hydrogen separator adapted to prevent solids and liquids in the reaction chamber from entering the hydrogen outlet line.
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 a liquid reagent to generate hydrogen gas and other nongaseous products.
The present invention provides apparatus for hydrogen generation by the hydrolysis of a solid fuel. In a preferred embodiment, the apparatus include a liquid reagent storage region, a reaction chamber bounded by at least one moveable wall and adapted to contain at least one solid fuel capable of generating hydrogen upon contact with the solid fuel, and at least one liquid outlet (distribution point) for contacting the solid fuel with the liquid reagent in the reaction chamber to produce hydrogen gas and a product. The apparatus further includes a hydrogen outlet line in communication with the reaction chamber, and a hydrogen separator adapted to prevent solids and liquids in the reaction chamber from entering the hydrogen outlet line.
The present invention further provides methods of generating hydrogen gas by a hydrolysis reaction utilizing a solid fuel capable of generating hydrogen and a product when contacted with an 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 moveable wall, wherein such contact generates hydrogen gas and a 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 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 moveable wall and in which at least one liquid outlet is disposed, wherein such contact generates hydrogen gas and a product and movement of the movable wall exposes unreacted solid fuel to the at least one liquid outlet in the reaction chamber.
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 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 bounded by at least one moveable wall, wherein such contact generates hydrogen gas and a product.
As used herein, the term “nongaseous” comprises solid and liquid forms; the nongaseous products may be a mixture of solid and liquid materials, and comprise a metal salt product.
The preferred chemical hydride fuel components of 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 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. Appl. 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(B10H18)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. These metal hydrides may be utilized in mixtures, but are preferably utilized individually. 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.n H2O, 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 % water) 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 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. 2, 2005, the entire disclosure of which is incorporated by reference herein in its entirety.
The liquid reagent may be water or may comprise a soluble catalyst in an aqueous solution. The liquid reagent may be an aqueous acidic solution, i.e., a reagent having a pH less than about 7. Suitable acidic reagents include, but are not limited to, both inorganic acids such as hydrochloric acid (HCl), sulfuric acid (H2SO4), and phosphoric acid (H3PO4), and organic acids such as acetic acid (CH3COOH), formic acid (HCOOH), maleic acid, malic 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 acidic reagent is a solution containing the acidic reagent in a range from about 0.1 to about 40 wt %. In some embodiments, the acidic 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 55 M water) and has a pH less than 7.
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.
A plurality of liquid reagents may be used in embodiments of the present invention. The plurality of liquid reagents may be fed concurrently; for example, a first liquid reagent comprising a metal salt solution can be fed in combination with a second liquid reagent comprising an acidic reagent; or a first liquid reagent comprising water may be fed in combination with a concentrated acidic reagent. Alternatively, the hydrogen generator may operate by initially feeding a first liquid reagent comprising a transition metal solution to the boron hydride fuel to accumulate metal particles or metal boride compounds in the solid fuel, and then feeding a second liquid reagent comprising water or an acidic reagent to the boron hydride 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 acidic reagent and the solid chemical hydride. The hydrogen generation reaction can be stopped by preventing contact between the acidic reagent and the solid chemical hydride.
Preferred embodiments of the present invention provide hydrogen generation systems in which relative movement between a liquid reagent outlet and a solid fuel is provided. For the methods of the present invention, it is preferred that the discharged fuel move away from and unreacted fuel move to stationary liquid reagent distribution points. 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.
Referring now to
Wall 130 can be configured to prevent contact of the solid hydride fuel with the liquid reagent, and is preferably a plunger or disk separator that will move along during the hydrogen generation process. Wall 130 can be replaced with or supplemented with acid adsorbents; suitable acid adsorbents include, but are not limited to, polymeric adsorbents such polyacrylic acid polymers, resins, silicates, carbons, and metal hydroxide salts such as sodium hydroxide and potassium hydroxide.
The hydrogen generator further comprises an actuator 170 configured to apply a bias to the reaction chamber 110.
Either or both of the chambers 110 and 120 may further comprise an inner container; wall 130 need not be used in such a design. Suitable liquid-tight materials for the inner container include, but are not limited to, nylon; polyurethane; polyvinylchloride (PVC); polyethylene polymers, such as low density polyethylene (LDPE), linear low density polyethylene (LLDPE), high density polyethylene (HDPE), and ethylene-vinyl acetate copolymers (EVA); natural rubber; synthetic rubber; and metal foil. The inner container for reaction chamber 110 may further comprise at least one gas permeable membrane or filter that is preferably substantially impermeable to liquids and solids. The term “substantially” 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. Ballast hydrogen may be stored within the hydrogen generator, for instance, in void space within reaction chamber 110, either internal or external to any optional inner container.
Reaction chamber 110 is bounded on one side by a porous wall 175 and preferably by a wall 130 on the opposite side. The porous wall 175 is configured to permit the passage of hydrogen gas from the reaction chamber 110 to a hydrogen outlet 160, and may be a screen plate.
Preferably, at least one liquid outlet 150 is initially at a position that is about ⅓ to ¼ of the total depth of reaction chamber 110 from wall 130. That is, in an exemplary example, for a 4 inch long reaction chamber measured from wall 175 to wall 130, at least one liquid outlet is initially positioned from about 3⅓ inches to about 2½ inches from the wall 130. As the reaction chamber 110 changes during operation, for example, moves and/or expands, at least one liquid outlet preferably remains fixed within the hydrogen generator (i.e., its absolute position is unchanged) while its relative position with respect to unreacted fuel within the reaction chamber 110 changes.
The hydrogen generator may further comprise a liquid reagent regulator to deliver the liquid reagent to the reaction chamber. Liquid reagent regulators include, for example, pumps such as, but not limited to, peristaltic pumps, piezoelectric pumps, piston pumps, diaphragm pumps, centrifugal pumps, and axial flow pumps, or valves such as, but not limited to, solenoid valves, ball valves, pinch valves, and diaphragm valves.
Hydrogen outlet 160 preferably connects to a power module comprising a fuel cell or hydrogen-burning engine to deliver hydrogen for conversion to energy, or to a hydrogen storage device, including balloons, gas cylinders or metal hydrides. Preferably, a gas permeable membrane is in communication with hydrogen outlet line 160, preferably at the inlet. The hydrogen generated in the reaction chamber 110 passes through the membrane to separate the hydrogen gas and maintain solids and liquids within the reaction chamber. 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.
Actuator 170 is in communication with wall 175 and provides mechanical energy to move the chemical hydride fuel in the reaction chamber 110 by either being expanded or compressed beyond its relaxed, neutral state. Nonlimiting examples of useful actuators include tension/extension springs, compression springs, helical springs, and coil springs. In some configurations, the actuator will be extended during operation, and will “push” the wall 175 such that the solid fuel contained within the reaction chamber 110 moves in relation to at least one liquid distribution point as provided in
An optional inlet may be included to allow water generated by the hydrogen fuel cell or collected from a condenser or source elsewhere within the power system to be fed to the liquid reagent reservoir 120, or to an optional water storage compartment. By feeding recovered water to the liquid reagent reservoir 120, a liquid reagent stored in a concentrated form can be diluted prior to contact with the solid chemical hydride. Alternatively, the recovered water could be fed concurrently with a liquid reagent as described herein.
The components of a hydrogen generation system in accordance with the teachings herein may be contained with an outer housing to form a fuel cartridge 200 suitable to use with a fuel cell power system, for example. In reference to
Referring now to
Water accumulated in a hydrogen fuel cell system 270 can be recycled to the hydrogen generation system 300 via conduit line 250. The conduit line 250 may connect directly to the liquid reagent storage chamber 120, or may transport the water to a mixing element 230. The mixing element 230 allows the water to be combined with the liquid reagent to provide the active concentration needed for hydrogen generation.
The mixing element 230 may comprise a chamber or junction in the conduits where the liquid reagent and the fuel cell water can be delivered using separate pumps and mixed to the desired active concentration. The mixing element 230 may alternately comprise a three-way valve, such as a solenoid, in which the desired water/liquid reagent ratio is controlled by the 3-way valve switching frequency.
In these and other embodiments of the fuel cartridge according the present invention, the liquid reagent regulator 240 comprises a separable pump wherein a pump head resides in one of the fuel cartridge or fuel cell power system and a controller resides in the other of the fuel cartridge or fuel cell power system. 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.
In reference to the previous figures, a preferred method for generating hydrogen using a generator as described herein comprises conveying a liquid reagent from a storage chamber 120 through conduit 140 using a liquid reagent regulator 240 to a reaction chamber 110 containing a solid chemical hydride fuel whereupon it reacts with the fuel to create hydrogen gas and a nongaseous product. Preferably, the solid chemical hydride fuel comprises a mixture of sodium borohydride and sodium hydroxide (preferably containing from about 87 to about 95 wt % sodium borohydride, and from about 5 to about 13 wt % sodium hydroxide). Preferably the liquid reagent is an acidic reagent comprised of hydrochloric acid, phosphoric acid, or sulfuric acid.
As the liquid reagent is removed from the chamber 120 and hydrogen is generated in the reaction chamber 110, the actuator 170 forces the reaction chamber to move into the volume created by removal of the liquid reagent while the absolute position of the at least one liquid reagent outlet 150 remains unchanged. Thus, unreacted solid fuel is moved to the liquid reagent outlet 150. As hydrogen is produced, it preferably passes through the bed of unconverted solid fuel (i.e., “fresh fuel”) before passing through the gas permeable membrane 260 and the hydrogen outlet 160. The unconverted solid fuel bed can thus act as a “scrubber” to remove any liquid reagent entrained in the gas stream via reaction with the unconverted solid fuel. The hydrogen generation system is configured in a volume exchanging configuration in which the reaction products can occupy the space originally occupied by the liquid reagent.
In a preferred embodiment of a method for hydrogen generation, the system further comprises a battery to control a liquid reagent regulator. For example, the liquid reagent is delivered using a liquid reagent regulator such as a peristaltic pump controlled by a PLC circuit. In operation, when the charge level of the battery drops to a defined level, for example, about 50% of its fully charged capacity and the system pressure drops below a set point, for example, less than about 1 psig, the liquid reagent regulator will be operated to provide the liquid reagent at a specified feed rate, for example, about 16 mL/h. When the system pressure exceeds a second set point, for example, above about 5 psig, the controller will signal the liquid reagent regulator to shut off.
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.
The invention was made with Government support under Contract N00164-06-C-6058 awarded by the United States Navy. The United States Government has certain rights in this invention.
