Patent Application
20080289951
The invention relates to the thermochemical generation of hydrogen and/or oxygen from water using a hybrid sulfur-ammonia cycle. The cycle can also be employed for the production of oxygen from air.
One of the leading alternatives to current fossil-based transportation fuels is hydrogen (H2). H2 satisfies energy needs from transportation to electric power generation, is least polluting, and lends itself to distributed production anywhere. H2 is currently the primary fuel used in most fuel cell systems. Fuel cells are being developed for a variety of applications including distributed energy use, backup power generation, as an alternative to batteries for portable power source for consumer electronics, and in automobile power plants. However, H2 is not an energy source; rather an energy carrier since H2 must be produced using primary energy sources, such as fossil fuels, nuclear and solar energy.
H2 can be produced from many feedstocks. For the reasons of availability, environmental acceptability and long-term energy security, it is highly desirable to produce H2 from renewable non-carbonaceous feedstocks and primary energy sources—e.g., water and solar energy. However, to date, the development of a cost-effective and energy efficient process for generating H2 from water using solar energy has remained a challenge hindering the realization of the future H2 economy.
Splitting water (H2O) for hydrogen production can be accomplished by direct thermolysis, electrolysis, photocatalysis, or photoelectrocatalysis of H2O. Obstacles to direct water splitting are that thermolysis requires very high temperatures (i.e. ΔH=285.9 kJ/mol, T>2500° C.) and problem with hydrogen (H2) and oxygen (O2) recombination. Electrolysis requires ΔE>1.23 V and is highly costly due to a need for Pt electrodes. Photocatalysis requires high band gap Eg>3.0 eV material as catalyst and plagued with poor solar UV availability and problems with the H2 and O2 recombination. Photoelectrocatalysis requires bias voltage and the efficiency of the photocatalysts available are low.
Thermochemical water splitting cycles (TCWSCs) offer an alternative to direct water splitting. TCWSCs employ two or more chemical reactions forming a closed cycle wherein the overall reaction is: H2O═H2+½O2. Typically, heat is the primary energy input. The basic Principle with the use of TCWSCs is to break down the total water splitting energy required into several steps with each step requiring only a part of the total energy necessary for water decomposition (ΔH=285.9 kJ/mol).
Pseudo TCWSCs have been looked at before that include: metal (M)/metal oxide (MO) systems such as MxOy=xM+y/2O2; ΔH1>ΔHw; (M=Zn, Li, Na, Mg, K, Ca, etc.), and xM+yH2O=MxOy+yH2; ΔG<0, as well as the CO2/CO system: CO2═CO+½CO; ΔH1>ΔHw, and CO+H2O═H2+CO2; ΔG<0. For these processes, the oxygen production step is tougher to carry out than direct H2O splitting.
There are three basic steps of any TCWSC: 1) H2 production; ΔH1, 2) O2 production; ΔH2, and 3) separation (ΔHs) and H2O pumping (ΔE). The efficiency of a TCWSC will be low if: ΔH1˜ΔHw>>ΔH2; or ΔH2˜ΔHw>>ΔH1, and will be high whenever: ΔH1˜ΔH2.
In order to achieve high cycle efficiency, the energy requirements for two major steps (i.e. hydrogen and oxygen production) should be close. In the sulfur-family TCWSCs, the thermal decomposition of H2SO4 (for oxygen production) is involved which is highly endodermic consuming 69% of the total water splitting energy required. In the solar driven TCWSCs, this step can be accomplished by consuming the thermal energy portion of solar radiation while the remaining 31% of solar spectrum can be utilized in the H2 production step. Therefore, matching a TCWSC to the solar spectrum is the critical element of the design of a superior water-splitting cycle. In other words, hydrogen generating step of a prospective solar thermochemical water splitting cycle should be able to utilize ΔE=0.52 V or about 33% of the total ΔE needed for the H2O splitting (about 1.52 V by water electrolysis). On the other hand, the oxygen production step of the cycle should be able to utilize the remaining 67% of the total energy needed for splitting water. For the solar spectrum, the wavelength border or split for this energy partitioning is at about 1=650 nm (i.e. 33% photonic energy falls in the wavelength region below 650 nm while 67% of solar thermal energy is at wavelengths longer than 650 nm). In some thermochemical water splitting cycles such as the Westinghouse hybrid cycle, the energy required for hydrogen evolution is low (only 0.17 V or about 1/10 of the total energy needed for water splitting). This mismatch of energy required for evolving H2 and O2 steps of this and some other disclosed cycles renders these water splitting cycles less efficient for hydrogen production.
A method for the production of H2 via thermochemical water splitting includes the steps of providing an ammonium sulfite compound, dissolving the ammonium sulfite in water, and oxidizing the ammonium sulfite in the presence of water, wherein H2 is produced as a reduction product associated with the oxidation of ammonium sulfite. The reaction temperature for the oxidizing step is generally between 275 K and 375 K, and is preferably between 275 K and 325 K.
In one embodiment, a catalyst is included. The catalyst can be selected from metal sulfides such as CdS, CdSe, CdTe, ZnS, Cu2S, RuS2 and mixtures thereof, metal oxides such as TiO2, RuO2, and WO3, and doped transition metal oxides such as Pt/CdS, Pt/FiO2 and Pt/TiO2-xNx. Other catalysts include metal sulfides together with particles comprising at least one noble metal selected from the group consisting of Pt, Pd, Ir, Ag, Au, Rh and Ru.
In one embodiment the oxidizing step comprises photooxidation. UV and/or visible light for the photooxidation can be provided by solar radiation. In this embodiment the method can further comprise the steps of splitting the infrared portion of solar radiation from its UV and visible light portions, directing the UV and visible light portions of the solar radiation into a photocatalytic reactor in order to drive the oxidizing step of the cycle, and directing the infrared portion of solar spectrum into a thermocatalytic reactor to provide heat required for the decomposition of ammonium sulfate (NH4)2SO4(aq) and sulfuric acid H2SO4(l) and generation of oxygen.
In one embodiment the oxidizing step proceeds exclusive of any catalyst, wherein the oxidizing step is performed in the presence of UV light. For example, the oxidizing step can comprise photooxidation, where electrical energy is used to generate UV and drive photooxidation reaction The electricity may come from off peak electricity, nuclear power, hydroelectric power, photovoltaic cells or wind turbines.
The method is preferably practiced as a closed cycle. When an oxidation product is ammonium sulfate, the method can further comprising the steps of thermally decomposing the ammonium sulfate to ammonia and sulfur dioxide and oxygen, and recovering and recycling the ammonia and sulfur dioxide to react with water to regenerate the ammonium sulfite.
In another embodiment of the invention, a method of generating oxygen (O2) is provided. The method comprises the steps of providing an ammonium sulfite compound, oxidizing the ammonium sulfite to produce ammonium sulfate in the presence of air, wherein O2 is separated from nitrogen in the air. A reaction temperature for the oxidizing step is generally between 275 K and 375 K and is preferably between 275 K and 325 K.
Ammonium sulfate can be decomposed into ammonia, water, sulfur dioxide, and oxygen. Separating oxygen from sulfur dioxide, the process can generate oxygen from air.
A system for generating hydrogen (H2) comprises a reaction vessel having an aqueous ammonium sulfite solution therein, at least one energy source coupled to the reaction vessel for providing energy to drive the oxidation of ammonium sulfite, wherein hydrogen is produced, and collected. The energy source can be provided exclusively by electricity. In another embodiment a photocatalyst is needed, wherein the energy source comprises solar radiation or a UV (e.g. mercury lamp) or visible (e.g. xenox lamp) light source. In this embodiment the system can include a broadband reflective coating for splitting the infrared portion from the UV and visible light portions of the solar radiation, and optics for directing the UV and visible light portions to drive the ammonium sulfite oxidation reaction, and optics for directing said infrared portion to provide heat required for decomposition of ammonium sulfate and sulfuric acid.
There is shown in the drawings embodiments which are presently preferred, it being understood, however, that the invention can be embodied in other forms without departing from the spirit or essential attributes thereof.
a) is a simple flow diagram for an exemplary cycle according to the invention.
b) shows the schematic including flow dynamics of an exemplary sulfur-ammonia closed cycle-based hydrogen generation system according to the invention.
A first embodiment of the invention is a method for the production of H2 via a hybrid sulfur-ammonia (S—NH3) thermochemical water splitting cycle. The method comprises the steps of providing an ammonium sulfite compound, dissolving the ammonium sulfite with water, and oxidizing the aqueous ammonium sulfite solution, wherein H2 is produced as a water reduction product associated with the oxidation of ammonium sulfite. As used herein, although the ammonium sulfite compound referred to herein is generally (NH4)2SO3, it may be possible to utilize other ammonium sulfites, such as ammonium bisulfite (NH4HSO3) In a preferred embodiment of the invention, the oxidation is a photo oxidation reaction. Light for the photoxidation can be provide by a direct light source, such as solar energy, or indirectly from conversion of electrical energy to light, such as using a UV or visible light lamp. Electrical energy can be provided by a variety of sources, including low cost sources comprising wind driven, water driven (hydroelectric), or nuclear power.
The inventive method is major modification and significant improvement over the Westinghouse hybrid cycle (WHC). The well known WHC is a two-step hybrid thermochemical water splitting cycle which includes the following reactions:
SO2+H2O→H2+H2SO4 (Electrolytic step; 350 K)
H2SO4(l)→SO2(g)+H2O(g)+½O2(g) (Thermochemical step; 1123 K)
The advantages of Westinghouse cycle are that it is an all-fluid cycle and constitutes only two reactions. The high temperature thermochemical step accepts heat over a large span of temperature that makes matching the cycle to the sensible heat of a power source more practical. The side reactions for the WHC are not significant and the thermodynamic properties of all chemical species involved are known. The main disadvantage of the WHC is just that, it is a hybrid cycle and as such suffers from the scale-up issues inherent to all electrochemical systems. Electrochemical plants can only be scaled-up in modular fashion. Furthermore, cycle has been studied extensively by both Westinghouse and others but no new improvement have been introduced since the last Westinghouse flowsheet was devised until the present modification. Also, the low solubility of SO2 in water renders the hydrogen production step not very efficient. The low water solubility of SO2 makes separation of H2SO4 from water after the thermochemical step an energy intensive process. Moreover, low pH of SO2 solution can lead to the generation of elemental sulfur, further reducing the overall process efficiency.
Modifying the WHC to incorporate ammonia according to the invention makes it possible to attain very high hydrogen (or oxygen as described below) production efficiencies, thus reducing the overall process costs. Several embodiments are described for the oxidation of aqueous ammonium sulfite, most of which can be performed at or near room temperatures, making the inventive method more efficient and flexible, and highly compatible for interfacing with a wide range of external power sources.
By modifying the WHC cycle to employ ammonia-sulfur compounds instead of SO2, problems associated with SO2 are largely eliminated. The invention converts SO2 into SO32− which can be handled in the aqueous phase rather than the need to separate it into the gaseous phase as in the WHC. Both solubility and solution pH of (NH4)2SO3 are substantially higher than those of the aqueous SO2 solution used in the WHC. High solubility of (NH4)2SO3 not only increases the efficiency of SO2 electrolysis but also makes simple room temperature and ambient pressure operation possible. Significantly, unlike SO2, (NH4)2SO3 solution can be either electrolytically, photolytically, photoelectrochemically or photocatalytically oxidized (at near room temperatures and one atm pressure conditions) to generate hydrogen as follows:
(NH4)2SO3+H2O+electricity=H2+(NH4)2SO4 (1)
(NH4)2SO3+H2O+photocatalyst+sunlight=H2+(NH4)2SO4 (2)
(NH4)2SO3+H2O+UV light=H2+(NH4)2SO4 (3)
(NH4)2SO3+H2O+TiO2+UV light=H2+(NH4)2SO4 (4)
(NH4)2SO3+H2O+photoelectrode+sunlight=H2+(NH4)2SO4 (5)
These five possible alternative processes for the oxidation of aqueous ammonium sulfite make the sulfur-ammonia water splitting cycle more practical than the conventional WHC for hydrogen production because of its flexibility in employing a variety of input energy options. Reactions (1), (3) and (4) can be advantageously used when low-cost electric power is available, such as that available from wind or hydroelectric. Reaction (1) is a highly efficient electrochemical oxidation process with efficiencies exceeding 75%. Reactions (3) and (4) require conversion of electricity to UV (or visible) light prior to photochemical oxidization of (NH4)2SO3. However, these reactions are very efficient in photolytic oxidation of ammonium sulfite because light from a suitable lamp can provide radiation in a precise wavelength range that can be directly taken up by the solution with higher efficiencies. Reaction (3) does not require any catalyst, simplifying the cycle by eliminating the separation step required to remove the photocatalyst powder from the reaction products. Results shown in
Photocatalysts used in Reaction (3) include most low bandgap semiconductor photocatalysts, such as CdS, CdSe, CdTe, RuS2, RuO2 and the solid solution of CdSxSe1-x, CdSxTe1-x and CdSxSeyTe1-x-y. Photoelectrodes useful for the practice of this invention according to Reaction (5) include metals (e.g. Pt) or carbon coated with nanoparticles of semiconductor photocatalysts: RuS2, RuO2, Ru1-xFexS2, CdS, CdSe, CdTe, TiO2-xNxa, TiO2, among others. Photocatalysts can be doped with noble metals such as Pt, Ru, Pd, Os, Ir, Au and Ag or base and/or other metals such as Cu, Cr, Fe or Zr.
a) provides a simple flow diagram showing four (4) steps believed to be taking place for an exemplary sulfur-ammonia cycle for the production of hydrogen according to the invention. The exemplary sulfur-ammonia cycle includes the following steps shown with the preferred reaction temperatures for the respective reactions with the overall reaction being the splitting of water into hydrogen and oxygen. None of the compounds involved in the cycle are consumed except water:
A key step in the inventive method is the production of H2 and ammonium sulfate via oxidation of an aqueous ammonium sulfite solution, for which five (5) alternative embodiments are provided above (Reactions 1-5). The cycle uses the ammonium sulfate that is generated and thermally decomposes it into oxygen, ammonia and SO2. Ammonia and sulfur dioxide are then recovered and recycled, and subsequently reacted via Reaction (6) with water to regenerate the ammonium sulfite. Reactions (6) and (9) are well understood and can be carried out using conventional chemical processes.
b) shows the schematic including flow dynamics of an exemplary sulfur-ammonia closed cycle-based hydrogen generation system 150 according to the invention. System includes photoreactor 155. Photoreactor 155 receives light from light source 160 powered by an electrical generator 165, such as based on nuclear power, hydroelectric power, or wind driven power. Hydrogen is generated (Reaction 7) by photoreactor according to (NH4)2SO3(aq)+H2O→(NH4)2SO4(aq)+H2(g), with H2(g) collected by structure for collecting 158. The (NH4)2SO4(aq) (Reaction 8) is then thermochemically separated into 2NH3(g)+H2SO4(l) in reactor 168. A thermochemical step is performed by reactor 170 (Reaction 9) which decomposes H2SO4(l)→SO2(g)+H2O(g)+½O2(g). Chemical absorption takes place at reactor 175 (Reaction 6) SO2(g)+2NH3(g)+H2O(l)→(NH4)2SO3(aq) thus regenerating (NH4)2SO3(aq) used by photoreactor 155 using solely reaction products.
A broadband antireflection coating 210 can be used for this purpose. Such a coating provides very low reflectance over a broad range of wavelengths within the UV and visible spectra. With this approach, the infrared (thermal) portion of the solar resource is resolved by the receiver/photoreactor units 210/230 in the mirror field and concentrated into a high temperature thermocatalytic reactor 240 located within the tower for the decomposition of (NH4)2SO4 and sulfuric acid to produce oxygen. The photonic (UV and visible light) portion of the solar light is utilized for the hydrogen production via photocatalytic oxidation of aqueous (NH4)2SO3 solution into (NH4)2SO4 while water is reduced into hydrogen. Hydrogen is collected by structure for collecting H2 255. The utilization of both thermal heat and photonic energy increases the solar to hydrogen energy conversion efficiency of the cycle (absorbed light to chemical energy of hydrogen) by up to 50% or more as compared to known cycles.
In a related embodiment of the invention, oxygen is produced via a sulfur ammonia thermochemical cycle according to the invention. Thus, the sulfur ammonia thermochemical cycle is extended beyond production of hydrogen via water splitting, to the generation of oxygen from purified air by oxidizing a sulfur ammonium compound such as aqueous ammonium sulfite solution in the presence of purified air. This process can be described in the following reactions believed to be occurring, provided along with their nominal reaction temperatures:
A major advantage of this oxygen generation process is that heat is the primary energy requirement in the process which is mainly used in the decomposition of sulfuric acid for oxygen production (Reaction 13). For example, the inventive process can be advantageously applied in metallurgical industries where large quantities of oxygen are needed for the manufacture of iron and steel. Such metallurgic processes are known to generate large amounts of high temperature “waste” heat. Using the invention, the “waste” heat can be utilized as a heat source to provide the oxygen (from the air) required for the manufacture of iron or steel.
Experiments have been carried out for the oxidation step, Reaction (11). The results obtained demonstrate that aqueous ammonium sulfite solutions can be oxidized by air at ambient temperature and pressure to produce ammonium sulfate, thus extracting oxygen from the other component of purified air (nitrogen). The oxidation was found to proceed at a relatively fast rate at room temperature without the need for any added catalysts.
The present invention can significantly contribute to enhancing the quality of the environment by allowing hydrogen to become more competitive with hydrocarbon-based fuels. As noted in the Background, due to reasons including environmental acceptability it is highly desirable to produce H2 from renewable non-carbonaceous feedstocks and energy sources. However, prior to the invention lack of a cost-effective and energy efficient process for generating H2 from non-carbonaceous energy sources has hindered progress toward realization of the future hydrogen economy. Through providing a cost-effective and energy efficient process for generating H2 from non-carbonaceous feedstocks and energy sources the present invention will advance the hydrogen economy and thus enhance the quality of the environment by allowing hydrogen to begin replacing hydrocarbon-based fuels.
It should be understood that the Examples described below are provided for illustrative purposes only and do not in any way define the scope of the invention.
In order to verify the photochemical hydrogen generation step shown in Reaction (7), a series of experiments involving visible-light photocatalytic production of H2 from aqueous ammonium sulfite solutions were carried out. Ammonium sulfite (Aldrich Chemicals) was used without further purification. A 250 mL and 0.25 M (or 0.50 M) aqueous slurry solution of ammonium sulfite and photocatalysts comprising 0.50 g of cadmium sulfide (CdS) powder (Alfa Aesar) and 2.0 mL poly platinum (as a photocatalyst) which was poured into a quartz photoreactor. The volume of hydrogen produced from the solution was measured by water displacement method. A Gas Chromatograph (GC) equipped with a thermal conductivity detector and argon carrier gas was used to determine the purity of product hydrogen. A solar simulator was employed equipped with a 1000-Watt xenon arc lamp (Schoeffel Instrument Corp.) and a water filter to absorb the undesirable excess IR radiation from the lamp. The spectral power distribution of light emanating from the xenon arc lamp was used as a solar simulator. The Pyrex glass window was found to absorb most of ultraviolet portion of the light generated by the xenon arc lamp. Radiation from the lamp at wavelengths greater than 800 nm is not utilized in the photocatalytic process and as such the light from the solar simulator used in the experiments provided a reasonable match to that of solar.
Two photoreactors were used for the photooxidation of ammonia sulfite aqueous solutions. One was a 250 mL quartz flask and the other used a Pyrex glass window to filter most of the ultraviolet light (λ<300 nm) emitted by the xenon arc lamp.
A series of experiments were conducted to study photolysis of ammonium sulfite aqueous solution using a 60 W low-pressure mercury vapor lamp (LPML) located within an annular batch reactor (500 mL) that contained 0.50 M aqueous ammonium sulfite.
Qualitative analyses of sulfite and sulfate ions were made both by High Performance Liquid Chromatograph (HPLC) and a chemical analysis. This example introduces the analytic method of sulfite and sulfate ions by the formation of barium sulfate. Similar analyses were carried out on freshly prepared ammonium sulfite solutions (before they were used in the photocatalytic reaction) and after they were used in the photocatalytic experiments. The suspended photocatalyst powders were filtered out of the solution prior to analyses. 10 mL solutions were first diluted to 100 mL by adding distilled water and then their pH was adjusted to 1.00 by slowly adding hydrochloric acid to the solution. The acidification of the solution facilitates the decomposition of sulfite ions into sulfur dioxide according to: H2SO3═SO2+H2O. The solutions were purged with argon gas while stirring vigorously. Small quantities (approximately 2 mL) were periodically sampled from the solution and subjected to barium chloride test to determine the presence of sulfite ions. The purging times were also recorded. The solution remaining after each photochemical reaction was also purged with argon gas for the same length of time and subjected to the barium chloride analysis.
In the inventive method, ammonium sulfate, which is produced in the course of photocatalytic oxidation of sulfite ions, decomposes into ammonia gas as noted by Reaction (8). The ammonia gas is then recycled to the ammonium sulfite synthesis reactor and used to form (NH4)2SO3. In the acid decomposition reactor, sulfuric acid is decomposed into sulfur dioxide and oxygen. Sulfur dioxide from the acid decomposition reactor is extracted and returned to the sulfite synthesis reactor where it combines with ammonia to form ammonium sulfite. A series of tests using a thermogravimetric/differential thermal analyzer (TG/DTA) instrument (Perkin Elmer Corp.) to determine the reaction kinetics and decomposition mechanisms of ammonium sulfate and ammonia sulfite solutions were also carried out. 0.50 M aqueous solutions of ammonium sulfite and ammonium sulfate were heated from 295 to 498 K at a rate of 1.0 K/min and from 498 to 653 K at a rate of 2.0 K/min. Furthermore, the prospects of separating ammonium sulfate from ammonium sulfite via TG/DTA analysis of mixed ammonium sulfite and sulfate solutions at a range of sulfite to sulfate ratios were also investigated.
During the photocatalytic oxidation of ammonium sulfite aqueous solutions, sulfite ions act as electron donors and consequently are oxidized to sulfate and/or dithionate ions, while water is reduced to hydrogen.
Ammonium sulfite can be also oxidized when irradiated with the UV light.
In another experiment, 250 mL of 0.5 M solution of ammonium sulfite was mixed with 0.5 g of CdS catalyst powder and added to a Pyrex photoreactor. Results of
In this experiment, a 0.5 M solution of aqueous ammonium sulfite and 0.5 g of 1.0 wt % Pt on CdS catalyst were added into a Pyrex reactor. The experiment ran continuously for 9 days, subject to about 7 hrs of illumination each day. Results shown in
It is important to be able to efficiently recover and recycle ammonia gas generated during the ammonium sulfate thermochemical decomposition step (8) of the inventive method. It is also noted that the photocatalytic oxidation of ammonium sulfite aqueous solution generates, in addition to hydrogen and un-reacted (NH4)2SO3, ammonium dithionate ((NH4)2S2O6) and ammonium sulfate as well. The decomposition patterns of (NH4)2SO3 and (NH4)2SO4 aqueous solutions were investigated by method of thermogravimetric/differential thermal analysis (TG/DTA). The results are depicted in
In order to verify the formation of ammonium dithionate (a dimer of ammonium sulfite) during photocatalytic reactions, a series of absorbance measurements were carried out using a UV-VIS spectrophotometer. The samples used for the analysis were, 0.5 M Na2SO3, 0.5 M Na2SO4, 0.5 M Na2S2O6 (dithionate), 0.5 M (NH4)2SO3, 0.5 M (NH4)2SO4, and the products of photocatalytic oxidation of aqueous ammonium sulfite solution after 64 hrs of operation. First, it was observed that there was virtually no cation effect on the absorbance of the solutions, and both ammonium and sodium sulfite solutions had almost identical absorbance spectra. Results of
TG/DTA results of
This invention can be embodied in other forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be had to the following claims rather than the foregoing specification as indicating the scope of the invention.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US06/24865 | 6/27/2006 | WO | 00 | 5/5/2008 |
| Number | Date | Country | |
|---|---|---|---|
| 60694273 | Jun 2005 | US |
