Method And Apparatus For Decomposing Nitrogen Oxide

Abstract

A method for decomposing nitrogen oxide includes: contacting a gas stream comprising nitrogen oxide with a device, the device comprising: a first electrode, an opposite second electrode, an electrolyte between the first and the second electrodes, and a power supply; and applying in a pulse mode an electrical current from the power supply to the first and the second electrodes to decompose nitrogen oxide. An associated apparatus is also described.

Description

BACKGROUND

[0001] Embodiments of the present invention relate generally to methods and apparatuses for decomposing nitrogen oxide.

[0002] Nitrogen oxide (NOx, including NO and/or NO2) is undesirable for the environment and has to be controlled. Some approaches have been proposed to decompose nitrogen oxide into nitrogen and oxygen. However, some approaches use hazardous compound such as ammonia, and/or cause secondary pollution by producing ammonium sulfate, besides being complex and expensive. Other approaches consume a relatively large amount of power while applying electricity in decomposing nitrogen oxide.

[0003] Therefore, while some of the proposed approaches have general use in various industries, it is desirable to provide new methods and apparatuses for decomposing nitrogen oxide.

BRIEF DESCRIPTION

[0004] In one aspect, the invention relates to a method for decomposing nitrogen oxide, comprising: contacting a gas stream comprising nitrogen oxide with a device, the device comprising: a first electrode, an opposite second electrode, an electrolyte between the first and the second electrodes, and a power supply; and applying in a pulse mode an electrical current from the power supply to the first and the second electrodes to decompose nitrogen oxide.

[0005] In another aspect, the invention relates to an apparatus for decomposing nitrogen oxide, comprising: a gas source for providing a gas stream comprising nitrogen oxide; and a device in fluid communication with the gas source and comprising: a first electrode, an opposite second electrode, an electrolyte between the first and the second electrodes, and a power supply comprising a controller for applying in a pulse mode an electrical current from the power supply to the first and the second electrodes to decompose nitrogen oxide.

DRAWINGS

[0006] These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings, wherein:

[0007] FIG. 1 illustrates a schematic cross sectional view of an apparatus of a first embodiment of the invention;

[0008] FIG. 2 illustrates a schematic cross sectional view of an apparatus of a second embodiment of the invention;

[0009] FIG. 3 illustrates a schematic cross sectional view of an apparatus of a third embodiment of the invention;

[0010] FIG. 4 illustrates a schematic cross sectional view of an apparatus of a fourth embodiment of the invention;

[0011] FIG. 5 shows the NO conversion percentage of a gas stream (80 ml/min, 400 ppm NO balanced with He) in the reactor using a La0.6Sr0.4Ni0.3Mn0.703--Zr0.89Sc0.1Ce0.01O2-x layer as the cathode at 600.degree. C. as a function of time applied with and stopped from 50 mA of direct current; and

[0012] FIG. 6 illustrates the NO conversion percentage of a gas stream (80 ml/min, 400 ppm NO balanced with He) at 600.degree. C. in reactors using a NiO--Zr0.89Sc0.1Ce0.01O2-x layer and a La0.6Sr0.4Ni0.3Mn0.7O3--Zr0.89Sc0.1Ce0.01O2-x layer as cathodes before applying and 5 hours after stopping 50 mA of electrical current, respectively.

DETAILED DESCRIPTION

[0013] Unless defined otherwise, technical and scientific terms used herein have the same meaning as is commonly understood by one of ordinary skill in the art to which this disclosure belongs. The terms "first", "second", and the like, as used herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. The use of "including", "comprising" or "having" and variations thereof herein are meant to encompass the items listed thereafter and equivalents thereof as well as additional items.

[0014] Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as "about" is not to be limited to the precise value specified. In some instances, the approximating language may correspond to the precision of an instrument for measuring the value. Here and throughout the specification and claims, range limitations may be combined and/or interchanged; such ranges are identified and include all the sub-ranges contained therein unless context or language indicates otherwise.

[0015] In the specification and the claims, the singular forms "a", "an" and "the" include plural referents unless the context clearly dictates otherwise. Moreover, the suffix "(s)" as used herein is usually intended to include both the singular and the plural of the term that it modifies, thereby including one or more of that term.

[0016] As used herein, the term "or" is not meant to be exclusive and refers to at least one of the referenced components (for example, a material) being present and includes instances in which a combination of the referenced components may be present, unless the context clearly dictates otherwise.

[0017] Reference throughout the specification to "some embodiments", and so forth, means that a particular element (e.g., feature, structure, and/or characteristic) described in connection with the invention is included in at least one embodiment described herein, and may or may not be present in other embodiments. In addition, it is to be understood that the described inventive features may be combined in any suitable manner in the various embodiments.

[0018] Embodiments of the present invention relate to methods and apparatuses for decomposing nitrogen oxide.

[0019] As used herein the term "nitrogen oxide" or the like refers to a gas comprising molecules including both oxygen and nitrogen, for example, nitrogen monoxide, nitrogen dioxide, or a combination thereof.

[0020] Please refer to FIGS. 1, 2, 3 and 4, an apparatus 10, 20, 30, 40 of embodiments of the invention includes a gas source 11, 21, 31, 41 for providing a gas stream 12, 22, 32, 42 comprising nitrogen oxide and a device 100, 200, 300, 400 in fluid communication with the gas source 11, 21, 31, 41.

[0021] The gas stream comprising nitrogen oxide may be from a variety of gas sources. In some embodiments, the gas sources are exhaust gas sources from gas turbines, internal combustion engines, or combustion devices. In some embodiments, the gas source comprises a conduit, a channel, or a tube for the passage of the gas stream.

[0022] In some embodiments, the device 100, 200, 300, 400 includes a first electrode 101, 201, 301, 401, an opposite second electrode 102, 202, 302, 402, an electrolyte 103, 203, 303, 403 between the first and the second electrodes, and a power supply 104, 204, 304, 404 having a controller 114, 214, 314, 414 for applying in a pulse mode an electrical current from the power supply 104, 204, 304, 404 to the first and the second electrodes to decompose nitrogen oxide.

[0023] In some embodiments, nitrogen oxide can be directly decomposed in the device 100, 200, 300, 400 before an electrical current is applied. When a gas stream comprising nitrogen oxide is contacted with the device, nitrogen oxide is decomposed in the cathode in a reaction such as: NO=1/2N2+1/2O2.

[0024] However, as can be seen from examples described hereafter, when the electrical current is applied, besides the direct decomposition of NO described above, nitrogen oxide can also be decomposed in the cathode in an electrochemical reaction of NO+2e.fwdarw.1/2N2+O2--. The oxygen ions produced thereby travel from the cathode through the electrolyte into the anode to be oxidized into oxygen in a reaction of O2-2e.fwdarw.1/2O2. A total reaction in the device is: NO=1/2N2+1/2O2. The decomposition rate of nitrogen oxide increases and after the electrical current is stopped, the conversion (decomposition) rate of nitrogen oxide is still higher for a long time period than the conversion rate of nitrogen oxide before applying the electrical current.

[0025] Therefore, by applying an electrical current in a pulse mode, the decomposition of nitrogen oxide can be achieved at a higher conversion rate than without applying an electrical current and with less power consumption than continuously applying an electrical current.

[0026] The decomposition of nitrogen oxide may be at any suitable temperature. In some embodiments, the step of applying the electrical current is at a temperature in a range from about 300.degree. C. to about 1000.degree. C.

[0027] As used herein, the term "pulse mode" refers to intermittently applying and removing electric current, in contrast to a continuous application of current during service. The manner and duration of respectively applying and removing the electrical current in the pulse mode may be dependent upon the specific apparatus, the specific gas stream, and the decomposition environment, as long as the conversion rate of nitrogen oxide and the power consumption are satisfactory in the specific circumstance.

[0028] In some embodiments, in the pulse mode the electrical current is applied and removed alternately. In some embodiments, in the pulse mode the electrical current is applied for a time period different from a time period when the electrical current is stopped. In some embodiments, in the pulse mode the electrical current is applied for a time period the same as a time period when the electrical current is stopped.

[0029] The electrical current may be any electrical current that can be used to decompose nitrogen oxide at a conversion rate higher than that of before an electrical current is applied. In some embodiments, the electrical current is direct current. In some embodiments, the electrical current is applied by jumping to the designed value directly. In some embodiments, the electrical current is applied by sweeping to the designed value slowly.

[0030] The controller 114, 214, 314, 414 may be any mechanism that controls the on and off and/or increasing and decreasing of the electrical current. In some embodiments, the controller is a switch for turning on and off the electrical current.

[0031] In some embodiments, the first electrode 101, 201, 301, 401 is an anode. The anode may include any material that oxidizes oxygen ions to oxygen, and any other materials that can be used in the anode. In some embodiments, the anode comprises a manganite, such as lanthanum strontium manganite (LSM), a non-limiting exemplary composition of which includes (La0.8Sr0.2)0.95MnO3; a combination of platinum and yttria stabilized zirconia; a combination of platinum and gadolinium-doped ceria; or any combination thereof.

[0032] In some embodiments, the second electrode 102, 202, 302, 402 is a cathode. The cathode may include any material that decomposes nitrogen oxide to nitrogen and oxygen ions, and any other materials that can be used in the cathode.

[0033] In some embodiments, the cathode includes catalysts catalyzing the decomposition of nitrogen oxide. In some embodiments, the cathode comprises catalysts catalyzing the decomposition of nitrogen oxide with little or no impact by the presence of oxygen. The oxygen coexisting with nitrogen oxide may be discharged from the cathode.

[0034] In some embodiments, the cathode has adsorption materials that adsorb nitrogen oxide. Examples of the adsorption material include, but are not limited to, magnesium oxide, calcium oxide, sodium oxide, potassium oxide, barium oxide, and strontium oxide.

[0035] In some embodiments, the cathode comprises a manganite, such as lanthanum strontium nickel manganite (LSNM), an exemplary composition of which includes, but is not limited to, La0.6Sr0.4Ni0.3Mn0.7O3; nickel oxide (NiO); a combination of LSNM and gadolinium doped ceria (GDC, e.g., Gd0.1Ce0.9O1.95); a combination of LSNM and scandia stabilized zirconia (SSZ, e.g., Zr0.89Sc0.1Ce0.01O2-x) (such as in a 50 wt % ratio); a combination of LSNM, NiO and SSZ (such as, a ratio of 40 wt %, 30 wt %, and 30 wt %); a combination of NiO and SSZ (such as in a 50 wt % ratio); a combination of platinum with yttria-stabilized zirconia; a combination of platinum with GDC; or any combination thereof.

[0036] In some embodiments, as is shown in FIGS. 3 and 4, the device 30, 40 comprises an adsorption layer 305, 405 disposed over the second electrode 302, 402, either directly, or with one or more intermediate layers therebetween. The adsorption layer may comprise any adsorption material that adsorbs nitrogen oxide, such as those described previously. The adsorption material may be distributed inside the cathode without forming an extra layer.

[0037] In some embodiments, the apparatus comprises a current collector (not shown). The current collector may be made of any electrically conductive materials such as metals or metal alloys and be in any forms suitable for use in supplying or withdrawing electrical current from the electrodes. In some embodiments, the current collector is made of nickel. In some embodiments, the current collector is in the form of mesh, porous film, foam, or any combination thereof. In some embodiments, the current collector is nickel foam. In some embodiments, a porosity of a porous metallic current collector is in a range from about 25% to about 99%.

[0038] In some embodiments, the current collector is a mechanical support for the first and the second electrodes.

[0039] In some embodiments, the device comprises a current collector disposed over the second electrode, either directly, or with one or more intermediate layers therebetween.

[0040] The electrolyte may include any material that has a suitable level of oxygen ion conductivity and any other suitable material. In some embodiments, the electrolyte comprises GDC, such as Gd0.1Ce0.9O1.95; SSZ, such as Zr0.89Sc0.1Ce0.01O2-x; oxide materials from the barium-zirconium-cerium-yttrium (BZCY) family, such as BaZr0.7Ce0.2Y0.1O3; or any combination thereof. In some embodiments, the electrolyte includes bismuth oxide, zeolite, alumina, silica, aluminum nitride, SiC, nickel oxide, iron oxide, copper oxide, calcium oxide, magnesium oxide, zinc oxide, aluminum, yttria stabilized zirconia, scandia stabilized zirconia, perovskite oxides, lanthanum strontium calcium manganese, lanthanum silicate, Nd9.33(SiO4)6O2, AlPO4, B2O3, and R2O (R stands for an alkaline metal), AlPO4--B2O3--R2O glass which carries out the main component of Na and the K, porous SiO2--P2O5 system glass, Y addition BaZrO3, Y addition SrZrO3 and Y addition SrTiO3, strontium doping lanthanum manganite, a lanthanum strontium cobalt iron oxide (La--Sr--Co--Fe system perovskite type oxide), a La--Sr--Mn--Fe system perovskite type oxide, a Ba--Sr--Mn--Fe system perovskite type oxide, or any combination thereof.

[0041] A dense electrolyte is, in an embodiment, used for mitigating the mixing of the gases of the cathode and the anode and reducing the ohmic resistance of the electrolyte. Low ohmic resistance is in an embodiment preferred for energy saving in the NOx reduction process.

[0042] Each of the electrode, the electrolyte, the current collector, and the adsorption layer may be a single layer or comprise more than one layer depending on the needed flexibility, gas diffusion capability, and porosity. Multiple layers may be the same as or different from each other and connected in suitable ways. In each single layer, the composition may be the same or different through at least one dimension thereof.

[0043] The device may be of any configuration suitable for decomposing nitrogen oxide. In some embodiments, as is shown in FIGS. 1 and 3, the device 100, 300 is of a planar configuration. In some embodiments, as is shown in FIGS. 2 and 4, the device 200, 400 is of a tubular configuration and comprises a space 206, 406 therein.

[0044] The device described herein may be prepared by providing a current collector and applying sequentially different layers on both sides thereof, or providing any of other layers and laminating different layers on either/both sides thereof. The layers may be formed/applied/laminated by any suitable means such as extruding, dip coating, spraying and printing.

EXAMPLES

[0045] The following examples are included to provide additional guidance to those of ordinary skill in the art in practicing the claimed invention. These examples do not limit the invention as defined in the appended claims.

Example 1

La0.6Sr0.4Ni0.3Mn0.7O3 Synthesis

[0046] La2O3, SrCO3, Mn(AC)2.4H2O and NiO were ball milled in EtOH and calcined at 1300.degree. C. for 8 hours to prepare La0.6Sr0.4Ni0.3Mn0.7O3. X-ray diffraction (XRD) analyses confirmed that a pure phase of La0.6Sr0.4Ni0.3Mn0.7O3 was obtained.

Example 2

Reactor Preparation

[0047] Two 7.5 cm long one-end open (La0.8Sr0.2)0.95MnO3 tubes were fabricated by extruding. The outer diameter of each tube was about 1 cm, and the inner diameter was about 0.7 cm.

[0048] A dense Zr0.89Sc0.1Ce0.01O2-x electrolyte film was coated on each (La0.8Sr0.2)0.95MnO3 tube and was co-sintered with the (La0.8Sr0.2)0.95MnO3 tube at 1250.degree. C.

[0049] A layer of La0.6Sr0.4Ni0.3Mn0.7O3 and Zr0.89Sc0.1Ce0.01O2-x (La0.6Sr0.4Ni0.3Mn0.7O3--Zr0.89Sc0.1Ce0.01O2-x layer, 50 wt % ratio) and a layer of NiO and Zr0.89Sc0.1Ce0.01O2-x (NiO--Zr0.89Sc0.1Ce0.01O2-x layer, 50 wt % ratio) were respectively deposited on the Zr0.89Sc0.1Ce0.01O2-x electrolyte films and sintered at around 900-1100.degree. C. to obtain two reactors. The active area of each of La0.6Sr0.4Ni0.3Mn0.7O3--Zr0.89Sc0.1Ce0.01O2-x and NiO--Zr0.89Sc0.1Ce0.01O2-x layers was about 10 cm2.

[0050] A layer of porous platinum paste was applied to each of La0.6Sr0.4Ni0.3Mn0.7O3--Zr0.89Sc0.1Ce0.01O2-x and NiO--Zr0.89Sc0.1Ce0.01O2-x layers to form a porous metallic current collector.

[0051] The microstructures of the reactors were analyzed. As a typical example, SEM images of the cross section of the (La0.8Sr0.2)0.95MnO3/Zr0.89Sc0.1Ce0.01O2-x/NiO--Zr0.89Sc0.1Ce0.01O2-x reactor show that the (La0.8Sr0.2)0.95MnO3 layer had a porous structure with a low porosity, the Zr0.89Sc0.1Ce0.01O2-x layer had a dense structure, while the NiO--Zr0.89Sc0.1Ce0.01O2-x layer had a porous structure with a high porosity.

Example 3

Decomposition of Nitrogen Oxide

[0052] The reactors were each put inside an alumina tube. The inner diameter of the alumina tube was about 2 cm. A gas stream (400 ppm NO balanced with He, 80 ml/min) was fed into the alumina tube passing through the outer surface of the reactor at a temperature of 600.degree. C. Direct current (DC) of 50 mA was applied on each reactor for about 900 minutes before being stopped.

[0053] The La0.6Sr0.4Ni0.3Mn0.7O3--Zr0.89Sc0.1Ce0.01O2-x layer and the NiO--Zr0.89Sc0.1Ce0.01O2-x layer were assigned as cathodes, where the direct decomposition of NO and electrochemical NO reduction took place. The (La0.8Sr0.2)0.95MnO3 layer was the anode, where the oxidation of oxygen ions took place. The corresponding voltage between anode and cathode was in the range of 1-1.5 V. Gas chromatography equipped with a PQ column and a RAE7800 gas sensor were used to detect NO and NO2 with an accuracy of 1 ppm and 0.1 ppm, respectively.

[0054] FIG. 5 shows the NO conversion percentage of the reactor using the La0.6Sr0.4Ni0.3Mn0.7O3--Zr0.89Sc0.1Ce0.01O2-x layer as the cathode layer at 600.degree. C. increased gradually from about 5% to about 40% in about 900 minutes after applying the direct current of 50 mA. The 5% NOx conversion rate before applying the DC is the direct catalytic NOx decomposition activity of the reactor. After the electrical current was stopped, the NO conversion rate gradually decreased to about 20% after 300 minutes, which is much higher than the initial 5% conversion rate before applying the electrical current. This suggests that the DC activated the reactor for the NOx decomposition. Therefore, this experiment demonstrates that nitrogen oxide may be decomposed at a higher conversion rate by applying and stopping the electrical current alternately in a pulse mode than without applying an electrical current.

[0055] NO conversion rates before applying and about 5 hours after stopping 50 mA of direct current in the reactors using the NiO--Zr0.89Sc0.1Ce0.01O2-x layer and the La0.6Sr0.4Ni0.3Mn0.7O3--Zr0.89Sc0.1Ce0.01O2-x layer respectively as cathode layers are shown in FIG. 6. Both of the two reactors show much improved NO conversion rate after stopping the 50 mA of electrical current than before applying the electrical current. The high NO conversion rate after stopping the electrical current might be related to the reduction of Ni species in NiO and Ni and Mn species in La0.6Sr0.4Ni0.3Mn0.7O3 while applying the electrical current, which could generate oxygen vacancies. These vacancies are potential active centers for the adsorption and further decomposition of NOx. This experiment further indicates that nitrogen oxide can be decomposed at a high conversion rate with less power consumption in a pulse mode of applying and stopping electrical current alternately than continuously applying an electrical current.

[0056] While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.

Claims

1. A method for decomposing nitrogen oxide, comprising: contacting a gas stream comprising nitrogen oxide with a device, the device comprising: a first electrode, an opposite second electrode, an electrolyte between the first and the second electrodes, and a power supply; and applying in a pulse mode an electrical current from the power supply to the first and the second electrodes to decompose nitrogen oxide.

2. The method of claim 1, wherein the step of applying the electrical current is at a temperature in a range of from 300.degree. C. to 1000.degree. C.

3. The method of claim 1, wherein in the pulse mode the electrical current is applied for a time period different from a time period when the electrical current is stopped.

4. The method of claim 1, wherein in the pulse mode the electrical current is applied for a time period the same as a time period when the electrical current is stopped.

5. The method of claim 1, wherein the electrical current is direct current.

6. The method of claim 1, wherein the first electrode is an anode.

7. The method of claim 1, wherein the first electrode comprises a material for oxidizing oxygen ions to oxygen.

8. The method of claim 1, wherein the second electrode is a cathode.

9. The method of claim 1, wherein the second electrode comprises a material for decomposing nitrogen oxide.

10. The method of claim 1, wherein the device comprises an adsorption layer disposed over the second electrode.

11. The method of claim 1, wherein the second electrode comprises an adsorption material for adsorbing nitrogen oxide.

12. The method of claim 1, wherein the apparatus is of a tubular configuration or a planar configuration.

13. The method of claim 1, wherein the device comprises a current collector.

14. An apparatus for decomposing nitrogen oxide, comprising: a gas source for providing a gas stream comprising nitrogen oxide; and a device in fluid communication with the gas source and comprising: a first electrode, an opposite second electrode, an electrolyte between the first and the second electrodes, and a power supply comprising a controller for applying in a pulse mode an electrical current from the power supply to the first and the second electrodes to decompose nitrogen oxide.

15. The apparatus of claim 14, wherein the first electrode is an anode.

16. The apparatus of claim 14, wherein the second electrode is a cathode.

17. The apparatus of claim 14, wherein the second electrode comprises an adsorption material for adsorbing nitrogen oxide.

18. The apparatus of claim 14, wherein the device comprises an adsorption layer disposed over the second electrode.

19. The apparatus of claim 14, wherein the device comprises a current collector.

20. The apparatus of claim 14, wherein the gas source is an exhaust gas source.