Ammonia gas sensor method and device

• US 20060194330A1
• Filed: 12/22/2005
• Published: 08/31/2006
• Est. Priority Date: 12/28/2004
• Status: Active Grant

• Current Assignee: EmiSense Technologies, LLC (Trelleborg AB)
• Original Assignee: Ceramatec, Inc.
• Inventors: Nair, Balakrishnan G., Nachlas, Jesse
• Granted Patent: US 7,442,555 B2
• Time in Patent Office: Days
• US Class Current: 436/113
• CPC Class Codes: G01N 33/0054 for ammonia
• CPC Class Codes: Y02A 50/20 Air quality improvement or preservation, e.g. vehicle emission control or emission reduction by using catalytic converters
• CPC Class Codes: Y10T 436/17 Nitrogen containing
• CPC Class Codes: Y10T 436/175383 Ammonia
• CPC Class Codes: Y10T 436/177692 Oxides of nitrogen
• CPC Class Codes: Y10T 436/179228 Both nitrogen oxide and dioxide
• CPC Class Codes: Y10T 436/25125 Digestion or removing interfering materials
• CPC Class Codes: Y10T 436/25875 Gaseous sample or with change of physical state

Abstract

A mixed potential sensor device and methods for measuring total ammonia (NH₃) concentration in a gas is provided. The gas is first partitioned into two streams directed into two sensing chambers. Each gas stream is conditioned by a specific catalyst system. In one chamber, in some instances at a temperature of at least about 600° C., the gas is treated such that almost all of the ammonia is converted to NO_x, and a steady state equilibrium concentration of NO to NO₂ is established. In the second chamber, the gas is treated with a catalyst at a lower temperature, preferably less than 450° C. such that most of the ammonia is converted to nitrogen (N₂) and steam (H₂O). Each gas is passed over a sensing electrode in a mixed potential sensor system that is sensitive to NO_x. The difference in the readings of the two gas sensors can provide a measurement of total NH₃ concentration in the exhaust gas. The catalyst system also functions to oxidize any unburned hydrocarbons such as CH₄, CO, etc., in the gas, and to remove partial contaminants such as SO₂.

Claims

1. A method of detecting the concentration of ammonia in a gas comprising the steps of:

receiving a source stream of gas;

splitting the source stream of gas into first and second streams of gas;

exposing one of said first and second streams of gas to a first catalyst system under conditions capable of converting NH₃ present in the gas to N₂;

exposing the remaining one of said first and second streams of gas to a second catalyst system under conditions capable of converting NH₃ present in the gas to NO;

detecting the levels of NO_x present in said first and second streams of gas; and

calculating the difference in NO_x concentrations between said first and second streams of gas.

2. The method of claim 1, wherein the first catalyst system comprises a low temperature catalyst selected from the group consisting of nickel aluminate (NiAl₂O₄), vanadium pentoxide (V₂O₅), Molybdenum Oxide (MoO₃), tungsten oxide (WO₃), iron oxide (FeO, Fe₂O₃, Fe₃O₄), cerium oxide (CeO₂), copper oxide (CuO), manganese oxide (MnO₂), ruthenium oxide (RuO₂), silver (Ag), platinum (Pt) and copper(Cu), and any mixture or composites thereof.

3. The method of claim 1, wherein the second catalyst system comprises a high temperature catalyst selected from the group consisting of:

nickel aluminate (NiAl₂O₄), vanadium pentoxide (V₂O₅), Molybdenum Oxide (MoO₃), tungsten oxide (WO₃), iron oxide (FeO, Fe₂O₃, Fe₃O₄), cerium oxide (CeO₂), copper oxide (CuO), manganese oxide (MnO₂), ruthenium oxide (RuO₂), silver (Ag), platinum (Pt) and copper(Cu), and any mixture or composites thereof.

4. The method of claim 1, further comprising the step of exposing said first and second streams of gas through a third catalyst system to establish a steady state equilibrium concentration ratio between NO and NO₂.

6. The method of claim 1, further comprising the step of absorbing SO₂ from the source stream of gas prior to the step of exposing one of said first and second streams of gas to a first catalyst system under conditions capable of converting NH₃ present in the gas to N₂.

7. The method of claim 1, wherein the step of detecting the levels of NO_x present in said first and second streams of gas is accomplished with mixed-potential-based sensing elements selective to NO_x.

8. The method of claim 7, wherein the mixed-potential-based sensing elements comprise sensing electrodes deposited on oxygen-ion-conducting electrolytes and wherein a potential is measured between the sensing electrode and a reference electrode corresponding to a function of the NO_x concentration in the gas.

9. The method of claim 1, wherein the step of detecting the levels of NOx present in said first and second streams of gas is accomplished with a sensing element comprising semiconductor metal oxide coatings, wherein adsorption of NO_x on the sensing element results in a change in a physical parameter of the sensing element such as resistance or capacitance, that is measurable and may be correlated with NO_x concentration in said first and second streams of gas.

10. The method of claim 7, wherein the mixed-potential-based sensing elements comprise NO_x mixed-potential electrodes with WO₃ as the NO_x sensing electrode.

11. The method of claim 10, wherein the mixed-potential-based sensing elements comprise electrodes that contain from about 5 to about 40 vol % electrolyte.

5. The method of claim 5, wherein the third catalyst system includes a catalyst selected from the group consisting of:

RuO₂, CuO, Ag, and Pt.

12. A sensor for measuring total ammonia (NH₃) concentration in a source stream of gas, comprising:

first and second flow paths for dividing the source stream of gas into first and second streams of gas;

a first catalyst system exposed to the first flow path for converting NH3 present in the first stream of gas to N2;

a second catalyst system exposed to the second flow path for converting NH3 present in the second stream of gas to NO; and

a sensor element for detecting the levels of NOx present in the first and second streams of gas.

13. The sensor of claim 12, wherein the first catalyst system comprises a catalyst selected from the group consisting of nickel aluminate (NiAl₂O₄), vanadium pentoxide (V₂O₅), Molybdenum Oxide (MoO₃), tungsten oxide (WO₃), iron oxide (FeO, Fe₂O₃, Fe₃O₄), cerium oxide (CeO₂), copper oxide (CuO), manganese oxide (MnO₂), ruthenium oxide (RuO₂), silver (Ag), platinum (Pt) and copper(Cu), and any mixture or composites thereof.

14. The sensor of claim 12, wherein the second catalyst system comprises a catalyst selected from the group consisting of:

nickel aluminate (NiAl₂O₄), vanadium pentoxide (V₂O₅), Molybdenum Oxide (MoO₃), tungsten oxide (WO₃), iron oxide (FeO, Fe₂O₃, Fe₃O₄), cerium oxide (CeO₂), copper oxide (CuO), manganese oxide (MnO₂), ruthenium oxide (RuO₂), silver (Ag), platinum (Pt) and copper(Cu), and any mixture or composites thereof.

15. The sensor of claim 12, wherein the sensor element comprises an amperometric sensor or a mixed-potential-based sensing element selective to NO_x.

16. The sensor of claim 15, wherein the mixed-potential-based sensing elements comprise sensing electrodes deposited on oxygen-ion-conducting electrolytes and a potential is measured between the sensing electrode and a reference electrode corresponding to a function of the NO_x concentration in the gas.

17. The sensor of claim 12, wherein at least one of the sensing elements comprise semiconductor metal oxide coatings, wherein adsorption of NO_x on the sensing element results in a change in a physical parameter of the sensing element such as resistance or capacitance, that is measurable and may be correlated with NO_x concentration in said first and second streams of gas.

18. The sensor of claim 12 further comprising a SO₂-absorbing stage.

19. The sensor of claim 18, wherein the SO₂-absorbing stage comprises CaO, MgO, or a perovskite.

20. The sensor of claim 12, further comprising an equilibrating including RuO₂, CuO, Ag, or mixtures thereof.