Method and apparatus for measuring NOx gas concentration

• US 6,214,208 B1
• Filed: 12/02/1997
• Issued: 04/10/2001
• Est. Priority Date: 12/02/1996
• Status: Expired due to Term

• Current Assignee: NGK Spark PLUG Company Limited (Niterra Co., Ltd.)
• Original Assignee: NGK Spark PLUG Company Limited (Niterra Co., Ltd.)
• Inventors: Ando, Masashi, Sato, Yoshikuni, Ishida, Noboru, Sugaya, Satoshi, Nadanami, Norihiko, Oshima, Takafumi, Okumura, Tatsuo, Ootuka, Takaki
• Primary Examiner(s): WARDEN, JILL ALICE
• Application Number:
  US08/982,869
• Time in Patent Office: 1,225 Days
• Field of Search: 422/83, 422/98, 73/233.1, 73/310.5, 436/116, 436/117, 436/118, 436/151, 436/152, 205/781, 204/424-427
• US Class Current: 205/781
• CPC Class Codes: G01N 27/4074 for detection of gases other than oxygen
• CPC Class Codes: Y10T 436/179228 Both nitrogen oxide and dioxide

Abstract

This is a method and apparatus for accurately determining a NOx concentration of a measurement gas that contains H₂O and/or CO₂, without being affected by a dissociation of H₂O and/or CO₂.

In this method and apparatus,

(1) the NOx-measurement gas containing H₂O and/or CO₂ is introduced to travel through a heated flow channel formed by an oxygen ion conductive solid ceramic electrolyte body that electrically control amount of O₂ from the flow channel,

(2) while said measurement gas travels, a residual gas including NO and H₂O and/or CO₂ is formed from the measurement gas,

(3) said residual gas is flowed to contact a catalytic electrode to which a negative polarity of a low voltage that does not dissociate H₂O and/or CO₂ but dissociates NO is applied, so that said electrode dissociates a NO gas of the residual gas into N ions and O ions and flows an electric current through the electrolyte body in proportion to an amount of the O ions dissociated from said residual gas at the electrode, and

(4) the NOx concentration in the measurement gas is determined based on the current.

The low voltage preferably applied in the above (3) is about 350-450 mV.

Claims

1. A method for determining NOx concentration in a measurement gas comprising NOx, O₂, CO₂ and H₂O, under a varying condition of the NOx concentration in the measurement gas in the course that the measurement gas is allowed to travel through a flow channel facing a ceramic body having an electrically controllable conducting rate of oxygen ion,

wherein the method comprises:

(1) introducing the measurement gas into the flow channel;

(2) forming an NO-concentrated residual gas in said flow channel under a condition that allows substantial decomposition of NOx, said NO-concentrated residual gas having a NOx concentration different from the measurement gas and extracting said O₂ from the measurement gas introduced into the flow channel;

(3) dissociating NO of said NO-concentrated residual gas into nitrogen and oxygen by applying across electrodes formed on the ceramic body a voltage to an extent that does not substantially dissociate said H₂O or said CO₂ in the NO-concentrated residual gas;

(4) measuring an electric current flowing through the ceramic body between the electrodes, said electric current being generated by an electrochemical action of the oxygen dissociated from the NO in step (3); and

(5) determining the NOx concentration of the measurement gas based on the electric current measured in step (4).

2. The method as defined in claim 1 wherein, in step (1), the flow of the measurement gas into said flow channel is restricted.

3. The method as defined in claim 1 wherein, between steps (2) and (3), the flow of the residual gas to the electrodes of the step (3) is restricted.

4. The method as defined in any one of claims 1 to 3 wherein said flow channel comprises a first flow channel and a second flow channel communicating with the first flow channel, said formation of the NOx residual gas of said step (2) is performed in the first flow channel, and wherein NOx in step (3) is dissociated in the second flow channel.

5. The method as defined in claim 1 wherein in step (2) the NO-concentrated residual gas is formed under a condition that allows and compensates for substantial decomposition of NOx in the flow channel.

6. The method as defined in claim 4 wherein in the first flow channel the NO-concentrated residual gas is formed under a condition that allows and compensates for substantial decomposition of NOx in the first chamber.

7. The method as defined in claim 1 wherein in step (3) the voltage is applied to an extent that does not substantially dissociate H₂O in the NO concentrated residual gas.

8. The method as defined in claim 1 wherein in step (3) the voltage is applied to an extent that does not substantially dissociate CO₂ in the residual gas.

9. A method for determining NOx concentration in a measurement gas comprising NOx, O₂, H₂O and CO₂, said method employing an electrically controlled electrochemical cell comprising at least one oxygen ion conductive solid electrolyte body and a catalytic electrode formed on the electrolyte body, comprising:

wherein the method comprises;

(1) introducing the measurement gas through a flow channel formed in the oxygen ion conductive solid electrolyte body that electrically controls an amount of O₂ in the flow channel;

(2) forming an NO-concentrated residual gas at a temperature of at least 700 C. by decomposition of NOx in the flow of the measurement gas;

(3) contacting said NO-concentrated residual gas with the catalytic electrode to which a negative polarity of a dc voltage that does no substantially dissociate said H2O or said CO₂ but dissociates NO contained in the NO-concentrated residual gas is applied;

(4) dissociating said NO contained in the NO-concentrated residual gas into N-ions and O-ions at the catalytic electrode and flowing an electric current through the electrolyte body in proportion to an amount of the O-ions dissociated from said NO at the catalytic electrode; and

(5) measuring the current flowing through the electrolyte body and determining the NOx concentration in the measurement gas based on the electric current measured.

10. The method as defined in claim 9 further comprising:

flowing the NO-concentrated residual gas into a reservoir formed adjacent to said flow channel, said catalytic electrode being formed on a wall of the reservoir.

11. The method as defined in claim 10, further comprising:

applying a negative polarity of a voltage to a second electrode formed on a wall of said flow channel and extracting said O₂ out of the flow channel to lower an oxygen concentration of the measurement gas flowed in the flow channel.

12. The method as defined in claim 10, further comprising:

applying a negative voltage to a third electrode formed around an inlet of the reservoir and detecting said O₂ concentration in the flow channel to control an oxygen concentration of the NO-concentrated residual gas flowed in the reservoir.

13. The method as defined in claim 11, wherein

said second electrode formed on the wall of the flow channel faces its surface in parallel to a surface of the catalytic electrode formed on the reservoir wall.

14. The method as defined in claim 10, wherein

said reservoir is formed in between two oxygen ion conductive solid electrolyte bodies that are electrically insulated from each other.

15. The method as defined in claim 9, further comprising heating said electrolyte body to a temperature of at least 700°

C.

16. The method as defined in claim 14, further comprising heating said oxygen ion conductive solid electrolyte body to a temperature of 700-900°

C.

17. The method as defined in claim 9, wherein

the dc voltage is applied across the catalytic electrode and an electrode that is formed outside of the electrolyte body, and said dc voltage is in the range of from dc 300 mV to dc 500 mV where H₂O in the NO-concentrated residual gas does not substantially dissociate on the surface of the catalytic electrode and a maximum gain of NOx detection is obtained.

18. The method as defined in claim 17, wherein

the dc voltage is about 450 mV.

19. A method for determining NOx concentration in a measurement gas comprising NOx, O₂, H₂O and CO₂, said method employing an electrically controlled electrochemical cell comprising at least one oxygen ion conductive solid electrolyte body and at least one catalytic electrode formed on the electrolyte body, comprising:

(1) introducing the measurement gas through a flow channel formed in the oxygen ion conductive solid electrolyte body that electrically controls an amount of O₂ in a first part of the flow channel,

(2) forming a NO-concentrated residual gas at a temperature under a condition that allows and compensates for substantial decomposition of NO in the first part of the flow channel, said NO-concentrated residual gas having a different NO-concentration than that of the measurement gas,

(3) contacting said NO-concentrated residual gas with the catalytic electrode to which a dc voltage is applied that does not substantially dissociate said H₂O or said CO₂ but dissociates NO contained in the NO-concentrated residual gas,

(4) dissociating said NO contained in the NO-concentrated residual gas in a second part of the flow channel into N-ions and O-ions to cause an electric current to flow through the electrolyte body, said current being being in proportion to an amount of the O-ions dissociated from said NO gas at the electrode, and

(5) determining the NOx concentration in the measurement gas based on the current with a compensation for the decomposed NO in the first part of the flow channel.

20. The method as defined in claim 19, wherein said dc voltage is controlled so that H₂O or CO₂ in the NO-concentrated residual gas do not substantially dissociate on the surface of the electrode and a maximum gain of NOx detection is obtained.

21. The method as defined in claim 19 or 20, wherein the temperature of the electrolyte is controlled so that H₂O or CO₂ in the NO-concentrated residual gas do not substantially dissociate on the surface of the electrode and a maximum gain of NOx detection is obtained.

22. The method as defined in claim 19, wherein said dc voltage is in the range of from dc 300 mV to dc 500 mV.

23. The method as defined in claim 19, wherein said dc voltage is in the range of from dc 300 mV to about dc 450 mV.

24. The method as defined in claim 19, wherein said high temperature is at least 700°

C.

25. The method as defined in claim 19, wherein said high temperature is 700-900°

C.