GB2285866A

GB2285866A - Oxygen analysis apparatus

Info

Publication number: GB2285866A
Application number: GB9401158A
Authority: GB
Inventors: William Charles Maskell; Mohieddine Benamor Benammar
Original assignee: BENAMMAR MOHIEDDINE B; MIDDLESEX SERV Ltd, University of
Current assignee: BENAMMAR MOHIEDDINE B; MIDDLESEX SERV Ltd, University of
Priority date: 1994-01-21
Filing date: 1994-01-21
Publication date: 1995-07-26
Also published as: GB9401158D0

Abstract

An oxygen analysis apparatus comprises a sensor having an internal volume 4 bounded by a solid electrolyte oxygen pump 6, 1, 7 and a solid electrolyte oxygen gauge 9, 2, 8, each of which has one electrode in the internal volume and the other in a sample gas to which the sensor is exposed. The device includes a pore or porous material to act as a diffusion path 10 between the internal volume and the exterior of the sensor. A heater maintains the temperature of the sensor at a desired operating value. Oxygen is electrochemically pumped continuously into the internal volume of the sensor using a constant current. The magnitude of the pumping current is chosen high enough so that excess oxygen is maintained within the internal volume for all possible compositions of the external gas including fuel-rich conditions where the sample gas is exhaust gas from a combustion system. A pseudo-reference gas is then generated within the internal volume and a measure of the gauge EMF enables determination of the oxygen concentration in the sample gas. The apparatus enables accurate determination of the stoichiometric point and distinguishes the excess oxygen and fuel-rich regions. <IMAGE>

Description

"Gas analysis apparatus and method" This invention relates to analysis of gases and, in particular, to the measurement of oxygen concentration in a sample gas.

The determination of the oxygen content of gases is important in many applications, including the control of the air-to-fuel ratio in fossil-fuelled combustion systems in dependence upon the oxygen content of the exhaust gases.

Electrochemical measurement devices incorporating solid electrolytes are preferred to devices with liquid electrolytes for such purposes because the former do not lose solvent by evaporation during extended periods of operation.

A known potentiometric electrochemical sensor using a solid oxygen ion-conducting electrolyte and a stable reference oxygen partial pressure enables measurement of oxygen partial pressure in the exhaust gases of a combustion system operating in the excess air region with an excess of air in the air/fuel mixture. The known sensor also enables accurate determination of the stoichiometric point of the air/fuel mixture and distinguishes between the excess air and fuel-rich regions. The disadvantage of this known sensor is that a reference gas providing the reference oxygen partial pressure must be piped to the sensor, which is not a trivial problem.

The present invention aims to provide an apparatus operating on the potentiometric principle without the need for a stable reference gas.

This is achieved by using an electrochemical sensor comprising a pump, a gauge and an internal volume. The device includes a pore or porous material to act as diffusion path between the internal volume and external atmosphere. Oxygen is electrochemically pumped continuously into the internal volume of the device using a constant current. The magnitude of the pumping current is chosen high enough so that excess oxygen is maintained within the internal volume for all possible compositions of the external gas including fuel-rich conditions. The internal oxygen concentration is then dependent upon the external concentration, the amplitude of current and the leak conductance. This internal gas acts as a pseudoreference for the potentiometric measurement of the oxygen concentration in the sample gas. This measurement is provided by the gauge EMF.

Accordingly, in a first aspect, the invention provides an apparatus for determining the oxygen concentration in a sample gas including: a sensor for exposure to the sample gas and comprising a pump, a gauge, an internal volume and a diffusion path between the internal volume and the exterior of the sensor, the pump and the gauge each comprising a pair of porous electrodes and a layer of solid oxygen ionconducting material therebetween, one electrode of each pair being disposed to contact gas in the internal volume and the other electrode being disposed to contact the sample gas outside the sensor; means for applying an electric pumping current to the pump electrodes to pump oxygen electrochemically into the internal volume so as to maintain excess oxygen inside the sensor; and means for measuring the gauge EMF to determine the oxygen concentration in the sample gas.

In second aspect, the invenlion provides a method of determining the -oxygen concentration in a sample gas comprising: exposing to the sample gas a sensor comprising a pump, a gauge, an internal volume and a diffusion path between the internal volume and the exterior of the sensor, the pump and the gauge each comprising a pair of porous electrodes and a layer of solid oxygen ion-conducting material therebetween, one electrode of each pair being disposed to contact gas in the internal volume and the other electrode being disposed to contact the sample gas outside the sensor; applying an electric pumping current to the pump electrodes to pump oxygen electrochemically into the internal volume so as to maintain excess oxygen in the internal volume; and measuring the gauge EMF to determine the oxygen concentration in the sample gas.

In a preferred embodiment the internal volume of the sensor is less than 5 mm3 and the pumping current is in the range 0.5 to 5mA.

This invention also envisages planar sensors made by thin and/or thick film techniques where the internal volume may result from porosity in one or more of the layers. Furthermore, in this case the inner two electrodes of the sensor may be opposite faces of a single electrode layer.

In order that the invention may be more readily understood, embodiments thereof will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a cross-sectional view illustrating the principle of a sensor used in an apparatus embodying the present invention; Figure 2 is a cross-section through a sensor made from discrete components; Figure 3 is a circuit diagram of an apparatus embodying the invention; Figure 4 is a graph showing in solid line the results of a computer simulation of the output EMF of the measuring apparatus embodying the invention versus oxygen concentration for an apparatus with typical parameters and in dashed line the theoretical output EMF of a known potentiometric sensor with piped air reference;; Figure 5 is a graph showing experimental results obtained when operating a sensor embodying the invention in air/nitrogen mixtures; Figure 6 is a graph showing experimental results obtained using a sensor in a fuel gas burning flue; and Figure 7 is a graph showing experimental results for the excess oxygen region of the fuel gas/air mixture being burnt.

A preferred form of a sensor for use in an apparatus embodying the invention is shown in Figures 1 and 2. The sensor consists of two discs 1 and 2 of solid oxygen ion-conducting material (e.g. stabilised zirconia) and a spacer component 3 which may be composed of a metal (e.g. Au or Pt), a glass or a ceramic. The discs 1 and 2 are assembled together with the spacer 3 therebetween. The spacer 3 has a hole through it so that when the sensor is assembled an internal volume 4 is enclosed. The components may be fixed together by any of a number of means including the use of a glass, glass-ceramic, a metal-ceramic bond or combination of these where appropriate.

Respective pairs of porous electronically conducting electrodes 6,7 and 8, 9 are disposed on the plane surfaces of the discs 1 and 2. If the spacer 3 is not an electronic conductor then one or more electronically conducting connections 11 and 12 are made through the spacer between the inner electrodes 7 and 9 and the outside 5. The components 6, 1, 7 constitute an oxygen pump of the sensor while the components 9, 2, 8 comprise a gauge. The sensor includes a diffusion path 10 connecting the internal volume 4 to the outside 5. This diffusion path may be a small hole or holes drilled through one or both of the two ceramic discs 1 or 2; alternatively, the spacer 3 may include a leakage path or leakage paths.

A temperature sensor (not shown) and a heater (not shown) may be provided to sense the operating temperature of the sensor to maintain the sensor at a predetermined operating temperature.

The sensor of Figures 1 and 2 is incorporated in an apparatus embodying the invention as shown in Figure 3 by connecting a current source 20 to the pump electrodes 6, 7 and a voltmeter 21 across the gauge electrodes 8, 9.

In use of the apparatus, the sensor is positioned in the sample gas, a current is applied to the pump 6, 1, 7 to pump oxygen electrochemically into the internal volume 4 and the gauge EMF is measured to determine the oxygen concentration in the sample gas. The signal from the gauge may be processed by suitable computing means (not shown) to provide a measurement of the oxygen concentration in the sample gas outside the sensor.

The theory below has been developed for a sensor embodying the invention operating in an oxygen-inert gas mixture and incorporating a diffusion path with a leak conductance, aO2. For a diffusion path of length L, and uniform cross-sectional area S, the leak conductance is given by a02 = DS/RTL where D is the oxygen diffusion coefficient, R is the gas constant and T is the absolute temperature of operation. When a current , is applied to the pump 6, 1, 7 oxygen is electrochemically pumped into the internal volume 4 and the resulting flux is given by Jcuu = I/4F (1) where F is the faraday. The convention adopted is that a positive current pumps oxygen into the internal volume.

Likewise, a positive oxygen diffusive flux represents oxygen transfer into the internal volume. Assuming linearity of the gradient of oxygen concentration within the diffusion path, the flux of oxygen leaking through the diffusion path is given by Jdiff = a02 (P02 - P02,v) (2) where P02,v and Po2 are respectively the oxygen partial pressures inside the inner volume 4 and in the surrounding outside atmosphere 5. At steady state the total effective flux into or out of internal volume is equal to zero, i.e.

J = Jdiff + Jcurr = (3) From eqns. (1)-(3), the internal oxygen partial pressure may be written Po2v = P02 + (I/4Fa02) (4) The Nernst EMF is given by E = (RT/4F)ln(2o2/po2v) Combining eqns.(4) and (5) reveals E = -(RT/4F) ln(l + CI/4Fa02 Pro2}) (6) For pure bulk diffusion the oxygen diffusion coefficient D (and a02 as a consequence) is inversely proportional to the barometric pressure. In this case, the gauge EMF is dependent on the oxygen concentration in the sample gas and not the barometric pressure.

Figure 4 shows the gauge EMF versus oxygen concentration for a sensor with a leak conductance a02 = 4.95 10-13 [m Kg-l s mol) operated in O2-inert gas mixtures with a pumping current I = 3 mA. The trend is similar to that of a known potentiometer sensor with a reference gas with a stable oxygen partial pressure.

Figure 5 shows typical results obtained in airnitrogen gas mixtures using a sensor with an internal volume of 1 mm3 in air-nitrogen gas mixtures at 953 K. The dashed lines show the theoretical curves plotted using the value of 4.95 10-13 [m Kg-l s mol) for the leak conductance v02. Values for the pumping current are as shown in Figure.

The experimental results were in good agreement with the theoretical curves (dashed lines).

Figure 6 and Figure 7 show the results obtained by operating a sensor with an internal volume of lmm3 in a gas burning flue at 953K. A is the normalised air-to-fuel ratio (A ={actual-air-to-fuel ratio}/ {stoichiometric airto-fuel ratio}). Again the sensor behaved like a conventional potentiometric sensor with a stable reference gas; the stoichiometric point may be determined accurately and, when using a current of 4 mA, the excess oxygen and fuel-rich regions of the air/fuel mixture are clearly distinguished. For low values of the air-to-fuel ratio (A < 0.85), excess oxygen was not maintained within the internal volume when using a pumping current of 2 mA: the oxygen pumped into the internal volume was insufficient to oxidise all CO (and H2) diffusing into the internal volume of the sensor.

When operated in the excess oxygen region of the gas mixture the sensor output E is typically below 200 mV at 1000 K. In the fuel-rich region, provided that the pumping current is sufficient to maintain excess oxygen within the internal volume of the device, the output EMF is typically higher than 600 mV.

Claims

CLAIMS:

1. An apparatus for determining the oxygen concentration in a sample gas including: a sensor for exposure to the sample gas and comprising a pump, a gauge, an internal volume and a diffusion path between the internal volume and the exterior of the sensor, the pump and the gauge each comprising a pair of porous electrodes and a layer of solid oxygen ionconducting material therebetween, one electrode of each pair being disposed to contact gas in the internal volume and the other electrode being disposed to contact the sample gas outside the sensor; means for applying an electric pumping current to the pump electrodes to pump oxygen electrochemically into the internal volume so as to maintain excess oxygen inside the sensor; and means for measuring the gauge EMF to determine the oxygen concentration in the sample gas.

2. An apparatus as claimed in claim 1, wherein the sensor comprises two oxygen ion-conducting laminae and a spacer assembled to define the internal volume.

3. An apparatus as claimed in claim 2, wherein the diffusion path is formed in at least one of the laminae.

4. An apparatus as claimed in claim 2, wherein the diffusion path is formed in the spacer.

5. An apparatus as claimed in any preceding claim, wherein the internal volume has a volume of less than 5mm3.

6. An apparatus as claimed in any preceding claim, wherein the applied pumping current is constant.

7. An apparatus a0 claimed in any preceding claim, wherein the magnitude of the pumping current is in the range of 0.5 to 5mA.

8. An apparatus as claimed in any preceding claim including means for processing the gauge EMF to provide a measurement of the oxygen concentration.

9. An apparatus as claimed in any preceding claim including means for sensing the operating temperature of the sensor.

10. An apparatus as claimed in any preceding claim including means for heating the sensor to maintain the sensor at a predetermined operating temperature.

11. A method of determining the oxygen concentration in a sample gas comprising: exposing to the sample gas a sensor comprising a pump, a gauge, an internal volume and a diffusion path between the internal volume and the exterior of the sensor, the pump and gauge each comprising a pair of porous electrodes and a layer of solid oxygen ion-conducting material therebetween, one electrode of each pair being disposed to contact the gas in the internal volume and the other electrode being disposed to contact the sample gas outside the sensor; applying an electric pumping current to the pump electrodes to pump oxygen electrochemically into the internal volume so as to maintain excess oxygen in the internal volume; and measuring the gauge EMF to determine the oxygen concentration in the sample gas.

12. A method according to claim 11, comprising applying heat to the sensor to maintain the sensor at a predetermined operating temperature.

13. An apparatus susbtantially as hereinbefore described with reference to the accompanying drawings.

14. A method substantially as hereinbefore described with reference to the accompanying drawings.

15. Any novel feature or combination of features described herein.