GB2208007A - Air/fuel ratio sensing apparatus - Google Patents

Abstract

The apparatus, which measures oxygen in the gas after combustion comprises a sensor having a pseudo reference chamber B and an amperometric chamber A, separated by solid electrolyte wall 1 and each having a further solid electrolyte wall 5,6 and a diffusion barrier 3,4. The gas in chamber B is normally maintained at a higher air/fuel ratio than the external gas by oxygen pumping current I2 and the current I1, flowing through dividing wall 1 due to applied potential 20 is a measure of the air/fuel ratio. Various operating arrangements are described. The output of current drive unit 21 may be controlled so that I1 and I2 is held constant. Alternatively, the voltage across wall 1 may be held constant by driving a current across wall 5 to pump oxygen out of chamber A. In another alternative, the pump current I2 is applied, the current due to a fixed voltage across wall 5 is measured and the EMF across wall 1 determines whether the mixture is rich or lean. A further alternative dispenses with the pump current I2 but makes barrier 4 more restrictive than barrier 3. <IMAGE>

Description

GAS ANALYSIS APPARATUS This invention relates to the measurement of oxygen partial pressure.

The determination of the oxygen content of gases is important in many applications including the control of air-to-fuel ratio in fossil fuel led boilers and the optimisation of internal combustion engines, in particular for operation in the lean-burn regime. Electrochemical devices incorporating solid electrolytes are preferred to devices with .liquid electrolytes because the former do not lose solvent by evapo- ration during extended periods of operation.

This invention describes an oxygen sensor of the amperometric type.

In the simplest form an amperometric oxygen sensor consists of a sheet of an oxygen-ion-conducting material with porous electrodes on each side.

A voltage is applied between the electrodes causing oxygen to be pumped electrochemically from one face to the other. A diffusion barrier is placed adjacent to the negative electrode to restrict the diffusive transport of oxygen to this electrode. Then by operating at an appropriate applied voltage to attain the limiting current typically 0.5V, the current flowing is proportional to the oxygen partial pressure of the surrounding sample gas in O2-inert gas mixtures or in exhaust gases containing excess oxygen.

As stoichiometry is approached from the excess air direction the current tends towards zero. However on traversing stoichiometry into the fuel-rich region the current rises and has the same sign as in the region with excess oxygen. Hence, using this simple device it is not possible to distinguish the regions on each side stoichiometry. This problem may be overcome by operating the positive electrode in a reference gas such as air. Then on traversing stoichiometry the direction of the measured current reverses. The disadvantage of this design is that the reference gas must be piped to the sensor which is not a trival Problem.

According to the present invention the regions on each side of stoichiometry may be distinguished while the need for provision of a reference gas is eliminated. This is achieved by the incorporation of a pseudo-reference electrode (PRE). This PRE contains gas which is generally maintained at all times with excess oxygen but the oxygen partial pressure is not precisely defined. In one embodiment of this invention the PRE contains gas which has a greater air-to-fuel ratio than the sample gas at all times but when the sample gas is fuel-rich so too is the gas in the PRE.

The PRE is a sealed chamber connected to the sample gas via a diffusion barrier. Consequently if left for sufficient time with no external influences the gas composition within and without the PRE chamber would become identical. However if oxygen is electrochemically pumped continuously into the PRE chamber then the internal gas will maintain an air-to-fuel ratio greater than that of the external sample gas. Further, if the pumping current is sufficiently great for a given diffusion barrier then the gas within the PRE chamber will contain excess oxygen for all possible compositions of the external sample gas including fuel-excess conditions. This gas within the PRE containing excess oxygen can then act as a pseudo-reference to provide a sufficiently stable Nernst voltage for the operation of an amperometric sensor.

The sensor in the present invention consists of two parts. The first part is the PRE as described above. The second is the amperometric sensor which is separated from the PRE by a piece of oxygen-ion-conducting material with an electronically conducting electrode on each side one in the PRE chamber and one in the amperometric chamber. This arrangement permits operation of the sensor in a number of modes each of which is superior to the simple amperometric sensor without a PRE.

Specific embodiments of the invention will now be described by way of example with reference to the accompanying drawings.

Figure 1 shows the sensor in cross-section.

Figure 2 shows a specific form of the sensor in cross-section.

Figure 3- shows the apparatus with a constant current -I2 pumping oxygen - into the PRE and a fixed voltage generating a variable current Ii o Figure 4 shows the apparatus with a constant current I2 pumping oxygen into the PRE and a current I1 pumping oxygen out of the amperometric half of the cell with I1 adjusted to keep the Nernst potential between the two halves of the cell constant.

Figure 5 shows the apparatus with a constant current I2 pumping oxygen into the PRE, a fixed voltage generating a variable current I1 pumping oxygen out of the amperometric cell and a means of monitoring the emf generated between the two chambers.

Figure 6 shows the characteristic of the apparatuses illustrated in Figs. 3 and 4 and a modification of Fig. 3 described below.

~ = actual air/fuel ratio stoichiometric air/fuel ratio Figure 7 shows the characteristics of the apparatus illustrated in Fig. 5.

Figure 8 shows the characteristic of the apparatus in Fig 3 for I2 = 0 where diffusion barrier 4 is more restrictive than 3.

Referring to Fig. 1, the sensor consists of two chambers A and B separate by an oxygen-ion-conducting material 1, such as zirconia.

Volumes A and B are in contact with gas 2 external to the sensor via diffusion barriers 3 and 4 which may be tubes or pores in any of the materials 5-8. Materials 7 and 8 are non-ionically conducting materials such as alumina, glass or metals. Components 5 and 6 are normally oxygen-ion conductors although in some forms of the apparatus described below they may be composed of non-ionically conducting materials. Components 9-14 are electronic conductors which act as electrodes: these may be composed of a variety of materials including electronically conducting ceramics, mixed ionic conductors or metals; porous platinum is often preferred as it has good catalytic properties so that gases are brought to thermodynamic equilibrium on the surface of the metal. Electrodes 9, 10, 13 and4 are optional in some forms of the apparatus.Where components 7 and 8 are not electronic conductors then connection to electrodes must be made by wires shown as 15-18. The part of the cell containing the volume A is the amperometric part while the part containing volume B is the PRE part. The device is brought to an appropriate operating temperature by a heater 19 which may be a separate component or an integral part of the sensor. Where the sample gas is within the range of temperature for suitable operation of the device then it may be possible to dispense with the heater 19.

A specific form of the sensor is shown in Fig. 2. It consists of three discs, 1, 5 and 6, of oxygen-ion-conducting materials such as zirconia with metal spacers 7 and 8 which may be gold foil through which are cut holes to form the volumes A and B. The cell is assembled with heat and pressure to produce hermetic ceramic-metal seals at the interfaces. Holes 3 and 4 form the diffusion barriers.

Sensors based upon planar technology are also encompassed within the scope of this~invention. In this case either volume A or volume B or both are normally defined by the space within a porous material having interconnected pores. A porous substrate can be used to form diffusion barriers 3 or 4 or alternatively these can be prepared using porous thick films.

Figure 3 shows an apparatus for the measurement of the oxygen partial pressure or the air-to-fuel ratio of the sample gas 2. Oxygen is pumped from chamber A to chamber B by a constant voltage supply 20 typically operating at 0.5V to achieve the limiting current condition.

When the sample gas 2 contains excess oxygen the principal reactions at electrodes 12 and 11 are

so that oxygen is consumed at electrode 12 and evolved at electrode 11.

The species 02- is mobile within the oxygen-ion conductor. However, when the sample gas 2 is derived from a fuel-excess mixture then the principal reactions are

because now there is a negligible amount of oxygen within the gas in chamber A and the reacting species are CO and CO2. The reactions as written indicate a reversal of current for this situation.

Oxygen is also pumped from sample gas 2 into chamber B by a constant current supply 21. The magnitude of the constant current I2 and the degree of restriction of the diffusion barriers 3 and 4 are chosen so that under all compositions of the sample gas 2, and hence under all possible air-to-fuel ratios, the gas within chamber B always contains excess oxygen. The direction of oxygen transport between chambers A and B and the direction of current I1 shown in Fig. 3 are those pertaining when sample gas 2 contains oxygen in excess: these directions reverse if the air-to-fuel ratio of sample gas 2 traverses stoichiometry and becomes fuel-rich.It is necessary with this apparatus to ensure that 1121 > Ill and that (I1+I2) is always large enough to oxidise all the CO diffusing into chamber B via barrier 4 when sample gas 2 is fuel-rich. This is achieved by making diffusion barrier 3 more restrictive than 4. The air-tofuel ratio can then be determined from a measurement of I1. In a refinement of this apparatus the Nernst potential is monitored between electrodes 13 and 14. This can be useful to indicate that the amperometric cell is operating in the limiting region: in this case the emf observed exceeds 50mV. The current I1 vs air-to-fuel ratio for this apparatus is shown in Fig. 6.

In a first modification of the apparatus in Fig. 3 a signal indicative of the magnitude and sign of I1 is fed to the unit 21 controlling the current to electrodes 9 and 10 which then adjusts the value of I2 so that the total current pumping oxygen into chamber B (I1+I2) is constant or approximately so. (I1+I2) is chosen to be large enough to ensure that the gas in chamber B always contains oxygen in excess. Clearly I2 may take positive or negative values. This modification has the advantage that it allows more freedom in the selection of the characteristics of diffusion barriers 3 and 4 but inevitably entails more complex electronic circuitry. The characteristic behaviour of this apparatus in shown in Fig. 6.

In a second modification of the apparatus in Fig. 3 a constant current supply 21 is not connected so that current 12 is zero.

Furthermore, diffusion barrier 4 is constructed to be more restrictive than barrier 3. The PRE, i.e., chamber B, contains gas at all times of greater air-to-fuel ratio than the sample gas 2 except at stoichiometry where the two regions approach the same composition.

The characteristic behaviour of this apparatus is shown in Fig. 8.

In this case there is no reversal of current or traversing stoichiometry.

However, by choosing diffusion barrier 4 to be considerably more restrictive than 3 the current I1 at all air-to-fuel ratios in the fuelrich region is small. Consequently the apparatus is-operated only at currents I1 in excess of the maximum value observed in the region for X < 1: then there is no doubt that the sample gas is in the region X > 1.

Thus, with this restriction oxygen partial pressure may be determined in the oxygen-excess region.

Figure 4 shows an apparatus for the measurement of the oxygen partial pressure or the air-to-fuel ratio of the sample gas 2. Oxygen is pumped into chamber B by unit 21 operating at constant current I2 which is sufficient to ensure that chamber B contains excess oxygen under all possible compositions of sample gas 2. The emf E between electrodes 11 and 12 is given by the Nernst equation E = RT ln 4F where R is the gas constant, T is the temperature (K), F is the faraday and PB and PA are the oxygen partial pressures in volumes B and A respectively. A voltage E1 is introduced into the circuit as shown and the unit 22 generates a current I1 to maintain the Nernst potential E equal in magnitude and opposite in polarity to E1. E1 is typically set in the range 50-500mV.In the oxygen-excess region I is porportional to the oxygen partial pressure within gas 2. The current I1 reverses when stoichiometry is traversed by gas 2. Thus I may be monitored to determine the air-to-fuel ratio of the sample gas as shown in Fig. 6.

Figure 5 shows an apparatus for the measurement of the oxygen partial pressure or the air-to-fuel ratio of the sample gas 2 in the oxygen-excess region only but also indicates definitively whether the sample gas 2 has a composition corresponding to the oxygen-excess or the fuel-excess region. As it is essential in most combustion applications to operate over a defined region within the oxygen-excess region this apparatus is adequate for control purposes. Oxygen is pumped into chamber B by a constant current unit 21 at a rate sufficient to ensure that the gas within volume B contains excess oxygen under all possible compositions of sample gas 2. Chamber A is operated in the normal amperometric mode by application of a fixed voltage, typically 0.2 - C.5V across electrodes 13 and 14 by a constant voltage unit 20.

As stoichiometry is approached from the oxygen-excess side within sample gas 2 the current I1 tends towards zero. Throughout this range the measured emf E at' 24 is small ( < 500 mV). On traversing stoichiometry into the fuel-rich region the current I1 rises without changing sign; also the emf E rises to large values ( > 500 mV) indicating that the sample gas 2 is now fuel-rich. Thus, with this apparatus the two sides of stoichiometry are distinguished by the value of the emf E at 24. The characteristics of this apparatus are shown in Fig. 7.

Definition For the purposes of the description of this invention, air-tofuel ratio of the sample gas should be taken to mean the air-to-fuel ratio of the mixture of gas from which the sample gas has been derived, i.e. ratio of air-to-fuel prior to combustion.

Claims (8)

1. An electrochemical apparatus for determining the air-to-fuel ratio of a sample gas derived from a combustion process includes a sensor comprising at least two enclosed volumes where the first volume is connected to the external sample gas via a first diffusion barrier and the second volume is connected to the external sample gas via a second diffusion barrier; the two volumes are separated by a first piece of an oxygen-ion-conducting material (OICM) such as zirconia with one electronically conducting electrode on each surface of the 01CAM within each volume; the first volume (B) is bounded by a 'second piece of an OICM which has one electronically conducting electrode on its surface in contact with the gas within the chamber and another in contact with the external sample gas; the second OICM iS separated from the first OICM by a material that does not conduct oxygen ions such as a metal, a glass or a non-conducting ceramic; the second volume (A) is constructed similarly to B including a third piece of 01CAM and associated electrodes; the second and/or third OICM may be replaced by non-ionicc51y conducting materials where appropriate and in these cases the associated pairs of electrodes become redundant.

one or more external circuits for voltage sensing and/or for pumping oxygen electrochemically between the first and second volumes and/or between the external sample gas and one or both of the first and second volumes a consequence being that the air-to-fuel ratio of the gas in volume B is raised above that in the external sample gas and the electrochemical potential of an electrode within volume B is modified compared with a similar electrode within the sample gas; the electrode on the surface of the first OICM and within volume B is used to provide a pseudo-reference potential for operation of the amperometric part of the cell which includes the second volume (A), the second diffusion barrier and the first OICM; the current flowing I1 to pump oxygen in or out of volume A together with, where appropriate, a measurement of the emf between electrodes on the first 01CAM, one in each volume A and B, provide a measurement of the air-to-fuel ratio of the sample gas.

a heater, which may be a separate component or an integral part of the sensor to maintain the sensor at an appropriate temperature to ensure adequate conductivity of the OICMs and suitably rapid kinetics of the electrode reactions; it is sometimes possible to dispense with the heater where the sample gas temperature is within a suitable range.

2. An electrochemical apparatus as claimed in Claim 1 wherein one external circuit supplies a constant current I2 to the pair of electrodes on the surfaces of the second OICM with appropriate polarity to pump oxygen into volume B and of sufficient magnitude to maintain the gas within this volume with excess air under all compositions of external sample gas.

3. An electrochemical apparatus as claimed in Claim 2 wherein another external circuit applies a constant voltage difference, typically 0.5V, across the pair of electrodes on the-surfaces of the first OICM with polarity such that the electrode within volume B is held positive with respect to the other electrode; the effect is to generate a variable current I1 in this circuit which pumps oxygen from volume A to volume B when the external sample gas contains excess oxygen and from B to A when the external sample gas is fuel-rich; the air-to-fuel ratio is determined from a measurement of I1 in both oxygen-excess and fuel excess regions.

4. An electrochemical apparatus as claimed in Claim 1 wherein one external circuit supplies a current I2 to the pair of electrodes on the surfaces of the second OICM.

another external circuit applies a constant voltage difference, typically 0.5V, across the pair of electrodes on the surfaces of the first OICM with polarity such that the electrode within volume B is held positive with respect to the other electrode; the effect is to generate a variable current Ii in this circuit, dependent upon the air-to-fuel ratio of the external sample gas information regarding the magnitude and direction of the current I1 is fed into the circuit supplying the current I2 which automatically adjusts I2 such that (I1 + 12) is maintained approximately constant and of sufficient magnitude to ensure that the gas within volume B contains excess oxygen under all possible compositions of the external sample gas; the air-to-fuel ratio is determined from a measurement of I1 in both oxygen-excess and fuel-excess regions.

5. An electrochemical apparatus as claimed in Claim 1 wherein an electrical circuit applies a constant voltage, typically 0.5V, across the-two electrodes on the first OICM so that the electrode within volume B is driven positive with respect to that in A with the effect that oxygen is pumped from A to B only, by a variable current Ii; the first diffusion barrier is constructed to be more restrictive than the second barrier; the maximum current for a sample gas with fuel-excess (X < 1) is then small and the air-to-fuel ratio is determined from a measurement of I1 in oxygen-excess mixtures provided the apparatus is operated at currents I1 in excess of the maximum in the x < l region and indicates that the sample gas is not fuel-rich.

6. An electrochemical apparatus as claimed in Claim 2 wherein another external circuit senses the emf generated between the electrodes on each surface of the first OICM and generates a current I which is applied to the electrodes on the surface of the third OICM to pump oxygen into or out of volume A in the appropriate direction to hold the emf across the first OICM constant, typically 0.2-0.5V; the air-to fuel ratio of the sample gas in both the oxygen-excess and fuel-excess regions is determined from a measurement of Ii.

7. An electrochemical apparatus as claimed in Claim 2 wherein another electrical circuit applies a constant voltage across the electrodes on the third OICM with polarity such that the electrode within the sample gas is held positive with respect to that within chamber A, typically by 0.5V and results in a current I1 in the circuit which varies with the air-to-fuel ratio of the external sample gas.

a third circuit monitors the emf across the electrodes on the first OICM; the emf E takes large values ( > 500mV) when the sample gas is derived from a fuel-excess mixture and takes small values ( < 500mV) when the sample gas is derived from an air-excess mixture.

the current I1 provides a measure of the air-to-fuel ratio of the sample gas in the oxygen-excess region and the emf E indicates when the sample gas is in the fuel-excess region.

8. An apparatus for gas analysis substantially as described herein with reference to Figures 1-5 of the accompanying drawings.