WO1999057558A1

WO1999057558A1 - Gas sensing assembly

Info

Publication number: WO1999057558A1
Application number: PCT/GB1999/001384
Authority: WO
Inventors: Bryan Stewart Hobbs; Constantine Dean Capetanopolous
Original assignee: Sem Corporation
Priority date: 1998-05-06
Filing date: 1999-05-04
Publication date: 1999-11-11
Also published as: GB9809653D0; AU3834799A

Abstract

A gas sensing assembly to which a gas stream is supplied. The sensing assembly comprises a catalyst converter (7) for converting one or more higher nitrogen oxides in the gas stream to NO; and an electrochemical gas sensor (8) for sensing at least one of the gases in the gas stream output from the converter, characterized in that the converter is a catalytic reactor which utilizes a catalyst comprising a molybdenum alloy.

Description

1 GAS SENSING ASSEMBLY

The invention relates to a gas sensing assembly.

It is common practice to monitor NO content of gases for example from automotive exhausts and other combustion sources due to the environmental pollution impact, producing acid rain and photochemical smog. There is also an increasing requirement to monitor other, higher oxides of nitrogen such as N0₂ to provide a total N0_X measurement from combustion sources. The need to detect N0₂ (and other higher oxides of nitrogen) is particularly important in respect of diesel engines where these oxides can represent a more significant component of the exhaust gas.

The measurement of NO_x gases in ambient air is also important for environmental monitoring for safety and air quality reasons.

Chemiluminescence analysers can be used to make these measurements but are very costly, not easily adapted into portable instruments and require specialist operators to use. In particular, they require ozonizers and optical equipment . Examples of known chemiluminescence sensors are described in US-A-5358874 and JP-A-53079793. Electrochemical sensors represent a considerably cheaper, more portable and simple alternative to the chemiluminescent analyser.

Both NO and N0₂ electrochemical sensors are used commercially to measure N0_X gases from stationary combustion sources and for environmental monitoring. The NO sensors are especially suitable for these measurements; N0₂ sensors however, generally exhibit a slower response than NO sensors due to the electrocatalyst employed and cannot be used in all applications, particularly automotive applications where a fast response is required.

Typical requirements which are now laid down by the relevant authorities are analyser response times for NO of T₉₀ of 4.5 seconds and T_g5 of 5.5 seconds . DE-A-4316970 describes an ammonia sensor in which the ammonia is converted to NO and then the NO is sensed using an electrochemical sensor. Although this describes the principle of operation of the sensor of that invention, it does not describe how apparatus would be constructed to implement it. In particular, the system described in DE-A- 4316970 is not for NO_x exhaust measurement but for ammonia gas determination and thus there is no consideration of response time. Sufficiently fast response N0₂ sensors are available with alternative electrode catalysts but these generally exhibit cross interferences to sulphur gases (S0₂) which can be present , dependent on the fuel being used by the automotive source; the slower response electrodes possess much lower responses to S0₂.

In order to decrease response time, it is desirable to position the converter as close as possible to the electrochemical sensor. However, this is difficult in practice since in the case of catalytic converters, the catalyst material (typically molybdenum metal) must be operated at high temperatures in the order of 450°C. If the converter is placed too close to the sensor then there is a risk of overheating the sensor causing it to be damaged and/or to perform poorly. Separately, there is also a need to remove nitrogen dioxide as an interfering gas from certain electrochemical sensors such as sulphur dioxide sensors .

In accordance with the present invention, a gas sensing assembly to which a gas stream is supplied comprises a converter for converting one or more higher nitrogen oxides in the gas stream to NO; and an electrochemical gas sensor for sensing at least one of the gases in the gas stream output from the converter and is characterized in that the converter is a catalytic reactor which utilizes a catalyst comprising a molybdenum alloy. With this invention, we overcome the problems mentioned above by utilizing a special catalyst which we have developed and which can be operated at much lower temperatures, typically in the order of 150-200°C. Thus, the converter can be sited much closer to the electrochemical sensor without overheating the sensor. This leads to a much faster response time than has been achieved conventionally.

Preferably, the catalyst comprises a molybdenum/ manganese alloy. A preferred alloy contains 20%-80% by weight molybdenum and a balancing amount of manganese. Currently, the most preferred alloy contains substantially 70% Mn and 30% Mo.

In operation, the converter converts one or more higher nitrogen oxides in the gas stream, for example nitrogen dioxide, before the gas stream reaches the electrochemical gas sensor. This improves the operation of certain electrochemical sensors such as an S0₂ sensor (and any other similar sensor) that exhibit large undesirable cross sensitivities to N0₂ gas that cannot be removed through filtration without removing the (target) S0₂ gas. Use of a converter in connection with an electrochemical S0₂ sensor removes this undesirable cross sensitivity by converting the interfering N0₂ gas to NO which does not produce a response on the electrochemical sensor.

The invention also enables the use of an electrochemical NO gas sensor which has a much faster response time than an electrochemical N0₂ sensor, for sensing N0₂. This is achieved by first converting the N0₂ or other nitrogen oxides to NO and then detecting the NO. In addition, the costly and complex equipment required for chemiluminescence is avoided.

The present invention enables much cheaper electrochemical NO gas sensors to be employed which also provide the fast response needed in automotive applications. Furthermore, this method makes possible the elimination of sophisticated conditioning systems that are necessary to maintain N0₂ sample integrity in the sample line .

The reactor is capable of reducing higher nitrogen oxides to NO in both anaerobic conditions which prevail in exhaust emissions and also in air samples where the oxygen content is much higher.

Typically, the catalytic reactor will be insensitive to NO. This has the advantage that the sensing assembly can be used to monitor the total N0_X (i.e. NO + other higher nitrogen oxides) in the gas stream.

In a simple application, the output signal from the electrochemical gas sensor can be used to indicate the presence of higher nitrogen oxides such as N0₂ and/or NO by either bypassing the converter or simply turning the converter heater ON or OFF. However, preferably the sensor further comprises a processor connected to the electrochemical NO sensor to generate an output, in response to the output signal, indicative of the concentration of NO. This enables the gas content to be more accurately monitored and recorded or displayed.

Another embodiment of this invention associated with the instrument's microprocessor operation is the documentation of the sensor's and converter's performance by independent means through an appropriate printout or other indication of proper calibration.

Still another feature of this invention places the converter in the exhaust system of an internal combustion engine so that N0₂ reduction takes place at the instrument's probe. This eliminates the need for a sophisticated sampling system such as a heated sample line or permeation drier, since there is no N0₂ present any longer in the extracted sample. This could also reduce the power requirement for the converter which can also derive some (or all) of its heat from the exhaust system. Some examples of gas sensing assemblies according to the invention will now be described with reference to the accompanying drawings, in which:- Figure 1 is a block diagram of a first example of a sensing assembly for use with an automobile exhaust system_;

Figure 2 illustrates graphically the results of testing the system's response to NO gas in the presence and absence of the catalytic reactor;

Figure 3 illustrates graphically the results of testing the system in the presence of N0₂;

Figures 4 to 6 illustrate alternative constructions of the gas sensing assembly in which the sensor and converter are formed together as a single unit;

Figure 7 is a view similar to Figure 1 but of a further example; and,

Figure 8 illustrates a typical output print.

The system comprises a probe 1 which can be removably located in an exhaust 2 of a fuel burning source such as a petrol driven or diesel driven internal combustion engine of an automobile (not shown) . A sample of the exhaust gas is extracted through the probe 1 by a pump 3, the sample being drawn through a treatment system 4 for removing moisture and for cooling the gas. Moisture removal is essential when acid gases such as N0₂ and S0₂ are present, to avoid these gases from being absorbed in condensing moisture and thus not being passed onto the subsequent analytical system. The treated gas sample then passes to a switch 5 which in one position causes the gas to pass along a bypass conduit 6 and in its other position to pass to a catalytic converter 7.

Any N0₂ or higher nitrogen oxides in the exhaust gas will be converted in the catalytic converter 7 to NO. For N0₂, this is according to the reaction:

N0₂ + catalyst = NO + X 0₂

As will be shown later, the NO in the exhaust gas will be unaffected by the catalytic converter 7.

The catalytic converter 7 includes a catalyst comprising a molybdenum alloy which in the most preferred arrangement contains substantially 70% Mn and 30% Mo. This material reduces N0₂ to NO at a much lower temperature than conventional catalysts and can be operated typically in the region of 150 to 200°C. The NO from the catalytic converter 7, either produced from conversion from N0₂ or higher oxides or having existed already in the exhaust gas, then passes to an electrochemical gas sensor 9 which is adapted to sense S0₂. This sensor benefits from the reduction of N0₂ to NO by having an interfering gas eliminated from the sample stream. The sensor 9 would also be coupled with a microprocessor in the same way as a sensor 8 (see below) so that an indication of S0₂ concentration can be provided.

The gas then passes to an electrochemical NO gas sensor 8 such as an AUTONO sensor made by City Technology Limited. The electrochemical gas sensors 8 and 9 both operate in accordance with the principles described in "Liquid Electrolyte Fuel Cells", B S Hobbs et al, Chapter 6 of Techniques and Mechanisms in Gas Sensing, Adam Hilger 1991 and GB-A-1571282. The sensor 8 generates an output signal relating to the concentration of NO in the gas and this signal is then processed, after conversion to digital form, by a microprocessor 30. The microprocessor 30 generates a control signal, which is applied to a display 31 such as a monitor, indicative of the concentration of NO in the sample of gas .

Since the catalyst reactor 7 is operated at a relatively low temperature, it can be placed close to the sensors 8,9 without affecting significantly their performance. This enables a much faster response to be obtained from the apparatus than with conventional apparatus .

Figures 4 to 6 illustrate alternative constructions of the gas sensing assembly and in particular the converter 7 and sensor 8, with the sensor 9 omitted.

Figure 4 shows a single NO sensor assembly consisting of a converter 11 housing a catalyst 12 and heated by a 7 heater 10 mounted on top of an electrochemical NO sensor 15 having a capillary opening 14 and possibly a filter 13.

Figure 5 shows an alternative single sensor assembly where thermoelectric elements 16 heat the converter while cooling the NO sensor.

Figure 6 shows an alternate design, where the capillary 14 is located in front of the converter 11. The benefit of this design is due to the fact that only a very small fraction of the gas enters the converter sensor assembly, thus requiring a much smaller converter with reduced power requirements. One possible operation of the system is to operate the thermoelectric element 16 in such a way that when the sensor 15 is in the OFF state, the converter is cooled while the sensor is heated to drive off any excess accumulated N0₂ (or higher oxides) from the electrolyte, resulting from the electrochemical reaction at the NO-sensor sensing electrode, namely:

NO + H₂0 = N0₂ + 2H⁺ + 2e NO + 3/2 H₂0 = 1/2 N₂0₅ + 3H⁺ + 3e

When the sensor is ON, the thermoelectric element 16 is switched polarity heating the converter for proper operation and cooling the sensor to make sure that no significant N0₂ gas is given off by the electrolyte to give a false response. Additional heating (not shown) may be needed for the converter. A temperature sensor 17 monitors, and enables control of, the thermoelectric elements . Figure 7 shows an instrument assembly that places the converter 11 inside the source's exhaust or stack 2. Since the N0₂ fraction is reduced to NO at the heated exhaust, the sample line leading to the pump 3 and sensor assembly 18 (similar to the sensors of Figures 4 to 6) is a simple hose made of any flexible material. Even though the converter is heated by the exhaust, an additional heater (not shown) may be required for proper converter operation. A sophisticated sampling system is not required.

Figure 8 demonstrates by means of the instrument's printer a preferred calibration protocol to ensure proper documentation of the sensor and converter operation as required by the EPA's Reference Method 20.

In order to illustrate the effect of the catalytic converter 7 on incoming NO gas, a test was carried out using apparatus similar to that shown in Figure 1 downstream of the pump 3 and excluding the sensor 9. The switch 5 was set to cause gas to pass along the bypass conduit 6. The output of the electrochemical sensor 8 was displayed as shown in Figure 2. The chart corresponds to 200μA full scale deflection while the chart speed was 2.5mm per minute (i.e. just over two chart divisions per minute) . The test gas used was lOOppm NO in nitrogen at 200ml/minute from a gas storage cylinder (not shown) . Initially, before the gas was switched on, the output signal was monitored to generate a zero base line as shown at 20. The test gas was then supplied directly to the electrochemical gas sensor 8 at a time 21 and it will be seen from Figure 2 that a large positive signal was generated 22. The gas supply was then terminated at time 23 and turned on again at 24. The switch 5 was then operated at time 25 so that the test gas first passed through the catalytic reactor 7 before reaching the electrochemical gas sensor 8. It will be seen that the resultant signal 26 is substantially the same as the signal obtained at 22 indicating that the catalytic reactor 7 has no effect on the NO. Figure 3 illustrates the results of an experiment using the same conditions as in Figure 2 but in this case with a test gas of lOOppm N0₂ in air at 200ml/minute. As before, a base line 30 was set and the N0₂ test gas switched on at 31 with the switch 5 set to pass gas to the bypass 6. At time 32, the switch 5 was operated to switch in the catalytic reactor 7 and it will be seen that a substantial output signal 33 was obtained. It can be seen, therefore, that the electrochemical gas sensor 8 itself is virtually unresponsive to N0₂ gas but on diverting the gas through the reactor, a full NO response occurs, indicating efficient conversion of N0₂ to NO in the reactor.

The small blips and transients on the traces are pressure effects arising from operation of the gas valve switching the reactor in and out respectively.

It will be appreciated therefore that in use, the output signal will provide a total N0_X concentration.

Claims

10 CLAIMS

1. A gas sensing assembly to which a gas stream is supplied, the sensing assembly comprising a converter for converting one or more higher nitrogen oxides in the gas stream to NO; and an electrochemical gas sensor for sensing at least one of the gases in the gas stream output from the converter, characterized in that the converter is a catalytic reactor which utilizes a catalyst comprising a molybdenum alloy.

2. An assembly according to claim 1, wherein the catalyst comprises a molybdenum/manganese alloy.

3. An assembly according to claim 2, wherein the alloy contains 20%-80% by weight molybdenum and a balancing amount of manganese.

4. An assembly according to claim 3, wherein the alloy contains substantially 70% Mn and 30% Mo.

5. An assembly according to any of the preceding claims, wherein the converter is a catalytic reactor.

6. An assembly according to claim 5, wherein the catalytic reactor is insensitive to NO in the gas stream.

7. An assembly according to any of the preceding claims, further comprising a processor connected to the electrochemical gas sensor to generate an output in response to the output signal, indicative of the concentration of the sensed gas in the gas stream.

8. An assembly according to any of the preceding claims, further comprising a second electrochemical gas sensor for sensing the concentration of S0₂.

9. An assembly according to any of the preceding claims, wherein the converter is adapted to convert all higher nitrogen oxides in the gas stream to NO.

10. An assembly according to any of the preceding claims, wherein the converter and electrochemical gas sensor are provided as a single unit. 11

11. A gas sensing assembly according to any of the preceding claims, arranged to monitor gas emitted from an exhaust system.

12. An assembly according to claim 11, wherein the converter is located in the exhaust system.

13. An assembly according to any of the preceding claims, wherein the electrochemical gas sensor senses the concentration of NO.