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Detection of NO and unburnt gases in combustion processes using SnO2 sensors operated at different temperatures
700
Sensors and Actuators
B, 7 ( 1992) 700&703
Detection of NO and unburnt gases in combustion processes using SnO, sensors operated at different temperatures J. Huusko, H. Torvela and V. Lantto Microelectronics
Laboratory,
University
of Oulu, SF-90570
Oulu (Finland)
Abstract Four SnO,-based semiconductor gas sensors are operated simultaneously at different temperatures and their conductance responses recorded. The sensors are manufactured using thick-film technology. A small concentration (0.05 mol%) of Pd is doped in the SnO, base material as a catalytic additive. This sensor type has already been found in laboratory tests to be sufficiently selective to CO at about 500 “C in the presence of NO (and SO& and hence could be used to monitor CO concentrations in combustion gases. The present study was undertaken in order to evaluate the possibility of using the same sensor type, operating at different temperatures, for the simultaneous monitoring of both NO and unburnt gas components like hydrocarbons, in addition to CO, present in combustion gases. A dry synthetic combustion gas is used as the carrier gas, which is mixed with CO, NO and CH,. The last gas component is intended to simulate the unburnt hydrocarbon fraction. The concentrations of the active gas components vary from 100 to 1000 ppm (v/v) and the operating temperatures of the sensors are 200, 300, 400 and 500 “C.
1. Introduction The development of semiconductor gas sensors for the detection of combustion gases has been in progress for several years [ l-61. The selectivity of these sensors has been one of the main aspects in the development work. Sensors have been tested both in laboratory and in field experiments. It was shown in our latest laboratory tests and in experiments conducted in real combustion processes that the SnO,-based thick-film sensors containing a small amount of Pd as a catalytic additive can follow the concentration of CO in the presence of NO with sufficient accuracy at an operating temperature of around 500 “C [ 11. The CO-selectivity of the sensors, however, decreases with decreasing operation temperature, but the sensitivity to NO becomes more dominant [ 1,4]. Hydrocarbons, existing in the emission gases as a result of incomplete combustion, are also a very important group of gases for testing with Pddoped sensors. Together with hydrocarbons, the CO level is usually also relatively high in combustion gases. In the present laboratory study, a new approach was introduced in order to test the detection of 0925-4005/92/%5.00
unburnt hydrocarbons, simulated by CH4, CO and NO in a gas mixture, by using four identical Pd-doped SnO, sensors operating at 200, 300, 400 and 500 “C, respectively. The purpose of the measurements was to check the capability of this sensor type for monitoring both unburnt gases and NO in combustion processes when sensors in a sensor array system are operating at different constant temperatures.
2. Experimental procedure The thick-film sensors used in the present study were screen printed on alumina substrates containing preprinted gold electrodes and a Pt heating resistor on the reverse side of the substrate. The Sn02 paste (P106) used in printing the gas-sensitive layer contained 0.05 mol% Pd. In laboratory tests, four identical sensors were mounted in a test chamber and the sensors were heated with their own heating resistors to 200, 300, 400 and 500 “C, respectively. The sensor temperatures were controlled by measuring and adjusting the current passing through the Pt heating resistor of the sensors. A dry synthetic combustion gas consisting of 80% NZ, 15% CO* @ 1992 -
Elsevier Sequoia. All rights reserved
701
TABLE I. Laboratory CH, in dry synthetic 300, 400 and 500 “C
10.0
test procedure used for detecting NO, CO and combustion gas. Test temperatures were 200, .
E NO concentrations 100, 500 and 1000 ppm (a) No other gases. (b) At a constant five minutes from (c) At a constant five minutes from
CH, concentration. start. CO concentration. start.
Introduction
of the gas after
Introduction
of the gas after
(d) At constant CH, and CO concentrations (200 ppm/500 ppm). Introduction of the gases before the registration of the response values. CH, concentrations 100, 500 and 1000 ppm (a) No other gases. (b) At a constant NO concentration the gas after five minutes from start.
(200 ppm).
Introduction
of
(c) At a constant CO concentration (500 ppm). Introduction of the gas after five minutes from start. (d) At constant NO and CO concentrations (200 ppm/500 ppm). Introduction of the gases before the registration of the response values.
and 5% O2 was used as the carrier gas. The carrier gas was mixed with CO, NO and CH4. The concentrations of the active gas components varied from 100 to 1000 parts per million by volume (ppm) and a gas blender was used for the regulation of concentrations. The test procedure is shown in Table 1.
I t 8 5 : ; a z
1.0
.l
.Ol
Al01
, 0
I
*
300
I
.
600
I
-
900
I
-
I
’
I
1200 1500 lsoo
-
I
-
2100 2400
Time Is] Fig. I. The relative conductance response of the PI06 sensor at 200, 300, 400 and 500 “C to NO at concentrations of 100, 500 and 1000 ppm. The carrier gas was dry synthetic combustion gas.
0
300
600
!I00
1200 1500 1800 2100 2400
Time ls]
3. Results and discussion 3.1. NO response
The NO response of the Pd-doped SnO* sensor (P106) was strongest at 200 “C and the conductance of the sensor at 1000 ppm of NO was less than l/500 of the initial value at zero NO concentration, as is shown in Fig. 1. A clear conductance change was observed at all NO concentrations used. At 200 and 300 “C, after the introduction of 100 ppm of NO into the test chamber, the conductance decreased to l/ 100 of the value in dry synthetic combustion gas. When the concentration of NO was increased from 100 to 500 ppm, the conductance decreased further to about half of the preceding value, and the concentration change from 500 to 1000 ppm still reduced the conductance by one third. At 400 “C, and especially at 500 “C, the change of NO from 100 ppm to 500 ppm and further to 1000 ppm did not significantly affect
Fig. 2. The relative conductance response of the PI06 sensor at 200, 300, 400 and 500 “C to NO at concentrations of 100, 500 and 1000 ppm in the presence of 200 ppm of CH,. The carrier gas was dry synthetic combustion gas.
the conductance of the sensor, as is shown in Fig. 1. The results of the relative conductance response of the sensor (P106) to NO, in the presence of CH4 and CO individually or in combination, are shown in Figs. 2, 3 and 4. The effect of NO on the conductance response of the sensor was smaller in the presence of either CH4 or CO as compared to the effect in pure synthetic combustion gas. This was the case at 200 and 300 “C, when the concentration of NO was changed from 0 to 100 ppm. Some effect of NO on the sensor conductance was now observed also at 400 and 500 “C, especially when CO was present. According to the laboratory tests, in a special case like this, where the
702
500 ppm 100.0
INO:
100
co 500
.’[ppml
.Ol , 0
.
1000 500
t
t
100
0
)
t
temperatures amplification would be necessary.
200-Jc , . , . , . , . , . , . , . 300 600 900 1200 1500 1800 2100 2400 Time [s]
Fig. 3. The relative conductance response of the PI06 sensor at 200, 300, 400 and 500 “C to NO at concentrations of 100, 500 and 1000 ppm in the presence of 500 ppm of CO. The carrier gas was dry synthetic combustion gas.
200ppmcm + +500DImIco
IO.04 NO: --
.OOl~ 0
,
loo
500
1000 500
100
0
I
of the sensor current
3.2. CH, response The introduction of CH4 into the test chamber decreased the conductance of the sensor at 200 and 300 “C, as is shown in Fig. 5. At these temperatures the conductance response of the sensor to CH, was not stable. The conductance values were different when the CH, concentration was increased from 0 to 100 and further to 500 and to 1000 ppm as compared to the case when the concentration was decreased back to zero using the same steps (Fig. 5). At 500 “C, CH4 increased the conductance of the sensor similarly to that of CO (Fig. 5). On the other hand, the conductance increase caused by CH4 was relatively low as compared to that caused by CO. In the presence of 500 ppm of CO, the introduction of 1000 ppm of CH4 increased the conductance by no more than about 15% (Fig. 6), and in the presence of NO the increase was practically the same. The fact that both CO and CH4, and obviously also other hydrocarbons, have the same kind of response characteristics at around 500 “C would be an asset from the point of view of the development of a sensor for combustion monitoring.
, , , , , . , , , . , . , . I 300 600 900 1200 1500 1SlM 2100 2400 Time [sl
Fig. 4. The relative conductance response of the P106 sensor at 200, 300, 400 and 500 “C to NO at concentrations of 100, 500 and 1000 ppm in the presence of 200 ppm of CH, and 500 ppm of CO. The carrier gas was dry synthetic combustion gas.
concentration of CO remains approximately constant and the concentration of NO varies from some hundreds to some thousands of ppm, it would be possible to use this sensor type for monitoring of the NO level at around 400 “C. At this temperature range, the recovery time of the sensor after exposure to NO is relatively short, and the conductance values are high enough to make amplifiers unnecessary in actual measurements. On the other hand, in combustion processes where the concentration level of NO is in the range of some hundreds of ppm, the sensor operating at 200-300 “C could be used for monitoring NO in the presence of non-polar hydrocarbons and CO. At these low
4. Conclusions The decreasing effect of NO on the sensor conductance was found to be strongest at 200 “C. The
.l!
. 0
,’
300
,
, . , . , . , . , . , . I 600 900 1200 1500 1800 2100 2400 Time Is]
Fig. 5. The relative conductance response of the P106 Sensor at 200, 300, 400 and 500 “C to CH, at concentrations of 100, 500 and 1000 ppm. The carrier gas was dry synthetic combustion gas.
703
appropriate operating temperatures would be around 500 “C for CO and CH4 and around 200300 “C for NO. The conductance increase caused by CH4 was quite small compared to that caused by CO. The monitoring of NO seems to be possible only at low coexisting concentrations of SO2 [l], as for instance in the case when sulphur dioxide is captured by limestone. o.o! . , , , . , . , . , . , . , . 0
300
600
900
1200 1500 1800 2100 2400
Time[s]
Fig. 6. The relative conductance response of the PI06 sensor at 500 “C to CH4 at concentrations of 100, 500 and 1000 ppm in the presence of 500ppm of CO. The carrier gas was dry synthetic combustion gas.
ratio of the conductance without NO to that with 1000 ppm NO was about 500 at this temperature. This ratio decreased when CH4 or CO was also present. The concentration change of NO could still be detected at 200 “C and even at 300 “C. The conductance of the sensor decreased on exposure to CH4 at low temperatures. When the temperature was raised to 500 “C, the conductance response was positive, as in the case of CO, so that CH4 increased the conductance at all testing concentrations. On the other hand, the CH4 response was much’weaker than the CO response and was also independent of the presence of NO. Some support is obtained for the conclusion that the simultaneous detection of both NO and unburnt gas components, i.e., CO and hydrocarbons, in some combustion gases could be possible using an array of Pd-doped SnO,-based gas sensors operating at different temperatures. The
Acknowledgements
This work was supported by the National Combustion Research Programme, LIEKKI, in Finland.
References I H. Torvela, J. Huusko and V. Lantto, Reduction of the interference caused by NO and SO2 in the CO response of Pd-catalysed SnO, combustion gas sensors, Sensors and Actuators B, 4 (1991) 479-484. 2 H. Torvela, A. Harkoma and S. Leppiivuori, Detection of the concentration of CO using SnO, gas sensors in combustion gases of different fuels, Sensors and Actuators, 17 (1989) 3699375. 3 H. Torvela, P. Romppainen and S. Leppavuori, Detection of CO levels in combustion gases by thick-film SnO, sensor, Sensors and Actuurors, 14 (1988) 19-25. 4 A. Harkoma, H. Torvela, P. Romppainen and S. Leppavuori, Detection of CO levels by semiconductor gas sensor in combustion gases containing NO, Cornbust. Sci. Technol., 62 ( 1988) 21-29. 5 P. Romppainen, H. Torvela, J. VllnIinen and S. Lepplvuori, Effect of CH,, SOs and NO on the CO response of an SnO,-based thick-film gas sensor in combustion gases, Sensors and Actuators, 8 (1985) 271-279. 6 V. Lantto, P. Romppainen and S. Leppavuori, The response of an SnO, gas sensor to CO and NO alone and in combination, Rep. S 96, Department of Electrical Engineering, University of Oulu, Oulu, Finland, 1987, 22pp.
Sensors and Actuators
B, 7 ( 1992) 700&703
Detection of NO and unburnt gases in combustion processes using SnO, sensors operated at different temperatures J. Huusko, H. Torvela and V. Lantto Microelectronics
Laboratory,
University
of Oulu, SF-90570
Oulu (Finland)
Abstract Four SnO,-based semiconductor gas sensors are operated simultaneously at different temperatures and their conductance responses recorded. The sensors are manufactured using thick-film technology. A small concentration (0.05 mol%) of Pd is doped in the SnO, base material as a catalytic additive. This sensor type has already been found in laboratory tests to be sufficiently selective to CO at about 500 “C in the presence of NO (and SO& and hence could be used to monitor CO concentrations in combustion gases. The present study was undertaken in order to evaluate the possibility of using the same sensor type, operating at different temperatures, for the simultaneous monitoring of both NO and unburnt gas components like hydrocarbons, in addition to CO, present in combustion gases. A dry synthetic combustion gas is used as the carrier gas, which is mixed with CO, NO and CH,. The last gas component is intended to simulate the unburnt hydrocarbon fraction. The concentrations of the active gas components vary from 100 to 1000 ppm (v/v) and the operating temperatures of the sensors are 200, 300, 400 and 500 “C.
1. Introduction The development of semiconductor gas sensors for the detection of combustion gases has been in progress for several years [ l-61. The selectivity of these sensors has been one of the main aspects in the development work. Sensors have been tested both in laboratory and in field experiments. It was shown in our latest laboratory tests and in experiments conducted in real combustion processes that the SnO,-based thick-film sensors containing a small amount of Pd as a catalytic additive can follow the concentration of CO in the presence of NO with sufficient accuracy at an operating temperature of around 500 “C [ 11. The CO-selectivity of the sensors, however, decreases with decreasing operation temperature, but the sensitivity to NO becomes more dominant [ 1,4]. Hydrocarbons, existing in the emission gases as a result of incomplete combustion, are also a very important group of gases for testing with Pddoped sensors. Together with hydrocarbons, the CO level is usually also relatively high in combustion gases. In the present laboratory study, a new approach was introduced in order to test the detection of 0925-4005/92/%5.00
unburnt hydrocarbons, simulated by CH4, CO and NO in a gas mixture, by using four identical Pd-doped SnO, sensors operating at 200, 300, 400 and 500 “C, respectively. The purpose of the measurements was to check the capability of this sensor type for monitoring both unburnt gases and NO in combustion processes when sensors in a sensor array system are operating at different constant temperatures.
2. Experimental procedure The thick-film sensors used in the present study were screen printed on alumina substrates containing preprinted gold electrodes and a Pt heating resistor on the reverse side of the substrate. The Sn02 paste (P106) used in printing the gas-sensitive layer contained 0.05 mol% Pd. In laboratory tests, four identical sensors were mounted in a test chamber and the sensors were heated with their own heating resistors to 200, 300, 400 and 500 “C, respectively. The sensor temperatures were controlled by measuring and adjusting the current passing through the Pt heating resistor of the sensors. A dry synthetic combustion gas consisting of 80% NZ, 15% CO* @ 1992 -
Elsevier Sequoia. All rights reserved
701
TABLE I. Laboratory CH, in dry synthetic 300, 400 and 500 “C
10.0
test procedure used for detecting NO, CO and combustion gas. Test temperatures were 200, .
E NO concentrations 100, 500 and 1000 ppm (a) No other gases. (b) At a constant five minutes from (c) At a constant five minutes from
CH, concentration. start. CO concentration. start.
Introduction
of the gas after
Introduction
of the gas after
(d) At constant CH, and CO concentrations (200 ppm/500 ppm). Introduction of the gases before the registration of the response values. CH, concentrations 100, 500 and 1000 ppm (a) No other gases. (b) At a constant NO concentration the gas after five minutes from start.
(200 ppm).
Introduction
of
(c) At a constant CO concentration (500 ppm). Introduction of the gas after five minutes from start. (d) At constant NO and CO concentrations (200 ppm/500 ppm). Introduction of the gases before the registration of the response values.
and 5% O2 was used as the carrier gas. The carrier gas was mixed with CO, NO and CH4. The concentrations of the active gas components varied from 100 to 1000 parts per million by volume (ppm) and a gas blender was used for the regulation of concentrations. The test procedure is shown in Table 1.
I t 8 5 : ; a z
1.0
.l
.Ol
Al01
, 0
I
*
300
I
.
600
I
-
900
I
-
I
’
I
1200 1500 lsoo
-
I
-
2100 2400
Time Is] Fig. I. The relative conductance response of the PI06 sensor at 200, 300, 400 and 500 “C to NO at concentrations of 100, 500 and 1000 ppm. The carrier gas was dry synthetic combustion gas.
0
300
600
!I00
1200 1500 1800 2100 2400
Time ls]
3. Results and discussion 3.1. NO response
The NO response of the Pd-doped SnO* sensor (P106) was strongest at 200 “C and the conductance of the sensor at 1000 ppm of NO was less than l/500 of the initial value at zero NO concentration, as is shown in Fig. 1. A clear conductance change was observed at all NO concentrations used. At 200 and 300 “C, after the introduction of 100 ppm of NO into the test chamber, the conductance decreased to l/ 100 of the value in dry synthetic combustion gas. When the concentration of NO was increased from 100 to 500 ppm, the conductance decreased further to about half of the preceding value, and the concentration change from 500 to 1000 ppm still reduced the conductance by one third. At 400 “C, and especially at 500 “C, the change of NO from 100 ppm to 500 ppm and further to 1000 ppm did not significantly affect
Fig. 2. The relative conductance response of the PI06 sensor at 200, 300, 400 and 500 “C to NO at concentrations of 100, 500 and 1000 ppm in the presence of 200 ppm of CH,. The carrier gas was dry synthetic combustion gas.
the conductance of the sensor, as is shown in Fig. 1. The results of the relative conductance response of the sensor (P106) to NO, in the presence of CH4 and CO individually or in combination, are shown in Figs. 2, 3 and 4. The effect of NO on the conductance response of the sensor was smaller in the presence of either CH4 or CO as compared to the effect in pure synthetic combustion gas. This was the case at 200 and 300 “C, when the concentration of NO was changed from 0 to 100 ppm. Some effect of NO on the sensor conductance was now observed also at 400 and 500 “C, especially when CO was present. According to the laboratory tests, in a special case like this, where the
702
500 ppm 100.0
INO:
100
co 500
.’[ppml
.Ol , 0
.
1000 500
t
t
100
0
)
t
temperatures amplification would be necessary.
200-Jc , . , . , . , . , . , . , . 300 600 900 1200 1500 1800 2100 2400 Time [s]
Fig. 3. The relative conductance response of the PI06 sensor at 200, 300, 400 and 500 “C to NO at concentrations of 100, 500 and 1000 ppm in the presence of 500 ppm of CO. The carrier gas was dry synthetic combustion gas.
200ppmcm + +500DImIco
IO.04 NO: --
.OOl~ 0
,
loo
500
1000 500
100
0
I
of the sensor current
3.2. CH, response The introduction of CH4 into the test chamber decreased the conductance of the sensor at 200 and 300 “C, as is shown in Fig. 5. At these temperatures the conductance response of the sensor to CH, was not stable. The conductance values were different when the CH, concentration was increased from 0 to 100 and further to 500 and to 1000 ppm as compared to the case when the concentration was decreased back to zero using the same steps (Fig. 5). At 500 “C, CH4 increased the conductance of the sensor similarly to that of CO (Fig. 5). On the other hand, the conductance increase caused by CH4 was relatively low as compared to that caused by CO. In the presence of 500 ppm of CO, the introduction of 1000 ppm of CH4 increased the conductance by no more than about 15% (Fig. 6), and in the presence of NO the increase was practically the same. The fact that both CO and CH4, and obviously also other hydrocarbons, have the same kind of response characteristics at around 500 “C would be an asset from the point of view of the development of a sensor for combustion monitoring.
, , , , , . , , , . , . , . I 300 600 900 1200 1500 1SlM 2100 2400 Time [sl
Fig. 4. The relative conductance response of the P106 sensor at 200, 300, 400 and 500 “C to NO at concentrations of 100, 500 and 1000 ppm in the presence of 200 ppm of CH, and 500 ppm of CO. The carrier gas was dry synthetic combustion gas.
concentration of CO remains approximately constant and the concentration of NO varies from some hundreds to some thousands of ppm, it would be possible to use this sensor type for monitoring of the NO level at around 400 “C. At this temperature range, the recovery time of the sensor after exposure to NO is relatively short, and the conductance values are high enough to make amplifiers unnecessary in actual measurements. On the other hand, in combustion processes where the concentration level of NO is in the range of some hundreds of ppm, the sensor operating at 200-300 “C could be used for monitoring NO in the presence of non-polar hydrocarbons and CO. At these low
4. Conclusions The decreasing effect of NO on the sensor conductance was found to be strongest at 200 “C. The
.l!
. 0
,’
300
,
, . , . , . , . , . , . I 600 900 1200 1500 1800 2100 2400 Time Is]
Fig. 5. The relative conductance response of the P106 Sensor at 200, 300, 400 and 500 “C to CH, at concentrations of 100, 500 and 1000 ppm. The carrier gas was dry synthetic combustion gas.
703
appropriate operating temperatures would be around 500 “C for CO and CH4 and around 200300 “C for NO. The conductance increase caused by CH4 was quite small compared to that caused by CO. The monitoring of NO seems to be possible only at low coexisting concentrations of SO2 [l], as for instance in the case when sulphur dioxide is captured by limestone. o.o! . , , , . , . , . , . , . , . 0
300
600
900
1200 1500 1800 2100 2400
Time[s]
Fig. 6. The relative conductance response of the PI06 sensor at 500 “C to CH4 at concentrations of 100, 500 and 1000 ppm in the presence of 500ppm of CO. The carrier gas was dry synthetic combustion gas.
ratio of the conductance without NO to that with 1000 ppm NO was about 500 at this temperature. This ratio decreased when CH4 or CO was also present. The concentration change of NO could still be detected at 200 “C and even at 300 “C. The conductance of the sensor decreased on exposure to CH4 at low temperatures. When the temperature was raised to 500 “C, the conductance response was positive, as in the case of CO, so that CH4 increased the conductance at all testing concentrations. On the other hand, the CH4 response was much’weaker than the CO response and was also independent of the presence of NO. Some support is obtained for the conclusion that the simultaneous detection of both NO and unburnt gas components, i.e., CO and hydrocarbons, in some combustion gases could be possible using an array of Pd-doped SnO,-based gas sensors operating at different temperatures. The
Acknowledgements
This work was supported by the National Combustion Research Programme, LIEKKI, in Finland.
References I H. Torvela, J. Huusko and V. Lantto, Reduction of the interference caused by NO and SO2 in the CO response of Pd-catalysed SnO, combustion gas sensors, Sensors and Actuators B, 4 (1991) 479-484. 2 H. Torvela, A. Harkoma and S. Leppiivuori, Detection of the concentration of CO using SnO, gas sensors in combustion gases of different fuels, Sensors and Actuators, 17 (1989) 3699375. 3 H. Torvela, P. Romppainen and S. Leppavuori, Detection of CO levels in combustion gases by thick-film SnO, sensor, Sensors and Actuurors, 14 (1988) 19-25. 4 A. Harkoma, H. Torvela, P. Romppainen and S. Leppavuori, Detection of CO levels by semiconductor gas sensor in combustion gases containing NO, Cornbust. Sci. Technol., 62 ( 1988) 21-29. 5 P. Romppainen, H. Torvela, J. VllnIinen and S. Lepplvuori, Effect of CH,, SOs and NO on the CO response of an SnO,-based thick-film gas sensor in combustion gases, Sensors and Actuators, 8 (1985) 271-279. 6 V. Lantto, P. Romppainen and S. Leppavuori, The response of an SnO, gas sensor to CO and NO alone and in combination, Rep. S 96, Department of Electrical Engineering, University of Oulu, Oulu, Finland, 1987, 22pp.