NOx sensor and method of measurement of NOx concentration

Abstract: An NOx sensor has a sensor element (8) made of an oxide, a resistance of which is varied in response to an NOx component in a gas to be measured, and a measuring portion (2) for measuring a resistance variation of the sensor element and for detecting an NOx concentration in the gas to be measured. A catalyst (6) is arranged at an upstream side of a flow of the gas to be measured with respect to the sensor element, which makes a partial pressure ratio of NO/NO 2 to an equilibrium state and removes a CO component from the gas to be measured. A heater (7) for adjusting a temperature is arranged at a position close to the sensor element, which keeps the temperatures of the sensor element and the catalyst constant. An O 2 sensor (9) for an adjustment is arranged at a position close to the sensor element.

Description: Background of the Invention (1) Field of the Invention The present invention relates to an NOx sensor having a sensor element made of an oxide, a resistance of which is varied in response to an NOx component in a gas to be measured, and measuring means for measuring resistance of the sensor element for detecting NOx concentration in the gas to be measured. (2) Related Art Statement As a method of measuring an NOx concentration in a gas to be measured such as exhaust gas from an incineration, which includes an NOx component such as nitrogen oxide, there is known a method of sampling a gas to be measured including an NOx component in for example a dust chimney, and measuring an NOx concentration of the sampled gas by means of an optical measuring apparatus. However, the optical measuring apparatus is expensive, and its responsiveness is poor since the sampling operation is necessary. In order to eliminate the drawbacks mentioned above, a direct insertion type semiconductor sensor is used recently. For example, in Japanese Patent Laid-Open Publication No. 6-222028, an NOx sensor comprising a response portion made of an oxide having a predetermined perovskite structure, and a conductivity measuring portion for measuring a conductivity of the response portion is disclosed. However, also in the direct insertion type semiconductor sensor mentioned above, there is no countermeasure for an influence of O 2 and CO components included in the gas to be measured with respect to the measured NOx concentration. Moreover, in the response portion, resistance varies in response to an amount of NOx (NO 2 +NO) i.e. a concentration thereof. However, if the ratio of amount (concentration) between NO 2 and NO i.e. the ratio of partial pressure between NO 2 and NO is varied, the resistance measured by the response portion is varied even for the same NOx amount. In this case, it is not the case that only the NOx component is selectively measured. Therefore, in the direct insertion type semiconductor sensor mentioned above, there is a drawback that the NOx concentration in the gas to be measured cannot be selectively measured in a highly precise manner, while the semiconductor sensor is cheap and shows excellent responsiveness as compared with the optical measuring apparatus. US-A-4840913 describes a NOx detector in which the gas being detected contacts a heated catalyst which oxidizes NO to NO 2 and oxidizes reducing gases such as CO, H 2 , hydrocarbons and alcohols, before contacting a non-selective oxide of nitrogen sensor which is made of SnO 2 or ZnO. Change of resistance of the sensor is sensed. Summary of the Invention An object of the present invention is to eliminate the drawbacks mentioned above and to provide an NOx sensor which can measure an NOx concentration in a gas to be measured selectively in a precise manner. According to the invention, there is provided a NOx sensor as set out in claim 1. In this construction, since the gas to be measured passes the catalyst which brings the partial pressure ratio of NO/NO 2 to an equilibrium state, and is then contacted with the sensor element under the condition that temperatures of the sensor element and the catalyst are maintained in a constant state by means of the heater, it is possible to perform a high precision measurement. Further, the relation between resistance measured by the sensor element and NOx concentration is determined in response to an O 2 concentration. Therefore, if the O 2 concentration is measured by the O 2 sensor for an adjustment and the NOx concentration is determined from the resistance value in response to the thus measured O 2 concentration, it is possible to perform a high precision measurement. Moreover, since the catalyst has a function for removing a CO component from the gas to be measured, it is possible to measure the NOx concentration with no CO influence. According to the invention, there is also provided a method of measurement of NO x concentration as set out in claim 7. Brief Description of the Drawing Fig. 1 is a schematic view for explaining one concept of an NOx sensor according to the invention; and Fig. 2 is a graph showing a relation between a resistance value measured in the NOx sensor and an NOx concentration according to the invention. Description of the Preferred Embodiments Fig. 1 is a schematic view for explaining one concept of an NOx sensor according to the invention. In Fig. 1, an NOx sensor according to the invention comprises a response portion 1 and a measuring portion 2. The response portion 1 is set in a dust chimney 3 through which a gas to be measured flows. The response portion 1 is constructed by arranging, from an upstream side of a flow of the gas to be measured, a catalyst 6, a heater for a temperature adjustment 7, a sensor element 8 and an O 2 sensor 9, all of which are arranged in an alumina protection tube 5 having a gas inlet portion 4. The measuring portion 2 is constructed by arranging a digital multimeter 10 for the sensor element 8, a digital multimeter 11 for the O 2 sensor 9 and a processing portion 12. Moreover, a numeral 13 is a constant-potential power supply of the heater for a temperature adjustment. The catalyst 6 is used for making a partial pressure ratio of NO/NO 2 to an equilibrium state and for removing a CO component from the gas to be measured. In this embodiment, the catalyst 6 is integrally formed, but it is possible to form the catalyst 6 separately corresponding to the objects mentioned above respectively. In the case of constructing the catalyst 6 separately, not only the same kinds of catalysts but also the other kinds of catalysts may be used for the catalyst 6. In order to achieve the objects mentioned above, it is preferred to use precious metals or oxides as the catalyst 6. As the precious metals, it is preferred to use platinum, rhodium or gold. As the oxides, it is preferred to use manganese oxide, cobalt oxide or tin oxide. The heater for a temperature adjustment 7 is used for maintaining the sensor element 8 and the catalyst 6 always at a constant temperature even if a temperature of the gas to be measured is varied. Therefore, it is preferred to arrange the heater for a temperature adjustment 7 between the sensor element 8 and the catalyst 6. The sensor element 8 is made of an oxide, a resistance of which is varied in response to an NOx component, if the oxide is contacted to the gas to be measured including an NOx component. As the oxide mentioned above, it is preferred to use metal oxide semiconductors. Among them, it is further preferred to use SnO 2 , TiO 2 or In 2 O 3 . If the sensor element 8 is made of the oxides mentioned above, it is possible to use the. same structure, shape and so on as those of the known sensor element. In the NOx sensor according to the invention having the construction mentioned above, an NOx concentration measuring is performed as follows. At first, the gas to be measured is supplied from the gas inlet portion 4 into the response portion 1 under such a condition that temperatures of the sensor element 8 and the catalyst 6 are maintained constantly by means of the heater for a temperature adjustment 7. The thus supplied gas is passed through the catalyst 6. If the gas to be measured is passed through the catalyst 6, a partial pressure ratio of NO/NO 2 in the gas to be measured becomes an equilibrium state and a CO component in the gas to be measured is burnt. Therefore, the gas to be measured, in which the partial pressure ratio of NO/NO 2 is an equilibrium state and a CO component is removed, can be contacted with the sensor element 8. In this case, a relation between a resistance of the sensor element 8 and NOx concentration can be determined one by one if an oxygen concentration is constant. However, the oxygen concentration in the gas to be measured is not constant actually. Therefore, in the present invention, the O 2 sensor 9 is arranged in the response portion 1 so as to always measure the oxygen concentration, and the NOx concentration is obtained from a relation between the resistance of the sensor element 8 based on the oxygen concentration and the NOx concentration. As one example, a relation between resistances at the oxygen concentrations of 1% and 20% and NOx concentrations, which is based on the results in the following experiment 1 of sample Nos. 1-10, is shown by Fig. 2. In Fig. 2, the relation is shown only at the oxygen concentrations 1% and 20%. However, if relations at the other oxygen concentrations are measured beforehand, the NOx concentration can be measured by using the relation corresponding to the oxygen concentration measured by the O 2 sensor 9. As a result, the NOx concentration can be measured without being affected by the partial pressure ratio of NO/NO 2 , the O 2 component, the CO component and the atmospheric temperature. Hereinafter, an actual embodiment will be explained. Experiment 1 As shown in Fig. 1, the NOx sensor was constructed by arranging the catalyst 6, the heater for a temperature adjustment 7, the sensor element 8 and the O 2 sensor 9. The sensor element 8 was produced according to the following steps. At first, tin chloride was subjected to a hydrolysis by using an ammonia solution to obtain a dissolved solution. Then, the dissolved solution was separated by a filtering. After that, the thus separated dissolved solution was subjected to a pyrolysis at 600°C for 2 hours to synthesize tin oxide powders. Then, the thus obtained tin oxide powders were mixed in a wet state in ethanol solution for 10 hours by using zirconia balls to obtain an tin oxide slurry for dipping. As a body of the sensor element 8, use was made of an alumina tube having a diameter of 1.5 mm and a length of 5 mm to which a platinum wire having a diameter of 3 mm was secured. Then, the body was dipped in the tin oxide slurry. After that, the thus dipped body was fired at 800°C for 2 hours to obtain the sensor element 8. Moreover, the heater for a temperature adjustment 7 was produced by working a platinum wire into a coil shape. Further, platinum powders were arranged on a cordierite honeycomb carrier by a washcoat method. After that, the cordierite honeycomb carrier was fired at 500°C for 2 hours to obtain the catalyst 6 which functions to control the partial pressure ratio of NO/NO 2 and remove the CO component. As the O 2 sensor 9, use was made of a zirconia O 2 sensor. The measurement was performed in such a manner that a resistance of the sensor element 8 and a current of the O 2 sensor 9 were detected respectively by the digital multimeters 10 and 11 via the platinum lead wires. As shown in the following Table 1, the gas to be measured including NOx such as NO 2 and NO having a predetermined concentration as well as the other components such as O 2 , CO 2 , H 2 O, CO and N 2 was prepared. In this case, a total of all components was 100%. Then, the thus prepared gas was flowed, under such a condition that a temperature of the sensor element 8 was maintained constantly, to measure a resistance of the sensor element 8 by using the NOx sensor having the construction mentioned above. Moreover, as a comparative example, a resistance of the sensor element 8 was measured in the same manner as the example mentioned above except that a temperature of the sensor element 8 is not controlled and the catalyst 6 is not used. The results were shown in Table 1. From the results shown in Table 1, when the oxygen concentration is constant, it is understood that the same resistance can be always obtained in the examples according to the invention even if a concentration ratio between NO 2 and NO is varied and also the CO component is included. On the other hand, it is understood that the resistances are largely varied in the comparative examples. Therefore, in the examples according to the invention, if the NOx concentration is measured from the resistance, the constant NOx concentration can be always obtained even if a concentration ratio between NO 2 and NO is varied and also the CO component is included. Accordingly, the precise measurement can be performed. On the other hand, in the comparative examples, even if the NOx concentration is measured from the resistance, the constant NOx concentration cannot be obtained, and thus the measurement accuracy becomes lower. Experiment 2 The NOx concentration measuring was performed in the same manner as that of the experiment 1 by using the substantially same NOx sensor as that of the experiment 1 except that an indium oxide obtained by subjecting a nitrate to a pyrolysis at 600°C for 2 hours was used as a material of the sensor element 8, a manganese oxide was used as the catalyst 6 for controlling the partial pressure ratio of NO/NO 2 , and a tin oxide was used as the catalyst 6 for removing the CO component. The results were shown in Table 2. Also from the results shown in Table 2, when the oxygen concentration is constant, it is understood that the same resistance can be always obtained in the examples according to the invention even if a concentration ratio between NO 2 and NO is varied and also the CO component is included. On the other hand, it is understood that the resistances are largely varied in the comparative examples. Experiment 3 The NOx concentration measuring was performed in the same manner as that of the experiment 1 by using the substantially same NOx sensor as that of the experiment 1 except that a titanium oxide obtained by subjecting a sulfate to a pyrolysis at 800°C for 1 hour was used as a material of the sensor element 8, a cobalt oxide was used as the catalyst 6 for controlling the partial pressure ratio of NO/NO 2 , and a gold was used as the catalyst 6 for removing the CO component. The results were shown in Table 3. Also from the results shown in Table 3, when the oxygen concentration is constant, it is understood that the same resistance can be always obtained in the examples according to the invention even if a concentration ratio between NO 2 and NO is varied and also the CO component is included. On the other hand, it is understood that the resistances are largely varied in the comparative example. As clearly understood from the above, according to the invention, since the gas to be measured passed through the catalyst which makes a partial pressure ratio of NO/NO 2 to an equilibrium state is contacted to the sensor element under such a condition that temperatures of the sensor element and the catalyst are maintained in a constant state by means of the heater, it is possible to perform a high precision measurement. That is to say, under such a condition mentioned above, a relation between a resistance measured by the sensor element and an NOx concentration is determined one by one in response to an O 2 concentration. Therefore, if the O 2 concentration is measured by the O 2 sensor for an adjustment and the NOx concentration is determined from the resistance value in response to the thus measured O 2 concentration, it is possible to perform a high precision measurement. Moreover, since the catalyst has a function for removing a CO component from the gas to be measured, a CO component can be removed from the gas to be measured if the gas is contacted with the sensor element, and thus it is possible to measure the NOx concentration with no CO influence.

Claims: A NOx sensor having a sensor element (8) made of an oxide, the resistance of which varies in dependence on the NOx concentration of a gas to be measured, measuring means (2) for measuring the resistance of the sensor element (8) and thereby detecting the NOx concentration of the gas to be measured, a catalyst (6) arranged upstream with respect to said sensor element in a flow direction of the gas to be measured, said catalyst being adapted, when contacted by the gas to be measured, to bring the partial pressure ratio of NO and NO 2 in the gas to equilibrium and to remove CO from the gas, a heater (7) arranged in relation to the sensor element (8) and catalyst (6) for maintaining constant temperature of the sensor element and the catalyst, and an O 2 sensor (9) for detecting the O 2 concentration of the gas to be measured, wherein said measuring means (2) is arranged to perform an adjustment of the measurement value from the sensor element (8) on the basis of the O 2 concentration detected by said O 2 sensor, to determine the NO 2 concentration of the gas. A NOx sensor according to claim 1, wherein said oxide of the sensor element (8) is a metal oxide semiconductor. A NOx sensor according to claim 2, wherein said metal oxide semiconductor is SnO 2 , TiO 2 or In 2 O 3 . A NOx sensor according to claim 1, 2 or 3 wherein said catalyst (6) is a precious metal or an oxide. A NOx sensor according to claim 4, wherein said catalyst (6) is a precious metal selected from platinum, rhodium and gold. A NOx sensor according to claim 5, wherein said catalyst (6) is a manganese oxide, a cobalt oxide or a tin oxide. A method of measurement of NOx concentration, using a sensor according to any one of the preceding claims.

Inventors: KATSUDA YUJI, OGAWA NAOYUKI, TAKAHASHI TOMONORI, YOSHIDA TOSHIHIRO, KATSUDA, YUJI, OGAWA, NAOYUKI, TAKAHASHI, TOMONORI, YOSHIDA, TOSHIHIRO, Katsuda, Yuji, 207, NGK Takeda-Minami-Shataku
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1996
Main classification: G01N27
IPC codes: G01N27, G01N33