Catalytic combustion sensors

Also for CO sensing, the present sensors are available only for the field of security not for environmental use because of the insufficient sensitivity and selectivity to monitor CO in the atmosphere. Examples of CO sensor which have been improved their sensitivity and selectivity are, for example, SnO semiconductor sensors operated under periodic temperature cycle[85-87], a electrochemical sensor using nafion membrane[88], a catalytic combustion sensor composed of catalysts and hydrophobic pol uner[89], a SnOj diode sensor doped with Pd[90] and an optical fiber catalytic sensor with Au/CogO as combustion catalyst[91]. 

The production of gas sensors. The production records of various types of gas sensors for past five years in Japan are listed in Table I except for the oxygen and humidity sensors. The sensors produced in the largest quantity are of the semiconductive type, followed by the catalytic combustion and thermistor types. These sensors have been mostly applied to domestic uses such as gas leakage alarms or gas control systems for LP gas and town gas which are extensively used for cooking and heating in Japanese houses. This is why these sensors are manufactured on a large scale. Other electrochemical sensors have been developed mainly to monitor other gases. 

Sasahara,T., H. Kato, A. Saito, M. Nishimura, and M. Egashira. 2007. Development of a ppb-level sensor based on catalytic combustion for total volatile organic compounds in indoor air. Sens. Actuat. B B126 536-543. 

One of the common limitations of catalytic-combustion-type analyzers is the poisoning of the filament by silicon, sulfur, chlorinated compounds, or lead compounds. A variety of filament protection means have been added to increase the poison resistance of the sensors. Life expectancies are usually defined in terms of exposure concentration hours. One high-concentration exposure of a poison has been known to knock out a sensor therefore, nonpoisoning techniques should be considered when poisoning is an issue. 

In the catalytic combustion design, the sensor consists of a measuring and a reference cell, with a filament in each. The measuring filament is provided... 

Catalytic bead sensors (combustible gases), electrochemical sensors (toxic gases oxygen), etc. 

Combustible gas detectors are required whenever there is a possibility of a hazard to life or property caused by the accumulation of combustible gases. There are mainly two types of combustible/flammable gas detectors that are often used in most industries. These are the low-cost catalytic bead sensor and infrared sensor [26,27]. 

Fig. 11.1 (a, b) Configurations of pellistor (catalytic bead) gas sensors and (c) temperature-dependent heat generation of a catalytic combustion of combustible gases in air... 

Deng YQ, Neved TG, Ewen RJ, Honeybonme CL, Jones MG (1993) Sulfur poisoning, recovery and related phenomena over supported padadium, rhodiinn and iridium catalysts for methane oxidation. Appl Catal A 101 51-62 Ehrhardt JJ, CoUn L, Jamois D (1997) Poisoning of platinum surfaces by hexamethyldisdoxane (HMDS) application to catalytic methane sensors. Sens Actuators B 40 117-124 Firth JG, Jones A, Jones TA (1973) The principles of the detection of flammable atmospheres by catalytic devices. Combust Flame 21 303-311... 

Han CH, Hong DW, Han SD, Gwak J, Singh KC (2007a) Catalytic combustion type hydrogen gas sensor using TiOj and UV-LED. Sens Actuators B 125 224—228... 

Wang Y, Tong MM, Zhang D, Gao Z (2011) Improving the performance of catalytic combustion type methane gas sensors using nanostructure elements doped with rare earth cocatalysts. Sensors 11 19-31... 

For any gas sensor of either a semiconductor-type or catalytic combustion-type, sufficient sensitivity, accuracy, selectivity, reproducibility, life, and stability are required, and the sensing mechanism starts by adsorption or reactive adsorption of the gas component on the surface of sensor materials. Metal oxides such as SnOi, AI2O3,... 

The sensor with the most important limitations is the traditional catalytic or pellistor type percent Lower Explosive Limit (LEL) combustible gas sensor. In spite of the millions of combustible sensor-equipped atmospheric monitors in service around the world, there is still a lot of misinformation and misunderstanding when it comes to the performance characteristics and limitations of this very important type of sensor. Understanding how combustible sensors detect gas is critical to correctly interpreting readings and avoiding misuse of instruments that include this type of sensor. 

Pellistor type combustible sensors and photoionization detectors represent complementary, rather than competing, detection techniques. Pellistor sensors are excellent for the measurement of methane, propane and other common combustible gases that are not detectable by means of a PID. On the other hand, PIDs can detect large VOC and hydrocarbon molecules that are effectively undetectable by pellistor sensors, even when the catalytic sensor is operable in ppm measurement ranges. The best approach for VOC measurement in many cases is to use a multi-sensor instrument equipped with both a pellistor LEL sensor and a PID sensor. 

Pellistors are used to detect flammable gases like CO, NH3, CH4 or natural gas. Some flammable gases, their upper and lower explosion limits and the corresponding self-ignition temperatures are listed in Tab. 5.1. This kind of gas sensor uses the exothermicity of gas combustion on a catalytic surface. As the combustion process is activated at higher temperatures, a pellistor is equipped with a heater coil which heats up the active catalytic surface to an operative temperature of about 500 °C. Usually a Platinum coil is used as heater, embedded in an inert support structure which itself is covered by the active catalyst (see Fig. 5.33). The most frequently used catalysts are platinum, palladium, iridium and rhodium. 

During the reaction of the hot catalyst surface with a flammable gas the temperature of the device increases. The Platinum coil itself serves at the same time as a resistance thermometer. The resistance increase of the coil then is a direct measure for the amount of combusted gas. Usually the amount of heat that develops during combustion is small and amounts to 800 kj/mol for methane, for example [8], Therefore the sensor is connected in a bridge circuit to a second resistor which shows the same setup as the pellistor but is catalytically inactive. The bridge voltage is then controlled by the temperature difference of the two sensors (see Fig. 5.34). 

Catalytic gas detection is based on the principal that oxidation of a combustible gas in air is promoted at the surface of a heated catalyst such as a precious metal. The oxidation reaction results in the generation of heat that provides a direct measure of the concentration of the gas that has been reacted. The sensing element embodying the catalyst is a small bead that is supported with the sensor. 

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