"1. introduction
With the development of petrochemical industry, the types and application scope of flammable, explosive and toxic gases have been increased. Once these gases leak in the process of production, transportation and use, they will cause poisoning, fire and even explosion accidents, which will seriously endanger people's life and property safety. Due to the diffusivity of the gas itself, after leakage, under the action of external wind force and internal concentration gradient, the gas will diffuse along the ground surface, forming a combustion and explosion or poison danger area at the accident site and expanding the hazard area. For example, in July 1995, a chlorine gas leak occurred in the liquid chlorine workshop of Chengdu chemical plant, Sichuan Province, killing three people and injuring six people on the spot. It took only about an hour. The pungent smell of chlorine gas can be smelled within dozens of square kilometers of the urban area. Therefore, this kind of accident has the characteristics of strong sudden, rapid diffusion, difficult rescue and wide range of harm. In case of gas leakage accident, corresponding measures must be taken as soon as possible to reduce the accident loss to the lowest level. Detecting the content of some gases in the air timely and reliably, taking effective measures to remedy in time, taking correct disposal methods and reducing accidents caused by leakage are the necessary conditions to avoid causing major property and casualties. This puts forward higher requirements for gas detection and monitoring equipment. As an important gas detector, gas sensor has been greatly developed in recent years. The development of gas sensor makes it more and more widely used. This paper introduces the development of gas sensor and its application in gas leakage accident disposal.
2. Overview of gas sensor
Foreign countries began to research and develop gas sensors in the 1930s. In the past, gas sensors were mainly used for the detection and alarm of gas, liquefied petroleum gas, natural gas and gas in mines. At present, the types of gases to be detected are expanded from the original reducing gases (H2, C4H10, CH4) to toxic gases (CO, NO2, H2S, no, NH3, PH3).
There are many kinds of gas sensors. According to different gas sensing materials and gas sensing characteristics, it can be divided into semiconductor type, solid electrolyte type, electrochemical type, contact combustion type, polymer type, etc.
2.1 semiconductor gas sensor
This sensor mainly uses semiconductor gas sensing materials. Since the semiconductor metal oxide gas sensor came out in 1962, it has been widely used because of its high sensitivity and fast response. At present, it has become one of the largest and most widely used sensors in the world. According to different ways of detecting gas sensing characteristic quantity, it is divided into resistance type and non resistance type.
Resistive semiconductor gas sensor works by detecting the change of gas sensor with gas content. Metal oxide ceramic gas sensing materials are mainly used. With the research and development of new materials such as composite metal oxide and mixed metal oxide in recent years, the characteristics and application range of this gas sensor have been greatly improved. For example, the concentration range of NH3 detected by WO3 gas sensor is 5ppm ~ 50ppm, and ZnO CuO gas sensor is very sensitive to 200ppm Co.
The non resistive semiconductor gas sensor works on the principle that the current or voltage of the gas sensor changes with the gas content. There are mainly MOS diode type, junction diode type and FET gas sensor. Most of the detected gases are combustible gases such as hydrogen and silane.
2.2 solid electrolyte gas sensor
Solid electrolyte gas sensor uses solid electrolyte gas sensing material as gas sensing element. The principle is that the gas sensitive material generates ions when passing through the gas to form electromotive force, and the electromotive force is measured to measure the gas concentration. Because of its high conductivity, good sensitivity and selectivity, this sensor has been widely used and has almost entered various fields such as petrochemical, environmental protection and mining, second only to metal oxide semiconductor gas sensor. For example, yst-au-wo3 for measuring H2S, NH + 4caco3 for measuring NH3, etc.
2.3 contact combustion gas sensor
It can be divided into direct contact combustion type and catalytic contact combustion type. Its working principle is: when the gas sensing material is energized, the combustible gas is oxidized and burned or oxidized and burned under the action of catalyst. The heat generated heats up the electric heating wire, so as to change its resistance value. Measure the resistance change, so as to measure the gas concentration. This sensor can only measure combustible gases and is not sensitive to non combustible gases. For example, the sensor made of active catalysts RH and PD coated on Pt wire has broad-spectrum characteristics, that is, it can detect various combustible gases. The contact combustion gas sensor is very stable at ambient temperature and can detect most combustible gases with lower explosion limit. It is widely used in the monitoring and alarm of combustible gases in petrochemical plants, shipyards, mine tunnels, bathrooms, kitchens and so on.
2.4 polymer gas sensor
Gas sensors using polymer gas sensing materials have been greatly developed in recent years. When polymer gas sensing materials encounter specific gas, their physical properties such as resistance, dielectric constant, surface acoustic wave propagation velocity and frequency, material weight and so on change. It mainly includes phthalocyanine polymer, LB film, phenylcyanine acetylene, polyvinyl alcohol phosphoric acid, polyisobutylene, aminoundecylsilane, etc. Polymer gas sensing materials play an important role in the detection of toxic gases and food freshness because of their easy operation, simple process, good selectivity at room temperature, low price and easy combination with microstructure sensors and surface acoustic wave devices. According to the gas sensing characteristics of the materials used, such sensors can be divided into: polymer resistance gas sensors that measure the gas concentration by measuring the resistance of gas sensing materials; According to the concentration cell formed when the gas sensing material absorbs gas, the concentration cell gas sensor measures the electromotive force to determine the gas concentration; A surface acoustic wave gas sensor is made according to the principle that the propagation velocity or frequency of sound wave on the material surface changes after the polymer gas sensing material absorbs gas; And a quartz vibrator type gas sensor made according to the weight change of polymer gas sensing material after absorbing gas. Polymer gas sensor has high sensitivity to specific gas molecules, good selectivity, simple structure, and can be used at room temperature, which can supplement the shortcomings of other gas sensors.
3. Development direction of gas sensor
At present, the research on new gas sensing materials and gas sensors at home and abroad is very active, and its main research and development directions mainly focus on the following points:
First, develop new gas sensing materials. The main measure is to doping some elements in the traditional semiconductor gas sensing materials SnO, SnO2 and Fe2O3. At present, there are many research reports in this field; The second is to develop composite and hybrid semiconductor gas sensing materials and polymer gas sensing materials, so that these materials have high sensitivity, high selectivity and high stability to different gases.
In addition, develop new gas sensors, apply new materials, new processes and new technologies, and further study the mechanism of gas sensors, so as to make the sensors more miniaturized and multifunctional, and have the characteristics of stable performance, convenient use and low price.
At the same time, computer technology is further used to realize the intelligence of gas sensor. With the combination of gas sensor and computer technology, an intelligent gas sensor - electronic nose appears. Electronic nose for identifying and detecting food and spices has been successfully developed at home and abroad. The development of a new bionic gas sensor - bionic electronic nose is the main direction of gas sensor development in the future.
4. Application of gas sensor in gas leakage accident disposal
4.1 It is used for combustible gas monitoring and alarm
At present, the development of gas sensing materials makes the gas sensor have high sensitivity, stable performance, simple structure, small volume and low price, and improves the selectivity and sensitivity of the sensor. Most of the existing gas alarms use tin oxide and noble metal catalyst gas sensors, but the selectivity is poor, and the accuracy of the alarm is affected by catalyst poisoning. The sensitivity of semiconductor gas sensing materials to gas is related to temperature. The sensitivity is low at room temperature. With the increase of temperature, the sensitivity increases and reaches the peak at a certain temperature. Because these gas sensitive materials have the best sensitivity at higher temperature (generally greater than 100 ℃), it will not only consume additional heating power, but also cause fire.
The development of gas sensors has solved this problem. For example, the gas sensor made of iron oxide gas sensing ceramics can create a gas sensor with high sensitivity, good stability and certain selectivity without adding noble metal catalysts. Reduce the working temperature of semiconductor gas sensing materials, greatly improve their sensitivity at room temperature, and make them work at room temperature. At present, in addition to the commonly used single metal oxide ceramics, some composite metal oxide semiconductor gas sensing ceramics and mixed metal oxide gas sensing ceramics have been developed.
Install the gas sensor in the production, storage, transportation, use and other places of flammable, explosive, toxic and harmful gases to detect the gas content in time and find the leakage accident as soon as possible. The gas sensor is linked with the protection system to make the protection system act before the gas reaches the explosion limit and control the accident loss to the minimum. At the same time, the miniaturization and price reduction of gas sensors make it possible to enter the family.
4.2 application in gas detection and accident disposal
4.2.1 types and characteristics of detection gas
After the gas leakage accident, the accident disposal will focus on sampling and detection, determining the warning area, organizing the evacuation of people in the dangerous area, rescuing poisoned personnel, plugging and decontamination. The first aspect of disposal should be to minimize the injury of leakage to personnel, which requires understanding the toxicity of leakage gas. The toxicity of gas refers to that the leakage of substances can disturb people's normal reaction, thus reducing people's ability to formulate countermeasures and reduce injuries in accidents. The American Fire Protection Association classifies the toxicity of substances into the following categories:
NH = 0 in case of fire, there are no other dangerous substances in short-term contact except general combustible hazards;
NH = 1 substances that can cause irritation and slight injury in short-term contact;
NH = 2 high concentration or short-term exposure may cause temporary incapacity or residual injury;
NH = 3 short term exposure can cause serious temporary or residual injury;
NH = 4 short contact can also cause death or serious injury.
Note: the above Toxicity Coefficient NH value is only used to indicate the degree of human injury, and cannot be used for the evaluation of industrial hygiene and environment.
Since toxic gas can enter human body through human respiratory system and cause injury, safety protection must be completed quickly when dealing with toxic gas leakage accidents. This requires the accident disposal personnel to understand the type, toxicity and other characteristics of gas in the shortest time after arriving at the accident site.
The combination of gas sensor array and computer technology constitutes an intelligent gas detection system, which can quickly and accurately identify the types of gas and detect the toxicity of gas. The intelligent gas sensing system consists of gas sensing array, signal processing system and output system. Multiple gas sensors with different sensitive characteristics are used to form an array, and neural network pattern recognition technology is used to identify and monitor the gas concentration of mixed gas. At the same time, the types, properties and toxicity of common toxic, harmful and flammable gases shall be input into the computer, and the accident disposal plan shall be prepared and input into the computer according to the nature of the gas. In case of leakage accident, the intelligent gas detection system will work according to the following procedures:
Enter the site → adsorb gas samples → signal generated by gas sensor → computer identification signal → computer output gas type, nature, toxicity and disposal scheme.
Due to the high sensitivity of the gas sensor, it can be detected when the gas concentration is very low without going deep into the accident scene, so as to avoid unnecessary injury caused by not knowing the situation. With computer processing, the above process can be completed quickly. In this way, effective protective measures can be taken quickly and accurately, and the correct disposal scheme can be implemented to minimize the accident loss. In addition, since the system stores information such as the properties of common gases and disposal plans, if you know the types of gases in leakage accidents, you can directly query the gas properties and disposal plans in this system.
4.2.2 find leakage points
When a leakage accident occurs, it is necessary to quickly find the leakage point and take appropriate plugging measures to prevent the further expansion of the accident. In some cases, it is difficult to find the leakage point due to long pipelines, more containers and hidden leakage points, especially when the leakage is light. Due to the diffusivity of the gas, after the gas leaks from the container or pipeline, it begins to diffuse around under the action of external wind force and internal concentration gradient, that is, the closer it is to the leakage point, the higher the gas concentration. According to this feature, the use of intelligent gas sensor can solve this problem. Different from the intelligent sensing system for detecting gas types, the gas sensing array of this system is composed of several gas sensing elements with overlapping sensitivity parts, so as to enhance the sensitivity of the sensing system to a certain gas. By using the computer to process the signal change of the gas sensing element, the change of gas concentration can be detected quickly, and then the leakage point can be found according to the change of gas concentration.
At present, the integration of gas sensors makes it possible to miniaturize the sensor system. For example, an integrated ultrafine particle sensor developed by Panasonic, Japan, can detect hydrogen, methane and other gases, concentrated on a 2mm Square silicon wafer. At the same time, the development of computer technology can make the detection speed of this system faster. Therefore, a small and portable intelligent sensor system can be developed. Combining this system with appropriate image recognition technology and using remote control technology, it can automatically enter the hidden space, toxic and harmful places where people are not suitable to work and find the location of leakage points.
5. Conclusion
Develop new gas sensors, especially develop and improve the intelligent gas sensing system, so that it can play the role of alarm, detection, identification and intelligent decision-making in gas leakage accidents, greatly improve the work efficiency and safety of gas leakage accident disposal, and play an important role in controlling accident losses.
With the emergence of new gas sensing materials, the intelligence of gas sensor has also developed rapidly. It is believed that in the near future, more mature intelligent gas sensing system will come out, and the disposal status of gas leakage accidents will be greatly improved.
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