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Aerosol Fire Extinguishing Technology


2012-11-22


Chapter 1 Development process of aerosol fire extinguishing technology  

Aerosol fire extinguishing agent is a new type of fire extinguishing agent developed in the past forty years. It is a solid mixture composed of oxidizing agent, reducing agent, burning rate control agent and binder. The release of hot aerosol fire extinguishing agent has undergone a combustion reaction, and the product contains both solid and gas. Most of them are fire extinguishing gases such as N2, CO2 and water vapor, and the solid particles are oxides of potassium and strontium. When the release product is cooled and condensed, it will generate extremely fine particles, the diameter of which is generally less than 0.45 microns. These extremely fine particles can efficiently absorb and neutralize the combustion free radicals in the flames, thereby achieving chemical inhibition of fire extinguishing. The aerosol formed by the solid particles wrapped in the fire extinguishing gas can be suspended for a long time, and can bypass obstacles and spread to every corner, effectively extinguishing the fire in a fully submerged manner. Simply put, aerosol fire extinguishing agent is a kind of micro-nano-scale dry powder particles that can be suspended in the air. It is the crystallization of the development of pyrotechnic technology and nanotechnology.  

Strictly speaking, aerosols have been developed through three generations so far.  

The first generation of aerosol fire extinguishing products was born as early as the 1960s. Liu Menghuan and other researchers from the Tianjin Fire Research Institute of the Ministry of Public Security of my country have conducted research on aerosol fire extinguishing devices and proposed to use pyrotechnic agents to burn and release products to extinguish fires. It was called "smoke fire extinguishing system" at that time, and it was mainly used on fire extinguishing devices for storage tanks of petrochemical products. 

The first generation of aerosol fire extinguishing products was born as early as the 1960s. At that time, the researchers of Tianjin Fire Research Institute conducted a lot of research on aerosol fire extinguishing agents and their devices. Class C liquid storage tank fire. This is a new fire extinguishing technology that is different from the past. It has both smoke and fog, fine solid particles, and aerosol substances formed by water vapor, N2, and CO2 fire extinguishing gases for fire extinguishing.  

Under the background of harmonious relations between China and the Soviet Union at that time, scientists in the former Soviet Union followed the guidance of this theory and took the lead in developing an aerosol fire extinguishing agent that could be used in ordinary places. However, due to the limited environment at that time, our country did not conduct in-depth applied research on this theory, and the products only stayed in the oil tank series. As a result, in the 1980s, my country's application in the field of aerosol fire extinguishing technology has lagged far behind the former Soviet Union. However, the theory of aerosol fire extinguishing was first proposed by us Chinese, which has been confirmed by various documents and records, and is indisputable!  

Although the first generation of aerosols appeared in the early 1960s, because people were more accustomed to using halon fire extinguishing agents at that time, the promotion and application of aerosol fire extinguishing products was slow. After entering the 1980s, people gradually realized the damage effect of halon fire extinguishing agents on the atmospheric ozone layer, and aerosol fire extinguishing agents gradually gained people's attention as a green and environmentally friendly halon substitute. Various types of aerosol fire extinguishing equipment researched and developed by various countries have made the application technology of aerosol develop rapidly. This technology has formed a series of products in Russia. The United States, Britain, Germany, Japan, Canada and other countries have also successively developed their own aerosol fire extinguishing products and promoted their application.  
In my country, in the mid-1990s, Beijing Institute of Technology developed the second generation of aerosol fire extinguishing products on the basis of learning foreign experience. Many domestic enterprises have been involved in the production of this product. However, because the temperature of the tank was not fully considered in the design, there were high temperatures and flame defects during spraying, which led to some major liability accidents.  

The problems in the second generation of products were caused by the fact that the R&D personnel did not know much about the market environment, and the production personnel did not conduct in-depth research on the technology. The solution to the problem is very simple. It can be solved by simple physical methods such as increasing the spacer of the box and adding some metal heat shields. The improved second-generation aerosol product has overcome the defect of high temperature in the box and has been gradually recognized by the market. However, because its formula is basically the potassium salt formula of Beijing Institute of Technology, and potassium nitrate is used as the main oxidant of the aerosol generator, the product after eruption is easily combined with water in the air to form a viscous conductive substance. This substance is very destructive to electronic equipment. The damage to electronic equipment caused by misspraying also had a very bad impact on the second-generation aerosol. For this reason, large national enterprises such as China Mobile, China Netcom, and China Telecom have expressly prohibited the use of aerosol fire extinguishing devices in their precision electronic equipment. Can aerosols be used to protect places with delicate electronic equipment? Does the sprayed product have any impact on electronic equipment? These issues, placed in front of many fire experts, have also caused widespread controversy in the industry. 

Problems keep popping up and technology keeps improving. At this time, the third generation of aerosol fire extinguishing products was born. The third-generation aerosol mainly uses strontium salt as the main oxidant, which is different from the second-generation potassium salt aerosol. It fundamentally solves the damage to electrical equipment caused by the spray products of this type of product. In 2001, after rigorous investigation and verification, China Mobile Communications Corporation finally chose strontium salt aerosols to protect its communication base stations and other places equipped with sophisticated electronic equipment. At present, strontium salt aerosol products have been used in thousands of engineering projects, and misspraying has also occurred, but so far there has been no accident that damaged electronic equipment. It is worth mentioning that in August 2004, in a communication room in Changsha, Hunan, the power supply caught fire due to aging wires, and the strontium salt aerosol fire extinguishing agent successfully extinguished the fire, during which the communication equipment operated as usual. This is the only "case" in China where aerosol extinguishes a fire without damaging electronic equipment. The third-generation aerosol has become more and more accepted by the majority of users.  

On June 4, 2004, the Ministry of Public Security promulgated the industry standard GA499.1 "Hot Aerosol Fire Extinguisher". The standard divides aerosol fire extinguishing devices into two categories. The first category is K-type aerosol, which refers to a fire extinguishing device filled with an aerosol generating agent containing more than 30% potassium nitrate. That is, the above-mentioned potassium salt aerosol, that is, the second generation aerosol. The second category is S-type aerosol, which refers to a fire extinguishing device filled with an aerosol generating agent containing 35% to 50% strontium nitrate and 10% to 20% potassium nitrate. That is, the above-mentioned strontium salt aerosol, that is, the third generation aerosol.  
Regarding the difference between S-type and K-type aerosols, it is clearly stipulated in this industry standard that "the insulation level of the sediment of the K-type fire extinguishing device shall not be lower than 1 megohm, and the insulation level of the sediment of the S-type fire extinguishing device shall not be lower than 20 megohm." . A substance with a resistance of 1 megohm is strictly a conductor, and so is the conductivity of water. And a substance with a resistance of 20 megohms is an insulator, equivalent to the conductivity of dry wood. This industry standard is the first in the world to divide aerosol into S type and K type according to the formula, and it is also the first time to use the scientific index of "aerosol deposition insulation level" to explain why S type can be used to protect the A place for precision electronics, and a reason the K-type cannot. 

The significance of the promulgation of this industry standard lies in the fact that aerosols are divided into two categories in terms of actual indicators, namely, S category and K category, which provides guidance for the formulation of relevant specifications and the use of the majority of users in the future. The promulgation of the industry standard "Hot Aerosol Fire Extinguishing Device" has been highly praised by industry experts. At the expert review meeting, it was praised as "internationally advanced" by experts, which is rare in the history of Chinese standard writing. Its promulgation will have a strong regulatory effect on the aerosol industry.

3. Cold aerosol fire extinguishing technology 

Hot aerosol fire extinguishing technology has the advantages of high fire extinguishing efficiency, good electrical insulation, and no damage to the atmospheric ozone layer. There are also defects such as: "thermal joint reaction", inability to apply partial protection, and slow fire extinguishing speed. How to make full use of the strengths of aerosol application for fire extinguishing and overcome its inherent defects, a large number of fruitful researches have been carried out at home and abroad. In recent years, the research and application of cold aerosol fire extinguishing technology is the new achievement of halon replacement technology research. 

1. Superfine dry powder fire extinguishing technology

Ultra-fine dry powder fire extinguishing agent is the latest "non-high temperature aerosol fire extinguishing technology" developed in China, also known as "cold aerosol". The fire extinguishing agent uses nitrogen (N2) and other inert gases as the driving body, and the ultra-fine dry powder is the dispersed body. After release, it forms a mist aerosol mixed with gas powder to extinguish the fire. The fire extinguishing mechanism is mainly chemical fire extinguishing, supplemented by physical fire extinguishing. It can be used for total submersion fire extinguishing or partial protection fire extinguishing. 

Superfine dry powder fire extinguishing agent can be divided into: superfine dry powder of ammonium phosphate salt, superfine dry powder of sodium bicarbonate, and superfine dry powder composed of other fire extinguishing materials according to the composition of the base material. The ultra-fine dry powder based on ammonium phosphate has a particle size of 10-20 μm. Due to the fine particle size, it can fully react during fire extinguishing, and the fire extinguishing effect is significantly increased. The raw material ammonium phosphate needs to be silicified during the preparation process, and the particle mass is relatively large. After being sprayed by nitrogen gas, most of the powder particles quickly fall to the ground, and no aerosol is formed to participate in the fire extinguishing reaction, so the effect of using it for total flooding fire extinguishing is not very ideal. . 

British scientific and technological workers use spray granulation technology to produce ultra-fine powder with a particle size of less than 5 μm with a small spray dryer, which is pressurized and sprayed to form a cold aerosol to extinguish the fire. -5 times. Due to the complex equipment and process requirements and the huge energy consumption of product production, it has not been popularized and applied so far. The researchers of Wuhan Green Fire Fighting Equipment Co., Ltd. applied the latest technology at home and abroad, and adopted new fire extinguishing materials, and successfully developed a new type of superfine dry powder high-efficiency fire extinguishing agent. The fire extinguishing mechanism of ultra-fine dry powder fire extinguishing agent is mainly chemical fire extinguishing, supplemented by physical fire extinguishing. 

A. Inhibitory effect on flaming combustion. Flaming combustion is a chain reaction process. Combustion molecules are activated under the high temperature of combustion or other forms of energy, and generate combustion free radicals or active groups in the presence of oxygen, and rely on these high-energy free radical propagation reactions to maintain continuous combustion. Burning free radicals have high energy and are very active. Once they are generated, the next reaction will occur immediately to generate more free radicals. The fire extinguishing components in the ultra-fine dry powder are inactive substances to the combustion reaction. When their particles enter the combustion zone and combine with the flame, they can capture the combustion free radicals at the same time. As a result, the free radicals in the flame are consumed faster than the produced ones. Speed, the burning free radicals are quickly exhausted, the process of the chain reaction is terminated, and the flame is extinguished. The above effects are called chemical inhibition and negative catalytic effects on combustion. 

B. The extinguishing effect on surface combustion. Ultrafine dry powder fire extinguishing agent can not only effectively extinguish flaming combustion, but also can extinguish surface combustion of general solid substances. When the ultra-fine dry powder crystal powder contacts the surface of the burning combustion material, a series of chemical reactions occur, and are melted under the high temperature of the solid surface to form a glassy covering layer, which separates the solid surface from the oxygen of the surrounding air , to suffocate the burning 

C. The shielding effect on heat. When using superfine dry powder to extinguish a fire, the dense cloud-like powder mist (aerosol) is mixed with the flame, which can reduce the radiant heat of the flame to the surface of the burning object, reduce the temperature of the flame, and slow down the combustion process. 

D. Dilution of oxygen in the combustion zone. When the ultra-fine dry powder fire extinguishing agent is extinguished, a series of decomposition reactions will occur under the high temperature of the flame. These reactions are generally endothermic reactions, which can absorb part of the heat of the flame and produce some inactive substances such as carbon dioxide (CO2). Dilute the oxygen concentration in the combustion zone to slow down the combustion. At the same time, it is used as the pressure gas nitrogen to drive the superfine dry powder to be released in the combustion zone along with the ultrafine dry powder, which further dilutes the oxygen in the combustion zone and inhibits combustion. to extinguish the combustion. 

Features of ultrafine dry powder fire extinguishing agent: 

(1), green and environmental protection. The ultra-fine dry powder fire extinguishing agent has zero ozone depletion potential (ODP) and zero greenhouse effect potential (GWP). 

(2) High fire extinguishing efficiency. The fire extinguishing rate of superfine dry powder fire extinguishing agent is 40 times that of water, 20 times that of hot aerosol fire extinguishing agent, and 6 times that of mixed gas fire extinguishing agent. The fire extinguishing efficiency is 30 times that of water, 2 times that of hot aerosol fire extinguishing agent, 2-3 times that of halon fire extinguishing agent, 4-8 times that of heptafluoropropane fire extinguishing agent, 12-16 times that of mixed gas fire extinguishing agent, is 6-10 times that of ordinary dry powder fire extinguishing agent. 

(3) Wide range of applications. Ultra-fine dry powder fire extinguishing agent can be fully submerged to extinguish fire in a closed space, and can also be applied locally in a large area to extinguish fire in an open place, which has the advantages of fully submerged and omnidirectional fire extinguishing. The fluidity and electrical insulation of the fire extinguishing agent are excellent, and it can be filled with commonly used portable fire extinguishers, fixed fire extinguishing devices (fire extinguishing systems without pipe network), fire extinguishing systems with pipe network, forest fire extinguishing bombs, etc., and are widely used in national defense and military And civil facilities, used to put out Class A, Class B, Class C fires and live equipment fires. 

As we all know, as a chemical fire extinguishing agent, its particle size is inversely proportional to its fire extinguishing efficiency, that is, the smaller the particle size, the higher the fire extinguishing efficiency. Due to the constraints of processing technology and cost, the ultra-fine dry powder fire extinguishing agent can achieve an average particle size of ≤5μm at present, and its fire extinguishing rate and fire extinguishing efficiency are already very good. If the particle size of the ultrafine dry powder is processed to be finer, so as to reach the nanometer level, and a series of technical problems in the application process are solved, the fire extinguishing efficiency will be dramatically increased by dozens or even hundreds of times.

2. Water mist fire extinguishing technology 

Water mist technology is firstly a fire extinguishing technology developed by a few developed countries such as Finland, the United States, and Canada. As the halon fire extinguishing agent is about to be eliminated, under the background of research and development of halon substitutes in various countries, water mist fire extinguishing technology has been paid attention to. In the 1990s, water mist fire extinguishing technology developed rapidly. In 1997, the NFPA Technical Committee on Water Mist Fire Extinguishing System was formed by American development and engineering departments, water mist fire extinguishing system manufacturers, insurance companies, legal institutions and customer representatives, and put forward relevant documents on design, installation and standardization, and officially Published the "Water Mist System Standard" (NFPA750). At the end of the 1990s, my country began to carry out research, development and testing of water mist fire extinguishing technology, and it was listed as a national "Ninth Five-Year" scientific and technological project. At present, various types of water mist fire extinguishing system products have been developed successively. 
At present, many enterprises at home and abroad have produced water mist fire extinguishing systems. According to the pressure of the system, the flow phase of the fire extinguishing agent in the pipeline, the application method, the formation and installation method of the system, etc. The fire extinguishing mechanism of the water mist fire extinguishing system is mainly physical fire extinguishing. 
1), gas phase cooling. Generally, the DV0.90 of the fine water mist is less than 400μm, and the specific surface area is larger than that of ordinary water spray. It can completely evaporate in the fire scene, absorb a large amount of heat, and make the combustion slow.

2), suffocation. After the fine water mist is sprayed into the fire site, it evaporates rapidly to form steam, the volume expands rapidly, and the air is removed, forming a barrier around the burning area or the burning object, preventing the entry of fresh air, reducing the oxygen concentration in the burning area, and suffocating the flame. 

3) Block the effect of radiant heat. After the fine water mist evaporates, the steam quickly covers the flame and smoke plume of the combustion material, which has a good barrier effect on the radiation of the flame, and can effectively inhibit the radiant heat from igniting other surrounding objects, so as to prevent the flame from spreading. 

Features of water mist fire extinguishing system: 

1) The water mist fire extinguishing system can extinguish Class A, Class B and Class C fires. If the droplet diameter is mostly in the range of 40-200μm, fine water mist can also effectively extinguish electrical fires. 

2) The water mist fire extinguishing system uses easily available and cheap water as the fire extinguishing agent, and its resource utilization rate and environmental protection indicators are superior to other fire extinguishing systems. The water mist fire extinguishing system greatly improves the utilization rate of water. Compared with the water spray fire extinguishing system, the water utilization rate is increased by more than 3 times; compared with the water spray fire extinguishing system, the water utilization rate is increased by more than 6 times. 

3) The fire extinguishing mechanism of the water mist fire extinguishing system is physical fire extinguishing, and the fire extinguishing speed is much slower than chemical fire extinguishing. 

4) The water mist fire extinguishing system has strict requirements on water and ambient temperature. The water mist storage tank should be filled with pure water, generally treated potable water. The water in the water reservoir should be replaced regularly to prevent the growth of the water from blocking the nozzle hole after a long storage time. The water mist fire extinguishing system should be installed in a special equipment room with an ambient temperature of 4-50 °C, because the temperature is too low to freeze the water, and the temperature is too high to increase the temperature of the water in the storage tank, resulting in gasification or heat exchange , and may produce scale or growth, affecting water quality. Because water is susceptible to contamination or other environmental influences, the water mist system should be located in a dedicated room. 

5) The water mist fire extinguishing system is used for total flooding fire extinguishing, and its fire extinguishing ability depends more on the selection of the technical parameters of the nozzle and the shape and position of the protected object. As far as the current technical level of the product is concerned, the fine water mist product cannot evenly distribute the water mist droplets to the entire protection space. It is very difficult for the water mist system to extinguish and block the fire. When there is an obstacle between the nozzle and the flame, the temperature near the flame cannot be lowered quickly, and the fire extinguishing time will be prolonged, so it cannot always be extinguished. This is due to the blocking effect of the obstacle, which makes the fine water mist deposit on the surface of the obstacle, reducing the quantity and momentum of the water mist. Especially the prefabricated suspension type fire extinguishing device, due to the limited quantity and momentum of its water storage, it cannot be fully submerged in fire extinguishing application, and the scope of its partial protection application is also very limited. In terms of technical application indicators, the current water mist fire extinguishing system is much worse than the ultrafine dry powder fire extinguishing system, and it is not as good as the gas fire extinguishing system. 

6) The performance of the water mist fire extinguishing system depends on two capabilities. One is the ability to produce water droplets that are small enough, and the other is the ability to distribute water in sufficient quantities throughout the space. These two capabilities are affected by factors such as the size, velocity, distribution, impulse, and geometric characteristics of the nozzle, as well as other objective factors such as the geometry of the protected object and the shape and size of the protected space. How to improve the above two capabilities will be the key to the application and development of the water mist fire extinguishing system. 

7) Water mist fire extinguishing technology is applied to portable fire extinguishers, and some products have been launched, but the effect is not ideal. The reason is that due to the impact of the technical parameters of the sprinkler head, the amount of water stored and the kinetic energy, the sprayed water mist is relatively close, and it is difficult to form an oxygen barrier effect on the fire. For Class B fires, the fire is mainly extinguished by the cover layer sprayed onto the oil surface to isolate oxygen. The spray distance of fine water mist is short, and the formation of a cover film on the oil surface is much slower than direct use of ordinary light water film fire extinguishers.