1. Characteristics of dust explosion

Dust explosion is an oxidation reaction that is completed instantaneously under specific conditions when the dust particles suspended in the air are in full contact with the oxygen in the air. A large amount of heat is released in the reaction, resulting in the phenomenon of high temperature and high pressure. Any dust explosion must have three conditions: ignition source; Combustible fine dust; The dust is suspended in the air and reaches the limit range of explosion concentration.

(1) Dust explosion is more complex than combustible substances and combustible gases. Generally, combustible dust is suspended in the air to form a dust cloud within the explosion concentration range. Under the action of the ignition source, part of the dust in contact with the ignition source is first ignited and forms a small fireball. Under the action of the heat released by the combustion of this small fireball, the surrounding dust will be heated, the temperature will rise, and the phenomenon of ignition and combustion will occur. In this way, the fireball will expand rapidly and form dust explosion.

The difficulty and severity of dust explosion are closely related to the physical and chemical properties of dust and the surrounding air conditions. Generally, the greater the heat of combustion, the finer the particles and the higher the activity of the dust, the greater the risk of explosion; Light suspended solids and combustible substances have great explosion risk; When the oxygen content in the air is high, the dust is easy to be ignited and the explosion is also violent. Because moisture can inhibit explosion, the drier the dust and gas, the greater the risk of explosion.

(2) After the occurrence of dust explosion, secondary explosion often occurs. This is because during the secondary explosion, many dust are deposited together, and their concentration exceeds the upper limit concentration of dust explosion and cannot explode. However, when the shock wave or air wave formed by the secondary explosion lifts up the deposited dust again, it will mix with the air in the air, and the concentration is within the dust explosion range, which may be followed by the secondary explosion. The disaster caused by the second explosion is often much more serious than the second explosion.

The direct cause of the dust explosion accident in a domestic aluminum product factory in 1963 was the friction fire between the exhaust fan impeller and the end face of the suction inlet. The shrimp bend and underpants tee at the suction inlet of the fan have poor air flow and are easy to accumulate dust. In particular, it is easier to retain dust during shutdown. Once started, the deposited dust will be lifted up and soon reach the lower explosion limit, causing dust explosion.

(3) The mechanism of dust explosion when combustible dust burns in the air, it will release energy, and the well will produce a large amount of gas, and the speed of releasing energy, that is, the combustion speed is related to the area exposed to the air. Therefore, for the powder of the same solid material, the smaller the particle size, the larger the specific surface area and the faster the combustion diffusion. If the particle size of this solid is very fine. It can even be suspended. Once it is ignited by an ignition source, it can release a lot of energy in a very short time. These energies have no time to escape into the surrounding environment, resulting in the heating and adiabatic expansion of the gas in the space. On the other hand, a large amount of gas will be produced when the powder is burned, which will form a local high pressure of the system, resulting in explosion and propagation, which is commonly referred to as dust explosion.

(4) The difference between dust explosion and combustion the combustion of large solid combustibles is pushed inward in a nearly parallel layer, such as the combustion of coal. The release of this combustion energy is relatively slow. The heat and gas generated can escape quickly. The pile combustion of combustible dust forms open fire combustion under the condition of good ventilation, but under the condition of poor ventilation. It can form smoke-free or flame hidden combustion.

There are several stages in the combustion of combustible dust: in the stage, the surface powder is also heated; In the second stage, the surface layer is gasified and the volatile matter overflows; In the third stage, volatilization and gas phase combustion.

The explosion of ultra-fine powder is also a relatively complex process. Because the scale of powder dust cloud is generally small, and the flame propagation speed is fast, hundreds of meters per second, a fire source ignition occurs in the dust center and burns the whole dust cloud in less than 0.1s. In this process, if the dust has been burned out, the pressure will be generated; If it is not burned out, a lower pressure is generated. Whether combustible particles can burn out depends on the particle size and combustion depth.

(5) Classification of combustible dust powder can be divided into two categories according to its flammability: one is combustible; One is non combustible. The classification methods and standards of combustible powders are different in different.

The United States classified combustible powder as class II dangerous goods, and included metal powder, carbon dust and grain dust in different groups. The classification method developed by the United States is to classify the measured powder according to the pressure rise speed obtained when the dust explosion occurs in the standard test device, and it is divided into three grades. At present, there is no ready-made standard for the classification of combustible dust in China.

2. Influence of dust concentration and particles on Explosion

(1) Dust concentration combustible dust explosion also has upper and lower limits of dust concentration. This value is affected by ignition energy, oxygen concentration, powder particle size, powder variety, moisture and other factors. The explosion limit can be estimated by using the simplified formula. Generally speaking, the lower limit concentration of dust explosion is 20 ~ 60g / m3 and the upper limit is 2 ~ 6kg / m3. The upper limit is affected by many factors. Its value is not as easy to determine as the lower limit, and it is usually not easy to reach the concentration of the upper limit. Therefore, the lower limit value is more important and useful.

Physically speaking, the upper and lower limits of dust concentration reflect the influence of the distance between particles on the propagation of particle combustion flame. If the distance between particles makes the combustion flame unable to extend to adjacent particles, the combustion cannot continue (spread) and the explosion will not occur; At this time, the dust concentration is lower than the lower limit of explosion concentration. If the distance between particles is too small, the oxygen between particles is not enough to provide sufficient combustion conditions and form explosion. At this time, the particle concentration is higher than the upper limit.

(2) When the powder particle size is larger than 400um, the dust cloud formed will no longer be explosive. However, for ultra-fine powder, when its particle size is less than 10um, it has great danger. It should be noted that sometimes even if the average particle size of the powder is greater than 400um, it often contains finer powder, and this small part of the powder is also explosive.

Although the regularity of the influence of the particle size of the powder on the explosion performance is not strong, the smaller the size of the powder, the larger its specific surface, the faster the combustion, and the speed of pressure rise increases linearly. It depends on the total energy released, because it has little to do with the total energy released under certain conditions.

3. Technical measures for dust explosion

Combustion reaction requires combustible substances and oxygen, as well as ignition source with certain energy. For dust explosion, it should have three elements: ignition source; Combustible fine dust; The dust is suspended in the air and forms a dust cloud within the explosion concentration range. These three elements exist at the same time to explode. Therefore, the explosion can be prevented as long as one of the conditions is eliminated. The following technical measures are often adopted in bag filter.

(1) Explosion proof structural design measures in the special design of the body structure, in order to prevent the accumulation of combustible dust in the internal components of the dust collector, all beams and partition plates shall be equipped with dust plates, and the slope of the dust plate shall be less than 70 degrees. The slip angle of the ash hopper is greater than 70 degrees. In order to prevent the accumulation of ash due to the small angle between the walls of the two buckets, the two adjacent side plates shall be welded with a slip plate to eliminate the deposition of dust. Considering the condensation and cold blocking of the ash hopper due to abnormal operation and high dust humidity, when designing the ash hopper, add steam pipe insulation or tubular electric heater to the high-temperature dust collector on the wall plate of the ash hopper. In order to prevent the canopy of ash hopper, each ash hopper shall also be equipped with bin arm vibrator or air monitor.

One dust collector has at least 2 ~ 3 ash hoppers and at most 5 ~ 8. When in use, it will produce deflection caused by uneven air volume, uneven amount of pulverized coal in each ash hopper and large amount of ash behind H.

In order to solve the problem of uneven air volume, the following measures can be taken in the structure:

① Add a wind deflector on the inclined partition of the air duct, as shown in figure 5-168. The size of baffle shall be determined according to the principle of equal air volume and equal air pressure;

② Considering the change of the actual situation on site, the adjustable valve rod is adopted between the lifting valve rod and the valve plate, so that the outlet high h is the change value for further correction;

③ The air volume regulating valve is set in the air inlet branch pipe to regulate the air volume of each box room after the equipment is in operation. Control the air volume difference of each box room within 5%.

(2) The anti-static filter bag is used inside the dust collector. Because the high concentration dust rubs with each other in the flow process, and the dust and filter cloth also rub with each other, static electricity can be generated, and the accumulation of static electricity will produce sparks and cause combustion. For the pulse ash cleaning method, the polyester needle felt is used for the filter bag. In order to eliminate the lack of static electricity easily generated by the polyester needle felt, the conductive metal wire or carbon fiber is spun into the filter bag cloth. When installing the filter bag, the filter bag is connected with the perforated plate through the steel frame and connected to the workshop grounding grid through the shell. Mp922 and other anti-static products have been developed for the filter bag of anti blowing and ash cleaning. The use effect is very good.

(3) Safety holes (valves) are set. In order to limit the explosion to the interior of the bag filter without expanding to other aspects, it is important to set safety holes and essential fire extinguishing equipment. The purpose of setting safety holes is not to prevent explosion, but to limit the scope of explosion and reduce the number of explosions. Most dust collectors dealing with explosive dust operate under the condition of setting safety holes. Because of this, the design of safety hole should ensure that it can play a practical role in case of explosion accident; The maintenance and management of safety holes shall be strengthened at ordinary times.

The rupture plate type safety hole is made of ordinary thin metal plate. Because the box body of the bag filter can not bear a lot of pressure, the strength of the broken plate should be designed so that the plate can be damaged under a lower pressure. Sometimes the aluminum plate is fatigued and deformed due to the long-term compression of the box, resulting in cracking. Even if this is normal, it is not allowed to replace the thick plate with high strength.

The spring door type safety hole adjusts the opening pressure by increasing or decreasing the spring tension. In order to ensure that the door hole can play a safe role in the event of an accident, the action test must be carried out regularly.

The area of the safety hole should be determined according to the relationship between the pressure during dust explosion, the speed of pressure increase and the compressive strength of the box, but there is no exact data at present. The size of the safety hole area should be determined according to the form and structure of the bag type dust collector. We believe that the ratio of the safety hole to the volume of the dust collector is 1 / 10 ~ 1 / 30 for small and medium-sized dust collectors and 1 / 30 ~ 1 / 60 for large and medium-sized dust collectors. In case of difficulties, it is necessary to properly refer to the actual conditions of safety explosion-proof holes reserved for other devices.

① Explosion proof plate explosion proof plate is an emergency pressure relief device driven by pressure difference and not automatically closed. It is mainly used for pipelines or dust removal equipment to avoid damage caused by overpressure or vacuum. Compared with safety valve, bursting disc has the advantages of large discharge area, sensitive action, high precision, corrosion resistance and not easy to be blocked. Bursting discs can be used alone or in combination with safety valves.

The explosion-proof plate device is composed of bursting disc and holder. The holder is made of Q235, 16Mn or OCr13 and other materials. Its function is to clamp and protect the explosion-proof plate to ensure the stability of blasting pressure. The explosion-proof plate is made of aluminum, nickel, stainless steel or graphite and has different shapes: the concave surface of the arched explosion-proof plate faces the compression side, and tensile or shear failure occurs during blasting; The convex surface of the reverse arched explosion-proof plate faces the pressure side. During blasting, it is suddenly overturned due to instability and cut by the blade or torn along the slot; Tensile or shear failure also occurs when the plane explosion-proof plate is blasted.

The pressure resistance of the explosion-proof plate selected for the dust collector shall be based on the working pressure of the dust collector. Because the pressure resistance of the dust collector body is required to be 8000 ~ 18000pa, check the data according to the set pressure resistance requirements to determine the rupture pressure of the explosion relief valve membrane.

② Explosion proof valve design there are two main types of safety explosion-proof valve design: one is explosion-proof plate; The other is heavy hammer explosion-proof valve. After the former one is broken, it is necessary to replace the new plate, and the production shall be interrupted. In case of high negative pressure, it is easy to blank and difficult to keep warm. The latter one is more advanced than the former one. In the closed state, it is pressed by heavy hammer, and the tightness is poor. High voltage pulse ash cleaning is not suitable for the above two methods. In order to solve the problem of tightness, an explosion-proof safety lock can be designed on the heavy hammer fat explosion valve. The utility model is characterized in that when closing, the safety door is locked mainly through this lock, which can be automatically opened and released in case of explosion, and its release force (safety force) can be adjusted by spring. In order to balance the stress of the safety door, it is generally necessary to set 4 ~ 6 locks according to the area of the safety door. In order to make the explosion-proof door tight without air leakage, it can be designed into a dual structure of explosion-proof plate and safety lock.

(4) Detection and fire-fighting measures in order to prevent accidents, necessary fire-fighting measures can be taken on the dust removal system.

① Fire fighting facilities. Mainly water, CO2 and inert extinguishing agent. For the cement plant, CO2 and CO2 are mainly used, while nitrogen can be used in the steel plant.

② Temperature detection. In order to relieve the temperature change of the dust collector and control the ignition point, several thermometers are generally installed at the inlet of the dust collector and on the ash hopper.

③ Detection of Co. For large dust removal equipment, due to its large volume, the installation of thermometer is very limited. Sometimes combustion occurs far away from the measuring point of thermometer, which is difficult to be reflected from the thermometer. A CO detection device can be installed at the outlet of the dust collector to help detect. As long as combustion occurs anywhere in the dust collector, the CO in the flue gas will rise. At this time, the alarm of rising CO concentration can be linked with the control of the dust removal system, so as to stop the operation of the system dust collector in time.

(5) Equipment grounding measures explosion-proof dust remover is often arranged in the open air due to the need of operation safety. Even if it is arranged on the tall steel structure in the open air, according to the equipment grounding requirements, equipment grounding lightning protection has become an essential measure, but the dust collector is generally not equipped with lightning rod.

Conductors with good conductivity are added between all connecting flanges of the dust collector, and the conductor form can be made into card type. It can also be made into line type. The line conductor is shown in the figure. The card type conductor is shown in the figure. No matter which form of conductor is adopted, the connection must be firm and buried on the surface, with certain corrosion resistance. Otherwise, the grounding and lightning protection effect of equipment will be affected.

(6) The selection of explosion-proof components is essential in the explosion-proof measures of dust collector. The explosion-proof dust remover is forbidden to cause explosion hazard when the dust in the operating condition flows into the electrical load. During the operation of the dust collector, electric shock and cremation will inevitably occur when the electrical load and components are in contact with the current transmission and even in the conduction. It is also very easy for the discharge spark to induce the explosion of dust source gas exceeding the limit concentration. All electrical load components must be explosion-proof components to eliminate the generation of explosion inducing factors. Ensure the safety of equipment operation and operation. For example, the pulse valve of the pulse dust collector and the solenoid valve for the poppet valve should be explosion-proof products.

(7) Measures to prevent Mars from mixing: in the bag type dust collector for treating waste gas from sawdust boiler, rice husk boiler, aluminum regeneration furnace and smelting furnace, the burned dust in the furnace may enter the box with the air flow of the air duct, so that the dust accumulated on the filter cloth will catch fire and cause accidents.

In order to prevent Mars from entering the bag filter, the following measures shall be taken:

① Pre dust remover and cooling pipeline shall be set. The coarse dust collector or cyclone is set as the pre dust collector. In this way, too fine particles are not easy to capture, and in most cases, the particulate dust can be burned out before entering the dust collector. Set the cooling pipe after the pre dust remover, and control the flow rate in the pipe to make it as low as possible. This is a relatively reliable technical measure, which can make the gas have sufficient residence time in the pipe.

② Lingque spray tower. Gas cooling method of direct water spray in advance. In order to ensure the safety and fire prevention of dusty gas in bag filter, the cooling water consumption is controlled. Most of the burning dust can be cooled after contacting with fine water droplets, but the water droplets are easy to vaporize. In order to cool the burning dust that has not been contacted with water droplets, there should be necessary space and residence time.

In special cases, the spray tower, cooling pipe and pre dust collector are used together to completely prevent the mixing of sparks.

③ The Mars capture device is shown in Fig. It is a simple and feasible method to install Mars capture device on the pipeline. In addition, when Mars passes through the trap, it can send an electrical signal to give an alarm. At the same time, stop the operation or change the gas circuit.

The design requirements of Mars trap are as follows:

a. When the spark catcher is used to collect spark particles in high-temperature flue gas, the main material of the equipment is generally 15Mo3 or 16mo, and Q235 is used for beams, columns and platform ladders. When the spark catcher is used as flue gas pre separator, Q235 can be used for other materials except 15mn for rotating blades;

b. The inlet and outlet speed of the equipment is generally between 18 ~ 25m / S;

c. Consider the separation effect of dust. The blade shall be provided with certain wear-resistant measures and proper rotation angle;

d. The equipment structure design shall take into account the equipment deformation caused by high temperature.

(8) When controlling the dust concentration at the inlet and adding incombustible powder, the internal concentration distribution of the bag filter will inevitably make a part within the explosion limit. In order to improve safety, avoid the concentration between the upper and lower limits of dust explosion in the pipeline. For example, in the dust collection work such as pneumatic conveying, crushing and grading, attention should be paid to the design to make it operate at a high concentration exceeding the upper limit; In case of local collection, the dust concentration in the pipeline shall be kept at a low concentration below the lower limit.

An example of using dilution method to prevent fire. When collecting explosive dust, due to the dust collection cover, the dust is diluted with air, and the concentration in the pipeline is far lower than the lower explosion limit. Non combustible powder, such as clay and bentonite, is continuously provided from the middle of the system to the pipeline, and the explosive dust is diluted inside the dust collector to prevent the risk of explosion and fire.