Working Principle and Application Classification of Industrial Boiler
Boiler is a kind of energy conversion equipment. The energy input to the boiler includes chemical energy and electric energy in fuel. The boiler outputs steam, high temperature water or organic heat carrier with certain heat energy. Boilers can be divided into power plant boilers, industrial boilers, vehicle and marine boilers, domestic boilers and so on. Industrial boilers are boilers that provide steam or hot water (hot water boilers) for industrial and mining enterprises to meet the needs of production process, power and heating.
The main working principle of the boiler is a kind of thermal equipment which uses the heat energy released after the combustion of fuel or the waste heat in industrial production to transfer to the water in the container so that the water can reach the required temperature or a certain pressure steam. After water enters the boiler, the heat absorbed by the heating surface of the boiler in the steam-water system is transferred to the water, so that the water is heated to a certain temperature and pressure of hot water or steam, which is introduced for application. In the part of combustion equipment, fuel combustion continuously emits heat. The high-temperature flue gas generated by combustion transmits heat to the heating surface of the boiler through heat transmission. The temperature of the boiler itself decreases gradually, and finally is discharged by the chimney.
Boiler is an important heat supply equipment in national economy. Electric power, machinery, metallurgy, chemical industry, textile, paper making, food and other industries, as well as industrial and civil heating need a lot of heat energy from boilers.
1. According to the structure, it can be divided into fire tube boiler and water tube boiler.
(1) Fire tube boilers
In addition to the fire drum, the utility model also installs several smoke pipes in the vertical or horizontal pot shell. The high temperature flue gas generated by the combustion of fuel in the combustion chamber passes through these flue pipes to heat the pot water and then discharges into the chimney. Compared with the firedrum boiler, it has many advantages, such as large number of flue gas tubes, larger heating area, lower exhaust temperature, higher thermal efficiency, smaller consumption of steel and boiler volume.
Fig. 1 is a horizontal tempering tube boiler
The evaporation of fire tube boilers is generally no more than 15 tons per hour, the working pressure is generally no more than 1.6 MPa, and the thermal efficiency of coal-fired boilers is generally about 70%. The thermal efficiency of all automated oil-fired boilers can reach 85%.
(2) Water pipe boiler
Most of them are used as industrial boilers with large capacity and high pressure. There are two types of water tube boilers: natural circulation boilers and direct current boilers. The natural circulation water tube boiler has the structure of single drum and double drum.
Figure 2 is a double drum water pipe boiler
This kind of boiler usually sets economizer after convection tube bundle, and sometimes air preheater to reduce the exhaust gas temperature. The efficiency of coal combustion can reach 80%, which is higher when fuel is used. There is no drum in a once-through industrial boiler. The heating surface consists of a boiler tube coiled around the inner wall of the furnace and a convective tube bundle behind it. Feed water enters from one end of the boiler tube and steam is exported from the other end. DC industrial boilers are small in size, usually with steam-water separators and automatic control devices.
2. According to the outlet working fluid pressure, it can be divided into atmospheric pressure boilers, micro-pressure boilers, low pressure boilers, medium pressure boilers, high pressure boilers, ultra-high pressure boilers, Sub-critical Pressure boilers, supercritical pressure boilers and ultra-supercritical pressure boilers. Boilers below 2.5 MPa are called low pressure boilers, and boilers above 6.0 MPa are called high pressure boilers. Boilers with pressure between them are called medium pressure boilers.
3. According to the form of boiler water circulation, it can be divided into natural circulation boilers and forced circulation boilers (including once-through boilers).
4. According to the fuel and energy used, it can be divided into solid fuel boilers, liquid fuel boilers, gas fuel boilers, waste heat boilers and waste material boilers.
5. According to the classification of boiler capacity: small boilers with evaporation less than 20 t/h, large boilers with evaporation greater than 75 t/h, and medium boilers with evaporation between them.
The problems to be considered in boiler selection are summarized as follows:
1. How much steam and hot water are needed by the enterprise itself in the production process or heating and heating.
2. Boiler steam or hot water parameters.
3. Analysis of the relationship between the selection of steam and hot water boilers.
4. Selection of industrial boiler type.
5. Coal types used in boilers.
6. Other factors such as shape, size, load change, etc.
Specific selection steps:
1. How to Determine the Capacity and Number of Boilers
How to determine the boiler capacity is an important issue when selecting and purchasing boilers. Generally speaking, the output or capacity of the boiler should be determined according to the steam volume or heating area used in the production process and the heat supply required for domestic hot water. Firstly, users need to know the total load range; secondly, the peak situation, including the duration of peak load, the frequency of peak, that is, the periodicity of peak occurrence, such as the number of peak occurrences per month, the difference between winter and summer in weekly, monthly and one year. After finding out the total load requirement and peak load, we can consider the selection of equipment capacity and number. When choosing, we should also consider the problem of emergency stopping standby. Standby quantity generally refers to a standby boiler. This needs to be determined according to the importance of users themselves, if the production and use of high continuity requirements must seriously consider the issue of standby equipment.
Generally, the economic load of the boiler is over 75% of the rated load, so the actual load of the boiler in use should be more than 50% of the rated load of the unit itself, that is to say, the equipment provided by the manufacturer should be stable and efficient between 50% and 100% of the rated output. In the past, users purchased excessive surplus in order to be safe, which resulted in the equipment running under low load for a long time, which was very uneconomical. Generally, the total capacity of the boiler room is 8 tons per hour or the thermal power is more than 6 MW, so it is more appropriate to select three sets of equipment capacity. Because of the difference between winter and summer, we can choose the equipment with different output. But it is more convenient and economical to choose the same equipment for the new boiler room in operation, maintenance and management. Such auxiliary equipment layout of boiler room can be reasonably arranged. If the difference between high and low peaks, winter and summer is large, two large and one small equipment can also be selected. According to the needs of the new boiler room, it is also possible to choose more equipment than three, but it should not be too many. Instead of using many equipment, it is better to choose the equipment with high-grade capacity. In the current domestic equipment quality situation, it is necessary to consider standby units.
The reliability utilization rate of foreign equipment is higher, but they generally consider standby equipment because of the small number of initial investment costs compared with future operation and maintenance costs. In case of accident shutdown, the standby equipment should be put on top, which ensures the use requirements. When foreign countries have higher requirements on equipment regulation ratio, they mainly focus on the selection of combustion equipment. At present, there are not many kinds of combustion equipment in China, and there is not much choice in this respect.
Overseas, it is recommended to use a heat accumulator or a large-capacity shell-type regenerative internal combustion boiler (Thermal Storage Boiler) to bear the load with a short peak time.
The regenerator is a device consisting of a large storage vessel, an auxiliary regulating valve, management, etc. It is a device for accumulating heat energy. Most of the storage media in the regenerator are water (and other medium vapors, etc.). The high temperature water stored in the regenerator can release steam at any time to meet the production needs. It can make the boiler run under high efficiency and stable load, so it is an energy-saving device. To some extent, it can reduce the investment of industrial and mining enterprises in boiler equipment. The steam consumption of many industrial and mining enterprises in China is uneven within 24 hours a day, with peaks and low loads. One-shift factory goes to work at 8:00 in the morning. After 9:00, the steam load of each workshop rises rapidly, decreases slightly at 12:00 noon, peaks again at about 1:00 p.m., and returns to low load or even pressing fire at about 4:00 p.m. The two-shift system and the three-shift system have different loads. Therefore, the operation and operation requirements of boilers are higher. If the plant has high and low load, the instantaneous change is very large, in order to maintain steam pressure stability, and a certain boiler efficiency, it is necessary to have a perfect combustion automatic control device. After using regenerator, the boiler can always operate under rated load or economic load, which simplifies the operation. When the plant heat load reaches its peak, the regenerator can release steam. When the heat load is low, the excess steam produced by the boiler can be stored in the regenerator. Before choosing regenerator, industrial and mining enterprises should make statistics on steam load of their departments and draw load curve. The load distribution and the method of balancing steam consumption are analyzed, and the necessity of choosing regenerator is finally determined.
2. How to Determine the Parameters of Steam and Hot Water
Users should determine the parameters of steam or hot water according to the use requirements. Most of the boilers that industrial and mining enterprises need in production are steam boilers. Steam is mostly used in the production of heat to meet the production process requirements, such as textile printing and dyeing, food sugar, rubber papermaking, petrochemical and other industries. Therefore, the saturated steam parameters of the boiler should be determined according to the requirements of the production process. The advantage of saturated steam is that it can ensure the constant temperature needed in production.
Generally, the pressure of boilers is always higher than the saturated steam pressure needed in production. The reason is to overcome the pressure loss in pipes or networks. At present, in many cases, the nameplate pressure of the boiler is far greater than the pressure required in production. For example, many users have purchased 13-pressure boilers, but in fact only 5 or 6 gauge pressures are used. Running under such low pressure has a bad effect on steam quality. Because the pressure decreases, the specific volume of steam increases, and the flow velocity at the outlet increases greatly, which makes the phenomenon of steam carrying water rise. It is enough to add 25%~30% pressure margin after the pressure needed in actual production plus the pressure drop needed to overcome the resistance of all pipelines.
The head of the feed pump is matched by the boiler nameplate. The actual operating pressure of the boiler is too low. The energy consumption of the feed pump is obviously a great waste.
The steam parameters of boilers used in small power plants can be matched according to the parameters required by steam turbines. Most of them are superheated steam at 25 meters of atmospheric pressure. If the steam turbine is superheated steam with 13 meters of atmospheric pressure, then the boiler should select superheated steam parameters with 16 meters of atmospheric pressure, which can make the steam quality better.
For the hot water boilers needed for heating in cities and factories, the commonly used temperature parameters are the supply and return water temperature of 95/70 degrees Celsius. At present, there are three types of urban heating in China: thermal power plant, regional boiler room and decentralized small capacity boiler room. The former two are centralized heating, with district boiler room heating as the development direction, and at present, scattered small capacity boiler room is the majority. For District Central heating, the temperature of high temperature hot water boiler, supply and return water will develop 130/70 parameters. At present, there are 95/70, 115/70 and 130/70 hot water boilers available for selection. The operating pressure of hot water boilers in actual use is mostly lower than the design pressure, which is mainly limited by the original pipe network system.
3. Selection Relation of Steam Boiler and Hot Water Boiler
Because hot water heating is more energy-saving and comfortable than steam heating, and energy-saving requires that steam heating be replaced by hot water heating. In many industrial and mining enterprises, steam heating is still needed in the production process, so enterprise users need both steam and hot water. In this case, how to choose the type is a more complex problem. What equipment should be selected should be analyzed concretely. It is roughly estimated that there are the following situations:
- A. Industrial and mining enterprises often use a large amount of steam while heating hot water is not much in progress. Steam boilers can be used, and surface heat exchangers can be used to produce hot water.
- B. When the heating system uses a large amount of hot water and requires a small amount of steam, but the pressure is high, it is advisable to set up a small steam boiler outside the hot water boiler as a steam source.
- C. When both hot water and steam are relatively small, steam-water boilers can be selected.
- D. When the quantity of hot water and steam is very large, it should be studied systematically.
4. Relation between Coal-fired Varieties and Type Selection
Coal is the fuel of most industrial domestic boilers in China. The characteristics of coal are more complex, such as anthracite is difficult to ignite, high ash and inferior coal is not easy to ignite, although lignite has a high volatility, but the moisture content is also high, ignition is also difficult. These difficult-to-ignite coals have special requirements for the design of the arch of the furnace, and the problems in this respect are quite complicated. In developed industrial countries, the combustion types of boilers are relatively stable after screening. Most of the raw coal supplied in China’s market is raw coal, and the quality of raw coal sources is difficult to maintain stability. In particular, the coal produced by small kilns has been put into the market, which makes the coal-fired varieties more complex. Therefore, before purchasing equipment, users must seriously study the development trend of coal-fired varieties with local fuel supply departments in order to ensure that the furnace type and arch type are suitable.
5. Selection of Industrial Boiler Types
The choice of combustion equipment depends first on the fuel type used by enterprise users, but the degree of fuel adaptability of combustion equipment is also staggered. A brief summary is as follows:
Chain grate with arch can burn a wide range of coal. But under the given arch, the adaptability of coal combustion is also in a certain range. Because the metal consumption of chain grate is large, coal with low calorific value needs large grate area, and the metal consumption increases accordingly. Therefore, the chain grate is suitable for medium or above coal combustion (calorific value is more than 4000 calories per hour), especially for high quality coal.
Reciprocating grate can make coal-fired production agitate, so coal is widely used. But because the reciprocating grate is not as good as chain grate in cooling condition, it is suitable for burning coal with more ash, because ash can protect grate pieces from overheating and burning. Suitable for burning coal with calorific value of 2700-4500 calories/kg.
The inverted grate of the coal thrower has the characteristics of wide adaptability to coal combustion, but it is not suitable for anthracite burning, because the long back arch is needed on the anthracite bed burner, and the coal thrower needs an open hearth with secondary air. Coal spreader has semi-suspension combustion property and is sensitive to load regulation. But the flue gas and dust content of the coal thrower are high. Therefore, in order to give full play to the advantages of the coal thrower stove, the first thing is to do a good job in reducing the dust content of the flue gas. For example, there are not many fine particles in raw coal granularity, the use of secondary air in the furnace, fly ash collection and reburning device and other measures to keep up, reduce the dust content of exhaust to a level close to or slightly higher than that of chain furnace, so that users can choose.
The fluidized bed furnace is a new type of furnace developed actively both at home and abroad. The development of fluidized bed boilers abroad focuses on environmental protection, while the domestic development focuses more on the burning of inferior coal. It can burn any kind of coal with a calorific value of more than 1300 calories per kilogram. When using superior coal and low-quality coal, the arrangement of heating surface and operation mode of combustion chamber are different. The metal consumption of the fluidized bed furnace is small and the cost is low, so it has a great development prospect.
Generally speaking, pulverized coal fired boilers are not suitable for small industrial boilers with capacity less than 20 tons per hour. This is due to the complexity and inexpensiveness of auxiliary devices. The capacity of coal-fired and bagasse dual-purpose furnaces in central and southern China is also more than 10 tons per hour.
Selection of Types of “Pot”
At present, there are three types of “pot”: shell type (fire pipe), water-fire pipe mixing type and water pipe type, each of which has its own characteristics. The shell type and water-fire tube type are suitable for smaller capacity boilers, such as 4 tons per hour or less. Water tube boilers are suitable for larger capacity boilers such as 4 t/h and above boilers. Generally speaking, the water pipe boiler has good safety. Under the same shape and size, the furnace of the water tube boiler can be slightly higher, and the arrangement of the arch is better than that of the shell type and the water-fire tube type, which is conducive to combustion.
6. The relationship between other factors and selection
In addition to the three basic factors of boiler output, working fluid parameters and coal-fired varieties, there are other aspects such as shape size, load fluctuation, water quality, air pollution, mechanization and automation, which are all related to the selection of main and auxiliary engines. For example, the same type of 10 tons/hour, 6 tons/hour and 6.5 tons/hour double drum horizontal (SHL) boilers have different sizes in different factories. Users need to understand the situation of the manufacturer’s products.
1. Maintenance methods
- 1. Boiler feed water must be treated qualified, which is an important matter to prolong the life of the boiler and ensure the safety of operation.
- 2. Feed water temperature should be as close as possible to furnace body temperature, preferably above 20 C. A kind of
- 3. Low water cut-off devices must be regularly inspected, maintained and cleaned. The water level controller is cleaned at least once a month and the water level meter is flushed at least once a day. The water level meter should be flushed at the highest level, and the water level after flushing should not be lower than the lowest safe level. If one flushing is not clean, it can be flushed many times, but it must be flushed at the highest level.
- 4. Regular sewage discharge operation is required, at least once a day (every shift). After sewage discharge, it is necessary to check whether the sewage valve leaks due to the influence of pollutants; if so, it must be eliminated. A kind of
- 5. When the boiler stops without maintaining the steam pressure, the steam space of the boiler will generate negative pressure and retain air. Therefore, the main steam valve should be opened when the combustion is re-ignited. A kind of
- 6. After soot deposit in flue pipe, the temperature of flue gas will rise, which will reduce the efficiency of the boiler and increase fuel consumption. Therefore, the front and rear door covers should be opened regularly, and soot deposit in flue pipe should be brushed or blown away.
2. Stopping Maintenance and Anti-Corrosion
- 1. After stopping the boiler, in order to prevent corrosion in the boiler, maintenance must be carried out. A kind of
- 2. Short-term maintenance. The method of wet maintenance can be adopted, that is, after stopping the furnace, the furnace water is filled up and the air is discharged completely. A kind of
- 3. Long-term maintenance. Dry maintenance method can be adopted, that is, after stopping the furnace, the furnace water is discharged, the lower hand hole is opened, the water accumulated in the pot is dried, and the drier is put into the pot through the lower hand hole, and it should be replaced regularly. It can be checked and replaced once a month at the beginning, and once a month later. It should be noted, however, that all desiccants must be removed before the boiler is commissioned.
3. Surveillance and Maintenance
- 1. During the operation of the boiler, the equipment is inspected every 1 hour, focusing on the operation of the boiler safety components and rotating equipment.
- 2. The inspection, test and regular operation of boiler equipment shall be specified in detail and loaded on site in the form of icons.
- 3. Operators fill in the records in time according to the situation of inspection.
- 4. Must always keep the site clean, so that there is no impurities, no ash, no water, no grease scale.
Matters needing attention
Matters needing attention in water treatment of industrial boilers
The main content of industrial boiler water treatment is water softening, that is, removing calcium and magnesium hardness salt from water, preventing boiler scaling. Soft water treatment equipment is also used to become water treatment equipment. In order to prevent the boiler from softening before, the salt and other impurities which can cause boiler scaling and corrosion in the water should be sent out first, so that the water supply meets the water quality standards stipulated by the state.
1. The commonly used boiler water treatment technology is boiler dosing and out-of-pot ion exchange method. Boiler dosing method is to add appropriate amount of chemical scale inhibitor to the boiler, so that it can react with calcium and magnesium ions in the boiler water to form chemical, physical or physical chemical precipitation of insoluble substances, and form loose scale, which can be removed by boiler blowdown, in order to prevent or alleviate internal scaling.
Deoxygenation of feed water is one of the important measures to prevent metal corrosion of boiler and ensure safe operation of boiler thermal system. At present, thermal deoxidation, vacuum deoxidation, chemical deoxidation and other new deoxidization methods are commonly used in deoxidization treatment. Thermal deoxidization is the most commonly used boiler water treatment technology.
2. The characteristics of boiler water treatment in boiler water treatment technology Boiler water treatment is that by putting a certain amount of water softener into the boiler, the scaling substances in the boiler feed water are transformed into scales, and then the scales are discharged from the boiler through sewage discharge, thus achieving the purpose of slowing down or preventing scales from forming. This kind of water treatment is mainly carried out inside the boiler, so it is called water treatment inside the boiler. Boiler water treatment has the following characteristics:
- 1. Boiler water treatment does not require complex equipment, so it has low investment, low cost and easy operation.
- 2. Boiler dosing treatment is the most basic water treatment method, and it is also the continuation and supplement of chemical water treatment outside the boiler. After the treatment of water outside the boiler, there may be residual hardness. In order to prevent scaling and corrosion of the boiler, some water treatment agents are added.
- 3. Boiler water treatment can not completely prevent the boiler from scaling, especially the generated scale. It is easy to form secondary scaling if the sewage is not discharged in time.
- 4. Boiler dosing method does not pollute the environment. Unlike ion exchange and other water treatment methods, it does not deal with the amount of impurities in natural water, how many impurities are discharged after regeneration, but also discharges a large number of residual regenerants and products after regeneration. Boiler dosing treatment method is to turn the main impurities in water into insoluble dirt, which will not cause pollution to nature.
- 5. The formulation used in the purifying method of boiler dosing should be matched with the water quality of feed water. When the hardness of feed water is too high, a large amount of water slag will be formed to accelerate the scaling speed of heat transfer surface. Therefore, it is generally not suitable for high hardness water quality.
3. Application Scope of Boiler Drug Addition
According to the characteristics of boiler water treatment, as long as the following conditions are met, the boiler plus chemical water treatment method can be adopted:
- 1. Boiler has no water wall tube;
- 2. In operation, the slag formed in the boiler can be eliminated reliably.
- 3. The sludge formed by adding chemicals will not affect the safe operation of the boiler.
- 4. Users do not require high quality steam.
1. Water shortage in the pot
When the water level in the pot is lower than the lowest permissible water level, it is called water shortage in the pot. It is also divided into slight water shortage and severe water shortage.
Slight water shortage: When the water level of the boiler disappears from the glass tube (plate) water level meter, the water level can reappear by means of flushing water level meter and “calling water”, which is called slight water shortage.
Serious water shortage: When the flushing water level meter and the “call water” are used, the water level in the pot still can not appear in the glass tube (plate) water level meter, which is called serious water shortage.
When there is serious water shortage in the pot, it will cause the blasting accident of the furnace tube. When the tube or drum is burnt red, if a large amount of water is added to the tube or drum by mishandling, a large amount of steam will be generated when the water contacts the tube or drum. Because of the sudden increase of steam pressure, the boiler explosion accident will occur. Especially when the pressure is high and the volume of water is large, the pot-shell boiler will be more powerful when it explodes. Therefore, when there is serious water shortage in the pot, it is strictly forbidden to add water to the pot, and emergency stopping measures should be taken.
There are many reasons for water shortage in the pot. According to the statistical data analysis of the state labor department, about 70% of them are mainly caused by the slack labor discipline and misoperation of the operators. For example, long-term forgetting water supply; forgetting to close sewage valve or not close tightly after sewage discharge; not flushing water gauge on time, so that water gauge cock blocked, forming false water level and so on. The remaining 30% are due to equipment defects or other malfunctions. If the water supply equipment suddenly breaks down, or the water source suddenly interrupts, stops the water supply and so on. Therefore, in order to eliminate water shortage accidents in the boiler, the key is to strengthen the education of boiler operators to abide by labor discipline. As long as the operators have a high sense of responsibility and master the operation technology skillfully, even if equipment failures occur, water shortage accidents in the boiler can be eliminated in time.
2. Boiler Overpressure
Boiler overpressure means that the working pressure of the boiler in operation exceeds the maximum permissible working pressure.
The causes of boiler overpressure and boiler explosion accidents are mostly the result of blindly increasing the working pressure of the boiler or leaving the position of the director without permission, and the boiler is in the unmanned management. Therefore, the working pressure of boilers can not be increased blindly. If it is necessary to increase the working pressure of the boiler, it must pass the strict technical appraisal of the relevant departments. In addition, from the case of overpressure explosion of boiler, we can see the importance of training the furnace personnel and strengthening the post responsibility system.
Sometimes pressure gauges and safety valves fail simultaneously, which may also cause overpressure of boilers.
3. Fill the pot with water
Domestic full water means that the water level in the pot exceeds the maximum permissible water level. When the water level is serious, the steam pipe emits water shock sound.
The overfilling of the pot is usually due to negligence of operators and excessive water supply. When water filling occurs in the pot, the drainage valve should be opened immediately, and excessive water should be released so that the water level can be maintained normal and the full water in the pot can be eliminated.
4. Coefficiencies of soda and water
The characteristic of water and water sharing is that the surface of the water level meter fluctuates violently up and down. The water in the boiler is bubbly and water in the steam is very large, causing serious water attack in the pipeline.
The main reason for the co-evaporation of soda and water is that the salt content of pot water is too high. Therefore, the main measure to prevent the co-evaporation of steam and water is to control the salt content of pot water within the critical salt content. Strengthening water treatment and increasing continuous sewage discharge are also effective measures to prevent co-evaporation of steam and water.
5. There are significant blasting and steam injection sounds in tube blasting.
At this time, the water level drops rapidly and the vapor pressure decreases obviously. Generally, the vapor pressure and water level can not be maintained, so the furnace must be shut down urgently.
The main cause of tube blasting in industrial boilers is the poor water treatment or no water treatment at all, resulting in scaling or corrosion. Scaling in the pot is due to the fact that the hardness of feed water exceeds the prescribed standard for a long time, and scales are deposited on the inner wall of the furnace tube, even blocking the furnace tube. Corrosion is caused by excessive oxygen content or acid value in feed water. According to the investigation, many units do not pay attention to water treatment work, resulting in tube blasting or tube blockage, boiler scrap accident is quite serious, enough to take warning. Therefore, only by strengthening the management of water treatment, can the blasting accident of furnace tube be fundamentally eliminated, and a great deal of fuel, steel, manpower and financial resources can be saved.
Secondly, because of the serious water shortage of the boiler, the furnace tube will be overheated, which will also cause the furnace tube blasting accident, and the furnace tube blasting accident will soon cause water shortage. Therefore, tube explosion and water shortage often cause and effect each other. Therefore, it is very important to closely monitor the water level in the operation of the boiler to prevent the boiler from water shortage and tube explosion accidents.
6. Furnace Explosion
When the concentration of combustible substances mixed with air in the furnace reaches the explosion limit, the tube explosion or deflagration will occur in the open fire. When the furnace is deflagrated, the flame sprays out from the ignition hole, the fire hole and other places of the boiler, which is very vulnerable to injury. Furnace explosion will cause furnace collapse, boiler damage, and seriously threaten personal safety.
When all kinds of combustible substances mix with air, there is a certain limit range of explosion concentration. It is not easy to explode under too low or too high concentration. The following table lists the limits of explosion concentration when some combustible substances mix with air at room temperature and atmospheric pressure.
Boilers using fuel oil (heavy oil), natural gas, dry gas, gas and pulverized coal are difficult to avoid the explosion concentration limit in actual operation. Especially in ignition, it is very easy to cause furnace deflagration or furnace explosion accidents due to improper operation.
Boilers fueled with fuel oil (heavy oil), natural gas, dry gas, gas and pulverized coal must be ventilated for more than 5 minutes before ignition in operation to prevent furnace explosion or deflagration; torches should be used for ignition instead of residual heat; the temperature of combustible substances in the furnace should not be too high during operation, and the blast and induced air volume should be balanced.
7. Secondary combustion
The phenomenon that the combustion material deposited at the tail of the boiler is re-ignited is called secondary combustion. When the boiler is burned twice, the economizer and induced draft fan will be burned down, and the tail of the boiler may be burned down in serious cases.
Combustible substances deposited at the tail of the boiler, such as carbon black, oil sluggish and pulverized coal, are mainly brought into the tail of the boiler due to incomplete combustion. Especially when ignition and shutdown occur, incomplete combustion is most likely to occur and a large amount of combustible material is deposited at the tail of the boiler. These sediments reduce the heat transfer effect of the tail heating surface and increase the exhaust gas temperature. When the exhaust temperature rises to a certain value and there is enough oxygen to support combustion, secondary combustion will occur.
Some secondary combustion occurs after several minutes or hours of shutdown, which is related to the ignition point and heat dissipation conditions of combustible materials. The flue gas velocity at the tail of the running boiler is faster, and the heat generated by the combustible material is quickly taken away by the flue gas. It can only be oxidized slowly and can not achieve ignition combustion. After stopping the furnace, the flue gas in the flue stops flowing, the heat generated by the oxidation of combustible materials is not easy to lose, and the temperature will rise. If the door or flue baffle is not closed tightly and fresh air is leaked to assist combustion, it will cause secondary combustion after stopping the furnace.
In order to prevent secondary combustion at the tail of the boiler, the fundamental measure is to eliminate the deposition of combustible substances at the tail of the boiler. Therefore, if ignition fails, fuel (such as coal, oil, gas, etc.) must be stopped immediately in the furnace to prevent the inflammable material from being brought into the tail of the boiler. In operation, soot blowing should be strengthened and air distribution should be appropriate so as to make the combustion complete and smoke free. After stopping the furnace, the temperature of the economizer should be monitored. If the temperature of the economizer suddenly rises by tens of degrees, it will cause spontaneous combustion. Fire extinguishing measures should be taken immediately.
In order to eliminate secondary combustion in time, carbon dioxide fire extinguisher can be used to extinguish fire, and steam fire extinguishing pipe can also be installed at the heating surface of the boiler tail. Steam fire extinguishing pipes should be evenly laid out, with less resistance to flue gas, no influence on soot blowing, tight and leak-tight, and sufficient exhaust capacity to asphyxiate secondary combustion.
8. Boiler Flameout
Boiler extinguishing is also called boiler extinguishing. It refers to the sudden extinguishment of the boiler in normal operation. Sudden flameout is a common fault in the operation of oil-fired boilers.
9. Damage of Furnace Wall
The phenomena of wall damage are as follows: there are falling bricks in slag remover; temperature of hoisting parts of furnace wall bracket, shell or arch brick suddenly rises, even burns red; asbestos filler drops in large quantities at the transfer point of furnace wall, and the contact point between furnace wall and steel frame, wall sleeve, etc., resulting in excessive cold air intruding into furnace, lower furnace temperature and lower boiler load; external furnace wall is seriously protruded and cracked, and falls down. The danger of collapse.
Reasons for wall damage: unreasonable structure of furnace arch and wall hinders the normal expansion of pressure components of boiler; less arrangement of water wall tubes; incorrect location of burner, which causes excessive temperature of some furnace walls and arch. In terms of operation, improper adjustment of combustion flame and deviation of flame center; long-term positive pressure combustion, excessive furnace temperature, low melting point of fly ash and serious coking in the furnace; excessive ignition, shutdown and load increase and reduction, resulting in sudden cooling and sudden heating of the furnace wall and arch; broken furnace wall when removing coke slag; explosive impact of combustible gas in the furnace. In the aspect of installation and maintenance, the brick edge is broken and the crack is too big; the proportion of mortar is not appropriate, the maintenance is not good, and the brick bonding is not strong; there is not enough expansion joint left; the drying time is too short, and the furnace wall is not completely dry, that is, the boost operation.
In the operation of the boiler, if cracks are found in the furnace wall, tightness inspection should be carried out, load should be reduced, air induction should be increased, and furnace negative pressure should be maintained. When there is a slight crack in the outer wall of the furnace, asbestos rope is usually used to fill it and fire-resistant cement slurry or cement-lime slurry is coated on the outside. If a small amount of refractory bricks fall down or the outer wall of the furnace is slightly protruded, the inspection in operation should be strengthened and the operation should be temporarily maintained until the boiler stops servicing. If the damage area of the furnace wall is large and the temperature of the furnace wall and the outer surface of the furnace frame rises, the furnace should be shut down urgently when there is a danger of collapse.
10. Positive Pressure in Furnace
Causes of combustion under negative pressure
- 1) The influence of full pressure deviation H of fan itself is not considered when selecting fan. When H is positive deviation, the air volume of induced draft fan increases and decreases when H is negative deviation.
- 2) The difference between the actual resistance and the calculated value of the pipeline network is too large, which leads to a lot of reduction in the air volume of the fan. From the general characteristic equation H = KQ2, it can be seen that when the actual K value is less than the calculated value K, the flow increases, and when the actual K value is greater than the calculated value K, the flow decreases. When selecting the type of induced draft fan, experience is used instead of calculation, ignoring the difference of furnace structure of boiler manufacturer, the length of air duct from the outlet of air preheater to the entrance of chimney, the number of elbows, the way of dust removal and desulfurization, the cross-section area of air duct and so on. If these differences make the actual K value increase a lot, the air volume of the induced draft fan will decrease a lot, which not only eats up the reserve coefficient of the air volume and the air pressure in the selection of the induced draft fan, but also causes the insufficiency of the air volume.
- 3) As a special equipment, some installation units are responsible for the required inspection items and the pressurized parts, and are careless about the auxiliary system. If the elbow is not made according to the standard, or the radius of change is too small or the guide vane should not be added; the inner wall of the duct is uneven; the flange installation is not parallel; the asbestos rope is sealed without filling, and so on, it is easy to cause air leakage or resistance increase (along the resistance and local resistance).
- 4) Due to the limitation of objective conditions, the actual flue resistance loss is often larger than the design value. At the same time, many boilers in the boiler room share one flue and chimney exhaust. For each induced draft fan, it is equivalent to feeding gas into a positive pressure space, which undoubtedly increases the resistance of the flue system. With the prolongation of service time, the aging of equipment, the wear and tear of fans, the leakage of air ducts and so on, the resistance of air ducts will increase. It is more advantageous to increase the air volume and pressure reserve of induced draft fans appropriately in design for future use and regulation.
Reasons for problems and Solutions
1. Positive Pressure in Furnace Caused by Operation
In the process of combustion, if the amount of flue gas discharged from the furnace equals the amount of flue gas produced by combustion, the pressure in the furnace keeps relatively constant because of the material balance in the furnace. If the amount of flue gas discharged is less than the amount of flue gas produced by fuel, it will inevitably cause positive pressure in the furnace. When the heat load increases, the air volume of the induced draft fan should be increased first, that is, opening the large adjusting valve, then increasing the coal consumption and the blast volume; otherwise, when the load decreases, the coal consumption and the blast volume should be reduced first, and then the induced draft volume should be reduced.
2. Positive pressure combustion in furnace caused by improper maintenance or damage of equipment
1) Blockage and abrasion of boiler tubes in air preheater are the main causes of positive pressure combustion of boilers. Once the boiler tube is blocked, the flue gas flow area becomes smaller and the resistance increases. When the number of blocked boiler tubes exceeds 5% of the total number of boiler tubes, the positive pressure combustion can not be avoided. After the air preheater boiler tube is worn and leaked, the blast and induced air directly form a short circuit, one side is positive pressure, the other side is negative pressure, which will divert a lot of invalid induced air. Comparing the changes of flue gas pressure at the inlet and outlet of air preheater (check and record) can predict whether blockage and wear. Therefore, all blocked boiler tubes must be dredged when stopping the furnace for maintenance; if the air leakage in the middle section of individual boiler tubes can seal both ends of the boiler tubes, the number of closed boiler tubes can not exceed 5% of the total number of boiler tubes. If more than one third of the total number of boiler tubes, the whole group should be renewed; the most serious wear at the end of the tube (at the entrance of flue gas), adding the protective sleeve at the end of the tube can prevent the wear at the end of the tube, and maintenance is more convenient.
2) Ash deposit in economizer can also cause positive pressure combustion of boiler. Ash deposit reduces the flow area of flue gas and increases resistance. Economizers are usually equipped with purging and ash cleaning facilities. Regular purging and ash cleaning is an effective measure to prevent the accumulation of ash in economizers, and should not be less than once a week.
3) For the positive pressure combustion after wet dust removal (e.g. marble dust collector, dust removal and desulfurization integrated equipment etc.), we should first consider whether the flue gas has water or not. The method is to compare whether the current of the induced draft fan is obviously higher at the same opening of the regulating valve, whether the vibration of the induced draft fan is increased, whether the impeller is sticky or not, and if the impeller is sticky, the dynamic balance is destroyed after the impeller sticks to the ash, resulting in the vibration of the induced draft fan and the increase of the motor current, resulting in the turbulence of the air flow and the decrease of the induced air flow. This situation is generally within the warranty period of the equipment, we should find the equipment manufacturer in time to solve the problem of incomplete gas-water separation. Secondly, we should consult the relevant data or carry out experiments to verify whether the local resistance is much higher than before.
4) For the old-fashioned cyclone dust collector, if the smoke quality deteriorates, the pressure loss increases and the positive pressure combustion occurs, it is likely that the dust accumulates at the lower part of the cyclone dust collector outer barrel, which causes the internal air flow disorder and brings the smoke and dust into the updraft. When the inner and outer barrels of the dust collector are worn through by dust and the gas lock device is not tight, the pressure loss decreases, but the short circuit of the flue gas occurs, which not only reduces the dust removal efficiency, but also causes the positive pressure combustion of the boiler.
In the maintenance and repair of the equipment, we should not only carefully clean the ash, but also check whether the sealing parts such as flange, ash removal device, air lock device are sealed and leaked. For serious wear and tear, repair and replacement should be arranged in time.
5) The smoke quality is poor and the temperature of the furnace can not rise, which makes the flue gas temperature at the outlet of the furnace low and increases the density of the flue gas. The design exhaust temperature of the induced draft fan is 180-200 C, and the pressure is a standard atmospheric pressure. When the exhaust temperature is lower than the design value, the density of the flue gas increases, and the fan works under the overdesign load. At the same time, in order to meet the external load, only the amount of coal is increased. It also increases the flue gas discharge. For example, it is difficult to establish furnace negative pressure if the amount of affluence in fan design and selection is small. Because of the positive pressure combustion caused by coal quality, the adaptability of furnace to coal can be improved by adding stratified combustion coal feeder.
The moisture of coal in front of the furnace should also be controlled. A large amount of water vapor increases the amount of flue gas produced in the furnace. The water content of coal is generally not easy to exceed 8%-12%. If it rains or snows, the coal in the dry coal grille should be used.
6) Dust collection at the bottom of chimney is too much. Failure to seal the inspection holes and ash clearing holes at the back of furnace in time can also cause resistance increase. Short circuit of induced air should be checked carefully before start-up. Attention should also be paid to avoiding the simultaneous operation of several positive-pressure burning boilers or positive-pressure and negative-pressure burning boilers and deteriorating positive-pressure burning.
7) In case of sudden positive pressure in the furnace for unknown reasons, it is necessary to check whether the water wall and the heating surface of economizer are damaged to prevent the situation from expanding.
3. Positive Pressure Combustion in Furnace Caused by Design Selection and Installation.
Source: China Boiler Tubes Manufacturer – Yaang Pipe Industry Co., Limited (www.metallicsteel.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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