Waste heat boiler refers to the boiler that uses the waste heat in the industrial production process to produce steam.It belongs to a heat exchanger with high temperature and pressure.The waste heat boiler was earlier used to produce some low-pressure steam. The heat recovered is limited, and it is only used as a general auxiliary equipment for production.With the development of production technology, the parameters of waste heat boiler are gradually improved. The waste heat boiler is transformed from a process boiler producing low-pressure steam to a power boiler producing high-pressure steam.New achievements of waste heat research are constantly emerging New achievements of research are constantly emerging New achievements of research in waste heat boiler design, manufacturing, use, safety management and other fields are constantly emerging.
Waste heat boiler can be divided into power plant waste heat boiler and chemical waste heat boiler according to the application situation.
1. Waste heat boiler of power station
The waste heat boiler of the power station isIGCCThe key equipment in the gasification island of the power plant system is very different from the waste heat boiler for chemical industry.Use waste heat boiler to control IGCCCoal gasificationSyngas aftersensible heatRecycling can improve the generating efficiency of IGCC power plant units.The sensible heat utilization of synthesis gas is divided into three parts: high temperature section, medium temperature section and low temperature section, which are respectively composed of radiant waste heat boilerconvectionType waste heat boiler and regenerative heater.Therefore, the waste boiler of the power station can be divided into radiation type and convection type. The convection type waste boiler is divided into fire tube type and water tube type, and the fire tube type is no longer used.In addition, the radiation waste boiler and convection waste boiler are designed as a whole, called radiation convection integrated waste boiler, which can greatly reduce the investment cost, but the domestic technology is not yet mature.
Today's worldIGCCPower station technology is the most promising power generation technology. The key to the commercialization of IGCC is the gasification island technology, and the waste heat boiler is its key equipment.Therefore, the waste heat boiler technology is one of the most critical technologies to solve the bottleneck of IGCC commercial application.
2. Chemical waste heat boiler
The heat exchanger uses the high-temperature logistics in the production process as the heat source to produce steam, which is not only the heat exchanger of high-temperature logistics in the process flowcoolerIt is also a power device that uses waste heat to provide steam.If inEthylene plantFrom theTubular furnaceComing outPyrolysis gas(temperature up to 800~900 ℃)QuenchThe temperature of the waste heat boiler is reduced to 350~600 ℃ within 0.1s, which can not only prevent the ethylene yield from being reduced due to excessive cracking, but also produce high-pressure steam of more than 10MPa, improving thethermal efficiencyAnd economic benefits.In nitric acid, acid, synthetic ammonia andPetroleum refining industryWaste heat boilers are widely used in many production processes, such as, to make the power and heat needed in the production process fully or partially self-sufficient.
According to the structure, the waste heat boiler can be divided into shell and tube type, flue type and double sleeve type.
Structure of shell and tube waste heat boilerShell and tube heat exchangerThere is no big difference, and there are also fixed tube sheet, floating head andU-tubeFormula, etc.flueThe structure of type B waste heat boiler is similar to that of ordinary boilerrefractory bricksBuild a furnace,chamber of a stove or furnaceThe tube bundle is installed inside, and high temperature gas passes through the furnace to heat the water flowing in the tube bundlevaporization。
The waste heat boiler is divided into fire tube waste heat boiler and water tube waste heat boiler according to whether the high-temperature gas passes through the inside or outside of the pipe.The fire tube waste heat boiler is a high temperature gas in the furnaceFlow in pipe。Boiling water fromSteam drumthroughDowncomerIt flows into the bottom of the boiler and is heated on the pipe wall to become a steam water mixtureRiserReturn to the steam drum.The structure of the fire tube waste heat boiler is simple, because the shell bearssteamPressure, suitable for producing steam with low pressure.Water tube waste heat boiler is that boiling water flows through the tube and high-temperature gas flows through the tube.As the boiler shell does not bear steam pressure, it is suitable for producing high-pressure steam.
According to the flow form of boiling water, water tube waste heat boiler can be divided into: ①natural circulation The driving force of boiling water flow in the tube is the weight difference between the water in the downcomer and the steam water mixture in the riser.To ensure a certain amount of circulation,Steam drumSufficient installation height is required. ②Forced circulationType, installed in the descending pipelineCirculating pumpTo increase the flow rate of boiling water in the tube and prevent the stagnation of bubbles from worsening heat transfer.Because this kind of boiler has many unfavorable factors in operation, maintenance and investment, it is only used in special occasions.[1]
Waste heat boiler structure
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Figure 1
1. Traditional waste heat boiler structure
The overall structure of the traditional waste heat boiler is shown in Figure 1. The steam drum and the waste heat boiler body are connected by 4 risers and 2 downcomers. The natural circulation is realized by using the density difference between steam and water and the high position of the steam drum, so that the system can operate safely.The trap in Figure 1 is set in the outlet pipe box of the waste heat boiler body, and the cooled process air flow is discharged and collected after the liquid is collected by the trap and cooled.Due to the structure of traditional waste heat boiler, two sets of waste heat boilers must be arranged before and after the reactor to realize full recovery.[2]
2. New structure of waste heat boiler
The most prominent innovation in the structure of the new waste heat boiler is to reduce the low temperature process gas cooler II (low temperature waste heat boiler).According to the design process parameters of the waste heat boiler, after the removal of the low temperature waste heat boiler, a heat exchanger capable of re cooling the process gas must be added in the high temperature waste heat boiler body or steam drum to cool the process gas after the reactor reaction, generate steam and recover resources.Generally, waste heat boiler is mainly composed of waste heat boiler body, steam drum, riser and downcomer.The waste heat boiler body is essentially an evaporation heat exchange equipment. The heat source is high-temperature process gas, and the heated medium is nearly saturated boiler water;The steam drum shall have sufficient steam volume and water volume, which is mainly used to enable the heat exchange tube in the body to continuously obtain appropriate water volume under the effect of heat load, and obtain certain circulating power from the high position system of the steam drum and the steam water density difference, so as to meet the requirements of water cycle reliability;The steam water separation device is set inside the steam drum to fully separate the steam water mixture in the steam drum to obtain steam with less water and impurities;The boiler feed water is heated to saturation in the steam drum before entering the waste heat boiler proper, which improves the circulating power;The saturated boiler water flows into the waste heat boiler body through the downcomer for heating and evaporation, and the steam water mixture in the body flows into the steam drum through the upcomer, so as to achieve the safe operation of the waste heat boiler system;According to the unique role of the steam drum, evaporation heat exchange tubes cannot be set in the steam drum generally, so as not to affect the safety and reliability of the water circulation and waste heat boiler proper.
Figure 2
The design parameters of waste heat boiler are compared with various schemes, and two integrated design schemes are proposed through heat transfer calculation, flow resistance calculation and water circulation calculation.That is, the scheme of directly adding a group of evaporators to the water storage part of the lower half of the drum cylinder and the scheme of directly adding a group of evaporators to the waste heat boiler body.Through heat transfer calculation, flow resistance calculation and equipment strength calculation, there are two major defects in directly adding a group of evaporators to the waste heat boiler body. On the one hand, the equipment structure is extremely complex;On the other hand, the temperature difference stress of the upper and lower parts of the tube plate of the waste boiler body is large due to the difference in the temperature difference between the two groups of evaporator media, which makes the design of the tube plate difficult, and even difficult to ensure the safety and reliability of the waste boiler during operation.While a group of evaporators is added to the water storage part in the lower half of the drum cylinder, although different from the design of the traditional drum, the detailed calculation is carried out according to the design parameters, and the results are completely feasible, and can ensure the normal operation of the system water circulation.The overall structure of the newly designed waste heat boiler system is shown in Figure 2.[1]
characteristic
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Compared with ordinary boilers, the main features of waste heat boilers are:
① High temperature and high pressure, such as synthetic ammonia productionheavy oilGasifierRear waste heat boiler, importedair temperature1350 ℃, pressure 3.2~4.0MPa;
②QuenchIn some technological processes (such as cracking petroleum hydrocarbon to produce ethylene), in order to reduce side reactions, the gas must be cooled as soon as possible, so the waste heat boiler should have a highconduct heatRate;
③ Strictly control the gas outlet temperature, high temperature gas (such asPyrolysis gas）When cooling, if the temperature is lower than highboiling pointThe dew point of the components that will condense and separate out.If it contains corrosive gas (such as SOthree）, will also formAcid mist, corrosive equipment;
④ High dust content, and some high-temperature gases contain dust (such as acid plantsFluidized bed furnaceOutlet furnace gasDust contentAbout 200g/m,Petroleum cracking gasIt contains carbon black and other particles), and dust blockage and erosion must be fully considered in the structure;
⑤ LowPressure dropThe pressure drop of gas must be controlled within the allowable range of process conditions;
⑥ Sealing. Some high-temperature gases are flammable, explosive or toxic. At this time, sealing often becomes a problem in the production design of waste heat boilers.[1]
influence factor
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1. Influence of water quality treatment on service life of waste heat boiler
(1) Water treatment outside the waste heat boiler
Water is one of the important factors affecting the service life and safe and normal operation of waste heat boiler. The quality of water added into the boiler often directly determines whether the waste heat boiler can operate normally.Therefore, before water is added to the waste heat boiler, a series of improvement measures such as softening, demineralization and thermal deoxidation are required to improve the water quality
(2) Water treatment in waste heat boiler
To fully extend the service life of the waste heat boiler, maximize its thermal efficiency, and ensure the safe and normal production of the boiler, it is not enough to rely only on the water treatment outside the boiler. At the same time, we should pay attention to the quality of the water treatment outside the boiler, control the quality indicators of the water inside the boiler, and focus on strengthening the water treatment inside the boiler.For example, the newly installed boiler shall be boiled before being put into operation, and acid cleaning shall be carried out after boiling. After being put into operation, the boiler shall be subject to regular blowdown while maintaining continuous blowdown, and chemicals shall be added to the boiler regularly.
2. Influence of water quality transportation on service life of waste heat boiler
Water is the lifeblood of the normal production of waste heat boiler. If the boiler runs dry due to water shortage, it will disrupt the normal production and cause serious consequences.Therefore, while paying attention to the water treatment in the boiler, it is also necessary to strengthen the operation of the multi-stage centrifugal water pump, which is the equipment for conveying water, so that it can operate continuously and stably, thus providing water source guarantee for the efficient production of waste heat boiler.In addition, when the characteristics of the multi-stage centrifugal pump body are certain, more attention should be paid to the working characteristics and working conditions of the recirculation pipeline at the outlet of the feed pump, the check valve installed on the outlet pipe of the feed pump and the bypass valve installed on the check valve on the outlet pipe of the pump.
In order to feed feed water into the boiler and makeWater supply regulating valveIt has good regulation characteristics,Boiler feed pumpThe outlet pressure shall not only overcome the working pressure of the steam drum, but also overcome the flow resistance of the feed water pipeline and the static pressure difference caused by the height difference between the outlet of the feed pump and the steam drum, and maintain a certain pressure difference before and after the feed water control valve.Generally, the outlet pressure of the feed pump is 30%~40% higher than the working pressure of the steam drum.
(2) Influence of check valve on outlet pipe of multistage centrifugal pump
In order to enable the standby pump to be started at any time and to supply water normally immediately after starting, the inlet and outlet valves of the standby pump must be in the fully open position.In order to prevent the water with high pressure in the main pipe at the outlet of the feed pump from returning to the treatment tank through the standby feed pump, which will cause the reverse of the standby feed pump and the pressure reduction of the main pipe at the outlet, and the water cannot be supplied to the boiler normally, the outlet of each feed pump should be installed with a check valve.
There are many other factors that affect the service life of waste heat boiler and normal and safe production. Only by adjusting and controlling the various indicators that affect its production within a certain range, can it give full play to its positive role and extend its service life.The design, manufacture, installation, operation and maintenance of boilers involve fluid mechanics, heat transfer, metallurgy, boiler principles, physics, chemistry, automatic control and measurement and other disciplines. It is of great significance to practically improve the theoretical and practical level of boiler operators for fuel saving, environmental protection and safe production.[1]
Existing problems
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(1) The quality of soft water for boiler is poor, which leads to scaling of heat exchange tubes and affects steam production.The high salt content in soft water forms scale on the outer wall of heat exchange tubes, which greatly hinders the heat transfer process.
(2) The failures caused by the welding quality of tubes and center tubes are mostly concentrated in some fillet welds and weld beads that cannot be inspected by X-ray flaw detection after welding. Most of them are slag inclusion, porosity, lack of fusion, lack of penetration and other defects, and their strength is difficult to adapt to the thermal expansion and contraction of equipment in production and operation, resulting in cracks and leaks.
(3) The problems in design and installation cannot ignore the changes of thermal expansion and cold contraction during equipment operation.
(4) The problems existing in the use process were not operated strictly according to the process indicators, and the faults were caused by the abnormal conditions such as over temperature, boiler water shortage, and "dry boiler".The internal heat-resistant lining of the waste heat boiler was not dried as required, and a large number of cracks were found in the internal heat-resistant lining of the boiler after replacement.[1]
take steps
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First of all, we should focus on the quality of boiler production.The manufacturer with the qualification to manufacture Class III containers must be selected for manufacturing. During the manufacturing process, no defects that violate the welding requirements are allowed.The proportion of casting materials for heat-resistant lining must be strictly controlled, special molds must be made, and dense casting must be carried out. Adequate maintenance must be carried out.All parts of the boiler shall be assembled according to the design requirements and completed under the guidance and supervision of the technician.For example, the assembly of heat resistant bushing, lining plate and protective small bushing of each row of pipes shall be strict, careful, standardized and firm.In particular, the assembly of the central control valve at the gas outlet should be more standardized. The size should be measured and the travel of the valve head should be adjusted to ensure that the valve is flexible and reliable.Overhaul of cracking and leakage of column pipe and central pipe.The treatment method for tube plate weld leakage shall first pass dye inspection to determine the crack, then grind and groove, and repair welding shall be carried out according to the welding process requirements.Cracks are found at the weld junction between the butt tube and the tube plate, so pipe plugging shall be avoided as far as possible. The method of re welding along the weld in the tube shall be used to maintain the heat exchange area as much as possible.
Operators shall have the ability to operate skillfully to avoid overheating, boiler water shortage, "dry boiler" and other accidents during normal operation of equipment.At the same time, the heating and cooling rate shall be strictly controlled during the heating and cooling process.Scales are easily formed during boiler operation.For systems with dosing equipment, pay attention to the ratio of drugs.After dosing, the sewage shall be discharged in strict accordance with the requirements to avoid the formation of new scale layer.For systems without dosing equipment, regular boiler cleaning is necessary.[1]
Operating Procedures
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（1） Hydrostatic test
After boiler installation and repair, hydraulic test must be carried out as follows:;
1. Check whether there are tools and sundries in the upper and lower furnace barrels, and then install the inlet holes.The safety valve shall be sealed with blind flange and removed after hydrostatic test.
2. Check the pressure gauge and conduit to ensure they are in good condition.Inject fresh water, and the temperature of test water should be about 20 ℃.
3. Open the upper vent valve to exhaust the air, and then close the vent valve.Gradually increase the pressure, about 0.35 MMPa per minute.
4. After reaching the test pressure of 3.0Mpa, keep it for 30 minutes and check whether there is leakage at all parts, especially the connecting water pipes of the upper and lower furnace drums. If there is any leakage, repair it, and then retest until it is qualified.
5. If there is no problem, gradually open the lower blowdown valve, and pay attention to avoid vacuum in the furnace drum.
6. If there is waste heat in the boiler, hydrostatic test is not allowed.
（2） Preparation before starting
1. Check whether all parts of the furnace drum are connected correctly, whether the tightness is appropriate, confirm that there is no person and sundries left in the furnace, then put on the manhole cover, and check whether the sealing gasket is damaged.
2. Check whether the water level gauge functions normally, open the main steam valve a little and then close it.
3. Check the installation of water supply valve, blow down valve, water level gauge valve and pipeline valve, operate them manually once and put them in the normal position.
4. Open the vent valve of the upper furnace drum.
5. Fill water with the feed pump and check the leakage of the drain valve. When the water level reaches the upper limit of the water level, stop the feed pump and check whether the joint seal of each component leaks.[1]
（3） Temperature rise and pressure rise
1. Check whether the instruments, meters, soot blowers and inlet holes on the furnace body are loose.
2. Check the performance of the pressure gauge and confirm whether the gauge valve is open.
3. Check the water level to make it slightly higher than the normal water level.
4. SetSteam stop valveOpen it a little, and then close it gently, otherwise it will not open after the air lift.
5. Burn the incinerator and gradually raise the temperature and pressure. When the temperature and pressure rise, gently twist the handle of the valve to prevent the steam valve from getting stuck. When the steam pressure reaches 0.1-0.2Mpa, close the vent valve.
6. Gradually increase the pressure to no more than 0.2-0.3Mpa per hour, further check, fasten the inlet, handhole cover, furnace valve and flange, and do it again when the pressure reaches 1.0Mpa.
7. When the water level exceeds the upper limit, gradually discharge the sewage, and the water level returns to normal.
8. Some low-pressure steam is discharged to warm the steam pipe system.
（4） Steam supply
1. Open the drain valve of steam pipe system, supply steam and fully discharge residual water.
2. Gradually open the main stop steam opening valveAir heaterAnd condenser. At this time, check the noise of the pipe system and avoid water hammer.If normal, fully open the stop valve for normal operation.
3. WhenPressure control valveAfter opening, when steam enters the condenser, it supplies steam to each gas consumption unit.
（5） Operation in operation
1. Monitor the water level gauge to keep the water level 100 mm above the center of the upper furnace drum, and the water level displayed by the water level gauge shall be within a small range of up and down;When using the automatic water level control device, do not rely too much on it. You must carefully observe the water level of the four water level indicators to identify the true and false water levels, and always keep the water level indicators clean.
2. ConfirmPressure gauge valveKeep it open. When the sensitivity decreases, it must be checked. Pay attention to antifreeze when stopping the boiler in winter.
3. Keep the heating surface clean, and decide whether to use soot blower according to the amount of steam generated, smoke exhaust temperature, and smoke exhaust pressure loss to ensure thermal efficiency (soot blowing is generally performed at least once a day).[3]
