Analysis of the cause of non-compliance of waste heat power generation in cement kiln and its solution

Date: 2019-06-24 Views:

In our country, cement kiln low temperature waste heat power generation technology has been widely used, whether it is technology, equipment, civil construction, installation and commissioning, or operation has been quite mature. According to the operation of some waste heat power stations, the power generation of some waste heat power stations exceeds the design value, and the waste heat resources are fully utilized, but there are still a considerable number of cement kiln waste heat power generation which can not reach the design value. There are many factors that restrict the power generation of waste heat of cement kiln, such as process, kiln type, fuel, raw material, calciner, preheater, castor cooler, fan and so on. According to the design rules, the author combines the operation parameters, operation and other aspects, from the operation point of view of the cement kiln common do not The cause of the achievement is analyzed, and some debugging operation experience is provided for the standard of the waste heat power generation.

1 Common causes of non-compliance


Failure to meet the standard refers to the design value of waste heat generating capacity of cement kiln. At this time, the key operating parameters shall be analyzed:power generation, boiler inlet and outlet flue gas temperature, negative pressure, boiler outlet steam parameters(flow, pressure, temperature), steam inlet parameters of steam turbine, etc.

The amount of electricity generated is not only related to the waste heat condition, process, equipment and fuel of the kiln itself, but also to the coordination degree of cement and power generation majors, the operation level and operation characteristics of kiln operation.

1.1 the actual utilization of waste heat smoke in cement kilns is on the low side.


The design average smoke volume of domestic 5 000 t cement kiln waste heat power generation kiln head waste heat boiler (Air Quenching Cooler Boiler, (AQC furnace) and kiln tail waste heat boiler (Suspension Preheater Boiler, (SP furnace) are 240000 Nm'/h and 340 000 Nm'/h, respectively. The actual available waste heat smoke volume does not reach the design parameters, the concrete performance is as follows.

1.1.1 The amount of flue gas in the inlet of SP furnace is small.


The exhaust gas from SP furnace should be used for raw material grinding and drying, and its temperature needs to meet the temperature needed for raw meal drying. In the actual operation of waste heat power station, if the exhaust gas temperature at the outlet of the boiler is lower than the designed value or the pressure difference between the inlet and outlet of the boiler is obviously small, it indicates that the amount of smoke entering the boiler may be too small.

The main reason why the amount of flue gas in the SP furnace has not reached the design value is that the bypass valve of the boiler is not completely closed. In operation, the bypass valve can adjust the temperature of entering the raw material mill flue gas at any time, the bypass valve of the boiler is opened, and the part of the flue gas directly enters the original process system of the cement kiln, and the heat exchange of the boiler is no longer passed, and the amount of flue gas actually entering the boiler is reduced.

the opening degree of the bypass valve is related to the type, the natural condition and the operation level of the raw material mill, the raw material grinding of the cement kiln is divided into a pipe mill, a vertical mill and a roll press 3, wherein the temperature of the pipe mill operation is the highest, the temperature is above 230 DEG C, the vertical mill needs to be 200-220 DEG C, the required temperature of the roller press is the lowest, The degree of opening of the bypass valve is different due to different natural conditions in different regions, especially in the rainy season, the water content of the raw materials is high, and the raw material grinding requires higher drying temperature; and the operation level of the kiln operation is different.

1.1.2 The amount of flue gas in the AQC furnace is small.


The outlet exhaust gas of kiln head AQC furnace is not used in production, and the exhaust gas is discharged after dust collection. In the design of waste heat boiler, the exhaust temperature is generally about 100 ℃. If the exhaust temperature of boiler is low, the waste gas volume is too small. At the same time, because the heating surface of AQC furnace can not be fully utilized, the feed water temperature of SP furnace is low, which affects the steam production of AQC furnace and SP furnace.

In addition, the pressure difference between the inlet and outlet flue gas of the designed boiler is 600800 Pa,. If the pressure difference between the inlet and outlet of the boiler shows that the inlet and outlet pressure difference of the boiler is obviously small, it indicates that the amount of flue gas entering the boiler is too small.

1.2 the actual waste heat flue gas temperature of cement kiln is on the low side.


The waste heat power generation of 5000 t cement kiln adopts double pressure technology. The average flue gas inlet temperature of kiln head AQC furnace and kiln tail SP furnace is 380 ℃ and 320 ℃, respectively. the low inlet temperature will lead to boiler unpacking. The main performance of low inlet flue gas temperature of cement kiln waste heat power generation boiler is as follows.

1.2.1 low flue gas temperature entering SP furnace


The smoke temperature at the entrance of SP furnace is mainly determined by the C1 outlet temperature of cement kiln preheater. According to the design code of cement plant, the smoke temperature at the entrance of SP furnace should not be higher than 320 ℃ when the fifth stage preheater is adopted, and the smoke temperature at the entrance of SP furnace is 320 ℃. In actual operation, some cement kilns are below 310 ℃, resulting in low waste heat utilization temperature.

1.2.2 low temperature entering AQC furnace


The residual air temperature of the kiln head of the cement kiln is 250-300 ℃, as shown in Figure 1, when the waste heat power generation is designed, the pipeline is led out from the front of the cold-cooling machine, and the waste gas at the temperature of about 380 ℃ is pumped to the AQC furnace after the large-particle dust is removed, and the reason that the flue gas temperature of the AQC furnace is low may be as follows.

a. The air intake point is rearward. The heat available for waste heat power generation is mainly concentrated in section I and I(section)of castor cooler(in order not to affect the temperature and air volume of secondary and tertiary air to ensure the normal production of cement kiln, and the air intake of waste heat power generation is located at the front of the second section of the castor cooler. If the air intake is set, the temperature of flue gas at the boiler inlet may be low.

b. The thickness of the material layer of the castor cooler is not enough. The original cement kiln generally requires "thin material fast burning". If the thickness of castor cooler material layer is not enough, the cooling air penetration material layer does not have time to heat up fully, which will lead to the low temperature of kiln head exhaust gas.

1.3 defects in the design and installation of residual thermal power stations

1.3.1 Pipeline design defects


a. Smoke pipe. The design of flue gas pipeline is unreasonable, for example, the angle is not enough, which will lead to ash accumulation in the pipeline, affect the ventilation volume, and cause the boiler evaporation to be insufficient.

b. Steam pipe. Unreasonable design of steam pipeline will affect drainage, steam, disalignment, single line assessment and so on. In addition, the inner diameter of the pipe is too large, and the steam flow rate bottom will cause the steam temperature drop to be large.

1.3.2 defects in thermal insulation design and installation


Heat loss is one of the main factors that affect the standard of waste heat power station project, so the heat preservation effect also determines whether the power generation of waste heat power station system is up to standard. The power generation is mainly determined by the air volume and exhaust gas temperature. Under the condition of ensuring that the air volume is large enough and the temperature is high enough, how to ensure that the waste heat of the system can be used to generate electricity as much as possible depends on the quality of the heat preservation measures.

a. Smoke pipe.

The development time of waste heat power generation is short, the heat preservation of early cement kiln only considers the requirements of scald prevention and condensation, and the standard is too low. After the matching waste heat power generation, the heat loss of the original equipment and flue gas is large, and the heat preservation of the new flue gas pipeline and equipment is improper, such as the valve does not carry on the heat preservation and so on, all of which affect the waste heat utilization.

The main heat preservation parts of flue gas pipeline and equipment are as follows: kiln tail preheater C1 outlet to SP furnace inlet pipe; high temperature fan outlet to raw material mill inlet; castor cooler exhaust port to AQC furnace population pipeline; flue gas pipeline valve; flue gas pipe support.

b. Steam pipe.

Steam pipe, valve heat preservation is not good, will lead to boiler steam outlet to turbine inlet temperature drop is too large.

1.4 process and equipment constraints of cement kilns


The excess production capacity of cement production line, fan performance margin and castor cooler structure will affect the power generation of waste heat.

The premise of the stability of the waste heat power generation system is the stability of the cement process system. The great fluctuation of the cement kiln will force the removal of the waste heat boiler, and the waste heat power station can not generate electricity normally, let alone reach the standard. In addition, some cement production lines selected in the early construction of the equipment is relatively old, and do not consider the matching waste heat power generation project reserved space.

The kiln cover of castor cooler, the air chamber of castor cooler, the thrust of castor plate hydraulic cylinder, the speed of castor, the deviation of blower and castor bed affect the thickness control of material layer and so on, which will directly affect the flue gas volume and flue gas temperature of kiln head AQC furnace. The insufficient capacity of kiln head fan will lead to insufficient smoke volume of AQC furnace.

2 Solutions

2.1 Do the preliminary design and installation work


The residual heat power station is attached to the cement kiln, and is familiar with the operation characteristics of the cement kiln and is of great importance to the organic connection between the cement kiln and the waste heat power generation system, which is the main basis for the process design and the workshop layout of the whole waste heat power station.

Thermal calibration is the basis of parameter design of waste heat power station. Waste heat power generation project follows the principle of "determining electricity by heat". Cement kiln can use residual heat to determine the installation scale, equipment selection and process design of waste heat power station.

Even cement kilns with the same production scale at home and abroad are different in technological process, equipment structure, dust collection mode, flue gas cooling mode and operation level due to different design, region and owner. According to the characteristics of each cement kiln, the best process system, parameters and equipment model should be selected to avoid design and installation defects.

Good heat preservation and antifreeze measures are particularly important for high and cold areas. In the design, it is necessary to do a good job of thermal insulation design according to the natural conditions of the project, to increase the insulation chamber for drum, water supply operation platform, sampling and dispensing device, to adopt insulation box for instruments and ducts, electric heating or steam heating, and to consider the heat preservation design of equipment, steam and water pipeline, flue gas pipeline and so on. Later find the pipeline valve and other easy to omit the location, re-heat preservation or replacement of thermal insulation materials. At the same time, the important point control is monitored and controlled.

2.2 drawing on the experience of transformation

2.2.1 blast retrofit of castor cooler


The blower in section I of the cold-cooling machine is replaced; the blowing pressure in the section I is increased from 10 kPa to 15 kPa; the blast volume in the section I is increased by 50%; the lower blower is a frequency-modulation fan, and the blast volume is adjusted under the premise of ensuring the cooling of the clinker; and the section I The whole of the n-section air chamber is modified into the form of an air-filled beam. The thrust of the hydraulic cylinder is modified to meet the operation requirements of the thick material layer.

2.2.2 Transformation of coal mill and air taking


When the cement kiln coal is ground in the kiln head, the high-temperature wind drawn from the front part of the cold-cooling machine and the residual wind before the bypass valve of the AQC furnace of the kiln head are mixed as the air for the coal grinding, so that the influence of the coal grinding on the waste heat generation can be reduced, the waste heat utilization efficiency of the system can be improved, and the power consumption of the system is reduced.

2.2.3 Circulating air modification


In Northeast China, the air temperature of the blower at the head of the kiln reaches 20-30 ℃ in winter. if the flue gas discharged from the boiler is mixed with proper amount of cold air and its temperature is reduced to 20 ℃ for recycling, the normal cooling of clinker can still be guaranteed, and the cooling air temperature of the inlet castor cooler will be increased by about 50 ℃, thus increasing the flue gas temperature at the inlet of the boiler.

2.2.4 renovation of exhaust Fan


With the addition of waste heat power generation system, the inlet resistance of kiln head exhaust fan increases by about 1200 Pa, resulting in insufficient fan capacity. Therefore, the transformation of head exhaust fan is beneficial to increase the inlet smoke volume of AQC furnace.

2.3 improve kiln operation level


The main reason for the operator to influence the non-compliance of the waste heat power generation project is the lack of experience or the lack of exchange cooperation with the operation personnel of the surplus heat power station. after the waste heat power generation is increased by the cement production line, the operation difficulty of the kiln is increased; for example, in order to improve the flue gas temperature of the AQC furnace, the cooling temperature of the clinker is difficult to be ensured; in order to improve the flue gas amount of the AQC furnace, the kiln head fan is enlarged, and the temperature of the flue gas is reduced, And can not guarantee the negative pressure of the cold-cooling machine, the pressure of the fan under the condensing bed and the thrust of the hydraulic cylinder can not be met, the speed is difficult to adjust, and the bed is thick and easy to press and die.

Therefore, the relevant measures should be taken to encourage the kiln operator to raise awareness and accumulate experience as soon as possible. At the same time, the best operation mode should be explored without affecting the quality and yield of the cement, and the waste heat will be sent to the boiler as much as possible, and the power generation will be generated as much as possible. the operation requirements are as follows: the feeding amount of the cement kiln is to be stable, the yield of the cement kiln is small and small, and the AQC furnace can not be operated; in general, the higher the clinker output is, the higher the power generation amount is; the lowest wind temperature of the raw material grinding is detected, and the bypass valve of the kiln tail SP furnace is completely closed as far as possible, so that the waste heat of the flue gas of the kiln tail can be fully utilized, the optimal material layer thickness and the blowing amount of the cold-cooling machine are obtained, and the kiln head after power generation of the matched waste heat is operated to In order to ensure the safe operation of the system equipment, avoid large adjustment and ensure the stability of exhaust gas temperature; strengthen the communication and cooperation between kiln operation and waste heat power generation operators, adjust the waste heat to waste heat power generation as much as possible, and gradually increase the power generation under the premise that both cement and power generation systems are stable.

Conclusion

Waste heat power generation is to make maximum use of the waste heat of cement kilns. Without affecting the heat consumption of clinker and the normal production of cement kilns, the electricity should be determined by heat.

a. Waste heat power generation system depends on the main line, waste heat power generation must not affect the main line production under any circumstances.

B. It is not possible to increase the power generation by increasing the heat consumption.

c. The principle of determining electricity by heat should be followed.

d. Under the premise of the same clinker heat consumption, the amount of electricity generated depends on the kiln operation. It is suggested that the kiln operation should transfer the waste heat to the waste heat power station system as far as possible to generate electricity without affecting the main line production.

e. The operation rate of the relative kiln of waste heat power station is improved, and the waste heat discharged from cement line is converted into electric energy synchronously.

In a word, the waste heat power generation project should be well built, but also operate well, and reach the standard according to the schedule, which requires the joint efforts of many parties. It is necessary to base on reality, draw lessons from experience, debug the key parameters, and make the power generation reach the standard as soon as possible.

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