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Discussion on the rationality of the configuration of waste Heat Power Generation system in Precalciner kiln

Date:2019-06-20 15:05 Source:未知Views:

1. Scale and production capacity of cement production system


The scale of cement production system should be the production capacity of the whole production line, but it often exceeds the production scale in the actual production of cement industry, so it is very common to improve the production capacity, and the change of production capacity directly affects the exhaust gas parameters, which leads to the matching problem of waste heat power generation system. On the surface, it is a production line of the same scale, and its production capacity may be different, which may be affected by many factors, such as raw fuel, altitude, equipment selection and so on. When configuring the waste heat power generation system, it should be designed according to the specific situation in order to achieve good results, rather than how much electricity must be generated on a certain production scale, which is just A general configuration, not absolute. It is not that much electricity generation is backward technology, more than how much is advanced technology, this statement is neither scientific nor comprehensive.

2 volatility of cement production process


The waste heat power generation system can adapt to the normal fluctuation of cement production process, but the large irregular fluctuation is very disadvantageous to the waste heat power generation system. For example, when the coal mill needs to be pumped from the kiln head cooler for drying, the operation of the coal mill will not be completely synchronized with the operation of the kiln system. According to the theoretical analysis, if the waste heat power generation system does not compete with the cement production system for the heat, the balance of the system should be maintained, and the air supply to the waste heat power generation system on the scale of 2500 ~ 5000 tons will be 16000 ~ 350. The air volume difference of 00 Nm3/ h and the temperature difference of about 10 ℃. The waste heat power generation system must have measures to deal with this fluctuation, and the power generation will also change. In the design of waste heat power generation system, the parameters of this fluctuation should be provided to the manufacturers of boilers and steam turbines in detail, and the corresponding adjustment measures should be taken in the design of the system. However, in the actual production and operation process, the system balance is not absolute, the actual operation is difficult to control the right. In order to maintain the stability of the system, it is possible for the waste heat power generation system to compete with the cement system for heat. Therefore, it is best to let the coal mill use the kiln tail waste heat boiler outlet. The waste gas is used as the drying heat source to eliminate this fluctuation from the source.

Position of air outlet of kiln head grate cooler


From the angle of heat balance, if the exhaust port of the cold-cooling machine is on the cold end, the relative temperature is low, the air amount is large, the exhaust port is on the hot end, the relative temperature is high, and the air amount is also small. According to the theoretical calculation, on the premise that the waste heat power generation system does not compete with the cement production system, the waste gas generating capacity with low gas consumption and high temperature is larger than that of the gas quantity and the temperature is low. Therefore, the exhaust port of the cold-cooling machine for the waste heat power generation should be closer to the hot end, and can be arranged at the position of the general coal mill exhaust port or close to the position of the coal mill exhaust port, and the conventional cold end exhaust port is only used as the exhaust passage when the waste heat power generation system is shut down. if it is, a plurality of exhaust ports are arranged on the cold machine to be used for generating the waste heat at the same time, so that the system is very complex, the investment is increased, the air leakage point is increased, the process arrangement and the operation difficulty are increased, And the probability of heat of the cement system can be easily increased.


Heat balance between waste heat power generation system and cement production system

There have been some disputes between waste heat power generation system and cement production system. First of all, the assessment mechanism is not clear, whether the heat competition should be based on what principles as the standard, only under the premise of clear assessment principles can we define whether there is a problem of heat competition. However, it is possible in theory to ensure that there is absolutely no competition for heat in industrial production. In practice, it will be very difficult to do so. It can only be considered reasonable to control this situation within a certain range of indicators.

It should be necessary to measure the heat consumption and power generation of cement production system, but the heat consumption of cement clinker is defined by unit clinker index. There are two problems here: one is that the design code has a range of requirements for heat consumption index, such as 2000 ~ 4000 t / d, 4000 t / d and above, and there is no subdivision of production scale. If the heat consumption index is to be subdivided, the heat consumption index is different; Second, there is a difference between the scale of production and the actual production capacity, which has been discussed earlier. Therefore, in addition to the heat consumption of clinker, the cement production system should also be considered. The actual production capacity, unit heat consumption index will vary according to the actual production capacity. In addition, the same production capacity and the different fuel used will also change the heat consumption of clinker. Therefore, it is very difficult to formulate a unified standard, and the specific project should be analyzed specifically, which cannot be generalized.

For a project, in order to measure whether the configuration of waste heat power generation system is reasonable, in addition to the efficiency of waste heat power generation system itself, we should also look at whether the matching between waste heat power generation system and cement production system is reasonable. The actual production capacity of cement system, clinker heat consumption and power generation should be examined together. For example, when the waste heat power generation system is not put into operation to assess the cement production system, the actual production capacity and clinker heat consumption have a corresponding index, at the same time, there is also a long-term operation index, such as the average index of continuous operation for one month; and then after the waste heat power generation system is put into operation, the production capacity, heat consumption and power generation also have a corresponding index. Also running for one month in a row, the monthly average index can be obtained. At this time, the index of unput waste heat power generation system is compared with that of waste heat power generation system, and it can be seen whether the waste heat power generation system competes with cement system for heat. Of course, the factors of waste heat power generation system itself will also affect the power generation, such as boiler efficiency, steam turbine efficiency, system comprehensive efficiency should be considered as the power generation index.

(5) make full use of waste heat as much as possible.


The waste heat discharged from most cement plants comes from the kiln system, and the waste heat discharge point of the kiln system is mainly the outlet of the kiln tail preheater and the cooler. If the heat dissipation on the surface of the equipment is to be recovered, it is technically feasible, but the input is not proportional to the income, and the focus should be on the full recovery and utilization of the heat in the waste gas.

The exhaust gas inlet temperature of the waste heat boiler is determined by the cement production system. The exhaust temperature of the kiln tail boiler shall consider the heat required for the drying of raw fuel. The exhaust gas temperature is generally above 200℃. There is no more room for excavation from the heat source. However, the exhaust gas of kiln head boiler is discharged after treatment, which is no longer used, and its heat should be fully recovered. Of course, the temperature is better as low as possible, but it must be constrained and restricted by some conditions.

5.1 Dew point temperature


If you only look at the dew point temperature, the exhaust temperature of the kiln head boiler should be 10 ~ 20 ℃ higher than the dew point temperature. If the gas contains sulfur, it will corrode the equipment. Therefore, it should be based on the acid dew point temperature, but the content of SO3 can not be considered in the kiln head grate exhaust gas, mainly based on the water dew point temperature. The dew point temperature of the gas is shown in Table 1.

Table 1 Dew point temperature of gas


5.2 Node temperature difference


in the determination of the exhaust temperature of the boiler, the dew point temperature of the gas can not be taken into account, and the difference between the temperature of the exhaust gas at the inlet of the evaporator at the inlet of the waste heat boiler and the saturated water temperature at the selected pressure (referred to as a node temperature difference) needs to be taken into account, and as the temperature difference of the node is reduced, The exhaust temperature of the boiler will also be reduced correspondingly, which will help to improve the thermal efficiency of the boiler, but at the same time, the heat exchange area and the resistance loss of the boiler are also increased, and the investment cost of the boiler is correspondingly increased. In addition, the higher the selected pressure, the higher the saturation water temperature, which also affects the node temperature difference. Obviously, the temperature difference of the node cannot be zero, otherwise the heat exchange area of the boiler will be increased to infinity Big, this is also unrealistic. Therefore, the node temperature difference should have a reasonable value, which can also explain that the exhaust temperature of the single pressure system is always higher than that of the corresponding double pressure system. Because the exhaust temperature of boiler is contradictory with the investment cost, the design needs to seek its best point and serve as the basis of selection, which is also the work that needs to be done to optimize the system.


5.3 system differences


When single pressure, double pressure and flash system are used respectively, the thermal recovery rate is different. Of course, the thermal recovery rate of double pressure system is higher, the investment is also increased, and the difficulty of operation and maintenance is increased. According to the calculation and comparison, the power generation of the double voltage system is about 1.5% and 1.9% higher than that of the single voltage system, and the thermal efficiency of the unit is increased by 1.7%. In the conventional power generation system, the thermal efficiency of the three-voltage system is about 0.6% higher than that of the double-voltage system, and the heat can be recovered more. However, in the pure low-temperature waste heat power generation system, the exhaust gas temperature is low, so it is not suitable for the three-voltage system and is economical. It's not good either. It's enough to use a double pressure system. The thermal recovery rate of flash system is between single pressure and double pressure system, but the investment is less, and its economy has certain advantages.

6 suggestions on reasonable configuration of the system


when the waste heat power generation system is arranged, the scale of the cement production system can not be simply used as the condition for the system configuration of the waste heat power generation system, the production capacity of the waste heat power generation system is to be taken into account of the actual production increase amplitude and the corresponding heat consumption index, and if the production capacity is not expected to increase, It is recommended that 10% of the normal production scale be considered.

Under the condition that the pumping point of the kiln head grate cooler does not affect the normal use of the kiln system and does not compete with the cement production system for heat, the pumping temperature should be increased. If the thermal balance is kept unchanged, the pumping volume will be reduced and the electricity generation will be increased. That is to say, the pumping point of grate cooler should be moved to the hot end as much as possible, which can be close to the air pumping point of coal mill or the common coal mill outlet. When considering the arrangement of the system, the heat source of coal grinding and drying should be extracted from the outlet of kiln tail boiler as far as possible.

When selecting the waste heat power generation system, we should consider the heat recovery rate, economy and the difficulty of operation and maintenance comprehensively, especially the coal economizer arrangement of kiln head boiler in the system has many combination schemes, and the arrangement with oxygen removal can be considered, which will have different effects on the full recovery of heat.

Fully communicating with the equipment manufacturers of boiler and steam turbine to balance the differences between different industries is also the premise to optimize the system parameters and ensure the reliable operation of the system.

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