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How to recover heat from tunnel kiln reasonably and effectively?

Category: Industry information

Time:2023-03-18

1. Current Situation

Almost all sintered brick factories use the heat recovered by the tunnel kiln as the heat source of the drying chamber, which is desirable as the renewable utilization of energy. However, because there are many drawbacks in the current process method of heat recovery in tunnel kiln, it causes many problems to product quality control and external environment, so it is necessary to deeply study the reasonable way of heat recovery in tunnel kiln and effectively recover the waste heat of tunnel kiln.

In particular, with the improvement of energy-saving and environmental protection requirements in the brick industry, the production technology of hollow block and insulation block produced by the secondary code firing process in the sintering brick factory is being fully promoted, which shows the urgency of solving this problem.

For the popular tunnel kiln heat recovery method, the medium and low temperature waste heat at the tail of the tunnel kiln is extracted as a drying heat source, and the insufficient part is extracted from high temperature and heat to supplement it. This method of waste heat recovery will have the following drawbacks:

(1) The double influence of the two sections of the tunnel kiln roasting belt after receiving money, the fire degree of the roasting belt drifts forward or backward, and the operation of the tunnel kiln is extremely difficult to control several streets.

(2) The flue gas contains water vapor, the drying medium entering the drying chamber is not dry hot air, drying to a certain degree, the partial pressure of water vapor on the surface of the product and the partial pressure of water vapor in the drying medium reach a balance, resulting in high drying residual moisture, less than 2% of the ideal index, and the water in the billet will enter the flue gas in the tunnel kiln pre-zone. Wet and hot flue gas body can not be completely dried when drying, such a vicious cycle is very harmful to the production process. Tunnel kiln

(3) Theoretically, after roasting the belt, the products from. The heat difference between the high fever temperature and the kiln temperature, excluding the heat dissipation of the kiln body, the heat taken away by the kiln car and the products when they are discharged from the kiln, the rest can be recovered. However, the above method of heat recovery in tunnel kiln failed to do so. Maximum recovery of heat, especially failed to recover high temperature heat, only the product temperature is high, the average temperature inside and outside the billet is about 150 ° C, and the kiln temperature is close to room temperature is more reasonable. In this way, on the one hand, it fails to provide enough heat to the drying chamber, and about 30% of the high temperature smoke heat needs to be added, and on the other hand, a lot of heat is lost when the kiln car and products are discharged from the kiln.

(4) This method of extracting residual heat makes the temperature curve of the cooling belt fail to meet the reasonable state of the process requirements, and the product is very likely to burst in the middle temperature segment.

(5) When a large amount of flue gas enters the heat delivery pipe system and dries. The acid gas in the flue gas seriously corrodes the drying car and the drying tray. The gas containing dust and acid mist overflowing outside the drying room poses a serious threat to the steel structure plant and the surrounding environment.

2, cooling belt reasonable temperature curve

According to the working principle of the tunnel kiln, the working system of the tunnel kiln is divided into two parts: preheat roasting belt (including insulation belt) and cooling belt, which should work separately and not interfere with each other, and separate the roasting belt and cooling belt. The effective method is to quench the air curtain. The cooling zone is divided into three sections according to the cooling rate.

2.1 Quench section

In order to separate the cooling zone of the cooked belt fish, the quench air curtain must be set at the beginning of the cooling zone. First, the quench air curtain effectively blocks the reverse flow of air flow from the insulation area to the cooling area, making the regulation of the smoke exhaust system more smooth; At the same time, the mechanical properties and the apparent texture of the products are greatly improved by quenching. Third, the high-temperature gas generated after the heat exchange between the quenched gas and the hot product is convenient for the recovery of high-temperature waste heat.

It must be noted that the position of the quench section must be. The high harvest temperature to about 800 ° C (some raw materials can be reduced to 700 ° C), then the product has good elasticity, the shrinkage deformation of the product is elastic deformation rather than plastic deformation, and the product is not in danger of bursting.

Therefore, it is very important to automatically control the quench air curtain according to the limited temperature value in the kiln. For large-section tunnel kiln, the quench air curtain is arranged in several groups on the kiln roof.

2.2 Slow cooling zone

The SiO₂ in the product changes from α quartz to β quartz at 573℃, and the volume change rate of the transformation is 0.82%. Although the transformation volume of this low-temperature type is not large, the conversion speed is fast, and the transformation is carried out under the condition of no liquid buffer, so it is extremely destructive, if there is carelessness, it will cause the product to burst. Therefore, in the range of 500~800 ° C, no cooling measures are allowed, and the cooling of the product must be very slow. In other words, in the tunnel kiln section, there must be a sufficient number of kiln cars.

2.3 Accelerating the cooling belt

From 500 ° C to the temperature of the product out of the kiln (about 50 ° C), the cooling products can be accelerated, and the method of blowing cold air into the kiln by the back pressure fan at the end of the kiln is generally adopted. This cooling process is longer, so the tunnel kiln must have enough length, and increasing the cooling air volume is also a good way to provide more hot air to the drying chamber.

3. Heat recovery model of cooling belt

In summary, the heat of the tunnel kiln cooling belt can be recovered in two parts, one is the high temperature heat generated by the quenching air curtain, and the other is the low temperature heat generated by accelerated cooling. The two parts of the heat medium are... The clean hot and dry air can be provided to the drying room for safe use.

After the quenching air curtain is blown into the kiln, the inside and outside of the kiln products will produce a large temperature difference, in order to... The uniformity of the temperature in the kiln and the temperature of the product, the high temperature gas generated by the extraction of the quench air curtain should be extracted countercurrent, that is, the high temperature gas extraction outlet is closer to the exit end of the tunnel kiln than the quench gas drum entrance.

Because the temperature of the high temperature gas extracted from the kiln is very high, the branch pipe of the exhaust air must be made of heat-resistant steel. At the same time, the exhaust branch pipe must be equipped with a special valve for distributing cold air, the hot air main pipe may be equipped with a cold air inlet valve, and the hot air main pipe must also be equipped with a cold air inlet valve. The hot air temperature that finally enters the high temperature heat recovery fan does not exceed 300℃.

A large amount of accelerated cooling air is pumped into the kiln by the end of the kiln, and gradually extracted from the top of the kiln after heat exchange with the product. The temperature of the hot air entering the low-temperature heat recovery fan is about 150℃.

The drying room requires the temperature and air volume of the drying medium to be completely different. Therefore, it is necessary to set up another drying heat supply fan and tunnel kiln preheating fan for use, and a container must be set up between the two to facilitate the mixing of high temperature hot air, low temperature hot air and cold air, so that the hot air supply machine can provide the drying medium with appropriate temperature and air volume to the drying chamber. This process can be fully automated. At the same time, the working system of the tunnel kiln and the working system of the drying chamber are completely separated, which is conducive to their separate control and regulation.

4. Other recoverable heat in tunnel kiln

4.1 Smoke Fever

Under the action of smoke exhaust fan, a large amount of flue gas produced by fuel combustion of burned products in the tunnel kiln kiln will flow to the preheat zone, preheat the billet, and gradually heat up the billet. If high temperature smoke heat is extracted at 350℃ in the residual zone, the preheat effect of the flue gas on the billet in the early stage of the preheat zone will be affected. And the use of high-temperature flue gas. The big disadvantage is that the acid gas in the flue gas will bring serious corrosion to the drying equipment and the escape of gas in the drying chamber will harm the environment. Therefore, the use of smoke heat as a drying medium is not desirable, the smoke should be fully exchanged with the billet in the tunnel kiln, so that its temperature drops to 100~150 ° C, discharged from the kiln, and then through the elimination of soot equipment centralized treatment, to achieve... Standard requirements after discharge.

4.2 Kiln cavity heat transfer

In the past, the kiln cavity heat transfer was used as a kind of heat preservation measure, but with the improvement of insulation material grade, this kind of heat transfer method has been gradually eliminated, especially the kiln wall cavity heat transfer. Now some tunnel kilns in order to reduce the temperature of the kiln roof hanging structure... Kiln roof... There is still a kiln top cavity heat exchange between the top insulation layer and the hanging beam, and the hot air temperature after heat exchange is very low, about 50 ° C, which can be used as the supplementary air volume of the drying medium.

4.3 Hot air after cooling of kiln car

Modern tunnel kilns are equipped with under vehicle pressure balancing system. The cooling fan under the kiln car is set at the large point to blow air to the steel structure of the kiln car to cool the kiln car, and the gas after heat exchange flows to the kiln head and the kiln tail. The gas flowing to the kiln head is extracted at the corresponding exhaust position to achieve the up-down pressure balance of the preheating belt. Because the seal between kiln cars is not very tight, there will be flue gas components in the extracted gas that goes under the kiln car, so it is generally processed together with the flue gas in the kiln exhaust system. The gas flowing to the kiln end enters the low-temperature heat recovery pipe through the pipe.

5. Conclusion

(1) The tunnel kiln can be divided into two parts through the quenching air curtain. As long as the heat recovery of the tunnel kiln is combined with the cooling process requirements, the crystalline dry hot air can be effectively recovered as a drying medium in the cooling belt of the tunnel kiln.

(2) Through calculation and practical experience, the heat recovered by the cooling belt of the tunnel kiln can account for 35%~45% of the total heat consumption of the tunnel kiln, which can basically meet the heat demand of the drying chamber. For example, the heat consumption index of 380kcal/kg tunnel kiln, the recovery of heat can be as high as 152kcal/kg, the heat consumption index of the drying chamber is about 1100kcal/kg water, when the moisture of the wet billet is dried by 20% and the residual moisture of the child is 2%, the heat is 160kcal/kg, and the supply and demand are basically balanced.

(3) For internal combustion tunnel kiln, as long as it is not super-hot roasting, this heat recovery method is also applicable. The internal combustion ratio should not exceed 80%.

(4) For the secondary code burning tunnel kiln, it must not be extracted as a drying medium, which is not conducive to the roasting of the tunnel kiln, but also conducive to environmental protection, and will also cause damage to the drying equipment and steel structure plant.

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