of kilns and calciners? In terms of reducing the cost of cement enterprises and reducing carbon emissions, what optimization and transformation can be done for rotary kilns and decomposition furnaces? Recently, China Cement Network consulted Qi Yanyong, an associate professor at Southwest University of Science and Technology, on these issues.
In view of the energy saving of the firing system of the precalcining kiln, Qi Yanyong proposed two paths to improve the thermal efficiency and the heat exchange efficiency of the grate cooler.
1. The way to
improve the thermal efficiency of decomposition furnace and rotary kiln is that there are two energy consumption areas in the precalciner kiln system. Firstly, it is the decomposition of carbonate in the decomposition furnace, the outlet temperature of the decomposition furnace is generally below 900 ℃, and the heat transfer mode is gas-solid convection. Because the particle size of raw material is only tens of microns, the heat transfer is very rapid and almost instantaneous. The decomposition of carbonate takes only a few seconds. The key to improve the efficiency is the dispersion state of raw meal and the combustion of pulverized coal. The other is the high temperature calcination of the raw meal in the rotary kiln, and the maximum calcination temperature is considered to be about 1450 ℃. The heat transfer mode is mainly radiation heat transfer (accounting for 80% of the total heat transfer). The radiation heat transfer capacity is proportional to the fourth power of the flame temperature, and increasing the flame temperature of the rotary kiln is the biggest energy-saving point of the firing system.
(1) Calciner
At present, the decomposition temperature of limestone used in most cement production lines in China is about 860 ℃ (determined by differential thermal analysis), but many enterprises control the outlet temperature of calciner at 880 ℃. At such a high temperature, the decomposition of carbonate is only about 93%. The reason lies in the poor dispersion of raw meal into the furnace, the slow combustion of pulverized coal, the 600 micron agglomerated "ash" of raw meal in the decomposition furnace, and even the slight collapse of raw meal in the decomposition furnace, resulting in low actual decomposition rate and large fluctuation. The efficient operation of the
decomposition furnace is mainly due to the dispersion of the material in the furnace and the fact that the pulverized coal cannot collide with the material flow, which is mainly due to the design. However, it is necessary to check whether there is a "dark shadow" at the necking of the smoke chamber and the cone of the decomposing furnace during operation.
(2) For the improvement of the efficiency of the rotary
kiln, the control of the excess air coefficient of the rotary kiln needs to be paid close attention to when the flame temperature is improved. This is the most important parameter for rapid combustion of pulverized coal! In terms of
operation control, (1) the fineness of pulverized coal shall be controlled at 80 micron sieve residue < 2.0%; (2) the excess air coefficient in the kiln shall be sufficient, and the oxygen content in the smoke chamber shall be controlled at > 3%; (3) the swirl air volume shall be properly increased, the axial air volume shall be reduced, the flame length shall be shortened, and the flame temperature shall be increased.
2. Improve the heat exchange efficiency
of the grate cooler. The heat exchange efficiency of the grate cooler is low, especially the heat recovery rate of high temperature air (secondary and tertiary air), which is generally lower than 65%. The heat recovery of secondary and tertiary air + AQC is generally lower than 80%. The air volume for cooling is generally around 1.6 nm3/kg. Cl. Wind short circuit is the main reason for low heat recovery rate at high temperature. The ventilation resistance of the material in the cross section is absolutely uneven, which leads to high cooling air volume and low heat exchange efficiency. Operation control:
(1) Reduce the blast volume in the area with low resistance (more large particles), and reduce the blast
volume in the area with high resistance (more fine particles). (2) Adjust the thickness of the material layer, reduce the thickness of the material layer in the area with high resistance, and increase the side ventilation of the fine material to avoid red river and large lumps.
(3) Due to the complexity of granular material flow, there are two distinct operations to reduce wind short circuit. If the pushing speed of the grate bed is accelerated, the thickness of the material layer in the area with high resistance can not be reduced, the wind short circuit is aggravated, and the heat exchange efficiency is reduced. The only way to achieve uniform ventilation is to reduce the grate speed, thicken the material layer and increase the resistance of the material layer in the low resistance area. The temperature of exhaust gas extracted from the
grate cooler waste heat power generation AQC boiler is less than 450 ℃. Heat of 11-15 kg standard coal is extracted per ton of clinker . If 450 ℃ low temperature air is changed into 1100 ℃ high temperature air, the coal consumption of the whole firing should be reduced by 6-8 kg standard coal.
In the aspect of optimization and transformation, Qi Yanyong pointed out three points:
1. Optimization
of raw material feeding point and coal feeding point in the decomposition furnace. The combustion temperature of pulverized coal in the decomposition furnace is low and the excess air coefficient is low. If the pulverized coal enters the decomposition furnace and is wrapped by raw material fine powder, the combustion of pulverized coal is slow and the concentration of Co is high. The decomposition rate of carbonate is low, resulting in a large loss of high temperature at the outlet of the primary cylinder. Efficient operation of the decomposition furnace requires good dispersion of raw materials, no collapse of materials, and no collision between pulverized coal flow and raw material flow, so as to ensure full combustion of pulverized coal in a low-oxygen environment.
Use computational fluid software to optimize the position of feed point and coal feeding point.
(1) ensure good dispersion of raw material without material
collapse; (2) ensure that pulverized coal doe not collide
with raw material flow; and (2) adopting oxygen-enriched combustion of a rotary kiln, if that temperature of a firing zone can be raised from the current 1600 deg C or so by oxygen-enriched combustion
, When the temperature is raised to 1900 ~ 2000 ℃, the flame radiation capacity will be increased by 1.8 ~ 2.1 times, the coal consumption will be greatly reduced, the clinker quality will be improved, and the raw material source will be expanded.
3. Grate cooler material distribution, air volume adjustment
, efficient material distribution, air regulation device and intelligent air volume control software. Reduce the cooling air volume of the grate cooler to 1.2 nm3/kg. Cl. The high temperature heat recovery rate will be increased to 75%, and the coal consumption and power consumption will be greatly reduced.
The cement industry has great potential for carbon reduction. It is a long way to go to accelerate transformation and upgrading, enter the stage of green, low-carbon and high-quality development, and effectively implement energy conservation and emission reduction.
How to improve the energy efficiency of the firing system? What are the ways to save energy and reduce consumption? China Cement Network will hold the "2024 China Cement Double Carbon Conference and the 12th Energy Conservation and Environmental Protection Technology Exchange Conference" in Wuhu Conch International Hotel on October 24-25. The conference will focus on the core issues of low-carbon development and green upgrading in the cement industry, discuss the application of energy efficiency improvement and energy-saving and carbon reduction technologies, and how the cement industry can use the carbon trading mechanism to promote its low-carbon transformation. At the meeting, Qi Yanyong will give a wonderful speech around the examples of energy efficiency improvement and consumption reduction technology of firing system. Sign up quickly!
