What are the reasons for high raw burning in lime kilns
As a key equipment for the thermal decomposition of calcium carbonate to produce calcium oxide, lime kilns often encounter the phenomenon of raw burning during the production process, where limestone is not completely decomposed, resulting in the production of raw lime containing undecomposed calcium carbonate. High raw firing not only reduces the quality of lime, but also affects the subsequent production process and product application.
1、 Raw material factors
1. Limestone particles are too large
The calcination rate of limestone is closely related to its particle size. The larger the particle size, the more difficult it is for heat to enter the interior of the stone, resulting in a slower calcination rate and a higher risk of sandwich burning. The particle size of limestone in ordinary vertical kilns should be controlled between 40-80mm, and for larger kilns, it can be relaxed to 50-150mm. For large limestone blocks, the use of gravel technology should be considered to adjust their particle size to an appropriate range.
2. Uneven quality of limestone
The uneven quality of limestone, such as containing a large amount of impurities such as silicon, aluminum, and iron, can easily form low melting point compounds at high temperatures, leading to nodules. Nodules can hinder the normal descent and ventilation of materials, causing some limestone to not be fully calcined and resulting in raw combustion.
2、 Fuel and air supply factors
1. Small fuel ratio or low calorific value
Insufficient fuel or low calorific value, unable to provide enough heat to completely decompose limestone. For example, coal used in mixed firing kilns generally requires a calorific value of 5500 kcal or higher. If the calorific value of the fuel is insufficient or the fuel ratio is unreasonable, it will lead to insufficient combustion temperature, resulting in raw combustion.
2. Unreasonable air supply
Insufficient or uneven air supply can lead to incomplete fuel combustion, uneven heat transfer, and localized burning. On the section inside the kiln, some areas have high air flow, while others have low air flow. The areas with high air flow have been burned, while the areas with low air flow have been burnt. Therefore, it is necessary to improve the air supply system to ensure sufficient and evenly distributed air volume, and to use probabilistic air caps and other devices to evenly distribute the air throughout the entire section of the kiln, promoting uniform combustion of the fuel.
3、 Operational and management factors
1. Improper control of calcination temperature
The calcination temperature is a key factor affecting the degree of limestone decomposition. If the temperature is too high, it will cause excessive decomposition of limestone, resulting in overburned lime; If the temperature is too low, the limestone will not decompose completely, resulting in burning. The normal combustion temperature of ordinary limestone is 1000-1200 ℃. If the temperature monitoring equipment malfunctions or the operator controls the temperature improperly, it can lead to the occurrence of burning phenomenon.
2. Failure to unload ash in a timely manner
Poor ventilation or delayed ash discharge in the kiln can cause limestone to stay in the kiln for too long. Some limestone may be excessively calcined due to delayed discharge, while others may not be fully calcined due to prolonged residence time, resulting in raw combustion. Therefore, it is necessary to strengthen the ventilation inside the kiln to ensure timely discharge of exhaust gas and reduce the residence time of limestone in the kiln.
3. Uneven fabric
Uneven fabric distribution can lead to uneven distribution of materials in the kiln, resulting in a situation where one side has more materials and the other side has less, known as kiln deviation. Deviation from the kiln can lead to poor local ventilation inside the kiln, and some materials may not be fully calcined, resulting in raw combustion. Therefore, it is necessary to strengthen operational management, improve the technical level of operators, and ensure uniform fabric distribution.
4、 Equipment and maintenance factors
1. The kiln body tilts or deforms
After long-term operation, the kiln body may tilt due to foundation settlement, kiln body deformation, and other reasons, resulting in uneven distribution of materials inside the kiln, and some materials may not be fully calcined, leading to raw burning. Therefore, it is necessary to regularly inspect and calibrate the kiln body to ensure that its verticality meets the requirements.
2. Sealing device failure
Sealing devices are usually installed at the edges of lime kilns to reduce heat and exhaust gas losses. If the sealing device fails, it will lead to heat loss and increased exhaust emissions, affecting the stability of the kiln temperature and the calcination effect of the materials. Therefore, it is necessary to strengthen the maintenance and inspection of the sealing device to ensure its normal operation.
5、 Factors affecting changes in the calcination zone
1. Move the calcination zone upwards
The upward movement of the calcination zone refers to the upward movement of the calcination zone inside the kiln, which causes the fuel to burn earlier in the upper part of the kiln, and when the furnace material drops to the calcination zone, the fuel loses its firepower, resulting in raw combustion. The reasons for the upward movement of the calcination zone include high top temperature control, excessive air pressure and volume, and large particle size of limestone. Therefore, it is necessary to adjust the combustion parameters and reduce the kiln top temperature; Reduce wind pressure and air volume to lower the fire layer; Adjust the particle size of limestone and control it within a reasonable range.
2. Downward movement of calcination zone
The downward movement of the calcination zone refers to the downward movement of the calcination zone inside the kiln, resulting in a decrease in the temperature at the kiln top and an increase in the ash temperature. Some materials are burnt due to insufficient calcination. The reasons for the downward movement of the calcination zone include excessive ash discharge, insufficient air flow, and excessively large fuel particles. Therefore, it is necessary to reduce the amount of ash unloading and increase the air supply appropriately; Replace fuel with smaller particle size; During the processing, attention should be paid to changes in kiln temperature to prevent over processing and upward movement.
6、 Improvement measures
In response to the above reasons, the following improvement measures can be taken to reduce the problem of high firing in lime kilns:
Control the quality of raw materials: Select limestone raw materials with uniform quality and appropriate particle size, and avoid using limestone containing excessive impurities.
Optimize fuel ratio: Ensure that the calorific value of the fuel meets the requirements, and control the fuel particle size reasonably. When using fine coal, add water appropriately to improve combustion efficiency.
Improve the air supply system: Ensure sufficient and evenly distributed air volume, use probabilistic air caps and other devices to evenly distribute air throughout the entire kiln section.
Strengthen temperature monitoring: Use temperature monitoring equipment to monitor the temperature inside the kiln in real time, avoiding overheating or overburning caused by high or low temperature.
Strengthen operational management: Improve the technical level of operators to ensure uniform fabric distribution, timely ash unloading, and kiln verticality meeting requirements.
Regular inspection and maintenance: Regularly inspect and maintain the kiln body, sealing devices, etc. to ensure their normal operation.
The reasons for the high firing rate of lime kilns involve multiple aspects such as raw materials, fuel and air supply, operation and management, equipment and maintenance, and changes in the calcination area. By taking targeted improvement measures, the burning rate can be effectively reduced, and the quality and yield of lime can be improved.
