Clinker production may not sound glamorous, but for those of us in the cement industry, it’s where the real magic happens. Beneath the clinker rotary kilns and precisely calibrated burners lies a process that transforms ordinary limestone into the vital core of cement. And at the heart of this transformation? Calcination—a deceptively simple yet scientifically intricate phase that defines everything from fuel efficiency to final product strength.
In today’s competitive and sustainability-driven market, clinker production is no longer just about turning up the heat. It’s about mastering heat transfer, optimizing material flow, reducing CO₂ output, and unlocking peak performance from every stage of the process. From preheater towers to rotary kilns, every component plays a crucial role, and even the smallest improvement in calcination efficiency can result in substantial gains in productivity and environmental performance.
In this in-depth exploration, we’ll delve into the entire journey of clinker production, with a particular focus on the calcination process. Whether you’re an engineer fine-tuning an existing cement manufacturing plant or a decision-maker planning your next plant investment, understanding the science and strategy behind calcination is essential to shaping the future of modern cement manufacturing.
New dry process cement production line
With suspension preheating and off-kiln decomposition technology as the core, it integrates the latest achievements of modern science and technology and industrial production to create a modern cement production process that is efficient, high-quality, energy-saving and environmentally friendly. Compared with the traditional wet and semi-dry rotary kiln processes, the new dry process shows significant advantages in preheating, decomposition and sintering. By transferring the preheating process of the material accumulation state in the kiln to the cyclone preheater for suspension preheating, the contact area between the material and the airflow is greatly increased, thereby significantly improving the heat transfer rate and efficiency. In addition, the use of off-kiln decomposition technology has increased the decomposition rate of raw materials from 30% to 85%-95%, which not only reduces the heat load of the calcining zone in the kiln, but also helps to reduce the scale of the kiln and large-scale clinker production, thereby saving investment and extending the life of the lining.
Preheater system
Next, we will explore the key elements of suspension preheating technology in depth. First of all, its superiority is reflected in the fundamental change of the heat transfer state of the material preheating process. Through suspension preheating technology, the material can be quickly and evenly heated in the high-temperature airflow, thanks to the significant increase in the contact area between the material and the airflow. Secondly, we will also introduce the composition and function of the preheater in detail. In the precalciner kiln, the cyclone preheater is the main preheating equipment used. Its heat exchange unit includes the cyclone cylinder and the connecting pipes (heat exchange pipes) between them, which together constitute an efficient heat exchange system.
The preheater system needs to meet three key functions: fully disperse the gas and solid phases, quickly exchange heat, and efficiently separate.
Cyclone preheater
Cyclone preheater, as a heat exchanger, consists of a cyclone and a connecting pipe (i.e., air duct). At present, five-stage preheaters are more common, and there are also designs with six-stage preheaters. The air duct plays an important role in the preheater system. It not only connects the upper and lower cyclones to convey the gas-solid flow, but also undertakes the tasks of material dispersion and uniform distribution as well as heat exchange between gas and solid phases. In addition, the preheater system also includes components such as the feeder pipe, the spreader, and the air lock valve, which together constitute a heat exchange unit.
The cyclone is mainly used for gas-solid separation, and its heat transfer effect is relatively small, accounting for only 6% to 5%. However, the heat exchange between gas and solid is mainly carried out in the air duct, mainly by convection heat exchange. The heat exchange process is very rapid, and it generally takes only about 0.5s to complete in the inlet pipe.
The separation efficiency of the highest level cyclone (C1) is crucial to the dust emission of the preheater system. In order to improve its separation efficiency and reduce external circulation, the first-level cyclone usually adopts a double cyclone design.
When selecting the wind speed of the heat exchange pipe of the cyclone preheater, it is generally controlled within the range of 12 to 20 m/s. In order to ensure that the raw material can be fully dispersed and suspended in the air flow in the pipe and accelerate the heat transfer between gas and solid, the following measures are usually taken: at the rising pipe where the raw material enters each stage of the preheater, a material dispersion device, such as a plate spreader or a box spreader, is set to prevent the downward rush of the descending material when entering the heat exchange pipe, and to promote the splashing and dispersion of the material.
The pipe between the two cyclones should be of sufficient length to ensure that the raw material can be fully suspended and stay in the pipe for a sufficient time, thereby improving the heat exchange efficiency.
The air-locking flap ash discharge valve installed on the cyclone discharge pipe should have a reasonable structure, flexible adjustment and good sealing. The valve is installed at a suitable position between the discharge pipe of the upper cyclone and the spreading box to maintain the sealing state of the discharge pipe, ensuring that the discharge is uniform and unobstructed, and preventing the airflow from entering the upper cyclone through the material pipe, ensuring that “the air goes through the air path and the material goes through the material path”, each going its own way. In this way, it can prevent the hot air in the heat exchange pipe from flowing up through the discharge pipe to the discharge port of the upper cyclone, causing the material to fly again and reduce the separation efficiency, and ensure that the hot air flow and the material are fully exchanged with heat, thereby improving the heat exchange efficiency.
Precalciner
Pre-calcination technology, or off-kiln calcination technology, partially burns fuel in the precalciner and preheats the raw meal, so that the raw meal is heated and decomposed in a suspended state, thereby converting calcium carbonate in the raw meal into calcium oxide. The introduction of this technology allows about 60% of the fuel required for clinker calcination to be transferred to the calciner for the decomposition reaction of carbonates. As a result, the decomposition rate of raw meal entering the kiln is significantly improved to about 85% to 95%, which greatly reduces the load of the rotary kiln, improves production capacity, and lays the foundation for large-scale production. For example, large-scale cement clinker production lines of more than 5000t/d have become possible.
Next, we will have a deeper understanding of the role of the calciner and the basic forms of air flow movement inside it. The core task of the calciner is to complete the decomposition of carbonates in the raw meal and improve the decomposition rate of raw meal entering the kiln. In the furnace, the raw meal and fuel are fully dispersed and mixed, the fuel is completely burned, and heat is transferred to the material, prompting the carbonates in the raw meal to quickly absorb heat and decompose. In addition, the basic forms of air flow movement in the decomposition furnace include vortex, jet, suspension, and fluidized bed etc. In these forms, raw meal and fuel are dispersed in the air flow by different effects. At present, the development trend of the decomposition furnace includes appropriately expanding the furnace capacity to extend the residence time of the air flow.
- Optimize the furnace structure and construct a reasonable three-dimensional flow field to extend the residence time of the raw meal in the furnace.
- Ensure uniform feeding so that the material can be quickly and evenly dispersed after entering the furnace.
- Improve the burner design and layout reasonably to achieve rapid ignition of the fuel.
- Reasonably match the position of the feeding point, coal feeding point, and tertiary air to promote the effective combustion of the fuel and the decomposition of carbonates.
- Reduce the emission of harmful gases such as NOx and SOx to ensure that environmental protection indicators meet the standards.
The process of raw meal in the precalciner kiln
The raw meal is suspended in the decomposition furnace, and its decomposition reaction is jointly affected by the chemical reaction kinetics and the diffusion rate of reactants and products. The key influencing factors include decomposition temperature, CO concentration in furnace gas, physical and chemical properties of raw meal, particle size and suspension dispersion degree. In addition, Table 1 also shows in detail the relationship between decomposition temperature, CO concentration, decomposition rate and decomposition time.
In production, the apparent decomposition rate of the raw meal discharged from the furnace is usually maintained between 90% and 95%. If a high decomposition rate is pursued excessively, the residence time of the raw meal in the furnace will be significantly prolonged, thereby increasing the design requirements of the furnace volume. At the same time, a high decomposition rate will also slow down the decomposition rate and reduce the decomposition heat absorption, which may cause material overheating and furnace gas overheating, and ultimately lead to the problem of crusting and blockage of the bottom-level preheater. On the contrary, if the decomposition rate is controlled too low, the advantages of decomposition outside the kiln cannot be fully utilized.
The combustion characteristics in the precalciner kiln.
When the pulverized coal enters the precalciner kiln, it will be suspended in the air flow, and after the process of preheating, decomposition and combustion, it will emit light and heat to form many small sparks. These small sparks are widely distributed in the furnace, and from the overall point of view, no tangible flame with a clear outline can be observed. Therefore, the combustion of pulverized coal in the precalciner kiln is classified as flameless combustion. This combustion method has the advantages of uniform fuel dispersion, full utilization of combustion space, and avoidance of local high temperature.
Since the pulverized coal in the decomposition furnace burns flamelessly and does not form a concentrated high-temperature “flame”, the pulverized coal must be quickly dispersed and fully contacted with the airflow in the furnace to obtain the necessary oxygen and ignition temperature. Poor dispersion of pulverized coal or uneven distribution in the furnace is a key factor that causes the pulverized coal to fail to ignite or only partially ignite.
In addition, the combustion rate of coal in the precalciner kiln has a direct impact on the temperature in the furnace and the decomposition rate of the raw materials entering the kiln. The faster the combustion rate, the more heat is released, and the higher the temperature in the furnace, which accelerates the decomposition rate. On the contrary, if the combustion rate decreases, the decomposition rate will decrease accordingly.
In the precalciner kiln, the combustion process is at the junction of low-temperature chemical kinetic control and high-temperature diffusion control. The chemical kinetic factors that affect the combustion rate include fuel type, properties, temperature, pressure, and reactant concentration. At the same time, the turbulence degree of the furnace gas, the relative rate of the fuel and airflow, and the dispersion of the fuel will also significantly affect the diffusion combustion rate.
In order to properly increase the combustion rate and control the furnace temperature, we can take the following measures: reasonably select the coal feeding point and adopt the multi-point coal feeding method; properly control the fineness of the coal powder; reasonably adjust the primary and secondary wind speeds and the position of the feeding point. Through the optimization and adjustment of these measures, we can better control the combustion process and temperature distribution in the decomposition furnace.
Heat transfer in the precalciner kiln
The heat transfer method in the decomposition furnace is mainly convection heat transfer, accompanied by radiation heat transfer. In the furnace, the fuel and raw materials are suspended in the air flow. The heat generated by the combustion of the fuel increases the temperature of the air flow. These high-temperature air flows transfer heat to the material by convection. Since the gas and solid phases are in full contact in the suspended state, the heat transfer rate is relatively high.
Gas flow in the precalciner kiln
The air flow in the decomposition furnace has multiple functions, including oxygen supply, combustion, floating material, and as a heat transfer medium. In order to ensure good combustion conditions and heat transfer effects, it is necessary to maintain appropriate wind speeds in various parts of the decomposition furnace to maintain the stability of fuel combustion and the uniform distribution of materials. At the same time, in order to prolong the time of fuel combustion and material decomposition, the airflow should form a swirl or spray state in the furnace to increase the residence time of the gas per unit volume. In addition, it is necessary to ensure that the fluid resistance in the decomposition furnace is small to reduce the power consumption of the system.
In general, the movement of the decomposition furnace gas has the following requirements:
- Appropriate rate distribution;
- Appropriate reflux and turbulence;
- Small pressure loss.
Next, we will discuss the calcination process of clinker.
Generally, the decomposition rate of the material discharged from the C5 discharge pipe can reach 85-95%, and the temperature is maintained at about 850℃. When the raw material enters the kiln, under the joint heating of the high-temperature airflow at the kiln tail and the kiln wall, its temperature will gradually rise from 850℃ to about 900℃. In this process, the incompletely decomposed carbonate will continue to decompose until the reaction is basically completed. Since the total decomposition amount of the material in the kiln is reduced, the length of the decomposition area in the kiln will be shortened compared with the suspension preheater kiln.
Solid phase reaction
As the decomposition reaction in the raw material is gradually completed, the material temperature gradually increases, and then the solid phase reaction is triggered. Usually, the primary solid phase reaction starts at 800℃ in the decomposition furnace. However, it should be noted that in the decomposition furnace, since the components are in suspension and the contact is not tight enough, the main solid phase reaction is carried out on a large scale after the raw material enters the rotary kiln and the material temperature rises further, and finally C2S, C3A and C4AF are generated.
In order to accelerate the process of solid phase reaction, in addition to controlling the reasonable raw material rate value, it is also very important to maintain the fineness and rate stability of the raw material. The solid phase reaction in the kiln is exothermic, and the heat released helps to increase the material temperature and make the material temperature reach the firing temperature, thereby promoting the firing of clinker.
Sintering reaction
The firing reaction in the pre-decomposition kiln is the core link in the entire clinker production process. In order to balance the output, quality and energy consumption in the production process, the conditions of the firing reaction are usually controlled at a temperature of 1300-1450℃, the liquid phase volume is controlled at 20-30%, and the reaction time is maintained at 10-15 minutes. Compared with the preheater kiln, the residence time of the material in the pre-decomposition kiln in the firing zone is shortened, which is mainly due to the faster kiln speed. Although the sintering time is shortened, the quality of the clinker can still be maintained at a high level.
The precalciner kiln adds a decomposition furnace as the “second heat source”, so that most of the carbonate decomposition process is completed in the preheating decomposition system, which greatly improves the production capacity of the rotary kiln and marks a major leap in clinker firing technology.
