In recent years, many cement companies have achieved increased production and reduced energy consumption by optimizing production processes and upgrading equipment. This article primarily introduces a retrofit project for a 2,500 t/d cement production line. The line’s clinker calcination system has an output of 3,000 t/d and consists of a five-stage cyclone preheater, a precalciner, a Φ4.0 m × 60 m rotary kiln, and a third-generation cooler. The cooler has an effective cooling area of 62.4 m².
Five Bottlenecks On The Cement Production Lines
1. Preheating & Pre-decomposition System: Currently, the sectional wind speed of the C1 cyclone is above 4 m/s, the C4 and C5 cyclones are 6 m/s, and the precalciner exceeds 11 m/s. Overall, the preheating and pre-decomposition system has reached the design capacity limit.
2. Rotary Kiln & Tertiary Air Duct: The designed capacity of this kiln is 2,500 t/d, with a maximum rotation speed of 4.07 r/min and a main motor power of 315 kW. However, its actual clinker output is 3,000 t/d, with a volumetric output of 4.9 t/(m³•d) and a sectional heat load of 4.5 MW/m². These values are relatively high for a Φ4.0 m × 60 m rotary kiln, and the material filling rate inside the kiln is also high, which is unfavorable for heat exchange. In addition, the outer diameter of the tertiary air duct is 2.2 m, with an effective inner diameter of 1.8 m. The airflow speed inside the duct is already high, and if production is further increased, the ventilation resistance will rise, affecting the balance of air usage in the kiln.
3. Cooler: The cooler has an effective cooling area of 62.4 m², but faces issues such as high clinker discharge temperature and poor heat recovery efficiency.
4. High-temperature Fan: The rated air volume of the high-temperature fan is 400,000 m³/h, and the rated air pressure is 8,896 Pa, which is already operating close to full capacity.
5. Other Auxiliary Equipment: Equipment such as the burner, raw material rotor scale, and raw material feeding elevator all require corresponding optimization and upgrades.
Retrofit Scheme
C1 Cyclone Retrofit
We need to replace the entire C1 cyclone, increasing its diameter from 4.6 m to 5.4 m. The design sectional wind speed of the new cyclone is around 3.5 m/s, which increases the dust settling time within the cyclone. In addition, we have added flow straighteners and vortex breakers at the cone section, along with optimized adjustments to the cyclone’s dimensions. These modifications will ensure a separation efficiency of over 95%, with a body resistance of less than 800 Pa.
C2-C5 Cyclone Retrofit
The sectional wind speed of the C2-C5 cyclones is relatively high (6–8 m/s). Only partial modifications will be made, with the goal of reducing cyclone resistance, ensuring an appropriate separation efficiency (86%–90%), and improving the heat exchange efficiency of the preheater.
(1) The C2-C5 cyclone resistance reduction modification will be achieved by expanding the inlet area and increasing the cyclone inner cylinder diameter. When expanding the inlet area, the width-to-height ratio of the inlet must be maintained between 0.5 and 0.65. We can achieve this by expanding the volute shell to increase the inlet width, while also raising the cyclone top cover to increase the inlet height.
(2) C2-C5 cyclone separation efficiency modification. After increasing the inner cylinder diameter of the cyclone, the risk of short-circuiting the dust-laden airflow also increases, which reduces the separation efficiency. Therefore, while expanding the inner cylinder diameter, it is necessary to ensure an appropriate distance between the inlet and the inner cylinder. During the cyclone modification, particular attention should be given to optimizing the flow field of the volute shell, which will help maintain the separation efficiency to a certain extent.
(3) Preheater heat exchange efficiency modification. After the modification, the wind speed in the rising ducts of each cyclone stage has increased. This not only reduces the heat exchange time of the materials in the rising ducts but also increases the resistance loss of momentum transfer in the material acceleration zone. Therefore, we need to carry out diameter expansion modifications for the rising ducts of each cyclone stage.
| Rising Duct Dimensions Comparison | ||
| Position | Before Retrofit | After Retrofit |
| C5-C4 | φ3.86m | φ4.56m |
| C4-C3 | φ3.86m | φ4.46m |
| C3-C2 | φ3.74m | φ4.26m |
| C2-C1 | φ3.52m | φ4.06m |
Precalciner Expansion & Flow Field Optimization
The volume of the precalciner is approximately 763 m³. According to calculations, the residence time of the gas inside the precalciner is less than 4 seconds, which is insufficient to meet the decomposition time of raw materials and the combustion time of coal powder for the corresponding output. Therefore, the precalciner expansion is crucial. We can achieve this expansion by increasing the height of the goose-neck pipe at the precalciner outlet.
The wind speed in the precalciner exceeds 13 m/s. Excessively high wind speeds reduce the residence time of raw materials and fuel in the combustion zone, leading to poorer heat exchange. Increasing the height of the goose-neck pipe increases the precalciner volume but reduces the volumetric load. Additionally, the sectional heat load is relatively high (1.88 × 10⁷ kJ/(m²•h)), which also affects the heat exchange and decomposition of the materials. Therefore, expanding the diameter of the precalciner can reduce the sectional heat load and increase the residence time of raw materials and fuel in the main combustion zone.
Flue Chamber Retrofit
The ventilation volume of the kiln tail flue chamber is a key factor affecting clinker production capacity. To meet the requirement of increasing clinker output, it is necessary to increase the minimum cross-sectional area of the flue chamber. Due to structural limitations, partial modifications cannot achieve the required design dimensions. Therefore, it is necessary to replace the entire flue chamber to increase the minimum ventilation cross-sectional area.
Rotary Kiln & Tertiary Air Duct Retrofit
According to the modification objectives, the main gearbox and main motor of the kiln should be replaced, increasing the rotary kiln speed from 0.41–4.07 r/min to 0.50–4.5 r/min. The main motor power is increased from 315 kW to 355 kW. In addition, the diameter of the tertiary air duct should be expanded from 2.2 m to 2.4 m.
Cooler Retrofit
The third-generation cooler should be replaced with a fourth-generation cooler, increasing the effective cooling area to 90.1 m². After the replacement, the number of cooler fans is reduced from 12 to 9, with a slight increase in the fan motor power.
High-temperature Fan Retrofit
The airflow and pressure of the high-temperature fan should be increased to meet the needs for increased production and reduced energy consumption. The modified fan has a pressure of 9,200 Pa, a rated airflow of 600,000 m³/h, and a motor power of 1,800 kW.
Other Auxiliary Equipment Retrofit
| Item | Before Retrofit | After Retrofit |
| Kiln Head Burner | 6.5-7.1(max:10)t/h | 9-10(max:12)t/h |
| Raw Material Rotor Scale | 60-250t/h | 80-280(max:400)t/h |
| Raw Material Feeding Elevator | 220t/h | 260(max:280)t/h |
The retrofit took approximately 53 days. After being put back into production, the clinker output can stabilize at 4,100 t/d, with the rotary kiln’s volumetric output at 6.71 t/(m³•d) and a sectional heat load of 5.79 MW/m². The clinker coal consumption decreased by 5 kg/t, showing a significant reduction in energy consumption. The main improvements include a decrease in the C1 outlet flue gas temperature, reduced flue gas volume and dust concentration, an increase in the cooler’s heat recovery efficiency, and a reduction in the unit clinker surface heat dissipation of high-temperature equipment after the production increase.
