The cement clinker plant has been using rotary kiln technology for many years, while the rapid popularization of the suspended preheating, external decomposition kiln technology in China began in the early 21st century. Over the past 25 years, although it may seem that there have been no significant changes on the surface, the suspended preheating, external decomposition kiln technology has actually undergone tremendous changes. This is because: energy consumption has been significantly reduced by enterprises with better energy management; the consumption of coal has decreased from the initial 110 kilograms of standard coal per ton of clinker to less than 94 kilograms of standard coal now; electricity consumption has also been reduced from more than 60 degrees per ton of clinker initially to less than 48 degrees now. These data undoubtedly demonstrate the significant improvement of the cement rotary kiln technology. Of course, technological progress covers multiple aspects such as process management, process optimization, and equipment upgrading. However, we cannot ignore the significant contributions made by the improvements in cyclone preheaters and precalciner as well as the advancement of fan technology to the reduction of energy consumption. Today, we will explore the process transformation and technological progress in depth.
Preheater Upgrade
In the high-energy consumption cement production system, the rotary kiln system is the core equipment, and its energy consumption accounts for a very important proportion. The cyclone preheater, as a key component of the rotary kiln system, has a decisive impact on energy consumption. In fact, in the cement production process, the most serious energy waste is the exhaust gas emitted from the first cylinder of the preheater. The higher the temperature of the emitted exhaust gas, the more serious the energy waste; conversely, the lower the temperature, the higher the energy utilization efficiency. The next is the heat recovery of the grate cooler, that is, the temperature of the secondary air. A higher secondary air temperature means a better heat recovery effect and lower energy consumption for the clinker; a lower secondary air temperature indicates a poorer heat recovery effect and higher energy consumption for the clinker.
At present, the preheaters of the rotary kilns in most cement manufacturing plants are of five stages, while a few are of four stages. In some newly-built production lines, six-stage preheaters are adopted. Some enterprises have upgraded from five-stage to six-stage, and a few even use seven-stage preheaters. The temperature at the outlet of the first cylinder of the four-stage preheater is over 380 degrees, which undoubtedly is the main factor causing high energy consumption. This indicates that a large amount of thermal energy is not fully utilized and is discharged, resulting in a huge waste of energy. The five-stage preheater, if designed properly, can reduce the outlet temperature of the first cylinder to below 300 degrees. The six-stage preheater can further reduce the outlet temperature of the first cylinder to 240 degrees. It can be seen that the reduction in energy consumption is quite significant with multiple cyclone cylinders. In fact, the technology of the six-stage preheater is relatively mature, and promoting the transformation of the six-stage preheater is imperative. The key to the transformation of the six-stage preheater lies in increasing the number of stages of the preheater. By increasing the first stage of the preheater, the number of heat exchanges between the material and the hot gas flow is significantly increased and the time is prolonged, thereby greatly improving the heat exchange efficiency. After the heat exchange efficiency is improved, the material can absorb more heat, resulting in a significant reduction in the outlet temperature of the preheater and a reduction in heat energy loss, achieving more efficient utilization of energy. At the same time, due to sufficient heat exchange time, the outlet temperature of the decomposition furnace can also be controlled more effectively, which is beneficial to the safe operation of the system. After the transformation, only the transformation of the preheater alone can save 4 kilograms of standard coal per ton of clinker, with extremely significant energy-saving effects, effectively promoting the green and low-carbon development of the enterprise.
Expansion of the Precalciner
The precalciner, as the core equipment of the cement rotary kiln production system, is responsible for a series of key processes such as fuel combustion, carbonate decomposition, gas-solid two-phase transportation, mixing, heat exchange and mass transfer. The performance of the precalciner directly affects the energy consumption and output of the entire production system. With the continuous expansion of the cement industry scale and the increasing requirements for energy conservation and consumption reduction, the expansion of the precalciner has become an important strategy to improve production efficiency and reduce energy consumption.
During long-term operation, traditional precalciner gradually revealed several problems due to design limitations and the expansion of production scale. Firstly, the limited space in the furnace resulted in insufficient residence time of fuel and raw materials within the furnace, leading to incomplete reactions. Taking a production line with a daily output of 5,000 tons of clinker as an example, if the volume of the precalciner was too small, the residence time of fuel and raw materials in the precalciner might only be a few seconds, which could not meet the requirements for complete calcium carbonate decomposition, resulting in a low decomposition rate of the raw materials entering the kiln, affecting the firing effect and quality of the clinker. Secondly, the uneven airflow distribution in the precalciner led to short-circuiting in some areas, which not only reduced the combustion efficiency of the fuel but also caused uneven temperature distribution in the furnace, further affecting the progress of the decomposition reaction and increasing energy consumption.
The expansion of the precalciner, that is, through reasonable design and renovation, increases the volume and reaction space of the furnace. During the expansion process, it is necessary to fully consider the flow field, temperature field and concentration field distribution in the furnace to ensure that fuel and raw materials can be fully mixed and uniformly heated within the precalciner, achieving efficient carbonate decomposition reactions. By increasing the diameter and height of the precalciner, the reaction space in the precalciner can be effectively increased, and the residence time of fuel and raw materials can be prolonged.
The effect of the expansion of the precalciner is remarkable. After widening the diameter, the carbon monoxide content in the first cylinder decreased significantly, generally dropping to below 400 ppm. The carbon monoxide content in the un-expanded cylinder was 3-4 ppm, and some were even higher; the decomposition rate significantly increased, with some reaching over 98%; the outlet temperature of the precalciner decreased, and some were controlled at around 860 degrees, with the decomposition rate still reaching over 96%. The lower precalciner temperature control is not only conducive to reducing the use of ammonia water and nitrogen oxide emissions, but also significantly reduces the possibility of system scaling.
In addition, due to more thorough decomposition reactions, the firing pressure of the rotary kiln decreases, and problems such as scaling and blockage in the kiln are effectively alleviated, improving the stability and service life of the equipment. Of course, these improvements are achieved under certain parameter matching conditions. Therefore, the expansion of the precalciner is a key measure to reduce energy consumption and stabilize the operation of the rotary kiln.
Optimization of Preheater Low-Pressure Resistance
In the cement rotary kiln system, the optimization of the low-pressure resistance structure of the preheater plays a crucial role in achieving energy conservation and reduction of consumption. It has a significant impact on the energy utilization efficiency of the entire production process. The cyclone tube, as the core equipment for gas-solid separation, has a decisive influence on the separation efficiency and system resistance in the process.
Although the structure of the cyclone tube seems simple, the design of the cyclone tube is closely related to the gas-solid separation efficiency and system resistance. If the wind speed is too high, it will lead to a significant increase in system resistance, thereby increasing energy consumption; conversely, if the wind speed is too low, it is not conducive to the dispersion and separation of materials, thus reducing production efficiency. In practical operation, the gas-solid separation efficiency is directly proportional to the system resistance, that is, the higher the separation efficiency, the greater the system resistance, and the corresponding increase in energy consumption. However, with the development of modern technology, through optimized design, such as changing the type and size of the inlet and outlet of the cyclone tube, the vortex interference can be reduced, the separation efficiency can be improved, and the system resistance can be reduced.
To further optimize the low-pressure resistance structure, a series of targeted measures can be taken. For example, adding guide vanes at the inlet or outlet of the cyclone tube to guide the airflow smoothly into and out of the cyclone tube, reducing airflow disorder and energy loss, thereby reducing system resistance. In addition, improving the structure of the cyclone tube discharge port, optimizing its shape and size, ensuring smooth discharge of materials, avoiding accumulation and blockage, also helps to reduce system resistance and improve production efficiency.
Through various improvement methods, optimizing the low-pressure resistance structure of components such as the cyclone tube, the system resistance is significantly reduced, and electricity and coal consumption are greatly decreased. At the same time, the gas-solid separation efficiency is also significantly improved, meaning more materials are effectively separated, reducing material loss and the energy consumption of subsequent processing, thus achieving significant comprehensive energy-saving effects, reducing production costs for enterprises and enhancing market competitiveness. The reason why the electricity consumption per ton of clinker of many enterprises can be reduced to below 48 degrees is largely due to the optimization of the low-pressure resistance structure of the preheater and pipelines.
High-efficiency fan
In the huge energy consumption system of the cement rotary kiln, the fan, as the core power equipment, consumes a significant proportion of electricity. Therefore, in the pursuit of the goal of energy conservation and reduction, the energy-saving renovation of the fan is an indispensable key link. For a long time, traditional fans have exposed many problems during operation, which seriously limit the energy efficiency of cement production and the economic benefits of enterprises.
The problem of low operating efficiency that is common in traditional fans is particularly prominent in actual production. Affected by various complex factors, the operating efficiency of large fans in many cement plants is lower than 80%. For example, some cement plants’ high-temperature fans, due to poor matching between the design and actual working conditions, have been operating under high load and low efficiency for a long time. This not only leads to a large amount of electricity consumption but also fails to meet the precise requirements of the production process for air volume and air pressure, thereby causing system instability and affecting production efficiency. In addition, the adjustment methods of traditional fans are relatively outdated, mostly using damper adjustment. This adjustment method, while adjusting the air volume, significantly increases the resistance of the air path, resulting in a waste of a large amount of energy.
To solve these fundamental problems, it is urgent to adopt new high-efficiency fans to replace traditional fans. New high-efficiency fans demonstrate significant advantages in design concepts, structural design, and energy efficiency performance. In terms of design concepts, new fans aim to improve operating efficiency and reduce energy consumption as the fundamental goal. Through comprehensive optimization of the structure, materials, and processes of the fan, efficient operation is achieved. In terms of structural design, especially the use of large axial flow fans theoretically can more reliably reduce energy consumption. From the perspective of energy efficiency performance, high-efficiency energy-saving fans have a significantly higher energy efficiency ratio than ordinary fans. During operation, they can effectively reduce air resistance and increase air flow speed, thereby improving the operating efficiency of the fan. In the design and manufacturing process, by applying new materials and processes, the loss of the fan is further reduced, thereby improving the energy efficiency ratio.
During the fan renovation process, it is necessary to carefully select a suitable manufacturer and reasonably set process parameters. This step should be based on the long-term operating performance of the fan as the evaluation standard, rather than merely reducing air volume as a means of saving coal and electricity. It is worth noting that it is essential to avoid cooperating with enterprises with poor reputation to prevent greater losses for the enterprise.
Efficient Utilization of Waste Heat
In the production process of cement rotary kilns, approximately 30% of the thermal energy is discharged into the atmosphere in the form of exhaust gas through the dust collectors at the kiln tail and kiln head. The core principle of the waste heat power generation system lies in recovering these medium-temperature exhaust gases to achieve waste heat power generation. This process does not require additional fuel consumption and does not produce pollutants during the power generation process. This system is an economically advantageous and environmentally friendly technology that complies with the national clean energy and energy conservation industry policies. However, the temperature of the exhaust gas after being treated by the boiler usually exceeds 200 degrees, so it is particularly important to find more efficient ways to utilize these low-temperature exhaust gases.
To improve the efficiency of waste heat power generation, measures such as increasing the heat exchange efficiency of the boiler, optimizing the boiler process design, and enhancing the utilization rate of low-temperature exhaust gases can be taken. Selecting efficient heat exchangers can improve the heat conduction efficiency, thereby enhancing the efficiency of waste heat power generation. At the same time, using high-quality heat conduction materials can also effectively improve the heat conduction efficiency.
Waste heat power generation efficiency enhancement plays a crucial role in energy conservation and consumption reduction of cement rotary kilns, and shows great potential, providing solid support for cement enterprises to achieve green and sustainable development.
Automatic Control
In the process of energy conservation and consumption reduction of cement rotary kilns, automatic control technology also plays a crucial role. With the help of an automatic control system, key parameters of the rotary kiln, such as temperature, pressure, and material flow rate, can be monitored and precisely adjusted in real time. In terms of temperature management, multiple temperature sensors are set up to continuously monitor the rotary kiln, and the PID control algorithm is used to adjust the heating system in real time, ensuring the stability of the internal temperature of the rotary kiln.
In terms of equipment operation status monitoring, the automatic control system can track the operation conditions of each equipment in the rotary kiln in real time, such as the current, voltage, speed of the motor, and the air volume, air pressure of the fan. Once an abnormality is detected, the system will immediately issue an alarm and take corresponding preventive measures to prevent equipment failure from causing downtime and energy waste.
The prerequisite for achieving automatic control is detailed data collection, storage, statistics, and analysis. The “Intelligent Management Platform” based on big data launched by Yuanxin Data Communication Technology Company is an ideal choice. The introduction of this platform enables the production process to achieve refined management, not only reducing energy consumption but also improving product quality and production efficiency, significantly enhancing the economic benefits and market competitiveness of the enterprise. This provides valuable experience for the green and sustainable development of the cement industry.
Technological progress goes beyond the aforementioned aspects and includes the renovation of grate cooler, the improvement of insulation materials, and the application of permanent magnet motors, among others. All these have endowed cement kilns with the potential for energy conservation and cost reduction. By implementing technological renovations rationally, the cement factory’s goal of reducing coal consumption to 90 kilograms of standard coal per ton of clinker and electricity consumption to 40 degrees in the future is not beyond reach. However, during the process of technological renovation, the technology itself is not the key issue; choosing the right partners is what is crucial. In practice, excessive promotion often has an adverse impact on decision-making. Managers must remain highly vigilant to avoid establishing partnerships with those who lack technology but rely on speculation and opportunism.
