A brief discussion on energy-saving measures for spray drying towers

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A Brief Discussion on Energy-Saving Measures for Spray Drying Towers Abstract: The energy-saving approaches for spray drying towers are discussed from several aspects, including the drying medium, the structure of the spray drying tower itself, and the properties of the ceramic slurry, with a summary of the commonly used energy-saving measures for such towers. Keywords: spray drying tower, energy saving, drying medium, slurry properties. In recent years, China’s ceramic industry has developed rapidly. In 2006, China ranked first in the world in terms of production of both tableware ceramics and sanitary building ceramics. The output of tableware ceramics reached 17 billion pieces, accounting for about 65% of the world’s total production, while the annual production of sanitary building ceramic tiles was around 3.5 billion pieces, accounting for approximately 55% of the world’s total production. At the same time, our country is also a major energy consumer, and the building sanitary ceramics industry is a significant user of both fuel and electricity. At present, the energy utilization rate in China’s ceramic industry is only 28%–30%, which is still a significant gap compared to the 50%–57% energy utilization rate in developed countries. Spray drying for powder production is one of the energy-intensive manufacturing processes in the ceramics industry. According to the energy audit data from ceramic factories, the energy consumption associated with spray drying for powder production accounts for 10–20% of the total energy consumption of these factories. With the energy crisis and fierce market competition, reducing the energy consumption in spray drying for powder production is of great significance for lowering corporate production costs, enhancing competitiveness, and promoting the sustainable development of the ceramics industry. 1 　Main measures for saving energy and reducing consumption in spray drying towers. Since the energy consumption during the spray drying process has a direct impact on a company’s economic performance and future prospects, ceramic manufacturers and industry experts have proposed numerous measures to save energy and reduce consumption in this process. These measures can be summarized as follows: firstly, adjustments to the performance and structure of the spray drying tower itself; secondly, control over the properties of the material being dried, issues related to fuel, and the characteristics of the drying medium. 1.1 Control of the drying medium 1.1.1 Increasing the temperature of the hot air entering the tower: Under conditions where the temperature of the air exiting the tower remains constant, the higher the temperature of the hot air entering the tower (also known as the inlet air temperature), the more total heat is delivered. Consequently, more heat is transferred to each unit mass of slurry droplets, and more moisture can be evaporated per unit volume of hot air. With constant production capacity, the amount of hot air required decreases (which means less heat is carried away by the hot air as it leaves the tower), thereby reducing the heat consumption in spray drying for powder production and improving the utilization rate of hot air as well as thermal efficiency. However, the temperature of the hot air entering the tower should not be too high (not exceeding 600 °C); if it is too high, it will damage the distributor at the top of the tower. 1.1.2 Reducing the temperature of the hot air exiting the tower: When the temperature of the hot air entering the tower remains constant, the lower the temperature of the hot air exiting the tower, the greater the temperature difference between the incoming and outgoing air. This results in more thermal energy being transferred from the hot air to the slurry for drying purposes, thereby improving the efficiency of use of the hot air. However, the exhaust temperature should not be too low either; below 75°C, the powder becomes too wet, which hinders proper drying. 1.1.3 Recycling of hot air (waste gas) exiting the tower: After the ceramic slurry is converted into powder through spray drying, if the hot air exiting the tower is released directly into the atmosphere, a significant amount of heat is lost (approximately 10% to 20% of the energy consumed in the powder production process). Therefore, this excess heat should be made full use of; for example, the hot air exiting the tower can be recycled in the preheating and drying process. In addition to being directly recycled, the hot air exiting the tower can also have its excess heat stored or exchanged using a heat exchanger before being reused. 1.2 Factors related to the spray drying tower itself 1.2.1 Choosing the appropriate model: Most manufacturers in the ceramics industry use the 4000 model of spray drying tower; some ceramic factories employ the 5000 and 6000 models. The largest model available is the 12000 model developed by SACMI, which is equipped with as many as 48 nozzles. The larger the model, the greater the production capacity, and the less energy is required to produce each ton of powder; manufacturers can choose the appropriate model based on specific circumstances. 1. 2. 2 Overall airtight control: Since this system operates under negative pressure, any air leakage will increase energy consumption. Therefore, all parts of the equipment as well as the connection flanges, the thermocouple sockets in the hot air furnaces and hot air ducts, the negative pressure measurement ports on the tower, and the discharge outlets of the conical flip-bottom feeders and the cyclone dust removal systems must be properly sealed to prevent air leakage. 1. 2. 3 Control of hot air generators: Hot air generators serve as the source of hot air for spray tower drying; their fuel consumption directly affects the drying costs, making them a key factor in energy savings in spray drying towers. The efficiency of a hot blast stove depends primarily on the fuel atomization nozzles; optimal thermal efficiency is achieved when the fuel is evenly atomized and burned completely. Therefore, it is necessary to strictly control the pressure and flow rate of the atomization air, as well as those of the fuel. In addition, the atomization angle, spray height, and gun angle of the atomization nozzle should all be kept within appropriate ranges. The atomization angle (α) of a typical atomizing nozzle is between 90° and 120°, the spraying height is 4 to 4.5 meters, and the angle of the spray gun is maintained between 110° and 120° to ensure sufficient heat exchange between the sprayed material and the hot air. The choice of fuel for hot blast stoves can directly affect the cost associated with fuel consumption. Using clean fuels such as liquefied petroleum gas or light diesel will increase costs, while when using heavy oil or mixed fuels, it is necessary to control their sulfur content; otherwise, it will be difficult to ensure that SO2 emissions remain within acceptable levels. Today, many ceramic factories use coal dust separated from coal gas to mix with coal ash in order to produce water-coal slurry. Since coal gas contains 10% to 20% unburned carbon, and in some cases even more than 20%, phenol water and tar generated from coal gas are injected into hot air furnaces for combustion, which prevents the emission of these harmful substances; they are converted into harmless water and CO2 during high-temperature combustion and then released. This not only helps to **reduce combustion costs**, but also allows for the full utilization of these waste residues and liquids, thereby saving energy and reducing consumption. 1. 2. 4 Use of linear burners: Traditional hot air systems in spray drying towers typically use fuel (gas) hot air generators, boiler steam heat exchangers, heat transfer oil heat exchangers, or electric heating systems for heating. All the above traditional heating systems use heat exchangers, and the efficiency of these heat exchangers determines the thermal energy utilization efficiency of traditional heating systems. Moreover, heat exchangers have a limited service life and high maintenance costs. A direct-fired hot air system centered around a linear burner. The linear burner has a relatively small size and is installed directly within the air duct; the drying medium can come into direct contact with it and quickly reach the desired temperature. The direct-fired hot air system centered around a linear burner boasts both energy-saving and environmental protection advantages. Firstly, the combustion mechanism of the linear burner is reasonable; a certain amount of excess air is maintained in the combustion zone, which not only ensures complete combustion but also helps to suppress the formation of nitrogen oxides. This direct-fired hot air system comes into direct contact with air without the need for a heat exchanger, ensuring effective transfer of combustion heat to the air and thus high thermal efficiency. Additionally, ease of use is another feature of linear burners; the temperature of the hot air can be adjusted by controlling the gas control valve. 1. 3 　Quality control of slurry 1) Reducing the moisture content of ceramic slurry requires less heat for drying; however, slurry with a low moisture content has poor fluidity, and poor fluidity leads to a poor atomization effect. To resolve this contradiction, appropriate diluents (water reducers) or electrolytes (such as water glass, soda ash, humic acid, etc.) are usually added during production to adjust the fluidity of the slurry while reducing its water content. In collaboration with Guangdong Xinmingzhu Group, the author used a composite water reducer, which reduced the moisture content of the slurry from 39.5% to 36%. The ball milling time was shortened by 5 hours; electricity costs were saved by 16.5 yuan per ton of powder produced, and production increased by 18.8%. The annual cost savings amounted to over 1.5 million yuan. 2) Raising the temperature of the ceramic slurry can effectively reduce its viscosity, improve its atomization performance, and prevent clogging of the atomization nozzle due to slurry crystallization. Therefore, the waste heat recovered from the hot air exiting the tower can be used to preheat the slurry, which is an effective way for energy recycling. 2 　Conclusion: In addition to the measures mentioned above for energy savings in spray drying towers, solutions can also be sought in terms of energy use, such as by utilizing new energy sources and properly controlling the combustion process. Of course, many problems still need to be identified and solved in actual production. Ceramic enterprises can enhance their economic and social benefits by rationally improving the energy efficiency of spray drying towers in line with the goals of sustainable development.