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China currently has over 550 converters, of which more than 10 use dry dust removal methods, while the rest use wet methods. Both wet and dry methods suffer from various issues to varying degrees, including high emissions, high energy consumption, high water consumption, and heavy maintenance requirements. To this end, Shanghai Waigaoqiao Spraying System Company applied for a semi-dry dust removal process for converter flue gas that is more suitable for the current national conditions, and obtained a Chinese invention patent in September 2006. The following is a comparison of some performance indicators for semi-dry dust removal of converter flue gas with dry dust removal and wet dust removal: At present, the semi-dry evaporation cooling tower technology used in this process has been applied in the renovation or new construction projects of more than 10 converters in China. Its significant advantages in terms of environmental protection, energy savings, water conservation, reduced maintenance needs, increased production and lower costs, as well as reduced construction investment, smaller land footprint, and shorter renovation times, have been recognized and approved by an increasing number of users. 1. Semi-dry high-efficiency spray cooling and dust removal tower: The temperature of the converter flue gas after being cooled in the vaporization cooling flue is usually between 800 and 1000°C; evaporation cooling towers are used for dry cooling, while wet methods are employed for cooling. This process uses one or more hollow, semi-dry high-efficiency spray cooling and dust removal towers for cooling, that is, it employs dry evaporation cooling technology ； The difference is that spray dust removal is still used for dust removal, and the resulting wastewater is still treated by flushing with water. Results from practical applications show that the most significant advantage of using a semi-dry high-efficiency spray cooling and dust removal tower in place of the traditional design is a 90% reduction in system resistance, from 3–5 kPa to around 300 Pa. This advantage can be utilized in the following ways: Environmental protection – by converting only the traditional unit into a tower, the saved pressure difference can be applied to the second unit, thereby reducing the concentration of dust in the exhaust gases to below 100 or 50 milligrams per cubic meter (depending on the capacity of the fan), which means that the system resistance can be reallocated ； Improving equipment capacity: It is possible to address issues such as insufficient fan capacity resulting from the expansion of converters, additional equipment installations, and increased oxygen supply rates, as well as problems with smoke emission from chimneys and furnace openings, all at a very low cost, thereby enhancing the system’s processing capacity ； Energy saving: If the dust concentration is already within the specified limits, the fan speed can be reduced; simply converting one unit into a tower can save approximately 1–2 kWh per ton of steel produced ； Water conservation: The amount of water used in circulation is reduced by at least 50%; in some cases, no water is needed for circulation after the ash is dried, which solves problems such as insufficient water treatment capacity, poor water quality, and high operating costs for water treatment. 2. Wet electrostatic dust removal: For the precise dust removal of converter flue gas, dry electrostatic dust collectors are currently used in dry processes. The main problems associated with these collectors are that the dust concentration cannot be kept stably below the designed level of 10 milligrams per cubic meter ； Secondly, anode plates/cathode wires tend to corrode, deform, and scale over time due to prolonged use, resulting in high maintenance costs ； Third, electrostatic precipitators must employ reliable explosion prevention measures. The main problems with the wet method are as follows: first, the emission concentration is high; the recovered gas needs to be further purified using a wet electrostatic precipitator before it can be used properly ； Second, the required pressure difference is extremely large, resulting in high energy consumption for the system ； Third, the circulating water volume is particularly large. This process uses a wet electrostatic precipitator (horizontal or vertical) for precise dust removal. Its main advantages are: a low outlet temperature, and the recovered gas can be fed directly into the gas tank without the need for cooling as is required in dry methods ； It provides the best purification effect, ensuring that the dust concentration remains below 10 milligrams per cubic meter or even lower ； 50% energy savings per ton of steel (~4kWh/t steel), the same as in the dry process ； Compared to the two-wet method, it reduces the amount of circulating water by 50–90%; essentially no circulating water is used, with only a small amount required for cleaning the equipment ； Rinsing the electrodes with water can also eliminate problems such as secondary dust generation during dry processing and dust accumulation on the electrode plates/cathode wires. 3. Nozzle spraying: The optimal differential pressure between the converter nozzle and the movable smoke hood is ±10 Pa; in other words, it is necessary that the system does not emit smoke outward, while at the same time minimizing the amount of air drawn in by the system in order to increase the calorific value of the gas. Currently, dry processes use AC variable-frequency speed control for the primary fan, while wet processes employ pressure difference regulation at the two nozzles to achieve micro-differential pressure control at the furnace outlet. In a considerable number of converters, the actual differential pressure is not under automatic control or is poorly controlled, resulting in excessive leakage of flue gas or an increased amount of air being drawn in. This leads to a decrease in the CO content in the gas and a sharp increase in the volume of flue gas processed by the system. At the same time, problems such as slag sticking to the furnace mouth, leaks in the smoke hood, and high maintenance costs are also quite common. This process uses a nozzle spray system to provide supplementary control of the slight pressure difference; its main function is to remove dust and reduce temperature when the pressure at the furnace outlet is too high ； When the pressure at the furnace opening is low, it can act as a soft seal, reducing the combustion of carbon monoxide caused by incoming air, as well as minimizing the size of dust particles and lowering the temperature ； It can also cool the smoke hood and furnace outlet equipment, reducing slag sticking and water leakage ； This system can also be extended to secondary dust removal during molten iron pouring, scrap addition, and steel tapping, thereby replacing or reducing the processing capacity of the secondary dust removal system. Due to the use of well-nebulated nozzles, there is no potential risk of explosion caused by mechanical water. 4. AC variable-frequency speed control of the fan: After the adoption of wet electrostatic precipitators, there is no secondary voltage regulation function available in the wet process ； Also, due to the large fluctuations in flue gas flow rate during the converter smelting cycle, in order to maintain a stable micro-differential pressure at the furnace mouth and save energy, the fans must be equipped with AC variable-frequency speed control ； Other pressure regulation methods can also be used to maintain a slight differential pressure at the furnace mouth. 5. Water flushing of ash and wastewater treatment: The dust removed by spray cooling and dust removal as well as wet electrostatic dust removal is flushed, in part or in full, using the existing system’s continuous or intermittent water spraying method; it is then sent to a wastewater sedimentation tank where sludge separation and wastewater treatment take place. The treated water is reused, with a small amount of industrial water being added periodically. 6. Atomizing gas for the spray gun: The atomizing medium required for the atomizing nozzle used in the evaporation cooling process can be superheated steam or saturated steam, or it can also be nitrogen. An automatic switching mechanism is provided between the two, allowing the system to prioritize steam or nitrogen atomization based on the production conditions of the steel plant. Domestic production experience shows that steam atomization should be given priority when there is an excess of steam to be discharged ； If steam is already fully utilized and there is no surplus, using nitrogen for atomization is more economical, with operating costs being less than 30% of those incurred by using steam for atomization. 7. Mechanism by which the semi-dry method reduces the amount of circulating water. It is well known that there are two different cooling mechanisms for cooling flue gas through direct water spraying: one is the so-called saturated cooling, in which the heat from the flue gas is absorbed by heating the water; theoretically, each kilogram of water can absorb only 50 kilocalories of heat. The second method is evaporative cooling, which involves using the latent heat of water vaporization to absorb the heat from the flue gas, thereby cooling it. Theoretically, the latent heat of vaporization for each kilogram of water is over 500 kcal, which is exactly 10 times that of saturated cooling; as a result, the water flow rate required to cool the same amount of flue gas is 1/10 of that needed for saturated cooling. Like the dry method, the semi-dry method also utilizes the evaporation cooling principle; therefore, not only can the flow rate of circulating water be significantly reduced, but there are no requirements regarding water temperature, and even heated circulating water can be used directly. 8. Mechanisms of energy savings and improved system performance in the semi-dry method: The total system resistance of the semi-dry dust removal process is the same as that of the dry method, at only 6–8 kPa ； In comparison, the total resistance of the wet-process system is between 22 and 28 kPa. The mechanism analysis for its energy savings and improvement in system processing capacity is as follows: N = QH / 102η (1), where N = power of the primary fan ； Q = air volume under operating conditions ； H = total pressure of the fan ； η = comprehensive efficiency of the fan. Assuming that the operating air volume and the overall efficiency of the fan remain constant, equation (2) can be derived; this equation shows that the proportion by which the total system resistance decreases is equal to the percentage decrease in the power required by the fan. ΔN/N1 = ΔH/H1 (2) Where: ΔN = the difference in power required by the fan before and after modification ~70% ； N1 = Power required by the fan before modification ； ΔH = the difference in resistance required for the fan before and after modification ~ 15–18 kPa ； H1 = total system resistance before modification. Similarly, assuming that the fan power and overall efficiency remain constant, equation (3) can be derived. This equation shows that the change in the volume of air that the fan can handle is inversely proportional to the change in system resistance; therefore, using the semi-dry method can significantly improve the system’s processing capacity. Generally, if only one component is modified while the other equipment remains unchanged, the air handling capacity of the fan can be increased by 15% to 20% ； If a wet electrostatic precipitator is used, the existing fan can be considered for use ； It is more economical to replace it with a fresh air unit having a total pressure of around 8 kPa. ΔQ/Q1 = H1/ΔH (3) Conclusion The semi-dry dust removal process for converter flue gas, for which a Chinese invention patent has been obtained, is suitable for new converter projects, and is particularly appropriate for the renovation of existing converter wet dust removal systems. Its main features are: 1. Short transformation time: The shutdown time required for the actual transformation has been reduced to as little as 5 days ； 2. Most of the existing process technologies and equipment can be used directly, which allows for a significant reduction in investment compared to introducing new technologies and equipment ； 3. It can address some of the issues associated with the introduction of process technologies and equipment, such as the excessive circulation water flow rate in saturated spray cooling towers. It is foreseeable that the further promotion and application of this technological process, along with its continuous improvement, will surely play a positive role in helping China reduce dust/wastewater emissions in converter dust removal, save energy and water, enhance overall treatment capabilities, and improve the level of circular economy. (end)