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Application of Spray Foam Tower in Boiler Flue Gas Treatment Authors: Li Xiaobing, Liu Jiongtian, Liu Jingming Abstract: A new type of spray foam desulfurization and dust removal tower for use in boiler flue gas treatment is introduced. Based on the multi-stage purification principle that combines centrifugation, spraying, and foaming, and through cyclone spraying and two-stage spray foam plate washing, the desulfurization efficiency is 91.4% and the dust removal efficiency is 98.7%. The spray foam tower features integrated dust and sulfur removal, low floor space requirements, and cost savings, making it suitable for sulfur removal and dust elimination from the flue gases of large and medium-sized industrial boilers. Keywords: spray foam tower, desulfurization, dust removal 1 Introduction In China’s primary energy consumption structure, coal accounts for over 70% of the total energy used, and the SO2 generated from coal makes up approximately 90% of the country’s total SO2 emissions. Therefore, desulfurizing the flue gas from coal-fired boilers and controlling SO2 emissions are urgent requirements for China’s economic and social development. A heat power plant in Beijing plans to treat the flue gas from a 35 t/h coal-fired boiler by removing the existing φ2500 mm Venturi marble water film dust collector, and replacing it with a new type of high-efficiency spray foam desulfurization and dust removal tower that offers stable purification efficiency, reliable operation, and an investment level suitable for the economic conditions in Beijing. According to the requirements of the desulfurization and dust removal system, corresponding high-efficiency dewatering equipment, water circulation systems, chemical dosing systems, aeration systems, and automatic control systems are installed. 2 Governance Solutions 2.1 Design Parameters The main design parameters were determined based on the test reports and data provided by the plant: flue gas volume ≤ 63,000 m3/h, flue gas temperature at the exit of the air preheater ≤ 180°C, dust concentration at the exit of the air preheater ≤ 2,500 mg/m3, sulfur content in the coal ≤ 0.8%, system resistance before the dust collector ≤ 1.0 kPa, desulfurization efficiency ≥ 90%, and dust removal efficiency ≥ 98.2%. 2.2 Governance Process This process includes a flue gas system, a water circulation system, a chemical dosing system, an aeration system, and an automatic control system; the process flow is shown in Figure 1. Figure 1 Flue gas desulfurization and dust removal process flow 2.2.1 Flue gas system In this process, the boiler’s flue gas is introduced into an air heat exchanger to be cooled to below 180°C, and then it enters the spray foam tower tangentially through pipes. Inside the tower, the flue gas is washed with a washing liquid before passing into a high-efficiency dehydrator. The flue gas containing mist is dehydrated and then sent to an exhaust fan, from where it is discharged through a chimney. The induced draft fan selected is a GDGYNo13–left 90°–132kW–60℃ corrosion-resistant induced draft fan. The flow rate is 75,000 m3/h, and the total pressure is 3.6 kPa. 2.2.2 Water circulation system: A circulation pump transports the circulating water containing desulfurizing agent (MgO powder) from the tank to the spray foam tower, where it reacts with the flue gas inside the tower; thereafter, it is discharged through an overflow channel and sent back to the tank (with a capacity of 2400 m3) via the ash water ditch. The total circulating water volume of this system is 252 t/h. Two 150UHB–ZK–250–35 (75kW) wear-resistant and corrosion-resistant water pumps are selected as the circulation pumps (one of which is used as a spare). 2.2.3 Chemical dosing system: The circulating water entering the tank has its pH value measured by an automatic pH meter. When pH < 6.5, the electric control valve on the Mg(OH)2 emulsion pipeline is automatically opened to inject Mg(OH)2 emulsion ； When the pH of the circulating water leaving the tower is adjusted to 6.5, the electric control valve shuts off automatically, and the pH of the water in the circulating water tank is adjusted to 9–11 using a pH meter. The MgO powder is added to the digestion tank, mixed with water for a few minutes to form an emulsion, which then flows by gravity into the Mg(OH)2 emulsion storage tank. The emulsion in the tank flows by gravity to the sedimentation tank for desulfurization. The dosage of MgO powder is 66.8 kg/h. 2.2.4 Aeration system: To oxidize MgSO3 in the sedimentation tank into MgSO4 that is soluble in water, aeration is required in the sedimentation tank. This not only helps to **reduce suspended solids in the circulating water, but also prevents scaling and blockages in the circulating water system and the desulfurization tower; it also reduces the amount of desulfurization slag generated. The aerated compressed air supply is provided directly by a Roots blower and delivered to the sedimentation tank through the aeration pipeline. Compressed air passes through the circulating water in the form of small bubbles from the aeration pipeline and escapes to the surface. The oxygen consumption rate is 4.6 m3/min. 2.2.5 Automatic control system: The induced draft fan in this system is controlled via frequency conversion, with the control panel located in the boiler control room. The water pump is also controlled via frequency conversion. The pH automatic controller controls the dosing electric valve using a 4–20mA signal based on the sampled data. 2.3 Working Principle The spray foam tower uses tangential air inlet to cause the airflow to rotate and rise. One or two layers of spiral nozzle assemblies are arranged above the flue gas inlet; above these nozzle layers lies a layer of porous foam tray plates, on which spray nozzles for distributing water are installed. The entire tower is divided into two sections, upper and lower, or three sections, upper, middle, and lower (when 2 layers of trays are used); the lower part of the lower section consists of a circulating water tank and a liquid seal drainage tank. The flue gas emitted by the boiler enters the swirl section of the spray foam tower tangentially; the dust particles with larger sizes, due to centrifugal force, experience an adhesion effect to the wall and merge with the water curtain provided by the tower plates, flowing to the bottom of the tower where it is discharged. The flue gas continues to rise inside the tower; it is first washed and absorbed by 2 layers of atomization nozzles, thereby removing some of the fine particulate matter and SO2. As the gas rises further, it passes through 2 layers of foam trays, where porous plates covered with absorbent liquid create a foam layer with a large surface area for liquid films. The aerosols with large liquid film surfaces on these trays also facilitate the dispersion of particulate matter, thus further removing fine particles and SO2, and ultimately achieving high efficiency in dust and sulfur removal. Both washing and absorption rely on the gas-liquid two-phase liquid film interface; the larger the area of this liquid film, the higher the efficiency of dust and sulfur removal. The aerosols in the flue gas are purified; they rise slowly within the upper tower, pass through the connection pipe between the tower and the dehydrator, and enter a high-efficiency multi-stage dehydrator. After dehydration, they proceed through the flue duct to the exhaust fan and are then discharged through the chimney in compliance with regulations. After absorbing SO2 in the tower, the pH value of the alkaline circulating water drops rapidly. It is then discharged into the circulation sedimentation tank where it mixes with the alkaline wastewater from the boiler. Using a dosing device, MgO powder with a particle size of over 200 mesh is converted into an Mg(OH)2 emulsion. An electric valve for dosing is controlled by a pH automatic regulator in order to adjust the pH value in the tank, so that the pH of the liquid exiting the tower for washing is around 6.5. The circulating water entering the tank is aerated by blower air, causing the desulfurization products to be ultimately oxidized into water-soluble MgSO4. Its chemical reaction equation is: To prevent supersaturation of sulfates in the tank, it is necessary to discharge a portion of the circulating water, with this amount accounting for approximately 2% of the total circulating water volume. 3 Analysis of operational performance The monitoring results for this process are shown in Table 1. Table 1 Data on flue gas treatment results. As shown in Table 1, the SO2 emission concentration after treatment is 95.0 mg/m3, with a removal rate of 91.4% ； The emission concentration of dust was 27.0 mg/m3, with a removal rate of 98.7%. The \"Comprehensive Emission Standards for Pollutants from Boilers\" (DB11/139–2002), jointly issued by the Beijing Environmental Protection Bureau and the Beijing Quality and Technical Supervision Bureau, specify the pollutant emission limits for Zones A and B during Period II as follows: SO2 ≤ 150 mg/m3, and smoke dust ≤ 50 mg/m3. This shows that desulfurization and dust removal using a spray foam tower can achieve very satisfactory results, with the emission concentrations of dust and SO2 being well below the standard limits. The total investment in this process system is 1.58 million yuan, the annual operating cost of the system is 486,000 yuan, and the cost for removing SO2 is 0.93 yuan/kg. 4 Conclusions 4.1 The spray foam tower utilizes a multi-stage purification principle that combines centrifugation, spraying, and foaming. It integrates two-stage spray washing with two-stage foam layer desulfurization and dust removal, allowing their advantages to complement each other and enabling the full utilization of the functions of these two dust removal and desulfurization units. This approach ensures that dust removal is carried out in a manner that progresses from larger particle sizes to smaller ones, while desulfurization operates from moderate efficiency to high efficiency, thereby guaranteeing excellent operating conditions and performance for the desulfurization and dust removal device. 4.2 This process system features stable purification efficiency and reliable operation, with desulfurization and dust removal efficiencies reaching 91.4% and 98.7% respectively. 4.3 The spray foam tower features integrated dust and sulfur removal, low floor space requirements, and reduced investment costs, making it suitable for sulfur removal and dust suppression of flue gases from large and medium-sized industrial boilers. The first author is Li Xiaobing, male, born in 1976, and a postgraduate at China University of Mining and Technology.