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Water film dust collectors and venturi condensers to reduce SOx emissions from power plant boilers – Author: Huang Shaoe, Fujian Electric Power Research Institute 1 Introduction At present, China’s energy structure is dominated by coal; coal used in domestic coal-fired power plants accounts for over 30% of the total coal production in the country. A significant characteristic of China’s coal is that more than 75% of it is high-sulfur coal with a sulfur content of over 1%. In fact, over 20% of the coal used in power plants across the country is high-sulfur coal. The pollutants emitted by coal-fired plant boilers include sulfur oxides SOx (primarily SO2), nitrogen oxides NOx, and dust. The combustion of high-sulfur coal produces large amounts of SO2, which is an important component of air pollution and poses a threat to people’s production and daily life. With the continuous and rapid development of China’s national economy, the extensive exploitation and use of coal have led to severe environmental pollution. The clean utilization of coal has become an important issue that cannot be ignored on a global scale. **Strict controls must be imposed on the total amount of pollution emissions across the country; higher environmental standards will be required for the power industry, which is a sector that operates ahead of others, and it is imperative to strengthen the development and application of clean coal power generation technologies. To improve the ecological environment, countries around the world have established strict limits on SO2 and NOx emissions from coal-fired power plant boilers. The current emission standard for SO2 set in our country is 1200 mg/Nm3, which is significantly lower than the requirements in developed countries, and it also represents a low level compared to those in developing countries. The goal is to guide and encourage the existing coal-fired power plants, most of which can meet the standards through equipment upgrades. There are various boiler flue gas desulfurization processes. Depending on the process flow, they differ significantly in terms of equipment investment, operating costs, desulfurization efficiency, and applicable scale. The desulfurization efficiency of calcium injection inside the boiler in China is not high (it is around 50% when Ca/S=2–3.5) ； There are still some technical issues in industrial applications. Flue gas desulfurization, which involves the chemical absorption of SO2 from flue gas at the exhaust end, is widely used in developed countries. Some pilot projects have also been carried out in China, but due to the high cost of equipment and operating expenses, it is likely difficult to implement this technology on a large scale given China’s current economic conditions. Wet desulfurization using specialized equipment offers high efficiency and can reduce SO2 emissions to below 200 mg/Nm3; however, due to the large investment required, it is difficult to implement it on a wide scale in the near future. This article describes the cost-effective flue gas desulfurization methods that have been developed in recent years by the All-Russian Institute of Thermotechnology (BTИ) and PAO “EEEC POCIИI” in Russia, through the use of simple systems integrated with existing boiler processes and equipment, thereby enabling sulfur compound emissions to meet regulatory standards [1]. One of the flue gas desulfurization methods used in the province is to directly inject alkali solution using the widely applied water film dust collector, namely the Venturi coagulator. It is particularly suitable for power plants that switch to burning high-sulfur heavy oil seasonally. 2 Flue gas desulfurization system using a water film dust collector and venturi coagulator. This technology was tested on a TP-230-2 type coal-fired boiler at the Shalavatsk Thermal Power Plant, with a rated capacity of 230 t/h and a superheated steam pressure of Po=100 kgf/cm2 (9.80 MPa) ； The superheated steam temperature is t=510°C; four water film dust collectors equipped with vertical Venturi condensers are used, with a diameter of 800 mm at the throat of the venturi tubes ； The drip catcher has a diameter of 3000 mm, and stable process water with a pipeline water pressure of 4.0 kgf/cm2 (0.39 MPa) is used as the spray water for the dust collector. When the boiler is switched to burning high-sulfur heavy oil, the original water film dust collector is still used for dust removal. The original dust collector operated with \"open\" spray water; the reagent used, namely caustic soda (NaOH), was not changed nor reused. During testing, a 43% NaOH aqueous solution was directly fed into the spray water pipes through the nozzle mixing tank, without the need for a separate tank for preparing the spray alkaline solution, in order to minimize investment costs. The required amount of reactant is fed into the mixing tank using a H-1000/10 type metering pump with a volume of 4 m3; the pump’s flow rate is 1000 l/h and the pressure is 10 kgf/cm2 (0.98 MPa). A schematic diagram of the test setup is shown in Figure 1. Figure 1 Schematic diagram of the system for introducing alkaline solution into the spray water of the Venturi coagulator 1 – Venturi coagulator ； 2-Droplet catcher ； 3-Spray nozzle ； 4-Process water pipeline ； 5- Mixing tank for reactant and water ； 6- Pipelines for transporting reactants ； 7-Quantitative delivery pump ； 8 – Flow metering box 3 of the reactant; Desulfurization efficiency of the Venturi coagulator. Tests were conducted when heavy oil, with a sulfur content Sy=2.92%, accounted for 50% of the boiler fuel, and the oil flow rate was determined based on the boiler’s heat balance and the flow characteristics of the oil gun nozzles. The amount of alkaline solution with spray water injected is measured based on the change in liquid level per unit time within the reactor flow meter box. A portable gas analyzer of the “electrochemical” type manufactured by the German company “TESTO” ； The gas analyzer produced by the British company “QUINTO”, equipped with a rapid flue gas cooler, is used to analyze the composition of flue gas in the inlet and outlet ducts of dust collectors. When calculating the desulfurization efficiency, errors that affect the accuracy of such calculations—such as the infiltration of cold air into the flue gas and the dilution of the gas composition due to the evaporation of water in the venturi—are taken into account, and special collection and processing are carried out on the samples. The main test results are shown in Table 1 and Figure 1. Table 1 Main test results of flue gas desulfurization using alkaline solution spray in a Venturi condenser. When the boiler load varied between 165 t/h and 200 t/h, the flue gas velocity at the throat of the venturi was 57 m/s ； The spray intensity of the unit spray water is 0.07 kg/Nm3 to 0.11 kg/Nm3. The flue gas is cooled and its temperature drops as it passes through the water film dust collector; after passing through the venturi tube, the temperature drops by 50°C to 60°C. Thus, as the boiler load changes, the flue gas temperature decreases from 160°C to 180°C to 106°C to 125°C. According to the boiler heat balance calculations, the evaporation rate of water in the venturi is 6.2 t/h to 8.1 t/h, and the volume of the flue gases increases by a factor of 1.054 to 1.067 due to this water evaporation. When the share of heavy oil burned accounted for 50% of the total fuel volume, the SO2 concentration before flue gas desulfurization was between 1.76 g/m³ and 31.88 g/m³ (based on an excess air coefficient of α=1.4); it dropped to 1.0 g/m³ after passing through the venturi tube (again with α=1.4). During the experiments, the alkalinity of the spray water varied within the range of 39 milliequivalents per liter to 500 milliequivalents per liter. The flue gas desulfurization efficiency is calculated based on the changes in SO2 weight before and after desulfurization, taking into account the influence of cold air leaking in from the flue ducts along the path and the evaporation of water within the venturi. When using pure water obtained through conventional processes for spraying the Frasch tube, the flue gas desulfurization efficiency is only 3%–4%. When using weakly alkaline water with a alkalinity of 30 milliequivalents per liter, the desulfurization efficiency is generally 5.2%. As the alkalinity increases to 250 milliequivalents per liter or 500 milliequivalents per liter, the desulfurization efficiency rises significantly, reaching 36%–44%. Tests have shown that the water film in the droplet trap has no actual effect on flue gas desulfurization, as the interaction between the flue gas and the liquid water mainly occurs within the venturi tube; therefore, future tests will focus solely on the effect of liquid injection through the venturi tube on flue gas desulfurization. Analysis of the test results shows that the key factor in desulfurizing flue gas is the weight ratio of \"reactant/sulfur dioxide\" (NaOH/SO2), in kg/kg, within the venturi tube. The relationship curve between the desulfurization efficiency ηSO2 and NaOH/SO2, kg/kg for the experiment is shown in Figure 2. The relationship shown in Figure 2 is consistent with the test results obtained previously by AO “Bashkirenergo” in collaboration with BTİ, using the same reactant in an industrial experimental desulfurization (“wet”) unit equipped in the Novosteryitamsk thermal power plant. When NaOH/SO2 is less than 0.4, the flue gas desulfurization efficiency of the venturi tube is the same as that of a specially designed absorber. At high NaOH/SO2 levels, the desulfurization efficiency of the former is inferior to that of the latter. Slurry analysis confirmed that the flue gas desulfurization efficiency of the absorber reached 70%–87%. The desulfurization efficiency of the venturi is lower than that of the absorber because the smoke velocity at the throat of the venturi is extremely high, and the contact time between the flue gas and water is too short. Although this desulfurization technology is not highly efficient, it still has practical value in achieving a moderate level of desulfurization under industrial conditions, and it is effective in most cases. Especially for power plants that switch to burning high-sulfur heavy oil seasonally. It has advantages such as low investment costs, simple equipment that can be supplemented with standard devices, easy operation and maintenance, and strong versatility. The startup and shutdown time for the desulfurization system is only 15 to 20 minutes. Figure 2 Curve showing the relationship between desulfurization efficiency ηSO2 and the weight ratio of \"reactant/sulfur dioxide\" 1 – Injection of alkaline solution in a Venturi coagulator (Shalavatsk Thermal Power Plant) ； 2 – Operation of the membrane dust collector in the industrial experimental alkaline mist absorber (Novosterytsk Thermal Power Plant) in an “open” system mode for flue gas desulfurization can affect the quality of the wastewater discharged by the power plant. According to the chemical analysis of the mortar: the sulfur content in the mortar amounts to 83%, with sulfates being the main components. The sulfate concentration in the mortar from the test dust collector is nine times higher than the allowable discharge standards for wastewater. However, after mixing with other wastewater from the power plant, the sulfate concentration in the wastewater actually discharged by the plant into the ash pond increased by only 1.5 times, remaining below the specified limit. When operating as an \"open\" system, the venturi tube enables stable desulfurization; however, the fact that the alkaline solution used for spraying is not reused requires improvement from both environmental protection perspectives and economic viewpoints, as it leads to waste of expensive reactants such as caustic soda and soda ash. 4 Conclusion 4.1 The flue gas desulfurization technology based on a simple alkaline solution delivery system using provincial investment has shown, through experiments, that by injecting the alkaline solution at a weight ratio of \"reactant / sulfur dioxide\" (NaOH / SO2) of 0.5–0.6, the venturi tube can remove up to 40% of the SO2 in the flue gas. 4.2 Under the operating conditions of the existing water film dust collector (flue gas velocity at the venturi throat of 60 m/s to 70 m/s, and unit spray water intensity of 0.11 kg/m3), increasing (NaOH/SO2) to above 0.6 has little effect on improving desulfurization efficiency; instead, it reduces the utilization rate of the reactants. 4.3 Using a water film dust collector with a venturi coalescer (Venturi tube) for flue gas desulfurization results in low investment costs, simple operation and maintenance, and high applicability. Author profile: Huang Shaoe, a professor-level senior engineer at the Fujian Electric Power Testing Institute, and a senior member of the Chinese Society for Electrical Engineering