Condensable Particulate Matter Removal and Its Mechanism by Phase Change Technology During Wet Desulfurization Process

Abstract

Limestone-gypsum wet flue gas desulfurization (WFGD) played a key role in SOx removal and clean emissions. However, it would also affect the condensable particulate matter (CPM) removal and compositions. The effects of the WFGD system on the removal of CPM and the contents of soluble ions in CPM were investigated in a spray desulfurization tower at varied conditions. The results indicate that the emission concentration of CPM decreased from 7.5 mg/Nm³ to 3.7 mg/Nm³ following the introduction of cold water spray and hot alkali droplet spray systems. This resulted in a CPM reduction rate of approximately 51%, reducing the percentage of CPM in total particulate matter and solving the problem of substandard particulate matter emission concentrations in some coal-fired power plants. The concentrations of NO₃⁻, SO₄²⁻, and Cl⁻ among the soluble ions decreased by 41–66.6%. As the liquid-to-gas ratio of the cold water spray and hot alkali droplet spray increased, CPM came into contact with more spray, which accelerated dissolution and chemical reactions. Consequently, the CPM emission concentration decreased by 17.4–19%. The liquid-to-gas ratio has a great effect on the ion concentrations of NO₃⁻, SO₄²⁻, Cl⁻ and NH₄⁺, with a decrease of 28–66%. The temperatures of the cold water spray and the hot alkali droplet spray primarily affect the ionic concentrations of SO₄²⁻ and Ca²⁺, leading to a decrease of 32.3–51%. When the SO₂ concentration increased from 0 mg/Nm³ to 1500 mg/Nm³, large amounts of SO₂ reacted with the desulfurization slurry to form new CPM and its precursors, the CPM emission concentration increased by 57–68.4%. This study addresses the issue of high Concentration of CPM emissions from coal-fired power plants in a straightforward and efficient manner, which is significant for enhancing the air quality and reducing hazy weather conditions. Also, it provides a theoretical basis and technical foundation for the efficient removal of CPM from actual coal-fired flue gas.

1. Introduction

In coal-fired power plants, the emitted particulate matter (TPM) includes both condensable particulate matter (CPM) and filterable particulate matter (FPM) [1,2,3]. CPM refers to substances that are in a gaseous state in the flue environment but form solid or liquid particles through condensation or reaction immediately upon cooling and dilution. Once CPM enters the atmosphere, it rapidly condenses into a large number of submicron particles, becoming precursors of secondary aerosols in ambient air and is considered one of the main causes of haze. In contrast, FPM means that it exists in the form of particulate matter under flue gas conditions, and that it itself is solid particulate matter, which can be directly intercepted by the filter membrane. FPM exists in the flue environment in the form of liquid or solid particles [4,5]. In recent years, with the implementation of ultra-low emissions in China’s coal-fired power plants, the proportion and emission levels of CPM in TPM have surpassed those of FPM [6], leading to increasingly prominent environmental issues. In recent years, Chinses environmental protection department has introduced policies to strengthen the detection technology and emission control of CPM in coal-fired power plants. Despite this, research on CPM in China is still in its initial stage, focusing mainly on the emission characteristics of CPM. There is a lack of profound understanding of the migration and transformation laws of CPM in existing pollution control equipment, which makes it difficult to provide a theoretical basis for using existing pollution control equipment to promote the synergistic removal of CPM.

Currently, the ultra-low emission dust removal retrofitting in domestic coal-fired power plants commonly employs technologies such as low-low temperature electrostatic precipitators [7], desulfurization synergistic high-efficiency dust removal technology [8], and wet electrostatic precipitators [9]. While these technologies achieve efficient removal of FPM, they also have a certain degree of synergistic removal of CPM [10]. However, due to the small particle size of CPM (generally less than 1 μm [11]) and its complex composition, it is challenging to capture [12], resulting in less than satisfactory removal efficiency of CPM by the aforementioned equipment and technologies. However, Wet Flue Gas Desulfurization (WFGD) systems are effective in controlling sulfur dioxide (SO₂) emission concentrations and also improve the removal of TPM to a certain extent [13,14,15,16]. Studies have shown that the efficiency of CPM removal by WFGD systems ranges from 20% to 69% [17], indicating a significant potential for CPM removal by WFGD systems [18]. The environment within WFGD systems is complex and variable, with high-temperature flue gas coming into counterflow contact with low-temperature desulfurization slurry, resulting in a substantial drop in flue gas temperature, which can quickly decrease from 85–130 °C to 40–50 °C [19]. According to reports [20], the formation of CPM is closely related to the rate of flue gas temperature decrease. Rapid cooling of the flue gas leads to the generation of a large number of small-sized CPM particles, making them difficult to capture. Additionally, the WFGD system may not achieve a supersaturated water vapor environment, which hinders the agglomeration and growth of CPM, thereby affecting its removal efficiency. This limitation affects the WFGD system’s ability to synergistically remove CPM.

If a supersaturated water vapor environment can be constructed within the WFGD system and the number of condensation nuclei in the environment is increased, it can significantly improve the removal efficiency of CPM. Wu [21] constructed a supersaturated water vapor environment by reducing the flue gas temperature entering the WFGD system and increasing the humidity. This caused the water vapor to condense on the surface of fine particulate matter, forming larger dust-containing droplets that were more easily removed. This is due to the fact that when the flue gas is cooled, gaseous CPM first forms extremely small droplet nuclei through homogeneous condensation. These primary droplet nuclei grow further through molecular aggregation and condensation, eventually transitioning into particulate CPM. Then the supersaturated water vapor environment causes the Water vapor to treat granular state CPM as condensation nuclei for condensation and growth, transforming them into larger dust-containing droplets, which are subsequently more easily captured and removed by the desulfurization and demisting systems. Wang [22] increased the removal efficiency of fine particulate matter by approximately 20% by inserting a fluoroplastics capillary condensing heat exchanger between the two-stage towers, creating a supersaturated water vapor environment. Cui [23] injected Ca(OH)₂ during the wet flue gas condensation process as both condensation nuclei and absorbent. The supersaturated water vapor created a Stefan flow on the surface of Ca(OH)₂ particles, and the slight solubility of Ca(OH)₂ promoted the proximity of some acidic components to Ca(OH)₂, thereby enhancing desulfurization efficiency. Mr. Zhang’s study [24] showed that non-homogeneous condensation can effectively promote the removal of sulfur trioxide aerosols, which can increase the removal efficiency of sulfur trioxide aerosols by 14.3%. Under non-homogeneous condensation, the size of sulfur trioxide aerosols increased to higher values. In addition, lower flue gas temperatures, favorable removal of sulfur trioxide aerosol.

In this study, hot alkali droplets (supernatant of calcium hydroxide) were sprayed at the bottom of the desulfurization tower to evaporate and precipitate calcium hydroxide particles, thus reacting with some of the acidic CPM. Adding cold water spray between the desulfurization slurry spray layers can reduce the flue gas temperature and increase the humidity, thus establishing a supersaturated water vapor field, which makes the CPM particle size larger through the non-homogeneous condensation reaction. In addition, the effects of liquid-to-gas ratio, temperature of the spray on the CPM emission concentration were investigated. The changes of soluble ion concentration in CPM were measured. Finally, the principle of the method of graded spray to promote the synergistic and efficient CPM removal by WFGD was proposed, and the key control parameters were further clarified, so as to provide a theoretical basis and technical foundation for realizing the emission control of CPM.

2. Materials and Methods

2.1. Experimental System

The experimental system, as shown in Figure 1, primarily consists of a heater, a desulfurization tower, a spray system, and a test control system. The simulated flue gas, which is formed by fully mixing SO₂ with air, is heated by the heater before it enters the desulfurization tower. Constructed from acrylic panels and polytetrafluoroethylene (PTFE) pipes known for their excellent heat resistance, the desulfurization tower is equipped with a three-stage spray system. It has a tower diameter of 160 mm and a height of 1400 mm. At the top of the tower, a high-efficiency demister is installed. Inside the desulfurization tower, there are cold water spray layers and a hot alkali droplet spray layer. The CPM emission concentration at the outlet of the desulfurization tower is measured using the EPA Method 202 from the United States.

During the experiment, the basic operating parameters of the desulfurization process are shown in

Table 1. Basic operating parameters during desulfurization.

Project Title	Typical Operating Conditions
liquid-to-gas ratio	20 L/m3
Flue gas flow rate	18 Nm3/h
Inlet flue gas temperature	120 °C
Desulfurization slurry concentration	15%
SO2 concentration	1500 mg/Nm3
Desulfurization slurry pH value	5.5
Desulfurization slurry temperature	45 °C

. The liquid-to-gas ratio for desulfurization is 20 L/m³, the flue gas flow rate is 18 Nm³/h, the inlet flue gas temperature is 120 °C, the desulfurization slurry temperature is 45 °C, the pH value of the desulfurization slurry is 5.5, the concentration of the desulfurization slurry is 15%, and the SO₂ concentration is 1500 mg/Nm³. The desulfurization slurry is a circulating slurry from an actual coal-fired power plant, with the pH value stabilized at 5.5 by continuously adding CaCO₃ solution to the slurry tank. The simulated flue gas in the experiment is a mixture of SO₂ and clean air, the hot alkali droplet spray is calcium hydroxide supernatant, and the cold water spray is industrial water, all of which have adjustable temperatures. The standard working conditions of the spray system are as follows: the hot alkali droplet spray temperature is 75 °C, liquid-to-gas ratio is 0.08 L/Nm³; cold water spray temperature is 15 °C, liquid-to-gas ratio is 0.08 L/Nm³. Simulated flue gas after the heater heated to 120 °C into the mixing room fully mixed into the bottom of the desulfurization tower, desulfurization slurry from the slurry tank through the inlet pump sprayed into the tower, hot alkali droplets and cold water droplets in the reservoir through the metering pump sprayed into the tower. The liquid-to-gas ratio for the cold water spray and hot alkali droplet spray in the experiment is set from 0.05 L/Nm³ to 0.11 L/Nm³, with the optimal ratio being 0.08 L/Nm³.

2.2. Measurement Method

To measure the concentration of FPM and CPM emissions, the U.S. EPA Method 202 sampling method was used [25,26,27]. Figure 2 shows a schematic diagram of the FPM/CPM sampling device. The FPM/CPM sampling device mainly consists of a FPM filter, a coil condenser, two empty impingers, a CPM filter, and two moisture trap impingers. The flue gas is heated to 120 °C in the heating chamber before entering the FPM filtration device. It then passes through the coil condenser and empty impingers, where a portion of the CPM is captured. The remaining CPM is collected by the downstream CPM filter. After sampling, the system is immediately purged with high-purity nitrogen at a flow rate of 10 L/min for 1 h to minimize the impact of water-soluble gases on the experimental results. The empty impingers, condenser, and connecting pipelines are then rinsed with deionized water and acetone-hexane. The rinsing liquid is transferred to clean evaporating dishes, and the total mass of the organic and inorganic components in the CPM is determined by drying and weighing the dishes. The difference between the weight of the empty dish after drying and the weight of the dish containing the dried rinsing liquid represents the mass of CPM in the rinsing liquid.

The FPM and CPM filters are placed in an oven at 105 °C for two hours, and the difference in filter weights before and after drying is used to determine the mass of CPM and FPM on the filters. The sum of the CPM in the rinsing liquid and the CPM on the filters represents the total mass of CPM. The emission concentrations of FPM and CPM are calculated using Equations (1) and (2), respectively.

$${\mathrm{C}}_{\mathrm{F}\mathrm{P}\mathrm{M}}={\displaystyle \frac{\mathrm{m}-{\mathrm{m}}_1}{\mathrm{V}}}$$

(1)

$${\mathrm{C}}_{\mathrm{C}\mathrm{P}\mathrm{M}}={\displaystyle \frac{{\mathrm{m}}_2+{\mathrm{m}}_3+{\mathrm{m}}_5-{\mathrm{m}}_4}{\mathrm{V}}}$$

(2)

In the formulas, ${\mathrm{C}}_{\mathrm{F}\mathrm{P}\mathrm{M}}$ (mg/m³) represents the emission concentration of FPM under standard conditions, $\mathrm{m}$ (mg) represents the mass of the FPM filter after sampling, ${\mathrm{m}}_1$ (mg) represents the mass of the FPM filter before sampling, and ${\mathrm{C}}_{\mathrm{C}\mathrm{P}\mathrm{M}}$ (mg/m³) represents the volume of the flue gas sampled under standard conditions. $\mathrm{V}$ (m³) represents the volume of dry harvested gas in standard condition. ${\mathrm{m}}_2$ (mg) represents the mass of the organic components of CPM after sampling, ${\mathrm{m}}_3$ (mg) represents the mass of the inorganic components of CPM after sampling, ${\mathrm{m}}_4$ (mg) represents the mass of the clean evaporating dish before sampling, and ${\mathrm{m}}_5$ (mg) represents the increase in mass of the CPM filter.

To measure the concentration of soluble ions in CPM, an ion chromatograph ICS-600 was used to measure the concentration of SO₄²⁻, NO₃⁻, Cl⁻ and F⁻ in CPM. A Plasma 3000 ICP spectrometer was used to measure the concentrations of Ca²⁺, K⁺, Na⁺ and Mg²⁺ in CPM. A UV spectrophotometer was used to measure the concentration of NH₄⁺ in CPM. Data were recorded by the instrument and tables were generated.

3. Results and Discussion

3.1. CPM Emission Characteristics

Figure 3 compares the emission characteristics and removal performance of the CPM formed by the flue gas after passing through the WFGD system and the flue gas after passing through the WFGD system with the addition of cold water spray and hot alkali droplet spray. The basic operating parameters in the experiment are shown in Section 2.1.

As can be seen from the figure, after adding cold water spray and hot alkali droplet spray, the concentration of CPM emission is greatly reduced, which can be reduced from 7.5 mg/Nm³ to 3.7 mg/Nm³, with a reduction rate of about 51%. On the one hand, high-temperature alkali droplets precipitate Ca(OH)₂ particles, inducing CPM to treat precipitated calcium hydroxide particles as condensation nuclei through non-homogeneous condensation to the granular state. At the same time, high-temperature alkali droplet evaporation reduces the rate of flue gas temperature drop, which lead to a larger particle size of the granular CPM formed by homogeneous condensation. In addition, the contact between the high-temperature hot alkali droplets and the desulfurization slurry increases the partial pressure of the surrounding water vapor, which can inhibit the evaporation of the desulfurization slurry and reduce the re-release of CPM. On the other hand, the low-temperature cold water spray can cool and humidify the flue gas, promoting the establishment of a supersaturated water vapor environment. It causes water vapor to treat granular CPM as a condensation nucleus, leading to condensation and growth, thereby transforming it into large-size dusty liquid droplets, which is easier to be removed by the subsequent desulfurization and mist elimination system. At the same time, the low-temperature cold water spray cools and humidifies the flue gas, inhibiting the evaporation of the desulfurization slurry. This action ensures that the slurry is converted from the liquid phase to the gas phase at a reduced rate, thereby preventing the release of CPM and its precursors in the desulfurization slurry.

Figure 3b shows the effect of cold water spray and hot alkali droplet spray on the concentration of soluble ions in CPM. In general [28], the SO₄²⁻ in the desulfurization slurry accounted for 60%, Ca²⁺ for 26%, Cl⁻ for 7% and Mg²⁺ for 3%. The main components of limestone-gypsum flue gas desulphurization are SO₄²⁻ and Ca²⁺, Cl⁻ and Mg²⁺ are the main impurities in gypsum [29]. The soluble ions in the CPM formed by adding the cold water spray and the hot alkali droplet spray are mainly SO₄²⁻, Ca²⁺, Na⁺, and Cl⁻, which are found to be similar to the CPM ions formed by the flue gas after WFGD. Given the similar composition, it can be inferred that most of the CPM is a result of the evaporation of the desulfurization slurry. As shown in Figure 3b, the concentrations of Ca²⁺, SO₄²⁻, Mg²⁺, NH₄⁺, and Cl⁻ were substantially reduced by adding hot alkali droplet spray and cold water spray, with decreasing rates of 33.3%, 32%, 54.5%, 76.9%, and 36%, respectively. This may be mainly due to the fact that the hot alkali droplet spray and cold water spray reduced the evaporation rate of the desulfurization slurry droplets, resulting in a decrease in the concentration of Ca²⁺ and SO₄²⁻ in the slurry droplets. Because NH₄⁺ and Cl⁻ are extremely soluble components, most of them are dissolved and absorbed by the sprayed-in cold water [30].

3.2. Influence of Operating Parameters on CPM Emissions

The emission characteristics of CPM during WFGD may be related to the cooling and humidification rate of the flue gas [31]. At the same time, the number of Ca(OH)₂ particles precipitated by evaporation may affect the reaction with the acidic CPM components to a certain extent [32]. In addition, the variation of SO₂ concentration may also greatly affect the CPM emission concentration [33]. Therefore, we controlled the rate of flue gas cooling and humidification by varying the temperature and the liquid-to-gas ratio of the cold water spray. The amount of precipitated calcium hydroxide particles was controlled by varying the liquid-to-gas ratio of the hot alkali droplet spray.

3.2.1. Effect of the Liquid-to-Gas Ratio of Hot Alkali Droplet Spray

Figure 4 shows the effect of hot alkali droplet spray liquid-to-gas ratio on the emission characteristics and removal performance of CPM. In the experiment, the temperature of hot alkali droplet spray was set at 75 °C, the liquid-to-gas ratio of hot alkali droplet spray was from 0.05 L/m³ to 0.11 L/m³, the cold water spray was turned off, and the SO₂ concentration was 1500 mg/m³.

Figure 4a shows the effect of hot alkali droplet spray liquid-to-gas ratio on CPM emission concentration. When the hot alkali droplet spray liquid-to-gas ratio increased from 0.05 L/m³ to 0.11 L/m³, the emission concentration of CPM gradually decreased from 4.2 mg/m³ to 3.4 mg/m³, with a decrease rate of about 19%. This is due to the fact that the flue gas cooling rate is reduced by the hot alkali droplet spray, resulting in a larger particle size of CPM formed by homogeneous condensation [34]. Secondly, the hot alkali droplet evaporates and precipitates Ca(OH)₂ particles in the tower, which induces the CPM to transform into the granular state by non-homogeneous condensation with the precipitated fine particles as the nuclei, and then it is more easily to be captured by the slurry. On the other hand, the hot alkali droplet also induces the acid-biased CPM to be dissolved and absorbed on its surface while reacting with SO₂. When the liquid-to-gas ratio of the hot alkali droplet spray is increased, the ability to reduce the flue gas cooling rate is enhanced, and the number of precipitated Ca(OH)₂ particles is increased. As a result, more CPM is transformed into the granular state through non-homogeneous condensation reactions with Ca(OH)₂ particles. At the same time, the Ca(OH)₂ particles increase in number, prompting more acidic CPM components to react with them and thus be removed by dissolution [35]. Zhou [36] constructed a SO₃/sulfuric acid mist mass transfer model for the WFGD desulfurization process and concluded that an increase in the spray flow rate promoted SO₃ removal during the spraying process in the desulfurization tower. In contrast to this paper, increasing the liquid to gas ratio of the hot lye spray also contributes to CPM removal.

Figure 4b shows the effect of liquid-to-gas ratio of hot alkali droplet spray on soluble ion concentration in CPM. When the liquid-to-gas ratio increased from 0.05 L/m³ to 0.11 L/m³, the decrease rates of NO₃⁻, SO₄²⁻ and Cl⁻ concentrations were 66.6%, 46.5% and 41%, respectively. These ions were all derived from acidic substances with certain acidity. With the increase of the liquid-to-gas ratio of the hot alkali droplet spray, a large number of Ca(OH)₂ particles precipitated, which prompted their reaction with the acidic. The concentration of Ca²⁺ increased slightly, which may be due to the presence of Ca²⁺ in the Ca(OH)₂ solution itself, and the release of Ca²⁺ into the tower through the evaporation of the hot alkali droplet spray.

3.2.2. Effect of the Temperature of Hot Alkali Droplet Spray

Figure 5 shows the effect of hot alkali droplet spray temperature on the emission characteristics and removal performance of CPM. In the experiment, the liquid-to-gas ratio of hot alkali droplet spray was set to be 0.08 L/m³, the temperature of hot alkali droplet spray was from 60 °C to 90 °C, the cold water spray was turned off, and the SO₂ concentration was 1500 mg/m³.

Figure 5a shows the effect of hot alkalidroplet spray temperature on CPM emission concentration. As shown in the Figure, when the hot alkali droplet spray temperature increased from 60 °C to 90 °C, the CPM emission concentration decreased from 4.6 mg/m³ to 3.2 mg/m³ with a decrease rate of about 30%. This is due to the fact that the competitive evaporation of high-temperature alkali droplets increases the partial pressure of water vapor near the desulfurization. According to the principle of saturation vapor pressure [37], when the partial pressure of water vapor in the desulfurization tower is close to or reaches the saturation vapor pressure of the slurry, the rate of evaporation of the slurry will decrease significantly, preventing the re-release of CPM in the slurry. When the temperature of the hot alkali droplet rises, it rapidly evaporates into water vapor upon contact with the flue gas. This process increases the overall water vapor partial pressure in the tower, which in turn inhibits the evaporation of the desulfurization slurry to a greater extent. Consequently, this leads to an improvement in the CPM removal efficiency. Figure 5b shows the effect of hot alkali droplet spray temperature on the concentration of soluble ions in CPM. As can be seen from the figure, when the hot alkali droplet spray temperature increased to 90 °C, the decrease rates of SO₄²⁻ and Ca²⁺ concentrations were 51% and 43.3%, respectively. This indicates that the high-temperature hot alkali droplet spray reduces the partial pressure of water vapor in the surrounding environment to a greater extent, thereby enhancing the ability to inhibit the evaporation of the desulfurization slurry. It also slows down the rate of the slurry from the liquid state into the gaseous state, which prevents the re-release of the CPM. Additionally, since the part of the Cl⁻ originated from the HCl in the desulfurization slurry [38], the above reasons also lead to a substantial decrease in the concentration of Cl⁻.

3.3. Effect of Cold Water Spray on CPM Emission Characteristics

3.3.1. Effect of the Liquid-to-Gas Ratio of Cold Water Spray

Figure 6 shows the emission characteristics and removal performance of CPM at different cold water spray liquid-to-gas ratios. In the experiment, the cold water spray liquid-to-gas ratio was from 0.05 to 0.11 L/m³, the cold water spray temperature was 15 °C. The hot alkali droplet spray was turned off, and the SO₂ concentration was 1500 mg/Nm³.

Figure 6a shows the effect of cold water spray liquid-to-gas ratio on CPM emission concentration. As can be seen from the figure, with the increase of cold water spray liquid-to-gas ratio, CPM emission concentration was gradually reduced. When the liquid-to-gas ratio rised from 0.05 L/m³ to 0.11 L/m³, the CPM emission concentration was reduced from 4.6 mg/Nm³ to 3.8 mg/Nm³, and the CPM removal rate was about 17.4%. The cold water spray in the desulfurization tower cools and humidifies the flue gas, establishing a supersaturated water vapor environment. Water vapor treats granular state CPM as condensation nuclei, leading to condensation and growth, so that the granular state CPM can transform into large particle size dust-containing droplets and then more easily removed by the subsequent desulfurization mist removal device. Secondly, cold water spray has a dissolving effect on some soluble CPM such as NH₄⁺ and Cl⁻. Since the temperature of the cold water spray is lower than the temperatures of both the desulfurization slurry and the flue gas in the tower, it has a cooling effect on the surrounding flue gas environment and reduces the evaporation rate of the desulfurization slurry. As the cold water spray liquid-to-gas ratio increases, the water vapor content in the flue gas rises, leading to more water vapor being agglomerated onto the CPM. This agglomeration promotes the condensation and growth of CPM particles. The cold water spray provides deep cooling and humidification to the surrounding flue gas environment, significantly reducing the evaporation rate of the desulfurization slurry. This effect enhances the ability to dissolve hydrophilic CPM components, thereby improving the CPM removal efficiency. Wang [39] cooled the flue gas through a fluoroplastic capillary condensing heat exchanger, which established a supersaturated water vapor environment, resulting in a decrease in CPM concentration of about 25%. Unlike Wang, I reduced the flue gas temperature by adding a cold water spray to achieve a supersaturated water vapor environment for heterogeneous water vapor condensation. The dissolving effect of the fine spray can also be utilized to remove some of the hydrophilic CPM.

Figure 6b shows the effect of different cold water spray liquid-to-gas ratios on the concentration of soluble ions in CPM, which shows that when the cold water spray liquid-to-gas ratio increased from 0.05 L/m³ to 0.11 L/m³, the concentrations of Ca²⁺, SO₄²⁻, NH₄⁺ and Mg²⁺ in CPM were substantially reduced, with a decrease rate of 42.8%, 30.8%, 30.3% and 28.8%, respectively. Since the desulfurization slurry originated from the actual power plant, NH₄⁺ may originate from the unreacted NH₃ in the SCR process. Those substances were carried over when it entered the WFGD system and subsequently mixed with the desulfurization slurry. The NH₄⁺ is extremely soluble in water, and when the liquid-to-gas ratio of the cold water spray is increased, the NH₄⁺ is further dissolved and absorbed. Additionally, an increased liquid-to-gas ratio also lowers the ambient temperature more effectively, suppressing the evaporation of the desulfurization slurry to a greater extent. This suppression leads to a substantial reduction in the concentrations of Ca²⁺ and SO₄²⁻. Furthermore, a portion of the Cl⁻ comes from HCl, while another portion may originate from the cold water spray. Given that the spray water itself contains Cl⁻, as the liquid-to-gas ratio increases, more fine droplets are introduced into the desulfurization tower. The evaporation of these sprayed droplets results in the release of Cl⁻, which in turn causes a slight increase in their concentration.

3.3.2. Effect of the Temperature of Cold Water Spray

Figure 7 illustrates the impact of cold water spray temperature on the emission characteristics and removal efficiency of CPM. During the experiment, the liquid-to-gas ratio of the cold water spray was maintained at 0.08 L/m³. The temperature of the cold water spray was varied from 5 °C to 25 °C, the hot alkali droplet spray was turned off, and the SO₂ concentration was held constant at 1500 mg/m³.

Figure 7a demonstrates the impact of cold water spray temperature on CPM emission concentration. As the temperature of the cold water spray was gradually lowered from 25 °C to 5 °C, the CPM emission concentration decreased from 4.9 mg/m³ to 4.2 mg/m³, representing a reduction of approximately 14.3%. This reduction is attributed to the cooling effect of the cold water spray on the surrounding environment. As the temperature decreases, the cooling effect becomes more pronounced, which inhibits the evaporation of the desulfurization slurry. This inhibition slows the rate at which the slurry transitions from the liquid phase to the gas phase, thereby enhancing the CPM removal efficiency. Figure 7b illustrates the impact of cold water spray temperature on the concentration of soluble ions in CPM. The data indicates that as the temperature of the cold water spray decreases, the evaporation of the desulfurization slurry is significantly suppressed. This suppression is evident in the reduced rates of SO₄²⁻ and Ca²⁺ concentrations, which were 32.3% and 39.3%, respectively. Furthermore, the concentrations of Cl⁻ and Na⁺ also decreased, by 36% and 53.2%, respectively. This reduction may be attributed to the fact that some of the Cl⁻ and Na⁺ originate from the spray water itself. At lower temperatures, the evaporation rate of the spray water is reduced, and most of these ions are carried into the desulfurization slurry without being released into the atmosphere due to evaporation [40]. The other part of Cl⁻ comes from HCl in the desulfurization slurry. The cold water spray effectively suppresses this evaporation, thereby reducing the concentration of Cl⁻.

3.4. Effect of SO₂ on CPM Emission Characteristics

Figure 8 shows the effect of SO₂ concentration on the emission characteristics and removal performance of CPM, with SO₂ concentrations ranging from 0 mg/m³ to 1500 mg/m³ in the experiments, with the cold water spray and hot alkali droplet spray turned off.

Figure 8a shows the effect of SO₂ concentration on CPM emission concentration. When the SO₂ concentration was reduced from 1500 mg/m³ to 0 mg/m³, the emission concentration of CPM was reduced from 7.6 mg/m³ to 2.4 mg/m³, with a decrease rate of about 68.4%. This occurs because, as the concentration of SO₂ in the flue gas increases, two main processes take place. Firstly, a significant amount of CPM is generated as SO₂ reacts with the desulfurization slurry. Secondly, it may also result from the re-release of residual NH₄⁺ within the slurry, which react with SO₂ to form new CPM and its precursors. Figure 8b shows the effect of SO₂ concentration on the concentration of soluble ions in CPM. As shown in the Figure, with the decrease of SO₂ concentration, the decrease rates of SO₄²⁻ and Ca²⁺ concentration were 92.3% and 86%, respectively. On the one hand, during the desulfurization process, SO₂ was firstly absorbed by alkaline substances in the slurry, and then converted to SO₄²⁻ through oxidation reaction. Secondly, the limestone in the slurry produces CaSO₄ when reacting with SO₂, which not only changes the concentration of SO₄²⁻, but also increases the concentration of Ca²⁺. The decrease rate of NH₄⁺ was 84.6%, and the NH₄⁺ may be originated from the residual unreacted NH₃ left in the desulfurization slurry, which makes the concentration of NH₄⁺ increases while the desulfurization slurry reacts chemically with SO₂.

Furthermore, the experiment also examined the impact of cold water spray and hot alkali droplet spray on CPM emission concentration under varying SO₂ concentrations. Figure 9a,b show the effect of SO₂ concentration on the CPM emission characteristics and removal performance after adding cold water spray. In the experiments, the temperature of the cold water spray was set to 15 °C, the liquid-to-gas ratio of the cold water spray was 0.08 L/m³, and the hot alkali droplet spray was turned off, the concentration of SO₂ was varied from 0 mg/m³ to 1500 mg/m³. As shown in Figure 9a, when the concentration of SO₂ was reduced from 1500 mg/m³ to 0 mg/m³, the emission concentration of the CPM was reduced from 4.3 mg/m³ to 1.5 mg/m³, and the reduction rate was about 65%. Figure 9b further indicates that the concentrations of SO₄²⁻, Ca²⁺, and NH₄⁺ experienced the most significant decreases, with reduction rates of 76%, 64%, and 69.9%, respectively. Figure 9c,d show the effect of SO₂ concentration on the emission characteristics and removal performance of CPM after adding hot alkali droplet spray. In the experiments, the hot alkali droplet spray temperature was set to 75 °C, the hot alkali droplet spray liquid-to-gas ratio was 0.08 L/m³, and the cold water spray was turned off, the concentration of SO₂ was varied from 0 mg/m³ to 1500 mg/m³. As shown in Figure 9c, when the concentration of SO₂ was reduced from 1500 mg/m³ to 0 mg/m³, the emission concentration of CPM was reduced from 3.8 mg/m³ to 1.6 mg/m³, and the reduction rate was about 57%. Figure 9d shows that the concentration of SO₄²⁻, Ca²⁺ and NH₄⁺ decreased by 89%, 76% and 70.5%, respectively, which are the three ions with the highest decrease rates. All these data confirm that as the SO₂ concentration increases, the desulfurization slurry reacts with SO₂ to produce a significant amount of CPM [41]. This reaction substantially raises the concentration of soluble ions in CPM [42], with the most notable increases observed in SO₄²⁻, Ca²⁺, and NH₄⁺.

3.5. Key Mechanism of Condensable Particulate Matter Removal

Figure 10 presents a key mechanism diagram of condensable particulate matter removal in the WFGD system. Firstly, the hot alkali droplet spray at the bottom of the tower precipitates calcium hydroxide particles, which induces the CPM to treat the precipitated calcium oxide particles as nuclei to be converted to granular CPM by non-homogeneous condensation reaction. And some of the acidic CPM reacts chemically with calcium hydroxide particles to be removed [43]. Subsequently, the cold water spray causes the flue gas temperature to decrease and the humidity to increase, thus causing the flue gas environment to reach a supersaturated state [44]. The water vapor is induced to treat the granular CPM as condensation nucleus to further condense and grow up, transforming it into dusty droplets with large particle sizes, which are easier to be captured by the collision of the desulfurization slurry [45].

4. Conclusions

In this work [29], the effects of the WFGD system on the removal of CPM and the contents of soluble ions in CPM were investigated in a spray desulfurization tower at varied conditions. The following conclusions were obtained:

(1) Hot alkali droplet spray and cold water spray significantly reduced the CPM emission concentration from 7.5 mg/Nm³ to 3.7 mg/Nm³. The decrease rates of Ca²⁺, SO₄²⁻, Mg²⁺, NH₄⁺ and Cl⁻ concentrations were 33.3%, 32%, 54.5%, 76.9%, and 36% respectively. When hot alkali droplet was sprayed, they precipitated Ca(OH)₂ particles, which induced the CPM to treat the precipitated calcium oxide particles as nuclei to be converted to granular CPM by non-homogeneous condensation reaction. Cold water cooled down and humidify the flue gas to establish a supersaturated water vapor environment, and induced the water vapor to treat granular state CPM as condensation nuclei for condensation and growth.

(2) When the liquid-to-gas ratio of hot alkali droplet was increased, more Ca(OH)₂ particles were evaporated, leading to the dissolution of acidic CPM on the surface, which reduced the NO₃⁻, SO₄²⁻, and Cl⁻ ion concentrations by 41–66.6%. When the temperature increased, the evaporation of the desulfurization slurry reduced, decreasing the Ca²⁺, SO₄²⁻ concentration by 43.3–51%. As the liquid-to-gas ratio of the cold water spray increased, NH₄⁺ were dissolved and absorbed by the cold water, resulting in a 30.3% decrease of concentration. In addition, lowering the temperature of the cold water spray can effectively inhibit the evaporation of the desulfurization slurry and reduce the concentrations of Ca²⁺ and SO₄²⁻ by 42.8% and 30.8%, respectively.

(3) As the SO₂ concentration increased from 0 mg/Nm³ to 1500 mg/Nm³, SO₂ reacted with the desulfurization slurry, leading to the generation of a large amount of CPM. During this process, the concentrations of all soluble ions increased significantly, with increase rates ranging from 64% to 92.3%. Correspondingly, the CPM emission concentration rose from 57% to 68.4%.

The experimental results show that this method can largely reduce the emission concentration of CPM, enabling the coal-fired power plant to meet the national emission standards and helping to realize the ultra-low emission target. In addition, the WFGD system improves the efficiency of desulfurization while making its ability to synergistically remove pollutants, especially fine particulate matter, improved. This means for environmental policy that multiple pollutants can be controlled more comprehensively to protect air quality. It also inspires more scholars to be able to utilize the ability of synergistic removal of WFGD systems, which makes the technology in the field of pollutant removal go further.

Future research directions are given for the limitations of this study: (1) This paper only investigated the synergistic removal of CPM by the WFGD system, and it can be in exploring the removal of other pollutants (such as heavy metals and other substances) by the WFGD system. (2) This paper only investigated the formation mechanism of CPM in the WFGD system, and it can be focused on exploring the formation process of CPM under different combustion conditions in the future, including the effects of coal type, combustion temperature, and oxygen concentration on the generation of CPM.