Enhancing SO₃ and Fine Particle Co-Removal in Low-Low Temperature Electrostatic Precipitation via Turbulent Agglomeration

Abstract

Fine particulate matter (PM) and sulfur trioxide (SO₃) from coal-fired flue gas pose significant environmental and health risks. While low-low temperature electrostatic precipitators (LLT-ESPs) enhance PM and SO₃ removal by cooling flue gas below the acid dew point, their efficiency is limited by incomplete agglomeration. This study proposes integrating turbulent agglomeration technology into LLT-ESP systems to improve collision and adhesion between droplets and particles. Experiments were conducted under three conditions: flue gas containing SO₃ alone, fly ash alone, and their mixture. Particle size distributions, mass concentrations, and removal efficiencies were analyzed using ELPI⁺ and PM samplers. Results showed that turbulent agglomeration reduced the number concentration of sulfuric acid droplets by 21.4% from 1.59 × 10⁷ cm⁻³ to 1.25 × 10⁷ cm⁻³ (SO₃-only case) and fine fly ash particles by 19.5% from 5.79 × 10⁶ cm⁻³ to 4.66 × 10⁶ cm⁻³ (fly-ash-only case). Although LLT-ESP combined with turbulent agglomeration has a certain removal effect in the case of individual pollutants, the overall effect is not unsatisfactory, especially for SO₃, whose mass-based removal efficiency was merely 16.2%. The value of the fly-ash-only case was 92.1%. Synergistic effects in the coexistence scenario (fly ash and SO₃) significantly enhanced agglomeration, increasing SO₃ and PM removal efficiencies to 82.9% and 97.6%, respectively, compared to 69.7% and 90.1% without turbulent agglomeration. The mechanism behind the efficiency improvement involved droplet–particle collisions, sulfate deposition, and improved particle charging. This work demonstrates that turbulent agglomeration optimizes multi-pollutant control in LLT-ESP systems, offering a feasible strategy for achieving ultra-low emissions in coal-fired power plants.

1. Introduction

Fine particulate matter (PM) and sulfur trioxide (SO₃) are two pollutants emitted from coal-fired power plants. Fine PM, typically with an aerodynamic diameter of less than 2.5 μm (PM_2.5), originates from incomplete combustion, mineral transformations, and secondary nucleation via gas-phase reactions [1,2]. These ultrafine particles are difficult to capture due to their low inertia and weak electrostatic attraction, leading to their persistent presence in flue gas emissions. SO₃ is primarily formed through the oxidation of SO₂ in high-temperature environments, catalyzed by metal oxides such as vanadium pentoxide (V₂O₅) in selective catalytic reduction (SCR) systems [3,4]. The presence of SO₃ in flue gas could accelerate equipment corrosion and induce visible plume formation, leading to environmental and operational concerns. Moreover, both SO₃ and PM pose serious health risks, as they can penetrate deep into the human respiratory system, exacerbating cardiovascular and pulmonary diseases [5]. Therefore, effective control strategies for PM and SO₃ emissions are essential for mitigating environmental and health impacts.

Various particulate emission control technologies, including electrostatic precipitators (ESP) and fabric filters, have been developed to mitigate PM from coal combustion, with distinct mechanisms and efficiencies. Although the PM removal efficiency of traditional ESP could already be up to 99.5%, its efficiency is highly dependent on operation parameters such as flue gas temperature [6,7]. Meanwhile, due to the penetration window [8], the capture efficiency of fine particles in the submicron range is relatively low. To address this issue, by operating at significantly lower temperatures, low-low temperature electrostatic precipitators (LLT-ESPs) have been recognized as an effective technology for enhancing fine PM removal in coal-fired power plants. Moreover, the implementation of LLT-ESP technology for achieving ultra-low emissions of fine particulate matter [9,10] while simultaneously removing SO₃ is widely recommended. LLT-ESP systems incorporate a heat exchange unit before conventional ESP, reducing the flue gas temperature at the ESP inlet from the typical 120~160 °C to below the acid dew point (around 90 °C). The temperature reduction enhances particle agglomeration and alters the physicochemical properties of fine particulate matter and SO₃, thereby improving overall removal efficiency. The phase state of SO₃ in the flue gas varies with temperature. In the LLT-ESP system, flue gas passes through a heat exchange unit, leading to the condensation of gaseous H₂SO₄ into sulfuric acid mist, which subsequently adheres to dust particles. This condensation process lowers the specific resistivity of the particles, thus effectively conditioning the flue gas. Consequently, the removal efficiency of fine PM was enhanced, which simultaneously facilitates the co-removal of a significant portion of SO₃ and other condensable pollutants including polycyclic aromatic hydrocarbons (PAHs) [11] and heavy metals [12,13] from the flue gas.

The condensation of SO₃ onto fly ash, as well as the collision efficiency between mist droplets and fine PM, significantly influence the overall pollutant removal efficiency. However, during the LLT-ESP process, some sulfuric acid droplets formed via homogeneous nucleation fail to effectively collide and adhere to fine fly ash particles. This incomplete interaction reduces the extent of heterogeneous condensation, thereby limiting the overall removal efficiency of PM and SO₃. To enhance the collision efficiency of fine particles, various agglomeration pretreatment technologies have been developed. These include chemical agglomeration [14,15], heterogeneous-condensation agglomeration [16], turbulent agglomeration [17,18,19,20], and acoustic agglomeration [17,21]. Among these approaches, turbulent agglomeration has garnered significant research interest. This technology makes use of the velocity gradients and turbulence fluctuations in a turbulent flow field, leading to increased particle collisions and agglomeration. Compared to other agglomeration techniques, turbulent agglomeration offers advantages such as a simple structure, easy installation and implementation, low failure rate, and minimal energy consumption. Therefore, employing turbulent agglomeration technology in LLT-ESP could further improve PM removal efficiency, while simultaneously facilitating SO₃ capture, making it a highly promising approach. However, research on the enhancement effects of turbulent agglomeration in the context of LLT-ESP remains limited.

In this paper, turbulent agglomerators made up of several deflectors, which induce turbulent vortex inside the flue duct, were installed downstream of the heat exchanger to enhance the collision and adhesion between sulfuric acid droplets and fly ash particles, thereby improving SO₃ removal efficiency of LLT-ESP. Experiments were conducted to investigate the agglomeration and removal characteristics under three different conditions: flue gas containing only SO₃, only fly ash, and a mixture of both SO₃ and fly ash. The results could provide theoretical and data support for the efficiency improvement of LLT-ESP in coal-fired power plants.

2. Methods

The experimental setup of the turbulent-agglomeration-coupled LLT-ESP system by placing the turbulent agglomerator downstream of the flue gas heat exchanger is shown in Figure 1. During the experiment, the flue gas sequentially passes through an air heater, a buffer tank, a flue gas heat exchanger, a turbulent agglomerator, an ESP, and an induced draft fan. The electrostatic precipitator is a plate-type dust collector. Inside the dust collector, there are two channels, and 10 needle-point electrodes are arranged as cathodes. The anode plate is made of stainless steel. The high-voltage power supply (TRC2025, Dalian Taisiman Technology, Dalian, China) could apply a maximum output voltage of −100 kV. The flue gas volume of this system is 300 Nm³/h. The gas flow velocity inside the electrostatic precipitator is approximately 1.5–2 m/s, while the flow resistance is about 150 Pa.

To generate the simulated flue gas, an aerosol generator (SAG410, Elmonite Scientific Instruments Co., Ltd., Dresden, Germany) is used to introduce coal fly ash into the buffer tank, which is dispersed by means of compressed air utilizing shear force, while the SO₃ generation system injects diluted sulfuric acid spray. To ensure the complete evaporation of diluted sulfuric acid, the outlet temperature of the air heater is maintained at approximately 300 °C. Additionally, to facilitate the condensation of gaseous H₂SO₄, the cooling water flow rate of the heat exchanger is adjusted to keep the outlet gas temperature at 90 °C. This operation temperature is much lower than the traditional ESP and optimizes fly ash resistivity through sulfate deposition and moisture condensation. First, the reducing temperature is beneficial to reduce the viscosity of the gas and thus improve the collision efficiency. Moreover, the temperature decrease also helps to increase the relative humidity and resistivity of the particle to the optimal range. This temperature-dependent synergy helps to improve PM removal efficiency.

The turbulent agglomerator used In the experiment Is the model shown in Figure 2, which incorporates both Z-type and cross-type deflectors. A bypass duct is installed alongside the turbulent agglomerator to examine SO₃ and fine particle removal characteristics under different operating conditions. The ELPI⁺ system is employed to measure the size distribution of fine particles and condensable sulfuric acid droplets in the flue gas. Furthermore, a dust sampler is used to collect fine particle samples before and after the heat exchanger. A SO₃ sampling and analysis system measures the mass concentration of SO₃ in the flue gas, and the schematic figure of the sampling method is presented in Figure 3. A PM_1.0/2.5/10 sampler is utilized to analyze the concentration of fine particles within the PM₁₀ range.

To evaluate the removal efficiency of fine particles, the overall removal efficiency of the electrostatic precipitator (ESP) and its size-classified removal efficiency are considered. The fine particle removal efficiency is assessed in terms of both number-based and mass-based efficiencies, defined as follows:

$${\eta }_N={\displaystyle \frac{N_0-N_2}{N_0}}\times 100\%$$

(1)

where η_N is the number-based fine particle removal efficiency (%), N₀ is the number concentration of fine particles in the raw flue gas (particles/cm³), and N₂ is the number concentration of fine particles after passing through the ESP (particles/cm³).

Similarly, the mass-based removal efficiency (η_M) represents the percentage reduction in the fine particle mass concentration after ESP:

$${\eta }_M={\displaystyle \frac{M_0-M_2}{M_0}}\times 100\%$$

(2)

where η_M is the mass-based fine particle removal efficiency (%), M₀ is the mass concentration of fine particles in the raw flue gas (mg/m³), and M₂ is the mass concentration of fine particles after passing through the ESP (mg/m³).

The size-classified removal efficiency (η_Ni) evaluates the removal efficiency for fine particles of different size ranges, as measured by the ELPI⁺ system:

$${\eta }_{Ni}={\displaystyle \frac{N_{i0}-N_{it}}{N_{i0}}}\times 100\%$$

(3)

where η_Ni is the removal efficiency of fine particles at the ith size channel of the ELPI⁺ (%), N_i0 is the number concentration of fine particles in the raw flue gas at the ith size channel (particles/cm³), and N_it is the number concentration of fine particles at the ESP outlet for the ith size channel (particles/cm³).

The mass concentration of SO₃ in fine particulate matter collected by PM_1.0/2.5/10 samplers, as well as in specific size fractions of ELPI⁺, is defined as the mass of SO₃ contained per unit mass of collected fly ash particles:

$$M_{{\mathrm{SO}}_3}={\displaystyle \frac{m_{{\mathrm{SO}}_3}}{m_{\mathrm{ash}}}}$$

(4)

where M_SO3 represents the mass concentration of SO₃ in the fine particles (μg/mg), m_SO3 is the detected mass of SO₃ in the collected fly ash sample for a given size fraction (μg), and m_ash is the total mass of fly ash particles (mg).

The removal efficiency of SO₃ is defined as the percentage reduction in the SO₃ mass concentration after passing through the ESP, relative to the concentration before the heat exchanger:

$${\eta }_{{\mathrm{SO}}_3}={\displaystyle \frac{M_{\mathrm{in}}-M_{\mathrm{out}}}{M_{\mathrm{in}}}}\times 100\%$$

(5)

where η_M is the mass-based SO₃ removal efficiency (%), M_in is the mass concentration of SO₃ in the raw flue gas (μg/m³), and M_out is the mass concentration of SO₃ after passing through the ESP (μg/m³).

3. Results and Discussion

3.1. Agglomeration and Removal Characteristics of SO₃

When the flue gas contains only SO₃, the cooling process in the heat exchanger induces homogeneous nucleation, leading to the formation of a large number of ultrafine sulfuric acid droplets. As these droplets pass through the turbulent agglomerators, collisions and agglomeration occur, altering their concentration and size distribution, which in turn affects their removal efficiency. Therefore, this section focuses on investigating the agglomeration and removal characteristics of SO₃ in the absence of fine fly ash particles.

When the aerosol generator was turned off and the SO₃ generation system was operating normally, ELPI+ was used to measure the change in the number concentration of sulfuric acid droplets formed by SO₃ condensation before and after turbulent agglomeration. The results are shown in Figure 4. It can be observed that after passing through the heat exchanger, the gaseous SO₃ in the flue gas condensed into a large number of sulfuric acid droplets, with a number concentration of 1.59 × 10⁷ 1/cm³. After passing through the turbulent agglomerator, the number concentration decreased to 1.25 × 10⁷ 1/cm³, a reduction of approximately 21.4%. This result indicates that sulfuric acid droplets underwent collisions and agglomeration in the turbulent flow field, where multiple droplets adhered to each other to form larger droplets, thereby reducing their number concentration.

Figure 5 shows the change in the size distribution of sulfuric acid droplets before and after turbulent agglomeration. The results indicate that before turbulent agglomeration, most of the sulfuric acid droplets formed by condensation were smaller than 0.1 µm, accounting for 99.2% of the total droplet count. After passing through the turbulent agglomerator, due to collisions and agglomeration, multiple droplets combined to form larger ones, resulting in a significant reduction in the number concentration of droplets below 0.1 µm. The proportion of droplets within this size range decreased to 89.4%, while the number of droplets larger than 0.1 µm increased accordingly.

These findings suggest that turbulent agglomeration also has a certain agglomeration effect on sulfuric acid droplets formed by SO₃ condensation, leading to a reduction in the number concentration and an increase in the droplet size. According to previous studies [17,19], smaller particles tend to enter the recirculation zone behind the vortex generators in the turbulent agglomerator, where they undergo agglomeration. In the case of SO₃ alone, most of the sulfuric acid droplets formed by homogeneous nucleation are smaller than 0.1 µm, making them highly responsive to the flue gas flow. Consequently, they can enter the recirculation zone of the turbulent agglomerator, experience agglomeration, and grow into larger droplets, thereby reducing their number concentration.

Figure 6 presents the concentration of SO₃ before and after LLT-ESP and its removal efficiency with and without turbulent agglomeration. When the experimental system was operated without turbulent agglomeration, the SO₃ concentration in the flue gas before the heat exchanger was 82.5 mg/m³, while the concentration after the ESP was 73.2 mg/m³, resulting in a removal efficiency of 11.3%. When a turbulent agglomerator was incorporated into the system, the SO₃ concentration before the heat exchanger was 81.6 mg/m³, while the concentration after the ESP decreased to 68.4 mg/m³, improving the removal efficiency to 16.2%. These results indicate that in the presence of SO₃ alone, the removal efficiency of SO₃ by LLT-ESP is relatively low. This is because the sulfuric acid droplets formed by homogeneous nucleation of SO₃ are extremely small, making them difficult to acquire sufficient charge within the electric field. As a result, their electrostatic migration rate is low, leading to poor capture efficiency.

When a turbulent agglomerator is introduced into the system, two key effects improve SO₃ removal. The first reason is that multiple sulfuric acid droplets collide and merge into larger droplets, reducing the overall droplet number concentration. Another reason is that as droplet size increases, their ability to acquire charge in the electric field improves, leading to a higher charge level and increased migration rate. Consequently, the turbulent agglomeration process enhances the capture efficiency of sulfuric acid droplets in the ESP, thereby promoting the removal of SO₃ even when present alone in the flue gas. To further improve the removal efficiency, one can consider regulating the flue gas by adding some vapor inside the flue gas, which can significantly enhance the tendency of the growth of sulfuric acid droplets by homogeneous condensation and thereby improve the overall removal efficiency.

3.2. Agglomeration and Removal Characteristics of Fly Ash Particles

The agglomeration and removal characteristics of fine PM were also tested in the presence of fly ash alone in the flue gas. This serves as a baseline for comparison with the agglomeration and removal characteristics when SO₃ and fly ash coexist.

When the SO₃ generation system was turned off and the aerosol generator was operating normally, ELPI⁺ was used to measure the change in the fine particulate number concentration before and after turbulent agglomeration, as shown in Figure 7. The results indicate that before turbulent agglomeration, the number concentration of fine particulates in the flue gas was 5.79 × 10⁶ 1/cm³, which decreased to 4.66 × 10⁶ 1/cm³ after turbulent agglomeration, achieving an agglomeration efficiency of 19.5% (Figure 8), illustrates the change in particle size distribution before and after turbulent agglomeration. Before turbulent agglomeration, the particle size distribution exhibited a bimodal pattern, with peak particle sizes at 0.04 µm and 1.25 µm. The number concentrations at these peak sizes were 2.20 × 10⁶ 1/cm³ and 2.19 × 10⁵ 1/cm³, respectively. After turbulent agglomeration, the particle size distribution remained bimodal, and the peak particle sizes did not change. However, an analysis of the distribution changes reveals that the number concentration of fine particles below 0.13 µm decreased to varying degrees, while that in the range of 0.13–1.25 µm slightly increased. This suggests that turbulent agglomeration facilitated the collision and adhesion of smaller fine particles, leading to the formation of larger particles. Consequently, the number concentration of smaller fine particles decreased, while that of larger particles increased.

The PM_1.0/2.5/10 samplers were used to measure the mass concentration of fine particulates before and after the low-low temperature electrostatic precipitator (ESP) under conditions with and without turbulent agglomeration. The removal efficiency was then calculated, and the results are shown in Figure 9. Without the turbulent agglomeration, the mass concentration of fine particulates in the flue gas before the heat exchanger was 245 mg/m³, and it decreased to 33.6 mg/m³ after passing through the ESP, achieving a removal efficiency of 86.3%. When a turbulent agglomerator was installed after the heat exchanger, the mass concentration of fine particulates before the heat exchanger remained at 245 mg/m³, but after the ESP, it further decreased to 19.4 mg/m³, with the removal efficiency increasing to 92.1%. These experimental results indicate that the turbulent agglomerator enhances fine particulate removal efficiency by promoting the agglomeration and growth of particulates in the flue gas.

3.3. Synergistic Agglomeration and Removal of SO₃ and Fine Fly Ash Particles

When both SO₃ and coal combustion fly ash coexist in the flue gas, SO₃ undergoes heterogeneous condensation on fine fly ash particles as condensation nuclei, along with homogeneous condensation during the cooling process in the heat exchanger. Subsequently, sulfuric acid droplets formed via homogeneous condensation interact with fine particulates through collisions and adhesion under the influence of turbulent vortices, thereby enhancing the combined removal efficiency of SO₃ and fine fly ash particulates by the ESP. Therefore, this section experimentally investigates the coagulation and removal characteristics of SO₃ and fine fly ash particulates in coexistence.

When both the aerosol generator and SO₃ generation system were operating normally, ELPI+ was used to measure the overall particle size distribution in the flue gas before and after turbulent agglomeration. Additionally, a comparison curve was plotted, representing the sum of the size distributions after turbulent agglomeration for cases with only SO₃ and only fly ash. The results are shown in Figure 10. The measurements indicate that before turbulent agglomeration, the flue gas after cooling in the heat exchanger contained both fine fly ash particles and sulfuric acid droplets. The overall particle size distribution exhibited a bimodal pattern, with peak particle sizes at 0.02 µm and 1.25 µm, and corresponding number concentrations of 5.85 × 10⁶ 1/cm³ and 8.25 × 10⁵ 1/cm³, respectively. After passing through the turbulent aggregator, the size distribution shifted to a unimodal form, with a peak at 0.02 µm and a number concentration of 3.79 × 10⁶ 1/cm³ at the peak. Furthermore, the particle number concentration decreased across all size ranges, with the most significant reduction occurring in the sub-micro range. In this range, the number concentration dropped from 1.67 × 10⁷ 1/cm³ before agglomeration to 9.77 × 10⁶ 1/cm³ after agglomeration, representing a 41.5% decrease. Given that most sulfuric acid droplets formed via homogeneous condensation of SO₃ fall below 0.1 µm and are present in large quantities, it can be inferred that the droplets and fine particles underwent collision and adhesion in the turbulent field, leading to a substantial reduction in the particle concentration within this size range. Moreover, when comparing the sum of the particle size distributions after turbulent agglomeration for the individual SO₃ and fly ash cases with the distribution obtained in their coexistence, it is evident that particle concentrations at all size ranges were significantly lower in the coexistence case. This suggests that not only did fine fly ash particles and sulfuric acid droplets aggregate among themselves respectively, but also that inter-particle agglomeration occurred between the fly ash and sulfuric acid droplets. Consequently, the overall agglomeration effect in the coexistence scenario was superior to the sum of the effects observed in the individual cases.

Additionally, PM_1.0/2.5/10 samplers were used to collect fine particulate samples before and after turbulent agglomeration. The SO₃ mass concentration in fine particles across four particle size ranges was measured, and the results are shown in Figure 11. It can be observed that after turbulent agglomeration, the SO₃ mass concentration within fine particles increased across all size ranges, with the most significant rise occurring in particles smaller than 1.0 µm. In this range, the SO₃ mass concentration increased from 154.2 μg/mg before agglomeration to 392.4 μg/mg after agglomeration. These results indicate that turbulent agglomeration enhances the adhesion and deposition of sulfuric acid droplets onto fine fly ash particles [22], with this effect being more pronounced for smaller particles. Particles with lower mass and inertia in a turbulent flow field can follow the gas stream into the recirculation zone behind the turbulence-generating elements, where agglomeration occurs. In the case of fly ash, fine particles smaller than 1.0 µm exhibit better gas-following behavior due to their lower mass, allowing them to accumulate in the recirculation zone. Since most sulfuric acid droplets are smaller than 0.1 µm, they can also fully follow the gas flow into the recirculation zone. Consequently, many submicron fly ash particles collide and aggregate with sulfuric acid droplets within this zone, leading to significant adhesion and a marked increase in the SO₃ mass concentration in particles smaller than 1.0 µm. In contrast, larger fly ash particles tend to maintain their original velocity and movement within the mainstream flow, reducing their probability of collision with sulfuric acid droplets. As a result, the enhancement effect of the turbulent aggregator on sulfuric acid droplet deposition weakens with increasing particle size.

To further investigate the agglomeration characteristics when SO₃ and fine fly ash particles coexist, SEM and EDS analyses were conducted on the morphology and elemental changes of fine particles before and after turbulent agglomeration. The results are shown in Figure 12. It can be observed that before the turbulent aggregator, only a small amount of agglomeration contact occurred between fine particles, with most particles unconnected (Figure 12a). However, after passing through the turbulent aggregator, multiple fine particles of different sizes were found to combine, forming larger agglomerates (Figure 12b). The main mechanism behind the synergistic effect was the heterogeneous condensation of SO₃ on the fly ash particles. Compared with the single pollutant scenario, the condensational growth of SO₃ was greatly enhanced, since the energy barrier of heterogeneous condensation was lower than that of homogeneous nucleation. Moreover, the SO₃ on the fly ash particles could induce surface chemistry modification, which increased the cohesiveness of PM and thus increased the turbulent agglomeration efficiency.

Furthermore, EDS analysis (Figure 12c) revealed that the surface sulfur content of the agglomerates was significantly higher than that of the fine particles before agglomeration. This indicates that during the turbulent agglomeration process, sulfuric acid droplets collided and aggregated with fine particles, leading to the deposition of sulfuric acid droplets onto the surface of the fine particles. This increased the surface interaction forces of the particles, promoting the adhesion of multiple fine particles to form larger agglomerates. Therefore, the deposition of SO₃ on the surface of fine particles also enhanced the turbulent agglomeration effect.

To investigate the removal characteristics of SO₃ and fine PM, concentrations were measured using an SO₃ sampling system and PM_1.0/2.5/10 samplers before and after the LLT-ESP. As illustrated in Figure 13, the SO₃ concentration measured 83 mg/m³ upstream of the heat exchanger. Without turbulent agglomeration, the LLT-ESP outlet concentration decreased to 25.1 mg/m³ (69.7% removal efficiency). With turbulent agglomeration, this further reduced to 14.2 mg/m³ (82.9% removal efficiency). For PM_2.5, the initial concentration of 245 mg/m³ decreased to 24.3 mg/m³ (90.1% removal efficiency) without turbulent agglomeration and 5.9 mg/m³ (97.6% removal efficiency) with turbulent agglomeration.

These findings indicate that turbulent agglomeration synergistically enhances LLT-ESP’s removal performance for both SO₃ and fine PM. The coexistence of SO₃ and fly ash yielded higher removal efficiencies than individual components. Mechanistic analysis reveals two pathways: (1) Sulfuric acid droplets coalesce onto particulates through turbulent collision, enabling their co-removal; (2) Sulfate deposition increases particle surface adhesion and reduces electrical resistivity, thereby improving charge acquisition and capture efficiency in the ESP field. The implementation of turbulent agglomeration technology in the LLT-ESP system demonstrates dual optimization for multi-pollutant control in the flue gas treatment process.

4. Conclusions

To enhance the synergistic removal of SO₃ and fine particulates from coal-fired flue gas in low-low temperature electrostatic precipitators (LLT-ESP), this study developed an experimental LLT-ESP system incorporating turbulent agglomeration technology. By implementing turbulent flow conditions upstream of the precipitator, the system promotes collision and adhesion between condensed sulfuric acid droplets and fly ash particulates, thereby improving co-removal efficiency.

Experimental investigations validated the enhancement effect of turbulent agglomeration technology on LLT-ESP systems for individual SO₃ and PM removal. In SO₃-only flue gas conditions, turbulent agglomeration enhanced collision-coalescence of sulfuric acid droplets, reducing their number concentration while decreasing the fraction of PM_0.1. This modification elevated SO₃ removal efficiency from 11.3% to 16.2%. Under fly ash-only conditions, turbulent agglomeration decreased the fine particulate number concentration by 19.5%. The results demonstrate effective enlargement through turbulent agglomeration. The differential growth mechanisms were quantitatively characterized: sulfate droplets exhibited Brownian motion-dominated coalescence, whereas fly ash particulates primarily underwent shear-induced aggregation.

When the fly ash and SO₃ coexist in the flue gas, the implementation of turbulent agglomerators in LLT-ESP systems greatly improves the removal efficiency by facilitating three-phase interactions: intra-particulate aggregation of fly ash, self-coalescence of sulfuric acid droplets, and agglomeration between fly ash and droplets. This multi-modal agglomeration enhances SO₃ removal through two synergistic mechanisms. On one hand, turbulent flow promotes droplet–particle collisions, enabling 82.9% SO₃ removal efficiency through ESP-coordinated deposition of sulfate-laden particulates. On the other, sulfuric acid deposition on fly ash, which was revealed by SEM-EDS analysis, increases surface adhesion while reducing surface resistivity. This dual modification enhances particulate charging efficiency and subsequent capture.

The post-heat exchanger installation of turbulent agglomerators thus demonstrates dual efficacy, simultaneously optimizing SO₃ abatement and particulate control through physiochemically coupled mechanisms. Therefore, the technology combination proposed in this work has a certain feasibility for the co-removal of multiple pollutants in coal-fired power plants.