Wenting Liu1, Xin Wang1, Bowen Zhao1, Jianyi Lu This email address is being protected from spambots. You need JavaScript enabled to view it.1,2 

1 Department of Environmental Science and Engineering, Hebei Key Lab. Power Plant Flue Gas Multipollutant, North China Electric Power University, Baoding 071003, China
2 College of Environmental Sciences and Engineering, MOE Key Laboratory of Resources and Environmental Systems Optimization, North China Electric Power University, Beijing 102206, China


Received: June 21, 2023
Revised: September 19, 2023
Accepted: October 4, 2023

 Copyright The Author(s). This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are cited.


Download Citation: ||https://doi.org/10.4209/aaqr.230145  

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Cite this article:

Liu, W., Wang, X., Zhao, B., Lu, J. (2023). New Insights into the Synergistic Effect on Condensable Particulate Matter Based on the Formation, Characteristics and Removal. Aerosol Air Qual. Res. 23, 230145. https://doi.org/10.4209/aaqr.230145


HIGHLIGHTS

  • Recent condensable particulate matter (CPM) removal methods were summarized.
  • The main components of CPM and the means of removal were investigated.
  • The efficiency of conventional dust remover on CPM removal was analyzed.
  • Effective means of CPM removal and future developments were discussed.
 

ABSTRACT


Emissions of total particulate matter (TPM) from stationary combustion sources consist of both filterable particulate matter (FPM) and condensable particulate matter (CPM). According to the study's findings, CPM emissions contributed significantly to the overall concentration of TPM. Therefore, there has been a growing focus on the physicochemical properties and control strategy of CPM. Firstly, this paper systematically reviewed the formation, composition, characteristics, and methods of removing CPM. Secondly, integrating the current removal methods, a cold electrode electrostatic precipitator (CE-ESP) based on the synergistic effect of multi-field coupled force on CPM coagulation and removal was put forward and installed. Thirdly, case studies of CE-ESP on CPM removal were conducted in a municipal solid waste incineration plant and a coal-fired power plant. The results showed that the removal efficiency of CPM could reach as high as 93%. The CE-ESP had a significant removal effect on both organic and inorganic substances in CPM.


Keywords: Condensable particulate matter, Formation mechanism, Main components, Removal methods, Cold electrode electrostatic precipitator


1 INTRODUCTION


Total particulate matter (TPM) emissions from stationary combustion sources include filterable particulate matter (FPM) and condensable particulate matter (CPM) (Peng et al., 2021b). Over the past few years, technologies have been rapidly developed to reduce FPM emissions, which have been effectively controlled. The current Chinese ultra-low emission (ULE) standard for particulate matter emissions is already one of the most stringent. Coal-fired boilers have achieved the ULE request for FPM in recent years (Liang et al., 2020). It has been difficult to effectively manage CPM, which refers to the particulate matter in a stationary source outlet that condenses to a liquid or solid state upon leaving the flue and reaching the sampling site under ambient conditions. It is classified as fine particulate matter and is a crucial precursor to PM2.5 and atmospheric aerosols (Cano et al., 2021). The studies revealed that CPM emissions constituted a significant proportion of the overall TPM levels (Wu et al., 2020). Reducing CPM emissions in flue gas is highly effective in curbing air pollution once conventional pollutants are under control. Reducing CPM emissions in flue gas is highly effective in curbing air pollution once conventional pollutants are under control. And in the face of the limited effectiveness of traditional APCDs in removing CPM, an effective new dust removal technology is required.

From the physical morphology perspective, CPM formation primarily occurs through the condensation process of gaseous precursors (Corio et al., 2000). CPM had a small particle size and high dispersion, which made it difficult to settle when floating in the atmosphere (Liu et al., 2022b).

CPM is highly adsorptive and tends to enrich organic pollutants, heavy metals, and inorganic ions, which can spread over long distances in the atmosphere and cause environmental problems. Regarding chemical composition, the surface of CPM is prone to the enrichment of heavy metals, which are mainly carcinogenic and genotoxic mutagens and highly hazardous (Fan et al., 2009). The harm of CPM to human health has two aspects: firstly, it is caused by the complex chemical composition of CPM. As a carrier of other pollutants, it can adsorb multiple chemical components into the human body through respiration and increase toxic substances' reaction and dissolution rates. Moreover, CPM retention time and atmospheric absorption rates increase as the particle size decreases (Yuan et al., 2021). Secondly, fine particles can enter the lungs and cause pneumoconiosis by invading histiocytes (Faiz et al., 2018). In addition, when the particle size decreases, CPM stays in the atmosphere for a longer time and is absorbed more quickly into the respiratory system (Liu et al., 2016). Current research on CPM is mainly focused on emission characteristics. The primary sampling methods are impingement condensation and dilution condensation. Li et al. (2021) used EPA method 202 to collect particulate matter from three ULE coal-fired power plants equipped with different air pollution control devices (APCDs). They found that the CPM could account for up to 83.2% of the TPM. Yang et al. (2018a) set sampling points at different stationary combustion sources and measured the CPM concentrations, which comprised 44–74% of the TPM. These studies indicated that the concentration and proportion of CPM emissions were high and should be given sufficient attention. The results showed that before 2018 there were only a dozen papers on the study of CPM, and few studies addressed CPM removal. Since 2018, there has been a significant increase in relevant literature, with over 70 articles now available. Nearly half of the articles examine migration patterns and removal effects of CPM, revealing a new research trend (Wang et al., 2022). The papers primarily examined the conversion and migration of CPM in conventional APCDs, as well as the efficiency of traditional control strategies like condensation and adsorption for removing CPM. The results showed some success in removing CPM through these methods. Still, there were also some negative efficiency data when studying the removal of CPM by wet electrostatic precipitator (WESP) and wet flue gas desulfurization (WFGD) (Zheng et al., 2018). The purpose of this paper was to review these works, discuss the main characteristic components of CPM and their removal methods, and review the main CPM removal methods and their removal efficiencies to facilitate further investigation of control methods for CPM. Based on these studies, an unique device called CE-ESP was proposed to efficiently capture CPM by combining condensation with an electrostatic precipitator, and it can achieve multi-field force coupled removal of CPM employing an applied temperature field. CE-ESP removal efficiency on CPM from flue gas could reach as high as 93%.

 
2 FORMATION AND CHARACTERISTICS OF CPM 


The formation pathways of CPM are shown in Fig. 1. Fig. 1 described the pathways, reaction modes, main components, and influencing factors on organic and inorganic components of the generation of CPM for three types of coal blended combustion materials with different fuel ratios. It is significant to study the chemical components and formation mechanism of CPM to develop CPM control technology. Subsequent studies can be conducted on the targeted removal of CPM based on its main features and properties.

Fig. 1. Formation pathways and characteristics of CPM.Fig. 1. Formation pathways and characteristics of CPM.

 
2.1 Formation of CPM

CPM has a small particle size and is categorized as fine particulate. Different methods of removing fine particulate can be explored for further study. The process of gasification-condensation was instrumental in forming CPM (Zou et al., 2023). In a high temperature combustion environment, some inorganic matter (0.2%–3%) in the coal first underwent gasification. The gasification products continuously diffused outwards and reacted with oxygen in the coke boundary region (Wu et al., 2021). Subsequently, when the inorganic vapor reached supersaturation, it formed many fine particles by homogeneous nucleation. Particles grew gradually in two ways. The first way involved particles colliding with each other, condensing, and uniting in a volume that equals the sum of the colliding particles' volumes. The resulting composition was a mixture of the particles (Yuan et al., 2022). 

Another possibility was that the inorganic vapor condenses unevenly onto the surface of pre-existing ash particles, leading to an increase in their volume. At lower temperatures, particle diameter growth slowed down. Eventually, colliding ash particles clumped together to form agglomerates with aerodynamic diameters greater than 0.36 µm (Kamiya et al., 2011) and discharged into the atmosphere with the boiler flue gas. Ott et al. (2021) concluded that fly ash particle formation in pulverized coal furnaces could be described by a three-mode particle size distribution, which included: a submicron flue gas zone and a small particle fragmentation zone with a particle size of approximately 2 µm and a large particle fragmentation zone. After entering the furnace, pulverized coal was subjected to thermal shock, and volatile matter rapidly precipitated, resulting in internal stress that caused pulverized coal particles to break into finer fragments (Kang et al., 2022). As combustion proceeded, the structure of coal char was destroyed, and the oxidation reaction of pulverized coal was not uniformly conducted, resulting in secondary breakage of coal char and the formation of many fine particles (Yan et al., 2023). As shown in Fig. 2, during the combustion process, the minerals in the coal powder also underwent gasification, condensation, and polymerization with the reaction, resulting in fine particulate matter. When particles of various sizes were formed, particles smaller than 1 µm were predominantly created through gasification and condensation reactions (Ngo et al., 2022), while most of the particles generated by fragmentation reaction were 1–10 µm particulate matter. In terms of the gasification-condensation mechanism, the inorganic components gasified at high temperatures, a part of the inorganic vapor condensed with the formed fly ash particulate matter as a core, and a part of the submicron particles agglomerated and coalesced to form ash particles as shown in Fig. 3 (Pan et al., 2015). These indicate the main characteristics of CPM, with its fine and complex composition and temperature sensitivity. One area of potential research could be exploring the impact of temperature on particle agglomeration, with the goal of increasing particle size and improving removal efficiency (Jung et al., 2020).

Fig. 2. Mechanisms of fly ash formation from coal combustion processes with different particle sizes.Fig. 2. Mechanisms of fly ash formation from coal combustion processes with different particle sizes.

Fig. 3. Formation of particulate matter during coal combustion.Fig. 3. Formation of particulate matter during coal combustion.

 
2.2 Composition of CPM

The main components of CPM include inorganic components: sulfate, nitrate, nitrite, and some of the heavy metals that can be harmful to our health, including mercury, arsenic, selenium, chromium, and cadmium (Zhang et al., 2023b). CPM organic components are complex in composition, including hundreds of organic compounds, mainly hydrocarbons, and esters. They are usually generated by non-combustion emission sources as well as gas turbines and engines (Xu et al., 2018). The concentrations and percentages of TPM and the main components of CPM in different combustion sources are collected by some national and international scholars. Current studies on sampling methods, control facilities, emission concentration, and inorganic composition of CPM are listed in Table 1.

Table 1. Current studies on sampling methods, control facilities, emission concentration and inorganic composition of CPM

The above studies showed that CPM from both boilers and other stationary sources were the main contributors to the TPM. This paper analyzes the main components of CPM, considering that it can extend from the removal of individual pollutants to the collaborative removal of other pollutants and CPM.

Regardless of the fixed pollution source of any fuel type, the highly volatile Ca, K, and Na are the main inorganic components of CPM (Xu et al., 2022). The main water-soluble ions are SO42, NO3, Cl, and NH4+. Zheng et al. (2020) collected CPM from a power plant that has achieved ultra-clean emissions after WFGD and WESP and detected the highest concentrations of Al3+, Ca2+, Na+, K+, Cl, and F. Song et al. (2020) studied the composition of CPM emissions from a coal-fired power plant that had completed an ultra-low emission retrofit and found that the major anions were SO42−, NO3, and Cl, while the main cations included Na+ and Ca2+. Yang et al. (2022b) arranged sampling points in different combustion sources, such as wood, coal, diesel, and heavy oil, and found that the main metal cations of CPM were Na+, Ca2+, and K+. Wang et al. (2021) and others measured by spectrophotometry that the content of NH4+ in the CPM discharged is high, with the main anions being SO42 and NO3, and the main cations being Na+, K+, and Ca2+ from the waste incineration process.

Many scholars have also detected heavy metals in samples of CPM. Although the concentrations of heavy metals in CPM are deficient, they should be given high priority due to their high toxicity. Yang et al. (2019) found heavy elements, including Cu, Zn, and Ni, in CPM emitted from steel mills. He also detected these elements in combustion sources from 5 different fuel types. Cano et al. (2019) measured mercury concentration in CPM emissions from industrial sources. Wang et al. (2022) identified minimal amounts of Mn, Ti, and Cr in CPM from waste incinerator emissions.

The properties of the heavy metals found in the particulate matter were identical. They were condensable, volatile, and gaseous in the flue and tended to collect on smaller particles (Feng et al., 2021). During coal combustion, heavy metal elements were converted, reflecting the condensability of heavy metals. All these observations highlighted the relevance of CPM in relation to heavy metals (Wu et al., 2020). This is a definitive guide to further exploring control methods for CPM.

To meet the emission standard of heavy metals, such as electrostatic precipitators, bag filters, WESP, electric bag filters, and WFGD, which can all be used to capture heavy metals with high efficiency (Zheng et al., 2020). The dust removal unit was highly efficient in eliminating particulate matter such as Cd, Pb, and Mn, with a success rate of over 94%. However, its ability to remove gaseous metals was lower, between 48.5% and 97.6% (Huang et al., 2020). Adsorption methods are commonly carried out with adsorbents, including mineral adsorbents, calcium-based adsorbents, activated carbon, and fly ash, to remove heavy metals. Solid sorbents can also be added to the combustion process to suppress heavy metal emissions (Sheng et al., 2023). Adsorption methods can remove a wide range of heavy metals from flue gases, particularly heavy metal vapors. For example, catalytic oxidation is widely used in mercury removal (Zou et al., 2023). In most cases, adsorbents, such as activated carbon, can be modified to improve the adsorption capacity. Qiao et al. (2023) suggested that MnOx/Al2O3 prepared by impregnation can remove elemental mercury (Hg0). Zhang et al. (2023b) observed that activated carbon could remove CPM and Hg from flue gas. Based on these findings, it appeared that adsorption could be used to initiate the removal of CPM, and the modification of adsorbent to enhance the adsorption effect, as well as the after-treatment and reuse of adsorbent, was the focus of subsequent research (Zhang et al., 2023a).

Inorganic anions in CPM are mainly transformed by gas-phase precursors (Tan et al., 2023). In general, SO42 is from H2SO4 vapor and sulfate gas. During coal combustion, a small quantity of SO2 was emitted and then underwent oxidation to become SO3, forming H2SO4 steam (Li et al., 2016b). At temperatures below 200°C, SO3 is converted to H2SO4 vapor. The catalyst in the SCR helps to convert SO2 to SO3. H2SO4 has an external vapor pressure and condenses quickly under ambient conditions. Due to the condensability of SO3/H2SO4, it has been suggested that the CPM inorganic components primarily comprise SO3. Li et al. (2016a) found that almost all the H2SO4 belongs to the CPM. Jiang et al. (2024) found that SO42 in the CPM is generated through the conversion of SO2, as the collection unit absorbs the SO2 in the flue gas. This factor could lead to a positive bias in the CPM analysis, and it is recommended that a nitrogen purge be carried out after the test to remove SO2 from the solution in the shock bottle to reduce this error. In coal combustion (Liang et al., 2023a), the reductant NH3 is usually added to remove NOx from the flue gas. Zheng et al. (2018) believed that F and Cl are highly volatile. With decreasing temperature, the gaseous states HF and HCl readily dissolve in water and condense, thereby converting to CPM. Most of these water-soluble anions in CPM are condensed from volatile gases. The overall removal of CPM can be investigated by drawing on methods for removing explosive gases from flue gas, such as adsorption and mixed combustion (Zheng et al., 2020).

CPM contains much organic matter. Li et al. (2021) researched the characteristics of CPM that came from coal-fired power plants and discovered the presence of polycyclic aromatic hydrocarbons (PAHs) in the CPM samples. Liu et al. (2022b) studied the characteristics of CPM emissions. They also found the highest levels of tricyclic and tetracyclic PAHs. Given the highly toxic nature of PAHs in CPM, studying their sources and formation is necessary to explore CPM control methods.

PAHs are present in CPM in the lower ring form. Currently, the removal of PAHs from flue gases has been extensively investigated (Zhang et al., 2023c). Current air control devices can be used to remove a wide range of pollutants, including PAHs, synergistically.

Due to the condensability and volatility, the organic matter in CPM is partially coincident with VOCs. Paraxylene was detected by Feng et al. (2021). Typical VOC control and removal technologies are condensation, adsorption, catalytic oxidation, combustion, and biological treatment (Yuan et al., 2023). The removal of these organics in CPM, which are highly volatile, should be synergistic and similar (Fan et al., 2009). The subsequent simultaneous removal of VOCs from CPM by synergistic action should be considered.

Overall emissions, particle size distribution characteristics, and the main components of CPM can be considered to have a significant impact on the environment (Pyo et al., 2017). At a time when the Chinese ultra-low emission standard has become the world's strictest emission standard, the CPM concentration of coal-fired units is still higher than the FPM emission standard (5 mg Nm3). CPM consists mainly of inorganic components, as most of the organic material in the fuel is burned during combustion, and the pollutants emitted should be mostly inorganic (Liu et al., 2022b). However, the application of multiple particulate matter control facilities has successfully decreased the concentration of inorganic elements in CPM, while the removal of organic components is less effective. As a result, their emissions of CPM with high organic component content which is more harmful to the environment (Song et al., 2020). Therefore, the key to developing collaborative air pollutant removal and reducing non-conventional pollutant emissions in China currently lies in controlling CPM emissions (Pan et al., 2015). The temperature of flue gas is a crucial factor that impacts the emission of CPM. Further control of CPM emissions requires synergistic removal and reduction of flue gas temperature.

 
2.3 Morphology of CPM

In addition to composition, the microscopic particle size of CPM has been investigated in terms of CPM properties. CPM usually exists in the form of condensation nuclei, with particle size mainly in the range of 20 nm–1 µm, and the largest particle size is generally below 100 nm, which is submicron particulate matter (Li et al., 2016b). Its particle concentration and specific surface area are more significant than conventional particles at the same mass concentration. Unlike conventional particles, CPM is more hygroscopic and can absorb moisture and increase (Kang et al., 2022).

The filter membranes of the CPM samples were observed by scanning electron microscope (SEM), and the CPM formation was more dispersed, with particle sizes mostly less than 2.5 µm (Zhang et al., 2018). Cano et al. (2017) passed the removed particulate flue gas into a residence chamber mixed with dry dust-free air to dilute it and collected the CPM with a filter membrane. After sampling and qualitative analysis of the CPM chemical composition by SEM/EDS, they observed that the CPM collected on the filter membrane by SEM was abundantly and uniformly distributed, the size of the particles ranged from 1–2 µm, and their surface had a porous structure. However, the limitations of the SEM method of observing CPM were that the SEM could only observe CPM particles collected on the filter membrane, which is a small sample. However, it remains representative (Qi et al., 2017). The pattern of CPM condensation occurring in the sampling device is inconsistent with the natural pattern of CPM condensation outside the flue, so the observed patterns may not be the same as the actual conditions. Huang et al. (2021) connected ELPI+ directly after the sampling device and monitored the CPM particle size. According to the findings, over 95% of the CPM were less than 0.2 µm, which proved that the CPM is PM2.5. Future research directions could be considered to collect the CPM particle size distribution at an actual time with a dilution sampling device connected to ELPI, as the cooling process of the dilution sampling method simulates the natural condensation process of CPM, thus avoiding some of the errors and providing adequate data for the study of the microscopic morphology of CPM (Tong et al., 2023).

CPM particles are small, and it is difficult to capture by conventional methods, and consideration needs to be given to increase the particle size before further removal. A comprehensive analysis of the ionic concentration, components, and microscopic morphology of CPM can provide the basis for accurate monitoring and efficient removal of CPM.

 
3 CURRENT REMOVAL METHODS FOR CPM


The reduction of CPM emission concentrations requires a combination of CPM generation, synergistic flue gas conventional pollutant treatment and CPM treatment, and economic and reasonable removal means of CPM (Jiang et al., 2024). The studies reported so far on the removal of CPM can be divided into two groups. One is the use of existing technologies, such as the collaborative removal of CPM by APCDs in power plants, and the main component characteristic adsorption method for CPM removal. Secondly, a number of specialized removal methods including condensation, adsorption, and fuel blending were employed (Guo et al., 2023). Table 2 lists the processes and efficiencies of CPM removal studies by national and international scholars.

Table 2. Methods and efficiencies of CPM removal.

 
3.1 Removal of CPM by APCDs

In recent years, to meet emission standards, power plants have been retrofitted with ULE and equipped with different APCDs, as shown in Fig. 4. These conventional pollutant treatment units are more effective in removing FPM. However, it is necessary to discuss further the efficacy of removing CPM. This paper summarizes the effectiveness of different APCDs on CPM removal through data results.

Fig. 4. Typical ultra-low emission power plant processes.Fig. 4. Typical ultra-low emission power plant processes.

The low-low temperature electrostatic precipitator (LLT-ESP) is a device that combines a gas heater in front of the traditional electrostatic precipitator. This device can lower the temperature of the flue gas, facilitating the condensation of gas pollutants and preventing the high dust specific resistance of soot in high temperatures. Therefore, it can enhance the effectiveness of removing dust. When the flow rate of flue gas decreases, the time it stays in the flue increases as the flue gas temperature in the dust collector reduces. Once the temperature of the flue gas drops to the acid dew point, SO3 exists in the form of sulphonic acid droplets. It can be removed by interacting with the particulate matter and adhering to the fly ash surface, so the low temperature influenced the synergistic removal of CPM.

The removal efficiency of the electrostatic bag filter (EBF) and electrostatic precipitator (ESP) has been discussed (Rabbat et al., 2023). In fact, the main difference between the EBF, ESP, WESP, and the LLT-ESP mentioned above is the operating temperature and the installation position, their central principle of operation is the separation of gas and dust employing a high voltage DC corona discharge. Huang et al. (2020) collected and measured CPM, CPM organic components, and CPM inorganic components concentrations before and after EBF and ESP. They found that EBF removed 77.34%, 78.97%, and 74.28% of CPM, CPM organic components, and CPM inorganic components, respectively. And ESP removed 79.23%, 79.23%, and 53.19%, respectively. Particulate matter control in cement clinker kilns was investigated by Cano et al. (2019). After conducting tests on their pollutant control facilities, it was found that EBF and SNCR had a high removal efficiency for CPM. The EBF is a combination of an ESP and bag filter, combining the advantages of both, electrostatics enhances the efficiency of the bag in trapping submicron particles while reducing the pressure drop to reduce resistance losses. The high-pressure discharge promotes the ionization of large organic molecules in the particulate matter, and the bag filters the condensed fine particulate matter, so it has a better effect on CPM removal. The study illustrated the mechanism of CPM removal by EBF and ESP.

WFGD is the process of removing SO2 by spray washing the gas with a limestone slurry. Liu et al. (2022b) determined 44.85% inorganic removal of CPM in their experiments. Li et al. (2017) collected CPM at the ESP, WFGD outlet, and the middle of the stack, and the removal rate of ESP reached 87.3% and below 40% for all others. The reason might be that the flow rate before the ESP was fast, and a large amount of gas-phase CPM was not fully formed into the particulate state, after entering the ESP, the gas velocity decreased significantly, the gas residence time became longer, and the gas-phase CPM was able to condense into the particulate state, which was removed by the ESP. Yang et al. (2018b) set up sampling points before and after WFGD and at the outlet of WESP and found that WFGD and WESP had a synergistic effect. The efficiency of WFGD ranged from 34.64%–91% in different studies (Yang et al., 2018c; Yang et al., 2019; Ngo et al., 2022). The CPM organic components and CPM inorganic components removal rates ranged from 3.7% to 67.3% and 13.36% to 92%, respectively (Zhang et al., 2018). The highest removal rate exceeds 90% (Lu et al., 2020). Yuan et al. (2021) concluded that the temperature drops sharply, and homogeneous and non-homogeneous condensation of CPM occurs thus being removed. In addition, negative efficiency data existed on the efficiency. Lu et al. (2019) discovered that the CPM present has risen from 9.73 mg Nm3 to 29.66 mg Nm3. This abnormal behavior might be caused by the evaporation of substances such as inorganic salts from the FGD slurry or measurement error. CPM was detected at the scrubber outlet using EPA Method 202, with CPM concentrations ranging from 27.8 to 208.0 mg Nm3 (Huang et al., 2021). In summary, the conversion processes of CPM are complex and need to be further explored.

The WESP works in a humid environment and removes fine particles, SO3/H2SO4 aerosol particles, and droplets of desulfurization slurry, thus achieving deep flue gas cleaning. Based on available information, researchers found that WESP removed 18.57% to 80.36% of CPM (Jiang et al., 2024), 10.42% to 76.61% of CPM organic components, and 9.05% to 88.06% of CPM inorganic components, respectively. This suggested that WESP has the potential to remove CPM efficiently and synergistically. It has been observed that after WESP, the concentration of induction cations in CPM is significantly reduced (Qi et al., 2017). It has also been found that WESP has some removal effect on volatile gases and SO3 in flue gas collected before and after WESP (Yang et al., 2022a). Liu et al. (2022a) and Song et al. (2020) found that WESP was effective in the removal of esters and n-alkanes from the organic in CPM. Studies have proven that mist eliminators also influence CPM removal, and some scholars (Liang et al., 2020) have collected flue gas before and after WESP and mist eliminators, and the removal efficiency of CPM removal was found to be 42% ± 10% and 60% ± 7% respectively. The principle of stripping has also been studied in the works of literature. It is indicated that gaseous precursors of CPM, such as SO3, mostly exist as aerosols that can be readily absorbed into liquid droplets dispersed in WESP. It has been shown (Cui et al., 2018) an increase in CPM concentration of more than 150% after WESP. Yang et al. (2022b) included that during the corona discharge of WESP, certain reactive substances could oxidize SO2 and NO to SO3 and NO2. Song et al. (2020) also found that when the flue gas passed through WESP, the number of phthalates in CPM organic components increased. It is theorized that the reheating of the MGH in front of the stack increased the concentration of gaseous precursors of CPM.

Yang et al. (2015) conducted a study on boilers for different fuels equipped with different particulate matter control facilities. Boilers with particulate matter control facilities installed had significantly lower CPM emission concentrations than those without, suggesting that conventional particulate matter control facilities have some control effect on CPM. Based on the studies, it appears that CPM exists in the gaseous state within the flue atmosphere. Additionally, the flue gas undergoes a transformation into FPM without releasing CPM during the cooling process in APCDs. The range of cooling for APCDs' flue gas is limited, and there are still fine particles of condensed CPM that prove challenging to eliminate, but some of the CPM is removed due to the porous structure of some FPM surfaces, which can adsorb some gaseous state CPM. Therefore, APCDs do not completely remove CPM. A comparison of these studies reveals that these particulate matter control techniques have different effects on CPM, and therefore multiple samples need to be selected for sampling to obtain a comprehensive and generalized pattern of in-flue conversion of CPM.

 
3.2 Removal of CPM by Adsorption

Adsorption is commonly used for the removal of conventional gaseous pollutants. With reference to adsorption control techniques for Hg and VOCs in flue gas, to effectively capture pollutants, it's essential to inject a suitable sorbent at the right spot in the flue, resulting in the direct removal of gas phase CPM. To verify the adsorption effect of activated carbon (AC) on CPM, Zhang et al. (2018) studied the effect of AC on CPM removal at different temperatures and showed that the best adsorption could achieve at 90°C, with adsorption rates ranging from 19%–22%. This removal effect was not satisfactory and was constrained by temperature. Gao et al. (2015) investigated the removal of SO3 in flue gas with different alkaline adsorbents and injection systems, showing that alkaline adsorbent injection techniques and dry powder injection systems were cost-effective removal methods. Although adsorption technology can effectively remove VOCs and SO3 in CPM, the existing Matter are that the adsorbents are difficult to reuse and may cause secondary pollution. The components of CPM are complex and difficult to be effectively removed with a single adsorbent, which will be the main obstacle to the large-scale industrial application of adsorption technology for the removal of CPM in the future. Therefore, the development of new regenerable and multi-effective adsorbents is a research direction for the adsorption and removal of CPM. However, this approach usually results in adsorbent waste and secondary contamination, which requires further treatment of the adsorbent, so the recovery technology and higher price of the adsorbent are the biggest limitations to its application (Liu et al., 2022b). More efficient adsorbents can be considered and modified to improve the adsorption effect and to increase the reusability of the adsorbent, with more consideration given to the post-treatment of the adsorbent at a later stage in future studies.

 
3.3 CPM Removal by Condensation

Removing CPM can be effectively achieved with the condensation method, which is possible due to condensability being the most crucial feature of CPM. The main cooling modes are indirect cooling and direct cooling. It has been studied that the concentration of CPM is correlated with the temperature of flue gas. Based on the research, it has been found that the concentration of CPM decreased with a decrease in the temperature of flue gas (Peng et al., 2021a). Gao et al. (2015) proposed a thermophoretic bumper and horizontal mist eliminator by arranging the WFGD scrubber tower outlet in series. The metal fins of the thermophoresis bonder acted as coolers, promoting the settling of CPM on their surface while also capturing droplets via the horizontal mist eliminator. Arto et al. (2009) applied an indirect cooling method with a heat exchanger to cool the flue gas. Jung et al. (2020) developed a filtration system consisting of two reduced graphene oxide (RGO) filters and a condenser to effectively remove CPM even under high temperatures and acidic conditions. Once it has gone through the initial RGO filter, the gas had its FPM component effectively removed. To facilitate CPM condensation, Wu employed direct cooling to evaporate water spray at the ESP inlet. This created a highly supersaturated water vapor environment that furnished ample condensation nuclei. Additionally, a low-temperature heat exchanger was installed at the WESP inlet to facilitate CPM removal through non-homogeneous condensation. Flue gas cooling was also employed to eliminate FPM (Wu et al., 2021). The combined effects of diffusive and thermophoretic forces effectively eliminate particulate matter, and this method can also be utilized for the removal of fine particulate matter during CPM coalescence. As the technology for utilizing waste heat from flue gas involves cooling it, there is potential to develop a technology that can simultaneously remove CPM and utilize waste heat from flue gas. This could be a promising direction for future development. To remove CPM through condensation, the typical approach involves lowering the temperature to change the gaseous CPM precursors into particles. These particles can then be filtered out to eliminate the particulate matter. However, effective removal was still difficult because the particles were small and produced by condensation. Zheng et al. (2018) studied gaseous SO3/H2SO4 and experimented with four methods to increase the concentration of condensed SO3/H2SO4 particles by adding ammonia, adding fly ash, lowering the temperature, and applying electrical discharge to enhance the efficiency of particle removal.

 
3.4 CPM Removal by Fuel Blending

Over time, various fuel blending techniques have been devised to effectively eliminate a wide range of pollutants (Zhang et al., 2023d). The impact of introducing biomass into pulverized coal on the generation of coal-fired CPM is being studied for its inhibitory effects. It showed that adding the right amount of biomass mixed with pulverized coal could inhibit the generation of CPM, with 30% corn straw being more effective in controlling CPM. This may be related to these factors: firstly, the sulfur level found in corn stover is significantly less than the amount present in pulverized coal. Secondly, the de-volatile fraction consumes more oxygen at the beginning of combustion, therefore, adding corn stover to increase the volatile matter content inhibits the oxidation process of SOx precursors. Thirdly, the combustion of corn stover results in a considerable sulfur fixation effect due to the high concentration of alkali metal elements present in it, thereby reducing SO42 emissions. To further reduce SO42 emissions, it is recommended to implement the findings from previous blending technologies to control CPM during combustion. In addition, new blending technologies should be developed with the specific goal of controlling CPM.

 
4 MULTI-FIELD COUPLED FORCE METHOD FOR CPM REMOVAL


With reference to control methods for the main components of CPM with adsorption, condensation, and applications of existing APCDs being the dominant means of CPM control. The CE-ESP is an innovative use of the electric field, temperature field, and concentration field multi-field coupling based on the current removal methods, and the circulating cooling water is connected to the power plant cooling water system, which has a certain economic value.

 
4.1 CE-ESP Construction and Synergistic Mechanism

The current approach to CPM removal focuses on the development of new dust removal techniques and technologies over traditional dust removal technologies, so improving removal efficiency requires the exploration of more effective and efficient methods for removing the dust that can replace the traditional APCDs. Liu et al. (2023) has developed a CE-ESP that simultaneously facilitates the coagulation, agglomeration growth and capture of CPM and FPM in a single device by modifying the structure and operation of the precipitator with multiple forces coupled synergistically.

By placing multiple parallel rows of anode plates in the ESP and arranging a tubular cooling bundle inside the plates, the cooling water is connected to the boiler supply water system piping. The cold electrode creates a field of low temperature to make the thermophoretic and diffusive forces coupled with stronger electric field forces to enhance the CPM agglomeration, and trapping effects, realizing simultaneous CPM agglomeration, and trapping in the same device. The temperature gradient generated by the circulating cooling water in the system generates thermophoretic forces on the CPM, the electric field works together with turbulent agglomeration to enhance particle bonding, while the concentration field generates diffusive forces that synergistically promote particle coagulation and agglomeration, promoting the condensation and growth of harmful gases in the flue gas, as well as volatile heavy elements.

Multi-field force-coupled CE-ESP differs from WESP where the anode plate is sprayed or with water, which cools the dust collection plate with tubular circulating water, causing a temperature difference in the vertical direction of the flue gas flow and reducing the local flue gas temperature. A low temperature field is formed on the surface of the plate electrodes, prompting water vapor to phase change and coalesce. During the cooling and condensing heat exchange process, as the water vapor phase changes and coalesces, when CPM meets numerous fine droplets, it condenses and increases the specific surface area for collision. The FPM in the flue gas also helps by providing condensation nuclei, which facilitate the heterogeneous condensation process of CPM. Moreover, the moist flue gas gets chilled and condensed while meeting the dust collection plates. A temperature gradient is created from the main body of the flue gas towards the surface of the pole plate due to the difference in temperature between them. Due to the temperature gradient, the plate surface generates a thermophoretic force on microfine particles. Turbulent agglomeration in the presence of coupled electric and temperature fields leads to velocity differences between particles due to flow field disturbances. Because of this phenomenon, particles tend to accumulate in the local flow field, causing highly irregular radial velocities and promoting the formation and growth of new particles. Ultimately, this leads to a concentration of fine particles that are directed toward the dust collection plates, where they become trapped and deposited. When the particles near the pole plate start to decrease, the concentration field's diffusive effect comes into play, which enhances the coagulation and agglomeration phenomena between the particles. As a result, the temperature and concentration fields' thermophoresis and diffusiophoresis, coupled with a stronger electric field, work together to promote the agglomeration, directional movement, and deposition of TPM, particularly CPM, within the electric field. The multi-field force coupling can significantly increase their agglomeration and trapping probabilities. The CPM removal mechanism of CE-ESP is shown in Fig. 5.

Fig. 5. CPM removal mechanism.Fig. 5. CPM removal mechanism.
 


4.2 Case Studies of CE-ESP for CPM Removal

Case studies of CE-ESP on CPM removal were conducted in a municipal solid waste incineration plant and a coal-fired power plant. The effectiveness of the device for CPM removal was studied by Liu et al. (2023) in a municipal solid waste incineration plant, where the efficiency in removing CPM was up to 76%, with the organic fraction being more efficient than the inorganic fraction. It also had a significant effect on the removal of SO42, F, Ni, Al, Cr, Pb, and As in CPM, while it had a poor performance on the removal of NO3 and Ni in CPM and a significant contribution to the reduction of esters in CPM. Liu employed the Impact Condensation Method based on EPA Method 202 to test the emissions of CPM and FPM at a coal-fired power plant stack inlet in a coal-fired power plant. Additionally, CE-ESP was installed to remove CPM. High efficiency of CPM removal by multi-field synergistic CE-ESP with temperature and electric fields coupled with concentration fields, with a removal rate of up to 93%. The average CPM concentration after removal was 2.8 mg Nm3, and the CE-ESP had a significant removal effect on SO42, Ca, As, esters, and olefins from CPM as well. What’s more, the CE-ESP showed higher efficiency for the inorganic substance as compared to the organic substance. The CPM concentration in the treated flue gas was significantly reduced and could reach the ULE standard before emission.

 
5 SUMMARY


With the implementation of ULE in power plants, the concentration of FPM emissions in flue gas has been gradually decreasing. As a result, there has been a gradual rise in the percentage of CPM within TPM. The issue of CPM measuring, and removal has attracted considerable attention as conventional dust removal devices cannot easily remove CPM before condensation. This paper summarizes the conceptual aspects of CPM, formation mechanisms, emissions, composition, and morphological characteristics. An effort has been made to provide a better overview of CPM removal methods and treatment prospects through the formation mechanism of CPM and the removal of the main components.

CPM is a type of particulate matter that can condense, so it has two states in the flue and atmosphere. The microscopic properties of CPM are small and porous, which makes it difficult to remove in industrial production. Further control of CPM is a crucial and complex area of air pollution control. The main inorganic components of CPM are volatile gases and inorganic cations, including heavy metals, and the organic components are complex and overlap with VOCs and PAHs. These conventional pollutants can be removed by reference to the methods used in the application.

CPM is a challenging substance to deal with due to its intricate makeup, small particle size, and sensibility to temperature. As a result, the standard approach is to employ advanced air pollution control devices like LL-ESP, WESP, and EBF, which are particularly effective in removing CPM. Despite the current emission levels being comprehensive, they still do not meet the ULE requirements. The adsorption and fuel blending methods are also effective in removing CPM, and more consideration should be given to economics and recyclability in the application process. Regarding the temperature sensitivity of CPM, the condensation method can potentially cause both particle size growth and agglomeration condensation. Based on the condensation method to modify ESP, multi-field force coupling CE-ESP for the removal of CPM was adequate, which had the ability to agglomerate and remove CPM in the same device, and the circulating cooling water could be connected to the power plant supply water, that could be considered in combination with the flue gas waste heat recovery technology and CPM removal of its synergistic development.

 
ACKNOWLEDGMENT 


This work was supported by the National Natural Science Foundation of China (Grant No. 51761125011).


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