Characterization and Removal of Condensable Particulate Matter in Flue Gas Studied with Cold Electrode Electrostatic Precipitator

The emissions of filterable particulate matter (FPM) from stationary source flue gases are well-controlled, but condensable particulate matter (CPM) is becoming a growing concern as it's more challenging to eliminate and poses a more significant environmental threat. Conventional air pollution control devices (APCDs) in coal-fired power plants have limited control over CPM. To address the issue, our research employed the Impact Condensation Method based on EPA Method 202 to test the emissions of CPM and FPM at the stack inlet in a coal-fired power plant. Additionally, a cold electrode electrostatic precipitator (CE-ESP) was installed to remove CPM. According to the study, CPM was emitted at the stack inlet with an average concentration of 30.2 mg Nm –3 , which far exceeds the concentration of FPM (3.7 mg Nm –3 ) and the Chinese ultra-low emission (ULE) standard for particulate matter. Regarding emission characteristics, the most significant proportion of total detected anionic water-soluble was SO 42– in CPM. Na and Ca were the most abundant elemental metal components, followed by Pb. The primary organic compounds found in CPM were hydrocarbons, olefins, and esters. This was further evidence of the environmental and human hazards of CPM. The removal of CPM is highly efficient with multi-field synergistic CE-ESP that combines temperature, electric, and concentration fields. The removal rate can reach up to 93%. The average CPM concentration after removal was 2.8 mg Nm –3 , and the CE-ESP had a significant removal effect on SO 42– , Ca, As, esters, and olefins from CPM. Moreover, the CE-ESP showed higher efficiency for the inorganic substance than the organic substance. The CPM concentration in the treated flue gas was significantly reduced and could meet the ULE standard before emission. This study indicated that the concentration of CPM was extremely high at the stack inlet. However, CE-ESP was able to remove CPM efficiently.


INTRODUCTION
In recent years, China has proposed a "Peak carbon, Carbon neutral" action plan, which will gradually reduce the share of thermal power generation.However, according to the "World Energy Statistical Review 2022", coal will continue to be the primary fuel for power generation, increasing from 35.1% in 2020 to 36% in 2021, with a 6.2% increase in power generation.As a simple and reliable resource, coal remains the most secure energy component for power generation.Coal-fired power plants (CFPPs) are widely recognized as significant contributors to air pollution due to their high emissions of pollutants.They are now subjected to stricter regulations (Jiang et al., 2024).Stationary combustion sources are a significant contributor to air pollution, particularly the formation of haze, due to the emission of particulate matter (Huang et al., 2021).There are two types of primary particulate matter: condensable particulate matter (CPM) and filterable particulate matter (FPM).CFPPs being stationary sources are known to emit a significant amount of primary particulate matter, and available air pollutant control systems (APCDs) already meet ultra-low emission (ULE) standards for the removal of conventional pollutants such as FPM (Liang et al., 2020).However, there are severe deficiencies in the removal of CPM.Processes for controlling contaminants in existing CFPPs have been in mature application for over a century (Yang et al., 2015).They were initially designed primarily for conventional pollutants such as FPM generated from coal combustion.CPM mainly consists of unconventional pollutants such as SO3, heavy metals, and VOCs, and it was not initially included in the pollutant removal list due to a lack of adequate knowledge (Cui et al., 2018).However, as control technologies continue to be optimized and the requirements for pollutant emission control increase, the problem of CPM emissions and the significant environmental risks they present are gaining the attention of more scholars (Li et al., 2020).
The combustion of coal as a fuel is currently a significant cause of PM2.5 pollution in China.Electricity production mainly relies on coal-fired power generation, but it has a substantial impact on air pollution and climate change (Liang et al., 2020).CPM particles typically have a size of less than 2 µm.However, it has a larger specific surface area, making it easier to concentrate harmful elements and their compounds, making it an essential component of PM2.5 (Lu et al., 2019a).There is a distinction between CPM and FPM.CPM exists in gas or vapor and is not efficiently eliminated by current treatment methods.Due to temperature changes after leaving the flue, CPM condenses into a liquid or solid state when it enters the atmosphere, making it difficult to accurately measure its concentration with existing detection equipment (Liu et al., 2019).Research has indicated that the percentage of CPM in the TPM emitted from CFPPs is over 50% (Wang et al., 2021;Liu et al., 2022;Yang et al., 2022b;Yuan et al., 2022), so a removal technology that can effectively remove CPM urgently requires development.
Most of the research on CPM at this stage has focused on emission characteristics.Researchers collected TPM emitted from ultra-low emission CFPPs, and they found that the percentage of CPM in TPM (CPM/TPM) ranged from 5% to 84% (Yang et al., 2018;Zheng et al., 2018).Researchers (Yang et al., 2022b) analyzed a sample of TPM from a steel mill.It showed that CPM/TPM could reach 99.6%.In addition, they investigated the issue of boiler emissions from different fossil fuels.Based on the results, the proportion of CPM/TPM levels was observed to vary from 25.7% to 96.5% (Yang et al., 2022a).Based on the studies conducted, it was evident that the levels and ratios of CPM emissions were substantial, and the environmental impact cannot be disregarded.However, without more severe emission problems, studies on the deeper removal of CPM have rarely been addressed.
Studies on CPM removal have focused on the efficiency of APCDs.The current APCDs vary in their effectiveness in removing CPM in flue gas (Huang et al., 2020).According to research, the LLT-ESP has demonstrated a CPM removal rate ranging from 60.9% to 78.89% (Yang et al., 2018;Zheng et al., 2020;Fujitani et al., 2023).Researchers (Li et al., 2016) sampled CPM at the electrostatic precipitator (ESP) inlet and outlet, the wet flue gas desulfurization (WFGD) outlet, and the middle of the stack.It showed that the removal efficiency of ESP reached 87.3%, while the effectiveness of WFGD and wet electrostatic precipitator (WESP) in removing CPM was lower at 36.7% and 22.2%, respectively.However, some researchers also found negative efficiency data for WFGD and WESP on CPM (Qi et al., 2017).Thus, it is of great practical importance to develop a removal technology that can efficiently and consistently remove CPM from the flue gas of CFPPs.
This study collected CPM and FPM samples from a domestic CFPP to study their emission characteristics.The current methods of removing non-conventional pollutants such as CPM from ULE power plants in China focus on the development of new dust removal efficiency enhancement techniques and technologies over traditional dust removal technologies, so improving removal efficiency requires discovering novel and efficient dust removal devices and methods to replace the conventional dust collectors.This study has developed a cold electrode electrostatic precipitator (CE-ESP) that effectively removes flue gas emissions from coal-fired power plants.The modification of the precipitator's structure and operation allows for the deep removal of these emissions while achieving the synergistic promotion of condensation, agglomeration growth, and trapping between CPM, CPM and FPM in a single unit by coupling multiple forces.The mechanism is shown in Fig. 1.A study was conducted on the CPM removal effect of CE-ESP at a waste incineration plant.The results showed an efficiency of up to 76% in removing CPM, with the organic fraction proving to be more effective than the inorganic fraction.It also showed effective removal of SO4 2-, F -, Ni, Al, Cr, Pb, and As in CPM, while the removal of NO3 -and Ni from CPM was poor, and there was a noticeable reduction in the esters present in CPM after removal.

EXPERIMENT AND METHODS
Currently, CFPPs in China are mainly 300 MW and 600 MW units.A 350 MW unit CFPP situated in Hebei Province has been chosen.The CFPP serves as a significant project for the city's three-year campaign to prevent and control air pollution, as well as to address the gap in heat supply.It is built with flue gas desulfurization, denitrification, dust removal and plume treatment, desulphurization wastewater disposal devices, etc.The principle of denitrification equipment is selective catalytic reduction, and the dust collector is an electric-bag dust collector.The power plant's flue gas treatment process and sampling points are illustrated in Fig. 2. The sampling point located in front of the stack was selected for this experiment.
In this study, flue gas was gathered from the power plant's stack and examined for CPM concentration, as well as physical and chemical properties.A CE-ESP, depicted in Fig. 3, was utilized to comprehensively cleanse the flue gas after the ultra-low emission power plant's APCDs initially treated it to capture and consolidate CPM resistant to conventional dust removal methods.This device is a product of independent research and development.Its principle is incorporating an additional condensation method based on the electric precipitator.This device features a durable stainless-steel casing and utilizes serrated mansard wire for its discharge electrode.It is a reliable  The electric precipitator of the tubular type utilizes a temperature-controlled mechanism to circulate cooling water outside the dust collection poles.To maintain the strength of the electric field, it's essential to distribute the water film evenly and consistently.The heat exchange tank is arranged on the outside of the dust collection poles of the electric precipitator to ensure a stable and uniform heat exchange of the dust collection poles during the stable operation of the system.
To control heat exchange, the flow rate of circulating cooling water is managed to create an external temperature field.This water flow regulation allows precise heat exchange control and effectively lowers the flue temperature.It Provides additional thermophoretic force coupling with existing electric field force and diffusion force to enhance the capture of particulate matter.By modifying the structure and operation of the dust collector, it is possible to simultaneously promote coagulation, agglomeration growth, and trapping between CPM, CPM, and FPM in a single unit using multiple force coupling.
The sampling system was built independently based on EPA Method 202, which could test FPM and CPM simultaneously, with selected sampling points, frequencies, and volumes according to Chinese GB/T 16157-1996, as shown in Fig. 3.The Laoying 3012H automatic flue gas analyzer was used to measure at selected sampling points in the flue with a steady flow rate.The front part of the fume tester was a standard sampling nozzle, cartridge, and sampling gun for particulate matter collection systems.The samples were dried at 130°C before being processed in a desiccator containing 20-25°C silica gel and weighed by an analytical balance.The sampling gun was connected to a homemade test system with a hose at the end of the gun.The pipeline receives a PM-laden flue gas that undergoes filtration through a filter cartridge, which traps the FPM.The removed FPM then enters a recirculating condenser tube where the condensed CPM is captured in a buffer bottle due to the impact of the flue gas.The CPM is thoroughly absorbed and collected.To maintain a sampling temperature below 30°C, a thermocouple thermometer has been installed in the water bath.The CE-ESP was available in three operating modes: temperature field only mode A1, the electric field only mode A2, and multi-field coupled mode A3 with temperature and electric fields, etc.The three operating modes were used to purify the power plant flue gas in depth, and the efficiency of each operating mode and the removal characteristics were investigated.After sampling, the cartridges and CPM membranes were removed with tweezers and stored in the appropriate sample storage boxes.Secondly, clean air was used to blow off the condensate at the sampling flow rate for 1 hour., after which the condensate was collected from the collection and buffer bottles, the glass parts were rinsed with deionized water, and the rinse solution was collected in the ultra-pure water rinse solution, followed by rinsing of the condensate tubes, condensate collection bottles and buffer bottles with hexane.The condensate was then rinsed with hexane and collected in a hexane rinse solution.The filter membrane was then used to collect ash samples from the top to the bottom of the cold electrode precipitator.
Samples collected on site included FPM cartridges, CPM membranes, ultrapure water rinse, hexane rinse, and cold electrode scrubber fly ash.They were processed separately and characterized, as shown in Fig. 3.
After sampling, the cartridge at the front of the sampling gun was dried and weighed to obtain the mass of FPM.The mass concentration of CPM was the sum of two components: the solid phase fraction on the filter membrane and the liquid phase fraction in the condensate and wash solution.
The weighed CPM membranes were sonicated using ultrapure water and n-hexane, respectively.The organic fraction of the CPM samples was extracted with n-hexane.The inorganic fraction of the CPM sample was also removed with deionized water in ultrasonication.The organic extract was evaporated to 1 mL.The organic fraction of CPM was tested semi-quantitatively using a gas chromatograph/mass spectrometer (GC/MS) with a GC/MS injection volume of 1 µL.
An ion chromatograph was used to quantify the anions (Cl -, F -, NO3 -, NO2 -, SO4 2-) in the inorganic component solution.The calibration curve correlation coefficients were all greater than 0.999.A visible spectrophotometer was used to quantify the concentration of ammonium ions (NH4 + ) based on the nano-reagent method, and the calibration curve correlation coefficients were more significant than 0.99.An inductively coupled plasma emission spectrometer was used to quantify the concentration of metal elements.The calibration curve correlation coefficients of the detected metal elements (Ca, Na, As, Cd, Se, Cr, Pb, Hg) were all greater than 0.999.
The filter membranes from the sampling system, microscopic characterization, composition, and particle size analysis were performed separately on the filter membranes, and dust collector fly ash samples were obtained from various areas.This also included the cold electrode samples.
Morphological observations of the particulate matter trapped on the filter membranes were carried out using a thermal field emission scanning electron microscope.A semi-quantitative elemental analysis of the particulate matter trapped on the filter membranes was also carried out using an equipped X-ray energy spectrum analyzer.

CPM and FPM Emission Concentrations and Removal Efficiency
The mass concentrations of the blank collected for analysis are shown in Table 1.The concentrations of FPM, CPM, organic CPM, and inorganic CPM in the flue gas at the sampling point were 3.7 mg Nm -3 , 30.2 mg Nm -3 , 5.9 mg Nm -3 , 24.3 mg Nm -3 , respectively.After treatment with conventional APCDs at the power plant, the concentration of FPM decreased significantly, meeting the ULE standard concentrations.Nevertheless, the total CPM mass concentration exceeded 30 mg Nm -3 , with the inorganic component of CPM accounting for 80.5%.Currently, CPM emissions cannot be ignored, and it is urgent to establish CPM emission limits and further remove CPM from flue gas by employing new dust removal equipment.
As shown in Fig. 4, Blank refers to the flue gas emitted by coal-fired power plants that have not been treated by cold electrode electrostatic precipitators.The CE-ESP was available in three operating modes: temperature field only mode A1, the electric field only mode A2, and multi-field coupled mode A3 with temperature and electric fields.The removal efficiency of the conventional electric field's A2 operating condition was significantly higher than the A1 with only a temperature field.However, the CPM inorganic removal efficiency was essentially the same at A1 and A2, probably due to the water-soluble substances of the CPM inorganic dissolving and being removed as the temperature decreases.This indicates that temperature had a more significant influence on the inorganic component of CPM.However, the removal rates of total FPM, total CPM, CPM organic, and CPM inorganic components by the cold electrode electrostatic precipitator under multi-field force coupling conditions reached 81.36%, 93.00%, 56.76%, and 87.02%, respectively.The results showed a noticeable increase compared to the previous two operating conditions.The CE-ESP had a slightly lower removal efficiency for FPM, which could be due to its lower concentration and smaller particle size.When particles decrease in size, they typically have a lower charge.However, this decrease is offset by an increase in mobility, resulting in a balance between the reduction in charge and the increase in mobility.The efficiency of removing the organic fraction of CPM was slightly lower compared to its inorganic counterpart.The development of CPM primarily resulted from the processes of evaporation, nucleation, and condensation.When the flue gas entered the CE-ESP, the circulating water cooled the temperature rapidly.The lower temperature promoted homogeneous nucleation and heterogeneous condensation of CPM.The thermal movement of flue gas molecules is weakened, which results in a reduction of the flue gas viscosity.And the multi-field force coupling further enhances the reaction of CPM with droplets and their adsorption on the FPM surface (Zhang et al., 2021).After treatment with CE-ESP, the CPM concentration reached 2.8 mg Nm -3 , which was basically at the same level as FPM and met the ULE standards well.

Composition and Proportion of Organic Substances in CPM Samples
Based on Feng's research, the organic component of CPM was composed of numerous organic substances, primarily alkanes, esters, and other large molecules (Feng et al., 2020).The primary source of the organic component in CPM was the incomplete combustion of coal.After conducting research, Feng et al. (2020) concluded that raw coals possessing lower volatile content and higher fixed carbon content tend to have slower burn-up rates.It would be challenging to fully decompose organic materials, which could harm the environment, but the current conventional dust removal equipment was ineffective in removing them.
The efficiency of removal CPM of CE-ESP was investigated in 3 modes.Sampling points are located after the conventional dust removal equipment in CFPPs.Semi-quantitative analysis of the organic fraction of CPM trapped on the condensate and filter membrane showed that alkanes accounted for approximately 60% of the peak area.For a more visual comparison, the percentage of organic species in the CPM for blank samples and A1, A2, and A3 operating conditions, respectively, were plotted in Fig. 5. Comparing the operating conditions of A1 and A2, the lower flue gas temperature, lower flow rate of flue gas and increased reaction time resulted in better removal of ketones and esters which were more sensitive to temperature.Researchers (Song et al., 2020) discovered that the significant n-alkanes in CPM were C24-C30 and the most abundant phthalate ester was dibutyl phthalate.In addition to the higher carbon alkanes, tetra-and pentacyclic PAHs were common with C21-C29 (Cano et al., 2019).It is possible that the application of external voltage could result in the removal of certain PAHs and meso-alkanes by CE-ESP.This might be due to the combination of organic matter, such as hydrocarbons and particles being carried away by fly ash.A2 was better for removing alkanes and aromatic hydrocarbons, and A3 significantly impacted alkanes, aromatic hydrocarbons, esters, and ketones.All of which were positively removed by the CE-ESP under multi-field force coupling conditions.According to the findings of Liang et al. (2020), dust collectors were effective in capturing CPM organics in flue gas by transferring them from the gas phase to the particulate state through fly ash adsorption.This was in accordance with the analytical findings of this paper.CE-ESP treated flue gas in the temperature field mode without electro-cooled plates, and the lower flue gas temperature and lower flow rate of flue gas When CE-ESP was used to treat flue gas in the temperature field mod, it resulted in lower flue gas temperature and lower flow rate.This led to a significant impact on the removal of organic compounds such as ketones and esters.

Composition and Proportion of Inorganic Substances in CPM Samples
Table 2 showed the concentrations of major anions, ammonium ions, and major metal ions in the inorganic fraction of CPM for each operating conditions of the blank and cold electrode precipitator.SO4 2-and Cl -were the samples' major contributors to anions.The presence of ammonium ions was significant and could not be ignored.The anion that was most commonly found in CPM was SO4 2-.The SO4 2-primarily originates from the elemental S in the raw coal.F -and Cl -were easily released during coal combustion because of their high volatility.During the condensation process in the sampling unit, NH3 in the flue gas reacted with H2O and SO2 to produce (NH4)2SO4 (Qi et al., 2017).At low temperatures, the ammonium bicolpate produced was condensed onto the surface of fly ash.Alternatively, it could form droplets that merge with other particles and combine to form CPM. The levels of Na + and Ca 2+ in CPM were notably elevated compared to the concentrations of other metallic elements, which was also consistent with previous tests at actual emission sources.Substances such as Ca were usually found in coal in combination with organic matter.This was due to their strong gasification properties, making them more likely to form particulate matter through a "gasification-condensation" mechanism.Removing more of these components from the flue gas would be highly advantageous for effectively managing the inorganic portion of CPM emissions.
In the desulphurization tower, when (NH4)2SO4 aerosol met the slurry droplets, the inertial sulfate ions might bypass the droplets and escape from the WFGD with the flue gas (Yang et al., 2019).The desulfurization slurry effectively removed water-soluble ions from the CPM through a synergistic effect.However, it also introduced new components to the flue gas.During the desulfurization process, there was speculation that the circulating slurry, mainly CaCO3, would evaporate into the gas phase.As a result, small droplets containing Ca could be carried into the flue gas.Compared to other soluble metal ions, sodium (Na + ) had a more excellent solubility in water.It can be readily transported into the flue gas, leading to inadequate or even counterproductive removal of CPM from the desulfurization tower (Faiz et al., 2018).Because of its small particle size and characteristics, it was difficult to remove by conventional power plant APCDs, which resulted in significant emissions.
Fig. 6 displayed that the removal of halogen elements in A1 was more effective due to its temperature sensitivity.The precipitator's gas flow rate is reduced because of lower flue gas temperature, which increases its residence time in the CE-ESP.Condensation caused the formation of numerous nodules, which increased the possibility of CPM contact with reactive substances on their surfaces.As shown in Table 3, according to the results, A3 showed the highest efficiency in removing SO4 2-.If the temperature at the CE-ESP inlet falls below 100°C, it quickly drops below the acid dew point.Most of the SO3 was converted to gaseous H2SO4, reducing some of the CPM and thus increasing the efficiency of SO4 2-removal in the CE-ESP.Furthermore, when the flue gas entered the CE-ESP, small particles of water-soluble ions in the form of submicron aerosols were    (Lu et al., 2019b).
The CPM also comprised metallic elements, including Na, Ca, and As, primarily in their ionic forms.And their emission into the atmosphere resulted in some environmental risks.As seen from Fig. 7 comparing A1 with A2, most metal cations were not particularly sensitive to temperature.Thus high voltage ionization was relatively effective in removing them.The CE-ESP was most effective for removing Ca and was suitable for the removal of the heavy metal ion As, as both Ca and As were highly valued cations.The Ca ion had a larger ionic radius.the higher the dispersion, the less likely it was to settle during the atmospheric drift, the longer the transport distance, and the greater the range of impact on the quality of the atmosphere.Also, the smaller the diameter of the particles, the easier it was to adsorb large amounts of toxic and harmful substances.

Microscopic Morphology of CPM Samples
As the PM moved towards the flue gas exit, it tended to have larger particle sizes.This also illustrated the role of thermophoretic forces coupled with electric field forces in promoting the agglomeration of PM.The presence of fine PM mainly in the form of spherical piles indicated that the collected fine particulate matter was dominated by fine particulate matter from the original coal combustion flue gas, with spherical particulate matter forming agglomerates of larger particle size with fine particulate matter.It also further indicated that the CE-ESP strengthened the condensation of CPM through multi-field force coupling and enhanced the force of the electrode plate on the particulate matter, thus achieving a strong CPM capture effect.

CONCLUSIONS
(1) Traditional APCDs successfully captured FPM but did not eliminate CPM from flue gas efficiently.
The concentration of CPM was measured at 30.2 mg Nm -3 , while that of FPM was 3.7 mg Nm -3 in flue gas at the stack inlet.It appears that the CE-ESP could effectively remove CPM under the synergy of coupled fields, and the efficiency could reach 93%.After using the CE-ESP at the stack inlet, the concentration of CPM dropped to 2.8 mg Nm -3 , meeting the requirements of ULE for particulate matter.
(2) The main organic substances in CPM included alkanes, aromatic hydrocarbons, olefins, and esters.The highest proportion of CPM organic were alkanes.Due to their low volatility, solid forms of high carbon alkanes could be challenging to remove using conventional APCDs.However, in A3 mode, the CE-ESP, which couples the temperature and concentration fields with the electric field, has a stronger coagulation and trapping effect on fine particles than a single electric field or a single temperature field dust collector, resulting in better removal of high carbon alkanes.CE-ESP more effectively removed olefins and esters than other substances.
(3) The main inorganic substances in CPM included SO4 2-, Cl -, NH4 + , Na + , and Ca 2+ , which were difficult to remove since the particle size was tiny and the condensation characteristics of CPM.However, under the synergistic action of the coupled fields, the CE-ESP had a removal efficiency of more than 70% for inorganic anions, up to 83.6%, and also had a high removal efficiency for ammonium ions, which were difficult to remove by conventional dust collectors.SO4 2-and Ca were more effectively removed by CE-ESP than other substances.(4) The CE-ESP facilitated the condensation as well as an agglomeration of CPM and strengthened the driving force of CPM to the electrode plate via multi-field force coupling to achieve further efficient capture of CPM.

Fig. 4 .
Fig. 4. CPM and FPM emission concentrations and removal efficiency at various operating conditions.

Fig. 5 .
Fig. 5. Organic components in CPM were captured by CE-ESP under different operating conditions.

Fig. 6 .
Fig. 6.Inorganic components in CPM were captured by CE-ESP under different operating conditions.

Fig. 8 .
Fig. 8. Micromorphology of the four electric field CPMs in the cold electrode.

Table 1 .
The mass concentration of FPM and CPM.

Table 2 .
The concentration of major ions in CPM inorganic at various operating conditions.

Table 3 .
Removal efficiency of major inorganic ions in CPM.Their removal by the CE-ESP was achieved by ionization, Brownian diffusion, inertial collisions, and thermophoretic effects.The high removal efficiency of SO4 2-was probably because of the small Stokes value of the submicron H2SO4 aerosol