Moisture Removal Techniques for a Continuous Emission Monitoring System: a Review

atmosphere

Review Moisture Removal Techniques for a Continuous Emission Monitoring System: A Review

Trieu-Vuong Dinh and Jo-Chun Kim *

Department of Civil and Environmental Engineering, Konkuk University, 120 Neungdong-ro, Gwangjin-Gu, Seoul 05029, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-2-450-4009; Fax: +82-455-2994

Abstract: A continuous emission monitoring system (CEMS) is a well-known tool used to analyze the concentrations of air pollutants from stationary sources. In a CEMS, the presence of a high moisture level in a sample causes a loss of analytes due to artifact formation or absorption. This issue brings about a bias in the measurement data. Thus, moisture removal is an important pretreatment step. Condensation and permeation methods have been widely employed to remove moisture from the CEMS for gaseous compounds. In terms of particulate matter, dilution methods have been applied to reduce the moisture level in the gas stream. Therefore, condensation, permeation, and dilution methods are critically reviewed in this work. The removal efﬁciencies and recovery rates of analytes are discussed, as well as the advantages and disadvantages of each technique. Furthermore, the suitable applications of each technique are determined. Condensation methods have not been well documented so far, while permeation and dilution methods have been continuously studied. Many types of permeation materials have been developed. The limitations of each method have been overcome over the years. However, the most reliable technique has not yet been discovered.

Keywords: moisture removal; CEMS; condensation; permeation; dilution; desolvator

Citation: Dinh, T.-V.; Kim, J.-C. 1. Introduction Moisture Removal Techniques for a The continuous emission monitoring system (CEMS) has been applied to monitor Continuous Emission Monitoring the air pollutants emitted from stationary sources. An extractive method, in which air System: A Review. Atmosphere 2021, 12, 61. https://doi.org/10.3390/ pollutants are delivered to analyzers located in a shelter, and an in situ method, in which atmos12010061 analyzers are attached directly to an emission stack, are used for the CEMS [1]. The CEMS is usually applied to detect the emissions of air pollutants such as carbon monox- Received: 2 November 2020 ide (CO), carbon dioxide (CO2), sulfur oxides (SOx), nitrogen oxides (NOx), hydrogen Accepted: 29 December 2020 chloride (HCl), hydrogen ﬂuoride (HF), ammonia (NH3), water vapor (H2O), particu- Published: 1 January 2021 late matter, etc. Spectroscopy analyzers have been widely installed in CEMS due to the advantage of continuous monitoring and good accuracy. As a spectroscopy analyzer, Publisher’s Note: MDPI stays neu- nondispersive infrared (NDIR) and Fourier transform infrared analyzers have been widely tral with regard to jurisdictional clai- used because they operate consistently with low energy consumption compared to other ms in published maps and institutio- spectroscopy technologies [2]. However, moisture (H2O) in the gas stream is a signif- nal afﬁliations. icant interference since it affects the accuracy of the NDIR analyzer [3]. Moisture can cause a bias of the NDIR analyzer up to 30% for NO2 (i.e., at a wavelength of 6.21 µm), 20% for SO2 (i.e., at a wavelength of 7.45 µm), and 5% for NO (i.e., at a wavelength of 5.25 µm) [4]. For particulate matter, the particle concentration can be monitored by light Copyright: © 2021 by the authors. Li- censee MDPI, Basel, Switzerland. scattering analyzers (e.g., optical particle counter or condensation particle counter) [5–9], This article is an open access article light absorption analyzers (e.g., spot meters, aethalometer, photoacoustic soot sensor, or distributed under the terms and con- laser-induced incandescence) [5,6,10], light extinction analyzers (e.g., cavity ring-down ditions of the Creative Commons At- or opacity meter) [5,6,11,12] and microbalance analyzers (e.g., tapered element oscillation tribution (CC BY) license (https:// microbalance or quartz crystal microbalance) [5,6,13]. Size distributions of particles can be creativecommons.org/licenses/by/ continuously determined using a fast-mobility particle sizer and an electric low-pressure 4.0/). impactor [5,6,14–16]. Among these methods, light-scattering meters, opacity meters, Beta

Atmosphere 2021, 12, 61. https://doi.org/10.3390/atmos12010061 https://www.mdpi.com/journal/atmosphere Atmosphere 2021, 12, 61 2 of 25

attenuation meters, and electrification devices have been widely applied for CEMS [17,18]. However, H2O has a significant effect on the light-scattering method used to measure particulate matter [18–21]. It was found that light-scattering ratios of sodium chloride particles increased from 1- to 10-fold when the relative humidity increased from 20% to 80%. On the other hand, these values for uranine dye particles were increased 1-2-fold due to their lower hygroscopicity [18]. Zieger et al. (2013) found that light-scattering ratios were increased approximately 5-, 7-, 9-, and 16-fold with respect to (NH4)2SO4, NaNO3, Na2SO4, and H2SO4 particles, respectively, at 85% relative humidity and a 589 nm wavelength [19]. It was also found that the number of particles increased by approximately 50%, and the PM10 concentration increased by as much as 46% when the experiment was conducted at 75% relative humidity [20]. For electrification devices, wet gas streams had significant effects on the probe electrification [17]. It is now well known that the moisture content in the flue gas is high, as shown in Table1. The United States Environmental Protection Agency (U.S. EPA) stated that moisture is one of the significant bias sources for extractive CEMS [22]. Moisture causes effects such as the absorption of water-soluble gases or artifact formation. For artifact formation in the presence of moisture, HCl may react with NH3 to produce ammonium chloride salt [23]. In a municipal waste incinerator, NH3 was found to react with HCl or SO2 to create ammonia salts such as ammonium chloride or ammonium sulfite [24]. For the absorption, a negative measurement bias for NH3 was found due to condensation [24]. Thirty percent of 10 ppmv SO2 was found to be lost at 20% absolute humidity [22]. The condensed water could also corrode the system and cause a leak [22]. In combination with particles in the gas stream, mud could form and plug the system [22]. Hence, moisture removal is an important issue for extractive CEMS. Reactions and dissociations of several analytes may occur in two ways as follows [25–29]:

Table 1. Humidity in the ﬂue gas of some industrial stacks.

Emission Source Temperature (◦C) Humidity (vol%) Blast furnace (coal fuel) [30,31] 72 5.6–10 Heating station [32] 150 0.9 Basic oxygen furnace [31] 54 5.7 Coke oven [31,32] 79–150 0.7–5.1 Electric arc furnace [31] 86.7 4.2 Heavy oil plant [31] 247 7.3 Cement (drying) [31] 103 19.4 Cement (pryo-processing) [31] 142 3.5 Boiler (heavy oil fuel) [33] 310 10 Power plant (coal fuel) [30–32,34,35] 100–177 0.9–8.7 Hazardous waste incinerator [23] – 50 Portland cement kiln [23] – 10–35

In the ﬁrst case, moisture is condensed as liquid droplets:

+ − CO2(g) + 2H2O(aq) ↔ H3O + HCO3 − 2− + HCO3 + H2O(aq) ↔ CO3 + H3O + − NH3(g) + H2O(aq) ↔ NH4 + OH − − NH3(g) + HCO3 ↔ NH2COO + H2O + − SO2(g) + 2H2O(aq) ↔ H3O + HSO3 - + − 2NO2(g) + H2O(aq) ↔ NO2 + 2H + NO3 − + − 2− 2NO2(g) + HSO3 + H2O(aq) ↔ 3H + 2NO2 + SO4 SO2(g) + H2O (aq) → H2SO3 Atmosphere 2021, 12, 61 3 of 25

In the second case, moisture remains in the gas phase:

2NH3(g)+SO2(g)+H2O(g) ↔ (NH4)2SO3(s) NH3(g)+SO2(g)+H2O(g) ↔ NH4HSO3(s) 2NH3(g)+NO2(g)+H2O(g) ↔ (NH4)2NO3(s) NH3(g) + NO2(g) + H2O(g) ↔ NH4HNO3(s)

To reduce the effect of water vapor, three methods have been used to date. In the first method, both extractive sampling lines and the gas cells of analyzers are heated to prevent water vapor from forming condensation. The high energy consumption caused by a heater is a disadvantage of this method. In addition, air pollutants in contact with a high-energy light source in the analyzer such as UV or IR can cause reactions among the pollutants. Another disadvantage of the method is the measurement bias because of the sample temperature [23]. It was found that the HCl measurement results involved a negative bias when the sample temperature was lower than the stack temperature. In contrast, when the sample temperature was higher than the stack temperature, particulate ammonium chloride might volatilize to produce HCl and NH3, which brought about a positive measurement bias for HCl and NH3 [23]. The second method involves the dilution of flue gas to reduce the concentration of water. However, dilution factors are dependent on the detection limit of the gas analyzer. Therefore, this method requires highly sensitive analyzers that increase the cost of the CEMS. The third method, which has been popularly employed, is the cool/dry method. In this approach, moisture in the flue gas is removed using a moisture removal system. The removal of moisture from the flue gas helps to reduce the energy consumption of a heater, the interference of target compound detection, and artifact formation. Due to the high concentration of moisture in the gas stream emitted from a stack, two types of moisture removal methods are commonly used: condensation and permeation [1]. Condensation processes are generally classified into homogeneous and heterogeneous condensation [36]. In homogeneous condensation, a liquid droplet embryo is formed within the supercooled vapor. On the other hand, in heterogeneous condensation, liquid droplet nucleation occurs at the interface of another phase (e.g., a cold solid wall or particle) at a low temperature [36]. For the permeation method, permeation membranes are divided into dense (i.e., pores of ~0.1 nm) and porous (i.e., pores of ~0.1 µm) types [37]. However, an ideal method has not yet been invented since each of these has limitations [22]. Although moisture removal is an important issue in CEMS, scientific research into this topic is lacking. Almost all moisture removal technologies are presented as patents and commercial products. However, many studies have been carried out on moisture control for other processes such as air conditioning [38–44], chemical analysis [45–52], clothes drying [53–57], gas turbines [58–60], and laundering [61–63]. Consequently, the aim of this study is to review the moisture removal methods for extractive CEMS. As for gaseous compounds, condensation and permeation methods are taken into account here. For particulate matter, dilution methods are discussed. The advantages and disadvantages of each gas and particle method will be addressed as well. Furthermore, future research will be suggested to develop the best moisture removal system for CEMS.

2. Methodology Moisture removal methods for CEMS are systematically reviewed in this study. Google Scholar, Google Patent, ScienceDirect, and the library database of Konkuk University were the databases used to search for references. The scope of the review was condensation and permeation methods for gaseous compounds and dilution methods for particulate matter in CEMS. Since the review work was carried out to determine the development of moisture removal methods for CEMS, publication years of references were not limited. Published patents, peer-review articles, conference papers, books, and standard methods were searched out in the English language. All searches were conducted by May 2020. Key- words including moisture removal, dehumidiﬁer, continuous emission monitoring system, cooler, moisture permeation, moisture membrane, Naﬁon® dryer, stack dilution sampling, Atmosphere 2021, 12, 61 4 of 25

and pretreatment device were used. First, the title and abstract of references were read to select references relevant to the scope of this study. Then, full-text copies of relevant papers were downloaded and read. Bibliographies of the selected references were also used to find more relevant papers. Condensation, permeation, and dilution techniques applied for the monitoring of gaseous compounds and particulate matter were taken into account. For the condensation technique, a schematic diagram of a device, condensation type, temperature, moisture removal efficiency, and loss ratio of an analyte were collected. For the permeation method, the structure of a device, membrane material, membrane selectivity, membrane permeability, temperature, purging rate, moisture removal efficiency, and loss ratio of an analyte were considered. In terms of the dilution method, the flowrate, dilution ratio, temperature, type of particulate matter, and measurement errors were summarized, and schematic diagrams of devices were collected. In addition, the advantages and disadvantages of each technique were studied. The bias of each technique was observed in the study. The advantages and disadvantages were assessed based on the references and the experience of our authors. Application scopes of each technique were recommended based on the characteristics of the technique.

3. Results and Discussion 3.1. Condensation Method On the basis of water physical properties, the temperature of the flue gas in the extractive line was reduced below the dew point to condense the water vapor. The condensation system must be suitable for the flow rate and moisture concentration of the sample gas. Moreover, the structure of the system must avoid contact between the condensed water and the dried flue gas. To decrease the temperature of the gas stream, two technologies have generally been employed: refrigeration using a coolant and Peltier. The typical structure of a refrigerated system is shown in Figure1. On the basis of this principle, several apparatuses have been developed. Dowling (1980) invented a moisture removal system based on the refrigerated method with an improvement to the air-to-air and air-to-refrigerant heat exchanger [64]. The exchanger consists of a bundle of vertical tubing in a housing (Figure2). Parallel sheet metal fins are used to hold the tubing. Moisture is condensed and deposited on the sheet and then drips down the bottom of the housing due to gravity. The advantage of this system is the absence of the water separator. Moreover, the fin sheets help to increase the turbulence of the air and liquid flow, which protects the surface of the sheet to stave off fouling [64]. Nanaumi and Baba (1980) also improved the heat exchanger for the refrigerated system [65]. The main novelty of this invention is the use of an extra heat exchanger to cover the heat exchanger of the refrigerated system to precool the inlet air [65]. These two inventions improved the cooling efficiency, which helped to remove more moisture with respect to a high loading capacity. The principle of both inventions is heterogeneous condensation. The moisture was removed under liquid phase, and most of the droplets were condensed on the cold surface of a device. Thus, the loss of highly water-soluble compounds would occur. A two-stage moisture removal system was developed by Basseen et al. (1988) [66]. First, the inlet air was precooled to −40 ◦C using a refrigerator to reduce the air temperature and remove some of the moisture. The flue gas was continued, introducing it to the second stage, which consists of dual desiccant beds to adsorb the remaining moisture content. These beds are alternatively working and regenerating. The system can reduce the humidity from about 15% to 1% [66]. In this invention, the moisture is also condensed by way of a heterogeneous process. The phase of water is solid, which is suitable for highly water- soluble compounds. Due to the two stages of moisture removal, the device can operate at high moisture loading. Moreover, the moisture removal efficiency of the device is also high. However, a desiccant could adsorb certain compounds, which affected the selectivity of the device. Atmosphere 2021, 12, x FOR PEER REVIEW 5 of 26 AtmosphereAtmosphere 20212021,, 1212,, x 61 FOR PEER REVIEW 55 of of 26 25

Figure 1. Typical structure of a refrigerated moisture removal device [1]. FigureFigure 1. 1. TypicalTypical structure structure of of a a refrigerated refrigerated moisture moisture removal removal device [1]. [1].

Figure 2. Structure of an air-to-air heat exchanger [64]. Figure 2. Structure of an air-to-air heat exchanger [64]. FigureIn 2. particular,Structure of Tosian air-to-air (2009) heat used exchanger a vortex [64]. tube to cool the ﬂue gas to the dew point temperatureA two-stage [ 67moisture]. In the removal tube, the system vortical was motion developed of air atby high Basseen speed et (i.e., al. (1988) a moving [66]. hot First,stream theA inlettwo-stage with air a differentwas moisture precooled direction removal to −40 to system that°C using of thewas a coldrefrigerator developed stream) toby induced reduceBasseen coldthe et airal. air, tempera-(1988) which [66]. was tureFirst,introduced and the remove inlet to air thesome was heat of precooled exchangerthe moisture. to to − reduce 40The °C flue using the gas temperature a wasrefrigerator continued, of theto reduce introducing inlet gas the (Figure air it tempera- to3 the)[ 67 ]. secondtureThe andstage, condensation remove which someconsists of water of theof vapor dual moisture. desiccant is a homogeneous The beds flue togas adsorb processwas continued, the in remaining this case. introducing Themoisture advantage it con- to the of tent.second These stage, beds which are alternatively consists of dualworking desiccant and regenerating. beds to adsorb The the system remaining can moisturereduce the con- Atmosphere 2021, 12, x FOR PEER REVIEWthis system is its low energy consumption compared to a refrigerated technology. However,6 of 26 humiditytent.due toThese from the directbeds about are contact 15% alternatively to with1% [66]. condensed working In this invention, water and regenerating. in a the heat moisture exchanger, The issystem also water-soluble condensed can reduce by gases the wayhumiditycan of bea heterogeneous absorbed from about in water, 15%process. to which 1% The [66]. might phase In affectthis of waterinvention, the accuracy is solid, the ofmoisturewhich the analyticalis suitableis also condensed results. for highly by water-solubleway of a heterogeneous compounds. process.Due to the The two phase stages of ofwater moisture is solid, removal, which theis suitable device canfor highlyop- eratewater-soluble at high moisture compounds. loading. Due Moreover, to the two the stagesmoisture of moisture removal efficiencyremoval, the of thedevice device can is op- alsoerate high. at However,high moisture a desiccant loading. could Moreover, adsorb the certain moisture compounds, removal efficiencywhich affected of the the device se- is lectivityalso high. of the However, device. a desiccant could adsorb certain compounds, which affected the se- lectivityIn particular, of the device. Tosi (2009) used a vortex tube to cool the flue gas to the dew point temperatureIn particular, [67]. In theTosi tube, (2009) the usedvortical a vortex motion tu ofbe airto atcool high the speed flue gas(i.e., to a themoving dew hotpoint streamtemperature with a different [67]. In thedirection tube, theto thatvortical of the moti coldon stream) of air at induced high speed cold (i.e., air, awhich moving was hot introducedstream with to the a different heat exchanger direction to reduceto that thofe the temperature cold stream) of the induced inlet gas cold (Figure air, which 3) [67]. was

Theintroduced condensation to the of heatwater exchanger vapor is ato homogene reduce thouse temperature process in ofthis the case. inlet The gas advantage (Figure 3) of[67]. thisFigureTheFigure system condensation 3. 3.Fundamental Fundamentalis its low of energy flowwater flow diagram diagramconsumptionvapor ofis of a avortexhomogene vortex compared tube tube ous[67]. [67 to ].process a refrigerated in this case. technology. The advantage How- of ever,this due system to the is directits low contact energy with consumption condensed compared water in toa heata refrigerated exchanger, technology. water-soluble How- The advantages of the refrigeration-based method are the high cooling efficiency, high gasesever, canThe due be advantages toabsorbed the direct in of water, contactthe refrigeration-based which with mightcondensed affect watermethod the accuracy in area heat the of exchanger, thehigh analytical cooling water-soluble efficiency,results. highgasesworking working can capacitybe absorbed capacity (i.e., (i.e.,in high water, high air which flowair flow loading), might loading), affect and and the typical typicalaccuracy cooling cooling of the technology. technology.analytical However, results. How- ever,a refrigerator a refrigerator is a complex is a complex system system (e.g., evaporator, (e.g., evaporator, pump, valves). pump, Therefore,valves). Therefore, it leads to it an leads to an increase in the dimension of the removal system and the maintenance cost. To overcome this problem, a Peltier chiller was used instead of a refrigerator. The moisture removal system based on refrigeration has more volume than the Peltier system. How- ever, the large volume of the system causes the dilution of the target compounds due to a mixing effect. Consequently, the accuracy of the analytical data will decline [68]. For a moisture removal system with a small volume, a Peltier is a good choice of cooling device. A Peltier has been used to cool the flue gas instead of a refrigerator or vortex tube. Chap- man et al. (1980) developed a moisture removal system using a Peltier block for the heat exchanger (Figure 4) [68]. Water vapor condensed under solid state and then settled to the bottom of the system by gravity. The diameter of the inlet embodiment was 1.58 to 4.76 mm, and that of the outlet embodiment was 3.17 to 6.35 mm [68]. The condensation of the moisture in this device is a heterogeneous process. The advantage of the device is that it is suitable for highly water-soluble compounds because moisture is removed in the solid phase. However, since the cooling efficiency of a Peltier is low, the device cannot be applied for high inlet temperature gas or high moisture loading. Groger (1988) combined a condensate coil using a Peltier cooling element with a fine membrane filter to remove the moisture from a sampling gas stream [69]. The flue gas entering the coil was cooled to about 4 °C. In the coil, the water vapor was condensed, 90% of which was removed by a drain pump. The distance between the outlet coil and the inlet pump was short. Hence, the condensate was continuously and immediately extracted to avoid the absorption of target gas in water. The remaining moisture in the form of aerosol (about 5%) continued to be filtered by the fine filter [69]. The condensation process of the moisture in this invention was heterogeneous. The advantage of this invention is the high removal efficiency of moisture. However, using a filter to get rid of water aerosol will also produce water droplets on the surface that can absorb target gases. Groger and Groger (1989) proposed an apparatus to remove moisture with a cooling tube designed as a conical taper [70]. The cone angle was 5°. The inlet gas entered the cooler at the bottom (above the condensate discharge socket) as the cross-section of the tube produced a countercurrent flow (i.e., the cyclone effect). The outlet of the gas was at the top of the tube. Due to this design, the retention time of the flue gas in the cooler was longer with a smaller surface area of the tube. This helped to simplify the manufacture and reduce the size of the moisture removal device [70]. In this invention, the phase of H2O was changed from a gas to a liquid by a heterogeneous condensation process. This invention only improved the contact surface in a cooling tube, which helped to improve the moisture removal efficiency. However, the device did not overcome the loss of target analytes due to an absorption effect. A probe assembly for sampling, coupled with a Peltier-based cooler, was manufactured by Bacha- rach, Inc., New Kensington, PA, USA. The device could work at a flow rate of 1.5 L/min (Figure 5) [71]. An advantage of this in situ cooler is the reduction in energy consumption for an extractive line because moisture is removed before entering the extractive line, which helps to reduce the heating temperature of the line. However, the loss of target

Atmosphere 2021, 12, 61 6 of 25

increase in the dimension of the removal system and the maintenance cost. To overcome this problem, a Peltier chiller was used instead of a refrigerator. The moisture removal system based on refrigeration has more volume than the Peltier system. However, the large volume of the system causes the dilution of the target compounds due to a mixing effect. Consequently, the accuracy of the analytical data will decline [68]. For a moisture removal system with a small volume, a Peltier is a good choice of cooling device. A Peltier has been used to cool the flue gas instead of a refrigerator or vortex tube. Chapman et al. (1980) developed a moisture removal system using a Peltier block for the heat exchanger (Figure4)[ 68]. Water vapor condensed under solid state and then settled to the bottom of the system by gravity. The diameter of the inlet embodiment was 1.58 to 4.76 mm, and that of the outlet embodiment was 3.17 to 6.35 mm [68]. The condensation of the moisture in this device is a heterogeneous process. The advantage of the device is that it is suitable for highly water-soluble compounds because moisture is removed in the solid phase. However, since the cooling efficiency of a Peltier is low, the device cannot be applied for high inlet temperature gas or high moisture loading. Groger (1988) combined a condensate coil using a Peltier cooling element with a fine membrane filter to remove the moisture from a sampling gas stream [69]. The flue gas entering the coil was cooled to about 4 ◦C. In the coil, the water vapor was condensed, 90% of which was removed by a drain pump. The distance between the outlet coil and the inlet pump was short. Hence, the condensate was continuously and immediately extracted to avoid the absorption of target gas in water. The remaining moisture in the form of aerosol (about 5%) continued to be filtered by the fine filter [69]. The condensation process of the moisture in this invention was heterogeneous. The advantage of this invention is the high removal efficiency of moisture. However, using a filter to get rid of water aerosol will also produce water droplets on the surface that can absorb target gases. Groger and Groger (1989) proposed an apparatus to remove moisture with a cooling tube designed as a conical taper [70]. The cone angle was 5◦. The inlet gas entered the cooler at the bottom (above the condensate discharge socket) as the cross-section of the tube produced a countercurrent flow (i.e., the cyclone effect). The outlet of the gas was at the top of the tube. Due to this design, the retention time of the flue gas in the cooler was longer with a smaller surface area of the tube. This helped to simplify the manufacture and reduce the size of the moisture removal device [70]. In this invention, the phase of H2O was changed from a gas to a liquid by a heterogeneous condensation process. This invention only improved the contact surface in a cooling tube, which helped to improve the moisture removal efficiency. However, the device did not overcome the loss of target analytes due to an absorption effect. A probe assembly for sampling, coupled with a Peltier-based cooler, was manufactured by Bacharach, Inc., New Kensington, PA, USA. The device could work at a flow rate of 1.5 L/min (Figure5)[ 71]. An advantage of this in situ cooler is the reduction in energy consumption for an extractive line because moisture is removed before entering the extractive line, which helps to reduce the heating temperature of the line. However, the loss of target compounds due to water droplets is a drawback of this invention. In general, the advantages of the Peltier-based heat exchanger are its low cost, simple construction, and reduced energy consumption. However, the cooling efficiency of the Peltier is lower than that of the refrigerator and of the vortex tube. Therefore, the Peltier tube could be applied for low-flow-rate sampling. Atmosphere 2021, 12, x FOR PEER REVIEW 7 of 26 Atmosphere 2021, 12, x FOR PEER REVIEW 7 of 26

compounds due to water droplets is a drawback of this invention. In general, the ad- compoundsvantages of due the toPeltier-based water droplets heat isexchanger a drawback are ofits thislow invention. cost, simple In construction,general, the ad-and vantagesreduced ofenergy the Peltier-based consumption. heat However, exchanger the coolingare its lowefficiency cost, simpleof the Peltier construction, is lower and than reducedthat of theenergy refrigerator consumption. and of However,the vortex the tube. cooling Therefore, efficiency the Peltier of the tubePeltier could is lower be applied than Atmosphere 2021, 12, 61 7 of 25 thatfor oflow-flow-rate the refrigerator sampling. and of the vortex tube. Therefore, the Peltier tube could be applied for low-flow-rate sampling.

FigureFigure 4.4. TheThe embodimentembodiment ofof aa heatheat exchangerexchanger usingusing aa PeltierPeltier [[68].68]. Figure 4. The embodiment of a heat exchanger using a Peltier [68].

Figure 5. Schematic diagram of a sampling probe coupled with a Peltier cooler [71]. Figure 5. Schematic diagram of a sampling probe coupled with a Peltier cooler [71]. FigureIn 5. ourSchematic research diagram (see Supplementaryof a sampling probe Materials), coupled with we founda Peltier that cooler the [71]. heat exchanger to removeIn our moisture research from (see a samplingSupplementary gas under Materials), liquid phasewe found reduced that the inlet heat SO 2exchangerby approx- to In our research (see Supplementary Materials), we found that the heat exchanger to imatelyremove 20%moisture (i.e., thefrom inlet a sampling concentration gas under was 1000liquid ppmv) phase andreduced that underinlet SO solid2 by phaseapproxi- it removereducedmately moisture20% inlet (i.e., HCl from bythe up inleta tosampling 50%concentration (i.e., gas the under inlet was concentrationliquid 1000 phaseppmv) wasreduced and 50 that ppmv). inlet under SO Therefore, 2solid by approxi- phase a cor- it matelyrectionreduced 20% factor inlet (i.e., for HCl the each by inlet targetup toconcentration gas50% should (i.e., the be was inlet applied 1000 concentration toppmv) the ﬁnal and analyticalwas that 50 under ppmv). results. solid Therefore, However,phase it a reducedthe concentration inlet HCl by of emissionup to 50% gas (i.e., is the varied, inlet andconcentration the removal was efﬁciency 50 ppmv). of theTherefore, moisture a removal system is not consistent. Hence, a more optimal measure should be taken. In the condensation method, water can be removed under solid or liquid phase. The advantage of the condensation method is its suitability for removing a high concentration of water from a high loading air volume. However, the condensate might affect the target gases. When the condensate is collected under the liquid phase, the absorption of soluble gases such as SOx, NOx, HCl, and NH3 can occur. It was reported that the loss of ozone was approximately Atmosphere 2021, 12, 61 8 of 25

10% at 30% relative humidity and 40% at 80% relative humidity when the condensation method was applied to remove moisture for ozone measurement. Furthermore, SO2 was found to be lost at rates of 19.3%, 29.3%, and 61.5% at relative humidities of 30%, 50%, and 80%, respectively, when a cooler was used to remove the moisture [72]. Lee et al. (2019) investigated the effect of a cooler as a moisture pretreatment device in the analysis of methyl ethyl ketone (MEK), isobutyl alcohol (i-BuAl), methyl isobutyl ketone (MIBK), butyl acetate (BuAc), and styrene [73]. These are very highly water-soluble odorous compounds. It was reported that the losses of i-BuAL, MIBK, BuAc, and styrene were approximately 19%, 4%, 5%, and 10%, respectively, in association with 80% relative humidity. In addition, the reproducibility of their concentrations was approximately 8–31%. This indicated that the water liquid in the cooler kept absorbing and desorbing the analytes [73]. The U.S. EPA stated that HCl and NH3 are lost in the H2O condensate in a condenser [23,24]. Measurement bias of total hydrocarbon emitted from a hazardous waste incinerator was found when a refrigerant moisture removal device was used because VOCs and organic air pollutants might comprise highly, poorly, and non-water-soluble compounds [23]. On the other hand, if the condensate is removed under the solid phase, the absorption of soluble gases can be avoided. Several studies addressed a good recovery rate for highly water-soluble compounds when the moisture was removed under solid phase. It was found that the loss of SO2 at 150 ppmv was less than 2% when the moisture was removed at the solid phase (i.e., frost) [72]. Likewise, the losses of MEK, i-BuAl, MIBK, and BuAc at sub-ppbv level were 0%, 3.4%, 0.5%, and 2.1%, respectively [73]. Son et al. (2013) found that H2S (23 ppbv), CH3SH (16 ppbv), dimethyl sulfide (13 ppbv), and dimethyl disulfide (8 ppbv) had a recovery rate over 97% when the moisture in the sample was removed at the solid phase [52]. Nevertheless, these studies were not conducted with respect to air pollutants in the ambient air with low moisture content rather than emission gases from a stack. Moreover, HCl or NH3 at the atmospheric conditions could still be absorbed on the ice surface due to the hydrogen bond [74,75]. Thus, more investigations should be implemented. In general, the advantages of the condensation method are the low cost, simple structure, and easy operation and maintenance. However, the loss of target analytes is a serious issue with the method. Moreover, the treatment of drain water is another drawback of the condensation method. Consequently, the condensation method is recommended to be used for the CEMS of poorly water-soluble compounds such as CO, CO2, NO, and CH4 when the moisture is removed under a liquid phase. On the other hand, if moisture is removed under a solid phase, higher water-soluble gases such as SO2 or NO2 could be applied. More comprehensive studies should be carried out to investigate the influence of the solid phase. A conversion process of highly water-soluble analytes to poorly water-soluble analytes would be an alternative when the moisture is removed under liquid phase. For example, the U.S. EPA suggested that a catalytic converter could be applied to convert the NH3 in the sample gas to NOx [24]. Then the sample gas could penetrate a condenser to remove moisture before moving into a NOx analyzer [24]. The summary of condensation methods with respect to different inventions is shown in Table2. The condensation process, advantages, disadvantages, and recommendations of applicable analyte are listed. Atmosphere 2021, 12, 61 9 of 25

Table 2. Development of condensation devices to remove moisture in the ﬂue gas.

Condensation Recommendation of No. Method Advantages Disadvantages Type Applicable Analytes - Reduced dimension of Refrigerated the device due to the - Poorly water-soluble - Complicated moisture removal absence of a gases such as CO, NO, structure device with water separator CO , and CH due to 1 Heterogeneous - Potential loss of 2 4 improvement of - High cooling efficiency water droplets absorbing highly water-soluble heat - Less foiling highly water-soluble compounds exchanger [64] - Allowable for high compounds loading of moisture. - High cooling efficiency - Poorly water-soluble Refrigerated - Bulky - Allowable for high gases such as CO, NO, moisture removal device temperature of the CO , and CH due to 2 device with an Heterogeneous - Potential loss of 2 4 inlet gas water droplets absorbing extra heat highly water-soluble - Allowable for high highly water-soluble exchanger [65] compounds loading of moisture. compounds -High cooling efficiency - Complex and bulky Two stages: - Allowable for high structure - Highly water-soluble refrigeration at loading of moisture - Potential loss of 3 Heterogeneous gases such as SO or NO −40 ◦C and - Suitable for highly certain compounds 2 2 under a solid phase desiccant bed [66] water-soluble due to the adsorption compounds. of desiccants. - Poorly water-soluble - Low energy gases such as CO, NO, - Potential loss of consumption CO , and CH due to 4 Vortex tube [67] Homogeneous highly water-soluble 2 4 - Less maintenance water droplets absorbing compounds required highly water-soluble compounds. - Compact size - Not operatable with - Low energy a high loading amount Peltier moisture - Highly water-soluble consumption of moisture 5 removal Heterogeneous gases such as SO or NO - Suitable for highly - Unsuitable for a high 2 2 device [68] under a solid phase. water-soluble temperature of the compounds inlet gas - Potential loss of certain compounds - Poorly water-soluble due to the selectivity Two stages: gases such as CO, NO, - Allowable for high of the membrane 6 Peltier and Heterogeneous CO , CH due to water loading of moisture. - Potential loss of 2 4 membrane [69] droplets absorbing highly highly water-soluble water-soluble compounds. compounds due to water droplets. - Compact size Poorly water-soluble gases - Potential loss of Conical cooling - Large contact surface such as CO, NO, CO , and highly water-soluble 2 7 tube using a Heterogeneous - Easy manufacturing CH due to water droplets compounds due to 4 Peltier [70] due to its simple absorbing highly water droplets. structure. water-soluble compounds - Hard to maintain due - Easy reduction in the Poorly water-soluble gases to in situ location temperature of an such as CO, NO, CO , and - Potential loss of 2 8 Peltier probe [71] Heterogeneous extractive line CH due to water droplets highly water-soluble 4 - Saving energy for absorbing highly compounds due to a CEMS. water-soluble compounds. water droplets. Atmosphere 2021, 12, 61 10 of 25

3.2. Permeation Method The permeation method is based on the selective permeability of water through a membrane. The flue gas-contained moisture is fed to the feed side of the membrane while Atmosphere 2021, 12, x FOR PEER REVIEWa dry gas (e.g., nitrogen) flows to the delivery side of the membrane with a counter-current10 of 26 flow direction in order to promote the permeation of water vapor through the membrane (Figure6)[ 76,77]. There are four flow directions of the air in a membrane: perfect mixing, cross-plug flow, co-current flow, and counter-current flow [37]. It was concluded that the counter-current flowflow systematicallysystematically hadhad the bestbest performance, followedfollowed by cross-plug, co- current, andand perfect perfect mixing mixing [78 [78].]. Therefore, Therefore, counter-current counter-current and and cross-plug cross-plug flow haveflow beenhave widelybeen widely used forused permeation for permeation devices devices [78]. For[78] optimal. For optimal operation, operation, a membrane a membrane should should have perfecthave perfect radial radial mixing mixing across across the substrate the substrat porosity,e porosity, which is which calculated is calculated by Equation by Equation (1) [79]: (1) [79]: RM = (ymax − y)/(ymax − ybore), (1) RM = (ymax − y)/(ymax − ybore), (1)

wherewhere ymax isis the the concentration concentration at at the the low low pressure pressure (computed (computed by by the the cross-flow cross-ﬂow model model of ofmembrane membrane performance); performance); y is y the is theactual actual concen concentrationtration at the at low-pressure the low-pressure side sideof the of sub- the substrate;strate; and and ybore y isbore theis concentration the concentration in the in fiber the ﬁber bore. bore.

Figure 6. Schematic of a permeation membrane tubetube [[76].76].

To evaluateevaluate thethe performanceperformance ofof aa permeationpermeation membrane, the permeability and selec- tivitytivity areare keykey parametersparameters [[37,80].37,80]. The permeabilitypermeability of the membrane to water vapor (i.e., thethe permeationpermeation rate)rate) shouldshould bebe ≥≥100,100, which which can can be be calculated calculated via via Equation Equation (2) (2) [81]: [81]:

J =J P[(p = P[(p1 −1 −p 2p)/L],2)/L], (2)(2)

where J is the permeation rate (cm3 3/cm2·s); p1 and p2 are the pressures of upstream and where J is the permeation rate (cm /cm ·s); p1 and p2 are the pressures of upstream and downstream of the membranemembrane (cmHg), respectively;respectively; L is thethe membranemembrane thicknessthickness (mm);(mm); and PP isis thethe permeability. permeability. The The common common unit unit for fo ther the permeability permeability is Barrer. is Barrer. Alternatively, Alternatively, the permeancethe permeance is usually is usually used used when when the membranethe membrane thickness thickness is unknown is unknown [82]. [82]. The The common com- unitmon forunit the for permeance the permeance is the is gas the permeation gas permeation unit (GPU). unit (GPU). The selectivityselectivity of thethe membranemembrane is thethe ratioratio ofof thethe permeabilitypermeability ofof waterwater vaporvapor andand thethe permeability of of another another compound compound throug throughh the the membrane. membrane. The The selectivity selectivity can canbe cal- be calculatedculated via via Equation Equation (3) (3)[37,82]: [37,82 ]: αij = P /P , (3) αij =i Pi/Pj j, (3) where P is the permeability of water vapor and P is the permeability of another compound. where Pi i is the permeability of water vapor andj Pj is the permeability of another compound.The mechanisms for the mass transfer of moisture in a permeation membrane are pore ﬂow and solution-diffusion. In terms of the pore ﬂow, H O is only enough small to The mechanisms for the mass transfer of moisture in a 2permeation membrane are penetrate some of the pores in the membrane to the permeate side. In solution-diffusion, pore flow and solution‒diffusion. In terms of the pore flow, H2O is only enough small to penetrate some of the pores in the membrane to the permeate side. In solution‒diffusion, H2O dissolves in the membrane and then diffuses from the higher H2O concentration side to the lower concentration side due to the difference in diffusivity [37,80]. Four membrane materials have been used: polymer, zeolite, mixed matrix, and supported liquid [37,80]. Among these materials, the polymeric membranes are the most widely employed to remove moisture due to their reasonable cost, lack of defects, reproducibility, and strength [37,80]. The polymeric materials consist of cellulose acetate, ethyl

Atmosphere 2021, 12, 61 11 of 25

H2O dissolves in the membrane and then diffuses from the higher H2O concentration side to the lower concentration side due to the difference in diffusivity [37,80]. Four membrane materials have been used: polymer, zeolite, mixed matrix, and supported liquid [37,80]. Among these materials, the polymeric membranes are the most widely employed to remove moisture due to their reasonable cost, lack of defects, reproducibility, and strength [37,80]. The polymeric materials consist of cellulose acetate, ethyl cellulose, natural rubber, PBT/PEO block copolymer, PEBAX® 1074, polyacrylonitrile, polyamide 6, polycarbonate, polydimethylsiloxane, polyether sulfone, polyethylene, poly- imide, polyphenylene oxide, polypropylene, polystyrene, polysulfone, polyvinyl alcohol, sulfonated polyether ether ketone, and sulfonated polyether sulfone [37,80]. PEBAX® 1074 shows the highest moisture permeability. However, the most selective polymer (i.e., between H2O and N2:H2O/N2) is sulfonated polyether ether ketone [37,80]. The zeolitic membrane is an inorganic one that is typically produced by the growing of zeolite on aluminum or stainless steel. The advantages of the zeolitic membranes are their thermal and chemical stability [37,80]. The combination of polymeric and zeolitic membranes is known as a mixed matric membrane. High thermal and chemical stability, easy and cheap fabrication process, high reproducibility, and high transportability are the main advantages of the mixed-matrix membranes [37]. Supported liquid membranes consist of a single laminate of a hygroscopic liquid (e.g., triethylene glycol, polyethylene glycol 400, ionic liquids) and a hydrophobic microporous membrane. The membrane reveals high moisture selectivity and permeability [37]. Membrane materials have continued to be developed. Hybridizing of polymeric materials has been widely carried out. Some representative studies are taken into account in this review. Makino and Nakagawa (1988) used an aromatic imide polymer generated from a tetracarboxylic acid and diamine component, exhibiting high solubility in an organic polar solvent [76]. The membrane could remove up to 10% of the water content from flue gas. The preferred dry gas flow rate was 0.5–8 m/s. The permeation rate was 0.1–10 (cm3/cm2·s) [76]. Keyser et al. (1990) developed a continuous membrane cartridge, coupled with an intermittent air compressor [83]. Some of the outlet dry air was reused to maintain the activity of the membrane. The membrane cartridge was also heated up to improve its efficiency [83]. The heating up of membranes to improve the moisture removal efficiency and prevent condensation on the surface is the advantage of this device. Using an air compressor to supply dry air instead of a gas tank is also a good aspect of this invention. However, a full permeation system would be bulky and consume a lot of energy. Norlien et al. (1991) developed a permeation moisture removal device using a perfluorinated polymer [77]. Lovelock (1992) invented a perfluorocarbon polymer with lithium sulfate group that could work as a membrane at 100 ◦C[84]. Bartholomew et al. (2002) compacted a permeation membrane, particle filter, and water droplet collector into an integral gas dryer and filtration device (Figure7)[ 85]. Eckerbom (2009) reported the Nomoline™ technology to remove moisture in the gas sampling line [86]. Nomoline™ works based on the properties of a polyether block amide cover. Polyamide and polyether help to sweat water from the gas sample flow to the outer surface of the Nomoline™ cover [86]. Zhao et al. (2015) combined polyacrylonitrile with 1 wt% polydimethylsiloxane. The polyacrylonitrile has high H2O/N2 selectivity (i.e., 1,875,000) [87]. However, its H2O permeability is only 300 Barrers. In contrast, the polydimethylsiloxane shows high H2O permeability and low H2O/N2 selectivity of 40,000 and 143, respectively [37,80]. Thus, they were combined to complement each other’s disadvantages. The membrane’s permeance was reported as 12,827 GPU. Its H2O/N2 selectivity was 220 [87]. Likewise, the polyacrylonitrile was combined with 0.5 wt% polydimethylsiloxane to produce a thinner membrane. However, the permeance and selectivity for H2O were only 3700 GPU and 13 (H2O/N2)[88]. Baig et al. (2016) fabricated a thin-film nanocomposite using carboxylated TiO2 and polyamide [89]. The amount of carboxylated TiO2 was 0.2 wt%. It was reported that the H2O/N2 selectivity of the membrane was 486. Its H2O permeance was 1.340 GPU [89]. Interfacial polymerization using m-phenylene diamine and trimesoyl Atmosphere 2021, 12, 61 12 of 25

chloride was conducted on the inner surface of the polysulfone hollow fiber membrane to improve the water permeation. The optimal mass ratios of m-phenylene diamine and trimesoyl chloride were 2 wt% and 0.25 wt%, respectively. The H2O permeance of the membrane was found to be 1500 GPU. In addition, the H2O/N2 selectivity was 500 [90]. Engelhard titanosilicate-4 was polymerized with polysulfone and polyamide. The amount of Engelhard titanosilicate-4 was 4 wt%. The membrane was revealed as having 1377 GPU moisture permeance and 346 for the moisture selectivity [91]. Ingole et al. (2017) combined 0.1 wt% 1,3-benzenedithiol with polyethersulfone to produce a permeation membrane [92]. It was reported that the H2O permeance was 2050 GPU and the H2O/N2 ratio was 119. Moriyama et al. (2019) fabricated a 1,2-bis(triethoxysilyl)ethane (BTESE) membrane by coating SiO2–ZrO2-derived sols onto an α-alumina porous tube [93]. It was found that the H2O permeance was 14,925 GPU and that the H2O/N2 ratio was 160 [93]. Ahhtar et al. (2019) developed two kinds of membranes using a sulfonated penta block copolymer [94]. For the first membrane, 2 wt% of the sulfonated penta block copolymer was casted into a tetrahydrofuran solution. Its H2O/N2 selectivity and H2O permeability were found to be approximately 420,000 and 553,000 Barrers, respectively. For the second membrane, 2 wt% of the sulfonated penta block copolymer was also used to cast into a solution comprising 85 wt% toluene and 25 wt% isopropanol. It was reported that the H2O/N2 selectivity was 24,000 and that H2O permeability was 850,000 Barrers [94]. Phosphonic acid-based membranes supported by Nafion® 212 were developed by Leoga et al. (2020) [95]. The substrate was pretreated by 100 W of plasma discharge. It was reported that the H2O permeance was approximately 1400 GPU [95]. Zhang et al. (2020) modified a NaA Zeolite membrane on a hollow fiber using a WS2 nanosheet to improve the removal of moisture at a low level [96]. It was reported that the membrane could remove the moisture well at 0.5 wt% [96]. Deimede et al. (2020) casted a membrane made of a pyridinium-based polymeric ionic liquid film containing a MeSO4 counter anion [97]. It was reported that the membrane showed very high H2O permeability and H2O/N2 selectivity compared to typical polymeric membranes. The highest permeability was found to be about 184,000 Barrers. The highest selectivity was 1,840,000 [97]. The H2O permeance of the membrane was 5000 GPU. The rest of the development process of membrane materials can be found elsewhere [37,80]. Nafion® membrane has been widely applied as a permeation method to remove moisture in CEMS, most recently by [98–100]. Nafion® ionomers were first invented by Dr. Walther Grot and produced as a commercial product by the DuPont company in the 1960s [101]. Perfluorinated vinyl ether comonomer was copolymerized with tetrafluo- roethylene to create Nafion® [101]. Jetter et al. (2002) developed a two-stage moisture removal device using a series of permeable tubes [100]. The difference between the two stages was the temperature of the tubes. In the first stage, permeable tubes were heated up to about 70 ◦C to increase the dew point of the gas stream. In contrast, the second stage was controlled at 0 ◦C to enhance the moisture removal efficiency of the device. The moisture level at the outlet of the device was reported as 700 ppmv with a 12 L/min purging rate. This low-moisture sample is suitable for the FTIR analyzer [100]. The advantages of this device are its high moisture removal efficiency and high moisture loading capacity. The device can be applied to a high-temperature gas stream due to a heater at the first stage. However, the energy consumption would be relatively high because of the temperature control process. Wang et al. (2013) wrapped the Nafion® tube in a winding cylinder to increase the length of the tube with a small-dimension device [99]. Nafion® tubes were combined with a cold trap to remove the high moisture content from the stack gas. Firstly, the sample gas was introduced into the Nafion® tubes, which were heated to the desired sample temperature. Then, the sample gas temperature was reduced to −25 ◦C in the cold trap to remove more moisture. The device could remove moisture up to 50% absolute humidity. The flow rate of sample gas was 50–1000 mL/min. That of dry gas should be 1–3 L/min. The device could operate with an inlet gas temperature of 25–65 ◦C. It was found that the outlet relative humidity could reach 15%. In addition, the device helped to Atmosphere 2021, 12, 61 13 of 25

improve the detection of NO down to 50 ppbv in a high-moisture sample gas using an ion transfer spectrograph [99]. The advantages of this device were its high moisture removal efﬁciency and high moisture loading capacity. Using a Naﬁon® membrane also resulted in ◦ Atmosphere 2021, 12, x FOR PEER REVIEWhigh selectivity for many gases. The maximum inlet temperature was only 65 C, which13 of 26 reveals the limitation of this device. Moreover, high energy consumption was another drawback of the device.

Figure 7. Schematic diagram using a gas dryer coupled with a filtration device [85]. Figure 7. Schematic diagram using a gas dryer coupled with a filtration device [85]. Compared to the condensation method, the permeation method does not require Nafion® membrane has been widely applied as a permeation method to remove a condensate trap. In addition, the absorption of the target gas by the condensate is moisture in CEMS, most recently by [98–100]. Nafion® ionomers were first invented by avoided. However, condensed droplets could appear due to the different pressures, and Dr. Walther Grot and produced as a commercial product by the DuPont company in the the membrane could be plugged by particles. Using a filter to remove the particles before 1960sthe membrane [101]. Perfluorinated could cause watervinyl toether condense comonomer on the filterwas copolymerized surface, which mightwith tetrafluoro- also absorb ® ethylenetarget gases to create [1]. The Nafion large dry [101]. gas consumptionJetter et al. (2002) is another developed disadvantage a two-stage of the moisture permeation re- movalmethod device [76]. Theusing selective a series property of permeable of the membranetubes [100]. is The also difference important. between Son et al. the (2013) two stagesreported was that the usingtemperature the permeation of the tubes. method In the for first sulfur stage, compound permeable analysis tubes were resulted heated in upsomewhat to about lower 70 °C waterto increase removal the efficiencydew point and of the higher gas lossstream. of target In contrast, compounds the second than usingstage wasa Peltier-based controlled coolingat 0 °C to tube enhance in certain the moisture conditions removal [52]. Lee efficiency et al. (2019) of the found device. that The Nafion mois-® turecaused level the at loss the ofoutlet methyl of the ethyl device ketone was and reported isobutyl as 700 alcohol ppmv (i.e., with >40% a 12 atL/min 20–100 purging ppbv rate.of the This analyte low-moisture and 50–80% sample relative is suitable humidity) for the [102 FTIR]. It wasanalyzer also found[100]. The that advantages artifact (such of thisas benzene) device are formation its high moisture occurred removal when the efficiency permeation and methodhigh moisture was applied loading to capacity. remove Themoisture device in can the measurementbe applied to processa high-temperature of hazardous gas volatile stream organic due to compounds a heater at [ 103the, 104first]. stage.In However, general, the the permeation energy consumption method had would a higher be relatively recovery ratehigh for because most ofof thethe analytes,temperaturea higher control moisture process. removal Wang efficiency, et al. (2013) and wrapped no generation the Nafion of by-products® tube in a winding compared cylinder to the to increase the length of the tube with a small-dimension device [99]. Nafion® tubes were combined with a cold trap to remove the high moisture content from the stack gas. Firstly, the sample gas was introduced into the Nafion® tubes, which were heated to the desired sample temperature. Then, the sample gas temperature was reduced to −25 °C in the cold trap to remove more moisture. The device could remove moisture up to 50% absolute humidity. The flow rate of sample gas was 50‒1000 mL/min. That of dry gas should be 1‒ 3 L/min. The device could operate with an inlet gas temperature of 25‒65 °C. It was found

Atmosphere 2021, 12, 61 14 of 25

condensation method. However, there are still certain limitations to the method such as high cost, loss of certain compounds, and high consumption of dry air. Thus, the continuous development of a high-selectivity membrane is necessary. Until now, the Naﬁon® membrane has been widely applied to CEMS for highly water-soluble gases such as SO2, HCl, NH3, and H2S. Although the membrane itself sometimes caused the loss of certain chemical compounds, the amount lost was in the acceptable range. A summary of permeation devices is given in Table3. The materials, advantages, disadvantages, and recommendations of applicable analytes are given. A summary of the development of membrane materials can be found in other review works [37,80].

Table 3. Development of permeation devices to remove moisture in the ﬂue gas.

Membrane Recommendation of No. Device Materials Advantages Disadvantages Type Applicable Analytes - A dry gas tank is - The device unnecessary. is bulky Any Any Target gases depended Heater - The condensation - It consumes a lot 1 commercial commercial on membrane’s cartridge [83] does not occur on of energy due to membrane membrane specification. the membrane the compressor surface. and heater. - The effect of particles on the Integral membrane is moisture Any Target gases depended reduced. -Maintaining the 2 membrane and Polymer commercial on membrane’s - The particle filter filters is costly. particle product specification. helps to enhance filter [85] the lifetime of the membrane. - The device has a - The device may high moisture consume a lot of removal efficiency. energy The authors Two-stage - Condensation Perfluorinated - Particles may recommend the device 3 permeable Polymer cannot occur on the polymer plastic damage the be used for an FTIR tubes [100] membrane surface. membrane surface analyzer. - It shows a high due to the absence loading amount of of a filter. moisture. - The device has a high moisture - The device may removal efficiency. - Nafion® membrane consume a lot of Two-stage - Condensation has been widely energy winding cannot occur on the applied for CO, CO , 4 Polymer Nafion® - Nafion® 2 cylinder membrane surface. NO, NO , HCl, NH , membrane is 2 3 device [99] - It has high loading H S, and SO sensitive to 2 2 of moisture. measurements. particles. - It shows a high selectivity.

3.3. Dilution Method Condensation and permeation methods are suitable for CEMS of gaseous compounds. For particulate matter, the two methods cannot be employed due to the physical characteristic of the particles. The U.S. EPA classified particulate matter into filterable and condensable particulate matter [105]. Filterable particulate matter refers to particles emitted directly by a source such as a solid or liquid at a stack or their release conditions and captured on the filter of a stack test train [105]. Condensable particulate matter includes materials that are in the vapor phase at stack conditions but condense and/or react upon cooling Atmosphere 2021, 12, 61 15 of 25

and dilution in the ambient air to form solid or liquid particulate matter immediately after discharge from the stack [105]. In terms of particle size, primary PM2.5 or PM2.5 is particulate matter with an aerodynamic diameter less than or equal to 2.5 µm [105]. PM2.5 consists of filterable PM2.5 and condensable particulate matter [105]. Primary PM10 or PM10 is particulate matter with an aerodynamic diameter less than or equal to 10 µm [105]. PM10 consists of filterable PM10 and condensable particulate matter [105]. Generally, particulate matter emitted from a stack is continuously measured by three main techniques, including direct extraction of high-temperature gasification coupled with laser scattering measurement, high-temperature extractive dilution coupled with laser scattering measurement, and alternating current coupled with charge induction, opacity meters, Beta attenuation meters, or electrification devices. The direct extraction and the extractive dilution methods are the most popular ones [17,106]. Furthermore, the dilution method is a standard method recommended by the International Organization for Standardization ISO 25597:2013 [107] as well as U.S. EPA CTM-039 [108] to measure primary PM2.5 and PM10. Only condensable particulate matter can be measured by the dry cooling impinger method [105,109] and the dilution method [109]. Therefore, the dilution method was also reviewed in this study, although it is not a direct moisture removal method. The simple principle of the dilution method is presented in Figure8. The ratio of dilution air to sample air should be at least Atmosphere 2021, 12, x FOR PEER REVIEW20:1 to maintain a relative humidity of the diluted sample of less than 70%. The16 of filter 26 temperature should be maintained at approximately 42 ◦C by either a high dilution ratio or a heater [107].

FigureFigure 8. 8.SchematicSchematic diagram diagram of ofa dilution a dilution module module [107]. [107 ]. The development of dilution systems has continued for decades. Hildemann et al. The development of dilution systems has continued for decades. Hildemann et al. (1989) invented a U-shaped dilution system for the measurement of organic aerosols emitted (1989) invented a U-shaped dilution system for the measurement of organic aerosols emit- from a stack [110]. The fundamental diagram of the invention is presented in Figure9. ted from a stack [110]. The fundamental diagram of the invention is presented in Figure The body of the dilution tunnel and the residence time chamber were made of stainless 9. The body of the dilution tunnel and the residence time chamber were made of stainless steel. The dilution ratio was 25–100-fold. The sampling flow rate was 30 L/min at 150 ◦C. steel. The dilution ratio was 25–100-fold. The sampling flow rate was 30 L/min at 150 °C. It was reported that the loss of particles 1 µm and 2 µm in diameter was approximately It was reported that the loss of particles 1 µm and 2 µm in diameter was approximately 7% and 15%, respectively. In the application of a field test at an oil-fired industrial boiler, 7% and 15%, respectively. In the application of a field test at an oil-fired industrial boiler, using the dilution system was found to collect 10 times more organic carbon than using usingU.S. the EPA dilution Method system 5 [110 ].was Although found to this collect dilution 10 times system more helped organic improve carbon the than particulate using U.S.matter EPA measurement, Method 5 [110]. its Alth limitationough this was dilution bulk (i.e., system >17 kg). helped Therefore, improve the the dilution particulate system matterhas kept measurement, improving toits becomelimitation more was compact. bulk (i.e In., >17 addition, kg). Therefore, the significant the dilution loss of system particles hasin kept the range improving of 1–2 toµm become was a drawbackmore compact. of this In device. addition, the significant loss of particles in the range of 1‒2 µm was a drawback of this device.

Figure 9. Dilution system invented by Hildemann et al. [110].

Lipsky and Robinson (2005) developed a compact dilution system (Figure 10) [111]. The dilution system was made of stainless steel. The length and diameter of the mixing chamber were 0.9 m and 0.15 m, respectively. The total air flow through the chamber was

Atmosphere 2021, 12, x FOR PEER REVIEW 16 of 26

Figure 8. Schematic diagram of a dilution module [107].

The development of dilution systems has continued for decades. Hildemann et al. (1989) invented a U-shaped dilution system for the measurement of organic aerosols emitted from a stack [110]. The fundamental diagram of the invention is presented in Figure 9. The body of the dilution tunnel and the residence time chamber were made of stainless steel. The dilution ratio was 25–100-fold. The sampling flow rate was 30 L/min at 150 °C. It was reported that the loss of particles 1 µm and 2 µm in diameter was approximately 7% and 15%, respectively. In the application of a field test at an oil-fired industrial boiler, using the dilution system was found to collect 10 times more organic carbon than using U.S. EPA Method 5 [110]. Although this dilution system helped improve the particulate

Atmosphere 2021, 12, 61 matter measurement, its limitation was bulk (i.e., >17 kg). Therefore, the dilution system16 of 25 has kept improving to become more compact. In addition, the significant loss of particles in the range of 1‒2 µm was a drawback of this device.

Figure 9. Dilution system invented by Hildemann et al.al. [[110].110]. Atmosphere 2021, 12, x FOR PEER REVIEW 17 of 26 Lipsky and Robinson (2005) developed a compact dilution system (Figure 10)[111]. Lipsky and Robinson (2005) developed a compact dilution system (Figure 10) [111]. The dilution system was made of stainless steel. The length and diameter of the mixing The dilution system was made of stainless steel. The length and diameter of the mixing chamber were 0.9 m and 0.15 m, respectively. The total air flow through the chamber was chamber were 0.9 m and 0.15 m, respectively. The total air flow through the chamber was 174174 L/min. L/min. The The inlet inlet air air temperature temperature could could be be varied varied from from 150 150 to to 300 300 °C.◦C. The The diluted diluted outlet outlet airair was was approximately approximately 27 27 °C.◦C. The The dilution dilution ratio ratio was was 20 20–350-fold.‒350-fold. It Itwas was found found that that the the measurementmeasurement errors errors were were less less than than 10% 10% with with regard regard to to a alaboratory laboratory test test using using a adiesel diesel generatorgenerator and and a awood wood stove stove [111]. [111 ].For For a afield field test, test, measurement measurement errors errors varied varied from from 5% 5% to to 18%.18%. The The change change in in particle particle shape shape with with respect respect to to dilution dilution ratios ratios was was also also addressed. addressed. The The 3 3 3 3 densitiesdensities of of PM PM2.52.5 werewere 0.5 0.5 g/cm g/cm andand 1.0 1.0 g/cm g/cm forfor on ondilution dilution ratios ratios of of20- 20- and and 160-fold, 160-fold, respectivelyrespectively [111]. [111 ].The The advantages advantages of of this this device device were were its its compact compact size size and and fast fast retention retention time.time. However, However, the the significantsignificant measurementmeasurement bias bias and and the the dependency dependency of the of particlethe particle shape shapeon the on dilution the dilution ratio ratio were were disadvantages disadvantages of the of device. the device.

FigureFigure 10. 10. A Acompact compact dilution dilution system system invented invented by by Lipsky Lipsky and and Robinson Robinson [111]. [111 ]. England et al. (2007) improved the compact dilution system and conducted field tests England et al. (2007) improved the compact dilution system and conducted field tests on gas-fired and oil-fired boilers [107]. The schematic diagram of this dilution system was on gas-fired and oil-fired boilers [107]. The schematic diagram of this dilution system was similar to that in [107]. The system consisted of a multiple parallel jet flow in order to similar to that in [107]. The system consisted of a multiple parallel jet flow in order to quickly mix the stack sample with air at a dilution ratio of 20:1, a mixing chamber to well quickly mix the stack sample with air at a dilution ratio of 20:1, a mixing chamber to well the sample mix with a retention time of 10 s; and a clean air generator to produce dilution the sample mix with a retention time of 10 s; and a clean air generator to produce dilution air (Figure 11). The device was designed for inlet sample temperatures below 175 ◦C. The airweight (Figure of 11). the dilutionThe device system was wasdesigned about for 9 kg. inlet It wassample reported temperatures that the minimumbelow 175 detection°C. The weightlimit ofof thethe systemdilution was system 0.13–1.2 was about mg/m 93 kgwith. It was respect reported to 1 h that sampling the minimum at 113 L/min. detection The 3 limitrelative of the measurement system was error0.13–1.2 of mg/m the method with wasrespect 27–34% to 1 h [ 112sampling]. The at advantages 113 L/min. of The this relative measurement error of the method was 27–34% [112]. The advantages of this invention were its compact size and low minimum detection limit. However, the device still led to significant measurement bias.

Figure 11. Schematic diagram of a compact dilution system invented by England et al. [112]. (TMF: Teflon membrane filter, QFF: quartz fiber filters, SMPS: scanning mobility particle sizer, MFM: mass flow meter).

Atmosphere 2021, 12, x FOR PEER REVIEW 17 of 26

174 L/min. The inlet air temperature could be varied from 150 to 300 °C. The diluted outlet air was approximately 27 °C. The dilution ratio was 20‒350-fold. It was found that the measurement errors were less than 10% with regard to a laboratory test using a diesel generator and a wood stove [111]. For a field test, measurement errors varied from 5% to 18%. The change in particle shape with respect to dilution ratios was also addressed. The densities of PM2.5 were 0.5 g/cm3 and 1.0 g/cm3 for on dilution ratios of 20- and 160-fold, respectively [111]. The advantages of this device were its compact size and fast retention time. However, the significant measurement bias and the dependency of the particle shape on the dilution ratio were disadvantages of the device.

Figure 10. A compact dilution system invented by Lipsky and Robinson [111].

England et al. (2007) improved the compact dilution system and conducted field tests on gas-fired and oil-fired boilers [107]. The schematic diagram of this dilution system was similar to that in [107]. The system consisted of a multiple parallel jet flow in order to quickly mix the stack sample with air at a dilution ratio of 20:1, a mixing chamber to well the sample mix with a retention time of 10 s; and a clean air generator to produce dilution air (Figure 11). The device was designed for inlet sample temperatures below 175 °C. The weight of the dilution system was about 9 kg. It was reported that the minimum detection Atmosphere 2021, 12, 61 17 of 25 limit of the system was 0.13–1.2 mg/m3 with respect to 1 h sampling at 113 L/min. The relative measurement error of the method was 27–34% [112]. The advantages of this invention were its compact size and low minimum detection limit. However, the device still led toinvention significant were measurement its compact bias. size and low minimum detection limit. However, the device still led to signiﬁcant measurement bias.

Atmosphere 2021, 12, x FOR PEER REVIEW 18 of 26

FigureFigure 11. Schematic 11. Schematic diagram diagram of a compact of a compact dilution dilution system system invented invented by England by England et al. et [112]. al. [112 (TMF:]. (TMF: TeflonTeflon membrane membrane filter, filter, QFF: QFF: quartz quartz fiber fiber filters, filters, SMPS: SMPS: scanning scanning mobility mobility particle particle sizer, sizer, MFM: MFM: mass flow meter). massA multipollutantflow meter). dilution sampling system was developed to measure not only particulate matterA multipollutant but also gaseous dilution pollutants sampling emitted system from was a stationary developed source to measure such as not carbon only partic- dioxideulate (CO matter2), carbon but monoxide also gaseous (CO), pollutants nitrogen emittedoxides (NO fromx), asulfur stationary dioxide source (SO2), such oxygen as carbon (O2), and total volatile organic compounds (TVOCs) [113–115]. A 90° elbow residence dioxide (CO2), carbon monoxide (CO), nitrogen oxides (NOx), sulfur dioxide (SO2), oxygen chamber made of stainless steel was used to increase the residence time up◦ to 28 s (O2), and total volatile organic compounds (TVOCs) [113–115]. A 90 elbow residence cham- (Figureber 12). made The air of stainlessflow ratesteel of the was inlet used sample to increase was 5 L/min. the residence That of timethe dilution up to 28 air s (Figure was 12). 125 L/min.The airThe flow inlet rate air oftemperature the inlet sample was 146 was‒174 5 L/min. °C. The That range of of the dilution dilution ratios air was was 125 20‒ L/min. 5-fold.The It was inlet found air temperature that the relative was 146-174measurement◦C. The error range was of less dilution than ratios5.1% [113–115]. was 20-5-fold. The It was inventionfound showed that the advantages relative measurementfor multiple func errortions. was Furthermore, less than 5.1% the [113 device–115 ].also The had invention a reasonableshowed measurement advantages error. for multiple However, functions. maintenance Furthermore, was an the issue device due also to its had compli- a reasonable cated nature.measurement error. However, maintenance was an issue due to its complicated nature.

Figure Figure12. A Schematic 12. A Schematic of the multipollu of the multipollutanttant dilution dilution system system[113–115]. [113 –115].

For ultra-low particulate matter emitted from a power plant, an electrical low-pressure impactor has been widely applied to investigate the particle size distribution [116]. However, moisture in the flue gas caused a significant bias in the analytical process. An increase in humidity resulted in an increase in measurement error. It was reported that the measurement error varied from 1000% to 2500% with respect to different particle sizes and humidity [116]. Thus, a hybrid dilution system was developed to reduce the moisture level in the sample gas (Figure 13). The system consisted of a dilution module and a drying module. The dilution ratio was 8:1. The dilution module had the same structure as in [107]. The temperature of the inlet gas should not be over 50 °C. The drying module was a Nafion® tube. It was claimed that the relative measurement errors of PM0.2, PM1, PM2.5, and PM10 were within 10% at a relative humidity of 73.7% [116]. The advantage of this invention was its enhanced measurement accuracy for ultra-low particulate matter. How- ever, using the Nafion® tube led to an increase in capital cost. Maintenance cost was also an issue for the system.

Atmosphere 2021, 12, 61 18 of 25

For ultra-low particulate matter emitted from a power plant, an electrical low-pressure impactor has been widely applied to investigate the particle size distribution [116]. How- ever, moisture in the flue gas caused a significant bias in the analytical process. An increase in humidity resulted in an increase in measurement error. It was reported that the measurement error varied from 1000% to 2500% with respect to different particle sizes and humidity [116]. Thus, a hybrid dilution system was developed to reduce the moisture level in the sample gas (Figure 13). The system consisted of a dilution module and a drying module. The dilution ratio was 8:1. The dilution module had the same structure as in [107]. The temperature of the inlet gas should not be over 50 ◦C. The drying module ® was a Nafion tube. It was claimed that the relative measurement errors of PM0.2, PM1, PM2.5, and PM10 were within 10% at a relative humidity of 73.7% [116]. The advantage of Atmosphere 2021, 12, x FOR PEER REVIEWthis invention was its enhanced measurement accuracy for ultra-low particulate19 matter. of 26 Atmosphere 2021, 12, x FOR PEER REVIEW 19 of 26 However, using the Nafion® tube led to an increase in capital cost. Maintenance cost was also an issue for the system.

Figure 13. Schematic diagram of the hybrid dilution system for measuring low particulate matter emittedFigureFigure from13. 13. SchematicSchematic a power diagramplant diagram [116]. of of the the hybrid hybrid dilution dilution system system for for measuring measuring low low particulate particulate matter matter emittedemitted from from a apower power plant plant [116]. [116]. The methods mentioned above were developed to monitor only filterable particulate The methods mentioned above were developed to monitor only filterable particulate matterThe or amethods combination mentioned of filterable above and were condensable developed toparticulate monitor onlymatter. filterable To monitor particulate only matter or a combination of filterable and condensable particulate matter. To monitor only condensablematter or a combination particulate matter, of filterable Cano and et al.condensable (2017) improved particulate the dilutionmatter. To method monitor CTM- only condensable particulate matter, Cano et al. (2017) improved the dilution method CTM-039 039condensable of U.S. EPA particulate (Figure 14) matter, [117]. ACano filter et module al. (2017) was improved employed the instead dilution of a PMmethod2.5 cyclone. CTM- of U.S. EPA (Figure 14)[117]. A filter module was employed instead of a PM cyclone. A039 quartz of U.S. fiber EPA filter (Figure was 14)used [117]. to remove A filter filter moduleable wasparticulate employed matter. instead Dilution of a PM ratios2.52.5 cyclone. were A quartz fiber filter was used to remove filterable particulate matter. Dilution ratios 10A‒ quartz40-fold, fiber and filter the inletwas usedair temperature to remove filter wasable in the particulate range of matter.190‒200 Dilution °C [117]. ratios The weread- were 10–40-fold, and the inlet air temperature was in the range of 190–200 ◦C[117]. The vantage10‒40-fold, of this and method the inlet is airthe temperature high accura cywas for in thethe measurementrange of 190‒ 200of only °C [117].condensable The ad- vantageadvantage of this of this method method is the is the high high accura accuracycy for for the the measurement measurement of of only only condensable condensable particulateparticulate matter. matter. However, However, it itis isdifficult difficult to tomaintain maintain the the filter filter holder holder at atthe the sampling sampling probe.particulate The selection matter. However,of filter size it mightis difficult also affectto maintain the accuracy the filter of the holder measurement. at the sampling probe.probe. The The selection selection of of filter filter size size might might al alsoso affect affect the the accuracy accuracy of of the the measurement. measurement.

Figure 14. General schematic diagram of a dilution system for measuring condensable particulate Figure 14. General schematic diagram of a dilution system for measuring condensable particulate matter [117]. matterFigure [117]. 14. General schematic diagram of a dilution system for measuring condensable particulate matter [117]. In general, the dilution method is useful for the measurement of particulate matter becauseIn general,it can reduce the dilution the effect method of the ishigh useful moisture for the without measurement causing of a particulate loss of particles. matter Anotherbecause advantage it can reduce of the the dilution effect of method the high is themoisture minimization without of causing the contamination a loss of particles. of the systemAnother because advantage the volume of the dilution of sample method gas extr is theacted minimization from the stack of theis small contamination [23]. However, of the thissystem method because has somethe volume limitations. of sample The dilutiongas extracted might from bring the about stack a is decrease small [23]. in theHowever, num- berthis of method particles has in somethe flue limitations. gas. If this The number dilution is lowermight than bring the about detection a decrease limit inof the num-analyzer,ber of bias particles will occur. in the Influe addition, gas. If this the number method isis lower not suitable than the for detection the high limit vacuum, of the high ana- pressure,lyzer, bias and will high occur. temperature In addition, (i.e., the >260 method °C) of theis not gas suitable stream [107].for the The high condensation vacuum, high of certainpressure, organic and high and temperatureacid compounds (i.e., >260could °C) occu of ther because gas stream of cooling [107]. Theby thecondensation dilution air of [23].certain Moreover, organic high and variationsacid compounds in the concentration, could occur because velocity, of and cooling temperature by the dilutionof the flue air gas[23]. can Moreover, influence high the measurementvariations in the results concentration, [107]. Finally, velocity, the stratification and temperature of the ofemission the flue gasgas also can resultsinfluence in anthe increase measurement in measurem results ent[107]. error Finally, when the the stratification emission gas of is the not emission mixed wellgas [107].also results The dilution in an increase method in is measuremstrongly affectedent error by whentemperature, the emission pressure, gas isand not sample mixed well [107]. The dilution method is strongly affected by temperature, pressure, and sample

Atmosphere 2021, 12, 61 19 of 25

In general, the dilution method is useful for the measurement of particulate matter because it can reduce the effect of the high moisture without causing a loss of particles. Another advantage of the dilution method is the minimization of the contamination of the system because the volume of sample gas extracted from the stack is small [23]. However, this method has some limitations. The dilution might bring about a decrease in the number of particles in the flue gas. If this number is lower than the detection limit of the analyzer, bias will occur. In addition, the method is not suitable for the high vacuum, high pressure, and high temperature (i.e., >260 ◦C) of the gas stream [107]. The condensation of certain organic and acid compounds could occur because of cooling by the dilution air [23]. Moreover, high variations in the concentration, velocity, and temperature of the flue gas can influence the measurement results [107]. Finally, the stratification of the emission gas also results in an increase in measurement error when the emission gas is not mixed well [107]. The dilution method is strongly affected by temperature, pressure, and sample gas molecular weight [22]. This is a limitation of the dilution method. Correction factors should be applied to counteract these effects. Since the dilution method reduced a high-temperature sample gas down to an ambient level, the condensable particulate matter could be generated [111]. This condensation process could occur in the dilution system when the phase equilibrium of condensable compounds was less than their residence time in the dilution system [111]. This equilibration time can be estimated by Equation (4) [111]:

1 τs = R ∞ (4) 4πDA 0 n(RP)RP f (Kn, α)dRP

2 where DA is the gas diffusivity (~0.06 cm /s), n(RP) is the number of particles at radius RP, f (Kn, α) corrects the mass flux for non continuum effects and imperfect accommodation, and α is the accommodation coefficient. The Fuchs and Sutugin approach could be used to evaluate f (Kn, α) and α [111]. Based on Equation (4), a dilution chamber could be designed to measure only the filterable particulate matter or both filterable and condensable particulate matter. To date, unfortunately, there is no reliable dilution system that can measure filterable and condensable particulate matter independently. Using two dilution systems with different volumes or different sampling ports with respect to retention time could be a potential method to separate filterable and condensable particulate matter. Various dilution systems are shown in Table4. The characteristics, advantages, disadvantages, and application of each device are given below.

Table 4. Development of dilution systems for particulate matter measurement.

No. Method Characteristics Advantages Disadvantages Application - Flow rate: 30 L/min - Bulky system - Gas temperature: Condensable plus U-shaped dilution Able to improve the - Significant loss of 1 150 ◦C filterable system [110] collection of organic carbon particle in the size - Dilution ratio: particulate matter range of 1–2 µm 25–100-fold - Compact size due to absence of residence time - Potential change - Flow rate: 174 L/min tank of particle shape - Gas temperature: Compact dilution - Retention time of sample according to Filterable 2 150–300 ◦C system [111] gas < 1 s dilution ratio particulate matter - Dilution ratio: - Able to operate at a high - Potential for 20–350-fold and wide temperature range significant bias of inlet gas Atmosphere 2021, 12, 61 20 of 25

Table 4. Cont.

No. Method Characteristics Advantages Disadvantages Application - Small dimensions of the - Flow rate: 113 L/min system - Potential for Condensable plus Compact dilution - Gas temperature: 3 - Low minimum detection significant bias filterable system [112] <175 ◦C limit of particles compared (27–34%) particulate matter - Dilution ratio: 20:1 to a standard method - Flow rate: 125 L/min - Compact size - Gas temperature: - Various functions for - Complicated Condensable plus 90◦ elbow dilution 4 146–174 ◦C particulate and gaseous structure filterable chamber [113–115] - Dilution ratio: measurements- Low - Hard to maintain particulate matter 20–50-fold measurement bias - Suitable for a wide range Hybrid dilution of particles, especially - Costly due to system coupled - Gas temperature: 50 ◦C Filterable 5 ultra-low particulate matter Nafion® dryer with Nafion® - Dilution ratio: 8:1 particulate matter - Low measurement bias - Hard to maintain dryer [116] caused by humidity Dilution system - Gas temperature: with a filter for - Highly accurate for - Hard to maintain 190–200 ◦C Condensable 6 filterable condensable particulate - Potential bias due - Dilution ratio: particulate matter particulate matter to pore filter size 10–40-fold matter [117]

4. Conclusions Condensation and permeation are the two general techniques used to remove moisture from flue gas in a CEMS for gaseous compounds. In condensation techniques, the moisture is removed under liquid or solid phase due to the decrease in flue gas temperature to a dew point. The correction factors or optimal operation conditions should be determined when employing these technologies. In addition, the condensation methods should only be applied for very poorly water-soluble compounds to prevent the loss of analytes. The permeation method seems to be a better technology in terms of moisture removal efficiency and selectivity. Polymeric membranes were found to be the most widely used membrane to remove moisture in flue gas besides zeolite, mixed matrix, and supported liquid membranes [37]. For decades, 19 main kinds of polymeric materials have been developed in order to remove the moisture. Various fabrication methods have been conducted with different substrates to improve the moisture removal efficiency and the membrane selectivity. However, the large amount of dry gas and the lower amount of sample gas are significant drawbacks of the permeation method. In addition, the loss of some target compounds, artifact formation, and contamination in the membrane were found when the permeation method was employed to remove moisture. In terms of particulate matter, the dilution method has been widely implemented. The dilution ratio should be carefully considered based on the detection limit of an analyzer to prevent measurement bias. The effect of dilution ratios on the shape of particulate matter should also be taken into account. The influence of the pressure, temperature, and molecular weight of target compounds should be corrected. These are also limitations of the method. Moreover, the measurement error of the particulate matter using the method is still significant. Accordingly, studies to find the best technology for moisture removal should be carried out. As for the condensation method, the removal of moisture at super-cooling conditions under a solid phase might be a potential way. The relationship between the polarities of water and target gases at different phases should be taken into account in order to predict the extent of their loss. The development of an effective catalytic converter to convert highly water-soluble analytes to poorly or non-water-soluble ones might be a Atmosphere 2021, 12, 61 21 of 25

promising solution for the application of the condensation method. In terms of permeation techniques, the influence of strong oxidative compounds and fine particles on membranes should be carefully investigated. Their lifetime should be considered as well. On the other hand, the dilution method is not the best technique for particulate matter measurement. Other methods that can remove the moisture without damaging particles may be an alternative. Furthermore, the development of a dilution system that can independently measure filterable and condensable particulate matter should be considered.

Supplementary Materials: The following are available online at https://www.mdpi.com/2073-443 3/12/1/61/s1: Table S1: Experimental results of water removal under solid phase (−20 ◦C) for HCl flue gas sample; Table S2: Experimental results of water removal for SO2 flue gas sample. Author Contributions: J.-C.K. and T.-V.D. conceptualized the work. T.-V.D. wrote the original draft, and the review and editing were done by J.-C.K. All authors have read and agreed to the published version of the manuscript. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: No new data were created or analyzed in this study. Data sharing is not applicable to this article. Acknowledgments: This paper was supported by Konkuk University Researcher Fund in 2019. Conflicts of Interest: The authors declare no conflict of interest.

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