Investigation on the Emission Characteristics with a Wet-Type Exhaust Gas Cleaning System for Marine Diesel Engine Application

Journal of Marine Science and Engineering

Article Investigation on the Emission Characteristics with a Wet-Type Exhaust Gas Cleaning System for Marine Diesel Engine Application

Younghyun Ryu 1 , Taewoo Kim 2, Jungsik Kim 2 and Jeonggil Nam 3,*

1 Division of Marine Mechatronics, Mokpo National Maritime University, Mokpo 58628, Korea; [email protected] 2 Techwin Co., Ltd., Cheongju 28580, Korea; [email protected] (T.K.); [email protected] (J.K.) 3 Division of Marine Engineering, Mokpo National Maritime University, Mokpo 58628, Korea * Correspondence: [email protected]; Tel.: +82-61-240-7220

Received: 22 September 2020; Accepted: 23 October 2020; Published: 28 October 2020

Abstract: Global air pollution regulations are becoming stricter for large diesel engines powering automobiles and ships. In the automotive sector, Euro 4 regulations came into force in January 2013 in accordance with European Union (EU) emission standards for heavy-duty diesel engines and are based on steady-state testing. In the marine sector, the International Maritime Organization (IMO) Maritime Environment Protection Committee (MEPC) is a group of experts who discuss all problems related to the prevention and control of marine pollution from ships, such as eﬀorts to reduce ozone-depleting substances and greenhouse gas emissions. To reduce the harmful emissions from marine diesel engines, a wet-type exhaust gas cleaning system was developed in this study. As a test, seawater, electrolyzed water, and sodium hydroxide were sequentially injected into the exhaust gas. SO2 was reduced by 98.7–99.6% with seawater injection, NOx by 43.2–48.9% with electrolyzed water injection, and CO2 by 28.0–33.3% with sodium hydroxide injection.

Keywords: IMO MEPC; marine diesel engine; emissions; seawater; electrolyzed seawater; sodium hydroxide

1. Introduction Global air pollution regulations for large diesel engines powering automobiles and ships are becoming stricter [1]. In the automotive sector, Euro 4 regulations came into force in January 2013 in accordance with European Union (EU) emission standards for heavy-duty diesel engines based on steady-state testing [1]. EU regulations for exhaust gas emissions have continually become more stringent starting from Euro 1 in 1992 to Euro 4 in 2013 [1]. In the marine sector, Tier 3 regulations enacted by the Marine Environment Protection Committee (MEPC) of the International Maritime Organization (IMO) entered into force on 1 January 2016 [2]. The MEPC is a group of experts who discuss all problems related to the prevention and control of marine pollution from ships, such as eﬀorts to reduce ozone-depleting substances and greenhouse gas (GHGs) emissions [2]. In particular, they review the prevention and regulation of marine pollution by ships and carry out functions related to the adoption and amendment of related international conventions [2]. The IMO headquarters in London, UK, holds the MEPC international conference once or twice a year [2–4]. Many researchers and stakeholders such as marine engine manufacturers, shipbuilding companies, and shipping companies are striving to reduce emissions from marine diesel engines. Marine diesel engines use the lowest-grade fuel and discharge more harmful exhaust gases than other transport diesel engines [5]. Therefore, studies have been conducted to improve the combustion process by modifying the fuel to reduce emissions from

J. Mar. Sci. Eng. 2020, 8, 0850; doi:10.3390/jmse8110850 www.mdpi.com/journal/jmse J. Mar. Sci. Eng. 2020, 8, 0850 2 of 12 marine diesel engines. Ryu et al. [6–8] attempted to improve the performance and lower the emissions of a diesel engine by mixing dimethyl ether with heavy fuel oil (HFO). Ryu et al. [9–11] also applied two types of fuel additives based on Ca and Fe to reduce the fuel consumption and emissions of marine diesel engines. Geng et al. [12] applied waste cooking oil to a marine diesel engine. Katayama et al. [13] applied Jatropha oil, which is a biofuel, to marine diesel engines. Hashimoto et al. [14] mixed palm oil, which is a biofuel, with gas oil and MDO (marine diesel oil) and investigated the combustion characteristics. Nam et al. [15] applied an emulsion fuel, which is a mixture of fuel and water, to a diesel engine and examined the combustion and emission characteristics. Panomsuwan et al. [16] reduced pollutant emissions by applying non-thermal plasma technology to a marine diesel engine. Exhaust gas recirculation (EGR) is widely used to reduce the NOx emissions of diesel and commercial vehicles [17,18]. There have also been many studies on using EGR in ships. Selective catalytic reduction (SCR) is being applied to large vehicles such as diesel engine trucks and in onshore power plants to reduce NOx [19]. It has also been installed in many ships. Ryu et al. [20] reported the installation of SCR to reduce NOx emissions from ships to meet the IMO MEPC Tier 3 regulations. Although there are regulations on the particulate matter (PM) emissions of automotive diesel engines, there are no such regulations for marine diesel engines yet. However, it is expected that PM regulations will be enacted in the future. Ntziachristos et al. [21] researched the reduction in PM emissions from marine diesel engines. In addition, there have been many discussions on regulations to reduce SOx emissions from ships. Ships operating in a sulfur environmental control area (SECA) are obligated to use fuel with low sulfur content. The use of 1.0% m/m low-sulfur fuel was enforced in 2010, and the regulation was strengthened in 2015 to the use of 0.1% m/m low-sulfur fuel. In other areas outside an SECA, the use of 3.5% m/m low-sulfur fuel has been enforced since 2012. According to Regulation 14.8 of the International Convention for the Prevention of Marine Pollution from Ships (MARPOL) Annex 6, the use of marine fuel oil with 0.5% m/m or less sulfur content was enforced for ships operating anywhere in the world from 1 January 2020 (Regulation 14.1.3 of MARPOL Annex 6) [4]. Many measures are being suggested to reduce SOx emissions, such as scrubbers. Table1 lists the sulfur content in fuel oil required by the IMO. The IMO also controls improvements in energy eﬃciency and GHGs emissions through various regulations. Table2 lists the regulations for CO 2 reduction. CO2 is a GHG that is cited as the main cause of global warming. Table2 lists the CO 2 emission factors by the type of fuel used in ships [4].

Table 1. IMO sulfur requirements [4].

Outside ECA Inside ECA (Global Requirement) 4.5% m/m prior to 1 January 2012 1.5% m/m prior to 1 July 2010 3.5% m/m on and after 1 January 2012 1.0% m/m on and after 1 July 2010 0.5% m/m on and after 1 January 2020 0.1% m/m on and after 1 January 2015 Emission Control Areas (ECA).

Table 2. CO2 emission factors (g/g fuel) [4].

Year Region Fuel Type 2012 2030 2050 HFO 3114 3114 3114 LSFO 3114 3114 3114 Global MGO 3206 3206 3206 LNG 2750 2750 2750 Heavy Fuel Oil (HFO), Low Sulphur Fuel Oil (LSFO), Marine Gas Oil (MGO), Liqueﬁed Natural Gas (LNG). J. Mar. Sci. Eng. 2020, 8, 0850 3 of 12

Various technologies are available for reducing NOx, SOx, and CO2. This study applied the wet-type exhaust gas cleaning system to diesel engines. This system is an after-treatment technology that can reduce the three harmful exhaust gas components simultaneously. Ryu et al. [22] suggested reducing NOx by injecting electrolyzed water to exhaust gas. In this study, however, seawater, electrolyzed seawater, and sodium hydroxide were sprayed sequentially onto exhaust gas from a diesel engine to reduce NOx, SOx, and CO2. The NOx emitted from a marine diesel engine contains 90–95% NO, which is insoluble. NO2, which is generated from the oxidation of NO, is known to have 20 times higher solubility in water compared to NO [23,24]. Therefore, NO was oxidized to NO2 through the gas–liquid contact of electrolyzed seawater (electrolyzed water) and exhaust gas to reduce NOx generation. An et al. [25] oxidized NO and absorbed NO2 in exhaust gas by spraying a negative strong acidic solution and positive strong basic solution sequentially. These solutions were obtained through membrane electrolysis, which is easily applicable to ships because of the ready supply of seawater. However, membrane electrolysis of seawater has the disadvantage of needing a strong acid with pH 2–3 and generating a large volume of chlorine gas. To address this disadvantage, Kim et al. [23,24] tried to reduce NOx through non-membrane seawater electrolysis. In Ryu et al. [22], Kim et al. [23,24] and An et al. [25], only NOx reduction was presented, but in this paper, NOx, SOx and CO2 reduction were also presented. An experimental apparatus was planned, designed, and installed for research on NOx and SOx reduction using real seawater in an indirect injection type diesel engine. Reducing CO2 with sodium hydroxide was also researched. This apparatus for reducing exhaust gas with seawater can be used not only on ships but also in thermal power plants by the sea.

2. Experimental Apparatus and Method

2.1. Engine Test In this study, a four-stroke diesel engine of the indirect injection (IDI) type was used. This is a small engine with four cylinders that has a compression ratio of 22:1. Table3 outlines the engine speciﬁcations.

Table 3. Test engine speciﬁcations.

Item Description Engine type D4BB-G3, four-stroke diesel engine Bore stroke 91.1 100 mm × × Combustion type Indirect injection No. of cylinders 4 inline Displacement volume 2607 cm3 MCR output 45 PS @ 1800 rpm Compression ratio 22:1 Fuel Diesel oil Maximum Continuous Rating (MCR); Pferde-Starke (PS).

Figure1 shows the entire experimental apparatus used in this study. To remove NOx, SOx, and CO 2 generated from engine combustion, real seawater that had not been electrolyzed, electrolyzed seawater (electrolyzed water), and a sodium hydroxide (NaOH) solution diluted to a certain concentration were sprayed sequentially onto exhaust gas discharged from a diesel engine. In general, nitrogen compounds refer to the combination of NO and NO2 [26]. The nitrogen compounds were measured by using 350-Maritime from TESTO. This model can measure the concentrations of NO, NO2, SO2, and CO2 separately with electrochemical cells. The removal rates in the reactor were calculated as follows [22]: ca,i ca,o Ya = − 100% (1) ca,i × J. Mar. Sci. Eng. 2020, 8, 0850 4 of 12

J. Mar. Sci. Eng. 2020, 8, 850 4 of 13 where a denotes a chemical species such as NO, NO2, or NOx and Ci and Co denote the concentrations atelectrolyzed the inlet and water outlet, was respectively, sprayed, of the the correspondinginlet and outlet chemical concentrations species. After of theNO/NO electrolyzed2/NOx waterwere wasanalyzed. sprayed, Figure the inlet 2 andshows outlet a concentrationsschematic diagram of NO/NO of2 /NOxthe wereexperimental analyzed. Figureapparatus.2 shows The a schematicpH/OPR/temperature diagram of thesensors experimental were installed apparatus. in Thethe pHpipe/OPR for/ temperatureelectrolyzed sensors water wereto examine installed the in theeffects pipe of for operating electrolyzed variables water such to examine as the temperature the effects ofand operating pH of the variables electrolyzed such aswater the temperatureon the NOx andremoval pH ofperformance. the electrolyzed The VSTAR12 water on themodel NOx of removal Orion was performance. used as the Thesensor. VSTAR12 A magnet-type model of pump Orion was(Yusung, used asNH-3024PH-CV-5HP) the sensor. A magnet-type was used pump to (Yusung, spray NH-3024PH-CV-5HP)the electrolyzed water, was and used an to area-type spray the electrolyzedflowmeter (COREA water, and flow, an HGF-1-P) area-type flowmeterwas used (COREAto measure flow, the HGF-1-P) flow of the was sprayed used to measureelectrolyzed the flowwater. of To the produce sprayed an electrolyzed oxidant through water. the To produceelectrolysis an oxidantof seawater, through which the was electrolysis one of the of seawater,purposes whichof this wasstudy, one a ofnon-membrane the purposes electrolysis of this study, tank a non-membrane with a capacity electrolysis of 1.25 kg Cl tank2/day, with produced a capacity by ofT 1.25company, kg Cl2 /wasday, used, produced and bya three-phase T company, 380 was V used, AC was and aapplied three-phase through 380 Va rectifier AC was applied(MK power, through MK- a rectifier50200G). (MK This power, electrolysis MK-50200G). tank was This designed electrolysis for tankthe waselectrolysis designed of for seawater the electrolysis and circulation of seawater of andelectrolyzed circulation water of electrolyzed with a structure water withthat apreven structurets the that accumulation prevents the accumulationof scaling or of particulate scaling or particulatecontaminants. contaminants.

Figure 1. PhotographPhotograph of test facility.

Figure 2. Schematic diagram of experimental apparatus.

A scrubberscrubber forfor sprayingspraying electrolyzed electrolyzed water water was was produced produced as aas packed a packed tower tower with with a residence a residence time oftime 6.5 of s, which6.5 s, which is given is ingiven Equation in Equation (2). The (2). ﬁlling The material filling material for improved for improved gas–liquid gas–liquid contact eﬃ contactciency wasefficiency 1-in Tripack was 1-in made Tripack of polypropylene made of polypropylen material,e andmaterial, a spiral-type and a spiral-type nozzle was nozzle used towas spray used the to cleaningspray the solution. cleaning Figure solution.3 shows Figure the 3 cleaningshows the solution cleaning spray solution nozzle spray and packing.nozzle and Table packing.4 lists the Table test 4 conditionslists the test of conditions the experimental of the experimental apparatus. apparatus. h i Volume𝑉𝑜𝑙𝑢𝑚𝑒 o f Packing 𝑜𝑓m 𝑃𝑎𝑐𝑘𝑖𝑛𝑔3 m 𝑅𝑒𝑠𝑖𝑑𝑒𝑛𝑐𝑒 𝑡𝑖𝑚𝑒 s= (2) Residence time [s] = 𝐹𝑙𝑜𝑤 𝑟𝑎𝑡𝑒h 𝑜𝑓 𝑔𝑎𝑠Nm /si (2) Flow rate o f gas Nm3/s Table 5 presents the solubility of gases used in this study based on the data provided by Haynes [27]. J. Mar. Sci. Eng. 2020, 8, 0850 5 of 12 J. Mar. Sci. Eng. 2020, 8, 850 5 of 13

FigureFigure 3. Photograph 3. Photograph of spray of spray nozzle nozzle and and packing. packing.

Table 4. Test conditions. Table 4. Test conditions.

ItemItem Description Description 1st stage:1st stage:seawater seawater SprayingSpraying order order 2nd stage:2nd stage:electrolyzed electrolyzed seawater seawater 3rd stage: NaOH (+Na2S) Absorber type3rd stage: NaOH Packed (+Na bed2S) AbsorberPacking type 1” Packed Tripack bed (polypropylene) ExhaustPacking gas ﬂow 1” Tripack (polypropylene)130 10 Nm3/h ± ExhaustRetention gas time flow 130 ± 10 Nm 6.3–6.93/h s 3 RetentionLiquid–gas time ratio 6.3–6.943–50 s L/m CNO 887–942 ppmv Liquid–gasx,i ratio 43–50 L/m3 CSO2,i 430–470 ppmv x,i CNOCCO2,i 887–9425.30–5.45 ppmv vol.% Cl2 concentrationCSO2,i 430–470 4.8–5.3 ppmv g Cl 2/L NaOHCCO concentration2,i 5.30–5.45 6.1–7.7 vol.% g NaOH /L Cl2 concentration 4.8–5.3 g Cl2/L Table5 presents theNaOH solubility concentration of gases used in this study 6.1–7.7 based g NaOH/L on the data provided by Haynes [ 27].

Table 5. Solubility of selected gases in water.

GasGas Solubility Solubility in Water in Water [mol/L] [mol /L] NO 1.51 × 10−6 6 NO 1.51 10− −5× 5 NONO2 2 2.44 ×2.44 10 10− −3× 3 SOSO2 2 1.25 ×1.25 10 10− × 5 CO 3.48−5 10 CO2 2 3.48 × 10 × − Cl 8.20 10 5 scientiﬁc notion Cl2 2 8.20 × 10−5 ×scientific− notion

2.2. Oxidation and Reduction Agent In this this study, study, the the oxidant oxidant was was produced produced in inreal real time time through through the theelectrolysis electrolysis of seawater. of seawater. The Thequantity quantity of oxidant of oxidant can canbe converted be converted to the to effective the eﬀective chlorine chlorine concentration concentration (g Cl (g2/L), Cl2 /whichL), which is the is thequantities quantities of chlorine of chlorine (Cl (Cl2), 2hypochlorite), hypochlorite ions, ions, and and hypochlorous hypochlorous acid acid (HOCl) (HOCl) converted converted to to the equivalent amount of Cl22.. This This was was measured measured with with the the following following titration titration method: (1)(1) InjectInject 1 1 mL mL of of the the sample sample into into 25 25 mL mL of of ultrapure ultrapure water. (2)(2) AddAdd 1 1 g g of of potassium potassium iodide iodide and 1 mL of acetic acid. (3)(3) AddAdd two two to to three three drops drops of 1% starch soluti solution.on. The The solution solution then then turns reddish brown. (4)(4) AddAdd 0.1 0.1 N sodium thiosulfate to the reddish-brown solution and measure the amount added untiluntil it it becomes becomes colorless. colorless. (5)(5) DetermineDetermine the the effective eﬀective chlorine concentration as follows: 𝐶mL × 3.545 C[ ] Available chlorineg 𝐶𝑙/L = mL 3.545 (3) Available chlorine[g Cl2/L] = 𝑆mL× (3) S[mL] where C is the titration amount of 0.1 N sodium thiosulfate and S is the sample injection amount. where C is the titration amount of 0.1 N sodium thiosulfate and S is the sample injection amount. In the No. 3 tower, sodium hydroxide diluted in water was sprayed as absorbent, and the concentration was measured according to the following procedure: J. Mar. Sci. Eng. 2020, 8, 0850 6 of 12

In the No. 3 tower, sodium hydroxide diluted in water was sprayed as absorbent, and the J. Mar.concentration Sci. Eng. 2020, was8, 850 measured according to the following procedure: 6 of 13

(1) (1)AddAdd 1 mL 1 mLof sample of sample to to25 25 mL mL of of ultrapure ultrapure water. (2) (2)AddAdd two twoto three to three drops drops of ofphenolphthalein. phenolphthalein. The solutionsolution turns turns red. red. (3) (3)AddAdd 0.1 0.1M hydrochloric M hydrochloric acid acid to to the the red red solutionsolution andand measure measure the the amount amount injected injected until until it it becomesbecomes colorless. colorless. (4) (4)DetermineDetermine the theconcentration concentration of of sodium sodium hydroxide hydroxide asas follows: follows: C[mL] 4 Sodium hydroxide[g NaOH/L] =𝐶mL ×4× (4) Sodium hydroxideg NaOH/L = S[mL] (4) 𝑆mL where C is the titration amount of 0.1 M hydrochloric acid and S is the sample injection amount. where C is the titration amount of 0.1 M hydrochloric acid and S is the sample injection amount. FigureFigure 4 shows4 shows the the calculated calculated concentrations of of the the oxidant oxidant and and absorbent absorbent according according to the to the operationoperation time. time.

FigureFigure 4. Concentrations 4. Concentrations of of oxidant oxidant and absorbentabsorbent against against the the operating operating time. time. 3. Results and Discussion 3. Results and Discussion 3.1. Sulfur Oxide (SOx) Emissions 3.1. Sulfur Oxide (SOx) Emissions SO2 gas was removed by using the alkalinity of seawater, as suggested by Andreasen et al. [28]. 2 AfterSO2 gas SO2 wasdissolves removed in water, by using it exists the as alkalinity bisulfate ionsof seawater, (HSO3−) as and suggested sulfate ions by (SO Andreasen3 ). As aresult, et al. [28]. the solution reaches equilibrium because of the decreased pH, and− the dissolved CO is discharged.2 After SO2 dissolves in water, it exists as bisulfate ions (HSO3 ) and sulfate ions (2SO3 ). As a result, In this study, the diluted solutions of seawater, electrolyzed water, and sodium hydroxide were the solution reaches equilibrium because of the decreased pH, and the dissolved CO2 is discharged. sequentially sprayed in each stage. The main purpose of the No. 1 tower, in which seawater was In this study, the diluted solutions of seawater, electrolyzed water, and sodium hydroxide were sprayed, was to remove SO2 and particulate contaminants. When a large amount of particulate sequentiallycontaminants sprayed in the in exhaust each stage. gas flows The into main the No.purpose 2 tower, of whichthe No. operates 1 tower, in a in closed which loop, seawater this can was sprayed, was to remove SO2 and particulate contaminants. When a large amount of particulate negatively influence the efficiency of the electrolysis tank. When SO2 is absorbed, it decreases the contaminantspH of the in cleaning the exhaust water. gas When flows the into pH the of the No. electrolyzed 2 tower, which water operates decreases in to a neutralclosed loop, or lower, this can negativelythe dissolved influence Cl2 canthe beefficiency discharged. of the This electrolysis implies the tank. loss ofWhen oxidant SO by2 is energy absorbed, input. it Therefore,decreases the pH ofprior the removal cleaning of water. and particulate When the contaminants pH of the el inectrolyzed the No. 1 towerwater can decreases minimize to theneutral above or adverse lower, the dissolvedeffects. Cl Figure2 can5 beillustrates discharged. the SO This2 removal implies rate the of thisloss system. of oxidant The SO by2 removalenergy rateinput. was Therefore, 98.7–99.6%, prior removalwhich of indicates and particulate relatively contaminants stable operation. in the This No. confirms 1 tower that can at minimize least 98% ofthe SO above2, which adverse has high effects. solubility in water, was removed with the No. 1 tower, as shown in Figure6. Figure 5 illustrates the SO2 removal rate of this system. The SO2 removal rate was 98.7–99.6%, which indicates relatively stable operation. This confirms that at least 98% of SO2, which has high solubility in water, was removed with the No. 1 tower, as shown in Figure 6. SO2 removal mechanism:

𝑆𝑂 ⇔𝑆𝑂 (5)

𝑆𝑂 𝐻𝑂 ⇔𝐻𝑆𝑂 𝐻 (6)

(7) 𝐻𝑆𝑂 𝐻𝑂 ⇔𝑆𝑂 𝐻 J. Mar. Sci. Eng. 2020, 8, 850 7 of 13 J. Mar. Sci. Eng. 2020, 8, 850 7 of 13

𝐻𝐶𝑂 𝐻 ⇔𝐶𝑂 𝐻𝑂 (8) 𝐻𝐶𝑂 𝐻 ⇔𝐶𝑂 𝐻𝑂 (8)

𝐶𝑂 ⇔𝐶𝑂 (9) 𝐶𝑂 ⇔𝐶𝑂 (9) J. Mar. Sci. Eng. 2020, 8, 0850 7 of 12

Figure 5. Removal rate of SO2. Figure 5. Removal rate of SO . Figure 5. Removal rate of SO2 2.

Figure 6. Average concentration of SO2 at each stage. Figure 6. Average concentration of SO2 at each stage.

SO2 removal mechanism:Figure 6. Average concentration of SO2 at each stage. 3.2. Nitric Oxide (NOx) Emissions SO SO (5) 2(g) ⇔ 2(aq) 3.2. Nitric Oxide (NOx) Emissions + Wet-type NOx removal involvesSO ( ) + theH2 oxidationO( ) HSO of− NO,+ whichH accounts for 90–95 vol%(6) of NOx. 2 aq l ⇔ 3(aq) (aq) 2Wet-type NOx removal involves the oxidation of NO, which accounts for 90–95 vol% of NOx. Cl was used as the oxidant of NO in this study; it was2 produced+ by non-membrane electrolysis of HSO−( ) + H2O(l) SO −( ) + H (7) Cl2NaCl was inused water. as the The oxidant Cl2 gas of produced NO in3 aqthis from study; the electrode it⇔ was3 producedaq surface(aq quickly )by non-membrane dissolves an electrolysisd exists as three of 2 + NaClchemical in water. species. The ClAs gasshown produced inHCO Figure− from 7,+ theeachH electrode chemicalCO surface species+ H quicklyO reaches dissolves equilibrium and accordingexists as(8) three to the 3(aq) (aq) ⇔ 2(aq) 2 (l) chemicalpH. When species. the AspH shownis lower, in Figurethe condition 7, each chbecomesemical speciesmore unstable, reaches equilibriumand gases can according be discharged. to the CO CO (9) pH.HOCl, When which the pH is predominantis lower, the incondition the weak becomes2 (acidaq) ⇔ domain, more2(g) unstable,has an oxidizing and gases power can beat leastdischarged. 80 times − HOCl,higher3.2. which Nitric than Oxide isthat predominant (NOx)of OCl Emissions[29]. in In the this weak study, acid therefore, domain, thehas conditionan oxidizing of pH power 6–7 wasat least maintained 80 times so higherthat thanHOCl that would of OCl not−[29]. be Indischarged this study, as therefore, gas while the maintainingcondition of pHthe 6–7oxidizing was maintained power of sothe Wet-type NOx removal involves the oxidation of NO, which accounts for 90–95 vol% of NOx. thatelectrolyzed HOCl would water, not which be wasdischarged the cleaning as gas solution while for maintaining the No. 2 tower. the Theoxidizing reducing power agent of sodium the Cl2 was used as the oxidant of NO in this study; it was produced by non-membrane electrolysis electrolyzedsulfide (Na water,2S) was which injected was inthe the cleaning No. 3 solutiontower to for im theprove No. the 2 tower. NOx removalThe reducing performance, agent sodium and its of NaCl in water. The Cl2 gas produced from the electrode surface quickly dissolves and exists as sulfide (Na2S) was injected in the No. 3 tower to improve the NOx removal performance, and its effectthree was chemical observed. species. As shown in Figure7, each chemical species reaches equilibrium according to effectthe was pH. observed. When the pH is lower, the condition becomes more unstable, and gases can be discharged. HOCl, which is predominant in the weak acid domain, has an oxidizing power at least 80 times higher than that of OCl− [29]. In this study, therefore, the condition of pH 6–7 was maintained so that HOCl would not be discharged as gas while maintaining the oxidizing power of the electrolyzed water, which was the cleaning solution for the No. 2 tower. The reducing agent sodium sulfide (Na2S) was injected in the No. 3 tower to improve the NOx removal performance, and its effect was observed. The results indicated that the total NOx removal rate was 43.2–48.9%, as shown in Figure8, and the injection of the reducing agent in the No. 3 tower showed a negative effect. To express the NO and NO2 removal rates separately, as shown in Figure9, the NO removal rate decreased after injection of the reducing agent. This can be interpreted that the strong reducing agent Na2S reduced NO2 or NO3− to NO. Figure 10 shows the average concentrations of NO and NO2 at the outlet of each stage. J.J. Mar.Mar. Sci.Sci. Eng.Eng. 20202020,, 88,, 850850 88 ofof 1313

J. Mar. Sci. Eng. 2020, 8, 0850 8 of 12

The oxidation reaction of NO was dominant in the No. 2 tower, whereas the absorption of NO2 was dominant in the No. 3 tower. In the case of the No. 3 tower, in which the NaOH solution was sprayed, the NO oxidation rate was around 17%, unlike the No. 1 tower. This appears to be because both NO andJ. Mar. NO Sci.2 Eng.were 2020 removed, 8, 850 by reaction with NaOH. 8 of 13

FigureFigure 7.7. ChlorineChlorine speciationspeciation profileprofile asas aa functionfunction ofof pH.pH.

TheThe resultsresults indicatedindicated thatthat thethe totaltotal NOxNOx removalremoval raterate waswas 43.2–48.9%,43.2–48.9%, asas shownshown inin FigureFigure 8,8, andand thethe injectioninjection ofof thethe reducingreducing agentagent inin thethe No.No. 33 towertower showedshowed aa negativenegative effect.effect. ToTo expressexpress thethe NONO andand NONO22 removalremoval ratesrates separately,separately, asas shownshown inin FigureFigure 9,9, thethe NONO removalremoval raterate decreaseddecreased afterafter injectioninjection ofof thethe reducingreducing agent.agent. ThisThis cancan bebe interpretedinterpreted thatthat thethe strongstrong reducingreducing agentagent NaNa22SS reducedreduced − NONO22 oror NONO33− toto NO.NO. FigureFigure 1010 showsshows thethe averageaverage concentrationsconcentrations ofof NONO andand NONO22 atat thethe outletoutlet ofof eacheach stage.stage. TheThe oxidationoxidation reactionreaction ofof NONO waswas domidominantnant inin thethe No.No. 22 tower,tower, whereaswhereas thethe absorptionabsorption ofof NONO22 waswas dominantdominant inin thethe No.No. 33 tower.tower. InIn thethe casecase ofof thethe No.No. 33 tower,tower, inin whichwhich thethe NaOHNaOH solution was sprayed, the NO oxidation rate was around 17%, unlike the No. 1 tower. This appears solution was sprayed, theFigure NO oxidation 7. Chlorine rate speciation was around profile proﬁle as 17%, a function unlike ofthe pH. No. 1 tower. This appears toto bebe becausebecause bothboth NONO andand NONO22 werewere removedremoved byby reactionreaction withwith NaOH.NaOH. The results indicated that the total NOx removal rate was 43.2–48.9%, as shown in Figure 8, and the injection of the reducing agent in the No. 3 tower showed a negative effect. To express the NO and NO2 removal rates separately, as shown in Figure 9, the NO removal rate decreased after injection of the reducing agent. This can be interpreted that the strong reducing agent Na2S reduced NO2 or NO3− to NO. Figure 10 shows the average concentrations of NO and NO2 at the outlet of each stage. The oxidation reaction of NO was dominant in the No. 2 tower, whereas the absorption of NO2 was dominant in the No. 3 tower. In the case of the No. 3 tower, in which the NaOH solution was sprayed, the NO oxidation rate was around 17%, unlike the No. 1 tower. This appears to be because both NO and NO2 were removed by reaction with NaOH.

x FigureFigureFigure 8. 8.8. RemovalRemoval raterate ofof NONOxx..

Figure 8. Removal rate of NOx.

FigureFigureFigure 9. 9.9. RemovalRemoval raterate ofof NONO/NONO/NO/NO222..

Figure 9. Removal rate of NO/NO2. J. Mar. Sci. Eng. 2020, 8, 0850 9 of 12 J. Mar. Sci. Eng. 2020, 8, 850 9 of 13

FigureFigure 10. 10.Average Average concentration concentration of NOof NO/NO/NO2 at2 at each each stage stage (without (without Na Na2S).2S).

NONO/NO/NO2 2removal removal mechanismmechanism by by electrolyzed electrolyzed seawater seawater [25,30–33]: [25,30– 33]:

NO(g) NO(aq) (10)(10) 𝑁𝑂⇔⇔𝑁𝑂

NO + OCl− NO + Cl− (11) (aq) (aq) → 2(aq) (aq) (11) 4NO 𝑁𝑂( ) + 2H 2 𝑂𝐶𝑙O( ) + →𝑁𝑂O ( ) 4 HNO 𝐶𝑙 ( ) (12) 2 aq l 2 g → 3aq 2NO(g) + O2(g) N2O4(g) (13) → (12) 4𝑁𝑂 N O 2𝐻 𝑂 N O 𝑂 →4𝐻𝑁𝑂 (14) 2 4(g) ⇔ 2 4(aq) N2O4(aq) + H2O(l) HNO3(aq) + HNO3(aq) (15) → (13) NO/NO2 removal mechanism by NaOH2𝑁𝑂 [25 ,30 𝑂–33]:→𝑁𝑂

NO + NO + 2NaOH NaNO + NaNO + H O (16) (aq) 2(aq) (l) → 2(aq) 3(aq) 2 (l) (14) 𝑁𝑂 ⇔𝑁𝑂 NO + NO + 2NaOH 2NaNO + H O (17) (aq) 2(aq) (l) ⇔ 2(aq) 2 (l) (15) 2NO2(aq) + 2NaOH(l) NaNO2(aq) + NaNO3(aq) + H2O(l) (18) 𝑁𝑂 ⇔ 𝐻𝑂 →𝐻𝑁𝑂 𝐻𝑁𝑂 NO/NO2 removal mechanism by Na2S[34–36]: NO/NO2 removal mechanism by NaOH[25,30–33]: 2 S − + H O HS− + OH− (19) (aq) 2 (l) ⇔ (aq) (16) 𝑁𝑂 𝑁𝑂 2𝑁𝑎𝑂𝐻 → 𝑁𝑎𝑁𝑂 𝑁𝑎𝑁𝑂 𝐻𝑂 NO + HS− NO− + HS (20) 2(aq) (aq) → 2(aq) 2NO (aq) + Na2S(s) N (g) + Na2SO (aq) (21)(17) 2 → 2 4 𝑁𝑂 𝑁𝑂 2𝑁𝑎𝑂𝐻 ⇔ 2𝑁𝑎𝑁𝑂 𝐻𝑂 3Na S + 8HNO 6NaNO + 3S + 2NO + H O (22) 2 (s) 3(aq) → 3(aq) (s) (g) 2 l (18) 3.3. Carbon Dioxide (CO ) Emissions 2𝑁𝑂2 2𝑁𝑎𝑂𝐻 ⇔ 𝑁𝑎𝑁𝑂 𝑁𝑎𝑁𝑂 𝐻𝑂 2 CO2 dissociates into bicarbonate ions (HCO3−) and carbonate ions (CO3 −) after being dissolved in water inNO/NO the carbonate2 removal system, mechanism and they by reachNa2S [34–36]: equilibrium according to the concentration of hydrogen ions. The hydrogen ion concentration of the cleaning solution was increased by the sodium hydroxide used to remove NO and NO . As a result, the equilibrium of the carbonate system moved in the(19) 2 𝑆 𝐻𝑂 ⇔𝐻𝑆 𝑂𝐻 2 direction of dissolving, and CO2 was converted to CO3 − [37,38]. Consequently, a CO2 removal rate of 28.0–33.3% was achieved, as shown in Figure 11. As shown by the average CO concentration at each 2 (20) stage in Figure 12, the CO2 concentration𝑁𝑂 did not 𝐻𝑆 change →𝑁𝑂 in the No. 𝐻𝑆 1 and No. 2 towers but decreased in the No. 3 tower. O2 absorption did not occur in the seawater spray tower and electrolyzed seawater 2 spray tower because HCO3− and CO3 − exist in general seawater from the hydrogen ions reaching(21) equilibrium with CO2 in the atmosphere.2N𝑂 𝑁𝑎𝑆 → 𝑁 𝑁𝑎𝑆𝑂

(22) 3NaS 8HNO → 6NaNO 3S 2NO HO J. Mar. Sci. Eng. 2020, 8, 850 10 of 13 J. Mar. Sci. Eng. 2020, 8, 850 10 of 13

3.3. Carbon Dioxide (CO2) Emissions 3.3. Carbon Dioxide (CO2) Emissions CO2 dissociates into bicarbonate ions (HCO3−) and carbonate ions (CO32−) after being dissolved CO2 dissociates into bicarbonate ions (HCO3−) and carbonate ions (CO32−) after being dissolved in water in the carbonate system, and they reach equilibrium according to the concentration of in water in the carbonate system, and they reach equilibrium according to the concentration of hydrogen ions. The hydrogen ion concentration of the cleaning solution was increased by the hydrogen ions. The hydrogen ion concentration of the cleaning solution was increased by the sodium hydroxide used to remove NO and NO2. As a result, the equilibrium of the carbonate sodium hydroxide used to remove NO and NO2. As a result, the equilibrium of the carbonate system moved in the direction of dissolving, and CO2 was converted to CO32− [37,38]. Consequently, system moved in the direction of dissolving, and CO2 was converted to CO32− [37,38]. Consequently, a CO2 removal rate of 28.0–33.3% was achieved, as shown in Figure 11. As shown by the average a CO2 removal rate of 28.0–33.3% was achieved, as shown in Figure 11. As shown by the average CO2 concentration at each stage in Figure 12, the CO2 concentration did not change in the No. 1 and CO2 concentration at each stage in Figure 12, the CO2 concentration did not change in the No. 1 and No. 2 towers but decreased in the No. 3 tower. O2 absorption did not occur in the seawater spray No. 2 towers but decreased in the No. 3 tower. O2 absorption did not occur in the seawater spray tower and electrolyzed seawater spray tower because HCO3− and CO32− exist in general seawater − 2− towerJ. Mar. Sci.and Eng. electrolyzed2020, 8, 0850 seawater spray tower because HCO3 and CO3 exist in general seawater10 of 12 from the hydrogen ions reaching equilibrium with CO2 in the atmosphere. from the hydrogen ions reaching equilibrium with CO2 in the atmosphere.

Figure 11. Removal rate of CO2. Figure 11. Removal rate of CO2.

FigureFigure 12. 12.Average Average concentration concentration of COof CO2 at2 at each each stage. stage. Figure 12. Average concentration of CO2 at each stage.

COCO2 removal2 removal mechanism: mechanism: CO2 removal mechanism: CO CO (23) 2(g) ⇔ 2(aq) CO + H O HCO + H + (24)(23) 2(aq) 2𝐶𝑂(l) ⇔𝐶𝑂3−(aq) (aq) (23) 𝐶𝑂 ⇔𝐶𝑂⇔

CO2(aq) + OH− HCO− (25) (aq) ⇔ 3(aq) (24) (24) 𝐶𝑂2 𝐻𝑂+ ⇔𝐻𝐶𝑂 𝐻 𝐶𝑂 CO − 𝐻+𝑂 H⇔𝐻𝐶𝑂HCO− 𝐻 (26) 3(aq) (aq) ⇔ 3(aq) 2 HCO− + OH− CO − + H O (27)(25) 3(aq) (aq) ⇔ 3(aq) 2 (l) (25) 𝐶𝑂 𝑂𝐻 ⇔𝐻𝐶𝑂 𝐶𝑂 𝑂𝐻 ⇔𝐻𝐶𝑂 4. Conclusions (26) In this study, a wet-type exhaust𝐶𝑂 gas cleaning 𝐻 system, ⇔𝐻𝐶𝑂 which is an after-treatment device(26) for 𝐶𝑂 𝐻 ⇔𝐻𝐶𝑂 reducing exhaust gases from a marine diesel engine, was developed and evaluated. An experiment was performed in which seawater, electrolyzed seawater, and sodium hydroxide were sequentially (27) sprayed onto the exhaust gas. The𝐻𝐶𝑂 following results𝑂𝐻 were⇔𝐶𝑂 obtained: 𝐻 𝑂 (27) 𝐻𝐶𝑂 𝑂𝐻 ⇔𝐶𝑂 𝐻 𝑂 (1) Electrolyzed water was sprayed into the second tower with NOx as the oxidizer, and the oxidizer concentration stayed within the range of 4.8–5.3 g Cl2/L during the operating period. Sodium hydroxide was injected into the third tower with NOx as the absorbent, and the concentration of the absorbent during the operation stayed within the range of 6.1–7.7 g/L. (2) SO2 in the exhaust gas from the diesel engine was decreased by 98.7–99.6%. Among the three towers, the first tower spraying seawater was confirmed to remove 98.7% of the SO2. (3) NOx decreased by 43.2–48.9%. The oxidation rate of NO was higher in the second tower spraying electrolyzed water, and NO2 was absorbed in the third tower spraying sodium hydroxide. Sodium hydroxide was found to be involved not only in NO2 absorption but also in direct absorption of NO. Na2S, used to improve the NOx removal rate, was found to have a negative effect. J. Mar. Sci. Eng. 2020, 8, 0850 11 of 12

(4) CO2 was reduced by 28.0–33.3%, and the third tower spraying sodium hydroxide was conﬁrmed to have an average removal rate of 29.3%. In the future, additional research on scrubbing technology using seawater will be conducted to increase the NOx reduction rate.

Author Contributions: Conceptualization, Y.R.; Methodology, Y.R., T.K., J.K. and J.N.; Validation, Y.R.; Formal Analysis, T.K., J.K. and J.N.; Investigation, Y.R., T.K. and J.K.; Writing—Original Draft Preparation, Y.R.; Writing—Review and Editing, T.K., J.K. and J.N.; Visualization, Y.R.; Supervision, J.N.; Project Administration, J.N. All authors have read and agreed to the published version of the manuscript. Funding: This study did not receive external funding. Conﬂicts of Interest: The authors declare that there is no conﬂict of interest.

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