Journal of Marine Science and Engineering
Article A Novel Method for Simultaneous Removal of NO and SO2 from Marine Exhaust Gas via In-Site Combination of Ozone Oxidation and Wet Scrubbing Absorption
Zhitao Han 1,2,* , Tianyu Zou 1,2, Junming Wang 1,2,3, Jingming Dong 1,2 , Yangbo Deng 1,2 and Xinxiang Pan 1,2,4,*
1 Marine Engineering College, Dalian Maritime University, Dalian 116026, China; [email protected] (T.Z.); [email protected] (J.W.); [email protected] (J.D.); [email protected] (Y.D.) 2 Liaoning Research Center for Marine Internal Combustion Engine Energy-Saving, Dalian Maritime University, Dalian 116026, China 3 Yantai Office, Qingdao Branch, China Classification Society, Yantai 264000, China 4 College of Ocean Engineering, Guangdong Ocean University, Zhanjiang 524088, China * Correspondence: [email protected] (Z.H.); [email protected] (X.P.); Tel.: +86-411-84723321 (Z.H.)
Received: 8 October 2020; Accepted: 16 November 2020; Published: 20 November 2020
Abstract: The stringent international regulations on marine emission abatement have exerted a huge push on the development of marine desulfurization and denitrification technologies. However, for the traditional vessels driven by large two-stroke diesel engines, simultaneous removal of NOx and SO2 is still a big challenge at present. Here, a one-stage ozone oxidation combined with in-situ wet scrubbing for simultaneous removal of NO and SO2 is proposed. A series of experiments were performed based on a bench-scale reaction system. The results showed that in-situ wet scrubbing could effectively decrease flue gas temperature, and then suppress the thermal decomposition of ozone, which was beneficial for improve oxidant utilization. Meanwhile, the in-situ combination of ozone injection and wet scrubbing was in favor of improving the selectivity oxidation of NO over SO2 by ozone, which was possibly due to the high aqueous solubility of SO2 in water. Aiming to reduce the electric power consumption by an ozone generating system, O3/NO molar ratio was kept as low as possible. A complete removal of SO2 and a high NOx removal efficiency could be achieved through the introduction of other oxidative additives in scrubbing solution. This integrated system designed for marine application was of great significance.
Keywords: marine exhaust gas; NO oxidation; ozone injection; wet scrubbing; in-situ combination
1. Introduction Increasingly stringent regulations on marine gaseous pollutant emissions have come into force in recent years [1]. The global shipowners are facing unprecedented pressure to take measures to abate multiple kinds of pollutants (such as SOx, NOx, CO2, and PMs) from marine exhaust gas, since these pollutants would impose very negative effects on global climate change and ecological environment [2]. International Maritime Organization (IMO) set a goal to reduce at least 50% of greenhouse gas emission by global marine industry for 2050 compared with that for 2008. A possible method is to develop novel propulsion technologies for ocean-going vessels. Currently, more interests are focused on non-fossil fuels, such as Liquified Natural Gas (LNG), NH3,H2, methanol, and biogas, as alternative fuels for marine propulsion plants. Undoubtedly, once these alternative fuels are applied extensively in marine
J. Mar. Sci. Eng. 2020, 8, 943; doi:10.3390/jmse8110943 www.mdpi.com/journal/jmse J. Mar. Sci. Eng. 2020, 8, 943 2 of 22 industry, they would greatly reduce the emissions of greenhouse gas together with other atmospheric pollutants. However, the majority of ship fleets are still powered by traditional diesel engines at present, and it requires a number of years for shipowners to replace the existing vessels by clean energy ships. Therefore, there are urgent needs for novel emission abatement technologies for ship operators to satisfy the strict regulations. Since 1 January 2020, the limit on sulfur content in marine fuel oil has been changed from 3.5 down to 0.5 wt.% for global navigation. In addition, newly built ships are required to reduce NOx emission concentration greatly when sailing in international emission control areas (ECAs). Thus, an exhaust gas treatment system that could control SOx and NOx simultaneously would be a good choice for shipowners as a short-term solution. Since sulfur oxides (SOx) mainly originate from the combustion of high-S fuel oil in diesel engines, it can be largely reduced to a very low level by taking low-S or no-S fuels, such as marine gas oil (MGO), LNG, biological methanol, et al., as alternative fuels [3,4]. Meanwhile, a wet scrubber, which is similar to a stationary sources desulfurization system, can also be used to remove SOx efficiently, especially open-loop seawater scrubbers have become a popular choice for international ship owners [5]. However, neither adopting low-S fuel or installing a wet scrubber could reduce NOx emission effectively. To date, selective catalytic reduction (SCR) and exhaust gas recirculation (EGR) techniques are two relatively mature denitration methods for marine diesel engines [6,7]. There are several possible solutions for ocean-going ships to control SOx and NOx emissions simultaneously: ultra-low-S fuel + SCR, ultra-low-S fuel + EGR, wet scrubber + SCR, wet scrubber + EGR, etc. However, there are some obvious drawbacks for such a combination system: high installation and maintenance cost, complex operation, large installation space, low operation economy [8]. The above-mentioned solutions have a lack of practicality. Thus, innovative multiple-pollutant abatement technologies are still in urgent demand for marine industry. A great deal of research work has been conducted to study how to remove SOx and NOx simultaneously during recent decades. However, most of it focused on land-based stationary sources. There were some obvious differences in technical demands between stationary sources and mobile sources. Ships are a typical kind of mobile source. Their engine space and energy supply are very limited, which require SOx and NOx removal systems to be as compact as possible. Since it is difficult to remove SOx and NOx simultaneously through a dry-method system, a great effort has been made to develop a wet-method system for marine application. The main component of NOx emitted from diesel engines is NO, whose proportion could be more than 95%. The solubility of NO in water is quite low, resulting in a low removal efficiency in traditional wet scrubber. A feasible way to improve NOx removal efficiency is to introduce some other oxidative additives into the scrubbing solution. Some common oxidants, such as NaClO [9], NaClO2 [10], H2O2 [11], Na2S2O8 [12], ClO2 [13], and O3 [14], have been investigated to oxidize NO into higher valance NOx. Since the solubilities of higher valance NOx are much higher than that of NO, a high NOx removal efficiency has been obtained in the previous research. Zhou et al. used Na2S2O8 as an oxidant to simultaneously absorb SO2 and NO from marine diesel engine exhaust gas. Their lab-scale experimental results showed that 900 ppm SO2 and 1000 ppm NO could be completely absorbed under nonoptimal conditions [12]. Zhang et al. conducted in a pilot-scale photochemical spraying tower to investigate the simultaneous removal of SO2 and NO from flue gas using H2O2/urea activated by vacuum ultraviolet (VUV) light, and the results showed that SO2 could attain complete removal, and NO removal efficiency was enhanced greatly through increasing the VUV irradiation intensity and H2O2 concentration [11]. Han et al. carried out a series of experiments to study the simultaneous removal of NO and SO2 from simulated exhaust gas by cyclic scrubbing and on-line supplementing pH-buffered NaClO2 solutions based on a bench-scale reactor. The results indicated that both SO2 and NO could be removed with a high efficiency, and the oxidant utilization rate was very high [10]. Similar research illustrated that simultaneous removal of SOx and NOx could be achieved easily by adding strong oxidants into scrubbing solutions. However, for marine application, it implies that a large amount of oxidant reagents would need to be stored onboard, taking up a large amount of space. In addition, the cost of oxidative reagents is relatively J. Mar. Sci. Eng. 2020, 8, 943 3 of 22
J. Mar. Sci. Eng. 2020, 8, x FOR PEER REVIEW 3 of 22 high, and some kinds of them might be not very accessible from ports. Thus, it is necessary to reduce theof usage oxidative amounts reagents of oxidative is relatively additives high, inand order some to kinds improve of them the practicalitymight be not of very similar accessible processes. from ports.It is knownThus, it thatis necessary the oxidative to reduce power the usage of ozone amounts (O3) of is oxidative very strong, additi andves itin couldorder to oxidize improve NO intothe higher practicality valance of NO similarx easily. processes. In recent years, ozone advanced oxidation process has been applied successfullyIt is known to remove thatNO thex oxidativein stationary power source of ozone areas. (O Especially3) is very strong, for the and cases it could with oxidize low-temperature NO into higher valance NOx easily. In recent years, ozone advanced oxidation process has been applied (below 200 ◦C) flue gas, ozone oxidation denitration would be a good choice. A traditional ozone oxidationsuccessfully denitration to remove process NOx isin tostationary combine source ozone areas. injection Especially with wetfor the scrubbing cases with absorption low-temperature in series, (below 200 °C) flue gas, ozone oxidation denitration would be a good choice. A traditional ozone as shown in Figure1a. It can be divided into two stages: O 3 is injected in gas duct to mix with flue gas at oxidation denitration process is to combine ozone injection with wet scrubbing absorption in series, the first stage, where NO is oxidized efficiently to higher valance NOx; NOx is removed through a wet as shown in Figure 1a. It can be divided into two stages: O3 is injected in gas duct to mix with flue scrubbing absorption process in a spraying tower at the second stage [15,16]. During recent decades, gas at the first stage, where NO is oxidized efficiently to higher valance NOx; NOx is removed through numerous researchers’ interests have been attracted for exploring the ozone oxidation denitration a wet scrubbing absorption process in a spraying tower at the second stage [15,16]. During recent mechanism and to improve NO removal performance [17,18]. To date, NO -O reaction processes are decades, numerous researchers’x interests have been attracted for exploringx 3 the ozone oxidation well known at the molecular level and comprise a small number of reactions [19,20]: denitration mechanism and to improve NOx removal performance [17,18]. To date, NOx-O3 reaction processes are well known at the molecular level and comprise a small number of reactions [19,20]: NO + O NO + O (1) 3 → 2 2 NO + O3 → NO2 + O2 (1) NO2 + O3 NO3 + O2 (2) NO2 + O→3 → NO3 + O2 (2) NO2 + NO3 N2O5 (3) NO2 + NO3↔ ↔ N2O5 (3) NO3(g) N2O5(l) (4) NO3(g)→ → N2O5(l) (4) N O + H O 2HNO (5) 2N5(l)2O5(l) + H2 2O→ → 2HNO3 (5)
FigureFigure 1. The 1. The process process diagrams diagrams of of ozone ozone oxidation oxidation denitrification denitrification processes:processes: ( (aa)) traditional traditional two-stage two-stage methodmethod and and (b) ( one-stageb) one-stage method. method.
AlthoughAlthough the the solubility solubility of NOof NO2 in2 waterin water is much is much higher higher than than that that of NO,of NO, it is it still is still much much lower lower than thatthan of N that2O5 .of When N2O5. the When molar the ratiomolar of ratio injected of injected O3 to NOO3 to in NO flue in gas flue is gas about is about 1, ozone 1, ozone could could oxidize oxidize most of NOmost molecules of NO molecules to NO2. to When NO2. further When further increasing increasing O3/NO O molar3/NO molar ratio toratio be moreto be more than 2,than ozone 2, ozone could furthercould oxidize further NO oxidizex to N 2NOO5.x Forto N practical2O5. For applicationspractical applications in stationary in stationary sources, a sources, high O3 /aNO high molar O3/NO ratio of moremolar than ratio 2 isof alwaysmore than chosen 2 is always to achieve chosen a high to achieve NOx removal a high eNOfficiencyx removal [21]. efficiency However, [21]. the However, generation processthe generation of ozone wouldprocessconsume of ozone awould great consume deal of electricala great deal energy, of electrical leading energy, to a sky-high leading operationto a sky- costhigh in stationary operation cost source in statio applicationnary source [22 ].application Meanwhile, [22]. moreMeanwhile, co-existing more SOco-existing2 in flue SO gas2 in would flue gas be oxidizedwould by be excess oxidized ozone, by excess which ozon woulde, which further would increase further the increase ozone usage the ozone [23]. usage These [23]. issues These still issues restrict thestill extensive restrict application the extensive of application ozone oxidation of ozone denitrification oxidation denitrification technology intechnology stationary in sources stationary to a sources to a large extent. In recent years, many efforts have been made to control O3/NO molar ratio large extent. In recent years, many efforts have been made to control O3/NO molar ratio as low as as low as possible [24,25]. Kang et al. studied simultaneous removal of NOx and SO2 by ozone possible [24,25]. Kang et al. studied simultaneous removal of NOx and SO2 by ozone oxidation and oxidation and wet scrubbing with sodium hydroxide in series mode, in which efficient NOx scrubbing wet scrubbing with sodium hydroxide in series mode, in which efficient NOx scrubbing could be could be observed when 60% of NO was oxidized to NO2 in the absence of SO2. Some researchers observed when 60% of NO was oxidized to NO in the absence of SO . Some researchers reported that reported that the simultaneous injection of2 ozone and ethanol 2mixtures could enhance ozone
J. Mar. Sci. Eng. 2020, 8, 943 4 of 22 the simultaneous injection of ozone and ethanol mixtures could enhance ozone oxidation denitration performance, and further reduce O3/NO molar ratio below 1 [24,26]. It was attributed that some organic radicals, generated through the ethanol oxidation by ozone, could oxidize NO to NO2, and finally to nitrate organics or aqueous nitrate acids. Liu et al. proposed to use a combination of catalytic ozonation and persulfate to remove SO2 and NOx, in which a lower ozone was needed for catalytic ozonation of NO over FeOx/SAPO-34 catalyst [27]. In order to enhance the absorption of NO2 together with SO2 after low temperature ozone oxidation for flue gas treatment, Shao et al. proposed a new kind of additive and tested the effects of key parameters in a lab-scale platform [28]. As above-mentioned, it could be seen that a lot of work had been done to lower O3/NO molar ratio to reduce ozone consumption. Most of the research aimed to solve the issues in stationary source applications, and the ozone oxidation denitration was based on a traditional two-stage mode. To date, research on ozone oxidation denitration for marine application has hardly been reported. Zhou et al. studied marine emission pollution abatement using ozone oxidation followed by a wet scrubbing absorption based on a lab-scale reactor, and the result indicated that urea was an excellent additive to reduce the consumption of ozone [14]. However, their experimental system was still a two-stage mode. For large cargo ships, low-speed two-stroke diesel engines are commonly adopted as the main powerplant, which emit a large amount of exhaust gas. NO concentration in marine exhaust gas is in the range of 200–1500 ppm, and SO2 concentration mainly depends on the sulfur content in fuel oil [2,29]. Note that several issues needed to be considered when introducing ozone oxidation denitration technology for marine diesel engine applications. On one hand, the exhaust gas temperature emitted from the funnel is usually up to 300 ◦C. It is known that the higher the diesel engine load is, the higher the exhaust gas temperature is and the higher NOx emission concentration is. However, a large fraction of ozone would be thermally decomposed when ozone is directly mixed with high temperature flue gas [30]. On the other hand, the electric power supply and engine space onboard are very limited; the traditional two-stage ozone oxidation denitrification process would require much more space and electric energy, which are far beyond ship’s practical capability [23]. To the best of our knowledge, hardly any published papers reported one-stage ozone oxidation denitrification process for marine application. Previously, Yamamoto et al. proposed a plasma combined semi-dry chemical process for a pilot-scale simultaneous removal of NOx and SOx for a glass manufacturing system [31]. Through their experiment, it was confirmed that simultaneous removal of NOx and SOx using semi-dry-type nonthermal plasma and chemical hybrid process was highly effective and promising for exhaust gas treatment in glass manufacturing. However, their system and process were quite complex, not suitable for marine application. Herein, we proposed a one-stage ozone oxidation denitrification process for marine multiple-pollutant abatement, as shown in Figure1b, in which ozone was injected directly in the wet scrubber, aiming to combine ozone oxidation and wet absorption in-situ. This one-stage ozone-based marine exhaust gas treatment system possessed some obvious advantages. For example, in-situ combination of ozone injection with wet scrubbing could efficiently cool flue gas temperature down to below 150 ◦C, which was beneficial to suppress the thermal decomposition of ozone, further improving the utilization of O3 oxidant. As the layout of the ozone generating system was flexible, it could be well integrated with the traditional wet scrubber, thus forming a very compact system. More importantly, extra oxidative additives could be easily introduced into scrubbing solution to combine with ozone to oxidize NO, which was in favor of reducing the ozone dosage, thus reducing the electric power consumption by ozone generating system. Based on the concept that we proposed, a series of tests were conducted based on a bench-scale experimental setup to explore the feasibility of such a one-stage process which combined ozone oxidation and wet absorption in-situ for treating marine exhaust gas. The marine diesel engine exhaust gas was simulated by mixing air with targeted pollutant gases (NO and SO2). A flue gas flow rate of 50 m3 h 1 at room temperature and a low O /NO molar ratio of 0.5 were chosen as basic test conditions. · − 3 Since the flue gas flow rate was relatively high, it would consume a great amount of material gas if NO concentration approached a high level which corresponded to a high load of diesel engine. In order to J. Mar. Sci. Eng. 2020, 8, 943 5 of 22 reduce the consumption of NO material gas, NO concentration of 200 ppm was chosen as a basic condition. In this work, the change of thermal decomposition of ozone with flue gas temperature was examined based on the bench-scale experimental system. Furthermore, NO oxidation performance by ozone was studied under different flue gas temperature. Then, simultaneous removal of SO2 and NOx by in-situ combination of ozone oxidation and wet scrubbing was investigated. Some common types of oxidative additives were introduced in order to enhance NOx removal efficiency. The results showed that in-situ wet scrubbing could efficiently suppress thermal decomposition of ozone and improve NO oxidation selectivity over SO . When O /NO molar ratio was 0.5 and 0.4 mol L 1 NaClO was added in 2 3 · − the in-situ scrubbing solution; NOx and SO2 removal efficiencies were 63.4% and 100%, respectively. This indicated that in-situ combination of ozone oxidation and wet scrubbing was of great potential for ocean-going vessels to simultaneously remove multiple pollutants from exhaust gas.
2. Materials and Methods Normally the exhaust gas emitted from a marine diesel engine contained multiple kinds of pollutants. Under various loads, NO concentrations in exhaust gas was about in the range of 200–1500 ppm. When residual fuel (heavy fuel) with high sulfur content was used onboard, SO2 concentration was in the range of approximately 300–650 ppm. In our experiments, air was used as a carrier gas and supplied by a high-pressure blower. The targeted pollutants, NO and SO2, were supplied from gas cylinders and injected into the flue duct to mix with air. Note that the flow rates of simulated flue gases and initial targeted-pollutant concentrations were adjusted to the set values at room temperature at the beginning of the tests. The air flow was kept at 50 m3 h 1. Considering that · − the consumption amount of NO material gas would be very high when NO concentrations in simulated flue gas approached the highest level of diesel engines, an inlet NO concentration of 200 ppm was chosen as a basic condition in the experiments aiming to reduce test cost. As a validation test, a set of experiments with inlet NO concentration of 800 ppm and SO2 concentration of 400 ppm were carried out in the end. The schematic diagram of our experimental setup was shown in Figure2a. The reaction system consisted of gas supplies, reactor, and gas analyzers. The air flow was monitored by a vortex flow meter and could be controlled by adjusting the blower’s working frequency. The injection amount of NO and SO2 pure gases from separate cylinders were controlled through separated mass flow meters. After NO and SO2 gases were injected into the flue duct, the gas mixtures entered a duct heater to be heated to a set temperature (50–300 ◦C). O3 was produced by an ozone generator (Qingdao Guolin Co. Ltd., Qingdao, China) with O as a gas source. O with a flow rate of 0.5 m3 h 1 was supplied from 2 2 · − an O2 cylinder to the ozone generator, and O3 yield was controlled through adjusting input power. The reactor was a typical spraying tower with a stainless nozzle at the top, and its structural diagram was shown in Figure2b. The reactor was made of stainless steel, and its inner diameter was 400 mm. The simulated exhaust gas entered the reactor through the inlet at the bottom. There were 7 sampling points for measuring the flue gas temperature and flue gas components. The perpendicular distance between the two neighboring sampling points was 200 mm. An O3/O2 gas mixture was injected into the reactor through a separate pipe. A gas distributor was located above the inlet ports to mix the flue gas uniformly. A spraying nozzle was installed at the top of the reactor. The total volume of scrubbing solution was 14 L in each test. Scrubbing solution was pumped to the nozzle and the flow rate was about 1 L min 1. A cyclic scrubbing mode was adopted, and the solution was collected in · − a tank. During the scrubbing process, the solution temperature could be kept at 50 ◦C through a thermostatic water bath. The pH value of the scrubbing solution in circulating tank was monitored by a pH meter and it could be maintained to the set value by adding 1 mol L 1 HCl or 1 mol L 1 NaOH · − · − solutions. O3 concentration in flue gas was measured by an ozone monitor (2B Technology Co., Boulder, CO, USA) with a sampling interval of 6 s. O2, NO, and NO2 concentrations in flue gas were monitored by a gas analyzer (MRU Co., Baden-Württemberg, Germany). Here, NOx concentrations referred to the sum J. Mar. Sci. Eng. 2020, 8, 943 6 of 22
of NO and NO2 concentrations. Each test proceeded for 30 min for 3 times, and the average values of gas concentrationsJ. Mar. Sci. Eng. were 2020 used, 8, x FOR tocalculate PEER REVIEW NO x and SO2 removal efficiencies through the following 6 of equation: 23
η = [(C Cout)/C ] 100% (6) η = [(Cin −in − Cout)/Cinin] × ×100% (6)
in which η was NOx or SO2 removal efficiency, Cin was the initial concentration of targeted pollutant in which η was NOx or SO2 removal efficiency, Cin was the initial concentration of targeted pollutant (NOx or SO2) before injecting O3/O2 gases and/or scrubbing any solution, Cout was the concentration (NOx or SO2) before injecting O3/O2 gases and/or scrubbing any solution, Cout was the concentration of of targeted pollutant (NOx or SO2) after injecting O3/O2 gases and/or scrubbing any solution. Both Cin targeted pollutant (NO or SO ) after injecting O /O gases and/or scrubbing any solution. Both C and Cout were measuredx at sampling2 point 7 as shown3 2 in Figure 2b. in and Cout were measured at sampling point 7 as shown in Figure2b.
FigureFigure 2. (a 2.) ( Schematica) Schematic diagram diagram of of our our experimental experimental setup: setup:1: variable 1: frequency variable air frequency blower; 2: vortex air blower; 2: vortexflow flow meter; meter; 3: air3: heater; air heater; 4–6: gas 4–6: cylinders; gas cylinders; 7: ozone generator; 7: ozone 8: generator; reactor; 9: circulating 8: reactor; pump; 9: circulating 10: pump; 10: solution tank; 11: ozone monitor; 12: electrical condenser; 13: gas analyzer; 14: mass flow meters. (b) Schematic diagram of the reactor. J. Mar. Sci. Eng. 2020, 8, 943 7 of 22
3. Results
3.1. The Oxidation Performance of NO by Ozone Injection
3.1.1. Thermal Decomposition of Ozone
After ozone is injected in hot flue gas, a part of ozone will be decomposed to O2 during the mixing process of O3/O2 gas mixtures with hot flue gas. That is because ozone is thermodynamically unstable. Therefore, it is necessary to investigate the effect of flue gas temperature on ozone self-decomposition properties in the reaction tower before in-situ combining ozone injection with wet scrubbing. Based on the home-made reaction system, the self-decomposition properties of ozone were investigated, and the results were shown in Figure3. The flow rates of simulated flue gases and initial O3 concentrations were adjusted to the set values at room temperature at the beginning of the tests. The air flow was kept at 50 m3 h 1 measured at room temperature. Initial ozone concentrations in · − carrier gas were adjusted to 50, 100, 150, and 200 ppm, respectively. In each test, flue gas temperatures measured at sampling point 6 increased step by step from room temperature (25 ◦C) to 300 ◦C. Note that the volumes of flue gas increased obviously with the increase in flue gas temperature, then the flow rates and residence time in reaction tower changed correspondingly, as shown in Table1.
Table 1. Gas flow rates and residence time of simulated flue gas in reaction tower under various inlet temperatures measured at sampling point 6.
3 1 1 Gas Temperatures (◦C) Gas Flow Rates (m h ) Gas Velocities (m s ) Residence Time (s) · − · − 25 50 0.111 9.01 50 55.12 0.122 8.20 100 63.66 0.141 7.09 150 72.18 0.160 6.25 200 80.72 0.178 5.62 250 89.25 0.197 5.07 300 97.78 0.216 4.63
The sampling points 1–5 correspond to various residence times of flue gas in the reaction zone. It could be seen from Figure3 that ozone concentration decreased obviously with the increase in flue gas temperature. When flue gas temperature was 50 ◦C, only a very small part of ozone decomposed. There was a decreasing trend of ozone concentration with the increase in residence time. In addition, the changing trend seemed the same for different initial ozone concentrations. This result agreed well with the previous studies, which indicated that ozone would be decomposed obviously when flue gas temperature was higher than 150 ◦C[19,31]. As shown in Figure3b, at sampling point 5, ozone concentrations were 161, 112, and 38 ppm when inlet flue gas temperatures were 200, 250, and 300 ◦C, respectively. This indicated that a large part of ozone would be decomposed when flue gas temperature was higher than 200 ◦C. Normally, the temperature of exhaust gas emitted from marine diesel engines was in the range of 180–300 ◦C, which could avoid the corrosion problem incurred by co-existing SOx in flue gas. Therefore, a nonnegligible part of ozone will be lost due to thermal decomposition if ozone is directly mixed with high-temperature marine exhaust gas. This will result in a very low oxidant utilization. It could also be seen from Figure3b that when inlet flue gas temperature was 300 ◦C, outlet ozone concentrations measured at different sampling points were more or less the same. This result was very interesting. Note that different sampling points corresponded to various residence times of flue gas in the reaction tower. Normally speaking, ozone concentration measured at different sampling points decreased obviously with the increase in residence time. However, our result did not exhibit the same trend. It was strange that ozone concentration sharply decreased from 200 ppm down to 43 ppm measured at sampling point 1, but it just further decreased to 38 ppm measured at sampling point 5. This result indicated that the majority of ozone molecules might be decomposed at the moment of J. Mar. Sci. Eng. 2020, 8, x FOR PEER REVIEW 8 of 22 J. Mar. Sci. Eng. 2020, 8, 943 8 of 22 sampling point 5. This result indicated that the majority of ozone molecules might be decomposed at the moment of injected ozone contacting with hot flue gas. The initial temperature of O3/O2 gas injected ozone contacting with hot flue gas. The initial temperature of O /O gas mixtures supplied mixtures supplied from the ozone generator was at room temperature.3 2 When inlet flue gas from the ozone generator was at room temperature. When inlet flue gas temperature was 300 C, temperature was 300 °C, there was a huge temperature difference between the inlet flue gas and inlet◦ there was a huge temperature difference between the inlet flue gas and inlet O3/O2 gas mixtures. Here, O3/O2 gas mixtures. Here, it was supposed that the rapid heating process was the main reason for it was supposed that the rapid heating process was the main reason for ozone thermal decomposition. ozone thermal decomposition. As shown in Figure 2b, there was a gas distributor located just under As shown in Figure2b, there was a gas distributor located just under the sampling point 1. It was the sampling point 1. It was believed that injected ozone mixed uniformly with flue gas with the help believed that injected ozone mixed uniformly with flue gas with the help of the gas distributor, and their of the gas distributor, and their temperature reached a stable state. Then ozone in flue gas only temperature reached a stable state. Then ozone in flue gas only decomposed slightly although the flue decomposed slightly although the flue gas temperature was still relatively high. This result also gas temperature was still relatively high. This result also indicated that the thermal decomposition indicated that the thermal decomposition reaction occurred within a very short time (<1 s). reaction occurred within a very short time (<1 s).
Figure 3. Figure 3.The The changechange ofof ozoneozone concentrationsconcentrations withwith flueflue gasgastemperatures temperatures at at di differentfferent inlet inlet initial initial ozone ozone concentration levels: (a) 100 and (b) 200 ppm. concentration levels: (a) 100 and (b) 200 ppm. 3.1.2. NO Conversion by Ozone Oxidation 3.1.2. NO Conversion by Ozone Oxidation The oxidative ability of ozone is very high, so O3 can oxidize NO to higher valence NOx easily. BasedThe on theoxidative home-made ability of reaction ozone system,is very high, NO conversionso O3 can oxidize performance NO to higher by ozone valence oxidation NOx easily. was Based on the home-made reaction system, NO conversion performance by ozone oxidation was investigated, and the results were shown in Figure4. Both initial NO and SO 2 concentrations in investigated, and the results were shown in Figure 4. Both initial NO and SO2 concentrations in simulated flue gas were adjusted to 200 ppm at room temperature. The injected O3 concentrations simulated flue gas were adjusted to 200 ppm at room temperature. The injected O3 concentrations were adjusted to 100 or 200 ppm at room temperature. The concentrations of NO, NO2, and O3 were monitored at sampling point 5.
J. Mar. Sci. Eng. 2020, 8, x FOR PEER REVIEW 9 of 23 were adjusted to 100 or 200 ppm at room temperature. The concentrations of NO, NO2, and O3 were monitored at sampling point 5. J. Mar.It could Sci. Eng. be2020 seen, 8, 943 from Figure 4a that when there was no co-existing SO2 in flue gas and9 of 22 flue gas temperature was relatively low, the injected ozone of 200 ppm could oxidize NO with a very high efficacy. NO conversion efficiency reached 95.5% when inlet flue gas temperature was 50 °C. It could be seen from Figure4a that when there was no co-existing SO 2 in flue gas and flue gas However,temperature NO was conversion relatively efficiency low, the injected was only ozone 31.2% of200 when ppm inlet could flue oxidize gas temperature NO with a very increased high up to 2 3 300effi °C.cacy. It was NO conversiongenerally ebelievedfficiency reachedthat oxidation 95.5% when of NO inlet to flue NO gas occurred temperature when was O 50/NO◦C. molar However, ratio was belowNO conversion 1.0, and further efficiency oxidation was only 31.2%of NO when2 to inletN2O5 flue occurred gas temperature when O increased3/NO molar up to ratio 300 ◦ C.was It wasabove 1.0. Thegenerally previous believed study thaton the oxidation influence of NO of reacti to NOon2 occurred temperature when indicated O3/NO molar that ratiothe rate was of below NO2 1.0,formation wasand very further fast oxidation and was of basically NO2 to N not2O5 affectedoccurred by when the Oreaction3/NO molar temperature ratio was above[19]. In 1.0. our The experiments, previous the residencestudy on time the influence of flue gas of reaction in the reaction temperature zone indicated was calculated that the rate to be of NO4.632 formations when flue was gas very temperature fast wasand heated was basically to 300 °C. not The affected low byNO the conversion reaction temperature efficiency implied [19]. In ourthat experiments, there was obvious the residence competition betweentime of flueozone gas thermal in the reaction decomposition zone was calculated and NO tooxidat be 4.63ion s whenby ozone. flue gas It was temperature very possible was heated that ozone to 300 C. The low NO conversion efficiency implied that there was obvious competition between thermal ◦decomposition reaction occurred faster than NO oxidation reactions. As mentioned above, ozone thermal decomposition and NO oxidation by ozone. It was very possible that ozone thermal when inlet flue gas temperature was 300 °C, the majority of ozone was decomposed thermally rather decomposition reaction occurred faster than NO oxidation reactions. As mentioned above, when inlet than reacting with NO at the moment of O3 injecting into the reaction tower. This explained well why flue gas temperature was 300 ◦C, the majority of ozone was decomposed thermally rather than reacting it withwas NOdifficult at the to moment obtain of a Ohigh3 injecting NO oxidation into the reaction efficiency tower. when This injecting explained ozone well why into it washot difluefficult gas. to obtainAs shown a high in NO Figure oxidation 3b, when efficiency inlet when flue gas injecting temperature ozone into was hot 300 flue °C gas. and there was no initial NO in flueAs gas, shown onlyin 38 Figure ppm3 ozoneb, when was inlet left flue in gas outlet temperature flue gas. was This 300 meant◦C and that there 162 was ppm no O initial3 had NO been lost duein flueto thermal gas, only decomposition. 38 ppm ozone was However, left in outlet when flue NO gas. wa Thiss introduced meant that into 162 the ppm flue O3 gas,had beenNO conversion lost efficiencydue to thermal of 31.2% decomposition. was obtained, However, as shown when in NO Figure was introduced4b. It seemed into that the flue the gas, introduction NO conversion of NO into flueeffi ciencygas could of 31.2% suppress was obtained, the thermal as shown decompositio in Figure4b. It seemedn of ozone that the to introduction some extent. of NO Since into fluethere was competitiongas could suppress between the ozone thermal thermal decomposition decomposition of ozone and to some NO oxidation extent. Since by there ozone, was both competition ozone thermal between ozone thermal decomposition and NO oxidation by ozone, both ozone thermal decomposition decomposition and NO oxidation proceeded simultaneously at the injection area. Furthermore, there and NO oxidation proceeded simultaneously at the injection area. Furthermore, there might be another might be another reason that a gas mixture of O3/O2 was injected into the reaction tower. As the ozone reason that a gas mixture of O3/O2 was injected into the reaction tower. As the ozone generator used in generator used in the experiments took O2 as gas source, the major part of O3/O2 gas mixture was O2. the experiments took O2 as gas source, the major part of O3/O2 gas mixture was O2. Thus, a small part 2 2 Thus,of NO a small in flue part gas couldof NO be in oxidized flue gas to could NO2 by be O oxidized2 in the reaction to NO tower. by O in the reaction tower.
Figure 4. Cont.
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Figure 4. The effects of ozone injection on (a) NO conversion efficiencies and (b) outlet
FigureSO 4.2 concentrations.The effects of ozone injection on (a) NO conversion efficiencies and (b) outlet SO2 concentrations. As shown in Figure4, co-existing SO 2 imposed a negative effect on NO conversion efficiency. The previous studies reported that ozone could selectively oxidize NO when there was SO2 co-existing As shown in Figure 4, co-existing SO2 imposed a negative effect on NO conversion efficiency. in the flue gas. Only a very small part (<0.2%) of SO2 could be oxidized to SO3 when flue gas 2 The previoustemperature studies was 150 reported◦C[17,19 ],that but inozone our bench-scale could selectively experiments, oxidize when inletNO Owhen3 concentrations there was SO co- existingwere in 100the andflue 200 gas. ppm, Only SO2 aconversion very small effi partciencies (< 0.2%) were 8.5% of SO and2 could 16.5%, be respectively. oxidizedIt to consumed SO3 when flue gas temperaturemore ozone was with 150 the °C increase [17,19], in but inlet in ozone our concentration. bench-scale Theexperiments, result showed when that co-existinginlet O3 concentrations SO2 consumed a nonnegligible ozone in gas phase reactions. There was an obvious competition between were 100 and 200 ppm, SO2 conversion efficiencies were 8.5% and 16.5%, respectively. It consumed NO and SO2 to be oxidized by ozone. Therefore, it was necessary to take measures to further improve more ozone with the increase in inlet ozone concentration. The result showed that co-existing SO2 the selective oxidation of NO over SO2. consumed a nonnegligible ozone in gas phase reactions. There was an obvious competition between 3.2. Simultaneous Removal of NO and SO by In-Situ Combination of Ozone Injection with Wet Scrubbing NO and SO2 to be oxidized by ozone. Therefore,2 it was necessary to take measures to further improve the selective3.2.1. Eff oxidationect of Wet Scrubbing of NO over on Solution SO2. Temperature Firstly, the effects of wet scrubbing on cyclic scrubbing solution temperatures were investigated 3.2. Simultaneousbased on our Removal self-built reactionof NO and system, SO2 and by theIn-Situ results Combination were shown inof FigureOzon5e. Injection Here, 30 L with deionized Wet Scrubbing water was used as cyclic scrubbing solution. Initial solution temperatures were at room temperature, 3.2.1. Effectand the of thermostat Wet Scrubbing water bath on wasSolution switched Temperature off. The flow rate through the spraying nozzle was about 1 L min 1. It can be seen from Figure5 that cyclic scrubbing solution temperature increased Firstly, the· effects− of wet scrubbing on cyclic scrubbing solution temperatures were investigated gradually, and then reached a stable state. When inlet flue gas temperatures were 200, 250, and 300 ◦C, based cyclicon our scrubbing self-built solution reaction temperatures system, after and scrubbing the resu forlts 60were min wereshown 43, in 46, Figure and 51 ◦ 5.C, Here, respectively. 30 L deionized water Atwas the used same as time, cyclic outlet scrubbing flue gas temperatures solution. Initial measured solution at sampling temperatures point 7 were were 51, 56, at androom 59 ◦temperature,C, and therespectively. thermostat It impliedwater bath that flue was gas switched temperature off. hadThe been flow controlled rate through to a very the low spraying level during nozzle the was about 1 L·minwet−1. scrubbingIt can be process. seen from This Figure result illustrated 5 that cyclic that the sc wetrubbing scrubbing solution process temperature could help to suppress increased the gradually, thermal decomposition of ozone efficiently. and then reached a stable state. When inlet flue gas temperatures were 200, 250, and 300 °C, cyclic scrubbing solution temperatures after scrubbing for 60 min were 43, 46, and 51 °C, respectively. At the same time, outlet flue gas temperatures measured at sampling point 7 were 51, 56, and 59 °C, respectively. It implied that flue gas temperature had been controlled to a very low level during the wet scrubbing process. This result illustrated that the wet scrubbing process could help to suppress the thermal decomposition of ozone efficiently.
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Figure 5. The effects of wet scrubbing on flue gas temperatures.
Figure3.2.2. E5.ff Theect of effects Wet Scrubbing of wet scrubbing on Outlet on Ozone flue Concentrationgas temperatures.
3.2.2. EffectIt wasof Wet normal Scrubbing that the contact on Outlet of spraying Ozone solution Concentration and flue gas in reaction tower could decrease flue gas temperature greatly. Furthermore, the effect of wet scrubbing on outlet ozone concentrations Itwas was examined, normal andthat the the results contact were of shown spraying in Figure soluti6. Anon amount and flue of 14gas L deionizedin reaction water tower was usedcould decrease flue gasas temperature scrubbing solution, greatly. and theFurthermore, cyclic solution the temperature effect of wet was scrubbing kept at 50 ◦ Con by outlet a water ozone bath at concentrations the beginning. Initial ozone concentrations were adjusted to 50–200 ppm at room temperature. Inlet flue was examined, and the results were shown in Figure 6. An amount of 14 L deionized water was used gas temperatures were heated to 200, 250, and 300 ◦C, respectively. Since there were no targeted as scrubbingpollutants solution, in inlet flue and gas, the there cyclic was solution no need to temperature maintain solution was pH kept to a at set 50 value °C byby adding a water any bath at the beginning.additives Initial during ozone the cyclic concentrations scrubbing process. were adjusted to 50–200 ppm at room temperature. Inlet flue gas temperaturesAs shown were in Figure heated6a, when to inlet200, flue250, gas and temperature 300 °C, was respectively. 250 ◦C, outlet Since ozone there concentrations were no targeted pollutantsapproached in inlet the flue initial gas, levels there with thewas combination no need ofto in-situ maintain scrubbing. solution When pH initial to ozone a set concentration value by adding any was 200 ppm, outlet ozone concentration was 195 ppm with only a 2.5% decrement. This result additiveswell during demonstrated the cyclic that in-situ scrubbing scrubbing process. was vital to suppress the thermal decomposition of ozone, Aswhich shown made in it Figure possible 6a, for when more ozone inlet toflue be leftgas to temp oxidizeerature NO in was the reaction 250 °C, tower. outlet It alsoozone could concentrations be approachedseen from the Figure initial6b that levels when fluewith gas the temperature combination was 300 ◦ofC, outletin-situ ozone scrubbing. concentration When increased initial ozone concentrationsharply from was 40 200 to 180ppm, ppm outlet after theozone introduction concentr ofation in-situ was wet 195 scrubbing ppm with deionizedonly a 2.5% water. decrement. This resultDuring well the demonstrated cyclic scrubbing that process, in-situ outlet scrubbing ozone concentration was vital changedto suppress slightly. the Thethermal result decomposition also showed that there was no obvious effect of scrubbing solution on outlet ozone concentration, indicating of ozone, which made it possible for more ozone to be left to oxidize NO in the reaction tower. It also that ozone would not be reduced by the scrubbing solution through physical absorption or chemical could reactions.be seen from It confirmed Figure 6b that that in-situ when combination flue gas oftemperature ozone injection was and 300 wet °C, scrubbing outlet ozone effectively concentration increasedsuppressed sharply the from thermal 40 to decomposition 180 ppm after of ozone. the intr Thus,oduction it was of of in-situ great potential wet scrubbing to improve with the deionized water.utilization During the of ozone cyclic oxidants scrubbing greatly process, in the denitrification outlet ozone process. concentration changed slightly. The result also showed that there was no obvious effect of scrubbing solution on outlet ozone concentration, indicating that ozone would not be reduced by the scrubbing solution through physical absorption or chemical reactions. It confirmed that in-situ combination of ozone injection and wet scrubbing effectively suppressed the thermal decomposition of ozone. Thus, it was of great potential to improve the utilization of ozone oxidants greatly in the denitrification process.
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Figure 6. The effects of wet scrubbing on outlet O3 concentrations: (a) various initial ozone Figure 6. The effects of wet scrubbing on outlet O3 concentrations: (a) various initial ozone concentrations with inlet flue gas temperature of 250 C and (b) various inlet flue gas temperatures concentrations with inlet flue gas temperature of 250◦ °C and (b) various inlet flue gas temperatures with initial ozone concentration of 200 ppm. with initial ozone concentration of 200 ppm.
3.2.3. NO Oxidation and SO2 Removal by In-Situ Ozone Oxidation and Wet Scrubbing 3.2.3. NO Oxidation and SO2 Removal by In-Situ Ozone Oxidation and Wet Scrubbing Experiments were performed to oxidize NO by in-situ combination of ozone oxidation and wet scrubbing,Experiments and the were results performed were shown to oxidize in Figure NO7. by NO in-situ concentration combination in inlet of ozone flue gas oxidation was adjusted and wet scrubbing, and the results were shown in Figure 7. NO concentration in inlet flue gas was adjusted to 200 ppm at room temperature at the beginning. Since there was about 21% O2 in carrier air gas, to 200 ppm at room temperature at the beginning. Since there was about 21% O2 in carrier air gas, there was 20–30 ppm NO2 in the initial flue gas due to the oxidation of NO by O2 during the NO there was 20–30 ppm NO2 in the initial flue gas due to the oxidation of NO by O2 during the NO injection process. An amount of 14 L deionized water was used as scrubbing solution. When SO2 was injection process. An amount of 14 L deionized water was used as scrubbing solution. When SO2 was introduced in inlet flue gas, the absorption of SO2 lowered the pH of scrubbing solution obviously duringintroduced the cyclic in inlet scrubbing flue gas, process. the absorption This required of SO2 supplementing lowered the pH alkaline of scru solutionbbing solution continuously obviously to maintainduring the cyclic cyclic scrubbing scrubbing solution process. pH This to a setrequired value. supplementing Considering that alkaline there weresolution no othercontinuously oxidative to additivesmaintain introducedcyclic scrubbing in the solution scrubbing pH solution, to a set value. the introduction Considering of that alkaline there species were no would other impose oxidative a additives introduced in the scrubbing solution, the introduction of alkaline species would impose a great effect on SO2 absorption. Therefore, pH value of cyclic scrubbing solution was not maintained in great effect on SO2 absorption. Therefore, pH value of cyclic scrubbing solution was not maintained these tests. The cyclic solution temperature was kept at 50 ◦C by a water bath at the start. Initial O3 and in these tests. The cyclic solution temperature was kept at 50 °C by a water bath at the start. Initial O3 and SO2 concentrations in flue gas were adjusted to 200 and 220 ppm, respectively, at room
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SO2 concentrations in flue gas were adjusted to 200 and 220 ppm, respectively, at room temperature when used. The temperatures of flue gas before entering the reaction tower were maintained at 250 ◦C throughout the tests. As shown in Figure7a, outlet NO concentration decreased quickly from 200 to 81 ppm, and outlet NO2 concentration increased from 29 to 152 ppm after injection of 200 ppm ozone into the reaction tower at 5th min. Then NO concentration further decreased to 40 ppm, and NO2 concentration increasedJ. Mar. Sci. to Eng. 182 2020 ppm, 8, x afterFOR PEER the REVIEW introduction of in-situ wet scrubbing of deionized water. As expected, 14 of 23 in-situJ. Mar. combination Sci. Eng. 2020, of8, x ozone FOR PEER injection REVIEW and wet scrubbing improved NO oxidation by ozone effi 14ciently. of 23
FigureFigure 7. 7.Changes Changes of of outlet outlet gas gas concentrationsconcentrations withwith reaction duration duration at at the the absence absence (a () aand) and presence presence Figure 7. Changes of outlet gas concentrations with reaction duration at the absence (a) and presence (b)(b of) of SO SO2 in2 in inlet inlet flue flue gas. gas. (b) of SO2 in inlet flue gas.
AsFigure shown 7b in Figureillustrated6b, when the inleteffects flue of gas co-existing temperature SO was2 on250 NO◦ Coxidation and initial and ozone NO concentrationx removal Figure 7b illustrated the effects of co-existing SO2 on NO oxidation and NOx removal wasperformance 200 ppm, outlet during ozone the reaction concentration process. was At the about beginning, 195 ppm. initial However, NO and one SO2 could concentrations see from Figurein inlet7 a performance during the reaction process. At the beginning, initial NO and SO2 concentrations in inlet thatflue NO gas conversion were adjusted efficiency to about was 200 only and 80% 220 with ppm, in-situ respectively. combination After the of ozoneintroduction injection of andO3/O wet2 flue gas were adjusted to about 200 and 220 ppm, respectively. After the introduction of O3/O2 mixture gases at the 5th min, outlet SO2 concentration obviously decreased to 179 ppm, while outlet mixture gases at the 5th min, outlet SO2 concentration obviously decreased to 179 ppm, while outlet NO concentration decreased from 198 to 102 ppm and outlet NO2 concentration increased from 9 to 98NO ppm. concentration Some previous decreased lab-scale from studies 198 toreported 102 ppm that and ozone outlet could NO selectively2 concentration oxidize increased NO rather from than 9 to 98 ppm. Some previous lab-scale studies reported that ozone could selectively oxidize NO rather than SO2 when O3/NO molar ratio was less than 1 [19]. However, in our bench-scale experiments, the SO2 when O3/NO molar ratio was less than 1 [19]. However, in our bench-scale experiments, the selectivity of NO oxidation by ozone was not high enough, and co-existing SO2 imposed an apparent negativeselectivity effect of NO on oxidation NO oxidation. by ozone This was result not high confirmed enough, thatand co-existingthere were SO competition2 imposed an oxidation apparent negative effect on NO oxidation. This result confirmed that there were competition oxidation reactions between NO and SO2 with ozone. Especially when O3/O2 mixture gases were injected out reactions between NO and SO2 with ozone. Especially when O3/O2 mixture gases were injected out
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scrubbing, which was a slightly lower than the expectation. The solubility of NO2 was much higher than that of NO: a part of NO2 reacted in water to form HNO3 and HNO2 through the following equations [32]. During the NO2 absorption process, NO was formed and escaped from scrubbing solution. This might be the reason for a relatively lower NO conversion efficiency. At the 25th min, ozone injection was stopped, and outlet NO concentration quickly increased to about 202 ppm. It showed that little NO could be removed by wet scrubbing deionized water due to extremely low solubility of NO in water. 2NO N O (7) 2(g) ↔ 2 4 (g) NO NO (8) 2(g) ↔ 2(l) N O N O (9) 2 4 (g) ↔ 2 4 (l) 2NO + H O HNO HNO (10) 2(l) 2 (l) ↔ 3(l)+ 2(l) N O + H O HNO HNO (11) 2 4 (l) 2 (l) ↔ 3(l)+ 2(l) 3HNO HNO + 2NO + H O (12) 2(l) ↔ 3(l) (l) 2 (l) NO NO (13) (l) ↔ (g) Figure7b illustrated the e ffects of co-existing SO2 on NO oxidation and NOx removal performance during the reaction process. At the beginning, initial NO and SO2 concentrations in inlet flue gas were adjusted to about 200 and 220 ppm, respectively. After the introduction of O3/O2 mixture gases at the 5th min, outlet SO2 concentration obviously decreased to 179 ppm, while outlet NO concentration decreased from 198 to 102 ppm and outlet NO2 concentration increased from 9 to 98 ppm. Some previous lab-scale studies reported that ozone could selectively oxidize NO rather than SO2 when O3/NO molar ratio was less than 1 [19]. However, in our bench-scale experiments, the selectivity of NO oxidation by ozone was not high enough, and co-existing SO2 imposed an apparent negative effect on NO oxidation. This result confirmed that there were competition oxidation reactions between NO and SO2 with ozone. Especially when O3/O2 mixture gases were injected out from the injection pipe, locally high-concentration of O3 existed near the injection area. This led to some side reactions through Equations of ((2),(3),(14)). SO + O SO + O (14) 2 3 → 3 2 Since the oxidation of SO2 consumes a certain fraction of O3, resulting in a low utilization of ozone, such side reactions should be avoided as far as possible. For practical application, it might be necessary to take some measures to improve the local high-level ozone distribution problem. For example, one could adopt a multiple-pipe injection method and/or lower O3 concentration by introducing extra air to mix with O3/O2 mixture before it was injected into the reaction tower. At the 14th min, in-situ scrubbing of deionized water was introduced: outlet NO concentration decreased further from 102 to 40 ppm while outlet NO2 concentration increased from 98 to 162 ppm. Outlet SO2 concentration only decreased from 179 to 150 ppm mainly due to physicochemical absorption by scrubbing solution. The results showed that in-situ scrubbing could not only improve NO oxidation through suppressing the thermal decomposition of ozone but also improve NO oxidation selectivity. Since the solubility of SO2 in water was very high, more SO2 was removed through the hydrolysis:
SO SO (15) 2(g) ↔ 2(l) SO SO (16) 2(l) ↔ 2(l) SO + H O H SO (17) 2(l) 2 (l) ↔ 2 2(l) + H SO H + HSO − (18) 2 3(l) ↔ 3 J. Mar. Sci. Eng. 2020, 8, 943 15 of 22
At the 20th min, the injection of O3 was stopped, and outlet SO2 concentration changed slightly. This suggested that SO2 was removed mainly through the physicochemical absorption by wet scrubbing solution. After stopping ozone injection, outlet NO and NO2 concentrations recovered to the initial level quickly. The cyclic scrubbing was stopped at the 30th min, and outlet SO2 returned to the initial level quickly. The results showed that in-situ combination of ozone injection with wet scrubbing could improve both ozone utilization and NO oxidation selectivity simultaneously, and co-existing SO2 imposed little effect on NO oxidation by O3.
3.2.4. NOx and SO2 Removal by Ozone Injection and In-Situ Scrubbing with Oxidant Additives The above-mentioned experiments illustrated that in-situ combination of ozone injection and wet scrubbing could oxidize NO to NO2 effectively, but it still required to take some measures to remove NO and SO2 from flue gas with a high efficacy. In the tests, O3/NO molar ratio was kept at 0.5 in order to achieve a low energy consumption for generating ozone onboard. Aiming to obtain a high NOx removal efficiency, it was necessary to introduce some oxidant additives into scrubbing solution to combine with ozone to oxidize NO, and then to remove NOx and SO2 simultaneously. This was an obvious advantage of this approach that we proposed. Here, effects of several common oxidant additives on NO and SO2 removal performance were investigated preliminarily, and the results were presented in Figure8. The air flow was kept at 50 m3 h 1 measured at room temperature. NO, SO , and O concentrations · − 2 3 in inlet flue gas were adjusted to 200, 220, and 100 ppm, respectively, at the beginning. The temperature of flue gas before entering the reaction tower was maintained at 250 ◦C during the whole tests. An amount of 14 L deionized water was used as scrubbing solution, and the solution pH was maintained to 6 by adding 1 mol L 1 NaOH solution continuously during the whole reaction process. · − The cyclic solution temperature was kept at 50 ◦C by a water bath from the beginning. Four kinds of common oxidants—NaClO, Na2S2O8,H2O2, and NaClO2—were chosen to add into scrubbing solution to work with ozone. The oxidative power of these oxidants was strong enough, and a large amount of interestsJ. Mar. Sci. had Eng. been 2020, attracted8, x FOR PEER from REVIEW numerous researchers to investigate their NO removal performance 16 of 23 through wet scrubbing [9–12]. The previous research indicated that these four kinds of oxidative additivesfollowing that tests, we initial chose concentrations could oxidizeand of oxidant remove additives NO with ain high solutions efficiency. were Inin thethe following range of tests,0–0.1 −1 initialmol·L concentrations. of oxidant additives in solutions were in the range of 0–0.1 mol L 1. · −
Figure 8. Cont.
Figure 8. Effects of various oxidant additives in in-situ wet scrubbing solutions on: (a) NOx and (b)
SO2 removal efficiencies.
It could be seen from Figure 8a that NOx removal efficiency was about 11% when ozone was in- situ combined using pH-buffered scrubbing solution with no oxidative additives. This indicated that online supplementing of alkaline species was in favor of absorbing NOx from flue gas. Generally, oxidants such as Na2S2O8 and H2O2 must be activated by heat, UV, or metal ions for generating strong oxidant species (free radicals) to effectively oxidize NO to higher valance NOx, thus obtaining a high NOx removal efficiency [12,33,34]. In our experiments, ozone was injected in-situ to contact with scrubbing solution droplets. As shown in Figure 8a, NOx removal efficiencies for Na2S2O8 + O3 and H2O2 + O3 systems were below 20%. This implied that ozone could not activate Na2S2O8 and H2O2 efficiently during the scrubbing process. When NaClO or NaClO2 solution was cyclic scrubbed in the reaction tower with ozone injection, a much higher NOx removal efficiency could be obtained. As shown in Figure 8a, with the increase
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following tests, initial concentrations of oxidant additives in solutions were in the range of 0–0.1 mol·L−1.
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Figure 8. Effects of various oxidant additives in in-situ wet scrubbing solutions on: (a) NOx and (b) SO2 Figure 8. Effects of various oxidant additives in in-situ wet scrubbing solutions on: (a) NOx and (b) removal efficiencies. SO2 removal efficiencies.
It could be seen from Figure8a that NO x removal efficiency was about 11% when ozone was It could be seen from Figure 8a that NOx removal efficiency was about 11% when ozone was in- in-situ combined using pH-buffered scrubbing solution with no oxidative additives. This indicated situ combined using pH-buffered scrubbing solution with no oxidative additives. This indicated that that online supplementing of alkaline species was in favor of absorbing NOx from flue gas. online supplementing of alkaline species was in favor of absorbing NOx from flue gas. Generally, oxidants such as Na2S2O8 and H2O2 must be activated by heat, UV, or metal ions Generally, oxidants such as Na2S2O8 and H2O2 must be activated by heat, UV, or metal ions for for generating strong oxidant species (free radicals) to effectively oxidize NO to higher valance NOx, generating strong oxidant species (free radicals) to effectively oxidize NO to higher valance NOx, thus thus obtaining a high NOx removal efficiency [12,33,34]. In our experiments, ozone was injected in-situ obtaining a high NOx removal efficiency [12,33,34]. In our experiments, ozone was injected in-situ to to contact with scrubbing solution droplets. As shown in Figure8a, NO x removal efficiencies for contact with scrubbing solution droplets. As shown in Figure 8a, NOx removal efficiencies for Na2S2O8 Na2S2O8 + O3 and H2O2 + O3 systems were below 20%. This implied that ozone could not activate + O3 and H2O2 + O3 systems were below 20%. This implied that ozone could not activate Na2S2O8 and Na2S2O8 and H2O2 efficiently during the scrubbing process. H2O2 efficiently during the scrubbing process. When NaClO or NaClO2 solution was cyclic scrubbed in the reaction tower with ozone injection, When NaClO or NaClO2 solution was cyclic scrubbed in the reaction tower with ozone injection, a much higher NOx removal efficiency could be obtained. As shown in Figure8a, with the increase in a much higher NOx removal efficiency could1 be obtained. As shown in Figure 8a, with the increase NaClO2 concentration from 0 to 0.1 mol L− , NOx removal efficiency increased from 11% to 72%. As for · 1 NaClO, with the increase in NaClO concentration from 0 to 0.06 mol L , NOx removal efficiency 2 · − increased from 11% to 65.8%. However, NOx removal efficiency decreased down to 62.2% when further increasing NaClO concentration to 0.1 mol L 1. This illustrated that in-situ combination of 2 · − ozone oxidation and wet scrubbing absorption using NaClO or NaClO2 as extra oxidative additives apparently improved NOx removal performance. Meanwhile, a complete removal of SO2 was reached when NaClO or NaClO concentration was higher than 0.04 mol L 1. 2 · − Though the oxidative power of NaClO2 was normally much higher than that of NaClO for wet oxidation of NO [35], NOx removal efficiency approached to each other when additive concentration was 0.04 mol L 1 used in our experiments. Considering that the reagent costs of NaClO was much · − 2 higher than that of NaClO, and for ocean-going vessels, there was another possible way to obtain NaClO through seawater electrolysis [36]. Thus, it was believed that ozone injection combined with in-situ scrubbing NaClO solution would be a good choice for marine application for simultaneous removal of NO and SO2 from exhaust gas. Most of the previous studies on simultaneous removal of SO2 and NO from simulated flue gas by using NaClO solution were based on lab-scale reactors, together with an extremely high liquid-gas ratio and gas residence time in reactor. NO oxidation rate rather than NOx removal efficiency was used to characterize the denitration performance. For instance, Mondal et al. studied NO oxidation by wet scrubbing using NaClO solution. Under their optimal conditions, maximum SO2 removal efficiency and NO conversion rate were 100% and 92%, respectively, at 40 ◦C, initial NaClO solution J. Mar. Sci. Eng. 2020, 8, 943 17 of 22 pH 5.8, and 0.01 mol L 1 NaClO concentration [37]. The similar results implied that it was difficult · − to obtain a high NOx removal efficiency by wet scrubbing using NaClO solution alone due to the relatively low solubility of NO and NO2. Thus, it was necessary to take measures to further improve NOx absorption performance of NaClO solution. Yang et al. proposed to enhance NO oxidation performance by introducing online UV irradiation, in which radical species were generated to oxidize NO in an efficient way. However, their NOx removal performance was still not high enough [38]. In our experiments, a very high NOx removal efficiency was achieved through in-situ combining ozone injection and wet scrubbing based on a bench-scale reactor.
J. Mar.For Sci. illustrating Eng. 2020, 8, x the FOR reaction PEER REVIEW process of in-situ combining ozone oxidation and wet scrubbing 17 of 22 absorption, the changes of outlet gas concentrations with the reaction duration were presented 1 inobtain Figure 9a. high Here, NO initialx removal NaClO efficiency concentration by wet scrubbing in solution using was NaClO 0.04 mol solutionL . alone NO, SOdue, to and the O · − 2 3 concentrationsrelatively low insolubility inlet flue of NO gas and were NO adjusted2. Thus, it towas 200, necessary 220, and to take 100 measures ppm, respectively, to further improve at room temperatureNOx absorption at the beginning.performance It canof NaClO be seen solution. from Figure Yang9 thatet al. after proposed the introduction to enhance of NO ozone oxidation injection performance by introducing online UV irradiation, in which radical species were generated to oxidize and in-situ wet scrubbing at the same time, both outlet NO and SO2 concentration decreased sharply NO in an efficient way. However, their NOx removal performance was still not high enough [38]. In down to 0 ppm. Outlet NO2 concentration increased from 5 to 75 ppm. A complete removal of SO2 and our experiments, a very high NOx removal efficiency was achieved through in-situ combining ozone NOx removal efficiency of 63.4% were achieved simultaneously. The addition of NaClO in scrubbing injection and wet scrubbing based on a bench-scale reactor. solution enhanced NO oxidation and NO2 absorption greatly. For illustrating the reaction process of in-situ combining ozone oxidation and wet scrubbing Simultaneous removal of high-concentration SO2 and NO by in-situ combination of ozone injection absorption, the changes of outlet gas concentrations with the reaction duration were presented in and wet scrubbing was also investigated, and the result was shown in Figure 10. Here, initial NaClO Figure 9. Here, initial NaClO concentration in solution was 0.04 mol·L−1. NO, SO2, and O3 concentration in solution was still 0.04 mol L 1, while NO, SO , and O concentrations in inlet flue gas concentrations in inlet flue gas were adjusted· − to 200, 220,2 and 1003 ppm, respectively, at room were adjusted to 800, 400, and 400 ppm, respectively, at room temperature at the beginning. The air temperature at the beginning. It can be seen from Figure 9 that after the introduction of ozone 3 1 flow was kept at 50 m h− measured at room temperature. The temperature of flue gas before entering injection and in-situ wet· scrubbing at the same time, both outlet NO and SO2 concentration decreased the reaction tower was maintained at 250 C throughout the tests. At the start, initial NO concentration sharply down to 0 ppm. Outlet NO2 concentration◦ increased from 5 to 75 ppm. A complete removal measuredof SO2 and at sampling NOx removal point efficiency 7 was about of 63.4% 770 ppm were rather achiev thaned 800simultaneously. ppm, because The with addition the increase of NaClO in flue gasin temperature scrubbing solution from room enhanced temperature NO oxidation to 250 ◦ andC, NO NO was2 absorption oxidized greatly. to NO2 by O2 in carrier air gas.
FigureFigure 9. 9.The The changes changes of of outlet outlet gas gas concentrationsconcentrations with the the reaction reaction duration duration in in the the test test ofof in-situ in-situ 1 combination of 100 ppm ozone injection and wet scrubbing 0.04 mol L−1 NaClO solution. combination of 100 ppm ozone injection and wet scrubbing 0.04 mol·L· − NaClO solution.
Simultaneous removal of high-concentration SO2 and NO by in-situ combination of ozone injection and wet scrubbing was also investigated, and the result was shown in Figure 10. Here, initial NaClO concentration in solution was still 0.04 mol·L−1, while NO, SO2, and O3 concentrations in inlet flue gas were adjusted to 800, 400, and 400 ppm, respectively, at room temperature at the beginning. The air flow was kept at 50 m3·h−1 measured at room temperature. The temperature of flue gas before entering the reaction tower was maintained at 250 °C throughout the tests. At the start, initial NO concentration measured at sampling point 7 was about 770 ppm rather than 800 ppm, because with the increase in flue gas temperature from room temperature to 250 °C, NO was oxidized to NO2 by O2 in carrier air gas. The result showed that after the introduction of 400 ppm ozone, outlet NO concentration decreased quickly down to about 440 ppm, while outlet SO2 concentration decreased slightly down to about 280 ppm. With the cyclic scrubbing of 0.04 mol·L−1 solution, both outlet NO and SO2 concentration decreased sharply down to 0 ppm. A complete removal of SO2 and NOx removal efficiency of 65.5% were achieved simultaneously. However, when ozone injection was shut off,
J. Mar. Sci. Eng. 2020, 8, x FOR PEER REVIEW 18 of 22
outlet NO concentration increased sharply from 0 to 405 ppm, while NO2 decrease from 286 ppm down to 101 ppm. At that moment, NOx and SO2 removal efficiencies were 39% and 100%. This result showed that both ozone and NaClO played important roles in NO oxidation and absorption in our reaction system. As long as the molar ratio of O3 to ozone was kept at 0.5, higher inlet NO concentration did not change NOx removal efficiency. Because ozone could efficiently oxidize a large part of NO to NO2, NaClO in solution could oxidize the left part of NO and absorb NOx in an efficient J. Mar. Sci. Eng. 2020, 8, 943 18 of 22 way.
FigureFigure 10. 10.The The changes changes of of outlet outlet gas gas concentrationsconcentrations with the the reaction reaction duration duration in in the the test test of ofin-situ in-situ 1 combination of 400 ppm ozone injection and wet scrubbing 0.04 mol L−1 NaClO solution. combination of 400 ppm ozone injection and wet scrubbing 0.04 mol·L· − NaClO solution.
4. TheDiscussion result showed that after the introduction of 400 ppm ozone, outlet NO concentration decreased quickly down to about 440 ppm, while outlet SO2 concentration decreased slightly down to about 2804.1. ppm. New With Concept the of cyclic an Integrated scrubbing System of 0.04 mol L 1 solution, both outlet NO and SO concentration · − 2 decreased sharply down to 0 ppm. A complete removal of SO2 and NOx removal efficiency of The conventional solution for removal of NO and SO2 from marine exhaust gas was the 65.5%combination were achieved of wet scrubbing simultaneously. and SCR However,in series, but when this method ozone injectionwas difficult was to shut realize off ,due outlet to the NO concentrationlimited engine increased space and sharply complicated from flue 0 to gas 405 condit ppm,ions. while Ozone NO 2wasdecrease a kind of from efficient 286 ppmNO oxidant, down to 101but ppm. the Attwo-stage that moment, method NO usuallyx and required SO2 removal O3/NO effi molarciencies ratio were higher 39% than and 2, 100%. and it This was result difficult showed to thatobtain both a ozone high oxidant and NaClO utilization played due important to serious roles thermal in NO decomposition oxidation and of ozone. absorption Here, inwe our proposed reaction system.the method As long of combing as the molar ozone ratio injection of O3 withto ozone in-situ was wet kept scrubbing at 0.5, oxidative higher inlet solution NO concentration to complete the did notsimultaneous change NOx removalremoval of effi NOciency. and SO Because2. The process ozone could diagram effi cientlywas shown oxidize in Figure a large 11, part in which of NO ozone to NO 2, NaClOwas ininjected solution into could the oxidizescrubbing the tower left part directly. of NO The and absorbin-situ NOsprayingx in an solution efficient cooled way. flue gas temperature quickly below 100 °C, which was beneficial to suppress the thermal decomposition of 4. Discussionozone, further improving the oxidant utilization greatly. The molar ratio of O3/NO was kept much lower than 1 in order to reduce the usage of ozone and to reduce the electricity consumption by ozone 4.1. New Concept of an Integrated System generating system. Additional oxidative species (such as NaClO) were used to collaborate with ozone
to Theoxidize conventional NO, and solutionabsorb NO for removalx and SO of2 NOin an and efficient SO2 from way. marine This exhaustnew method gas was was the in combination favor of of wetexploiting scrubbing the advantages and SCR in of series, ozone butoxidation this method and we wast scrubbing difficult processes, to realize ma dueking to theit more limited practical engine spacefor andindustrial complicated applications. flue gas conditions. Ozone was a kind of efficient NO oxidant, but the two-stage method usually required O3/NO molar ratio higher than 2, and it was difficult to obtain a high oxidant utilization due to serious thermal decomposition of ozone. Here, we proposed the method of combing ozone injection with in-situ wet scrubbing oxidative solution to complete the simultaneous removal of NO and SO2. The process diagram was shown in Figure 11, in which ozone was injected into the scrubbing tower directly. The in-situ spraying solution cooled flue gas temperature quickly below 100 ◦C, which was beneficial to suppress the thermal decomposition of ozone, further improving the oxidant utilization greatly. The molar ratio of O3/NO was kept much lower than 1 in order to reduce the usage of ozone and to reduce the electricity consumption by ozone generating system. Additional oxidative species (such as NaClO) were used to collaborate with ozone to oxidize NO, and absorb NOx and SO2 in an efficient way. This new method was in favor of exploiting the advantages of ozone oxidation and wet scrubbing processes, making it more practical for industrial applications. J. Mar. Sci. Eng. 2020, 8, 943 19 of 22 J. Mar. Sci. Eng. 2020, 8, x FOR PEER REVIEW 19 of 22
FigureFigure 11. 11.The The proposedproposed processprocess diagram diagram for for simultaneous simultaneous removal removal of of NO NO and and SO SO2 2from from marine marine exhaustexhaust gas gas by by ozone ozone injection injection combined combined with with in-situ in-situ wet wet scrubbing scrubbing oxidant oxidant solution. solution.
4.2.4.2. Application Application Prospect Prospect of of the the Integrated Integrated System System SinceSince 1 1 January January 2020, 2020, the the limit limit on on sulfur sulfur content content in marinein marine fuel fuel oil hasoil has been been lowered lowered greatly greatly from from 3.5 down3.5 down to 0.5 to wt.%. 0.5 wt.%. Instead Instead of adopting of adopting low sulfur low sulfur fuel oil, fuel thousands oil, thousands of sets of of se wetts of scrubbers wet scrubbers have been have installedbeen installed onboard onboard in recent in years.recent Mostyears. of Most the scrubbersof the scrubbers were based were onbased open-loop on open-loop mode, whichmode, madewhich usemade of the use alkalinity of the alkalinity of seawater of seawater to remove to SOremovex from SO marinex from exhaust marine gas exhaust with aga highs with effi acacy. high Once efficacy. the priceOnce di thefferences price betweendifferences high-S between and low-S high-S fuel and oils low- wereS fuel high oils enough, were thehigh shipowners enough, the would shipowners be paid backwould within be paid 2–5 back years. within Generally, 2–5 years. it would Generally, cost a couple it would of millioncost a couple dollars of to million install dollars an open-loop to install wet an DeSOopen-loopx systems wet onboardDeSOx systems an ocean-going onboardvessel. an ocean-going At present, vessel. such At a wet present, scrubber such could a wet not scrubber remove could any NOnotx removefrom marine any NO exhaustx from gas marine due toexhaust the ultralow gas due solubility to the ultralow of NO insolubility water. of NO in water. ItIt was was known known that that it it was was hard hard to to abate abate NO NOx xemission emission through through improving improving the the performance performance of of dieseldiesel engine engine itself. itself. ToTo date, date, stringent stringent regulations regulations on on NO NOx xemissions emissions havehavecome comeinto into force force in in some some internationalinternational emission emission control control areas. areas. The The demands demands for for simultaneous simultaneous removal removal of of SO SOx xand and NO NOx xfrom from marinemarine exhaust exhaust gas gas became became increasingly increasingly imperious. imperious. The The integrated integrated system system that that we we proposed proposed could could be easilybe easily realized realized by combining by combining an ozone an ozone generating generating device device with with a wet a wet DeSO DeSOx scrubber.x scrubber. Though Though such such an integratedan integrated system system required required extra extr equipmenta equipment and and operation operation cost, cost, it wasit was of of great great potential potential to to remove remove SOSOx xand andNO NOxx simultaneouslysimultaneously withwith aa highhigh eefficiency.fficiency. ComparedCompared withwith otherothersolutions, solutions, suchsuchas as low low sulfursulfur oil oil+ +SCR, SCR, lowlow sulfur sulfur oil oil+ +EGR, EGR,and and wet wet scrubber scrubber+ +SCR, SCR, ourour integrated integrated system system had had some some advantages: high removal efficiency, low investment cost, and easy operation. For future application, there were still some issues to be further explored with this integrated system, in order to reduce
J. Mar. Sci. Eng. 2020, 8, 943 20 of 22 advantages: high removal efficiency, low investment cost, and easy operation. For future application, there were still some issues to be further explored with this integrated system, in order to reduce energy consumption and the secondary pollution risk. However, it was believed that in-situ combination of ozone oxidation and wet scrubbing for simultaneous removal of SOx and NOx, even as a transitional solution, was of great potential for marine application.
5. Conclusions A new process of in-situ combining ozone injection and wet scrubbing oxidative solution was proposed for simultaneous removal of NO and SOx from marine exhaust gas. The results showed that in the absence of in-situ wet scrubbing, a large amount of ozone was thermally decomposed when it was directly injected into hot flue gas, and co-existing SO2 consumed a nonnegligible amount of ozone during the mixing process. The introduce of in-situ wet scrubbing not only suppressed the thermal decomposition of ozone, further improving the oxidant utilization, but also improved NO oxidation over SO2. The addition of other oxidants into scrubbing solution could simultaneously remove NO and SO2 in an efficient way. When O3/NO molar ratio was 0.5 and initial NaClO concentration in 1 solution was 0.04 mol L , NOx removal efficiency of 63.4% and a completed removal of SO could · − 2 be achieved at the same time. The integrated system had some obvious advantages such as compact structure, low oxidant usage, high pollutant removing efficiency, and easy accessibility of oxidants. This offered useful reference for marine multi-pollutant abatement technologies.
Author Contributions: Conceptualization, validation, supervision, investigation, writing—original draft, writing—review and editing, project administration, funding acquisition, Z.H.; investigation, validation, formal analysis, T.Z. and J.W.; methodology, J.D.; data analysis, Y.D.; project administration, funding acquisition, X.P. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by National Natural Science Foundation of China, grant number 51779024, and Fundamental Research Funds for the Central Universities, grant number 3132019330. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
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