Simultaneous Removal of SO₂ and NO by O₃ Oxidation Combined with Seawater as Absorbent

Abstract

Aiming at NO_x (NO 90%, NO₂ 10%) and SO₂ in simulated vessel emissions, denitration and desulfurization were studied through ozone oxidation combined with seawater as absorbent. Specifically, the different influencing factors of denitration and desulfurization were analyzed. The results indicated that the oxidation efficiency of NO can reach over 90% when the molar ratio of O₃/NO is 1.2. Ozone oxidation and seawater washing in the same unit can decrease the temperature of ozone oxidation of NO, avoid high temperature ozone decomposition, and enhance the oxidation efficiency of NO. When NO inlet initial concentration is lower than 800 ppm, the NO_x removal efficiency can be improved by increasing NO inlet concentration, and when NO inlet initial concentration is greater than 800 ppm, increasing the concentration of NO would decrease the NO_x removal efficiency. Increasing the inlet concentration of SO₂ has minor effect on desulfurization, but slightly reduces the absorption efficiency of NO_x due to the competition of SO₂ and NO_x in the absorption solution. Besides, final products (${\mathrm{NO}}_2^{-}$, ${\mathrm{NO}}_3^{-}$, ${\mathrm{SO}}_3^{2-}$, and ${\mathrm{SO}}_4^{2-}$) were analyzed by the ion chromatography.

1. Introduction

Marine shipping has made an important contribution to the development of the world economy, but vessels also emit a large amount of SO₂ and NO_x into the atmosphere, causing serious pollution. At present, more and more countries/organizations have issued strict laws to limit exhaust gas pollution from shipping.

In general, wet scrubbing technology has been widely used in ships to remove SO₂, and the removal effect can reach 95%. Among the mature nitric oxide (NO) removal technologies, selective catalytic reduction (SCR) has always been considered to be the most effective technology, which converts nitrogen oxides into nitrous gas by reducing catalyst. As common sense, independent desulfurization and denitration operation require more operating cost. Therefore, simultaneous removal of SO₂ and NO_x technology is a better choice. At present, oxidation absorption methods (OAM) are a viable direction, which oxidizes the NO to a high valence mixture (NO₂, and N₂O_5, which have a higher solubility in water), and then SO₂ and NO_x were absorbed together by washing liquid.

In the first stage of OAM (oxidation), many researchers have investigated the oxidation effect of NO. Many oxidants have been used to oxidize NO, such as Fe (II) EDTA [1,2,3], NaClO₂ [4,5,6], and NaClO [7,8]. The oxidation of the above methods has ideal effects, but the products will cause secondary pollution. Amongst these strong oxidizers, ozone has the advantages of fast reaction, small dosage, easy to produce on site, convenient operation, and no secondary pollution [9,10,11]. With the maturity of ozone preparation, ozone has been widely used in denitrification.

In the second stage of OADD (absorption), usually, absorption technologies can be classified into alkaline absorbents and advanced material adsorbents. Alkaline absorbent such as NH3 [12,13], NaHCO₃ [14,15], Ca(OH)₂ [16,17,18], red mud [19,20,21,22,23], has the advantages of low cost. The advanced material adsorbents have a very good absorption effect, such as graphene being extensively used by its single layer and associated band structure [24]. Carbon nanotubes are also often used in the removal of pollutants from water due to their excellent absorption properties [25]. Considering the huge SO₂ and NO_x emissions, desulfurization and denitration of ship exhaust gas need to carry abundant absorbents (alkaline absorbents or advanced material adsorbents), which will take up a lot of storage space and reduce the transport capacity. Therefore, it is of great significance to develop a kind of absorbent that can be produced on-site for marine ships [26].

Since seawater is readily available from around ships, seawater scrubbing for sulfur dioxide removal has been widely used [27,28,29,30]. At present, there are several reports about synergistically absorbed SO₂ and NO_x by seawater.

Hence, this paper proposes a simultaneous desulfurization and denitrification technology with ozone as the oxidant and seawater as the absorbent. Different influencing factors of inlet flue gas temperatures, O₃ concentrations, seawater scrubbing temperature, NO concentrations, SO₂ concentrations, and pH for desulfurization and denitrification were carried out. Then, the reaction mechanisms were also discussed [31].

2. Experimental Setup

2.1. Experimental Materials

Figure 1 shows the structure and schematic of the exhaust gas treatment system. The main equipment includes a simulated gas blending unit, an oxidation (ozone), a reactor, and a flue gas analysis unit.

Marine diesel exhaust gas is replaced by 4 kinds of gases: O₂ (99.99%), N₂ (99.999%), SO₂ (9.80% SO₂ balanced N₂), and NO (9.98% NO balanced N₂). In the experiments, the gas flow was kept at 1.5 L/min by mass flow controllers (MFC, Beijing Seven Star Electronics Co., Ltd., Beijing, China). The initial gas concentration of ozone is 15%. Ozone is prepared by an ozone generator (Qingdao Guolin Co. Ltd., Qingdao, China), and ozone concentration is controlled by a mass flow controller to meet the experimental needs.

In the oxidation reaction furnace, the simulated flue gas is mixed with O₃ and oxidizes NO into higher oxidation states (NO₂ and N₂O₅).

The reactor (inner diameter 60 mm, height 500 mm) was a spray tower.

The O₃ and simulated flue gases were mixed in advance at the bottom of the spray tower. A gas distributor was located above the inlet ports to mix the flue gas uniformly. The liquid sprayed by the nozzle at the top of the spray tower is artificial seawater (salinity of 30 PSU, initial pH value was 8), and the flow rate was about 1.0 L·min⁻¹ [32]. For each run, the initial pH values of the solution were adjusted to the desired values by adding HCl solution (1 mol L⁻¹) or NaOH solution (1 mol L⁻¹). The solution temperature was adjusted to 50 °C by the water bath (Julabo F34–ED Refrigerated/Heating Circulator, Seelbach, Germany).

An ozone analyzer (2B Technology Co., Boulder, CO, USA) was used to detect the concentration of ozone in the simulated flue gas with a sampling interval of 6 s. The flue gas analysis unit has a gas analyzer (MGA 6, MRU, Neckarsulm, Germany) to monitor the import and export of NO_x and SO₂ [33].

Besides, samples were taken to measure the determine concentrations of ${\mathrm{NO}}_2^{-}$, ${\mathrm{NO}}_3^{-}$, ${\mathrm{SO}}_3^{2-}$, and ${\mathrm{SO}}_4^{2-}$ anions by ion chromatography (Dionex ICS 1500, Waltham, MA, USA).

2.2. Date Processing

To show the effects of desulfurization and denitration treatment by ozone combined with electrolyzed seawater, the removal efficiency was defined as:

$${\zeta }_i=\frac{{\mathrm{C}}_{i-\mathrm{in}}-{\mathrm{C}}_{i-\mathrm{out}}}{{\mathrm{C}}_{i-\mathrm{in}}}$$

(1)

where ${\mathrm{C}}_{i-\mathrm{in}}$ and ${\mathrm{C}}_{i-\mathrm{out}}$ are the import/export concentrations of SO₂, NO, and NO_x.

3. Results and Analysis

3.1. Effect of Inlet Flue Gas Temperatures and O₃ Concentrations on the Oxidation of NO

Figure 2 depicts the effect of inlet flue gas temperatures (T₃) on the oxidation of NO. The inlet concentrations of SO₂ and NO were 200 ppm and 300 ppm. ${\mathrm{n}}_{{\mathrm{O}}_3}/{\mathrm{n}}_{\mathrm{NO}}$ denotes the molar ratio of O₃ and initial concentration of NO.

The efficiencies of NO oxidation under different inlet flue gas temperatures can be seen in Figure 2. When the NO inlet concentration is 200 ppm, and inlet flue gas temperatures is 50 to 150 °C, the NO oxidation efficiencies have no significant difference; the average efficiencies of the oxidation of NO are above 87% when the molar ratio of O₃ and initial concentration of NO is 1.0.

The efficiencies of NO oxidation begin to decrease when inlet flue gas temperatures exceed 200 °C; the oxidation efficiency decreased obviously after 250 °C. The reason may be the ozone decomposition [34,35].

In the above experiment, when inlet flue gas temperatures are fixed, the increase of ozone concentration can improve the NO removal efficiency; when the inlet flue gas temperature was 150 °C and ${\mathrm{n}}_{{\mathrm{O}}_3}/{\mathrm{n}}_{\mathrm{NO}}$ (the ${\mathrm{O}}_3/\mathrm{NO}$ molar ratio) was 0.4 and 0.8, the oxidation efficiency is 35% and 75%, respectively. When the O₃/NO molar ratio was lower than 1.0, NO₂ was the main product (Equation (2)). When ${\mathrm{n}}_{{\mathrm{O}}_3}/{\mathrm{n}}_{\mathrm{NO}}$ is 1.0 or 1.2, the oxidation efficiency was more than 87% or 95%, respectively, and NO₃ (Equation (3)) and N₂O₅ (Equation (4)) were the major product [36]. Considering the economic benefit of O₃ dosage, and the actual marine ship’s exhaust gas temperature, this paper chooses the most suitable ${\mathrm{n}}_{{\mathrm{O}}_3}/{\mathrm{n}}_{\mathrm{NO}}$ of 1.2.

$${\mathrm{NO}+\mathrm{O}}_3\to {\mathrm{NO}}_2{+\mathrm{O}}_2$$

(2)

$${\mathrm{NO}}_2{+\mathrm{O}}_3\to {\mathrm{NO}}_3{+\mathrm{O}}_2$$

(3)

$${\mathrm{NO}}_2{+\mathrm{NO}}_3\to {\mathrm{N}}_2{\mathrm{O}}_5$$

(4)

3.2. Effect of Seawater Scrubbing Temperature for Desulfurization and Denitrification

In Figure 3, the efforts of seawater scrubbing temperature were tested. In the experiment, 30 L artificial seawater was a cyclic scrubbing solution. The initial temperature of the scrubbing solution was 23 °C. The spraying nozzle flow rate was about 1 L·min⁻¹. As shown in Figure 3, the scrubbing solution temperature (Figure 1, T₂) will gradually rise, and the temperature of the scrubbing solution will rise gradually at the beginning and eventually stabilize. In the experiment, inlet gas temperatures (Figure 1, T₃) were set at 200 °C, 250 °C, 300 °C, 350 °C, the duration of seawater washing was 60 min, the scrubbing solution temperature (Figure 1, T₂) was 41 °C, 45 °C, 49 °C, 54 °C, respectively. The temperatures of outlet gas (Figure 1, T₁) were 50 °C, 54 °C, 60 °C, and 65 °C, respectively. It can be seen, through seawater scrubbing, that the reaction temperature of the flue gas can be controlled at a lower value. The seawater scrubbing absorption method can reduce the oxidation reaction temperature of flue gas, inhibit the decomposition of ozone at high temperatures [37], and promote the denitrification effect.

3.3. Effect of Inlet NO Concentrations for Desulfurization and Denitrification

The inlet concentration of NO on the removal of SO₂ and NO_x was studied from 200 ppm to 1000 ppm.

Figure 4a shows that the SO₂ removal efficiency decreases with increasing NO concentration. This is mainly due to a large amount of NO competing with SO₂ for sea water washing solution, and removal efficiencies of SO₂ will reduce. From Figure 4b, increasing the inlet concentration of NO, when the initial inlet concentration of NO is 200 ppm to 800 ppm, the NO removal efficiency of seawater scrubbing gradually increased, and can be maintained at 80%. That is because NO belongs to insoluble gas, and the solubility in the seawater absorbent is very small. The inlet NO concentration increase is equivalent to increasing the NO partial pressure in the gas phase. Mass transfer driving force of NO gas will be increased, which is conducive to NO oxidation absorption. However, when the initial concentration of NO was further increased to 1000 ppm, the ozone oxidation rate in the liquid film became the main controlling factor, the molar ratio of oxidant and NO decreased gradually, and the denitration efficiency decreased gradually.

3.4. Effect of Inlet SO₂ Concentrations on Desulfurization and Denitration

Figure 5a shows the effect of inlet SO₂ concentration is varying from 300 to 1500 ppm. When the initial concentration of SO₂ is 300 ppm to 600 ppm_, the SO₂ removal efficiency of seawater scrubbing can be maintained above 96%, while at 1500 ppm, efficiency has reduced to 80%, which might suggest that SO₂ removal was controlled by liquid mass transfer. The flow rate of scrubbing solution was about 1 L·min⁻¹, which means that when the initial concentration of SO₂ is low, the amount of scrubbing solution can ideally absorb SO₂; when the inlet concentration of SO₂ increases to 900 ppm, the pH value decreases rapidly, and the mass transfer resistance will be increased, so the desulfurization will decrease.

According to Figure 5b, the NO_x removal efficiencies increased slightly when SO₂ concentration increased from 300 ppm to 900 ppm. This may be due to the SO₂ concentration being low, which the chemical reactions (5), (6), and (7) will undergo to produce more ${\mathrm{HSO}}_3^{-}$ and ${\mathrm{SO}}_3^{2-}$ ions, which can react with NO₂ (formula (11), formula (12)) to remove NO_X. However, when this exceeds 1200 ppm, the removal of SO₂ used up a large amount of scrubbing solution, and denitrification efficiency will be reduced.

3.5. The Influence of pH and Mechanism Analysis of Desulfurization and Denitrification

In the spraying tower, No is oxidized to a high valence mixture (NO₂, and N₂O₅) by O₃, and then, NO₂, N₂O₅, and SO₂ are absorbed by the seawater solution (The flow rate of the spraying nozzle was about 1 L·min⁻¹) 30 L. The initial pH value of artificial seawater was 8. The cyclic seawater solution temperature is kept at 50 °C by a water bath. The inlet concentrations of NO and SO₂ are 300 and 1000 ppm, respectively; ${\mathrm{n}}_{{\mathrm{O}}_3}/{\mathrm{n}}_{\mathrm{NO}}$ are 1.2. The absorption time is 60 min. pH values, desulfurization, and denitrification efficiency were recorded at intervals of 1 min to 5 min. The experimental results are shown in Figure 6.

As shown in Figure 6, the pH gradually decreases from 8 to 7.11 before 25 min. Between 25 and 40 min, the pH decreases rapidly from 7.11 to 4.13. After 40 min, the pH goes down to 3.97.

In the first 40 min, the desulfurization effect stays at about 98%. After 40 min, the desulfurization efficiency decreases because the pH value is less than 5. The seawater washing SO₂ principle is as follows.

$${\mathrm{SO}}_2+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_2{\mathrm{SO}}_3$$

(5)

$${\mathrm{H}}_2{\mathrm{SO}}_3\to {\mathrm{H}}^{+}{+\mathrm{HSO}}_3^{-}$$

(6)

$${\mathrm{HSO}}_3^{-}\to {\mathrm{H}}^{+}+{\mathrm{SO}}_3^{2-}$$

(7)

$${\mathrm{SO}}_3^{2-}+\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.{\mathrm{O}}_2\to {\mathrm{SO}}_4^{2-}$$

(8)

When the NO inlet concentration is 300 ppm, ${\mathrm{n}}_{{\mathrm{O}}_3}/{\mathrm{n}}_{\mathrm{NO}}$ is 1.2, NO is oxidized by ozone, and the main product is N₂O₅. As shown in Figure 6, the denitration efficiency is affected by pH value. Within the first 25 min, the pH value is above 7.11, the removal efficiency of NO_x can keep above 95%; in the 25 to 40 min, when the pH value drops to 4.13, the removal efficiency of NO_x drops to 75%; in the 40 to 60, the pH value goes down to 3.97, the denitration efficiency is increased to 80%. The main reason is that sulfur dioxide dissolved in water produces sulfite and bisulfite, and these two ions will react with nitrogen dioxide, thus improving the removal rate of nitrogen dioxide. However, when the SO₂ concentration was too high, the large amount of SO₂ will consume too much alkaline scrubbing solution, leaving relatively insufficient scrubbing solution for NO_x, which will reduce the NO_x removal effect.

$${3\mathrm{NO}}_2+{\mathrm{H}}_2\mathrm{O}\to {2\mathrm{HNO}}_3+\mathrm{NO}$$

(9)

$${2\mathrm{NO}}_2+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{HNO}}_3+{\mathrm{HNO}}_2$$

(10)

$${2\mathrm{NO}}_2+{\mathrm{SO}}_3^{2-}+{\mathrm{H}}_2\mathrm{O}\to {2\mathrm{H}}^{+}+{\mathrm{SO}}_4^{2-}+{2\mathrm{NO}}_2^{-}$$

(11)

$${2\mathrm{NO}}_2+{\mathrm{HSO}}_3^{-}+{\mathrm{H}}_2\mathrm{O}\to {3\mathrm{H}}^{+}+{\mathrm{SO}}_4^{-}+{2\mathrm{NO}}_2^{-}$$

(12)

4. Analysis of Products in Solution

Analysis of desulfurization and denitrification products were shown in Figure 7. Sampling every 5 min, desulfurization products were mainly ${\mathrm{SO}}_3^{2-}$ and ${\mathrm{SO}}_4^{2-}$. During the reaction, O₃ could also oxidize ${\mathrm{SO}}_3^{2-}$ to ${\mathrm{SO}}_4^{2-}$. Therefore, ${\mathrm{SO}}_4^{2-}$ was the final main existence forms of sulfur in the solution. Denitrification products were ${\mathrm{NO}}_3^{-}$ and a small amount of ${\mathrm{NO}}_2^{-}$. Although ${\mathrm{NO}}_2^{-}$ is highly toxic, the content is small. Furthermore, it can be seen that the trend of the experimental values was close to the theoretical values. The errors mainly come from sampling and statistical errors.

5. Conclusions

A new process of O₃ oxidative injection and seawater scrubbing was proposed for the desulfurization and denitrification of marine diesel exhaust gas. The experimental results show that the seawater scrubbing absorption method can suppress the decomposition of O₃ and promote NO oxidation. When ${\mathrm{n}}_{{\mathrm{O}}_3}/{\mathrm{n}}_{\mathrm{NO}}$ increased to 1.3, the NO removal rate also gradually stabilized at 95%. Considering ozone energy consumption, ${\mathrm{n}}_{{\mathrm{O}}_3}/{\mathrm{n}}_{\mathrm{NO}}$ was set at 1.2 in this paper. When the initial NO inlet concentration was lower than 800 ppm, the NOx removal efficiency can be improved by increasing the NO inlet concentration; when the NO inlet initial concentration is greater than 800 ppm, increasing the NOx concentration will reduce the NOx removal efficiency. Increasing SO₂ inlet concentration is not very useful for desulfurization, but the absorption efficiency of NOx was suppressed due to the competition between SO₂ and NOx in the absorption liquid. The reaction products are mainly ${\mathrm{SO}}_3^{2-}$ and ${\mathrm{NO}}_3^{-}$, which were less acidic. The results show that the method proposed in this paper (O₃ oxidation combined with seawater as absorbent) has great potential in ship exhaust gas treatment.