Study of nitrogen oxide absorption in the calcium sulfite slurry

Abstract

Experiments were conducted using a bubbling reactor to investigate nitrogen oxide absorption in the calcium sulfite slurry. The effects of CaSO₃ concentration, NO₂/NO mole ratio and O₂ concentrations on NO₂ and SO₂ absorption efficiencies were investigated. Five types of additives, including MgSO₄, Na₂SO₄, FeSO₄, MgSO₄/Na₂SO₄ and FeSO₄/Na₂SO₄, had been evaluated for enhancing NO₂ absorption in CaSO₃ slurry. Results showed that CaSO₃ concentration had significant impact on NO₂ and SO₂ absorption efficiencies, and the highest absorption efficiencies of SO₂ and NO₂ could reach about 99.5 and 75.0 %, respectively. Furthermore, the NO₂ absorption was closely related to the NO₂/NO mole ratio, and the existence of NO₂ in flue gas may promote NO absorption. The presence of O₂ in simulated flue gas was disadvantage for NO _x removal because it can oxidize sulfite to sulfate. It was worth pointing out that FeSO₄/Na₂SO₄ was the best additive among those investigated additives, as the NO₂ removal efficiency was significantly increased from 74.8 to 95.0 %. IC and in situ FTIR results suggest that the main products were NO₃ ⁻ and NO₂ ⁻ in liquid phase and N₂O, N₂O₅ and HNO₃ in gas phase during the CaSO₃ absorption process.

Introduction

Nitrogen oxides (NO _x ) generated from fuel combustion and waste incineration in stationary sources are precursors of acid rain, photochemical smogs and fine aerosols, have received unprecedented public attention [1]. In China, a series of NO _x emission standards for different industries, which are regarded as the strictest standards in history, have been successively issued by the Chinese government to limit NO _x emissions. The emission limit value of NO _x for waste incineration has changed from 400 to 250 mg m⁻³ [2]. Therefore, an urgent issue of air quality improvement is NO _x emission control. Currently, various removal technologies for control of NO _x have been widely used, including combustion modification technology, selective catalytic reduction (SCR) method, selective non-catalytic reduction (SNCR) method and wet absorption process [3]. However, most of these single-function unit operations suffer high operating costs. Emerging cost-effective technologies for multi-pollutants control are necessary. One promising candidate is the chemical absorption process for the co-capture of pollutants.

For the chemical absorption deNO _x process, sufficient NO₂ is the precondition. Accordingly, numerous aqueous oxidants such as KMnO₄, H₂O₂, Fe(II)-EDTA and NaClO₂ [4–6], some gas phase oxidations including ozone, ClO₂, non-thermal plasma and transition metal oxides catalysts [7, 8] have been investigated to convert insoluble NO to soluble NO₂. However, higher NO oxidation efficiency is limited by higher operating cost. If a certain amount of NO could be co-captured in absorption process, the overall treatment cost will be greatly slashed. Numerous aqueous absorbents, such as H₂O, NaOH, Ca(OH)₂, aqueous sulfite, ammoniacal cobalt(II) solutions, ferrate(VI), Fe(II)-EDTA and other complex component solutions, have been investigated in the removal of NO and NO₂ [9–18]. Compared with other approaches, the aqueous sulfite absorption method is one of the promising processes due to its high removal efficiency, moreover sulfite is an abundant reaction intermediate in the typical wet flue gas desulfurization [19].

CaSO₃ slurry can be a reasonable absorbent for the co-capture of multi-pollutants, as it is the leading byproduct in the limestone scrubbing solution and reacts with NO₂ in preference to O₂ [17]. Nevertheless, the low solubility of CaSO₃ slurry limits the reaction of NO₂ and SO₃ ²⁻, thus scientists are currently investigating alternative cost-effective additives for enhancing NO₂ absorption through enriching the SO₃ ²⁻ concentration. Tang et al. [13] indicated the MgSO₄ as an effective additive could significantly increase the NO₂ absorption efficiency from 70.8 to 86.0 %. Some metal and non-metal additives for enhancing NO₂ absorption in insoluble calcium sulfite slurry were also investigated. Wang et al. [17] reported that the FeSO₄ was the most effective additive with absorption efficiency of 95 %. In addition, with 0.5 mol L⁻¹ ammonium sulfate added, the NO₂ removal efficiency increased significantly from 67 to 90 %.

To the best of our knowledge, the chemical behavior of NO₂ absorption process in CaSO₃ slurry has not yet been systematically investigated. This study investigated the effects of CaSO₃ concentration, NO₂/NO mole ratio and SO₂ and O₂ concentrations on NO _x absorption in CaSO₃ slurry. The liquid and gas products formed in CaSO₃ slurry were investigated, and the chemical behavior of NO₂ absorption process in CaSO₃ slurry was discussed. In addition, different additives employed to enhance the absorption process were also studied.

Experimental section

Experimental setup

The schematic of the experimental system setup is illustrated in Fig. 1. It consists of a reaction gas supply unit, a cylindrical flue gas treatment reactor and a set of analytical instruments.

Fig. 1

Schematic diagram of the experimental setup

A cylinder with an inner diameter of 300 mm and a length of 600 mm (a volume of 5 L) was used as the bubbling reactor, redundant solution was stored in the overflow tank, and outer layer of the reactor was filled with water from water bath to control reaction temperature. The liquid stirring speed was controlled at 110 rpm. A cooling pipe was used to cool and dehydrate the outlet flue gas to protect the flue gas analyzer.

Experimental methods

The simulated flue gas was prepared with air, N₂, CO₂, SO₂, NO, and NO₂. A set of mass flow controllers was used to adjust the flow rate. All of the experiments were performed at atmospheric pressure and 325 K. The simulated gas consisted of 12 % CO₂, 300 ppm SO₂, 0–200 ppm NO, 0–200 ppm NO₂, 0–10 % O₂, using N₂ as balance gas, with a total flow rate of 4 L min⁻¹. While in the study of reaction products of NO₂ absorption in CaSO₃ slurry (see “Reaction products of NO2 absorption in CaSO3 slurry”), the simulated flue gas consisted of 5 % O₂, 260 ppm NO₂, and inert gas Ar, which is marked red in Fig. 1. The concentrations of NO _x , O₂ and SO₂ were monitored by a flue gas analyzer (Testo 350, Germany). Sulfite ions (SO₃ ²⁻) in the liquid phase were measured by an iodometric titration method. The concentrations of NO₃ ⁻ and NO₂ ⁻ in the liquid phase were tested by ion chromatogram (Metrohm 792, Switzerland; column: Asupper5/250). The qualitative analysis of gaseous products after reactor was performed by a Fourier transform infrared spectrometer (In situ FTIR, Nicolet 6700, USA).

And the initial pH (5.5–6) was moderated by the additional HCl. The CaSO₃ used in this study is CaSO₃·2H₂O (98 % purity) and deionized water, and the total CaSO₃ slurry volume in reactor was 2 L. Additives tested in this study include MgSO₄, Na₂SO₄, FeSO₄, MgSO₄/Na₂SO₄, FeSO₄/Na₂SO₄, which were introduced to the slurry with a concentration ranging from 0 to 0.5 mol L⁻¹. Meanwhile, the mole ratio of complex additives was 1/1.

The absorption efficiencies of SO₂ and NO _x were calculated as

$${\text{Removal}}\;\;{\text{efficiency}}\;(\% ) = \frac{{C_{\text{in}} - C_{\text{out}} }}{{C_{\text{in}} }}\; \times \;100\;\% ,$$

(1)

here C _in (ppm) is the concentrations of SO₂ or NO _x measured at inlet of the bubbling reactor; C _out (ppm) is the outlet concentrations of SO₂ and NO _x .

Results and discussion

Effect of CaSO₃ concentration on SO₂ and NO₂ absorption

Some experiments were carried out to investigate the effect of CaSO₃ concentration on NO₂ absorption rate, and the results were shown in Fig. 2. From Fig. 2a, b, it is obvious that the higher CaSO₃ concentration is favorable to the absorption of NO₂. It can be found from Fig. 2a that the absorption efficiency of NO₂ increased with CaSO₃ concentration from zero to 0.05 mol L⁻¹, thereafter, remained stable with increasing CaSO₃ concentration at above 0.05 mol L⁻¹, and the highest absorption efficiency of NO₂ about 75.2 % was obtained. This phenomenon is not only the cause of the existence of more SO₃ ²⁻ ions, but also more likely attributed to the absorbent particles that were suspended in the CaSO₃ slurry which could provide more reactive surface; the absorption rate would increase with solid content within a certain range, which is in accordance with the conclusion drawn by Dagaonkar et al. [20]. Also, when CaSO₃ concentration was changed from zero to 0.1 mol L⁻¹, the absorption efficiency of SO₂ kept stable, different from NO₂, the removal efficiency of SO₂ is slightly affected by CaSO₃ concentration and almost retains 99.0 %, and it indicates that SO₂ is easier removed. Figure 2b shows the relation of the NO₂ removal efficiency and CaSO₃ concentration with reaction time at different CaSO₃ concentrations. As shown in Fig. 2b, when CaSO₃ concentration was above 0.05 mol L⁻¹, the presence of more CaSO₃ absorbent (0.05–0.2 mol L⁻¹) resulted in a significant improvement in NO₂ absorption performance. Although the variety trend of NO₂ absorption was similar, a longer efficient time was kept. However, a sharply declining happened in a short time, and NO₂ removal efficiency finally retained at around 50 %. This trend is due to the fact that the SO₃ ²⁻ concentration decreases can lower chemical reaction rates. Moreover, the decreases of SO₃ ²⁻ concentration influence the gas–liquid mass transfer.

Fig. 2

a Effect of CaSO₃ concentration on SO₂ and NO₂ absorption. b Variation of NO₂ removal efficiency and CaSO₃ concentration in absorption solution with reaction time. (The reaction temperature is 325 K, the CaSO₃ slurry pH is 6.0, flue gas flow rate was 4.0 L min⁻¹, and the concentrations of O₂, CO₂, SO₂ and NO₂ are 5, 12 %, 300 and 200 ppm, respectively)

The results indicate that compared with solubility SO₃ ²⁻ absorbent, although the low solubility of CaSO₃ will limit the NO₂ absorption efficiency, the maximum removal efficiencies for NO₂ reached 75.2 %, so CaSO₃ slurry is a suitable absorbent, because CaSO₃ can also be oxidized by NO₂ prior to by O₂, which is consistent with the conclusion drawn by Tang et al. [13].

The overall reaction of the NO₂ absorption in CaSO₃ slurry can be written as follows:

$$4{\text{NO}}_{2} \left( {\text{aq}} \right) + {\text{SO}}_{3}^{2 - } \left( {\text{aq}} \right) + 2{\text{H}}_{2} {\text{O}}\left( {\text{aq}} \right) \to 3{\text{NO}}_{2}^{ - } \left( {\text{aq}} \right) + {\text{NO}}_{3}^{ - } \left( {\text{aq}} \right) + {\text{SO}}_{4}^{2 - } \left( {\text{aq}} \right) + 4{\text{H}}^{ + } .$$

(2)

According to Eq. (2), when NO₂ is absorbed by CaSO₃ slurry, SO₃ ²⁻ is oxidized to SO₄ ²⁻, and NO₂ is reduced to NO₃ ⁻ and NO₂ ⁻. Meanwhile, SO₂ absorption in CaSO₃ slurry will be complementary to SO₃ ²⁻, and then, desulfurization and denitrification processes will be combined reasonably.

Effect of O₂ concentration on SO₂ and NO₂ absorption

The effect of O₂ concentration on the SO₂ and NO₂ absorption were investigated, and the results are displayed in Fig. 3.

Fig. 3

Effect of O₂ concentration on NO₂ absorption. The reaction temperature is 325 K, the CaSO₃ slurry pH is 6.0, the initial CaSO₃ concentration is 0.1 mol L⁻¹, flue gas flow rate was 4.0 L min⁻¹, and the concentrations of CO₂, SO₂ and NO₂ are 12 %, 300 and 200 ppm, respectively

In Fig. 3, it is obvious that the presence of O₂ resulted in a significant decrease of NO₂ absorption performance in CaSO₃ slurry, and it is partly due to the fact that SO₃ ²⁻ can also be oxidized to SO₄ ²⁻ by O₂. The effective operating time in absence of O₂ was obviously longer than that of 5 % O₂ concentration, the result revealed that O₂ concentration is an important parameter during the CaSO₃ absorption process. In this study, there is almost no influence of the variation of O₂ concentration on the SO₂ absorption.

Figure 4 shows the effect of O₂ concentrations on NO₂ ⁻, NO₃ ⁻ and the DO concentrations in CaSO₃ slurry. NO₃ ⁻ concentration steadily increased with the increase of O₂ concentration, while NO₂ ⁻ concentration decreased. That is because the NO₂ ⁻could also be oxidized to NO₃ ⁻ in the presence of O₂. Also, the formation of more NO₃ ⁻ is due to the reaction of NO₂ and H₂O. The reactions with the participation of oxygen in this study can be summarized by the following chemical equations.

$$2{\text{CaSO}}_{3} \left( {\text{aq}} \right) + {\text{O}}_{2} \left( {\text{g}} \right) \to 2{\text{CaSO}}_{4} \left( {\text{aq}} \right),$$

(3)

$$4{\text{NO}}_{2} \left( {\text{aq}} \right) + 2{\text{H}}_{2} {\text{O}}\left( {\text{aq}} \right) + 2{\text{O}}_{2} \left( {\text{g}} \right) \to 4{\text{HNO}}_{3} \left( {\text{aq}} \right),$$

(4)

$$2{\text{HNO}}_{2} \left( {\text{aq}} \right) + {\text{O}}_{2} \left( {\text{g}} \right) \to 2{\text{HNO}}_{3} \left( {\text{aq}} \right).$$

(5)

Fig. 4

NO₂ absorption product compositions and DO in CaSO₃ slurry. The reaction temperature is 325 K, the CaSO₃ slurry pH is 6.0, the initial CaSO₃ concentration is 0.1 mol L⁻¹, flue gas flow rate was 4.0 L min⁻¹, and the concentrations of O₂, CO₂, SO₂ and NO₂ are 5, 12 %, 300 and 200 ppm, respectively; the sampling time is 30 min

Figure 4 also showed that the DO concentration in CaSO₃ slurry was almost invariable at about 0.46 mg L⁻¹ as the O₂ concentration increased from 0 to 10 %. The result indicates that the mass transfer resistance of O₂ dissolving may lie on liquid phase.

Effect of NO₂/NO ratio on SO₂ and NO _x absorption

Since the effect of NO₂/NO ratio on the removal of SO₂ and NO _x is an important factor, NO₂/NO ratios were investigated and the results are shown in Fig. 5.

Fig. 5

Effect of NO₂/NO ratio on SO₂ and NO _x absorption in CaSO₃ slurry. The reaction temperature is 325 K, the CaSO₃ slurry pH is 6.0, the initial CaSO₃ concentration is 0.1 mol L⁻¹, flue gas flow rate was 4.0 L min⁻¹, and the concentrations of O₂, CO₂, SO₂, NO and NO₂ are 5, 12 %, 300, 0–200 and 0–200 ppm, respectively. The reaction time is 30 min

As shown in Fig. 5, the SO₂ and NO _x absorption in CaSO₃ slurry is closely related to NO₂/NO ratio, because the SO₂ removal efficiency in the absence of NO₂, which is much lower than that in the presence of NO₂ (99.5 %), was 94.2 %. This result reveals that the existence of NO₂ can facilitate SO₂ absorption. It could be ascribed to that the dissolved NO₂ reacted with CaSO₃ slurry, which leads to decrease of the SO₃ ²⁻ concentration. On the other hand, the increasing of SO₃ ²⁻concentration, due to the products of SO₂ dissolution into CaSO₃ slurry, may promote the reaction Eq. (2) moving to the right. However, the SO₂ removal efficiency had a slight increase with the NO₂ concentration from 50 to 200 ppm. Also, with the increase of the NO₂/NO ratio, the concentration of NO₂ had a slight decrease, which indicates that NO₂ absorption reaches equilibrium and further NO₂ absorption is inhibited. Figure 5 also shows that the NO removal efficiency kept low range without NO₂ in CaSO₃ absorbent system, which is decided by the insoluble property of NO. Moreover, NO removal rate increased firstly and then decreased, according to reaction (6), the NO₂ is advantageous to the absorption of effect on the absorption of NO at lower concentration of NO₂; then when the NO₂ concentration is large enough, the NO absorption rate shows decreasing trend. This change is consistent with the study of Gao et al. [11]. They investigated the effect of the presence of NO₂ on NO absorption and indicated that the existence of NO₂ may promote NO absorption in (NH₄)₂SO₃ absorbent; this result was also similar with the case of NO _x absorption into CaSO₃ slurry. When NO and NO₂ coexist in inlet flue gas, the reaction between NO, H₂O and NO₂ occurred in the liquid phase.

$${\text{NO}}\left( {\text{g}} \right) + {\text{H}}_{2} {\text{O}}\left( {\text{aq}} \right) + {\text{NO}}_{2} \left( {\text{g}} \right) \to 2{\text{HNO}}_{2} \left( {\text{aq}} \right).$$

(6)

Effect of additive agents on SO₂ and NO₂ absorption

To improve the dissolution of CaSO₃ slurry, the additives, including individual FeSO₄, MgSO₄, Na₂SO₄, and complex MgSO₄/Na₂SO₄, FeSO₄/Na₂SO₄ had been selected to enhance the deNO _x performance (Fig. 6).

Fig. 6

Effects of individual and complex additives on NO₂ absorption in CaSO₃ slurry. The reaction temperature is 325 K, the CaSO₃ slurry pH is 6.0, the initial CaSO₃ concentration is 0.1 mol L⁻¹, flue gas flow rate was 4.0 L min⁻¹, and the concentrations of O₂, CO₂, SO₂ and NO₂ are 5, 12 %, 300 and 200 ppm, respectively; the reaction time is 30 min

According to the preceding results, about 75 % of NO₂ was removed in the CaSO₃ system without additives. As shown in Fig. 6, with the concentration of additives varied from 0 to 0.5 mol L⁻¹, all individual and complex additives had contributed to NO₂ absorption at different levels, and the bubble was finer and smoother when additives were added into absorption system. Meanwhile, complex additives were more effective additives with absorption efficiency reaching about 95 %. While for individual additives, the maximum absorption efficiency reached around 87 %. The performance of additives varies typically as the following order MgSO₄ < Na₂SO₄ < FeSO₄ < MgSO₄/Na₂SO₄ < FeSO₄/Na₂SO₄. It is possible that SO₄ ²⁻ could preferentially react with Ca²⁺ in CaSO₃ slurry to form CaSO₄, and the existence of SO₄ ²⁻ increases the concentration of dissociated SO₃ ²⁻, The related reactions appear as following [17]. On the other hand, another reason of enhancing the deNO _x performance by additives is that the metal ions Mg²⁺ might react with SO₃ ²⁻ to form other soluble sulfite species, and Na⁺ might improve the gas–liquid mass transfer of absorption system. Fe²⁺ has catalytic effects on the oxidation of aqueous sulfur dioxide solutions, which may also benefit the contribution of soluble SO₃ ²⁻, all individual and complex additives had contributed to shift the reaction (2) to the right, and the SO₃ ²⁻ ions in the system are enriched.

$${\text{Ca}}^{2 + } + {\text{SO}}_{4}^{2 - } \leftrightarrow {\text{CaSO}}_{4} ,$$

(7)

$${\text{CaSO}}_{3} \leftrightarrow {\text{Ca}}^{2 + } + {\text{SO}}_{3}^{2 - } .$$

(8)

Reaction products of NO₂ absorption in CaSO₃ slurry

The potential reaction products of NO₂ absorption in CaSO₃ slurry were NO₃ ⁻ and NO₂ ⁻; using IC analysis, SO₄ ²⁻, NO₃ ⁻ and NO₂ ⁻ were detected. NO₃ ⁻ and NO₂ ⁻ were formed in solution because of the reactions between NO₂ and H₂O and between NO₂ and SO₃ ²⁻, respectively.

To further investigate the products of NO₂ absorption and nitrogen species fate in reaction system, a typical measurement of NO₂ absorption in CaSO₃ solution was carried out (Fig. 7).

Fig. 7

a Typical experiment of NO₂ absorption in CaSO₃ solution; b ion chromatogram of NO₂ absorption in CaSO₃ solution; c nitrogen equilibrium of NO₂ absorption in CaSO₃ slurry; d in situ FTIR spectra of all nitrogen individuals of outlet flue gas. The reaction temperature is 325 K, the CaSO₃ slurry pH is 6.0, the initial CaSO₃ concentration is 0.06 mol L⁻¹, flue gas flow rate was 4.0 L min⁻¹, and the concentrations of O₂ and NO₂ are 5 % and 260 ppm, respectively, Ar as the balance gas, the reaction time is 70 min

As shown in Fig. 7a, the absorption efficiency of NO₂ remained stable at around 70 % before 20 min, however, a sharp decline occurred from 20 to 30 min; thereafter, the NO₂ removal efficiency was originally stabilized. Also, when the reaction time was 15, 30 and 60 min, respectively, the obtained samples of the absorption liquid were tested by the ion chromatograph. The ion chromatogram of absorption solution (reaction time is 30 min) is presented in Fig. 7b. Figure 7c points out that the amount of nitrate and nitrite concentrations showed an increasing trend as time went on; nevertheless, the experimental sum of total nitrate and nitrite concentrations were not equivalent to the calculated values, about 10 % other nitrogen species existed in this system. This is mostly due to the fact that N–S compound and N–S compound decomposed [15, 21]. Figure 7d shows the in situ FTIR spectra of all chemical individuals of outlet flue gas, especially for nitrogen species. It can be found that the absorbance peaks at 2930, 2287, 1720, 1600, 1326, and 887 cm⁻¹ were presented; the bands were assigned to NO₂, N₂O, N₂O₅, NO₂, HNO₃, HNO₃, respectively [22]. According to the IC and in situ FTIR results, the main NO₂ absorption products in CaSO₃ absorption system are nitrate and nitrite in liquid phase; meanwhile, the consideration of gaseous products, including N₂O, N₂O₅, HNO₃ and so on, is indispensable.

Conclusions

A series of experiments were carried out to study the absorption of SO₂ and NO _x in CaSO₃ slurry with or without additives and the formation of liquid and gas products. The experimental results can be summarized as follows:

• 1. NO₂ removal efficiency increases with CaSO₃ concentration at the range from 0 to 0.2 mol L⁻¹, and the highest NO₂ absorption efficiency is about 75 %, while SO₂ removal efficiency will not be affected.

• 2. The existence of O₂ in flue gas is disadvantage to the sulfite concentration in solution in the system. NO₃ ⁻ is the major absorption product in the presence of O₂, while NO₃ ⁻and NO₂ ⁻ concentrations are almost equilibrium in the absence of O₂.

• 3. SO₂ and NO _x absorption in CaSO₃ slurry is strongly related to NO₂/NO ratio. When NO and NO₂ coexist in inlet flue gas, the existence of NO₂ may promote NO absorption in CaSO₃ absorbent, the best NO₂/NO ratio for NO and NO₂ co-absorption is 1/1.

• 4. The individual and complex additives have contributed to NO₂ absorption at different levels, complex additives are more effective additives, and the performance of additives varies typically in the following order MgSO₄ < Na₂SO₄ < FeSO₄ < MgSO₄/Na₂SO₄ < FeSO₄/Na₂SO₄.

• 5. In addition to the liquid phase products, gaseous N₂O₅, N₂O and HNO₃ products in outlet flue gas can be obviously detected. The new reaction of NO _x absorption and conversion in CaSO₃ slurry could be explained.