Removal of SO₂ and NO from flue gas by wet scrubbing using an aqueous NaClO₂ solution

Abstract

This study used a NaClO₂/NaOH solution as the additive/absorbent to determine the extent of NO_x removal in a wet scrubbing system. A combined SO_x/NO_x removal system was also tested. The experiments were performed in a bench-scale spraying sieve tray wet scrubber in a continuous mode. The operating variables included NO and SO₂ concentrations, L/G ratio, molar ratio, and initial pH.

The results of the individual DeNO_x experiments show that the maximum DeNO_x efficiencies ranged from 3.1 to 12.6%. The results of the combined DeSO_x/DeNO_x experiments show that the maximum DeNO_x and DeSO_x efficiencies ranged from 36.6 to 71.9% and from 89.4 to 100.0%, respectively. The major parameters affecting NO_x removal efficiencies are the L/G ratio and the dosage of additive. The major parameter influencing DeSO_x efficiencies is the L/G ratio.

Introduction

Acid precipitation is a current air pollution problem caused mainly by SO₂ and NO_x. Conventionally, engineers use wet scrubbing SO₂; the most widely used process is flue gas desulfurization (FGD) process. Although the wet FGD system has experienced high SO₂ removal efficiencies, that is not so for NO_x. The reason for this process failure is that NO, which accounts for more than 90% of NO_x in the flue gas, is quite insoluble in water. Since the wet scrubbing system currently dominates the FGD market, a minor adjustment of the system may work for the combined SO_x/NO_x removal system and should be cost-effective. The DeSO_x wet scrubbing SO_x removal system is not new to us. It has, however, many unknowns if it is to be used as a DeNO_x system. In general, additives have to be added to the scrubbing system to convert relatively water insoluble NO to soluble NO₂ which can then be removed by alkaline absorbents.

Possible NO oxidants are ClO₂ or O₃ [1], [2]. However, these additives are quite expensive as well as very dangerous in equipment operating in the gas phase. Therefore, chemical reagents added to the liquid phase have been widely used lately. There are many reagents such as FeSO₄/H₂SO₄, Fe(II) EDTA, KMnO₄/NaOH, NaClO₂/NaOH, Na₂S/NaOH, Na₂S₂O₄/NaOH, H₂O₂, Na₂SO₃, FeSO₄/Na₂SO₃, CO(NH₂)₂, NaOH, Na₂CO₃ and P₄ (yellow phosphorus) that have been tested for NO_x absorption [3], [4], [5], [6]. Of this group, NaClO₂ was the most effective reagent [7], [8], [9], [10], [11], [12], [13], [14], [15].

The absorption of NO_x in NaClO₂ solution was studied by Teramoto et al. [7] and Sada et al. [8], [9], [10] in the late seventies. Teramoto et al. investigated the effect of various operating conditions on the absorption rates in mixed aqueous solutions of NaClO₂ and NaOH, using a semi-batch agitated vessel with a flat gas–liquid interface. They found that the absorption of NO proceeded in the region of the fast pseudo mth order reaction and the absorption rate was not affected by liquid-phase mass transfer coefficient. The NO concentration dependence of the absorption rate was more pronounced in the higher concentration range than that in the lower concentration range. The absorption rate increased greatly with NaClO₂. However, the absorption rate decreased markedly with an increase in NaOH. The effect of temperature on the absorption rate was found to be much greater in the lower concentration range on NO.

Sada et al. [8] also performed a series of kinetic studies using NaClO₂ as the additive in a stirred tank absorber. The concentration of NO in the gas stream ranged from 0.8 to 15%. The rate of reaction was found to be second order in NO and first order in NaClO₂. They also did some work on the absorption of dilute NO concentrations (<1%) with the same NaClO₂ concentrations as above. The results showed that the order of reaction in NO varied from 2 to 1 because of the lower interfacial concentration of NO [9]. In both cases, they claimed that the oxidation power of NaClO₂ increased with a decrease in pH value of the solution. However, the reaction product, NO₂, may desorb from the solution in the case with less OH⁻. Therefore, the addition of OH⁻ to the solution is required in order to fix NO₂ as NO₃⁻. That suggested that the reaction proceeds as follows:

$$\text{4}\text{NO+3ClO}{}_{\mathrm{<mtext>2</mtext>}}{}^{\mathrm{-}}\text{+4}\text{OH}{}^{\mathrm{-}}\text{→4}\text{NO}{}_{\mathrm{<mtext>3</mtext>}}{}^{\mathrm{-}}\text{+3}\text{Cl}{}^{\mathrm{-}}\text{+2}\text{H}{}_{\mathrm{<mtext>2</mtext>}}\text{O}$$

In addition to the above kinetic studies, some researchers published their performance investigations on the absorption of NO_x. The experimental equipment, operating conditions and results of these studies [11], [12], [13], [14], [15] are summarized in Table 1. Chan [11] found that the liquid and gas flow rates had little effect on NO_x removal, while increasing liquid chlorite concentrations enhanced NO_x absorption. Absorption of NO_x tended to be totally gas-film controlled at all NO_x input levels. The experimental results of Yang and co-workers [12], [13] showed that NO can be quantitatively oxidized by NaClO₂ in an aqueous solution. During scrubbing, NO was oxidized to NO₃⁻ and ClO₂⁻ was converted to Cl⁻. Due to the production of HNO₃, the pH value decreased sharply from 10 to 3 within minutes of operation. Low concentrations of NaOH increased the effectiveness of NO absorption in the NaClO₂ aqueous scrubbing solution by 7%, while higher NaOH concentrations decreased or inhibited the absorption. They also suggested that the reactions proceed under acidic condition as follows:

$$\text{4}\text{NO+3NaClO}{}_{\mathrm{<mtext>2</mtext>}}\text{+2}\text{H}{}_{\mathrm{<mtext>2</mtext>}}\text{O}\text{→}\text{4HNO}{}_{\mathrm{<mtext>3</mtext>}}\text{+3}\text{NaCl}$$

$$\text{5}\text{NO+4HCl}\text{→4}\text{ClO}{}_{\mathrm{<mtext>2</mtext>}}\text{+5}\text{NaCl+2H}{}_{\mathrm{<mtext>2</mtext>}}\text{O}$$

$$\text{5NO+3ClO}{}_{\mathrm{<mtext>2</mtext>}}\text{+4}\text{H}{}_2\text{O}\text{→}\text{5HNO}{}_{\mathrm{<mtext>3</mtext>}}\text{+}\text{NaCl}$$

Brogren et al. [14] found that the pH value of the absorbing liquid had a significant impact on the absorption efficiency. The major fraction of the nitrogen oxides was absorbed via the hydrolysis of N₂O₃ and N₂O₄. Hsu et al. [15] performed the NO absorption study using a packed column. Their results indicated that the NO oxidation efficiency and removal efficiency could reach 98.8 and 61.5%, respectively.

So far, no one has attempted to determine the removal efficiencies of NO and SO₂ simultaneously by using a NaClO₂ solution in a spraying sieve tray wet scrubbing system. In this work, we used a low concentration NaClO₂ solution under acidic condition in a bench-scale spraying sieve tray column to investigate the effect of various parameters on SO₂ and NO_x absorption efficiency.