Study on NO₂ absorption by ascorbic acid and various chemicals

Abstract

Study on NO₂ absorption aimed at seeking a better NO₂ absorption chemical at pH 4.5~7.0 for application to existing wet flue gas desulfurization (FGD). The results from the double-stirred reactor indicated that ascorbic acid has very high absorption rate at this pH range. The rate constant of ascorbic acid reaction with NO₂ (0~1000×10⁻⁶ mol/mol) is about 3.54×10⁶ mol/(L·s) at pH 5.4~6.5 at 55 °C.

INTRODUCTION

Nitrogen oxide (NO_x) is one of the main air pollutants found in the flue gases from chemical and power plants belching acid rain and photochemical smog. Over 90 percent of all man-made nitrogen oxides that enter our atmosphere are produced by the combustion of various fuels. Compared with the advanced stage of SO₂ gas removal technologies development, the removal of NO_x gases is still in the initial stages of development as roughly 90 to 95 percent of NO_x emitted in combustion processes appears in the form of NO which is comparatively inactive and insoluble. To reduce the generation and emission of NO_x to the atmosphere, it is necessary to use low nitrogen fuel, low NO_x burners and implement more stringent air pollutant emission standards, it is necessary to setup and operate NO_x control devices for further reduction of pollutions. Some processes have been discussed, such as SCR (selective catalytic reduction), SNCR (selective non-catalytic reduction), NSCR (non-selective catalytic reduction) and adsorption. Wet FGD (flue gas desulfurization) scrubbers are very efficient to remove SO₂ from flue gas, but cannot remove NO because of its low solubility in aqueous solution. Research efforts to modify existing wet FGD processes for simultaneous control of SO₂ and NO_x emissions led to several new approaches to enhance NO_x absorption in scrubbing liquors.

One promising approach to simultaneously remove SO₂ and NO_x from flue gas is the combination of NO oxidation to NO₂ and conventional wet scrubbing. The possible NO oxidants include KMnO₄ (Kobayashi et al., 1977), hydrogen peroxide (Thomas and Vandescheren, 1998), NaCl₂O (Chien and Chu, 2000), P₄ (Chang and Lee, 1992), etc. The NO oxidation product, NO₂, can be absorbed by several chemical reagents, although most chemical reagents for NO₂ absorption are effective only at high pH range. In this work, our research is aimed at finding a better NO₂ absorption chemical effective at pH 4.5~7.0, which is the application pH range for existing popular wet limestone/lime FGD.

EXPERIMENTAL DETAILS

Chemicals

L-ascorbic acid (99%), thiourea (99%), 2.4-pentanedione (99%), 2.3-pentanedione (97%), barbituric acid (99%), oxalic acid (98%), glyoxal (40 wt%), di(ethline glocal) (99%), were all obtained from Shanghai Chemical Reagent Co., China and used without further purification. The gases used included 5.0% NO₂ in N₂ and 99.99% N₂. All other chemicals were analytical grade reagents.

Apparatus and procedure

Experiments on NO₂ absorption were carried out in a highly characterized double-stirred reactor with a planar gas-liquid interface, which was mentioned in our previous papers (Shi et al., 1996; 1997). The double-stirred reactor had a gas-liquid interface that was planar to a close approximation. The reactor was 80 mm in diameter and equipped with four vertical baffles and two stirring blades. A water jacket of the reactor is used to maintain the desired temperature by circulating water from a constant temperature water bath. The reactor had continuous gas flow rate of 1000 ml/min. Five percent (V/V) of NO₂ was diluted by carrier nitrogen gas. The inlet concentration of NO₂ in this experiment ranged from 0~1000×10⁻⁶ mol/mol. All the experiments were conducted with liquid stirring speed of 150 r/min and gas stirring speed of 900 r/min. The experimental setup includes a flue gas simulation system, a double-stirred reactor, and analysis system shown in Fig.1.

Fig. 1.

Experimental setup for measurement of NO absorption

1, 2: NO₂, N₂ cylinder; 3: Attemperator; 4: Gas flowmeters; 5: Gas/liquid control valve; 6: Driving motors; 7: Stirred cell reactor; 8: Stirring speed controller; 9: Cold trap; 10: Gas analyzer; 11: Entrance of liquid

The liquid phase was a deionized water solution of chemical reagents. The solution pH was adjusted by adding small amounts of concentrated NaOH or H₂SO₄. The volume of the aqueous solution in the reactor was always 310 ml. For the case of SO₃ ²⁻ solution preparation, the deionized water was boiled for 10 min to remove the dissolved oxygen, then cooled down no oxygen atmosphere. All the experiments were performed at 55 °C (normal temperature of conventional wet FGD) in stirring reactor and operated in batch mode. The gas film mass transfer coefficient of the apparatus (K _GNO2) was 2.04×10⁻⁵ kmol/(m²·s) and the liquid-side mass transfer coefficient (K _KNO2) was 3.96×10⁻⁶ kmol/(m²·s) (Shi et al., 1996).

The inlet and outlet gas concentration of NO₂ were measured with a chemiluminescent NO_x analyzer with independent outputs for NO, NO₂, and NO_x. A cold trap was used to remove moisture before the gas entered the analyzers. During the experiments, NO₂, and NO_x in the simulated flue gas were examined.

RESULTS AND DISCUSSION

Comparison of NO₂ absorption rate by various chemical reagents

To search for ideal chemical to absorb and reduce NO₂, numerous chemicals were tested in the stirring reactor. The NO₂ absorption rates of various chemicals, including thiourea, urea, sodium sulfide, 2.4-pentanedione, 2.3-pentanedione, barbituric acid, oxalic acid, sodium sulfite, sodium thiosulfate, L-ascorbic acid, glyoxal, di(ethline glocal) and water, were determined with different pH. The results of comparison of the rate of NO₂ absorption into these chemical reagents with various pH values are depicted in Fig.2.

Fig. 2.

Effect of the aqueous solution pH on the rate of NO₂ absorption by various chemicals

Gas flowrate: 1000 ml/min; NO: 500×10⁻⁶ mol/mol; C _B0: 0.05 mol/L; T: 55 °C

Effect of ascorbic acid concentration on the absorption rate

Fig.3 shows the effect of the square root of ascorbic acid concentration on the absorption rate. The absorption rate increases linearly increasing ascorbic acid concentration (C _B0) from 0~25 mmol/L. On the basis of mass transfer model with chemical reaction, there should be a linear relationship between NO₂ absorption rate and the square root of the ascorbic acid concentration in a second order reaction (first order with respect to ascorbic acid and first order with respect to NO₂). The prediction was confirmed in the range of low concentration of ascorbic acid, 0~25 mmol/L. When concentration was higher than 25 mmol/L, the absorption rates remained almost steady at 1.4×10⁻⁵ mol/(m²·s), which probably indicated that the reaction was a fast gas-liquid surface reduction during the concentration range.

Fig. 3.

Effect of ascorbic acid concentration on the absorption rate

T: 55 °C; pH: 5.4~6.0; NO: 500×10⁻³ mol/mol; Gas flow rate: 1000 ml/min

Chemical and physical properties of NO₂

The kinetics and chemistry of NO₂ absorption into aqueous solution is complicated. Schwartz (1983) reviewed the chemical equilibria in detail, including the Henry’s law (solubility) equilibria of the reactive oxides NO₂ and N₂O₄, etc. Henry’s coefficient, H _NO2, and the rate of the second-order hydrolysis reaction, k ₂, are two key parameters required for characterizing heterogeneous process involving NO₂ (g). However, even after numerous studies, uncertainty still exists in the values of these two parameters. Literature values range from 7×10⁻⁸ to 2×10⁻⁷ mol/(L·Pa) for H _NO2 and from 10⁷ to 10⁸ mol/(L·s) for k ₂ at 25 °C. These uncertainties are mainly due to the complexity of NO₂ aqueous interaction.

In our research, the diffusion coefficient of dissolved NO₂(D _NO2) was taken to be 3.28×10⁻⁹ m²/s (Shen and Rochelle, 1998; Wilke and Chang, 1955) at 55 °C. Henry’s coefficient of NO₂, H _NO2, was taken to be 1.4×10⁻⁷ mol/(L·Pa) at 25 °C in recent research results of Cheung et al.(2000). No existing data on H _NO2 at 55 °C was found a way must be found to estimate it.

The solubility of gases decreases with increasing temperature. According to the relation of Henry’s coefficient and the temperature, H _NO2 at 55 °C could be estimated by the following Clausius-Clapeyron equation (Manahan, 1999):

(1)

where C ₁ and C ₂ denote the gas concentration in water at absolute temperatures of T ₁ and T ₂, respectively; ΔH is the heat of solution; and R is the gas constant. The value of R is 8.314 J/(K·mol), which gives ΔH units of J/mol. However, due to the complexity and uncertainty of ΔH, we have to take the data from Cheung et al.(2000),H _NO2=1.4×10⁻⁷mol/(L·Pa) at 20 °C and 2.3×10⁻⁷ mol/(L·Pa) at 0 °C, and the Clausius-Clapeyron equation to extrapolate Henry’s constant at temperature of 55 °C. Henry’s constant of NO₂ at 55 °C is estimated to be 6.8×10⁻⁸ mol/(L·Pa) by the above means.

Estimate of NO₂ reaction rate constant

According to Danckwerts theory (Danckwerts, 1970), in fast pseudo-m-order reaction, there is linear relationship between the absorption rate and square root of the ascorbic acid concentration if C _B0 the concentration of ascorbic acid in the bulk liquid, is much larger than the NO₂ concentration at the interface, that is: N _A∝C _B0. To a close approximation, the absorption rate for a pseudo-mth order reaction is given by

(2)

, where D is the diffusivity in liquid phase, m²/s; m is the reaction order; N is the absorbing rate; k is the rate constant; A is dissolved gas (NO₂); B is the liquid-phase reactant; i is the gas-liquid interface and 0 is initial value.

Fig.3 shows a linear relationship between the absorption rate and square root of the ascorbic acid concentration of 0~25 mmol/L, which verifies that the value of m in Eq.(2) should be 1.0. Hence, this absorption is the fast pseudo-first order reaction when the ascorbic acid concentration is 0~25 mmol/L. The reaction rate constants between NO₂ and ascorbic acid can be obtained from Eq.(2) using the experimental data and the above estimated chemical and physical parameters. The calculating results on reaction rate constants are shown in Table 1. The rate constant of ascorbic acid reaction with NO₂ is about 3.54×10⁶ mol/(L·s) at pH 5.4~6.5 at 55 °C.

Table 1.

Values of NO₂ reaction rate constant, k ₂

Reagent	Investigator	k2, mol/(L·s)
Ascorbic acid (pH 5.4~6.5)	This work	3.54×106
SO32−/HSO32− (pH 5.4~6.5)	This work	1.05×106
Sodium thiosulfate (pH 5.4~6.5)	This work	3.37×105
SO32−	Shen and Rochelle (1998)	1.12×106
HSO32−	Shen and Rochelle (1998)	2.8×104
S2O32−	Shen and Rochelle (1998)	5.4×103
SO32−	Takeuchi et al.(1977)	6.6×105

The chemistry of NO₂ absorption into aqueous phase of SO₃ ²⁻ or other reagents is complicated. The reduction of NO₂ was found to be first order with respect to SO₃ ²⁻ and first order with respect to NO₂ by Takeuchi et al.(1977). Shen and Rochelle (1998) indicated that the reactions between NO₂ and sulfite (SO₃ ²⁻), bisulfite (HSO₃ ²⁻) and thiosulfate (S₂O₃ ²⁻) were first order in NO₂ concentration. Their rate constants at 55 °C in this work and in other reports are also listed in Table 1.

CONCLUSION

The results from the double-stirred reactor indicated that ascorbic acid has very high absorption rate at the wet limestone/lime FGD pH 5.0~7.0, the order of NO₂ absorption rate is as follows, ascorbic acid>2.4-pentanedione>SO₃ ²⁻/HSO₃ ²⁻>S₂O₃ ²⁻>2.3-pentanedione>barbituric acid>thiouera>oxalic acid>glyoxal>citrate>H₂O. The calculated results on reaction rate constant of ascorbic acid reaction with NO₂ is about 3.54×10⁶ mol/(L·s) at for pH 5.4~6.5 at 55 °C.