54
Energy & Fuels 2004, 18, 54-62
Effect of Mn2+ on Sulfite Oxidation in Limestone Scrubbing Atsushi Tatani* Mitsubishi Heavy Industries, Ltd., Minatomirai, Nishi-ku, Yokohama 220-8401, Japan
Tetsuya Imai Mitsubishi Heavy Industries, Ltd., Nishi-ku, Hiroshima 733-8553, Japan
Yukihisa Fujima Nagoya University, Chikusa-ku, Nagoya 464-8603, Japan Received March 5, 2003. Revised Manuscript Received July 23, 2003
Although it has been already reported that manganese plays an important role in the oxidation of sulfite ion, the influence has not yet been understood exactly and the oxidation rates reported in the existing investigations are much lower than those estimated from the performance of actual FGD systems. Manganese compounds are usually contained in raw limestone as the trace element, and concentration of Mn2+ in the absorption slurry is of several ppm. It was found in this investigation that Mn2+ ion is inevitable for the oxidation of Ca(HSO3)2, the oxidation rate in the homogeneous liquid phase is proportional to the [Mn2+]2 and 10(0.1038pH), and the oxidation rate of Ca(HSO3)2 is about 16 times as high as that of H2SO3. It is further predicted that the limited absorption rate of oxygen into the absorption calcium slurry makes the oxidation rate appear linearly dependent on the partial pressure of oxygen in the flue gas and does not depend on Mn2+ concentration as the Mn2+ concentration is so high that the chemical reaction rate is larger than the oxygen absorption rate into the slurry. The result agrees well with practical experience with FGD systems.
Introduction
1. Wet Limestone Gypsum FGD System
The wet limestone scrubbing system with forced oxidation is one of the most reliable flue gas desulfurization (FGD) processes and is used most commonly all over the world. SO2 in the flue gas is absorbed into powdered limestone slurry, and oxidation takes place with simultaneously absorbed oxygen (O2). Sulfate ion formed reacts with calcium to form calcium sulfate and is removed as byproduct gypsum crystal from the slurry. The performance of the FGD absorber is occasionally insufficient, for which the mechanism has not yet been fully understood. It has been known that Mn2+, contained in raw limestone in a small amount, catalyzes and propels the oxidation extensively, as it dissolves in the slurry. However, it is still impossible to predict accurately the oxidation rate, even in the homogeneous liquid phase, and the rate in connection with the absorption and diffusion of O2 in flue gas into the limestone slurry across the gas-liquid interface of actual FGD system. The purpose of this investigation is to clarify experimentally the influences of the important chemical factors on the sulfite ion oxidation rate and to show that the FGD absorber performance can be predicted with the experimental results as required for the practical engineering.
Figure 1 is the flowchart of a typical wet limestone scrubbing FGD system with forced oxidation in the absorption tower tank. Flue gas comes in contact with the absorption slurry in the 4-m-high grid-packed tower. SO2 gas is absorbed in the slurry which flows down along the grid surface in wetted-wall manner at the temperature of 50 °C. Sulfite ion is formed in the absorption slurry and further oxidized partly to sulfate by the dissolved O2 of both the excess air blown in the absorption tank and the flue gas in the grid bed. The slurry is collected in a tank at the tower bottom, and air (oxygen) is blown into the slurry to oxidize all the sulfite ions in the absorption tower tank. Powdered limestone slurry is fed into the tank in order to keep the slurry weakly acidic, about 5 in pH, for the calcium sulfate precipitation as gypsum crystals (CaSO4‚2H2O) and to compensate for the loss of calcium ion out of the slurry. The gypsum crystals are collected in the separator and the filtrate is returned into the tank. The limestone slurry concentration is generally about 10 wt %. The flue gas and the slurry flow down through the grid-packed tower in about 1 s and 10 s, respectively. The mean slurry residence time in the tank is about 300 s which is usually fed at the rate of 7-14 L per 1 m3 of N of flue gas.
* Corresponding author. E-mail:
[email protected].
10.1021/ef0300500 CCC: $27.50 © 2004 American Chemical Society Published on Web 11/15/2003
Effect of Mn2+ on Sulfite Oxidation in Limestone Scrubbing
Figure 1. The wet limestone gypsum FGD system.
2. Reactions in the Absorption Tower Reactions in the tower are given by eqs 1-5.
SO2 + H2O f H+ + HSO3-
(1)
CaCO3 + 2H+ + 2HSO3- f Ca(HSO3)2 + CO2 + H2O (2) Ca(HSO3)2 + O2 f CaSO4 + 2H + + SO42+
CaCO3 + 2H + SO4
2-
(3)
f CaSO4 + H2O + CO2 (4)
CaSO4 + 2H2O f CaSO4‚2H2O
(5)
The SO2 equilibrium partial pressure at the surface of the slurry is related strictly to the HSO3- concentration and pH which are governed by the oxidation and neutralization reactions of eqs 2 and 3. Therefore, reaction 3 should be understood for the appropriate design and operation of an FGD absorber.1 Although sulfite ion concentration (bisulfite ion as well as sulfite ion are called sulfite ion in this paper) in the slurry increases to about 3 mM (mmol/L) by absorbing SO2 in flue gas during the flow down along the grid surfaces, it reacts with limestone or CaCO3, added to neutralize the slurry to form liquid Ca(HSO3)2. Sulfite ion is oxidized partially in the grid and fully in the tank. Raw material limestone contains manganese compounds as an impurity with an average concentration 0.005% (as MnO). Since the limestone slurry concentration is adjusted to about 10 wt %, Mn2+ ion concentration in the slurry is around 4 ppm (weight) assuming it to be well dissolved in the acidic slurry according to the following equation:
MnOx + 2H+ f Mn2+ + H2O (x ) 1 for the simplicity) (6) Huss et al.2 reported that sulfite ion cannot be oxidized in solution without such metal catalysts as (1) Muramatsu, K.; Shimizu, T.; Shinoda, N.; Tatani, A. Development of Mitsubishi Wet Flue Gas Desulfurization System. Chem. Econ. Eng. Rev. 1984, 16, 15-22. (2) Huss, A., Jr.; Lim, P. K.; Eckert, C. A. On the “Uncatalyzed” Oxidation of Sulfur(IV) in Aqueous Solutions. J. Am. Chem. Soc. 1978, 100, 6252-6253.
Energy & Fuels, Vol. 18, No. 1, 2004 55
Mn2+ and Fe2+. Coughanowr and Krause3 and PasiukBronikowska et al.4-6 reported also that Mn2+ catalyzes the oxidation of sulfite ion. However, their oxidation rate (about 0.002 M/h) of the sulfite, including several ppm of Mn2+ ion, was much lower than the estimated (0.1 M/h) from the performance of an FGD system.1 They ignored the difference. Ulrich et al.7 made an experimental investigation of the catalytic role of Mn2+ in the oxidation of sulfite under conditions similar to that in a typical FGD absorber which had already been in use, but with Mn2+ concentration much higher than that in the absorption slurry of the actual FGD system. Lancia et al.8,9 pointed out that the situation in the absorption slurry is too complex in their elaborate investigations of the influences of various parameters on the oxidation rate. They developed a computation model to predict the O2 absorption rate into calcium sulfite solution as a function of the Mn2+ concentration. However the model gave an oxidation rate too low as compared to that in an actual FGD system. Concerning the oxidation rate, various numerical values have been reported for the powers in the following expression (eq 7) from experiments under conditions subtly different from each other:
oxidation rate ) kCjSmOxn
(7)
where C, S, and Ox mean concentration of catalyst, sulfite ion, and dissolved O2, respectively. Table 1 shows the results of the existing investigations on sulfite oxidation kinetics. All the experiments were made under conditions different from the actual FGD system condition as noted in Table 1. It seems that the difference among these experiments in such essential reaction-rate controlling factors as temperature, pH, and the manner of mixing reactants results in considerably wide distribution of powers of j and m. It was reported that n is normally 0 and increases to 1 as the overall reaction rate becomes controlled by the absorption rate of oxygen into reaction liquid.4,8 The numbers in the right row, reaction rate, were chosen from the plots of the existing literature at the experimental condition supposedly nearest to an actual FGD system, but at each experimental temperature, pH, and 2 ppm Mn2+ ion concentration. The reaction rate by Lancia et al.8 was measured without Mn2+ ion. The existing investigations did not give any definite number at a given definite common condition. The row implies that the figures are significantly lower than that estimated from the performance of an actual FGD system, which may be attributed to (3) Coughanowr, D. R.; Krause, F. E. The Reaction of SO2 and O2 in Aqueous Solutions of MnSO4. Ind. Eng. Chem. Fundam. 1965, 4, 61-66. (4) Pasiuk-Bronikowska, W.; Bronikowski, T. The rate equation for SO2 autoxidation in aqueous MnSO4 solutions containing H2SO4. Chem. Eng. Sci. 1981, 36, 215-219. (5) Pasiuk-Bronikowska, W.; Ziajka, J. Oxygen Absorption in Aqueous Sulphur Dioxide Solutions. Chem. Eng. Sci. 1985, 40, 1567-1572. (6) Pasiuk-Bronikowska, W.; Ziajka, J. Kinetics of Aqueous SO2 Oxidation at Different Rate Controlling Steps. Chem. Eng. Sci. 1989, 44, 915-920. (7) Ulrich, R. K.; Rochelle, G. T.; Prada, R. E. Enhanced Oxygen Absorption into Bisulphite Solutions Containing Transition Metal Ion Catalysts. Chem. Eng. Sci. 1986, 41, 2183-2191. (8) Lancia, A.; Musmarra, D.; Pepe, F. Uncatalyzed Heterogeneous Oxidation of Calcium Bisulfite. Chem. Eng. Sci. 1996, 51, 3889-3896. (9) Lancia, A.; Musmarra, D.; Pepe, F.; Prisciandaro, M. Model of oxygen absorption into calcium sulfite solutions. Chem. Eng. J. 1997, 66, 123-129.
56
Energy & Fuels, Vol. 18, No. 1, 2004
Tatani et al.
Table 1. The Results of the Existing Investigations on Sulfite Oxidation Kineticsa literature
sulfite
T [°C]
pHb
jb
m
n
reactor
rateb,c [M/h]
Coughanowr3 Huss11 Pasiuk5 Ulrich7 Pasiuk6 Lancia8 this study
H2SO3 H2SO3 H2SO3 H2SO3 CaSO3 Ca(HSO3)2 H2SO3 Ca(HSO3)2
25 25 26 25-75 26 25-63 50 50
1-4 1.5 4-5 4.5-6 2.5-3.5 5 5-3
2 1-2 2 1/2 1/2 2 -
0 0-1 0 0 3/2 3/2 0 0
0 0 1 0-1 0 0 0 0
pool, O2 room pool, O2 room pool, O2 room pool, O2 room pool, O2 room pool, O2 bubble pipe flow, full liquid pipe flow, full liquid
0.002 0.002
