WO2001041902A1

WO2001041902A1 - A process to increase the oxidation rate of dissolved sulphur dioxide in seawater

Info

Publication number: WO2001041902A1
Application number: PCT/NO2000/000401
Authority: WO
Inventors: Otto Morten Bade
Original assignee: ABB Fläkt AB
Priority date: 1999-12-08
Filing date: 2000-12-01
Publication date: 2001-06-14
Also published as: NO996030D0; NO996030L; AU1742301A; NO314129B1; MY143670A

Abstract

Process to increase the rate of oxidation of dissolved sulphur dioxide in seawater, where the sulphur dioxide rich seawater is produced in an absorber unit where sulphur dioxide is scrubbed from flue gas with seawater, thereby performing the oxidation with molecular oxidation in the presence of iron or soluble iron salts added to the seawater.

Description

A process to increase the oxidation rate of dissolved sulphur dioxide in seawater

The present invention relates to a process of removing sulphur dioxide from flue gas by using seawater to scrub the sulphur dioxide from the gas, and specifically to a method of increasing the rate of oxidation of dissolved sulphur dioxide to sulphate in the seawater.

The Flakt-Hydro process for flue gas desulphurization (FGD) utilises seawater as the once-through absorbent. Seawater is by nature alkaline with a pH of 8,0 to 8,3. The flue gas containing sulphur dioxide is scrubbed in an absorber with seawater. The absorbed sulphur dioxide needs to be oxidised to sulphate before discharge into the sea for environmental reasons. Accordingly, the sulphur containing water is transferred to an aeration basin in which air and fresh seawater is added, and the oxidation of dissolved sulphur dioxide to sulphate is carried out. The aeration basin may either be located in the bottom of the absorber, in the cooling water channel, in interconnecting pipes, or as a separate plant.

The rate of oxidation is strongly temperature dependent, and especially for cold water cases the oxidation is slow. To ensure complete oxidation, the residence time and thus the aeration basin volume has to be large. The aeration basin is an expensive part of the FGD unit.

Generally, the mechanisms and kinetics concerning sulphite and bisulphite oxidation are not as yet entirely clear. Although the oxidation with molecular oxygen can be expressed in the simple way:

1 catah st S0X² + -0-, → SO₄ ^2"

2 -

1 catah s t

HSO; +-0., → H⁺ + so₄ ^2~

2 ^'

the reactions are highly complex.

It is well known that catalytic autoxidation of dissolved sulphur dioxide in water occur. Ions of transition metals, such as Mn, Fe, Co and Cu are described in the literature as catalysts for sulphite oxidation. Also some heterogeneous catalysts are known.

In fresh water, the reaction rate of the Fe-catalysed oxidation process in general exhibits a bell-shaped pH dependence with a maximum rate around pH 2-4. At higher pH values, dissolved iron is not active because of the very fast precipitation of Fe³⁺ hydroxides.

This invention describes a method to increase the rate of oxidation of dissolved sulphur dioxide in seawater, and thus being able to decreases the size of the aeration basin. The inventor of this process has observed that the pH influence on Fe-catalysed oxidation in seawater is different from fresh water. In seawater, the oxidation has to be carried out in the pH range between 4 and 7, and the concentration of iron ions needed is much lower. The reason for this difference is that seawater contains large amounts of Cf ions, which stabilises the active Fe-complexes toward precipitation during the time needed for oxidation. With the same amount of added iron (Fe ⁺, Fe³⁺), the oxidation rate in seawater is found to be more than one order of magnitude higher than in fresh water.

By adding 10 -20 ppb of Fe^" or Fe to sulphite containing seawater, the rate of S(IV) oxidation is about 5 times higher than oxidation rates observed in seawater with no catalyst added. Discharge of seawater with this low concentration of iron has no negative impact on the environment.

The iron may be added to the seawater in the form of soluble iron salts, for instance as sulphate or chloride. The iron salts may be added to the absorber effluent, or to the seawater upstream the absorber or upstream the aeration basin. The oxidation state of added iron (Fe²⁺ or Fe³⁺) has no influence on the rate of S(IV) oxidation provided the catalyst is added in acidic effluent. However, only ferrous iron should be used for the seawater upstream the absorber or the aeration basin. The reason for this is the very fast precipitation of Fe ⁺ in neutral or basic solutions. Alternatively, the iron ions may be added to the system through dissolution of iron materials in an acid solution or in the acidic absorber effluent. By using the principles of cathodic protection and an anode of ferrous material (iron, steel, etc), the rate of dissolution of iron ions to the seawater or to the absorber effluent can be controlled by the applied current. The novelty of this pro cess is the very small amount of Fe ions needed, because of the very active complexes of iron formed in seawater in the pH range 4-7. This observation has not been reported before.

Several oxidation experiments have been performed in a small-scale pilot plant. Ferric or ferrous iron (in the form of soluble salts) is added to a reaction basin containing fresh seawater acidified with dissolved S0 . Only a very slow oxidation is observed when the pH of the solution is below 4, however, the oxidation reaction starts and proceeds very rapid when more seawater is added to the reaction basin and the pH of the solution is in the range 4-7. To achieve this specified pH, the quantity of added seawater is dimensioned so that the naturally occurring bicarbonate in seawater is sufficient to act as a buffer. It is also possible to increase the alkalinity of the seawater, and thereby the absorption capacity of sulphur dioxide, by adding lime or limestone or solutions of these to the seawater.

Air or oxygen is bubbled through the basin in order to ensure the presence of dissolved oxygen during the oxidation reaction. pH and dissolved oxygen are continuously monitored during the reaction, while samples for sulphite analyses are taken at intervals of a few seconds. The sulphite concentrations were determined by iodine/thiosulphate titration, using starch as indicator. The chemicals used were analytical reagent grade. The basin was cleaned with HC1 between runs to minimise the effect of catalysts from previous experiments.

The results show that the rate of S(IV) oxidation is about 5 times faster in experiments with 20 ppb Fe ions than with no catalyst added.

Claims

P a t e n t C l a i m s

1. Process to increase the rate of oxidation of dissolved sulphur dioxide in seawater, where the sulphur dioxide rich seawater is produced in an absorber unit where sulphur dioxide is scrubbed from flue gas with seawater, characterised by per- forming the oxidation with molecular oxidation in the presence of iron or soluble iron salts added to the seawater.

2. Process according to claim 1 , characterised by adding a soluble Fe²⁺ or Fe ⁺ containing salt to the water upstream the aeration basin, in the amount of 1 -200 μg Fe/1 liquid.

3. Process according to claim 1 , characterised by adding a soluble Fe²⁺ or Fe³⁺ containing salt to the liquid inside the absorber or to the absorber effluent, in the amount of 1-200 μg Fe/1 liquid.

4. Process as claim 1 , characterised by using corrosion and dissolution of iron material as a method for adding dissolved iron to the liquid phase in the absorber, to the absorber effluent or to the seawater upstream the absorber or aeration basin.

5. Process as claim 1 , characterised by using the principles of cathodic protection to dissolve iron to the seawater in the absorber sump or upstream or downstream the absorber, and by using a material containing iron compounds as the anode material.

6. Process as claim 1 , characterised by that pH during the oxidation reaction should be in the range 4-7.

7. Process as claim 1 , characterised by adding air or oxygen to the liquid phase during the oxidation reaction.