ANTI-FOAMING MAGNESIUM HYDROXIDE SUSPENSION

Abstract

The present invention relates to an aqueous magnesium hydroxide suspension comprising: a) at least 40 wt. %, relative to the total weight of the suspension, of magnesium hydroxide particles; b) from 0.001 to 5 wt. % relative to the total weight of the magnesium hydroxide of at least one dispersant and c) from 0.001 to 5 wt. %, relative to the total weight of the suspension, of at least one anti-foaming agent. The present invention further relates to the use of this aqueous suspension for the treatment of gas washing water in an on-board wet or semi-wet flue gas desulfurization system, the treatment of effluent at a municipal or industrial wastewater treatment plant, in industrial transformation processes or bioprocesses involving a fermentation or respiration process, or in the paper production process or in agriculture.

Claims

1-11. (canceled)

12. An aqueous magnesium hydroxide suspension comprising: a) at least 40 wt. % relative to the total weight of the suspension, of magnesium hydroxide particles; b) from 0.001 to 5 wt. % relative to the total weight of the magnesium hydroxide, of at least one dispersant, and c) from 0.001 to 5 wt. % relative to the total weight of the suspension, of at least one anti-foaming agent.

13. The aqueous suspension according to claim 12, wherein the at least one dispersant is a polyether polycarboxylate or a polyacrylate.

14. The aqueous suspension according to claim 12, wherein the at least one anti-foaming agent is a dispersion of organic particles in an organic oil.

15. The aqueous suspension according to claim 12, wherein the magnesium hydroxide particles have a volume average size D.sub.90 less than 200 ?m measured by a laser particle size analyzer.

16. The aqueous suspension according to claim 12, wherein the magnesium hydroxide particles have a volume average size D.sub.50 less than 40 ?m measured by a laser particle size analyzer.

17. The aqueous suspension according to claim 12, the viscosity of which measured at 1.7 s.sup.?1 at 20? C. with a Brookfield viscometer, is less than 1000 mPa.Math.s.

18. The aqueous suspension according to claim 12, wherein it is stable in the rest phase at room temperature.

19. The aqueous suspension according to claim 12, wherein the specific surface area of the magnesium hydroxide particles is comprised between 5 and 100 m.sup.2/g.

20. The aqueous suspension according to claim 12, wherein the at least one anti-foaming agent does not comprise silicone oil.

21. A method of treatment of gas washing water in an on-board wet or semi-wet flue gas desulfurization system or of effluent at a municipal or industrial wastewater treatment plant comprising the addition of the aqueous suspension according to claim 12.

22. An alkaline anti-foaming agent comprising the aqueous suspension according to claim 12.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0062] FIG. 1 represents the pH curve as a function of time of acidified water in which a standard magnesium hydroxide suspension (MH1, MH2 and MH3) or a magnesium hydroxide suspension according to the invention (MH1AM4, MH2AM4 and MH3AM4) is added to neutralize the effluent under the conditions of example 2.1.

[0063] FIG. 2 represents a block diagram of an on-board desulfurization system.

EXAMPLES

Example 1: Testing Different Anti-Foaming Agents

[0064] The tests were carried out in order to come as close as possible to the foaming conditions encountered in an on-board wet or semi-wet flue gas desulfurization system.

[0065] The material used is: [0066] acidified sea water (pH=2) (EDMA); [0067] a magnesium hydroxide suspension (MH1) containing: [0068] 53 wt. % relative to the total weight of the suspension of magnesium hydroxide particles with granulometric distribution D.sub.10=0.67 ?m, D.sub.50=2.18 ?m and D.sub.90=10.34 ?m (measured by the laser particle size analyzer Mastersizer 2000 from Malvern Instruments after dilution of the suspension to 20000.sup.th and passage through the particle size analyzer), and [0069] 0.25 wt. % relative to the weight of particles of a Polyether polycarboxylate dispersant in aqueous solution having a solid content of 40%; [0070] a standard magnesium hydroxide suspension (MH2) containing: [0071] 58 wt. % relative to the total weight of the suspension of magnesium hydroxide particles with granulometric distribution D.sub.10=2.36 ?m, D.sub.50=16.2 ?m and D.sub.90=66.4 ?m (measured by the laser particle size analyzer Mastersizer 2000 from Malvern Instruments after dilution of the suspension to 20000.sup.th and passage through the particle size analyzer), and [0072] 2.3 wt. % relative to the weight of particles of a sodium polyacrylate dispersant in aqueous solution having a solid content of 40%; [0073] a magnesium hydroxide suspension (MH3) containing: [0074] 53 wt. % relative to the total weight of the suspension of magnesium hydroxide particles with granulometric distribution D.sub.10=0.96 ?m, D.sub.50=4.86 ?m and D.sub.90=37.42 ?m (measured by the laser particle size analyzer Mastersizer 2000 from Malvern Instruments after dilution of the suspension to 20000.sup.th and passage through the particle size analyzer), and [0075] 5.0 wt. % relative to the weight of particles of a dispersant belonging to the category of lignosulfonates and mixtures thereof in aqueous solution having a solid content of 40%; [0076] 5 different anti-foaming agents; [0077] Plastic stirring container % filled: [0078] Volume: 80 mL [0079] Cylinder with a height: h=8.3 cm.

[0080] The shape of the container and the amount of liquid added have been selected so as to accentuate foaming phenomena.

[0081] The different anti-foaming agents used are indicated in Table 1 below.

TABLE-US-00001 TABLE 1 Anti- foaming agent Supplier Chemical family AM1 ArrMarz (USA) Polyester(s) polyol(s) esterified with natural fatty acids AM4 SNF (FR) (Flofoam D) Hydrocarbon dispersion of organic components AM5 ArrMarz (USA) Polyester(s) polyol(s) esterified with natural fatty acids AM6 SNF (FR) (Flofoam H) Mixtures of polyether polyols esterified with natural fatty acids AM7 Kurita (FR) Polydimethylsiloxanes

[0082] The test methodology is as follows: [0083] 1.20 mL of EDMA is added to the container. [0084] 2. An amount of 6 to 24 mg/L of magnesium hydroxide suspension is then added to neutralize the water to pH 6. [0085] 3. An amount of 5 ppm of the amount of liquid present in medium of the anti-foaming agent is then added to the system. [0086] 4. The container is sealed then shaken manually for 30 seconds. [0087] 5. The height of the foam formed is measured as a % of the total height of the cylinder. [0088] 6. A second dose of 15 ppm relative to the amount of liquid present in the medium of anti-foaming agent is then added and then steps 4 and 5 are repeated. [0089] 7. A third dose of 30 ppm relative to the amount of liquid present in the medium of anti-foaming agent is then added and then steps 4 and 5 are repeated.

[0090] The results are gathered in the following table 2.

TABLE-US-00002 TABLE 2 total dosage in ppm relative to Anti- the amount of height of Suspen- foaming liquid present residual sion agent in the medium foam in % Comments MH1 AM1 5 8.3 Low unstable froth - Very rapid coalescence MH1 AM1 15 0 No foaming MH1 AM4 5 0 No foaming MH1 AM5 5 8.3 Light non-homogeneous froth - Rapid coalescence MH1 AM5 15 0 No foaming MH1 AM6 5 0 No foaming MH1 AM7 5 8.3 Light non-homogeneous froth - Medium coalescence MH1 AM7 15 8.3 Light non-homogeneous froth - Medium coalescence MH1 AM7 30 0 No foaming MH2 AM4 5 0 No foaming MH3 AM4 5 0 No foaming

[0091] A lot more anti-foaming agent AM7 (30 ppm) must be used to obtain no foaming. It is therefore the least effective anti-foaming agent. Although all other anti-foaming agents had an effect, anti-foaming agents AM4 and AM6 demonstrated optimal effectiveness in this test. Indeed, they prevent the appearance of foaming for the minimum dose added to the effluent.

[0092] Therefore, the experiments with these agents were repeated using process water (pH=5, presence of sulfates and residues of incomplete combustion of heavy fuel oil) previously acidified to evaluate whether the effectiveness of these agents is maintained in a context closer to reality.

[0093] The results are summarized in Table 3 below.

TABLE-US-00003 TABLE 3 total dosage in ppm relative to Anti- the amount of height of Suspen- foaming liquid present residual sion agent in the medium foam in % Comments MH1 AM4 5 0 No foaming MH1 AM6 5 5 Slight peripheral froth - Fairly rapid coalescence MH2 AM4 5 0 No foaming MH3 AM4 5 0 No foaming

[0094] Therefore, it appears that the anti-foaming agent AM4 is the agent which has the most anti-foaming power for the system in question. The other examples were carried out with this anti-foaming agent.

Example 2: Testing Reactivity and Stability of the Suspension According to the Invention Containing the Anti-Foam AM4

[0095] These tests aim at evaluating the anti-foaming properties of the suspension according to the invention containing the anti-foam AM4 while quantifying the impact of the addition of this anti-foam on its reactivity and stability.

Example 2.1: Reactivity of the Suspension

[0096] The material used is: [0097] drinking water acidified at pH=3; [0098] a standard magnesium hydroxide suspension (MH1) containing: [0099] 53 wt. % relative to the total weight of the suspension of magnesium hydroxide particles with granulometric distribution D.sub.10=0.67 ?m, D.sub.50=2.18 ?m and D.sub.90=10.34 ?m (measured by the laser particle size analyzer Mastersizer 2000 from Malvern Instruments after dilution of the suspension to 20000.sup.th and passage through the particle size analyzer), and [0100] 0.25 wt. % relative to the weight of particles of a Polyether polycarboxylate dispersant in aqueous solution having a solid content of 40%; [0101] a magnesium hydroxide suspension according to the invention (MH1AM4) containing: [0102] 53 wt. % relative to the total weight of the suspension of magnesium hydroxide particles with granulometric distribution D.sub.10=0.67 ?m, D.sub.50=2.18 ?m and D.sub.90=10.34 ?m (measured by the laser particle size analyzer Mastersizer 2000 from Malvern Instruments after dilution of the suspension to 20000.sup.th and passage through the particle size analyzer), 0.25 wt. % relative to the weight of particles of a Polyether polycarboxylate dispersant in aqueous solution having a solid content of 40% and [0103] 1000 ppm of anti-foaming agent AM4; [0104] a standard magnesium hydroxide suspension (MH2) containing: [0105] 58 wt. % relative to the total weight of the suspension of magnesium hydroxide particles with granulometric distribution D.sub.10=2.36 ?m, D.sub.50=16.2 ?m and D.sub.90=66.4 ?m (measured by the laser particle size analyzer Mastersizer 2000 from Malvern Instruments after dilution of the suspension to 20000.sup.th and passage through the particle size analyzer), and [0106] 2.3 wt. % relative to the weight of particles of a sodium polyacrylate dispersant in aqueous solution having a solid content of 40%; [0107] a magnesium hydroxide suspension according to the invention (MH2AM4) containing: [0108] 58 wt. % relative to the total weight of the suspension of magnesium hydroxide particles with granulometric distribution D.sub.10=2.36 ?m, D.sub.50=16.2 ?m and D.sub.90=66.4 ?m (measured by the laser particle size analyzer Mastersizer 2000 from Malvern Instruments after dilution of the suspension to 20000.sup.th and passage through the particle size analyzer), and [0109] 2.3 wt. % relative to the weight of particles of a sodium polyacrylate dispersant in aqueous solution having a solid content of 40% and 1000 ppm of anti-foaming agent AM4; [0110] a standard magnesium hydroxide suspension (MH3) containing: [0111] 53 wt. % relative to the total weight of the suspension of magnesium hydroxide particles with distribution D.sub.10=0.96 ?m, D.sub.50=4.86 ?m and D.sub.90 =37.42 ?m (measured by the laser particle size analyzer Mastersizer 2000 from Malvern Instruments after dilution of the suspension to 20000.sup.th and passage through the particle size analyzer), and [0112] 5.0 wt. % relative to the weight of particles of a dispersant belonging to the category of lignosulfonates and mixtures thereof having a solid content of 40%; [0113] a magnesium hydroxide suspension according to the invention (MH3AM4) containing: [0114] 53 wt. % relative to the total weight of the suspension of magnesium hydroxide particles with granulometric distribution D.sub.10=0.96 ?m, D.sub.50=4.86 ?m and D.sub.90=37.42 ?m (measured by the laser particle size analyzer Mastersizer 2000 from Malvern Instruments after dilution of the suspension to 20000.sup.th and passage through the particle size analyzer), and [0115] 5.0 wt. % relative to the weight of particles of a dispersant belonging to the category of lignosulfonates and mixtures thereof having a solid content of 40% and [0116] 1000 ppm of anti-foaming agent AM4; [0117] a pH meter.

[0118] The test methodology is as follows: A dose of 1700 mg/L of magnesium hydroxide suspension is added to neutralize the effluent. The evolution of the pH of the solution is followed as a function of time and plotted.

[0119] The results are shown in FIG. 1. Considering the results, the reactivity of a suspension according to the invention containing 1000 ppm of anti-foaming agent AM4 is little impacted by its presence. The kinetic behavior of the two suspensions becomes identical again after 3.5 minutes of reaction. The times allocated for alkalis to react within an industrial system are orders of magnitude greater.

Example 2.2: Suspension Stability

[0120] The material used is: [0121] a standard magnesium hydroxide suspension (MH1) as described in Example 2.1; [0122] a magnesium hydroxide suspension according to the invention (MH1AM4) containing: [0123] 53 wt. % relative to the total weight of the suspension of magnesium hydroxide particles with granulometric distribution D.sub.10=0.67 ?m, D.sub.50=2.18 ?m and D.sub.90=10.34 ?m (measured by the laser particle size analyzer Mastersizer 2000 from Malvern Instruments after dilution of the suspension to 20000.sup.th and passage through the particle size analyzer), 0.25 wt. % relative to the weight of particles of a Polyether polycarboxylate dispersant in aqueous solution having a solid content of 40% and [0124] 200 to 500 ppm of anti-foaming agent AM4; [0125] a standard magnesium hydroxide suspension (MH2) as described in example 2.1; [0126] a magnesium hydroxide suspension according to the invention (MH2AM4) containing: [0127] 58 wt. % relative to the total weight of the suspension of magnesium hydroxide particles with granulometric distribution D.sub.10=2.36 ?m, D.sub.50=16.2 ?m and D.sub.90=66.4 ?m (measured by the laser particle size analyzer Mastersizer 2000 from Malvern Instruments after dilution of the suspension to 20000.sup.th and passage through the particle size analyzer), and [0128] 2.3 wt. % relative to the weight of particles of a sodium polyacrylate dispersant in aqueous solution having a solid content of 40% and [0129] 200 to 500 ppm of anti-foaming agent AM4; [0130] a standard magnesium hydroxide suspension (MH3) as described in example 2.1; [0131] a magnesium hydroxide suspension according to the invention (MH3AM4) containing: [0132] 53 wt. % relative to the total weight of the suspension of magnesium hydroxide particles with granulometric distribution D.sub.10=0.96 ?m, D.sub.50=4.86 ?m and D.sub.90=37.42 ?m (measured by the laser particle size analyzer Mastersizer 2000 from Malvern Instruments after dilution of the suspension to 20000.sup.th and passage through the particle size analyzer), and [0133] 5.0 wt. % relative to the weight of particles of a dispersant belonging to the category of lignosulfonates and mixtures thereof having a solid content of 40% and [0134] 200 to 500 ppm anti-foaming agent; [0135] Brookfield RV series viscometer.

[0136] The viscosity of the suspensions tested is measured at 1.7 s.sup.?1 and at 20? C.

[0137] The results are presented in Table 4 below.

TABLE-US-00004 TABLE 4 AM4 dose Viscosity No Suspension (ppm) (mPa .Math. s) Comments 1 MH1 0 180 Original standard suspension MH1 2 MH1AM4 200 180 No macroscopic changes visible to the naked eye 3 MH1AM4 300 170 No macroscopic changes visible to the naked eye 4 MH1AM4 400 150 No macroscopic changes visible to the naked eye 5 MH1AM4 500 150 No macroscopic changes visible to the naked eye 6 MH2 0 130 Original standard suspension MH2 7 MH2AM4 200 130 No macroscopic changes visible to the naked eye 8 MH2AM4 300 120 No macroscopic changes visible to the naked eye 9 MH2AM4 400 120 No macroscopic changes visible to the naked eye 10 MH2AM4 500 120 No macroscopic changes visible to the naked eye 11 MH3 0 100 Original standard suspension MH3 12 MH3AM4 200 100 No macroscopic changes visible to the naked eye 13 MH3AM4 300 110 No macroscopic changes visible to the naked eye 14 MH3AM4 400 110 No macroscopic changes visible to the naked eye 15 MH3AM4 500 110 No macroscopic changes visible to the naked eye

[0138] The viscosity of the suspension according to the invention remains within the preferential viscosity scale obtained after formulation.

Example 2.3: Foaming Test with the Anti-Foaming Agent Integrated into the Suspension

[0139] The material used is: [0140] IL beaker; [0141] High shear rate mixer; [0142] a standard magnesium hydroxide suspension (MH1) as described in example 2.1; [0143] a magnesium hydroxide suspension according to the invention (MH1AM4) containing: [0144] 53 wt. % relative to the total weight of the suspension of magnesium hydroxide particles with granulometric distribution D.sub.10=0.67 ?m, D.sub.50=2.18 ?m and D.sub.90=10.34 ?m (measured by the laser particle size analyzer Mastersizer 2000 from Malvern Instruments after dilution of the suspension to 20000.sup.th and passage through the particle size analyzer), 0.25 wt. % relative to the weight of particles of a Polyether polycarboxylate dispersant in aqueous solution having a solid content of 40% and [0145] 300 ppm of anti-foaming agent AM4; [0146] a magnesium hydroxide suspension without dispersant (MH1) containing: [0147] 53 wt. % relative to the total weight of the suspension of magnesium hydroxide particles with granulometric distribution D.sub.10=0.67 ?m, D.sub.50=2.18 ?m and D.sub.90=10.34 ?m (measured by the laser particle size analyzer Mastersizer 2000 from Malvern Instruments after dilution of the suspension to 20000.sup.th and passage through the particle size analyzer); [0148] a dispersant solution (D1) containing Polyether polycarboxylate in aqueous solution concentrated at 2 wt. % relative to the weight of water; [0149] a standard magnesium hydroxide suspension (MH2) as described in example 2.1; [0150] a magnesium hydroxide suspension according to the invention (MH2AM4) containing: [0151] 58 wt. % relative to the total weight of the suspension of magnesium hydroxide particles with granulometric distribution D.sub.10=2.36 ?m, D.sub.50=16.2 ?m and D.sub.90=66.4 ?m (measured by the laser particle size analyzer Mastersizer 2000 from Malvern Instruments after dilution of the suspension to 20000.sup.th and passage through the particle size analyzer), and 2.3 wt. % relative to the weight of particles of a sodium polyacrylate dispersant in aqueous solution having a solid content of 40% and [0152] 300 ppm of anti-foaming agent AM4; [0153] a dispersant solution (D2) containing sodium polyacrylate in aqueous solution concentrated at 2 wt. % relative to the weight of water; [0154] a standard magnesium hydroxide suspension (MH3) as described in example 2.1; [0155] a magnesium hydroxide suspension according to the invention (MH3AM4) containing: [0156] 53 wt. % relative to the total weight of the suspension of magnesium hydroxide particles with granulometric distribution D.sub.10=0.96 ?m, D.sub.50=4.86 ?m and D.sub.90=37.42 ?m (measured by the laser particle size analyzer Mastersizer 2000 from Malvern Instruments after dilution of the suspension to 20000.sup.th and passage through the particle size analyzer), and 5.0 wt. % relative to the weight of particles of a dispersant belonging to the category of lignosulfonates in aqueous solution having a solid content of 40% and [0157] 300 ppm of anti-foaming agent AM4; [0158] a dispersant solution (D3) containing sodium lignosulfonate in aqueous solution concentrated at 4 wt. % relative to the weight of water.

[0159] The test methodology is as follows: [0160] Dilution of the suspension evaluated by 10. The pH of this solution is such that: pH=10; [0161] 400 mL of suspension or solution are agitated vigorously for 10 minutes; [0162] The incremental volume rate is measured and determined in volume %. [0163] The evolution of the incremental volume after the end of agitation is evaluated qualitatively

[0164] The results are presented in Table 5 below.

TABLE-US-00005 TABLE 5 Volume increment No Test typology (%) Evolution of foam 1 Diluted MH1 standard 8% Stable foam suspension Slow coalescence 3 Standard MH1 suspension 7.5%.sup. Stable foam aged one year Slow coalescence 4 Dispersant solution D1 2% Light foaming Fairly rapid coalescence 5 Diluted MH1 suspension 0% No foaming without dispersant 6 Diluted MH1AM4 suspension 2% Unstable foam according to the invention Rapid coalescence 7 Diluted MH1AM4 suspension 2% Unstable foam according to the invention Rapid coalescence aged for 2 months 8 Diluted standard MH2 4% Light foaming suspension Fairly rapid coalescence 9 Dispersant solution D2 2% Light foaming Fairly rapid coalescence 10 Diluted MH2AM4 suspension 0% No foaming according to the invention 11 Diluted standard MH3 4% Light foaming suspension Fairly rapid coalescence 12 Dispersant solution D3 2% Light foaming Fairly rapid coalescence 13 Diluted MH3AM4 suspension 0% No foaming according to the invention

[0165] The results show that the anti-foaming agent AM4 is effective in reducing the foaming phenomenon caused by the simultaneous presence of solid particles and dispersant molecules in a system which may correspond to an overdose situation in magnesium hydroxide suspension.

[0166] What is more, there is no aging effect over the first 2 months of the anti-foaming properties of the suspension according to the invention.

Example 3: Full-Scale Test of the Suspension According to the Invention Containing the Anti-Foam AM4

[0167] The protocol is as follows: Ship A is a container carrier 400 m long and 60 m wide. Consuming heavy fuel oil (>2 wt. % of Sulfur), it complies with the regulations of the International Maritime Organization (IMO) by carrying out washing of its exhaust gases, wet desulphurization with Close Loop operating cycles.

[0168] Ship A therefore needs an alkaline agent in order to be able to neutralize the acidity provided by the absorption of SO.sub.2 in its washing water. The alkaline chosen is a magnesium hydroxide suspension.

[0169] During operation, an overflow of the washing solution storage tank (labeled holding tank) was observed in the diagram in FIG. 2. The origin of the causes of this overflow was investigated and determined.

[0170] Indeed, the storage tank is equipped with vents allowing the depressurization of the tank following the arrival of air in the latter due to transport in pipes that are not completely submerged. The extraction capacity of these vents is such that they are capable of extracting foam, much less dense than the carrier liquid, to the outside of the tank. This foam overflows and settles on the deck of the ship. Once deposited, it coalesces and gives way to a polluted liquid having a risk for passengers and staff on board.

[0171] Before using a magnesium hydroxide suspension according to the invention as a means for remediation, several solutions were investigated without giving convincing results. These solutions correspond to modifications of the on-board desulfurization system: [0172] Modification of the water level setpoint within the tank. [0173] Reduction in the velocity of the washing solution arriving within the tank while ensuring the preserved effectiveness of the treatment. [0174] Modification of the regulation algorithm at the pH thresholds in order to minimize overdose problems.

[0175] Although the implementation of these solutions yielded encouraging results, they were not sufficient to eliminate the foaming problem in all usage scenarios of the onboard desulfurization system. Overflow problems were always observed when the main scrubber treated the exhaust gases coming from the main motor when the latter was operating at more than 20% of its maximum speed.

[0176] Thus, the use of a magnesium hydroxide suspension with anti-foaming properties was considered, said suspension containing: [0177] 53 wt. % relative to the total weight of the suspension of magnesium hydroxide particles with granulometric distribution D.sub.10=0.67 ?m, D.sub.50=2.18 ?m and D.sub.90=10.34 ?m (measured by the laser particle size analyzer Mastersizer 2000 from Malvern Instruments after dilution of the suspension to 20000.sup.th and passage through the particle size analyzer), 0.25 wt. % relative to the weight of particles of a Polyether polycarboxylate dispersant in aqueous solution having a solid content 40% and [0178] 300 ppm anti-foaming agent AM4.

Test Results

[0179] The use of the magnesium hydroxide suspension according to the invention allowed to eliminate the foaming problem not only for previously problematic conditions of use (motor load>20%) but also in extreme scenarios of stress on the main motor (up to motor load>65%).

[0180] Additionally, the use of this suspension had no significant impact on: [0181] The amount of magnesium hydroxide suspension consumed; [0182] The pH levels reached in the tank; [0183] The efficiency of the desulfurization process and [0184] The smooth running of post-use water treatment operations.

[0185] It is therefore possible to conclude that the magnesium hydroxide suspension according to the invention effectively responds to the problem of foaming during the treatment of gas washing water in an on-board wet or semi-wet flue gas desulfurization system.