GB2050325A

GB2050325A - SO2 scrubbing system for flue gas desulfurization

Info

Publication number: GB2050325A
Application number: GB8008551A
Authority: GB
Original assignee: Research Cottrell Inc
Current assignee: Research Cottrell Inc
Priority date: 1979-03-30
Filing date: 1980-03-13
Publication date: 1981-01-07
Also published as: CH638404A5; DE3011592C2; AT379322B; ZA801607B; DK152177B; DK152177C; ES489860A0; JPS55142529A; ATA162880A; SE8002289L; IT1193525B; IT8021045A0; JPS6057364B2; ES8102833A1; CA1129181A; AU5657380A; BE882464A; DK132480A; AU532453B2; FR2452466A1

Abstract

Sulfur dioxide is scrubbed from boiler flue gases in a double-loop alkali scrubber, the first a quencher loop and the second an absorption loop. The reagent flow of the two loops is cycled to the absorber loop and a portion of the make-up water for the quencher loop is received from the absorber loop. By controlling recycled water from a dewatering system and selective utilization of high and low solids streams from the absorber system, the quencher slurry concentration may be controlled and the requirement for new make-up water reduced. <IMAGE>

Description

SPECIFICATION SO2 scrubbing system for flue gas desulfurization I. DESCRIPTION Technical Field This invention is directed to a unique double-loop SO2 scrubbing system for flue gas desulfurization which optimizes use of make-up water while controlling slurry concentration for SO2 scrubbing efficiency and quenching.

Background of Prior Art Scrubbing of boiler flue gases with slurries of limestone (CaC03) or calcined limestone products, lime and hydrated lime, is a known method for the removal of sulfur dioxide (SO2) from these combustion gases. The standard system does, however, require significant amounts of make-up water for operation and therefore increases the total plant requirements for water. As suitable quality water is often available to the plant only in limited quantities, it is essential that the scrubbing system use a minimum of high quality make-up or reused water.

Make-up water is required in sulfur dioxide scrubbing systems to replace water lost principally in two areas: (1 ) water lost through evaporation by the quenching and lowering the temperature of flue gases into the scrubber; and (2) water lost with the discharge of the solid waste product composed of a slurry of unreacted reagent, calcium sulfite hydrates and calcium sulfate hydrates that are discharged from the system. The total make-up water requirements for the system can, therefore, be minimized by reducing these water losses.

Brief Summary of Invention Applicants have discovered that by controlling recycled water from a dewatering system and selective utilization of high and low solids streams from the absorber system, the quencher slurry concentration may be controlled and the requirement for new make-up water reduced. The two-loop process isolates the main absorber system including the demisters, which are prone to scaling and corrosion, from the evaporative quencher portion of the process. All the recycled water is returned to the evaporative quencher loop and none to the loop containing the demisters and the primary absorber sections.

However, under varying SO2 feed rates, the recycled water to the quencher loop may be in excess or insufficient for the evaporative material balance of the quencher loop. To compensate for either imbalance, it is necessary to utilize a separator and flow control between the two loops to decrease or increase the water balance flow to the quencher loop while maintaining the absorber loop at the proper operating balance. Normally, the slurry composition should be controlled to allow greater than 3% reaction products of combined calcium sulfite and calcium sulfate in the quencher loop. Further, to achieve an effective absorption of sulfur dioxide, the calcium base must be maintained above a minimum level. Typically, an absorber operates between 6% and 14% solid content.

The two-loop process, then, permits the operation of the demisters and the primary absorber sections in a complete open loop, whereupon contaminated recycled water is avoided and use of additional demister wash water is maximized. Eliminating recycled water in these sections reduces the encrustations, scale formation and uncontrolled crystallization which hampers continuous operation of the 502 removal system. This also permits the use of less expensive materials which would be required to prevent attack by dissolved chemicals such as chlorides.

The use of recycled water from the dewatering system in the second loop, namely the quencher section, permits the operation of the total system in a closed loop such that all the water available is utilized in the process ratherthan being disposed.

Brief Description of Drawings The invention will be more particularly described with reference to the drawings, wherein: Figure 1 is a simplified flow diagram of the system; Figure 2 is a partial sectional diagrammatic view of a multi-stage quencher-absorber tower useful for carrying out the system of the invention.

Detailed Description of Invention Referring to Figure 1 of the drawings, 10 generally designates a system of the invention and the system includes a multiple-stage quencher-absorber tower 12, to be more fully described with reference to Figure 2, which includes a quencher 14 and an absorber 16.

Arrows 18a, band c designate, respectively, the gas flow to the quencher 14, gas flow from the quencher 14 to the absorber 16 and the SO2-stripped flue gas from the absorber 16.

Other primary components of the system include an absorber tank 20, a quencher tank 22, a dewatering system 24, an absorber separator 26, pumps 28, 30 and 32, each of which has a pump seal water inlet designated 34a, band cfor pumps 32,30 and 28, respectively.

The primary liquid/slurry lines for the system are: a line 36 from the absorber tank 20 to pump 28; a line 38 comprising the primary absorber feed line from the pump 28 to the absorber 16; a secondary absorberfeed line 40 from absorber tank 20 to pump 30; a secondary absorber feed line 42 from pump 30 to absorber 16; a line 44 comprising a branch of line 42 from the secondary absorber feed to the absorber separator 26; a line 46 from the absorber 16 to the absorber tank 20, the demister wash water line 48 for the multi-stage quencher-absorber tower 12; a line 50 the reagent feed line for absorber tank 20; an exit line 52 from the absorber separator 26 to absorber tank 20; exit line 54 from the absorber separator 26; absorber tank (20) overflow line 56 to the quencher tank 22; a line 58 from quencher tank 22 to pump 32; a line 60 comprising the quencher feed line from pump 32 to the quencher 14; a quencher return line 62 from quencher 14to the quencher tank 22; a line 64 comprising the discharge from the quenchertank 22 to the dewatering system 24; a line 66 from the dewatering system 24 to the quencher tank 22; and line 68 comprising the dewatering system blowdown line.

Referring now to Figure 2, the multi-stage quencher-absorber tower 12 has a vertically extending shell 70 with a flue gas inlet 72 adjacent at the lower and a SO2-stripped flue gas outlet 73 at the upper end. Below the flue gas inlet 72 is a sump 22 or quencher tank provided with a sump stirring mechanism generally designated 76.

In Figure 2, the quencher section is generally designated 14 and the absorber section is generally designated 16. The quencher section includes a plurality of headers 80 which are connected to line 60 from the pump 32, with each of the headers being provided with a plurality of spray outlet nozzles 82.

In a preferred embodiment, the quencher section 14 is of the cyclonic type as the flue gases entering inlet 72 are caused to flowtangentially upwardly. Return line 62 depicted in Figure 1 is the return by gravity of the treating fluid from the headers 80 to the quencher tank 22.

Between the quencher section 14 and the absorber section 16 is a gas, liquid bowl separator generally designated 84. This separator 84 collects the water from the demister and the slurry fluid from the absorber and the collected fluid is directed from the separator 84 for return to the absorber tank, via line 46. Above the bowl separator 84 is a plurality of headers designated 86 and 86a, each having connection to lines 38 and 42 comprising the primary and secondary absorber feed lines.

Each of the headers 86 and 86a is provided with a plurality of spray-type outlet nozzles 88 and between the headers 86 and 86a is a conventional packed tower arrangement 90. Above the header 86a is arranged lower demister 92 and upper demister 94.

Wash watç r for the lower demister is provided via header 96 having spray-type outlets 98. The upper demister 94 is also provided with a wash water means including a header 100 provided with spraytype outlets 102.

The absorber separator 26 and the dewatering system 24 designated in Figure 1 may take the form of hydrocyclones, thickeners, centrifuges or vacuum filters.

Referring again to the drawing of the present invention, the two-loop system includes a quencher loop A wherein almost all of the evaporative water losses occur, and an absorber loop B (which includes the demisters 92-94) wherein gases pass first through the quencher loop, then through the absorber loop. Reagent flow is counter-current to the gas flow, passing first through the absorber loop. Solids are removed from the system as follows: Solids products of the reaction between the caicium-based reagent and sulfur dioxide, as well as some unreacted reagent, are fed from the absorber tank 20 of the absorber loop B to the quencher loop A along with some water through line 56 at a rather constant concentration. Slurry is also fed to loop A by line 44, absorber separator 26 and line 54.The solids are then circulated through the quencher loop A wherein more reaction products are formed as the concentration of unreacted reagent decreases. The solids are then discharged from the quencher to the dewatering system 24 and ultimate waste disposal.

Make-up water enters the absorber loop B as (1) water entering with the reagent at 50; (2) small amounts of water for slurry pump packing glands at 34c and b; and (3) demister wash water at 48.

Make-up water enters the quencher loop A as: (1) small amounts of fresh make-up water for slurry pump and agitator packing glands at 34a and 34d, respectively; (2) quencher make-up water (recycled water) at 66, which replaces most of the evaporative losses; and (3) water accompanying the absorber loop discharge solids at 56'.

When the process requirement for quencher make-up water is satisfied by the recycled water generated by the sludge dewatering system and the absorber loop discharge, optimum water utilization is achieved.

Where the recycled water generated exceeds the quencher make-up water requirement, the concentration of solids in the absorber loop discharge slurry must be increased. This is accomplished by standard controls at the absorber separation 26 which direct the high solids stream containing solids content in the range of 10% to 50% to flow through line 54 to mix with the slurry flowing through line 56 while the low solids stream containing solids content in the range of 3% to 10% is to be returned to the absorber tank 20. The absorber loop then operates as an open system discharging high solids content slurry into the quencher loop. Of course, both loops considered together constitute a closed loop system.

For those operating conditions when the recycled water generated is less than the quencher make-up water requirement, it is desirable to increase the water content or decrease the solids content in the absorber discharge by directing the low solids stream exiting from the separator 26 to mix with the absorber tank discharge slurry in line 56'. At the same time, water may be added to the absorber loop as demister wash water relative to the total fresh make-up water requirement.

As described above, it is necessary to vary the amount of water exiting with the absorber loop discharge solids, that is, the concentration of the discharge slurry, without affecting the solids and chemical balance of the absorber loop. This is done through the use of solid liquid separating device 26 and controls which may be commercially purchased.

This device treats a portion of the absorber loop slurry to generate two streams, a high solids stream and a low solids stream. Either or both of these streams can be combined through one or more lines 54 with an appropriate quantity of untreated absorber loop slurry 56 to produce a stream 56' containing the desired concentration in the slurry flowing into quencher tank 22. With this approach, a wide range of solids content is attainable from the absorber discharge stream.

Because plant operating conditions, i.e., load factor and 502 gas content, are constantly varying, the rate of solids generation in the absorber circuit, and the amount of water required to enter the quencher circuit are also changing. To allow the absorber system to react to the process require ments for water versus solids in the slurry, a process signal is utilized. This signal relates the SOP mass flow into the absorber tower, absorber slurry density, or any other process variable which changes with changing SO2 mass flow to the absorber tower to the required concentration of slurry discharge from the absorber loop to the quencher tank. The signal is fed to a controller, not shown, which maintains the concentration of solids discharge to the quencher tank at the desired level.

Claims

CLAIMS 1. A method of scrubbing sulfur dioxide from a gas stream comprising the steps of: (a) quenching the gas stream with a first water slurry containing alkali solids; (b) scrubbing the quenched gas stream with a second water slurry containing alkali solids; (c) maintaining the alkali solids content in the scrubbing slurry at a pre-determined level relative to the sulfur dioxide content in the gas stream by adding alkali solids and water as required; (d) maintaining the alkali solids content in the quenching slurry at a pre-determined level relative to the sulfur dioxide content in the gas stream by selectively withdrawing from the scrubbing slurry, slurry portions having higher or lower alkali solids content than said predetermined level, thereby raising or lowering the alkali solids content in the quenching slurry. 2. The method as defined in claim 1 wherein fresh water and concentrated slurry of alkali solids are introduced into the scrubbing slurry and introduced into the quenching slurry to avoid build-up of dissolved calcium sulfate compounds in the scrubbing slurry and to provide a substantially open loop system. 3. The method as defined in claim 1 or 2 wherein a portion of the said first slurry is withdrawn and water is extracted from said withdrawn slurry and returned to said first slurry. 4. The method as defined in claim 1,2 or 3 wherein the alkali solids in the concentrated slurry introduced into the scrubbing slurry are maintained in the range of 30% to 40% and the alkali solids in slurry withdrawn from the said second slurry are maintained in the range of 5% to 15%. 5. The method as defined in claim 1 wherein the quantity of water added to both the scrubbing loop and the quenching loop is controlled by selectively withdrawing slurry portions from the scrubbing slurry loop having a higher alkali solids content and returning slurry portions to the scrubbing slurry loop having a lower alkali solids content thereby reducing the amount of water required to be added to the scrubbing loop to maintain the solids content at a desired level, and increasing the amount of water added to the scrubbing loop by withdrawing slurry portions having lower alkali solids content and returning slurry portions having higher alkali solids content. 6. The method as defined in claim 5 wherein the scrubbing of the gas stream occurs in a closed vessel in which the gas enters from the bottom and exits from the top and the quenching slurry is sprayed into the gas stream near the bottom of the vessel and the absorber slurry is sprayed into the gas stream near the top of the vessel and each sprayed slurry is collected separately. 7. A method of scrubbing sulfur dioxide substantially as herein described with reference to and as shown in the accompanying drawings. New claims or amendments to claims filed on 20 Auk. 1980 Superseded claims 1 to 7 New or amended claims: CLAIMS

1. A method of operating a two-loop gas stream quencher-scrubber for scrubbing sulfur dioxide from a gas stream comprising the steps: (a) quenching in a first loop the gas stream with a first water slurry containing an alkali; (b) directing the discharge from the quencher to a quencher liquid storage tank; (c) scrubbing in a second loop the quenched gas stream with a second water slurry, isolated from the first water slurry and containing an alkali in a sulfur dioxide absorber; (d) controlling recycled water and selective utilisation of high and low solids streams from a separator by:: (i) separating the water slurry discharges from the sulfur dioxide absorber into a low solids overflow stream and a high solids underflow stream in a liquid-solids concentrator; (ii) directing the high solids portion of the scrubbing slurry to the quencher liquid storage tank; (iii) continuously dewatering a portion of the quencher slurry from the quencher liquid storage tank; and (iv) disposing of the solids from the dewatering step while returning the water to the quencher liquid storage tank.

2. A method as claimed in claim 1 wherein the scrubbed gas stream is passed through a demister; the demister is washed with water and the wash water from the demister is added to the discharge liquid from the scrubber in the second loop.

3. A method as claimed in claim 1 or 2, including directing the low solids overflow stream from step (d) to a scrubbing liquid holding tank; and as needed, adding water and an alkali reagent to said holding tank.

4. A method as claimed in Claim 3 wherein the alkali reagent is lime or limestone.

5. A method as claimed in any preceding claim, wherein the solids content of the water slurry in the sulfur dioxide absorber is maintained between about 6% and 14% solid content.

6. A method as claimed in any preceding claim, wherein the high solids stream from the separation step has a solids content in the range of 10% to 50% and the low solids stream has a solids content in the range of 3% to 10%.

7. A method defined in claim 1 wherein the alkali solids comprise CACAO3 and the solids disposed of in step (iv) are primarily calcium sulfate.

8. A method as claimed in any preceding claim, wherein the high solids stream from step (d) is primarily water and calcium carbonate.

9. A method as claimed in any preceding claim, wherein the high solids underflow stream is primarily water, calcium sulfite and calcium sulfate.

10. A method of operating a two-loop gas stream quencher-scrubber for scrubbing sulfur dioxide from a gas stream comprising the steps of: (a) quenching in a first loop the gas stream with a first water slurry containing alkali solids; (b) scrubbing in a second loop the quenched gas stream with a second water slurry isolated from the first water slurry and containing alkali solids; (c) maintaining the alkali solids content in the scrubbing slurry at a level between 6% and 20% relative to the sulfur dioxide content in the gas stream by adding alkali solids and water as required;; (d) maintaining the alkali solids content in the quenching slurry at a level greater than 3% relative to the sulfur dioxide content in the gas stream by selectively withdrawing from the scrubbing slurry slurry portions having higher or lower alkali solids content than said level, thereby raising or lowering the alkali solids content in the quenching slurry.

11. A method as claimed in claim 10, wherein the alkali solids comprise CaCO3 and fresh water and concentrated slurry of the alkali solids are introduced into the scrubbing slurry and cycled slurry is withdrawn from the scrubbing slurry and introduced into the quenching slurry to avoid build up of dissolved calcium sulfate compounds in the scrubbing slurry and to provide a substantially open first loop.

12. A method as claimed in claim 11, wherein a portion of the said first slurry is withdrawn and water is extracted from said withdrawn slurry and returned to said first slurry.

13. A method as claimed in claim 11 or 12, wherein the alkali solids in the concentrated slurry introduced into the scrubbing slurry are maintained in the range of 30% to 4000 and the alkali solids in slurry withdrawn from the said second slurry are maintained in the range of 5% to 15%.

14. A method as claimed in claim 11, 12 or 13, wherein the alkali solids in the quenching slurry are maintained in the range of 10% to 20To.

15. A method as claimed in claim 14, wheren slurry having a uniform alkali solids content is continuously fed from the scrubbing slurry to the quenching slurry partially to substitute for the slurry lost in the quenching process.

16. A method as claimed in any of claims 10 to 15, wherein a portion of the quenching slurry is withdrawn and dewatered and the water is returned to the quenching slurry while the solids are discarded.

17. A method as claimed in any of claims 10 to 16, wherein the quenching slurry and the scrubbing slurry are circulated in separate scrubbing loop and quencher loop systems.

18. The method as claimed in claim 11, wherein the quantity of water added to both the scrubbing loop and the quencher loop is controlled by selectively withdrawing slurry portions from the scrub bing slurry loop having a higher alkali solids content and returning slurry portions to the scrubbing slurry loop having a lower alkali solids contentthereby reducing the amount of water required to be added to the scrubbing loop to maintain the solids content at a desired level, and increasing the amount of water added to the scrubbing loop by withdrawing slurry portions having lower alkali solids content and returning slurry portions having high alkali solids content.

19. A method as claimed in claim 18, wherein the alkali solids content in the scrubbing slurry is maintained within the range of 5% to 15% and the content in the quencher slurry is maintained within the range of 10% to 20%.

20. A method as claimed in claim 19, wheren the scrubbing of the gas stream occurs in a closed vessel in which the gas enters from the bottom and exits from the top and the quenching slurry is sprayed into the gas stream nearthe bottom of the vessel and the absorber slurry is sprayed into the gas stream near the top of the vessel and each sprayed slurry is collected separately.

21. A method as claimed in claim 20, wherein water is sprayed into the gas stream in the vessel above the scrubbing spray and the sprayed water is collected with the scrubbing slurry.

22. A method of operating a two-loop gas stream quencher-scrubber for scrubbing sulfur dioxide from a gas stream comprising the steps of: (a) passing the gas stream through a vessel from the bottom and out of the top in which are located lower level quencher sprays, middle level scrubbing sprays and upper level demister sprays; (b) quenching the gas stream with a first water slurry containing CaCO3 solids and collecting the quenching slurry in a first separate loop system; (c) scrubbing the quenched gas stream with a second water slurry containing CaCO3 solids and collecting the scrubbing slurry in a second separate loop system isolated from the first loop system; (d) spraying fresh water into the gas stream adjacent the top of the vessel and collecting the water with the quenching slurry;; (e) maintaining the CaCO3 solids content in the scrubbing slurry at a level between 5% and 15% relative to the sulfur dioxide content in the gas stream by adding concentrated slurry of CaCO3 solids as required; (f) continuously withdrawing a portion of the circulated absorber slurry to avoid build-up of dissolved calcium sulfate compounds in the second loop system and introducing such withdrawn portion into the first slurry loop; (g) withdrawing a portion of the quencher slurry, removing the solids from such withdrawn portion and returning the remaining water to the first slurry loop thereby defining a substantially closed loop system; and (h) maintaining the CaCO3 solids content in the quencher slurry at a level greater than 3% relative to the sulfur dioxide content in the gas stream by selectively withdrawing scrubbing slurry portions having higher or lower CACAO3 solids content from the second loop system thereby raising or lowering the CaCO3 solids content in the quenching slurry.

23. A method as claimed in claim 22, wherein the concentrated slurry introduced into the second loop system has an alkali solids content within the range of 30% to 40% and the alkali solids content in the second loop system is maintained within the range of 5% to 15% and the alkali solids content in the first loop system is maintained within the range of 10% to 20%.

24. A method of operating a two-loop gas stream quencher-scrubber substantially as herein described with refernece to and as shown in the accompanying drawing.