IE44986B1 - Process and apparatus for reducing the so2 content of a hot flue gas - Google Patents

Description

i ^4S86 CASE 569,5a.

This invention relates to a process and an apparatus for reducing the sulfur dioxide content of a hot flue gas. ' i > A tubular rotating scrubber for removing sulfur dioxide from stack gases by tumbling contact of a lime slurry has 5 been described in the 'following U.S.A. publications and various oral presentations of similar contexts (1) Lewis, G.J., "Mew Concept for Scrubbing Gases", Brick and Clay Record, June 1973? ---- 10 (2) Lewis, C.J^ "Lime, Limestone Employed in Mew Gas Scrubbing Concept", 1»it and guarrj?, July 1973? (3) Lewis, C.Jo, "Aqueous Lime Scrubber Simplifies Sulfur Dioxide Removal", 15 Rock Products, July 1974» The Lewis system effectively removes sulfur dioxide from a flue gas, but the waste sludge product normally contains from about 50 to 80 per cent by weight of free moisture. This sludge poses a waste solids disposal problem because of its 20 semi-solid nature and substantial quantity. An aim of this invention is to provide a process and a scrubber for removing ; ? sulfur dioxide from flue gas which minimizes the amount of free moisture in the waste product.

According to the invention, there is provided a pro-25 cess for reducing the sulfur dioxide content of a hot flue gaS, comprising continuously contacting the flue gas with a counter-current stream of lime slurry in contiguous scrubbing and dry-' ing zones whereby sulfur dioxide is removed from the flue gas -2- 44986 by said lime slurry in the scrubbing zone and the waste sludge product formed is simultaneously dewatered by evaporation in the drying zone downstream of said scrubbing zone by contact with the incoming flue gas.

We also provide a flue gas scrubber for reducing the sulfur dioxide/consent of a hot flue gas, comprising an elongated tubular drum rotatably drivable about its longitudinal axis and having a scrubbing zone extending inwardly from one end thereof contiguous with a drying zone extending inwardly from 10 the other end thereof, said drum including slurry retaining means at the interface of said zones, a gas inlet disposed at the drying zone end for introducing hot flue gas into the drum for flow through it from the drying zone end to the scrubbing zone end, a gas outlet at the scrubbing zone end, slurry feeding 15 means on the drum for introducing lime slurry into the drum for flow through it from the scrubbing zone end to the drying zone end,, tumbling means in the scrubbing zone for intimately mixing the slurry with the gas to cause the slurry to combine with sulfur dioxide components of the gas and be transformed into a 20 waste sludge product, and collecting means at the drying zone end of the drum for conducting the waste sludge product out of the drum after it has been dewatered by evaporation from contact in the drying zone with the hot flue gas introduced at the drying zone end and flowing through the drying zone.

Preferably, the moisture content of the waste sludge product is reduced to about 5 to 25, especially about 5 to 15, per cent by weight. Agitating and substantially smooth noduliz-ing portions of the drying zone may advantageously be separated by a low annular baffle or dam. An advantageous example of the 30 scrubber of this invention has a drum which is about 75 to 100 -3- / feet long with a diameter of about 10 to 15 feet? the scrubbing zone is about 30 to 50 feet long with the remainder of the drum comprising the drying zone, In.order that the invention may be more fully under-S stood, it will now be described in Conjunction with the accompanying drawings/ in whichf Fig. 1 is a diagrammatic side view in elevation partially in cross section of a scrubber installation which incorporates one embodiment of a scrubber capable of carrying.out 10 the process of this invention; Fig. 2 is a cross-sectional view taken through Fig. 1 along the line 2-2; and Fig. 3 is another cross-sectional view taken through Fig. 1 along the line 3-3.

In Fig. 1 is show flue gas scrubber system 30 incor porating an elongated tubular drum 32 mounted to rotate about a slightly inclined longitudinal axis 34. The inclination is, for example, 1/8 inch par foot. Tubular drum 32 is about 88 feet long, with an internal diameter of about 11 feet. The scrub-20 bing section 36 is about 40 feet long and a drying section 38 comprises the remainder of drum 32, The drying-section 38 in-{ eludes an agitating portion 40 and a substantially smooth' noduliaing portion 42 adjacent the exit 44. Drum 32 is rotated by pinion gear 46 engaged by bull gear 48 mounted to the outside 25 of the drum 32. Electric motor 50 connected to pinion gear 46 supplies the rotating force. The drum 32 is rotatably supported or cradled in tires 47 mounted to engage rim 49, Although two such tire and rim support systems are shown in Fig. 1, more such support systems could be used depending on the- length of 30 drum to be supported and rotated. -4- «49ββ Lime, or limestone, slurry 19 is introduced in the direction of arrow 52 through supply pipe 54 into entrance 56 of scrubbing section 36, Slurry 19 is prepared in a mixing tank 57 into which lime or limestone is fed from storage tower 5 58 and water is introduced through piping 60 in amounts regulat ed by automatic control valve 62, Flue gases designated by arrow 16 are introduced through inlet duct 64 and bonnet 66 into the end 68 of drying section 38 and flow through drum 32. The treated gases 70 flow out of the end 72 of scrubbing section 36 10 and through bonnet 74, outlet duct 76 and fan 78 to exhaust to atmosphere. S02 gas sampler and control unit 77 is connected to exhaust gas duct 76. This unit controls the sulfur dioxide content in the treated gas that is escaping into the atmosphere by increasing or decreasing the amount of slurry volume through 15 regulation of slurry flow control valve 53 mounted in slurry supply line 54.

Scrubbing section 36 includes tumbling means 80 including a cage 82 to which chain assembly 84 is Connected. Cage 82 is removably inserted within scrubbing section 36 to facil-20 itate cleaning. Chain assembly 84 is connected to cage 82 for tumbling and agitating slurry 19 into intimate contact with flue gas 16 to remove the sulfur dioxide from it and to form waste product 86, which passes through drying section 38 as later described. Cage 82 includes longitudinal ribs which are formed as 25 scoops 88, as shown in Fig. 2, which disperse and distribute slurry 19 throughout the interior of scrubbing section 36 during drum rotation. Scoops 88 and chain assembly 84 tumble lime slurry 19 within the flow of flue gas 16 to intimately mix and react them with each other to scrub and substantially remove 30 sulfur dioxide components from gas 16 before it flows out of end -5- 44®8δ 72 of scrubbing -section 36 in the form .of -treated or scrubbed...... gas 70 which is exhausted to the atmosphere. The scrubber system 30 reduces the sulfur dioxide content, for example, from about 2000-4000 ppm.in flue gas 16 to about 500 ppm or less in 5 treated gas 70. \ Scrubbing section 36 is separated from drying section 38 by annular dam 92 which ensures that a substantial height of slurry 19 is retained within scrubbing section 36o Drying section 38 includes an agitating portion 40 and 10 a substantially smooth or nodulizing portion- 42 separated from each other by another annular dam 98. The nodulizing portion 42 retains waste product 86 within drying section 38 for a sufficient time to be nodulized and to be dried by flue gas 16, which flows through the entire length of drum 32. Hot flue gases 16 15 are removed from stack 104 by inlet duct 64 connected below transverse sealing baffle 106. Stack 104 is, for example, the flue for boiler 108 and conducts a flow of flue gases having a substantial sulfur dioxide content when coal or fuel oil of high sulfur content is burned in boiler 108.

An anchored agitating chain assembly 110 is mounted in agitating portion 40 of drying section 38. Agitation chain 110 is of an inclined configuration and is connected, for example, as shown in Figs. 1 and 3 to help agitate wet waste product 86 throughout the flow of hot flue gas 16 to help lower the moisture 25 content of waste product 86 before it flows out of exit 44 of drying section 38. The chain assembly 110 keeps scale from building up on the inside of drum 32 by its constant innerwall contact. In addition, the chains are used as supplemental heat conductors to the waste product 86 since they will conduct the 30 heat from the flue gas 16. Waste product 86 has a free moisture \ -6- 1 J 44986 content of, for example» from about 50 to 80 per cent by weight as it passes from scrubbing section 36 to drying section 38, The hot flue gases 16 introduced through duct 64 are at about 400°?= and remarkably reduce the moisture content of waste 5 product 86 passing through drying section 38, The water transfer to the flue gas 16 as it passes through drying section 38 remarkably reduces the temperature of the flue gas 16 before it reaches scrubbing section 36, Temperature sensor 112 disposed within drying section 10 38 is connected through control system 114 to spray pipe 116 in nodulizing portion 42 to maintain dried waste product 102 at a temperature of about 170°F, and a free moisture content of about 5 to 25, especially 5 to 15, per cent by weight as it is discharged in the direction of arrow 118 into collecting hopper 120.

Waste product 102 is removed through collecting and discharging system 122 and transported away from the scrubbing system by truck 124o The only controlled variable in the drying section 38 is the free moisture content of the discharged solids 102. The 20 desired free moisture content is estimated to be between about 5 and 25 weight per cent. A moisture content of less than 5 par cent tends to create fugitive dusting in the solids handling systems. A moisture content much in excess of 25 per cent causes the solids td be excessively fluid in addition to increasing the 25 amount of solids for disposal.

The free moisture content of the discharged solids 102 is controlled by an empirical correlation between the moisture content and the solids temperature. Solids temperature at the point 44 at which solids leave the drying section 38 is monitor-30 ed by an infrared temperature sensor 112, An output signal from -7- this sensor controls the addition of water to the drying section 38 via a spray pipe 116° Thus, once the entire scrubber system has been startea, a solids discharge temperature corresponding to the desired free moisture content is selected.

This moisture content is then maintained by the automatically controlled addition of water to the drying section 38.

The flue gas scrubber as described herein, because of its method of operation in which hot flue gas is continuously contacted with a countercurrent stream of lime slurry in con-10 tiguous scrubbing and drying zones, is remarkably efficient, not only for removing the sulfur dioxide from the flue gas but also from the standpoint of energy conservation. The sulfur dioxide is effectively removed in a scrubbing zone by the lime slurry and the waste product sludge which is formed is simultaneously 15 dewatered in the drying zone, downstream of the scrubbing zone in the direction of flow of the lime slurry, upon contact with t the incoming hot flue gas. The flue gas enters the drying zone J i at from about 350 to 550°F. and leaves the scrubbing zone at from about 120 to 18b°F. Fly ash present in the incoming hot 20 flue gas is also removed, in the drying as well as the scrubbing zone, and discharged^ in the dewatered sludge,‘Which leaves the drying zone at from about 100 to 200aF. Thus a readily disposable waste product, suitably with a free moisture content of about 5 to 25 weight per cent, is obtained with no additional 25 energy consumption.

This waste product is also readily adaptable for use as landfill, A prior: process for producing landfill from scrubber sludge is described in a brochure entitled "Poz-O-Tee©' distributed by XU Conversion Systems, Xnc., Philadelphia, 30 Pennsylvania. As described in that brochure, the Poz-O-Tec -8- system treats hot flue gas, after electrostatic precipitation of its fly ash content, in a sulphur dioxide scrubber, and the resultant sludge is then dewatered by thickening and filtration steps and recombined with the separated fly ash and other 5 additives in the production of wet stabilized material suitable for landfill.

In the process of the present invention such a landfill product can be produced in a single step. The sulfur dioxide and fly ash are effectively removed from flue gas in the 10 novel process of the present invention and the sludge which forms is simultaneously dewatered in the drying zone thereof.

This process affords the ability to discharge a solid waste product of whatever moisture content may be desired, for example, from about 5 to 25 weight per cent as already set forth herein-15 before. Such solid-waste is eminently suitable for use as landfill. Other substances such as boiler bottom ash may be incorporated in the solid waste if desired for the use as landfill.

Claims (13)

1. 2. A process according to claim 1, wherein the free moisture content of said dewatered sludge leaving said drying zone is controlled to be from about 5 to 25 weight per cent. 15 3. A process according to claim 2, wherein said moisture content is maintained by controlled addition of water in said drying zone.

2. 4, A process according to any of claims l'to 3, wherein said flue gas enters said drying zone at from about 350 to 20 550°F. and leaves said scrubbing zone at from about 120 to 180°F. .

3. 5, A process according to any of claims 1 to 4, wherein said dewatered sludge leaves said drying zone at a temperature of from about 100 to 200°F. 25 6. A process according to.any of claims 1 to 5, wherein said flue gas enters said drying zone with a sulfur dioxide content of from about 2000 to 4000 ppm and leaves said scrubbing zone with a sulfur dioxide content of about 500 ppm or less. -10-

4. 7. A process according to any of claims 1 to 6, wherein 4^-9 8 6 fly ash is removed from said flue gas by said slurry in said scrubbing and drying zones and is incorporated in the dewatered waste sludge product to form landfill therewith. 5 8, A process according to claim 7» wherein boiler bottom ash is added to the landfill formed by the incorporation of the fly ash in the dewatered waste sludge product.

5. 9, A flue gas scrubber for reducing the sulfur dioxide content of a hot flue gas, comprising an elongated tubular drum 10 rotatably drivable about its longitudinal axis and having a scrubbing zone extending inwardly from one end thereof contiguous with a drying zone extending inwardly from the other end thereof, said drum including slurry retaining means at the interface of said zones, a gas inlet disposed at the drying zone 15 and for introducing bot flue gas into the drum for flow through it from the drying zone end to the scrubbing zone end, a gas outlet at the scrubbing zone end, slurry feeding means on the drum for introducing lime slurry into the drum for flow through it from the scrubbing zone end to the drying zone end, tumbling 20 means in the scrubbing zone for intimately mixing the slurry with the gas to cause the slurry to combine with sulfur dioxide components of the gas and be transformed into a waste sludge product, and collecting means at the drying zone end of the drum for conducting the waste sludge product out of the drum after it 25 has been dewatered by evaporation from contact in the drying zone with the hot flue gas introduced at the drying zone end and flowing through the drying zone.

6. 10. A flue gas scrubber according'to claim 9, wherein the longitudinal axis of the tubular drum slopes slightly downward-30 ly from the scrubbing zone end to the drying zone end to cause -11- -4&906 the slurry and waste sludge product to flow through the tubular drum.

7. 11. A flue gas scrubber according to claim 9 or 10, wherein the drying zone and gas inlet are so arranged that the • t 5 dewatered waste sludge product is discharged with a moisture content of about 5 to' 15 per cent by weight.

8. 12. A flue gas sepubber according to any of claims 9 to 11, wherein liquid-spraying means and temperature-sensing means are disposed within the end of the drying zone and are connected by control means for controlling the moisture content of the dis-charged waste sludge product.

9. 13. A flue gas scrubber according to any of claims 9 to 12, wherein the drying zone includes an agitating portion disposed adjacent the scrubbing zone and a substantially smooth portion 15 disposed adjacent the drying zone end.

10. 14. A flue gas scrubber according to claim 13, wherein an annular dam is disposed between the agitating and substantially smooth portions of the drying zone.

11. 15. A flue gas scrubber according to any of claims 9 to 14, 20 wherein the slurry retaining means is an annular dam.

12. 16. A process for reducing the sulfur dioxide content of a hot flue gas, substantially as herein described.

13. 17. A flue \jas pcrubber for reducing the sulfur dioxide content of a hot flue gas, constructed and arranged substantial- 25 ly as herein described with reference to the accompanying drawings. -12- PFIZER INC. -Sheet 1 449{J6 : . tLO-i I <\£K I \A»ΪΙ' i \ -y ^Vl ^ »Ψ"f «v^pcSEfii « tt^x^s-ls x U gg F|Jl-> lip Jiuf $ w ? -. j a 5 \νΐΗ ^ rlr1 ,f "18^=^ ΐ V$^'^W=i\V—\ t s ,& 84 fllps'i vTx -rSf— -ri—f—* \ ® C==]%^H F.' R. KELLY & CO. '.......