CA2190868A1

CA2190868A1 - Improved wet scrubbing method and apparatus for removing sulfur oxides from combustion effluents

Info

Publication number: CA2190868A1
Application number: CA002190868A
Authority: CA
Inventors: Jonas S. Klingspor; Even Bakke; Gerald E. Bresowar
Original assignee: Individual
Current assignee: Alstom Power Inc
Priority date: 1994-06-09
Filing date: 1995-06-07
Publication date: 1995-12-14
Also published as: PH31493A; RU2149679C1; AU2943295A; BR9507951A; FI964891A0; KR970703798A; WO1995033547A1; BG63154B1; PL317931A1; HU221181B1; FI964891A7; SK151696A3; BG101099A; CZ353396A3; HU9603356D0; HUT77896A; GEP20002319B; TW349876B; SI9520071A; EP0765187A4

Abstract

Sulfur oxides (SOx) are scrubbed from combustion effluents with aqueous limestone slurries single-loop, open-tower countercurrent limestone wet scrubbers. Effluent flow rates are greatly increased while L/G values and reaction tank (150) residence times are decreased. Improved entrainment eliminator design, nozzle (112) placement and spacing, and the use of a hydrocyclone (181) to separate and recycle smaller particles of limestone from the byproduct gypsum, facilitate these advantages. Limestone is reduced to very fine particles, e.g. about 8µ or less with more than 99 % of the particle by weight less than 44µ, and introduced into a scrubbing slurry which is contacted with SOx-laden effluent. Reactivity of the scrubbing slurry is maintained, even at reduced pH, by continuously operating a hydrocyclone to assure a molar ratio of calcium-containing to sulfur-containing compounds of greater than about 1.3 to 1 while keeping both a low chloride and low non-reactive solids content. The hydrocyclone removes large particles of calcium sulfate and provides a recycle stream (184) of fine calcium carbonate and non-reactive solids which is bled off as necessary to maintain both the desired low chloride and non-reactive solids levels.

Description

0 9S133547 PCr~US95/07167 2~ q~)868 CR-r I ION
IMPROVED WET SCRUBBING METHOD
AND APPARATUS FOR REMOVING
SULFUR OXIDES FROM COMBUSTION EFFLUENTS
5 Technical Field The invention relates to improvements enabling the removal of sulfur oxides (SOx) from combustion effiuents with greater efiiciency and with e,.u"u~ ,s in capltal and operating costs.
The combustion of ..d,l,olla,,~us materials containing significant amounts 10 of sulfur, including fossil fuels and waste, is being closely regulated by governments around the world. Combustion of these materials causes free radi-cals of sulfur and oxygen to combine at the elevated temperatures involved to produce a variety of oxides of suifur which are referred to as a group as SOx.
Regulations are in place in many countries to reduce the amounts of sulfur 15 oxides released to the ' "u~ul~e,~ to alleviate the problems ~co~ I with acid rain.
Numerous strategies are being employed to reduce the discharge of SOx to the ~ u "~ . Among these are methods for cleaning sulfur from fuels prior to combustion, methods for chemically tying up the sulfur dunng combustion, and 20 methods for removing the sulfur oxides from combustion effiuents. Among the WO 9~/33547 2 1 9 0 8 6 8 PCT/US95/07167 ~, ` , ? ` ~
methods for treating combustion effluents to remove sox1 are wet and dry scrubbing. Wet scnubbing technology is well developed and effective; however, very large equipment has been required and costs are plul~olliulldl.
The technology for wet scrubbing combustion effluents to remove SOx 5 provides gas-liquid contact in a number of different configurations. Among themost prominent are the single- and double-loop countercunrent spray towers and towers which employ both cocurrent and countercurrent sections.
The single-loop, open-tower systems employing calcium carbonate to react with the SOx are the simplest in construction and operation. These systems10 are often preferred because they can be operated with low pressure drop and have a low tendency to scale or plug. The advantages of their simplicity and reliability have, however, been offset in some situations by their large size. For example, because they do not employ any trays or packings to improve contact between the effluent and the scrubbing liquid, tower heights are typically high 15 and many levels of spray nozles have been employed to assure good contact.
In open spray towers, the ability of the scrubbing liquid to absorb SOx from the gas depends on the availability of alkalinity in the liquid. The most cost effective source of alkalinity for wet scrubbing systems is generally accepted to be calcium carbonate. Unfortunately, calcium carbonate solubility usually 20 decreases with increasing alkalinity in the scrubbing liquid. Towers with packings and trays improve absorption by retaining calcium carbonate longer in the gas-liquid contacting zone, thereby providing a ",e.,l,d"is", for more dissolution and, as a result, more effficient use of the scrubbing liquid. Open spray towers, on the other hand, are typically designed relatively taller to provide for as long a contact 25 time as possible, often with multiple spray levels to facilitate the most efficient introduction of scrubbing liquid into the tower.

~W095133547 ~ Q~8 Pcr~usssm7l67 It would be desirable to improve single-loop, open-tower wet scrubbing employing calcium carbonate for treating SOx-laden combustion effluents, by improving process efficiency with a co~ ".ùl Idif ,yly higher process economy while ~ asi"y the overall size requirements of the tower, improving calcium 5 carbonate utilization, Illdillldillilly high reliability, reducing energy consumption, and achieving high throughputs with high p~ lltayt: SOx reduction.
It would also be desirable to improve single-loop, open-tower wet scrubbing employing calcium carbonate for treating SOx-laden combustion effluents, by increasing reactivity in the scrubbing slurry without reliance on 10 chemical additives.
Bd~.hy.u.~ l Art The design and operation of single-loop, countercurrent spray towers utilizing limestone is discussed by Rader and Bakke, in I~ UI,UUIdljIIIJ Full-Scale E~,u~ ce Into Advanced Limestone Wet FGD Designs, presented at the IGCI
Forllm 91, September 12, 1991, Wdsl,i"y~u,~, D.C. ( fonmerlythe Industrial Gas Cleaning Institute, now the Institute of Clean Air Companies, Wdsl ,i, Iytul), DC) Open spray towers (i.e., those not having packings, trays or other means for facilitating gas-liquid contact) are simple in design and provide high reliability.
They are especially useful in coal-fired power stations where the evolution of 20 chlorides has caused a number of problems, including reduced reactivity of the scrubbing solution and severe corrosion of scrubber internals. Another factor favoring the use of open spray towers is their inherent low pressure loss and resulting fan power economy.
The use of a variety of reagents has been s' ~gest~ but the most 25 preferred are those which are effective without high additive levels and can be WO 95133547 , . ` r~ 67 ~ `}~08`68 purchased at low cost and stored and ~Idll::~,UUI Lt:d with minimal special handling.
Calcium carbonate (cu, ,,,,,~I.,i..:!y available in a number of fomms including limestone) is a material of choice because it meets these criteria and, when properly processed, yields process byproducts that can be easily disposed of as 5 landfill or sold as gypsum.
In single-loop, countercurrent, open scrubbing towers of the type discussed by Rader and Bakke, a scrubbing liquid based on calcium carbonate flows t' ..., dly while the SO~-laden efffluent flows upwardly. They summarize historical values for a range of pdl dl I ,~l~r:" including absorber gas velocity 10 (giving a mlnimum of 6 and a maximum of 15 feet per second, i.e. about 2 to less than 5 meters per second), indicating that absorber gas velocity has a weak influence on the liquid-to-gas ratio (UG), a key factor in both capital and operating expenses. The height of the spray contacting zone in these towers is not given, but typical values will be on the order of from about 6 to about 15 15 meters, historically ,u"~ d an important factor in ellyi"~li"g an effficientsystem which can be expected to reliably remove at least 95% of the SOx from combustion effuents.
In conventional towers of this type, the ratio of the quantity of slurry to the quantity of gas (L /G) is said to be arguably the single most significant design20 parameter. The UG affects the cost of pumping, the cost of holding tanks and other U~ l dLiUI Idl and economic factors. The cost of pumping the limestone slurry increases ,ulu,uolliull.~.'y with the tower height. It would be desirable to decrease L/G requirements and height for open spray towers.
Sulfur oxides (SO"), principally SO2, are absorbed in the d~:,c~l ~di"y25 scrubbing slurry and collected in a reaction tank where solid calcium sulfite and solid calcium sulfate are fonmed. Desirably, the reaction tank is oxygenated to force the production of the sulfate. Once the crystals of sulfate are grown to asufficient slze, they are separated from the slurry in the reaction tank.

~wossl33s47 21~`~bi~6~` P~ ./167 In a paper by K. R. I l~ lldl 11~, et al., entitied THE EISCHOFF FLUE GAS
DESULFURIZATION PROCESS (presented at the EPA and EPRI cua,uOl~ule~d First Combined FGD and Dry S02 Control Symposium, October 25-28, 1988) a scrubbing tower is depicted as including a hydrocyclone loop which separates a 5 gypsum slurry from a wet scrubber into a coarse solids stream and a fine solids stream, with the fine solids stream being returned to the scrubber. In U.S. Patent No. 5,215,672, Rogers, et al. describe a process similar to that of I l~e" ,d"", et al. in that it employs a hydrocyclone as a primary ~ _' i"y device. In the latter case, after se:,ua, " ,9 a fine solids stream from a coarse solids stream rich in 10 gypsum, water as part of a thickened fines stream is disposed of along with at least a portion of the fines removed. Neither of the des-,,iuliu,~s of these d~u~uacl)~sl however, indicates how the use of a hydrocyclone as a primary J~ ....'~,. il l9 device can be employed to improve overall process efficiency with a col l~,uù~di~ ,yi~i higher process economy while d~ul~:dail ,g the overall size 15 requirements of the tower, improving reagent utilization, Illdillldil~illy high reliability, reducing energy consumption, and achieving high throughputs with high p~ l lLdy~ SOx reduction.
The art has also provided packed towers. Rader and Bakke point out that while these types of towers have some advantage in temms of decreased 20 operating costs, they present additional risks. The packings or other gas-liquid mixing means can become clogged or corroded and cause ~ c~ ,l bypass or pressure drop, resulting in prolonged periods of downtime. It would be advantageous to have an open tower which had the advantages of the packed towers, but which did not require the packings, and was smaller than open 25 towers of conventional constnuction.
.

The prior art does not directly address the points necessary to achieve improvements that, in the context of single-loop, open-tower, countercurrent limestone wet scrubbers for SOx reduction, permit results CUlll,UdldLJI~ to W09S/33S47 2~ 9r,~ i8 achieved with packed towers but without the use of packings or the problems cl with them.
In single-loop, countercurrent, open scrubbing towers of the type discussed by Rader and Bakke, a scrubbing slurry composed of calcium 5 carbonate, calcium sulfate, calcium sulfite, and other non-reacting solids flows ~'o~, l .lly while the SOx-laden eflfluent gas flows upwardly. The SOx, principally SO2, is absorbed in the desct:"ui"y scnubbing slun-y and is collected in a reaction tank where calcium sulfite and calcium sulfate are fommed. Desirably,the reaction tank is oxygenated to force the produdion of sulfate over sulfite.
10 Once the crystals of sulfate are grown to a sufficient size, they are removed from the reaction tank and separated from the slurry. Soluble impurities, such as chlorides, are also withdrawn. These scrubbing towers are relatively t .,u, lUllli~, to construct and operate, but costs in both areas are d~,uel1d~ on the reactivity of the scrubbing slurry. Indeed, the costs are Jc:Llilll~llt~:lJ impacted by high 15 dissolved chloride ~.UI~c~ ,i, dLiul ,s in the scrubbing slun-y which suppress the reactivity of the calcium carbonate.
It is known to reduce the chloride content of the scrubbing slurry by the use of a blow down stream. Typically. the blow down is taken from the reaction tank or from water recovered from gypsum recovered from the process.
For example, in U.S. Patent No. 3,995,006, Downs, et aL withdraw slurry from an absorber sump, pass the slurry to a hydl ucy~,lul ~e separator, to separate a stream high in fine particles of calcium sulfite from a stream high in relatively larger particles of calcium carbonate. Following a second separation of the calcium sulfite, a thickened stream containing the calcium sulfite is d;~ d~y~d.In most situations, the discharge of large amounts of water in this manner controls the buildup of chloride in the system. However, the discharge of large amounts of water is ~"de:,i, dL,le from both the envi, u"" ~ Ldl and economic ~Ldl l~I,uui~ ,ts.
_ WO 95133S47 PCT/IJS9~/07167 '2 i 9G868 ln U.S. Patent No. 5,215,672, Rogers, et a~. describe a process similar to that of Downs, et al. in that it employs a h~d~ucy~,lune to separate unreacted calcium carbonate from calcium salts formed by reaction with the SOx scrubbed 5 from a combustion effuent. In this case, after separating a fine solids streamfrom a coarse solids stream rich in gypsum, water as part of a thickened fines stream is disposed of along with at least a portion of the fines removed. While blow down in this fashion is suffcient to control the buildup of chloride in thesystem if sufficient water is removed, this scheme will eliminate a ~,u,uu~ Lio~ :J
10 high amount of hne solids. Rogers, et al. seek to dispose of the hnes as waste.
However, it will be apparent from the des~ ;UI~ of the present invention, that reversing this strategy, while still blowing down a portion of the water to control chlorides, can facilitate increased reactivity in the system.
In a paper by Rosenberg and Koch, published in the 93rd Bimonthly 15 Report of the Stack Gas Emissions Control Coor~li, Id~iUi) Center Group, .~uly 1989, a hydrocyclone loop installed on an FGD (flue gas desulfurization) plant in the N~ dl Ida, like that in Rogers, et al., separates a gypsum slurry from a wetscrubber into a coarse solids stream and a fine solids stream, with all of the hne solids stream being returned to the scrubber. By operating in this manner, blow 20 down is not taken from this stream and must be taken elsewhere. The process diagram of Figure 2 of that paper, shows blowdown being taken from a vacuum belt hlter. Removal of water from this point in the process will control chloride, but it does so by removing higher amounts of water than necessary, since the water so removed has been diluted by fresh makeup water used to wash the 25 gypsum.
The prior art does not directly address the points necessary to achieve reactivity improvements in the context of single-loop, open-tower, countercunrent limestone wet scrubbers for SOx reduction.

` . t`:,,,~.;~lq(~86,8 Disclosure of th~ Invention It is an object of the invention to provide improved processes and apparatus for wet scrubbing combustion effluents, especially from coal-fired boilers, to remove sulfur oxides.
It is another object of a preferred ~ L~u~ l of the invention to provide improved single-loop, open-tower, countercurrent limestone wet scrubbers for Sx reduction.
It is a further object of the invention to enable operation of single-loop, open-tower, countercurrent limestone wet scrubbers at reduced UG values.
It is a yet further object of the invention to reduce the size of single-loop, open-tower, countercurrent limestone wet scrubbers.
It is another specific object of the invention to increase the velocity of the flue gas through single-loop, open-tower. countercunrent limestone wet scrub-bers.
It is yet another object of the invention to improve the design and location of l~ ldilllllelll St~ dl ' ::~ and mist ~,i.llilldLul~ in single-loop, open-tower, coun-tercurrent limestone wet scnubbers to effectively demist scnubbed effluents and change their direction away from the roof of the scrubbing tower.
It is a yet further object of the invention to improve the operation of single-loop, open-tower, countercurrent limestone wet scrubbers to reduce the residence time of gypsum crystals in the scrubber and enable the use of a hyd~uc~,lul1e to separate them from smaller particles of limestone.

O 95/33S47 Pfr`Tn-fS95;fO7167 '' "i ,' ~f ' t2 'tf f9 fnf 8 68 -f3-lt is still another object of a preferred ~l l Ibo~i,, ,~l IL of the invention to im-prove the operation of single-loop, open-tower, countercurrent limestone wet scrubbers by reducing the residence time of gypsum crystals in the scrubber and enabling the use of a hydrocyclone to maintain operation at high a~uiulliulll~lif 5 ratios of calcium to sulfur while fostering high utilization of calcium carbonate It is a still further object of a preferred tllllbOl.lilll~fllL of the invention to im-prove the process efliciency of single-loop, open-tower, countercurrent limestone wet scrubbers by achieving eflective liquid to gas contact within a scrubbing zone of reduced height utilizing a reduced number of spray levels.
It is a yet another object of a preferred ~,,ILo~i,,,e:,,l of the invention to im-prove the operation of single-loop, open-tower, countercurrent limestone wet scrubbers by improving the dlldlly~llle:llL of the spray nozzles to minimize theamount of gas passing through without being treated and to achieve eflective gas-liquid contact with a reduced number of spray nozzles.
It is a further object of a preferred c~ uodi~ lll of the invention to improve the operation of single-loop, open-tower, countercurrent limestone wet scrubbersby Illdil l~dil lil l9 a high reactivity in the scnubbing slurry, improving limestone utilization, and providing an overall improvement in process efliciency.
It is still a further object of the invention to improve the operation of single-20 loop, open-tower, countercurrent wet scrubbers by providing an efficient means for purging chloride from the scnubbing liquor.
These and other objects are accf"" I,uli~l ~ed by the invention which provides both improved processes and apparatus for wet scrubbing, particularly the scrubbing of effluents from the combustion of sulfur-containing fuels such as 2~ coal and solid waste.

W0 95/33547 ~ r~ q [} 8 6 8 r ~ 67 ,~

ln one aspect, the invention improves a single-loop, open-tower, counter-current limestone wet scrubbing process for reducing the ~vu, ~v~, r~ of SOx (principally SOz) in flue gases. In another, the invention provides an improved apparatus capable oF achieving the noted improvements and will be described in 5 detailinthefollowingd~ 1". Theprocess,insummary,cu",~ s. (a) directing a flow of flue gas containing SOx upwardly through a vertical scrubbing tower at a bulk flow velocity of greater than about 4.5, and preferably up to about 6, meters per second; (b) introducing into a vertical scrubbing section within said tower, a spray of droplets of an aqueous slurry of finely divided calcium 10 carbonate, calcium sulfate, calcium sulfite, and other non-reactive solids, the calcium carbonate preferably having a weight median diameter of 6,u or less with99% by weight less than 44,u, and a total molar ratio of calcium-containing to sulfur-containing compounds in the solids of at least 1.1 to 1.2, to contact the flue gas while clesv~l,di"~ through the tower countercurrently to the flow of flue gas;
15 (c) collecting the slurry in a reaction tank after contact with the flue gas; (d) withdrawing slurry from the reaction tank, preferably after an average residencetime of eight hours or less; (e) subjecting slurry withdrawn from the reaction tank to a dewatering treatment, preferably in a hydrocyclone, to provide a recycle stream composed of the hydrocyclone overflow rich in hne particles of calcium 20 carbonate and having a total molar ratio of calcium-containing to sulfur-containing compounds of 1.3 or greater and another stream composed of the hydrocyclone underflow rich in calcium sulfate particles, preferably having a weight median diameter of from about 25 to about 55,u; (fl returning to the process a major portion of the recycle stream rich in calcium carbonate; and (g)25 introducing fresh calcium carbonate and other non-reactive solids as Feed into the system in amounts sufficient to replace the calcium withdrawn and not recycled as well as that dissolved and reacted with the SO~ absorbed in the liquid phase in the scrubber tower, said finely-divided calcium carbonate having a weight median particle size of less than about 10,u as introduced.

~WO 95~33547 ,-, ri, k, ? ~1 ~ D 8 6 8 PCT/US9~ilO7167 It is preferred that the slurry be introduced from spray nozles, alternating between upward and downward orientation from two spray levels spaced from about 1 to about 2 meters apart. ~t is also preferred that the total tower height in the spray contacting zone be less than about 6, and preferably less than about 4, 5 meters in height, as it has been d~ . ",i"ed that height is not so important for reliably removing 95% or more of the SOx from combustion effluents. It is an advantage of the invention that the tower diameter can be relatively small, so that the operating bulk velocity of flue gas passing vertically through the spray contacting zone, based only on the cross sectional area and neglecting the area 10 taken up by spray headers and nozzles, be no less than 4.5 and preferably up to 6 meters per second. This higher velocity provides a means of suspending liquid in the tower without increasing tower height and without adding packing or traysfor liquid holdup, and the liquid so suspended is more reactive due to the increased time for ~icsr~ of the calcium carbonate. Hence a distinct 15 advantage of the invention is to increase tower contacting time without adding tower height, while at the same time " Idil lldil lil ,g the simplicity of design, construction, operation, and I l ldil IL~I ldl ~ce of an open spray tower.
In the more preferred ,:",uodi",t:"ts, the median size of the calcium carbonate particles in the reaction tank is ",di"~cli"ed within the range of from 20 about 2 to about 6,u, and the weight median particle size of the finely-divided calcium carbonate as introduced is less than about 8,u, with at least 99%
(e.S7.,99.5 %) by weight of the particles being less than 44,u.
It is advantageous for all countercurrent, open spray towers, packed towers, or towers with trays, that the molar ratio of calcium-containing to sulfur-25 containing compounds in the solid phase of the scrubber slurry be high. Highratios make more alkalinity available for SOx removal, thus improving the absorptive capacity of the liquid. However, in current processes, a high ratio is not ecu,,u,,,i.,al because valuable calcium-containing compounds, ~r~ '~ 'l~r calcium carbonate, will be wasted with the removal of sulfur compounds via the WO95133547 ~1 9~868 1 111~ ~ /167 i"~ system. The invention permits operation with a scrubbing slurry in the spray tower for which the solid calcium carbonate conut:"~, dLiUI I is much higher than ecu"u"liua'lJ viable for other systems. When utilizing the preferredconditions of particle size and gas-iiquid contact, the hydrocyclone is effective in 5 increasing the relative cu,)c~"l,dliù11 of available calcium and alkalinity in the tank.
In the preferred t " ,~udi" ,~, Its, the scnubbing tower comprises at least a first e~ Ill ~.;. ll l l~l ,L separator to remove a significant amount of the entrained moisture and to turn the direction of flow of the flue gases by at least 30 ' from 10 the vertical axis of the tower. In its preferred form, the majority of droplets having diameters less than about 100,u are eliminated either by dropping them out of the efffluent or cu,~ ' ' " ,9 them to form larger droplets which can more easily beremoved by a ' .~ dl11 mist eliminator. The first ~"t, ail1ll ~e"I separator is preferably followed by a generally vertical mist eliminator.
1~ In another aspect, the invention provides an improved wet scrubbing process for reducing the Cull~,~lllldLiull of SOx in a flue gas, cu,,,,u,i:,i,,u (a) directing a flow of flue gas upwardly through a scrubbing tower; (b) introducing a spray of an aqueous slurry of finely-divided calcium carbonate, calcium sulfate,calcium sulfite, and non-reactive solids to descend through the tower 20 countercurrently to the flow of flue gas, the weight median size of the calcium carbonate particles being within the range of from about 2 to about 6,u; (c) following contact with the flue gas, collecting the slurry in a reaction tank; (d) Illdillldillilly a high reactivity in the slurry by v~:'h.ll ,y slurry from the reaction tank and subjecting slurry withdrawn to treatment in a hydrocyclone to provide a25 recycle stream rich in fine particles of calcium carbonate and non-reactive solids and another stream rich in calcium sulfate, both of said streams containing dissolved chlorides, and d;:~,lldly;llg calcium sulfate as solids and a portion of the recycle stream rich in calcium carbonate and non-reactive solids to remove soluble chlorides and non-reactive solids; and (fl introducing fresh calcium wo ssl33s47 r~ /167 ¢~r) ¢~'lj~! ! `~?1 90868 carbonate as feed into the system in amounts sufficient to replace the calcium withdrawn due to said separation of said calcium sulfate and said portion of said recycle stream dia,_l,dlyad, said finely-divided calcium carbonate having a weight median particle size of less than about 1 Ou as introduced.
The process pemmits operation at pH values that also enhance reactivity.
Preferably the pH of the slurry in the reaction tank is within the range of fromabout 5.0 to about 6.3, and most preferably in the range of from about 5.8 to about 6.3.
Desirably, the molar ratio of calcium-containing to sulfur-containing compounds in the recycle stream is Illdil llclil ,ed at a value greater than about 1.3, preferably above about 1.4. Also, it is preferable to maintain a suspended solids cul~ut:llLldliull of less than about 15%, and most preferably less than about 5%, in the recycle stream. Preferably, the process further includes d~'~""i"i"y the chloride content of the slurry, and ~ ,l ldl yil lg a portion of the recycle stream should the value exceed a p~d~ ""i"ad maximum allowable chloride content.
Even more preferably, the process includes de:t~ l l lil lil l9 the solids density of the recycle stream, and ~jD~I IdlY;I IY a portion of the recycle stream whenever thesolids density exceeds a ,ul~:d~t~:llllill~d control value. In this last matter, the fraction of non-reactive solids are controlled.
In another of its aspects, the invention provides an improved wet scrubbing apparatus for reducing the t,ull~ lllldliull of SOx in flue gases, ~u" I,u~ iail Iy. (a) a scrubbing tower c~" ,u, ;D;I 19 a gas inlet duct, a gas outlet duct, and a vertical scrubbing section, configured to direct a flow of flue gas containing Sx upwardly through said scrubbing section; (b) an array of spray devices positioned within said scrubbing section conhgured to introduce a spray of an aqueous slurry of finely-divided calcium carbonate, calcium sulfate, calcium sulfite, and non-reactive solids to descend through the tower countercurrently to the flow of flue gas; (c) a reaction tank located below said array of spray devices WO 95/33547 r~ 67 to enable collection of the slurry after a period of contact with said flue gas within said vertical scnubbing section, said reaction tank being of a size suitable to permit reaction of the SOx with the calcium carbonate to fomm crystals of gypsumhaving a weight median particle diameter at least two times that of the particles 5 of calcium carbonate added as feed; (d) means for supplying calcium carbonate with a weight median particle size of less than about 1 0,u with 99% or more of the particles less than 44,u as feed to said reaction tank; (e) a spray sluny supply means cu~ ul iail ly at least one pump and ~co~ d conduit for ~ ,L..:. ,y slurry from the reaction tank and delivering slun y to said array of spray devices 10 positioned within said scnubbing section; (fl a slurry quality Illdill~ dllu~ system including a hydrocyclone capable of separating said slurry in said reaction tankinto a stream rich in small particles of calcium carbonate and non-reactive solids and relatively larger particles of calcium sulfate, at least one pump and O~ t~ .~l conduit for ~ lldl : Iy slurry from the reaction tank and delivering 15 slurry to a hydrocyclone, a recycle conduit leading from said hydl ucy~,lu~ ~e to said reaction tank to can y a recycle stream rich in calcium carbonate and non-reactive solids from said hydrocyclone, a ca!cium sulfate slurry recovery conduit leading from said hydrocyclone to remove slurry rich in calcium sulfak from saidhydrocyclone, and a discharge conduit in communication with said recycle 20 conduit and adapted to remove a portion of said recycle stream from said recycle conduit.
One effect of these improvements is a tower which is about one half the weight and volume of the current open-tower scrubbers. Process effciency is improved with a ~ù"~,c u, Idi, ~y~y higher process economy while reagent 25 utilization is improved, high reliability is ~di~di~l~d, energy consumption is reduced, and high throughputs with high p~lu~llldy~ SOx reduction are achieved.
.

WO ~5133547 PCT/US95A)7167 r ~ a 8 6 8 Brief D~ of the Drawings The inYention will be better Ulldt~l~LUOd and its advantages will be better d,U~ from the following detailed des~ iUi 1, especially when read in light of the accu" I,Udl ,ying drawings, wherein:
Figure 1 is a schematic view of a preferred t~",,oJi",~"l of the process of the invention employing a single-loop, open-tower, countercurrent limestone wet scrubber;
Figure 2 is a more detailed schematic Yiew of a scrubbing tower of the type shown in Figure 1;
Figure 3 is a partial side elevational view of the dl I dl Iytll llt:l 11 of spray nozzles in two spray levels shown in the tower of Figure 2;
Figure 4 is a bottom plan view of the spray no~zles in the two spray levels for a spray tower of the type shown in Figure 2; and Figure 5 is a perspective view of the e:lllldilllllt:lll separator shown in the 15 spray tower of Figures 1 and 2.
Industrial A~, " '~ :y The i" "., u /~" ,~"~ of the invention have prefenred ,, " " , to coal-flred utility boiler flue gases, and in some aspects are particularly effective for high chloride operations such as i"~ e~ ~. While the adVdl ILd~ S may be the 20 greatest in these types of operations, the invention is by no means limited to them. Efffluents from the combustion of all types of .,d,L~ol,a~t:ous materials can be treated, also including natural gas, synthetic gas, fuel oils, bitumens and WO 95/33547 P ~. 1 / U ~ r, v / 167 ~ . j.i --residual fuel oils, domestic and industrial solid or other combustible waste, and the like.
The following .Ids~ ,Liull is centered on the preferred ~IllL,odi,,,~ of Figure 1 which is a single-loop, open-tower, countercurrent limestone wet scrub-5 bing operation for removing sulfur oxides, principally as SO2, from combustioneffluents.
Limestone is the preferred form of calcium carbonate but can be replaced with another form, if desired. In addition to limestone, other fomms of calcium carbonate include oyster shells, aragonite, calcite, chalk, marble, marl, and 1 0 travertine. It can be mined or manufactured. In this cles.,, i,: " the terms calci-um carbonate and limestone are used illLt:l~lldl,yedLJly.
It is important to note that almost all accessible fomms of calcium carbonate found in nature contain minor quantities of relatively inert materials, such as free silica, magnesium carbonàte or dolomite, iron oxides, alumina, and 15 so forth. In principle, it is always desired to find very pure fomms for the limestone wet scrubbing process, but in practice, some impurities are always present whichform non-reactive solids in the wet scrubbing process. Other sources of non-reactive solids entering the process are fly ash escaping the particulate collector 10 and caught by the scrubber 100.
The limestone is finely divided, preferably by grinding as described below, to achieve a weight median diameter of about 1 0,u or less, with 99% below 44~u.This is extremely fine for wet scrubbing in an open tower with a countercurrent flow of limestone slurry. The more typical grind size of the prior art is a weight median diameter of 1 5,u or less with no more than 95% of the particles less than 44,u. In further contrast to the prior art, it is noted that the preferred grind size of the invention will yield particles with a weight median particle size of less than WO 95~33547 PCT/US95/07167 about 8u with 90% (e.g. 99.5%) by weight of the particles being less than 441~.
The use of a grind of the preferred size has several advantages.
The preferred process dlldll~ llL of Figure 1 shows an effluent such as from a coal-fired industrial or utility boiler entering a suitable means 10 for 5 removingparticulates suchasanel~L,ubldlk.~ Jildlul orfabricfilter which removes entrained solids to a practical extent. The cleaned flue gas is then passed via duct 20 to wet scrubbing tower 100 wherein it flows upwardly countercurrent to a spray of an aqueous slurry which contains finely-divided limestone ~ia~ lIdlyt:d within a vertical scrubbing section 110 from two levels of 10 spray nozles. From the scrubbing section 110, the gas continues through gas outlet duct 120. The tower is configured to direct a flow of flue gas upwardly through the vertical scrubbing section. The scrubbing slurry falling through thevertical scrubbing section 110 is collected in reaction tank 130. The reaction tank 130 is preferably of a size suitable to permit reaction of the SO2 with the calcium 15 carbonate to form crystals of gypsum having a weight median diameter at least 2, and preferably from 5 to 1û times as large as the particles of calcium carbonateadded as feed.
Mdil ,I~,~a~ of this differential in particle sizes facilitates the preferred udi~ l which calls for ~ d~ g a stream of slurry from the reaction tank, 2û preferably after an average residence time of about 6 hours and cullc~lllldlilly it in terms of calcium carbonate (as fine particles, preferably having a weight median diameter of less than about 6,U) and removing gypsum.
The vertical scrubbing section 110 contains an array of spray devices positioned within it. The array is configured to introduce a spray of an aqueous25 slurry of finely-divided calcium carbonate to descend through the tower countercurrently to the flow of flue gas. The Figure illustrates a bank of spraynozles which is shown to include two levels 112 112'of nozles. Each of the nozles 114 (see Figure 2) is fed slurry from a header 116 116', or 116a. It is WO 95/33547 . PCI/US9510 ~ trllJt~ 0868 7167 ~

typical to also include a third level to pemmit one level to be off line for repair or cleaning while two remain in operation.
The nozzles are preferably arranged with a spacing between levels of from about 1 to less than about 2 meters and with the direction of flow from adja-5 cent nozles in a given level alternating between upward and downward. Thepreferred ~"l~od;,"t:"ts of the invention reduce the spacing between the nozles, reduce the number of levels in use at any time (preferably to 2), and increase the rate of gas flow upwardly through the vertical scrubbing section. The preferred flow patterns of both the slurry being sprayed and the effluent passing upward 10 through the tower are illustrated in Figure 4.
The preferred form of nozle is a centrifugal nozle which fomms a spray at an angle of within the range of from about 90 to about 140 ', preferably about120. One suitable nozle is a Whirqet 300 gallon per minute nozle available from Spraying Systems Co., Wheaton, Illinois. Droplet sizes are preferably in the 15 range of from about 100 to about 6000,u, typically about 2000,u, Sauter mean diameter as measured by a Malvern Particle Analyzer.
Each of the headers 116 is oriented at an angle with respect to the header in the next upper or lower rack. The angle is preferably 90 when two or three racks are employed.
It is one of the novel and improved features of the invention that the resi-dence time in the reaction tank is reduced from the typicaD_ulllllle,~idl value of about 15 hours or more down to less than about 8 hours, more typically about 6 hours. This is facilitated by the improved dissolution rate of fine calcium carbonate particles and, to some extent, the relatively fast p, t:l,i~lJi~d~i~Ji, rate of calcium sulfate to form gypsum particles The reactive properties of the slurry are, in tum, enhanced by the separation of calcium sulfate from calcium carbonate in the slurry and recycling the calcium carbonate to the slurry as very 0 95133547 r~
~W (1t~ 2~q3868 fine particles which dissolve rapidly in the reaction tank. The reduction of theresidence time in the reaction tank has a positive impact on overall process efficiency as well as a number of advantages in terms of ,uluu~aailly ease, equip-ment sizing and quality of the byproduct gypsum.
The bulk gas velocities of the flue gas moving through the vertical scnubbing section 110 are above 4.5, and preferably up to about 6, meters per second. These gas velocities are high in the context of single-loop, open-tower wet limestone scrubbers and are employed by the invention preferably in CUIll~i,ldliuil with other innovative d~,~J,uaul,es to improve overall process efficiency. The preferred scnubbing towers of the invention enable the treatmentof flue gases with practical, low pressure drops and lower relative amounts of aqueous slurry, e.g. Iower L/G ratios.
The sulfur oxides in the efffluent are absorbed into the aqueous phase of the slurry, fomming bisulfite and hydrogen ions. Some bisulfite oxidizes to sulfate, releasing even more hydrogen ions. As the droplets become saturated with hydrogen ions, calcium carbonate begins to dissolve at an increasing rate, thus forming calcium ions and bica,L,u,,dLc:. The finely-pulverized calcium carbonateis very effective at absorbing hydrogen ions, thereby improving the absorptive capacity of the aqueous phase in the tower spray zone. The high gas velocities employed according to the preferred e"lL,u~i",~"la, and the preferred spray pattern, tend to maintain the slurry droplets suspended with a degree of fluidization to achieve enhanced contact.
Figure 1 shows limestone being finely divided in a mill 170, classified by cyclone 172, captured by bag house 174 and metered through air lock 176 into the pressurized flow of air in line 178. By pulverizing the limestone i""" " ' ly before introduction into the scrubber, the limestone which is introduced into the reaction tank to replenish the calcium carbonate can be made within well-definedparticle size ranges, free from large particles, those greater than about 44,u. In WO 95J33547 ~ . _ 1 167 ~<' f~ i 9 3 ~ 6 8 fact, it is typically possible and routinely achieved with dry pulverizing calcium carbonate particles of weight median size less than about 8,u and with 99% or more less than 44,u. The exclusion of the large particles from the limestone introduced into the reaction tank is a principal feature pemmitting the reaction tank 5 of the invention to be made substantially smaller than is presently employed in conventional scrubbers.
The air in line 178 facilitates supplying oxygen for the oxidation of sulfite and bisulfite ions to sulfate ions. The tank is preferably stirred by conventional means which are not illustrated in the Figure.
On the other side of the process as illustrated in Figure 1, slurry is withdrawn from reaction tank 130 for collcc:"L,_~;.,g the reactive calcium carbonate for recycle and reducing the level of solids, principally by removing gypsum. Figure 1 shows slurry being withdrawn from reaction tank 130 via line 183 and passed to hydrocyclone 181. The hydrocyclone is espedally effective in 15 the operation of the invention because it can rapidly and effectively separate the very fine particles of limestone from the larger particles of calcium sulfate. The particles of the calcium sulfate preferably have a weight average diameter of from about 25 to about 55,u. The S~:~IJdl d~iOI~ of the smaller particles of limestone provides a recycle stream 174 rich in calcium carbonate and a discharge stream 20 176 rich in calcium sulfate. Preferably, the weight average particle size of the calcium carbonate in the reaction tank and therefore in the recycle stream 184 is in the range of from about 2 to about 6,u.
Figure 1 shows the preferred form of the invention wherein the recycle 25 stream is COIlCt~ ,dL~d in temms of calcium carbonate and useful process water in hydrocyclone 181. The preferred sizes for the calcium carbonate particles will have a weight median diameter in the range of from about 2 to about 6u. The calcium sulfate particles will have a weight median diameter within the range offrom about 25 to about 55,u.

WO 95133547 PC'T/US95/07167 . ?~1 90868 Reaction tank 130 is located below the array of spray devices to enable collection of the slurry after a period of contact with the flue gas within the vertical scrubbing section 110. The reaction tank 130 is of a size suitable to permit reaction of the SO2 with the calcium carbonate to fomm crystals of gypsum having5 a weight median diameter at least 2, and preferably from 5 to 10, times as large as the particles of calcium carbonate added as feed.
By virtue of the difference in particle sizes between the calcium carbonate and the gypsum, and the means employed for separating the gypsum and C~l~C~ ldlilly the calcium carbonate as will be explained in detail below, the 10 solids col1c~ ld~iUi, of calcium carbonate can be increased by about 20 to about 50% above the cul)c~llLldliuns attainable in countercurrent designs of the priorart. It is a further advantage of the invention that the slurry will have a higher :,~uiulliu,,,~LIi., ratio of calcium-containing to sulfur-containing compounds than prior art systems, typically being at least 1.3 and preferably being about 1.4 or 15 greater. This system includes at least one pump 182 and a~so~ I conduit 183 for ..:" ~d~ _ ..;. ,9 slurry from the reaction tank and delivering slurry to the h~dl ucy~,lu~ ,e.
The sulfur oxides in the efffluent are absorbed into the aqueous phase of the slurry in vertical scrubbing section 11 û and react with available alkalinity in 20 the form of hydroxide ions to form bisulhte, which can be partially oxidized in the scrubbing section 110 and almost fully oxidized in the reaction tank 130 to fommsulfate. The alkalinity is principally derived from the dissolution of calcium carbonate to fomm biudl L,u~ and hydroxide ions, which occurs both in the scnubbing section 110 and in the reaction tank 130. An oxygen sparge, as 25 cul,~ iul)dl in the art, is preferably employed to assure suffhcient reaction, although some oxygen can be obtained from the flue gas itself in the scrubbing section 110. The reaction occurs to some extent in the falling droplets, but is ef-fected main~y in reaction tank 130 which collects the slurry. It is one of the novel and improved features of the invention that the residence time in the reaction WO 95/33547 PCI/llS9!i/07167 ~'t''i~ '2`1 qO~68 ~

tank is reduced from the typical Cu~ ,idl value of about 15 hours down to about 6 hours The reduction of the residence time in the reaction tank has a number of advantages in temms of ,u,ucc~.~;, Ig ease, equipment sizing and quality of the byproduct gypsum.
The pH of the slurry in the reaction tank 130 is preferably in the range of from about 5.0 to about 6.3, most preferably from about 5.8 to about 6.3. HigherpH indicates a higher available alkalinity in the slurry liquid and a ,ullt:auulldillyly higher capacity of the liquid to absorb SO2. It is an advantage of the inventionthat, because the calcium carbonate is supplied as fine partic!es and is recycled 1 û as will be explained later, also in the form of fine particles, a higher available alkalinity is possible. Low pH is typically employed on systems of prior art to increase the rate of reaction of calcium carbonate, but this normally reduces the absorption of SO2 in the scrubbing section because the decreased available alkalinity. The small particle size of the present invention offers increased 15 available alkalinity even at lower than desired pH, thereby offsetting to a large extent the impact of low pH on the scrubbing capacity of the slurry.
Associated with the reaction tank 130 and the array of spray devices positioned within the vertical scrubbing section 110, is a spray slurry supply means cu",~ .i"g at least one pump 122 and ~ '`'J' ;~ d conduit 124 for 20 v.;:lldl :.,9 slurry from the reaction tank 110 and delivering slurry to the array of spray devices positioned within the scrubbing section.
Figure 1 shows iimestone being fnely divided in a mill 170, classified by cyclone 172, captured by bag house 174 and metered through air lock 176 into the pressurized flow of air in line 178, which in turn is injected directly into the 25 scrubber 100 or into the duct 20 il l ll l le~lidlt~ly upstream ûf the scrubber.
Alternatively, the limestone from the baghouse 174 may be mixed in a tank and pumped to the reaction tank 130. By pulverizing the limestone at or near the point of injection, the size of the pulverized material can be closely controlled.

~WO95133547 PCTIUS95r07167 l' q O 8 6 8 The size of the particles is particularly critical to the invention. Preferably, the makeup stream of calcium carbonate has a weight median particle size of about 8,u or less with 99% or more of the particles less than 44,u, as fed to replenish the calcium carbonate lost to the reaction with SOx and to the byproduct gypsum and 5 with soluble chlorides as will be explained later.
The air in line 178 facilitates supplying oxygen for the oxidation of calcium sulfite to calcium sulfate. The tank is preferably stirred by ,~ o~,.,liu,,al means which are not illustrated in the Figure.
Also ' -' with the reaction tank 130 is a slurry quality Illdilll~:lIdll~,~
10 system depicted generally as 180. To maintain a high reactivity in the system, calcium carbonate is supplied as finely-divided particles as described, and a hydrocyclone 181 is employed to remove a portion of the slurry in reaction tank 130 for the purposes of cu"~ "I, d~ f ne particles of calcium carbonate for recycle as well as for di~l ldi~ gypsum. The hydrocyclone 181 separates the 15 slurry from the reaction tank into a recycle stream 184 rich in small particles of calcium carbonate and non-reactive solids and another containing a majority of relatively larger particles of calcium sulfate. The preferred sizes for the calcium carbonate and non-reactive solids particles will have a weight median diameter in the range of from about 1 to about 8,u, preferably from about 2 to about 6,u. The 20 calcium sulfate particles will have a weight median diameter within the range of from about 25 to about 55~u. Preferably, the weight median diameters of particles of calcium sulfate will be at least 2, and more preferably from 5 to 10, times greater than those of calcium carbonate. This syskm includes at least one pump 182 and _ ,c ' conduit 183 for ~ 1, ,y slurry from the reaction tank and 25 delivering sluny to the hydrocyclone.
A recycle conduit 184 is shown to lead from the h~dluGy~,lc,l,e 181 to the reaction tank 130 to carry a recycle stream rich in calcium carbonate from the hydrocyclone. An important feature of the system is achieving blow down from WO 95133547 ~ 67 , q 0 8 6 8 the recycle overflow, namely from recycle stream 184. A dischar~qe conduit 185 in communication with the recycle conduit 184 which is adapted to remove a portion of the recycle stream from the recycle conduit. It is preferred to provide a monitor for the chloride content of the slurry in line 183 or elsewhere, and to 5 control the amount of slurry to blow down from line 185 to control the chloride content in the slurry within It~dSUI Idbl~ values, e.~., below about 30,000 mg/l, and preferably below 20,000 mgll. Higher chloride contents tend to slow the dissolution of calcium carbonate and lower the available alkalinity in the scrubbing slurry. Stream 185 has the highest Gul1C~IILldliull of chlorides, being 10 equal to the .ull.,~ dliol~ in the reaction tank, and therefore is the best source of chloride purge in the system.
It also can occur that non-reactive solids in the reaction tank 130 which enter the system with the calcium carbonate or as entrained solids in the gas stream 20 and are composed of relatively small particles, with weight median 15 sizes ranging from about 4 to about 12,u, will tend to accumulate p,~r~,c:"li..l'J in the recycle stream 184, with their co,~c~ l dliUI I growing in the recycle tank 130.
Monitoring of these non-reactive solids in the recycle stream can be a~.~u,,,~ ed by chemical means (i.e., analysis for a ulldldult~ , specie, e.g., silica, iron, or others) or by physical means (i.e., either by particle size distribution 20 analysis, total solids cu,~"~, dliul~, or some other suitable method). It is a feature of the invention to adjust the blow down stream 185 in such a manner to control chlorides as described above, control the cu, ,~,~l ILI dliUi, of non-reactive solids in the reaction tank, or to simultaneously control both. The preferred means of control is to adjust the rate of stream 185 up or down as required to 25 meet the most stringent limit for either chlorides or non-reactive solids. It is desirable to maintain the level of non-reactive solids generally below about 20%by weight, and p,~r~,~"li~.:'y below 15% of the total solids in the reaction tank 130.
.

~W095133547 ~ ~?.~.-.9..n868 P~ 167 r~

Solids thus removed from the reaction tank via conduit 185 may be disposed with the blow down liquid, separated from the liquid, or in some other way treated and made suitable for disposal or other uses. The blow down liquid may also be treated in some manner to make the stream suitable for disposal or 5 for some other use. It is not the intention of this invention to limit in any way the possible di~,uùsi~iu,~s for the blow down stream 185, but rather to a.,h" lu '~;lge that there are numerous methods for treating the stream, S~::pdl d~ it into fractions, recycling all or a portion of it, and so forth. Such methods and means for treating stream 185 are beyond the scope of the present invention.
1û Also provided is a calcium sulfate slurry recovery conduit 186 leading from the hydrocyclone to remove calcium sulfate slurry from the hydrocyclone wherein the calcium sulfate is present as particles larger in size than the particles ofcalcium carbonate.
Figure 1 shows the preferred fomm of the invention wherein the recycle 15 stream 184 is fed back to the reaction tank 130. An advantage of operating inthis manner according to the invention is the ability to greatly increase the available alkalinity in the liquid droplets which come into contact with the SOx-laden effluent. By utilizing the recycle stream directly from the l~yd~u~,y..lol~e, at which point it is highly enriched with very fine particles of calcium carbonate and 20 a high pH and a high .Lui~,lliulll~llh, ratio of calcium to sulfur, it is possible to treat effluents rich in sulfur oxides in very short contact times.
Preferably, the ~Lui~llio,,,~l,i., ratio of calcium-containing to sulfur-containing compounds in recycle stream 184 will be in the range of from about 1.2 to about 2.0, most preferably from about 1.3 to about 1.4. The 25 COI~ la~l-ull of suspended solids in the recycle stream will typically be in the range of from about 1 to about 10%, by weight, most typically from about 2 to about 6%. Separation of the majority of the calcium sulfate from the limestone by .

~1~08-`68 l~d~ucy~,lu~e 182, in addition to raising the noted slui,_l,iu,,,t:~,iu ratio and the available alkalinity, also decreases the solids content of the slurry.
One advantage of the ~,UIIIbill " 1 of techniques employed in the process of the invention, is that the reaction tank has a high ~oi(.l ,iu" Ittl i., ratio of 5 calcium-containing to sulfur-containing compounds, e.g. on the order of from about 1.1 to about 1.6, preferably from about 1.2 to about 1.3. v'vhen this advantage is coupled with a further feature of the calcium carbonate being present as very small particles, it becomes possible to achieve better overall process efficiency with ecol ,u, I ,i~s of equipment sizing and raw material 1 0 utilization.
Preferred solids content of stream 183 coming from the reaction tank 130 is preferably within the range of from about 10 to about 20%, preferably betweenabout 13 to about 17%. And, the solids content of stream 186 is preferably within the range of from about 30 to about 55%. Stream 186 is fed to filter 188 15 or other suitable device to dewater the slurry. The solid gypsum is of high quality and can be utilized for building materials. The filtrate is drawn off by line 189 and can be recycled to the reaction tank 1 3û or any portion can be .li~:l Idl y~d as blow down, but it is an advantage of the invention that this stream need not be dia~,lldly~d to control the buildup of chloride in the system.
The scrubbed effluent is 5iyll ~ I~ly freed of entrained droplets of liquid and diverted in direction of flow by ~ ldilllllt~ separator 14û. At the high gasvelocities enabled by the invention, problems of encrustation of the roof 102 ofthe tower and of mist e~;, l li, IdlUI S of conventional construction would be ~p~ ced unless measures were taken. The use of a more ef~icient mist eliminator in lieu of the t:llLldilllllelll separator 14û is not feasible, since at operating bulk velocities of 4.5 to 6 meters per second, no practical, high-ef~iciency mist Ol;. l lil ldi~ are available, and cul l l~ idl unlts which could be specified for this location tend to drain -oorly and flood, thus increasing the WO gsl33547 PCT/US9~/07167 2il :9 08 ~8 potential for pluggage and low reliability. Hence, the t~ ldilllll'3llL separator 140 is designed for the specific purposes required by this invention.
Preferably, the e"~lhi"l"~"~ separator 140 removes a significant amount of the entrained moisture and turns the direction of flow of the flue gases by at least 5 30 from the vertical axis of the tower, aiso producing a more unifomm velocity profile into the vertical mist eliminator 150. In its preferred fomm, the majority (by weight) of droplets having diameters less than about 1 OO~u are eliminakd eitherby dropping them out of the effluent or cu,. ' ' ,9 them to fomm larger dropletswhich can more easily be removed by a ~' ~u . I~ dl l I mist eliminator.
The~lllldil""~lllseparator140ispreferablyfollowedbyagenerally vertical mist eliminator, shown in the Figures as 150. The bulk of the efffluentflow is changed from vertical to near horizontal by the ellllhilllllt:lll separator 140.
This has several advantages including the reduced i",,ui"~",t:"l of slurry onto the roof 102 of the scrubbing tower, with prevention of the fommation of deposits 15 there which tend to grow larger in time, to an extent that they can break off in large pieces, often as much as a meter or more in diameter, and either damage the nozle headers or fall through to the reaction tank 130 and ultimately cause plugging of the spray nozzles in 112 and 112'. Also, and importantly, it permitshigh-efFiciency demisting of an essentially horizontal flow by vertical mist 20 eliminator 150. The high-effficiency horizontal flow mist eliminator 150 inherently drains well, thus allowing operation at higher velocities than for a similarly designed, vertical flow mist eliminator. It also achieves superior demisting in the horizontal flow orientation. A high degree of demisting is an important feature of the invention, although not nel,t:,sd, ily unique, since horizontal flow mist 25 ~I;,IIilldlUI:~ are commonly used in FGD systems and other industries where high-efficiency demisting is required. However, it is a unique feature that the cul "~i"..~;al1 of the ~"~, dil 111 It:l 1I separator 140 with the high-efficiency mist eliminator 150 provides superior demisting by providing a relatively unifomm velocity profile into the mist eliminator and by Cul, ' ' " Ig the majority of .

smaller droplets into larger droplets in the tc~ dil 111 It~ separator prior to final demisting in the high-efficiency mist eliminator.
Figure 5 illustrates a preferred fomm of an improved ~"L~dil""e"~ separator 140 which can effectively remove or co" ' ' a majority of the smaller droplets 5 (i.e., less than 1 00,u diameter) and redirect the vertical flow of the efffluent away from the upper wail surfaces of the tower. E"~l di"" "c"l separator 140 is illustrated in Figure 2 as oriented at an angle y relative to the horizontal in scrub-bing tower 100. This angle will preferably be within the range of from about 10 to about 45", e.g. about 20.
The separator 140 utilizes single pass separator blades 142 to collect droplets by i,,~Ji,,g~,,,,c~,l and to turn the gas in a direction most sultable for further mist ~li" ,i, lalioll The individual blades 142 are oriented at an angle o with regard to the lower surface of dS~ lll' '' 144, 144', 144", etc., of the blades 142. Typically, a blade of this type will be a ~d~ " ~(." d",-shaped piece 15 of from about 0.15 to about 0.23 meters in minor dimension and from about 0.6to about 1.5 meters in major dimension. Spacing between individual blades will typically be from about 40 to about 70% of the minor dimension of the individualblades. Angle o will preferably be wlthin the range of from about 20 to about 40D, the exact value depending on the angle o and the desired degree of flow 20 direction of the efffluent stream.
The ass,_"~' " 144, etc., are constructed and oriented in a fashion that facilitates excellent drainage. The individual d~ '' are arranged in a pattern of chevrons as illustrated. The d:. ttCII ' '' 144, etc., are preferablyoriented with respect to one another at an angle e, typically in the range of from 25 about 125 to about 145, and preferably about 140. The ,cl,;"ii"",~"l separator structure is supported by members 146 which run the lengths of each of the ~SSdl11' '- Other dl Idl 19~ - of supporting structures are possible.

0 95133547 PCT,'US9~i~07167 ~w . si ,.2;190868 The structure of the ~ dil ~ separator 140 permits direct contact washing of the blades by means of f xed nozzle lances 147 haYing spray nozzles 148 capable of spraying wash water directly onto the blades from both the top and the bottom. Washing is typically done by operating each washer header separately and sequentially with the others. The wash water is of sumcient quality and is used in sufficient quantity to reduce the level of saturated, dis-solved salts on the separator surfaces. Together with the good drainage afford-ed by the chevron-shaped a"dlly~"l~llL of daat:lll' ' 144, etc., the use of highquality wash water and frequent washing affords practically deposit-free opera-1 0 tion.
~t is a feature of the invention that the separation efficiency of the first t:llLldilllllellL separator 140 need not be as high as multipass separators employed in the prior art because the ability to redirect the flow from vertical to horizontal enables the use of a high-efficiency, vertically-oriented mist eliminator 150. Thus, even though the t:"L, dil 1111~1 IL removal efficiency is lower than might be thought desirable for wet scrubbing towers, the ~:"l,..;.""~"L separator causes very low pressure drops, e.g. Iess than about 0.15 inches water column. and has other advantages in temms of cleanability, drainage, high bulk gas velocities, and direction of the gas flow from the upper wall surfaces of the tower and toward ahighly-efficient, vertical mist eliminator 150. The mist eliminator 150 is preferably of the bafffle type, e.g. a zig-zag baffle.
The scrubbed and demisted effluent can then be dial,lldly~d to the air such as by stack 160. In the an altemate t:lllLJo.li",~"l, the demisted effluent is heated prior to discharge such as in a gas-to-gas heat e:~-;l Idlly~l in a vertical confguration as described in copending, commonly-assigned U.S. Patent Application S.N. 08/257,158 (attomey's docket number 1930-P0020), fled on June 9,1994, filed in the names of the inventors named herein.

The effect of the improvements of the invention in l,UlllLJilldliUI~ is to enable construction of a single-loop, wet-scrubbing, open spray tower which is about one half the empty weight of current open spray towers. This difference in si~e coupled with improved SOx absorptive capacity afforded by the slurry results in 5 an improvement in total process efficiency oF roughly 30% or more over conventional systems. Total process efficiency is measured by the value of all resources expended to remove a unit of SO~ from the untreated gas. These include both capital and operating resources.
The above des~,,i,u~iu,~ is for the purpose of teaching the person of 10 ordinary skill in the art how to practice the invention, and it is not intended tû
detail all of those obvious 1, ' ~ " ~s and variations of it which will become apparent to the skilled worker upon reading the ddsu,i~,tlul1. It is intended, however, that all such obvious Illùdi~i~,dliulla and variations be included within the scope of the invention which is defined by the following claims. The claims are 15 meant to cover the claimed elements and steps in any dl I dl Iyel l l~l IL or sequence which is effective to meet the objectives there intended, unless the context ~-e,_i~iua,'y indicates the contrary.

Claims

1. A single-loop, open-tower, countercurrent limestone wet scrubbing process forreducing the concentration of SOx in flue gases, comprising:
(a) directing a flow of flue gas containing SOx upwardly through a vertical scrubbing tower at a bulk flow velocity of greater than about 4.5 meters per second;
(b) introducing into a vertical scrubbing section within said tower, a spray of droplets of an aqueous slurry of finely-divided calcium carbonate, calcium sulfate, and inert solids to contact the flue gas while descending through the tower countercurrently to the flow of flue gas;
(c) collecting the slurry in a reaction tank after contact with the flue gas;
(d) withdrawing slurry from the reaction tank;
(e) subjecting slurry withdrawn from the reaction tank to a treatment effective to provide a recycle stream rich in fine particles of calcium carbonate and another stream rich in calcium sulfate particles;
(f) returning to the process a major portion of the recycle stream rich in calcium carbonate; and (g) introducing fresh calcium carbonate as feed into the system in amounts sufficient to replace the calcium withdrawn and not recycled, as well asthat dissolved and reacted with the SOx absorbed in the liquid phase in the scrubbing section.

2. A process according to claim 1 wherein finely-divided calcium carbonate introduced as feed has a weight median particle size of less than about 8µ asintroduced.

3. A process according to claim 1 wherein the pH of the slurry as introduced into the scrubbing tower is within the range of from about 5.0 to about 6.3.

4. A process according to claim 1 wherein a bulk flue gas flow rate through the scrubbing tower is up to about 6 meters per second.

5. A process according to claim 1 wherein the tower comprises a single pass entrainment separator effective to reduce the quantity of droplets and to turn the direction of flow of the flue gases to an orientation effective for efficient utilization of a vertically-oriented mist separator.

6. A process according to claim 5 wherein the tower further comprises a vertically-oriented mist eliminator, and said entrainment eliminator being effective to turn the direction of flow of the flue gases by at least 30° from the vertical axis of the tower.

7. A process according to claim 1 wherein the slurry withdrawn from the reactiontank is passed to a hydrocyclone to provide a recycle stream rich in fine particles of calcium carbonate having a weight median diameter of about 6µ or less and a molar ratio of calcium-containing to sulfur-containing compounds of at least 1.3, and a discharge stream rich in relatively larger particles of calcium sulfite having a weight median diameter of from about 25 to about 55µ.

8. A process according to claim 1 wherein the slurry is withdrawn from the reaction tank after an average residence time of less than about 8 hours.

9. A process according to claim 1 wherein at least a portion of the slurry in the recycle stream is fed back the reaction tank at a molar ratio of calcium-containing to sulfur-containing compounds of at least 1.3 and a solids concentration of less than 10%.

10. A process according to claim 9 wherein the molar ratio of calcium-containingto sulfur-containing compounds in the recycle stream is greater than about 1.4.

11. A process according to claim 9 wherein the recycle stream comprises less than 5% suspended solids.

12. A process according to claim 1 wherein the slurry is introduced by spray nozzles, arranged in two levels with a spacing between levels of less than about2 meters, and with the direction of flow from adjacent nozzles alternating be-tween upward and downward.

13. A process according to claim 1 wherein the median size of the calcium carbonate particles in the reaction tank is maintained within the range of from about 2 to about 6µ, and the weight median particle size of the finely-divided calcium carbonate as introduced is less than about 8µ, with 99 % by weight ofthe particles being less than 44µ.

14. A process according to claim 1 wherein the pH of the slurry in the reaction tank is within the range of from about 5.8 to about 6.3.

15. A single-loop, open-tower, countercurrent limestone wet scrubbing process for reducing the concentration of SOx in flue gases, comprising:
(a) directing a flow of flue gas containing SOx upwardly through a vertical scrubbing tower at a bulk flow velocity of from greater than about 4.5 meters per second up to about 6 meters per second;
(b) introducing into a vertical scrubbing section within said tower, a spray of droplets of an aqueous slurry of finely-divided calcium carbonate, calcium sulfate, and inert solids to contact the flue gas while descending through the tower countercurrently to the flow of flue gas;
(c) collecting the slurry in a reaction tank after contact with the flue gas;
(d) withdrawing slurry from the reaction tank after an average residence time of less than about 8 hours;

(e) subjecting slurry withdrawn from the reaction tank to a treatment effective to provide a recycle stream rich in fine particles of calcium carbonate and another stream rich in calcium sulfate particles;
(f) returning to the process a major portion of the recycle stream rich in calcium carbonate; and (g) introducing fresh calcium carbonate as feed into the system in amounts sufficient to replace the calcium withdrawn and not recycled as well as that dissolved and reacted with the SOx absorbed in the liquid phase in the scrubbing section, the finely-divided calcium carbonate introduced as feed having a weight median particle size of less than about 10µ as introduced.

16. A process according to claim 15 wherein the pH of the slurry as introduced into the scrubbing tower is within the range of from about 5.0 to about 6.3.

17. A process according to claim 16 wherein the pH of the slurry in the reactiontank is maintained within the range of from about 5.8 to about 6.3.

18. A process according to claim 15 wherein the tower comprises a single-pass entrainment separator effective to reduce the quantity of moisture droplets and to turn the direction of flow of the flue gases to an orientation effective for efficient utilization of a vertically-oriented mist separator.

19. A process according to claim 18 wherein the tower further comprises a vertically-oriented mist eliminator, and said entrainment separator is effective to turn the direction of flow of the flue gases by at least 30° from the vertical axis of the tower.

20. A process according to claim 15 wherein the slurry withdrawn from the reaction tank is passed to a hydrocyclone to provide a recycle stream rich in fine particles of calcium carbonate having a weight median diameter of about 8µ orless and a molar ratio of calcium-containing to sulfur-containing compounds of at least 1.3, and a discharge stream rich in relatively larger particles of calciumsulfite having a weight median diameter of from about 25 to about 55µ.

21. A process according to claim 20 wherein at least a portion of the slurry in the recycle stream is fed back the reaction tank at a molar ratio of calcium-containing to sulfur-containing compounds of at least 1.3.

22. A process according to claim 21 wherein the molar ratio of calcium-containing to sulfur-containing compounds in the recycle stream is greater than about 1.4, and the recycle stream comprises less than 5% suspended solids.

23. A process according to claim 15 wherein the calcium carbonate is milled immediately prior to being supplied as feed to the slurry to maintain 99% of thecalcium carbonate particles less than 44µ, the weight median size of the calcium carbonate particles in the reaction tank is maintained within the range of from about 2 to about 6µ, and the weight median particle size of the finely-divided calcium carbonate as introduced is less than about 8µ, with 99% by weight of the particles being less than 44µ.

24. A single-loop, open-tower, countercurrent limestone wet scrubbing process for reducing the concentration of SOx in flue gases, comprising:
(a) directing a flow of flue gas containing SOx upwardly through a vertical scrubbing tower;
(b) introducing into a vertical scrubbing section within said tower, a spray of droplets of an aqueous slurry of finely-divided calcium carbonate, calcium sulfate, and inert solids, preferably having a weight median diameter of calciumcarbonate of about 6µ or less and a molar ratio of calcium-containing to sulfur-containing compounds of at least 1.1, to contact the flue gas while descending through the tower countercurrently to the flow of flue gas;
(c) after contact with the flue gas, collecting the slurry in a reaction tank maintained at a pH of from about 5.0 to about 6.3;

(d) withdrawing slurry from the reaction tank after an average residence time in the reaction tank of less than about 6 hours;
(e) subjecting slurry withdrawn from the reaction tank to treatment in a hydrocyclone to provide a recycle stream rich in fine particles of calcium carbonate having a weight mean particle size of less than about 6 µ and another stream rich in calcium sulfate particles having a weight median diameter of fromabout 25 to about 55µ;
(f) returning to the process at least a portion of the recycle stream rich in calcium carbonate having a molar ratio of calcium-containing to sulfur-containing compounds of at least 1.4; and (g) introducing fresh calcium carbonate as feed into the system in amounts sufficient to replace the calcium withdrawn and not recycled as well as that dissolved and reacted with the SOx absorbed in the liquid phase in the scrubbing section, said finely-divided calcium carbonate having a weight median particle size of less than about 8µ as introduced.

25. A process according to claim 24 wherein the tower comprises a single pass entrainment separator effective to turn the direction of flow of the flue gases to an orientation effective for efficient utilization of a vertically-oriented mist separator.

26. A process according to claim 24 wherein the slurry is introduced into the vertical scrubbing section by spray nozzles, arranged in two levels with a spacing between levels of less than about 2 meters, and with the direction of flow from adjacent nozzles alternating between upward and downward.

27. A process according to claim 24 wherein the calcium carbonate is milled immediately prior to being supplied as feed to the slurry to maintain 99% of thecalcium carbonate particles less than 44µ, the weight median size of the calcium carbonate particles in the reaction tank is maintained within the range of from about 2 to about 6µ, and the weight median particle size of the finely-divided calcium carbonate as introduced is less than about 8µ, with 99% by weight of the particles being less than 44µ.

28. A single-loop, open-tower, countercurrent limestone wet scrubbing process for reducing the concentration of SOx in flue gases, comprising:
(a) directing a flow of flue gas containing SOx upwardly through a vertical scrubbing tower at a bulk flow velocity of greater than about 4.5 meters per second;
(b) introducing into a vertical scrubbing section within said tower, a spray of droplets of an aqueous slurry of finely-divided calcium carbonate, calcium sulfate, and inert solids, to contact the flue gas while descending through the tower countercurrently to the flow of flue gas, said slurry being introduced by spray nozzles, arranged in two levels with a spacing between levels of less thanabout 2 meters, and with the direction of flow from adjacent nozzles alternatingbetween upward and downward;
(c) collecting the slurry in a reaction tank after contact with the flue gas;
(d) withdrawing slurry from the reaction tank;
(e) subjecting slurry withdrawn from the reaction tank to a treatment effective to provide a recycle stream rich in fine particles of calcium carbonate and another stream rich in calcium sulfate particles;
(f) returning to the process at least a portion of the recycle stream rich in calcium carbonate; and (g) introducing fresh calcium carbonate as feed into the system in amounts sufficient to replace the calcium withdrawn and not recycled as well as that dissolved and reacted with the SOx absorbed in the liquid phase in the scrubbing section.

29. A process for reducing the concentration of SOx in a flue gas by wet scrubbing, comprising:
(a) directing a flow of flue gas containing SOx upwardly through a scrubbing tower;

(b) introducing a spray of an aqueous slurry of finely-divided calcium carbonate, calcium sulfate, calcium sulfite, and non-reactive solids to descend through the tower countercurrently to the flow of flue gas, the weight median size of the calcium carbonate particles being within the range of from about 1 to about 8µ;
(c) following contact with the flue gas, collecting the slurry in a reaction tank;
(d) maintaining a high reactivity in the slurry by withdrawing slurry from the reaction tank and subjecting slurry withdrawn to treatment in a hydrocyclone to provide a recycle stream rich in fine particles of calcium carbonate and anotherstream rich in calcium sulfate, both of said streams containing dissolved chlorides, and discharging the calcium sulfate as solids and a portion of the recycle stream to remove either soluble chlorides or non-reactive solids, or both;
and (f) introducing fresh calcium carbonate as feed into the system in amounts sufficient to replace the calcium withdrawn due to said separation of said calcium sulfate and said portion of said recycle stream discharged, said finely-divided calcium carbonate having a weight median particle size of less than about 10µ as introduced.

30. A process for reducing the concentration of SOx in combustion effluents, comprising:
(a) providing a scrubbing tower comprising a gas inlet duct, a gas outlet duct, and a vertical scrubbing section, configured to direct a flow of flue gas upwardly through said vertical scrubbing section;
(b) positioning an array of spray devices within said scrubbing section, said array being configured to introduce a spray of an aqueous slurry of finely-divided calcium carbonate, calcium sulfate, calcium sulfite, and non-reactive solids to descend through the tower countercurrently to the flow of flue gas;
(c) supplying calcium carbonate with a weight median particle size of less than about 8µ as feed;

(d) providing a reaction tank located below said array of spray devices to enable collection of the slurry after a period of contact with said flue gas within said vertical scrubbing section, said reaction tank being of a size suitable to permit reaction of the SOx with the calcium carbonate to form crystals of calcium sulfate having a weight median particle diameter at least 2 times larger than the particles of calcium as added as feed;
(e) withdrawing slurry from the reaction tank and delivering slurry to said array of spray devices positioned within said scrubbing section; and (f) maintaining a low chloride content in the slurry in the reaction tank by withdrawing slurry from said reaction tank, passing the slurry withdrawn from the reaction tank to a hydrocyclone to provide a recycle stream rich in small particles of calcium carbonate and a stream rich in relatively larger particles of calciumsulfate, determining the chloride content of the recycle stream, and discharging a portion of the recycle stream in response to the determined chloride content.

31. A process for reducing the concentration of SOx in flue gases by wet scrubbing, comprising:
(a) directing a flow of flue gas containing SOx upwardly through a scrubbing tower, (b) introducing a spray of an aqueous slurry of finely-divided calcium carbonate, calcium sulfate, calcium sulfite, and non-reactive solids to descend through the tower countercurrently to the flow of flue gas, the pH of the slurry in the reaction tank being within the range of from about 5.0 to about 6.3, (c) collecting the slurry in a reaction tank, (d) maintaining a low chloride content in the slurry in the reaction tank by withdrawing slurry from said reaction tank, passing the slurry withdrawn from the reaction tank to a hydrocyclone to provide a recycle stream rich in small particles of calcium carbonate and a stream rich in relatively larger particles of calciumsulfate, determining the chloride content of the recycle stream, and discharging a portion of the recycle stream in response to the chloride content;

(e) returning a portion of the recycle stream, having a molar ratio of calcium-containing to sulfur-containing compounds greater than about 1.3, to thereaction tank;
(e) withdrawing the stream rich in calcium sulfate from the hydrocyclone to recover calcium sulfate; and (f) introducing fresh calcium carbonate into the system in amounts sufficient to replace the calcium withdrawn, said finely-divided calcium carbonate having a weight median particle size of less than about 10µ.

32. A wet scrubbing apparatus for reducing the concentration of SOx in flue gases, comprising:
(a) a scrubbing tower comprising a gas inlet duct, a gas outlet duct, and a vertical scrubbing section, configured to direct a flow of flue gas upwardly through said scrubbing section;
(b) an array of spray devices positioned within said scrubbing section configured to introduce a spray of an aqueous slurry of finely-divided calcium carbonate to descend through the tower countercurrently to the flow of flue gas;(c) a reaction tank located below said array of spray devices to enable collection of the slurry after a period of contact with said flue gas within said vertical scrubbing section, said reaction tank being of a size suitable to permit reaction of the SO2 with the calcium carbonate to form crystals of gypsum havinga weight median particle diameter at least 2 times larger than the particles of calcium carbonate added as feed;
(d) means for supplying calcium carbonate with a weight median particle size of less than about 10µ as feed to said reaction tank;
(e) a spray slurry supply means comprising at least one pump and associated conduit for withdrawing slurry from the reaction tank and delivering slurry to said array of spray devices positioned within said scrubbing section;
(f) a slurry quality maintenance system including a hydrocyclone capable of separating said slurry in said reaction tank into a stream rich in small particles of calcium carbonate and relatively larger particles of calcium sulfate, at least one pump and associated conduit for withdrawing slurry from the reaction tank and delivering slurry to a hydrocyclone, a recycle conduit leading from said hydrocyclone to said reaction tank to carry a recycle stream rich in calcium carbonate from said hydrocyclone a discharge conduit in communication with said recycle conduit and adapted to remove a portion of said recycle stream fromsaid recycle conduit and a calcium sulfate slurry recovery conduit leading from said hydrocyclone to remove calcium sulfate slurry from said hydrocyclone.