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Separation and Purification Technology 34 (2004) 143153Modeling flue gas desulfurization by spray-dry absorptionFabrizio Scalaa, Michele DAscenzob, Amedeo LanciabaIstituto di Ricerche sulla Combustione C.N.R., P.le Tecchio, 8080125 Napoli, ItalybDipartimento di Ingegneria Chimica, Universit degli Studi di Napoli Federico II, P.le Tecchio, 8080125 Napoli, ItalyAbstractA detailed model for flue gas desulfurization by spray-dry absorption with a lime slurry is presented. The model combinesa steady state one-dimensional spray-dryer model with a single-drop model for SO2absorption with instantaneous irreversiblereactioninarigiddropletcontaininguniformlydispersedfinelimeparticles.Thefateofthedropletsisfollowedfromatomizationuntil formation of a porous coherent shell around the drying droplets. The model results were validated against availableexperimental spray-dry FGD results, showing excellent agreement at low to medium Ca/S feed ratios. The model was thenused to study the relevance of the different resistances to SO2absorption and to predict the influence of the main operatingvariables on the spray-dryer desulfurization performance. Analysis of variables profiles along the spray-dry column showed thatthe initial droplet velocity has no influence on model results and that the initial droplets decelerating phase always accounts fornegligible SO2capture. Results further showed that the controlling resistance to SO2absorption shifts from a liquid-phase onenear the atomizer to a gas-phase one at the column exit. The operating variables that exert the largest influence on the overalldesulfurizationefficiencyaretheCa/Smolarfeedratio,themeaninitialdropletsizeandthemeanlimeparticlesize.Inparticular,careful control of the last two variables is critical in order to obtain a good spray-dryer performance. 2003 Elsevier B.V. All rights reserved.Keywords: Spray-dry; Absorption; Desulfurization; Modeling; Drop; Slurry1. IntroductionSpray-dry flue gas desulfurization (FGD) in con-junction with baghouse particulate collection repre-sents a viable alternative to wet scrubbing in boilersburning low to medium sulfur coal or fuel oil 14.The advantages of spray-drying over other technolo-gies include: the production of a dry waste byproductnot requesting sludge handling equipment; no scalingand corrosion problems enabling the use of cheaperCorresponding author. Tel.: +39-081-768-2969;fax: +39-081-593-6936.E-mail address: r.it (F. Scala).materials; smaller space needed and possibility ofeasily retrofitting existing plants; no requirement offlue gas reheating; flexibility in operation with re-gard to varying boiler load; low energy consumption;reduced installation and operating costs. On the con-trary, spray-dryers hardly exceed 70% SO2removalefficiency at 12 calcium to sulfur ratios (Ca/S), asopposed to values higher than 90% for wet scrub-bing, making this technology attractive when SO2concentration in the flue gas is relatively low.Inthespray-dryFGDprocessthehotfluegasiscon-tacted with a fine spray of alkaline suspension, usuallylime, in a reaction chamber where a droplet residencetime of 1015 s is provided. During their life-time the1383-5866/$ see front matter 2003 Elsevier B.V. All rights reserved.doi:10.1016/S1383-5866(03)00188-6144F. Scala et al./Separation and Purification Technology 34 (2004) 143153sprayed droplets simultaneously evaporate and absorbSO2. The absorbed SO2reacts in the alkaline aqueousphase with the dissolved lime following the overallreaction:SO2+ Ca(OH)2H2O CaSO312H2O +12H2O(1)where the resulting calcium sulfite precipitates as aconsequence of its low solubility in water.The spray-dry FGD process implies a complex in-terplay of two phase fluid dynamics, heat and masstransfer, liquid phase dissolution, ionic reactions andprecipitation.Broadlyspeaking,thedesulfurizationef-ficiencyinaspray-dryeristheresultofthecompetitionbetween the SO2absorption rate in the slurry dropletsand the water evaporation rate from the droplets 58.Both processes are enhanced near the atomizer wherehigh slip velocities result in increased mass and heattransfer rates. Lower transfer rates are achieved furtheron in the spray-dry chamber, decreasing significantlyafter the precipitated solids form a porous coherentshell (crust) around the drying droplets. On the wholethe process can be divided into three steps: a first shortphase after atomization in which the droplets deceler-ate until they reach their terminal velocity; a secondconstant rate drying phase, accounting for most ofthe sulfur removal, until the solid shell starts to format the surface of the drops; a third falling rate dryingphase until the particles are dried 5,8,9. The driedsolid product leaves the spray-dry chamber holdingtypically only few percents of free moisture and isseparated in a downstream collection device, usuallya baghouse.In spite of the considerable experimental activitycarried out on the spray-dry FGD process 3,4,813, alimited number of modeling works can be found in theliterature. Simulation of the spray-dry FGD process isof extreme practical importance in order to understandthe influence of the different operating variables onthe overall plant performance, without expensive andtime-consuming pilot or full scale experimentation.Karlsson and Klingspor 9 proposed two simplifiedmodels for the constant rate drying phase under thelimiting cases of gas-phase SO2diffusion resistancecontrol and lime dissolution control. Experimental re-sults were found to compare satisfactorily with the twomodels predictions under large excess of lime and un-der shortage of lime, respectively. Partridge et al. 6and Dantuluri et al. 14 developed a model for theconstant rate drying period based on the film theoryusing a simplified expression for gas absorption in aslurry 15. Newton et al. 7 derived a comprehen-sive model for the spray-dry FGD process during theconstant rate drying phase devoting particular atten-tion to the detailed microscopic behavior of the singledroplets. The calculation procedure relied, however,on time consuming trial-and-error iterations in whichat each step the reaction front position in the dropletmust be checked. Hill and Zank 8 described the ab-sorption process with a simplified mechanistic modelwhich was able to handle also the falling rate dry-ing period after the solid crust formation. The model,however, was only solved for the constant rate dryingphase and neglecting liquid-phase diffusion resistance.Model simulations were compared with a large num-ber of laboratory scale experimental results showingfair qualitative agreement.Recently Scala and DAscenzo 16 presented a de-tailed single-drop model for gas absorption followedby an instantaneous irreversible chemical reaction fora rigid droplet containing sparingly soluble fine reac-tant particles. The model takes into account externaland internal mass transfer resistances together withslurry particles dissolution. Under suitable assump-tions, the model was solved analytically giving a sim-ple and easy to handle expression for the instantaneousgas absorption rate to the droplet.In the present paper the expression derived byScala and DAscenzo 16 is applied to the SO2-limeslurry system and combined to a steady stateone-dimensional spray-dryer model. Combination ofthe two models allows to easily carry out the materialbalance on sulfur dioxide along the column in orderto calculate the desulfurization efficiency profile. Thefate of the droplets is followed from atomization untilformation of the crust (i.e. during the deceleratingand constant rate drying phases). The model hasbeen used to predict the influence of the main operat-ing variables on the spray-dryer desulfurization per-formance. In particular the following variables havebeen investigated: inlet gas temperature, approach toadiabatic temperature, stoichiometric calcium to sul-fur molar feed ratio, average droplet initial size andvelocity, average suspended calcium hydroxide par-ticle size, sulfur dioxide inlet concentration. ModelF. Scala et al./Separation and Purification Technology 34 (2004) 143153145results have been analyzed in the light of outputvariables profiles along the spray-dry column and ofoverall desulfurization performance. Comparison ofmodel predictions with experimental data available inthe literature has been used for model validation.2. Model descriptionThe sulfur dioxide absorption process can beschematized as a series of steps:(i) gas phase diffusion of SO2from the gas bulk tothe droplets surface;(ii) dissolution of SO2at the droplet surface;(iii) formation of sulfurous acid and dissociation intoionic sulfur species following the scheme:SO2(aq)+ H2O H2SO3(aq)H2SO3(aq) H+ HSO3(aq)(2)HSO3(aq) H+ SO32(aq)(iv) liquid phase diffusion of sulfur species towardsthe droplet center;(v) parallel dissolution of calcium hydroxide parti-cles;(vi) liquidphasediffusionofalkalinespeciestowardsthe droplet surface;(vii) neutralization by reaction between acid and al-kaline species.In order to simplify the liquid phase diffu-sion/reaction process it is assumed that all the sulfurspecies can be lumped into a single species with anaverage diffusivity. This assumption is based on theobservation that HSO3is likely to be the dominantsulfur species in the liquid phase 7.The two-phase flow inside the spray-dry column isdescribed with a steady state one-dimensional model,allowing variables change only along the axial direc-tion. The column is schematized as a constant sectionduct starting at the tip of the atomizer. The followingassumptions are made:1) The flue gas is in plug flow.2) The gases have ideal behavior.3) The spray-dry column is adiabatic.4) Drying of the slurry droplets can be described asdrying of pure water droplets 8.5) The simultaneous diffusion of sulfur dioxide andwater vapor in the gas phase has no significantinfluence on the single fluxes of the two species,so that water evaporation and sulfur dioxide ab-sorption processes can be de-coupled 8.6) The droplets are spherical, rigid and isothermal8.7) In each column section the droplets are uniformin size, uniformly dispersed in the flue gas and donot collide between each other (i.e. the numberof drops does not change along the column).8) Thermodynamic equilibrium holds at the dropletssurface (Henrys law).9) Heats of reaction and dissolution are small andcan be neglected.10) Both gaseous and solid reactants have low solu-bility in the liquid.11) The pseudo-stationary assumption as regardschanges in concentration profiles in the dropletsis valid 7.12) Ionic reaction between sulfur species and calciumhydroxide is irreversible and instantaneous 17.13) The product species is assumed to precipitate in-stantaneously and its influence on the process isnegligible 7.14) Solid particles are spherical, uniform in size, uni-formly dispersed in the liquid droplets and do notagglomerate.15) Water and lime particles do not circulate withinthe droplet 7,8.16) The presence of CO2in the flue gas has no influ-ence on the desulfurization process. This assump-tion is based on the experimental finding that for-mation of CaCO3under spray-dry conditions istypically negligible 1,5 and that SO2absorptionis not influenced by the CO2concentration in theflue gas 18.Momentum, heat and mass balances (on water va-por and sulfur dioxide) are carried out separately onboth liquid phase (droplets) and gas phase along thespray-dry column, taking into account friction, heatand mass transfer by convection and evaporation. Nus-selt and Sherwood numbers for droplets moving inthe flue gas have been calculated by means of cor-relations by Ranz and Marshall 19. Equations havebeen derived for the continuous phase with the Eu-lerian approach and for the dispersed phase with the146F. Scala et al./Separation and Purification Technology 34 (2004) 143153Lagrangian approach. With a suitable change of vari-ables, using the instantaneous droplets velocity, thedispersed phase equations have been changed to anEulerian reference system. Solution of the above setof equations describes the steady state droplet size,velocity and temperature profiles, the flue gas veloc-ity, temperature and humidity profiles together withthe SO2removal efficiency profile along the column.The calculation ends when the critical solids con-centration for the crust formation is reached in thedroplets. In this condition the solid particles insidethe droplets touch each other and a coherent shellstarts to form at the droplets surface. This criticalsolids concentration is estimated to be 60% by vol-ume fraction 7,20. In principle two different dry-ing patterns can happen. In the first pattern, as thedroplet evaporates the suspended solids tend to con-centrate at the drop surface as they are not able tomove effectively towards the droplet center and thefinal result is a hollow solid sphere. In the second pat-tern, the suspended solids effectively move towardsthe droplet center, maintaining an approximately con-stant volume fraction across the droplet radius, anda dense solid sphere is eventually formed. Theoret-ical and experimental results showed that in typicalspray-dry FGD conditions the second pattern is rele-vant 20. This is confirmed by electron microscope(SEM) analysis of spent particles cross-sections, thatclearly showed formation of dense solid agglomerates5. As a consequence in the present model a con-stant solids volume fraction across the droplet radius isassumed.Under the pseudo-stationary assumption (assump-tion 11), the instantaneous sulfur dioxide absorptionrate to a single droplet has been expressed followingthe model developed by Scala and DAscenzo 16.These authors presented a detailed single-drop modelfor gas absorption followed by an instantaneous irre-versible chemical reaction in a rigid droplet contain-ing uniformly dispersed and sparingly soluble fine re-actant particles. The model considers the formation ofa macroscopic spherical reaction front concentric tothe droplet surface dividing the droplet itself into twozones: an inner zone where no sulfur is present andan outer shell where sulfur species concentration fallsfrom the equilibrium value (at the surface) to zero (atthe reaction front). As in the outer shell zone dissolv-ing lime particles are present, around each of theseparticles a microscopic spherical reaction front es-tablishes. The diffusion/reaction equations are solvedanalytically giving a simple expression for the instan-taneous gas absorption rate to the droplet without hav-ing to know a priori the location of the reaction fronts.Applied to the SO2-lime system the SO2instantaneousabsorption rate (per unit interface area) to a singledroplet (J*A) reads:JA=DApG/H + DBBSRD/ + DA/(kGH)if : pG DBBS/kGRDJA= kGpGif : pG DBBS/kGRD(3)where DAand DBare the liquid phase diffusivitiesof sulfur and alkaline species respectively, pGis theSO2partial pressure in the gas bulk, H is the Henrysconstant, BSis the lime saturation solubility, kGthegas phase mass transfer coefficient and RDthe dropletradius. The parameter is defined as: = (1 P)?tanh() 1?(4)where: =RDrP3?3P(1 P)?1 3P?(5)In Eqs. (4) and (5) Pis the solids volume frac-tion and rPthe lime particles radius. If the condi-tion in Eq. (3) is satisfied, then the droplet surfaceconcentration of sulfur species goes to zero, the re-action front shifts to the gasliquid interface and theabsorption rate is entirely controlled by gas phaseresistance.Application of this model is based on the simplify-ing assumption that lime particles inside the dropletsdo not change in size, but deplete in number becauseof dissolution. As a consequence, the particles num-ber depletion in the droplets as well as the solidsvolume fraction increase (due to water evaporationand product precipitation) with time have been takeninto account in the material balances. Justificationfor this assumption is given as follows. As has beenshown by Scala and DAscenzo 16, for all operat-ing conditions of practical interest for spray-dry FGDthe macroscopic reaction front stands very close toF. Scala et al./Separation and Purification Technology 34 (2004) 143153147the droplet surface, being the solids dissolution ratevery fast with respect to the gas absorption rate. Typ-ical experimental evidences support this theoreticalresult: SEM/energy dispersive X-ray (EDX) analysisof cross-sections of exhausted sorbent particles showalways a sharp core-shell behavior, where the coreof the particles is formed by unreacted lime and theshell by calcium sulfite 5. As a consequence thelime particles near the gasliquid interface are boundto be dissolved much more rapidly than the others,so that after a relatively short time a particle-freezone would establish near the droplet surface. On theother hand, water rapidly evaporates from the droplet,whose surface recedes as the droplet decreases insize. This process tends to compensate particles de-pletion so that during the whole droplet life-timefresh lime particles are present near the dropletsurface.3. Model resultsThe first step was to validate the model resultsagainst experimental spray-dry FGD data available inthe literature. Subsequently, the model has been usedto predict the influence of the main operating vari-ables on the spray-dryer desulfurization performance.The procedure followed to analyze model results hasbeen that of selecting a set of operating variables asa base case for computations and to assess the in-fluence of the relevant input variables on the pro-cess by varying them one at a time. Values assignedto the operating variables for the base case and therange of variation of each variable are reported inTable 1.Table 1Operating variables values for the base case and range of variationVariableBase caseRangeTIN150C100200CCa/S?TAS15C1020CR0D30 ?m10100 ?mrP3 ?m15 ?mpG1000 ppm1002000 ppmv0D50 m/s2100 m/s3.1. Model validationAlthough a large quantity of experimental data havebeen reported in the literature ranging from lab-scaleto full-scale facilities, for most of them no goodcharacterization of droplet atomization and/or of limeslaking characteristics is available. In particular, thedroplets initial mean size and the lime particles meansize are seldom measured; as it will be shown laterthese two quantities exert a considerable influence onmodel results. Laboratory scale experimental resultsreported by Hill and Zank 8 are the only ones foundin the literature where characterization of both theabove quantities was carried out; for this reason thisset of data was chosen for validation of the presentmodel. It is important to note that once the operatingvariables are set the model has no adjustable parame-ter to enhance fitting of results to experimental data.As regards experimental data by Hill and Zank 8 thefollowing variables values are reported by the authorsand have been used in all model calculations: initialmean droplets radius R0D= 12.5 ?m; mean lime par-ticle radius rP= 1.0 ?m; average inlet sulfur dioxideconcentration pG= 500 ppm. The initial droplet ve-locity was not reported by the authors but, as will beshown later, this quantity has a negligible influenceon model results.Fig. 1 reports the comparison between the exper-imental and model data, shown as the overall sulfurdioxide removal efficiency () in the spray-dryer asa function of the calcium to sulfur molar feed ratio(Ca/S), for different flue gas inlet temperatures (TIN)and approaches to adiabatic saturation temperature atthe outlet (?TAS). In the figure it is also reported thereference line corresponding to complete calcium con-version. It can be seen that excellent agreement be-tween model and experimental results is found for lowto medium Ca/S ratios. For high stoichiometric ratios,instead, the model underpredicts the SO2removal ef-ficiency, especially for low approaches to adiabaticsaturation temperature. This result, however, was ex-pected as in these conditions the additional SO2ab-sorption due to the falling rate drying phase (after thesolid crust is formed) can be significant 9. Removalof the uniform droplet and lime particle size assump-tions by taking into account the actual size distribu-tions should further enhance the agreement betweenmodel and experimental data.148F. Scala et al./Separation and Purification Technology 34 (2004) 143153Fig. 1. Comparison between experimental data of Hill and Zank 8 and model data (continuous lines). Dashed line is the reference linefor complete calcium conversion.3.2. Model predictions3.2.1. Variables profiles along the columnFig. 2 reports droplet radius, velocity and temper-ature variation as well as the SO2removal efficiencyalong the column axis for the base case. The first twovariables have been adimensionalized with the ini-tial droplet radius (R0D) and velocity (v0D) respectivelywhile the axial coordinate has been adimensionalizedwith the total column length (L). The figure clearlyshows that the droplets reach their terminal velocityand temperature very rapidly and that during this firstphase both evaporation and sulfur absorption are neg-ligible. This finding is true whatever the droplet initialsize and velocity in the range investigated (Table 1).Two general conclusions can be drawn: firstly, thedeceleratingphaseafteratomizationalwaysaccountsfor negligible SO2capture with respect to the constantdrying phase; secondly, the initial droplet velocity haspractically no influence on the model results.Fig. 3 reports the variation along the column of theproduct between the gas phase mass transfer coeffi-cient and the specific droplets interface area (a) andof the ratio between the actual SO2absorption fluxand the maximum theoretical flux (if absorption ratewere entirely controlled by gas phase resistance) forthe base case. The first curve (kGa) is an indicatorof the gas-phase resistance to absorption and resultslargely dependent on the evolution of the droplets spe-cific surface area: in the first column section (corre-sponding to the decelerating phase) a increases rapidlybecause of the increase of the number of drops perunit volume, while afterwards a decreases becauseof the reduction of droplets radius upon evaporation.On the contrary the mass transfer coefficient first de-creases upon droplets deceleration and then slightlyincreases upon droplets radius reduction, but its influ-ence on the product kGa is overcome by the variationof the droplets specific surface area. The small bumpin the curve near the maximum is a consequence ofthe change of sign of the gas-droplets slip velocity justbefore the drops reach their terminal velocity. The sec-ond curve (J*A/kGpG) is an indicator of the relativeimportance of the different resistances to SO2absorp-tion. The curve clearly shows that, under the base caseconditions, just after atomization liquid-phase resis-tance mainly controls the process; during the constantrate drying phase both liquid-phase and gas-phase re-F. Scala et al./Separation and Purification Technology 34 (2004) 143153149Fig. 2. Adimensional droplet radius and velocity, droplet temperature and SO2removal efficiency profiles along the spray-dryer axis forthe base case.Fig. 3. Variation along the spray-dryer axis of the product kGa and of the ratio between the actual SO2absorption flux and the maximumtheoretical flux for the base case.150F. Scala et al./Separation and Purification Technology 34 (2004) 143153sistances are relevant, while when approaching thecolumn exit absorption is completely controlled bygas-phase resistance (condition in Eq. (3) is satisfiedas the SO2partial pressure and droplet radius decreaseand solids volume fraction increases).The relative importance of the different resistancescan also be analyzed by means of the liquid phase con-centration profiles of the sulfur and alkaline speciesin the droplets. Fig. 4 reports the concentration of thesulfur species (A) and of alkaline species (B) alongthe adimensional droplet radius for different positionsalong the column axis for the base case. The alka-line and sulfur species concentrations have been adi-mensionalized respectively with the lime saturationsolubility (BS) and with the sulfur dioxide concentra-tion that would be in equilibrium with the actual gaspartial pressure (pG/H). The figure shows that the re-action front (where both species concentration goesto zero) always stands very close to the droplet sur-face, being the solids dissolution rate very fast withrespect to the gas absorption rate. The reaction frontapproaches the droplet surface as the absorption pro-cess proceeds eventually reaching the surface near thecolumn exit. It is interesting to note that the adimen-Fig. 4. Adimensional concentration profiles of the sulfur species and of alkaline species along the droplet radius for different positionsalong the spray-dryer axis for the base case.sional sulfur species concentration at the surface isalways much lower than one, indicating that even inthe first stages of the process the gas phase resistanceexerts a non-negligible influence.3.2.2. Influence of operating variables on the overallperformanceLimited influence on the overall desulfurization ef-ficiency was found for the inlet gas temperature andfor the inlet SO2gas concentration in the range stud-ied, as reported in Fig. 5. The curves in the figure showthat the overall SO2removal efficiency () slightlyincreases with the gas inlet temperature, as a conse-quence of the larger water injection flow rate that inturn increases the droplets interface area. On the con-trary, removal efficiency slightly decreases with theinlet SO2gas concentration, because in order to keepa fixed Ca/S ratio a more concentrated slurry must beinjected leading to a shorter droplets life-time. Fig. 5also shows the influence of the approach to adiabaticsaturation temperature at the column exit: a larger?TAScorrespond to a lower desulfurization efficiencybecause of the lower water injection flow rate and ofthe shorter droplets life-time. While it is clear that theF. Scala et al./Separation and Purification Technology 34 (2004) 143153151Fig. 5. Overall SO2removal efficiency as a function of inlet gas temperature and inlet SO2gas concentration at different approaches toadiabatic saturation temperature at the outlet.lower the approach to adiabatic saturation tempera-ture the better the spray-dryer performance, the lowestoperating value for ?TASis determined by safety con-siderations: accidental water condensation on the bag-house fabrics that may lead to formation of mud andconsequently to fabric plugging has to be avoided.This would result in the necessity of costly plant shutdown for baghouse regeneration.The variables that exert the largest influence onthe overall desulfurization efficiency are the Ca/Smolar feed ratio, the mean initial droplet size andthe mean lime particle size. Figs. 6 and 7 report theoverall SO2removal efficiency as a function of theCa/S molar feed ratio for different droplet and par-ticle sizes, respectively. In the figures the referenceline corresponding to complete calcium conversion isalso reported. The desulfurization efficiency alwaysincreases with Ca/S, as the liquid-phase resistance toabsorption decreases; at large stoichiometric ratios,however, the efficiency tends to approach an asymp-totic value, when the absorption process is completelycontrolled by gas-phase resistance during the wholedroplets life-time. A reduction of the droplets meansize (Fig. 6) considerably decreases the overall desul-furization efficiency as a consequence of the reducedlife-time in the spray-dryer; this influence is especiallystrong for small droplet sizes. It must be underlined,however, that the largest droplet size that can be usedin practice is limited by the length of the spray-drycolumn: an important design criterion is, in fact, thatat the spray-dryer outlet only solid material must bepresent in order to avoid deposition and corrosion onthe duct walls. A critical influence is exerted by themean lime particle size in the slurry (Fig. 7). Smallerparticles correspond to much larger desulfurizationefficiencies as a consequence of the enhancement oflime dissolution that leads to a lower liquid-side re-sistance. This result indicates that particular attentionmust be paid to the lime slaking process in order toget the smallest particle size possible.152F. Scala et al./Separation and Purification Technology 34 (2004) 143153Fig. 6. Overall SO2removal efficiency as a function of the stoichiometric calcium to sulfur molar feed ratio for different mean initialdroplet sizes.Fig. 7. Overall SO2removal efficiency as a function of the stoichiometric calcium to sulfur molar feed ratio for different mean limeparticle sizes.F. Scala et al./Separation and Purification Technology 34 (2004) 1431531534. ConclusionsIn this paper a detailed model for flue gas desul-furization by spray-dry absorption with a lime slurryis presented. The model combines a steady stateone-dimensional spray-dryer model with a recentlypresented single-drop model for gas absorption withinstantaneous irreversible reaction in a rigid dropletcontaining uniformly dispersed and sparingly solublefine reactant particles. The fate of the droplets is fol-lowed from atomization until formation of a porouscoherent shell (crust) around the drying droplets. Themodel was first validated against available experi-mental spray-dry FGD results. Comparison betweenmodel and experimental results was excellent at lowto medium Ca/S feed ratios, while for high stoichio-metric ratios the model underpredicted the SO2re-moval efficiency. This result, however, was expectedas the model does not consider the additional SO2absorption due to the falling rate drying phase (afterformation of the crust) that in these conditions can besignificant.The model has then been used to study the rele-vance of the different resistances to SO2absorptionand to predict the influence of the main operating vari-ables on the spray-dryer desulfurization performance.Analysis of variables profiles along the spray-dry col-umn showed that the initial droplet velocity has noinfluence on model results and that the initial dropletsdecelerating phase always accounts for negligibleSO2capture. Results further showed that just
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