Modeling flue gas desulfurization by spray-dry absorption

Separation and Purification Technology 34 (2004) 143–153

Modeling flue gas desulfurization by spray-dry absorption

Fabrizio Scala a,∗, Michele D’Ascenzo b, Amedeo Lancia b

a Istituto di Ricerche sulla Combustione – C.N.R., P.le Tecchio, 80–80125 Napoli, Italyb Dipartimento di Ingegneria Chimica, Università degli Studi di Napoli Federico II, P.le Tecchio, 80–80125 Napoli, Italy

Abstract

A 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 SO2 absorption with instantaneous irreversiblereaction in a rigid droplet containing uniformly dispersed fine lime particles. The fate of the droplets is followed from atomizationuntil 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 SO2 absorption 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 SO2 capture. Results further showed that the controlling resistance to SO2 absorption 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 overalldesulfurization efficiency are the Ca/S molar feed ratio, the mean initial droplet size and the mean lime particle size. In particular,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; Slurry

1. Introduction

Spray-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 [1–4].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 cheaper

∗ Corresponding author. Tel.: +39-081-768-2969;fax: +39-081-593-6936.

E-mail address:[email protected] (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% SO2 removalefficiency at 1–2 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.

In the spray-dry FGD process the hot flue gas is con-tacted with a fine spray of alkaline suspension, usuallylime, in a reaction chamber where a droplet residencetime of 10–15 s is provided. During their life-time the

1383-5866/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/S1383-5866(03)00188-6

144 F. Scala et al. / Separation and Purification Technology 34 (2004) 143–153

sprayed droplets simultaneously evaporate and absorbSO2. The absorbed SO2 reacts in the alkaline aqueousphase with the dissolved lime following the overallreaction:

SO2 + Ca (OH)2H2O−→ CaSO3 · 1

2 H2O + 12 H2O

(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. Broadly speaking, the desulfurization ef-ficiency in a spray-dryer is the result of the competitionbetween the SO2 absorption rate in the slurry dropletsand the water evaporation rate from the droplets [5–8].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 second‘constant 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 drying’phase 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 [3,4,8–13], 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 SO2 diffusion 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. [6]and 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 D’Ascenzo [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 D’Ascenzo [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 ‘decelerating’and ‘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. Model

F. Scala et al. / Separation and Purification Technology 34 (2004) 143–153 145

results 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 description

The sulfur dioxide absorption process can beschematized as a series of steps:

(i) gas phase diffusion of SO2 from the gas bulk tothe droplets surface;

(ii) dissolution of SO2 at the droplet surface;(iii) formation of sulfurous acid and dissociation into

ionic sulfur species following the scheme:

SO2(aq) + H2O ⇒ H2SO3(aq)

H2SO3 (aq)⇔ H+ + HSO−3 (aq) (2)

HSO3−(aq)⇔ H+ + SO3

2−(aq)

(iv) liquid phase diffusion of sulfur species towardsthe droplet center;

(v) parallel dissolution of calcium hydroxide parti-cles;

(vi) liquid phase diffusion of alkaline species towardsthe 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 HSO−

3 is 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 as

drying 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 isothermal[8].

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 (Henry’s 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 CO2 in the flue gas has no influ-ence on the desulfurization process. This assump-tion is based on the experimental finding that for-mation of CaCO3 under spray-dry conditions istypically negligible [1,5] and that SO2 absorptionis not influenced by the CO2 concentration 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 the


Lagrangian 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 SO2 removal 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 agglomerates[5]. 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 D’Ascenzo [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 these

particles 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 SO2 instantaneousabsorption rate (per unit interface area) to a singledroplet (J*A) reads:

J∗A = DApG/H + DBBS

RD/β + DA/ (kGH)

if : pG > βDBBS/kGRD

J∗A = kGpG

if : pG ≤ βDBBS/kGRD

(3)

where DA and DB are the liquid phase diffusivitiesof sulfur and alkaline species respectively, pG is theSO2 partial pressure in the gas bulk, H is the Henry’sconstant, BS is the lime saturation solubility, kG thegas phase mass transfer coefficient and RD the dropletradius. The parameter β is defined as:

β = (1 − εP)

(α

tanh (α)− 1

)(4)

where:

α = RD

rP

3

√3εP

(1 − εP)(1 − 3

√εP

) (5)

In Eqs. (4) and (5) εP is the solids volume frac-tion and rP the 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 gas–liquid 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 D’Ascenzo [16], for all operat-ing conditions of practical interest for spray-dry FGDthe macroscopic reaction front stands very close to


the 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 gas–liquid 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 results

The 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 variation

Variable Base case Range

T IN 150 ◦C 100–200 ◦CCa/S 1.5 0.1–2.4�TAS 15 ◦C 10–20 ◦CR0

D 30 �m 10–100 �mrP 3 �m 1–5 �mpG 1000 ppm 100–2000 ppmv0

D 50 m/s 2–100 m/s

3.1. Model validation

Although 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 R0

D = 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 (η) 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 SO2 removal ef-ficiency, especially for low approaches to adiabaticsaturation temperature. This result, however, was ex-pected as in these conditions the additional SO2 ab-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.


Fig. 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 predictions

3.2.1. Variables profiles along the columnFig. 2 reports droplet radius, velocity and temper-

ature variation as well as the SO2 removal efficiencyalong the column axis for the base case. The first twovariables have been adimensionalized with the ini-tial droplet radius (R0

D) and velocity (v0D) respectively

while 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, the‘decelerating’ phase after atomization always accountsfor negligible SO2 capture 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 SO2 absorption flux

and 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 SO2 absorp-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-


Fig. 2. Adimensional droplet radius and velocity, droplet temperature and SO2 removal 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 SO2 absorption flux and the maximumtheoretical flux for the base case.


sistances are relevant, while when approaching thecolumn exit absorption is completely controlled bygas-phase resistance (condition in Eq. (3) is satisfiedas the SO2 partial 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 overallperformance

Limited influence on the overall desulfurization ef-ficiency was found for the inlet gas temperature andfor the inlet SO2 gas concentration in the range stud-ied, as reported in Fig. 5. The curves in the figure showthat the overall SO2 removal efficiency (η) 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 SO2 gas 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�TAS correspond to a lower desulfurization efficiencybecause of the lower water injection flow rate and ofthe shorter droplets life-time. While it is clear that the


Fig. 5. Overall SO2 removal efficiency as a function of inlet gas temperature and inlet SO2 gas 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 �TAS is 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 SO2 removal 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.


Fig. 6. Overall SO2 removal efficiency as a function of the stoichiometric calcium to sulfur molar feed ratio for different mean initialdroplet sizes.

Fig. 7. Overall SO2 removal efficiency as a function of the stoichiometric calcium to sulfur molar feed ratio for different mean limeparticle sizes.


4. Conclusions

In 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 SO2 re-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 SO2 absorptionand 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 negligibleSO2 capture. Results further showed that just after at-omization liquid-phase resistance mainly controls theprocess, during the constant rate drying phase bothliquid-phase and gas-phase resistances are relevant,while at the column exit absorption is completelycontrolled by gas-phase resistance. The operatingvariables that exert the largest influence on the overalldesulfurization efficiency were shown to be the Ca/Smolar feed ratio, the mean initial droplet size and themean lime particle size. Careful control of the lasttwo variables is the key to obtain a good spray-dryerperformance.

Acknowledgements

Useful discussion on numerical model solution pro-cedure with Dr M. Grosso is gratefully acknowledged.

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Modeling flue gas desulfurization by spray-dry absorption