Sulfation Behaviour of Lime Stone Partciles in AFBC Test Rig

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Paper No. FBC99-0101

Transient Sulfation Behavior of Limestone Particles in an

AFBC Test Rig: Data For Validation Studies

Proceedings of the 15th International Conference on

Fluidized Bed Combustion

May 16 - 19, 1999

Savannah, Georgia

Copyright 1999 by ASME


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TRANSIENT SULFATION BEHAVIOR OF LIMESTONE PARTICLES IN

AN AFBC TEST RIG: DATA FOR VALIDATION STUDIES

Levent Organ and Nevin Seluk *

Department of Chemical EngineeringMiddle East Technical University

06531 Ankara, TURKEY

Tel: +90 (312) 210 2603, Fax: +90 (312) 210 1264E-mail: [email protected]

ABSTRACTIntraparticle conversion profiles and evolution of

pore structure with time were investigated byexposing essentially non-porous limestone particleswith an average size of 0.655 mm to reaction

conditions in a 0.3 MWt atmospheric fluidized bedcombustor (AFBC) operating under steady stateconditions with and without limestone addition.Structural analyses of the partially sulfated particlesshow that both the surface area and pore volumeincrease at the early stages of the reaction but later

pore volume remains nearly constant while BETsurface area progressively decreases due to the

blockage of smaller pores by the bulky reactionproduct, calcium sulfate. Correspondingly, the poresize distributions indicate a progressive shift toward

larger pore diameters. The particles were alsoexamined by means of SEM-EDX analysis techniquein order to determine their intraparticle sulfation

profiles. The analyses demonstrate that theconversion profile starts from a maximum at the

particle surface and decreases progressively towardthe product layer-core interface.

________*Corresponding author

INTRODUCTIONLimestone particles, when injected into an

atmospheric fluidized bed combustor, experiencesimultaneous calcination and sulfation. Thesereactions result in considerable changes in the pore

structure of the particles, which affect their reactivityand ultimate conversion. Hence, a complete physicaland chemical characterization of the particles must

be undertaken at various stages of the reaction inaddition to obtaining the usual conversion-time data.Such data are particularly valuable for particlereaction models which consider the change of porestructure with time.

Some of the experimental studies on the evolutionof pore structure with time have been carried out forlimestone injection processes involving smaller

particles, shorter residence times and highertemperatures compared to fluidized bedcombustion (Gullett and Bruce, 1987; Mahuli et al.,1997). Krishnan and Sotirchos (1994) haveinvestigated the variation of particle porosity withthe extent of sulfation for calcined limestone

particles. The particle size and the reactiontemperature of this experimental work fall in therange commonly employed in fluidized bedcombustors. However, as emphasized by Haji-


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Sulaiman and Scaroni (1992), instantaneouscalcination is not possible for such large particlesdue to the presence of carbon dioxide in thecombustion gases. In a study carried out by Dam-Johansen and stergaard (1991), highly porouslimestone particles of the size relevant to fluidized

bed combustion have been sulfated and pore sizedistributions of the original, calcined and sulfatedsorbents were compared. However, the commercialsorbents employed for in-situ desulfurization influidized bed combustors are usually non-porous.

Therefore, in this study, in an attempt to provideexperimental data for validation of various

mathematical models of the limestone sulfationreaction, the transient evolution of pore structure ofnon-porous limestone particles and variation ofintraparticle conversion profiles were investigated ina 0.3 MWt AFBC test rig burning low quality, highsulfur content lignite in its own ash.

EXPERIMENTALTest Rig

This study is based on experimental data collected

as part of a complete experimental research programfor the investigation of combustion and in-situdesulfurization characteristics of low quality Turkishlignites. The experimental work was carried outon a 0.3 MWt AFBC test rig designed andconstructed within the scope of a cooperationagreement between Middle East TechnicalUniversity (METU) and Babcock and WilcoxGAMA (BWG) under the auspices of CanadianInternational Development Agency (CIDA). Thedetailed description of the rig, its operation and dataacquisition and control system can be foundelsewhere (Kirmizigl et al., 1995; Seluk et al.,1997).

Sulfation characteristics of limestone wereinvestigated by carrying out two experiments. Table1 lists the operating conditions for the runs. The firstrun was carried out without limestone addition.During the experiments a typical Turkish lignite,namely Beypazari, and Bartin limestone were used.Coal and limestone were fed to the combustor

through an under-bed feeding port by a variablespeed screw feeder. Characteristics of Beypazarilignite and its ash are summarized in Tables 2 and 3,respectively. As can be seen from Table 2, the ligniteis characterized by its high ash and combustiblesulfur contents. Physico-chemical properties ofBartin limestone fed to the combustor are given inTable 4.

Sample HolderIn order to study the changes occurring in

limestone particles at various stages of the reaction,

the particles were placed into a sample holder, andexposed to reaction conditions of both run 1 (withoutlimestone addition) and run 2 (with limestoneaddition) at the steady state operation of the rig. Thetip of the sample holder, where the particles arefluidized by the upcoming combustion gases, is acylindrical stainless steel wired mesh (ASTM 50) 18mm in diameter and 100 mm in length (Organ,1997). In each run, particles were allowed to reactfor different time intervals. In each case, 3.510-3 kgof raw limestone sample with a size range of -25/+30

(ASTM), corresponding to an average size of 0.655mm was placed into the holder. This wasapproximately the same as the average particle sizeof the limestone fed to the combustor, which wasdetermined by considering that very small particleslead to excessive elutriation losses while too large

particles lead to underutilization. The sample holderis introduced to the combustor through a hole located43.5 cm above the expanded bed height. Table 5shows the reaction conditions to which the particleswere exposed during each run at this location. After

being removed from the holder, the partially sulfatedparticles were kept under dry conditions. Theseparticles were, in turn, subjected to various physicaland chemical analyses.

Structural AnalysesSurface area development of partially sulfated

particles during the reaction was studied by usingMicromeritics ASAP 2000 gas adsorption/desorption


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Table 1. Operating conditions for runs 1 and 2

Run 1 Run 2Coal flow rate, kg/s 3.210-2 3.210-2

Limestone flow rate, kg/s 0 4.210-3

Bed drain flow rate, kg/s 7.510-3 7.510-3

Carryover flow rate, kg/s 6.910-3 9.410-3

Air flow rate, kmol/s 510-3 510-3

Excess air, % 22 22

Superficial velocity, m/s 2.4 2.4

Average bed temperature, K 1130 1135

Combustion efficiency, % 97 97

Ca/S molar ratio (includes Ca in Coal) 2 3.8

Sulfur retention efficiency, % 42 65

Calcium Utilization, % 21 17

Table 2. Characteristics of Beypazarlignite

Sieve Analysis Proximate Analysis Ultimate Analysis (dry)

Size, mm Weight (%) Comp. Weight (%) Comp. Weight (%)

4.00-2.36 27.18 Mois. 9.00 C 31.35

2.36-1.70 34.35 Ash 45.36 H 2.23

1.70-1.40 9.08 VM 40.32 O 12.96

1.40-1.18 9.66 FC 5.32 N 1.22

1.18-0.85 11.86 S (org) 1.39

0.85-0.60 3.07 HHV: 1.06104 kJ/kg S (prt) 1.00

0.60-0.43 1.14 dp mean: 1.26 mm S(slf) 0.74

0.43-0.00 3.64 density: 1920 kg/m3 Ash 49.85


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Table 3. Ash Analysis of Beypazarlignite

Component Weight, % (dry)

SiO2 37.19

Al2O3 19.31

Fe2O3 7.24

CaO 16.71

MgO 4.98

SO3 13.83

Na2O 3.05

K2O 1.25

Table 4. Characteristics of Bartn limestone*

Sieve Analysis Chemical Analysis (dry)

Size, mm Weight, % Component Weight, %

1.40-1.00 7.39 CaCO3 94.54

1.00-0.85 23.71 MgCO3 2.89

0.85-0.71 25.73 SiO2 2.27

0.71-0.60 19.33 Na2O 0.07

0.60-0.50 13.71 K 2O 0.09

0.50-0.00 10.12 R 2O3 0.21

*Average Diameter: 0.64 mm

Table 5. Sample holder conditions

Run 1 Run 2

SO2, ppm (dry) 1133 773

CO2, % (dry) 11 11

O2, % (dry) 8 8

Temperature, K 1131 1147

equipment. Surface area of the particles wascalculated from the BET method using nitrogenadsorption data. The particles were, then, removedfrom the analyzer and subjected to mercury

porosimetry. A Micromeritics 9310 mercuryporosimeter was used for the determination of porevolume and pore size distribution.

SEM-EDX AnalysesIn order to prepare pellets for SEM-EDX analysis,

sulfated particles were mixed with transoptic powder(dialylphthalate) and placed into a stainless steel

mold. The mold was then heated from the roomtemperature to about 433 K under a pressure of 28MPa in a hydraulic press equipped with an electricalheater. The sample was hardened by slow cooling

back to the room temperature while the pressure wasstill kept constant at 28 MPa. The pellet was, then,removed from the mold, sectioned by using variousgrit papers (320, 400, 600) to expose the crosssections of the particles and polished by usingalumina and diamond pastes. The surface of the

pellets were in turn coated with a thin, uniform

conductive carbon layer. The samples were, then,analyzed with a JEOL JSM 6400 scanning electronmicroscope combined with a NORAN/Series II X-ray analyzer.

Intraparticle conversion profiles were obtained bydetermination of the molar ratio between sulfur andcalcium in a large number of points on a straight line

bisecting the particle cross-section.

S and Ca Analyses

The overall conversion of the particles wascalculated by using the sulfur and calcium contentsof the particles determined by a Leco analyzer andgravimetric method, respectively.

RESULTS AND DISCUSSIONOverall Conversion

Figure 1 shows the overall conversions of thelimestone particles introduced to the test rig in the


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two runs. The conversion values are very low forboth runs. Higher values for the first run areconsidered to be due to the higher sulfur dioxideconcentration to which the particles are exposed inthis run. As can be seen from the figure, there is alittle increase in conversion after a reaction time ofabout 60 minutes. Due to the bulky reaction product,calcium sulfate, plugging of pore openings takes

place and pore and product layer diffusionalresistances become significant at the early stages ofthe reaction for large particles. Therefore, thereaction is restricted to an outer shell surrounding the

particles, thereby decreasing the ultimate conversion

to values much lower than the theoretical maximumdetermined by the pore volume of the sorbent.Sotirchos and Zarkanitis (1992) argued that even inthe absence of intraparticle diffusional limitations,formation of inaccessible pore space limits theultimate capacity of a sorbent to a level below theone corresponding to complete plugging of the porespace with the solid product. Furthermore, when it isconsidered that the reaction is carried out withlimestone rather than calcined stone, the lowconversion values in Fig. 1 are not unexpected and

complies with the findings of Haji-Sulaiman andScaroni (1992).

BET Surface Area and Pore VolumeFigures 2 and 3, respectively, show BET surface

area and pore volume of the particles exposed toreaction conditions for different time intervals duringrun 1 (without limestone addition) and run 2 (withlimestone addition) at the steady state operation ofthe rig. Pore volumes are intrusion volumes above an

applied pressure of 551 kPa corresponding to porediameters less than 1 m. As can be seen from thefigures, both BET surface area and pore volumeincrease with steep gradient for the first 10 minutesof the reaction time, indicating faster rate ofcalcination than sulfation (Fig. 1) at the early stagesof the reaction. After this time period, the BETsurface area of the particles decreases continuouslywhile the pore volume remains nearly constant,suggesting preferential decline of the pore volume in

smaller pores. This finding is in contradiction withthat of Gullett and Bruce (1987) who studied thesulfation of precalcined sorbents and found that bothBET surface area and pore volume decrease withreaction time. This can be mainly due to very small

particles (< 3 m) used in their study as opposed tolarger ones (655 m) in this study.

Figures 4 and 5 illustrate pore size distributions ofthe particles exposed to reaction conditions fordifferent time intervals during run 1 and run 2,respectively, at the steady state operation of the rig.The common feature between the two figures is thatthe pore size distribution shifts to larger pore sizes as

the reaction proceeds. This supports the blockage ofthe smaller pores indicated by the variation of BETsurface area and pore volume with time (Fig. 2 and3). However, the progressive shift of the peak withreaction time for the experiments with limestoneaddition is not as significant as that for theexperiment without limestone addition.

Intraparticle Conversion ProfilesFigure 6 shows typical EDX mappings of sulfur in

the particles reacted for 10, 30 and 60 minutes,respectively, for the experiment without limestoneaddition. Each mapping is a collection of EDXanalyses performed on a matrix of 128128 data

points throughout a particle cross-section. The figureclearly indicates the presence of a sulfate shellsurrounding each particle, the thickness of the shellincreasing with the time of exposure of the particlesto the reaction conditions. Comparison between theintraparticle conversion profiles for the particlesreacted for 10, 30, 60 minutes for the experimentwithout limestone addition is illustrated in Fig. 7.The position and direction of the lines along whichthe profiles were determined are indicated by thearrows on the corresponding mappings of Fig. 6. Ascan be seen from the figure, the fractional conversionfor different reaction times start from a maximum atthe surface of the particles and decrease

progressively with the distance from the surface.Fractional conversion values less than unity withinthe reacted shell reveal that the reaction cannot be


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described by a simple shrinking core model asdepicted by the EDX mappings at first glance. Thiscomplies with the findings of Dam-Johansen andstergaard (1991).

Figure 7 also shows the effect of reaction time onthe intraparticle conversion profiles. As can be seenfrom the figure, degree of conversion at any distancefrom the particle surface increases with reactiontime. Typical EDX mappings of sulfur in the

particles reacted for 10, 30 and 60 minutes,respectively, for the experiment with limestoneaddition are illustrated in Fig. 8. The trend of theintraparticle profiles corresponding to Fig. 8 are

similar to those obtained for the experiment withoutlimestone addition (Fig. 9). However, localconversion values are found to be lower due to lowersulfur dioxide concentration the particles areexposed to in this run.

CONCLUSIONSTransient behavior of pore structure and

intraparticle conversion profiles were investigated byintroducing non-porous limestone particles into a 0.3

MWt AFBC test rig during steady state experimentswith and without limestone addition. The followingconclusions were reached from the observations inthis study:(1) During the sulfation of non-porous limestone

particles both BET surface area and pore volumeincrease with steep gradient at the initial stages ofthe reaction but later pore volume remains nearlyconstant while surface area progressivelydecreases due to the preferential blockage ofsmaller pores.

(2) Peak of the pore size distribution curves of theparticles exposed to reaction conditions forincreasing reaction times shifts toward larger porediameters, the extent of the shift being moresignificant for the experiment without limestoneaddition.

(3) The reaction cannot be described by a simpleshrinking-core model as the conversion startsfrom a maximum at the particle surface anddiminishes progressively within the reacted shell.

Experimental data presented in this paper can beused for validation of various mathematical modelsfor the sulfation of non-porous limestone particles ofsizes typically employed in AFBCs.

REFERENCESDam-Johansen, K., and stergaard, K., 1991,

High Temperature Reaction Between SulfurDioxide and Limestone-II. An Improved Basis for aMathematical Model, Chemical EngineeringScience, Vol. 46, No. 3, pp. 839-845.

Gullett, B.K., and Bruce, K.R., 1987, PoreDistribution Changes of Calcium-Based SorbentsReacting with Sulfur Dioxide,AIChE Journal, Vol.33, No. 10, pp. 1719-1726.

Haji-Sulaiman, M., Z., and Scaroni, A.W., 1992,The Rate Limiting Step in the Sulfation of NaturalLimestones During Fluidized Bed Combustion,

Fuel Processing Technology, Vol. 31, pp. 193-208.Kirmizigl ., Barlas D., and Seluk N., 1995,

METU 0.3 MW AFBC Test Rig, Preprints ofInternational Symposium on Coal Fired Power

Generation, The Environment and PublicAcceptance, Ministry of Energy and NaturalResources and Turkish Electricity GenerationTransmission Corporation, pp. 257-295.

Krishnan, S., V., and Sotirchos, S., V., 1994,Effective Diffusivity Changes During Calcination,Carbonation, Recalcination, and Sulfation ofLimestones, Chemical Engineering Science, Vol.49, No. 8, pp. 1195-1208.

Mahuli, S.K., Agnihotri, R., Chauk, S., Ghosh-Dastidar, A., Wei, S. -H., and Fan, L.-S., 1997,

Pore-Structure Optimization of Calcium Carbonatefor Enhanced Sulfation, AIChE Journal, Vol.43,

No. 9, pp. 2323-2335.Organ, L., 1997, Sulfation Characteristics of

Limestone in 0.3 MW AFBC Test Rig, M.S. Thesis,Middle East Technical University, Ankara, Turkey.

Seluk, N., Degirmenci, E., and Oymak, O., 1997,Simulation of 0.3 MWt AFBC Test Rig BurningTurkish Lignites, Proceedings, 14th International


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Combustion on Fluidized Bed Combustion, F. D. S.Preto, ed., Vol. 2, pp. 1163-1174.

Sotirchos, S.V., Zarkanitis, S., 1992, InaccessiblePore Volume Formation During Sulfation ofCalcined Limestones, AIChE Journal, Vol. 38, No.10, pp. 1536-1550.


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Figure 1. Variation of overall conversion with time for runs 1 and 2

Figure 2. Variation of BET surface area with time for runs 1 and 2


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Figure 3. Variation of pore volume with time for runs 1 and 2

Figure 4. Pore size distributions of particles exposed to reactionconditions of run 1 for different time intervals


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Figure 5. Pore size distributions of particles exposed to reactionconditions of run 2 for different time intervals

a) 10 min. b) 30 min. c) 60 min.

Figure 6. Sulfur mappings of particles exposed to reaction conditions of run 1 for differenttime intervals


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Figure 7. Intraparticle conversion profiles of particles exposed toreaction conditions of run 1 for different time intervals

a) 10 min. b) 30 min. c) 60 min.

Figure 8. Sulfur mappings of particles exposed to reaction conditions of run 2 for differenttime intervals


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Figure 9. Intraparticle conversion profiles of particles exposed toreaction conditions of run 2 for different time intervals

Sulfation Behaviour of Lime Stone Partciles in AFBC Test Rig

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