Regeneration of Sulfided Dolomite with Steam and Carbon Dioxide

The regeneration process of a sulfided dolomite with steam and carbon dioxide was analyzed in a thermogravimetric system at temperatures between 475 a...
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Energy & Fuels 2001, 15, 85-94

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Regeneration of Sulfided Dolomite with Steam and Carbon Dioxide J. Ada´nez,* F. Garcı´a-Labiano, A. Abad, L. F. de Diego, and P. Gaya´n Instituto de Carboquı´mica (C.S.I.C.), Department of Energy and Environment, Marı´a de Luna, 12, 50015, Zaragoza, Spain Received April 17, 2000. Revised Manuscript Received September 25, 2000

The regeneration process of a sulfided dolomite with steam and carbon dioxide was analyzed in a thermogravimetric system at temperatures between 475 and 650 °C. Different gas concentrations from 5 to 40% and different H2O/CO2 ratios were used to isolate the effect of the different gases on the regeneration process. The morphological changes of the sorbent during regeneration and cyclic sulfidation/regeneration reactions were followed by SEM-EDX analysis, which indicated a grain structure inside the particle. The changing grain size model with a variable diffusivity in the CaCO3 product layer was used to predict the experimental data, and to obtain the kinetic parameters of the regeneration reaction. The sintering of the CaCO3 has showed to be responsible of the changes in the solid diffusivity along the time, the H2O being the more important sintering agent on the process. Finally, the dolomite was subject to the calcination/ sulfidation/regeneration process during 20 cycles and no activity loss was detected along the successive cycles.

Introduction Increasing requirements to decrease the environmental impact from coal-based power plants has resulted in intensive research in order to develop new processes with low emission of air pollutants. In many cases the gaseous pollutants, for example, the sulfur-derived compounds, are retained by a sorbent, generally based on calcium, such as limestone or dolomite. In the case of processes using coal gasification, the solid generated must be stabilized before disposing, which produces additional costs in the whole process. The most typical alternative is to oxidize CaS to produce the stable CaSO4,1,2 although recent studies have shown that this process is not so straightforward,3-5 and specially when using millimeter-size sorbent particles. The oxidation reaction stopped due to the increase in the intraparticle diffusion resistance as a consequence of the formation of a CaSO4 product layer around the particle with an internal core of CaS. In the last years several studies have been directed toward the possibility to regenerate the calcium-based * To whom correspondence should be addressed. Phone: (34)976733977. Fax: (34)976733318. E-mail: [email protected]. (1) Yrjas, P.; Hupa, M.; Iisa, K. Pressurized Stabilization of Desulfurization Residues from Gasification Processes. Energy Fuels 1996, 10 (6), 1189-1195. (2) Marban, G.; Garcia-Calzada, M.; Fuertes, A. B. Kinetics of Oxidation of CaS Particles in the Regime of Low SO2 Release. Chem. Eng. Sci. 1999, 54 (1), 77-90. (3) Abbasian, J.; Rehmat, A.; Banerjee, D. D. Sulfation of Partially Sulfided Calcium-based Sorbents. Ind. Eng. Chem. Res. 1991, 30, 1990-1994. (4) Ninomiya, Y.; Sato, A.; Watkinson, A. P. Oxidation of Calcium Sulfide in Fluidized Bed Combustion/Regeneration Conditions. Proc. 13th Int. Conf. Fluid. Bed Combust. 1995, 1027-1033. (5) Qiu, K.; Mattison, T.; Steenari, B.-M.; Lindqvist, O. Thermogravimetric Combined with Mass Spectrometric Studies on the Oxidation of Calcium Sulfide. Thermochim. Acta 1997, 298, 87-93.

sorbents, which will reduce the sorbent requirement during fuel gas desulfurization as well as waste handling. The oxidative procedures may be preferred if sulfuric acid is an acceptable product, while the reductive regeneration procedure is advantageous if sulfur is to be marketed in elemental form. From the thermodynamic consideration, the oxidative reaction of the CaS is favorable at atmospheric or lower pressures and high temperatures to give SO2-rich exhaust gas.6 However, Schewerdtfeger and Barin7 concluded that it is impossible to convert CaS quantitatively to CaSO4 by a simple one-step oxidation with air at elevated temperatures and suggested a two-step process at high temperature or one-step process operating at low temperature and involving reaction acceleration by a catalyst. Jagtap and Wheelock8 proposed a regenerating process for calciumbased sorbents by subjecting particles to repeated cycles of oxidation and reduction at temperatures between 950 and 1100 °C. van der Ham et al.9 proposed a three steps process which includes several sulfidation-oxidationregeneration cycles. The CaS formed during sulfidation is partially oxidized with SO2 in a second step to obtain CaSO4 and elemental sulfur. In a third step, the solid mixture of CaS and CaSO4 remaining after the oxidation step is decomposed to produce CaO and SO2 which are (6) Turkdogan, E. T.; Olsson, R. G. Desulphurization of Hot Reducing Gases with Calcined Dolomite. Ironmaking Steelmaking 1978, 4, 168-176. (7) Schwerdtfeger, K.; Barin, Y. Problems in Hot Desulfurization of Coal Gas with Lime. Erdo¨ l Kohle 1993, 46 (3), 103-110. (8) Jagtap, S. B.; Wheelock, T. D. Regeneration of Sulfided CalciumBased Sorbents by a Cyclic Process. Energy Fuels 1996, 10 (3), 821827. (9) van der Ham, A. G. J.; Heesink, A. B. M.; Prins, W.; van Swaaij, W. P. M. Proposal for a Regenerative High-Temperature Process for Coal Gas Cleanup with Calcined Limestone. Ind. Eng. Chem. Res. 1996, 35, 1487-1495.

10.1021/ef000079t CCC: $20.00 © 2001 American Chemical Society Published on Web 11/18/2000

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reused in previous steps. Brooks and Lynn10 proposed the production of fresh CaCO3 from an aqueous slurry of CaS by the use of methyldiethanolamine (MDEA) or other alkanolamines. Abbasian11 regenerated samples of sulfided dolomite with water and carbon dioxide at low temperature. Qiu et al.12 proposed a regenerative process that involves the exposure of CaS to alternating oxidizing and inert atmospheres. All the regeneration and disposal schemes described above include complicated processes and some of them require further processing because of the diluted streams of sulfur compounds produced. To justify the complex processes proposed some of them argue that the direct regeneration of the sulfidated calcium-based sorbent with a mixture of H2O and CO2 under pressure and at moderate temperatures (500-700 °C), as suggested by Squires et al.13 and Keairns et al.,14 was not feasible. This process was tested on pilot plant scale by Conoco Coal Development Co.15 They used two interconnected fluidized bed reactors as desulfurizer and regenerator reactors. The main problems attributed to this method were the low H2S concentrations obtained upon regeneration (1-3 vol %), the sorbent attrition, and the decreasing regenerability of the sorbent along the cycles.16,17 The thermodynamic equilibrium of the reaction

CaS + H2O + CO2 T CaCO3 + H2S given by the following equations,18

Kr ) log Kr )

PH2S PH2OPCO2 6093 - 8.056 T

(1)

(2)

indicate that the H2S content of the exhaust gas increases, and the quantity of CO2 and H2O used per mole of H2S produced decreases, with increasing total pressure and decreasing temperature. It is seen that by adjusting the temperature, pressure and composition of (H2O + CO2) mixture, it is possible to achieve a desired concentration of H2S in the exhaust gas to suit the (10) Brooks, M. W.; Lynn, S.Recovery of Calcium Carbonate and Hydrogen Sulfide from Waste Calcium Sulfide. Ind. Eng. Chem. Res. 1997, 36 (10), 4236-4242. (11) Abbasian, J. Stabilization and Regeneration of Spent Sorbents. In Desulfurization of Hot Coal Gas; Atimtay, A. T., Harrison, D. P., Eds.; NATO ASI Series; Springer: Berlin, Germany, 1998; pp 283296. (12) Qiu, K.; Lindqvist, O.; Mattisson, T. Regeneration of Calcium Sulfide under Alternating Oxidizing and Inert Conditions: Kinetics and Mechanism. Ind. Eng. Chem. Res. 1998, 37 (3), 923-928. (13) Squires, A. M.; Graff, R. A.; Pell, M. Desulfurization of Fuels with Calcined Dolomite. Part 1. Introduction and First Kinetic Results. Chem. Eng. Prog., Symp. Ser. 1971, 67 (115), 23-34. (14) Keairns, D. L.; Newby, R. A.; O’Neill, E. P.; Archer, D. H. HighTemperature Sulfur Removal System Development for the Westinghouse Fluidized Bed Coal Gasification Process. Am. Chem. Soc. Prepr.Div. Fuel Chem. 1976, 21 (4), 91-113. (15) O’Neill, E. P.; Keairns, D. L. Selection of Calcium Based Sorbents for High-Temperature Fossil Fuel Desulfurization. AIChE Symp. Ser. 1977, 73, 100-107. (16) Furimsky, E.; Yumura, M. Solid Adsorbents for Removal of Hydrogen Sulfide from Hot Gas. Erdo¨ l Kohle 1986, 39 (4), 163-172. (17) Dobner, S.; Kan, G.; Graff, R. A.; Squires, A. M. A Thermobalance for high-Pressure Process Studies. Thermochim. Acta 1976, 16, 251-265. (18) Barin, I. Thermochemical Data of Pure Substances; VCH Verlag: Weinheim; 1989.

Figure 1. Equilibrium H2S content of wet effluent gas as a function of temperature for indicated pressures.

requirements in the Claus process for the recovery of sulfur. Figure 1 shows the equilibrium H2S concentrations as a function of temperature and pressure for an equimolar ratio of 30% of H2O-CO2 during sorbent regeneration. For instance, a stream containing about 24% H2S can be achieved working at 500 °C and 20 atm of pressure. Even if diluted streams of H2S would be obtained working at higher temperatures to increase the reaction rate, the recent advances in the sulfur industry treatment19 would make feasible the use of this regeneration method. On the other hand, the sorbent attrition that takes place in fluidized beds could be drastically reduced by the use of moving or fixed bed reactors. However, the reasons that produce the decreasing regenerability of the sorbent by increasing the number of sulfidation-regeneration cycles are not clear. A reaction mechanism which postulates the migration and sintering of MgO on CaS crystals20,21 and the decrease of crystallographic defects by sorbent recrystallization16 are the main reasons given for the regenerability decrease of sulfided dolomite with repeated regeneration. Nevertheless, the regeneration of sulfided dolomite could be accomplished more readily than that of sulfided limestone without presence of MgO.16 This fact makes necessary the search for other reasons besides the given above. This paper analyzes the regenerative reaction of a sulfided dolomite with H2O and CO2, and the causes limiting the sorbent regenerability. A grain model with changing grain size was used to obtain the kinetic parameters of the sorbent. Moreover, several sulfidation-regeneration cycles were carried out under atmospheric pressure to investigate the long-term behavior of the sorbent. Obviously, the industrial process will need pressures above 15 atm to obtain acceptable H2S concentrations in the produced stream during sorbent regeneration although this work is focused on the (19) Nehb, W.; Vydra, K. Sulfur. In UllmannÅs Encyclopedia of Industrial Chemistry; Elvers, B., Hawkins, S., Russey, W., Eds; VCH: New York, 1994;Vol A25, pp 507-567. (20) Curran, G. P.; Pasek, B.; Pell, M.; Gorin, E. Reaction of H2S with Half-Calcined Dolomite in a Regenerable Process. Am. Chem. Soc. Prepr.- Div. Fuel Chem. 1976, 114-128. (21) Sun, C. C.; O’Neill, E. P.; Keairns, D. L. The Sulfidation and Regeneration of Half-Calcined Dolomite. Thermochim. Acta 1978, 26, 283-296.

Regeneration of Sulfided Dolomite

Energy & Fuels, Vol. 15, No. 1, 2001 87 Table 1: Chemical Analysis and Physical Characteristics of the Dolomite composition (wt %)

dolomite (Sierra Arcos)

CaCO3 MgCO3 Na2O K2O SiO2 Al2O3 Fe2O3 Soa (m2 m-3) 0a Fra (g cm-3) VMa (cm3 mol-1) a

Figure 2. Diagram of the experimental setup for thermogravimetric analysis.

52.5 40.5 0

C ) C0 dC )0 dR

at R ) 0 and t > 0

(

)

(6)

CCO2,2 )

at r ) r1

(8)

C ) C2

at r ) r2

(9) at r ) r2 (10)

The analytical solution of this equation allow us to

[ [

( )]

(11)

( )]

(12)

CCO2,2r2kr Dr CCO2,1

1+

CH2O,2r2kr Dr

r2 1r1

r2 1r1

The grain size r1 and the unreacted core size r2 at each time and position inside the particle, are calculated with the equations:

dr2 ) krCH2O,2CCO2,2VM dt

(13)

r31 ) Zrr30 + (1 - Zr)r32

(14)

The initial radius of the sulfided grains r0 within a certain particle can be obtained from the relationship:

r0 )

3(1 - 0) S0

(15)

where the values of 0 and S0 were derived from calcined sorbent data assuming complete sulfidation of the sorbent. The effective diffusivity inside the pores is calculated as a function of the particle porosity by

De ) Dg2

(16)

Because of the variation in the size of the pores during the regeneration reaction, the gas diffusivity was calculated as a combination of the molecular23 and Knudsen diffusions: -1 -1 Dg ) [D-1 m + Dk ]

(17)

The different molecular diffusivity values of the reacting gases, H2O and CO2, produces different concentration profiles inside the particle. Assuming the particle as a solid matrix composed of uniformly sized spherical grains of radius r2, the effective Knudsen diffusivity is24

DkA )

C ) C1

CH2O,1

1+

(7)

with the following boundary conditions and assuming the same value of solid diffusivity Dr for the reacting gases H2O and CO2,

dCH2O dCCO2 ) Dr ) krCH2O,2CCO2,2 Dr dr dr

CH2O,2 )

(5)

The reaction rate per unit of particle volume is proportional to the chemical reaction rate constant and to the gas concentration of CO2 and H2O at the reaction interface. Because of the possible diffusional resistance through the solid product layer, the gas concentration in the pores could be not the same as that existing at the reaction interface. To find that relation, a mass balance in spherical coordinates was performed:

dC 1d Drr2 )0 2dr dr r

express the concentration of the reacting gases H2O and CO2 at the reaction interface by the following equations, which must be solved simultaneously:

( )(

1 8RgT 8 πMA

1/2

)( )( )

r2 π  1 + π/8 1 -  τs

(18)

where the tortuosity factor was calculated as

τs ) 1/

(19)

The porosity changes inside the particle with regeneration conversion were calculated as a function of the (23) Reid, R. C.; Prausnitz, J. M.; Poling, B. E. The Properties of Gases and Liquids, 4th ed.; McGraw-Hill: New York, 1987; pp 587597. (24) Torres-Ordon˜ez, R. J.; Longwell, J. P.; Sarofim, A. F. Physical Transformations during CaS(s) Oxidation. Energy Fuels 1989, 3, 595603.

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initial porosity 0 and the expansion ratio Zr:

(t) ) 0 - (Zr - 1)(1 - 0)X(t)

(20)

(R,t) ) 0 - (Zr - 1)(1 - 0)X(R,t)

(21)

where Zr is defined as

Zr ) 1 +

Fr(VMMgO.CaCO3 - VMMgO.CaS) MMgO.CaS

(22)

For modeling purposes, it has been assumed that the grains are composed by Ca and Mg in a ratio given by the chemical composition. The grain properties so calculated are shown in Table 1. Finally, the local conversion at each time and position inside the particle was calculated with the following equation:

X(R,t) ) 1 -[r2(R,t)/r0]3

(23)

and the mean conversion at each time in the whole particle was calculated by integration of local conversions:

∫0R 4πR2X(R,t)dR 0

X(t) )

4 3 πR 3 0

(24)

Discussion Determination of the Chemical Reaction Constant. The chemical reaction constant of the regeneration reaction was determined by the initial reaction rates method with the experiments carried out at different temperatures (Figure 3) with dolomite particles of 0.4-0.63 mm. To really ensure kinetic control, only the first instants of the reaction, with conversions below 5%, were considered. Assuming an Arrhenius type dependence with temperature for the chemical reaction constant, a preexponential factor of 0.247 m4 mol-1 s-1 and an activation energy of 96.3 kJ mol-1 were obtained. Determination of the Product Layer Diffusivity. An attempt was initially made to reproduce the experimentally observed behavior of the sorbent using the grain model described above with a constant diffusivity in the product layer. Forcing a good match with experimental data at large times and high conversions often resulted in underpredicting the extent of reaction at small times. Conversely, matching experimental data early in the reaction frequently caused the final extension of reaction to be overpredicted. A sensitivity analysis of the model to the main variables affecting the conversion predictions to search for possible errors in the model formulation was then made. Different grain radius, initial sorbent porosity and specific surface area, and different equations to define the Knudsen diffusion into the sorbent were tested. However, none of them were sufficient to explain the variations in the reaction rate experimentally observed. The results led us to conclude that the resistance for mass transport in the product shell increased with the progress of the reaction. This fact has been already detected in different gas-

solid reactions by other authors. Ranade and Harrison25 achieved a significant improvement in the match of their experimental results from the reaction of hydrogen sulfide with zinc oxide using a variable property grain model. Marban et al.2 in their studies on CaS oxidation described the diffusion-controlled process by means of a solid state diffusion mechanism, in which the effective diffusivity decreased with conversion, probably via a grain boundary diffusion mechanism. Hajaligol et al.26 and Khrisnan and Sotirchos27 used a conversion dependent diffusivity in order to reproduce their experimental data related with the direct sulfation of limestone. Khrisnan and Sotirchos27 justify this fact because of the formation of progressively denser layers of solid product during reaction as a consequence of the limited space inside the sorbent and the important differences in molar volume from reactant, CaCO3, to product, CaSO4. In the regeneration reaction here analyzed the space limitations are not so important as in the direct sulfation of limestone because the high porosity of the half-calcined dolomite will allow the grain to expand during the reaction inside the sorbent. The other possibility for a decreasing solid diffusivity was that the reaction could be affected by a sintering phenomena changing the CaCO3 product layer structure during the reaction. The sintering of CaCO3 has been observed by de Diego et al.28 in a previous work on sulfidation of noncalcined limestones and half-calcined dolomites, and also by Illerup et al.29 in their pressurized experiments of limestone sulfation. On the other hand, the important sintering effect of the two gases, H2O and CO2, used in this work for sorbent regeneration has been observed by different authors working with calcium-based sorbents.30,31 To determine whether the space limitations or the sintering was responsible of the changes in solid diffusivity during reaction, a new experiment was carried out. Sulfided particles of 0.4-0.63 mm were partially regenerated at 550 °C with an equimolar H2O/CO2 of 15% up to 60 min. At this stage, the flow of reacting gases was stopped and the sample was heated to 850 °C in an atmosphere containing nitrogen and hydrogen. This atmosphere allowed the calcination of the CaCO3 previously formed and avoided the CaS decomposition. After complete calcination, the dolomite sample was returned to the reacting temperature, 550 °C and a flow of CO2 was introduced allowing the quick and complete recarbonation of the CaO. Once a fresh layer of CaCO3 (25) Ranade, P. V.; Harrison, D. P. The Variable Property Grain Model Applied to the Zinc Oxide-Hydrogen Sulfide Reaction. Chem. Eng. Sci. 1981, 36, 1079-1089. (26) Hajaligol, M. R.; Longwell, J. P.; Sarofim, A. F. Analysis and Modeling of Direct Sulfating of CaCO3. Ind. Eng. Chem. Res. 1988, 27, 2203-2210. (27) Khrisnan, S. V.; Sotirchos, S. V. A Variable Diffusivity Shrinking-Core Model and its Application to the Direct Sulfation of Limestone. Can. J. Chem. Eng. 1993, 71, 734-745. (28) de Diego, L. F.; Garcı´a-Labiano, F.; Ada´nez, J.; Palacios, J. M. Factors Affecting the H2S Reaction with Noncalcined Limestones and Half-calcined Dolomites. Energy Fuels 1999, 13 (1), 146-153. (29) Illerup, J. B.; Dam-Johansen, K.; Lunden, K. High-Temperature Reaction Between Sulfur Dioxide and Limestone. VI. The Influence of High Pressure. Chem. Eng. Sci. 1993, 48 (11), 2151-2157. (30) Borgwardt, R. H. Calcium Oxide Sintering in Atmospheres Containing Water and Carbon Dioxide. Ind. Eng. Chem. Res. 1989, 28, 493-500. (31) Milne, C. R.; Silcox, G. D.; Pershing, D. W.; Kirchgessner, D. A. Calcination and Sintering Models for Application to High-Temperature Short-Time Sulfation of Calcium-Based Sorbents. Ind. Eng. Chem. Res. 1990, 29, 139-149.

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Energy & Fuels, Vol. 15, No. 1, 2001 91

Figure 7. Effect of the CaCO3 sintering on the regeneration process of the dolomite. (dp ) 0.4-0.63 mm, 550 °C, 15% H2O15% CO2).

was formed, the H2O and CO2 were again introduced in the required concentration and the sorbent continued the regeneration. Figure 7 shows the results so obtained together with the sample regenerated in one step. It can be observed how the formation of a new layer of CaCO3 produced an important increase in the regeneration reaction rate. If the spatial limitations were the responsible of the changes in the solid diffusivity, no change in the reaction rate should have been observed. These results confirmed that the sintering of the CaCO3 layer was the responsible of the loss of reactivity during the regeneration process. Other experiment was carried out to detect the effect of the gases on the sintering. The sorbent regeneration was stopped at 60 min of reaction, maintained under nitrogen flow during 5 h, and afterthat the regeneration conditions were again introduced. As can be observed in Figure 7 no sharp changes of reactivity were observed after the regeneration renewal, which implies that the sintering is due to the presence of the reacting gases, H2O and CO2, although the effect of these gases on the sintering can change with temperature. To quantify the effect of the different operating variables on the sintering of the CaCO3 layer, a more detailed analysis of the experimental results was carried out. As the chemical reaction constant and the reaction order of the regeneration process have been previously determined, and the pore diffusion is given by the eqs 16-19, the only unknown parameter of the model is the product layer diffusivity. Therefore, the value of Dr necessary to reproduce the experimental conversion data was obtained as a function of time at different operating conditions. An exponential decay for the product layer diffusion coefficient with time was observed from the experimental data, and the following equation was therefore used to determine the model parameters,

Dr ) D0r exp(-At)

(25)

The value of the initial solid diffusivity Dr0 depended only from temperature while the solid diffusion decay constant A depended on the temperature and on the CO2 and H2O concentrations. a Cb A ) KsCH 2O CO2

(26)

The regeneration data obtained at 550 °C with variable gas concentrations shown in Figures 4-6 were used to determine the exponents of the sintering process, a and

Figure 8. Effect of CO2 concentration on the sintering of the CaCO3 product layer.

Figure 9. Effect of H2O concentration on the sintering of the CaCO3 product layer.

b, with respect to the H2O and CO2, respectively. In a semilogarithmic plot of Dr versus time, the ordinate in the origin corresponds to the value of Dr0 at 550 °C and the slope shows the sintering effect of the different gases. An average value for Dro at 550 °C, 2.3 × 10-16 m2 s-1 was used for all the experiments carried out at this temperature. Figures 8 and 9 show the effect of the CO2 and H2O concentration, which varied between 5 and 40%, when the other gas was kept constant at 15%. A value of 1.5 for the dependence of sintering on the H2O concentration (exponent a) and 1 for the dependence on the CO2 (exponent b) were obtained from a semilogarithmic plot of the above slopes versus the gas concentration. It can be observed how the effect of H2O was more important on the sintering than the CO2, producing lower values of Dr at the same reaction time for a same total gas concentration. These results are in fair agreement with those found by other authors as Borgwardt30 on CaO sintering and Furimsky et al.16 who observed a quicker solid deterioration during regeneration in the presence of H2O and CO2 than the obtained during the regeneration in the presence of CO2 only. However, it must be taken into account that, in the latter case, the regeneration product is CaO instead CaCO3 and the reaction mechanism can be different. A special case was detected for the highest H2O concentration of 40% used in this work. As can be observed in Figure 9, the solid diffusivity reached a value nearly constant, ∼1.3 × 10-17 m2 s-1, after 65 min of reaction. This would correspond to a limit value of solid diffusivity beyond which the value of Dr does not decrease at the temperature used. This fact was also detected by Mess et al.32 in their experiments of CaO carbonation. They found that the effective diffusivity (32) Mess, D.; Sarofim, A. F.; Longwell, J. P. Product Layer Diffusion During the Reaction of Calcium Oxide with Carbon Dioxide. Energy Fuels 1999, 13 (5), 999-1005.

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Figure 10. Effect of H2O/CO2 concentration on the sintering of the CaCO3 product layer.

decreased exponentially until it reached a constant value, which depended on temperature with an activation energy of 57 kcal mol-1. This limit value was also observed when higher reacting gas concentrations were used for regeneration. Figure 10 shows the effect of the gas concentration on the value of Dr when equimolar concentrations of H2O and CO2 were used. It can be observed how the limit diffusivity was reached at early times as the H2O + CO2 concentration was increased. This limit value was about 1.5 × 10-17 m2 s-1 in all cases at the temperature tested, 550 °C, although probably it changes with temperature in a similar way to the found by Mess et al.32 These experiments were also valid to check the validity of the individual values of the exponents, a and b, previously determined. A global exponent, a + b, of 2.5 was obtained from a plot of the slopes obtained in Figure 10 versus the gas concentration when equimolar concentrations of H2O and CO2 were used. In view of the strong dependence of product layer sintering on water pressure, increasing the pressure of the CO2-H2O regenerating mixture in order to increase H2S concentration in the product gas could produce a detrimental effect on solid conversion. However, the magnitude of the limit value to be obtained during highpressure operation would be high enough to produce high reaction rates of sorbent regeneration, which is also favored by the higher partial pressures of the reactants. Experiments carried out at different temperatures from 475 to 575 °C with an equimolar concentration of 15% for CO2 and H2O were used later to determine the variation of the initial solid diffusivity Dr0 with temperature. Experiments above 600 °C were not taken into account because the existence of important pore diffusion resistance inside the sorbent particle. The values of D0r at the different temperatures were obtained from the ordinate in the origin of a semilogarithmic plot of the variation of Dr with time. Assuming an Arrheniustype dependence with temperature, the value of the initial solid diffusivity was determined to be D0r ) 2.43 × 10-6 exp(-19000/T), as can be observed in Figure 11. On the other hand, Figure 12 shows the semilogarithmic plot of the variation of Dr/Dr0 with time at different temperatures. The slopes of the lines allow to determine the dependence with temperature of the sintering constant Ks, once the values of the exponents a and b have been determined. Assuming again an Arrhenius dependence with temperature, the value of Ks was determined to be Ks ) 29.33 [(m3 mol-1)2.5‚s-1] exp(-11000/T), as observed in Figure 11. The decrease of Ks at temperatures above 575 °C is due to the

Ada´ nez et al.

Figure 11. Temperature dependence of initial solid diffusivity and sintering for the sorbent regeneration process.

Figure 12. Effect of temperature on the variation with time of the normalized solid diffusivity.

existence of concentration profiles inside the particle because the gas pore diffusion begin to be important at these conditions, and therefore have not been considered. The limit value of the product layer diffusivity was also observed for the highest temperatures used, as can be observed in Figure 12. In this case, a constant value of ∼1.0 × 10-16 m2 s-1 was obtained at 575 °C after 65 min of reaction when using an equimolar concentration H2O/CO2 of 15%. Figures 3-6 present a comparison between experimental conversions versus time data and that predicted by the model using the model parameters obtained in this work. A good fitting of the experimental results was observed in all cases. It must be remarked that the deviations present in the Dr values determined (Figures 8-10), in the first moments of reaction, scarcely affect the model predictions on conversion because, at those times, the reaction is mainly controlled by the chemical reaction resistance. Sulfidation-Regeneration Cycles. The kinetic of the regeneration process and the changes of sorbent reactivity by sintering was determined over samples obtained in the first cycle of sulfidation. However, it is important to evaluate the durability of the calcium sorbent in a cyclic process of sulfidation and regeneration. The main problems related with the solid behavior in this regeneration method were the sorbent attrition and the decreasing regenerability of the sorbent along the cycles.6,16 The use of a moving bed as regeneration reactor or a fixed bed in successive cycles sulfidation/ regeneration could decrease the sorbent attrition observed by Conoco Coal Development Co. in their fluidized bed experiments on pilot plant scale.15 On the other hand, the sorbent sintering, the recrystallization of the adsorbent leading to a decrease of the crystallographic defects,16 the migration and crystallite growth of MgO on CaS crystals21 or the reduction in number but

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Energy & Fuels, Vol. 15, No. 1, 2001 93

Figure 13. Regeneration cycles of the dolomite. (20 cycles, dp ) 0.8-1.0 mm, 650 °C, 15% H2O-15% CO2).

enlargement in size of pores, and partial development of a relatively dense mosaic microstructure on the outer surface33 have been proposed to be the causes of the regenerability decrease with repeated sorbent regeneration. To better analyze the problem, a sample of dolomite was subject to the process calcination/sulfidation/ regeneration during 20 cycles at 650 °C and an equimolar concentration H2O/CO2 of 15%. After finishing each regeneration cycle, the sorbent was calcined in such a way that the sulfidation took place over fully calcined dolomite. The regeneration in each cycle was stopped after 2 h of reaction, which corresponded to a solid conversion of about 80%. Both the sulfidation and regeneration conversion are calculated taken as reference the total calcium existent in the sorbent. Figure 13 shows the regeneration conversion obtained with the dolomite in the successive cycles. A deviation lower than 5% was obtained in the regeneration process on the 20 cycles. The dolomite regenerability did not decrease with the successive regeneration cycles and the sorbent reactivity with respect to the sulfidation reaction was maintained also constant. The sorbent sulfidation in the first cycle was complete in about 40 min. In the later cycles, the sulfidation took place on the regenerated zones of the sorbent, maintaining always an internal core (about 20% of the total calcium of the sorbent) of CaS, which is not active for the repeated sulfidation/ regeneration cycles in order to avoid too long regeneration times at these operation conditions. These results are quite different to those found by Dobner et al.17 who found a decline in capacity of the solid to about 23% of the total calcium after about 6 cycles working at atmospheric pressure and 750 °C. To follow the internal structure changes suffered by the sorbent along the sulfidation-regeneration cycles, several samples of dolomite were analyzed by SEM and EDX at different stages of the process. No differences of structure were found between similar samples at the same stage of a cycle depending only on the regeneration cycle number. The EDX analysis showed a uniform distribution of calcium and magnesium within the particles and no changes were observed along different cycles. Figure 14 shows the general aspect of the sample regenerated at 50% in the cycle 20. It can be observed how the external part of the particle shows a uniform aspect with smooth regions and low grain frontiers, (33) Harvey, R. D.; Kan, G.; Graff, R. A.; Squires, A. M. Behavior of Dolomite in Absorption of H2S from Fuel Gas, World Mining and Metals Technology; American Institute of Mining, Metal & Petroleum Engineers: New York, 1976; Vol. 1, Chapter 11, pp 163-177.

Figure 14. SEM pictures of a partially regenerated particle and a detailed view of a cracked grain.

mainly due to the carbonate sintering. Conversely, the grain structure is evident at the central part of the particle. This fact confirmed that the dolomite follows a grain reaction model during the regeneration and the existence of some conversion gradients in the particle at this temperature. Figure 14 also shows an internal part of the dolomite where a grain was cracked. It can be observed a typical structure of the CaCO3 surrounded the grain of CaS. Further studies are required to probe the validity of the regeneration process here analyzed, specially at high pressures where the H2S concentration in the produced stream will be high. However, the results herein obtained are promising in order to be used in a reactor where the sorbent attrition could be avoided. Conclusions The regeneration process of a sulfided dolomite with H2O and CO2 to obtain a stream concentrated in H2S has been analyzed from the point of view of the sorbent behavior. It was observed that dolomite particles of 0.40.63 mm can be completely regenerated in 100 minutes at 650 °C with an equimolar gas concentration H2O/ CO2 of 15%. The grain model with changing grain size was used to obtain the kinetic parameters of the dolomite regeneration, which was found to be of first order with respect to the H2O and CO2 concentrations. A preexponential factor of 0.247 m4 mol-1 s-1 and an activation energy of

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96.3 kJ mol-1 were obtained for the chemical reaction of CaS with H2O and CO2. An initial attempt to reproduce the experimentally observed behavior of the dolomite during regeneration using the grain model with a constant diffusivity in the product layer was failed. This fact led us to conclude that the resistance for mass transport in the product shell increased with the progress of the reaction. The sintering of the CaCO3 layer demonstrated to be the main cause of the sorbent reactivity decrease during the regeneration. This sintering phenomena depended on the H2O and CO2 concentration for a given temperature, being the former the most important sintering agent. An equation to calculate the product layer diffusion coefficient as a function of time has been obtained. Twenty sulfidation-regeneration cycles were carried out under atmospheric pressure to investigate the longterm behavior of the dolomite. It was observed that the dolomite regenerability and the sulfidation reactivity did not decrease with the successive regeneration cycles. Therefore, the regeneration of sulfided dolomite with steam and carbon dioxide can be feasible in an installation where the sorbent attrition could be avoided. Acknowledgment. This research was carried out with the financial support from the Comisio´n Interministerial de Ciencia y Tecnologı´a (CICYT) (Project AMB 98-0883). The authors thank Dr. Diego Alva´rez for his assistance with the SEM technique. Nomenclature a,b A Ci C0 C1

exponents for the sintering effect of the H2O and CO2, respectively solid diffusion decay constant (s-1) local concentration of the reacting gas i (mol m-3) bulk H2S concentration (mol m-3) concentration at the external part of the grain (mol m-3)

Ada´ nez et al. C2 De Dg Dk Dm Dr D0r E kr Kr Ks M n1,n2 Pi r r0 r1 r2 R R0 Rg (r)r S0 t T VM X Zr

concentration at the reaction interface of the grain (mol m-3) effective diffusivity within the sorbent particles (m2 s-1) gas diffusion coefficient (m2 s-1) Knudsen diffusion coefficient (m2 s-1) molecular diffusion coefficient (m2 s-1) product layer diffusion coefficient (m2 s-1) preexponential factor of the product layer diffusion coefficient (m2 s-1) activation energy (J mol-1) chemical reaction rate constant (m3 m-2 s-1) thermodynamic constant for the equilibrium of the regeneration reaction (atm-1) constant for the sintering of the CaCO3 product layer (m3 mol-1)2.5 s-1 molecular weight (kg mol-1) reaction orders with respect to H2O and CO2, respectively partial pressure of gas i (atm) radial position within the grain (m) initial grain radius (m) grain radius after some reaction (m) radius of unreacted grain core (m) radial coordinate within the particle (m) particle radius (m) ideal gas constant (J mol-1 K-1) reaction rate of the sorbent regeneration (mol m-3 s-1) initial specific surface area of the sulfided sorbent (m2 m-3) time (s) temperature (K) molar volume (m3 mol-1) regeneration conversion (-) expansion ratio for the regeneration reaction (-)

Greek Symbols 0 initial particle porosity of the sulfided sorbent (-) p particle porosity (-) Fr true density of the sulfided sorbent (kg m-3) τS tortuosity factor (-) EF000079T