Reactivation by Water Hydration of Spent Sorbent for Fluidized-Bed

5692

Ind. Eng. Chem. Res. 2004, 43, 5692-5701

Reactivation by Water Hydration of Spent Sorbent for Fluidized-Bed Combustion Application: Influence of Hydration Time Fabio Montagnaro,† Fabrizio Scala,‡ and Piero Salatino*,§ Dipartimento di Chimica, Universita` degli Studi di Napoli Federico II, Complesso Universitario del Monte di Sant’Angelo, 80126 Napoli, Italy, Istituto di Ricerche sulla Combustione, Consiglio Nazionale delle Ricerche, Piazzale Vincenzo Tecchio 80, 80125 Napoli, Italy, and Dipartimento di Ingegneria Chimica, Universita` degli Studi di Napoli Federico II, Piazzale Vincenzo Tecchio 80, 80125 Napoli, Italy

A spent limestone sorbent, generated by fluidized-bed desulfurization under simulated combustion conditions, was reactivated by means of hydration with water at 25 °C for times ranging from 10 min to 24 h. A bench-scale fluidized-bed reactor was used to assess sulfur uptake of the original and reactivated sorbent. A combination of XRD, porosimetric, and computer-controlled SEM analyses was used to assess hydration-induced microstructural and chemical modifications of sorbent samples. The ultimate degree of calcium conversion of the exhausted sorbent was 28%. The ultimate degrees of calcium conversion of reactivated/resulfated sorbents ranged from 45% to 53%. A nonmonotonic influence of the hydration time on the ultimate sulfur uptake of reactivated sorbents was observed, which was explained in the light of the competition between the following phenomena: particle swelling, change of pore volume, sulfur redistribution, particle attrition/fragmentation. Two optimal hydration times were found, one on the order of minutes and the other on the order of some hours. Samples hydrated for shorter time showed less propensity to attrition/fragmentation than samples hydrated for long times. Introduction One of the main advantages of fluidized-bed (FB) combustion lies in the possibility of achieving in situ removal of sulfur oxides by injection of limestone-based sorbents. This process is attractive, as limestone is relatively cheap and display peak sulfur capture efficiencies at or around the typical range of FB combustion temperatures (800-900 °C).1-3 Under overall oxidizing conditions and at atmospheric pressure, limestonebased sorbents undergo the following reactions:

CaCO3(s))CaO(s)+CO2(g)

∆H≈44kcal/gmol

(1)

CaO(s)+SO2(g)+1/2O2(g))CaSO4(s) ∆H≈-116kcal/gmol (2) i.e., the sorbent first calcines (eq 1) to yield porous calcium oxide, which, in turn, is able to remove SO2 (eq 2), yielding compact calcium sulfate as the product. Calcination has a much shorter time scale than sulfation, so that for all practical purposes the two reactions can be considered in series.4 Sorbent sulfation has been investigated in detail in the past.5-8 Sulfation most typically conforms to the core-shell pattern: the reaction front in the sorbent particle divides the porous unreacted CaO inner core from the dense reacted CaSO4-rich outer shell. Extensive sulfation of the unreacted CaO core is prevented * To whom correspondence should be addressed. Tel.: +39081-7682258. Fax: +39-081-5936936. E-mail: [email protected]. † Dipartimento di Chimica, Universita` degli Studi di Napoli Federico II. ‡ Consiglio Nazionale delle Ricerche. § Dipartimento di Ingegneria Chimica, Universita` degli Studi di Napoli Federico II.

by the onset of a strong diffusional resistance to SO2 migration across the CaSO4-rich outer layer. Degrees of calcium conversion seldom exceed 30-40%, and overstoichiometric Ca/S feed molar ratios (about 2-3) are required to obtain sulfur capture efficiencies larger than 90%. Substantial changes in the particle size distribution of sorbents can be brought about by particle attrition/ fragmentation. As highlighted by Scala et al.,9,10 comminution phenomena may significantly impact particle sulfation and elutriation. The comminution phenomena were classified as (i) primary fragmentation, due to CO2 release and thermal shocks to which particles are subjected immediately after feeding to the reactor, which may generate coarse angular fragments and elutriable fines; (ii) secondary fragmentation, mostly associated with impact loading, yielding coarse fragments; and (iii) attrition by abrasion, induced by surface wear, which is responsible for the generation of elutriable fine particles. Mutual effects of sulfation and attrition were recognized: the buildup of a hard sulfated shell upon sulfation makes attrition vanishingly small in the long term. FB combustion generates larger quantities of solid residues with respect to suspension firing. Moreover, they are of worse quality with respect to possible reuse in the cement and concrete industry.11 This has driven research toward processes for sorbent reactivation and reutilization in order to limit ash disposal and sorbent consumption: different methods have been envisaged, as surveyed by Anthony and Granatstein.12 The present work addresses reactivation of spent sorbents by water hydration. This method is currently considered as one viable technique for sorbent reactivation. Previous studies on the subject7,13-17 highlighted the following features:

10.1021/ie034188h CCC: $27.50 © 2004 American Chemical Society Published on Web 06/09/2004

Ind. Eng. Chem. Res., Vol. 43, No. 18, 2004 5693

(i) As exhausted sorbent particles become wet, the unreacted inner core of CaO fast reacts, yielding calcium hydroxide,

CaO(s)+H2O(l))Ca(OH)2(s) ∆H≈-16kcal/gmol (3) while CaSO4 remains essentially anhydrous. (ii) Hydration (eq 3) induces swelling of the particle core that, in turn, favors the breakup of the sulfate layer. (iii) When the hydrated-reactivated sorbent, composed of Ca(OH)2 and CaSO4, is fed again to the combustor, it loses its chemically bound water

Ca(OH)2(s))CaO(s)+H2O(g) ∆H≈26kcal/gmol

(4)

This process is responsible for further shell breakage. (iv) CaO formed after dehydration (eq 4) is characterized by smaller crystals when compared with the freshly formed lime. Accordingly, recrystallization of CaO results in larger specific surface area. (v) In experiments with one limestone, it was observed that hydration brought about a pronounced sulfur redistribution within the particle. This was attributed to a combined solubilization-precipitation mechanism.17 (vi) On the whole, hydration enhances sulfur-capture ability. Particle swelling and sulfur redistribution might cooperate in determining this effect. In addition to Ca(OH)2 formation, the generation by pozzolanic reactions of large-surface-area calcium silico/ sulfo aluminate hydrates,18-20 such as ettringite,21-24 is another factor relevant to spent FB residues reactivation by hydration. The formation of these binding phases positively affects sulfur capture upon reinjection in FB reactors, but it may adversely influence the operation of regenerators due to some agglomeration tendency of hydrated particles. Proper design and operation of the regenerator is required to overcome operational issues. The presence in FB ashes prior to reactivation of OCC (other calcium compounds) has also been highlighted: 22,25 their calcium content could be partly exploited, upon water hydration, to produce materials able to capture SO2 once reinjected into the combustor, but the reactivation potential of the OCC appears to be limited by their poor reactivity. The aim of this work was to assess the influence of the hydration time on the regeneration of the sulfurcapture ability of spent limestone. Characterization of sample microstructure and of particle attrition/fragmentation complemented the analysis and was directed to shed light on the mechanism of sorbent reactivation. Experimental Section Materials. The sorbent used in the experiments was a high-calcium (96.8% CaCO3) Italian limestone (Massicci). The bed material consisted of mixtures of limestone and silica sand, prepared starting from batches of the two materials independently sieved in given particle size ranges prior to mixing. Streams of SO2, oxygen, and nitrogen used in FB desulfurization experiments were prepared by mixing air of technical grade with SO2-N2 mixtures (3000 ppmv SO2, balance N2) supplied in cylinders. Apparatus. Sulfation and resulfation tests were carried out in a stainless steel atmospheric bubbling FB reactor, represented in Figure 1. The reactor is a

Figure 1. FB reactor (not to scale).

laboratory-scale facility, 40 mm i.d., consisting of three sections: (a) the preheater/premixer of the fluidizing gas (height ) 66 cm) at the bottom, (b) the fluidization column (height ) 95 cm), and (c) the two-exit head, made of brass and mounted on top of the fluidization column. The solids feeding hopper and exhaust line are connected to the top of the fluidization column. A 3 mmthick perforated plate (49 holes of 0.5 mm i.d. in a triangular pitch) served as gas distributor. The reactor is electrically heated by two hemicylindrical ovens (2.4 kW each, 60 cm height, o.d. ) 22 cm) located around the upper part of the preheater/premixer and the lower part of the fluidization column. Heat losses from the ovens are prevented by ceramic fiber insulation. A type-K thermocouple measures the temperature at 4 cm above the gas distributor, flush to the inner surface of the fluidization column. The thermocouple is connected with a PID temperature controller modulating electrical power supply to the ovens. The upper part of the fluidization column is unlagged so as to favor rapid cooling of gases and prevent further reactions of elutriated fines. Gas flow rate and compositions were adjusted by means of mass flow meters/controllers (Brooks, 0154/ 5850). The two-exit head is purposely designed to convey flue gases through either of two 2.5-cm (i.d.) cylindrical sintered brass filters (height ) 3.5 cm, filtration efficiency ) 1 for >10-µm particles). Alternated use of filters enabled time-resolved capture of elutriated fines at the exhaust. On-line analysis of flue gas is accomplished. In particular, SO2 was detected by a nondispersive infrared analyzer (Hartmann & Braun, Uras 3G) whose precision and reproducibility was better than (50 ppmv. Concentration signals are logged on a PC at a sampling rate of 1 Hz. Procedures. (a) S Experiments: Sulfation of Fresh Limestone. The reactor was heated to the process temperature of 850 °C. Then a bed of 150 g of silica sand in the size range 0.85-1 mm was charged to the reactor and fluidized at the gas superficial velocity of 0.8 m/s with the SO2-oxygen-nitrogen mixture (1800

5694 Ind. Eng. Chem. Res., Vol. 43, No. 18, 2004 Table 1. Operating Conditions for Sulfation (S) and Resulfation (RS) Experiments S bed material sorbent size range sand size range temperature time gas inlet composition gas superficial velocity

RS

sorbent (20 g) + sorbent (25 g) + sand (150 g) sand (150 g) 0.4-0.6 mm 0.85-1 mm 850 °C 180 min SO2 (1800 ppmv) + O2 (8.5% v) + N2 (balance) 0.8 m/s

ppmv SO2, 8.5% v O2, balance N2). Once steadiness of operation was achieved, 20 g of limestone in the particle size range of 0.4-0.6 mm was added to the bed and the test started. The tests lasted 180 min, after which the limestone was easily sieved out of the bed material because of its smaller particle size. Operating conditions of S experiments are summarized in Table 1. The degree of calcium conversion during sulfation was calculated as a function of time by working out the SO2 concentration at the exhaust according to

XCa(t) )

in ∫0t[FSO

2

out - FSO (t)] dt 2

nCa

(5)

in out where FSO and FSO (t) are, respectively, the molar 2 2 rates of SO2 fed to the reactor and in the exhaust gas, and nCa is the overall moles of calcium fed to the reactor. Reproducibility of data was assessed by repeating the whole sulfation/reactivation procedure for selected hydration times. XCa(t) was reproducible within XCa(t) ( 1%. SO3 concentration at the exhaust was not monitored. However it was noted9 that, when the fluidizing gas containing SO2 and O2 passed through the reactor (without any sorbent) at 850 °C, a loss of SO2 was recorded. This loss was attributed to the SO2/SO3 oxidation inside the reactor by homogeneous and/or surface-catalyzed (at the preheater/reactor walls) reaction26 and was accounted for when processing SO2 concentration. (b) H Experiments: Hydration/Reactivation of Sulfated Limestone. Fully deactivated sorbent particles were reactivated by liquid-phase hydration in a thermostatic bath (Haake, DC50). The thermostatic bath was equipped with internal recirculation using water as the thermal carrier (accuracy ) (0.001 °C, heating supply ) 2 kW). Batches of sulfated samples (10 g) blended with a large excess of distilled water (water/solid weight ratio ) 25) were charged to sealed polyethylene bags and put in the thermostatic bath at 25 °C. Curing times were 10 and 30 min and 1, 2, 3, 6, 10, and 24 h. At the end of the H experiments, samples were retrieved from the bags, vacuum filtered, washed with acetone, dried with ether, and stored in a desiccator. The effect of the reactivation cycle on the particle size distribution was also assessed. To this end, sieve analysis was performed on two samples, namely, (a) a 3 h hydrated sample and (b) a 3 h hydrated sample sieved in the size range 0.4-0.6 mm and further subjected to FB calcination/dehydration in air at 850 °C for 5 min. (c) RS Experiments: Resulfation of Reactivated Sorbents. The operating procedure of RS tests was the same as that followed in S tests as regards the temper-

Figure 2. Assessment of sorbent sulfation patterns from probability density functions of relative sulfur content.

ature, gas superficial velocity and composition, bed material mass and size, and duration of the experiments (Table 1). After steadiness of operation was achieved, 25 g of H samples, sieved in the size range 0.4-0.6 mm, was added to the bed of sand. This mass of sorbent corresponded to feeding the same amount of free calcium as in S tests. As in S experiments, RS samples were easily sieved out of the bed material at the end of the experiments, because of their smaller particle size. The degree of calcium conversion as a function of time during resulfation was calculated using eq 5. Addition of the limestone marked the time at which time-resolved collection of elutriated material started. As fines were absent in the sorbent and sand feedings and comminution of sand was negligible, elutriated material was only that generated by attrition/fragmentation of in-bed sorbent. Microstructural Characterization. (a) SEMEDX Analysis. Particle cross sections of sulfated and hydrated sorbent material (polished and embedded in epoxy resin) were analyzed by means of a scanning electron microscope (SEM) (Philips, XL30, magnification ) 50×) with a LaB6 filament and equipped with an energy-dispersive X-ray (EDX) probe (Edax, DX-4, resolution ) 138 eV) for elemental mapping of sulfur and calcium throughout the particle cross section. (b) CCSEM-EDX Mapping Technique. An algorithm was purposely set up for automated analysis of EDX elemental maps with the aim of obtaining semiquantitative indicators of sulfur distribution throughout the particle cross section.8,17,23 This computer-controlled SEM-EDX (CCSEM-EDX) procedure, worked out in G programming language (National Instruments, IMAQ Vision), generates probability density functions of pointwise sulfur contents that can be directly related to the sorbent particle sulfation pattern. A relative sulfur content (RSC) is defined as the fractional intensity of the signal generated by the EDX sulfur map. The RSC value obtained by the mapping procedure could not be quantitatively related to the actual local sulfur content. However, since it was obtained according to a strictly reproducible and standardized protocol, it provided a reliable tool for semiquantitative comparative analysis of samples of different nature. Figure 2 exemplifies the interpretation of RSC plots: (i) Bimodal probability density functions of sulfur content are indicative of a core-shell particle sulfation pattern: one peak corresponds to the low-sulfur-content


Figure 3. Degree of calcium conversion as a function of time during S and selected RS tests.

zone (core zone), another peak corresponds to the highsulfur-content zone (shell zone). (ii) Unimodal distribution functions correspond to a uniform sulfur distribution throughout the particle. (c) XRD Analysis. Hydrated sorbent particles were characterized by X-ray diffraction (XRD, Philips, PW1710). XRD aimed at qualitative speciation of the main crystalline phases in hydrated-reactivated samples. The diffraction angle ranged from 5° to 60° 2θ (Cu KR) with a 0.02° increment. (d) Porosimetric Analysis. H samples selected for porosimetric analysis were calcined/dehydrated in an electric oven at 850 °C for 2 min. Mercury porosimetry was carried out by means of a Carlo Erba P2000 porosimeter operated at pressures ranging from 1 to 2000 bar (corresponding to pore radii in the range from 6800 to 4 nm). Porosimetric data were worked out in terms of cumulative pore undersize distributions. Results S and RS Experiments. (a) Sulfur Capture. Figure 3 gives the degree of calcium conversion XCa as a function of time during sulfation and resulfation tests. When considering plots in Figure 3, it must be remembered that operating conditions were such that the reactor could not be considered differential with respect to SO2. In fact, SO2 dropped to vanishingly small values at the beginning of the test, to increase thereafter until the inlet concentration was approached in the long term. This feature prevented the assessment of the kinetics of sulfur capture from differential analysis of XCa vs t curves.

Figure 4. Values of the final degree of calcium conversion for S and RS tests.

The ultimate degree of calcium conversion of the original limestone (S experiment) was only 28%. Water reactivation of the spent sorbent was very effective in promoting further sulfur uptake upon resulfation: the ultimate degree of calcium conversion Xf ranged from 45% to 53%, depending on reactivation time. Figure 4 reports the Xf values as a function of hydration time tH. The Xf vs tH trend is nonmonotonic and exhibits two relative maxima: Xf is 50% for samples reactivated for 10 and 30 min, it decreases to 45% for the sample hydrated for 1 h, it increases again and reaches a maximum (53%) for sorbents hydrated at 3 and 6 h, and it decreases thereafter to 46% for the sample reactivated at 24 h. (b) Elutriation Rate. Elutriation of sorbent material during resulfation was determined in experiments with sorbent samples hydrated for 10 min and 6 and 24 h. Figure 5 shows the specific fines elutriation rate E as a

5696 Ind. Eng. Chem. Res., Vol. 43, No. 18, 2004

Figure 5. Fines elutriation rate during S and RS tests.

Figure 6. Cumulative particle undersize distribution of 3 h hydrated and of 3 h hydrated/FB dehydrated spent sorbent samples.

function of time. The corresponding values for the first sulfation of Massicci are also reported for comparison purposes. E varies with time according to a monotonically decreasing pattern that resembles those typically observed in batch sulfation tests.9,10 The elutriation rate results from the interplay of the following processes: (i) Calcination/dehydration might bring about particle fragmentation as a consequence of thermal shocks and/ or water release. (ii) Particle rounding off and removal of surface asperities are significant in the early stage of the particle processing in FB. (iii) The course of sulfation affects attrition by strengthening the particle outer layer and makes E(t) vanishingly small in the long term. It is worth noting that elutriation of the 10 min and 24 h hydrated materials was much less extensive than that of the 6 h hydrated one, and that elutriation was more pronounced for the sulfation than for the resulfation step. Particle Size Distribution of the Hydrated and Hydrated/Dehydrated Samples. Further tests were directed to check the propensity of samples to undergo fragmentation upon hydration and upon reinjection into the bed. Figure 6 compares the cumulative particle undersize distributions of the 3 h hydrated and of the 3 h hydrated/dehydrated samples. It must be remembered that the parent spent sorbent before hydration belonged to the size interval 0.4-0.6 mm. After hydration, less than 5% of the particles mass was found in the range 0-0.4 mm, and the population of particles finer than

0.15 mm was negligible (mean Sauter diameter ) 0.473 mm). This indicates that soaking in water did not promote any significant particle size reduction effect. On the contrary, nonnegligible fragmentation of reactivated particles was observed once they were dehydrated in FB: 22% of the particle mass was in the size range 0-0.4 mm, and particles finer than 0.05 mm could also be observed (mean Sauter diameter ) 0.358 mm). It is recalled here that, after hydration, the sorbent was resieved in the default size range (0.4-0.6 mm) prior to FB dehydration. Microstructural Characterization. (a) XRD Analysis. Figure 7 reports results of XRD analysis for the sulfated 0.4-0.6 mm sorbent hydrated for 10 and 30 min and 6 and 24 h. Only portlandite was detected as hydration product, and anhydrite and quartz were detected as the main unconverted species. At all hydration times (even for the shortest one), the absence of lime indicates complete conversion to Ca(OH)2. On the other hand, the presence of anhydrite (even at the longest curing period) is consistent with the negligible rate of reactions yielding gypsum-like phases (CaSO4‚ nH2O) under the operating conditions tested. The amount of quartz and alumina is instead related to some unavoidable sand impurities in the samples. Specimens hydrated for times different from those reported in Figure 7 showed similar XRD spectra. Data in Figure 7 indicate that the chemical process of hydration is complete within 10 min, yielding no hydrated species but Ca(OH)2. (b) Porosimetric Analysis. Figure 8 shows the cumulative pore undersize distribution of samples hydrated at different times and dehydrated in an electric oven at 850 °C for 2 min. For comparison, the pore size distributions of the fresh, calcined (at 850 °C in the FB reactor fluidized by air), and sulfated (S sample) sorbent are also reported. Figure 9 summarizes values of the total specific pore volume, along with the contributions relative to the fraction of pores with radius less than 100 nm,27 hereinafter referred to as “finer pores”. The limestone at hand was a relatively nonporous one. Calcination induced a strong development of both finer porosity (from 11 to 167 mm3/g) and total porosity (from 34 to 281 mm3/g), while the sulfation process determined a sharp decrease in the accessible finer porosity (43 mm3/g) and total porosity (85 mm3/g). Moreover, it can be observed that, as far as the finer pores region is concerned, the porosity decreased with reactivation time for tH < 10 h. After 10 min of hydration the total porosity was 291 mm3/g, fairly equal to that observed after calcination of the fresh sorbent; thus, the combined effect of swelling/fissuring induced by hydration and of subsequent dehydration led to the development of the same accessible porosity of the calcined sample. Upon increasing the hydration time, the total porosity reached a maximum (348 mm3/g) at 3 h hydration to decrease thereafter to 202 mm3/g at 24 h hydration. (c) CCSEM-EDX Analysis. Figure 10 reports SEM micrographs and EDX sulfur maps of polished cross sections of multiparticle sorbent samples sulfated and hydrated at all curing times investigated. Multiparticle samples were selected in order to ensure statistical significance of the maps. Inspection of the Figure 10 reveals the prevailing establishment of a core-shell particle structure for the sulfated sample, in line with findings reported by other authors.5,7,27 Moreover, and regardless of the hydration time, a marked redistribu-


Figure 7. XRD spectra of sulfated 0.4-0.6 mm Massicci sorbent hydrated for 10 and 30 min and 6 and 24 h (P ) portlandite, Alu ) alumina, Q ) quartz, A ) anhydrite). (ASTM-JCPDS reference cards employed: Ca(OH)2, portlandite, 4-0733; Al2O3, alumina, 26-31; SiO2, quartz, 33-1161; CaSO4, anhydrite, 37-1496). Table 2. Core-Zone and Shell-Zone Areas for the Samples Analyzed by the CCSEM-EDX Technique samplea

core-zone shell-zone area (%) area (%)

S H (10 min) H (30 min) H (1 h) H (2 h) a

35 26 20 25 13

65 74 80 75 87

samplea H (3 h) H (6 h) H (10 h) H (24 h)

core-zone shell-zone area (%) area (%) 7 6 6 6

93 94 94 94

S ) sulfated. H ) hydrated.

tion of sulfur between the shell and core zones becomes apparent for all the hydrated samples. The latter finding confirms and extends preliminary results reported by Scala et al.17 Semiquantitative assessment of sulfur redistribution was accomplished with the aid of the CCSEM-EDX technique (Figure 11 and Table 2): all H samples showed a bimodal sulfur distribution pattern similar to that of the sulfated sorbent, but the relative significance of peaks associated with the core zone and the shell zone changed significantly. A more detailed analysis of the CCSEM-EDX results shows that, the longer the hydration time (between 10 min and 6 h), the smaller the corezone area (from 35% for the S sample and 26% for the 10 min hydrated sample to 6% for the 6 h hydrated sample). Sulfur redistribution is particularly strong in the range of hydration times 1 h < tH < 3 h. No further redistribution occurred to samples hydrated for times longer than 6 h. Discussion The following phenomenological features can be recognized when samples of exhausted sorbent hydrated for different times tH are compared with each other: (i) The ultimate degree of calcium conversion of the reactivated sorbent is significantly larger than the degree of calcium conversion achieved upon the first

sulfation. As previously discussed, the trend of Xf versus tH is nontrivial (Figure 4). (ii) Conversion of CaO into Ca(OH)2 takes place in the early stage of hydration, within about 10 min, as indicated by XRD characterization (Figure 7). (iii) Sulfur redistribution takes place along with hydration. It is particularly pronounced in the range 1 h < tH < 3 h (Table 2). (iv) The porosity of reactivated/dehydrated samples changes along with hydration. The total porosity somewhat increases as tH increases up to 3 h, to decrease thereafter. The finer porosity decreases along with hydration (Figure 9). (v) Elutriation of attrited/fragmented material during resulfation of the reactivated sorbent is moderate for the 10 min hydrated sample, peaks for the 6 h hydrated sample, and decreases for the 24 h hydrated sample. On the whole, a nonmonotonic trend of elutriation rate with tH is suggested. Analysis of the mutual relationships between the above recalled phenomena is now in order. Swelling of the inner core and breakage of the outer shell take place as CaO is converted to Ca(OH)2. Consistently with the mechanism proposed by Shearer et al.,13 these phenomena are responsible for the large porosity left behind by sorbent hydration/dehydration. Swelling is completed within minutes from the beginning of hydration. Sulfur redistribution upon hydration has been recently addressed by Scala et al.17 They attributed sulfur redistribution to ion mobility in the aqueous phase associated with a solubilization-precipitation mechanism driven by concentration gradients. Sulfur redistribution is slower than particle swelling and takes place essentially in the time interval 1 h < tH < 3 h. It is likely that the same ion mobility invoked to explain sulfur redistribution throughout the particle be responsible for the observed changes of pore volume


Figure 8. Cumulative pore undersize distribution of fresh, calcined, and sulfated sorbent and of selected hydrated/dehydrated samples.

along with hydration. The phenomenology, consisting of coalescence of fine pores into larger ones, broadly resembles pore volume changes associated with hightemperature sintering upon prolonged calcination.28 Whereas sintering is a thermally activated process that only involves solid-state transformations, the low-temperature “cramming” observed within the present work should be closely associated with the presence of an aqueous phase and ion mobility due to solubilizationprecipitation dynamical processes. If sulfur redistribution is looked at as a recrystallization process, it should bring about an increase of crystallites size according to the Ostwald ripening process.29 The change of crystallite

size distribution would be reflected by the pore size distribution, especially in the fine pore size range. Cramming is apparent since the very early stage of hydration and is active over longer time scales than sulfur redistribution. At the longest hydration times a remarkable tendency of particles to agglomerate with each other was noticed. This observation further reinforced the hypothesis that structural reorganization via aqueous phase ion mobility is at work. Trends of elutriation rate as a function of tH closely reflect attrition/fragmentation of the reactivated material. The existence of a maximum of E versus tH is speculatively related to the competitive effects of sulfur


Figure 9. Specific pore volume of the samples.

redistribution and cramming. Sulfur redistribution is responsible for the transformation of the harder CaSO4 into the softer CaO at the particle outer shell, once the whole reactivation/dehydration cycle is completed. Accordingly, the reactivated particle would be increasingly prone to surface wear and breakage.9,10 On the other hand, long-term particle cramming is responsible for the compaction of the particle microstructure, reflected by decreased propensity to surface wear and breakage. All reactivated samples showed less extensive elutriation with respect to the fresh sorbent, especially during the first stage of sulfation: this could be related to the higher CaSO4 concentration at the outer shell for all the sulfated/reactivated particles with respect to the fresh ones. Altogether, the trend of the sulfur capture ability of reactivated sorbent as a function of hydration time tH results from the interplay of the above processes, as hereby summarized: (i) At short curing times (up to 30 min), the enhancement of the sulfur capture ability is essentially related to swelling due to CaO hydration. Some contribution from attrition/fragmentation of the reactivated material might also be expected. Sulfur redistribution and particle cramming play a minor role at this stage; (ii) Beyond tH ) 10 min, particle swelling is over. For 30 min < tH < 1 h finer pore volume decreases, whereas the total pore volume stays relatively constant. In the same range of tH, the ultimate sulfur uptake of the reactivated sorbent Xf decreases. It can be speculated that the reduced effectiveness of hydration in this range be determined by some cramming in the finer pore range. Sulfur redistribution is still negligible in this range of tH. (iii) A dramatic increase of ultimate sulfur uptake of the reactivated sorbent Xf is observed as tH is increased

Figure 10. SEM micrographs (upper rows) and EDX sulfur maps (lower rows) of cross sections of sulfated and hydrated multiparticle samples.

from 1 h up to 6 h. Pronounced sulfur redistribution is observed in the same range of tH. Cramming of finer pores occurs with some increase of larger pore volume (up to 3 h), which should favorably affect particle accessibility. To complete the picture, it is recalled that tH ) 6 h corresponds to the peak occurrence of attrition/ fragmentation, and this might provide another key for the enhanced sulfur capture ability. (iv) At hydration times even longer than 6 h the detrimental effects of cramming reduce sulfur uptake. It is recalled that interparticle cramming occurs at this stage together with intraparticle microstructural reorganization. Also attrition/fragmentation of reactivated material becomes lower. In conclusion, as far as SO2 uptake is concerned, two optimal hydration times are found, one on the order of minutes and the other on the order of a few hours. The shorter curing time has the obvious advantage of involving a rapid process, but the enhancement of SO2 capture is slightly smaller than that achieved at longer


Figure 11. CCSEM-EDX analysis results for sulfated and selected hydrated multiparticle samples.

hydration times. On the other hand, prolonged hydration gives rise to interparticle bonding, which might be responsible for the troublesome and unreliable operation of the reactivator. It is worth noting that the two optimal hydration times give rise to samples that behave differently as regards to attrition. The smaller propensity of material reactivated for short curing times to undergo attrition must be analyzed in the light of its 2-fold effect: attrited fines are characterized by shorter residence times in the combustor when compared with their mother particles but exhibit enhanced kinetics of sulfur uptake. The optimal choice of reactivation time might require consideration of combustion conditions and reactor design, which influence the residence times of sorbent material and the axial profiles of sulfur oxides concentration along the combustor. Conclusions Regeneration of sulfur uptake ability of FB spent sorbent by water hydration was assessed with a focus on the influence of hydration time on the ultimate degree of calcium conversion. It was observed that the total sulfur uptake of reactivated sorbent was nearly twice that of the spent sorbent, as hydration was carried out at 25 °C for times ranging from 10 min to 24 h. A more detailed analysis highlighted a nonmonotonic trend of the degree of sorbent regeneration versus hydration time: the degree of calcium conversion peaked

at 50% after 10 min hydration time, while a second maximum (53%) was recorded after 3-6 h hydration time. Microstructural characterization of sorbent samples was accomplished by XRD, SEM-EDX, and mercury porosimetry. It proved to be helpful in shedding light on mechanisms of sorbent reactivation. It was complemented by quantitative assessment of attrition/fragmentation of reactivated samples as a function of hydration time. A mechanistic pathway was developed to follow the nontrivial dependence of the ultimate degree of calcium conversion on hydration time. Accordingly, particle swelling due to conversion of CaO into Ca(OH)2 controlled reactivation at short curing times. Changes of pore volume due to microstructural reorganization and sulfur redistribution across the particle became active over longer time scales. These latter processes can be explained in the light of the enhancement of ion mobility due to the presence of an aqueous phase. Attrition/ fragmentation was maximum for samples hydrated for 6 h and may further contribute to the second peak of sulfur capture. Acknowledgment The authors wish to thank Dr. E. J. Anthony, for precious discussion, and are also indebted to Clelia Zucchini and Sabato Russo, for the SEM-EDX particle


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Received for review October 16, 2003 Revised manuscript received March 12, 2004 Accepted March 24, 2004 IE034188H

Reactivation by Water Hydration of Spent Sorbent for Fluidized-Bed

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