Reactivation by Steam Hydration of Sorbents for Fluidized-Bed

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Reactivation by Steam Hydration of Sorbents for Fluidized Bed Calcium Looping Antonio Coppola, Lucia Palladino, Fabio Montagnaro, Fabrizio Scala, and Piero Salatino Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b00413 • Publication Date (Web): 03 Jun 2015 Downloaded from http://pubs.acs.org on June 8, 2015

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Reactivation by Steam Hydration of Sorbents for Fluidized Bed Calcium Looping Antonio Coppolaa, Lucia Palladinob, Fabio Montagnarob,*, Fabrizio Scalac, Piero Salatinoc a

b

Istituto di Ricerche sulla Combustione, Consiglio Nazionale delle Ricerche, Piazzale Vincenzo Tecchio 80, 80125 Napoli (Italy).

Dipartimento di Scienze Chimiche, Università degli Studi di Napoli Federico II, Complesso Universitario di Monte Sant’Angelo, 80126 Napoli (Italy).

c

Dipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale, Università degli Studi di Napoli Federico II, Piazzale Vincenzo Tecchio 80, 80125 Napoli (Italy).

ABSTRACT. The present study addresses steam hydration as a tool for reactivation of the CO2 capture potential of spent limestone-based sorbents from fluidized bed calcium looping systems. A reference, high-calcium, reactive limestone was deactivated by carrying out lab-scale fluidized bed calcium looping process tests (calcination at 940°C in a 70% CO2 atmosphere; carbonation at 650°C in a 15% CO2 atmosphere), and then steam hydrated (at 250°C in a 50% steam atmosphere) in the same fluidized bed, for times ranging from 10 to 60 min. On-line flue gas analysis, continuous capture of the elutriated fines and evaluation of particle size distribution were performed during additional calcium looping process tests after sorbent reactivation. Thermogravimetric analysis, scanning electron microscopy and porosimetry were directed to characterize the microstructural features of the spent and of the steam hydrated sorbents. Moreover, the different materials were subjected to ex situ impact fragmentation tests. In this way, it was possible to investigate the effect of the hydration time on: the changes in the physico–chemical and microstructural properties induced by the hydration treatment; the reactivation of the sorbent CO2 capture capacity; the attrition/fragmentation tendency of the reactivated materials.

*

Corresponding Author. E: [email protected]. T: +39 081 674029. F: +39 081 674090.

ACS Paragon Plus Environment

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INTRODUCTION. The calcium looping process is among the most promising and mature techniques for post-combustion CO2 capture. It is most typically carried out in dual interconnected fluidized bed systems,1–4 by means of Ca-rich sorbents (typically limestone). The process is based on the alternated temperature-swing CO2 uptake (in an exothermal carbonator, operated at 650–700°C, see Eq. (1) forward) and concentrated release (in an endothermal calciner, operated as oxyfuel combustor at 900–950°C, see Eq. (1) backward):5–9

+ ⟺

(1)

Accordingly, a CO2-lean gaseous stream leaves the carbonator, and the exhausted sorbent (composed by a mixture of reacted CaCO3 and unconverted CaO) is regenerated in the calciner. Here a stream of nearly pure CaO is produced, which is recirculated to the carbonator, and a CO2-rich gas is emitted, ready for further processing and final storage. Possible drawbacks of this process are: a) sorbent thermal sintering/deactivation, that progressively decreases the CO2 capture capacity and rate of the sorbent upon iterated looping,10–12 and b) sorbent attrition/fragmentation in both fluidized bed units, which affects the sorbent particle size distribution, hence, its residence time distribution within the system, thus resulting in a net Ca loss from the circulating loop as elutriable fines.13–23 It is important to underline that in a real plant particle fragmentation is also related to impact damage, promoted by high-velocity collisions between fluidized particles and targets, which can be either bed solids or reactor walls/internals. The optimal management of the calcium looping cycle, therefore, implies continuous make-up of fresh limestone and purge of spent sorbent, to compensate for sorbent deactivation and attrition. Landfilling of the spent limestone is problematic due to the CaO-rich composition of this residue.24 The potential of the spent sorbent from calcium looping process as source of raw material in the cement manufacture is under scrutiny and seems to be promising.25,26 An alternative strategy is the regeneration of its CO2 uptake capacity by hydration-induced reactivation.27–35 Liquid water- and steam-hydration/reactivation were already studied in the past for applications related to in situ fluidized bed desulphurization.36–38 In the calcium looping context (“double looping process”, Figure 1), the technique is based on the conversion of CaO (in the spent sorbent retrieved from the calciner) to Ca(OH)2 (Eq. (2) forward, exothermal, typically carried out at room temperature or at around 200–300°C for water or steam hydration, respectively). Swelling of CaO due to hydration takes place (the molar volumes and the densities are 16.9 and 33.7 cm3 mol–1, and 3.32 and 2.20 g cm–3, for CaO and Ca(OH)2, respectively), and it is followed by Ca(OH)2 dehydration as the reactivated material is re-injected into the carbonator (Eq. (2) backward, endothermal):


2

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+ , ⟺

(2)

The resulting material is essentially again CaO, but with a recrystallized structure which is characterized by larger specific surface area and porosity (thus, more reactive toward CO2) with respect to the parent CaO-based spent sorbent.33–35 From both the energetic and the operational standpoint, steam hydration is more attractive than liquid water hydration, and may be implemented as a continuous double looping process using three interconnected fluidized beds. The optimal steam hydration temperature results from a trade off between kinetics and energetic/economic constraints on the one hand, which would push toward a high steam temperature, and the thermodynamic constraint, which would call for lower temperature, as discussed with reference to desulphurization.37 Moreover, the presence of steam has been reported to affect both calcination and carbonation stages.39,40 This paper reports on reactivation of spent sorbent by steam hydration. Calcium looping and steam hydration tests were carried out in bubbling fluidized bed reactors. The sorbent was deactivated by iterated Ca-looping, and then steam hydrated for different times. The attention was focused on the changes in the physico–chemical and microstructural properties induced by the hydration treatment, taking into account the effect of the hydration time, on reactivation of the sorbent CO2 capture capacity, and on the effect of reactivation on the attrition/fragmentation tendency of the sorbent. Since the bubbling fluidized bed test conditions are not likely to emphasize sorbent fragmentation by impact damage, this phenomenon was investigated in a separate purposely-designed ex situ apparatus. The sorbent used in the present investigation is a very reactive German high-calcium limestone. Results on reactivation by liquid water hydration of the same sorbent have been reported recently.33, 34

DESCRIPTION OF THE EXPERIMENTAL ACTIVITY.

Calcium looping process tests on raw and reactivated sorbent. The sorbent used in the experiments was a high-calcium limestone from South Germany. Its CaCO3 content is 99.20% wt. (only SiO2, MgO and Al2O3 % are larger than 0.10), and its good reactivity to CO2 capture19 and liquid water reactivation potential33 have been previously demonstrated. The limestone was supplied by Energie Baden–Württemberg AG (EnBW), and it was preliminary sieved in the 0.4–0.6 mm particle size range. The calcium looping process experiments were carried out in a lab-scale fluidized bed reactor, operated at atmospheric pressure, made of stainless steel and having internal diameter of 40 mm. The reactor was electrically heated, fluidized with gas of pre-set composition, and equipped with a head bearing two sintered brass filters, for the


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continuous capture of the sorbent fines elutriated from the bed. The flue gas analysis was performed on-line. Further details on the apparatus have been given in ref. 19. The operating conditions for the calcium looping process tests on the raw sorbent are reported in Table 1. The bed was fluidized under moderately slugging conditions at gas superficial velocities around 2–2.5 times the incipient fluidization velocity. A sorbent mass m0=25 g was loaded into the reactor, diluted with 150 g of coarser (0.85–1 mm) and inert silica sand. Under the operating conditions of the test, the limestone belonged to the Geldart B group, and the sand to the B–D groups. The coarser size of the sand, together with its negligible attrition, enabled easy sand/sorbent separation by mechanical sieving at the end of each experiment. The same fluidized bed reactor served both as a calciner and as a carbonator: at the end of each stage, the material was retrieved from the bed, the sorbent was separated from the sand, analyzed and fed back to the reactor for the subsequent stage after setting the new reactor operating conditions. Further details on the experimental procedure are reported in ref 19. The duration of each stage was such that the course of the chemical reaction (either calcination or carbonation) were practically complete. For each carbonation stage, the CO2 capture capacity (ξmol) was calculated on the basis of the flue gas analysis, as the moles of CO2 cumulatively captured in the carbonation stage divided by the moles of Ca initially fed to the system:

=

!" # $ #% &

(3)

where the limestone was assumed to be pure CaCO3 and W indicates the mass flow rates. By measuring the amount of elutriated fines collected in the brass filters during each calcination/carbonation stage, the specific elutriation rate was calculated as:

'( = )* +,

(4)

where mef is the mass of elutriated fines collected in the filter in the time interval ∆t. In the context of the present investigation, the elutriated fines can be considered to be entirely generated by sorbent attrition, since attrition of the sand and the presence of fines in the sorbent/sand feed that could be directly entrained out from the bed could be ruled out.


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The extent of size reduction of sorbent particles remaining into the bed after each stage (sorbent in-bed fragmentation) was quantified by measuring the particle size distribution of the material once separated from the sand. After the mechanical sieving in different particle size ranges (spanning from 0 to 0.6 mm), the particle size distribution of the sorbent was calculated in terms of x(di), where x is the mass fraction of particles having a certain mean diameter di. Correspondingly, the Sauter mean diameter for each distribution can be defined as:

-̅ =

/

∑2

(5)

1"2 "2

As reported in Table 1, the spent sorbent was collected after 4 complete calcination/carbonation cycles, followed by a 5th calcination stage. Then, the spent sorbent was steam hydrated (the details are reported in the next section). Subsequently, the steam hydrated sorbent was re-subjected to calcium looping cycles, under the same operating conditions held before hydration, performing the 5th carbonation, and then 3 more complete calcination/carbonation cycles, ending up with the 8th carbonation. Care was taken to calculate ξmol (Eq. (3)) with respect to the moles of Ca present in the hydrated sorbent initially fed to the system for the 5th carbonation, after characterization of the hydrated sorbent. The elutriation rate E(t) was expressed according to Eq. (4), with reference to the actual mass of hydrated sorbent used in the looping tests (again, starting from the 5th carbonation). In this way, a direct comparison of the pre- and post-hydration values assumed by these two parameters was possible.

Steam hydration tests on spent sorbent. The sorbent retrieved from the bed after the 5th calcination stage was considered as “spent”, and it was steam hydrated (in batches of 5 g, diluted again in a bed of sand) in the fluidized bed reactor according to the operating conditions reported in Table 1. The fluidizing gas (steam+N2) was generated through a Bronkhorst Controlled Evaporator and Mixer (CEM) apparatus. Three different samples were obtained: shy_10, shy_30 and shy_60, steam hydrated for 10, 30 and 60 min, respectively. Samples were characterized by means of: i) thermogravimetric analysis, carried out in a Netzsch STA 409C/CD system by heating the sample from room temperature to 1000°C, under Ar and at a heating rate of 10°C min–1; ii) scanning electron microscopy (SEM), using a FEI Inspect apparatus for magnifications up to 6000×; iii) N2 porosimetry, carried out in a Quantachrome Autosorb instrument, for the analysis of the microporosity (shy_60>shy_10, cf. Table 3), possibly for the same reasons. Altogether, reactivation brought about a moderate weakening of the particle structure since, for the raw material cycled before hydration, the Sauter diameter was never


8

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finer than 0.45 mm.33 Even if the particle size reduction associated with steam hydration of the samples was limited, the relevance of in-bed fragmentation to the generation of fine elutriable fragments (shy_60>shy_10 is confirmed. The latter two findings can be related to the same reasons invoked to explain the attrition/in-bed fragmentation propensity. For a detailed discussion of the impact fragmentation patterns, the following definitions will be used:14,17 the particle chipping is associated with the generation of a limited number of fragments of a size much smaller than that of the parent particle; the particle splitting is associated with the breakage of a particle into a relatively small number of fragments of size comparable with that of the parent particle; the particle disintegration is associated with an extensive


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loss of particle connectivity, which results in the generation of a large number of small fragments. These patterns are sketched in Figure 9, where the respective PDF of the particle size distributions are also qualitatively reported. From the analysis of the PDF curves reported in Figure 7 for the raw and spent sorbent (only di values up to 0.4 mm were reported, for the sake of the clarity), it is observed, in both the cases, that samples mostly underwent particle splitting, possibly combined with moderate chipping, with a prevailing generation of relatively coarse fragments. This is suggested by the nonuniform, monotonically increasing ψ(di) functions, with pronounced maxima close to the lower limit of the parent particle size (0.4 mm). While this finding was expected for the raw limestone (composed by brittle/semibrittle CaCO3 characterized by splitting/chipping fragmentation patterns),14 the behaviour was fairly new for the spent material. The spent sorbent, mostly composed by soft CaO, shows an impact fragmentation pattern similar to that typical of CaCO3, at odds with previous findings14 that soft CaO-based sorbents generally display a disintegration failure mode. Indeed, the CaO-based spent sorbent here analyzed is not freshly calcined lime (like in the previous study), but has undergone extensive sintering. It is likely that this feature enhanced the particle “brittleness” affecting the impact fragmentation pattern accordingly. This result is consistent with the finding that the attrition rate of the spent sorbent during the calcination stage is of the same order of that observed during the carbonation stage (Table 3), while a freshly calcined limestone (in air) was reported to have a much larger attrition rate.42 Figure 8 reports the ψ (di) functions for the steam hydrated samples after impact. A quite different fragmentation pattern was observed with respect to raw and spent sorbents. In fact, the PDF was that typical of softer materials (Figure 9): some splitting/chipping, related to the partly sintered nature of the spent material subjected to steam hydration, was superimposed to the disintegration, arising when the soft CaO derived from decomposition of Ca(OH)2 is present within the particle structure. These failure modes were already observed when steam hydrating sorbent samples derived from fluidized bed combustion/desulphurization,17 and have been recently observed also in the context of calcium looping.22

THE EFFECT OF THE HYDRATION TIME. The effect of steam hydration on the sorbent properties is outlined in Figure 10. Increasing steam hydration time is reflected into larger degrees of hydration and more extensive cold sintering/molecular cramming phenomena. The competition between these two effects results into a non-monotonic trend of porosity vs. hydration time. The sample hydrated for 30 min turns out to be the most micro- and meso-porous. Even if this large porosity makes this material reactive towards CO2 upon further looping, the greater extent of hydration for longer hydration times suggests the use of the sample hydrated for 60 min, as this is the most reactive. Furthermore, this sorbent has another advantage: it is less prone to undergo attrition/fragmentation as compared with the sample hydrated for 30 min, due to a somewhat


10

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more sintered (i.e., harder) structure. This aspect was also confirmed by results of ex situ impact fragmentation tests, which shed light on a general relationship that can be established between surface wear, in-bed and impact fragmentation. In other words, under these operating conditions, the hydration time of 60 min would be optimal in terms of a compromise between porosity, CO2 capture capacity and attrition tendency. The short-time hydrated sample (10 min) is the less attractive one, due to its lower reactivity to CO2, though being the most resistant to attrition among the steam hydrated sorbents. When comparing liquid water and steam hydration, the former yields larger sorbent hydration, due to faster liquid-phase hydration kinetics.33,34 On the other hand, soaking in liquid water makes the hydrated sorbent particles much more prone to the attrition than in the case of the steam hydration.33,34 As a final remark, it is important to highlight the relevance of the microstructural/petrographic study of the limestone under scrutiny. These properties can significantly differ also when comparing sorbents with a very similar chemical composition. Only through a thorough characterization of the limestone properties it is possible to perform an optimal “tailoring” of the double looping process, that must be coupled with a cost/energy estimation connected with the addition of the reactivation stage in a calcium looping process, Figure 1, for example in terms of the optimal hydration time. These aspects, similarly to what highlighted with reference to liquid water hydration, appear to be crucial and very sensitive to the nature of the reactivation treatment (why/shy) and of the native limestone.

CONCLUSIONS. The effectiveness of sorbent hydration by steam as a mean to reactivate the CO2 uptake potential of a limestone for Ca-looping applications has been demonstrated. Steam hydration followed by dehydration of the reactivated sorbent in the hot fluidized bed develops an increased porosity, hence improved rate and extent of CO2 uptake. At the same time the attrition and fragmentation propensity of the reactivated sorbent is increased. For the given limestone, the optimal trade off between sorbent reactivity/uptake and mechanical strength is achieved after 60 min hydration of the spent sorbent, but it is expected that this result cannot be generalized, as it is critically dependent on the sorbent texture. The comparison between water and steam hydration of spent sorbents as reactivation means indicated that the steam hydration is more favourable. Albeit the liquid water hydration gives rise to larger water uptake, the prolonged soaking in liquid water makes the reactivated sorbent more susceptible to attrition and fragmentation.

ACKNOWLEDGMENTS.


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The Authors wish to express their thankfulness to Mr. Luciano Cortese and Dr. Luciana Lisi (IRC–CNR) for their help in some solid characterization, and to Mr. Michele Allocca and Mr. Liberato Gargiulo (UniNa) for their support in the experimental campaign.

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Table 1. Main operating conditions adopted for the calcium looping and steam hydration process experiments. Calcination Carbonation Number of cycle [#] 1st to 5th 1st to 4th Temperature [°C] 940 650 Duration of the test [min] 20 15 CO2 inlet concentration [%v] 70 (balance air) 15 (balance air) Fluidization velocity [m s–1] 0.7 0.6 Steam hydration Temperature [°C] 250 Duration of the test [min] 10, 30, 60 min Steam inlet concentration [%v] 50 (balance N2) Fluidization velocity [m s–1] 0.7 Carbonation Calcination Number of cycle [#] 5th to 8th 6th to 8th Other conditions Same Same


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Table 2. Hydration degree (XH) and results of the porosimetric analysis for the spent material and the steam hydrated (shy) samples. Sample XH [%] Microporosity (

Reactivation by Steam Hydration of Sorbents for Fluidized-Bed

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