Fuel 87 (2008) 3344–3352

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Fuel

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Steam hydration of sorbents from a dual fluidized bed CO2 looping cycle reactor

Vasilije Manovic, Dennis Lu, Edward J. Anthony *

CANMET Energy Technology Centre-Ottawa, Natural Resources Canada, 1 Haanel Drive, Ottawa, Ontario, Canada K1A 1M1

a r t i c l e i n f o

Article history:Received 6 November 2007Received in revised form 7 February 2008Accepted 28 April 2008Available online 2 June 2008

Keywords:Fluidized bedCO2 captureLimestoneHydrationSorbent reactivation

0016-2361/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.fuel.2008.04.035

* Corresponding author. Tel.: +1 (613) 996 2868; faE-mail address: banthony@nrcan.gc.ca (E.J. Anthon

a b s t r a c t

Results are presented on steam hydration of spent residues obtained from a 75 kWth dual fluidized bedcombustion (FBC) pilot plant unit operating in a CO2 looping cycle mode. The samples were collectedfrom the unit under various conditions, which included electrical heating of the reactor, as well as firingwith coal, and biomass under oxy-fuel combustion conditions. In addition, different operating times, i.e.,number of cycles (25 min–455 min/1–25 cycles) were examined, with the carbonator operating at tem-peratures of 600–700 �C and the calciner at 850–900 �C. The samples collected came from the calciner,carbonator and cyclone. Steam hydration itself was done under atmospheric pressure in saturated steamat 100 �C for periods of 15, 30 and 60 min. The original limestone sample, as well as the spent samplesfrom the pilot plant and the hydrated samples were examined to determine their hydration and carbon-ation levels, as well as their unreacted CaO content using TGA and XRD analysis. In addition, sampleswere characterized for pore distribution (nitrogen adsorption/desorption: BET and BJH), skeleton charac-terization, with density by He pycnometry and particle surface area morphology (SEM/EDX), as well aschanges in sample volume during hydration (sample swelling). The results obtained showed successfulhydration (typically only �10% unreacted CaO) even for hydration periods as short as 15 min, and veryfavorable sample properties. Their pore surface area, pore volume distribution and swelling duringhydration are promising with regard to their use in additional CO2 capture cycles or SO2 retention. How-ever, their predisposition to fracture is the main disadvantage observed with these samples. This mayresult in difficulties in terms of their handling in FBC systems, due to intensified attrition and consequentelutriation from the reactor.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Fluidized bed combustion (FBC) is well known as an environ-mentally benign process in comparison with other processes forcombustion of fossil fuels. In particular, FBC has the ability to cap-ture SO2 in situ by use of a sorbent that is typically either a calciticlimestone or dolomite [1]. Despite the numerous benefits of in situSO2 capture there are some disadvantages that have not been re-solved in spite of decades of commercial application of FBC, includ-ing the relatively low sorbent utilization, which is in the range of30–45%. This demands the use of extra sorbent, which reducesthe economic and environmental advantages of the process.Numerous studies on improvement of sorbent utility have beenperformed but there are still no commercial applications of reacti-vation technology given in the open literature [2].

Recently, there has been significant effort devoted to the goal ofdeveloping FBC technology for CO2 capture [3–10]. The CO2 captureprocess is based on the (exothermic) carbonation reaction:

CaOðsÞ þ CO2ðgÞ ¢Carbonation

CalcinationCaCO3ðsÞ ð1Þ

ll rights reserved.

x: +1 (613) 992 9335.y).

The reverse reaction, calcination, enables sorbent regeneration andproduction of a concentrated stream of CO2, ready for final seques-tration. For a practical system, CO2 capture requires a looping cyclethat enables use of the same sorbent over many carbonation/calci-nation cycles. However, similar to SO2 capture, research has shownthat CaO utilization is far from stoichiometric. This arises due to sin-tering and the resulting decay of CaO utilization over even a fewcycles [11–15]. The loss of activity is also accelerated in the pres-ence of SO2, i.e., blockage of reacting surface by CaSO4 [16,17].

Thus, an outcome of operating a CO2 looping cycle would be theproduction of a spent sintered sorbent, with transport, storage andrelated environmental problems. This also results in increasedcosts for extra quantities of fresh sorbent and this appears to beone of the key factors affecting the economics of a lime-basedCO2 capture cycle [18,19]. Enhancement of the conversion duringthe various cycles, resulting in decreasing consumption of freshsorbent and disposal of spent sorbent, is imperative. Currently, ef-forts are being devoted to studying the effects of sorbent dopingwith additives (NaCl and Na2CO3) [5], pretreatment of fresh sor-bent by steam [20], steam reactivation of spent sorbent [21–24],and reactivation by moist air [25], amongst others.

Experience gained from studies on reactivation of partially sul-phated sorbent, i.e., sorbent ‘‘spent” in SO2 retention [2,26–31],

Fig. 1. CANMET mini dual fluidized bed Ca looping facility.

V. Manovic et al. / Fuel 87 (2008) 3344–3352 3345

provides insight into the reactivation of spent sorbent from CO2

capture cycles [24]. Namely, both carbonation and sulphation aregas–solid reactions with solid product formation at the surface ofthe reactant (CaO), and deactivation by the product layer formed(CaCO3 or CaSO4). This product layer limits diffusion for both sul-phation [32] and carbonation [14,33]. Moreover, in the case ofSO2 retention, the bulky product layer of CaSO4 can fill pores nearthe particle surface and form an outer shell that surrounds theunreacted core [34]. The pattern of partially sulphated sorbent par-ticles may also influence the behavior of sorbent during hydration[35–38].

In the case of carbonation, a somewhat different pattern of par-tially carbonated particles can be expected because the CaCO3

product has a smaller molar volume than does CaSO4. Hence, for-mation of an unreacted core/shell pattern is less probable andthere is evidence that only the product layer thickness on the sor-bent surface defines sorbent reactivity [39]. This means that uni-form carbonation should occur. This type of reaction pattern isless suitable for hydration because all the surface of unreactedCaO grains can be expected to be covered by CaCO3. Published re-sults on reactivation of spent sorbent from CO2 looping cyclesshowed that hydration of partially carbonated sorbent is difficultor impossible [21,24]. However, as carbonation is a reversible reac-tion, it is possible to remove product layer. The results of hydrationof sorbent in calcined form have demonstrated that hydration is avery promising method for reactivation of such spent sorbents[24,25].

Previous investigations on reactivation of spent sorbent fromCO2 capture cycles were performed on samples cycled in non-flu-idized bed reactors—tube furnace (TF) and thermogravimetricanalyzer (TGA), under idealized conditions and with pure sor-bents (without fuel ash). However, numerous investigations onhydration of sorbent spent in SO2 retention showed that idealizedexperiments cannot be easily applied to real systems [2] and sim-ilar conclusions can be expected for reactivation of sorbent fromCO2 cycles. The main contribution from this study is an investiga-tion of samples obtained, in conditions quite close to those ex-pected in real FBC CO2 capture systems, from a pilot plantoperating in a continuous mode as a CO2 looping cycle reactor.The samples, for which hydration is examined here, have experi-enced attrition, thermal stresses, sulphation by SO2 obtained bycombustion of coal in the calciner, and influence of biomass andcoal ash compounds. They are quite different than samples ob-tained in the idealized conditions examined earlier with regardto hydration, which greatly increases the significance of theirinvestigation.

2. Experimental

2.1. Spent sorbent samples

Havelock limestone from eastern Canada was used in this inves-tigation and its properties are well described elsewhere [40]. Thecrushed limestone sample was sieved and a particle size fractionbetween 0.4 and 0.8 mm was used in the in situ CO2 capture exper-iments with a 75 kWth pilot-scale atmospheric dual fluidized bedcombustion system, which is illustrated in Fig. 1. Both reactors(calciner/regenerator and combustor/carbonator) have an ID of100 mm and can be operated at up to 1000 �C under atmosphericpressure conditions. Here, the sorbent calciner/regenerator unit isa circulating fluidized bed combustor. This unit has been upgradedfor operation with oxy-fuel firing with flue gas recycle. The com-bustor/carbonator itself is divided into two parts and is designedto allow separation of the combustion/sulphation and carbonationprocesses.

Prior to heat-up, the combustor is preloaded with limestone.External electric heaters are used to heat the dense bed region ofthe combustor and when the bed temperature reaches 650 �C,the primary fuel supply, in this case either biomass or bituminouscoal, is initiated to bring the bed temperature to the desired calci-nation conditions. Calcination is relatively slow until the bed tem-perature reaches �875 �C, but quickly finishes once the bedtemperature exceeds 900 �C. Preheating and the first calcinationtypically last longer than 3 h, depending on the weight of the pre-loaded sorbent batch. Once calcination is complete, the sorbent canbe pneumatically transferred to the carbonator, allowing the solidsto be continuously looped between the two reactors.

The carbonator was fluidized by a mixture of air and CO2, pre-heated to 250 �C, prior to entering the reactor. The electrical heat-ers were used to bring the carbonator to the desired carbonationtemperature. CO2 from a high-pressure cylinder was introducedinto the carbonator’s fluidizing medium, at a volume concentrationof 15% to simulate flue gas composition.

Samples obtained with three operating modes for providingheat to the calciner were investigated: using only external electricheaters; burning biomass with air; and oxy-fuel combustion withbiomass and bituminous coal. The biomass used is a low-ashwood pellet material and the coal is a high-volatile, medium-sul-phur eastern bituminous (EB), (Table 1). More details about theexperimental pilot-scale reactor, operating conditions and proce-dures are published elsewhere [40]. In this study samples ob-tained under different conditions, using different types ofcalciner heating, and collected from different reactor positions(calciner/carbonator, bed/cyclone) are investigated. Sample desig-nations giving their characteristics and their source are presentedin Table 2.

Table 1Analysis of the solid fuels

Wood pellets EB coal

Moisture, wt% (as analyzed) 2.83 1.08

Proximate, wt% (dry basis)Ash 0.72 8.66Volatile matter 83.56 35.78Fixed carbon 15.72 55.56

Ultimate, wt% (dry basis)Carbon 52.04 77.81Hydrogen 5.97 5.05Nitrogen 0.10 1.49Sulphur 0.01 0.95Ash 0.72 8.66Oxygen (by difference) 41.16 6.04

Heating value, MJ/kg 19.53 32.51

3346 V. Manovic et al. / Fuel 87 (2008) 3344–3352

2.2. Hydration by steam

The samples of spent sorbent obtained from the dual pilot-scalefluidized bed reactor were hydrated by steam in a 2-dm3 Parr4522M pressure reactor (PR) [41] under atmospheric pressureand at 100 �C. Samples were hydrated as they were obtained fromthe FBC CO2 capture device. The aim of this work was to investigatethe possibility of steam hydration of sintered/spent sorbent fromCO2 looping cycles performed in a real FBC reactor. Moreover,atmospheric pressure was chosen to simulate atmospheric pres-sure in situ hydration. The ultimate goal of this work is to achievehydration in the dual FBC reactor itself. It should be noted that, in areal reactor a higher temperature would be employed, for thermalefficiency, than that examined here and hydration would be per-formed with superheated steam that cannot be simulated in thePR employed here. Hydration was performed for different dura-tions, 15, 30 and 60 min, to obtain data for process optimizationin the FBC, i.e., for the reduction of hydration time and costs.

The samples (1.0 g) were placed on an aluminum sample dishwithin the vapor space of the PR. The experiments were performedwith saturated steam. When a temperature of 100 �C was achieved,the valve on the bomb was opened periodically throughout theexperiment, enabling purge air to be introduced into the bomb atthe beginning of the experiment, to prevent pressure in the reactor

Table 2Spent samples from FBC CO2 capture pilot plant dual reactor used for hydration

Sample (time) Cycling time(h:min)/no. ofcycles

Heating of calciner

Carbonator #1 (6:10pm, April 16, 07) 2:45/6 EHs/biomass (100 rpm

Carbonator #4 (3:24pm, April 17, 07) 7:35/25 EHs/biomass (300 rpmCalciner #1 (6:10pm, April 16, 07) 2:45/6 EHs/biomass (100 rpmCalciner #2 (11:45am, April 17, 07) 3:55/9 EHs/biomass (125 rpm

Calciner #3 (1:20pm, April 17, 07) 5:30/13 EHs/biomass (175 rpmCalciner #4 (3:24am, April 17, 07) 7:35/25 EHs/biomass (300 rpmOxy-fuel, biomass (3:10pm, April 23, 07) 0:25/1 EHs/biomass

(1400 rpm)Oxy-fuel, EB coal (April 26, 07) �1:25/3 EHs + oxy-fuel, coal

Cyclone (April 26, 07) �1:25/3 EHs + oxy-fuel, coal

EHs – External electric heaters.

rising and to ensure the desired temperature was maintained.When the hydration process was completed, to avoid additionalhydration by free water adsorbed at the particle surface, the sam-ples were transferred to a vacuum oven and dried at 50 �C for 2 h.This effectively removed all the free water from the samples, asconfirmed by TGA analyses. The samples were weighed after dry-ing and their volumes (i.e., their height in a glass cylinder) weremeasured to provide data on sample swelling due to hydration.The samples themselves were kept in small, capped jars for subse-quent characterization experiments.

2.3. Sample characterization

All samples of spent and hydrated sorbent (hydrated for differ-ent durations) were analyzed using a Perkin Elmer TGA-7 thermo-gravimetric analyzer. The temperature program involved heatingat 30 �C/min to 850 �C in a nitrogen atmosphere and it was con-trolled by Pyris software. During this period samples were cal-cined, and H2O from Ca(OH)2 and CO2 from CaCO3 were released.The degrees of hydration and carbonation were determined onthe basis of mass loss due to evolution of H2O and CO2. The identi-fication of compounds present and their quantification in some se-lected hydrated samples were obtained by X-ray diffraction (XRD).XRD data were collected on a Bruker D500TT diffractometer overthe angular range 10–70� (2-theta) in 0.02� steps and 20 s per step.The phases were identified and quantitative analyses of the sam-ples were done using alpha-alumina (Al2O3), as an internal spikingstandard.

The ratio of sample volumes after and before reactivation (V2/V1) was determined to quantify visible sample swelling as a resultof hydration. The V2/V1 volume ratio was determined by measuring‘‘sample height” before (h1) and after (h2) reactivation in a glasscylinder (V2/V1 = h2/h1). To ensure uniformity of packing of samplewithin the cylinder (i.e., repeatability of results), the measuringcylinder was tapped repeatedly to obtain a flat sample surfaceand constant height.

The sample morphologies were observed with a Hitachi S3400scanning electron microscope (SEM) with 20 kV of acceleratingvoltage under high vacuum. SEM images from selected spent andhydrated samples coated by gold/palladium were obtained at dif-ferent magnifications, from 20� to 10,000�. The microscope wasequipped with energy-dispersive X-ray (EDX) analyzers, which

T (�C) carbonator/calciner

Taken from Note

) 617/882 Carbonator bed Biomass feeder rate 100 rpm;carbonator was heated by EHs

) 662/877 Carbonator bed) 617/882 Calciner bed Experiment started with 12.3 kg) 622/875 Calciner bed 2 kg additional sorbent was added in

the morning of April 17) 644/889 Calciner bed) 662/877 Calciner bed

678/880 Calciner bed

705/915 Calciner bed Experiment started with 10 kg ofsorbent; 5 kg of coal is combustedand coal ash is expected to beretained in the sorbent samples;sample was collected next day aftersystem shutdown

682/886 Carbonatorcyclone

Coal ash is expected to be retained inthe sorbent samples; sample wascollected next day after systemshutdown

V. Manovic et al. / Fuel 87 (2008) 3344–3352 3347

enabled examination of elemental composition at points of intereston the sample surface.

Pore surface area (BET) and pore volume distribution (BJH) ofselected spent and hydrated sorbent samples were determinedby the nitrogen adsorption/desorption method using a Micromeri-tics Tristar 3000 N2 sorption analyzer. To avoid comparison of poreareas and volumes for samples with different levels of carbonationand hydration, nitrogen adsorption/desorption tests were per-formed with samples calcined at 850 �C. In addition, changes tothe sample skeleton, i.e., skeletal densities, were investigated withan AccuPyc 1330 V3.00 He pycnometer, and the average density foreach sample was obtained from more than 10 runs.

3. Results and discussion

The results of TGA analyses of Ca(OH)2 and CaCO3, i.e., levels ofsample hydration and carbonation, and results on sample swellingafter hydration are given in Table 3. The levels of sample hydrationand carbonation were calculated on the basis of mass loss due toH2O and CO2 release during TGA runs. A typical TGA curve is pre-sented in Fig. 2. It should be mentioned that samples were non-

Table 3Results of TGA analyses of samples and their swelling during hydrationa

Sample Hydration timeb

(min)H2O(mol%)

CO2

(mol%)CaO(mol%)

V2/V1

Carbonator #1 0 3.66 20.92 75.42 n.a.15 63.47 25.94 10.59 2.3230 57.15 23.13 19.72 2.0960 64.22 22.63 13.15 2.27

Carbonator #4 0 3.91 16.21 79.88 n.a.15 70.78 19.69 9.53 2.4630 70.50 17.49 12.01 2.1760 69.70 21.35 8.95 2.67

Calciner #1 0 4.15 11.91 83.94 n.a.15 78.40 11.34 10.26 1.5830 72.05 13.59 14.36 1.5460 80.80 7.86 11.34 1.58

Calciner #2 0 10.19 11.75 78.06 n.a.15 81.71 13.80 4.49 1.6330 81.52 10.91 7.57 1.5460 72.83 15.73 11.44 1.58

Calciner #3 0 1.30 15.20 83.50 n.a.15 76.66 17.38 5.96 1.4630 70.31 18.95 10.74 1.4760 73.48 17.11 9.41 1.38

Calciner #4 0 5.06 10.23 84.71 n.a.15 73.47 14.96 11.57 1.5030 75.61 13.86 10.53 1.3860 82.93 8.00 9.07 1.46

Oxy-fuel,biomass

0 14.04 11.47 74.49 n.a.15 70.05 19.95 10.00 2.3230 69.68 14.49 15.83 2.0960 73.76 15.27 10.97 2.09

Oxy-fuel, EBcoal

0 0.00 6.89 93.11 n.a.15 80.74 9.42 9.84 2.0030 75.51 13.01 11.48 1.8260 79.28 9.64 11.08 2.05

Cyclone 0 1.53 31.23 67.24 n.a.15 49.71 38.76 11.53 2.0530 49.47 33.83 16.70 1.8260 56.13 34.22 9.65 2.09

a H2O represents the level of sample hydration, i.e., (mol H2O)/(mol CaO) � 100%;CO2 represents the level of sample carbonation, i.e., (mol CO2)/(mol CaO) � 100%; andCaO is unreacted CaO (100–H2O–CO2). It is assumed as an approximation that Ca inthe sample appears only as CaO, Ca(OH)2 and CaCO3.

b Hydration time = 0 means that sample is spent but not treated by steam.

uniform because of relatively large particles (0.4–0.8 mm) with re-spect to sample mass used in TGA runs (�20 mg) and all runs wererepeated at least twice and only mean values are presented in Ta-ble 3.

The very high levels of sample hydration achieved show effi-cient hydration under the conditions chosen here. This is also con-firmed by the small amount of unreacted CaO present (typically�10%). Carbonation levels of samples are generally low, and moreimportantly, the differences between samples from the carbonatorand calciner are very small (<5%). This means that samples werenot fully carbonated during CO2 cycles in the FBC carbonator; i.e.,there was an excess of sorbent enabling faster CO2 capture andlower CO2 concentration leaving the carbonator. The higher car-bonation levels obtained for cyclone material may be due to itssmaller particles and more probably due to its longer exposure tocarbonation conditions in the cyclone where this material wastrapped.

It was expected that CaCO3 in the samples would hinder hydra-tion and result in a larger amount of unreacted CaO with increaseof sample carbonation. This is shown in Fig. 3, and it can be seenthat the correlation is not significant; i.e., the influence of CaCO3

is not a predominant factor that affects hydration. We believe thatthis is a consequence of low levels of carbonation and that the thinCaCO3 product layer at the sorbent surface is insufficient to preventhydration. This is different than what was done in earlier research[21,24], and it should be noted that the carbonation of samplesexamined here is not ‘‘complete” (typically <25%). In earlier re-search, the samples were hydrated after carbonation levels weresignificantly higher (typically >50%), i.e., after the slow carbonationstage was achieved.

Fig. 4 shows the relation between carbonation levels and sam-ple swelling. It can be seen that samples from the carbonator,regardless of higher levels of carbonation, swell more than samplesfrom the calciner. This may result from some sorbent particle sizeseparation during CO2 cycles, as it was noticed visually that parti-cles from the calciner are larger. In addition, samples from the cal-ciner were notably harder than those from the carbonator and theskeletal densities are different (see below).

These results are in agreement with our earlier results [23,24]on hydration of samples from CO2 looping cycles in a tube furnace.It can be seen from Fig. 5 that the best swelling occurred in the caseof samples after the carbonation stage with low carbonation con-versions. These samples were obtained after 20 cycles and carbon-ation was at 650 �C, with �20% carbonation of sample in the lastcycle, which is similar to conversions of samples from the FBCexamined in this paper. A small degree of swelling was obtainedwith samples after 10 cycles at 700 �C, but it was clearly a conse-quence of high carbonation levels (�50%). It is also interesting thatcalcined limestone samples swell less than samples after cycling,which indicates a favorable effect of cycling on sample hydrationand resulting swelling. This effect is most likely a result of particlefracture, which causes large pores and cracks to be produced in thespent sorbent particles.

The results for sample hydration levels obtained by TGA wereconfirmed by XRD quantitative analyses (presented in Table 4).For easier comparison of XRD results with TGA results (Table 3),wt% from XRD analyses are recalculated and presented in bracketsin Table 4 as mol%. The Ca(OH)2 content in the samples is very highand appears to depend on the content of CaCO3 in the samples.Sorbent chemically available for hydration (in the form of CaO)was almost totally hydrated and less than 5 wt% of sorbent(7 mol% of CaO) remained in the hydrated sorbent in the form ofunreacted CaO; this is especially favorable given that analyzedsamples were hydrated only for 15 min in steam. The small differ-ences in carbonation levels between samples from the carbonatorand calciner were also confirmed.

TGA

12

13

14

15

16

17

18

19

0 5 10 15 20 25 30Time, t [min]

Mas

s, m

[mg]

0

100

200

300

400

500

600

700

800

900

1000

Tem

pera

ture

, T [ o C

]

mdm/dtT

Δ1m(H2O)

Δ2m(CO2)

0.50

-0.5-1.0

dm/d

t [m

g/m

in]

T1(H2O)

T2(CO2)

Fig. 2. Typical TGA test to determine sample hydration and carbonation levels.

R2 = 0.0724

0

5

10

15

20

25

5 10 15 20 25 30 35 40

Carbonation [%]

Un

reac

ted

Ca

O [%

]

Fig. 3. Relation between sample carbonation and unreacted CaO.

1

1.5

2

2.5

3

0 10 20 30 40 50Carbonation [%]

Sam

ple

swel

ling,

V 2

/V1.

Calciner (#1, #2, #3, #4)

Carbonator (#1, #4)

Oxy-fuel (EB Coal, Biomass)

Cyclone

Fig. 4. Relation between sample swelling during hydration and carbonation levelsfor samples taken from different positions in the dual fluidized bed reactor systemafter CO2 looping cycles.

0

1

2

3

4

5

0 10 20 30 40 50 60

Carbonation [%]

Sam

ple

swel

ing,

V 2

/V1

CO2 looping [23]

CO2 looping (sulphated) [23]

Calcination [23]

CO2 looping [24]

Hydrated after carbonation stage

Calcined before hydration

Hydrated after carbonation stage

Fig. 5. Relation between sample swelling during hydration and carbonation levelsfor calcined samples and samples from CO2 looping cycles performed in a tubefurnace [23,24].

Table 4Quantitative XRD analyses of samplesa hydrated for 15 min

Compound Carbonator#4

Calciner#4

Oxy-fuel,biomass

Oxy-fuel,EB coal

Cyclone

Calcite, CaCO3,wt% (mol%)

18.7 (14.6) 11.9 (9.2) 19.8 (15.4) 22.0 (17.1) 35.5 (28.6)

Lime, CaO, wt%(mol%)

1.4 (1.9) 0.33 (0.5) 4.3 (6.0) 5.0 (6.9) 5.1 (7.3)

Portlandite,Ca(OH)2, wt%(mol%)

79.2 (83.5) 86.6 (90.4) 75.0 (78.7) 72.2 (75.9) 59.0 (64.1)

Quartz, SiO2, wt% 0.70 1.2 1.0 0.84 0.4

a wt% from XRD analyses are recalculated and presented in brackets as mol%.

Table 5BET surface area of spent samples and those hydrated for 15 min

Sample BET (m2/g)

Original 12.16

Carbonator #4 Spent 4.60Reactivated 8.45

Calciner #4 Spent 3.88Reactivated 7.88

Oxy-fuel, biomass Spent 6.86Reactivated 8.96

Oxy-fuel, EB coal Spent 2.15Reactivated 7.28

3348 V. Manovic et al. / Fuel 87 (2008) 3344–3352

The excellent hydration, i.e., high content of Ca(OH)2 in samples,and their swelling were promising with regard to particle surfacearea morphologies. Results of the analysis of the BET surface areaof spent samples and those hydrated for 15 min are given inTable 5. It can be seen that there is a significant loss of particle sur-face area after CO2 cycles. The original sample after calcination hasa specific BET surface area of 12.16 m2/g, but after cycling this

value drops noticeably. Despite the fact that samples from thecarbonator and calciner (#4) were from the same FBC CO2 experi-ment and both were calcined before BET tests (same level, i.e., 0%CaCO3), they have different BET surface areas (4.60 vs. 3.88 m2/g).This may indicate that some particle separation occurred and thatthis is the cause for different swelling of samples taken from thecalciner and carbonator (Table 3). Moreover, after reactivation,samples from the carbonator have higher BET surface area valuesthan samples from the calciner. It is interesting to note the verylow surface area (2.15 m2/g) of the spent sample when coal wasused in oxy-fuel combustion. This may be due to the influence ofcoal ash, i.e., compounds from coal ash may promote sinteringand loss of surface area. However, it should be noted that onlyone experiment with three cycles is represented here and it maybe premature to conclude a negative influence of coal oxy-fuelcombustion in the calciner on the loss of sorbent surface area.

Table 6Skeletal densities of spent samples and those hydrated for 15 min

Sample Skeletal density(g/cm3)

Standard deviation(g/cm3)

Original 3.30 0.02

Carbonator #4 Spent 3.39 0.02Reactivated 3.20 0.01

Calciner #4 Spent 3.49 0.05Reactivated 3.32 0.02

Oxy-fuel, biomass Spent 3.29 0.03Reactivated 3.25 0.01

Oxy-fuel, EB coal Spent 3.40 0.03Reactivated 3.30 0.02

V. Manovic et al. / Fuel 87 (2008) 3344–3352 3349

However, it is a significant result that the sample after reactivationrecovered its surface area.

The BJH pore analyses show similar results as the BET analyses.In comparison with original calcined sorbent, the BJH cumulativepore volumes of samples after cycles in the FBC are lower and thiscan be seen in Fig. 6 (values in brackets). BJH pore volume distribu-tions presented in the same figure show two peaks: a small peak ataverage pore diameter �4 nm and a tall peak at �50 nm. This typeof pore volume distribution was observed in our previous research[24]. The main loss of pore volume after CO2 cycles is in the area ofthe 50 nm peak. It is interesting that there is no significant porevolume change in the area of small pores, before the second peak(<20 nm) and pore volume in that range mainly increased afterCO2 cycles. In that area of small pores, improvement is significantafter sample hydration, but the main increase of pore volume is ob-served around the 50-nm peak.

The pore surface areas and pore volumes obtained in this workare in close agreement with those for samples obtained in the caseof CO2 cycles performed in the TF [22–24]. In addition, an impor-tant finding presented here is the possibility of regeneration of sor-bent particle morphologies for spent sorbent, produced underconditions that may be expected in industrial-scale FBC systems.Samples are successfully reactivated under atmospheric conditionsin as little as 15 min; this enables consideration of atmosphericpressure reactors and in situ reactivation by introducing steam intoan atmospheric FBC.

Apart from changes in sorbent morphologies during CO2 cyclesand hydration, changes in the sorbent particle skeleton that are notdetectable by nitrogen adsorption/desorption measurements (BETand BJH), may occur. Namely, in hydrated sorbent particles, crys-tals of Ca(OH)2 were seen [23] and recrystallization during hydra-

Carbonator #4

0

0.001

0.002

0.003

0 50 100 150 200Pore Diameter, D [nm]

Por

e V

olum

e, d

V/d

D [

cm 3 /g

nm

]

Original (0.175)Spent (0.034)Reactivated (0.067)

Oxy-fuel, Biomass

0

0.001

0.002

0.003

0 50 100 150 200Pore Diameter, D [nm]

Por

e V

olum

e, d

V/d

D [

cm 3 /g

nm

]

Original (0.175)Spent (0.064)Reactivated (0.088)

Fig. 6. BJH desorption pore volume distributions in spent sorbent samples before and afteCarbonator #4, (b) Calciner #4, (c) Oxy-fuel, biomass and (d) Oxy-fuel, EB coal. The num

tion and changes in the sorbent particle skeleton can be expected.Also, during CO2 cycles migration of Ca ions in the crystals andrecrystallization may occur, which fill pores inaccessible to gasand increase measured skeleton density. Skeletal densities of sor-bent samples examined in this study were determined by heliumpycnometry and the results are given in Table 6. Skeletal densityof the original sample after calcination is 3.30 g/cm3, a characteris-tic value that is usually attributed to non-porous CaO material. It isvery interesting that spent samples have noticeably higher densi-ties, with the exception of the biomass oxy-fuel sample; however,this sample experienced only one cycle (Table 2). Skeletal densitiesafter reactivation decrease to the values of the original sample oreven less. This means that in CaO sorbent there are ‘‘hidden” spacesbut during carbonation cycles they disappear as a result of ionmigration, recrystallization and sintering. After hydration (and

Calciner #4

0

0.001

0.002

0.003

0 50 100 150 200Pore Diameter, D [nm]

Por

e V

olum

e, d

V/d

D [

cm 3 /g

nm

]

Original (0.175)Spent (0.026)Reactivated (0.082)

Oxy-fuel, EB Coal

0

0.001

0.002

0.003

0 50 100 150 200Pore Diameter, D [nm]

Por

e V

olum

e, d

V/d

D [

cm 3 /g

nm

]

Original (0.175)Spent (0.008)Reactivated (0.058)

r hydration for 15 min, compared with sorbent before use in CO2 looping cycles: (a)bers in brackets are BJH cumulative pore volumes (cm3/g).

3350 V. Manovic et al. / Fuel 87 (2008) 3344–3352

calcination) pores inaccessible to helium (because its entry isblocked) reappear, resulting in lower skeleton densities. Apartfrom sorbent morphology, skeletal density also may play a rolein determination of sample activity for CO2 capture, especially be-cause a consistent correlation between sorbent morphologies andactivity has not been established. A clear difference in densitiesmay be seen for samples from the carbonator and calciner (#4),(spent and reactivated). This also supports the presumption that

Fig. 7. SEM of surface area of investigated samples: (a,b) Oxy-fuel, EB coal; (c,d)

the sorbent particles in the carbonator and calciner had somewhatdifferent properties following the CO2 capture experiment.

SEM analyses of samples show the presence of pores/crackswith diameters >150 nm, which are not observable by BET/BJH,but they are also most likely responsible for the enhanced hydra-tion of spent samples (in comparison with the original samplesafter the first calcination [22,23]). Spent samples from FBC CO2 cy-cles investigated in this study also have large cracks and this can be

Oxy-fuel, EB Coal – hydrated and calcined; (e, f) Carbonator #1 – hydrated.

V. Manovic et al. / Fuel 87 (2008) 3344–3352 3351

seen in Fig. 7a and b). These cracks are very important for hetero-geneous reactions such as hydration of CaO when product mayblock contact between reactants and form a shell surroundingthe unreacted core. After hydration the cracks become even larger(Fig. 7c and d), which is most likely important for sample swellingduring hydration. Our previous research [23] showed that KellyRock limestone after CO2 cycles and hydration had large Ca(OH)2

crystals but in this study Ca(OH)2 crystals are visible only occasion-ally and they are small (Fig. 7e). EDX elemental analysis was doneand confirmed that small crystals visible by SEM were Ca(OH)2.The main difference in these experiments, that may be the causefor the different quantity and size of Ca(OH)2 crystals (visible bySEM), is the hydration conditions. Namely, hydration of Kelly Rocklimestone was done at 200 �C under high pressure, while hydrationof samples examined here was done at 100 �C and atmosphericpressure. That suggests that the pressure of steam and, moreimportantly, temperature accelerate ion migration, recrystalliza-tion and formation of Ca(OH)2 crystals.

Despite the favorable properties of spent sorbent after hydra-tion, reactivated material from the FBC has a major disadvan-tage—particle fragility. Due to large cracks in the spent sorbent(Fig. 7a and b), that became even larger after hydration (Fig. 7cand d), the particles are very fragile, which is illustrated in Fig.7f. Despite gentle handling, they very easily crush, forming smallerparticles; a significant portion of the reactivated sorbent is trans-formed into powder. This means that we can expect difficultiesduring their handling in a FBC reactor, with attrition and sorbentelutriation expected. However, significant enhancement of othersorbent properties provides a rationale for the continued investiga-tion of steam reactivation of sorbent spent in CO2 capture loopingcycles in FBC systems.

4. Conclusions

Spent samples from a dual FBC reactor pilot plant, operating asa CO2 looping combustor, were hydrated with the aim of enhancingtheir CO2/SO2 capture capacities. Hydration by steam under atmo-spheric pressure and at 100 �C for as little as 15 min produced sam-ples with a very small amount (10%) of unreacted CaO. Moreimportantly, samples have improved morphology (pore surfacearea and distribution) and it appears that further work on enhanc-ing the properties of such sorbents by hydration may yield evenbetter results. Particles having undergone CO2 looping cycles havelarge cracks that grow during hydration and contribute to sampleswelling and particle fragility. Better results are obtained withsamples from the carbonator than those from the calciner. It hasbeen noted that samples obtained from the oxy-fuel combustionexperiments tended to lose surface area and pore volume morereadily; however, these samples also reactivated efficiently.

Acknowledgements

The authors acknowledge Mr. John Blamey from the Depart-ment of Chemical Engineering, Imperial College London for provid-ing the BET/BJH and He pycnometry results presented here. Inaddition, they would like to thank Dr. Ashleigh Cousins from Nat-ural Resources Canada, CETC-O for operating the scanning electronmicroscope and providing the SEM images presented here.

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