Article pubs.acs.org/EF
Performance of Natural Sorbents during Calcium Looping Cycles: A Comparison between Fluidized Bed and Thermo-Gravimetric Tests Antonio Coppola,† Fabrizio Scala,*,† Grigorios Itskos,‡ Panagiotis Grammelis,‡ Halina Pawlak-Kruczek,§ Stelios K. Antiohos,∥ Piero Salatino,⊥ and Fabio Montagnaro# †
Istituto di Ricerche sulla Combustione, Consiglio Nazionale delle Ricerche, Piazzale Vincenzo Tecchio 80, 80125 Napoli, Italy Centre for Research and Technology Hellas/Institute for Solid Fuels Technology and Applications, 357-359 Mesogeion Avenue, Halandri GR-15231, Athens, Greece § Wroclaw University of Technology, Institute of Heat Engineering and Fluid Mechanics, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland ∥ Titan Cement Company S.A., Group R&D and Quality Department, Kamari Plant, 19200, Elefsina, Greece ⊥ Dipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale, Università degli Studi di Napoli Federico II, 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 ‡
ABSTRACT: Six European limestones and one European dolomite were tested for CO2 capture by simulating calcium looping cycles both in a lab-scale fluidized bed (FB) and in a thermo-gravimetric (TG) apparatus. The FB tests were carried out under severe conditions representative of a process with calcination in an oxy-firing environment (T = 940 °C, 70% CO2). The TG tests were performed with a somewhat milder calcination environment (T = 900 °C, 15% CO2). Carbonation conditions were the same for the two types of experiments (T = 650 °C, 15% CO2). The effect of the presence of SO2 (during both calcination and carbonation) at two different concentration levels was also studied in both devices. The aim of these experiments was to select the best sorbents for further larger scale testing and to establish if TG testing is representative of results obtained under more realistic FB conditions, with regards to CO2 capture capacity and overall ranking of the sorbents. Results showed that the CO2 capture capacity of the sorbents measured in TG tests was generally larger than that measured in FB experiments. This was attributed to the severer calcination conditions in the latter tests, implying an increased effect of sintering. A notable exception was the dolomite sorbent, which was less subject to sintering and more prone to fragmentation during the FB tests. The presence of SO2 depressed the CO2 capture capacity of all the sorbents in both types of experiments, due to the sulfate layer formation around the particles. SEM/EDX and porosimetric analyses of the spent sorbents confirmed the above findings. Interestingly, the ranking of the sorbents toward CO2 capture was similar under FB and TG conditions and was not significantly influenced by SO2.
1. INTRODUCTION Calcium looping consists of the removal of CO2 from a gas stream by means of a Ca-based solid sorbent which is continuously circulated between two fluidized bed (FB) reactors. In the first reactor (the carbonator) the sorbent is fluidized by the CO2-containing gas stream and CO2 capture occurs at a temperature of 650−700 °C according to the exothermic carbonation reaction: CaO(s) + CO2 (g) = CaCO3(s)
addition, the possible presence of SO2 in the gas further depresses the sorbent CO2 capture capacity, since part of the Ca is permanently sulfated:6,7 CaO(s) + SO2 (g) +
(2)
In fact, the sulfation reaction is irreversible at typical temperatures used in the calcium looping process.3,5 Finally, particle attrition in the FB also causes a net loss of calcium from the circulating loop, as elutriable fine particles that leave the cyclone with the gas stream. This loss of material adds to the deactivation of the sorbent and contributes to the required fresh sorbent makeup.8,9 The calcium looping performance of many natural and synthetic sorbents has been tested to date in both fluidized bed8−11 and thermo-gravimetric12−15 devices, under more or less realistic operating conditions. A direct comparison of these
(1)
The spent sorbent is then circulated to a second reactor (the calciner) where concentrated CO2 is released at 850−950 °C according to the reverse endothermic calcination reaction. This concentrated CO2 stream is then ready for further compression and sequestration. Unfortunately, the Ca-based solid sorbent progressively loses most of its CO2 capture capacity along cycling between the two reactors. The main reason has been identified in severe particle sintering during the high-temperature calcination stage, leading to a reduction of the available surface for reaction.1−5 In © XXXX American Chemical Society
1 O2 (g) = CaSO4 (s) 2
Received: July 10, 2013
A
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Table 1. Chemical Composition of Sorbents (% by Weight) sample
origin
SiO2
Al2O3
Fe2O3
CaO
MgO
K2O
Na2O
SO3
TiO2
LOI
Massicci Schwabian Alb EnBW Xirorema Sand Tarnow Opolski Czatkowice Dolomite Redziny
Italy Germany Germany Greece Poland Poland Poland
1.11 3.51 0.30 0.83 1.73 3.91 0.91
0.37 0.50 0.13 0.26 0.34 0.39 0.22
0.14 0.18 0.08 0.36 0.39 0.31 0.25
54.53 53.64 56.01 55.13 54.04 52.88 31.80
0.44 0.51 0.26 0.56 0.94 0.99 20.90
0.06 0.08 0.00 0.00 0.00 0.00 0.00
0.02 0.02 0.02 0.01 0.02 0.02 0.00
0.04 0.00 0.00 0.00 0.00 0.00 0.00
0.02 0.02 0.01 0.02 0.02 0.03 0.02
43.13 41.94 43.50 42.87 42.64 41.43 45.12
two techniques has not been carried out yet on the same sorbents, so that it is difficult to establish if the TG results are truly comparable or not to the more realistic FB tests. Available evidence suggests that sorbent reactivity is lower during FB testing than during TG testing.7 In 2010 an EU-funded project (CAL-MOD) was started with the aim of modeling and experimentally validating the calcium looping process. One task of the project consisted of the evaluation of a number of European Ca-based natural sorbents on a lab-scale basis, in order to select those with the best CO2 capture performance for further larger-scale testing. This labscale activity was carried out in parallel in a FB reactor and in a TG balance on the same sorbents. The time-consuming FB testing allowed simulating the process under more realistic conditions with regards to the reaction environment (temperature, CO2 concentration, impact of attrition). On the other hand, the TG testing was easier to perform and allowed one to carry out longer tests with a larger number of cycles. One further objective of the experimental campaign was that of comparing the results of the two kinds of tests, in order to establish if TG testing is representative or not of the results obtained in the FB reactor with regards to the CO2 capture capacity and the relative ranking of the sorbents. Finally, the effect of the presence of SO2 in the gas on the CO2 capture capacity was also tested in both devices. The spent sorbents from FB and TG tests were further subjected to SEM/EDX and porosimetric analyses to better understand the CO2 capture behavior. The most important results of this extensive experimental activity are reported in this paper.
SO2 during both carbonation and calcination) simulating CO2 capture from uncontrolled flue gas and regeneration in an oxy-fired calciner burning high-sulfur coal. Low-SO2 conditions (75 ppm SO2 during carbonation and 750 ppm SO2 during calcination) simulating CO2 capture from already desulfurized flue gas and regeneration in an oxyfired calciner burning medium-sulfur coal. For each TG run, a sorbent sample of around 50−80 mg with a particle size between 400 and 600 μm was introduced in the holder. This large mass most likely introduced mass transfer limitations to the chemical reactions but allowed very precise capacity measurements. No kinetic analysis was attempted on these results, as this was out of the scope of this experimental activity. Twenty-four subsequent cycles were performed with calcination and carbonation at 900 and 650 °C, respectively. Each calcination stage lasted for five minutes, with the exception of the first one lasting 10 min, while each carbonation stage lasted for 10 min. The sample heating rate from 650 to 900 °C in each calcination step was 140 °C/min, while the sample cooling rate from 900 to 650 °C in each carbonation step was 30 °C/min. Additional 5-cycles tests were also performed in order to obtain samples for further analysis with the same number of cycles than those obtained in the FB tests. The operating conditions of the experiments are summarized in Table 2.
2. EXPERIMENTAL SECTION
FB Reactor. The calcium looping tests were performed in a stainless steel lab-scale bubbling FB reactor, 40 mm ID operated at atmospheric pressure. Fluidizing gas mixtures are supplied to the reactor by means of three mass flow controllers. The gases are preheated and mixed in a first reactor section, 0.66 m high. A perforated plate (55 holes with 0.5 mm diameter in a triangular pitch) is used as the gas distributor. The fluidization column, 0.95 m high, is electrically heated with two semicylindrical ovens. On the top of the column a brass two-exit head is placed which is connected to a hopper for solids feeding and to the exhaust line. Flue gases in the two-exit head pass through either of two sintered brass filters (filtration efficiency is 1 for >10 μm particles). The filters are used in an alternated way in order to measure the timeresolved fines elutriation rate from the reactor. CO2 and SO2 concentrations are continuously measured in the exhaust gas with a NDIR analyzer. Batch FB Tests Procedure. Each FB experiment consisted of five calcination-carbonation cycles. A sorbent sample (20 g) was sieved in the 0.4−0.6 mm size range and diluted in 150 g of silica sand, sieved in the 0.85−1.0 mm size range. This procedure was necessary to avoid excessive bed temperature variations due to calcination and carbonation reactions. The bed was fluidized at 0.7 m/s during calcination stages and at 0.6 m/s during carbonation stages. Table 3 reports the operating conditions of the tests. The following procedure was used in all the experiments. First the reactor was charged with sand and heated up in air to 940 °C. Then
Table 2. TG Tests Experimental Conditions
a
2.1. Materials. Seven Ca-based natural sorbents have been employed in this work, namely six high-calcium limestones (calcite >94% by weight) and one dolomite, coming from several European countries (Italy, Germany, Greece, and Poland). Table 1 shows the chemical composition of the sorbents, as obtained by X-ray Fluorescence (Instrument: SRS 3400, Bruker). Fused beads were prepared for the analysis. The SO3 content of the sorbents was measured using wet chemical analysis according to EN 196-2. In the TG and FB tests the gas flows consisted of mixtures of air, CO2, and SO2/N2. 2.2. Experimental Setup. TG Apparatus and Procedure. The calcium looping cycles were simulated in a Perkin-Elmer Diamond Thermo-gravimetric (TG) analyzer, adapted for long tests with multiple carbonation-calcination cycles in different gas compositions. The TG apparatus is able to work at temperatures up to 1000 °C, but only up to 900 °C in a stable way, with a heating rate in the range 5− 200 °C/min. A platinum basket (6 mm diameter) was used as the sample holder. A PC continuously recorded the temperature and the sample weight. Mass flow controllers were used to set the reacting gas mixture flow at 200 mL/min. The composition of the gas flow in the tests without SO2 was CO2 = 15% and N2 = balance. The effect of SO2 on calcium looping was studied under two concentration levels (being the CO2 concentration fixed at 15%). High-SO2 conditions (1500 ppm B
calcination/carbonation
no SO2
low SO2
high SO2
duration [min] temperature [°C] inlet CO2 [%v/v] inlet SO2 [ppm]
5a/10 900/650 15/15 0/0
5a/10 900/650 15/15 750/75
5a/10 900/650 15/15 1500/1500
First calcination t = 10 min.
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3. RESULTS AND DISCUSSION 3.1. TG Tests. Figure 1a−c reports the CO2 capture capacity (in grams of captured CO2 per gram of initial sorbent) as a function of the number of cycles for the seven sorbents during TG experiments in the three conditions investigated (Table 2). In addition, Table 4 summarizes the CO2 capture capacity of the seven sorbents after 5 and 24 cycles (gCO2/gsorbent) in TG tests, both with and without SO2, as well as the (molar) Ca conversion to CaSO4 after 5 and 24 cycles during tests with SO2. As expected, the capture capacity decreased with the number of cycles for all the sorbents in all conditions. Asymptotic capture capacities after the 24th cycle lie in the range 0.05−0.07 gCO2/gsorbent in the tests without SO2 (Figure 1a). These values compare well with those typically reported in the literature (of the order of 0.1−0.2 gCO2/gsorbent as reported by Blamey et al.5). The ranking of the sorbents is as follows: Dolomite Redziny > EnBW > Czatkowice ≈ Tarnow Opolski ≈ Xirorema Sand > Massicci ≈ Schwabian Alb. Interestingly, after the 10th cycle the CO2 capture capacity of the dolomite was slightly larger than that of the limestones. This is a nontrivial result if one considers that the dolomite has a much lower Ca content than limestone (Table 1). In addition, the dolomite and the EnBW limestone show a lower decay of the capacity with the number of cycles, indicating that these two sorbents are somewhat less affected by sintering. In the tests with a high SO2 concentration (Figure 1c) a significant decrease of the CO2 capture capacity (of the order of 2−3 times) was found for all the sorbents. The residual capture capacity was 0.02−0.03 gCO2/gsorbent in this case. This result can be explained by the progressive build-up of a calcium sulfate layer at the particle surface that hinders intraparticle CO2 diffusion in the sorbent pores.9 However, when a lower SO2 concentration was used in the tests (Figure 1b), results were closer to those obtained in tests without SO2. It is probable that in this case a thinner and less compact sulfate shell is formed around the particles leaving more open pores for CO2 diffusion. It is interesting to note that the relative ranking of the sorbents toward CO2 capture was not significantly altered by the presence of sulfur dioxide. In the high-SO2 case the dolomite performance was slightly worse than that of EnBW and Czatkowice limestones. In particular, the dolomite exhibited the highest conversion to CaSO4 after 24 cycles and the largest loss of CO2 capture capacity (i.e., 0.048 gCO2/ gsorbent) compared to the test without SO2.
Table 3. FB Tests Experimental Conditions calcination/carbonation
no SO2
low SO2
high SO2
duration [min] temperature [°C] inlet CO2 [%v/v] inlet SO2 [ppm]
20/15 940/650 70/15 0/0
20/15 940/650 70/15 750/75
20/15 940/650 70/15 1500/1500
the gas was switched to a mixture containing 70% CO2. These conditions were chosen to simulate calcination in an oxy-firing environment. In fact, preliminary tests showed that at 70% CO2 a bed temperature of at least 940 °C was needed for calcination to proceed at a reasonable rate (the equilibrium CO2 concentration at this temperature is approximately 195 kPa). After stationary conditions were reached, the sorbent sample was fed to the bed through the hopper. The progress of the calcination reaction was followed by measuring the CO2 concentration at the exhaust. When the fast calcination stage was over (∼20 min), the bed was rapidly discharged, cooled down, and sieved to separate the sorbent from the sand. The sand was reinjected in the reactor, and the temperature set point was changed to 650 °C. The gas was switched to a mixture containing 15% CO2. After stationary conditions were reached, the calcined sorbent sample was fed again to the bed. The progress of carbonation reaction was also followed by measuring the CO2 concentration. When carbonation was complete, the bed was rapidly discharged, cooled down (in 100% CO2 to avoid possible calcination), and sieved to separate the sorbent from the sand. This procedure was then repeated in all the cycles. For the tests with SO2 the same experimental conditions and procedures were used. The only difference consisted of the presence of SO2 in the inlet gas mixture (both during calcination and carbonation). The same two SO2 concentration levels reported for TG experiments were used in these tests (Table 3). Calcium conversion to sulfate during the calcination/carbonation stages was evaluated from the SO2 concentration profiles at the exhaust. During all the FB tests the attrition and fragmentation propensity of the sorbents were also characterized in detail. These results have been reported extensively in previous publications16−18 and will not be repeated here. 2.3. Characterization of Sorbents. The following characterizations were performed on raw, carbonated, and sulfated-carbonated sorbents (in FB or TG, after five cycles). X-ray Diffraction (XRD, Bruker D8 Advance) was used to examine the sample mineralogy. Scanning Electron Microscopy (SEM, 6300 JEOL) was used to examine the sorbent microstructure. Energy-Dispersive X-ray Spectroscopy (EDS, installed on the SEM instrument) was used to determine the chemical composition of selected sorbent areas. Nitrogen adsorption measurements were carried out using an Autosorb-1, Micropore version, static volumetric system (Quantachrome instruments), at 77 K. The samples were outgassed at 623 K under high vacuum prior to each measurement test.
Figure 1. CO2 capture capacity of the seven sorbents as a function of the number of cycles for experiments performed in TG without SO2 (a), under low-SO2 conditions (b), and under high-SO2 conditions (c). See Table 2 for detailed experimental conditions. C
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Table 4. CO2 Capture Capacity and Ca Conversion to Sulfate after 5 and 24 Cycles in TG Tests 5 cycles CO2 capture capacity, g CO2/g sorbent
24 cycles Ca conversion to CaSO4, (molar)
CO2 capture capacity, g CO2/g sorbent
Ca conversion to CaSO4, (molar)
sorbent
no SO2
low SO2
high SO2
low SO2
high SO2
no SO2
low SO2
high SO2
low SO2
high SO2
Massicci Schwabian Alb EnBW Xirorema Sand Tarnow Opolski Czatkowice Dolomite Redziny
0.093 0.085 0.099 0.099 0.102 0.105 0.089
0.083 0.079 0.101 0.093 0.095 0.099 0.086
0.075 0.059 0.092 0.076 0.076 0.081 0.085
0.0004 0.0003 0.0001 0.0004 0.0001 0.0013 0.0002
0.038 0.061 0.042 0.045 0.045 0.044 0.068
0.049 0.049 0.066 0.056 0.053 0.057 0.074
0.043 0.046 0.060 0.050 0.047 0.050 0.069
0.026 0.022 0.030 0.021 0.022 0.028 0.026
0.0038 0.0039 0.0010 0.0030 0.0022 0.0061 0.0041
0.172 0.267 0.185 0.191 0.193 0.186 0.286
Figure 2. CO2 capture capacity of the seven sorbents as a function of the number of cycles for experiments performed in FB without SO2 (a), under low-SO2 conditions (b), and under high-SO2 conditions (c). See Table 3 for detailed experimental conditions.
high temperature and CO2 concentration during calcination. The residual capture capacity after the fifth cycle for the dolomite was 0.12 gCO2/gsorbent. This result highlights the positive effect of the large magnesium content of the dolomite in preserving Ca reactivity and reducing particle sintering.15 A further aspect influencing the CO2 capture capacity of the dolomite consists of the much larger propensity of this sorbent to undergo attrition with respect to limestones.18 Particle attrition exposes new reactive surface area and, in turn, enhances CO2 capture. The larger CO2 capture capacity of dolomite in FB tests with respect to TG tests (Figure 1) is most likely due to the effect of attrition in the fluidized bed, which by contrast is absent during TG experiments. The relative ranking of the sorbents, from the best to the worst capture capacity, is as follows: Dolomite Redziny > EnBW > Czatkowice > Xirorema Sand ≈ Tarnow Opolski > Schwabian Alb ≈ Massicci. It is noteworthy that this ranking is similar to that found in TG tests. When a high SO2 concentration was present in the gas a significant decrease of the CO2 capture capacity (of the order of 3−6 times) was found for all the sorbents (Figure 2c). The residual capture capacity was 0.004−0.04 gCO2/gsorbent in this case. The CO2 capture capacity of the dolomite remained always larger than that of the limestones along the cycles. When a lower SO2 concentration was used in the tests (Figure 2b), results were closer to those obtained in tests without SO2, similarly to what was observed in TG tests. Again, the relative ranking of the sorbents toward CO2 capture was not significantly altered by the presence of sulfur dioxide. Table 5 summarizes the CO2 capture capacity of the various sorbents after 5 cycles in FB tests, with and without SO2, as well as the cumulative calcium conversion degree to sulfate after five cycles, as calculated from the SO2 concentration profiles at the
Table 5. CO2 Capture Capacity and Ca Conversion to Sulfate after 5 Cycles in FB Tests 5 cycles CO2 capture capacity, g CO2/g sorbent
Ca conversion to CaSO4, - (molar)
sorbent
no SO2
low SO2
high SO2
low SO2
high SO2
Massicci Schwabian Alb EnBW Xirorema Sand Tarnow Opolski Czatkowice Dolomite Redziny
0.023 0.027 0.067 0.024 0.026 0.041 0.123
0.036 0.039 0.036 0.029 0.024 0.037 0.060
0.004 0.005 0.019 0.012 0.005 0.010 0.037
0.173 0.176 0.140 0.136 0.165 0.133 0.130
0.258 0.253 0.192 0.209 0.246 0.211 0.210
3.2. FB Tests. Figure 2a−c reports the sorbent capture capacity as a function of the number of cycles for the six limestones and the dolomite tested in the FB apparatus under the operating conditions reported in Table 3. Also in these tests the capture capacity declined with the number of cycles for all the sorbents, typically reaching an asymptotic value already after the fourth cycle. The residual capture capacity of the six limestones (0.02−0.07 gCO2/gsorbent in the tests without SO2 − Figure 2a) was generally lower than that found in the TG tests, both after 5 and 24 cycles (Figure 1a). The obvious explanation for this result lies in the experimental evidence that the combination of high temperature (940 °C) and high concentration of CO2 (70%) during calcination significantly enhances sintering.19 A similar result was also observed in a recent pilot-scale FB study.7 The CO2 capture capacity of the dolomite was larger than that of the limestones and remained relatively large along the cycles in spite of the lower Ca content of the sorbent and of the D
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Table 6. Porosimetric Data of Raw, TG-Tested and FBTested Samples after Five Calcination-Carbonation Cyclesa sorbent/reactor/conditions M/fresh M/TGA/sulfated-carbonat ed M/FB/sulfated-carbonated M/TGA/carbonated M/FB/mildly sulfated T/fresh T/TGA/sulfated-carbonat ed T/FB/sulfated-carbonated T/TGA/carbonated T/FB/mildly sulfated Cz/fresh Cz/TGA/sulfated-carbonat ed Cz/FB/sulfated-carbonated Cz/TGA/carbonated Cz/FB/mildly sulfated Sw/fresh Sw/TGA/sulfated-carbonat ed Sw/FB/sulfated-carbonat ed Sw/TGA/carbonated Sw/FB/mildly sulfated En/fresh En/TGA/sulfated-carbonat ed En/FB/sulfated-carbonated En/TGA/carbonated En/FB/mildly sulfated Xr/fresh Xr/TGA/sulfated-carbonat ed Xr/FB/sulfated-carbonated Xr/TGA/carbonated Xr/FB/mildly sulfated D/fresh D/TGA/sulfated-carbonat ed D/FB/sulfated-carbonated D/TGA/carbonated D/FB/mildly sulfated
BET area (m2/g)
porosity (%)
microporosity (%)
0.85 6.77
0.5 5.4
0.039 0.611
1.00 6.25 2.01 1.73 6.95
0.6 4.1 1.1 0.9 5.8
0.064 0.693 0.148 0.08 0.63
0.93 7.74 1.83 2.39 8.78
0.5 4.3 1.0 0.3 7.9
0.06 0.77 0.12 0.19 0.83
1.64 7.68 1.78 2.57 6.30
0.9 4.7 1.4 1.4 4.0
0.11 0.83 0.20 0.20 0.05
2.11
1.0
0.08
7.01 3.96 0.50 10.8
2.5 1.9 0.7 4.3
0.40 0.29 0.10 0.77
1.43 7.01 2.58 0.86 9.83
0.8 4.7 1.3 0.6 9.2
0.09 0.70 0.33 0.1 0.92
1.96 8.38 2.79 7.03 16.4
1.0 6.7 1.5 2.10 12.9
0.08 0.40 0.24 0.30 1.30
4.10 17.0 3.98
2.50 12.6 2.60
0.37 1.40 0.34
Figure 3. BET area (a), total porosity (b), and microporosity (c) of fresh, mildly sulfated, and strongly sulfated FB sorbents (M: Massicci; T: Tarnow-Opolski; Cz: Czatkowice; Sw: Schwabian Alb; En: EnBW; Xr: Xirorema Sand; D: Dolomite Redziny).
level on the sorbent properties. Table 6 summarizes the results of the porosimetric characterization of all the sorbent samples from TG and FB tests, with or without SO2. All the samples have been subjected to five calcination-carbonation cycles. The porosimetric characterization of the raw sorbents is also reported for comparison. The spent sorbents from 5-cycles TG experiments showed clear evidence of sintering, which reflected on their measured porosity and surface area. The dolomite samples presented both larger porosity and larger surface area than the limestone samples, highlighting the positive effect of Mg in reducing sintering. The samples obtained from FB tests showed a much severer effect of sintering than TG samples for all the sorbents: the properties of the samples measured after five FB cycles were similar to those of the raw sorbents. For the sulfated samples SEM/EDX results showed the formation of sulfated zones on the particle surface, suggesting that part of the CaO remained unreacted in the core. A network sulfation pattern was also observed in the particles, typically close to fractures and small craters. It was found that the macroporosity (%) of the sulfated particles increased, with respect to the nonsulfated particles, indicating less sintering in the presence of SO2, as also suggested by the BET results. The strong sintering conditions in the FB tests produced sulfated FB material which presented much lower reactivity and
a Sulfated = high-SO2; mildly sulfated = low-SO2. M: Massicci; T: Tarnow-Opolski; Cz: Czatkowice; Sw: Schwabian Alb; En: EnBW; Xr: Xirorema Sand; D: Dolomite Redziny).
exhaust. Concerning conversion to CaSO4, a comparison of data in Tables 4 and 5 indicates that after 5 cycles sorbent sulfation in TG tests was substantially lower than in FB experiments. This was likely a consequence of the shorter cycle time and of the lower calcination temperature in TG tests and possibly of additional diffusional resistances through the sorbent layer in the TG device. 3.3. Characterization of Sorbents. A thorough XRD, SEM/EDX, and porosimetric characterization of some preliminary samples of limestone and dolomite spent material obtained after TG and FB calcium looping tests was recently published.20 Hereafter we report the main findings relevant to this work together with results of additional new characterizations carried out to study the effect of the SO2 concentration E
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Figure 4. SEM picture of (a) mildly sulfated and (b) strongly sulfated -in FB tests- Xirorema Sand sorbent.
Figure 5. SEM picture of (a) mildly sulfated and (b) strongly sulfated -in FB tests- Dolomite Redziny sorbent.
4. CONCLUSIONS Analysis of the TG and FB experiments results together with the spent sorbents characterization allow us to find some general conclusions: • TG and FB devices are characterized by much different fluid-dynamic and mass transfer conditions, which result in different sample heating and reaction rates. However, as far as the CO2 capture capacity is concerned, our results suggest that the type of reactor has a lower influence than the calcination environment on the sorbent performance. • Factors that enhance sintering (e.g., high temperature and CO2 concentration) severely impact on the sorbent CO2 capture capacity. On the other hand, factors that slow down sintering (e.g., the presence of MgO, like in the dolomite) improve the sorbent performance. • The presence of SO2 also affects the sorbent CO2 capture capacity, as already documented in the literature,6,7 but this influence appears to be strongly dependent on the SO2 concentration level. High SO2 concentrations significantly depress the CO2 capture capacity due to the formation of an impervious sulfate layer that hinders CO2 penetration into the sorbent particle. By contrast, low SO2 concentrations have a rather limited influence on the sorbent performance. • The ranking of the sorbents appears to be similar from results of TG and FB experiments. However, this result might change if extensive sorbent attrition occurs in FB tests. In fact, attrition exposes a new reactive surface area of the sorbent and accordingly enhances CO2 capture. So, it is here suggested that TG tests can be reasonably used to rank sorbents for their CO2
porosity than the TG one. In particular, microporosity of the sulfated FB samples was negligible. Figure 3a−c compares the specific surface area (BET, m2/g), macroporosity (%), and microporosity (%) of fresh, low-SO2 sulfated, and high-SO2 sulfated sorbents, after five carbonationcalcination cycles in FB tests. Results reveal that the specific surface area of sorbents constantly decays with the increase of sulfur concentration in gas phase. As detailed before, the phenomenon is basically ascribed to the formation of CaSO4 anhydrite at the surface of the CaO particle, leaving part of its core unable to react. A useful remark is that mild sulfation conditions do not affect reactivity of sorbents that severely, consistent with CO2 capture results. The trend showed concerning BET area is also depicted in the case of macroand microporosity, as such properties are strongly interconnected and together they demonstrate the overall carbon capture capacity of the tested sorbents. It is noted that this decay is limited in the case of dolomite. Figure 4a shows a SEM picture of the mildly sulfated Xirorema Sand sorbent. The brighter formations identified upon its surface are attributed to anhydrite (as we can conclude by combining the EDX and XRD data). It is noted that the respective microphotograph of the strongly sulfated Xirorema Sand sorbent (Figure 4b) reveals much larger amounts of anhydrite, supporting the porosimetry findings on the connection between gas phase sulfur concentration-sorbent efficiency. The same observations also apply to all the other limestone sorbents tested. Figure 5a,b shows selected SEM pictures of the mildly sulfated and strongly sulfated dolomite sorbent, respectively, revealing particle-mode sulfation. F
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for CO2 capture. Proc. of 2nd Int. Conf. on Chemical Looping, Darmstadt, Germany, 2012, paper 93. (18) Coppola, A.; Montagnaro, F.; Salatino, P.; Scala, F. Fluidized bed calcium looping cycles for CO2 capture: a comparison between dolomite and limestone. Proc. of 14th Int. Conf. on Fluidization, Noordwijkerhout, The Netherlands, 2013, paper 233. (19) Borgwardt, R. H. Calcium oxide sintering in atmospheres containing water and carbon dioxide. Ind. Eng. Chem. Res. 1989, 28, 493−500. (20) Itskos, G.; Grammelis, P.; Scala, F.; Kruczek, H.; Coppola, A.; Salatino, P.; Kakaras, E. A comparative characterization study of Calooping natural sorbents. Appl. Energy 2013, 108, 373−382.
capture capacity, but only if these sorbents do not fragment significantly during FB operation.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +39 081 7682969. Fax: +39 081 5936936. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was funded by EU RFCS project CAL-MOD, Contract Number: RFCR-CT-2010-00013.
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REFERENCES
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dx.doi.org/10.1021/ef401876q | Energy Fuels XXXX, XXX, XXX−XXX
