"Reprinted (adapted) with permission from (Environ. Sci. Technol., 2012, 46 (2), pp 1262–1269
DOI: 10.1021/es202679w).
Copyright (2011) American Chemical Society."
Final Author’s paper after peer review
The influence of high-temperature steam on the
reactivity of CaO sorbent for CO2 capture
Felix Donat1, Nicholas H. Florin1*, Edward J. Anthony1,2, Paul S. Fennell1
Department of Chemical Engineering, Imperial College London, South Kensington Campus, London
SW7 2AZ, UK; 2. CanmetENERGY, Natural Resources Canada, 1 Haanel Drive, Ottawa, Canada K1A
1M1
RECEIVED DATE
Influence of steam on the reactivity of CaO sorbent
*
CORRESPONDING AUTHOR FOOTNOTE: n.florin@imperial.ac.uk
ABSTRACT Calcium looping is a high-temperature CO2 capture technology applicable to the post-
combustion capture of CO2 from power station flue gas, or integrated with fuel conversion in precombustion CO2 capture schemes. The capture technology uses solid CaO sorbent derived from natural
limestone and takes advantage of the reversible reaction between CaO and CO2 to form CaCO3; that is,
to achieve the separation of CO2 from flue or fuel gas, and produce a pure stream of CO2 suitable for
geological storage. An important characteristic of the sorbent, affecting the cost-efficiency of this
technology, is the decay in reactivity of the sorbent over multiple CO2 capture-and-release cycles. This
work reports on the influence of high-temperature steam, which will be present in flue (about 5-10 %)
1
and fuel (~ 20 %) gases, on the reactivity of CaO sorbent derived from four natural limestones. A
significant increase in the reactivity of these sorbents was found for 30 cycles in the presence of steam
(from 1–20 %). Steam influences the sorbent reactivity in two ways. Steam present during calcination
promotes sintering that produces a sorbent morphology with most of the pore volume associated with
larger pores of ~ 50 nm in diameter, and which appears to be relatively more stable than the pore
structure that evolves when no steam is present. The presence of steam during carbonation reduces the
diffusion resistance during carbonation. We observed a synergistic effect, i.e., the highest reactivity was
observed when steam was present for both calcination and carbonation.
KEYWORDS CaO, calcium looping, CCS, high-temperature steam, solid sorbent
INTRODUCTION
Carbon capture and storage (CCS) is an important part of the strategy for mitigating the risks of
climate change, recognizing the continuing global reliance on fossil fuels. CCS technology is in the
early demonstration stages, likely to last at least a decade, with governments currently developing and
implementing strategies to accelerate commercial deployment (including significant activity in the US,
EU and Australia).1 This work is focused on Calcium Looping CO2 capture technology.
Calcium Looping is based on the reversible reaction between CaO and CO2, according to Equation
(1):
(1)
Shimizu et al.2 described a system consisting of two interconnected fluidized bed reactors which used
the reversible reaction to achieve CO2 separation from a power plant flue gas and produce a pure stream
of CO2 suitable for storage (Figure 1).
2
Figure 1. Calcium looping technology applied to capturing CO2 from a combustion flue gas (after
Shimizu et al.)2
Flue gas enters the first fluidized bed reactor (carbonator), where CO2 is captured by CaO and
converted to CaCO3. CaCO3 is then transported to the second reactor (calciner/oxy-fuel combustor)
where it is decomposed to regenerate CaO and produce a concentrated stream of CO2. The regeneration
step requires the input of energy to increase the temperature (from about 650 °C to about 900 °C) and
drive the endothermic calcination reaction. This is achieved by burning additional fuel in pure oxygen,
to avoid dilution of the CO2 stream with N2 from the air. The advantages of the proposed scheme
compared to technologies closer to market include: (1) use of cheap and nontoxic sorbent derived from
limestone; (2) use of mature fluidized bed technology; (3) lower energy penalty and operating costs; and
(4) potential synergy by using the spent CaO in cement manufacture. There are drawbacks, most
significantly the decay in the reactivity of the sorbent through multiple CO2 capture-and-release cycles.
Factors known to influence the sorbent reactivity include sintering, attrition and reaction with impurities
in the flue or fuel gas, primarily sulphur species (SO2, H2S).
3
Steam will be present in combustion flue gas (5–10 %), in the oxy-fuel/calciner and fuel gases (~ 20
%) and so the influence of steam during calcination and carbonation is relevant for determining the
reactivity through multiple cycles. However, to date, most previous work in the field has examined the
reactivity of CaO sorbents using dry gas mixtures. That said, there is a significant body of relevant
literature from previous studies carried out in different contexts and there is general agreement that the
presence of steam, even at low concentrations: (i) increases the rate of sintering of CaO;3–5 (ii) increases
the rate of calcination;6,7 and (iii) influences the conversion of CaO to Ca(OH)2, CaS, CaSO4 and
CaCO3.6,8–19
There is no consensus in the literature in terms of the effect of steam on the carbonation reaction,
summarized in Table 1. Clearly, further work is required to better elucidate the effect of hightemperature steam on the reactivity of CaO for CO2 capture. To this end, we investigated the influence
of steam on the reactivity of four limestones in a fluidized bed reactor. We present results for
experiments conducted with steam concentrations from 0–20 % for calcination and/or carbonation up to
30 cycles.
Table 1. Selected studies from the literature investigating the influence of steam on the carbonation of
CaO derived from limestone or dolomite
Author (Date)
Manovic and
Anthony 11
Dobner et al. 12
Experimental details
Carbonation of calcines from
seven limestones (250–425 µm)
using a TGA; carbonation
temperature varied from 350–800
°C with steam concentrations of
10 and 20 % and 20 % CO2;
Calcination under N2 (800 °C) or
CO2 (950 °C)
Carbonation of dolomite particles
(~ 60 µm) in a TGA in the
presence of steam; during
calcination without steam
Comments
Reported improved reactivity observed
(most significant at lower temperatures and
with most sintered samples); authors
observed that steam accelerated the solidstate diffusion-controlled reaction phase
Reported a ‘catalytic’ effect of steam on the
carbonation of ‘dolomite’ observing an
increase in conversion. They demonstrated
that the influence was not permanent as it
was not sustained when the steam was
switched off, i.e., there was no ‘memory
effect’
4
Bandi et al. 13
Carbonation and calcination of
‘dolomite’ sorbent (< 20 µm) in a
TGA with 70 % N2, 10 % CO2
and 20 % steam; sorbent
continuously cycled between 480–
830 °C.
Lu et al. 14
Carbonation of synthetic sorbent
(< 10 µm) derived from calcium
acetate in a TGA at 700 °C with
10 % steam, 30 % CO2 and
balance He; calcination in He
Carbonation of calcined limestone
(212–250 µm) in a TGA at 850 °C
under pure CO2; calcination in 95
% H2O, 5 % N2 at 850°C and 900
°C
Carbonation of limestone particles
(250–500 μm) in a FBR in 100 %
CO2; calcination in the presence
of steam from 20–100 %
Sun et al. 15
Wang et al. 16
Dou et al. 17
Yang and Xiao 18
Lu et al. 19
Carbonation of commercial CaO
(450-1000 µm) in a fixed bed
reactor at 550 °C with steam at 5
% or 10 %; calcination in N2 at
900 °C
Carbonation of commercial CaO
(150–250 μm) in pressurized
TGA at steam partial pressures of
about 0.5 MPa (PTotal 3.0 MPa),
partial pressure of CO2 at 0.5 MPa
Carbonation of Havelock
limestone (250-425 µm) in FBR
at 620 °C with steam (17 %) in a
simulated syngas
Reported improved reactivity of dolomite
through multiple cycles; they asserted that
the formation of hydrogen carbonates
allowed the binding of two CO2 molecules
to an adsorption centre, as a possible
explanation for the much higher conversion
in an atmosphere containing steam
Reported a negative influence of steam on
the carrying capacity
Reported negligible effect on carrying
capacity. No effect on carrying capacity
reported with steam present during
carbonation only (no results presented)
Reported an increase in the calcination
conversion and improved reactivity of the
calcine corresponding with increasing steam
concentration during the previous
calcination
Reported improved carrying capacities and
argued that improvements were due to the
conversion of CaO to Ca(OH)2 which
reacted directly with CO2
Reported significantly improved
carbonation performance, with conversions
of about 50 % after 30 min with steam
present, compared to 10 % without steam
Reported an increase in the rate of reaction
and improved long-term reactivity.
Performance accounted for by increase in
macro-porosity observed by SEM
EXPERIMENTAL SECTION
In this work an atmospheric pressure bench-scale bubbling fluidized bed (BFB) reactor (quartz tube)
was used (Figure S1) to simulate continuous operation of a Calcium Looping CO2 capture process. This
was achieved by periodically changing the reaction temperature between that required for the
carbonation and calcination of a bed of limestone mixed with sand. (Sand was used to ensure
temperature uniformity owing to the exothermic and endothermic reactions, respectively.) An external
5
resistance-heated tube furnace was used – controlled using a K type thermocouple positioned within the
reactor bed. The fluidizing gas, which consisted of a mixture of N2, CO2 and steam, was metered using
rotameters. The gas was introduced at the base of the quartz liner, via a heated line to avoid
condensation. The lower section of the quartz liner, below a sintered glass disc distributor (with average
porosity ~ 70 µm), was stuffed with quartz wool to aid in pre-heating the gas and to reduce temperature
gradients within the bed. Water vapour was introduced using a heated bubbler (saturator) system
whereby the dry gas (N2 and CO2) was passed through a gas washing bottle filled with de-ionised water.
The amount of water vapour was controlled by varying the temperature of the bubbler (using heating
tape) and using a rotary valve and three solenoid valves. This arrangement enabled the diversion of
some, or all, of the dry gas past the bubbler, such that steam injection could be switched on and off
during an experiment without significant up-stream changes in pressure. This system achieved partial
vapour pressures of about 60–70 %, relative to the theoretical saturated vapour pressure. For calibration
of the humidity measurements, a syringe pump was used which injected known quantities of de-ionised
water directly through a heated line into the quartz liner.
A fraction of the exhaust gas was continuously sampled using a stainless steel tube inserted into the
top of the quartz liner; the sampled gas passed through a heated section of tubing (about 110 °C)
housing a relative humidity probe (Vaisala HMP75), then through an ice bath to condense out the water
and a static particle trap (quartz wool). A non-dispersive infrared CO2 gas analyser (ADC SB100) was
used to measure the CO2 concentration every second and the relative humidity probe was used to
measure the steam concentration.
The sorbent reactivity was determined by regularly measuring and recording the concentration of CO2
(and/or H2O in the exhaust gas) and is presented in terms of the carrying capacity per cycle N (C N),
defined as the number of moles of CO2 captured (nCO2) per mole of CaO (nCaO) loaded in the reactor,
according to Equation 2 (Here,
is the molecular weight of CaCO3,
is the
6
mass of limestone initially loaded to the reactor and
is the corresponding content of Ca, as shown in
Table S1). An increase in the CO2 concentration corresponded with calcination and a decrease
corresponded with carbonation.
(2)
Typical experiments were carried out at atmospheric pressure with an inlet gas stream containing
15 % (v/v) CO2. The steam concentration was varied from 0–20 % and the N2 flow rate was adjusted to
maintain a constant total flow rate. The total flow rate for the inlet gas stream was 170 mL/s (800 °C)
for all experiments, such that the U/Umf for the limestone was about 8, calculated for calcination
conditions. Calcination was conducted at 900 °C and carbonation at 650 °C. The duration of the
carbonation and calcination cycles was 600 s, including time for heating and cooling at an average rate
of about 0.9 °C/s.
CaO-sorbents derived from four limestones were tested: Havelock and Cadomin from quarries located
in Canada, and Purbeck and Longcliffe from the UK. The elemental compositions, determined by X-ray
fluorescence (XRF) analysis (Bruker AXS S4 Explorer) are given in Table S1. For the majority of
experiments, the fresh limestone was sieved to a size fraction of 500–710 µm, however a number of
additional experiments were carried out using limestone particles sieved to two smaller size fractions
(250–355 μm and 150–250 μm). About 4 g of limestone was used per experiment; mixed with about
12.5 g of sand (355–425 µm). A known mass of limestone and sand was loaded into the reactor at 900
°C and the final bed mass was measured to determine the loss of sorbent material owing to attrition,
assuming only limestone fines were elutriated. Based on this method the results reported for cycles 2–
10, unless otherwise stated, are likely slightly lower than the actual carrying capacity. (Figure S2 shows
carrying capacities, normalized for mass loss, for the sorbents tested with and without steam.) Particle
size distributions were measured using an optical microscope (Seben Stereo Microscope Incognita III)
and an electronic eyepiece (Hangzhou Opto Electronics MD 130) for Longcliffe sorbent after the initial
7
calcination, and after 10 and 30 cycles – with and without steam. Surface area and pore volume
measurements were taken for a selection of calcined sorbents by N2 adsorption/desorption
(Micromeritics TriStar 3000), in all cases, the sand and the calcined limestone were separated by sieving
out particles smaller than 425 μm (assumed to be sand) from those of a greater diameter (assumed to be
limestone). This means that the measurements shown reflect the results for the primary un-fragmented
limestone particles.
RESULTS AND DISCUSSION
To demonstrate the experimental reproducibility, three experimental runs were conducted with steam
concentrations at 0, 0.1 and 10 % – the results are presented for sorbent derived from Longcliffe
limestone in Figure 2. Here the error bars represent twice the standard deviation. The good repeatability
observed for Longcliffe was also observed for the other sorbents tested, with and without steam present,
and also for a steam concentration (~ 0.1 %) which was below the concentration level where the
maximum improvement in reactivity was observed.
The results in Figure 2 show the significant improvement in the carrying capacity for 10
carbonation/calcination cycles in the presence of steam. This improvement was seen for the four
sorbents tested, shown in Figure 3, and is consistent with previous work, e.g.,11–13,16–19. Also consistent
with previous work was an increase in the rate of calcination, e.g. 7, (Figure S3). Figure 3a shows the
carrying capacity of the four sorbents tested without steam present, showing the usual decay in
reactivity with a final carrying capacity after 10 cycles from 11–19 % for Havelock and Purbeck,
respectively (Here the data has not been corrected for mass loss owing to the elutriation of fines.). The
addition of 10 % steam during calcination and carbonation, presented in Figure 3b, shows an average
improvement of more than double the carrying capacity after 10 cycles, the highest capacity of 30 %
(molar basis) was observed with sorbent derived from Purbeck limestone. Interestingly, greater
consistency in performance of the four limestone-derived sorbents is observed when steam is present at
10 % for calcination and carbonation.
8
Variations in the performance of the different sorbents can, to some extent, be explained in terms of
their mechanical stability; this characteristic was quantified in terms of the amount of bed material
retained in the reactor throughout a cycling experiment (as fine limestone particles, < 100 μm at 900 °C,
generated by abrasion and/or fragmentation were elutriated). Table 2 compares the mass loss data for
the sorbents after 10 cycles with (0.1–20 %) and without steam during calcination and carbonation. No
significant differences in mass loss were observed with varying steam concentration for Havelock,
Longcliffe and Cadomin, however a decrease in mass loss when steam was present was observed for
sorbent derived from Purbeck. The unique behavior observed for Purbeck may be explained in terms of
the range of different impurities in the natural limestones (Table S1). Additionally, the attrition
behaviour of Longcliffe throughout 10 cycles with steam present was investigated – and, consistent with
previous work without steam (e.g.20,21) – most of the mass loss occurred after the initial calcination with
negligible loss of material for the following cycles. Particle size distributions for the primary particles
observed for Longcliffe sorbent did not reveal any significant differences with and without steam
present (Figure S4). However, during 30 reaction cycles a shift towards smaller particles was observed,
which given the small mass loss measured for Longcliffe sorbent (Table 2), can be largely attributed to
the densification of the particles, consistent with previous work.22
9
100
Longcliffe, no steam
Longcliffe, 0.1 % steam
Longcliffe, 10 % steam
90
70
80
70
50
CN [%]
60
50
40
40
30
30
g-CO2 / g-sorbent [%]
60
20
20
10
10
0
0
2
4
6
8
0
10
Cycle no. [-]
Figure 2. Carrying capacity of sorbent derived from Longcliffe limestone through 10 reaction cycles
with steam present at 0, 0.1 and 10 % for calcination and carbonation; error bars represent 2 standard
deviations calculated for three experiments
10
100
Havelock, no steam
Longcliffe, no steam
Cadomin, no steam
Purbeck, no steam
90
80
70
70
50
CN [%]
60
50
40
40
30
30
g-CO2 / g-sorbent [%]
60
20
20
10
10
0
0
2
4
6
8
0
10
Cycle no. [-]
(a)
11
100
Havelock, 10% steam
Longcliffe, 10% steam
Cadomin, 10% steam
Purbeck, 10% steam
90
80
70
70
50
CN [%]
60
50
40
40
30
30
g-CO2 / g-sorbent [%]
60
20
20
10
10
0
0
2
4
6
8
0
10
Cycle no. [-]
(b)
Figure 3. Carrying capacities of the four sorbents through 10 reaction cycles (a) without steam, (b) with
10 % steam present during calcination and carbonation; for clarity, error bars representing 2 standard
deviations are included only for cycle 5
Table 2. Mechanical stability of the four sorbents represented in terms of the mass lost during 10
reaction cycles with and without steam present; average data given for 0, 0.1 and 10 % steam
Final mass loss after 10 cycles (% of initial mass of CaO )
Cadomin
Havelock
Longcliffe
Purbeck
no steam
9.6
19.9
4.7
12.6
steam, 0.1 %
8.9
20.7
4.0
13.6
steam, 2.5 %
7.2
24.0
4.2
10.2
steam, 6 %
10.8
19.8
4.7
9.1
12
steam, 10 %
7.3
16.2
4.3
7.9
steam, 20 %
8.3
17.6
5.8
7.0
The steam concentration was varied from 0–20 % (present during calcination and carbonation) and
results are shown in Figure 4. In the case of Loncliffe, an increase in the carrying capacity was observed
through 10 cycles with only 0.1 % steam present; a greater increase was observed with 1.0 %, while no
significant further improvement was achieved with the higher concentrations tested up to 20 %. It
appears that at low concentrations the effect of steam on the carrying capacity is dependent on the
amount of steam available; however above some concentration/saturation point – which varied slightly
for the different sorbents (e.g., for Longcliffe ~ 1.0 %)—there was no further improvement. Manovic
and Anthony11 reported a small increase in conversion of one limestone during carbonation at 500 ˚C
with increasing steam concentration from 10 to 20 %, with steam present for carbonation only.
13
40
35
C10 [%]
30
25
20
Havelock
Longcliffe
Cadomin
Purbeck
15
10
0
2
4
6
8
10
12
14
16
18
20
Steam concentration [%]
Figure 4. Final carrying capacities for sorbents (i.e., after 10 cycles) with different steam concentrations,
from 0–20 %
To assess the relevance of these results to larger facilities that use smaller particles in circulating
fluidized beds, we carried out additional experiments with two smaller particle size fractions, i.e. sieved
to 250–355 μm and 150–250 μm. Figure S5 compares the carrying capacities of sorbent derived from
Longcliffe, with and without steam (10 %), for three size fractions, indicating that the improvement is
independent of the particle size fraction, with minor variability owing to different rates of elutriation.
(Figure S5 (b) shows carrying capacities, normalized for mass loss)
Experiments were conducted with steam present during only calcination or carbonation by switching
steam on and off during the experiments. Figure 5 compares the carrying capacity of sorbent derived
from Longcliffe, with and without steam (10 %), and with steam present during carbonation and
calcination only. A higher carrying capacity was observed when steam was present for carbonation only,
14
compared to when steam was present for calcination only. There appears to be a synergistic effect when
steam is present during both carbonation and calcination; thus, the presence of steam during calcination
is also very important in terms of the subsequent reactivity of the calcine.
These results are better elucidated with the analysis of the sorbent morphology determined by N2
adsorption/desorption. Figure 6 compares the evolution of the pore volume of CaO sorbent derived from
Longcliffe limestone after the initial calcination, and 5, 10 and 30 carbonation-calcination cycles – with
and without steam. (In this case, when steam was present, this was for both carbonation and calcination)
The presence of steam appears to effect the pore structure of the sorbent in two ways: The shift in the
maxima in the plot of dV/dlog(D) vs. pore diameter shows that most of the pore volume is associated
with slightly larger pores, with a diameter of about 50 nm when steam was present compared to about
30 nm without steam. Thus we observe a reduction in the BET surface area when steam is present after
1 calcination (Table 3). Secondly, although we observed a reduction in the pore volume and BET
surface area through 30 cycles, the pore volume associated with the pores with a diameter of about 50
nm appears relatively stable and the BET surface area is greater compared to when no steam was
present from 5–30 cycles. Interestingly, this pore diameter of 50 nm is the same as what was identified
in the context of thermal pre-treatment23; and, it is also consistent with critical carbonate product layer
thickness determined by Alvarez and Abanades24 to be associated with the transition of the carbonation
reaction to the slow diffusion-limited reaction phase. Figure 7 compares the evolution of pore volume of
CaO sorbent derived from Longcliffe samples after 10 cycles when steam was present during
calcination and carbonation only. These results show an increase in the pore volume when steam was
present for carbonation only, which is owing to the higher conversion in the previous cycle, and thus
greater pore volume is evolved during the subsequent calcination. The pore size distribution remains in
the same range compared to when no steam was present. For the case when steam was present for
calcination only, we observed a shift towards larger pores (~50 nm diameter) likely owing to the
enhanced sintering during calcination when steam is present. When steam is present during both
15
carbonation and calcination there is a shift towards the larger pores and an increase in the pore volume
relative to when steam is present for calcination only.
Figure 8 represents a potential explanation for the results shown. When no steam is present (Figure
8a) the finer pores are retained that are known to be susceptible to pore blockage/plugging. When steam
is present during carbonation only, the fine pores are retained and the higher conversion is owing to a
reduction in the diffusion resistance through the carbonate later, consistent with previous work11. When
steam is present for calcination only there is a shift towards larger pores due to steam enhanced
sintering. These larger pores are less susceptible to pore blockage and thus allow for the higher
conversion. Finally, the synergistic effect may be explained owing to the combined influence of a shift
to larger pores that are less susceptible to pore blockage and the lower diffusion resistance when steam
is present during carbonation, allowing better exploitation of larger pores.
100
Longcliffe,
Longcliffe,
Longcliffe,
Longcliffe,
90
80
no steam
10 % steam
10 % steam during carbonation only
10 % steam during calcination only
70
70
50
CN [%]
60
50
40
40
30
30
g-CO2 / g-sorbent [%]
60
20
20
10
10
0
2
4
6
8
10
Cycle no. [-]
16
Figure 5. Carrying capacity for experiments conducted with sorbent derived from Longcliffe, with and
without steam (10 %), and with steam present for calcination and carbonation only; for clarity, an error
bar representing 2 standard deviations is included only for cycle 5
Figure 6. Evolution of pore volume of CaO sorbent derived from Longcliffe limestone, through 30
reaction cycles, with (●) and without (○) steam; final carrying capacity (CN) and steam concentration
given; repeat measurements shown for 1st calcination and cycle 10 with and without steam
17
10 cycles
0.1
dV/d(log D) [cm3 g-1]
0.08
0.06
Longcliffe,
Longcliffe,
Longcliffe,
Longcliffe,
no steam
10 % steam
10 % steam during carb.only
10 % steam during calc.only
C10 =13.8% (0%)
C10 =29.1% (10%)
C10 =25.2% (10% carb.only)
C10 =21.7% (10% calc.only)
0.04
0.02
0
10
Average pore diameter D [nm]
100
Figure 7. Evolution of pore volume of CaO sorbent derived from Longcliffe limestone, after 10 reaction
cycles, with and without steam, and with steam during carbonation or calcination only and steam during
both carbonation and calcination; final carrying capacity (CN) and steam concentration given
18
Figure 8. Schematic representation of the relationship between sorbent morphology and conversion: (a)
no steam present, (b) steam present during carbonation only, (c) steam during calcination only, and (d)
steam present for carbonation and calcination
Table 3. BET surface area of CaO sorbent derived from Longcliffe limestone, through 30 reaction
cycles, with and without steam
19
BET surface area (m2/g)
no steam
steam (10 %)
1 calcination
16.6
14.7
5 cycles
6.1
6.6
10 cycles
3.7
5.2
30 cycles
1.8
3.4*
* steam concentration 5.5 %
The measured carbonation reaction rates for sorbent derived from Longcliffe with steam present (10 %)
for calcination and/or carbonation support our explanation of the relationship between sorbent
morphology and conversion. In Figure 9 the reaction rates are shown for the 10th carbonation calculated
on a molar basis, i.e. dX/dt (with carbonation conversion X, considering the number moles of CaO
according to the amount of sorbent initially loaded). The reactor bed temperature is also shown on the
secondary vertical axis, indicating that all of the conversion during the ‘rapid-reaction-phase’ occurs
within about 200 seconds while the temperature ramps down to 650 °C. When steam is present during
the carbonation reaction (i.e. steam during carbonation only and steam during carbonation and
calcination) the reaction proceeds more rapidly in the initial stages of the reaction and the maximum
reaction rate (represented by the height of the peaks) is higher compared to experiments without steam
during the carbonation. The higher maximum reaction rates, which may be interpreted as higher capture
efficiencies, clearly demonstrate the reduction in the diffusion resistance during carbonation.
Furthermore, the faster initial reaction rate confirms the notion of a ‘catalytic effect’ and the higher
maximum reaction rates is consistent with Manovic and Anthony 11 who reported an acceleration of the
solid-state diffusion mechanism when steam was present during carbonation. Uniquely, when steam was
present for calcination only, there is a dramatic increase in the rate of carbonation when the temperature
was ramped back up towards the calcination temperature. Although the open pore structure is less
susceptible to pore blockage, due to the lower surface area, the formation of a significant coverage of
20
product on the surface (i.e., a critical product layer thickness) greatly diminishes the rate of reaction. In
this case an increase in temperature, rather than the presence of steam, leads to a significant reduction in
the diffusion resistance. These findings are consistent with previous work in the context of sulphation.25
Figure 9. Carbonation reaction rate vs. time for sorbent derived from Longcliffe with steam present (10
%) for calcination and/or carbonation in the 10th carbonation; bed temperature shown on the secondary
vertical axis
ACKNOWLEDGMENT The research leading to these results has received funding from the European
Community's Seventh Framework Programme (FP7/2007-2013) under GA 241302-CaOling Project.
We thank the German Academic Exchange Service for the financial support of FD.
SUPPORTING INFORMATION Five additional plots are included as SI including: a schematic of the
BFB reactor (Figure S1); carrying capacities with and without steam, normalized for mass loss (S2);
calcination reaction rates for Longcliffe sorbent for the 2nd and 6th calcinations, with and without steam
present during carbonation and/or calcination (S3); particle size distributions for Longcliffe sorbent
given after 1, 10 and 30 cycles, with and without steam (S4); and carrying capacities for different
particle sizes of Longcliffe sorbent with and without steam, with data normalized for mass loss (S5).
21
Element composition of the four limestones is also given (Table S1).
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