Article pubs.acs.org/EF
Durability of CaO−CaZrO3 Sorbents for High-Temperature CO2 Capture Prepared by a Wet Chemical Method Ming Zhao,*,† Matthew Bilton,† Andy P. Brown,† Adrian M. Cunliffe,‡ Emiliana Dvininov,§ Valerie Dupont,‡ Tim P. Comyn,† and Steven J. Milne† †
Institute for Materials Research, School of Process, Environmental and Materials Engineering (SPEME), and ‡Energy Research Institute, School of Process, Environmental and Materials Engineering (SPEME), University of Leeds, Leeds LS2 9JT, United Kingdom § MEL Chemicals, Swinton, Manchester M27 8LS, United Kingdom S Supporting Information *
ABSTRACT: Powders of CaO sorbent modified with CaZrO3 have been synthesized by a wet chemical route. For carbonation and calcination conditions relevant to sorbent-enhanced steam reforming applications, a powder of composition 10 wt % CaZrO3/90 wt % CaO showed an initial rise in CO2 uptake capacity in the first 10 carbonation−decarbonation cycles, increasing from 0.31 g of CO2/g of sorbent in cycle 1 to 0.37 g of CO2/g of sorbent in cycle 10 and stabilizing at this value for the remainder of the 30 cycles tested, with carbonation at 650 °C in 15% CO2 and calcination at 800 °C in air. Under more severe conditions of calcination at 950 °C in 100% CO2, following carbonation at 650 °C in 100% CO2, the best overall performance was for a sorbent with 30 wt % CaZrO3/70 wt % CaO (the highest Zr ratio studied), with an initial uptake of 0.36 g of CO2/g of sorbent, decreasing to 0.31 g of CO2/g of sorbent at the 30th cycle. Electron microscopy revealed that CaZrO3 was present in the form of ≤0.5 μm cuboid and 20−80 nm particles dispersed within a porous matrix of CaO/CaCO3; the nanoparticles are considered to be the principal reason for promoting multicycle durability.
■
INTRODUCTION
particles but also differential thermal expansion effects and differential sintering rates of the two components, resulting in stresses that inhibit densification.21 Table 1 shows examples of the multicycle performance of modified CaO sorbent powders under various looping conditions, which can be broadly categorized as “mild” calcination (≤800 °C, requiring very low CO2 partial pressures for decarbonation) and “severe” calcination (>900 °C in the presence of high partial pressures of CO2) conditions. Calcination conditions have a major effect on durability,8 with increased calcination temperatures and dwell times giving increased densification, lower porosity, and lower CO2 uptake within the time scale of each cycle (≥5 cycles); however, the preceding carbonation conditions also affect the degree of densification during calcination. Carbonate decomposition initially produces a porous structure: the more extensive the carbonation reaction, the greater the degree of porosity generated upon initial decarbonation, which can result in less densification at the completion of the calcination cycle. Near full carbonation (100% conversion ratio of CaO) is achievable within each cycle using relatively high temperatures, prolonged dwell times, and CO2-rich atmospheres. Therefore, carbonation and decarbonation (calcination) conditions should both be considered when comparing durability performance data from different laboratories (Table 1). The multicycle reactivity of CaO is also affected by the steam content of the feed stream; steam treatments or other forms of hydration have been shown to improve performance because of structural
Powder sorbents for capturing CO2 at high temperatures find applications in a number of areas. Calcium oxide is of interest for post-combustion capture (PCC) from fossil-fuel-fired power plants and other single-point industrial emitters.1 Calcium-looping PCC based on the reversible reaction CaO + CO2 ⇆ CaCO3 can be implemented using two parallel fluidized beds operated as a carbonator and a regenerative calciner, typically at ≥650 °C in ∼15% CO2 and ≥950 °C in ∼100% CO2, respectively.2 Another area of application of CaO powder sorbents lies in steam reforming for the production of H2, whereby removal of the CO2 co-product shifts the reaction equilibrium in favor of greater H2 yields and purity.3 Conditions for carbonation are comparable to those of PCC, i.e., ∼600 °C in ∼15% CO2; however, when oxygen looping is employed in sorption-enhanced steam reforming (SESR), overall enthalpy is reduced by the exchange of oxygen through a metal catalyst and the regeneration of the sorbent is carried out in air at temperatures of ≥800 °C.4 Hence, sorbent regeneration conditions for SESR are considerably less severe than those for PCC, but nevertheless, unmodified CaO shows a serious loss of CO2 capacity after repeated calcination cycles, leading to lower H2 yields.5,6 Thus, there is an economic need to develop high-reactivity CaO-based sorbents that maintain performance between 0.3 and 0.78 g of CO2/g of sorbent over multiple CO2 capture cycles for SESR applications.7,8 Improvements to CaO sorbents have been achieved by the addition of second-phase refractory “spacer” particles to inhibit densification of the CaO particle matrix.6,9−20 It should be noted that refractory additives to CaO inhibit densification because of not merely physical separation of the sorbent © 2014 American Chemical Society
Received: October 19, 2013 Revised: January 17, 2014 Published: January 17, 2014 1275
dx.doi.org/10.1021/ef4020845 | Energy Fuels 2014, 28, 1275−1283
Energy & Fuels
Article
Table 1. Summary of Previous Reports on Spacer−CaO Sorbents carbonation authors (reference)
sorbents (mass ratio)
Li et al.9
75% CaO/25% Ca12Al14O33
Martavaltzi and Lemonidou10
85% CaO/15% Ca12Al14O33 75% CaO/25% Ca12Al14O33 nAl/nCa = 3:10 nAl/nCa = 1:9
Koirala et al.11 Broda and Muller12 Yu and Chen13 Wu and Zhu14 Li et al.15 Derevschikov et al.16 Zhao et al.6 Lu et al.17
Koirala et al.18
Radfarnia and Iliuta19 Broda and Muller20
76% CaO/10% Al2O3/14% TiO2 90% CaO/10% TiO2 58% CaO/42% MgO 20% CaO/80% Y2O3 65% CaO/35% SiO2 nZr/nCa = 3:10, by FSPa nZr/nCa = 3:10, by FSPa nZr/nCa = 3:10, by FSPa nZr/nCa = 3:10, by wet impregnation nZr/nCa = 3:10 (41 wt % CaZrO3)b,c nZr/nCa = 5:10 (56 wt % CaZrO3)b,c nZr/nCa = 5:10 (56 wt % CaZrO3)b,c nZr/nCa = 3.03:10 (by a surfactant template ultrasound method) nZr/nCa = 20:80 (36% ZrO2/64% CaO)d nZr/nCa = 10:90 (20% ZrO2/80% CaO)d nZr/nCa = 5:95 (10% ZrO2/90% CaO)d
net CO2 uptake (gofCO2/gofsorbent)
calcination
atmosphere
T (°C)
t (min)
atmosphere
T (°C)
t (min)
number of cycles
initial
final
14% CO2 14% CO2 15% CO2 15% CO2 30% CO2 55% CO2 100% CO2 100% CO2 20% CO2 100% CO2 25% CO2 67% CO2 30% CO2 100% CO2 30% CO2 30% CO2 100% CO2 100% CO2 100% CO2 100% CO2
690 690 690 690 700 750 850 750 600 758 740 600 700 850 750 750 700 700 850 600
30 30 30 30 10 20 10 60 10 30 10 60 30 10 10 10 30 30 10 30
100% N2 20% CO2 15% CO2 15% CO2 30% CO2 100% N2 30% CO2 100% N2 100% N2 100% He 100% Ar 100% N2 100% He 30% CO2 100% He 100% He 100% He 100% He 30% CO2 100% Ar
850 950 850 850 700 750 950 750 750 758 740 700 700 950 750 750 700 700 950 750
10 10 5 5 10 20 10 30 10 30 10 5 30 0 10 10 30 30 10 30
13 13 45 45 100 30 100 15 10 50 120 50 100 23 50 50 100 100 100 15
0.40 0.42 0.45 0.35 0.39 0.52 0.36 0.40 0.20 0.45 0.06 0.48 0.30 0.25 0.31 0.31 0.34 0.23 0.21 0.19
0.45 0.33 0.36 0.3 0.39 0.55 0.25 0.36 0.24 0.43 0.10 0.34 0.30 0.23 0.32 0.15 0.33 0.23 0.21 0.14
20% CO2 20% CO2 20% CO2
650 650 650
20 20 20
100% CO2 100% CO2 100% CO2
900 900 900
10 10 10
10 10 10
∼0.33e >0.49f >0.57f
0.21 0.31 0.36
a FSP denotes “flame spray pyrolysis”. bPrepared by the FSP method. cPhase percentage in mass was determined by quantification of powder XRD patterns. dPrepared by the sol−gel method: the precursor of CaO was Ca(OH)2, and gelling time was 2 h. eThe initial uptake was calculated by an average decay rate of 0.5% for a 10 cycle experiment. fThe initial uptakes were not indicated in either the paper or the Supporting Information but are estimated to be >0.49 and >0.57 net CO2 uptake (g of CO2/g of sorbent) based on the average decay rate of >0.5% for a 10 cycle experiment.
changes associated with CaO ↔ Ca(OH)2 interconversion.22−24 There has been considerable focus on the CaO/Al2O3 sorbent system because the in situ formation of Ca12Al14O33 particles hinders densification and sintering of the CaO phase during calcination.9−13 A number of other “spacer” particle additives are also reported, for example, CaO/CaTiO3,14 CaO/ MgO,15 CaO/Y2O3,16 and CaO/SiO2.6 The performance of these materials is also summarized in Table 1. Here, we report on the development of ZrO2 as a potential stabilizer for CaO sorbents because the two components react at elevated temperatures to form a thermodynamically stable binary mixture of CaO and CaZrO3.17−20 Lu et al. employed a flame spray pyrolysis (FSP) technique to prepare zirconiamodified CaO sorbents.17 They identified an optimum molar ratio nZr/nCa = 3:10 that retained a stable 64% molar conversion ratio (equal to a net CO2 capture capacity of 0.30 g of CO2/g of sorbent) after 23 cycles for calcination at 700 °C in He (carbonation at 700 °C in 30% CO2 for 30 min). For a more severe calcination temperature of 950 °C, a greater change in uptake was observed, with the molar conversion ratio falling to 50% after 23 cycles.17 Wet chemical synthesis of the same powder composition, nZr/nCa = 3:10, using co-precipitation from calcium nitrate and zirconyl nitrate solutions resulted in inferior performance compared to the FSP powder, such that the conversion ratio decreased from ∼60 to 30% after 23 cycles. The difference was attributed to the lower surface area of the wet chemical powder.17 The same group later reported that
nZr/nCa = 5:10 powders prepared by the same method but containing 56 wt % CaZrO3 gave optimum performance.18 For very mild calcination conditions at 700 °C for 10 min in He (after carbonation in 100% CO2), a conversion ratio of ∼60% was retained after >100 cycles, equating to ∼0.23 g of CO2/g of sorbent. Increasing the calcination temperature to 950 °C (in 30% CO2 after carbonation at 850 °C in 100% CO2) showed a drop in performance relative to mild calcination, but nevertheless, a conversion ratio of ∼55% was retained after 100 cycles.18 Radfarnia and Iliuta produced Zr-modified CaO sorbents using a surfactant template ultrasound synthesis method.19 The best powders, nZr/nCa = 3.03:10, exhibited a drop in the molar conversion ratio from ∼32% in cycle 1 to ∼24% in cycle 15. This corresponded to a CO2 uptake of 0.19 g of CO2/g of sorbent in cycle 1 and 0.14 g of CO2/g of sorbent in cycle 15, for mild calcination at 750 °C in Ar, following carbonation at 600 °C in 100% CO2.19 Most recently, Broda and Muller have reported a sol−gel method using zirconium propoxide as the gelling agent.20 Their best ZrO2-stablized CaO sorbents were obtained using Ca(OH)2 as a precursor for CaO and a gelling time of 2 h. Under relatively severe conditions (carbonation in 20% CO2 at 650 °C for 20 min and calcination in 100% CO2 at 900 °C for 10 min), the sorbents with the highest mass fraction of CaO (80 wt % CaO/20 wt % ZrO2 and 90 wt % CaO/10 wt % ZrO2) showed CO2 uptakes at cycle 10 of 0.31 and 0.36 g of CO2/g of sorbent, respectively. In contrast, the sorbents with a higher Zr mass fraction had a lower initial uptake but were more stable over multiple cycles, 1276
dx.doi.org/10.1021/ef4020845 | Energy Fuels 2014, 28, 1275−1283
Energy & Fuels
Article
The microstructure of carbonated and decarbonated sorbents was characterized by scanning electron microscopy (SEM) using LEO 1530 Gemini field emission gun (FEG)-SEM to scan the surface of the powder samples at 3 keV and a 3.7 mm working distance using an inlens secondary electron detector. All samples for SEM were sputtercoated with a layer of platinum, ∼5 nm thick. Transmission electron microscopy (TEM) was employed to gain insights into the detailed structure of the sorbent powders by selected area electron diffraction (SAED), atomic lattice imaging, energy-dispersive X-ray (EDX) spectroscopy, and elemental mapping. For this purpose, Philips CM200 FEG-TEM was operated at 197 keV and fitted with a Gatan imaging filter (GIF 200) and an Oxford Instruments 80 mm2 X-Max silicon drift detector (SDD) EDX spectrometer running the AZTEC processing software. Samples were prepared by dispersing sorbents in ethanol and drop casting onto holey carbon TEM support films (Agar Scientific, Ltd.).
for example, the uptake for 64 wt % CaO/36 wt % ZrO2 decayed from an initial value of 0.33 to 0.21 g of CO2/g of sorbent after 10 cycles.20 The present work concerns an alternative wet chemical powder synthesis method for CaZrO3/CaO sorbent powders, tested over 30 carbonation−decarbonation cycles. We show that the overall performance is superior to other Zr-modified CaO sorbents produced by solution-precipitation routes: nearstable multicycle CO2 capture performance, after an initial rise, is demonstrated for mild conditions relevant to SESR involving carbonation at 650 °C in 15% CO2 and calcination in air at 800 °C. The sorbents are also evaluated under severe conditions relevant to PCC. Here, the capture capacity declines by
