Stability and Carbon Capture Enhancement by Coal-Fly-Ash-Doped

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Stability and Carbon Capture Enhancement by Coal-Fly-Ash-Doped Sorbents at a High Temperature B. Sreenivasulu,† I. Sreedhar,*,† B. Mahipal Reddy,‡ and K. V. Raghavan§ †

Department of Chemical Engineering, Birla Institute of Technology & Science (BITS)-Pilani, Hyderabad Campus, Shameerpet, Hyderabad, Telangana 500 078, India ‡ Inorganic and Physical Chemistry Division and §Reaction Engineering Laboratory, Indian Institute of Chemical Technology, Tarnaka, Hyderabad, Telangana 500 007, India ABSTRACT: Carbon capture using various technical options, viz., adsorption, absorption, chemical looping combustion, membrane separation, and cryogenic separation in either pre- or post=combustion modes, has been found to be the effective solution to tackle a serious concern of global warming. Although adsorption has been widely employed in carbon capture as a result of its economically and environmentally benign nature, it could not be commercialized as a result of the highly energyintensive regeneration process involved. The major challenge in carbon capture is its sustainability at a high temperature, therefore enabling an easy integration with power generation to make it commercially viable, and also in the production of hydrogen using sorption-enhanced steam reforming. In this work, various combinations of sorbents based on CaO, MgO, zeolites H-Beta and H-ZSM 5, and Al2O3 with and without doping of coal fly ash (C and F types) have been employed in carbon capture using a lab-scale fixed-bed reactor system. After initial screening of numerous sorbents, those based on CaO, MgO, and fly ash have been selected for rigorous standardization with reference to various critical process parameters, such as the sorbent combination, sorbent quantity, temperature, carbonation time, gas composition, and flow rate. It has been found that 50% CaO, 10% MgO, and 40% fly ash gave the highest capture besides exhibiting cyclic stability up to 15 cycles, and a positive influence of coal fly ash in stability and carbon capture enhancement has been reinforced.

1. INTRODUCTION Carbon capture (CC) to mitigate global warming employing various technical options, viz., adsorption, absorption, membrane separation, chemical looping combustion, and cryogenic separation in pre- or post-combustion modes, has been widely reported.1−3 Many reviews on different CC technologies mentioned above are available in the literature.4−7 Although absorption has been commercialized with few solvents, it suffers from serious intrinsic problems, such as expensive solvent recovery and the corrosion and environmental disposal issues associated with the solvents.8 Adsorption seems to be next promising technology because there is a wide variety of microand mesoporous sorbents, viz., activated carbon from various sources, nitrogen-enriched carbon, silica gel, advanced membranes, amine-incorporated zeolites, and amine-modified sorbents, which are cost-effective, efficient, and environmentally benign.9,10 Because power generation is poised to be based on fossil fuels, such as coal, for the next 2−3 decades, any attempt to use wastes generated from the power sector, such as coal fly ash, would be welcome because it contains many inert aluminosilicates that are highly stable even at high temperatures.11,12 Also, the CC technology could be commercialized only if it could be successfully integrated with power generation industries that demand thermally stable sorbents possessing the desired attributes, such as high sorption capacity, selectivity to CO2, low regeneration energy, and easy synthesis protocols.13−15 Many sorbents, such as activated carbons, zeolites, alkali oxides, ceria-based sorbents, amine-doped sorbents, etc., have been reported to be applied only for low-temperature CC © XXXX American Chemical Society

applications because these showed rapid decay in sorption rates and capacities after multiple cycles at high temperatures besides poor mechanical strengths and high regeneration temperatures required.16,17 Ceramic-based materials, hydrotalcites, titanium silicates, etc. were reported to be suitable for high-temperature applications as a result of their thermal stability, rapid sorption−desorption kinetics, and high CO2 uptake capacity.18−21 Although CC studies have been reported employing CaOand MgO-based sorbents and coal fly ash individually, not many are available on a combination of these sorbents. Attempts have been made with a limited success to enhance thermal stability of these CaO- and MgO-based sorbents using pretreatments, such as hydration, thermal treatment, surfactant addition, and chemical modification.22,23 However, dispersion of these materials with inert precursors with high Tammann temperature (Tr) using wet or dry processes had some promising results.1,24 Hence, the coal fly ash, an abundantly available industrial waste, as a result of its composition of aluminosilicates, favorable kinetics in the temperature range of 400−800 °C, and thermodynamic conditions would enhance thermal stability and CO2 uptake of the otherwise vulnerable materials, such as CaO and MgO.25,26 In this work, CC studies have been conducted using different blends of CaO, MgO, and coal fly ash at high temperatures using cyclic carbonation−regeneration tests. Coal fly ash has Received: October 19, 2016 Revised: December 16, 2016 Published: December 21, 2016 A

DOI: 10.1021/acs.energyfuels.6b02721 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 1. XRF Analysis of the Two Coal Fly Ash Samplesa

a

sample

Al2O3

SiO2

Fe2O3

CaO

MgO

K2O

TiO2

SO3

CFA-C (wt %) CFA-F (wt %)

17.344 24.01

30.786 50.295

0.999 5.029

36.271 8.892

11.541 7.63

0.413 1.449

0.976 1.717

1.67 0.966

CFA-C, coal fly ash class C type; CFA-F, coal fly ash class F type.

been reported to be an ideal doping material not only as a result of its inherent thermal stability up to multiple cycles but also to use this widely abundant industrial waste beneficially, which otherwise leads to disposal and environmental problems.3,27 After initial screening of various sorbents along with fly ash type, the CC process has been rigorously standardized with reference to many critical parameters, such as the temperature, gas flow rate, sorbent type, sorbent composition, and time, to maximize CC using a fixed-bed quartz reactor using the Taguchi method of experimental design. Stability tests up to many carbonation−regeneration cycles have been performed at the optimal process conditions to verify the sustainability of the CC up to multiple cycles besides understanding the physicochemical changes, if any occur in the process.

Figure 1. Schematic diagram of the packed-bed reactor. ≤950 °C are attained. The temperature controllers of the preheater and furnace are operated simultaneously. CO2 gas in the desired proportion with N2 gas is made to flow through the reactor at a predetermined flow rate adjusted by the mass flow controller. The exit gas composition is measured by an online gas analyzer with data logging facility. The CC data as a function of time is recorded to plot the required breakthrough curves.32 The CC achieved is estimated using eq 1

2. MATERIALS AND METHODS 2.1. Materials and Analytical Methods. The various materials required for the synthesis of sorbents, viz., CaO, MgO, Al2O3, NaHCO3, MgCO3, etc., are of laboratory reagent (LR) grade with a purity of >99.7% and are procured from SD Fine-Chem, Ltd., Mumbai, India. Coal fly ash (on the basis of lignite coal) was procured from the National Thermal Power Corporation (NTPC), Ramagundam, India. The composition of the two fly ash samples taken from Xray fluorescence (XRF) analysis are given in Table 1. The analytical tools employed to understand the physicochemical changes in these studies are D5000 Siemens X-ray diffractometer using Ni-filtered Cu Kα radiation (λ = 1.506 Å) for assessing crystalline nature, SEM Carl Zeiss AG-EVO, 40 Series at an accumulation voltage of 10 kV and system resolution of 98 eV for morphological studies, and Brunauer− Emmett−Teller (BET) surface analyzer Smart instruments for surface area and pore volume measurements. The coal fly ash samples have been named as CFA-C and CFA-F types because their composition matches with standard C and F types with marginal deviation in minor constituents. The typical composition of standard C type is Al2O3 (15−20 wt %), SiO2 (20− 50 wt %), Fe2O3 (5−10 wt %), CaO (10−40 wt %), MgO (3−10 wt %), and alkalis (≤8 wt %), and that of F type is Al2O3 (20−30 wt %), SiO2 (45−65 wt %), Fe2O3 (4−20 wt %), CaO (400 °C). The length and diameter of the quartz tube are 55 and 2.54 cm, respectively. Gas leakage is prevented using an inert graphite gasket ring installed in the reactor. The reactor temperature is raised in steps of 50 °C at a rate of 16 °C/ min to avoid any damage to the internal circuit. Thermocouples are installed at various locations of the reactor to record the temperatures as a function of the time and position during the CC studies. The desired amount of adsorbent is loaded into the detached quartz tube and then fixed it to the experimental setup to function as a fixedbed reactor. The power is switched on; the inert N2 gas is purged at atmospheric pressure; and heating is continued until the desired temperatures of ≤250 °C of the preheater and furnace temperature of

Q ads =

Fo(ρ /M wt)(Co − Ca)Tq (1)

W

where Fo is the feed flow rate (cm /min), ρ is the density of CO2 gas at room temperature (mg of CO2/cm3), Mwt is the molecular weight of CO2 gas, Qads is the CO2 capture capacity of the adsorbent (mmol of CO2/gads), W is the sorbent weight (g), Tq is the breakthrough time taken to capture CO2 by the adsorbent (min), which is given by eq 2 3

Tq =

0.95 ⎛

∫0.05

C ⎞ ⎜1 − a ⎟ dt C ⎝ o⎠

(2)

where Co and Ca are feed and outlet concentrations at time tq (min), respectively.

3. RESULTS AND DISCUSSION OF SORBENT SELECTION 3.1. First Level Screening. The initial sorbent selection is governed by factors such as abundant availability, desirable oxide combinations, affordable cost, environmental acceptability, and high thermal, mechanical, and attrition characteristics. The active metal oxides (limestone, CaO, Fe2O3, CuO, NiO, MnO2, etc.) and supporting metal oxides (Al2O3, SiO2, coal fly ash, etc.) are blended with surfactants or dispersant polymers, pelletized, and regenerated at temperatures of >1050 °C to make them sustainable at rigorous operating conditions, as shown in Figure 3. Eight samples (Table 2) of CaO in compatible combination with other metal oxides doped with two types of fly ash (C and F types) are selected for the study. The eight candidate metal oxides are synthesized using the dispersion method. All of the adsorbents are synthesized in powder form. The parameters considered for the first level screening of the nine candidate metal oxides are cost, pore volume, surface area, and CC capacity. The fixed-bed reactor facility as shown in Figure 2 B


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Energy & Fuels

Figure 2. Experimental packed-bed reactor and its accessories.

sintering resistance because of fly ash addition beyond a critical value. Our observations are found to be in agreement with the reported literature.36 Hence, the sample CaO (30%)/MgO (30%)/FA-C (40%) containing CaO, MgO, and fly ash (C type) has been chosen as the potential candidate for further optimization, owing to its reasonably high CC and, more significantly, the high thermal stability as a result of the presence of fly ash. From these results, it could be understood that efficacy of fly ash in enhancing the CC is not a simple phenomenon but is highly complex, as reported with contradicting results depending upon the composition of fly ash, particle size, ash content, ash−metal oxide interactions, etc.4,12 In our studies, FA-C has given favorable results compared to FA-F, owing to the difference in the composition, where C type had more CaO content aiding in achieving higher CC. This fly ash has been reported to form an inert material, gehlenite, which has a very high melting point, exhibiting good thermal durability, acting as a physical barrier to prevent sintering and agglomeration of CaCO3 nanoparticles during calcination, and thereby giving cyclic stability at high temperatures.37,38 It was also interesting to note from the initial screening results that sample CaO (30%)/FA (70%) gave a low CC of 5 mmol/gads, and this could be clearly attributed to a large amount of fly ash taken. Although fly ash acts as a thermal stabilizer, leading to a minimal CC, there has to be an optimum amount, beyond which it leads to the decrease in the active capturing sorbent material and below which sorbent loses sintering resistance.4,39,40 Also, greater ash content leads to a larger heat consumption in the calciner. The optimum FA/CaO ratio could be predicted by the three-dimensional percolation theory to maximize CO2 uptake.41 3.2. Process Optimization with the Taguchi Method. The Taguchi method is based on orthogonal array experimentation to provide reduced variance for the optimized control parameters.42,43 The composition variation of the chosen mixed metal oxides at 350−650 °C, contact time (50− 60 min) of carbonation, and sample quantity (20−50 g) have been chosen as the main process variables. The following mixed

Figure 3. Flow sheet of sorbent synthesis.

is employed for the screening process. The inlet gas, consisting of 40% CO2 and 60% N2, is fed to the reactor at a flow rate of 250 mL/min. The exit gas composition is measured by an online gas analyzer. All of the first level screening experiments have been conducted at 650 °C with 20 g of sorbent for a duration of 60 min. Table 2 presents the outcome of the first level screening process employing single-, two-, and threecomponent sorbent systems. Table 2 shows that the composite metal oxide samples of this work CaO (50%)/H-ZSM-5 (50%) > CaO (50%)/H-Beta (50%) > CaO (50%)/MgO (50%) > CaO (30%)/MgO (30%)/FA-C (40%) > CaO have exhibited relatively higher CC capacity in that order. Samples CaO (50%)/H-ZSM-5 (50%) and CaO (50%)/H-Beta (50%) containing zeolite constituents are not only expensive but have been reported to possess inferior surface characteristics beyond 300 °C.12,29,33 From cost considerations, they are the most expensive for large tonnage CC operations. They are, therefore, not considered for further screening. It has been reported that MgO and MgO− CaO binary mixtures experienced pore blocking during repeated carbonation−regeneration cycles and are reported to exhibit rapid decay rates and unfavorable thermodynamic conditions at high temperatures.1,5,12,19,34−36 The surface area of the CaO−MgO combination is higher as a result of the high surface area of the MgO constituent. However, in combination with fly ash, the surface area is reduced as the MgO content is reduced and the surface areas of CaO and fly ash are low. With regard to the performance, the sample of the CaO−MgO combination is found to undergo sintering and pore blocking with successive cycles, reducing its performance, while that with fly ash, this phenomenon was negligible as a result of enhanced C


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Table 2. Comparison of Doped Fly Aash Adsorbents with Outcome of the First Level Screening of the Metal Oxide Candidatesa feed mixture (% CO2)

car T (°C)

cal T (°C)

SA (m2/g)

PV (cm3/g)

pore size (Ǻ )

CC (mmol of CO2/gads)

40

650

950

3.1

0.00316

40.8

9.2

CaO (100%)b RHA (4.1:1 Li2CO3/SiO2) FA−Li4SiO4 (1:2 SiO2/Li2CO3) 1:1 K−FA diatomite (2:1 Li4SiO4/SiO2) 2:1 K−FA 2:1 K−FA 2:1 K−FA 1:1 K−FA CaO (90%)/Yb2O3 (10%) CaO (30%)/FA-F (70%)

15 100 100 100 50 100 100 100 100 15 40

650 710 500 500 700 500 600 700 700 650 650

900 800 950 500 700 500 600 700 700 900 950

10.41 6.2

500−1000 0−200

7.15 4.2

0.0057

54.29

15.1 7.36 11 mmol of CO2/g of sorbent at 40% CO2 gas mixture in 60 min. At 650 °C, employing pure CO2, the CC

composition (%)

temperature (°C)

time (min)

quantity (g)

flow rate (cm3/min)

CCa (mmol/gads)

C1 C1 C1 C1 C2 C2 C2 C2 C3 C3 C3 C3 C4 C4 C4 C4

350 450 550 650 350 450 550 650 350 450 550 650 350 650 550 450

15 30 45 60 30 15 60 45 45 60 15 30 45 60 30 15

20 30 40 50 40 50 20 30 50 40 30 20 30 20 50 40

100 150 200 250 100 150 200 250 100 150 200 250 100 150 200 250

4.5 7.2 6.2 5.6 3.9 3.5 11.1 8.1 6.8 3.9 7.1 8.6 4.9 11.2 8.3 6.8

a

CC values correspond to 40% CO2 with N2.

achieved is much higher than that reported (2.4 mmol of CO2/ g) for fly ash alone but slightly lower than that reported for D


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Figure 4. (a−d) Effect of process parameters on sorbent performance: (a) at 650 °C with 20 g of sample quantity, (b and c) at 20 g sample and 1 h of contact time, and (d) at 650 °C and 1 h of contact time. E


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the gas with the sorbent. The sorbent C4 provided CC of >11 mmol of CO2/g at a 20 g level for our laboratory setup. It could be attributed to the availability of enhanced alkalinity and sorption sites and also the presence of a larger amount of MgO, the divalent cationic basic oxide. The reported literature has shown the need up to 300 g of sorbent loading in the case of laboratory-scale packed-bed reactors.14,52−54 However, caution needs to be taken because larger amounts would contribute to greater pressure drops and the undesired channeling effects. The adsorbent amount is strongly dependent upon the reactor capacity and design. 3.4. Sorbent Performance during Carbonation−Regeneration Cycles. The performance was compared for 15 cycles for the powdered sorbents, C2 and C4, at both 550 and 650 °C, respectively (Figure 5). Regeneration in the presence

CaO−MgO [15.9 mmol of CO2/g with 100% CO2 feed gas in thermogravimetric analysis (TGA)], owing to the difference in composition. It has been reported that the rate of carbonation increases with an increase in the MgO content in a CaO−MgO mixture, and hence, maximum CC could be attained faster in the C4 sample as a result of its higher MgO content under identical conditions. Hence, our results are in perfect agreement with those reported.44,45 Because MgO is an expensive material, its maximum content in the mixed oxides employed in this work was kept at 30%. The positive influence of FA-C could be attributed to its improved surface attributes, inherent CC ability by the presence of alkaline metal oxides, viz., CaO, MgO, Al2O3, and SiO2, besides others. A high-temperature CC process is very much desired to successfully integrate it with the power generation sector, such as the integrated gasification combined cycle (IGCC) process, for enhancing the economic viability and also in hydrogen production using sorption-enhanced steam reforming. In this work, CC is performed in lab scale at relatively high temperatures. Figure 4b shows the effect of the carbonation temperature (350−650 °C) on the performance of the sorbents. A maximum CC of 11.2 mmol of CO2/g has been achieved by them at 550 °C. The increased CC at higher temperatures is due to the enhanced carbonation rate.36,46,47 It has been reported that the carbonation temperature and the partial pressure of CO2 on CC are very critical and interrelated.48 It was also reported that the increase in the temperature leads to the increase in CC, but beyond a certain temperature, this is offset by the negative impact of sintering of the sorbents. Hence, there is always an optimum temperature for achieving maximum CC, which was found to be a function of the CO2 partial pressure/concentration in the feed gas. In our studies too, a similar trend has been observed, where CC reached a maximum between the temperatures of 550−600 °C, beyond which it dropped. It has been reported that the optimum carbonation temperatures were around 800 °C for pure CO2 and 660−710 °C for 15% CO2.49,50 The TGA and atomic force microscopy (AFM) trapping mode studies of these samples have shown that, at temperatures beyond 550− 600 °C, the adsorption surface was covered with the carbonated product operating under a diffusion-controlled regime dominated by grain boundary and lattice diffusional processes.46 Figure 4c shows the effect of gas flow rates on the carbonation of sorbents C1−C4 for a duration of 60 min. It has been reported that the carbonation occurs in two stages: fast carbonation stage up to 1 min and slow carbonation stage up to 120 min.49 Because our time taken was 60 min, an increase in CC was observed because the study was within this overall carbonation time frame. It was also reported that maximum CC achievable increases with the CO2 partial pressure.48 Hence, as the gas flow rate was increased, the amount of CO2 to be captured increased as well as this trend at the temperature taken. It was to be noted that high temperatures and high CO2 concentrations would lead to undesired sintering effects, resulting in a decrease in the surface area and pore volume.51 Accordingly, the gas flow rate and the carbonation times need to be adjusted carefully to avoid this phenomenon. The influence of high gas flow rates at much longer carbonation times needs to be studied to gain further insight into this phenomenon and the mechanisms. Figure 4d establishes the need for providing optimum sorbent quantity for the carbonation process, beyond which it would be counterproductive as a result of improper contact of

Figure 5. Comparison of the sorbent performance under carbonation−regeneration cycles.

of N2 gas was chosen to mitigate the sintering of the sorbents. The heating rate was set at 16 °C/min for a regeneration temperature of 950 °C. A pre-regeneration time of 20 min was maintained. A sample of 20 g has been used for each regeneration−carbonation cycle with 60 min duration. Both C2 and C4 samples registered an increase in CC during the initial few cycles before becoming stabilized for the samples C2 and C4. The increase in CC initially was due to the selfreactivation effect, which would be counteracted by sintering under rigorous regeneration conditions.1 This was further reinforced by our X-ray diffraction (XRD) studies that showed enhanced crystallinity during the initial cycles, thereby the enhanced diffusion-controlled reaction of CO2 along the grain boundaries.55−57 It may also be due to the enhanced surface area availability for carbonation during the initial 3 cycles and calcium silicate decomposition with CO2 as a result of the synergistic effect of alkaline metals present in the coal fly ash.44,58−62 Our sorbent showed much better cyclic stability than those reported with fly ash and CaO, which showed a decay of more than 50% in 15 cycles.4 Our trends were similar to those reported with fly ash, with higher CC achieved under the given conditions.1,4 Among the sorbents taken, the C2 sample performed better than the C4 sample throughout the 15 cycle operation in terms of CC and the stability. This was in line with reported results, where the increase in the MgO content led to a decrease in the surface area for CaO to become carbonated.36,63 Both the samples performed better at 650 °C than that at 550 °C, even at the end of 15th cycle, which could F


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Figure 6. (a) XRD, carbonated C2 sample during carbonation−regeneration cycles. (b) XRD, regenerated C2 sample during carbonation− regeneration cycles.

throughout the 15 cycles when compared to that with fly ash, C2. To understand the physicochemical changes during the CC process, the better performing C2 adsorbent has been subjected to powder XRD, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX) analyses. The XRD patterns of the used C2 samples from 1st to 15th cycles are shown in panels a and b of Figure 6. The decrease in the crystalline peak for dolomite and the increase in the crystalline peak of CaCO3 is observed from 10th to 15th cycle carbonation as a result of the synergetic effect of alkaline metals in fly ash for the sustainable CC. There is a XRD line peak from 5th to 15th cycles pertaining to the crystalline formation of CaO, CaCO3, and MgCO3. It is due to the decomposition of CaSiO3 with CO2, emitted by regeneration of other alkaline carbonates at higher temperatures of >400 °C. It is the synergetic contribution of the sorbent alkaline ratio (CaO/SiO2) of >5 for the carbonation of calcium silicates and magnesium silicates at higher temperatures (>400 °C). The surface areas (SAs) of carbonated samples after 1st, 5th, 10th, and 15th cycles are 2.01, 3.87, 2.44, and 1.81 m2/g, respectively. These areas increased from 1st to 5th cycle carbonation, in line with XRD peaks of carbonated samples from 1st to 5th cycles, and the capture capacity has increased as a result of chemical reaction over the surface of the adsorbent. From the 5th to 15th cycles, the SA has decreased, but XRD peaks increased from 5th to

be ascribed to the increased surface area, pore diameter, and pore volume and absence of the sintering effect at higher temperatures. Figure 5 clearly demonstrates the stability of CC capacity up to 15 cycles with marginal fluctuation. Although studies have been reported on sorbents based on CaO and MgO, very few studies have been reported using fly-ash-doped sorbents in high-temperature CC.15,64−70 This adsorbent proved to be a good performing adsorbent, giving almost an equivalent capture capacity as of CaO−MgO combinations as a result of the beneficial role played by the components of the fly ash. A sustainable higher capture capacity of coal-fly-ash-based sorbents was due to the synergistic effect of alkali and alkaline earth metal compounds present in it that would form an inert thermally resistant phase. It has been reported that the presence of alkaline carbonates and SiO2 enhances regeneration− carbonation looping of CaO-based sorbents. Experimental data presented in Figure 5 corroborate the reported finding.53,71,72 This is due to the positive contributions made by CaCO3, MgCO3, SiO2, and other constituents of fly ash in terms of lower activation energy and accelerated rate of carbonation.45,58,63,73,74 Figure 5 also elucidates the positive influence of fly ash addition by comparing the performance of CaO−MgO with and without fly ash, shown as C2 and C2*, respectively. C2* is found to give inferior performance G


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Figure 7. (a−j) SEM and EDX analyses of sample C2 during carbonation−regeneration cycles. H


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20 g, gas flow rate of 250 mL/min (superficial velocity of 0.822 cm/s), temperature of 650 °C, and powder form are found to be best set of parameters to maximize CC. This work establishes the superior performance and advantages of employing coal fly ash as a major constituent in conjunction with CaO and MgO as the sorbent mixture for CC. With fly ash being a thermally resistant material that could withstand up to 1100 °C, high-temperature CC could be achieved using it. Fly ash could promote CC by chemisorption by its alkaline metal constituents present in it. If employed as a support material, this could further enhance the CC value as a result of the large dispersion possible, and future research efforts should be focused in this direction.

15th cycle carbonation, and this is due to the diffusion reaction as the dominant contributor for the carbonation reaction. Various crystalline phases identified in different peak intensities include CaSiO 3 , SiO 2 , MgCO 3 , CaCO 3 , MgCO 3 ·CaCO3(dolomite), MgO, CaO, and other unidentified phases. Most of these peaks are relatively broad at first cycle, indicating the amorphous or poorly crystalline nature of the materials. Interestingly, there was a gradual increase in the intensity and sharpening of peaks from 1st to 15th cycles, indicating better crystallization of the materials with repeated cycles. From 5th to 10th cycles, there is a gradual increase of dolomite formation and then a decrease of the dolomite crystallinity peak at 15th cycle carbonation. Further, no new XRD line pertaining to the formation of new crystalline compounds or disappearance of the peaks as a result of the decomposition and formation of amorphous compounds is observed. However, the increase in the intensity of peaks pertaining to CaCO3 and MgCO3 is more as a result of effective carbonation of the corresponding alkaline metal oxides. The XRD results clearly conform to the stable performance of the C2 adsorbent, owing to the stable nature of the constituent materials. From the comparison of panels a and b of Figure 6, it is clearly evident that the sorbents retained the structural stability, carbonation−calcination mechanism, and performance stability up to 15 cycles, reinforcing the effectiveness of our regeneration protocol.65−68,70,75 The savings of >24% capital cost are higher than 78% reduction in operational relative to microelectrode array (MEA) technology applied for a high-temperature CO2 capture in a 500 MW coal-fired power plant with 90% CO2 capture that produces nearly 419 000 kg of CO2/h.67,76,77 The SEM micrographs of regenerated and carbonated C2 samples in 1st to 15th cycles are shown in panels a−h of Figure 7. The corresponding EDX patterns of the C2 sample carbonated in 1st and 15th cycles are shown in panels i and j of Figure 7. It shows the sustainable CC trend with an increase during the initial 3 cycles and stable CC with marginal change for the remaining number of cycles. This is in agreement with the results presented in panels a−d of Figure 4. The SEM images of all samples reveal uniform distribution of the particles, irrespective of the number of cycles. There is a gradual increase in the crystalline size from 1st to 15th cycles as a result of better crystallization of the materials, in line with XRD results. This is due to the mixed effect of the alkaline metals for carbonation and resistance to the agglomeration by the doped fly ash. The corresponding EDX patterns also support the uniform distribution of component metals, metal oxides, and metal carbonated as expected in these samples. On the whole, the XRD, SEM, and EDX results support the stable nature of the C2 adsorbent for repeated cycles of CC.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +91-4066303512. Fax: +91-4066303998. E-mail: [email protected]. ORCID

I. Sreedhar: 0000-0003-1706-5735 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors express their gratitude to the management of BITS-Pilani for funding and other contributions toward the research work.

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REFERENCES

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4. CONCLUSION High-temperature CC studies have been conducted using various sorbents doped with two different types of coal fly ash. After initial screening of more than 20 sorbents, the relatively best performing combinations have been subjected to rigorous standardization, employing the Taguchi method. The CC has been optimized with reference to critical process parameters, such as the adsorbent composition and amount, synthesis protocol, adsorption temperature, gas flow rate, carbonation time, and morphology. It has been found that the CaO−MgO− FA-C combination of 50:10:40 wt % has given the highest CC of more than 9 mmol of CO2/gads after the 15th cycle, which is higher than the reported values for coal fly ash. The weight of I


Article

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