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Insights into carbonation kinetics of fly ash from Victorian lignite for CO2 sequestration Long Ji, Hai Yu, Bing Yu, Ruijie Zhang, David French, Mihaela Grigore, Xiaolong Wang, Zuliang Chen, and Shuaifei Zhao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03137 • Publication Date (Web): 08 Jan 2018 Downloaded from http://pubs.acs.org on January 9, 2018

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Insights into carbonation kinetics of fly ash from Victorian lignite for CO2 sequestration Long Jia, b, c, Hai Yub*, Bing Yub, Ruijie Zhangc, David Frenchd, Mihaela Grigoree, Xiaolong Wangf, Zuliang Cheng, Shuaifei Zhaoa**

a

b

c

d

e

Department of Environmental Sciences, Macquarie University, Sydney, NSW 2109, Australia CSIRO Energy, Newcastle, NSW 2304, Australia China University of Mining & Technology Beijing, Beijing, 100083, China University of New South Wales, Sydney, NSW 2052, Australia CSIRO Energy, North Ryde, NSW 2113, Australia

f

Huaneng Clean Energy Research Institute, Beijing, 102209, China

g

Fujian Key Laboratory of Pollution Control and Resource Reuse, School of Environmental Science and

Engineering, Fujian Normal University, Fuzhou 350007, China

*Corresponding author Ph: +61-2-4960-6201 Email: [email protected] **Corresponding author Ph: +61 2 9850-9672 Email: [email protected];

Abstract Mineral carbonation of fly ash can both capture and store CO2 permanently in a single process without long-term monitoring. Previous studies indicate that fly ash with high calcium and magnesium contents exhibit promising CO2 fixation capability. However, the reaction mechanisms and kinetics involved in the carbonation reaction of fly ash is still not fully understood. In this study, a 1

ACS Paragon Plus Environment

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typical Victorian brown coal fly ash from Hazelwood power plant was selected to sequestrate CO2 in a direct aqueous carbonation process. Experiments were conducted in a vessel reactor at various temperatures (40, 50, 60, and 70 °C), stirring rates (900, 1050, 1200 and 1350 rpm) and CO2 pressures (3, 4, 5, 6 and 7 bar) to investigate the reaction kinetics and identify the rate-limiting steps of carbonation. The results show that both the carbonation rate and the maximum carbonation efficiency could be improved by optimizing parameters and by the introduction of NaHCO3. Also, the complex effects of the operating parameters on the carbonation rate and the maximum carbonation efficiency were investigated. The kinetic data can be well fitted by the surface coverage model with the R2≥0.98, indicating that the carbonation of fly ash can be physically expressed by this model. The maximum carbonation efficiency of fly ash could also be well predicted by the model. In addition, the assumed mechanisms of the carbonation reaction were validated by particle size, surface area and porosity changes of the fly ash particles after carbonation reactions. The observation of scanning electron microscope equipped with energy-dispersive X-ray spectroscopy before and after carbonation also confirmed that the newly formed precipitates were not only deposited on the active surface, but also filled the pores of the fly ash particles.

1 Introduction Global warming due to the emission of greenhouse gases over the past few decades has become of increasing concern. 1 Carbon dioxide (CO2) is widely accepted as the major greenhouse gas and about 40% of the total CO2 gas is emitted by fossil fuel combustion in large industrial sectors. 2 CO2 mineral carbonation, the accelerated process of natural rock weathering,

3

is an effective method for both

capturing and storing CO2 both permanently and safely. 4-6 The process was initially developed for the carbonation of natural silicates rich in calcium or magnesium, such as serpentine, olivine and 2


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wollastonite due to their widespread occurrence.

7

However, the effectiveness of this process using

natural minerals has been constrained by the requirement of energy intensive pre-treatments prior to mineral carbonation, such as mechanical, chemical, or thermal pre-treatment to activate the feedstocks. 8

Compared with natural minerals, fly ash, a by-product of coal fired power stations, has several advantages as a feedstock for CO2 mineral carbonation. It has low material costs, high reactivity, does not require pre-treatment, and is readily available near CO2 emission sources.

9

In the state of

Victoria, Australia, brown coal is the major energy source and provides over 85% of the state’s electricity.

10

Up to 1.3 million tons of fly ash are produced annually from the combustion of

Victorian brown coal, with the bulk being dumped in fly ash dams.

11

Disposal of the fly ash has

become a serious problem because of its toxicity and strong alkalinity. The high contents of calcium and magnesium in fly ash make it an excellent feedstock for CO2 mineral carbonation. 12 CO2 mineral carbonation can not only reduce the CO2 emission of power plants, but can also increase fly ash stability, thus expanding its utilization in construction material production.

Previous studies have discussed the technical feasibility of direct CO2 mineral carbonation with fly ash in both gas-solid and aqueous routes. 13 Mineral carbonation processes conducted by the aqueous route are more effective than those by dry gas-solid method. The carbonation efficiency of different fly ashes at various operation conditions have been evaluated. These experiments showed that the carbonation efficiency can be improved by the optimisation of experimental parameters, such as temperature, CO2 pressure, stirring rate, solid to liquid ratio and reaction time. 14-18 However, there are limited studies on the effects of operation parameters on the carbonation rate. Given that the reaction 3


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rate is not fast enough for large-scale application of this technology, kinetic studies of carbonation reaction with fly ash is important for improving the understanding of reaction mechanisms and for further promoting the carbonation reaction. More recently, several studies used a pseudo-second-order kinetic model developed

19

to simulate the carbonation reaction of fly ash in batch reactors.

20-22

This

mathematical model provided a good fit to the experimental data of long period carbonation reactions, especially when the reactant was from heterogeneous materials and chemical reactions were involved. 20

The model also provided a promising method for the calculation of the apparent activation energy

of the carbonation reaction, by which the reactivity of different fly ashes can be evaluated.

22

However, it failed to explain how operation parameters affected the kinetics of carbonation and the determination of the rate-determining step of the carbonation reactions. Chang et al.

23

studied the

carbonation kinetics of fly ash by the shrinking core model to determine the rate-determining step, but the model could not match the carbonation experiments after 10 min, indicating that multi-controlling steps were involved in the whole carbonation process. Despite the above studies, there is still a knowledge gap in our understanding of the reaction mechanisms involved in carbonation reaction. The effects of operation parameters and additives such as NaHCO3 on carbonation kinetics have not been critically evaluated. Moreover, a more valid model is required to improve the kinetic study of the carbonation reaction and to predict the maximum carbonation efficiency.

In this work, a typical Victorian brown coal fly ash from the Hazelwood power plant was selected to sequestrate CO2 in a direct aqueous carbonation process. Our aim was to improve the understanding of the mechanisms and kinetics of fly ash in mineral carbonation. Experiments were conducted in a vessel reactor at various temperatures (40, 50, 60, and 70 °C), stirring rates (900, 1050, 1200 and 1350 rpm) and CO2 pressures (3, 4, 5, 6 and 7 bar) to investigate the reaction kinetics and identify the 4


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rate-limiting steps of carbonation. A wide range of models based on various mechanisms were compared and introduced to investigate the effect of various parameters on the carbonation rate and efficiency. The selected model was also used to predict the maximum carbonation efficiency. The compositional changes of the fly ash after carbonation were analysed by X-ray diffraction (XRD). The morphology of the fresh and carbonated fly ash was characterized using a scanning electron (SEM) equipped with energy-dispersive X-ray spectroscopy (EDS), which provides a deeper understanding of the carbonation mechanisms.

2 Materials and methods 2.1 Materials Fly ash (FA) used in the present study was collected from the electrostatic precipitator (ESP) of the Hazelwood thermal power plant in Australia, which uses Victorian lignite as fuel. All the fly ash samples were dried overnight at 105 °C prior to any test. NaHCO3 (≥99.7%) was purchased from Sigma-Aldrich. Carbon dioxide (CO2, 99.99%) and nitrogen gases (N2, 99.99%) were purchased from coregas Australia. The chemical analysis of the fresh samples was determined by X-ray fluorescence spectroscopy (XRF). Particle size distribution of the fresh samples was measured by a laser particle analyser (LS-POP (VI)). Brunauer, Emmett and Teller (BET) surface area, total pore volume and average pore size were determined using a physisorption analyser (JW-BK 122W). The summary of physicochemical properties of the fly ash is shown in Table 1.

2.2 Aqueous carbonation experiments in a vessel reactor

5


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The accelerated carbonation reactions were carried out in a closed vessel reactor made by Swagelok. The internal volume of the vessel was calibrated prior to the experiments using N2 gas. For each carbonation experiment, 25 mL high-purity water and 2.5 g of fly ash was added into the vessel.

Ultrapure N2 gas (99.99%) was repeatedly introduced into the vessel to remove the impurities from the gas lines and the vessel. After N2 gas dosing was stopped, the vessel reactor was heated up

to a given temperature using a water bath. High purity CO2 gas (99.99%) was injected into the vessel directly from a Swagelok CO2 cylinder to a given pressure. This initial pressure creates a moderately pressurized CO2-rich environment for the carbonation reaction to proceed at a favourable rate. Magnetic stirring was then initiated and fly ash particles were immediately dispersed. This time was recorded as the starting time of the carbonation reaction. Once the carbonation reaction started, the pressure drop inside the vessel as a function of time was monitored by a pressure sensor. The dissolution of CO2 in the solution during carbonation produced a global pressure drop in the vessel reactor. To estimate the pressure-drop produced by the carbonation process, two complementary tests were conducted for each experiment. Firstly, the pressure drop by the dissolution of CO2 into pure water was measured (Pblank solution). Secondly, the pressure drop by the dissolution of CO2 in the solution mixed with fly ash was measured (Ptotal). Thus, the CO2 consumed by the carbonation of the fly ash can be calculated by equation (1) to (2), 24 = − (1)

(g CO /kg FA) =

!"#$%!&'$% ∙)∙** +,-∙./0

(2)

where V is the reactor volume occupied with gas (L), T is the operation temperature (K), Z is the compressive factor of carbon dioxide collected from property database of Aspen 8.0, which depends 6


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on the pressure and temperature, R is the gas constant (8.314 m3/mol/K), and mFA is the weight of fly ash (kg).

After 20 hours, the vessel reactor was cooled with cold water and depressurised. The suspension was immediately filtered through a 0.2 µm membrane filter unit equipped with a vacuum pump. The filter cake was dried overnight in an oven at 105 °C, while the fresh samples and selected carbonated samples were analysed by XRD and SEM-EDS.

It might be possible that the carbonation reaction would continue during the water cooling and depressurizing step. This may result in inaccurate estimation of the carbonation efficiency. However, before water cooling and depressurizing, the total pressure in the reactor became stable, suggesting very little carbonation occurred. With rapid drop of temperature and CO2 pressure, we would expected the carbonation rate further decreased. So the effect of the water cooling and depressurizing step on the measurement of carbonation efficiency is minimal.

The theoretical CO2 sequestration capacity in fly ash can be calculated by equation (3), 25

Th_ =

** ( 45

−

45 **

× ,8 −

45 × 98

:; ) +

** *8 =>

(3)

where Th_ (g-CO2/g-FA) is the theoretical CO2 sequestration capacity, (g-CaO/g-FA), :; (g-SO3/g-FA), and => (g-MgO/g-FA) are the weight fraction of CaO, SO3 and MgO in fresh samples, respectively. ,8 (g-CO2/g-FA) is the weight fraction of CO2 in the fresh fly ash determined by Titration.

The carbonation efficiency is defined by equation (4). 7


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.CD

? @%B = EF_. × 100

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(4)

CD

2.3 Reaction pathways It is widely accepted that the carbonation reaction pathways can be described by equation (5) to (11), 26

IJ (K) + L J → L IJN (OP) (5)

QRS

L IJN (OP) TU LIJNV + LW (6) QR

LIJNV TU L W + IJNV (7)

IOJ(X) + L J → IO(JL) (8)

IO(JL) (X) → IOW + 2JL V (9)

IOW + IJNV → IOIJN (Z[\]^_) (10)

IOIJN (Z[\]^_) → IOIJN (\O]\_`^) (11)

where Ka1 and Ka2 are the equilibrium constant of equation (6) and (7), respectively.

2.4 Characterization of fly ash samples The crystalline phases present in fresh and carbonated fly ash samples were determined by X-ray diffraction (XRD) analysis. Fly ash samples were ground in an agate mortar and pestle and then packed into an aluminium holder. All samples were run on an Empyrean PANalytical X-Ray Diffractometer using CuKα radiation at 40 kV and 40 mA. Step scans were conducted from 2 to 90° 8


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2θ, with a step interval of 0.02° 2θ. Mineral phase identification was performed by use of the Bruker Eva software package. Quantitative phase analysis was performed using the SiroquantTM software package following the Rietveld method, which generates a synthetic pattern first and then matches this to the experimental data using a least squares minimisation fitting procedure. 27 The advantage of this method is that the complete diffraction pattern is used to derive the quantitative results rather than relying on one or two peaks for identification. Although the application of the Rietveld method usually requires that the phases are crystalline to calculate the synthetic XRD pattern, SiroquantTM makes use of experimentally derived structural data (observed hkl files) for amorphous or poorly crystalline phases and thus can be used to determine the amount of amorphous material present in the sample. 27, 28

Morphological investigations of fresh and some carbonated fly ash samples were performed with a scanning electron microscope (SEM). Samples were mounted in epoxy resin, polished then mounted with double-sided carbon tape on an aluminium stub, and then coated with a thin-layer of carbon. The operating conditions were kept at a constant/accelerating voltage of 10 kV and a current density of 45 µA/cm2, with the electron beam directed at 90° to the specimen. In addition, the distributions of elements in the fresh and carbonated fly ash were determined using energy dispersive X-ray spectroscopy (EDS) equipped to the SEM.

3 Results and discussion 3.1 Physical and chemical properties Previous studies indicate the particle size and surface area significantly affect both carbonation rate and efficiency. Table 1 summarises the physicochemical properties of the fly ash selected for this 9


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study. The fly ash displayed a large proportion of fine particles, with the main statistical parameters (diameter at 10%, 50% and 90%, volume and surface mean diameters) of the particle size distribution as following: D10 = 3.51 µm, D50 = 8.40 µm, D90 = 14.66 µm, D[4,3] = 8.84 µm, D[3,2] = 6.03 µm. The size of the largest particles did not exceed 40 µm, which is finer than the materials reported in literature (≤ 100 µm). 29 Also, the fly ash displayed 30.29 m2/g BET surface area, which is larger than the materials reported in literature. 30 The physical properties of the fly ash suggest that pre-treatment grinding was not required prior to mineral carbonation. With respect to the chemical composition, the fly ash consisted predominantly of CaO (32.4 wt%) and MgO (29.3 wt%), indicating its potential capacity for CO2 sequestration. The high SO3 content (12.8 wt%) suggests the calcium of this fly ash might be partially present as calcium sulphate, which would result in low calcium availability for mineral carbonation. Only minor amounts of silica and alumina bearing crystalline phases are present, indicating that most of the silica and alumina is present in the amorphous phase.

31

The carbonate

fraction was 0.166 g/g FA. Based on chemical compositions, the theoretical capacity of the fly ash for CO2 sequestration was calculated by equation (3) to be 256.4 g-CO2/kg-FA.

Table 1 Summary of physicochemical properties of fly ash Items

Chemical properties

Fresh ash

µm

8.84

D[3,2]b

µm

6.03

D10

µm

3.51

D50

µm

8.40

D90

µm

14.66

BET surface area

m2/g

30.29

D[4,3]

Physical properties

Unit a

SiO2

wt%

5.8

Al2O3

wt %

3.0

Fe2O3

wt%

14.0

CaO

wt%

32.4

MgO

wt%

29.3

TiO2

wt%

0.7 10


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Na2O

wt%

0.2

K2O

wt%

0.2

P2O5

wt%

0.4

MnO

wt%

0.7

SO3

wt%

12.8

CO2 (Titration)

g-CO2/g-FA

0.166

Theoretical CO2 sequestration capacity

g-CO2/g-FA

256.4

a

b

: volume average particle diameter. : surface area average particle size

3.2 Mineralogical analysis Investigating the changes in mineral phases may improve the understanding of carbonation mechanisms. Figure 1 and Table 2 show a comparison of the XRD patterns between the fresh and carbonated samples after a carbonation reaction. The XRD patterns of fresh and carbonated fly ash are complex because of sample heterogeneity and complex mineralogy. The fresh fly ash was dominated by calcium- and magnesium-bearing crystalline phases. Calcite (CaCO3), basanite (CaSO4·0.5H2O), srebrodolskite (Ca2Fe2O5), gypsum (CaSO4·2H2O), and augite ((Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6), with weight fractions of 23.0%, 10.7%, 3.0%, 3.0%, and 2.2%, respectively, were the crystalline calcium-bearing phases identified, while the magnesium-bearing phases included brucite (Mg(OH)2), periclase (MgO) and pyroaurite (Mg6 Fe2CO3(OH)16·4H2O), with weight fractions of 4.8%, 3.2% and 25.6%, respectively. Of these, srebrodolskite, brucite and periclase were minerals with high reactivity in the carbonation reaction. 29 Coal fired power plants are usually operated with a slight excess of air over the required quantity for the complete combustion of coal. Thus, the flue gas generally contains H2O, O2, CO2 and SO3. The considerable fractions of calcite and gypsum were formed by the reaction between CaO and CO2 and SO3 due to rapid cooling of flue gases after the combustion.

After carbonation, two newly formed carbonates were identified in carbonated samples including aragonite (CaCO3) (2.6%) and Mg-calcite (MgCO3) (10.5%). The significantly increased peak 11


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intensity of calcite after carbonation suggests the formation of calcite in the carbonation reaction. The disappearance of phases of brucite and periclase indicates that they reacted with CO2 and produced Mg-calcite during the carbonation reaction. Moreover, the peak intensity of gypsum and basanite reduced after carbonation. We speculated that part of gypsum and basanite could react with CO2 and be converted into calcite or aragonite.

Table 2 The relative concentrations (wt.%) of the phases identified in the fresh and carbonated fly ash Id.

Chemical formula

Fresh ash

Carbonated ash

Quartz

SiO2

1.3

1.0

Augite

(Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6

2.2

5.3

Srebrodolskite

Ca2Fe2O5

3.0

2.2

Gypsum

CaSO4·2H2O

3.0

0.4

Bassanite

CaSO4·0.5H2O

10.7

4.8

Calcite

CaCO3

23.0

36.8

Aragonite

CaCO3

-

2.6

Mg-calcite

MgCO3

-

10.5

Brucite

Mg(OH)2

4.8

-

Periclase

MgO

3.2

-

Hexahydrite

MgSO4·6H2O

-

2.1

Pyroaurite

Mg6 Fe2CO3(OH)16·4H2O

25.6

11.6

Amorphous

-

23.2

22.4

12


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Figure 1: X-ray diffraction patterns of fresh and carbonated ash

3.3 Kinetics of the carbonation reaction 3.3.1 Effects of operation parameters on the carbonation reaction Figure 2 (a) to (c) present the carbonation efficiency of fly ash in the carbonation reaction as a function of time at different temperatures (40, 50, 60 and 70 °C), stirring rates (900, 1050, 1200 and 1350 rpm) and CO2 pressures (3, 4, 5, 6 and 7 bar), respectively. Figure 2 (d) shows the carbonation efficiency of fly ash in the carbonation reaction as a function of time with the introduction of 0.5 mol/L NaHCO3

26

at 40, 50 and 60 °C. The carbonation efficiency in Figure (a) to (d) increased

rapidly in the first 2 h and reached a maximum value after 8 h reaction. Around 70% of the maximum carbonation efficiency was achieved in 2 h. This might be attributed to the formation of a precipitate layer at the early stage of the carbonation reaction, which hindered the reactant inside the particles

13


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from diffusion and reaction.

32

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The carbonation rate, which can be calculated from the slope of the

carbonation efficiency curve, decreased as the reaction time elapsed.

In Figure 2 (a), the carbonation efficiency within 2 h at different temperature increased in the following order: 60 °C ＞ 70 °C ＞ 50 °C ＞ 40 °C. But the carbonation efficiency after 2 h displayed a reverse trend that the carbonation efficiency decreased with the elevated temperature from 40 °C to 70 °C. This is due to the complex effects of temperature on the carbonation reaction. Previous studies indicate that elevated temperatures increased the reaction rate by improving the mass transfer rate, promoting the thermal motion of molecules and increasing their average kinetic energy which helped speed up the carbonation reaction.

26

Raising the reaction temperature also reduced the

solubility of carbon dioxide in the solution. The equilibrium concentration of dissolved CO2 in solution follows Henry’s law: @L IJN B = ab /K d

(12)

Where is the CO2 partial pressure, which was kept constant in the experiments of Figure 2 (a); and KH (bar/mol/L) is the Henry’s law constant, which increases with elevated temperature. The concentration of the carbonate ion in the solution can be evaluated by equation (13). The detailed derivation of equation (13) can be found in the literature. 33

log@IJNV B = log K K g P /K i + 2pH

(13) 33

Given that Ka1, Ka2, KH and pH values were functions of temperature, and increased with elevated temperatures,

34, 35

the overall impact of elevated temperatures was to increase the concentrations of

carbonate ions in the solution slightly, which benefited the carbonation reaction. This result is 14


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consistent with findings in the literature.

34

On the other hand, elevated temperatures also influenced

the precipitation of the product. The precipitation of the CaCO3 and MgCO3 products was related to the solubility product constant of Ca/Mg-carbonate, Ksp:

K l = @IOW /mKW B × @IJNV B (14)

where [Ca2+/Mg2+] is the concentration of calcium or magnesium ions, and [CO32-] is the concentration of carbonate ions. It is well known that elevated temperatures lower the Ksp of Ca/Mg-calcite, which benefits the precipitation of Ca/Mg-carbonate.

14, 26

Ca/Mg-carbonate product also decreased with the increase in temperature.

The solubility of the 34

More newly formed

precipitates might deposit on the surface of the fly ash particles at higher temperatures than that at lower temperatures, which would hinder the reaction of the reactant inside the particles. This explains the lower maximum carbonation efficiency at 70 °C than at 40 °C, as observed in Figure 2 (a). Thus, it can be concluded that the enhanced mass transfer rate at elevated temperatures might have a dominating effect on the carbonation reaction in the first 2 h of the carbonation reaction, while the fast precipitation of the Ca/Mg-carbonate product at elevated temperatures lowered the carbonation rate thereafter. Due to the rapid carbonation reaction at elevated temperatures, the fly ash particles were quickly covered by the rapidly formed product layer, which resulted in a low maximum carbonation efficiency at elevated temperatures.

In Figure 2 (b), the carbonation efficiency of the carbonation reaction increased as the stirring rate increased from 900 to1050 rpm. A further increase in the stirring rate to 1350 rpm lead to a reduction in the carbonation efficiency. The stirring rate of 1050 rpm provided the most rapid carbonation reaction, although the maximum carbonation efficiencies at different stirring rates were in fact very 15


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similar. Similar results have been reported in the direct aqueous carbonation of steel slag, 32 in which the carbonation reaction was promoted by elevating the stirring rate from 500 to 1000 rpm and 1000rpm provided the highest carbonation efficiency.

In Figure 2 (c), as the initial CO2 pressure increased from 3 to 7 bar, both the carbonation rate and the maximum carbonation efficiency increased, indicating the significant impact of CO2 pressure on the carbonation reaction. Similar results were observed in the study conducted by Ukwattage et al.,

20

in

which fly ash was subjected to aqueous carbonation, with the CO2 pressure ranging from 20 to 60 bar. It is well known that increased CO2 pressure lowers the pH value, and this would help to leach calcium and magnesium from the fly ash particles. According to Henry’s law (equation (12)), increasing CO2 pressure also increases the CO2 solubility in the solution, and thus increases the carbonate ions available for the carbonation reaction.

Since elevating the CO2 pressure was highly energy intensive, 0.5 mol/L NaHCO3 was introduced into the solution to accelerate the carbonation. The introduction of NaHCO3 increases the concentration of the carbonate ion (CO32-) in the aqueous system. Figure 2 (d) shows that the addition of NaHCO3 can significantly increase the carbonation efficiency at 40, 50 and 60 °C. Compared with the carbonation without additive, the maximum carbonation efficiency in the presence of 0.5 mol/L NaHCO3 was improved from 27.7% to 33.4%, with the CO2 sequestration capacity improved from 71.0 g-CO2/kg-FA to 85.6 g-CO2/kg-FA under the same operating conditions.

16


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17


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Figure 2 Model fitting results of carbonation experiments at (a) various temperatures (30, 40, 50, 60 and 70 °C) (stirring rate, 900 rpm; CO2 pressure, 4 bar); (b) various stirring rates (900, 1050, 1200 and 1350 rpm) (temperature, 40 °C; CO2 pressure, 4 bar); (c) various CO2 pressures (3, 4, 5, 6 and 7 18


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bar) (temperature, 40 °C; stirring rate, 900 rpm); and (d) various temperatures (40, 50 and 60 °C) (stirring rate, 900 rpm; CO2 pressure, 4 bar; additive, 0.5 mol/L NaHCO3)

3.3.2 Kinetic modelling In order to determine the kinetic mechanisms and identify the rate controlling step of the carbonation reaction of fly ash, 10 most commonly used kinetic models in solid-liquid reactions to the experimental data by the integral analysis method

38

36, 37

were fitted

. The multiple regression correlation

coefficients (R2) are shown in Table 3. None of the equations could well describe the carbonation reaction as the R2 value was less than 0.90 for the 10 models. Thus, a more accurate model that could physically describe the carbonation kinetics was required.

Table 3 Data fitting of models No. 1

Mechanism

kt=(1−δ)−1/2−1

Three-halves-order kinetics

0.7779

Second-order kinetics

0.7929

Three-dimensional advance of the reaction interface

0.7518

Jander; three-dimensional

0.8710

−1

2

kt=(1−δ) −1

3

1/3

kt=1−(1−δ)

1/3

4

kt=[1−(1−δ) ]2

5

kt=1−2δ/3−(1−δ)2/3

6

kt=[1−(1−δ)1/2]2 1/3

7

kt=1/(1−δ) −1

8

kt=1−3(1−δ)2/3+2(1−δ)

9

kt=(1−δ)Ln(1−δ)+δ

10

R2

Equations

kt=(−ln(1−δ))

1/3

Crank-Ginstling-Brounshtein, mass transfer across a nonporous product layer

0.8646

Jander; cylindrical diffusion

0.8663

Dickinson, Heal, transfer across the contacting area

0.7728

Shrinking core, product layer (different form of Crank-Ginstling-Brounshtein)

0.8646

Two-dimensional diffusion

0.8612

Avrami-Erofeev; three-dimensional

0.5994

The surface coverage model was originally developed

39

to simulate the reaction of Ca(OH)2 with

SO2/CO2 at low temperatures. Previous study indicated that the kinetic data of the carbonation of steel slag could be fitted by the surface coverage model with R2 ranging from 0.98 to 0.99. The model 19


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assumed that the precipitates formed in a carbonation reaction would coat the active site of the surface of the reactant particles, which would hinder the reactant inside the particles from diffusion and reaction. The fraction of the active/un-covered surface sites (Φ) changed along with the reaction time and depended on the reaction rate. The reaction would thus reach a maximum carbonation efficiency (δmax). The rate of the carbonation efficiency (δ) and the fraction of the active/un-covered surface sites (Φ) can be described by equations (15) and (16), respectively,

no np

−

= q> m ∙ r = q> m ∙ s t (15)

nu np

= sl t vVg ∙ r = sl s t v (16)

where Sg (m2/g) is the initial specific surface area of the fly ash particles, M (g/mole) is the weight of fly ash per mole of the reactive species, rs (mole/min/m2) is the carbonation rate per initial surface area of the fly ash, ks (mole/min/m2) is the rate constant, and kp (m2/mole) is a proportional constant reflecting the fraction of the surface that is reactive and not covered by the reaction product. These two kinetics rate equations can be further simplified by assuming k1 (min−1) and k2 (dimensionless) as shown in equation (17) and (18), respectively. According to the definitions of k1 and k2, they are functions of the solid specific surface area, temperature and concentration of CO2.

sg = s q> m (17)

s =

wx yz {

(18)

20


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Energy & Fuels

By substituting equation (17) and (18) into equation (15) and (16), the integration of equation (15) can be used to describe the relationship between the carbonation efficiency and reaction time in terms of k1 and k2 as shown in equation (19). 32

? = @1 − exp(−sg s `)B/s

(19)

The two parameters, k1 and k2, in equation (19) can be obtained accordingly by least-squares fitting of the equation to the experimental data. If the carbonation efficiency of the fly ash reached a maximum value after a long time of carbonation, the exponential term (exp (−k1k2t)) in equation (19) approaches zero, and the carbonation efficiency of the fly ash can be simplified as a constant value of k2. Therefore, equation (19) could also be described as equation (20). 40

? = ?~ 1 − exp−s sl ` (20)

The reaction kinetics of the carbonation of fly ash in a vessel reactor was studied by the use of the surface coverage model as described in equations (15) to (20). Figure 2 (a) to (d) present the model fitting results of the carbonation of fly ash at the experimental conditions. The experimental data are highly correlated with the surface coverage model, with R2 values more than 0.98 (Table 4), which indicates that the carbonation reaction of fly ash in a vessel reactor can be physically expressed by the model. Table 4 also presents the values of ks, kp, k1, and k2 for the carbonation experiments. The values of k1 and k2 were obtained by least-squares fitting of equations (19) and (20) to the experimental results. Based on the content of CaO and MgO in the fly ash (Table 1), the value of M is estimated to be 89.0 g-FA/mole according to the assumption of the surface coverage model. The measured value of Sg was 30.29 m2/g (Table 1). ks and kp were calculated by equation (17) and 21


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equation (18). ks is the overall rate constant as expressed in equation (15). It is affected by several factors, such as the rate of CO2 dissolving into the solution, the rate of calcium ions leaching from the solid matrix into solution, the rate of calcium carbonate precipitation, and the concentration of calcium ions and CO2 in the solution. 39

In Table 4, ks increased as the temperature and CO2 pressure were elevated, and increased as the stirring rate increased from 900 to1050 rpm. A further increase in the stirring rate led to a reduction in ks. The stirring rate of 1050 rpm provided the largest ks. The fact that ks is affected by the operating conditions such as temperature, CO2 pressure and stirring rate, is consistent with the trend of the carbonation rate, indicating that ks reflected the carbonation rate. kp is a proportional constant, which is a function of temperature and the concentrations of the reacting species. 39 In Table 4, kp increased with elevated temperatures, and decreased with elevated CO2 pressures, which is converse to the trend of maximum carbonation efficiency. It was nearly constant as the stirring rate increased from 900 to 1350 rpm. Thus, kp reflected the fraction of the surface area of fly ash particles covered by the precipitates.

According to the assumptions of the surface coverage model, the active surface site of the fly ash would be gradually covered by the newly formed precipitates (CaCO3/MgCO3) during the carbonation reaction. Once the product layer covered the surface of fly ash particles, the diffusion of reactants through the product layer would be hindered and become the rate-limiting step of carbonation. This action would hinder the fly ash particles from further carbonation, resulting in a maximum carbonation efficiency. Also, the porous structure of the fly ash particles would be filled by the product during carbonation, which would further increase the resistance of the reactant diffusion. The 22


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above phenomena were confirmed by the morphology changes of the fresh and carbonated fly ash (Figures 4 and 5).

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Table 4 Kinetic parameters of the surface coverage model at various operation conditions Operation conditions Stirring

Carbonation efficiency

Model parameters

Temp.

PCO2

Additive

Exp.

Model

ks (×10−6)

kp (×102)

k1 (×10−3)

k2 (×10−2)

ks*kp (×10−4)

R2

rpm

°C

bar

mol/L

%

%

mole/s/m2

m2/mol

s−1

-

min−1

-

1

900

40

4

-

27.7

29.0 ±0.087

1.773 ±0.018

0.929 ±0.002

4.780 ±0.042

3.447 ±0.010

1.649 ±0.018

0.9915

2

900

50

4

-

26.3

26.4 ±0.076

1.866 ±0.020

1.023 ±0.002

5.030 ±0.049

3.793 ±0.011

1.909 ±0.022

0.9894

3

900

60

4

-

24.6

24.6 ±0.037

2.356 ±0.019

1.097 ±0.002

6.350 ±0.042

4.070 ±0.006

2.586 ±0.019

0.9944

4

900

70

4

-

23.1

23.3 ±0.044

2.170 ±0.019

1.158 ±0.001

5.850 ±0.047

4.296 ±0.008

2.515 ±0.023

0.9916

No.

rate

5

900

40

4

-

27.1

26.7 ±0.060

1.743±0.016

1.008 ±0.003

4.700 ±0.033

3.739 ±0.008

1.757 ±0.015

0.9942

6

1050

40

4

-

27.4

26.6 ±0.043

2.311 ±0.018

1.012 ±0.003

6.230 ±0.041

3.753 ±0.006

2.339 ±0.018

0.9947

7

1200

40

4

-

27.3

27.0 ±0.071

1.714 ±0.016

0.998 ±0.002

4.620 ±0.037

3.703 ±0.010

1.711 ±0.017

0.9927

8

1350

40

4

-

27.4

26.8 ±0.050

1.910 ±0.018

1.006 ±0.002

5.150 ±0.033

3.731 ±0.007

1.920 ±0.015

0.9952

9

900

40

3

-

23.1

23.1 ±0.075

1.213 ±0.012

1.166 ±0.002

3.270 ±0.026

4.327 ±0.014

1.413 ±0.015

0.9929

10

900

40

4

-

27.1

26.7 ±0.059

1.751 ±0.015

1.009 ±0.002

4.720 ±0.033

3.741 ±0.008

1.765 ±0.015

0.9945

11

900

40

5

-

28.8

28.4 ±0.062

2.211 ±0.014

0.948 ±0.003

5.960 ±0.047

3.518 ±0.008

2.097 ±0.019

0.9927

12

900

40

6

-

30.8

31.2 ±0.067

2.511 ±0.012

0.864 ±0.002

6.770 ±0.055

3.205 ±0.007

2.170 ±0.020

0.9923

13

900

40

7

-

32.2

32.1 ±0.071

2.437 ±0.010

0.840 ±0.004

6.570 ±0.052

3.115 ±0.007

2.047 ±0.019

0.9926

14

900

40

4

0.5

33.4

31.9 ±0.053

2.819 ±0.024

0.845 ±0.002

7.600 ±0.052

3.133 ±0.005

2.381 ±0.018

0.9940

15

900

50

4

0.5

31.0

29.1 ±0.055

2.901 ±0.029

0.926 ±0.002

7.820 ±0.066

3.436 ±0.006

2.686 ±0.025

0.9907

16

900

60

4

0.5

28.0

26.8 ±0.053

2.986 ±0.029

1.007 ±0.002

8.050 ±0.078

3.735 ±0.007

3.006 ±0.032

0.9877


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Figure 3 Comparison of the predicted carbonation efficiency of fly ash by the surface coverage model with experimental carbonation efficiency

Figure 3 presents the comparison of the predicted carbonation efficiencies by the surface coverage model with experimental measurements in the vessel reactor with an error of less than ±10%, suggesting that carbonation efficiency of the fly ash operated under various conditions could be well predicted by the surface coverage model.

3.4 Morphology characterization of the fresh and carbonated fly ash 3.4.1 Particle size, surface area and porosity Figure 4 shows the particle size distribution, surface area and porosity of the fly ash before and after carbonation. The fresh ash displayed a unimodal trend and consisted of a major particle population, with grain sizes in the range of 5-20 µm. The number of medium size (5-20 µm) particles was reduced by carbonation, while larger (≥20 µm) and finer (≤5 µm) particles displayed an opposite trend.

41

As

the temperature decreased, a further reduction in the number of medium particles and an increase in finer and larger particles was observed. This result indicates a strong relationship between the particle size change and the carbonation efficiency achieved. The increased particle size might be attributed to the newly formed precipitates that covered the fly ash particles.

41, 42

The surface area and total pore 25


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volume of the fly ash increased slightly after the carbonation at 40 °C, but decreased significantly after the carbonation at 70 °C. This result indicates that the active surface of the fly ash particles was more easily deposited at 70 °C than 40 °C, consistent with the discussion in Sections 3.3.1 and 3.3.2. The increase in the average pore size of the particles indicates that the precipitates formed in the carbonation reaction probably filled the small pores. 30

Figure 4 Particle size distribution of the fresh and carbonated fly ash (stirring rate, 900 rpm; CO2 pressure, 4 bar; temperature, 40, 50, 60 and 70 °C) fly ash

3.4.2 SEM-EDS Particles within the fresh and carbonated fly ash samples have irregular shapes, with a porous and uneven surface (Figure 5 (a) to (d)). Compared with the fresh sample, it was clearly visible that the newly formed precipitates have covered the surface of the unreacted fly ash particles. With a semi-quantitative chemical analysis by EDS, it is possible to identify the elemental composition of the precipitates and the unreacted ash particles. The EDS analysis as shown in the supporting information (Figure S1) suggests that Ca−Mg−C−O surrounded the surface of the unreacted fly ash particles, 26


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which were mainly composed of a Ca−Mg−Fe−O phase. Since the XRD results (Table 2 and Figure 1) already indicated the newly formed Ca/Mg calcite in the carbonated sample, the Ca−Mg−C−O phases in Figure 5 (b) to (d) likely presents as Ca/Mg carbonates. The growth of newly formed crystals on the external particle surface increased the mean particle diameter as a function of carbonation efficiency. In addition, parts of the magnesium product precipitated as isolated and separated fine particles with a size of approximately 3 µm. Figures 5 (b) to (d) also show that the precipitates contained not only calcium carbonates, but also magnesium carbonates, consistent with the XRD results. The SEM results confirmed the reaction mechanism proposed in Sections 3.3.1 and 3.3.2. The calcium and magnesium diffusion would be impeded by the formation of the precipitate layer, which would reduce the reactive surface of the ash particles and fill the small pores. The results were in good agreement with the findings reported in the literature,

23, 32

where the carbonation

reaction resulted in a reacted solid with a lower porosity, tortuosity, and pore area, due to the calcite precipitation formed on the surface of the carbonated solid.

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Figure 5 SEM images of (a) fresh fly ash, and (b) to (d) carbonated fly ash (temperature, 40ºC; stirring rate, 900 rpm; CO2 pressure, 4bar)

4 Conclusions In this study, the carbonation experiments of fly ash from Victorian brown coal were conducted in a vessel reactor at various temperatures (40, 50, 60, and 70 °C), stirring rates (900, 1050, 1200 and 1350 rpm) and CO2 pressures (3, 4, 5, 6 and 7 bar). The kinetics of the carbonation reaction were investigated by a wide range of models. The results show that both the carbonation rate and the maximum carbonation efficiency can be improved by optimizing parameters and by the introduction of NaHCO3. In particular, the complex effects of temperature on the carbonation rate and the maximum carbonation efficiency were clearly discussed. Particularly, the enhanced mass transfer rate at elevated temperatures might have a dominating effect on the carbonation rate in the first 2 h of the carbonation reaction, while the fast precipitation of the Ca/Mg-carbonate product at elevated 28


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temperatures lowered the carbonation rate thereafter. Due to the large carbonation rate at elevated temperatures, the fly ash particles were quickly covered by the rapidly formed product layer, which resulted in a low maximum carbonation efficiency at elevated temperatures. The kinetic data can be well fitted by the surface coverage model with the R2≥0.98, indicating that the carbonation of fly ash can be physically expressed by this model. The maximum carbonation efficiency of fly ash could also be well predicted by the model. In addition, the assumed mechanisms of the carbonation reaction were confirmed by morphology changes of the fly ash samples before and after carbonation. The newly formed precipitates were not only deposited on the active surface, but also filled the pores of the fly ash particles.

Acknowledgments Long Ji is grateful to Macquarie University for the Cotutelle-iMQRES scholarship, to China University of Mining & Technology (Beijing) for funding from the ‘Creating Outstanding Innovative Talent Project’, and to CSIRO Energy for the opportunity to work in their laboratories and use of resources.

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10. Sun, Y.; Parikh, V.; Zhang, L. J. Hazard. Mater. 2012, 209-210, 458-466. 11. Hosseini, T.; Selomulya, C.; Haque, N.; Zhang, L. Energ. Fuel 2014, 28, 6481-6493. 12. Hosseini, T.; Selomulya, C.; Haque, N.; Zhang, L. Energ. Fuel 2015, 29, 7483-7496. 13. Wee, J.-H. Appl. Energ. 2013, 106, 143-151. 14. Tamilselvi Dananjayan, R. R.; Kandasamy, P.; Andimuthu, R. J. Clean. Prod. 2016, 112, 4173-4182. 15. Siriruang, C.; Toochinda, P.; Julnipitawong, P.; Tangtermsirikul, S. J. Environ. Manage. 2016, 170, 70-78. 16. Mazzella, A.; Errico, M.; Spiga, D. J. Environ. Chem. Eng. 2016, 4, 4120-4128. 17. Mayoral, M. C.; Andrés, J. M.; Gimeno, M. P. Fuel 2013, 106, 448-454. 18. Grace, M. N.; Wilson, G. M.; Leslie, P. F. J. Environ. Sci. Health, Part A: Toxic/Hazard. Subst. Environ. Eng. 2012, 47, 245-259. 19. Ho, Y. S., McKay, G. Process Biochem. 1999, 34, 451-465. 20. Ukwattage, N. L.; Ranjith, P. G.; Yellishetty, M.; Bui, H. H.; Xu, T. J. Clean. Prod. 2015, 103, 665-674. 30


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Supporting information

33


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34


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Figure S1 EDS patterns for the spots of carbonated sample in Figure 5

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Figure S1 shows the EDS patterns of the 12 spots of the carbonated sample in Figure 5 (temperature, 40ºC; stirring rate, 900 rpm; CO2 pressure, 4bar). With the semi-quantitative chemical analysis provided by EDS, it is possible to identify the elemental composition of the precipitates and the unreacted ash particles.

36


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List of figures

Figure 1: X-ray diffraction patterns of fresh and carbonated ash

37


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Figure 2 Model fitting results of carbonation experiments at (a) various temperatures (30, 40, 50, 60 and 70 °C) (stirring rate, 900 rpm; CO2 pressure, 4 bar); (b) various stirring rates (900, 1050, 1200 and 1350 rpm) (temperature, 40 °C; CO2 pressure, 4 bar); (c) various CO2 pressures (3, 4, 5, 6 and 7 39


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bar) (temperature, 40 °C; stirring rate, 900 rpm); and (d) various temperatures (40, 50 and 60 °C) (stirring rate, 900 rpm; CO2 pressure, 4 bar; additive, 0.5 mol/L NaHCO3)

Figure 3 Comparison of the predicted carbonation efficiency of fly ash by the surface coverage model

Figure 4 Particle size distribution of the fresh and carbonated fly ash (stirring rate, 900 rpm; CO2 pressure, 4 bar; temperature, 40, 50, 60 and 70 °C) fly ash

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“

Figure 5 SEM images of (a) fresh fly ash, and (b) to (d) carbonated fly ash (temperature, 40ºC; stirring rate, 900 rpm; CO2 pressure, 4bar)

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List of tables Table 1 Summary of physicochemical properties of fly ash

Physical properties

Chemical properties

Items

Unit

Fresh ash

D[4,3]

µm

8.84

D[3,2]

µm

6.03

D10

µm

3.51

D50

µm

8.40

D90

µm

14.66

BET surface area

m2/g

30.29

SiO2

wt%

5.8

Al2O3

wt %

3.0

Fe2O3

wt%

14.0

CaO

wt%

32.4

MgO

wt%

29.3

TiO2

wt%

0.7

Na2O

wt%

0.2

K2O

wt%

0.2

P2O5

wt%

0.4

MnO

wt%

0.7

SO3

wt%

12.8

CO2 (Titration)

g-CO2/g-FA

0.166

Theoretical CO2 sequestration capacity

g-CO2/g-FA

256.4

Table 2 The relative concentrations (wt.%) of the phases identified in the fresh and carbonated fly ash Id.

Chemical formula

Fresh ash

Carbonated ash

Quartz

SiO2

1.3

1.0

Augite

(Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6

2.2

5.3

Srebrodolskite

Ca2Fe2O5

3.0

2.2

Gypsum

CaSO4·2H2O

3.0

0.4

Bassanite

CaSO4·0.5H2O

10.7

4.8

Calcite

CaCO3

23.0

36.8

Aragonite

CaCO3

-

2.6

Mg-calcite

MgCO3

-

10.5

Brucite

Mg(OH)2

4.8

-

Periclase

MgO

3.2

-

Hexahydrite

MgSO4·6H2O

-

2.1

Pyroaurite

Mg6 Fe2CO3(OH)16·4H2O

25.6

11.6

Amorphous

-

23.2

22.4

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Table 3 Data fitting of models No. 1

Mechanism

kt=(1−δ)−1/2−1

Three-halves-order kinetics

0.7779

Second-order kinetics

0.7929

Three-dimensional advance of the reaction interface

0.7518

Jander; three-dimensional

0.8710

−1

2

kt=(1−δ) −1

3

1/3

kt=1−(1−δ)

1/3

4

kt=[1−(1−δ) ]2

5

kt=1−2δ/3−(1−δ)2/3

6

kt=[1−(1−δ)1/2]2 1/3

7

kt=1/(1−δ) −1

8

kt=1−3(1−δ)2/3+2(1−δ)

9

kt=(1−δ)Ln(1−δ)+δ

10

R2

Equations

kt=(−ln(1−δ))

1/3

Crank-Ginstling-Brounshtein, mass transfer across a nonporous product layer

0.8646

Jander; cylindrical diffusion

0.8663

Dickinson, Heal, transfer across the contacting area

0.7728

Shrinking core, product layer (different form of Crank-Ginstling-Brounshtein)

0.8646

Two-dimensional diffusion

0.8612

Avrami-Erofeev; three-dimensional

0.5994

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Table 4 Kinetic parameters of the surface coverage model at various operation conditions Operation conditions Stirring

Carbonation efficiency

Model parameters

Temp.

PCO2

Additive

Exp.

Model

ks (×10−6)

kp (×102)

k1 (×10−3)

k2 (×10−2)

ks*kp (×10−4)

R2

rpm

°C

bar

mol/L

%

%

mole/s/m2

m2/mol

s−1

-

min−1

-

1

900

40

4

-

27.7

29.0 ±0.087

1.773 ±0.018

0.929 ±0.002

4.780 ±0.042

3.447 ±0.010

1.649 ±0.018

0.9915

2

900

50

4

-

26.3

26.4 ±0.076

1.866 ±0.020

1.023 ±0.002

5.030 ±0.049

3.793 ±0.011

1.909 ±0.022

0.9894

3

900

60

4

-

24.6

24.6 ±0.037

2.356 ±0.019

1.097 ±0.002

6.350 ±0.042

4.070 ±0.006

2.586 ±0.019

0.9944

4

900

70

4

-

23.1

23.3 ±0.044

2.170 ±0.019

1.158 ±0.001

5.850 ±0.047

4.296 ±0.008

2.515 ±0.023

0.9916

No.

rate

5

900

40

4

-

27.1

26.7 ±0.060

1.743±0.016

1.008 ±0.003

4.700 ±0.033

3.739 ±0.008

1.757 ±0.015

0.9942

6

1050

40

4

-

27.4

26.6 ±0.043

2.311 ±0.018

1.012 ±0.003

6.230 ±0.041

3.753 ±0.006

2.339 ±0.018

0.9947

7

1200

40

4

-

27.3

27.0 ±0.071

1.714 ±0.016

0.998 ±0.002

4.620 ±0.037

3.703 ±0.010

1.711 ±0.017

0.9927

8

1350

40

4

-

27.4

26.8 ±0.050

1.910 ±0.018

1.006 ±0.002

5.150 ±0.033

3.731 ±0.007

1.920 ±0.015

0.9952

9

900

40

3

-

23.1

23.1 ±0.075

1.213 ±0.012

1.166 ±0.002

3.270 ±0.026

4.327 ±0.014

1.413 ±0.015

0.9929

10

900

40

4

-

27.1

26.7 ±0.059

1.751 ±0.015

1.009 ±0.002

4.720 ±0.033

3.741 ±0.008

1.765 ±0.015

0.9945

11

900

40

5

-

28.8

28.4 ±0.062

2.211 ±0.014

0.948 ±0.003

5.960 ±0.047

3.518 ±0.008

2.097 ±0.019

0.9927

12

900

40

6

-

30.8

31.2 ±0.067

2.511 ±0.012

0.864 ±0.002

6.770 ±0.055

3.205 ±0.007

2.170 ±0.020

0.9923

13

900

40

7

-

32.2

32.1 ±0.071

2.437 ±0.010

0.840 ±0.004

6.570 ±0.052

3.115 ±0.007

2.047 ±0.019

0.9926

14

900

40

4

0.5

33.4

31.9 ±0.053

2.819 ±0.024

0.845 ±0.002

7.600 ±0.052

3.133 ±0.005

2.381 ±0.018

0.9940

15

900

50

4

0.5

31.0

29.1 ±0.055

2.901 ±0.029

0.926 ±0.002

7.820 ±0.066

3.436 ±0.006

2.686 ±0.025

0.9907

16

900

60

4

0.5

28.0

26.8 ±0.053

2.986 ±0.029

1.007 ±0.002

8.050 ±0.078

3.735 ±0.007

3.006 ±0.032

0.9877


Insights into Carbonation Kinetics of Fly Ash from ... - ACS Publications

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