Scholarly article on topic 'The CO2 -binding by Ca-Mg-silicates in direct aqueous carbonation of oil shale ash and steel slag'

The CO2 -binding by Ca-Mg-silicates in direct aqueous carbonation of oil shale ash and steel slag Academic research paper on "Chemical sciences"

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Abstract of research paper on Chemical sciences, author of scientific article — Mai Uibu, Rein Kuusik, Lale Andreas, Kalle Kirsimäe

Abstract This study was focused on carbonation of waste materials having low water-solubility in which Ca and Mg are generally bound as silicates. Here, pulverized firing oil shale ash (PFA from Narva Power Plants, Estonia), electric arc furnace slag (EAFS, types 1 and 2 from Uddeholm Tooling, Sweden) and ladle slag (LS from Uddeholm Tooling, Sweden) were studied as sorbents for binding CO2 from flue gases in direct aqueous mineral carbonation process. The experiments were carried out at room temperature and atmospheric pressure. Results showed that Ca-Mg-silicate phases bound up to 9 g of CO2 per 100 g of initial ash, which formed 30% of the total CO2 bound in direct aqueous carbonation of PFA. The CO2 uptakes for steel slags (EAFS1, EAFS2 and LS) were 8.7 g CO2/100 g EAFS1, 1.9 g CO2/100 gEAFS2 and 4.6 g/100 g LS. Quantitative XRD analysis indicated that Ca2SiO4 and Ca3Mg(SiO4)2 were the main CO2 binding low solubility components of oil shale ash as well as steel slags. The main carbonation product was calcite (CaCO3), indicating that Mg-compounds were not reactive towards CO2 at these mild conditions. Based on multifaceted studies on carbonation of oil shale ash, a new method for eliminating CO2 from flue gases by Ca-containing waste material was proposed. The process includes contacting the aqueous suspensions of Ca-containing waste material with CO2 containing flue gas in two steps: in the first step the suspension is bubbled with flue gas keeping the pH levels in the range of 10–12 and in the second step keeping the pH levels in the range of 7–8. The water-soluble components such as free lime are carbonated in the first step and the components of low solubility, in which Ca is generally contained in the form of silicates, are carbonated in the second step.

Academic research paper on topic "The CO2 -binding by Ca-Mg-silicates in direct aqueous carbonation of oil shale ash and steel slag"

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Energy Procedía 4 (2011) 925-932

Energy Procedía

www.elsevier.com/locate/procedia

GHGT-10

The CO2-binding by Ca-Mg-silicates in direct aqueous carbonation

of oil shale ash and steel slag

Mai Uibua1*, Rein Kuusika, Lale Andreasb, Kalle Kirsimaec

aLaboratory of Inorganic Materials, Tallinn University of Technology, 5 Ehitajate S.,Tallinn 19086, Estonia bDivision of Waste Science &Technology, Lulea University of Technology, SE 971 87 Lulea, Sweden cDepartment of Geology, University of Tartu, 14A Ravila St., 51014 Tartu, Estonia

Abstract

This study was focused on carbonation of waste materials having low water-solubility in which Ca and Mg are generally bound as silicates. Here, pulverized firing oil shale ash (PFA from Narva Power Plants, Estonia), electric arc furnace slag (EAFS, types 1 and 2 from Uddeholm Tooling, Sweden) and ladle slag (LS from Uddeholm Tooling, Sweden) were studied as sorbents for binding CO2 from flue gases in direct aqueous mineral carbonation process. The experiments were carried out at room temperature and atmospheric pressure.

Results showed that Ca-Mg-silicate phases bound up to 9 g of CO2 per 100 g of initial ash, which formed 30% of the total CO2 bound in direct aqueous carbonation of PFA. The CO2 uptakes for steel slags (EAFS1, EAFS2 and LS) were 8.7g CO2/100 g EAFS1, 1.9 g CO2/100 gEAFS2 and 4.6 g/100g LS. Quantitative XRD analysis indicated that Ca2SiO4 and Ca3Mg(SiO4)2 were the main CO2 binding low solubility components of oil shale ash as well as steel slags. The main carbonation product was calcite (CaCO3), indicating that Mg-compounds were not reactive towards CO2 at these mild conditions.

Based on multifaceted studies on carbonation of oil shale ash, a new method for eliminating CO2 from flue gases by Ca-containing waste material was proposed. The process includes contacting the aqueous suspensions of Ca-containing waste material with CO2 containing flue gas in two steps: in the first step the suspension is bubbled with flue gas keeping the pH levels in the range of 10-12 and in the second step keeping the pH levels in the range of 7-8. The water-soluble components such as free lime are carbonated in the first step and the components of low solubility, in which Ca is generally contained in the form of silicates, are carbonated in the second step. © 2011 Published by Elsevier Ltd.

Keywords: PF ash; EAF slag; ladle slag; aqueous carbonation; quantitative XRD analysis

1. Introduction

Atmospheric emissions of CO2 originating from the fossil fuels based heat and power production is a serious problem worldwide. Fixation of CO2 in the thermodynamically stable form of inorganic carbonates, also known as mineral carbonation is a prospective option for CO2 storage [1]. Although the CO2 storage capacity of the natural

* Corresponding author. Tel.: +372-620-2812; fax: +372-620-2801. E-mail address: maiuibu@staff.ttu.ee.

doi:10.1016/j.egypro.2011.01.138

Ca-Mg-silicate minerals is sufficient to fix the CO2 emitted from the combustion of the fossil fuels, the technological carbonation of these minerals (for instance serpentinite, olivine) is slow and energy demanding. One way to evade some of the negative aspects of the technological carbonation of natural minerals is to utilize some alkaline waste residues (ashes from coal- and oil shale-fired power plants [2-5], steel slags [6-8], MSWI ashes [9, 10], APC residues [11], etc) as CO2 sorbents. These materials are often associated with CO2 point source emissions and tend to be chemically more active than geologically derived minerals. Consequently, they require not as much of pre-treatment and less energy-intensive operating conditions to achieve sufficient carbonation rates [6].

Combustion of low-grade carbonaceous fossil fuel oil shale is characterized by high specific carbon emissions as well as huge amounts of waste ash. Carbonation of oil shale ash has previously been investigated in the context of its relatively high content of free lime (up to 30% depending on combustion technology). In addition to free lime, PFA also contains up to 30% of Ca-Mg-silicates (CaSiO3, Ca2SiO4, Ca3Mg(SiO4)2) as potential CO2 binders [12]. Previous experiments with synthetic model compounds (CaSiO3, Ca2SiO4, Ca3Mg(SiO4)2 and (Ca,Na)2(Mg,Al)(Si,Al)3O7) have shown that Ca-silicates displayed a good CO2-binding efficiency under mild operating conditions (atmospheric pressure and room temperature): CaSiO3 reached up to 88.7% and Ca2SiO4 up to 76.4% of their theoretical CO2-binding potential. The CO2-binding ability of Ca3Mg(SiO4)2 was considerably lower and (Ca,Na)2(Mg,Al)(Si,Al)3O7 was not active toward CO2 [13, 14].

Iron and steel slags are byproducts from iron and steel manufacturing, and consist mainly of calcium, magnesium, and aluminum silicates in various combinations [15]. Accelerated carbonation of steel slags is in most cases carried out in water slurry phase (S/L<1 w/w) at elevated pressure and temperature [16]. CO2 uptake is influenced by residue composition (iron and steel slags are highly variable with respect to their composition [17]) and operational parameters (pressure, temperature, particle size distribution) [16].

This study was focused on comparative carbonation of industrial wastes in which Ca and Mg are generally bound as silicates. Waste materials such as pulverized firing oil shale ash, electric arc furnace slag (types 1 and 2) and ladle slag were studied as sorbents for binding CO2 from flue gases in direct aqueous mineral carbonation process at mild operating conditions (room temperature and atmospheric pressure).

2. Materials and Methods

2.1. Characterization of the samples

The samples were characterized by chemical analysis and quantitative X-ray diffraction (XRD) methods as well as by BET (absorption theory by Brunauer, Emmet and Teller) and scanning electron microscopy (SEM) methods. For XRD analysis in a Dron-3M diffractometer using Ni-filtered Cu-Ka radiation, powdered non-oriented preparations were made. Diffractograms were digitally registered within 2-50o 29 range and analyzed by Sirquant [18] code using full-profile Rietveld analysis [19]. Specific surface area (SSA) was estimated with BET method at Sorptometer KELVIN 1042 (Costech Microanalytical Ltd.). Scanning electron microscope Jeol JSM-8404 was used for surface observations.

As a pre-treatment, the EAFS types 1 and 2 were ground in a ball mill (d <100 ^m). LS and PFA were used as received basis (dmean=24 ^m and dmean=42 ^m, respectively).

2.2. Experimental setup

The aqueous suspensions of initial samples (Table 1) were treated at S/L = 0.1 with CO2 containing model gas (50 L/h; 15% CO2 in air) in an absorber for 65 minutes (Figure 1a). The carbonation process was carried out at room temperature and atmospheric pressure. Carbonation products (cPFA, cEAFS1, cEAFS2 and cLS) were characterized by chemical analysis and quantitative XRD methods as well as by observations with scanning electron microscope. Theoretical extent of carbonation (g/100g) was calculated according to Huntzinger et al. [20]:

ThCO2=0,785(%CeO-0,56■CeCO3-0,7■%SO3)+1,091■%MgO+0,70■%Ne2O+0,468(%K2O-0,632■%KCl) (1)

To estimate the CO2-binding potential of oil shale ash components of low water-solubility (in which Ca and Mg are bound as silicates) exclusively the aqueous carbonation process was also carried out with the lime depleted material (LDM). PFA (20 g) was repeatedly (7 times) treated with distilled water (1000 mL) to prepare LDM. The

aqueous suspension of PFA was stirred for 15 minutes and filtered to separate the solid phase. The separated solid phase was then contacted with distilled water again to repeat the procedure (7 times). The final product (LDM) was analyzed for its chemical and phase composition. As a next step the aqueous suspension of LDM (S/L=0.1) was treated with CO2 containing model gas (50 L/h; 15% CO2 in air) in an absorber for 37 minutes (Figure 1a) at room temperature and atmospheric pressure. Carbonated lime depleted material (cLDM) was separated by filtering and analyzed for its chemical and phase composition as well as by observations with scanning electron microscope.

Carbonation treatment

pH, TDS

CO2 AIR

cEAFSl

cEAFS2

Flue gas

Reactor 1

Reactor 2

Characterization of solid samples: q-XRD; chemical analysis; SEM; BET

Separator

—¥-

Neutralized ash

~Aqueous~ phase

^Treated gas

^Aqueous _ phase

Figure 1. Laboratory batch setup (a) and process diagram for continuous mode aqueous carbonation of Ca-containing waste material (b)

3. Results and discussion

3.1. Characterization of the initial samples

The initial samples were characterized by chemical analysis (Table 1) and quantitative XRD methods (Table 2) as well as by BET (Table 1) and SEM methods (Figures 2a,e,g).

PFA contained 51% of total CaO, from which 44% (22.4 abs-%) was in the free form (CaO, Ca(OH)2) and 37% bound into Ca-Mg-silicates (CaSiO3, Ca2SiO4, Ca3Mg(SiO4)2) (Tables 2, 3). Minor amounts of CaO were bound into sulphates and carbonates. Previous studies about the composition of oil shale ash have shown relatively good correlation between the chemical and quantitative XRD analysis and the latter can be used for preliminary express analysis [12]. However, some inconsistencies may occur (content of CaO, Table 2). The particles of PFA were characterized by regular spherical shape with smooth surface (Figure 2a) and relatively low SSA (Table 1).

The mineralogy of the EAFS and LS samples was very complex and consisted of a number of silicate phases. EAFS1 contained up to 60% of various Ca-Mg-silicates such as merwinite (Ca3Mg(SiO4)2), montecilillite (CaMgSiO4) and cuspidine (Ca4Si2O7(F,OH)2) (Table 2). EAFS2 contained predominantly Mg-compounds: pyroxene ((Mg,Fe)2Si2O6), spinel (MgAl2O4) and Mg-olivine ((Mg,Fe)2SiO4) (Table 2). Total CaO content was the highest in the case of LS (42%), which contained mainly calcium silicate (Ca2SiO4), mayenite (Ca12Al14O33) and akermanite (Ca2MgSi2O7) (Tables 1, 2).

The particles EAFS1 and LS were characterized by sharp edges and smooth non-porous surface (Figure 2e,g). The specific surface areas of LS and ground EAFS was on the same level with PFA (SSA=1.23-1.71 m2/g).

Table 1 Chemical composition* and physical characteristics of the initial materials

SiO2t, % Al2O3t, % CaOt, % CaOf, % MgOt, % Fe2O3t, % CO2, % SSA, m2/g

PFA 21.90 5.25 51.19 22.40 4.93 3.98 5.41 1.84

EAFS 1 32.34 5.28 36.12 0.15 18.95 2.68 1.47 1.71

EAFS 2 39.76 18.98 26.91 0.13 18.95 2.87 1.06 1.63

LS 15.02 22.34 42.22 0.37 14.99 0.79 1.69 1.28

*: Chemical analysis performed at the accredited Laboratory of the Geological Survey of Estonia

Figure 2. SEM images of the initial (a - PFA, e - EAFS1, g - LS) and treated materials (b - cPFA, c - LDM, d - cLDM, f - cEAFS1, h - cLS).

Table 2 Phase composition of the initial materials

Component, % PFA EAFS 1 EAFS 2 LS

Calcite CaCÜ3 9.55 2.5 2.3 1.9

Dolomite CaMg(CO3)2 3.34

Portlandite Ca(OH)2 1.42 tr

Lime CaO 29.52

Periclase MgO 4.27 3.8 tr 11.3

Brucite Mg(OH)2 0.9 2.2 tr

alpha-Ca2SiO4 Ca2SiO4 alpha 1.99 18.3

beta-Ca2SiO4 Ca2SiO4 beta 16.92 14.8

Melilite (Ca,Na)2(Mg,Al)(Si,Al)3O7 4.99

Merwinite Ca3Mg(SiO4)2 6.81 19.8 1.4 1.5

Anhydrite CaSO4 4.48

Gypsum CaSO4*2H2O 0.76

Wollastonite 2M CaSiO3 2M 3.88 1.4 2.3 1.5

Hematite FeA 1.19 tr tr tr

Quartz SiO2 7.38 0.7 1.4 tr

Orthoclase KAlSi3Os 3.51 1.5 2.2 1.0

Montecillite CaMgSiO4 32.9

Cuspidine Ca,Si2O7(F,OH)2 15.8 14.8

Akermanite Ca2MgSi2O7 4.0 3.3 10.1

Mayenite Cai2Ali4O33 20.2

Pyroxene (Mg,Fe)2Si2O6 5.4 42.0 1.8

Spinel MgAl2O4 9.7 13.6 6.4

Mg-olivine Mg2SiO4 10.6

Brownmillerite Ca2(Al,Fe)2O5 1.4 2.3 0.7

Bredigite Ca7Mg(SiO4)4 3.5

Garnet Ca3Al2(SiO4)3 6.1

alfa-Fe 0.8

Z 100.00 99.8 99.2 99.1

3.2. The CO2-binding by Ca-Mg-silicates in direct aqueous carbonation of oil shale ash

The quantitative chemical and phase composition as well as distribution of Ca-compounds for initial (PFA) and treated materials (cPFA, LDM, cLDM) was determined (Table 3, Figure 3a,b). The Ca-compounds consistent in the initial and treated ash were divided into four groups: 1) free CaO (CaO, Ca(OH)2), 2) CaO bound into silicates (a-Ca2SiO4, p-Ca2SiO4, (Ca,Na)2(Mg,Al)(Si,Al)3O7, Ca3Mg(SiO4)2, CaSiO3), 3) CaO bound into sulfates (CaSO4, CaSO4^2H2O), 4) CaO bound into carbonates (CaCO3, CaMg(CO3)2). To compare the CO2-binding efficiency of different groups the quantitative changes in every step of the treatment were recalculated on the basis of the initial material (PFA).

Results of quantitative XRD indicated that the main CO2 binding component of oil shale ash was as expected CaO (16.2 g CO2/100 g PFA) (Table 3, Figure 3a). An additional amount of CO2 was bound by Ca-silicates (9.6 g CO2/100 g PFA) which formed 33% of the total CO2 bound in direct aqueous carbonation of PFA (29 g CO2 /100 g

PFA, counting also CO2 bound into CaMg(CO3)2 and K2Mg(CO3)2 by Mg and K compounds). The theoretical extent of carbonation (Eq.1.) was 35 g C02/100g PFA.

a)60 50 40 ^30 20 10 0

jLl&Jl,

b)60 50 40 ¡£30 20 10 0

□ PFA

] cPFA

□ LDM

] cLDM

Figure 3. Distribution of Ca-Mg compounds in the initial (PFA) and treated materials cPFA, LDM, cLDM) according to quantitative XRD measurements.

According to the results of quantitative XRD the LDM contained residual lime only in fractional quantity. After carbonation treatment (cLDM) most of the C02 was bound by Ca-silicates (5.4 g C02/100g PFA), predominantly on account of Ca2Si04 (Table 3, Figure 3b). The portion of Mg-compounds participating in C02 binding process was insignificant: about 0.15 g of C02 per 100 g of PFA was bound by Mg0 and Ca3Mg(Si04)2. The amount C02 bound by Ca-silicates in the cycle of PFA^LDM^cLDM was to some extent lower as compared to direct carbonation of PFA^cPFA (Table 3). Intensive chemical reactions like slaking and carbonation of lime trigger reaction heat and the internal expansive forces cause ash particles to fracture and disintegrate [21], creating thereby more favorable conditions for deeper carbonation of ash (including carbonation of Ca-Mg-silicates).

Table 3 Distribution of Ca0 bound into different groups of Ca-compounds in initial (PFA) and treated materials (cPFA, LDM, cLDM) and bound C02 (according to chemical analysis and quantitative XRD measurements).

Content of CaO bound into different groups of Ca-compounds PFA cPFA Bound CO2 LDM cLDM Bound CO2

g/100g PFA

Free Ca0 22.40 2.92 16.15 1.15 0.58 0.4

Sulphates 2.09 0.64 0.00 0.00 0.00 0.0

Silicates 19.24 7.00 9.61 17.53 10.71 5.4

Carbonates 6.36 40.12 Sum:25.8 13.00 20.43 Sum: 5.8

Leaching and carbonation treatment caused significant changes in the structure and surface characteristics of PFA: the particles of cPFA and LDM were fractured and disintegrated and covered with porous and permeable product layer (Figure 2b,c), as SSA increased from 1.8 to 13-16 m2/g. The particles of cLDM were partly covered with tighter layer of CaC03 crystals (Figure 2d).

Based on recent studies on the carbonation of oil shale ash, a new method for eliminating C02 from flue gases by Ca-containing waste material was proposed [22]. The process includes contacting the aqueous suspensions of Ca-containing waste material with C02 containing flue gas in two steps: in the first step the suspension is bubbled with flue gas keeping the pH levels in the range of 10-12 and in the second step keeping the pH levels in the range of 7-8 (Figure 1b). The water-soluble components such as free lime are carbonated in the first step and the components of low solubility, in which Ca is generally contained in the form of silicates, are carbonated in the second step. This enables optimal conditions for treating different phases of multicomponent waste materials. As another process route, the free lime could without difficulty be separated from ash by leaching it into the aqueous solutions in order to produce precipitated calcium carbonate as a commercial product.

3.3. The CO2-binding by Ca-Mg-silicates in direct aqueous carbonation of steel slag

Quantitative XRD analysis of the carbonation products indicated that Ca3Mg(SiO4)2 was the main CO2 binding component in EAFS1 (Figure 4a). The main carbonation product was calcite (CaCO3), indicating that Mg-compounds were not reactive towards CO2 at these mild conditions (Figure 4a). Consequently, the CO2 binding ability of EAFS2 was also marginal. The total amount of CO2 bound by EAFS1 was 8.7g CO2/100 g EAFS1, which formed 18% of the theoretical extent of carbonation calculated according to Eq. 1. EAFS2 bound only 1.9 g CO2/100 gEAFS2 (5% of the theoretical extent of carbonation).

Although LS contained substantial amount of CaSiO4 (Figure 4b) which according to the model experiments [14] showed quite good CO2-binding characteristics under atmospheric pressure and room temperature, the actual carbonation extent remained low. The CO2 uptake of LS was 4.6 g/100g LS (10% of the theoretical extent of carbonation). As LS contained a number of Ca-Mg-Al-silicate phases, it was not clear which ones were the main participants in the CO2 binding reactions. Grinding of the EAFS1 prior to carbonation treatment probably worked as a mechanical activation method and enhanced the carbonation process [23]. This would explain the higher carbonate contents of cEAFS1 as compared to cLS (LS was used as received basis). Carbonation treatment changed considerably the structure and surface characteristics of EAFS1: the particles were covered with spindle-shaped product layer (Figure 2f) and SSA increased from 1.7 to 13.9 m2/g. Changes in the shape and surface of cLS particles were not as noticeable (Figure 2h).

□ EAFS1 □ cEAFS1 DLS □ cLS

Figure 4. Distribution of Ca-Mg compounds in initial (EAFS1 and LS) and treated materials (cEAFS1, cLS) according to quantitative XRD measurements.

4. Conclusions

Direct aqueous carbonation of Ca and Mg silicates which were derived from the actual industrial wastes like PF oil shale ash and steel slags (EAFS types 1 and 2 and ladle slag) was demonstrated. The experiments were carried out at mild operating conditions: room temperature and atmospheric pressure. Comprehensive mineral composition of the initial samples as well as the carbonation products was presented.

Quantitative XRD analysis indicated that Ca2SiO4 and Ca3Mg(SiO4)2 were the main CO2 binding low solubility components of PF oil shale ash as well as steel slags. The main carbonation product was calcite (CaCO3), indicating that Mg-compounds were in most cases not reactive towards CO2 at these mild conditions.

Results showed that Ca-Mg-silicate phases bound up to 9 g of CO2 per 100 g of initial ash, which amounted to 30% of total CO2 bound in direct aqueous carbonation of PFA. The total amount of CO2 bound by PFA was 29 CO2 g/100 g PF, which formed 83% of the theoretical extent of carbonation (46% bound by lime and 27% bound by Ca-silicates). The CO2 uptakes for steel slags (EAFS1, EAFS2 and LS) were 8.7g CO2/100 g EAFS1, 1.9 g CO2/100 gEAFS2 and 4.6 g/100g LS, which formed 18%, 5% and 10% of the theoretical carbonation extents, respectively. Comparative carbonation of different Ca-Mg-silicates containing waste materials confirmed that the CO2-binding ability depends significantly on the origin of the material as well as on the pretreatment conditions.

Based on multifaceted studies about carbonation of oil shale ash, a new method for eliminating C02 from flue gases by Ca-containing waste material was proposed. The process includes contacting the aqueous suspensions of Ca-containing waste material with C02 containing flue gas in two steps: in the first step the suspension is bubbled with flue gas keeping the pH levels in the range of 10-12 and in the second step keeping the pH levels in the range of 7-8. The water-soluble components such as free lime are carbonated in the first step and the components of low solubility in which Ca is generally bound as silicate are carbonated in the second step.

Acknowledgements

Authors express their gratitude to Dr. V. Mikli for SEM analysis. The research was supported by the Estonian Ministry of Education and Research (SF0140082s08) and the Estonian Science Foundation (Grant No 7379).

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