Assessment of marble waste utilization as an alternative sorbent to limestone for SO2 control

Fuel Processing Technology 128 (2014) 461–470

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Assessment of marble waste utilization as an alternative sorbent to limestone for SO2 control N. Emre Altun ⁎ Middle East Technical University, Department of Mining Engineering, 06800 Ankara, Turkey

a r t i c l e

i n f o

Article history: Received 10 April 2013 Received in revised form 11 July 2014 Accepted 6 August 2014 Available online xxxx Keywords: Marble waste Limestone Wet flue gas desulphurization (WFGD) SO2 Dissolution rate Evolved gas analysis (EGA)

a b s t r a c t The possibility of marble waste utilization in wet flue gas desulfurization (WFGD) as an alternative to limestone was assessed. Chemical compositions, specific surface areas, grindabilities, dissolution liabilities and SO2 capture capabilities of five different marble wastes were determined and compared with three limestones used in WFGD systems. Chemical compositions of marble wastes were comparable with limestones, including significant CaCO3 and lower fractions of MgCO3. All limestones and marble wastes had low specific surface areas. Marble wastes had lower Bond Work Indices (BWI) than limestones, implying a higher liability to size reduction. Dissolution behavior was sample specific. Some marble waste types showed higher or comparable dissolution rates with limestones while some resulted in a relatively slower dissolution. A higher calcite favored the dissolution rate while an increase in dolomite reduced the liability to dissolution. Assessment of the FTIR spectra obtained by evolved gas analysis (EGA) revealed better or comparable SO2 retention with some marble waste types as compared to limestones. The capability of SO2 control was a function of the extent of calcite in the sorbents. In view of these, a significant opportunity was anticipated for the wastes of some marble types as an alternative sorbent in WFGD. © 2014 Elsevier B.V. All rights reserved.

1. Introduction The increase in the global energy demand is a well-known fact. Coal remains to be an outstanding energy resource featured by its extensive amount and availability of reserves as well as stable market conditions [1,2]. Coal based thermal power production is a significant solution to meet growing energy needs. The share of coal fired thermal plants in electricity production is expected to increase gradually [2–5]. On the contrary, these plants are anticipated as a big matter of environmental concern. A major problem in coal based power production is the postcombustion sulphur dioxide (SO2) emissions. Sulphur dioxide is toxic, harmful to human health and causes serious air and environmental pollution. Hence an effective SO2 abatement in these plants is critical for sustainable production of power. In coal fired thermal power plants the options for controlling SO2 are either during (in-situ) or after combustion (post-combustion) [2,6]. Insitu processes work effectively in plants based on fluidized bed combustion [5–7], however post-combustion desulphurization is the most commercialized process [8]. Among the available post-combustion methods, wet flue gas desulphurization (WFGD) accounts for 84% of the active desulphurization systems in the world. Overall, 70% of the WFGD systems use calcium-based material, mostly limestone, as SO2 sorbent [8]. In these systems, limestone is finely ground (95% passing ⁎ Tel.: +90 312 210 2656. E-mail address: [email protected].

http://dx.doi.org/10.1016/j.fuproc.2014.08.009 0378-3820/© 2014 Elsevier B.V. All rights reserved.

45 μm), and mixed with water to obtain a sorbent slurry. The overall reaction of post-combustion SO2 control by this slurry is [9,10]: CaCO3 ðsÞ þ SO2 ðgÞ þ ½O2 ðgÞ þ 2 H2 OðsÞ→CaSO4 2H2 OðsÞ þ CO2 ðgÞ ð1Þ Owing to technical limitations and geological conditions, the overall efficiency in marble quarrying and processing rarely exceeds 10%. In countries with outstanding marble production, like China, Turkey, Italy, Spain and Greece, huge amounts of marble wastes are generated in the form of fines from cutting or in the form of waste blocks and slabs from quarrying. These are encountered as an environmental problem as well as economic losses. Despite the recognition of possible opportunities associated with the utilization of marble wastes, their usage for SO2 control in the WFGD systems as an alternative sorbent is lacking of a broad evaluation and comparison with limestones. Except Davini's work [11,12] which assessed the SO2 adsorption capabilities of wastes from the marbles of Northern Tuscany, Italy, no other investigations were conducted to evaluate this opportunity. A number of physical and chemical characteristics, including mineralogical composition, porosity, grindability and dissolution behavior or reactivity, directly affect the selection and use of limestones for SO2 control in WFGD systems. The SO2 control capacity of a specific marble type is also one of the most significant features to consider. Evaluation of all these critical parameters is necessary to propose the waste of a specific marble type as a potential SO2 sorbent and to identify whether an opportunity exists

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for the utilization of marble wastes as an alternative to limestone in WFGD systems. This work aims to address this question, i.e. to determine the potential of marble wastes as an alternative SO2 sorbent to limestones with respect to these critical parameters. In this respect, the mineralogical compositions, specific surface areas, grindabilities, dissolution behavior (reactivities) and SO2 capture capabilities of five different types of marble wastes were determined. Three different limestones, practically used for SO2 control in the WFGD systems were also characterized in the same manner and used for comparisons with the marble wastes. 2. Materials and methods 2.1. Samples and characterization Five types of marble wastes and three different limestones were used in this study. The wastes were selected from the major marble types produced in Muğla, Turkey, which is one of the biggest natural stone production districts in Europe and the Middle East. Due to commercial restrictions, the marble wastes were presented with codes reflecting their geological formation periods rather than trade names (Table 1). The limestones were obtained from Yatağan, Kemerköy and Yeniköy thermal power plants, located at the Aegean region of Turkey and are practically in use in the WFGD systems as sorbent. All samples were subjected to XRD and XRF analyses for mineralogical characterization. For both analyses the samples were crushed and ground dry down to − 150 μm size, representatively. XRD spectra were obtained using XRD-2000 Scintag Powder instrument in 5–70° range, at a scan rate of 0.02°/min. XRF analyses were done with an X-Lab 2000 PED-XRF instrument. 2.2. Specific surface area measurements and grindability tests Specific surface areas of the samples were determined by measuring the quantity of gas adsorbed in the form of a single molecular layer, based on the Brauner–Emmett–Teller (BET) theory. A singlepoint determination of the surface area from adsorption isotherms of nitrogen at 77 °K and over a range of pressures was applied. The measurements were done with 2 g of representative samples with −45 μm particle size, using a Micromeritics ASAP 2020 instrument. Prior to the surface area measurements, the samples were degassed for 4 h at 125 °C using helium to remove any moisture and other possible contaminants. The grindability of the samples was determined in accordance with the standard Bond Grindability Test and the Bond Work Index values were found. The detailed procedure of this well-established test was described elsewhere [13]. The test consists of using a standard laboratory scale Bond Mill (12 × 12 in.) with designated size and amount of steel grinding balls. The test simulates dry grinding conditions based on a closed circuit with 250% circulating load. Following several grinding periods to achieve a steady amount of undersize material through a test sieve size, the Bond Work Index value (Wi) is calculated according to the following formula: Wi ¼ ðP1 Þ

0:23




44:5  0:82  10 10 pffiffiffi − pffiffiffi Gbp P F

ð2Þ

where; P1 G F P

Test opening of the test sieve (μm) Net weight of the test sieve undersize product per mill revolution (g rev−1) d80 size of the test feed (μm) d80 size of the mill product (μm)

Table 1 Codes and geological periods of formation of marble wastes. Sample code

Geological period

PK-MW T-MW1 T-MW2 T-MW3 C-MW

Permo-carboniferous Triassic Triassic Triassic Cretaceous

2.3. Determination of dissolution behavior (reactivity) The measurement of the dissolution rate of a calcium based sorbent as it reacts with a strong acid simulates the mechanism of SO2 control in WFGD systems. This is the most commonly applied and reliable procedure for the determination of the liability to dissolution [14–18]. In this method, the pH of the sorbent solution is maintained constant at a predetermined level by the addition of a strong acid. The dissolution rate or fractional conversion of the sorbent with time is calculated stoichiometrically from the acid volume used to maintain the predetermined pH level. The tests for determining the dissolution rates were conducted using the setup shown in Fig. 1. The setup consists of a pH pump for pH control and acid titration, a stirred reactor containing the sorbent solution, a beaker with acid solution, an analytical scale and a logger-computer system for recording the data from the scale. For the tests, the samples were ground dry down to −45 μm, which is the common sorbent size in the WFGD systems. 4 g of representative sample was dissolved in 2.5 l distilled water in a 5 l stirred reactor. The pH of the sorbent solution was fixed at 5.0, to simulate the pH level in the WFGD systems. For precise pH control, the pH pump and the 0.1 M HCl solution was used. The pH pump automatically controlled and maintained the pH in the stirred reactor at 5.0 by titrating with the 0.1 M HCl solution when the designated pH was exceeded. The dissolution rate of the sorbent was related stoichiometrically to the volume of the HCl used according to the following reactions [14,17–19]: CaCO3 þ 2HCl→CaCl2 þ H2 O þ CO2

ð3Þ

In view of the previous findings that the dissolution of the calcareous sorbents occurred in accordance with the shrinking core model and the reaction rate was controlled by surface reaction [16–18], the dissolution kinetics and the reaction rate constants of the samples were identified using the following equations; t ¼

i 1 ρk R h 1 − ð1 − XÞ3 kS CA

ð4Þ

Simplifying Eq. (4) yields; 1

kt ¼ 1 − ð1 − XÞ3

ð5Þ

In these equations; t R X ρk CA k

Reaction period (min) Universal gas constant (kJ mol−1 K−1) Conversion (dissolution) fraction of the sorbent Molar density of the sorbent (kg mol m−3) Concentration of HCl in solution (mol cm−3) Reaction rate constant (min−1) 1

A plot of the 1 − ð1 − XÞ3 values vs. time yields a straight line with its slope equal to the reaction rate constant, k, for each sample. The reaction rate constants were evaluated as a direct measure of the liability of the sorbent for reactions occurring in WFGD systems.

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Fig. 1. Laboratory setup for determining dissolution rates.

2.4. Evolved gas analysis In order to determine the effectiveness of SO2 control with the marble wastes and for comparison with limestones, evolved gas analysis (EGA) was conducted. For EGA, calcined marble wastes and limestones were mixed with raw coal at a fixed amount of 10% by wt. The absorbance intensities of SO2 emission bands on the FTIR spectra of the raw and sorbent added samples were used for qualitative comparison of the effectiveness of marble wastes in SO2 control. The SO2 emission rate profiles were used for quantitative evaluation of SO2 retention using marble wastes and limestones. Evolved gas analysis was performed using a Perkin Elmer TG7 thermal analyzer coupled with a Perkin Elmer 2000 FTIR spectrometer. Prior to the tests the system was purged with N2 to avoid contamination effects. Approximately 25 mg of sorbent added coal samples were combusted through a controlled program at a constant heating rate of 10 °C/min from ambient to 900 °C with a fixed airflow of 50 ml/min. The spectral range was 400 cm−1–4000 cm−1 for the FTIR spectra, providing the three dimensional emission profiles of the evolved gases. 3. Results and discussion 3.1. Compositional characteristics of the sorbents The results of the XRF analysis for the limestones and marble wastes are shown in Table 2. All limestones are high-calcium sorbents and Kemerköy limestone was distinguished with the highest calcite (CaCO3) content (% 98.45). The four times higher dolomite (MgCO3) in Yatağan limestone (% 1.72), as compared to other limestones should be noted. The iron-oxide and aliminium oxide contents of all limestones were fairly low and the SiO2 fraction in Yatağan and Yeniköy were more significant than Kemerköy limestone (Table 2). For the marble wastes, it

was seen that some samples had higher, some had lower calcite and dolomite contents compared to limestones (Table 2). The cretaceous, C-MW type marble waste had an outstanding amount of calcite (98.92%) among all limestone and marble wastes (Table 2). Also, C-MW had the second lowest dolomite content (0.45%) after Yeniköy limestone. Both the calcite and dolomite amounts of the permocarboniferous, PK-MW type marble waste were relatively high. The triassic marble wastes (T-MW1, T-MW2, T-MW3) had varying calcite and dolomite fractions (Table 2). The relatively low calcite (87.7%) and quite high dolomite (10.15%) in T-MW2 is noteworthy. Also, despite high calcite in T-MW3, its dolomite content ranked as the second highest (Table 3). All marble wastes had lower SiO2 contents than limestones (Table 2). The compositional characteristics of the limestones and marble wastes were also compared by XRD analyses. The X-Ray diffractograms of Yeniköy, C-MW and T-MW2, representing high and low calcite and dolomite conditions are presented in Fig. 2. The peaks at 23°, 30°, 36°, 40°, 47° and 49° show calcite and the characteristic peak at 31° correspond to dolomite. The strength of the peaks indicated the extent of the calcite and/or dolomite. The characteristic calcite peaks were apparently observed in the X-ray diffractograms of all limestones and marble wastes, confirming the XRF results. Only Yatağan limestone, T-MW2 and PK-MW marble wastes revealed observable dolomite peaks, justifying the relatively higher presence of MgCO3 in these samples shown by XRF analysis (Table 2). The intensity of the Mg peak of the T-MW2 type marble waste is particularly high corresponding to the highest dolomite fraction (10.15%) in the studied samples (Fig. 2). XRF and XRD analyses showed that the chemical compositions of marble wastes are quite similar to those of the limestones used in WFGD. Similar to the differences in the calcite and dolomite contents of the limestone samples, the fractions of calcite and dolomite also varied in different types of marble wastes.

Table 2 Chemical compositions of limestones and marble wastes. Sample

Limestone

Marble waste

Amount (%)

Yatağan Yeniköy Kemerköy PK-MW T-MW1 T-MW2 T-MW3 C-MW

CaCO3

MgCO3

SiO2

NaO

Fe2O3

Al2O3

K2 O

TiO2

P2O5

SO3

Cl

95.29 97.10 98.45 94.47 98.79 87.70 95.46 98.92

1.72 0.41 0.37 3.51 0.9 10.15 4.17 0.45

1.54 1.29 0.54 0.50 0.06 0.42 0.06 0.11

0.04 0.05 0.02 0.04 0.01 0.02 – 0.02

0.38 0.37 0.38 0.84 0.13 1.42 0.22 0.34

0.75 0.61 0.14 0.43 0.03 0.22 0.04 0.08

0.12 0.08 0.02 0.06 0.00 0.01 0.00 0.00

0.05 0.04 – 0.05 – – – 0.03

0.01 0.01 0.01 0.01 0.05 0.02 0.02 0.00

0.07 0.03 0.02 0.08 0.02 0.03 0.01 0.03

0.01 0.01 0.02 0.01 0.01 0.01 0.01 0.01

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Table 3 BET specific surface areas and bond work ındices of limestones and marble wastes. Limestone Yatağan Sp. surf. area (m2/g) BWI (kWh/t)

Marble waste Yeniköy

Kemerköy

T-MW1

T-MW2

T-MW3

1.66

1.92

1.95

2.26

2.06

1.03

2.14

2.14

9.78

11.33

12.28

8.69

9.76

9.33

9.33

11.03

It is well established that, higher calcite content of the sorbent corresponds to better performance in controlling SO2 in WFGD systems. On the contrary, dolomite has an adverse effect on SO2 control, since the presence of magnesium inhibits and/or decelerates the reactions between the sorbent and SO2, as previously shown [14–16,20,21]. Although all limestones in this study are high calcium sorbents (N95% CaCO3), Yatağan limestone is likely to provide a relatively lower SO2

PK-MW

C-MW

capture efficiency in the limestone group, due to its higher dolomite (1.72%) than the other two limestones. This also applies for the marble wastes; On the basis of high calcite and insignificant dolomite contents, C-MW and T-MW1 type marble wastes would be expected to provide an effective SO2 retention. Relatively lower effectiveness in SO 2 retention would be anticipated for PK-MW, T-MW3 and particularly for T-MW2 type marble wastes, owing to the relatively high MgCO3.

Fig. 2. X-ray diffractograms of Yeniköy limestone, T-MW2 and C-MW type marble wastes (Ca = Calcite, Mg = Dolomite).

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3.2. Specific surface areas and grindabilities of the sorbents

3.3. Dissolution behavior of the sorbents

The BET specific surface areas of the limestone and marble wastes are shown in Table 3. The surface areas ranged between 1.66– 1.95 m2/g for limestones and 1.03 m2/g–2.26 m2/g for the marble wastes. Generally, relatively low specific surface areas were recorded for both groups and all samples could be considered as low porosity materials. Among limestones, Yatağan, among the marble wastes T-MW2 had the lowest specific surface areas. It should also be noted that all marble wastes except T-MW2 had higher specific surface areas than limestones (Table 3). Thus, the marble wastes had at least similar or superior characteristics for use in WFGD systems than limestones with respect to the parameters related to the specific surface areas. The Bond Work Indices of the samples, indicating the grindabilities, (i.e. the liability to comminution) are presented in Table 3. Due to the high energy requirements of grinding as well as the intensive grinding required for reducing the size of sorbents to allowable limits in WFGD systems (90% b 45 μm), grindability is a critical aspect for sorbent selection. With the highest BWI among the limestones (12.28 kWh/t), grinding of Kemerköy limestone is relatively disadvantageous and requires higher energy as compared to Yatağan and Yeniköy. Yeniköy limestone (11.33 kWh/t) and C-MW marble waste (11.03 kWh/t) were other two samples with relatively low grindabilities (Table 3). All other samples had moderate to low BWI's (b 10 kWh/t) corresponding to favorable grindability. In terms of grindability characteristics, marble wastes were suggested as a favorable alternative to limestones. Even the C-MW marble waste, which had the highest BWI in the waste group, revealed higher grindability than Kemerköy (12.28 kWh/t) and Yeniköy (11.33 kWh/t) limestones (Table 3). In addition, the lower BWI's of all other marble wastes than limestones implied that the use of wastes in WFGD systems could provide great advantages in terms of grinding related energy consumption and costs. Lower energy consumption with the use of marble wastes is associated to additional advantages such as reduced environmental footprint and contributions to the overall sustainability of coal based thermal power production.

In the WFGD the liability of the sorbent particles to reaction with SO2 has great importance [14,21]. The sorbent should be sufficiently reactive to convert into calcium sulphate for effective control of SO2 [12,15]. The liability of the sorbent to dissolution or conversion indicates the extent the sorbent would provide alkalinity and react with the acid resulting from the dissolution of SO2 in water [15]. The liability to dissolution is not only an important criterion for sorbent selection, but also for the proper design of the WFGD unit [15,22]. A favorable dissolution behavior provides an effective SO2 abatement, less sorbent consumption and reduced waste generation. The liability of the sorbents to dissolution are expressed in terms of conversion profiles with time, i.e. the dissolution rates, and presented in Figs. 3 & 4. The fractional conversions of the limestone and marble wastes at specific instants are compared in Table 4. Kemerköy limestone provided an outstanding extent of conversion; It revealed the highest dissolution liability among the limestones with conversions of 51% and 90% of at the 15th and the 120th minutes. In the limestones group Yatağan limestone had the lowest rate of dissolution (Table 4). Kemerköy limestone also yielded a higher fractional conversion at all instants in the limestone group (Fig. 3 & Table 4). In the marble wastes group, more than half of C-MW was dissolved after the 15th minute and C-MW provided the highest dissolution rate with 91% conversion after 120 min. On the contrary, T-MW2 type marble waste had a quite slower dissolution as compared to other wastes, with a conversion of 21% at the 15th minute and 61% at the 120th minute (Table 4). As seen in Fig. 4 and Table 4, T-MW1 type marble waste ranked as the second, followed by the PK-MW and T-MW3 in terms of dissolution rate. Accordingly, for marble wastes, C-MW and T-MW1 were characterized as highly-reactive, PK-MW and T-MW3 as moderately-reactive and T-MW2 as poorly-reactive sorbents. An overall assessment showed that C-MW type marble waste had the highest liability to dissolution as compared to all limestones and wastes (Figs. 3 & 4, Table 4). Kemerköy and Yeniköy limestones revealed lower, but comparable dissolution rates to C-MW and these two limestones could also be classified as highly-reactive sorbents. Being a

Fig. 3. Fractional conversion of limestones with time.

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Fig. 4. Fractional conversion of marble wastes with time.

waste type with a higher dissolution rate than Yatağan limestone, the reactivity of T-MW1 was remarkable. Yatağan limestone, PK-MW, T-MW3 and T-MW2 type marble wastes were anticipated to be less favorable for use in WFGD systems, yielding relatively slower dissolutions and limited conversions. The reaction rate constant is also an indication of how fast the sorbents were dissolved [17,23] and determined by plotting the dissolution kinetics vs. time graphs for the limestones and marble waste types. Two of these plots representing a high-reactivity and a low-reactivity condition, are shown in Fig. 5 for C-MW and T-MW2 type marble wastes. The reaction rate constants of the samples are presented in Table 4 and compared with the fractional conversions of the samples. C-MW yielded the highest reaction rate constant (32 × 10−4 min−1).

It was followed by Kemerköy and Yeniköy limestones and T-MW1 type marble waste with reaction rate constants ranging between 28 × 10−4 and 31 × 10−4 min-1 (Table 4). T-MW2 type marble waste had the lowest reaction rate constant (19 × 10−4 min−1). The results confirm the dissolution characteristics of the samples. The ranking with respect to reaction rate constants entirely imitates the ranking with respect to dissolution rates: C-MW type marble waste, Kemerköy and Yeniköy limestones, which were classified as rapidly-dissolving and highly-reactive sorbents, yielded the highest reaction rate constants. The poor reactivity of T-MW2 type marble waste was justified with the lowest reaction rate constant (Table 4). Possible correlations between the dissolution behavior and compositional characteristics, of the samples were assessed. In Table 4 the

Table 4 Reactivity characteristics (in decreasing order) of limestones and marble wastes vs. compositional characteristics. Sorbent

C-MW Kemerköy Yeniköy T-MW1 Yatağan PK-MW T-MW3 T-MW2

Conversion, X (%)

Waste Limestone Limestone Waste Limestone Waste Waste Waste

15th min

30th min

60th min

100th min

120th min

53 51 50 46 41 32 30 21

65 63 61 59 53 45 42 33

78 76 73 71 65 59 56 47

89 86 84 82 76 69 67 58

91 90 88 85 79 72 70 61

Reaction rate constant (min−1) 32 31 29 28 24 22 22 19

× × × × × × × ×

10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4

CaCO3 (%)

MgCO3 (%)

98.92 98.45 97.10 98.79 95.29 94.47 95.46 87.70

0.45 0.37 0.41 0.90 1.72 3.51 4.17 10.15

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(b)

(a)

Fig. 5. Dissolution kinetics plots for the C-MW and T-MW2 type marble wastes: a) high-reactivity, and b) low-reactivity condition.

samples were presented in the order of decreasing liabilities to dissolution. It was seen that CO3, MgCO3 contents and the extents of conversion were correlated. C-MW type marble waste, which showed the highest dissolution liability, had the highest calcite (98.92%). The least reactive T-MW2 marble waste had the lowest calcite (87.70%). In general, an increase in the calcite fraction positively influenced the sorbent's liability to dissolution with slight exceptions in T-MW1 and T-MW3 type marble wastes (Table 4). The link between the MgCO3 content and dissolution behavior was stronger. Dolomite adversely affected the dissolution of marble wastes and limestones (Table 4). The highly-reactive sorbents, C-MW type marble waste, Kemerköy and Yeniköy limestones involved fairly low MgCO3 (0.37–0.45%). T-MW2 type marble waste, which revealed the lowest fractional conversion, had the highest MgCO3 (10.15%). This link between MgCO3 and reactivity applied to all other sorbents ranking in between. An increase in MgCO3 corresponded to reduced dissolution rate and decrease in fractional conversion (Table 4). These findings suggested that the dissolution liabilities of marble wastes and limestones were determined mostly by the calcite and dolomite contents and the influence of dolomite was more dominant. Further, for two sorbents including similar fractions of calcite, the difference

in dissolution rates were determined by the difference in dolomite contents. A comparison of the T-MW1 type marble waste vs. Yeniköy limestone as well as T-MW3 vs. PK-MW type marble wastes clearly shows this situation. Despite higher calcite in T-MW1 type marble waste than Yeniköy limestone, T-MW1 had lower fractional conversion and reaction rate constant due to its higher MgCO3 content. The same was observed for the T-MW3 vs. PK-MW type marble wastes (Table 4). The favoring effect of calcite as well as the adverse influence of dolomite on the dissolution of limestones was emphasized by several researchers [14,17,24]. This was attributed to the higher liability of calcite to dissolution than dolomite, particularly in acidic conditions [16, 23]. Calcite and dolomite had different dissolution rates, the latter being significantly slower [21,23,25]. The higher liability of calcite to dissolution than dolomite provided faster neutralization rates in acidic systems. The observations in this study comply with the previous findings regarding the CaCO3, MgCO3—dissolution rate correlation. The findings of this study also implied that marble wastes revealed the same behavior with limestones with regard to the influence of calcite and dolomite content on the dissolution of sorbents.

Fig. 6. (a) Characteristic bands of emitted gases (at 260 °C), and (b) EGA profile of raw coal.

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Table 5 Relative peak ıntensities of SO2 emission with respect to sorbent type. Peak temp.

260 °C 320 °C 520 °C

Absorbance cm−1 Raw coal

Kemerkoy LS

C-MW

T-MW1

Yenikoy LS

Yatagan LS

PK-MW

T-MW3

T-MW2

0.826 0.552 0.607

0.118 0.078 0.086

0.121 0.080 0.089

0.129 0.086 0.095

0.135 0.090 0.100

0.179 0.119 0.134

0.188 0.149 0.168

0.271 0.170 0.190

0.367 0.263 0.296

3.4. SO2 control capability of the sorbents In order to determine and compare the effectiveness of SO2 control using limestone and marble wastes, samples were subjected to evolved gas analysis. The FTIR spectra provided an insight about the temperatures and intensities of the emitted gases during the combustion of raw and sorbent added coal samples. Fig. 6 shows the evolved gases profile of the raw coal from ambient to 900 °C in the form of a three dimensional FTIR spectrum as well as the characteristic band positions corresponding to the emitted gases. In this spectrum, Y axis shows the temperature, X axis shows the characteristic wavelength of emitted gases and Z axis shows the relative absorbance intensities of the emitted gases. During the combustion of raw coal emissions of CO2, CO, H2O vapor, SO2, volatiles consisting of CH groups were clearly detected (Fig. 6). CO2 emission was exhibited in three distinct bands, between 524–745 cm−1, 2266–2472 cm−1 and 3461–3682 cm−1. Characteristic CO band was observed between 1976–2164 cm− 1. The emission of organic volatiles was identified by successive peaks between 2855 and 3157 cm−1. The band between 1611 and 1810 cm−1 corresponded to the evolution of H2O. The characteristic band in the range of 1202– 1466 cm− 1 shows the emission of SO2, which is the major focus in this study. The FTIR spectrum of emitted gases is a complex profile with numerous absorbance data and peak points, making it difficult to distinguish the difference between absorbance intensities. Thus, the relative peak absorption intensities for SO2 emissions, obtained from the spectra, are also given in Table 5 with respect to the sorbent used. The characteristic SO2 emission bands in the spectrum showed that, SO2 evolution started to emerge at 220 °C and proceeded until 600 °C. Within this temperature range three major SO2 emission peaks were recorded at 260 °C, 320 °C and 520 °C. As seen in Table 5 the highest SO2 absorption intensities were recorded at 260 °C for all samples. The band of peaks corresponding to SO2 absorbance at this temperature were extracted from the emission profiles and compared in Fig. 7. In the figure, the reduction in the intensities of the SO2 absorption bands with the addition of limestone and marble wastes are clearly seen. The extent of reduction, however, varies depending on the sorbent used.

Among the limestones, Kemerköy provided the highest reduction in SO2 emissions. The abatement of SO2 was also significant with Yenikoy limestone. Control of SO2 emissions was relatively limited with Yatagan limestone compared to Yeniköy and Kemerköy. Similar to the limestones, marble wastes provided varying effectiveness in the control of SO2 (Table 5 & Fig. 7). C-MW was a very effective sorbent, with the highest reduction in the SO2 peak intensities among marble wastes and with a retention performance equivalent to Kemerköy limestone. With T-MW1 type marble waste, the reduction in the intensity of SO2 emission bands was comparable to C-MW type marble waste and Kemerköy limestone. Also, with T-MW1, lower SO2 emission intensities than Yeniköy and Yatağan limestones were recorded. PK-MW, T-MW3 and T-MW2 type marble wastes clearly provided SO2 retention. Among these wastes, use of PK-MW provided comparable reduction in the SO2 peak intensities to Yatağan limestone (Table 5 & Fig. 7). For T-MW3 and T-MW2 notable reductions in SO2 peak intensities were recorded, however the reductions with the use of these two waste types were relatively lower. SO2 emission rates were also detected and plotted for quantitatively evaluating SO2 control with respect to sorbent type. SO2 emission rates for the raw and limestone and marble waste added coal are compared in Fig. 8. The reduction in the amount of SO2 emission was seen at different extents depending on the sorbent used. The highest reductions in the SO2 emission rate were obtained when C-MW type marble waste and Kemerköy limestone were used; At the peak points, the SO2 emission rates were approximately 0.5 wt.%/min and 0.36 wt.%/min for the raw coal. These rates were as low as 0.1 wt.%/min, 0.07 wt.%/min with the use of C-MW type marble waste and 0.12 wt.%/min and 0.09 wt.%/min with the use of Kemerköy limestone (Fig. 8). The decreases in SO2 emission rates were also significant with the use of T-MW1 type marble waste and Yeniköy limestone. It should be noted that T-MW1 provided a more effective SO2 control than both Yeniköy and Yatağan limestones (Fig. 8). The use of Yatağan limestone and PK-MW type marble waste also provided reductions in SO2 emission rate, but at lower extents. Although the least effective SO2 control was obtained with the use of T-MW3 and T-MW2 type marble wastes, the reductions at SO2 emission

Fig. 7. SO2 emission ıntensites of raw and sorbent added coal samples (at 260 °C).

N.E. Altun / Fuel Processing Technology 128 (2014) 461–470

rates were still visible. SO2 emission rates were reduced to approximately 0.22 wt./min and 0.16 wt./min with the use of T-MW3 and 0.27 wt./min and 0.20 wt./min with T-MW2 marble wastes (Fig. 8). The relationship between SO2 emission rates and calcite contents of the limestone and marble waste samples were presented in Fig. 9. The figure shows almost a direct relationship between the calcite content of a sorbent and its effectiveness in SO2 control. C-MW type marble waste and Kemerköy limestone, with their high calcite fractions, were distinguished as outstanding sorbents. As a marble waste, the performance of T-MW1 in SO2 abatement was also promising and relatively

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better than practically used Yeniköy and Yatağan limestones. Despite its comparable calcite content to Kemerköy limestone and C-MW type marble waste (Table 2 & Fig. 9), the effectiveness of SO2 control with T-MW1 was slightly lower. This could be attributed to the higher dolomite in T-MW1 type waste as compared to Kemerköy limestone and C-MW (Table 2). With a relative decline in the calcite content of the sorbents, increases in the SO2 emission rates were observed (Fig. 9). Overall, a comparison of marble wastes with limestones practically used in the WFGD systems showed that the wastes of some marble types possessed comparable or superior features in terms of calcite

Fig. 8. SO2 emission profiles of raw and sorbent added coal samples.

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References

Fig. 9. SO2 emission rates vs. CaCO3 and MgCO3 amounts in sorbents.

content, specific surface areas, grindability and liability to dissolution. The evolved gas analysis, related FTIR spectra and assessment of SO2 emission rates with respect to sorbent type also confirmed that the wastes of some marble types could provide a more effective or comparable SO2 capture with the limestones. Particularly, as marble wastes C-MW as well as T-MW1 had the distinction of being highly reactive and yielding significant reductions in SO2 emission rates. The results also showed that the use of some marble waste types, such as T-MW2 and T-MW3 for WFGD were relatively unlikely, mainly due to slower rates of dissolution and relatively lower capture of SO2. For these waste types the unfavorable reactivity characteristics could be attributed to the higher dolomite fractions in these wastes while the lower efficiency of SO2 control was due to their lower calcite contents. 4. Conclusions This study evaluated the possibility of marble waste utilization as an alternative sorbent in WFGD with respect to physical and chemical characteristics as well as SO2 capture capability. The reactivity tests showed that the dissolution behavior of a marble waste is a strong function of its calcite and dolomite content. It was seen by evolved gas analysis that the performance of marble wastes in SO2 abatement depended on the extent of calcite, and the higher the calcite content is, the higher the effectiveness of a marble waste type was in reducing SO2 emissions. In this respect, a favorable dissolution rate combined with high calcite, low dolomite, comparable surface area and higher grindability characteristics make the wastes of some marble types outstanding sorbent alternatives with potentially equivalent or better SO2 retention as compared to limestones. However, the availability of marble wastes as a sorbent is a specific feature and it would be misleading to consider and suggest the waste of any marble appropriate for WFGD systems without detailed investigation of particularly its reactivity and SO2 capture performance. Acknowledgment The financial support for this study through MU-BAP10/18 research project is gratefully acknowledged.

[1] EIA, International Energy Outlook 2008 report, US Department of Energy, Washington, DC, 2008. 20585 (DOE/EIA-0484). [2] D.O. Ogenga, et al., Sulphur dioxide removal using South African limestone/siliceous materials, Fuel 89 (2010) 2549–2555. [3] S.H. Mohr, G.M. Evans, Forecasting coal production until 2100, Fuel 88 (2009) 2059–2067. [4] M. Hook, K. Aleklett, Historical trends in American coal production and a possible future outlook, International Journal of Coal Geology 78 (2009) 201–216. [5] V. Manovic, E.J. Anthony, Competition of sulphation and carbonation reactions during looping cycles for CO2 capture by CaO based sorbents, Journal of Physical Chemistry A 114 (2010) 3997–4002. [6] S. Tia, T. Bunjungla-eid, Characterization of Thai limestone for desulphurization in a fluidized bed combustor, International Journal of Energy Research 27 (2002) 781–793. [7] E.J. Anthony, D.L. Granatstein, Sulfation phenomena in fluidized bed combustion systems, Progress in Energy and Combustion Science 27 (2001) 215–236. [8] J. Kaminski, Technologies and costs of SO2-emissions reduction for the energy sector, Applied Energy 75 (2003) 165–172. [9] H.N. Soud, Developments in FGD, IEA Coal Research, CCC 29, London, 2000. [10] J. Zhao, B. Jin, Z. Zhong, The degree of desulphurization of limestone/gypsum wet FGD spray tower using response surface methodology, Chemical Engineering Technology 30 (4) (2007) 517–522. [11] P. Davini, Behavior of certain by-products from the manufacture of marble in the desulphation of flue gases, Resources, Conservation and Recycling 6 (2) (1992) 139–148. [12] P. Davini, Investigation into the desulphurization properties of by-products of the manufacture of white marbles of Northern Tuscany, Fuel 79 (11) (2000) 1363–1369. [13] F.C. Bond, Crushing & grinding calculations Part I, British Chemical Engineering 6 (6) (1961) 378–385. [14] C. Hosten, M. Gulsun, Reactivity of limestones from different sources in Turkey, Minerals Engineering 17 (2004) 97–99. [15] S.R. Brown, R.F. DeVault, P.J. Williams, Determination of Wet FGD Limestone Reactivity, Electric Power 2010, Babcock & Wilcox Power Generation Group, Baltimore, USA, 2010. 1–8. [16] J. Ahlbeck, et al., Measuring the reactivity of limestone for wet flue-gas desulfurization, Chemical Engineering Science 50 (7) (1995) 1081–1089. [17] Z.O. Siagi, M. Mbarawa, Dissolution rate of South Australian calcium-based materials at constant pH, Journal of Hazardous Materials 163 (2009) 678–682. [18] M. Gulsun, Reactivity of limestones of different sources for flue gas desulfurization application, (MSc Thesis) Graduate School of Natural and Applied Sciences, Mining Engineering, Middle East Technical University, Ankara, 2003. [19] S.-M. Shih, J.-P. Lin, G.-Y. Shiau, Dissolution rate of limestones of different sources, Journal of Hazardous Materials B79 (2000) 159–171. [20] ASTM C1318-95, Standard Test Method for Determination of Total Neutralizing Capability and Dissolved Calcium and Magnesium Oxide in Lime for Flue Gas Desulphurization, 2009. [21] L. Yin, J. Guo, Study on Wet FGD Limestone Quality, Third International Conference on Measuring Technology and Mechatronics Automation, Shangai, China, 2011, pp. 605–608. [22] N. Ukawa, et al., Effects of particle size distribution on limestone dissolution in wet FGD process applications, Environmental Progress 12 (3) (1993) 238–342. [23] C. Carletti, et al., Modeling Limestone Reactivity and Sizing the Dissolution Tank in Wet Flue Gas Desulfurization Scrubbers, Environmental Progress & Sustainable Energy (2012), http://dx.doi.org/10.1002/ep.11683. [24] Th. Stumpf, A. Roeder, H.W. Hennicke, The reaction behavior of carbonate stone dusts in acid solution, more particularly sulphurous acid. Part II: Important influence parameters, and measurement on various carbonate stone dusts for flue gas desulphurization (in German), Zement-Kalk-Gips 37 (9) (1984) 454–461. [25] Z. Hu-feng, Impact of limestone powder quality on capability of wet process of FGD, China Environmental Protection Industry 9 (S2) (2008) 24–27.