**5. Results and discussion**

## **5.1 Physicochemical characteristics of oyster shells**

The physicochemical compositions of the waste oyster shells, mussel, clam, seashell, and commercial Jungsun limestone were summarized in Table 6 [Jung et al., 2007, 2005, 2000; Kwon et al., 2003]. To perform XRF (X-ray fluorescence spectroscopic) analysis, samples were powdered after dehydrated in a drying oven at 105.0°C for 24 hours. From the composition analysis, it was found that oyster-shells consist of mostly CaO, some of SiO2,

Reuse of Waste Shells as a SO2/NOx Removal Sorbent 311

53.81 52.94

Mussel 0.20 0.13 0.03 53.70 0.33 45.61 0.0129 Clam 0.46 0.20 0.04 53.92 0.22 45.16 0.1025 Seashell 0.66 0.40 0.04 53.58 0.20 45.12 0.0888 Jungsun limestone 2.43 0.25 0.14 53.80 0.85 42.50 0.0697 Table 6. Physicochemical properties of tested absorbents and seashells [Jung et al, 2005].

Fig. 6 shows the change of specific surface area of calcined hydration limestone. As shown in Fig. 6A, specific surface area of calcined hydration limestone particle increases rapidly with increasing hydration time up to 6 hours and then slowly increases to 12 m2/g. Effects of hydration temperature on the specific surface area of the calcined limestone has been shown in Fig. 6B. When the initial water temperature was in the range of 30∼90°C with every 10°C, up to the 70°C BET surface area was not changed largely. However, rapid increase of BET surface area of absorbent appears above the temperature of 80°C, indicating the rapid hydration reaction occurs. We can conclude the optimum range of temperature and time for hydration reaction of absorbent is about 80∼90°C and 24 hours, respectively

Fig. 7A∼B shows the adsorbed volume with relative pressure on the waste oyster shells and limestone. This figure shows the difference of amount of nitrogen gas according to the change of relative pressure absorbed to waste shell and limestone. We can conclude each of

Fig. 8 shows pore size distribution of waste oyster shells, clam, and seashell according to pore diameter. The results lead to the finding that shells have larger average pore size with the value of pore volume of 0.013∼0.024 cm3/g, which is higher than that of limestone. And, other result is pore size distribution according to the change of pore diameter of oyster manufactured as an absorbent with calcination and hydration. It was appeared that the average pore diameter of oyster was much bigger than that of raw materials. This result also indicates that calcination and hydration processes can enhance the removal capacity of acid

To clarify the effect of temperature, TGA analysis of the waste oyster shells and limestone were examined under N2 atmosphere by Automatic Derivative Differential Thermo-balance (Fig. 9). Heating rate in the calcination, which was conversion from calcium carbonate to calcium oxide, was 10.0 °C/min in the temperature range of 600.0∼ 900.0°C. As can be seen in Fig. 9, calcinations of the oyster-shells started at 645.0°C and completed at 780.0°C, whereas Jungsun limestone started over the temperature 680.0°C and completed at 845.0°C. The mass of waste oyster shells and limestone in the calcinations under N2 atmosphere

the sorbent shows a similar BET value in the condition of raw material.

Chemical composition [wt.%] Pore volume

44.87 44.02 0.0869 -

[cc/g] SiO2 Al2O3 Fe2O3 CaO MgO Ignition Loss

1.70 0.78

Sorbents

Waste oyster shells (WOS)

[Jung et al, 2000].

gases [Jung et al, 2005].

0.40 0.62 0.22 -

0.04 0.32

**5.2 Characteristics of calcination and hydration reaction with oyster shells** 

MgO, Al2O3, and Fe2O3. The composition of CaO in the oyster-shells was around 53.81 ∼ 52.94 wt.% which is comparable to that of commercial Jungsun limestone and was in good agreement with the results of Yoon et al. (2003) reporting the CaO content of oyster-shells was about 53.7 wt.% [Jung et al., 2007, 2005, 2000].

X-ray diffraction (XRD) patterns of waste oyster shells, limestone, and calcined waste oyster shells (C-WOS) are shown in Fig. 5. Fig. 5 shows the results of XRD analyses of the waste oyster shells and limestone with and without calcination. The patterns for waste oystershells and limestone were nearly similar and the diffraction peaks of CaCO3 as major phases are identified (Fig. 5a, 5b). The patterns for calcined waste oyster shells were exhibited peaks characteristics of CaO (Fig. 5c) [Jung et al, 2007]. And, the intensity of these peak increases with increasing the calcination temperature indicating CaO phase has been formed enough after calcination followed by hydration reaction. Comparing the results of Fig. 5 with Table 6, the waste oyster shells can be utilized as FGD absorbent [Jung et al, 2000].

Fig. 5. XRD profiles of oyster-shells and limestone; (a) limestone, (b) oyster shells, and (c) calcined oyster shells.

MgO, Al2O3, and Fe2O3. The composition of CaO in the oyster-shells was around 53.81 ∼ 52.94 wt.% which is comparable to that of commercial Jungsun limestone and was in good agreement with the results of Yoon et al. (2003) reporting the CaO content of oyster-shells

X-ray diffraction (XRD) patterns of waste oyster shells, limestone, and calcined waste oyster shells (C-WOS) are shown in Fig. 5. Fig. 5 shows the results of XRD analyses of the waste oyster shells and limestone with and without calcination. The patterns for waste oystershells and limestone were nearly similar and the diffraction peaks of CaCO3 as major phases are identified (Fig. 5a, 5b). The patterns for calcined waste oyster shells were exhibited peaks characteristics of CaO (Fig. 5c) [Jung et al, 2007]. And, the intensity of these peak increases with increasing the calcination temperature indicating CaO phase has been formed enough after calcination followed by hydration reaction. Comparing the results of Fig. 5 with Table

2-Theta

Fig. 5. XRD profiles of oyster-shells and limestone; (a) limestone, (b) oyster shells, and

20 40 60 80

6, the waste oyster shells can be utilized as FGD absorbent [Jung et al, 2000].

a

a

a

a

a

b b

a

a

a

a

a

a

a

a

<sup>a</sup> aaa <sup>a</sup> <sup>a</sup>

a

b b

a

aa a a a a

<sup>a</sup><sup>a</sup> <sup>a</sup> a a a

a : CaCO3(Calcite) b : CaO(Lime)

a : CaCO3(Calcite) b : CaO(Lime)

aa a <sup>a</sup>

a : CaCO3(Calcite) b : CaO(Lime)

b

a

a

a

<sup>a</sup> <sup>a</sup>

b

was about 53.7 wt.% [Jung et al., 2007, 2005, 2000].

Intensity(Counts)

Intensity(Counts)

Intensity(Counts)

0

400

0

400

0

(c) calcined oyster shells.

100

200

300

100

200

300

100

200

300

400

(a) Limestone

(b) Waste oyster-shell

a

(c) Calcined waste oyster-shell

a


Table 6. Physicochemical properties of tested absorbents and seashells [Jung et al, 2005].
