Hybrid Fuel Preparation Combining Glycerol-Derived Hydrogel and

Oct 31, 2013 - A number of countries are mixing wood biomass with coal in existing coal-fired power plants according to the implementation of renewabl...
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Hybrid Fuel Preparation Combining Glycerol-Derived Hydrogel and Coal and Its Characterization Se-Joon Park,¶,† Dong-Wook Lee,‡,† Young-Joo Lee,¶ Jong-Soo Bae,¶ Jai-Chang Hong,¶ Joeng-Geun Kim,§ Jaehyeon Park,⊥ Jae Hyeok Park,⊥ Jong-Seon Shin,⊥ and Young-Chan Choi*,¶ ¶

Clean Fuel Department, ‡Energy Materials and Convergence Research Department, §Energy Efficiency Department, ⊥Greenhouse Gas Department, Korea Institute of Energy Research, 71-2 Jang-dong, Yuseong-gu, Daejeon, 305-343 Republic of Korea ABSTRACT: A number of countries are mixing wood biomass with coal in existing coal-fired power plants according to the implementation of renewable portfolio standard (RPS) and cap-and-trade systems; problems arise due to mixing of the two fuels which have different combustion reactivities. In the previous work, research on glycerol impregnated hybrid fuel (Hybrid Coal by Korea Institute of Energy Research; HCK) was conducted for the diversification of bioliquid and issues to be resolved through further study in the application of glycerol to Hybrid Coal were noted. As a solution, a hydrogel having a small quantity of poly vinyl alcohol (PVA) added to glycerol was applied in the present work. PVA allowed the solidification of glycerol and contributed to the binding energy being stronger among glycerol derivatives and the surface of coal pore; thus, the fuel loss by readsorption of water can be inhibited by setting hydrogel into coal pores.

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lower than 200 °C; this temperature is not sufficient for decarboxylation and hydrophobicity of coal. This inevitably causes a fuel (glycerol) loss because liquid phase glycerol that fills coal pores can be diluted or come away from the pores by readsorption of water. To solve this problem, poly vinyl alcohol (PVA), a water-soluble polymer, is added to glycerol to prepare solid phase hydrogel at room temperature. This study characterizes Hybrid Coal modified with hydrogel and investigates its combustion behavior, functional groups, and ignition temperature.

INTRODUCTION In 2008, the European Commission adopted legislation to raise the share of renewable energy, such as wind, solar, and biomass, to 20% by 2020.1 Among the renewable sources, carbon-neutral biomass, particularly solid fuel, is immediately applicable in coal fired power plants, so a number of approaches to utilize biofuel by cocombustion with coal have been investigated.2−4 In particular, woody biomass such as wood chips and pellets has relatively greater reactivity than coal and leads to a thermal loss in a pulverized-coal power plant due to the heat imbalance in boilers. Potassium and sodium in woody biomass leads to slagging and fouling on the walls and steam tubes in the boiler as well as corrosion by chlorine.5,6 In addition, a feeding system for biomass and oxidizer must be retrofitted due to its intrinsic reactivity and wide variation in properties. We previously reported on Hybrid Coal by Korea Institute of Energy Research (HCK) combined with bioliquid derived from sugar cane juice or molasses as a two-in-one fuel.7 Hybrid Coal provides a single coal combustion pattern, which is unlike the simple blend of biomass and coal that has dual combustion; therefore, existing power plants could use the fuel without expending extra capital for altering equipment. However, an ethical problem related to an eatable substance might arise if a sugar cane-derived bioliquid is used. To address this concern, glycerol, which is an affordable bioliquid derived from the biodiesel process, was used in this study. Commonly, coal becomes strongly hydrophobic when it is just dried at 250 °C in an inert atmosphere, but Hybrid Coal with glycerol cannot be treated at that desired temperature because substantial glycerol inside coal pores might evaporate during the residence time (1 h) for preparation; that is, although coal and the glycerol phase of hybrid coal burn simultaneously in a combustion atmosphere, glycerol could gradually evaporate at low temperature below 250 °C during long thermal treatment time for preparation of Hybrid Coal. For this reason, HCK with glycerol needs to be treated at much © 2013 American Chemical Society

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EXPERIMENTAL SECTION Materials. Kideco coal classified as sub-bituminous coal was obtained from Korea East−West Power Co., Ltd., and its proximate and ultimate analyses are listed in Table 1. The coal sample was crushed to less than 10 mm using a hammer crusher and ground to a particle diameter of below 2.8 mm by pin milling. The mean particle diameter of the coal sample was 0.4 mm. Glycerol (99.5%) with a molecular weight of 92.09 g/ mol and PVA with a molecular weight of 89 000−98 000 g/mol and 99% hydrolyzed were procured from Sigma-Aldrich. Preparation of Hybrid Coal with Hydrogel. PVA powder was dissolved in deionized water for 5 min at 80 °C using a magnetic stirrer, and then, glycerol preheated in an oil bath at 80 °C was added to the aqueous solution with different weight ratios; glycerol−PVA solutions containing PVA of 1 (G99/P1), 3 (G97/P3), and 5 (G95/P5) wt % were employed in this study. For economy, the amount of PVA to glycerol added to the solution was limited to within 5 wt %. Since the solutions begin gelation or solidification at atmosphere temperature, they were kept warm and mixed with dried coal, Received: Revised: Accepted: Published: 16206

July 30, 2013 October 29, 2013 October 31, 2013 October 31, 2013 dx.doi.org/10.1021/ie402459v | Ind. Eng. Chem. Res. 2013, 52, 16206−16210

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Table 1. Proximate and Ultimate Analyses of Coal Samples proximate analysisa (wt %)

a

ultimate analysisb (wt %)

sample

MC

VM

ash

FC

C

H

N

O

S

HHVb (MJ/kg)

Kideco coal HCK/G10 HCK/H10/P5

31.12 2.19 3.81

30.18 47.09 47.32

7.79 8.84 8.37

30.91 41.88 40.50

60.75 60.60 62.37

4.38 5.02 5.22

0.78 1.26 1.23

26.17 24.06 22.57

0.13 0.22 0.24

22.80 22.13 23.30

As received basis. bDry basis.

10 wt % of coal, on a dry basis through an in situ method. The mixtures of coal and glycerol/PVA solution were stored in air for 24 h to impregnate the hydrogel solution into the coal pores sufficiently, and they were dried in an oven at 105 °C for 24 h for the hydrogel to settle on the surface of the pores firmly by removing the remaining water in the hydrogel. Hybrid Coal samples were obtained via these processes for this study. Characterization of Hybrid Coal. To study the characteristics of Hybrid Coal, proximate and ultimate analyses, calorific value analysis, combustion behavior (TGA or DTG), functional groups investigation, and an ignition test were conducted. The proximate analysis was carried out using a TGA-701 (LECO Co., USA), and the ultimate analysis was conducted using a TruSpec Elemental Analyzer (LECO Co., USA) and SC432DR Sulfur Analyzer (LECO.Co., USA). A Parr 6320EF Calorimeter (PARR Co., USA) and FT-IR spectrometer (FTS175C, Bio-Rad Lab., Inc., USA) were used for the calorific value analysis and the investigation of functional groups, respectively. Additionally, a combustion behavior analysis of each sample was carried out with a TGA Q500 (TA Inst., USA) at a heating rate of 10 °C/min and air flow rate of 100 mL/min. For the determination of the coal ignition temperature, cross point temperature (CPT) was employed as some investigations related to the spontaneous combustion of coal have been conducted.8

Figure 1. TGA weight loss curves for dried raw coal, glycerol, and HCK/G10.

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RESULTS AND DISCUSSION Combustion Behavior. Kideco coal is classified as subbituminous coal, and proximate and ultimate analyses of the coal sample are listed in Table 1, which contains a moisture content higher than 30%; its higher heating value is 22.80 MJ/ kg. In addition, the higher heating value of Hybrid Coal impregnated with glycerol of 10 wt % (HCK/G10), which is based on dried coal, is 22.13 MJ/kg. The heating value of the Hybrid Coal was slightly less than that of dried coal because glycerol has a relatively lower heating value, 18.12 MJ/kg, while the volatile matter of Hybrid Coal increased 5.45% for dried coal on a dry basis due to the addition of glycerol. The combustion behavior of Hybrid Coal impregnated with glycerol was investigated in a previous study. The results revealed that the glycerol in coal pores plays the role of volatile matter, while its intrinsic evaporation property disappears. In other words, raw glycerol rapidly starts to evaporate at 130 °C, but the glycerol in Hybrid Coal has a similar combustion behavior as the volatile matter of dried coal as shown in Figure 1. The combustion behavior of HCK/G10 is slightly promoted by an increase in volatile matter in the coal. That means that Hybrid Coal is literally a two-in-one fuel, not only a mixture of coal and biomass that has different reactivities. Hydrogels prepared with PVA showed no particular changes in combustion behavior compared with glycerol, and only the TG curve for G95/P5 was a little shifted to high temperature as shown in Figure 2. Furthermore, the PVA of 1 and 3 wt %

Figure 2. TGA weight loss curves for hydrogels and PVA.

added to hydrogels exhibited a semisolid phase, while complete gelation occurred in the hydrogel with that of 5 wt %. The addition of PVA of about 5 wt % to glycerol led to many more hydrogen bondings between the two substances and made the hydrogel firm.9 This solidified hydrogel similarly effected Hybrid Coal and its combustion behavior (Figure 3). The combustion behavior of HCK/H10/P5, which is impregnated with G95/P5 of 10 wt %, was delayed about 150 °C in the high temperature region compared to the other Hybrid Coal samples, HCK/H10/P1 and HCK/H10/P3. This indicates that the solid phase hydrogel facilitated hydrogen bonding with the hydrophilic surface of the coal pores and inhibited oxygen diffusion into the pores. According to a previous investigation, the pores in coal mostly lie scattered in ash, and oxygen is 16207

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Figure 3. TGA weight loss curves for HCKs. Figure 5. FT-IR spectra of dried raw coal and HCK samples.

adsorbed into the interface between the ash and fixed carbon phase; therefore, the well-dispersed ash phase contributes to improving coal combustion reactivity.10 FT-IR Analyses. Functional groups of hydrogels and Hybrid Coals were investigated by FT-IR spectroscopy. As in previous combustion behavior with TGA, hydrogels showed absorption peaks similar to glycerol on wavenumbers (Figure 4), but the

C−O groups. However, it is remarkable that a strong absorption peak attributable to the CO functional group is visible at 1600 cm−1; nevertheless, carbonyl groups are not included in glycerol. This is due to the selective oxidation of glycerol on the carbon surface in coal; glycerol converts to glyceric derivatives such as glyceric acid, dihydroxyacetone, tartronic acid, and mesoxalic acid when it is dried in an oven at 105 °C.11−14 Furthermore, the absorption peaks of all Hybrid Coals impregnated with hydrogel responded to HCK/G10 at the same wavenumbers, yet the intensity of each peak decreased with an increase of in the PVA ratio. In particular, HCK/H10/P5 exhibited peak intensity similar to dried raw coal. Ignition Temperature. The rank of coal is a decisive factor in spontaneous combustion, and the tendency of coal to selfheat is raised with an increase of oxygen functional groups. Dehydration and decarboxylation of coal occurs through coalification, and then, the oxygen functional groups are able to decrease. CPT is chosen to evaluate spontaneous combustion of the Hybrid Coal by measuring ignition temperature. Sato15,16 reported that spontaneous ignition behavior can be predictable by measuring the peak temperature through differential thermal analysis (DTA); however, the CPT employed for this work is a more realistic method to evaluate the ignition temperature of coal. The inversion temperature (or CPT), in which the temperature of the sample exceeds that of the atmosphere of the reactor, was 215 °C for the dried coal (Figure 6a). While HCK/G10 started to burn at 199 °C, which is rather lower than dried raw coal (Figure 6b), the ignition temperature for HCK/H10/P5 increased by 9 and 25 °C over dried raw coal and HCK/G10, respectively (Figure 6c). The differences in ignition temperature were brought about by their intrinsic reactivity related to hydroxyl and carbonyl functional groups. These polar functional groups comprising reactive oxygen greatly affected the ignition temperature for each coal sample as the results of TGA and FT-IR revealed. Additionally, the carbonyl groups resulting from the selective oxidation of glycerol made hydrogen bonding stronger with the surface of coal pores or PVA than the single bond, which is O−H in glycerol, and then, the ignition temperature was delayed. Determination of Glycerol Desorption. The pore surface of coal that is filled with water is hydrophilic; thus,

Figure 4. FT-IR spectra of glycerol, PVA, and hydrogels.

peak intensity of the C−O functional group at 1050 cm−1 tended to slightly decrease with an increase of PVA content from 1 to 5%. This resulted from a decrease of the composition ratio of glycerol in the hydrogel, and then, the C−O group also decreased. There are four major absorption peaks in dried Kideco coal as shown in Figure 5. An absorption peak attributable to the hydroxyl group (O−H) of the coal is present between 3200 and 3600 cm−1. The vibrational band observed at 2900 cm−1 is due to the stretching vibrations of the C−H alkyl group. The absorption peaks at 1600 and 1050 cm−1 refer to the stretching vibration of CO and C−O functional groups, respectively. On the other hand, the absorption peaks intensity attributable to O−H and C−O functional groups of HCK/G10 increased from the addition of glycerol containing abundant O−H and 16208

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to HCK/G10 and HCK/H10/P5. The samples were allowed to stand for 24 h to dilute them sufficiently with water and were filtered by filter paper. Then, solid samples were obtained after drying in an oven for 24 h at 105 °C. As shown in Figure 7,

Figure 7. DTG curves to evaluate glycerol desorption by water readsorption.

glycerol or hydrogel impregnated in coal pores played the role of the volatile matter of coal and began to burn over 200 °C, while neat glycerol began to evaporate by 130 °C. The weight loss rates of both HCK/H10/P5 and HCK/G10 were greatest near 350 °C, but the combustion peak of HCK/H10/P5 was much higher than that of HCK/G10 in DTG curves. This indicates that the glycerol filling the coal pores was diluted with water, but hydrogel adhered to the surface of the coal pores. The binding energy between the hydrogel and the surface of the coal pores was stronger than that between the hydrogel and water. Carbon supported metal composition in coal ash promoted the selective oxidation of glycerol as a catalyst and created a carbonyl functional group. This carbonyl group made the hydrogen bonding stronger among glyceric derivatives, PVA, and coal; consequently, the fuel loss by water readsorption was inhibited.

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CONCLUSIONS Hybrid Coal is a novel technology, in which hydrophilic bioliquid is impregnated by diffusion and capillary forces in coal pores instead of water, and able to cope with a renewable energy regulation or policy by promoting biomass consumption in a coal power plant. We focused on the diversification of bioliquid for Hybrid Coal as mentioned in previous work, and hydrogel was employed to make up the single use of glycerol in the present work. The combustion behavior of HCK was delayed by hydrogel filled up the coal pores since the solid phase hydrogel facilitated hydrogen bonding with the hydrophilic surface of the coal pores and inhibited oxygen diffusion into the pores. PVA allowed the gelation or solidification of glycerol, and hydrogel impregnated in coal pores created a carbonyl functional group by its selective oxidation. This carbonyl group contributed to the binding energy being stronger among glycerol derivatives, PVA, and the surface of coal pores; thus, the fuel loss by readsorption of water can be inhibited by setting hydrogel into coal pores. Previous HCKs with sugar and glycerol as additives tended to only promote the

Figure 6. CPT for the respective coal samples: (a) dried raw coal; (b) HCK/G10; (c) HCK/H/P5.

hydrophilic liquid biofuels can easily fill the pores. Glycerol is one of the hydrophilic liquid biofuels, but Hybrid Coal modified by glycerol leads to fuel loss by readsorption of water in pores, as mentioned earlier. To evaluate the desorption of glycerol or hydrogel in coal pores, excessive water was added 16209

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(12) Carrettin, S.; McMorn, P.; Johnston, P.; Griffin, K.; Kiely, C. J.; Hutchings, G. J. Oxidation of glycerol using supported Pt, Pd and Au catalysts. Phys. Chem. Chem. Phys. 2003, 5, 1329−1336. (13) Brett, G. L.; He, Q.; Hammond, C.; Miedziak, P. J.; Dimitratos, N.; Sankar, M.; Herzing, A. A.; Conte, M.; Lopez-Sanchez, J. A.; Kiely, C. J.; Knight, D. W.; Taylor, S. H.; Hutchings, G. J. Selective oxidation of glycerol by highly active bimetallic catalysts at ambient temperature under base-free conditions. Angew. Chem., Int. Ed. 2011, 50, 10136− 10139. (14) Katryniok, B.; Paul, S.; Belliere-Baca, V.; Rey, P.; Dumeignil, F. Glycerol dehydration to acrolein in the context of new uses of glycerol. Green Chem. 2010, 12, 2079−2098. (15) Sato, Y.; Kushiyama, S.; Kondo, Y.; Takagi, H.; Maruyama, K.; Yoshizawa, N. Property of upgraded solid product from low rank coal by thermal reaction with solvent. Fuel Process. Technol. 2007, 88, 333− 341. (16) Sato, Y.; Kushiyama, S.; Tatsumoto, K.; Yamaguchi, H. Upgrading of low rank coal with solvent. Fuel Process. Technol. 2004, 85, 1551−1564.

combustion behavior, and it is limited in application for low rank coal which has high reactivity in nature. Consequently, Hybrid Coal with hydrogel can be the desired coal from combustion control for coal fired power plants, and spontaneous combustion and dustiness could be inhibited as well. For a demonstration plant for HCK, we are currently investigating the pore-filling method of coal without the excessive water as a solvent.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +82-42-860-3784. Fax: +82-42-860-3134 E-mail: [email protected]. Author Contributions †

S.-J.P. and D.-W.L. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a primary project of the Korea Institute of Energy Research (KIER) and the Korea Institute of Energy Technology Evaluation and Planning (KETEP)

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