Comparative Study of the Aromaticity of the Coal Structure during the

Article pubs.acs.org/EF

Comparative Study of the Aromaticity of the Coal Structure during the Char Formation Process under Both Conventional and Advanced Analytical Techniques Andrew O. Odeh* Coal Research Group, Unit of Energy Systems, School of Chemical and Minerals Engineering, North-West University, Potchefstroom Campus, Private Bag X6001, Potchefstroom 2520, South Africa ABSTRACT: Six coal samples of different rank, five southern hemisphere and one northern hemisphere, were studied using both conventional and advanced analytical techniques: scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), carbon nuclear magnetic resonance (C NMR), and X-ray diffraction (XRD). Apart from SEM that was used to study the coal to char morphology, the other analytical techniques were used to determine the molecular structural parameter of coal, specifically, the aromaticity, fa. The application of these techniques to the simulation of coal formation revealed the aromaticity to be between 0.86 and 1.03 for lignite, between 0.86 and 1.03 for sub-bituminous, between 0.87 and 1.03 for bituminous, between 0.88 and 1.03 for semi-anthracite, and between 0.94 and 1.03 for anthracite. This reported value for the aromaticity was obtained from the conventional method of analyses. Similar values showing the same consistencies in coal rank were obtained using the FTIR (between 0.66 and 0.79 for lignite, between 0.58 and 0.90 for sub-bituminous, between 0.84 and 1.00 for bituminous, between 0.94 and 1.00 for semi-anthracite, and between 0.97 and 1.00 for anthracite). The values obtained using both C NMR and XRD were at variance from those obtained using both the conventional and FTIR, which call into question the reliability, authenticity, and dependability of these sophisticated and expensive analytical techniques. It is therefore proposed for researchers and coal scientist to rely on the conventional analysis technique of determining aromaticity, which serves as a predictive index for char reactivity, to understand the data obtained from these advanced analytical techniques. realignment of the carbon molecules.19−24 The change in the carbonaceous structure because of the modification of the organic and inorganic constituents in coal and its subsequent char is stated to be one of the key factors that affect the reactivity of coal/char in coal conversion processes.9,22,25−27 The chemical transformation involves the change in the organic chemical structure, while the physical transformation involves a change in the char morphology and porosity. Several analytical techniques have been used extensively for the study of the coal molecular structure and the determination of the aromaticity of coal. Although some findings have demonstrated similarity in the results obtained from nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy (FTIR) of low-rank coals, the outcome for highrank coals has always been different. Other investigators have found it difficult to obtain the aromaticity of some types of coal using the X-ray diffraction (XRD), because they find the analytical technique challenging.27−29 They concluded that XRD is good for the determination of the structural sizes (interlayer spacing, crystallite diameter, crystallite height, etc.) and not for the determination of aromaticity.2,30−32 The NMR technique is not available to all research laboratories because the cost of operation is high. Coupled with the cost implications is the fact that there are inconsistencies in the aromaticity obtained from this technique.2,29−31

1. INTRODUCTION The increased focus on renewable sources of energy has made current research efforts on coal geared toward clean coal technology. The research efforts on this subject over the past 2 decades have focused on the chemical treatment of coal (coal demineralization) and, recently, on carbon capture and storage (CCS).1−10 Although prospects for full-scale commercialization of coal chemical treatment before utilization in the foreseeable future have been indirectly suggested by some investigators, current studies are limited to laboratory scale in the determination of the molecular and structural parameters that defines the technical performance of coal during coal conversion processes.11−14 The essence of demineralization is to remove or reduce the mineral content in coal because it has been reported that the mineral content in coal melts when coal is subjected to heat treatment during coal conversion processes, which results in blocking the carbon active sites, thereby reducing the reactivity of the coal and decreasing the emission of pollutants: NOx, SO2, and particulates.15−17 Coal is known to be the most abundant hydrocarbon on earth, and its heterogeneous aggregate is made of organic polymeric material with some inorganic impurities. The organic materials are known as macerals, while the inorganic impurities are considered as the minerals.5 When exposed to heat treatment, the physical, chemical, thermal, mechanical, and electrical properties of coal undergo transformation.18 One of the parameters that is used in measuring the chemical stability of this transformation is the aromaticity; it gives a good representation of the maceral to char transformation, which stands as a good indicator of coal maturity because of the © 2015 American Chemical Society

Received: December 3, 2014 Revised: March 3, 2015 Published: March 4, 2015 2676

DOI: 10.1021/ef502672d Energy Fuels 2015, 29, 2676−2684

Energy & Fuels

Article

In this work, NMR was applied to the characterization and quantitative structural study of only the parent coal because of the high cost of operation. The structural (morphology) changes that take place during the coalification process (coal to char) were observed with the aid of scanning electron microscopy (SEM), while the FTIR, XRD, and conventional analyses were employed in the characterization and quantitative structural study of the coalification process applying the structural parameter, aromaticity. Aromaticity as coal rank or maturity index may be defined as the measure of the degree of disorderliness of carbon within a coal matrix.20 The pathway for the formation of char is affected by the heating rate; therefore, in this study, chars produced at a low heating rate were used.

Table 1. Proximate Analysis, Ultimate Analysis, Calorific Values, and Calculated H/C and Aromaticity Values for Untreated Coal

2. EXPERIMENTAL SECTION 2.1. Sample. Six coals of varying rank were studied: a lignite coal from Germany, coded in this study as GER; a sub-bituminous coal from Nigeria, coded in this study as NGR; two bituminous coals from South Africa (in the case of the bituminous coals, one is low-volatile bituminous coal, coded in this study as BCH, and the other is highvolatile bituminous coal, coded in this study as SSL); a semi-anthracite from South Africa, coded in this study as SM; and an anthracite from South Africa, coded in this study as SPL. The coal samples were subjected to coal preparation and pulverized to a coal particle size of −75 μm. All of the samples were stored under argon prior to analysis. The prepared coal samples were demineralized to reduce the amount of mineral matter present in them as well as to minimize their influence during quantitative analysis. The procedure used for the chemical cleaning of the coal followed the sequential leaching with hydrofluoric acid (HF) and hydrochloric acid (HCl); a more detailed procedure is reported in the study by Strydom et al.11 2.2. Apparatus and Procedure. The char production sequence from the parent coal samples are as follows: The measured coal samples (40 g) were placed in a boat and put in a horizontal tube furnace at 60 °C and for 10 min, so that the condensed moisture is driven off. The sample temperature was equilibrated to ambient temperature and pressure in a flow of nitrogen at a flow rate of 1 L/ min. The furnace was then heated non-isothermally at 20 °C min−1 to the target temperature and held isothermally at the target temperature for 60 min. The target temperature varies from 450 to 700 °C, a temperature regime considered for char formation. The conventional chemical analyses (both proximate and ultimate analyses) of the untreated coal and acid-treated and heat-treated samples were performed according to the international ASTM 3172 and ASTM 3176 methods, respectively. The coal morphology and that of the resultant char obtained in the transition of coal to char were observed using SEM model FEI Quanta 250 with a field emission gun (FEG) emitter, equipped with an energy-dispersive spectrometry (EDS) detector that does the elemental composition analysis. The surface areas of the various samples were determined using the carbon dioxide adsorption Brunauer−Emmett−Teller (BET) method on a Micromeritics ASAP 2020 surface area analyzer. XRD was used to study the carbon crystallite properties of both the coal and char samples. The XRD scans were conducted on a PANalytical XRD X’Pert Pro powder diffractometer using both Co and Cu Kα radiation;a detailed procedure is reported by Wang et al.29 The spectra used in obtaining the structural properties of both the coal and char were obtained from the FTIR spectrometer equipped with attenuated total reflectance (ATR), model PerkinElmer spectrum 400. More information on the procedure and advantages of using the FTIR−ATR technique has been reported by Li et al.32 The results of the proximate, ultimate, and calorific values of the parent and demineralized coal samples are presented in Tables 1 and 2, respectively. Table 3 gives the calculated values of the atomic H/C and aromaticity (theoretical calculated values from conventional analysis and experimental values from FTIR and XRD techniques).

coal

SPL

SM

BCH

SSL

NGR

GER

inherent moisture (air dried) (wt %) ash (air dried) (wt %) volatile matter (air dried) (wt %) fixed carbon (air dried) (wt %) carbon (daf) (wt %) hydrogen (daf) (wt %) nitrogen (daf) (wt %) oxygen (daf) (wt %) sulfur (daf) (wt %) gross calorific value (MJ/kg) H/C fa

1.5

1.0

2.1

4.2

9.6

15.4

11.2

17.3

16.2

29.1

9.0

12.4

5.3

7.6

26.7

21.4

37.6

45.7

82

74.1

55.0

45.3

43.8

26.4

90.2

90.4

81.6

77.5

75.6

70.5

2.7

3.5

4.6

4.5

5.2

6.6

2.2

2.0

2.0

2.2

1.7

0.6

2.7

3.3

10.7

15.4

16.9

18.5

2.3

0.9

1.2

0.4

0.7

3.7

29.6

28.7

26.8

20.0

24.6

21.2

0.36 0.91

0.46 0.85

0.67 0.73

0.69 0.72

0.82 0.65

1.12 0.49

Table 2. Proximate Analysis, Ultimate Analysis, Calorific Values, and Calculated H/C and Aromaticity Values for Acid-Treated Coal coal

SPL

SM

BCH

SSL

NGR

GER

inherent moisture (air dried) (wt %) ash (air dried) (wt %) volatile matter (air dried) (wt %) fixed carbon (air dried) (wt %) carbon (daf) (wt %) hydrogen (daf) (wt %) nitrogen (daf) (wt %) oxygen (daf) (wt %) sulfur (daf) (wt %) gross calorific value (MJ/kg) H/C fa (CA) fa (FTIR) fa (C NMR) fa (XRD)

2.5

2.3

2.7

1.3

1.9

1.7

1.5

1.8

1.2

3.3

2.0

0.8

6.8

9.6

27.2

25.0

43.2

60.3

89.2

86.3

68.9

70.4

53.0

37.3

85.6

89.0

83.4

80.9

75.1

69.2

2.4

3.3

4.6

4.2

5.2

6.2

2.0

1.8

2.0

2.3

1.8

0.6

7.7

5.0

9.1

12.3

17.4

20.3

2.1

0.7

1.0

0.3

0.1

2.7

32.7

33.3

32.0

30.0

29.3

28.9

0.34 0.92 0.98 0.98 0.89

0.45 0.86 0.84 0.94 0.87

0.66 0.74 0.72 0.76 0.78

0.62 0.76 0.74 0.80 0.74

0.83 0.65 0.58 0.58 0.70

1.08 0.52 0.40 0.43 0.66

3. RESULTS AND DISCUSSION The coal samples are referred to as GER450, GER700, etc. to clearly indicate the sample identity and the temperature to which it was heat-treated. The coal samples are of different rank, and as such, it is expected that the chemical composition 2677


Energy & Fuels

Article

and aromatic hydrogen, while its aliphatic hydrogen decreases during coalification. It was also observed that there were slight changes in the values obtained for both the parent coal samples and the acid-treated coal samples for the lower rank coals. As expected, the increase in the calorific value between the parent and demineralized coal with increasing coalification is as a result of the decrease in the ash content (Tables 1 and 2). It can therefore be established that, although there was no noticeable impact on the structural parameters of the lower rank coal by acid treatment, it does impact the amount of chemical energy that can be potentially released because the calorific values tend to increase with coals subjected to acid cleaning. The anticipation that, during the coalification process, devolatilization, aromatization, and rearrangement of the basic structural units (BSUs) would have taken place made the need to extend the calculation of the aromaticity to the char formation of the process. Significant transformational changes were observed in the values of aromaticity obtained. The trend obtained for the aromaticity was consistent for all coal samples used, in that the aromaticity was increasing with the increase in the coalification temperature: lignite to anthracite. The aromaticity was determined to be in the range of 0.86−1.03 for lignite, 0.86−1.03 for sub-bituminous, 0.86−1.03 for bituminous, 0.88−1.03 for semi-anthracite, and 0.94−1.03 for anthracite. The ranges of the aromaticities obtained in this study are within the limits of error associated with the measurements used in calculating the values. The H/C values decrease with the increase in the heating temperature, while the aromaticity increases with the increase in the heating temperature (Table 3). This could be attributed to the chemical modification (devolatilization, removal of aliphatic groups, and heteroatoms) that takes place during carbonization, which results into the ordering of the internal structure of the samples.5,7,8,10,33 It has been presumed that all carbon atoms present in coal are either aliphatic or aromatic in nature, neglecting the contribution that comes from the carbon in carboxylic acids. However, from this study, the contribution that comes from the high proportion of the carboxylic group in the low-rank coals seems to account for the differences in the aromaticity between the lower ranks and the high-rank coals. Everson et al.33 observed that the aromaticity of heat-treated coal increases with the decrease in the intrinsic reaction rate, which implies that more structurally ordered and oriented carbon crystallites are obtained as the coalification process proceeds to higher temperatures. Everson and co-workers33 concluded that reactivity of coal decreases with the increase in the coal aromaticity. The morphology of the coalification process was observed using SEM−EDS (Figure 1, using the NGR coal for illustration), from where the atomic mass O/C ratio was obtained and the surface area data generated from the BET facility both give a good account of the physical changes that take place during the coalification process. The values of atomic O/C obtained from SEM−EDS were used in correlating the surface area from BET because it has been confirmed that oxygen determined from the conventional analyses method is always underestimated because inorganic sulfur is included in the estimation of oxygen (by difference) from the ultimate analysis data, even though the calculation is performed on a dry and ash-free basis.34 The relationship showed that, for every decrease in atomic O/C, there is a corresponding increase in the surface area during the coalification process (Figure 2). It is generally known that the diffusion of oxygen to and within the

Table 3. Calculated H/C and Aromaticity Values for HeatTreated Coal temperature (°C)

450

H/C fa (CA) fa (FTIR) fa (XRD)

0.45 0.86 0.66 0.66


0.45 0.86 0.75 0.67


0.42 0.87 0.84 0.91


0.45 0.86 0.83 0.93


0.40 0.88 0.94 0.96

H/C fa fa (FTIR) fa (XRD)

0.31 0.94 0.97 0.96

500 GER 0.40 0.89 0.69 0.67 NGR 0.38 0.90 0.78 0.69 SSL 0.36 0.91 0.88 0.94 BCH 0.39 0.89 0.86 0.94 SM 0.40 0.89 0.95 0.98 SPL 0.28 0.95 0.98 0.97

550

600

650

700

0.28 0.95 0.73 0.68

0.28 0.95 0.74 0.72

0.21 0.99 0.76 0.74

0.13 1.00 0.79 0.76

0.32 0.93 0.81 0.70

0.26 0.96 0.84 0.74

0.16 1.00 0.87 0.78

0.13 1.03 0.90 0.80

0.31 0.93 0.90 0.96

0.27 0.96 0.93 0.97

0.20 1.00 0.97 0.97

0.09 1.05 1.00 0.97

0.34 0.92 0.89 0.97

0.29 0.95 0.92 0.98

0.22 0.98 0.95 0.99

0.14 1.03 1.00 0.99

0.34 0.92 0.98 0.99

0.28 0.95 1.00 0.99

0.21 0.99 1.00 0.99

0.13 1.03 1.00 0.99

0.28 0.95 1.00 0.98

0.25 0.97 1.00 0.99

0.22 0.98 1.00 0.99

0.13 1.03 1.00 0.99

and physical properties will show some considerable differences in their behavior under heat treatment. The reason to reduce or eliminate the interference of minerals in coal utilization processes and to focus on the coal (maceral) to char transformation can be considered as the motivation for demineralizing all of the coal samples used in this study. It has been reported that the structural transformation of maceral to char is as a result of the realignment of the carbon molecules, which could lead to swelling, while the minerals are transformed to ash.16 As expected, the atomic mass ratio of hydrogen/carbon (H/C) increases as the coal rank decreases, with the lignite coal sample having the highest ratio of 1.1 (Table 1). A similar trend was observed for the acid-treated coal samples in that the atomic ratio (H/C) increases as the coal rank decreases, but the values obtained for the higher rank coal (from bituminous to anthracite) were a bit lower when compared to the parent coal samples (Table 2). However, for the lower rank coal, lignite and sub-bituminous, there were no changes in the values obtained for the H/C ratio, indicating that there was little or no impact of demineralization on the lower rank coals.1,8,11 The theoretical apparent aromaticity was estimated using the relationship by Orrego-Ruiz et al.5 fa = −0.5438H/C + 1.16

(0.95)

(1)

The values obtained using this scheme revealed that the aromaticity increases with increasing the coalification temperature. In other words, coal increases both its aromatic carbon 2678


Energy & Fuels

Article

Figure 1. SEM micrographs of the transition of NGR coal to char.

Figure 2. Variation of the surface area with atomic O/C for lignite (GER).

2679


Energy & Fuels

Article

Table 4. Calculated Atomic O/C and BET Surface Area Values from SEM and ASAP 2020 for Heat-Treated Coal temperature (°C)

450

500

550

600

650

700

0.092 230.41

0.073 241.82

0.064 262.61

0.056 268.56

0.083 183.19

0.075 234.10

0.067 238.14

0.061 239.74

0.063 199.72

0.052 200.38

0.051 214.46

0.042 224.19

0.044 183.89

0.039 206.40

0.037 215.40

0.029 224.95

0.033 170.35

0.033 186.54

0.037 194.60

0.032 196.99

0.063 136.74

0.039 150.98

0.037 162.47

0.036 164.40

GER O/C BET surface area (m2/g)

0.132 169.96

0.103 193.97

O/C BET surface area (m2/g)

0.130 155.78

0.110 182.61


0.081 136.60

0.076 153.47


0.064 130.17

0.057 158.68


0.039 137.94

0.042 148.17


0.039 113.93

0.048 135.18

NGR

SSL

BCH

SM

SPL

−1 A A ar = 900 − 700 cm A al A3000 − 2815 cm−1

char particle influences the rate of char burning and, hence, the char morphology (Figure 1). This transformational change could be attributed to be as a result of the removal of all of the heteroatoms and light compounds from the coal samples and could also be related to the changes in the amount, dimensions, and shape of the pores of the coal because of the heat treatment.27,35 The surface area reported ranges from 169.96 to 268.56 m2/g for lignite, from 155.78 to 239.74 m2/g for subbituminous, from 130.17 to 224.95 m2/g for bituminous, from 137.94 to 196.99 m2/g for semi-anthracite, and from 113.93 to 164.40 m2/g for anthracite, which shows the trend of decreasing the surface area with increasing the rank in the coalification process. That is, the surface area increases with the increase in the pyrolysis temperature, which confirms the removal of light compounds and heteroatoms. The deceasing atomic O/C ratio with increasing the pyrolysis temperature corroborated this physical transformational change of coal to char, which implies that this change in surface area would lead to a change in the pore structure, resulting in the proportional change in the char conversion rate36 (Table 4). Earlier work by Solum and co-workers37 also revealed that the decreasing O/C with increasing the temperature could be attributed to the release or elimination of oxygen functional groups, such as ketones and esters, which could result in structural rearrangement and increased alignment of clusters that lead to a more densely packed structure with reduced porosity. The atomic structure of carbonaceous materials has been extensively examined by XRD, FTIR, NMR, high-resolution transmission electron microscopy (HRTEM), and other advanced techniques but not in the temperature range considered in this study (1−7). The aromaticity of the coal to char transition was derived using a FTIR apparatus equipped with ATR, which has the advantage of reducing the sample preparation time, no scattering of radiation, good signal-tonoise level, and a good optical contact of the sample. The peaks obtained from the spectra were assigned to different functional group bands, from which the band regions of 2800−3100 and 900−700 were deconvulated. From the area under the deconvulated band, the ratio of the aromatic stretching to aliphatic stretching, Aar/Aal, given in eq 25 was used to calculate the aromaticity (Table 3).

(2)

The values obtained from the FTIR technique showed a similar trend to the values obtained from the conventional technique (ultimate analysis) but slightly lower value for lower rank coals (GER and NGR), which could be attributed to instances of coplanar coalescence of smaller nuclei because of pressureinduced orientation and packing; the aromaticity is in the range of 0.66−0.79 for GER, 0.75−0.90 for NGR, 0.84−1.00 for BCH, 0.83−1.00 for SSL, 0.94−1.00 for SM, and 0.97−1.00 for SPL. It is worth noting here that aromaticity increases as the coalification temperature increases, indicating the structural rearrangement of the carbonaceous coal material. In comparison to the theoretical values obtained from the conventional technique, the results showed very good agreement. The XRD technique is a known and tested non-destructive method of analyzing carbonaceous materials, such as graphite, coal, and char. Different structural parameters, such as interlayer spacing, aromaticity, crystallite sizes (average crystallite diameter and average crystallite height), degree of disorder, and other structural properties of carbonaceous materials, have been derived from the XRD spectrum. The aromaticity of the coal to char transition was computed using the following expression:20 fa =

Car A 002 = Car + Cal A 002 + A γ

(3)

The aromaticity obtained using the XRD technique was in the range of 0.66−0.76 for GER, 0.67−0.80 for NGR, 0.91−0.97 for SSL, 0.93−0.99 for BCH, 0.96−0.99 for SM, and 0.96−0.99 for SPL. These values were obtained after much spectral correction and profile fitting, which makes the technique not suitable in the temperature regime considered in this study. Similar challenges have been reported by other researchers because the γ band that represents the aliphatic carbon chains seems not to exists in most of the spectra generated or rather the non-aromatic carbon proved difficult to be detected in the spectra.3,15,25,28,30 The results obtained showed a similar trend and consistency with the results derived from both the conventional and FTIR techniques because the changes 2680


Energy & Fuels

Article

structural size parameter was observed with a corresponding increase in the heating temperature for all of the coal samples investigated. The C NMR of the char formation was not performed in this study, but the comparison of the values of the aromaticity obtained for the parent coal to other analytical techniques discussed in this study shows a variation in the values obtained using C NMR. The values obtained using this technique were a bit higher than the values derived using other analytical techniques, which makes this analytical technique to be considered a complicated technique for this study (Table 2). It is observed that the aromaticity of all of the coal samples gave a good linear relationship with the elemental parameters (Figures 3−7).

observed were an increase in the proportion of aromatic carbon through the lower rank coals (GER and NGR), an increase in the proportion of aromatic carbon through the low-temperature range in the higher rank coals, and an increase in the size of the aromatic structure through the high-temperature range, that is, from 600 to 700 °C. The average structural sizes of the coal investigated are presented in Table 5. A slight increase in the Table 5. Calculated Structural Parameters from XRD Analysis temperature (°C)

450

500

550

600

650

700

GER crystallite height, Lc (Å) interlayer spacing, d002 (Å)

14

crystallite height, Lc (Å) interlayer spacing, d002 (Å)

14


18


16

3.67

14

14

3.68

3.70

13 3.74

13 3.76

13 3.78

NGR

3.66

14

14

3.66

3.66

14 3.68

13 3.71

13 3.78

SSL

3.54

17

16

3.56

3.60

15 3.61

15 3.63

13 3.64

BCH

3.55

16

16

3.55

3.56

15 3.57

15 3.58

13 3.60

Figure 4. Trend of atomic H/C with the temperature.

SM crystallite height, Lc (Å) interlayer spacing, d002 (Å)

20


20

3.47

19

18

3.48

3.51

17 3.51

15 3.52

14

Figure 3 gives the relationship between aromaticity and atomic mass H/C ratio. As seen, the aromaticity increases as the atomic H/C decreases, which confirms the removal of small aliphatic carbon chains in the coal matrix as a result of increasing the temperature. A similar trend of decreasing the atomic H/C ratio with the pyrolysis temperature is observed, as revealed in Figure 4, with all of the coals approaching the same value of 0.13 at the final pyrolysis temperature of 700 °C. A good linear relationship as reported of the aromaticity with

3.52

SPL

3.47

19 3.51

18 3.51

18 3.51

17 3.52

13 3.52

Figure 3. Transitional changes of aromaticity with H/C. 2681


Energy & Fuels

Article

Figure 5. Variation of aromaticity with H/C for lignte (GER).

Figure 6. Variation of aromaticity with O/C for lignite (GER).

Figure 7. Variation of aromaticity with the carbon content. 2682


Energy & Fuels

■

ACKNOWLEDGMENTS The work presented in this paper is based on the research supported by the South African Research Chairs Initiative of the Department of Science and Technology and the NRF (Coal Research Chair Grant 86880).

elemental parameters, such as the atomic mass H/C and O/C, with the regression value indicated using the lignite coal for illustration are depicted in Figures 5 and 6, respectively. Figure 7 shows the plot of aromaticity against percent mass carbon, which reveals that, as the aromaticity increases, the percent carbon increases, which could be attributed to the aromatization of the coal structure.

■

REFERENCES

(1) Ibarra, J. V.; Munoz, D.; Moliner, R. Organic FTIR study of the evolution of coal structure during the coalification process. Geochemistry 1996, 24, 725−735. (2) Li, Y.; Cao, X.; Zhu, D.; Chappel, M. A.; Miller, L. F.; Mao, J. Characterisation of coals and their laboratory-prepared black carbon using advanced solid-state 13C nuclear magnetic resonance spectroscopy. Fuel Process. Technol. 2012, 96, 56−64. (3) Lu, L.; Sahajwalla, V.; Harris, D. Characteristics of chars prepared from various pulverized coals at different temperatures using drop-tube furnace. Energy Fuels 2000, 14, 869−876. (4) Lunden, M. M.; Yang, N. Y. C.; Headley, J.; Shaddix, C. R. Mineral−char interactions during char combustion of a high volatile coal. Symp. (Int.) Combust., [Proc.] 1998, 27, 1695−1702. (5) Orrego-Ruiz, R. A.; Cabanzo, R.; Mejia-Ospino, E. Study of Colombian coals using photoacoustic Fourier transform infrared spectroscopy. Int. J. Coal Geol. 2011, 85, 307−310. (6) Painter, P.; Sobkowiak, M.; Heidary, S.; Coleman, M. Current status of FT-IR in the analysis of coal structure. Fuel 1994, 39, 49−53. (7) Pusz, S.; Duber, S.; Kwiecinska, B. The study of textural and structural transformations of carbonized anthracites. Fuel Process. Technol. 2002, 77, 173−180. (8) Rodrigues, S.; Suarez-Ruiz, I.; Marques, M.; Camean, I.; Flores, D. Microstructural evolution of high temperature treated anthracites of different rank. Int. J. Coal Geol. 2011, 87, 204−211. (9) Sekine, Y.; Ishikawa, K.; Kikuchi, E.; Matsukata, M.; Akimoto, A. Reactivity and structural change of coal during steam gasification. Fuel 2006, 85, 122−126. (10) Supaluknari, L.; Larkins, F. P.; Redlich, P.; Jackson, W. R. Determination of aromaticities and other structural features of Australian coals using solid state 13C NMR and FTIR spectroscopies. Fuel Process. Technol. 1989, 23, 47−61. (11) Strydom, C. A.; Bunt, J. R.; Schobert, H. H.; Raghoo, M. Changes to the organic functional groups of an inertinite rich medium rank bituminous coal during acid treatment process. Fuel Process. Technol. 2011, 92, 764−770. (12) Tsai, C.-T.; Scaroni, A. W. The structural changes of bituminous coal particles during the initial stages of pulverized-coal combustion. Fuel 1986, 66, 200−206. (13) van Niekerk, D.; Pugmire, R. J.; Solum, M. S.; Mathews, J. P. Structural characterisation of vitrinite-rich and inertinite-rich Permainaged South African bituminous coals. Int. J. Coal Geol. 2008, 76, 290− 300. (14) Zhu, W.; Song, W.; Lin, W. Effect of particle size on pyrolysis and char reactivity for two types of coal and demineralised coal. Energy Fuels 2008, 22, 2482−2487. (15) Cousins, A.; Paterson, N.; Dugwell, D. R.; Kandiyonti, R. An investigation of the reactivity of chars formed in fluidized bed gasifiers: The effect of reaction conditions and particle size on coal char reactivity. Energy Fuels 2006, 20, 2489−2497. (16) Oboirien, B. O.; Engelbrecht, A. D.; North, B. C.; du Cann, V. M.; Verryn, S.; Falcon, R. Study on structure and gasification characteristics of selected South African bituminous coal in fluidised bed gasification. Fuel Process. Technol. 2011, 92, 735−742. (17) Zhang, H.; Pu, W.; Ha, S.; Li, Y.; Sun, M. The influence of included mineral on the intrinsic reactivity of chars prepared at 900 °C in a drop tube furnace and muffle furnace. Fuel 2009, 88, 2303−2310. (18) Neavel, R. C. Origin, petrography and classification. In Chemistry of Coal Utilization: Second Supplementary Volume; Lowry, H. H., Elliott, M. A., Eds.; Wiley, New York, 1981. (19) Jasienko, S.; Matuszewska, A.; John, A. Properties and structure of hard coals from the borehole Niedobczyce IG-1 in the Rybnik Coal

4. CONCLUSION Aromaticity has been considered as a key parameter in the analysis of coal maturity. Different analytical techniques have been employed in the determination of aromaticity, such as XRD, FTIR, and C NMR, but aromaticity determination from conventional analysis has not really been given some seriousness by researchers because this technique is considered as the basis in analyzing carbonaceous materials. However, in this study, the aromaticity of coal subjected to heat treatment was determined by both the conventional (ultimate analysis) and advanced (FTIR and XRD) analytical techniques. The results obtained from all of the techniques employed showed a similar trend of increasing aromaticity with coal rank and increasing aromaticity with increasing the heating temperature in the coalification process. The trend could be explained from the conventional analysis results; in that, it is obvious with the release of volatiles associated with saturated hydrocarbons during heat treatment that there would be a decrease in aliphatic side chains, which inevitably leads to increases in the aromaticity of the coal. With so much uncertaintities still surrounding the application of some of these advanced analytical techniques in the determination of the aromaticity of coal, it is noted that research could still be performed with the basic conventional facilities in the determination of the structural parameters of coal. It is therefore suggested that researchers can still rely on the conventional technique in the determination of aromaticity in understanding the molecular structure of coal. In other words, it is a call for coal scientist to reconsider the use of elemental analysis data as the basis for structural analysis. Again, all of the analytical techniques to some extent seem to comprehend to one another in obtaining an understanding of the heterogeneous nature of coal: functional groups are better interpreted using FTIR; the aromaticity is better interpreted using C NMR; molecular weight distribution is better interpreted using HRTEM; morphology is better interpreted using SEM; and atomic O/C and H/C are better interpreted using the conventional technique. However, the technique employed by an individual researcher is subject to cost and availability of facilities. Finally, a good linear relationship between the aromaticity and atomic H/C and O/C was established, which reflects the dependency of the coal structures upon their ranks.

■

Article

AUTHOR INFORMATION

Corresponding Author

*Telephone: +27-78-207-5496. Fax: +27-86-654-5434. E-mail: [email protected]. Notes

Disclaimer: Any opinion, finding, conclusion, or recommendation expressed in this material is that of the author, and the National Research Foundation of South Africa (NRF) does not accept any liability in this regard. The authors declare no competing financial interest. 2683


Energy & Fuels

Article

District, Upper Silesian Coal Basin, their petrographic and group constituents. 2. Variations in petrographic composition of the coals along the depth of borehole and alterations in structure of the coals characterized by vitrinites spectroscopic analyses (X-ray, IR). Fuel Process. Technol. 1995, 41, 221−232. (20) Lu, L.; Sahajwalla, D.; Kong, C.; Harris, D. Quantitative X-ray diffraction analysis and its application to various coals. Carbon 2001, 39, 1821−1833. (21) Hurt, R.; Sun, J. K.; Lunden, M. A kinetic model of carbon burnout in pulverized coal combustion. Combust. Flame 1998, 113, 181−197. (22) Rouzand, J. N.; Oberlin, A. Structure, microtexture, and optical properties of anthracene and saccharose-based carbons. Carbon 1989, 27, 517−529. (23) Senneca, O.; Salatino, P.; Masi, P. Microstructural changes and loss of gasification reactivity of chars upon heat treatment. Fuel 1998, 77, 1483−1493. (24) Senneca, O.; Salatino, P.; Menghini, D. The influence of thermal annealing on oxygen uptake and combustion rates of a bituminous coal char. Proc. Combust. Inst. 2007, 31, 1889−1895. (25) Li, C. Z. Some recent advances in the understanding of the pyrolysis and gasification behaviour of Victorian brown coal. Fuel 2007, 86, 1664−1683. (26) Suarez-Ruiz, I.; Garcia, A. B. Optical parameters as a tool to study the microstructural evolution of carbonized anthracites during high-temperature treatment. Energy Fuels 2007, 21, 2935−2941. (27) Maity, S.; Choudhury, A. Influence of nitric acid treatment in different media on X-ray structural parameters of coal. Energy Fuels 2008, 22, 4087−4091. (28) Wang, J.; Du, J.; Chang, L.; Xie, K. Study on the structure and pyrolysis characteristics of Chinese western coals. Fuel Process. Technol. 2010, 91, 430−433. (29) Matsuoka, K.; Akahane, T.; Aso, H.; Sharma, A.; Tomita, A. The size of polyaromatic layer of coal char estimated from elemental analysis data. Fuel 2008, 87, 539−545. (30) Matsuoka, K.; Suzuki, Y.; Eylands, K.; Benson, S.; Tomita, A. CCSEM study of ash forming reactions during lignite gasification. Fuel 2006, 85, 2371−2376. (31) McBeath, A. V.; Smernik, R. J.; Schneider, M. P. W.; Schmidt, M. W. I. Determination of the aromaticity and degree of aromatic condensation of a thermosequence of wood charcoal using NMR. Org. Geochem. 2011, 42, 1194−1202. (32) Li, Z.; Fredericks, P. M.; Rintoul, L.; Ward, C. R. Application of attenuated total reflectance micro-Fourier transform infrared (ATR− FTIR) spectroscopy to the study of coal macerals: Examples from the Bowen Basin, Australia. Int. J. Coal Geol. 2007, 70, 87−94. (33) Everson, R. C.; Okolo, G. N.; Neomagus, H. W. J. P.; dos Santos, J.-M. X-ray diffraction parameters and reaction rate modeling for gasification and modelling of chars derived from inertinite-rich coals. Fuel 2013, 109, 148−156. (34) Perry, S. T.; Hambly, E. M.; Fletcher, T. H.; Solum, M. S.; Pugmire, R. J. Solid-state C NMR characterization of matched tars and chars from rapid coal characterization. Proc. Combust. Inst. 2000, 28, 2313−2319. (35) Choudhury, N.; Biswas, S.; Sarkar, P.; Kumar, M.; Ghosal, S.; Mitra, T.; Mukherjee, A.; Choudhury, A. Influence of rank and macerals on the burnout behaviour of pulverised Indian coal. Int. J. Coal Geol. 2008, 74, 145−153. (36) Ahmed, I. I.; Gupta, A. K. Particle size, porosity and temperature effects on char conversion. Appl. Energy 2011, 88, 4667−4677. (37) Solum, M. S.; Pugmire, R. J.; Jagtoyen, M.; Derbyshire, F. Evolution of carbon structure in chemically activated wood. Carbon 1995, 33, 1247−1254.

2684


Comparative Study of the Aromaticity of the Coal Structure during the

Recommend Documents