Indian Journal of Pure & Applied Physics Vol. 40, March 2001, pp. 207-212

FT-IR analysis of N eyveli lignite and fly ash samples R Venkatachalapathy* & C Manoharan

Department of Physics, Faculty of Engineering & Technology, Annamalai University, Annamalainagar 608 002

and

T Sridharan & C M Basilraj

Department of Physics, Annamalai University, Annamalainagar 608 002

Received 24 Jul y 2001; revised 19 November 200 I; accepted 31 December 200 I

Infrared spectroscopy has been recognised as a promising technique for the study of chemical structure and components present in the lignite samples. The extinction coefficient (K) values for aromatic-CH (3030 cm-1

) and aliphatic -CH (2920 and 2850 cm-1

) groups present in the lignite samples were calculated and compared with atomic H/C and 0 /C ratios. It is found that Neyveli ligni te formation belongs to Tertiary period of very low rank coals . The probable period of formation is found to be miocene and oligocene-palaeocene period. The flyash spectrum reveals the removal of all organic compounds present in li gnite and the presence of small amounts of refracting materials of inorganic nature like Si02,Fe0 and FeS.

1 Introduction

Large lignite deposits are found at Neyveli in Tamilnadu. The lignite deposits extend over an area of 330 km2 and the estimated lignite reserve is of the order of 3300 million tonnes. The Neyveli lignite field lies between latitudes II o 15' and II 0 45' N and longitudes 79° 15' to 79°46' E. Presently, the lignite is excavated from two mine-cuts namely, mine-cut I and mine-cut II by Neyveli Lignite Corporation through open-cast mining. Neyveli Thermal Power Stations I and II are the major consumers of this lignite producing power of about 600 MW and 1470 MW, respectively .

To identify the essential components, ranking and also to fix the probable age of formation of the Neyveli lignite deposits, Ff-IR spectral analysis of lignite samples collected at different depths from mine-cuts I and II and flyash samples has been carried out.

2 Experimental Details

The thickness of lignite layers both in mines I and II extends to a depth of 15 m and 16 m, respectively. Representative lignite samples were collected at three different depths I m (m1-l), 9 m (m 1-9), and 15 m (M 1-15) from the top surface to bottom of the lignite seam from mine-cut I.

Similarly at depths I m (M2-I ), 9 m (M 2-9) and 16 m (m2-16) from mine-cut II.

The Ff-IR spectra of the lignite and flyash samples were recorded by using Bruker IFS 66V Infrared spectrometer in the region 4000 to 400 em 1 by standard KBr pellet technique. The specific extinction coefficient (k) values were calculated for the bands at 3450, 2900, 2835, 1695, 1600 cm 1 etc.

3 Results and Discussion

Coal is a metamorphosed degradation product of an initially complex biological material, which has no single unique molecular structural unit. Although, a coal sample may contain more than one repetition of a structural unit (eg. benzene, naphthalene, ring system), that unit is believed to have different groups randomly connected to it, all or most of the time. However, average molecular parameters have been derived from a variety of chemical and spectroscopic measurements.

In the present investigation, an attempt has been made to study the structural analysis of the functional groups present in the Neyveli lignite samples and to establish the rank, mode of formation and to fix the period of formation by Ff­IR spectroscopic studies.

208 INDIAN J PURE & APPL PHYS, VOL 40, MARCH 2002

Table I - Ff-IR absorpt ion frequencies, relative intensity estimate and probable assignments of Neyveli lignite samples at different depths from mine-cuts I and II

M1-l M 1-9 M 1-l5 MT I Mr9 Mr16 Tentative Assign-ments< 1.5.6.7.R. IIIl

Freq lnt Freq lnt Freq lnt Freq lnt Freq In! Freq lnt em·' em· ' em· ' em· ' em· ' em· '

3676 vw OH-Stretching

3596 vw OH-Stretching

3400 s 3400 s 3400 s 3400 s 3400 s 3400 s OH-Stretching (0. 171 )* (0 .1 33) (0. 119) (0.098) (0.1 04) (0. 121)

3300 M 3300 w 3300 w OH-Stretching

3030 w 3030 w 3030 w Aromatic CH (0. 161 ) (0.075) (0.1 99) Stretching

2920 s 2920 s 2920 s 2920 s 2920 s 2920 s Aliphatic CH3

(0. 141 ) (0.087) (0.059) (0.073) (0. 165) (0.161 )

2850 M 2850 M 2850 vs 2850 M 2850 M 2850 M Aliph at ic CH2

(0.23 1) (0.186) (0. 132) (0.098) (0.222) (0. 16 I)

1610 vs 1610 vs 1610 VS 16 10 vs 1610 vs 16 10 vs Aromatic C=C (0.429) (0.435) (0.250) (0.399) (0.435) (0.43) Stretching

1500 M Benzene ring

1450 s 1450 s 1450 M 1450 s 1450 s 1450 s Methylene (0.268) (0262) (0. 170) (0.24 1) (0.280) (0.271) groups

1370 M 1370 M 1370 M 1370 M 1370 M 1370 M Methyl groups (0.235) (0.239) (0.299) (0.249) (0.250)

1273 s 1273 M 1273 s 1273 M 1273 M 1273 M C-0 Stretching of phenols

1117 M 1117 M 1117 M I i 17 M 1117 M 1117 M C-0 Stretching (0.238) (0.254) (0. 1 50) (0.299) (0.243 ) (0. 197) of phenols

1030 vw 1030 vw 1030 vw Si -0 Stretching

942 vw CH out of plane bending

870 vw 870 vw 870 vw 820 vw 820 vw 820 vw Aromatic CH (0.029) (0.033) (0.0 17) (0.020) (0.033) (0.011) out of plane

bending

766 vw 766 vw 766 vw 766 vw 766 vw 766 vw Aromatic CH (0.012) (0.0 17) (0.0 11 ) (0.008) (0.020) (0.01 I) out of plane

bending

535 vw 535 vw 535 vw 535 vw 535 vw 535 vw Fe-0 Stretching (0.0 17) (0.005) (0.008) (0.014) (0.002) (0.071 )

422 vw 422 vw 422 vw Fe-S Stretching

406 w 406 w Fe-S Stretching

YS =Very strong; S=Strong; M=Medium; W=Weak; YW=Very weak* K-values

Ff-IR spectra of lignite samples collected at coefficient values were calculated for all the bands

three different depths from mine-cuts I and II are by using the relation 1:

shown in Fig. I and the observed IR absorption D = Km/A (g/cm2)

bands are given in Table I . The specific extinction

VENKATACHALPATHY et al.:Ff-IR ANALYSIS OF LIGNITE & FLY ASH SAMPLES 209

where D is optical density, m, mass of the lignite sample in KBr disc (2 mg of lignite dispersed in 40 mg of KBr) of diameter 13 mm and A, the area of the KBr disc. Weight-percentage of carbon, hydrogen, nitrogen and oxygen present in the samples from their as received state by chemical analysis are given in Table 2 and calculated specific extinction coefficients (K) are given in Table I.

All the samples from mine-cut I and mine-cut II show a broad and strong absorption band at 3400 cm 1 which can be attributed to hydroxyl stretching of water molecules. This is well reflected from the observation of Osava & Shih 1

, Zubukova2, Subkowiak et al. 3 and Chomanti et al. \ in the case of coal oil-shale and lignite samples.

A weak broad absorption band appeared at 3030 cm·1 the case of M 1- l sample, which indicates the presence of aromatic CH-hydrogen . This absorption

band becomes clear when the carbon content of the coal is 81 % and as the rank increases. This can be ascribed to the development of aromatic structures in coal of high rank and also to the replacement by hydrogen of aliphatic side chains attached to aromatic carbon on the lines suggested by Fuji et al. 5 who observed thi s band at 3030 cm 1 in the Japanese coal, which supports the characteristic of samples.

The absence of the weak band at 3030 cm 1

characteristic of aromatic CH-groups in the Neyveli lignite samples M1-9, Mr15 and Mr 16 is also indicative of the fact that the aromatic condensation has not set in a clear indication of early stage of formation of wood peat to lignite stage. All these samples show remarkable similarity with the wood peat spectra.

40 00 3500 3000 2500 2000 1500 1000 500 Wavenumber (em -1

)

Fig. I - FT- IR spectra of Neyveli lignite samples from mine-cuts I and 2

210 INDIAN 1 PURE & APPL PHYS, VOL 40, MARCH 2002

Table 2- Chemi cal analys is of Neyvel i lignite samples from mine-cut I and mi ne-cut II and their H/C and 0/C ratios

Sample No. % C % H % N % 0 H/C 0 /C M I - I 56.9 4.9 1 1.00 37.19 1.0355 0.4902 M I - 9 58.5 5.10 0.9 35.5 1.0461 0.455 1 M1 - 15 61.2 6.3 1. 1 31.4 1.2352 0 3848 Mr I 54.3 4.6 0.9 40.2 1.01 65 0 5552 M2 - 9 5.5.8 4.4 38.8 0.9462 0 52 15 Mr 16 56.8 4.5 1.1 37.6 0.9507 0.4964

·The K-value of a ll the lignite samples from mine-cut I and mine-cut II has an average 0.2185 , fo r average value of carbon content 56.8%. The present values may favourabl y be compared with the va lue of 0.2 in the case of low-rank coals (73%) supported by Fuji et aP . From the chemical analys is, the H/C va lues for a ll Neyveli lignite sample ranging from 0 .94 to 1.23 compare favourably well with the values of 0.792 to 1.22, in the case of Austra lian brown coal samples (68% C to 70.5% C) reported by Supaluknari et af6 .

There is a sharp med ium band observed at 2850 cm·1 in a ll the samples which can be attributed to the presence of the aliphatic CH2 group. Chomnanti et a /. 4 observed simil ar sharp bands at 2830 cm·1 in the case of tertiary lignites. The sharpness of the bands in t.h e present samples ind icates that the probable age of the li gnites as be longing to tertiary

for mati ons.

A very pronounced absorption band at 1600 cm·1 is present in a ll the Neyveli ligni te samples and it can be attributed to aromatic ring stretch of small , discrete aromati c groups present in Ney ve li lignite samples.

T here is a medium band which appears at 1500 c tl1" 1 due to benzene rings present in the mine-cut II (M 2- I ). T hi s medium band indicates that the presence of benzene rings in phenyl propane type structures and also indicates that these samples be long to ple istocene-miocene age. The samples from mine-cut-I and Mr9, M,- 16 from mine-cut II :1t deeper depths show no peak at 1500 cm·1

• T he absence of thi s band shows that these samples have been subjected to aromati zation due to long peri od after fo rmation and hence be longing to an earlier period, prefe rab ly they may be safely ass igned to Oi lgocene-Palaecocene period .

4000 3500 3000 2500 2000 1500 1000 500 Wavenumber (em -t)

Fig. 2- FT-IR spectru m or Neyveli tlyash sample

Buravas et al. 7 studied the infrared spectra of tertiary lignites and woody peat. They observed th at the bands at 1500, 1450, 14 10, 1370 cm 1

, and to a lesser ex tent around 1200 cm 1

, whe re li gnite shows a pl ateau of more or less un reso lved absorption bands. From the ir observati ons they concluded that, band at 1500 c m·1 can be associated with the presencP. of lignin i.e with the presence of phenyl propane type structures and it can be used to he lp in the dating of so lid fuels. P leistocene lignites and woody peat showed absorption bands at 1500, 1450, 14 10, cm 1

• Pil ocene-pa laeocene (66-74%) lign ites showed no peak at 1500 em 1 and broad abso rption throughout 1450- 1410 cm·1

•

There is a strong and mediu m band observed at 1200- 1300 cm·1 are assigned as C-0 stretching of phenols in a ll the li gnite sample . T hi s is we ll refl ected by Burvas et af.

VENKATACHALPATHY et al.:Ff-IR ANALYSIS OF LIGNITE & FLY ASH SAMPLES 211

The medium band observed in the region 1100-1200 em' in all the lignite samples is assigned to C­O stretching of phenolic groups. Cannon et al. R

found the IR bands in the case of Rawhide sub­bituminous and Texas lignite samples and assigned the bands to C-0 stretching of phenolic hydroxyl groups. The plateau like unresolved absorption region 1100-1200 em' indicates the absence of the aromatic CH-groups and the complex nature of the polymeric material s present in the samples.

Table 3-Ff-IR absorpti on freq uencies, rel ative intensity and probable assignments of Neyveli tlyash sample

Ash Tentative Assignment( 1•5•

6•10

>

Frequency (em·') Intensity 3500 vw OH-stretching 3250 vw OH-stretching 2929 vw Aliphatic CH3

2853 vw Alipahtic CH2

2832 vw Aliphatic CH2

2806 vw Aliphatic CH2

1636 vw C=C aromatic stretching 11 03 vs Si - 0 stretching 889 w Si- 0- Al 1v 566 w Fe-0 stretching 450 M Fe-S stretching

VS=Very strong; M=Medium; W:.:Weak; VW=Very weak

The lignite samples from mine-cut II (MT 16) ex hibits band near 1100 em·' showing the presence of Si-0-Si band . The presence of clay in the sample can also be well establi shed from the sharp bands appeared at 3676 and 3596 em·' which are attributed to hydroxyl stretching of kaolinite c lay mineral present in the sample (M 2-16).

The weak band appears at 942 em' in mine-cut II (M 2-6) which is attributed to CH out of plane bending vibrations . This is well reflected from Colthup~ . Brown 111 observed a band at 900-700 em·' in the coals and suggested that these bands can ari se due to aromatic hydrogen out of plane bending. In the present study the very weak bands appear at 820 and 766 em ' in all the samples are due to aromatic hydrogen in out-of-plane bending modes.

Baruah et al. 11 studi ed the iron-su lphur band in Assam coals and assigned 340 and 540 em' are due to FeS and FeO stretching respectively. They observed that FeS moist in the most abundant

component of organic sulphur in lignite Assam coal. The very weak band appears at 535 and near 420, 422 em·' are assigned to Fe-0 and Fe-S stretching respectively which indicates the presence of marcacite, pyrite mineral s in the I ignite samples. These minerals will affect the combustor during firing and machineries used for pulverising. They also affect the environment during firing.

4 Flyash Sample

The FT-IR spectrum of the flyash sample collected from Neyve li is shown in Fig. 2. The absorption band energies, their relative intensit ies and their tentative assignments are given in Table 3.

The region 1500 em' due to the presence of aromatic material predominantly present in li gnite is absent in the flyash, indicating the decompos iti on (total) of all organic compounds. The presence of very strong intense band 1003 em·' with a shoulder around 1250 em·' indicates the presence of sulphur dioxide (SOrstretching) and silicon dioxide (S i02)

as prominent compounds in the flyash. Probabl y the sulphur component present in li gni te has been transformed into sulphur dioxide during the process. The presence of S02 in good amount in flyash is a potential health hazard and environmental pollution. The band at 889 em' is predominant where it practically very weak in lignite sampl e. The origin of band is not clear. The presence of strong band at 566 em·' and 450 em·' are indicati ve of the presence of FeO and FeS in the fly ash, respectively.

The methylene and hydroxyl groups predominant in li gnite samples are practica ll y absent in flyash spectra indi cating the decomposition of organic compounds of lignite while _process ing. The absence of absorption band in the region 1400 cnY ' also confirms the above assumption.

The presence of intense absorpti on band at I I 00 em·' show the presence of silicon dioxide (S i02)

band removed out of lignite, but present as inorganic compounds in the flyash. The other inorganic compounds containing iron , which are present in the li gnite also persist in the flyash in good amount.

212 INDIAN J PURE & APPL PHYS, VOL 40, MARCH 2002

Acknowledgements

The authors are thankful to the Director of Mines, Neyveli Lignite Corporation and Chief Geologists of mine-cuts I and II, Neyveli Ligni te Corporation, for the necessary permission and help to collect the samples.

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