Physico-Chemical Characterization and Speciation of Sulphur of Nigerian Coal Samples

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Journal of Minerals and Materials Characterization and Engineering, 2012, 11, 965-969

Published Online October 2012 (http://www.SciRP.org/journal/jmmce)

Physico-Chemical Characterization and Speciation of

Sulphur of Nigerian Coal Samples

Folahan A. Adekola*, Alafara A. Baba, Sherifat Buhari

Department of Chemistry, University of Ilorin, Ilorin, Nigeria

Email: *[email protected], [email protected]

Received July 27, 2012; revised August 29, 2012; accepted September 10, 2012

ABSTRACT

Studies of physico-chemical characterization, and chemical speciation of sulphur of seven Nigerian coal samples have

been undertaken. The seven coal samples originated from South-east, North-central and North-east of Nigeria where

there are proven economic deposits of this mineral. Moisture content, bulk density, percentage ash and loss of mass on

ignition of all the coal samples were determined. The ultimate analysis of the raw coal, the corresponding coal ash as

well as sulphur content of all samples were carried out with the aid of X-ray fluorescence technique (XRF). The major

elements were Fe, Ca and S while the minor metals were K, Sc, Zn, Ni, Ti and Zr. Other metals including Ga, Cu, Mn,

Cr and V were found in traces. The Nickel/Vanadium ratio which is a means of providing information on the source

rock depositional environment ranged between 8.8 - 32.9. Three different source rock depositional environments were

deduced for the coals from their nickel/vanadium ratios, while the calculated values of V/(V+Ni) suggested that they

were all formed under oxic condition. The three ch emical species of sulphur, su lph ate, pyritic and organic su lphu r in the

seven coal samples were determined using the ASTM method.

Keywords: Sulphur Speciation; Nigerian Coal; X-Ray Fluorescence Analysis; Heavy Metals

1. Introduction

Coal can be defined as a chemically and physically het-

erogeneous, “combustible”, sedimentary rock consisting

of both organic and inorganic materials. Organically,

coal consists primarily of carb on, hydrogen, and oxygen,

with lesser amounts of sulphur and nitrogen. Inorgani-

cally, coal consists of a diverse range of ash-forming

compounds distributed throughout the coal [1].

Coal is classified into four general categories, or

“ranks”. They range from lignite through subbituminous

and bituminous to anthracite, reflecting the progressive

response of individual deposits of coal to increasing heat

and pressure. The carbon content of coal supplies most of

its heating value, but other factors also influence the

amount of e ner gy it cont ai ns p e r unit of wei g ht [2] .

The sulphur content of coal varies considerably with

the nature and origin of the fossil depo sits [3]. Sulphur in

coal is present in both inorganic and organic forms. The

inorganic sulphur in coal consists predominantly of sul-

phides (pyrite (FeS2), sphalerite (ZnS), galena (PbS),

arsenopyrite (FeAsS) and others) and sulphates (barite

(BaSO4), gypsum (CaSO4·2H2O), anhydrite (CaSO4),

and a number of iron sulphates and others) [4]. Pyrite is

generally the preponderant inorganic sulphur in coal.

Particles of pyrite are randomly distributed as crystals

throughout th e coal but are not bound to it [5].

The organic sulphur in coal is covalently bound into its

large complex structure and is d ifficult to remove ph ysic-

cally or chemically, in contrast to pyritic or inorganic

sulphur [6]. The organic sulphur in coal exists as both

aliphatic and aromatic or heterocyclic forms, which can

be classified into four groups [7]: aliphatic or aromatic

thiols (mercaptans, thiophenols); aliphatic, aromatic, or

mixed sulphides (thioethers); aliphatic, aromatic, or mixed

disulphides (dithioethers) and heterocyclic compounds or

the thiophene type (dibenzothiophenes).

The utilization of co als for both energy productio n and

various coal conversion processes is limited by the pre-

sence of sulphur in the coal. The high sulphur dioxide

emissions caused by the utilization of coals as a major

fossil fuel leads to worldwide environmental problems

[8]. When coal is burnt its sulphur con tent combines with

oxygen to form sulphur dioxide (SO2), which contributes

to both pollution and acid rain [9]. Acid rain resulting

from SO2 has a harmful effect on agriculture and de-

stroys the ecological balance [10]. The presence of sul-

phur in coal also reduces the quality of metallurgical coal

[11,12]. Therefore, it is necessary to remove the mineral

matter and sulphur from coal prior to its utilization.

Nigeria has major coal resources that have not been

*Corresponding author.

F. A. ADEKOLA ET AL.

966

well explored or exploited. The coal reserves in Nigeria

are estimated to be in excess of 2.5 billion tonnes and

they are mainly of sub-bituminous type. The government

has prioritized the use of these resources in order to deal

with the country’s power supply problems [13].

Several studies have been carried out on the trace

metal characterization of coals from various locations in

Nigeria using mainly Atomic Absorption spectrometry

(AAS). There has been important underestimation of the

levels of metals due to incomplete wet digestion of the

coal materials on one hand and as a result of limited ca-

pacity of this technique when compared to modern in-

strumental techniques such as XRF. Moreover, most of

the works reported concerned the raw coal samples and

with lack of information on the coals’ ashes residues.

This study has therefore b een conceived with the view of

carrying out comprehensive trace metal analysis of not

only the coal materials but in addition, their correspond-

ing ashes residues.

Furthermore, there is dearth of information on the

chemical forms of sulphur in Nigerian coals which is a

pre-condition for assessing its suitability for use in a

coal-fired plant. The objectives of the present investiga-

tion were therefore to carry out physico-chemical char-

acterization of some Nigerian coal samples, determine

the various chemical forms of sulphur in these coals.

2. Experimental Section

2.1. Sample Collection

All coal samples used were obtained either from the Ni-

gerian Coal Corporation, Enugu or the National Metal-

lurgical Research Centre, Jos, Nigeria. The coal samples

originated from the following seven coal mining fields in

Nigeria:

Onyeama(OYM), Okpara (OKP), Okaba (OKB), Iva

(IVA), Chikila (CKL), Lafia Obi (LFB) and Jankwa Sha-

kodi (JKS).

2.1.1. Sample Pre-Treatment

The selected lumps of coal samples were air-dried and

then pulverized with clean mortar and pestle and sieved

into selected mesh powder. The powdered samples with

particles diameter less than 50 mm were retained for all

investigations.

2.1.2. Physico-chemical ch a racterization of Material s

The physic-chemical parameters analysed for in the coal

samples were moisture content, bulk density, loss of

mass on ignition, ash content, sulphur and metals.

Moisture content was determined by heating to a con-

stant weight using a MINO/53/CLAD oven at a tem-

perature of 105˚C for three hours [14]; The bulk density

of coal samples was determined following a simple dis-

placement procedure based on the Archimede’s principle

[14]; The loss of mass on ignition and the ash content

were determined by heating the sample to constant weight

at 650˚C for three hours in the carbolite ELF 11/148

muffled furnace [14]. The elemental composition of the

coal samples were carried out with the aid of Energy

Dispersive-X-ray fluorescence spectrophotometer (ED-

XRF), Philip’s model 12045 B4/3. The total sulphur in

coal samples was determined with ED-XRF. While Sul-

phate-S and Pyritic-S in the coals were carried out using

standard ASTM methods [15]. Organic sulphur was de-

termined by difference.

3. Results and Discussion

Physico-chemical characteristics of seven different coal

samples from Nigeria are summarized in Table 1.

Chikila coal has the highest moisture content with

value of 7.1% while Jankwa Shankodi has the lowest

moisture content of 1.3%. Onyeama, Okpara and Iva

coals have moisture contents of 2.6%, 3.1% and 4.1%

respectively which are very close to the value of 3.5%

earlier reported by Onwu [16] for Enugu coals.

3.1. Total Elemental Analysis

The results of elemental analysis of the coal obtained

with the aid of ED-XRF are given in Table 2.

It is evident from the ED-XRF results that there is

variation in abundance of various elements in all the coal

samples investigated. In Onyeama coal, sulphur occurred

Table 1. Proximate analysis of Nigerian coal samples.

Samples Moisture content (%) Bulk density (g/cm3) Loss of mass on ign i t i o n (%) Ash content (%)

Onyeama 2.6 0.71 83.7 16.3

Okpara 3.1 0.69 93.4 6.7

Okaba 4.3 0.73 91.0 9.4

Iva 4.1 0.69 93.4 6.7

Chikila 7.1 1.37 75.7 23.4

Lafia obi 1.5 1.29 87.4 12.9

Jankwa shankodi 1.3 1.20 79.9 20.1

F. A. ADEKOLA ET AL. 967

Table 2. Sulphur and metals analysis of some Nigerian coal samples.

Element Onyeama Okpara Okaba Iva Chikila Lafia obi Jankwa S.

S (ppm) 3015 ± 249 8614 ± 1414 7002 ± 12042996 ± 2557811 ± 398 3503 ± 28 1 4511 ± 286

K (ppm) 213 ± 12.1 667 ± 73.4 792 ± 76 106 ± 12401 ± 19 185 ± 16 291 ± 20

Ca(ppm) 533 ± 41 9751 ± 100 8201 ± 101.83852 ± 10.21.604 ± 0.01355 ± 41 5966 ± 121

Sc(ppm) 32.8 ± 4.6 1223 ± 176 1061 ± 159 168.9 ± 19.1414 ± 40.3 64.2 ± 11 84 ± 9

Ti (ppm) 784 ± 17.1 15,010 ± 0.03 13,000 ± 0.021559 ± 261342.1 ± 34356.1 ± 15 691.7 ± 21

V(ppm) 8.11 ± 1.7 137 ± 7.81 106.3 ± 7 15 ± 3 15.2 ± 1 9 ± 3 9.1 ± 1

Cr(ppm) 11 ± 1.4 86 ± 8.03 50.6 ± 6.1 13.1 ± 1.121.1 ± 4 14. 9 ± 2 13. 0 1 ± 3

Mn(ppm) 45.81 ± 2.4 130.2 ± 8 86 ± 5.8 155.8 ± 569 ± 3 62.3 ± 2 51.97 ± 3

Fe (%) 7.11 ± 0.051 5.02 ± 0.051 5.585 ± 0.077.771 ± 0.047.1 ± 0.03 9.07 ± 0.04 8.72 ± 0.031

Ni(ppm) 87.1 ± 10 1450.8 ± 69 933.1 ± 53.02149.2 ± 14. 1277 ± 21 296 ± 21 158 ± 17

Cu(ppm) 63 ± 5.2 290.3 ± 21 279.7 ± 18.474.8 ± 6.388.1 ± 5.9 64.9 ± 6.2 81 ± 9

Zn(ppm) 231 ± 17.2 861 ± 55 1081 ± 57 199 ± 18529.93 ± 29463 ± 27 465 ± 27

Ga(ppm) 77 ± 5.6 345.3 ± 25.2 290.8 ± 22 84.01 ± 8.869.2 ± 7 69 ± 6.4 50.6 ± 4.4

Zr(ppm) 178 ± 8 219 ± 21 144.6 ± 17.899.02 ± 942.01 ± 5.1 24.1 ± 4 33.1 ± 5

as the major element, while K, Ca, Ti, Zn and Zr oc-

curred as minor elements and Sc, V, Cr, Mn, Fe, Ni, Cu

and Ga occurred in trace elements. S, Ca, Sc and Ni are

present as major elements while K, V, Mn, Cu, Zn, Ga,

Zr occurred as minor elements and Ti, Ga and Fe oc-

curred as trace elements in Okpara coal. Okaba coal from

Kogi state has S, Ca, Sc, and Zn as major elements while

K, V, Ni, Cu, Ga and Zr occurred as minor elements and

Ti, Cr, Mn and Fe are the trace elements present. In Iva

coal, S, Ca, and Ti occurred as major elements while K,

Sc, Mn, Ni and Zn occurred as minor and V, Cr, Fe, Cu,

Ga and Zr occurred as trace elements.

Chikila has S and Ti as major elements while K, Sc, Ni

and Zn are the minor elements and Ca, V, Cr, Mn, Fe, Cu,

Ga and Z. r are the trace elements present.

Sulphur is the only major element while K, Ca, Ti, Ni

and Zn are present as minor element and Sc, V, Cr, Mn,

Fe, Cu and Ga are present as a trace element in the Lafia

obi coal. In Jankwa shankodi S and Ca are major ele-

ments present while K, Ti, Ni, and Zn are present as the

minor elements and Sc, V, Cr, Mn, Fe, Cu, Ga and Zr are

the trace element present. It is important to no te that sig-

nificantly higher values were measured for all metals

when compared to some previous studies [17-19]. This

could be adduced to the inefficiency of the digestion

procedures adopted by these authors prior to AAS analy-

sis. This was unlike the XRF technique used in th is study

which enabled a direct analysis of solid materials and

thus, avoiding a preliminary acid digestion of coal sam-

ples. It is also worthy of note that some coal samples (Iva,

Okpara, Chikila, Okaba and Onyeama) are very rich in

titanium metal and this could be an indication of pre-

sence of titanium-rich minerals such as TiO2 and FeTiO3

[19,20]. Okaba and Okpara also exhibited high concen-

tration of zinc and nickel. The high concentration of zinc

could be an indication of the presence of sphalerite min-

eral in thess areas [21] In terms of the degree of the ele-

mental analysis using, it is observed that Jankwa Shan-

kodi has the highest total metal concentration (102600.0

ppm) while Onyeama has the least (12,382 ppm). In

terms of degree of mineralization, the results show that

the three coal samples (Chikila, Lafia obi and Jankwa

shankodi) from the northern region of the country are

highly mineralized based on their total metal conten t.

The presence of Vanadium in traces (9.0 - 106.3 ppm)

for all the coal samples is an indication of low maturity

and marine/terrestrial sou r c ed coal [19].

The values of Ni/V ratio are presented in Table 3 and

they range between 8.8 - 32.9. Ni /V ratio has been u sed

by several authors as a means of providing information

on the source rock depositional environment [18,19].

The seven coal types investigated in this work appear

to fall into three distinct groups based on Ni/V ratio.

Onyeama, Okpara, Okaba and Iva coals with lowest and

practically the same Ni/V ratio can be put in the same

group. These coal samples can therefore be considered to

Table 3. The ratios of Ni/V and V/(V+Ni).

Samples Ni/ V V/V+Ni

Onyeama 10.7 0.09

Okpara 10.6 0.09

Okaba 8.8 0.1

Iva 9.9 0.1

Chikila 18.2 0.05

Lafia obi 32.9 0.04

Jankwa shankodi 17.4 0.05

F. A. ADEKOLA ET AL.

968

have the same depositional environment.

The second group consists of Chikila and Jankwa

shankodi with Ni/V ratio of almost twice the Ni/V ratio

of the first group would also be considered to the same

depositional enviro nment. And lastly, Lafia obi coal with

Ni/V ratio of 32.9 which is thrice the value of the first

group can be said to have a distinct depositional envi-

ronment.

It is important to mention that the Ni/V ratio calculated

for Onyeama, Okpara and Okaba were much greater than

the value earlier reported [19].

Futhermore V/V + Ni ratios are generally low for all

the coal samples and this suggest that they were all

formed under toxic conditions [19].

3.2. Sulphur and Metal Analysis of Ashes from

Coal Samples

The results of elemental analysis of the ashes of the coal

samples are presented in Table 4.

The coal ash elemental analysis gives the following

concentrations.

Vanadium concentration ranges from 8 to 137 ppm

with an average of 42.71 ppm. Chromium concentration

ranges from 12 to 86 ppm with an average value of 29

ppm. The concentration of manganese ranges between 16

and 154 ppm with an average value of 78.14 ppm and the

highest concentration was found in Iva coal while the

lowest concentration was recorded in Lafia obi coal.

Concentration of Nickel ranges from 86 to 1451 ppm

with the average value of 475.57 ppm; highest concen-

tration of Nickel was recorded in Okpara coal and the

least in Oyeama coal. The concentration of Zinc ranges

between 197 to 1078 ppm with the average of 541 ppm.

Iron concentration ranges from 4.92% to 8.26% with an

average value of 6.93%. It was revealed from this work

that coal beds are very rich source of some metals such

as Ni, Ti, Cu, Fe and Cu. This indicates that the ash resi-

due after burning would be a very good source of these

metals and they can also be used to manufacture some

other chemicals [19].

The sulphur content of the ashes is generally lower

than values measured in the raw coal samples. The va-

lues in the ashes rang e from 2441 - 8247 ppm. The varia-

tion follows same trend as observed for the raw coal

samples. The highest value was recorded for Okpara coal

while the lowest was recorded for Lafia obi coal. The

high level of Sulphur in the ash is an indication of the

predominance of inorg anic Sulphur (Sulphate and Pyritic

Sulphur) in all the coal samples.

The analytical results obtained for the three chemical

forms of sulphur in the coal samples are summarized in

Table 5.

Table 4. Total metal analysis of coal ash.

Elements Onyeama Okpara Okaba Iva Chikila Lafia obi Jankwa shankodi

S (ppm) 2577 ± 222 8247 ± 1372 6476 ± 1157 2841 ± 236 6219 ± 387 2441 ± 256 3325 ± 255

K (ppm) 200 ± 11 660 ± 73 791 ± 76 102 ± 8 3 29 ± 1 9 178 ± 13 285 ± 14

Ca(ppm) 532 ± 40 9749 ± 100 8195 ± 100 3 847 ± 1 00 15,200 ± 0.013 3 8 ± 37 5643 ± 100

Sc(ppm) 33 ± 5 1221 ± 175 1059 ± 158 168 ± 19 413 ± 40 63 ± 11 83 ± 10

Ti (ppm) 784 ± 17 14,200 ± 0.03 1 2,400 ± 0.02 1557 ± 26 1341 ± 32 356 ± 13 691 ± 18

V (ppm) 8 ± 1 137 ± 8 106 ± 7 15 ± 1 15 ± 1 9 ± 1 9 ± 1

Cr (ppm) 10 ± 1 86 ± 8 50 ± 6 13 ± 1 18 ± 2 14 ± 1 12 ± 1

Mn (ppm) 45 ± 2 128 ± 8 85 ± 6 154 ± 3 67 ± 3 16 ± 2 52 ± 2

Fe(%) 8.21 ± 0.03 4.92 ± 0.05 5.50 ± 0. 05 7.76 ± 0. 03 6.22 ± 0.03 8.26 ± 0.04 7.61 ± 0.03

Ni(ppm) 86 ± 8 1451 ± 69 932 ± 53 150 ± 14 275 ± 18 294 ± 18 141 ± 11

Cu(ppm) 51 ± 5 289 ± 19 278 ± 18 74 ± 6 85 ± 6 65 ± 6 72 ± 6

Zn(ppm) 206 ± 16 854 ± 53 1078 ± 57 197 ± 16 528 ± 26 461 ± 25 463 ± 25

Ga(ppm) 75 ± 5 345 ± 25 290 ± 22 83 ± 8 69 ± 7 69 ± 6 50 ± 5

Zr(ppm) 177 ± 8 219 ± 21 144 ± 17 99 ± 9 42 ± 5 24 ± 4 33 ± 4

Table 5. Concentration (%) of various chemical forms of sulphur in Nige rian c oal sample s.

Samples Total sulphur Sulphate-S Pyritic-S Organic-S

Onyeama 0.302 0.082 0.176 0.054

Okpara 0.861 0.469 0.356 0.036

Okaba 0.700 0.337 0.311 0.052

Iva 0.300 0.008 0.276 0.016

Chikila 0.781 0.012 0.610 0.159

Lafia obi 0.350 0.002 0.242 0.106

Jankwa S. 0.451 0.047 0.286 0.118

F. A. ADEKOLA ET AL. 969

The results of suphur speciation revealed that pyritic-

sulphur is by far the most important form in Onyeama

(58.2%), Iva (92%), Chikila (78.1%), Lafia obi (69.1%)

and Jankwa shankodi (63.4%) coals. Sulphate-sulphur

constitutes 54.5% and 48.1% of total sulphur in Okpara

and Okaba coals respectively. Organic-sulphur is gener-

ally very low in the coal samples except in Chik ila, Lafia

Obi and Jankwa Shankodi where it constitutes 20.4%,

30.3% and 26.2% of the total suphur respectively.

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