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Removal of trace elements from Thar coal to minimize its hazardous effect on the environment.

Byline: Zafar Khan and Saqiba Bibi

Abstract

Four lignite coal samples from Thar coal fields Pakistan were selected for the trace elemental analyses studies. The data was obtained with the use of X-ray Fluorescence (XRF) Spectrometer. It was observed that (Aluminum, Arsenic, Barium, Bromine, Calcium, Cerium, Cobalt, Chromium, Cesium, Copper, Iron, Gallium, Hafnium, Lanthanum, Manganese, Molybdenum, Niobium, Neodymium, Nickel, Lead, Rubidium, Antimony, Scandium, Selenium, Silicon, Tin, Strontium, Samarium, Tantalum, Titanium, Uranium, Vanadium, Tungsten, Ytterbium, Zink and Zirconium) elements were present in all the coal samples under investigation. These trace metals are found to have an adverse effect on the health of the living organisms intact with coal utilization.

Some of these elements such as Arsenic, Barium, Beryllium, Chromium, Cobalt, Copper, Lead, Molybdenum, Nickel, Selenium, Uranium, Vanadium and Zinc are extremely harmful to human health, if released to the environment during coal combustion. Therefore, the coal samples were being demineralized with washing the coal samples with standard solution of 2N HCl. The demineralized coal samples were then subjected in the same way to (XRF) Spectrometer for elemental analyses. The elements identified in raw samples were compared with the elements identified in the respective demineralized samples. It was found that demineralization caused better effect, in terms of metals removal from coal.

Keywords: Acid washing; Hazardous effect; Pakistani coal; Thar lignite; Trace elements; X-ray Fluorescence (XRF) Spectrometer.

1. Introduction

Coal is a major alternative naturally occurring energy source and is abundantly available as fossil fuel throughout the world. Coal is a black or brown solid combustible substance used as fuel from centuries which is formed under the earth crust from the decay of plants material (Damste et al., 1992) due to the very high pressure and temperature and due to the absorption of moisture from soil. Pakistan has plentiful supplies of coal. The estimated deposits of it are 186 billion tons. Thar has the largest reserve of coal in Pakistan, which alone has 175 billion tons coal reserves. To fulfill the world energy demand in future, one should utilize all the available energy resources including solid fuel such as coal (Izquierdo and Querol, 2012). It has been reported in the literature that at present 30.1% primary energy and 40% electrical energy is produced from coal world, whereas 70% steel production also rely on coal (Feng et al., 2015).

The production of total world coal was 7822.7 metric tons (Mt), according to the 2013 survey. The regional share to this total can be easily obtained from the International Energy Agency and the BP Statistical Review of World Energy (Key World Energy Statistics, 2014; Statistical Review of World Energy, 2014).

Coal is a heterogeneous substance consists of organic and inorganic minerals components (Ahmad et al., 2009). The organic components of coal determine its rank and the appropriateness for its use as renewable energy (Saeed et al., 2010). The inorganic mineral elements present in coal acts as catalysts during coal conversion processes such as liquefaction, gasification and pyrolysis (Tian et al., 2002). Thus, they have beneficial role for the useful utilization of coal at this regard. On the contrary, heavier and trace mineral elements can have a deep adverse effects on the health of people using coal in houses and offices or dwelling close to coal reserves, coal mines, and coal-combustion plants. These trace metals are found to have a very harmful effect on the health of the living organisms intact with coal utilization.

Some of these elements like Arsenic, Barium, Beryllium, Cadmium, Chromium, Cobalt, Copper, Lead, Molybdenum, Nickel, Selenium, Uranium, Vanadium and Zinc are extremely harmful to human health, if released to the environment during coal combustion and coal thermochemical processes (Franco et al., 2009; Swaine, 2000; Wang et al., 2010). The harmful mineral matter emissions during burning process have thoroughly studied recently (Zhang et al., 2008; Kizilshtein and Kholodkov, 1999; Goodarzi, 2006; Johnson et al., 2008; Gibbs et al., 2008; Goodarzi et al., 2008; Yudovich and Ketris, 2005; Wang et al., 2008).

Severe health problems in Asia are being reported from the emission of these elements, such as arsenic and selenium to the environment during coal burning and utilization. In the developing countries the health problems are severe, where the coal is being used in homes for heating and cooking purposes. The peoples of such localities are mostly suffering from arsenism and fluorosis (Finkelman, 1999). The brick makers and brick furnaces owners burn the coals as such, without knowing their qualities, thus putting the public in great trouble, dwelling in the vicinity of that furnaces, for being effected from coal dust as well as from the harmful health hazard effects of environment being polluted with harmful health hazardous trace elements. Prior to coal utilization, knowledge about the coal quality can help to reduce the health problems, such as knowledge about the concentration and distribution of toxic metal in coal may help the coal to be avoided to use as such.

Trace elements are being reported to be incorporated in the surrounding by natural and anthropogenic sources (Andriono, 1986). Knowledge about the elemental composition for the reduction of harmful effect of the environment and also for the effective utilization of coal is very important (Liu et al., 2001). The role of the trace metals is dependent upon the types (Sekine et al., 2008; Iwashita et al., 2004) and combustion parameters of coal. Reducing atmosphere and elevated combustion temperature enhances the release of trace elements (Guo et al., 2004).

The work presented in this manuscript is taken into consideration of the elemental composition of Thar coal, as the people are not eager to use Pakistani coal for power production due to high concentration of ash and the release of deleterious chemicals and hazardous gases. Due to the shortage of electricity and petroleum, there are golden chances for the locally available coal utilization especially in energy production but due to the lake of worthy of reliance or trustful information about quality of coal, the coal deposits are not fully utilized. The present work is therefore an attempt to explore the quality of Thar coal, which is a huge reserve of coal in Pakistan, to analyze its toxic metals, to cease or reduce its release into the environment with the help of acid treatment and finally to attract the users for its full utilization.

2. Experimental

2.1. Chemicals and reagents used

Analytical grade reagents (Merck) and standard hydrocarbon gases were supplied by Matheson Company, Germany.

2.2. Coal sample and its preparation

The representative raw coal (RC) samples were accumulated from Tharparkar coalfield, Sindh, Pakistan supplied by Sindh Coal Authority, Karachi, which were tightly closed in polyethylene bags. The as received coal samples were opened, crushed and sieved to a size of 106 um, warmed to make them dry at 100 C for 2 h in an oven, then chilled to room temperature (300 K) in a desiccators, and then stocked in a petri dish for further studies.

2.3. Demineralization of coal sample

Demineralization of coal was performed in a vat. A definite portion (1 g) of each of the prepared coal was slurred in 50 ml of 2M hydrochloric acid. The contents were kept in contact for three weeks at room temperature (27C), and filtered. The residual coal was washed several times with de-ionized water in order to remove last traces of the acid. The demineralized residual coal retained on the filter paper was subjected to dry in an oven at 100 C for 2 hours and then utilized for trace elemental analyses.

2. 4. Trace elemental investigations

The trace elemental investigations of all the Thar coal samples were explored at National Centre of Excellence in Geology, University of Peshawar. The elemental investigations were pursued by employing X-ray fluorescence (XRF) spectrometer, Company Model No. PAN alytical XRF Spectrometer PANalytical B. V. Lelyweg 1. 7602 EA, Almelo, the Netherland.

3. Results and discussion

Comparisons of the results of trace elemental analyses of the virgin and acid washed (demineralized) coal sample was made utilizing X-ray fluorescence (XRF) analyzer technique. Four coal samples collected from the Tharparkar coal deposits, marked as Thar 31-2, Thar 11-2, Thar 7-4 and Thar 3-2-3 were examined for the elemental analyses. The elements identified in Thar 31-2 coal sample, were Aluminum, Arsenic, Barium, Bromine, Calcium, Cerium, Cobalt, Chromium, Cesium, Copper, Iron, Hafnium, Lanthanum, Manganese, Molybdenum, Niobium, Neodymium, Nickel, Rubidium, Antimony, Scandium, Selenium, Tin, Strontium, Samarium, Tantalum, Titanium, Uranium, Vanadium, Tungsten, Yttrium, Zink and Zirconium.

The elements identified in case of Thar 11-2 coal sample, were Arsenic, Barium, Bromine, Calcium, Cerium, Cobalt, Chromium, Cesium, Copper, Iron, Hafnium, Lanthanum, Manganese, Molybdenum, Niobium, Neodymium, Nickel, Lead, Rubidium, Antimony, Scandium, Selenium, Strontium, Titanium, Uranium, Vanadium, Tungsten, Yttrium, Ytterbium, Zink and Zirconium. The elements identified in Thar 7-4 coal sample, were Aluminum, Arsenic, Barium, Bromine, Calcium, Cerium, Cobalt, Chromium, Copper, Iron, Gallium, Hafnium, Lanthanum, Manganese, Molybdenum, Niobium, Neodymium, Nickel, Lead, Rubidium, Antimony, Scandium, Selenium, Silicon, Tin, Strontium, Samarium, Titanium, Uranium, Vanadium, Tungsten, Yttrium, Zink and Zirconium.

The elements identified in case of Thar 3-2-3 coal sample, were Aluminum, Arsenic, Barium, Bromine, Calcium, Cobalt, Chromium, Copper, Iron, Gallium, Hafnium, Lanthanum, Manganese, Molybdenum, Niobium, Neodymium, Nickel, Rubidium, Antimony, Scandium, Selenium, Silicon, Tin, Strontium, Titanium, Uranium, Vanadium, Ytterbium, Yttrium, Zink and Zirconium. It has been observed that almost all the Thar coal samples contained the same trace elemental composition with some minor differences. For example, Al is absent in Thar 11-2 coal sample where as it is present in relative higher concentration in Thar 7-4 coal sample. Ytterbium is present in Thar 11-2 and Thar 3-2-3 while absent in Thar 31-2 and Thar 7-4. Similarly, Tantalum is present only in Thar 31-2 sample whereas it is absent in the rest of the coal samples.

The reason for the similarity of the entire coal samples of Tharparkar under investigations for the elemental composition may be due to the fact that these samples belong to the same region and due to the same morphological, geological and atmospheric changes during coalification; the coal samples attained the same mineral compositions. The original (RC) were then slurred in 2M hydrochloric acid and made them demineralize and the existence of trace mineral elements before and after demineralization of raw samples were determined with the application of X-ray fluorescence (XRF) spectrometer. The elements identified in the raw coal samples were matched with the elements identified in its demineralized counterpart. The comparison of the results are given are given in Tables 1-4.

Table 1. Comparison of elemental analysis of raw and demineralized Thar 31-2 coal sample.

###Values of Trace Elements in Thar 31-2 Coal

###Elements###Raw sample Sample dem- Units

###ineralized

###with

###2N HC1

###Al###0.21###0.10###%

###As###1.0###0.9###ppm

###Ba###15.6###6.4###ppm

###Br###3.4###2.8###ppm

###Ca###3.7###0.1###%

###Ce###4.9###LLD###ppm

###Co###1.7###1.0###ppm

###Cr###3.5###2.8###ppm

###Cs###9.8###8.3###ppm

###Cu###7.7###5.4###ppm

###Fe###2.24###1.19###%

###Hf###3.4###LLD###ppm

###La###14.7###4.8###ppm

###Mn###161.1###7.2###ppm

###Mo###0.3###0.2###ppm

###Nb###0.6###0.5###ppm

###Nd###2.5###2.4###ppm

###Ni###7.8###6.5###ppm

###Pb###0.1###0.1###ppm

###Sb###1.4###1.3###ppm

###Sc###3.9###1.4###ppm

###Se###14.2###11.6###ppm

###Sn###3.8###2.8###ppm

###Sr###343.5###10.4###ppm

###Sm###2.8###LLD###ppm

###Ta###1.7###LLD###ppm

###Ti###0.1###LLD###%

###U###3.5###1.4###ppm

###V###6.1###1.6###ppm

###W###2.0###1.9###ppm

###Y###5.9###0.8###ppm

###Zn###8.6###0.5###ppm

###Zr###19.3###13.3###ppm

Table 2. Comparison of elemental analysis of raw and demineralized Thar 11-2 coal sample.

###Values of Trace Elements in Thar 11 -2 Coal

###Elements###Raw sample###Sample demi-###Units

###neralized with

###2N HC1

###As###1.3###0.2###ppm

###Ba###33.8###9.6###ppm

###Br###10.1###7.8###ppm

###Ca###0.4###0.2###%

###Ce###6.6###4.2###ppm

###Co###5.3###4.1###ppm

###Cr###291.8###12.3###ppm

###Cs###8.6###LLD###ppm

###Cu###10.7###2.9###ppm

###Fe###0.3###0.2###%

###Hf###6.8###1.3###ppm

###La###10.7###7.6###ppm

###Mn###0.2###0.1###ppm

###Mo###1.7###1.1###ppm

###Nb###1.5###0.9###ppm

###Nd###4.6###3.4###ppm

###Ni###18.5###8.8###ppm

###Pb###1.2###0.8###ppm

###Rb###1.4###0.6###ppm

###Sb###1.8###0.6###ppm

###Sc###0.3###0.2###ppm

###Se###10.3###9.5###ppm

###Sr###337.6###14###ppm

###Ti###7.3###3.8###%

###U###3.3###2.6###ppm

###V###8.0###6.6###ppm

###W###2.2###1.0###ppm

###Y###5.9###1.7###ppm

###Yb###1.6###LLD###ppm

###Zn###0.8###LLD###ppm

###Zr###39.7###23.0###ppm

Table 3. Comparison of elemental analysis of raw and demineralized Thar 7-4 coal sample.

###Values of Trace Elements in Thar 7-4 Coal

###Elements###Raw sample###Sample demi-###Units

###neralized with

###2N HC1

###Al###3.3###3.0###%

###As###1.1###0.6###ppm

###Ba###28.2###20.5###ppm

###Br###2.9###1.8###ppm

###Ca###4.4###0.3###%

###Ce###11.4###4.7###ppm

###Co###4.8###2.5###ppm

###Cr###19.8###17.8###ppm

###Cu###23.3###17.3###ppm

###Fe###1.4###0.7###%

###Ga###7.3###4.9###ppm

###Hf###0.4###LLD###ppm

###La###13.5###12.0###ppm

###Mn###155.4###11.6###ppm

###Mo###1.5###1.3###ppm

###Nb###3.7###3.1###Ppm

###Nd###9.4###8.2###ppm

###Ni###10.3###8.1###ppm

###Pb###1.4###0.9###ppm

###Rb###1.9###1.7###ppm

###Sb###3.2###1.4###ppm

###Sc###8.5###1.1###ppm

###Se###3.7###2.9###ppm

###Si###1.2###1,1###%

###Sm###0.9###LLD###ppm

###Sn###7.7###3.5###ppm

###Sr###221.7###15.5###ppm

###Ti###0.4###0.3###%

###U###1.2###0.8###ppm

###V###43.9###14.4###ppm

###W###0.5###0.2###ppm

###Y###17.5###4.6###ppm

###Zn###7.7###6.0###ppm

###Zr###47.9###36.2###ppm

Table 4. Comparison of elemental analysis of raw and demineralized Thar 3-2-3 coal sample.

###Values of Trace Elements in Thar 3-2-3 Coal

###Elements###Raw sample###Sample demin-###Units

###eralized with

###2N HC1

###Al###0.5###0.4###%

###As###1.5###1.4###ppm

###Ba###17.6###17###ppm

###Br###2.8###1.7###ppm

###Ca###3.8###0.6###%

###Co###0.5###0.3###ppm

###Cr###12.1###9.2###ppm

###Cu###10.6###8.3###ppm

###Fe###2.0###1.3###%

###Ga###0.4###0.3###ppm

###Hf###2.5###1.1###ppm

###La###11.1###9.4###ppm

###Mn###228###24.8###ppm

###Mo###5.9###0.7###ppm

###Nb###2.4###2.2###ppm

###Nd###5.1###4.8###ppm

###Ni###3.4###2.7###ppm

###Rb###0.7###LLD###ppm

###Sb###3.4###3.3###ppm

###Sc###1.8###1.3###ppm

###Se###14.2###11.1###ppm

###Si###1.8###LLD###%

###Sn###3.7###2.4###ppm

###Sr###211.9###27.9###ppm

###Ti###0.2###0,1###%

###U###2.2###LLD###ppm

###V###8.3###4.6###ppm

###Y###2.7###1.8###ppm

###Yb###1.0###LLD###ppm

###Zn###40.5###1.7###ppm

###Zr###27.0###23.4###ppm

By matching the residues of each and every mineral element found in the virgin as well as in its demineralized form coal, it was observed that demineralization induced to remove the trace mineral elements, reduced in weight % and in ppm of almost each inorganic element. Among the elements determined, Ca, Hf, Mn, Sr, Sm, Ta, Ti, U, Y, Yb and Zn were found to remove in high concentration.

From the whole discussion here, we came to the conclusion that demineralization of raw coal removed substantial amount of inorganic mineral elements from raw coal sample. Present determinations are in conformity with the results presented earlier (Ahmad et al., 2009).

4. Conclusions

The coal is an important, easily available source of world energy, which can be directly used as heating sources in homes and for burning source for the cooking purposes. The coal is a complicated substance owing to its heterogeneous nature but, however, the study about its structure is very important for its effective utilization, due to its global copious nature. From the close examination of the structure of coal, it has been observed that the coal consists of organic and inorganic constituents. From the study of the organic portion, the nature, such as rank and type of coal is elucidated and also determines the appropriateness of the coal for its use as renewable energy, whereas, the study of the inorganic matter, provide the idea of the type of major and trace elements present in the coal.

The presence of some of the toxic mineral elements, which are released to the environment during coal conversion processes, such as liquefaction, gasification and combustion, causes a severe threat to human lives. A deep knowledge about the quality of coal is needed, which may help to reduce or minimize the health problems associated with coal utilization. Knowledge about the quantity and distribution of these undesirable and hazardous elements can help to avoid the direct use of these coal reserves, for being presence of toxic elements. The strength of the toxic mineral elements must be reduced or abolished by washing with acid solutions or applying different analytical techniques. Characterization and quality of coal can provide help to the biomedical researchers to protect the health of the people from the harmful effect of the toxic minerals in coal.

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Publication:Journal of Himalayan Earth Sciences
Geographic Code:9PAKI
Date:Jun 30, 2016
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