Beneficiation of Indian Coal

EFFECT OF ADDITIVES FOR BENEFICIATION OF

INDIAN COAL BY SOLVENT EXTRACTION

Thesis submitted in partial fulfillment of the requirements for award of the degree of

Master of Engineering in Chemical Engineering

Submitted by

SHASHWATA GHOSH

Class Roll No.: 001310302004

Examination Roll No.: M4CHE15-04

Registration No.: 124701 of 2013-14

Under the guidance of

Dr. ChanchalMondal

&

Dr. SudeshnaSaha

Department of Chemical Engineering

JADAVPUR UNIVERSITY

Kolkata- 700032

2015

Declaration of originality and compliance of academic ethics

I hereby declare that this thesis contains literature survey and original research work by

the undersigned candidate, as part of his Master of Chemical Engineering studies. All

information in this document have been obtained and presented in accordance with

academic rules and ethical conduct.

I also declare that, as required by these rules and conduct, I have fully cited and

referenced all material and results that are not original to research work.

Name: ShashwataGhosh

Examination Roll Number: M4CHE15-04

Thesis Title: Effect of additives for beneficiation of Indian coal by solvent extraction

Signature:

Date:

CERTIFICATION

To whom it may concern

This is to certify thatShashwataGhosh, final year Master of Chemical Engineering

(M.ChE) examination student of Department of Chemical Engineering, Jadavpur

University (Exam Roll No.M4CHE15-04; Reg. No. 124701 of 2013-14), has completed

the project work titledEffect of additives for beneficiation of Indian coal by solvent

extraction under the guidance of Dr. Chanchal Mondal and Dr. Sudeshna Saha in the

stipulated time during his post graduatecurriculum. This work has not been reported

earlier anywhere and can be approved for submission in partial fulfillment of the course

work.

___________________________

Dr. SudeshnaSaha

Assistant Professor

Chemical Engineering Department

Jadavpur University

_________________________

Prof. ChandanGuha

Head, Chemical Engineering Department

Jadavpur University

_________________________

Dr. SivajiBandyopadhyay

Dean, Faculty of Engineering & Technology

Jadavpur University

iv

ACKNOWLEDGEMENTS

It is a great pleasure to express my gratitude and thanks to each and everyone who have

helped me to complete my research and compile this thesis in the stipulated time. I am

grateful to all faculty members of Chemical Engineering Department, Jadavpur

University for allotting this project to me. I would like to express my respect to Dr.

Chanchal Mondal for his valuable suggestions and guidance, which have helped me

throughout the entire tenure of the research work. I am certainly indebted to Dr. Sudeshna

Saha for her resourcefulness and guidance, which undoubtedly has helped me complete

this research in time. In fact, I owe her more than I can even mention.

I would like to express my heartfelt gratitude towards my parents and friends Mr.

Santanu Ghosh and Mr. Abhik Das, whose have always inspired me and helped me

develop a positive attitude. I am also indebted to Mr. Amritanshu Banerjee, a great friend,

whose noble company has always enriched me. Without his help, my thesis would have

remained incomplete.

I would also express my thanks to all my fellow classmates Mr. Victor Sarkar, Ms.

Sudeepta Bhattacharya and research scholars, Ms. Sujata Sardar, Ms. Epika Mandal.

Without their cooperation and inspiration, I could not complete my research in time. I am

also grateful Mr. Ashok Kumar Seal for his help and co-operation in providing me with

all necessary equipments I required for this work.

v

ABSTRACT

High ash content of Indian coals not only reduces the efficiencies of boilers and furnaces,

but also affects the quality of metallurgical coke for use in blast furnaces. The present

study investigates the role of solvents and additives for extraction of coals and reduction

of ash content in the extracts obtained from an Indian coal. Toluene, a non-polar solvent

does not give significant yield of coal extract. Hence, a polar solvent like n-methyl

pyrrolidone (NMP) was chosen and the yields of extracts were observed for different

times of extraction and various additives. The additives used in the experiments were p-

nitro benzoic acid, quinoline, calcium fluoride and calcium chloride. It was observed that

the maximum yield of extract was 13.313% and was obtained by using Calcium chloride

as an additive with NMP. This yield was obtained with an additive concentration of 5%

(by weight of raw coal) of Calcium chloride under 1 hour of thermal extraction at a

temperature of 150C. However, the reduction in ash content of the coal extract in case of

Calcium chloride was 82.25%, which was lower than that obtained by the use of other

additives, or, NMP alone.

vi

TABLE OF CONTENTS

Title Page No.

Chapter 1 INTRODUCTION 1

1.1. Coal 1

1.2. Types of coal 1

1.3. Uses of coal 2

1.4. Chemical composition of coal 5

1.5 Proximate analysis of coal 7

1.6. Ultimate analysis of coal 8

1.7. Disadvantages of ash content of coal 9

1.8. Beneficiation of coal 10

1.9. Background of the project 10

1.10. Problem statement 11

1.11. Objectives 11

1.12. Originality of the work 12

Chapter 2 LITERATURE REVIEW 13

2.1. The chemical structure of coal 13

2.2. Solvent extraction of coals 14

2.3. Effect of rank of coal on solvent extraction 15

2.4. Effect of temperature and pressure on extraction of coal 16

2.5. Effect of nature of solvents on extraction of coal 18

2.6. Effect of hot filtration and room temperature filtration

on extraction

18

2.7. Effect of using mixtures of solvents for coal extraction 19

2.8. Effect of various additives on extraction of coals with

solvents

20

2.9. Kinetics of diffusion of solvent into the coal structure 21

2.10. FT-IR Analysis of raw, residue and extracted coals 22

vii

TABLE OF CONTENTS (Contd)

Title Page No.

2.11. Coal beneficiation by agglomeration techniques 23

2.12. Beneficiation by the use of inorganic chemicals 23

2.13. Reduction of ash content of coal with solvents without

coal extraction

24

Chapter 3 EXPERIMENTAL MATERIALS & PROCEDURE 25

3.1. Materials 25

3.2. Experimental procedure 25

3.2.1. Proximate analysis of coal sample 25

3.2.2. Thermal extraction of coals 26

3.2.3. Extraction of coal by Ultrasonic irradiation 27

3.2.4. Recovery of extract 29

3.2.5. Analysis of the extract and residue 29

Chapter 4 RESULTS & DISCUSSIONS 31

4.1. Characterization of raw coal sample 31

4.1. 1. Proximate analysis of raw coal 31

4.1.2. FT-IR Analysis 31

4.2. Effects of different solvents 32

4.3. Comparison of methods of extraction 32

4.4. Determination of optimum time of extraction 34

4.4.1. Comparison of percentages of extract recovered 34

4.4.2. Comparison of ash reduction in extracts 34

4.4.3. FT-IR spectra of coal extract and residue obtained

using fresh NMP

36

4.5. Comparison of results obtained using reused and fresh

(unused) NMP

37

4.5.1. Comparison of percentages of extract recovered 37

4.5.2. Comparison of ash reduction in extracts 38

viii

TABLE OF CONTENTS (Contd)

Title Page No.

4.5.3. Comparison of FT-IR spectra of extracts obtained

using fresh and reused NMP

39

4.6. Effect of reusing the residue for thermal extraction 40

4.6.1. Percentages of extract recovered from fresh and

reused coal residue

40

4.6.2. Percentages of ash reduction in extract 41

4.7. Experimental results for use of additives along with

NMP

41

4.7.1. p-Nitro benzoic acid 42

4.7.1.1. Percentage of extract recovered 42

4.7.1.2. Ash content of extract 43

4.7.2. Quinoline 44



4.7.3. Calcium fluoride 46



4.7.4. Calcium chloride 48

4.7.4.1. Percentage of extract recovered 48 4.7.4.2. Ash content of extract 49

4.8. Comparison of the experimental results for the different

additives

51

4.8.1. Comparison of the yields of extracts 51

4.8.2. Comparison of ash reduction in the extracts 52

4.8.3. Comparison of FT-IR results of the extracts for the

different additives

52

Chapter 5 CONCLUSIONS 54

5.1. Conclusions 54

5.2. Future prospects of the work 55

REFERENCES 57

ix

LIST OF TABLES

Table No. Title Page No.

3.1 Particulars of all experiments

28

4.1 Proximate analysis of raw coal

31

4.2 Comparison of percentages of extract recovered from Toluene

& NMP

32

4.3 Comparison of percentages of extract recovered by Thermal

extraction & Ultrasonic irradiation

33

4.4 Percentages of extract recovered and ash removed under

different times of extraction for fresh NMP

35

4.5 Percentages of extract recovered and ash removed under

different times of extraction for reused NMP

39

4.6 Percentages of extract recovered and ash removed from fresh

coal and reused residue

41

4.7 Percentages of extract recovered and ash removed for using p-

nitro benzoic acid as an additive with NMP

44

4.8 Percentages of extract recovered and ash removed for using

Quinoline as an additive with NMP

46


Calcium fluoride as an additive with NMP

48


Calcium chloride as an additive with NMP

50

x

LIST OF FIGURES

Fig. No. Title Page No.

1.1 Sources of energy in India in 2013

3

1.2 Sources of electricity in the United States in 2011

4

1.3 Sources of electricity generation in India, 2011

4

1.4 Example of chemical structure of coal

6

2.1 Representative structure of coal 14

2.2 Effect of rank of coal on extraction yield

16

2.3 Variation of extraction yield with temperature

17

2.4 Variation of extraction yield with initial applied pressure

17

2.5 Flow diagram of the Hyper-coal process

19

2.6 The effect of solvent nature on the extraction efficiency at

ambient conditions.

21

3.1 Schematic representation of experimental procedure

27

3.2 Thermal extraction set up 29

3.3 Thermal distillation set up 30

3.4 Dried Extract in a petridish 30

4.1 Comparison of percentages of extract recovered by Thermal

extraction & Ultrasonic irradiation

33

4.2 Percentages of extract recovered under different times of

extraction for fresh NMP

34

4.3 Percentages of ash removal of extract under different times of

extraction for fresh NMP

35

4.4 FT-IR spectra of coal extracts obtained using fresh and reused

NMP

36

xi

LIST OF FIGURES (Contd)


4.5 FT-IR spectra of raw coal and residue obtained using NMP 37

4.6 Percentages of extract recovered under different times of

extraction for fresh & reused NMP

38

4.7 Percentages of ash removal of extract under different times of

extraction for fresh &reused NMP

39

4.8 Percentages of extract recovered from fresh &reused residue

40

4.9 Percentages of ash reduction in extract from fresh coal & reused

residue

41

4.10 Percentages of extract recovered for different concentrations of p-

nitro benzoic acid

42

4.11 Percentages of ash reduction in extract for different

concentrations of p-nitro benzoic acid

43

4.12 Percentages of extract recovered for different concentrations of

Quinoline

45


concentrations of Quinoline

45


Calcium fluoride

47


concentrations of Calcium fluoride

48


Calcium chloride

49


concentrations of Calcium chloride

50

xii

LIST OF FIGURES (Contd)


4.48 Comparison of percentages of extract recovered for different

concentrations of the additives

51

4.19 Comparison of the percentages of ash reduction in extract for

different concentrations of the additives

52

4.20 Comparison of the FT-IR spectra of the extracts for the different

additives

53

1

Chapter: 1

Introduction

_____________________________________________

1.1. Coal

Coal is an important fossil fuel & has immense reserves in various parts of the world. It is a

solid, brittle, combustible, carbonaceous rock formed by the decomposition and alteration

of vegetation by compaction, temperature, and pressure. The vegetation that form coals had

been buried millions of years ago and may be mosses and other low plants; although some

varieties of coal contain significant amounts of materials derived from woody precursors.

The plant precursors that eventually formed coal were compacted, hardened, chemically

altered, and metamorphosed by heat and pressure over time.

1.1.1. Types of coal

According to degree of metamorphism, several types of coal are available, which vary in

color from brown to black and are usually stratified. Peat is considered to be the precursor

of coal consisting of partly decomposed plant material that has accumulated in situ under

temperate marshy conditions. It is associated with large quantities of moisture. Near the

surface of deposit, peat is light brown & highly fibrous. As depth increases, the colour

darkens & finally becomes black. Peat represents the first stage of formation of coal. The

other major stages are lignite, bituminous coal &anthracite.

Lignite is the lowest rank of coal. It is often referred to as brown coal for its brownish black

colour and woody texture. Its moisture content is quite high, ranging from 30-50% and is

used almost exclusively as fuel for steam-electric power generation. The heat content of

lignite ranges from 9 to 17 million Btu/ton on a moist, mineral-matter-free basis [1].

Subbituminous coal occupies the next rank. The properties of this type of coal range from

those lignite to those of bituminous coal. It may be dull, dark brown to black, and soft and

2

crumbly at the lower end of the range, to bright, black, hard, and relatively strong at the

upper end with a moisture content of 20-30% by weight. It is primarily used as fuel for

steam-electric power generation. The heat content of this coal usually varies from 17 to 24

million Btu per ton on a moist, mineral-matter-free basis [1].

The next rank is occupied by Bituminous coal, which is black in colour with bands of

bright and dull materials and is harder than lignite. Its moisture content is usually below

20% by weight and is primarily used as a fuel for steam-electric power generation and for

manufacturing coke. The heat content of Bituminous coal ranges from 21 to 30 million Btu

per ton on a moist, mineral-matter-free basis [1].

Anthracite is the most mature & hardest form of solid fossil fuel, with a sub-metallic luster,

or, graphitic appearance. It is hard and brittle and is referred to as hard coal. It has a

moisture content below 15% by weight with low quantity of volatile matter and high fixed

carbon content and is mostly used for electricity generation and metallurgical applications.

The calorific value of this type of coal ranges from 22 to 28 million Btu per ton on a moist,

mineral-matter-free basis [1].

1.2. Uses of coal

Coal is an important source of energy and has a wide range of industrial as well as

domestic applications. According to World Coal Association (WCA) statistics, the total

global coal production in 2013 was 7823Mt, of which India produced 613Mt of coal and

occupied third rank among the worlds top 10 coal producers.

In 2013, India ranked 4th largest energy consuming nation in the world according to a

report released by U.S Energy Information & Administration. United States was the biggest

Energy Consumer followed by China and Russia. The major sources of energy in India in

2013 were coal, which supplied about 41% of the energy demand, followed by petroleum

and solid biomass and waste products (Fig. 1.1). Natural gas, nuclear power and renewable

sources also contributed to meet Indias energy demand.

3

Fig.1.1: Sources of energy in India in 2013

(Source: U.S. Energy Information Administration, International Energy Statistics, 2013)

The major industrial uses of coal are as follows:

(i) Use as a source of energy in thermal power plants: When coal is used for electricity

generation, it is usually pulverized and then combusted (burned) in a furnace with a boiler.

The furnace heat converts boiler water to steam, which is then used to spin turbines which

turn generators and create electricity. As per WCA statistics, in 2013 coal was used to

generate over 40% of the world's electricity and 70% of the electricity in India was

generated from coal.

According to U.S. Energy Information Administration (EIA) report, about 68% of the

countrys electricity in 2011 was generated by coal, natural gas, petroleum and oil (Fig.

1.2). The next was nuclear energy at about 20% and about 13% was contributed by

renewable sources, like solar, hydropower, wind, geothermal and biomass.

4

Fig.1.2: Sources of electricity in the United States in 2011

(Source: U.S. Energy Information Administration, 2011 data)

According to the EIA report, India had an installed electricity generating capacity of

211GW as of September, 2012. The various sources through which India produces its

electricity are given in Fig 1.3.

Fig. 1.3: Sources of electricity generation in India, 2011

(Source: U.S. Energy Information Administration, International Energy Statistics, 2013)

5

(ii) Use in blast furnace: Coke is a solid carbonaceous residue derived from low-ash, low-

sulfur bituminous coal from which the volatile constituents are driven off by baking in an

oven without oxygen at temperatures as high as 1,000 C, so the fixed carbon and residual

ash are fused together. Metallurgical coke is used as a fuel and as a reducing agent in

smelting iron ore in a blast furnace. The result is pig iron and is too rich in dissolved carbon

as a result it must be treated further to make steel. According to WCA, approximately 15%

(over 1.2 billion tonnes) of total coal production in the world is currently used in the steel

industry and roughly 70% of total global steel production is dependent on coal.

(iii) Gasification to produce Synthesis Gas: Coal gasification can be used to

produce synthesis gas, a mixture of carbon monoxide (CO) and hydrogen (H2) gas.

Synthesis gas can be used to produce methanol & ammonia.

1.3. Chemical composition of coal

Coal consists of an organic mass with some quantities of inorganic substances, like, water

& mineral matter.

Moisture: Coal is always associated with moisture due to its nature, origin & occurrence.

Some amount of moisture is derived from the vegetable matter from which coal is formed.

Varying amounts of this water is present in different stages of coalification process. This is

evident from a comparison of the moisture contents of different ranks of coal, from lignite

to anthracite. Lignite has the maximum moisture content of 30-50%, which decreases to

less than 15% in anthracite. However, moisture content of coal is also due to washing of

coals as well as due to rain during transportation and storage of coal.

Complex organic mass: The exact organic structure of coal cannot be isolated & identified.

Coal composition is, therefore, studied by indirect methods. Fig. 1.4 shows an example of

the complex organic structure. The organic mass is a mixture of complex organic

compounds of carbon, hydrogen, oxygen, nitrogen & sulphur.

Mineral matter: Coal contains inorganic mineral substances, which are converted into ash

during the combustion of coal. Mineral matter may be of two types: inherent & extraneous.

6

The inorganic materials of the original vegetable substances are responsible for the inherent

mineral matter. The extraneous mineral matter is due to (i) the substances which get

associated with the decaying vegetable matter during the formation of coal and (ii) rocks &

dirt getting mixed during mining & transportation.

The former type of extraneous mineral matter is intimately associated with the organic

mass of coal & hence, difficult to remove by mechanical methods. This type of mineral

matter includes mainly clay, shale, sand and gypsum. The second type, comprising mainly

rocks and dirt is more amenable to coal cleaning methods. Inherent mineral matter cannot

be removed by any mechanical means. Indian coals suffer from the great disadvantage that

the mineral matter is high as well as is intimately associated with the coal structure due to

their drift origin [2].

Fig. 1.4: Example of chemical structure of coal

The bulk of the mineral matter is due to shale, or, clayey substances and consists of

aluminosilicates of different compositions. Most common clay minerals are kaolinite and

mixed-layer illitemontmorilloniteKaolinite-rich clay is commonly associated with coals in

most of the coal basins of the world. Other major constituents are calcite (calcium

7

carbonate) & pyrites. Among sulfide minerals that are present in coal, dimorphs pyrite

(FeS2) and marcasite (FeS2) are the dominant sulfide minerals in coal, pyrite being more

abundant. Sulphate minerals are present in coal, but their quantities in fresh, unoxidised

coal samples is insignificant. The sulfates, gypsum (CaSO4,2H2O) and barite (Ba2SO4) are

found in fresh coals, while a number of hydratedsulfates (FeSO4,xH2O) have been reported

in weathered coals. Carbonate minerals like Calcite (CaCO3) and ankerite (a mixed crystal

composed of Ca, Mg, and Fe carbonates) are abundant in some coals. Silica is also present

in coals while the most dominant form of silica being Quartz.[j] Among other minerals,

Authigenic apatite [calcium fluorochlorohydroxyphosphate, Ca5(PO4)3.F.Cl.OH] has been

found in coal produced at widely separated areas of the world [1].

During combustion of coal, the shale & other hydrated minerals lose water of hydration,

while sulphides, sulphates & carbonates decompose, or, get oxidized, leaving their basic

radicals to combine with excess silica (if any)[2]. As a result, there is a net loss in weight,

so that the ash of coal is less than the mineral matter content.

1.4. Proximate analysis of coal

The proximate analysis of coal determines the percentages of moisture, ash, volatile matter

and fixed carbon of coal.

Moisture: The amount of moisture present varies according to the rank of coals. Moisture

is of two types: external & inherent. The external moisture depends upon the mode of

occurrence & handling of coal. Inherent moisture may be referred to as equilibrium, air-

dried, or, hygroscopic moisture & can only be removed by heating coal above 100. The

total moisture in coal is the determination of the moisture (in all forms except water of

crystallization of the mineral matter) that resides within the coal matrix. Air-dried moisture

is determined by observing the loss in weight of a coal sample on heating above 105C.

Air-dried moisture of coal decreases with increasing rank of coal and ranges from 30-50%

in lignite to less than 15% in anthracite.

Ash content: Ash refers to the residue left after combustion of coal under specified

conditions. The changes that occur during combustion of coal include loss of water from

hydrated silicate minerals, liberation of CO2 from carbonates, oxidation of iron pyrites to

8

iron oxide and reaction between sulfur oxides and bases like calcium and magnesium[j].

Generally, more than 90% of ash for Indian coals consists of silica, alumina, iron oxide &

lime. The remaining are oxides, sulphates & phosphates of sodium, potassium &

magnesium. Trace elements, like, Gallium, Germanium, Nickel, Beryllium & Boron may

also be present as trace amounts in Indian coals [2].

Volatile matter and fixed carbon: These denote the volatile and non-volatile products of

thermal decomposition of coals under specified conditions. Volatile matter does not include

moisture present in coal, but includes moisture formed by hydrogen and oxygen during

decomposition of coal [2]. When represented in air-dried and d.a.f. bases, it includes the

water of hydration of mineral matter, which vaporizes due to heat. However, on d.m.m.f.

basis, volatile matter includes only volatile products from organic matter of coal. With

increase in rank, or, maturity of coal, volatile matter decreases. The volatile matter of

Anthracite coal ranges from 3 to 10% d.m.m.f., while that in bituminous coals usually lies

between less than 20 to about 45% on d.m.m.f. basis [2]. Fixed carbon does not, however,

include ash content of coal. It is the non-volatile residue of the organic mass of coal. The

higher the volatile matter, the lower is the amount of fixed carbon of the coal.

1.5. Ultimate analysis of coal

The ultimate analysis determines the carbon, hydrogen, sulfur, nitrogen and oxygen in the

pure coal.

Carbon content is determined by Liebigs method by completely burning the coal in pure

oxygen and finding the amount of carbon dioxide formed. However, correction is made for

carbondioxide formed due to decomposition of carbonates.

Hydrogen content of coal is also determined using the same procedure of burning the coal

in pure oxygen. Amount of hydrogen is calculated after estimating the amount of water

formed. Corrections are used for moisture in the coal and water of hydration of minerals.

The hydrogen content of lignite to bituminous coals vary in the range of 4.5-6.5%, while

hydrogen content lies in between 1-2% in anthracite [2].

9

Nitrogen is estimated by Kjeldahls method. The sample is digested with oleum containing

a catalyst to convert nitrogen into ammonium sulfate. The ammonia is then estimated and

nitrogen content is determined. In most coals, the nitrogen content varies between 1-2%

[2].

Three forms of sulfur, namely pyritic, organic and sulphate are common in most coals. The

total sulphur can be estimated by Eschka, or, Bomb method. In the former method, the

entire sulfur content is converted into soluble sulfates by heating the coal with an oxidizing

mixture of magnesium oxide and sodium carbonate. Then sulfate is estimated. In the Bomb

method, total sulfur is converted into sulfate in a Bomb calorimeter. The pyritic and

sulfates are detrmined by methods of analytical chemistry, while organic sulur is obtained

by subtracting inorganic sulfur from total sulfur. The amount of sulfur in Indian coals is

usually low (0.7%, or, less), but some coals may have sulfur content around 4% [2].

Oxygen content is obtained by subtracting the percentages of carbon, hydrogen, nitrogen

and sulfur (on a d.m.m.f. basis) from 100.

Phosphorous content of Indian coals is usually less than 0.15% [2]. The estimation of

phosphorous content becomes important during production of metallurgical coke. But the

low phosphorous content of Indian coal shows no problems for iron and steel production

process.

1.6. Disadvantages of ash content of coal

Mineral matter does not contribute to the calorific value of coal, but creates difficulties in

the efficient utilization of coal. The thermal efficiency of coal is reduced due to high ash

content as the latter not only interferes with the combustion of coal but also reduces

temperature of the combustion zone. High ash content of coal leads to large heat losses and

carbon losses in boilers and furnaces. High ash content in coal also results in boiler

deposits & clinkering. Due to these reasons, burning of coal takes place very slowly

compared to low ash coal under identical conditions of feed rate and excess air supplied.

Consequently steam output of boiler decreases, thereby reducing efficiency of a boiler.

10

Efficiency of blast furnace is also reduced due to high ash in coking coals because rate of

combustion gets reducedas ash restricts passage of air in furnace grates. Transportation and

handling costs are also increased if the mineral content of the coal is high.

1.7. Beneficiation of coal

Depending on the composition of the coal seam, in which several types of inorganic

intrusions may be present due to sedimentation and volcanic activity, and depending on the

mining method employed, the run-of-mine coal will commonly contain rock, shale and

other undesirable contaminants[2].

Coal beneficiation is the process of removal of the contaminants and the lower grade coal

to achieve a product quality which is suitable to the application of the end user - either as

an energy source or as a chemical agent or feedstock. A common term for this process is

coal "washing" or "cleaning".

Chemical beneficiation of coal refers to the use of chemicals to remove the mineral matter

of coal. While physical beneficiation processes mainly remove mineral matter, which get

mixed during mining & handling operations, chemical beneficiation aims at removing the

mineral matter, which is intimately associated with the coal structure. Various inorganic as

well as organic chemicals can be employed for this purpose. Organic solvents mainly

dissolve organic constituents of coal, leaving behind mineral matter obtained as residue.

Amount of coal extracted by each solvent is not same for every solvent. The percentage of

coal, extracted by the solvent, therefore, depends both on coal structure as well as

properties of the solvent.

1.8. Background of the project

Coal is an important source of energy and is used in many industries like coal-fired thermal

power plants. Coal is also converted into coke, which is used as a fuel as well as a reducing

11

agent in smelting iron ore in a blast furnace. But presence of high percentage of ash-

forming minerals in coal is disadvantageous. Though extraneous mineral matter can be

removed by physical coal cleaning methods, inherent mineral matter cannot be removed by

easy physical methods. Mainly chemical treatment is required to remove those minerals.

Indian coals consist of high percentage of ash-forming minerals, which are converted into

ash during combustion of coal. Mineral matter mainly consists of aluminosilicates apart

from pyrites and few other compounds. Presence of high ash not only interferes with

effective utilization of coals due to heat losses, but disposal of large amounts of ash is also

an important problem. Many studies have been conducted to reduce the ash content of coal

by use of various solvents and chemicals. This project also deals with the removal of ash

using solvents and additives.

1.9. Problem statement

The ash content of Indian coals is very high, seldom exceeding 50%. This makes them

unsuitable for use in boilers and blast furnaces, as high ash results in large heat losses,

thereby reducing efficiencies of boilers. Also, ash content of coal interferes with the

combustion of coal in blast furnaces. Moreover, disposal of large quantities of ash is also a

problem. All these disadvantages of ash content of coal have been discussed in details in

section 1.6. This project deals with extraction of Indian coal by the using different solvents

and additives, which brings down ash content of extracted coal. This extracted coal can be

used in boilers and furnaces and also for making metallurgical grade coke for blast

furnaces. However, the quantity of extracted coal remains quite little and studies are being

undertaken to increase the percentage of coal extracted by solvent. This project focuses on

the effectiveness of various additives in increasing extraction yield of coal.

1.10. Objectives

The main objective of this project is to determine the effect of additives during extraction

of an Indian coal with solvents on extraction yield. However, the extent of this project also

includes studying the effect of additives to decrease in ash content of the extract from that

of the original coal sample. For this, measurement of ash content of the raw coal, residue

12

and extract will be done. Additionally, to study organic functional groups, extracted by the

solvent-addtive mixture, FT-IR analysis will also be conducted.

1.11. Originality of the work

There are previous studies of coal extraction with the solvents used in this present research

work. But the originality lies in choosing the additives used in this work. For the present

work, additives have been selected after studying the chemistry of coal extraction process

and experimental work of coal extraction using these additives are not available in

literature as per knowledge.

13

Chapter: 2

Literature review

Many studies have been conducted to reduce the ash content of coal by the use of various

chemicals. Most of the studies include extraction of coal using organic solvents which in

turn reduce ash content of the extracted coal to a large extent. The effects of various

parameters like temperature, pressure, effects of additives, etc. have been studied. The

main aim of this review is to determine the various factors which decrease the ash content

of coal as well as increase extraction of coal. The effects of the various chemicals used in

these studies provide important information as to how ash content of coal can be reduced

to a significantly low percentage.

2.1. The chemical structure of coal

A number of workers have attempted to give a representative structure of coal that is

consistent with the observed chemistry of coal. In Fig. 2.1, a representative structure of

coal has been presented. According to this structure, coal consists of highly substituted

aromatic rings with a number of functional groups. The figure shows coal as a highly

cross-linked amorphous polymer, consisting of a number of stable aggregates connected

by relatively weak cross-links. These cross-links have marked by arrows in the figure.

This highly cross-linked structure fragments into radicals at high temperatures in

presence of hydrogen-donor solvents. In absence of hydrogen donor solvents, these

radicals may recombine to form char, or, coke. So, coal becomes highly reactive in

presence of hydrogen donors and liquefies easily [3].

It has also been observed that aromaticity varies with the rank of coal and can be low for

sub-bituminous coals, which contain significant amounts of polycyclic aliphatic rings.

14

High aromaticity of coal products is due to the processes used to convert coal and does

not imply high aromaticity of the starting product. Presence of hydrogen donor solvents

can increase solubility of coal fragments in solvent, but hydrogen is not necessary for

coal solubility. Temperatures above 750F are required for rapid conversion [3].

Fig. 2.1: Representative structure of coal [3]

2.2. Solvent extraction of coals

Solvent extraction of coals is accomplished by contacting the coal with a solvent under

specified conditions of temperature and pressure. After extraction, the residual coal

material is separated from the solvent containing the extracts. The extraction solvent is

well mixed with the coal to allow soluble constituents of coal to transfer to the solvent

phase. The residual coal and solvent are then separated by physical methods, such as

gravity decanting, filtering, or centrifuging. Distillation may be done to recover the

solvent from the extracts.

15

The solvents used to extract coal can be classified as follows [4]:

Non-specific solvents: Non-specific solvents can extract a small amount of coal (up to

about 10%) for temperatures up to 100. They are low boiling liquids, like methanol,

ethanol, acetone, ether, etc. The extract is believed to be occluded in the coal matrixdue

to waxes & resins. These resins and waxes donot form a significant portion of the coal.

Specific solvents: They extract up to 40% coal at temperatures below 200. They are

non-selective in nature & the nature of the extract is similar to that of the parent coal.

These are, generally, nucleophilic in nature due to the presence of a lone pair of electrons

on the nitrogen atom. e.g. NMP, Pyridine, Dimethylformamide, Dimethylacetamide, etc.

Degrading solvents: They can extract up to 90% of coal at temperatures of about 400.

They degrade coal thermally into smaller fragments. After extraction, the solvent can be

recovered without change in its chemical form. E.g. phenanthrene, diphenyl, etc.

Reactive solvents: These solvents react with the coal chemically. They are generally

hydrogen donors. The smaller fragments formed by thermal disintegration of coal are

stabilized by hydrogen which is donated by the solvent. Both the solvent & coal undergo

appreciable changes during extraction. e.g. tetralin.

2.3. Effect of rank of coal on solvent extraction

Rank of coal has a considerable influence on the chemical nature and quantity of extracts

obtained by the solvent extraction of coal. The soluble products of the extraction, referred

to as extracts vary according to the means by which they are obtained.

For higher rank Bituminous coals, it has been observed that the extraction yields increase

with an increase in temperature and there is a peak temperature at which a maximum

extraction yield is obtained. Beyond this peak temperature, the extraction rate again

drops. This is related to the thermal relaxation of molecules. At the peak temperature,

relatively small molecules may be released from the cross-linking coal structure to the

solvent, resulting in dissolution of coal in the solvent [5].

For lower rank coals, like sub-bituminous and lignite coals, extraction yield has been

observed to increase with increase in temperature. A suitable solvent and high

temperature of about 673K gives an extraction yield of over 70% and very low, or,

negligible ash content, as shown in figure 2.2. In figure 2.2, BD, BL and POP are lignite

16

coals, while CV and GEN are sub-bituminous coals. A higher proportion of vitrinite and

lower value of MMVR (mean maximum vitrinite reflectance) has been reported to give

higher extraction yield as lower MMVR values signify higher reactivity of coal.[6]

Fig. 2.2: Effect of rank of coal on extraction yield

(Rahman et al., 2013)[6]

2.4. Effect of temperature and pressure on extraction of coal

For higher rank coals, like Bituminous coals, it has been observed that the extraction

yield increases with an increase in temperature and there is a peak temperature at which a

maximum extraction yield is obtained, as shown in figure 2. Beyond this peak

temperature, the extraction rate again drops. The peak temperature at which the extraction

yield becomes maximum has been related to the softening temperature of coal. When the

coal softening point is closer and closer to the extraction temperature, the coal extraction

becomes higher and higher [5]. At the softening point, the structure becomes relaxed and

beyond the softening point, the coal is restructured by cross-linking, which results in a

decrease in extraction yield [7]. This is possibly due to the enhanced coalsolvent

interaction at this temperature and consequently, higher solvent induced thermal

relaxation of coal molecules is occurring and releasing mainly small molecules and free

radicals from the cross-linking coal structure to the solvent .[5] In figure 2.3, Kideko and

Roto south are sub-bituinous coals, while Sunhwa is a Bituminous coal.

17

Fig. 2.3: Variation of extraction yield with temperature

(Kim et al., 2007)[7]

For lower rank coals, such as lignite and sub-bituminous coals, there is no softening

temperature at which the coal structure becomes relaxed and so a polar solvent is required

to breakdown the structure. The ash content of coal extracts from both Bituminous as

well as sub-bituminous coal decreases as the extraction temperature increases. The initial

applied pressure has no significant effect on the extraction yield and ash reduction;

extraction yield and ash reduction can be enhanced more by increasing extraction

temperature rather than the initial pressure, as shown in Fig.2.3 [7].

Fig. 2.4: Variation of extraction yield with initial

applied pressure (Kim et al., 2007)[7]

18

2.5. Effect of nature of solvents on extraction of coal

For higher rank coals, like Bituminous coal, it has been reported that non-polar aprotic

solvents like 1-methylnaphthalene can give satisfactory yields of about 70% by weight

(d.a.f.) and ash content reduced to several hundreds of ppm at 340-360. However, it has

also been observed that if a hydrogen donating solvent like tetralin is used, the extraction

rate becomes more than that obtained when an aprotic solvent is used at the same

extraction temperature. This has been attributed to intermolecular hydrogen transfer from

the solvent to coal to stabilize small molecules derived by thermal decompositionof coal

which brings about due to coal softening phenomenon [5]. Polar solvents, like NMP have

been reported to give extraction yield above 60% for Bituminous coals at temperatures

above 350 [7].

However, in case of lower rank coals, such as lignite and sub-bituminous coals, there is

no softening temperature at which the coal structure becomes relaxed and so a polar

solvent is required to breakdown the structure. In case of sub-bituminous coals, it has

been observed that a polar solvent like NMP can give an extraction yield of over 80% at a

temperature of about 400 [7]. Hydrotreated aromatic hydrocarbons have given higher

extraction yields for low rank coals than 1-methyl naphthalene because latter contains

polar components [6]. NMP has been reported a better solvent than non-polar ones for the

low rank coals which have higher amount of polar sites [8].

Yoshida et al. [9] have reported that nitrogen containing solvents are effective for giving

higher yields of extracts during solvent extraction of coals. Thus, nitrogen containing

solvents, like amines may be used for enhancing extraction yield.

2.6. Effect of hot filtration and room temperature filtration on extraction

Coals of various ranks have been extracted with a variety of organic solvents, viz tetralin,

1- methyl naphthalene, dimethyl naphthalene and light cycle oil (LCO -a by-product of

cracking of vacuum gas oil to gasoline). It has been observed that high extraction yield

can be obtained if a suitably high extraction temperature is maintained; but, if the

separation of residue and solution is conducted at room temperature, the extraction yields

will not achieve the required specification of Hyper-coal [10].

19

The term Hyper-coal refers to ash-free coal [5]. This has been attributed to the separation

conducted at room temperature as the extract components soluble at high temperature

might have deposited while quenching and decreased the percentage of coal extract. [10]

Hence, filtration after extraction should be performed at a high temperature in order to

maintain a satisfactory extraction yield.

Fig. 2.5: Flow diagram of the Hyper-coal process

(Okuyama et al., 2004)[5]

2.7. Effect of using mixtures of solvents for coal extraction

Beneficiation of coal by mixtures of solvent, like CS2 & NMP (in the ratio 1:1) has been

reported to give a higher extraction yield than that obtained when NMP is used alone.

NMP is a polar solvent, but when CS2 is added, a synergistic effect is obtained. It has

been reported that with CS2 addition, the viscosity of the solvent mixture decreases and

the ability of the mixed solvent to penetrate the cross-linked coal structure increases. As a

result mixed solvent can interact with solvent-soluble molecules which in turn increase

the extraction yield [11].

Large synergistic effects have been observed for coals used with NMP/HHA (1,4,5,8,9,10

Hexahydroanthracene) mixed solvents. Also dissolution yield increases while using

20

mixed solvents. This is due to the fact that the extent of synergistic effect is highly

dependent on the kind of coal used. It has been reported that hydrogen donation from

HHA to the coal radicals is the key reaction [8].

2.8. Effect of various additives on extraction of coals with solvents

It has been observed that addition of strong bases, like NaOH, or, sodium tertiary

butoxide can increase the degree of dissolution of coal in solvents like NMP, DMF, etc.

Depending on the extraction conditions, carbon extraction efficiencies of up to 90% have

been obtained, as shown in Fig. 2.6. Sodium, or, potassium hydroxide is added as a

solution with water. However, addition of water has been observed to be detrimental for

DMF due to base catalyzed hydrolysis of DMF to dimethyl amine and formic acid in

presence of water. Addition of sodium sulfide has been observed to further reduce the ash

content of the extracted coal. It has also been reported that as the NaOH/Na2S molar ratio

became high, coal extraction yield also increases. [12]

The addition of a small amount of polar compound such as methanol to nitrogen-

containing polar solvent has been observed to have greatly increased the thermal

extraction yield for subbituminous coals at 360 [13].

Addition of salts like lithium and tetrabutylammonium salts with various anions to polar

aprotic solvents, or, solvent mixtures have also been observed to increase the extraction

yield for several coals. The yields increase in the order F->Cl

->Br

->I

-, implying that

smaller ions with large electronegativity are responsible for increase in yields. Hard bases

like F- & Cl

- attract proton and since, the solvent is polar and aprotic, these anions will

not be solvated and can interact with some hard acidic sites in coal. However, soft bases,

like Br- & I

- get solvated in the soft acid like CS2-NMP mixture and cannot interact with

the coal. It has also been reported that hard bases, like acetate ion can increase extraction

yield. It has also been reported that the addition of alkali metal salts, like LiCl can

increase the polarity of the solvents-salt solutions which in turn increase the extraction of

coal [14].

It has been observed that if polar components of an industrial solvent like CMNO (crude

methyl naphthalene oil) can be separated, the extraction yields obtained with the

extracted polar materials are 20-30% higher than that with CMNO (Kashimura et al.,

21

2006). It has been reported that the major polar component of CMNO was quinoline and

the minor constituents were isoquinoline, indole and methylquinoline. Indole has greater

ability to extract coal constituents compared to quinoline. But if both are used with a non-

polar solvent, then a higher extraction yield compared to quinoline-nonpolar solvent

mixture is obtained [13].

Fig. 2.6: The effect of solvent nature on the extraction efficiency at ambient conditions. Solvent:coal:KOH=100:10:1.56 on a mass basis. (Makgato et al., 2008)[12]

2.9. Kinetics of diffusion of solvent into the coal structure

Coal has a cross-linked structure. Due to this structure, swelling of coal is the first stage

in processes like solvent extraction. As the solvent penetrates the coal matrix, coal-coal

interactions are replaced by coal-solvent interactions. This is why, coal swells due to

contact with a solvent during extraction. Pande et al.[15] studied the kinetics of swelling

of a bituminous, non-coking coal in two solvents namely NMP and Ethylenediamine

(EDA) and their mixture (1:1)(vol/vol) at temperatures ranging from 15C to 60C. For

the swelling experiments, solvent, or, solvent mixture at the desired temperature was

added to centrifuged samples of coal and that temperature was maintained in a thermostat

for the desired time. Swelling ratios were calculated for the different solvents/solvent

mixture for the particular time and temperature. It was observed that the swelling ratio in

22

the mixed solvent was highest and that in NMP was lowest. It was reported that the

activation energy for swelling of coal in the mixed solvent system was found to be more

than that in either of the solvents. Mixed solvent has greater ability to disrupt more

number of stronger non-covalent interactions. A comparison of the extraction yields of

the coal at room temperatures showed that the mixed solvent not only has greater

swelling power, but also has ability to break other types of coal-coal interactions, such as

stronger hydrogen bonds and - aromatic interactions. The bonds cannot be broken by

either of the solvents alone. Again, the rate of swelling in EDA was faster than that in

NMP. As a result, when the mixed solvent was used, faster swelling solvent EDA opened

the coal structure for penetration by NMP. It was also reported that the swelling kinetics

was characterized by a Fickian diffusion process.

2.10. FTIR Analysis of raw, residue and extracted coals

Rahman et al. [6] have reported FTIR spectra of raw, residue and ash-free coals (extracts)

obtained after solvent extraction. The FTIR spectra is shown in Fig. 6. The peaks indicate

presence of C-H stretching in phenyl groups, C=C double bonds in the aromatic rings,

C=O stretching in the samples and also C-H stretching in aromatic rings and/or from

branched aliphatic chains. However a pair of sharp peaks in the range of 1000-1100 cm-1

and 3600-3700 cm-1

, observed in case of the raw and residue coal samples were missing

in the spectra of ash-free coal. These two peaks have been reported to designate presence

of mineral matters in the raw coal and residue, thereby showing that ash-free coal

contains no or, significantly low quantities of mineral matter.

Similar observations have been reported by Yoshida et al [10]. They have also observed

peaks in extracts, which are due to extraction solvents remaining in the former.

According to their study, for coals extracted at 350C and 370C, the other peaks were

similar to those of raw coal and residue; but for the extract obtained at 380C, the ratio of

intensity of aliphatic C-H stretching to that of aromatic C-H stretching was higher than

that at 350C, indicating that chemical reactions like gas evolution became significant at

380C.

23

2.11. Coal beneficiation by agglomeration techniques

Beneficiation of bituminous and lignite coals can be done by agglomeration using

binding oils containing either p-xylene, or deodorized rectisol naphtha and the reduction

in ash content has been studied with respect to particle size, mixing speed, mixing time,

oil to coal ratio and oil characteristics. It has been observed that the ash reduction of 320

mesh bituminous coal was more than that of 200 mesh coal showing reductions of 17%

and 14%, respectively. Thus, the ash reduction of smaller sized particles is greater than

that of the larger ones. Higher mixing speeds and longer mixing times has been reported

to have removed greater percentage of ash, but smaller flocs are formed, which are

difficult to handle. Hot water dried lignite shows greater removal of ash compared to as

received lignite. [16] However, this method does not remove ash to a much larger extent

as done by other chemicals.

2.12. Beneficiation by the use of inorganic chemicals

Beneficiation of Bituminous coals has also been done with aqueous HF followed by

HNO3 at temperatures of about 65. It has been observed that HF, if used alone reduces

the ash content to about 2.6% by weight, while further treatment with HNO3 reduced the

ash content to about 0.6% by weight. It was reported that prior to treatment with HNO3,

compounds such as AlF3, NaAlF4, CaF2, MgF2 formed during leaching and pyrites (FeS2)

does not react with HF. If HNO3 is used, then the fluoride ions react with the H+ ion of

HNO3 to form HF. However, HNO3 only reacts with pyrites above a particular HNO3

concentration, which suggests that it reacts with the organic coal structure to a certain

extent. Some amount of sulphur in the coal has also been observed to have decreased.[17]

Chowdhury, et al. [18] treated high ash Indian Western Coal Fields-Nagpur coal,

containing over 50% ash with 25% (w/w) aqueous ammonia solution under ambient

conditions. Then the treated coal was washed and made to react with concentrated

sulfuric acid and small quantities of calcium fluoride at 350 over four hours. Calcium

fluoride was added to promote in situ production of hydrofluoric acid in the reaction

mixture. The coal was dried and further boiled with dilute hydrochloric acid. The ash

content of the treated coal was reduced to 9.6% from 32.9% of the untreated coal. The

first treatment was aimed at disrupting the bonding forces in the coal structure (where

24

majority of the mineral matter was located) to aid in enhanced demineralization during

the second treatment. After the first treatment, the ash content reduced from 32.9% to

28.5%. After the second treatment with concentrated sulphuric acid, the ash content

increased due to the formation of calcium sulphate. However, subsequent treatment with

dilute hydrochloric acid was done to dissolve the calcium sulphate and reduction of ash

content to 9.6%.

2.13. Reduction of ash content of coal with solvents without coal extraction

It has been reported that ash content of the extract reduces after treatment of coal with

solvents. Saha, et al. [19] treated samples of coal with different solvents such as N-

Methyl-2-Pyrrolidone (NMP), Furfural, Aniline, Acetic Acid and Toluene by varying the

amounts of solvents. Coal was treated with the above mentioned solvents for a particular

amount of time and then the solvent was removed from the coal by thermal distillation,

followed by drying. Comparative study of different solvents show NMP is the best

solvent with respect to the removal of ash content from coal. The maximum reduction of

ash content of coal is 72 % at 1:10 coal to solvent ratio, 120 and 1 atmospheric

pressure in presence of NMP as an extractant which has high chemical and thermal

stability. The same experiment, when performed using with Crotonaldehyde, Acetonitrile

and Benzene showed that the highest percentage of ash removal was 53.84% when the

coal to solvent ratio was 1:70 [20].

It can be concluded from these literature review that ash content of coals can be reduced

with both inorganic chemicals as well as organic solvents. Solvent extraction of coals is

an effective method of reducing the ash content of coals, but the extraction yields remain

low. Most experiments aim at increasing the extraction yield of coals. Since polarity of

the solvents greatly influence the extraction yield of coals, various additives can be mixed

with the solvents to enhance their polarities and extraction yield as well.

25

Chapter: 3

Experimental Materials &

Procedure

____________________________________________

3.1. Materials

A coal sample of Indian origin was used in all experiments carried out during this study.

The coal was pulverized to 60 mesh (

26

3.2.1.2. Determination of volatile matter: The volatile matter of the coal sample

was determined by taking 1 g of the sample in a silica crucible and was covered

with a lid. Thereafter, the sample was heated in a muffle furnace at 925C for

exactly 7 minutes. The warm crucible was cooled on a cold iron plate and then

transferred to a desiccator. The percentage of volatile matter in the coal was

calculated from loss in weight of the sample.

3.2.1.3. Determination of ash content: About 1 gram of the coal was taken in a

silica crucible and placed in a muffle furnace at 400C without lid. The sample

was heated from 400C to 450C in about 30 minutes and this temperature was

maintained for another 30 minutes. Subsequently, the incineration was completed

by heating the coal sample for 1 hour at about 775C. The crucible was taken out

of the muffle furnace, covered with the lid and cooled in a desiccator. The

percentage of ash of the coal was determined from the loss in weight of the

sample.

3.2.1.4. Determination of fixed carbon: Percentage of fixed carbon in the coal was

calculated by subtracting the percentages of moisture, volatile matter and ash

from 100.

3.2.2. Thermal extraction of coals

Fig. 3.1 shows a schematic diagram of the experimental procedure. Approximately 20 g

of the pulverized coal was taken in a beaker and 200 g solvent and additives with

different weight percentages were added to it. Then thermal extraction of the coal was

carried out by heating coal-solvent slurry with constant stirring. After reaching the

prerequisite temperature, it was maintained for desired period of time. The temperature of

extraction was selected such that it was lesser than the boiling point of the solvent. The

beaker was covered with a piece of aluminium foil during thermal extraction to prevent

loss of solvent by evaporation. The ratio of coal to solvent was 1:10 (by weight) in all

the experiments. The speed of rotation of the stirrer was also kept constant throughout all

the experiments carried out during this research work. The percentages of additives, time

and temperature of extraction for all the experiments are represented in Table 3.1.

27

After thermal extraction, filtration was carried out to separate the liquid phase and the

undissolved solid coal residue using filtration. The residue was washed several times with

fresh solvent to remove extract that remained with the residue. Solvent containing extract

was kept aside for recovery of the extract which is explained in details later in this thesis

at section 3.4. The residue was dried in a hot air oven to remove excess solvent. The

thermal extraction set up is shown in Fig. 3.2.

3.2.3. Extraction of coal by Ultrasonic irradiation

Approximately 20 g of pulverized coal was extracted with 200 g of NMP without any

additive under ultrasonic irradiation (333 kHz) for 30 minutes at room temperature.

After extraction, filtration was carried out to separate the extract and the residue by using

the same procedure as described in section 3.2.2. The residue was dried in a hot air oven

and the solvent containing the dissolved extract was keep aside for recovery of the

extract.

Fig. 3.1. Schematic representation of experimental procedure

Pulverized coal

Thermal extraction of

coal with solvent and

additive with constant

stirring

Solvent

Separation of extracted

coal from solvent by

Thermal distillation

Removal of remaining

solvent from extract in

Hot air oven

Removal of

unextracted part

of coal (residue)

by Filtration

Additive

Coal Extract

Coal

Residue

Recovered solvent

Drying of

residue in

Hot air oven

28

Table 3.1: Particulars of all experiments

Sl.

No.

Solvent Additive Concentration

of additive

(in weight %

of raw coal)

Time of

extraction

(hours)

Temperature

of

Extraction

(C)

Method of

extraction

1 Toluene - - 1 90 Thermal

2 NMP - - 0.5 Room

temperature

Ultrasonic

irradiation

3 NMP - - 1 150 Thermal



6 NMP

(recycled)

- - 1 150 Thermal

7 NMP

(recycled)

- - 2 150 Thermal

8 NMP

(recycled)

- - 3 150 Thermal

9 NMP p-nitro benzoic acid

0.1 1 150 Thermal


0.5 1 150 Thermal


1.0 1 150 Thermal


2.5 1 150 Thermal


5.0 1 150 Thermal

14 NMP Quinoline 0.1 1 150 Thermal

15 NMP Quinoline 0.5 1 150 Thermal 16 NMP Quinoline 1.0 1 150 Thermal

17 NMP Quinoline 2.5 1 150 Thermal 18 NMP Quinoline 5.0 1 150 Thermal 19 NMP Calcium fluoride 0.1 1 150 Thermal 20 NMP Calcium fluoride 0.5 1 150 Thermal

21 NMP Calcium fluoride 1.0 1 150 Thermal 22 NMP Calcium fluoride 2.5 1 150 Thermal

23 NMP Calcium fluoride 5.0 1 150 Thermal 24 NMP Calcium chloride 0.1 1 150 Thermal 25 NMP Calcium chloride 0.5 1 150 Thermal 26 NMP Calcium chloride 1.0 1 150 Thermal 27 NMP Calcium chloride 2.5 1 150 Thermal 28 NMP Calcium chloride 5.0 1 150 Thermal

29

Fig. 3.2. Thermal extraction set up

3.2.4. Recovery of extract

The solvent containing extract was subjected to thermal distillation (Fig. 3.3) for

separation of the extract and the solvent. Complete recovery of the solvent was not

possible by distillation. For this reason distillation was continued until volume of solvent

containing dissolved extract reaches approximately 15 ml. After that, the extract was

recovered by evaporating the solvent from the extract in a hot air oven. The weight of the

dried extract was calculated thereafter. The extract obtained is shown in Fig. 3.4.

3.2.5. Analysis of the extract and residue

3.2.5.1. Determination of ash content of the extract and residue: The percentage

of ash in the extract and in the residue was evaluated using the same method as

described in section 3.2.1.3.

3.2.5.2. FT-IR analysis: The FT-IR spectra of the raw coal, residue and extract

after extraction were carried out by a FT-IR spectrophotometer (Perkin Elmer

30

Spectrum 2) at a resolution of 4 cm-1

in the wave number range of 450-4000 cm-1

.

50 scans were carried out each time prior to Fourier transformation. Samples of

FT-IR experiments were carried out by KBr pellet method using approximately 5

mg sample in 200 mg of KBr. All spectra were analyzed using Spectra Manager

software.

Fig. 3.3. Thermal distillation set up

Fig. 3.4. Dried Extract in a petridish

31

Chapter: 4

Results & discussions

4.1. Characterization of raw coal sample

4.1. 1. Proximate analysis of raw coal

The proximate analysis of raw coal was performed to determine its characteristics. All

the experiments for proximate analysis were performed at least thrice and the arithmetic

means of the data are given in Table 4.1.

Table 4.1: Proximate analysis of raw coal

Properties Moisture Ash Volatile matter Fixed carbon

Percentages

(%)

2.290

54.064

10.557

33.089

The proximate analysis shows that this coal sample contains a high amount of ash which

corresponds to mineral matter present in coal sample although moisture and volatile

matter contents are low. The fixed carbon content of this coal is 33.089% which falls

between 25-35%. According to the ASTM standards as reported by National Energy

Technology Laboratory, U.S. Department of Energy, this coal can characterized as

lignite in nature [21].

4.1.2. FT-IR Analysis

FT-IR spectroscopy was performed and is given in Fig. 4.5. In the spectra, noise

indicates the presence of high amount mineral matter in raw coal sample.

32

4.2. Effects of different solvents

Two solvents, Toluene and NMP have been used in the experiments. Percentage of

extracts recovered by thermal extraction of coal for 1 hour of extraction time is

presented in Table 4.2.

Table 4.2: Comparison of percentages of extract recovered from Toluene & NMP

Solvent Toluene NMP

Percentages of extract

recovered (%)

0.0495 4.428

From Table 4.2, it can be observed that the percentage of extract recovered in case of

Toluene is much less than that recovered by using NMP. This is why, NMP was chosen

for further experimentation.

Higher percentage of extract recovered in case of NMP is because of the fact that NMP

is a polar solvent. Kim et al. [7] found similar results for polar solvents and reported that

use of polar solvents give higher extraction yield. Toluene, on the other hand, is non-

polar and this little amount of coal extract may be attributed to its non-polar nature. It

has also been reported that bituminous coals can give high extraction yields for non-

polar solvents [5]. But, here, the coal is Lignite in nature as characterized by proximate

analysis, the extract recovered is very which is similar to the results reported by other

researchers [6]. It can be concluded from these results that polar solvents has higher

ability to extract large amount of coal compared to the non-polar solvent for low rank

coals.

4.3. Comparison of methods of extraction

To study the effect of different methods, thermal and ultrasonication were chosen for

extraction of coal samples in the present research work. Figure 4.1 compares

percentages of extract recovered by using thermal extraction at 150C and by using

33

ultrasonic irradiation at room temperature. NMP was used as the extracting solvent for

both the methods of extraction.

Fig. 4.1: Comparison of percentages of extract recovered by

Thermal extraction & Ultrasonic irradiation

From Fig. 4.1, it can be seen that the percentage of extract recovered in case of thermal

extraction is greater than that recovered by ultrasonic irradiation at room temperature

(Table 4.3). This indicates that thermal energy needs to be supplied for higher yields of

extracts. For this reason, thermal extraction was chosen for the other experiments.

Table 4.3: Comparison of percentages of extract recovered by Thermal extraction

& Ultrasonic irradiation

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

% o

f e

xtra

ct r

eco

vere

d

INDEX

Thermal extraction

INDEX

Ultrasonic irradiation

Method of extraction Thermal

extraction

Ultrasonic

irradiation


recovered (%)

4.428 3.386

34

4.4. Determination of optimum time of extraction

To study the effect of time on the yield of extract and reduction of ash content in the

extract, raw coal was thermally extracted for 1, 2 and 3 hours with NMP. The

temperatures were maintained at 150C for all the experiments

4.4.1. Comparison of percentages of extract recovered: The percentages of extract

recovered for 1, 2 and 3 hours of extraction time with NMP are given in Figure 4.2.

Fig. 4.2: Percentages of extract recovered

under different times of extraction for fresh NMP

The percentages of extract recovered are nearly same for extraction times of 1 hour and

2 hours. However, for 3 hours of extraction time, percentage of extract increases, but the

increase is not very significant. On the other hand, energy consumption for 3 h heating is

three times more than 1h heating. That is why, 1 hour has been chosen as the optimum

time of extraction for all the experiments.

4.4.2. Comparison of ash reduction in extracts: The reduction of ash content in the

extract for the different times of extraction are shown in Fig. 4.3 It can be seen that in all

three cases ash contents of the extracts lie in the range of 4-6%. These values have

1.0 1.5 2.0 2.5 3.01.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

% o

f extr

act

recovere

d

Time of extraction (hours)

35

fluctuations because the distribution of mineral matter in coal is not uniform. As a result,

it can be said from the results obtained during this study that time of extraction does not

affect ash content in the extracts. The ash content of residue and extracts of coal after

extraction with NMP are compared in Table 4.4.

Fig. 4.3: Percentages of ash removal of extract

under different times of extraction for fresh NMP

Table 4.4: Percentages of extract recovered and ash removed under different times

of extraction for fresh NMP

Time of extraction 1 hour 2 hours 3 hours


recovered (%)

4.428 4.268 5.629

Ash content of extract (%) 4.167 3.571 4.762

Percentage reduction of ash

content in extract (%)

92.292 93.395 91.190

Ash content of residue (%) 54.321 52.778 53.086

1.0 1.5 2.0 2.5 3.080

82

84

86

88

90

92

94

96

98

100%

re

du

ctio

n o

f a

sh

co

nte

nt

in e

xtr

act


36

4.4.3. FT-IR spectra of coal extract and residue obtained using fresh NMP: The FT-IR

spectra of the coal extract obtained by using fresh NMP is given in Fig. 4.4. From the

spectra, peak that appeared at 1675 cm-1

and 1124 cm-1

denote C=O (carbonyl)

stretching and =C-H bending in alkenes respectively. A peal that appeared at 604 cm-1

is

due to presence of C-H bending in alkynes.

The FT-IR spectra of raw coal and residue obtained after extraction with NMP are

shown in Fig 4.5. The noise in the spectra denotes presence of high amount of mineral

matter in both raw coal as well as residue. In the spectra, peaks obtained at 1592 cm-1

and

1034 cm-1

, signifies the presence of C-C bond stretching in aromatics and =C-H bend in

alkenes respectively. However, the peaks, which are observed in case of the raw coal

sample have diminished in the spectra obtained for the residue. This denotes that some

of the functional groups present in the raw coal have been extracted by NMP and can be

observed in the extract.

Fig. 4.4: FT-IR spectra of coal extracts obtained using fresh and reused NMP

----Fresh NMP

----Reused NMP

37

Fig. 4.5: FT-IR spectra of raw coal and residue obtained using NMP

4.5. Comparison of results obtained using reused and fresh (unused) NMP

To study the effect of using reused solvent on thermal extraction, raw coal was extracted

with reused NMP at 150C for 1, 2 and 3 hours.

4.5.1. Comparison of percentages of extract recovered: The percentages of extract

recovered for extraction with reused NMP under different times of extraction are shown

in Fig.4.6. The percentages of extract recovered are comparable for both fresh and

recycled solvents. However, a minor reduction in the percentage of extract recovered are

observed for extraction times of 1 hour and 3 hours. As a result, it can be concluded that

reusing the solvent lowers the ability of NMP to extract coal, but not to a very large

extent.

----Raw coal

----Residue obtained with NMP

38


under different times of extraction for fresh & reused NMP

4.5.2. Comparison of ash reduction in extracts: The effect of reused NMP on reduction

of ash content in the extract is shown in Fig.4.7. It can be observed that the ash content

of the extract increases for reused solvent i.e., ability of ash removal from coal is

reduced for reused solvent. This could be due to the presence of leached out mineral

matter in recycled solvents used for extraction which is attributed to lower ash removal

in case of recycled solvent compared to fresh solvent. It has been reported previously

that solvent has ability to leach away some amount of mineral matter of coal [19,20].

The ash content of the residue remains almost same as that of the raw coal as shown in

Table 4.5.

1.0 1.5 2.0 2.5 3.01

2

3

4

5

6

% o

f extr

act

recovere

d


For fresh NMP

For reused NMP

39

Fig. 4.7: Percentages of ash removal of extract

under different times of extraction for fresh &reused NMP

Table 4.5: Percentages of extract recovered and ash removed under different times

of extraction for reused NMP

Time of extraction 1 hour 2 hours 3 hours


recovered (%)

4.199 4.440 5.464

Ash content of extract (%) 13.984 14.700 14.000



74.134 72.810 74.105

Ash content of residue (%) 54.025 53.211 55.294

4.5.3. Comparison of FT-IR spectra of extracts obtained using fresh and reused NMP:

The FT-IR spectra of the extracts obtained using fresh NMP and reused NMP from

previous experiments are shown in Fig 4.3. From the spectra, it can be observed that the

groups extracted in both the cases are almost similar. The peaks corresponding to

various functional groups are less pronounced in case the extract, obtained using reused

1.0 1.5 2.0 2.5 3.050

55

60

65

70

75

80

85

90

95

100

% r

edu

ctio

n o

f a

sh

co

nte

nt

in e

xtr

act


For fresh NMP

For reused NMP

40

solvent. This may be due to the fact that due to reuse, some amount of carbonaceous

matter and minerals from coal may have remained in the matrix of the solvent. This

reduces the solvents ability to extract coal and reduce the ash content of the extract. In

Fig. 2, peaks at 3728 cm-1, 3695 cm-1 and 1031 cm-1are designated as presence of

mineral matter in the extract obtained in case of reused NMP. Rahman et al. [6] have

reported sharp peaks within the range 3600-3700 cm-1

and 1000-1100 cm-1

are due to

mineral matter in coal which is also observed in the present study.

4.6. Effect of reusing the residue for thermal extraction

The residue formed during thermal extraction of coal by NMP for 1 hour was reused

once again for extraction using fresh NMP as solvent for 1 hour without any additive.

The results are discussed in details below.

4.6.1. Percentages of extract recovered from fresh and reused coal residue: The results

for percentages of extract recovered from fresh coal and reused coal residue are shown

in Fig. 4.8. It can be observed that yield of extract decreases when residue coal is used

from previous experiments. This is because of fact that during thermal extraction of the

coal for the first time, most of the NMP-soluble part of coal already got dissolved and

extracted. Consequently, a smaller quantity of NMP-soluble portion was left behind in

the residue to be extracted by NMP. In this method, the total amount of extract

recovered from the raw coal was 7.70%. The results are summarized in Table 4.6.


from fresh &reused residue

0

1

2

3

4

5

% o

f e

xtra

ct

reco

vere

d

Fresh coal

INDEX

Reused residue

41

4.6.2. Percentages of ash reduction in extract: The percentages of ash reduction in

extracts obtained from fresh coal and reused residue are shown in Fig. 4.9. It can be

observed that there is no significant effect reutilization of coal on the percentages of ash

reduction.

Fig. 4.9: Percentages of ash reduction in extract from fresh coal &reused residue

Table 4.6: Percentages of extract recovered and ash removed from fresh coal and

reused residue

Coal used (fresh/residue) Fresh coal Residue


recovered (%)

4.428 3.630

Ash content of extract (%) 4.167 3.448



92.292 93.524

Ash content of residue (%) 54.321 55.961

4.7. Experimental results for use of additives along with NMP

To study the effect of additives on the extraction of coal with NMP, four different

additives, viz. p-Nitro benzoic acid, Quinoline, Calcium fluoride and Calcium chloride

were used to extract raw coal. Raw coal was extracted for 1 hour at 150C with NMP

0

10

20

30

40

50

60

70

80

90

100

% o

f as

h r

ed

uct

ion

in e

xtra

ct

Fresh coal

INDEX

Reused residue

42

and different concentrations of the additives. The concentration of the additives in

weight per cent of raw coal are 0.1%, 0.5%, 1.0%, 2.5% and 5%.

4.7.1. p-Nitro benzoic acid:

4.6.1.1. Percentage of extract recovered: The percentages of extract recovered

for various percentages of p-Nitro benzoic acid are shown in Fig. 4.10. It can be

observed that with increase in concentration of this additive, the yield of extract

increases. When additive concentration is minimum, i.e. 0.1% (by weight of

coal), the yield of extract obtained by using p-nitro benzoic acid is nearly same

as that obtained when NMP is used alone. But, for all the higher concentrations

of the additives, the yield of extract is higher than that obtained when no additive

is used. This enhancement of yield of extract takes place due to the fact that p-

nitro benzoic acid is a polar compound and it enhances the polarity of NMP.

Thus, the combined effects of the polarities of NMP and p-nitro benzoic acid

bring about the increase of yield of extracts.

Fig. 4.10: Percentages of extract recovered for

different concentrations of p-nitro benzoic acid

0 1 2 3 4 51.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

8.0

% o

f extr

act

recovere

d

% of additive (p-nitro benzoic acid)

43

4.7.1.2. Ash content of extract: The reduction in ash content of the extract for

different concentrations of p-Nitro benzoic acid is shown in Fig. 4.11. It can be

observed that decrease in ash content of the extracts take place with higher

concentration of additive. The percentage of ash reduction of extract for higher

concentration of this additive is thus, more than that obtained when NMP is used

alone. For lower concentrations of p-nitro benzoic acid, the ash content of the

extract is same as that when no additive is used. Thus, it may be concluded that

with increase in concentration of the additive, the polarity of the solvent

increases and thus, the ability of the solvent to leach away some portion of the

mineral matter also increases. This might have possibly resulted in a reduction of

ash content of the extract. However, the ash contents of the residues are

comparable with that of the raw coal (Table 4.7).

Fig. 4.11: Percentages of ash reduction in extract for

different concentrations of p-nitro benzoic acid

0 1 2 3 4 580

82

84

86

88

90

92

94

96

98

100

% a

sh r

eduction in e

xtr

act

% of additive (p-nitro benzoic acid)

44

Table 4.7: Percentages of extract recovered and ash removed for using p-nitro

benzoic acid as an additive with NMP

Concentration of p-nitro

benzoic acid

(in weight % of raw coal)

0%

0.1% 0.5% 1.0% 2.5% 5%


recovered (%)

4.428 4.577 4.822 4.935 5.405 7.396

Ash content of extract

(%)

4.167 4.348 4.310 2.821 2.934 1.149

Percentage reduction

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Beneficiation of Indian Coal