Petrographic controls on combustion behavior of inertinite rich coal and char and fly ash formation

JFUE 7928

No. of Pages 11, Model 5G

18 March 2014 Fuel xxx (2014) xxx–xxx 1

Contents lists available at ScienceDirect

Fuel journal homepage: www.elsevier.com/locate/fuel 5 6

Petrographic controls on combustion behavior of inertinite rich coal and char and fly ash formation

3 4 7 8 9 10 11

Q1

Atul Kumar Varma a,⇑, Mrityunjay Kumar a, Vinod Kumar Saxena b, Ashish Sarkar c, Santanu Kumar Banerjee a a b c

Coal Geology and Organic Petrology Lab., Dept. of Applied Geology, Indian School of Mines, Dhanbad 826004, India Dept. of Chemical Engg., Indian School of Mines, Dhanbad 826004, India Dept. of Applied Chemistry, Indian School of Mines, Dhanbad 826004, India

13 12 14 1 6 317 1 18 19 20 21 22 23 24 25 26 27 28 29 30

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Burn out level and unburnt carbon

amount influenced by rank and inertinite content.  Organo-petrographic constituents in char is controlled by coal microlithotype.  The intrinsic reactivity of inertinite is low with respect to other macerals.  High-density char may be mainly derived from inertinite dominated microlithotypes.  A new type of char named vitrosphere is reported.

33

a r t i c l e 3 4 5 8 36 37 38 39 40 41 42 43 44 45 46 47

i n f o

Article history: Received 11 October 2013 Received in revised form 3 March 2014 Accepted 4 March 2014 Available online xxxx Keywords: Coal petrography Coal combustion Fly ash petrography Plerosphere Char petrography

a b s t r a c t The inability to yield heat up to the expected and desired level by a few of the coal fired thermal power plants can primarily be attributed to the ineffectual combustion of coal. In an intensive endeavour to understand the role of petrographic characteristics in combustion behavior, the authors have collected inertinite rich feed coal and fly ash from six different power plants in India. The technological characteristics, petrographic make up (maceral – and microlithotype composition) and vitrinite reflectance of the feed coals were studied in details. The variation in char types were identified in the various fly ashes. The chars from different feed coals were prepared under controlled laboratory conditions. The burn out level and the unburnt carbon amount in fly ash appear to be controlled by rank and inertinite content. The good correlation between the high-density chars and inertinite dominated microlithotype suggests that high-density chars are mainly derived from these related microlithotypes. The presence of a new type of char, named vitrosphere is reported. The fly ash with little unburnt carbon is predominantly comprised of plerospheres. Ó 2014 Published by Elsevier Ltd.

49 50 51 52 53 54 55 56 57 58 59 60 61 62 63

64

⇑ Corresponding author. Tel.: +91 326 2235449 (O), 2235549 (R); fax: +91 326 2296563. E-mail address: [email protected] (A.K. Varma).

1. Introduction

65

Of late, owing to a growing energy demand the importance of coal as a source of energy has grown by leaps and bounds [1,2]. The abundance and vast distribution of coal in different parts of

66

http://dx.doi.org/10.1016/j.fuel.2014.03.004 0016-2361/Ó 2014 Published by Elsevier Ltd.

Please cite this article in press as: Varma AK et al. Petrographic controls on combustion behavior of inertinite rich coal and char and fly ash formation. Fuel (2014), http://dx.doi.org/10.1016/j.fuel.2014.03.004

67 68

JFUE 7928

No. of Pages 11, Model 5G

18 March 2014 2

A.K. Varma et al. / Fuel xxx (2014) xxx–xxx

72

the world provides significant guarantee of supply stability to countries with inadequate local resources of fuel [3]. Coal is, however, an extremely complex and heterogeneous material whose physical and chemical properties are not easy to categorize [4–8].

73

1.1. Previous compilations

74

The distribution of the various macerals within a particular coal, along with rank changes, to a large extent controls the coal properties and as a consequence, affects the combustion behavior of the coal [9–19]. During the coal combustion process, the potential energy stored is released in the form of heat and power. The coal combustion process can be divided into four stages viz. induction, devolatilization, char combustion and residual combustion. These stages last from a few microseconds to minutes [12,20–22]. Char combustion entails char burning by the diffusion of reacting gas molecules (mainly oxygen) from the furnace to the char surfaces and pore walls where they are temporarily adsorbed and react with active sites [23]. The characteristics of the char that generally influence the progress of burnout, includes (a) the external dimensions of the particles, (b) the volume, size and distribution of pores within the particles, (c) the total internal surface area of the char available for reaction and (d) the intrinsic reactivity [24].

69 70 71

75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90

1.2. Physical and chemical properties of ash

101

Several investigations have been undertaken in an effort to interpret the physical properties of ash [25–27]. Some studies have investigated feed coal, bottom ash as well as fly ash and they have demonstrated a high variability in terms of trace element distribution in the feed coal and related bottom and fly ashes [28,29]. The ignition propensity for various ranks of coal has also been tested [30]. Ash deposition was studied during pulverized coal combustion and oxy-firing [31,32]. The fly ash shows properties that depend upon the coal characteristics, the burning conditions and the collection system. These various properties are used in industrial applications [32–36].

102

1.3. Anisotropy, porosity and wall thickness of char

103

Anisotropy of char refers to the size of discernible coke anisotropy domains in the particle walls [37]. The char classification system is based on the physical properties that govern char reactivity and carbonaceous residues of coals produced in both inert and oxidizing atmospheres at temperatures P800 °C. Observations from various power stations support the classification system reasonably well and factually describe the unburnt carbon in the fly ash [38]. Genetic terms for char particles denoting origin from vitrinite or liptinite are seldom used, since after exposure to temperatures of around 800 °C in the furnace, these macerals can no longer be easily recognized [8]. Presently char particle type is defined following Bailey et al. [37]. However, the particle shape of char is related both to the rank and type of feed coal [38]. Elongate and sub parallel orientations of vesicles (taken into account in char classification) may allow greater access to the diffusing gases within the char particle, and it may influence the extent of internal burning [39]. The degree of development of graphitic texture is related to the increase of aromaticity in the char that depends on the liberation or retention of volatiles (during the plastic stage) that are believed to play an important role in char reactivity [40]. The char combustion is the rate-determining step in the overall combustion of the pulverized fuel. It is, therefore, of great consequence to compare char from the various macerals of coal and of different rank to ascertain which char types require longer residence time for burning to completion. An understanding of the coal combustion properties helps in designing and maintenance of boilers, aids in

91 92 93 94 95 96 97 98 99 100

104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128

maximizing burning efficiency, and assists in reducing carbon particle emissions. In this work, the authors have investigated petrographic characters of feed coals from six power plants in India and also attempted to characterize the fly ash of the same power plants. The impact of petrographic composition and rank of coal on char formation have been studied by preparing char in a muffle furnace under controlled combustion conditions.

129

2. Materials and methods

137

2.1. Collection of feed coal, fly ash and char samples for various investigations

138

Six samples of feed coal (FC-A, FC-B, FC-C, FC-D, FC-E and FC-F) and fly ash (FA-G, FA-H, FA-I, FA-J, FA-K and FA-L) were collected from six different Indian power plants viz., A (Damodar Valley Corporation (DVC) Power Plant; Unit-2, Waria, Durgapur, West Bengal), B (Jamadoba TISCO Colliery Power Plant, Jharia, Jharkhand), C (Dishergarh Power Plant, West Bengal), D (Barauni Thermal Power Station, Barauni, Bihar), E (Kahalgaon Super Thermal Power Station, Bhagalpur, Bihar) and F (Bokaro Thermal Power Station, Jharkhand) respectively (Table 1). The feed coal and the fly ash collected from various power plants were subjected to macroscopic, technological and micropetrographic investigations. The technological characteristics including moisture content (Wa), volatile matter yield (Vdaf), ash yield (Ad) and fixed carbon content (FCdaf) were determined (Table 2) by following procedures described in [41]. The micropetrographic study of coal was carried employing a Leitz MPV2 reflected microscope with a fluorescence attachment in both white and ultraviolet light. A Swift Point Counter was used to determine the volume percentages of the various

140

Table 1 Sample codes for feed coal, fly ash and chars prepared in the laboratory. Sl. no.

Sample code

Description

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

FC-A FC-B FC-C FC-D FC-E FC-F FA-G FA-H FA-I FA-J FA-K FA-L MC-M MC-N MC-O MC-P MC-Q MC-R MK-1

20

MK-2

21

MK-3

Feed coal-A from power plant No. A Feed coal-B from power plant No. B Feed coal-C from power plant No. C Feed coal-D from power plant No. D Feed coal-E from power plant No. E Feed coal-F from power plant No. F Fly ash –G from power plant No. A Fly ash –H from power plant No. B Fly ash –I from power plant No. C Fly ash –J from power plant No. D Fly ash –K from power plant No. E Fly ash –L from power plant No. F Char prepared in laboratory by pyrolysing FC-A Char prepared in laboratory by pyrolysing FC-B Char prepared in laboratory by pyrolysing FC-C Char prepared in laboratory by pyrolysing FC-D Char prepared in laboratory by pyrolysing FC-E Char prepared in laboratory by pyrolysing FC-F Unburnt component sample prepared from fly ash, FA-I of power plant No. C for FTIR analysis; unburnt carbon extract solution eluted with petroleum ether Unburnt carbon component sample prepared from fly ash, FA-I of power plant No. C for FTIR analysis; unburnt carbon extract solution eluted with benzene Unburnt carbon component sample prepared from fly ash, FA-I of power plant No. C for FTIR analysis; unburnt carbon extract solution eluted with dichloro-methane

Explanations: A is the Damodar Valley Corporation (DVC) power plant; Unit-2, Waria, Durgapur, West Bengal; B is the Jamadoba Tisco colliery power plant, Jharia, Jharkhand; C is the Dishergarh power plant West Bengal; D is the Barauni thermal power station, Barauni, Bihar; E is the Kahalgaon super thermal power station, Bhagalpur, Bihar; F is the Bokaro thermal power station, Jharkhand.

Please cite this article in press as: Varma AK et al. Petrographic controls on combustion behavior of inertinite rich coal and char and fly ash formation. Fuel (2014), http://dx.doi.org/10.1016/j.fuel.2014.03.004

130 131 132 133 134 135 136

139

141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157

JFUE 7928

No. of Pages 11, Model 5G

18 March 2014 3

A.K. Varma et al. / Fuel xxx (2014) xxx–xxx Table 2 Technological properties of the feed coals.

Table 4 Microlithotype composition (vol.%) of feed coal samples.

Sa

Wa (wt%)

Vdaf (wt%)

Ad (wt%)

FCdaf (wt%)

FR

FC-A FC-B FC-C FC-D FC-E FC-F

1.60 3.10 2.64 0.90 1.30 2.31

27.07 31.63 22.38 41.94 29.14 32.31

41.02 59.57 39.60 51.16 49.07 26.51

72.93 68.37 77.62 58.06 70.86 67.69

2.70 2.16 3.46 2.36 2.09 1.38

ML

Vitritea Claritea Vitrinertite-Va Vitrinertite-Ia Inertitea

Explanations: Sa is the sample; Wa is the moisture content (analytical state); Vdaf is the volatile matter yield (dry ash free basis); Ad is the ash yield (dry basis); FCdaf is the fixed carbon content (dry ash free basis); FR is the fuel ratio.

158 159 160 161 162 163 164 165 166 167 168

2.2. Size fractionation of fly ash and separation of unburnt coal particle

170

For the size analysis, the fly ash was first washed with acetone and then dried in air to remove the moisture present in the fly ash. A representative sample (1 kg dried mass) of fly ash was taken by employing the coning and quartering method. The fly ash was sieved using sieves of sizes BSS 85 mesh (180 lm), BSS 200 mesh (75 lm) and BSS 300 mesh (45 lm) and was separated into following four different size fractions: (a) +180 lm, (b) +75 lm to 180 lm, (c) +45 lm to 75 lm, and (d) 45 lm (Table 6 and Fig. 1). Separation of unburnt coal particles from the fly ash was done using the density gradient separation method [46]. For this purpose, a mixture of benzene and water was prepared in the ratio of (1:2.5) in order to obtain a density of 0.95 g/cc for the benzene water solution. Fifty grams of fly ash were taken in a glass tube apparatus and a-benzene water solution containing 100 ml of benzene and 250 ml of water was added to the glass tube. The glass tube was shaken thoroughly with its upper and side opening closed. Air was blown into the mixture using a glass pipe. The glass apparatus was kept stationary for 2 h in order to allow the separation of organic and inorganic matter. The organic matter was floated and collected at the top, whereas, the inorganic matter settled in the form of tailings near the bottom of the tube. The process was repeated till the fractionation was complete and the intervening solution between the organic and inorganic fractions was clear. Next, the stop cork was removed from the side opening of the tube

171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193

FC-A

FC-B

FC-C

FC-D

FC-E

FC-F

44.8 0.0 5.3 15.3 34.6

38.6 0.0 13.9 10.6 37.0

21.0 0.2 25.1 24.9 28.6

34.3 0.0 14.2 23.1 28.4

18.3 0.0 16.1 29.2 36.4

23.9 0.0 23.6 14.1 38.4

Explanations: CS is the coal sample and ML is the microlithotype. a Carbominerite and rock free basis.

macerals and microlithotypes of the feed coals adhering to ICCP methods of examination [42,43]. The petrographic investigations of fly ash and laboratory prepared char and its mineroid (>50% mineral matter) under microscope were undertaken following the procedures described in [5,8,37,44]. The maceral and microlithotype composition of the coal samples are given in Tables 3 and 4 respectively. The mean maximum vitrinite reflectance (in oil), Rom max vt % of the feed coal was measured with a Leitz MPV2 microscope using leucosapphire standard (reflectance = 0.59%) and is presented in Table 5 following ICCP procedures and ASTM standard [42,45].

169

CS

and the organic matter along with benzene water solution was collected in a vessel. The floatation products were filtered to remove water and benzene. The remaining organic matter was dried and weighed. The tailings were again treated in similar manner to separate the organic matter from tailings. The total organic matter was obtained by adding up the organic matter recovered in two stages (Figs. 1 and 2 and Table 6).

194

2.3. Burnout levels

201

The burnout levels for the feed coal of different power plants were calculated using the following equation (Table 6):

202

Burn out ð%Þ ¼ 100  100ðAd C a Þ=ð100  C a Þð100  AÞ;

ð1Þ

206

where Ca = % combustibles in residue (unburnt carbon) and Ad = initial ash yield of dry coal.

207

2.4. Char preparation

209

The char was prepared in a muffle furnace under controlled conditions to investigate the role of the petrographic constituents of the feed coal. The feed coals (FC-A, FC-B, FC-C, FC-D, FC-E and FC-F) of power plants viz. A, B, C, D, E and F were crushed and sieved to get coal particles of size <125 lm. The coal sample was put in silica dish and kept in the muffle furnace at a temperature of 800 °C for 5 h in air. The char (MC-M, MC-N, MC-O, MC-P, MCQ and MC-R) so formed was then taken out and pellets were made for the micropetrographic analysis of the sample. The petrographic constituents of the fly ash and char were carried out following the nomenclature of Bailey et al. [37]: the petrographic constituents of the fly ash and char are given in Table 7 and Table 8 respectively.

210

2.5. Scanning Electron Microscopy (SEM) of feed coal, fly ash and char

222

The Scanning Electron Microscopy (SEM) of the feed coal and char was carried out using a Hitachi, H-600 scanning electron microscope for characterization of the surface topography and identification of intricately associated minerals with coal and char mainly because of the large depth of focus available and the high

223

Table 3 Maceral and mineral distribution in feed coals (vol.%). Sa

FC-A FC-B FC-C FC-D FC-E FC-F

Vtmmf

Immf

Lmmf

TE

CT

VD

Total

SF

F

Mi

Ma

ID

Total

Total

2.6 3.3 1.3 5.3 3.1 5.0

37.2 33.1 39.1 40.2 29.7 35.6

8.8 10.9 10.0 8.0 4.7 5.6

48.6 47.3 50.4 53.5 37.5 46.2

32.9 35.4 36.1 31.6 38.8 32.3

12.6 11.3 11.2 9.9 6.2 15.8

1.2 0.9 0.1 0.0 0.4 0.5

1.3 1.5 1.0 1.3 5.2 2.0

3.3 3.6 1.2 3.7 11.6 3.2

51.3 52.7 49.6 46.5 62.2 53.8

– Traces – Traces – Traces

VMM

20.9 41.7 21.3 21.1 28.2 41.7

Explanations: Sa is the samples; Vt is the vitrinite; I is the inertinite; L is the liptinite; mmf is the mineral matter free basis; TE is the telinite; CT is the collotelinite; VD is the vitrodetrinite; SF is the semifusinite; F is the fusinite; Mi is the micrinite; Ma is the macrinite; ID is the inertodetrinite; VMM is the visible mineral matter (e.g. clay minerals, pyrite, quartz, calcite, etc.).

Please cite this article in press as: Varma AK et al. Petrographic controls on combustion behavior of inertinite rich coal and char and fly ash formation. Fuel (2014), http://dx.doi.org/10.1016/j.fuel.2014.03.004

195 196 197 198 199 200

203

204

208

211 212 213 214 215 216 217 218 219 220 221

224 225 226 227

JFUE 7928

No. of Pages 11, Model 5G

18 March 2014 4

A.K. Varma et al. / Fuel xxx (2014) xxx–xxx

Table 5 Maximum vitrinite reflectance in oil (Rom max vt %) analysis of feed coals. Range, Rom max vt %

d

d2

(Rom max vt %)

m%

W%

(+180microns)

FC-A FC-B FC-C FC-D FC-E FC-F

0.51–0.79 0.63–0.98 0.51–0.73 0.63–0.84 0.51–0.86 0.48–0.71

0.06 0.01 0.05 0.05 0.01 0.04

0.0036 0.0001 0.0025 0.0025 0.0001 0.0016

0.60 0.80 0.61 0.68 0.69 0.50

10.16 1.28 9.67 7.67 1.58 7.00

0.62 0.01 0.57 0.39 0.02 0.29

(-180+75 microns)

Wt.%

Sa

Explanations: Sa is the sample; (Rom max vt % is the maximum vitrinite reflectance in oil; (Rom max vt % is the mean maximum vitrinite reflectance in oil; d is the standard deviation; d2 is the variance; m (coefficient of variation) is the standard deviation divided by mean maximum reflectance in oil and expressed in modal percent; W (coefficient of non-equality) is the variance divided by mean maximum reflectance in oil and expressed in modal percent.

233

2.6. Fourier transform infrared (FTIR) analysis of fly ash

234

256

The C-3 power plant fly ash consisted of large amounts of unburnt carbon and, therefore, was selected for doing the Fourier Transform Infrared (FTIR) analysis. The carbonaceous matter separated from the finest fraction of the fly ash (45 lm) was subjected to crushing using an agate mortar in order to reduce its size further. About 5 g of sample was taken in a beaker and 100 ml of N-methyl-2-pyrollidon solvent was added to the charge. The round bottom flask was fitted with a reflux condenser through which water was circulated using rubber tubes. A few beads of china clay were added to the solution in order to prevent bumping during heating at about 50 °C for 10 h for extracting soluble carbon compounds from the fly ash. After refluxing was complete, a fraction of solution that remained in the round bottom flask was mixed with silica gel. The mixture was homogenized and poured into a 1 in. diameter burette with a cotton plug at its bottom and a solution of petroleum ether and silica gel. The stopper of the burette was opened and the liquid was collected in a conical flask. The solution was eluted using petroleum ether, benzene and dichloro-methane. This was done continuously until three different components viz. MK-1; MK-2 and MK-3 were collected. Each of the three components was stored in separate conical flask. The three fractions were distilled to remove any solvent. The residue was then mixed with KBr and pellets were made for FTIR analysis.

257

3. Results

258

3.1. Macroscopic properties of feed coal and fly ash samples

259

The colour of the feed coal samples was dull black. Coal samples (FC-A, FC-B, FC-C, FC-D, FC-E and FC-F) have a dull lustre with

229 230 231

235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255

260

microns)

(-45 microns)

Fly ash samples Fig. 1. Size fraction distribution in various fly ash samples.

+180 micron -180 to +75 micron -75 to +45 micron

wt.%

232

magnification power of the SEM. The fly ashes were gold coated to prevent charging of the specimen for doing SEM. The SEM was used to determine the porosity of the samples studied. The lowest magnification of this instrument was 30X while the highest magnification was 150,000 with a resolution of 70 Å.

228

(-75 + 45

-45 micron whole flyash

Fly ash samples Fig. 2. Unburnt carbon distribution in various size fractions of fly ash.

occasional vitreous bright bands, and conchoidal to uneven fractures. The hardness (Moh’s scale) of the feed coal ranges from 1.5 to 2.5. The fly ash samples of FA-H, FA-I, FA-J and FA-K were moist and agglomerated. They contain significant amounts of big coarse grained particles and also appear to contain large amounts of carbonaceous matter. The FA-G and FA-L fly ash were relatively fine grained, agglomerated and contained few big particles. In addition, they appeared to contain relatively lesser amounts of unburnt carbon as compared to the other fly ashes (Table 1).

Table 6 Unburnt carbon (combustible residues) distribution in different size fractions of fly ash and burn out level. Sa

Fraction I (+180 lm) (wt%)

Fraction II (180 to +75 lm) (wt%)

Fraction III (75 to +45 lm) (wt%)

Fraction IV (45 lm) (wt%)

Ca (wt%)

Burn out level (%)

FA-G FA-H FA-I FA-J FA-K FA-L

32.83 29.67 33.14 32.01 32.93 32.29

28.93 26.26 25.66 26.86 26.94 27.92

23.61 23.69 23.58 23.09 24.27 23.98

14.62 20.38 17.62 18.05 15.86 15.82

30.68 37.31 32.06 32.68 35.35 23.59

67.28 27.80 60.98 53.75 47.31 88.86

Explanations: Sa is the sample; Ca is the combustible residues in whole fly ash; Burn out level (%) is the 100(Ad Ca)/(100  Ca) (100  Ad); Ad is the ash yield (on dry basis; Table 2).

Please cite this article in press as: Varma AK et al. Petrographic controls on combustion behavior of inertinite rich coal and char and fly ash formation. Fuel (2014), http://dx.doi.org/10.1016/j.fuel.2014.03.004

261 262 263 264 265 266 267 268 269

JFUE 7928

No. of Pages 11, Model 5G

18 March 2014 5

A.K. Varma et al. / Fuel xxx (2014) xxx–xxx

Q2

Table 7 Char type distribution in fly ash (vol.%). Char type

Sample FA-G

FA-H

FA-I

FA-J

Low density char

Tenuisphere Tenuinetwork Fragments Crassisphere

8.10 25.56 1.46 5.27

19.80 7.86 3.18 9.75

4.70 20.36 3.91 4.40

15.64 5.83 1.51 6.36

FA-K 6.90 5.80 0.50 6.80

FA-L 7.07 16.50 2.16 3.44

High density char

Mixed porous Mesosphere Inertoid Mixed dense Solid Mineroid

2.73 3.41 33.85 1.56 8.88 9.17

4.08 16.02 22.68 4.10 8.56 3.98

3.90 6.65 30.93 7.24 7.05 10.86

5.61 15.75 20.93 9.06 6.58 12.73

2.90 31.74 18.20 9.10 10.59 7.60

7.17 8.06 18.47 12.18 13.36 11.59

Table 8 Char type distribution (vol.%) in char prepared in laboratory by pyrolysis of feed coal at 800 °C. Low density char/high density char

Char type

Sample MC-M

MC-N

MC-O

MC-P

MC-Q

MC-R

Low density char

Tenuisphere Tenuinetwork Fragments Crassisphere

5.27 32.04 0.50 1.39

26.74 3.88 0.20 9.74

2.30 27.80 1.50 2.50

18.00 5.00 1.30 10.20

10.90 1.90 0.40 6.90

1.20 22.80 2.20 1.50

High density char

Mixed porous Mesosphere Inertoid Mixed dense Solid Mineroid

0.80 7.46 32.54 1.49 10.15 8.35

1.09 18.99 23.96 2.88 9.94 2.58

0.98 8.80 34.60 3.40 11.20 7.10

1.20 18.60 23.40 4.40 11.10 6.80

4.90 29.74 22.18 5.20 12.58 5.30

8.40 10.30 15.80 5.40 20.30 12.10

270

3.2. Technological and petrographic characteristics

271

In Table 2 the technological properties of feed coals are presented. In these coals, moisture (Wa), volatile matter yield (Vdaf), ash yield (Ad) and fixed carbon (FCdaf) of these coals varied from 0.90–3.10, 22.38–41.94, 26.51–59.57 and 58.06–77.62 wt% respectively. The fuel ratio (FR) of the coal samples were in the range of 1.38–3.46 (Table 2). The maceral composition of the feed coal samples reveals a significant amount of inertinite macerals (Immf = 46.5–62.2 vol.%) making them inertinite rich (Table 3) [47]. Semifusinite was found to be the most inertinite maceral. The vitrinite macerals content (Vtmmf) in the feed coals were in the range from 37.5–53.5 (vol.%) and collotelinite was the most dominant among the vitrinite macerals. The maceral liptinite (Lmmf) was present in traces with maximum amounts up to 0.02 (vol.%). The visible mineral matter of the feed coal (VMM) varied from 20.9 to 41.7 (vol.%) (Table 3). The VMM was comprised of clay minerals, pyrite, quartz, calcite, etc. The microlithotype composition of the feed coals indicated the presence of mainly two groups of microlithotype: (a) vitrinite dominated microlithotype, VtM (vitrite, vitrinertite-V, clarite) and (b) inertinite dominated microlithotype, InM (inertite, vitrinertite-I). The VtM content ranged from 34.40 to 52.40 (vol.%) whereas the InM content varied from 49.90 to 65.60 (vol.%) (Table 4). The mean maximum vitrinite reflectance in oil (Rom max vt %) was in the range of 0.50–0.80%. The standard deviation for the measurements varied from 0.01 to 0.06. The m (coefficient of variation) ranged from 1.28% to 10.17% and W (coefficient of non-equality) varied from 0.01% to 0.62% (Table 5).

272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297

298

3.3. Size fractionation makeup

299

The size fractionation of the fly ash showed that the coarsest fraction (retained by 85 mesh BSS sieve, +180 lm) was the most

300

abundant, followed by the second fraction (85 to +200 BSS mesh; 180 to +75 lm). These two fractions jointly accounted for 70% (approx.) of the whole fly ash (Table 6). The finest size fractions were the least abundant. The wt% distribution in the fly ash indicates a reduction in unburnt carbon amount with decreasing size of the fly ash fractions (Fig. 2). The separation of unburnt carbon from the different size fractions of the fly ash showed that the coarsest fraction had the maximum amount of unburnt carbon in wt% (Fig. 2). The amount of unburnt carbon decreased from coarsest to the finest size fractions. The total unburnt carbon ranged from 23.59 to 37.31 wt% (Table 7). The burn out level for the different feed coals ranged from 27.80 to 88.86 wt% (Table 6).

301

3.4. Petrography of fly ash and char

313

The char types present in the fly ash are morphologically similar to the char prepared in the laboratory (Tables 7 and 8). Apart from the char types mentioned in the ICCP classification, a unique char type was observed that was spherical in shape and showed the incipient development of the vesicles. This char type is described as vitrosphere in this work (Plate 1:Photo 1). Several petrographic constituents with various types of vesicles and pores in fly ash are given in Plate 1:Photos 2–6.

314

3.5. Scanning Electron Microscopy (SEM) results

322

The Scanning Electron Microscopy (SEM) of the coarsest fraction of the fly ash revealed that both spherical and irregular particles were present and resulted in agglomeration amongst them. The plerospheres are more abundant in the low carbon fly ash, whereas partially reacted spherical particles with well-developed pores are more abundant in the high carbon fly ash. The average pore size in the low carbon fly ash ranged from 6.0 to 7.5 lm, whereas in the high carbon fly ash it ranged from 2 to 56 lm.

323

Please cite this article in press as: Varma AK et al. Petrographic controls on combustion behavior of inertinite rich coal and char and fly ash formation. Fuel (2014), http://dx.doi.org/10.1016/j.fuel.2014.03.004

302 303 304 305 306 307 308 309 310 311 312

315 316 317 318 319 320 321

324 325 326 327 328 329 330

JFUE 7928

No. of Pages 11, Model 5G

18 March 2014 6

A.K. Varma et al. / Fuel xxx (2014) xxx–xxx

332

The largest particle size observed during the SEM examination was 84 lm and smallest 0.8 lm (Plate 2:Photos 7–10).

333

3.6. Fourier Transform Infrared analysis (FTIR) consequences

334

The Fourier Transform Infrared (FTIR) spectra of the three compounds extracted from the unburnt carbon of the fly ash revealed the following:

331

335 336 337 338 339 340 341 342

(a) C-3 power plant fly ash Fraction MK-1: 2957 cm1 band indicated the presence of aromatic carbon (aromatic CAH structure). The strong aliphatic CH2 structure bands were also present (2925 and 2854 cm1). A band at 1733 cm1 indicated the presence of aliphatic ester, which was rather unusual: bands at 1282 and 1133 cm1 confirmed the pres-

ence of aromatic OAH (CAO) and aliphatic OAH (CAO). Different aromatic bands were present (968, 918, 742 cm1, etc.). (b) C-3 power plant fly ash fraction MK-2: it was more or less identical to the first one (MK-1). (c) C-3 power plant fly ash fraction MK-3: it contained a large number of aromatic bands. Carboxylic acid band was present at around 1705 cm1. Aromatic CAH (structure and banding) bands were less abundant.

343 344 345 346 347 348 349 350 351 352

4. Discussion

353

The technological properties and the mean virinite reflectance in oil (Rom max vt %) indicate that the rank of the feed coals varies from sub-bituminous to bituminous (Tables 2 and 5). The maceral

354

Photo 1. Photomicrograph showing a vitrosphere intermediate stage of char formation) observed in FA-J fly ash under oil immersion, reflected light

Photo 4 Photomicrograph of an iso-inertoid char showing development of tunnel type pores observed in FA-L fly ash under oil immersion, reflected light

Photo 2. Photomicrograph showing isomesosphere present in the FA-H fly ash having both primary as well as secondary vesicles under oil immersion, reflected light

Photo 5.Photomicrograph showing tenuisphere with well developed primary and secondary vesicles present, observed in the FA-G fly ash under oil immersion, reflected light

Photo 3. Photomicrograph showing fusinoid char with inherited porosity due to cellular structure present and partly fused inertinite in FA-G fly ash under oil immersion, reflected light

Photo 6. Photomicrograph showing a solid char consisting of large piece of inertinite from the FA-G fly ash under oil immersion, reflected light

Plate 1. Photo 1. Photomicrograph showing a vitrosphere (intermediate stage of char formation) observed in FA-J fly ash under oil immersion, reflected light; Photo 2. Photomicrograph showing isomesosphere present in the FA-H fly ash having both primary as well as secondary vesicles under oil immersion, reflected light; Photo 3. Photomicrograph showing fusinoid char with inherited porosity due to cellular structure present and partly fused inertinite in FA-G fly ash under oil immersion, reflected light; Photo 4. Photomicrograph of an iso-inertoid char showing development of tunnel type pores observed in FA-L fly ash under oil immersion, reflected light; Photo 5. Photomicrograph showing tenuisphere with well developed primary and secondary vesicles present, observed in the FA-G fly ash under oil immersion, reflected light; Photo 6. Photomicrograph showing a solid char consisting of large piece of inertinite from the FA-G fly ash under oil immersion, reflected light.

Please cite this article in press as: Varma AK et al. Petrographic controls on combustion behavior of inertinite rich coal and char and fly ash formation. Fuel (2014), http://dx.doi.org/10.1016/j.fuel.2014.03.004

355 356

JFUE 7928

No. of Pages 11, Model 5G

18 March 2014 7

A.K. Varma et al. / Fuel xxx (2014) xxx–xxx

Photo 7. Scanning electron micrograph showing spherical (plerospere) and irregular porous particles present in FA-H fly ash. Pore size = 7.5µm (diameter) (Detector: SE; Accelerating voltage = 25 KV)

Photo 9. Scanning Electron Micrograph Showing different types of particles as well as their associations in FA-I fly ash. Agglomeration of spherical and irregular particles can be clearly seen (Detector: SE; Accelerating voltage = 25KV).

Photo 8. Scanning electron micrograph showing a spherical plerosphere in FA-H fly ash (Detector: SE; Accelerating voltage= 25 KV).

Photo 10. Scanning electron micrograph showing plerosphere formation stage (Detector: SE; Accelerating voltage = 25KV).

Plate 2. Photo 7. Scanning electron micrograph showing spherical (plerosperes) and irregular porous particles present in FA-H fly ash. Pore size = 7.5 lm (diameter) (detector: SE; accelerating voltage = 25 kV); Photo 8. Scanning electron micrograph showing a spherical plerosphere in FA-H fly ash (detector: SE; accelerating voltage = 25 kV); Photo 9. Scanning electron micrograph showing different types of particles as well as their associations in FA-I fly ash. Agglomeration of spherical and irregular particles can be clearly seen (detector: SE; accelerating voltage = 25KV); Photo 10. Scanning electron micrograph showing plerosphere formation stage (detector: SE; accelerating voltage = 25KV).

40

60

50

Combustible residue in fly ash, wt%

Coarse fraction (+180 micron)

y = 0.3917x + 14.517 r2 = 0.89

y = -0.365x + 65.413 r2 = 0.85

55

45 40 35 30 25 20 20

40

60

80

100

Burn out level (wt %) Fig. 3. Relation between burn out level (wt%) and coarse fraction of fly ash (wt%). 357 358 359

composition indicates that the feed coals are inertinite rich containing large amounts of mineral matter [47]. Microlithotypically these coals may be termed as vitrinertitic–inertitic (Table 4).

20 20

40

60

80

Ash content (wt %,db) Fig. 4. Relation between ash content (wt%, db) and combustible residue in fly ash (wt%).

360

4.1. Burnout examination

361

The size fractionation and separation of unburnt carbon particles from the different fly ash samples reveal the presence of larger

362

amounts of unburnt carbon in the coarser fractions of the fly ash (Fig. 2). The proportion of the unburnt carbon in the fly ash depends to a large extent on the properties of the coal used as feed-

Please cite this article in press as: Varma AK et al. Petrographic controls on combustion behavior of inertinite rich coal and char and fly ash formation. Fuel (2014), http://dx.doi.org/10.1016/j.fuel.2014.03.004

363 364 365

JFUE 7928

No. of Pages 11, Model 5G

18 March 2014

367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384

stock, in particular, the varying reactivity of its macerals (considering other technological conditions such as boiler design- and reaction conditions). The correlation coefficient (r = 0.92) between the coarse fraction of the fly ash (+180 lm) and burn out level wt% is apparent from Fig. 3. The correlation may be an artifact of the fact that upon combustion the larger coal particles had lesser access to oxygen in comparison to the finer particles. It appears that when the burnout percentage is higher, there might be more fragmentation of particles and the amount of coarser particles in fly ash would be less. In addition, with an increase in the initial ash yield of the coal the amount of the unburnt carbon (combustible residue) in the derived fly ash was found to increase (Fig. 4). It appears that mineral matter present in coal inhibits combustion reactions. The amount of unburnt carbon decreases with increasing burn out level for the feed coals. The burnout level also shows a systematic variation with ash yield (db wt%). The increase in ash yield of the feed coal results in a decreasing burn out level (Fig. 5). Accordingly, the increasing ash yield inhibits access of oxygen to organic matter to enable combustion.

385

4.2. Two distinctive char types

386

The petrographic analysis of the char prepared in the laboratory indicates that the char types belong to two distinct groups:

387 388 389 390 391 392 393

(a) High density char (HDC): – having less pores and relatively thick walls e.g. mixed porous, mesosphere, mixed dense, inertoid, solid and mineroid. (b) Low density char (LDC): – having very high porosity and relatively thin walls e.g. tenuisphere, tenuinetwork, fragments and crassisphere.

394

400

Among the low density char the tenuisphere and tenuinetwork are dominant and together they constitute about 70% of the lowdensity char. The volume percent of low density char ranges from 59.20 to 79.90 with an average value of 66.03. Among the high density chars the mesosphere and inertoid together are the dominant char types (Table 8).

401

4.3. Effect of rank

402

With an increase in the rank (mean maximum vitrinite reflectance in oil, (Rom max vt %) of the coal, the burn out level is observed to decrease (correlation coefficient, r = 0.99; Fig. 6). Similarly, unburnt carbon content (wt%) of the fly ash also indicates good correlation (correlation coefficient, r = 0.94; Fig. 7) with the rank (mean maximum vitrinite reflectance in oil, (Rom max vt %) of the coal. Release of volatiles and formation of porosity depends largely on the rank. The low rank coal consists of the depolymerized remnants of humified vegetable source material and, as the coal is subjected to increasing temperatures and pressures in the coalification process, repolymerization process of the humic acids and other degraded biopolymers occurs [8]. This brings about the formation of a macromolecular network of carbon-rich aromatic and hydro-aromatic ring clusters (micelles of Hirsch, 1958) which are arranged in stacks of well-ordered pregraphitic layer planes and cross-linked by ether, thioether, methylene chains, carboxyl and other oxygenbased bridges [48–53]. This may be due to an increase in the rank of the coal up to the sub-bituminous/bituminous stage, the porosity of the coal decreasing with increasing rank. Consequently, the overall char porosity reduces. The net effect of such a change is a decrease in burning rates and, hence, a low burnout percentage and higher amounts of unburnt carbon. This also indicates that rank is one of the most reliable methods to predict and compare the burnout behaviors of inertinite rich coals.

395 396 397 398 399

403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425

4.4. Influence of petrographic composition

426

The coal composition is found to have an important bearing on the combustion process. A reasonable correlation (correlation coefficient, r = 0.74; Fig. 8) between the vitrinite content in the feed coal and the low density char is observed. Abundance of vitrinite dominated microlithotype exhibits a very good correlation with low density char (correlation coefficient, r = 0.91; Fig. 9) and suggests that the low density chars appear to be predominantly derived from the vitrinite dominated microlithotype (VtM). The intrinsic reactivity of inertinite is low with respect to other macerals and, hence, during the combustion process its low reactivity inhibits the combustion reaction leading to an increase in contents of unburnt carbon. The mixed dense, inertoid, solid and fusinoid char types dominates in high density char and the presence of such char types indicates the poor efficiency of combustion. There is a reasonable correlation (correlation coefficient, r = 0.73; Fig. 10) between the inertinite content of coal and the high density char. The good correlation (correlation coefficient, r = 0.80) between inertinite dominated microlithotype (InM) in coal and the high density char indicates that high density chars can be predominantly derived from these microlithotypes (Fig. 11). Release of volatiles and formation of porosity in inertinite depends exclusively on the rank and coking properties of the coal. The inertinites of bituminous coals have greater aromaticity and lower H/C ratio than corresponding vitrinites [37]. Therefore, coals with a greater

427

y = -1.7395x + 135.05 r2 = 0.94 Burn out level, wt%

366

A.K. Varma et al. / Fuel xxx (2014) xxx–xxx

Ash content.(db, wt%) Fig. 5. Relation between ash content (db, wt%) and burn out level of fly ash (wt%).

Mean maximum virinite reflectance in oil,%

8

y = -0.0049x + 0.9308 r2 = 0.99

Burn out level (wt %) Fig. 6. Relation between burn out level (wt%) and mean maximum vitrinite reflectance in oil, Rom max vt %.

Please cite this article in press as: Varma AK et al. Petrographic controls on combustion behavior of inertinite rich coal and char and fly ash formation. Fuel (2014), http://dx.doi.org/10.1016/j.fuel.2014.03.004

428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450

JFUE 7928

No. of Pages 11, Model 5G

18 March 2014 9

A.K. Varma et al. / Fuel xxx (2014) xxx–xxx

45

0.9

40

y = 0.0203x - 0.0002 r2 = 0.89

0.8

Low density char, vol.%

Mean maximum vitrinite reflectance, %

0.85

0.75 0.7 0.65 0.6 0.55

35

30

25

y = 1.1146x - 19.115 r2 = 0.83

20

0.5

15 0.45

30

40

50

60

Vitrinite dominated microlithotype, vol.%

0.4 20

25

30

35

40

Unburnt carbon, wt %

Fig. 9. Relation between vitrinite dominated microlithotype (VtM) content (vol.%) and low density char (vol.%) prepared in laboratory.

Fig. 7. Relation between unburnt carbon (wt%) and mean maximum vitrinite reflectance in oil, Rom max vt %.

Low density char, vol.%

High density char, vol.%

80

75

70

65

y = 1.0543x + 11.794 r2 = 0.54

60

y = 1.0408x -16.47 r2 = 0.55

55 45

50

55

60

65

Inertinite content,vol.%

Fig. 8. Relation between vitrinite content (vol.%) and low density char (vol.%) prepared in laboratory.

465

amount of inertinite dominated microlithotypes favour formation of high density chars. A similar role of microlithotypes has been observed in coal liquefaction, coking and solvent extraction [54– 59]. The fly ash petrography reveals several significant aspects of char combustion. A char type in the intermediate stage of combustion has been observed (Plate 1:Photo 1). This char appears to have derived from the vitritic mass of coal and it shows the incipient stage of development of the degassing vesicles. This char type does not figure into the ICCP classification of char types outlined in the previous Section 3.4 and has been given the name vitrosphere due to its spherical shape and resemblance with vitrinite (Plate 1:Photo 1). Also, the distinct morphology and the surface features of various char types present in the fly ash have been observed under the optical microscope and are depicted in the plates (Plate 1:Photos 1–6).

466

4.5. Plerospheres and porous fly ash particles

451 452 453 454 455 456 457 458 459 460 461 462 463 464

467 468 469

The SEM analysis of the fly ash samples reveals the morphology, surface features and the minute surface details of the fly ash particles (Plate 2:Photos 7–10). Both spherical and irregular particles

Fig. 10. Relation between inertinite content (vol.%) and high density char (vol.%) prepared in laboratory.

have been observed to be present. The SEM analysis of the fly ash shows the plerospheres (Plate 2:Photos 7 and 8) and solid spheres with very little or no porosity to dominate in the fly ash with the smallest amount of unburnt carbon (Plate 2:Photos 9 80

y = 0.5373x + 40.352 r2 = 0.64

75

High density char,vol.%

Vitrinite content, vol.%

70 65 60 55 50 20

30

40

50

60

70

80

Inertinite dominated microlithotype,vol.% Fig. 11. Relation between inertinite dominated microlithotype content (vol.%) and high density char (vol.%) prepared in laboratory.

Please cite this article in press as: Varma AK et al. Petrographic controls on combustion behavior of inertinite rich coal and char and fly ash formation. Fuel (2014), http://dx.doi.org/10.1016/j.fuel.2014.03.004

470 471 472 473

JFUE 7928

No. of Pages 11, Model 5G

18 March 2014 10

A.K. Varma et al. / Fuel xxx (2014) xxx–xxx

487

and 10). The plerosphere (sphere in sphere) is mainly a particle of empty and thin-walled spherical fly ash, enclosing a number of the smaller fly ash particles of different sizes. Formation of plerospheres may be due to a difference in the melting points of particles and the effect of gases formed with (a) decomposition of CaCO3 and carbon present on the surface or inside the ash slag particles and/or (b) dehydration of clay minerals (c) the coal particles with high dispersed mineral matter during the combustion and a fused silicate cover (d) fine ash particles entrained inside char and ash cenospheres suspended in the flue gas [60,61]. The significance of plerospheres in reducing emission is discussed [60]. On the other hand, fly ash with a large amount of unburnt carbon indicates the presence of greater amounts of highly porous rounded and partially reacted particles in it (Plate 2:Photos 7, 9, 10).

488

4.6. FTIR characteristics

489

501

The FTIR result of MK-1 (fraction-I, prepared from fly ash, FA-I, Table 1) shows peaks at 2957 cm1, 2925 cm1, and 2854 cm1. These bands actually represent the aromatic compounds present in coal. The peak at 1733 cm1 is unusual as it indicates the presence of aliphatic ester in coal. Apart from these the other compounds present in the coal are aromatic OAH (CAO) and aliphatic OAH (CAO) indicating differing aromatic fractionation. Presence of aromatic bands in char indicates that the increasing aromaticity or rank of the coal appears to impede combustion reactions. The MK-2 (fraction II, prepared from fly ash, FA-I) is more or less identical with MK-1. MK-3 (fraction-III, prepared from fly ash, FA-I, Table 1) contains a large number of bands (carboxylic acid) at around 1705 cm1. Aromatic bands are less abundant in MK-3.

502

5. Conclusions

503

518

The amount of unburnt carbon is greater in the coarser fractions of the fly ash which may be due to less fragmentation of particles with increasing rank (aromaticity) therefore resulting in a higher content of coarser particles in the fly ash. The increase in unburnt carbon contents in the coarse fly ash may be due to reduced access to oxygen, thereby inhibiting combustion. The formation of organo-petrographic constituents in char is controlled by the microlithotype of the feed coal. The high density chars appear to be predominantly derived from an inertinite dominated microlithotype and the low density chars seem to be principally obtained from vitrinite dominated microlithotypes. The above mentioned conclusions are based on a limited number of samples. Therefore, it is recommended to carry out further research works in similar areas on a larger number of coal samples of various ranks having different physical, chemical and petrographical characteristics.

519

Acknowledgements

520

526

The authors are indebted to the Deputy Director General, Coal Wing, Geological Survey of India, Kolkata for providing permission to use his laboratory facilities for virinite reflectance measurements. The authors also wish to thank anonymous learned reviewers and Dr. John William Patrick, Principal Editor for their critical evaluation and valuable suggestions to improve the quality of paper.

527

References

474 475 476 477 478 479 480 481 482 483 484 485 486

490 491 492 493 494 495 496 497 498 499 500

504 505 506 507 508 509 510 511 512 513 514 515 516 517

521 522 523 524 525

528 529 530 531

[1] Stewart B. Coal combustion product (CCP) production and use. In: Proceedings of 15th international ACAA symposium on management & use of coal combustion products (CCPs), Pittsburgh. Pittsburgh (PA): American Coal Ash Association, Aurora, CO, and Department of Energy; 2003. p. 1-1–1-8.

[2] ACAA. Coal combustion product (CCP) production and use survey. American Coal Ash Association. 2011. . [3] Hirano M. Perspective of coal power generation in Japan. Austral. Coal Assoc., Brisbane, Qld: Coal Utilisation Workshop; 1993. p. 1–2. [4] Francis W. Coal: its formation and composition. 2nd ed. London: Edward Arnold; 1961. [5] Stach E, Mackowsky MH, Teichmüller M, Taylor GH, Chandra D, Teichmüller R. Stach’s textbook of coal petrology. 3rd ed. Berlin, Stuttgart: Gebrüder Bornträeger; 1982. [6] Diessel CFK. Coal – bearing depositional systems. New York, Berlin, Heidelberg: Springer-Verlag; 1992. [7] van Krevelen DW. Coal typology, chemistry, physics and constitution. 3rd ed. Amsterdam: Elsevier; 1993. [8] Taylor GH, Teichmüller M, Davis A, Diessel CFK, Littke R, Robert P. Organic petrology. Berlin, Stuttgart: Gebrüder Bornträeger; 1998. [9] Shibaoka M. An investigation of the combustion process of single coal particles. J Inst Fuel 1969;42:59–66. [10] Ubhayaker SK, Stickler DB, Von Rosenburg CWJ, Gannon RE. Rapid devolatilisation of pulverised coal in hot combustion gases. In: Proc. C.R. 16th symp. (int.) on combustion. The Combustion Inst; 1977. p. 427–36. [11] Smith IW. Flash pyrolysis of Australian coals. 3rd ed. Sydney: Annual Hydrogenation Workshop; 1978. [12] Smith IW. Coal chars: A review. In: Proc. C.R. 19th symp. (int.) on combustion. The Combustion Inst; 1982. p. 1045–1065. [13] Melia PF, Bowman CT. A three zone model for coal particle swelling. Combust Sci Technol 1983;31:195–201. [14] Oka N, Murayama T, Matsuoka H, Yamada S, Yamada T, Shinozaki S, et al. The influence of rank and maceral composition on ignition and char burnout of pulverised coal. Fuel Process Technol 1987;15:213–24. [15] Skorupska NM. Coal specifications-impact on power station performance. London: IEA Coal Research; 1993. [16] Hower JC, Robertson DJ, Rathborne RF, Peterson G, Trimble AS. Petrology, mineralogy and chemistry of magnetically separated sized fly ash. Fuel 1999;78:197–203. [17] Jak E, Degterov S, Pelton AD, Happ J, Hayes PC. Thermodynamic modelling of the system Al2O3–SiO2–CaO–FeO–Fe2O3 to characterize coal ash slags. In: Gupta RP, Wall TF, Baxter L, editors. The impact of mineral impurities in solid fuel combustion. New York: Kluwer Academic/Plenum Press; 1999. [18] Qiu JA, Li F, Zhang CG. Mineral transformation during combustion of coal blends. Int J Energy Res 1999;23:453–63. [19] Su S, Pohl JH, Holcombe D, Hart JA. A proposed maceral index to predict combustion behavior of coal. Fuel 2000;80:699–706. [20] Shibaoka M, Thomas CG, Young BC, Oka N, Matsuoka H, Tamaru K, et al. The influence of rank and maceral composition on combustion of pulverised coal. Sydney: Int. Conf. on Coal Sci.; 1985. p. 665–68. [21] Smoot LD, Smith PJ, editors. Pulverised-coal combustion and gasification. New York: Plenum Press; 1985. [22] Saito M, Sadakat M, Sakai T. Measurements of surface combustion rate of single coal particles in laminar flow furnace. Combust Sci Technol 1987;51:109–28. [23] Smoot LD, Pratt DT, editors. Pulverised coal combustion and gasification. New York: Plenum Press; 1979. [24] Cloke M, Lester E. Characterisation of coals for combustion using petrographic analysis – a review. Fuel 1994;73:315–20. [25] Matsuoka K, Suzuki Y, Eylands KE, Benson SA, Tomita A. CCSEM study of ash forming reactions during lignite gasification. Fuel 2006;85:2371–6. [26] Aineto M, Acosta A, Rincon JM, Romero M. Thermal expansion of slag and fly ash from coal gasification in IGCC power plant. Fuel 2006;85:2352–8. [27] Song W, Tang L, Zhu X, Wu Y, Rong Y, Zhu Z, et al. Fusibility and flow properties of coal ash and slag. Fuel 2009;88:297–304. [28] Pires M, Querol X. Characterization of Candiota (South Brazil) coal and combustion by-product. Int J Coal Geol 2004;60:57–72. [29] Levandowski J, Kalkreuth W. Chemical and petrographical characterization of feed coal, fly ash and bottom ash from the Figueira power plant, Paraná, Brazil. Int J Coal Geol 2009;77:269–81. [30] Man CK, Gibbinsa JR. Factors affecting coal particle ignition under oxyfuel combustion atmospheres. Fuel 2011;90:294–304. [31] Carbonea F, Berettaa F, D’Annab A. A flat premixed flame reactor to study nano-ash formation during high temperature pulverized coal combustion and oxygen firing. Fuel 2011;90:369–75. [32] Fryda L, Sobrino C, Glazer M, Bertrand C, Cieplik M. Study of ash deposition during coal combustion under oxyfuel conditions. Fuel 2012;92:308–17. [33] Sear LKA. The properties and use of coal fly ash: a valuable industrial byproduct. London, UK: Thomas Telford; 2001. [34] Dai S, Zhao L, Peng S, Chou C-L, Wang X, Zhang Y, et al. Abundances and distribution of minerals and elements in high-alumina coal fly ash from the Jungar power plant, Inner Mongolia, China. Int J Coal Geol 2010;81: 320–32. [35] Hower JC. Petrographic examination of coal-combustion fly ash. Int J Coal Geol 2012;92:90–7. [36] Dai S, Seredin VV, Ward CR, Jiang J, Hower JC, Song X, et al. Composition and modes of occurrence of minerals and elements in coal combustion products derived from high-Ge coals. Int J Coal Geol 2014;121:79–97. [37] Bailey JG, Tate A, Diessel CFK, Wall TF. A char morphology system with applications to coal combustion. Fuel 1990;69:225–39.

Please cite this article in press as: Varma AK et al. Petrographic controls on combustion behavior of inertinite rich coal and char and fly ash formation. Fuel (2014), http://dx.doi.org/10.1016/j.fuel.2014.03.004

532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617

JFUE 7928

No. of Pages 11, Model 5G

18 March 2014 A.K. Varma et al. / Fuel xxx (2014) xxx–xxx 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646

[38] Sakorafa M, Michailidis K, Burragato F. Mineralogy, geochemistry and physical properties of fly ash from the Megalopolis lignite fields, Peloponnese, Southern Greece. Fuel 1996;75:419–23. [39] Rosenberg P, Petersen HI, Thomsen E. Combustion char morphology related to combustion temperature and coal petrography. Fuel 1996;75:1071–82. [40] Crelling JC, Thomas KM, Marsh H. Aspects of the combustion reactivity of coal macerals. Ironmaking Conf Proc 1992:126–30. [41] IS 1984. Indian Standard: 1350 – (Part 1) Proximate Analysis of Coal (reaffirmed 1996). [42] ICCP. International Committee for Coal Petrography, International handbook of coal petrology. 2nd ed. Paris: C.N.R.S. 1971. [43] ICCP. International Committee for Coal Petrography, Vitrinite classification. Paris: C.N.R.S. 1994. [44] Hower JC, Robertson DJ, Thomas GA, Wong SA, Scram WH, Graham UM, et al. Characterisation of fly ash from Kentucky power plants. Fuel 1995;75:403–11. [45] ASTM. Standard test method for microscopical determination of the reflectance of vitrinite in a polished specimen of coal. (D2798–96). Annual Book of ASTM Standards: Gaseous Fuels: Coal and Coke; Sec. 5, V. 5.05: 1994. p. 280–283. [46] Maroto-Valer MM, Taulbee DN, Hower JC. Novel method of separation of differing forms of unburnt carbon present in fly ash using density gradient centrifugation. Energy Fuels 1999;13:947–53. [47] Varma AK. Facies control on the petrographic composition of inertitic coals. Int J Coal Geol 1996;30:327–35. [48] Hirsch PB. Conclusions from X-ray scattering data on vitrain coals. Sheffield: Conf. Sci. Use of coals; 1958. p. A 29–33. [49] van Krevelen DW. Coal typology, chemistry, physics and constitution. 1st ed. Amsterdam: Elsevier; 1961. [50] Given PH. An essay on geochemistry of coal. Coal Sci 1984;3:63–252.

11

[51] Byrne JF, Menendez R, Marsh H. Chemistry of coal to coke transformation. In: Proc. Iron Making Conf. 1988. p. 325–330. [52] Solomon PR. Can coal science be predictive? keynote address. 4th Austral. Coal Sci. Conf. Proc. 1990. p 17–87. [53] Sakurovs R, Lynch L, Barton WA. Molecular conformation and stability of coal macerals. In: Schobert HH, editor. Coal science II. Amer. Chem. Soc.; 1991. p. 111–26. [54] Gabzdyl W, Varma AK. Influence of petrographical factors on hydrogenation of inertinite rich coals from the upper Silesian coal basin of Poland. In: IEA Coal Research, editor. Proceedings of 1991 international conference on coal science. Oxford: Butterworth Heinemann; 1991. p. 719–22. [55] Gabzdyl W, Varma AK. Thermogravimetric studies in prediction of coal behavior during coal liquefaction. J Therm Anal 1993;39:31–40. [56] Varma AK. Geological and petrological controls on coal liquefaction. Proceedings of the International symposium on coal and organic petrology. Fukuoka, Japan: 1994. p. 70–73. [57] Varma AK. Influence of petrographical composition on coking behavior of inertinite rich coals. Int J Coal Geol 1996;1996(30):337–47. [58] Varma AK. Thermogravimetric investigations in prediction of coking behavior and coke properties derived from inertinite rich coals. Fuel 2002;81:1321–34. [59] Varma AK, Kumar BN, Saxena VK, Sarkar A. Petrographic controls for solvent extraction of coal. In: Proc international seminar on mineral processing technology [MPT-2005]. New Delhi: Tata McGraw-Hill Publishing Company Limited; 2005. p. 319–24. [60] Shibaoka M, Paulson CAJ. Genesis of char and ash plerospheres. Fuel 1986;65:1020–2. [61] Goodarzi F, Sanei H. Plerosphere and its role in reduction of emitted fine fly ash particles from pulverized coal-fired power plants. Fuel 2009;88:382–6.

Please cite this article in press as: Varma AK et al. Petrographic controls on combustion behavior of inertinite rich coal and char and fly ash formation. Fuel (2014), http://dx.doi.org/10.1016/j.fuel.2014.03.004

647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675