Physico-chemical Characteristics of Pulverized Coals and Their Interrelations- A Spontaneous Combustion and Explosion Perspective

7 Characteristics of pulverized coals have significant influence on the spontaneous combustion and explosion 8 processes. This paper presents an experimental and theoretical framework on physico-chemical characteristics of 9 coal and analyzes their interrelations from spontaneous combustion and explosion perspectives. The chemical 10 properties, morphology, bulk density, particle size and specific surface area of pulverized coals from nine different 11 coal subsidiaries of India are vividly investigated in five distinct sizes. Moreover, the effects of particle size on bulk 12 density, specific surface area and N 2 adsorption capacity of pulverized coals are critically analyzed. The 13 micrographs revealed that the coal particles are mostly irregular in shape with angular outlines and sharp edges. 14 With decrease in particle size, the bulk density of pulverized coals decreased and the specific surface area and N 2 15 adsorption capacity increased. The relationships of bulk density and specific surface area of pulverized coals with 16 particle size are established. Moreover, the specific surface areas determined by both the particle sizing and BET 17 methods are compared and correlation factors between them are determined. This study led to the generation of 18 insightful coal characteristic data which can be used as reference material for furthering researches on spontaneous combustion and explosion involving pulverized coals. and 22.97-58.54%, respectively. The results of ultimate analysis show that the carbon, hydrogen, nitrogen and oxygen contents of the coal samples varied in the ranges of 36.01-63.34, 3.06-4.51, 1.29-2.71 and 30.41-59.32%, respectively. Moisture is an important parameter affecting the handling, storage and


24
Coal still maintains its legacy as the largest source of energy across the globe. Pulverized coal finds applications as 25 raw material in several process industries, wherein its physical properties, viz. bulk density, shape, size, surface 26 texture, moisture content, etc. play a major role. Bulk density is a useful parameter in the characterization and 27 handling of coal. Under a given processing condition, the bulk density of coal is significantly affected by the particle 28 size distribution and moisture content among many other factors (Yu et al., 1995;Braga et al., 2019). In contrast, 29 coal dust explosion and spontaneous combustion of coal leading to fire are the two major threats to the safety of coal 30 mines, process industries and utilities sector (Amyotte et al., 2003;Yuan et al., 2015;Yong et al., 2019). Apart from 31 imposing direct threat, they pollute the workplace and surrounding environment and adversely affect the human 32 health by releasing harmful toxic gases, combustion residues and significant amounts of particulates (Li et al., 33 2020). Coal dust produced during the mining process constitutes one of the major causes of explosion (Cashdollar, 34 1996;Mishra and Azam, 2018) and health hazards in coal mines (Laney and Weissman, 2014). Yuan et al. (2015) 35 reported that coal dust contributes to 35% of the dust explosions in China and one of the main causes of the 36 explosions is high dependence on coal for energy consumption.

37
The particle size and surface area of pulverized coal are often correlated and greatly influence the 38 spontaneous combustion susceptibility and explosion severity of coal dust (Cashdollar, 1996;Mishra and Azam, 39 2018;Azam and Mishra, 2019;Li et al., 2020;Pan et al., 2020). The explosibility and spontaneous combustion 40 susceptibility or oxidation rate of coal increase with increase in the fineness and exposed surface area of coal. The 41 finer the pulverized coal particle size, the greater the exposed specific surface area available for oxygen adsorption, 42 the stronger the coal oxidation and coal-oxygen recombination abilities, and the greater the susceptibility of coal to 43 spontaneous combustion (Pan et al., 2020). Hence, characterization of particle size and surface area of pulverized 44 coal is important from the spontaneous combustion susceptibility and explosion severity assessment perspectives.

45
In recent years, the effects of particle size and surface area on ignition sensitivity (Amyotte et al., 1993; 46 Mishra and Azam, 2018;Azam and Mishra, 2019), explosibility (Cashdollar, 2000;Gao et al., 2010;Harris et al., 47 2015;Li et al., 2016;Cao et al., 2012;Liu et al., 2018) and spontaneous combustion susceptibility (Rifella et al., where, Vi is the relative volume by particle size class di, ρ is the material density and Ds is the mean 76 diameter based on surface area, also known as Sauter mean diameter. The above equation relates the specific surface 77 area of a collection of smooth spherical particles with the average diameter of an equivalent spherical particle (Ds) of 78 density (ρ).

79
Gas adsorption is the preferred technique for specific surface area determination, as it takes into account the 80 surface roughness and crevices of particle exterior and porous interior of the particles. Among several methods 81 developed, Brunauer-Emmett-Teller (BET) adsorption method (Brunauer et al., 1938) is commonly used to measure 82 the specific surface area of particulate matter by physisorption of N2 gas molecules at the boiling point temperature 83 of liquid nitrogen of about 77 K (-196°C) ) (Clarkson and Bustin, 1999;Cheng et al., 2015;Zhao et al., 2016).

84
The volume of gas adsorbed on a monolayer over the particles surface is determined as per the BET 85 isotherm equation (Brunauer et al., 1938)

86
where, p and p0 are the equilibrium and saturation pressure of adsorbate gas, 0 p p is the relative pressure, v is the 88 total volume of absorbed gas, vm is the volume of gas required to form a complete unimolecular adsorbed layer, and 89 c is the BET constant. In Eq. (2), the plot of The constants vm and c can be evaluated from the slope and intercept. In BET method, the specific 91 surface area (S) of solid particles is determined by dividing the total surface area (Stotal) by mass of the solid sample 92 or adsorbent (w). It is given by equation (Thommes et al., 2015) where v is the molar volume of the adsorbate gas, vm is the monolayer volume of the adsorbed gas, N is 96 Avogadro's number (6.023 × 10 23 mol -1 ), Acs is the cross-sectional area of the adsorbate (16.2 Å 2 for nitrogen).

97
the surface area of internal pore spaces. Conversely, particle sizing method assumes the particles as smooth, Several researchers have measured the particle size and specific surface area of different types of 102 pulverized coals using various methods (Table 1). From the table, it may be observed that N2 gas adsorption is the most commonly used method among the others. Linge (1989) compared the surface area of coal particles measured 104 by different methods, including, particle sizing, photoextinction, methylene blue dye adsorption and gas adsorption 105 (both N2 and CO2). For the coal particles in size range of 15-9 µm, he reported the specific surface area by particle 106 sizing and N2 gas adsorption methods in the ranges of 0.16-0.30 and 1-7 m 2 g -1 , respectively. Cheng et al. (2015) 107 studied the effect of different experimental conditions on the specific surface area calculation and reported that BET 108 theory more accurately calculates the specific surface area of coal. Dubois et al. (2011) studied the dependency of 109 BET surface area on particle size for some granitic minerals of different particle sizes. They observed a linear 110 relationship between the BET surface area and inverse of the particle size, up to a certain particle size. Table 1 112 This paper aims at investigating the (1) important physico-chemical properties of pulverized coals, such as 113 proximate and ultimate analyses, morphology, bulk density, particle size and specific surface area, (2) effect of 114 particle size on bulk density, specific surface area and N2 adsorption capacity of pulverized coals and (3)   The bulk density of coal is influenced by its physical characteristics, such as relative density, shape, particle size 138 distribution, surface properties and moisture content, and on the dimensions of the measuring container. The bulk 139 density of pulverized coals was determined using a fixed volume stainless steel container as per ISO 23499:2013, to 140 study the effect of particle size on bulk density of coal. Sufficient amount of coal powder was filled in the container 141 and compacted. Excess coal powder was scraped carefully from top of the vessel with the sharp edge of a spatula 142 and the weight of the coal powder in the container was taken. The bulk density of the sample was determined by 143 dividing the sample weight by container volume. The bulk density of each sample was determined thrice and the 144 average value was considered as the bulk density of the sample.

146
2.4 Measurement of particle size and specific surface area of pulverized coals

147
The particle size of pulverized coals of different sizes was determined with a particle size analyzer. However, the 148 specific surface area was determined using both particle size analyzer and surface characterization analyzer particles. In this study, the refractive index of coal was taken as 1.6 for particle size analysis (Mengüç et al., 1994). 181 Table 2 182 The fuel ratios (ratio between the FC and VM) of coal samples were calculated to determine the rank of 183 coals. Based on the fuel ratio (FR), Frazer (1877)

203
The variation of average bulk density of pulverized coals with particle size shown in Fig. 4 depicts an 204 increasing trend, which best fitted the polynomial trend with R 2 = 0.989. The polynomial regression equation 205 obtained between the bulk density and particle size (y = -0.007x 2 + 0.136x + 0.492) in Fig. 4 can be used to predict 206 the bulk density of pulverized coals of known sizes. The average bulk density of pulverized coals increased from 207 0.63 to 0.97 g/cm 3 , or increased by 1.54 times, with increase in the particle size from <38 to 425-850 µm. The 208 reason behind decrease in bulk density with decrease in the particle size is attributed to the fact that with decrease in 209 particle size, the total surface area and voids between the particles are increased, which results in decrease of bulk   226 Table 3 227 Fig. 5 228 The variations of median particle diameter (D50) and specific surface area of pulverized coals with particle coals with particle size are depicted in Fig. 7. From these figures, it is evident that D50 increases with increase in the 231 coal particle size. Generally, the specific surface area of pulverized coals increased with decrease in the particle size.

232
With decrease in particle size from 425-850 to <38 µm, the mean value of D50 decreased by 27 times and the mean 233 specific surface area increased by 41.5 times. The finer coal particles with greater exposed surface area interact more 234 with oxygen resulting in higher oxygen adsorption. Moreover, size reduction of coal due to crushing generates more 235 free radicals and accelerates the coal oxidation reactivity (Xu et al., 2020). Therefore, the finer coal particles are 236 more prone to spontaneous combustion. Moreover, they require lesser ignition energy and consequently, increase the

240
In order to establish relationship between the specific surface area and particle size of pulverized coals, a 241 graph between the average median particle size (D50) and average specific surface area was plotted as shown in Fig.   242 8. It may be observed that the specific surface area of pulverized coals decreased with increase in the particle size,   surface areas determined by both the particle sizing and BET methods, and the results are presented in Table 4. The

252
BET method includes the pore and external areas to compute the total specific surface area and provides critical 253 information regarding the effects of particle size and porosity on N2 adsorption and specific surface area.
254 Table 4 255 The N2 adsorption isotherms of pulverized coal samples of A, H and I for different particle sizes are presented in Fig. 9. From the figure it is evident that N2 adsorption capacity of coals increases with decrease in the  277 Figure 10 also depicts that the variation of BET surface area with coal particle size is consistent with the N2 278 adsorption. BET surface area increased with decrease in the particle size of coal. 287 Hou et al. (2017) reported that the decrease in particle size makes some inaccessible mesopores accessible 288 to N2 molecules and increases the mesopore specific surface area and volume. In contrast to mesopore 289 characteristics, the micropore characteristics are independent of particle size. Liu et al. (2010) also observed that the 290 BET specific surface area increases with decreases in the coal particle size, while the mean pore size shows an 291 approximately opposite trend. The reason they cited for this is that, when the particle diameter is reduced, more and 292 more small pores inside the matrix are exposed, resulting in the increase of specific surface area.

293
3.6 Comparison between particle sizing and BET surface areas

294
A comparative analysis of surface areas of coal particles determined by both the particle sizing and BET methods 295 was done as shown in Fig. 11. It may be observed that the specific surface areas of coal samples determined by both 296 the methods decrease in the order H > A > I. Among the three samples, the specific surface area of sample H was 297 found to be highest in all the particle sizes. This indicates that sample H is highly porous in nature among the three 298 samples.

Fig. 11
300 An attempt was also made to find the relationships and determine correlation factors between the specific 301 surface areas determined by both the particle sizing and BET methods (Fig. 12). Good linear correlation between the 302 specific surface area values determined by both the methods was observed. In case of samples A and I, the 303 correlation factor was determined about 7 and 5, respectively, which means the BET specific surface area is 304 approximately 7 and 5 times, respectively greater than the particle sizing surface area. Gómez-Tena et al. (2014) 305 determined correlation factor of 6 for ceramic materials. In contrast, in case of sample H, though a good correlation 306 was observed between the surface area values measured by both the methods, the correlation factor was found to be 307 approximately 2 with a greater y-intercept of 3.17. The deviation signifies the highly porous nature of the sample. It is worth mentioning here that the correlation between the specific surface areas determined by both the methods 309 depends on the characteristics of the coal, and especially on the coal particle size and porosity. The correlation 310 factors determined in this study are specific to the coals analyzed and can be used as a reference for comparison 311 purpose by other studies.  density, specific surface area and N2 adsorption capacity of pulverized coals with particle size were obtained. The 323 study revealed that the bulk density, specific surface area and N2 adsorption capacity of pulverized coals greatly 324 influenced by the particle size. The bulk density of pulverized coals increased with increase in the particle size. The 325 bulk density of the smallest (<38 µm) and coarsest size (425-850 µm) coals varied in the ranges of 0.39-1.05 g/cm 3 326 and 0.76-1.35 g/cm 3 , respectively. With increase in the particle size from <38 to 425-850 µm, the average bulk 327 density of pulverized coals increased from 0.63 to 0.97 g/cm 3 , or it increased by 1.54 times. The polynomial 328 regression equation obtained between the bulk density and particle size (y = -0.007x 2 + 0.136x + 0.492) can be used 329 to predict the bulk density of pulverized coals of known sizes.

330
The specific surface area of pulverized coals increased with decrease in the particle size. With decrease in 331 particle size from 425-850 to <38 µm, the mean D50 value decreased by 27 times and the mean specific surface area 332 increased by 41.5 times. The relationship obtained between the specific surface area (S) and median particle size 333 (D50) of pulverized coals [S = 18.39 (D50) -1.14 ] can be used to predict the specific surface area of pulverized coals of size of pulverized coals. With decrease in particle size from 425-850 to <38 µm, the BET surface area for coal 336 samples of A, H and I increased from 0.22 to 2.80, 3.14 to 3.94 and 0.15 to 1.93 m 2 /g, or it increased by 12.73, 1.25 337 and 12.87 times, respectively. This signifies that the finer coal particles are more prone to spontaneous combustion 338 and explosion due to exposure of greater surface area for oxygen sorption. Linear correlation was obtained between 339 the particle sizing and BET specific surface areas of pulverized coals. In case of samples A, H and I, the correlation 340 factors between the particle sizing and BET specific surface areas were determined 7, 2 and 5, respectively.