Characterization of Synthetic‐Coal Char Particles using fractal dimension analysis

Abstract

The development and change of surface ruggedness in chars was studied at conditions typical in a pulverized coal furnace. The fractal dimension, a measure of surface ruggedness, of chars was measured using physisorption techniques. By adjusting the temperature encountered (1173 to 1773 K) and residence time (0.1 to 1.5 s) of the synthetic coal (sized to 46–106 μm diameter), chars at different stages of combustion were prepared in a laminar flow (drop‐tube) furnace. The particles were quickly cooled and quenched in an inert atmosphere. The samples were examined using a scanning electron microprobe, and their fractal dimensions were determined using gas physisorption. The adsorption data were used to test if the char surface was fractal on a molecular scale, to determine the fractal dimension, and to quantify changes in the fractal dimension during combustion. The fractal dimension of the unburned synthetic coal was approximately 2. The fractal dimension increased as high as 2.85 as the carbon matrix burned away and exposed mineral moieties. However, as combustion continued the carbon burned completely away leaving a mineral fly ash particle with a fractal dimension as low as 2.47.

Limitation of Determination of Surface Fractal Dimension Using N2Adsorption Isotherms and Modified Frenkel−Halsey−Hill Theory

“…At this region, the surface fractal dimension, D S , can be obtained from the slope using eq 6. At the highest magnification (Bragg region), broad peaks that correspond to the smallest real-space features are observed and can be analyzed using Bragg's law …”

“…The surface fractal dimension, D S , can be determined by gas adsorption, − ,− light scattering, − and image analysis. ,− The determination of surface fractal dimension by image analysis is very tedious. This method is only suitable when a sufficient number of particles can be collected and analyzed, the images have good contrast, and the particles are not overlapping.…”

“…In the second method, the particles do not have to be fractionated. However, several adsorbates, whose molecules have different cross-sectional areas, have to be used. ,, The surface fractal dimension is then determined as 34,35 where σ is the adsorbate molecular cross-sectional area.…”

“…The boundary fractal dimension, D L , can then be determined as follows: 11,12 D L , which varies from 1 to 2, describes the boundary line of a fractal object, 13 while D S describes a surface morphology. The relationship between D L and D S is 14 The surface fractal dimension, D S , can be determined by gas adsorption, [1][2][3][4][5][6][7][15][16][17][18][19][20][21][22] light scattering, [23][24][25] and image analysis. 13,[26][27][28][29][30][31][32][33] The determination of surface fractal dimension by image analysis is very tedious.…”

Homogeneous and Heterogeneous Reaction and Transformation of Hg and Trace Metals in Combustion Systems

“…Both produce synthetic char after periods of resin formation and carbonization. It has been reported that char from furfuryl alcohol resembles a low rank coal based on studies of weight loss during combustion [19], while the phenol and formaldehyde mixture produces a char more similar to a high volatile bituminous coal [20]. The phenol-formaldehyde mixture was therefore chosen for this study.…”