Pores in coal

The effect of mineral matter is so small that it is completely negligible compared to the combined specific surface of the macropores and transition pores in coals with scattering curves which have a shoulder. 

Qin Yueping Fu Gui 2000. Study on fractal characteristic of pore in coal and moisture-absorbing property of coal. Journal of China Coal Society 25(1) 55-59. 

The measmement of coal densities is complicated by the presence of pores in coals. A number of techniques for... 

The pore structure can markedly affect char reactivity. Most coals in general, and coal chars in particular, are highly porous and contain a polymodal pore size distribution. Pores normally are classified into macropores (>500 A in diameter), transitional pores (20-500 A in diameter), and micropores (< 20 A in diameter). Upon pyrolysis, pores in coals open up but still contain microporosity. Coal chars, in general, and lignitic chars, in particular, retain the polymodal pore distribution. The surface areas of coal chars can range between 100 and 800 m /g. Most of this surface area and therefore the active surface area resides inside the char particles so the accessibility of the reactive gases to the active sites is very important. 

C02 in gaseous form is injected into the coalbed, 300 to 600 metre depth, and adsorbed on the matrix pores, releasing the existing CH4 in the same (two molecules of C02 adsorbed by each CH4 molecule that travels). This has led to the possibility of storing C02 in coal seams, while CH4 recovered is valued. This technique is called enhanced coalbed methane production (ECBM). 

In liquid phase oxidations diffusivities are about 1/104 times those existing in the gas phase, and from the pores humic acids and benzenoid acids are removed whose mean diameters are very large compared with the average pore diameter in coal and molecules such as nitrogen and methanol. 

Figure 16, Shape and number of degassing pores in relation to the rate of heating, Volatile matter of the raw coal—23,8% daf sieve size 5-3 mm. Magnification 1.6 X...

For porous solids such as coal, there are five different density measurements true density, apparent density, particle density, bulk density, and in-place density. The true density of coal is the mass divided by the volume occupied by the actual, pore-free solid in coal. However, determining mass of coal may be deemed as being rather straightforward, but determining volume presents some difficulties. Volume, as the word pertains to a solid, cannot be expressed universally in a simple definition. Indeed, the method used to determine volume experimentally, and subsequently, the density, must be one that applies measurement rules consistent with the adopted definition. 

To obtain an equation describing the scattering pores in the coals, we assumed that... 

After we had determined I A and calculated I for the coal samples, we evaluated the specific surfaces S° and S for the macropores and transition pores in the 15 coals. These specific surfaces are listed in Table 1. We estimate that the uncertainty in the specific surfaces is about 40%. 

For coarse coal particles with high ash content, diffusion of oxygen through the ash layer often determines the overall combustion rate. The inner pores in the ash layer can be divided into micropores of 0.004-0.0012 /im, transition pores of 0.0012-0.03 fim and macropores of 0.03-1 fim diameter. Experiments showed that the macropores constitute the main channels for mass transfer. 

As our earlier investigations of coal suggested that the scattering was due to pores in the coal (7), we analyzed our scattering data under the assumption that the scattering was produced by three classes (15) of pores—macropores, with dimensions of 1 micron or more transitional pores, which have dimensions of a few hundred A, and micropores, which were not larger than about 20 or 30 A. 

At the time we wrote the manuscript for Reference (7), we were unable to relate the high fraction of transitional pores found in many of the coals to any other property of the coals. We have now concluded (11) that there are inflections in the scattering curves only for coals with fixed carbon contents in the interval from about 72% through 83% per cent (dry, mineral-matter free) and that for these coals, the specific surface of the transitional pores is appreciably larger than the specific surface of the macropores. As would be expected from our interpretation of the inflection (11), the specific surfaces of the transitional pores in all of the subbltumlnous coals in Table 1 except PSOC 138 are much larger than the specific surfaces of the macropores, and there is a tendency for the total x-ray specific surfaces [i. e., the sum of the specific surfaces of the macropores and transitional pores, as calculated by... 

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