Pore structure of coals

Coconut-shell-based GACs These have a high portion of micropores and present surface areas generally over 1000 m2/g and apparent densities of about 0.50 g/cm3. Being manufactured mainly from vegetative material, they do not exhibit the fully developed pore structure of coal-based carbons. They are used in both vapor- and liquid-phase applications. Coconut shell-based carbon is slightly more expensive to produce than coal-based GAC, since about only 2% of the raw material is recoverable as GAC, versus 8-9% for coal-based carbons. In Table 4.1, the basic properties of common materials used in the manufacture of activated carbon ate presented. 

In 1933, Francis (4) measured the rate of the alkaline permanganate oxidation of coal at screen cuts of through 60 on 100 and through 100 on 200 mesh. He found that the rate of reaction was directly proportional to the external area of the different coal samples, thus inferring that there is little internal area available for reaction. In 1936 Cauzelin and Crussard (8) reached the same conclusion in similar experiments. Since 1933 a vast and complicated pore structure of coal has been discovered. However, the discovery of this pore structure does not invalidate the experimental results of Francis as some authors (19) have suggested. 

We therefore suggest that the x-ray scattering method can be useful in studies of the pore structure of coals. In particular, the scattering curves from medium-rank and low-rank coals can provide an estimate of the specific surfaces associated with the macropores and the transition pores. This information is difficult to obtain by other techniques. 

Research on the fractal characteristics of pore structure of coal particles based on low temperature nitrogen adsorption method... 

ABSTRACT Based on low-temperature nitrogen adsorption principle, the pore structure of coal particles is tested and adsorption isotherms of coal particles with different size are obtained by Quantachrome Autosorb-iQ automatic specific surface area and pore size distribution analyzer. Then, microstructure characteristic parameters such as specific surface area, pore volume and average pore size of coal particles are calculated. Besides, fractal dimension of the internal surface of coal particles is calculated with FHH fractal theory. The relationship between fractal dimension and pore structure parameters together with the adsorption capacity of coal particles is analyzed. Studies show that fractal dimension can characterize the variation of characteristic parameters such as specific surface area and total pore volume of coal particles. In addition, with the increase of fractal dimension, the surface heterogeneity of pore structure is strengthened and so is adsorption capacity. The findings can provide a certain theoretical foundation for mechanism study on coal gas adsorption, desorption and seepage. 

THE FRACTAL CHARACTERISTICS OF PORE STRUCTURE OF COAL PARTICLES... 

According to the data in Table 3 and Table 4, the relation between pore structure of coal particles and fractal dimension can be obtained, as shown in Table 5. 

Table 5. The relation between pore structure of coal particles and fractal dimension.

SAXS has been extensively developed to characterize the pore structure of coals, carbonaceous materials and activated carbons, on a scale from 10 to 1000 A (refs. 7-11). It is a non intrusive technique which does not interact chemically with the sample and is able to probe both open and closed pores. 

There are two major types of filtration "cake" and "filter-medium" filtration. In the former, solid particulates generate a cake on the surface of the filter medium. In filter-medium filtration (also referred to as clarification), solid particulates become entrapped within the complex pore structure of the filter medium. The filter medium for the latter case consists of cartridges or granular media. Among the most common examples of granular materials are sand or anthracite coal. 

Examination of ultrathin tactions of coal in tko aloctron microscope hat revealed that one type of vitrinite (vitrinite A) it homogeneous, while the remaining vitrinite (vitrinite B) it a two-component material, the components having similar properties to vitrinite A and exinite, respectively. The material similar to exinite occurs in sheets no more than 1000 A. thick and is responsible for the lower reflectance and higher volatile matter yield of vitrinite B. Exinite, micrinite, and semifusinite have been identified in ultrathin sections. By using a technique of impregnation with a lead salt the ultrafine pore structure of vitrinite has been made visible. 

Physical properties Density Specific gravity Pore structure True density as measured by helium displacement Apparent density Specification of the porosity or ultrafine structure of coals and nature of pore structure between macro, micro, and transitional pores... 

The size and distribution of pores and the size, distribution, and identity of minerals in coal specimens from an eastern Kentucky splint coal and the Illinois No. 6 coal seam were determined by means of transmission electron microscopy (TEM) and analytical electron microscopy (AEM). The observed porosity varies with the macerals such that the finest pores (<2-5 nm) are located in vitrinite, with a broad range of coarser porosity (40-500 nm) associated with the macerals exinite and inertinite. Elemental analyses, for elements of atomic number 11 or greater, in conjunction with selected area diffraction (SAD) experiments served to identify the source of the titanium observed in the granular material as the mineral rutile. Only sulfur could be de-tected in the other coal macerals. Dark-field microscopy is introduced as a means for determining the domain size of the coal macerals. This method should prove useful in the determination of the molecular structure of coal. 

Parameters that can characterize the structure of coal particles include specific surface area, pore volume and average pore size. And they have different degrees of influence on the adsorption capacity of the adsorbent. According to the characteristics of the samples adsorption isotherms, the specific surface area of the adsorbent is calculated by BET method, while the total pore volume and pore size distribution of sample should be calculated by the Kelvin equation (Brunauer. S et al. 1938 ). 

It can be knovm from Fig. 3 that, in a certain coal particle size, the number of holes inside coal particles increase with the decrease of coal particle size. Besides, with the decrease of coal particle size, the pore structure become more and more complex, and the heterogeneity of pore distribution of coal particles is stronger, moreover, the surface structure of coal particles also tends to become complex, indicating that the surface of coal particles become more rough. As a result, the fractal dimension of coal particles increases to be more and more close to 3, which conclusion is similar to literature (Jiang Xiumin et al. 2003). There is a simple linear relationship between the two ... 

Ren Gengpo, et al. 2007. Analysis of Surface Area and Pore Structure of Datong Coal. Journal of Combustion Science and Technology, 13(3) 265-268 (in Chinese). 

There are also indications that the adsorption of small molecules on coal, such as methanol, occurs by a site-specific mechanism (Ramesh et al., 1992). In such cases, it appears that the adsorption occurs first at high-energy sites but with increasing adsorption the (methanol) adsorbate continues to bind to the surface rather than to other (polar) methanol molecules and there is evidence for both physical and chemical adsorption. In addition, at coverages below a monolayer, there appears to be an activation barrier to the adsorption process. Whether or not such findings have consequences for surface area and pore distribution studies remains to be seen. But there is the very interesting phenomenon of the activation barrier which may also have consequences for the interpretation of surface effects during coal combustion (Chapters 14 and 15). As an aside, adsorption studies of small molecules on coal has been claimed to confirm the copolymeric structure of coal (Milewska-Duda, 1991). 

The most appropriate manner in which to represent the structure of coal is through the use of the chemical and physical properties (Levine et al., 1982). Such a representation can be made in terms of both the chemical and physical bonding processes responsible for its structural integrity and the extensive network of pores that permeate the organic material. Information on the microscopic chemistry of coal and its relationship to coal s physical structure and reactivity is an essential component in the successful development of the next generation of coal conversion technologies. 

ACs are the most commonly used form of porous carbons for a long time. Typically, they refer to coal and petroleum pitch as well as coconut sheUs-based AC. In most cases, ACs are processed to be filled with rich micropores that increase the surface area available for gas sorption and separation. For this category, to get a definite classification on the basis of pore structure is difficult due to their countless products as well as their complex pore features. Based on the physical characteristics, they can be widely classified into the following types powdered, granular, extruded, bead ACs, etc. For the pore structure of ACs, actually, all the three types of pores (micropore, mesopore, and macropore) are included in one product (Fig. 2.1), with a wide pore size distribution [1, 2]. Up to now, many kinds of ACs have been well commercialized in gas sorption/separation including CO2 capture. For example, the BPL type with specific area of 1,141 m g is able to adsorb 7 mmol g CO2 under the conditions of 25 °C and 35 bar, while under the same conditions MAXSORB-activated carbon with specific area of 3,250 g can capture up to 25 mmol g [3]. 

The water in coal is bound in different forms to its constituents. It can be divided into three types (1) Free moisture, also referred to as external moisture, superficial moisture, or the primary moisture fraction, which is present in large cracks and capillaries. Water bound in this way retains its normal physical properties. (2) Inherent moisture, also referred to as internal moisture or the secondary moisture fraction, whose vapor pressure is lower, since it is absorbed within the pore structure of the coal. (3) Water of constitution, which is mainly combined with mineral matter normally present in coal. This water is generally driven off only at temperatures higher than those normally used for the determination of moisture content. Standard methods do not make use of these terms and define (1) the total moisture content of a coal and (2) the moisture content of the coal analysis sample. Total moisture determination must be made over the sample as received in the laboratory, in an air-proof recipient. The determination consists in drying in an oven at 105 °C till constant weight. Its value is of huge interest both in international and domestic coal trade (ISO 589, ASTM D3173). 

These are the reasons why the reservoir engineers are not interested in the detailed molecular structure of coals, their attention is rather concentrated on the problem How can flow the methane in the inside structure of coals, that is, they think in micropores, cavities, fractures giving possibility to transport of methane through the coal. It is indifferent to them how these pores, cavities etc. taken shape from the concrete molecular structure. To demonstrate this statement only two examples, from the recent literate, are shown here. King and Ertekin [7] supposed a desorption process from internal coal surfaces then diffusion through matrix and micropores and finally a fluid flow in network (See Figure 3). 

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