The influence factors for gas adsorption with different ranks of coals

Abstract

In this paper, scanning electron microscopy, low-temperature N 2 adsorption and CH 4 isothermal adsorption experiments were performed on 11 coal samples with R o,max between 0.98 and 3.07%. The pore structure characteristics of coals (specific surface area, total volume distribution) were studied to assess the gas adsorption capacity. The results indicate that there is significant heterogeneity on coal surface, containing numerous channel-like pores, bottle-shaped pores and wedge-shaped pores. Both Langmuir volume (V L ) and Langmuir pressure (P L ) show a stage change trend with the increase of coalification degree. For different coalification stages, there exist different factors influencing the V L and P L values. For low-rank coals (R o,max < 1.1%), the increase of V L values and decrease of P L values are mainly due to the abundant primary pore and fracture within coal. For middle-rank coals (1.1% < R o,max < 2.1%), the moisture content, vitrinite content and total pore volume are all the factors influencing V L , and the reduction of P L is mainly attributed to the decrease of moisture content and inertinite content. Meanwhile, this result is also closely related to the pore shape. For high-rank coals (R o,max > 2.1%), V L values gradually increase and reach the maximum. When the coal has evolved into anthracite, liquid hydrocarbon within pore begins pyrolysis and gradually disappears, and a large number of macropores are converted into micropores, leading to the increase of specific surface area and total pore volume, corresponding to the increase of V L . In addition, the increase of vitrinite content within coal also contributes to the increase of V L . P L , reaches the minimum, indicating that the adsorption rate reaches the largest at the low pressure stage. The result is mainly controlled by the specific surface area and total pore volume of coal samples. This research results will provide a clearer insight into the relationship between adsorption parameters and coal rank, moisture content, maceral composition and pore structure, and it is of great significance for better assessing the gas adsorption capacity.

Effects of Pore Structures of Different Maceral Compositions on Methane Adsorption and Diffusion in Anthracite

“…The higher the vitrinite content, the larger the adsorption surface area. Our results are in contrast to those obtained by Laxminarayana [49], whose research showed that the Langmuir volume decreased with the increase of vitrinite content in anthracite, but are consistent with the results of Guo [50] and Lamberson and Bustin [51]. The potential reason is that vitrinite is characterized by higher SSA and TPV of super-micropores [52], which supply more adsorption sites [53].…”

A Study on the Effect of Coal Metamorphism on the Adsorption Characteristics of a Binary Component System: CO2 and N2

“…The adsorption amounts of CO 2 and N 2 in the mixed gas environment by three coal molecules are ranked as DH > FGZ > DSC, which closely agrees with the simulation result from Gao et al that the high metamorphism of coal is negative to the adsorption amounts of CO 2 and N 2 . This is contrary to the fact that researchers generally believe that the adsorption amount of coal is positively correlated with the degree of metamorphism , However, many researchers have found that the coal metamorphism is not linearly related to the adsorption amount. ,,, According to our research, DH is a lignite with low metamorphism, and its oxygen-containing functional groups (OCFGs) are much higher than that of the other two coal samples. There are more OCFGs, including 2 CO, 16 −O–, 1 −COOH, and 5 C­(O)O; FGZ is a medium metamorphism bituminous coal, and its molecular structure consists of six CO and four −OH; DSC is anthracite with high metamorphism, including two CO, two −OH, one −O–, and one C­(O)O.…”

“…Therefore, it is necessary to convert the simulated value into the actual value of a unified unit and introduce the conversion formula, 19 = × A 1000 In the pure CO 2 adsorption environment (the component of CO 2 is 100%), the adsorption amount for DH is 7.03 mmol/g, that for FGZ is 5.67 mmol/g, and that for DSC is 4.95 mmol/ g; in the pure N 2 adsorption environment (the component of CO 2 is 0%), the adsorption amount for DH is 2.59 mmol/g, that for FGZ is 2.38 mmol/g, and that for DSC is 1.64 mmol/ g. The adsorption amounts of CO 2 and N 2 in the mixed gas environment by three coal molecules are ranked as DH > FGZ > DSC, which closely agrees with the simulation result from Gao et al 28 that the high metamorphism of coal is negative to the adsorption amounts of CO 2 and N 2 . This is contrary to the fact that researchers generally believe that the adsorption amount of coal is positively correlated with the degree of metamorphism 34,35 However, many researchers have found that the coal metamorphism is not linearly related to the adsorption amount. 27,29,36,37 According to our research, DH is a lignite with low metamorphism, and its oxygen-containing functional groups (OCFGs) are much higher than that of the other two coal samples.…”

Crystallite Structure Characteristics and Its Influence on Methane Adsorption for Different Rank Coals

“…72,73 The presence of oxygen-containing functional groups is not conducive to the adsorption of methane molecules. 74 Although there are large amounts of primary pores in the coalification stage in which the sample MTG can be located in theory, 75 aliphatic structures and partial oxygen-containing functional groups play a controlling role in the formation of these primary pores, 64 i.e., primary pores are dominated by aliphatic structures and oxygen-containing functional groups. Therefore, for the sample MTG, V L is the smallest, P L is the largest, the saturated adsorption amount of methane is the smallest, and methane is mainly present in these primary pores in a free state.…”