Correlation between Drying Behaviors of Brown Coal and Its Pore Structures

Abstract

Drying behaviors of brown coal are affected by internal water diffusion, which is controlled by pore structures. To diminish the effect of functional groups on drying, drying behaviors and pore structures of only one kind of coal dried at different heating rates and atmospheres were investigated. The final drying temperature was kept constant. The relationships between pore structures, including specific surface area (S BET), volume of mesopores (V meso), volume of macropores (V macro), volume of total pores (V total), and two fractal dimensions (D I and D II), and drying characteristics, including maximum drying rate (v max), activation energy of drying surface water (E 1), activation energy of drying pore water (E 2), apparent diffusion coefficient surface water (D eff‑1), and apparent diffusion coefficient pore water (D eff‑2), were correlated. The relationship between basic pore parameters and fractal dimensions was examined. For coals dried in N2, the S BET was confirmed as one of the key factors influencing D I. The v max increased with heating rates, resulting from the higher temperature at the same drying time and larger S BET and V macro. The E 1 was higher at faster heating rates, because of the higher temperature gradient and the bigger D I. The E 2 increased as heating rates increased from 3 to 20 °C/min and then decreased for flash drying (directly dried at 200 °C). The increase could be also due to the higher temperature gradient at higher heating rates. The decrease could be because of the effect of V macro on D eff‑2. For coals dried in different atmospheres, the higher E 1 and E 2 in air were due to oxidation reaction. The relatively large heat conductivity of N2 led to the lower E 1. The small molecular diameter of CO2 led to the lower E 2. There was no consistent relationship between S BET and D I and between pore structures and drying characteristics for coals dried in different atmospheres.

Effect of Water Evaporation on the Inhibition of Spontaneous Coal Combustion

“…Therefore, the pore structure distribution of coal gradually transitions from mesopores to micropores as the water evaporation time increases. Micropores are not conducive to methane diffusion in coal. , Thus, after treatment with the HWPI, the pore structure of coal changes, and the dense pore structure significantly reduces the risk of spontaneous coal combustion. However, the amount of water in the HWPI decreases as the drying time increases, weakening or annulling the function of the original liquid membrane isolation, gradually exposing the surface of the pore structures, and increasing the risk of spontaneous combustion .…”

Effect of Division Methods of the Adsorption Isotherm on the Fractal Dimension of Clay Minerals Calculated Based on the Frenkel–Halsey–Hill Model

“…An adsorption isotherm is sufficient to calculate the fractal dimension. The models of Brunauer–Emmett–Teller (BET), Langmuir, and Frenkel–Halsey–Hill (FHH) can be used to calculate fractal dimension values based on the gas adsorption isotherm. − In these models, the FHH model is very concise and has been proved to be the most efficient.…”

Moisture removal behaviour of single hard lignite particle during drying and quantitative characterization for its surface damage

“…He et al [ 26 ] investigated the relationship between water release behaviour and pore evolution during lignite thermal drying and concluded that most of the water exists in pores of 26–100 nm in size, and that the macroporous structure is more likely to be damaged than other pore structures. Feng et al [ 27 ] studied the drying behaviour and pore structure of lignite under different heating rates and drying atmospheres and discussed the relationship between the specific surface area, volume of mesopores and macropores, total pore volume, fractal dimension, and drying characteristics. Wang et al [ 28 ] observed the pore structure of Xilingol lignite during microwave drying and found that the failure mechanism of the pore structure mainly includes (a) destruction of the pore structure caused by shrinkage force during moisture removal, (b) opening and crosslinking of blind and closed pores, and (c) thermal decomposition of coal organic macromolecules into small molecular structures at high temperatures.…”