The particle size of coal particles is an important factor affecting the physical and mechanical properties of coal. In this study, uniaxial and triaxial compression tests were conducted to investigate the effects of coal particle size on the physical and mechanical properties of briquettes and their impact mechanism using a rock mechanics test-150B servo system (RMT-150B). The results showed that the uniaxial compression strength, elastic modulus and deformation modulus of briquettes increase when particle size is decreased. The deformation characteristics of the briquettes directly prepared by raw tectonic coal were similar to those of coal specimens with a particle size of 0.18–0.25 mm. The cohesion and strength of coal specimens increased when particle size was decreased, and the plastic deformation capacity decreased when particle size was decreased, showing a strong correlation. The f briquette directly prepared by the raw tectonic coal had a strength between that of coal specimens with a particle size of 2–6 mm and those with a particle size of 0.18–0.25 mm. The mechanical properties of briquettes mainly depend on the meshing force between the coal particles. The smaller the particles, the greater the mechanical meshing force. The “floating particles”, generated in the voids between coal particles during the preparation process, are a significant factor affecting the plasticity characteristics. The research results may be used as a basic reference in the study of the mechanical properties of tectonic coal, gas migration and coal and gas outburst mechanisms.
Briquette coals with different cement contents are frequently used to study the coal body’s properties. In this study, the deformation and strength of briquette coal samples with 0, 5, 10, and 20% cement contents were experimentally and theoretically investigated using the acoustic emission (AE) characteristics monitored during the uniaxial compression tests. The results show that the uniaxial compression process of raw coal and briquette coal samples can be subdivided into compaction, elastic, plastic (yield), and brittle failure stages. With an increase in cement content, briquette coal samples undergo the elastic and plastic stages, and their postpeak stress drop rate gradually grows, and their plastic deformation is followed by brittle failure. The uniaxial compressive strength and elastic modulus of briquette coal samples show a linearly increasing relationship with cement content, while their Poisson’s ratio decreases gradually. During the uniaxial compression, raw coal and briquette coal samples produce the AE signals. The overall AE signal of briquette coal samples is relatively low, and there are no obvious AE events in raw coal samples. The uniaxial compressive strength, elastic modulus, and Poisson’s ratio of briquette coal samples with a 20% cement content and their AE signal cumulative amplitude, count, and energy values are very close to the corresponding parameters of raw coal samples. Therefore, they can be used for simulating raw coal samples in laboratory tests.
Simulation of raw coal using briquette coal samples with similar mechanical properties and acoustic emission (AE) characteristics is quite instrumental in various analog models. Uniaxial compression with AE monitoring of raw coal and briquette coal samples with a 7% content of different types of binders was conducted using an RMT-150B electrohydraulic test bench. The compression process could be split into compaction, elastic, plastic (yield), and failure stages, with intrinsic AE features. Except for the MA group briquette coal samples, the AE signal average values of briquette coal samples always exceeded those of the raw ones. The maximal and minimal cumulative values of uniaxial compressive strength, peak strain, and AE signal were observed in briquette coal groups, containing 7% of coal tar or water, respectively. Measurements via the similarity method based on the Euclidean distance were used to construct space vectors, with the peak strength, peak strain, and elastic and deformation moduli of briquette and raw coal samples as characteristic values. The mechanical characteristics and deformation patterns of the briquette coal group with 7% rosin as a binder had the best compliance compared to those of raw coal samples, which makes them lucrative for further analog modeling of the raw coal behavior.
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