Experimental investigation on spontaneous combustion oxidation characteristics and stages of coal with different metamorphic degrees.

Coal spontaneous combustion (CSC) is a major disaster threatening coal mine safety; therefore, the investigation of coal spontaneous combustion and oxidation characteristics has been a hot topic in the long term. In this paper, the experimental temperature programmed system is used to carry out the simulation experiment of coal spontaneous combustion and oxidation of three kinds of coal with different metamorphic degrees under three oxygen concentrations (9%, 15%, 21%). The effects of metamorphic degree and oxygen concentration on coal oxidation characteristics were analyzed, and the variation laws of crossing point temperature, three characteristic point temperature, and apparent activation energy were qualitatively discussed. Finally, coal oxidation reaction stages were evaluated and divided. The results show that the concentrations of CO and C2H4 are negatively correlated with the degree of deterioration but increase with the increase of oxygen concentration. High metamorphic coal corresponds to high crossing point temperature (CPT). The average error between the CPT value calculated from the BM empirical correlation and the experimental data is very small, which is 6.42%. The higher the metamorphic degree of coal, the higher the three characteristic temperature points (critical temperature, xerochasy temperature, and activity temperature). The oxidation process of the three coal samples is divided into four stages: surface oxidation, oxidation self-heating, accelerated oxidation, and deep oxidation. The apparent activation energy of each stage exhibits significant variability, with varying patterns displayed with the degree of metamorphism.

Evaluation on explosion characteristics and parameters of pulverized coal for low-quality coal: experimental study and analysis.

Fully utilizing the energy generated by the explosion of pulverized coal will contribute to realize the clean and efficient exploitation of coal resources. The pulverized coal explosion characteristics will be a far-reaching and important task to explore. In this paper, ten kinds of low-quality coals such as high sulfur, high ash, and low metamorphic degree coals were investigated and the minimum ignition energy (MIE), lower explosion limit (LEL), and explosion intensity (EI) parameters under different particle sizes and coal powder concentration conditions were also analyzed combined with a 1.2-L Hartmann tube and a 20-L explosion sphere experimental system. Finally, the morphological characteristics of the exploded coal powder surface were evaluated by scanning electron microscopy (SEM). The results show that the particle size is positively correlated with MIE. LEL shows an inverted "U"-shaped trend with the increasing degree of coal deterioration. The low-rank coal is more flammable and explosive. The maximum pressure PMax at the LEL concentration and maximum pressure rise rate (dP/dt)Max overall value is small. Here, optimum pulverized coal particle size (75µm) for explosive utilization of low-quality coal was determined. Within 50-225 g/m3 of pulverized coal concentration range, the explosion intensity increases with increasing concentration. The smaller the particle size of pulverized coal, the greater the possibility of agglomeration of pulverized coal particles. The surface of the exploded coal particles produces more developed pores. They are irregularly shaped and have more rounded edges than the original coal.

Performance of Zernike polynomials in reconstructing raw-elevation data captured by Pentacam HR, Medmont E300 and Eye Surface Profiler.

PURPOSE: To investigate the capability of Zernike polynomials fitting to reconstruct corneal surfaces as measured by Pentacam HR tomographer, Medmont E300 Placido-disc and Eye Surface Profiler (ESP). METHODS: The study utilised a collection of clinical data of 527 participants. Pentacam HR raw elevation data of 660 eyes (430 healthy and 230 keratoconic) were fitted to Zernike polynomials of order 2 to 20. Same analyses were carried out on 158 eyes scanned by Medmont E300 Placido-disc and 236 eyes were scanned by ESP for comparison purposes. The Zernike polynomial â€‹fitting was carried out using a random 80% of each individual eye surface's data up to a corneal radius of 5 mm and the root means squared fitting error (RMS) was calculated for the unused 20% portion of the surface data. The process was carried out for the anterior and posterior surfaces of the corneal measurements of the Pentacam HR and the anterior surfaces only with the ESP and the Medmont E300 measurements. RESULTS: Statistical significances in reduction of RMS were noticed up to order 14 among healthy participants (p < 0.0001 for right eyes, p = 0.0051 for left eyes) and up to order 12 (p < 0.0001 for right eyes, p = 0.0002 for left eyes) in anterior surfaces measured by the Pentacam. Among keratoconic eyes, statical significance was noticed up to order 12 in both eyes (p < 0.0001 for right eyes, p = 0.0003 for left eyes). The Pentacam posterior corneal data, both right and left, healthy and keratotic eyes recorded significance (p < 0.0001) in reduction of RMS up to order 10 with same RMS values of 0.0003 mm with zero standard deviation. RMS of fitting Zernike polynomials to Medmont data up to order 20 showed a consistent reduction in RMS with the increase of the fitting order with no rise at high fitting orders. Minimum RMS = 0.0047 ± 0.0021 mm, 0.0046 ± 0.0019 mm for right and left eyes respectively were recorded at order 20 and were more than 15 times the minimum RMS of the Pentacam. RMS of fitting Zernike polynomials to ESP data also showed a consistent reduction in RMS with the increase of the fitting order with no sign of any rise at high fitting orders. Similar to the Medmont, minimum RMS of 0.0005 ± 0.0003 mm, 0.0006 ± 0.0003 mm was recorded at order 20 for right and left eyes respectively and was 2 times the minimum RMS of the Pentacam for right eyes and 1.7 times the minimum RMS of the Pentacam for left eyes. CONCLUSIONS: Orders 12 and 10 Zernike polynomials almost perfectly matched the raw-elevation data collected from Pentacam for anterior and posterior surfaces, respectively for either healthy or keratoconic corneas. The Zernike fitting could not perfectly match the data collected from Medmont E300 and ESP.