Simulation of spontaneous heating in longwall gob area with a bleederless ventilation system
2008; Society for Mining, Metallurgy & Exploration; Volume: 60; Issue: 8 Linguagem: Inglês
ISSN
0026-5187
Autores Tópico(s)Safety and Risk Management
ResumoAlthough only used in a few U.S. longwall mines, bleederless ventilation systems can be an effective spontaneous combustion control method in mines having a demon strated history of spontaneous combustion. To provide insights for the optimization of bleederless ventilation systems for U.S. underground coal mines, a computational fluid dynamics (CFD) study was conducted to model the spontaneous heating in longwall gob areas using a bleed erless ventilation system. A single longwall panel with a bleederless ventilation system was simulated, and typical longwall mine ventilation data were used in the simulations. The permeability and porosity profiles for the longwall gob were estimated using a geotechnical model and were used as inputs for the CFD modeling. The effects of gob permeability and resistance of the collapsed entries on the spontaneous heating were studied. The effectiveness of using nitrogen injection to prevent spontaneous heating in the gob was also examined.1 tion in underground coal mines. Be cause of the difficulty in conducting full-scale spontaneous combustion tests in underground coal mines, computational fluid dynamics (CFD) model ing has become an important method to study the spon taneous combustion in underground coal mines. Saghafi and coworkers did numerical modeling of spontaneous combustion in underground coal mines with a back return U-ventilation system (Saghafi et al., 1995; Saghafi and Car ras, 1997), but their work was limited to two dimensions. Balusu et al. (2002) conducted a CFD study of gob gas flow mechanics to develop gas and spontaneous combus tion control strategies for a highly gassy mine. Rosema et al. (2001) also simulated spontaneous combustion using a two-dimensional model. To understand the fire hazard caused by spontaneous combustion in a gob area, a computational fl uid dynamics (CFD) study was carried out by the National Institute for Occupational Safety and Health (NIOSH) to model the spontaneous heating in longwall gob areas under realistic mine ventilation conditions and methane generation rates. In previous NIOSH research, a CFD model was developed to describe the ventilation pathways through the immedi ate gob under different ventilation schemes (Yuan et al., 2006) and to simulate the spontaneous heating of coals in a two-panel gob area using a bleeder ventilation system (Yuan and Smith, 2007). In this paper, the CFD model was used to study the spontaneous heating of coals in a longwall gob area utilizing a bleederless ventilation system. Gob layout and ventilation system In a bleederless ventilation system, the previously mined-out panels are usually isolated from the active gob and the remainder of the mine. In this study, only the ac tive panel was simulated. The layout of the panel and the ventilation system is shown in Fig. 1. The simulated gob area is 2,000 m (6,562 ft) long, 300 m (984 ft) wide and 10 m (33 ft) high starting from the bottom of the coal seam. The ventilation airways are 2 m (6.6 ft) high and 5 m (16.4 ft) wide. The ventilation scheme is a simple “U” bleederless ventilation system. In the model, all entries inby the longwall face were treated as though they were collapsed. FIGURE 1 Typical ventilation pressures for the bleederless ventilation system Layout of longwall panel and ventilation system. were used in the simulation. The pres sure was -76.2 mm (-3 in.) water gauge at the intake inlet and -88.9 mm (-3.5 in.) water gauge at the return outlet. To control the airflow quantity to the longwall face, the wall roughness was adjusted to have a realistic intake airNumerical modeling A commercial CFD program, FLUENT2, from Fluent Inc., was used in this study to simulate the gas fl ow and spontaneous heating in the longwall gob areas. The gas flow in the longwall gob area was treated as laminar fl ow in a porous media using Darcy’s law, while the gas fl ow in the ventilation airways was simulated as fully developed turbulent fl ow. The permeability and porosity distributions of the gob area were based on geotechnical modeling of longwall min ing in the Pittsburgh coal seam and the associated stressstrain changes using Fast Lagrangian Analysis of Continua (FLAC)) code (Esterhuizen and Karacan, 2005). For a Pittsburgh coal seam longwall panel, the permeability val ues in the gob area were estimated to vary from 3.0 x 104 to 8.5 x 105 millidarcies (md), while the porosity value varies from 0.17 to 0.41 based on the modeling result from FLAC. Around the perimeter of the gob and immediately behind the face shields, the permeability and porosity values were the largest, while near the center of the gob, these values were the smallest due to compaction. A detailed descrip tion of the permeability calculation is given in Esterhuizen and Karacan (2007). The porosity profile in the gob was similar to the permeability profile except the maximum and minimum values are 0.41 and 0.17, respectively. It is assumed that these permeability and porosity fi les do not change with the gob height.The detailed permeability pro files are presented in Yuan and Smith (2007).
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