| Description |
Spontaneous combustion is one of the main sources for mine fires in underground coal mines. Most of these fires are initiated in the longwall gob (caved area) by coal oxidation. Because coal oxidation generates heat, this phenomenon is called the selfheating process. This process will eventually create hot spots under conditions, i.e., oxygen concentrations of at least 5% (by volume) and gob temperatures of 100°C. Coal properties, gob permeability, self-heating characteristics, and the ventilation system are the key variables for the formation of these hot spots. A study was carried out to identify the location of hot spots. The study is based on mine ventilation surveys, laboratory experiments, and gob simulations using Computational Fluid Dynamics (CFD). Ventilation surveys were conducted in an existing longwall mine located in the western United States; the laboratory experiments were performed on a physical gob model to investigate permeability (k) and airflow distribution; and the CFD models were simulated to investigate the flow behavior in the gob, the oxidation of coal, and heat transfer phenomena. Four CFD models were formulated and solved, three utilized a bleeder ventilation system, and the fourth a bleederless ventilation system. For these models, the gob length varied from 912 m to 2,445 m. The gob of each model was divided into 3 zones of different permeability: unconsolidated (k = 4.68 xlO"7 m2), semi-consolidated (k= 3.15 x 10"8 m2), and consolidated (k = 7.98 x 10"9 m2). The simulation results showed that in the models ventilated by a bleeder system, the hot spot was located in the consolidated zone near the return side of the gob. Once initiated, it propagated along the tailgate side as the gob progressed. The leakage flow through the gob played an important role in determining the size and location of the hot spot. In models ventilated by a bleederless system, the hot spot was located in the gob by the face line. This is mainly caused by the air leakage from the headgate T junction (face) and between the shields. It may extend further into the gob depending on the gob permeability and the fan pressure. In addition, these gob simulation exercises have shown that the hot spot areas in all cases can be located accurately. This information can be used to develop suitable control methods. The parametric studies have indicated that the ventilation system and gob permeability are the major contributing factors for the formation of hot spots. Although the gob models were developed for specific dimensions and ventilation system, the results can be applied to other schemes with minor adjustments. |
