Mining Topic: Spontaneous Combustion

Nitrogen concentration distribution in the gob after 20 days with a nitrogen injection rate of 500 cfm (1=100%).

What is the health and safety problem?

Spontaneous combustion continues to pose a hazard for U.S. underground coal mines, particularly in western mines where the coal is generally of lower rank. The risk of an explosion ignited by a spontaneous combustion fire is also present in those mines with appreciable levels of accumulated methane.

What is the extent of the problem?

Approximately 15% of the 164 total reported fires for underground coal mines for the period 1978–1990 were caused by spontaneous combustion. For the period 1990–2006, more than 20 U.S. coal mines suffered underground spontaneous combustion events, and 25 total reported fires for underground coal mines were caused by spontaneous combustion. The incidence of spontaneous combustion fires and the resulting explosion hazard in underground coal mines is expected to increase with the projected increased mining of lower rank coals, deeper mines with more methane, and the growth in the dimensions of longwall panels.

How is OMSHR addressing the problem?

The Office of Mine Safety and Health Research (OMSHR) is working to reduce spontaneous combustion risk in underground coal mines through the development of control methods and nitrogen injection strategies to prevent spontaneous heatings. The control methods include: (1) Optimizing the ventilation system to eliminate the possible critical velocity zones where the spontaneous heating will likely occur; (2) Selecting an appropriate face advance rate to prevent the coal oxidation behind the shields not to reach a thermal runaway; (3) Early detecting of spontaneous heating to allow for enough time to suppress the heating.

What are the significant findings?

OMSHR research has developed a computational fluid dynamics (CFD) model that can be used to simulate spontaneous heating of coal in the longwall gob area under realistic mine ventilation conditions and methane generation rates. The critical velocity zones for spontaneous combustion in the longwall gob area were identified for different gob conditions and coal properties based on the CFD simulations. The research also investigated the effects of longwall face advance rate, gob seal leakage, and the barometric pressure changes on the spontaneous combustion process in these longwall gob areas. The research results can help mine ventilation engineers and mine safety personnel better prevent and control spontaneous combustion.

What are the next steps?

OMSHR will continue to use CFD modeling to develop strategies for nitrogen injection for underground coal mines using bleeder or bleederless ventilation systems. Guidelines for location and flow rates using nitrogen injection will be developed.

Noteworthy Publications & Products

• CFD Modeling of Spontaneous Heating in a Large-Scale Coal Chamber (2012-01)
  This paper discusses a study in which a three-dimensional CFD modeling of spontaneous heating of coal, based on the U.S. Bureau of Mines (USBM) large-scale coal chamber tests, was conducted.
• CO and CO2 Emissions from Spontaneous Heating of Coal Under Different Ventilation Rates (2012-08)
  This study evaluated CO and CO2 emissions from spontaneous heating of a U.S. coal sample in an isothermal oven under different airflow ventilation rates from 100 to 500 cm3/min. Experiments were conducted at oven temperatures of 70, 90, and 100 °C.
• Computational Fluid Dynamics Modeling of Spontaneous Heating in Longwall Gob Areas (2012-08)
  Computational Fluid Dynamics simulations were conducted to investigate the spontaneous heating of coals in longwall gob areas with different apparent activation energies and reaction surface areas. The results are reported in this paper.
• Effect of Longwall Face Advance on Spontaneous Heating in Longwall Gob Areas (2010-06)
  In this paper, Computational Fluid Dynamics modeling of the effect of longwall face advance on the spontaneous heating of coals in a two-panel gob area using a bleeder ventilation system is presented.
• Effects of Ventilation and Gob Characteristics on Spontaneous Heating in Longwall Gob Areas (2012-08)
  The results of field tests to understand how ventilation and gob characteristics affect the spontaneous heating process causing spontaneous combustion fires in longwall or worked-out gob areas are presented.
• Modeling the Effect of Barometric Pressure Changes on Spontaneous Heating in Bleederless Longwall Panels (2012-08)
  The effect of inflow and outflow of gas on the potential spontaneous heating in the gob was investigated using the computational fluid dynamics (CFD) model.
• Modeling the Effect of Seal Leakage on Spontaneous Heating in a Longwall Gob Area (2012-08)
  In this NIOSH study, a computational fluid dynamics (CFD) model developed in previous work is used to model the effect of seal leakage on spontaneous heating of coal in longwall gob behind the seal.
• Numerical Study on Effects of Coal Properties on Spontaneous Heating in Longwall Gob Areas (2012-08)
  A computational fluid dynamics (CFD) study was conducted to model effects of coal properties on the potential for spontaneous heating in longwall gob (mined-out) areas.
• Simulation of Spontaneous Heating in Longwall Gob Area with a Bleederless Ventilation System (2012-08)
  To provide insights for the optimization of bleederless ventilation systems for U.S. underground coal mines, a computational fluid dynamics study was conducted to model spontaneous heating in longwall gob areas using a bleederless ventilation system.
• The Effect of Ventilation on Spontaneous Heating of Coal (2012-08)
  Study results of the role of various ventilation rates in the spontaneous heating of coal in underground mines found that there is an optimum ventilation flow to produce the maximum rate of temperature rise at the critical ambient temperature.