Analysis of the Coal Spontaneous Combustion Risk Space Evolution in the Goaf Under Ventilation and Extraction Coupling

ABSTRACT

The risk of coal spontaneous combustion in goaf is one of the disasters in coal mining, ventilation and extraction often aggravate air leakage from the goaf. Due to the unclear rule of expansion law and air leakage, the risk space for coal spontaneous combustion continues to occur. To investigate the influence of ventilation and extraction on the evolution of the risk space of coal spontaneous combustion (CSC) in goaf, further reveal the characteristics of air leakage and extraction in goaf and obtain the expansion law of CSC risk space. A three-dimensional model of goaf under surface drilling and gas extraction was developed. Using FLUENT numerical simulation software, taking a coal mine in Anhui Province, this study analyzes the influence factors, evolution law, and influence characteristics of CSC through field observations and numerical simulation. The research indicates that ventilation keeps the risk space of CSC expanding. The extraction of single drilling showed a local space expansion, while the extraction of double drilling shows the overall space expansion of the goaf. The air leakage flow path in goaf, air flow velocity vector in working face, gas concentration loss, and gas extraction characteristic parameters are determined. The loss of air leakage along the working surface meets four-stage: “strong in – flow - mixed flow – steady inflow – sudden outflow.” It can be determined that the extraction intensity of single drilling should be greater than 34.8 m³/min, and the double drilling should be greater than 18.7 m³/min. The relationship between the coupling influence of ventilation and extraction and the expansion law of CSC risk space is determined, and the maximum risk space width of CSC can be predicted. The research results are of guiding significance for CSC prevention and gas efficient extraction.

KEYWORDS:

• Drilling gas extraction
• coal spontaneous combustion
• evolution of risk space
• air leakage characteristics
• numerical analysis

Nomenclature

$\vec{u}$	=	velocity vector (m s−1)
$\mathit{k}$	=	porous media permeability (m2)
$\mathit{p}$	=	gas pressure (Pa)
$\mathit{\mu }$	=	gas dynamic viscosity (pa s)
$\mathit{n}$	=	coal porosity
$D_p$	=	average particle size (m)
$\mathit{T}$	=	coal temperature (℃)
$K_p$	=	compaction expansion coefficient
$\mathit{\rho }$	=	air density (kg m3)
${\rho }_k$	=	mixed density of methane and air (kg m3)
$\mathit{\kappa }\left(1,2\right)$	=	methane or air components (%)
$S_k$	=	methane source term
$\mathit{m}$	=	methane content
$m_f$	=	free methane content in matrix or fracture
$D_{\mathit{ij}}$	=	viscous resistance coefficient
$C_{\mathit{ij}}$	=	inertial resistance coefficient
$\mathit{C}$	=	oxygen concentration (%)
$C_0$	=	standard state oxygen concentration (%)
$W_{O_2}$	=	oxygen consumption rate coefficient
$b_0$	=	empirical constant
$W_{C{H}_4}^0$	=	methane strength released uniformly from coal walls (mol m−2 s−1)
$W_{C{H}_4}^1$	=	methane strength released from decay of residual coal (mol m−2 s−1)
$\mathit{\lambda }$	=	decay coefficient of residual coal
$\mathit{X}$	=	distance from working face (m)
$\mathit{V}$	=	advancing speed of working face (m/d)
$Q_f$	=	ventilation rate (m3 min−1)
$Q_c$	=	extraction rate of single drilling (m3 min−1)
$Q_{\mathit{c}-\mathit{c}}$	=	extraction rate of double drilling (m3 min−1)

Acknowledgements

The authors wish to thank these organizations for the support provided. They also wish to thank the viewers and editors for their constructive comments and suggestions in improving the manuscript.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was carried out with funding from the National Natural Science Foundation of China [Grant No. 52174169, 51674103, 52004084].