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ABSTRACT
Simulations and experiments have been carried out to investigate heat transfer and thermodynamic processes in coal-bearing strata in order to quantitatively understand the development of underground coal fires under the spontaneous combustion condition. With the controlled temperature and under lean oxygen conditions, the thermodynamic parameters for coal oxidation at different stages are experimentally determined in combination with the simultaneous thermal analysis. A combined heat transfer model of conduction, convection, and radiation with finite reactions is developed for the porous coal and rocks. The temperature distributions in the coal and roof strata at different times are simulated based on the single- and two-stage kinetic models, respectively, and compared with field geophysical prospecting. Effects of oxidation kinetic properties due to coal metamorphism on propagation of coal fires are examined. It reveals that a significant step change exists during the thermal process of coal fire caused by two-stage oxidation, and the coal rank of occurrence directly determines the spontaneous combustion period of underground coal fire.
Nomenclature
| A | = | frequency factor, Hz |
| Co | = | local oxygen concentration, kg/m3 |
| Cp | = | specific heat, kJ/(kg · K) |
| D | = | diffusion coefficient of oxygen air, m2/s |
| Ea | = | activation energy, kJ/mol |
| = | gravitational vector, m/s2 | |
| k | = | permeability, m2 |
| Or | = | theoretical oxygen requirement for combustion, kg/kg coal |
| p | = | hydrostatic pressure, Pa |
| S | = | compressibility, 1/Pa |
| q | = | heat release rate, W/g |
| qr | = | heat flux of radiation, W/m2 |
| Q | = | reaction heat, J/g |
| Qnet | = | net calorific value of coal, MJ/kg |
| R | = | gas constant |
| t | = | time, s |
| T | = | absolute temperature, K |
| v | = | seepage velocity, m/s |
| w | = | quantity of local oxygen consumption. Kg/(m3 · s1) |
| β | = | attenuation coefficient, m−1 |
| σ | = | Stephan–Boltzmann constant |
| λ | = | thermal conductivity, W/(m · K) |
| μ | = | air dynamic viscosity, Pa · s |
| ρ | = | density, kg/m3 |
| φ | = | porosity |
| Subscripts | = | |
| c | = | remaining coal |
| f | = | leakage air |
| s | = | solid material |
Nomenclature
| A | = | frequency factor, Hz |
| Co | = | local oxygen concentration, kg/m3 |
| Cp | = | specific heat, kJ/(kg · K) |
| D | = | diffusion coefficient of oxygen air, m2/s |
| Ea | = | activation energy, kJ/mol |
| = | gravitational vector, m/s2 | |
| k | = | permeability, m2 |
| Or | = | theoretical oxygen requirement for combustion, kg/kg coal |
| p | = | hydrostatic pressure, Pa |
| S | = | compressibility, 1/Pa |
| q | = | heat release rate, W/g |
| qr | = | heat flux of radiation, W/m2 |
| Q | = | reaction heat, J/g |
| Qnet | = | net calorific value of coal, MJ/kg |
| R | = | gas constant |
| t | = | time, s |
| T | = | absolute temperature, K |
| v | = | seepage velocity, m/s |
| w | = | quantity of local oxygen consumption. Kg/(m3 · s1) |
| β | = | attenuation coefficient, m−1 |
| σ | = | Stephan–Boltzmann constant |
| λ | = | thermal conductivity, W/(m · K) |
| μ | = | air dynamic viscosity, Pa · s |
| ρ | = | density, kg/m3 |
| φ | = | porosity |
| Subscripts | = | |
| c | = | remaining coal |
| f | = | leakage air |
| s | = | solid material |