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The application on investigations of thermal behaviour of two-phase coal-gas systems for the determi |
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The application on investigations of thermal behaviour of two-phase coal-gas systems for the determination of properties of gas contained in the pore system Authors: Gawor, M. ; Rysz, J.
The investigations of the influence of gas pressure on the magnitude of coefficient of thermal diffusion of coal saturated by gas make us possible to determine the properties of coal which contains gas in the pore system. Taking into account that the thermal diffusivity of gas is several orders of magnitude grater than the thermal diffusivity of coal, gas contained in the pore system of coal increases the magnitude of diffusivity of two phase gas-coal system in comparison to pure coal. The thermal diffusivity of the perfect gas is reduced in the increase of the pressure. The term 'perfect gas' is understood here to be the gas for which the free path of molecules is much grater than the dimensions of pores. The recorded increase of thermal diffusivity of the system in the increase of pressure indicates that the gas should be regarded as the rarified gas which differs significantly from the perfect gas. In the paper the authors describe the construction of measuring stand. The results of empirical investigations of thermal diffusivity of coal saturated by sorptive gases (CH4, CO2), and non-sorptive gases (Ar, Kr). For the experiments the gas pressures from the interval 0.004-1.1 MPa was applied. The computer program was applied to control: the supply of the Peltier cells, the acquisition of signals from thermocouples, and the decomposition of signals by means of the Fourier series. Basing of the phase shift and the damping of amplitude the thermal diffusivity coefficients were determined (cf. Eqs. (5),(6),(7),(8). The algorithm used for the determination of the coefficient of thermal diffusivity based on two different values (the ratio of the amplitude and the phase shift) recrorded in one survey enabled the authors to check whether the investigated material satisfies the rational assumptions which has been applied to formulate Eqs. (7),(8). For the solid coal the thermal diffusivity coefficient determined from the phase shift was different form that determined from the amplitude variation. Such a result imposes the conclusion that the solid coal is a highly non-homogeneous material containing fractures which imply discontinuous variations of the amplitude and phase of the thermal wave. Consequently, for that case the heat conduction cannot correctly be described by the well-known parabolic conduction equation (1) with constant K coefficient. The increase of the thermal diffusivity in the increase of the pressure indicates that gas contained in pores of the briquette should be regarded as the rarified gas. The access of the non-pressure-dependent value for the coefficient of thermal diffusivity for the pressure grater than 0.5 MPa may be regarded as the critical state above which gas may be regarded as the perfect gas. The carbon dioxide achieves the critical state for the magnitude of pressure less than for other gases. The influence of the sorption process on the thermal properties of the gas-coal system may be evaluated by the comparison of the recorded values of the thermal diffusivity coefficient for two gases of the same thermal diffusivity and different sorption intensity. The coefficients mentioned above for the 'couple' of gases Ar and CH4 are equal to [...], whereas for Kr and CO2 are equal to [...]. The diffusivity coefficients for coal from the Nowa Ruda Mine saturated by the sorbing gases are about 1.2 times greater than the diffusivity coefficients for coal from the Nowa Ruda Mine saturated by the non-sorptive gases.
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2011