| Underground Coal Gasification(UCG) converts unmineable coal deposits in situ into a combustible gas using controlled thermal effects and chemical reactions. The temperature field plays a key role in UCG process, which can affect the chemical reactions between char and CO2 or H2 O, and micro-crack and pore structure. The seepage, which affects the transport of heat and mass, is influenced by the change of coal structure. The concentration field is formed by the transport of gas component in micro-crack. The value of temperature field increase due to the heat release of gas combustion in coal seam. The evolution of thermal field is controlled by the diffuse of gasifying agent from gasification channel to micro-crack or pore structure, and the porosity of coal changes with temperature increasing, which formed the nonlinear coupling between the evolution of temperature field and concentration field. The pressure field is formed due to the gas generated during gasification which affect the pressure in the gasifier. Therefor, the evolution of pressure field is highly influenced by the evolution of concentration field. As a result of high temperature field, thermal stress is generated by thermal expansion in roof. In addition, some properties, such as coal porosity, fluid properties, chemical reaction rate, and the thermal physical parameters of coal and roof, change with temperature varying. All in all, the phenomena of multi-coupling exists among the temperature field, concentration field, pressure field, and thermal stress field in roof, the fields should be studied for their interaction by the coupling method which connect the fields by the coupling variables.Based on the above analysis, the similar material as roof in experimental model were obtained by similarity principle based on the mechanical condition of field roof. The influence of temperature on thermal physical and mechanical parameters of similar material and lignite were discussed in detail, and the relationship between temperature and thermal physical parameters were fitted by the polynomial fitting. The kinetic parameters of pyrolysis and gasification of large scale lignite(5cm 10 cm, and 15cm) were studied. The effect of gasification process parameters, which includes oxygen concentration and flow rate of gasification agent, on the evolution of characteristic fields(temperature field, concentration field, pressure field and thermal stress field of roof) in underground coal gasification were investigated based on the model experiment platform. The theory of chemical reaction, heat and mass transfer and fluid flow in porous media were used for numerical simulation, meanwhile, the multi-field coupling of characteristic fields was analyzed by COMSOL Multiphysics software. The results of simulation were compared with those of model experiment, and the model of multi-field coupling of characteristic field were verified with the model test. The main results obtained are as follows:1. The main component of similar material as soft roof in model experiment were determined by means of orthogonal experiment based on the similarity principle, and the measured value was close to theoretical one when the ratio of river sand to clay was 3:1.2. 100-300 oC, the growth of thermal conductivity was relatively slow, 300-600 oC, the increase rate was fast, 600-900 oC, the variation of thermal conductivity increase gently. The thermal conductivity of the roof similar materials decreased with the increase of temperature, and the decrease was 33.4%, and between 25-400 oC and 700-900 oC, the decrease rate was great. In the temperature range of 400-700 oC, the decrease was relatively slow. The specific heat capacity of the similar materials and lignite increased with the increase of temperature. The elastic modulus and compressive strength of similar material increased linearly with the increase of temperature. The porosity of lignite increased with temperature and reached the maximum stable value of 32.97% at 800 oC. and the influence of temperature on the permeability of lignite was obtained by using the relationship between porosity and permeability.3. When the heating rate was 3 oC/min, the hysteresis of heating rate, which increased from 1 to 2.33, of coal sample center increased with the increase of scale, and the coal samples of 5cm, 10 cm and 15 cm reached the maximum heating rate at 300 oC, 300 oC and 400 oC, respectively. The weight loss rate of lignite becomed smaller with the increase of the scale which improved the diffusion resistance.4. The influence of the scale on the activation energy and the pre-exponential factor were more complicated in temperature range of 300-500 oC. While in the range of 500-700 oC, The activation energy and the pre-exponential factor increased with the increase of lignite scale. The conversion rate reached the maximun at 1000 oC in the atmosphere of CO2, which showed the decline with increase of temperature at high temperature. The activation energy is 21.5kJ/mol in the range of 30%-80% of conversion rate. However, the conversion rate of carbon increased with temperature in the atmosphere of H2 O, and the activation energy is 54.31kJ/mol which is higher than that of reaction of CO2 with char, which demonstrated that the temperature was more sensitive to the reaction of H2 O with char.5. Thermal expansion of lignite existed in the temperature range of 100-600 oC, and reached the maximum value at 100 oC, Due to the release of volatile, the new cracks and pores were formed in coal seam, which provided the flow channel for the diffusion of the gasification agent and gas component in coal seam.6. With the increase of temperature, the precipitation rate of H2 from large scale lignite is higher than that of small scale lignite, when the temperature exceeded 800 oC, the precipitation rate of H2 in small scale lignite decreased, while the precipitation rate of H2 from large scale coal continued to grow. The CO component of large scale lignite increased from 5% to 30% among 500-900 oC. The CO component of small scale lignite remained stable with the increase of temperature. Among 500-700 oC, the precipitation rate of large scale lignite CH4 decreased linearly with the increase of temperature, which was 5 times of small scale lignite.7. The main direction of temperature field was affected by the fracture of coal and the gasifying agent. Under the condition of model experiment, the velocity ratio of the temperature field along the lateral side and channel was 3.11:1 at 65% of oxygen concentration, the main direction of the evolution of temperature field extended along the axial direction of the gasification channel at 75% of oxygen concentration. When the concentration of the gasifying agent was 85% of oxygen concentration, the expansion area of temperature field increased. When the flow rate increased from 1Nm3/h to 5 Nm3/h, the region of temperature field extended, and the direction of evolution moved only along the axial of gasification channel, the velocity of lateral side was very small.8. Under the condition of model experiment, the direction of evolution of temperature field in roof was along the height of roof, and the evolution velocity of roof temperature delayed that of coal temperature field at the flow rate of 1Nm3/h in the concentration of 65%, When the concentration was 75%, the temperature field of roof was about 2 times of that of 65%, and the roof temperature field extended along the gasification channel at 85%. The temperature field of roof was accelerated along the direction of the gasification channel, however, decreased in the height from 1Nm3/h to 5Nm3/h.9. Under the condition of model experiment, when the flow rate of the gasifying agent and the concentration were 1Nm3/h and 65% respectively, the evolution of concentration field was stable, when the concentration was 75%, the concentration field extended outside, when the concentration was 85%, the H2 and CO concentration fields were enlarged, and the centen position of CH4 concentration moved,meanwhile, the value of H2 concentration and CO concentration declined. When the concentration of gasifying agent was 75%, and the flow rate was 1Nm3/h, The seepage diffusion of gasifying agent in coal seam promoted the second expansion of the concentration field. When the flow rate was 5Nm3/h, the pressure in the gasifier increased, and the evolution of the concentration fields was suppressed.10. Under the condition of model experiment, when the flow rate was 1Nm3/h, and the oxygen concentration was 75%, the evolution of pressure field extended along the gasification channel. When the oxygen concentration of the gasifying agent was 85%, the pressure zone disappeared. The pressure of the gasifier increased, and the diffusion of gas from coal seam to gasification channel was suppressed when the flow of the gasifying agent increased from 1Nm3/h to 5Nm3/h.11. Before the temperature field of coal seam had not been established, The evolution rate of stress field in roof was slower than that of coal seam. With the stability of temperature field of coal seam, the evolution rate of roof temperature field was consistent with the evolution of temperature field in coal seam by the effect of thermal dispersion, which was less than the evolution rate of temperature field in coal seam.12. Under the condition of nonlinear coupling, the evolution of characteristic fields by numerical simulation are basically consistent with that of characteristic fields derived from model experiment, the mathematic description for the fields can meet the basic needs of experimental model, meanwhile, the evolution of all characteristic fields are close to the real conditions under the coupling of temperature-pressure-concentration fields. In additon, the numerical model has more complex errors with the increase of the number of coupling variables. In the evolution of the concentration field, because of less data measuring points, which are partial choked by tar dust in the experimental model, and affected by the complex pore structure of coal seam, the results of the concentration field between model experiment and numerical simulation exists error.13. The evolution of roof temperature field influenced the thermal stress of roof. In the early stage of gasification, the thermal stress was about 0.45 MPa. With the gasification process conducted, the thermal stress gradually increased, the thermal stress extended along the gasification channel with the temperature field of coal seam. Under the 1400 K, the thermal stress was 1.1 MPa when the length of cavity was 200 mm, the thermal stress was 1.3 MPa when the length of cavity equaled 400 mm, the value of thermal stress were 1.5 MPa when the length of cavity were 600 mm and 800 mm. When the length of cavity was 1000 mm, thermal stress was 1.6 MPa, according to the result of model test, the roof collapsed. The length of cavity was relatively stable with the length of 600-800 mm under the condition of model test. |
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