Roof subsidence law and related parameters study of gasifier under thermodynamic coupling action

In order to qualitatively and quantitatively study the subsidence law of seam roof and its related parameters in the process of underground coal gasification, a thermally coupled solid support beam model of key layer was established according to the spatial position morphology of combustion air zone, overlying rock layer and coal seam on both sides after underground gasification. On the basis of simplifying the model, and analyzing and calculating the force of the model, the equation of the temperature change of the position of the neutral layer of the critical layer rock beam under the action of thermodynamic coupling was derived by using the composite rock beam theory. Using the static equilibrium theory, the differential equation of the rock beam torsion curve was derived. After solving this equation, the equation of critical layer deflection curve was obtained. After assigning the equation's parameters, the rock beam's deflection law with time was theoretically analyzed. The changes in the neutral shaft position, temperature force, and thermal bending stiffness with time and their influence on the subsidence of essential layers were quantitatively analyzed. The results show that the deformation of the critical layer is a continuous subsidence process during the underground gasification process. Compared with the deformation degree of the rock beam at room temperature, the degree of deformation depends on the elastic modulus of the rock beam with temperature. When the elastic modulus decreases with the increase of temperature, the deformation degree is greater than the deformation of the rock beam at room temperature;otherwise, it is smaller than the deformation of the rock beam at room temperature. The neutral axis position of the rock beam is mainly affected by the temperature characteristics of the elastic model. When the elastic modulus of the rock beam decreases with the increase in temperature, the neutral axis position rises first and then falls. Conversely, if it falls first and then rises, the rise of the neutral shaft position will reduce the thermal bending stiffness. Descending will increase the thermal bending stiffness. Thermal bending stiffness plays a decisive role in the sinking of rock beams. The thermal bending stiffness increases, the deformation degree of the critical layer decreases, and conversely, the deformation degree increases. The additional temperature force caused by the temperature field propagation inside the rock beam shows a trend of "increase-decrease-increase". However, its influence on the subsidence process of rock beams is too tiny that the degree of influence is negligible. The thermal physical property parameter of rock beams that affects the change law of thermal bending stiffness is the elastic modulus. Its temperature-dependent properties have a superimposed effect on thermal flexural stiffness. The established model was used to predict the surface subsidence in the Huating underground gasification project. The difference between the predicted and measured values is only 6.2 mm, indicating that the model established in this paper can accurately predict the final shape of the surface subsidence.