Influence of coal types on overlying strata movement and deformation in underground coal gasification without shaft and prediction method of surface subsidence

Under the action of high temperature, the mechanical properties of coal will change significantly. After gasification, different types of coal will form different surrounding rock mechanical characteristics, which may have different impacts on the movement characteristic of combustion space areas overlying strata and surface. Without considering the influence of coal types or degree of coalification, the actual underground coal gasification (UCG) projects may have issues, such as instability of surrounding rocks in the combustion space zones, damage to surface buildings and structures. At present, the surface subsidence prediction method for UCG and underground gasifier design haven't considered the effects of different coal types. Therefore, this paper studies the influence of coal types on the movement characteristics of the combustion space area overlying strata and surface through field measurement, theoretical analysis and numerical simulation. The research results are as follows: 1) The principle of the mechanical property change of different types of coal is different after UCG; 2) Different types of coal have an effect on the surrounding rock movement and deformation around combustion space area, the vertical stress distribution of coal pillar and the surface subsidence. The strong-caking coal underground gasification is more useful for controlling overlying strata movement in the combustion space area and reducing the surface subsidence; 3) The prediction method of surface subsidence for UCG without shaft is proposed and the method is applied to the Ulanqab UCG industrial experiment field; 4) Suggestions for the design of gasifiers and isolated coal pillars considering the impacts of different coal types are proposed. The research results have important guiding significance and practical value for underground gasifier and isolated coal pillar design, surface subsidence prediction and UCG industrialization development.