Rubbling and structural stability of underground coal gasification reactors

Underground Coal Gasification (UCG) is a method which combines mining and gasification of coal. Field tests in the United States, over the past several years, have established the feasibility of this process. Currently, this method is capable of producing low to medium BTU gas. In this process a cavity is created in the coal seam. Coal on the roof and the sidewalls of this cavity spall into the cavity and form a pile of rubble.; Formation of the rubble plays a significant role in recovery of the energy of the coal and the success of the UCG process. Mechanisms leading to the spalling of the coal face in UCG are not completely known and those which are not known are not fully understood.; In this research several spalling mechanism for coal are identified, analyzed, and modeled. These models are solved by finite element method or finite difference method. The models are applied to conduct numerical experiments and parametric studies to determine the significance of the spalling mechanisms and determine the important characteristics of each one. Phenomenological models for failure of coal are proposed and applied to compute the regression rate of the coal face. Moreover, the significant spalling mechanisms are incorporated in a comprehensive computer model to describe sidewall and roof regression in the UCG reactor.; The spalling mechanisms include: thermal stresses, moisture induced stresses, heterogeneity of the coefficient of thermal expansion of the coal, variation of the structural properties of the coal due to temperature rise, and mechanical stresses. It is found that the thermal stresses are mostly compressive, they have a skin effect, and they may lead to compressive crushing of the sidewalls. The moisture induced stresses have a very significant effect in the growth of the UCG reactor for low rank coals (sub-bituminous and lower) and they lead to tensile failure of the coal face. The heterogeneity of the coefficient of thermal expansion is not a dominant factor in failure of the coal in UCG. Variation of structural properties of coal with temperature rise leads to the collapse of a stable arch. Mechanical stresses play a major role in growth of UCG reactors.; The computer model describes sidewall and roof regression given various thermal attack descriptors determined by combustion and gasification phenomena within the cavity. This model is used to simulate the growth of UCG reactors in rubble free and partially rubble-filled situations. The effects of coal properties, coal seam depth, strength of the bond between the coal and the underburden rock, separation of the coal from the overburden rock, and formation of hot and cold rubble on growth of the cavity are studied.