Structural Mechanics Simulations Associated With Underground Coal Gasification.

This study describes analytical simulations of the structural and thermal responses associated with the Longwall Generator concept of underground coal gasification (UCG). The work includes characterization of the mechanical properties of coal and shale overburden at elevated temperature, preliminary thermal and structural response modeling, finite element model formulations, and master model analysis. The characterization of mechanical properties of coal and shale overburden includes thermorheological representation of Pittsburgh coal and shale overburden at elevated temperature and the swelling characteristics of bituminous coal. For the preliminary thermomechanical modeling, dynamic thermal and stress response solutions are obtained for boundary value problems representing a radially expanding circular cavity and a line drive in the coal seam. Two dimensional static stress, displacement, and temperature fields are presented for cylindrically circular and elliptic cavities in a homogeneous isotropic medium. Field stresses, displacements, and temperature profiles for various structural model representations characterizing the Longwall Generator concept are obtained using finite element model formulations. The effects of layering, coke/softened layered regimes, and roof collapse are investigated. Subsidence and roof collapse resulting from in situ gasification are computed employing v elasto-plastlc finite element formulations. The results are compared to empirical and classical results from conventional mining. Master model analyses by use of the canned NASTRAN program are conducted to provide a general simulation of the structural mechanisms in the vicinity of burn cavity and the gross strata movements during UCG process. The results from the NASTRAN program are calibrated with a finite element computer program developed here. It is concluded that the structural mechanics investigations presented here provide interpretive and quantitative insight on UCG process mechanisms, parameters, and their eventual optimization. This research demonstrates that structural responses are important in the consideration of the UCG processes