ABSTRACT
Dynamic temperature and stress solutions are obtained for the boundary value problem posed by a combustion front uniformly translating in a thin coal seam. Static stress profiles are obtained for a layered, plane strain model represented by an elliptical cavity in a coal seam and subjected to tectonic stress, overburden stress, internal pressure, and temperature gradient loading.
NOMENCLATURE
A = K/pc Thermal diffusivity
E Elastic modulus
K Thermal conductivity
S Laplace transform variable
T Time
V Burn front velocity
X Axial co-ordinate
a Coefficient of thermal expansion
¿ Temperature
si Stress component
¿ Poisson's ratio
INTRODUCTION
Coal currently comprises 85% of the U.S. energy reserves but contributes only about 20% of the national energy requirements. Underground coal gasification (UCG) appears to be a viable avenue for providing low BTU gas for electric power generation [1]. Although the Soviet Union has successfully applied this technology at the commercial level [2], various UCG concepts and mechanisms are being field tested in the U.S. These concepts include (i) the Vertical Well to Well Linking experiments by Laramie Energy Research Center, at Hanna, Wyoming for medium coal seams [3], (ii) the Packed Bed investigations [4] on thick coal seams at Powder River Basin, Wyoming (Lawrence Livermore Laboratory) and (iii) the Longwall Generator UCG studies [5] on thin coat seams at Pricetown, West Virginia (Morgantown Energy Research Center). This paper investigates the structural and thermal responses associated with the Longwall Generator concept. This concept involves directional drilling of holes from the ground surface with horizontal holes entering through the coal seam (1000 ft. overburden and 6 ft. thick) and returning vertically back to the surface (Figure 1). The gasification reaction zone propagates horizontally (line drive) in the coal maximum permeability direction between the parallel horizontal holes drilled approximately in the coal butt cleat direction. Both forward combustion and reverse combustion experiments are planned. The key thermomechanical responses such as temperature stresses, displacements provide basic information on UCG global and micro/macro mechanisms. Transient solutions are derived for the moving boundary value problem posed by a rectilinear line drive of the coal seam. Static stress magnitudes and profiles are computed for two dimensional linear thermoelastic models represented by an elliptic cavity in layered media. The effects of tectonic stress, overburden, internal pressure, and surface temperature gradient are investigated.
LINE DRIVE MODEL
The rectilinear propagation line drive model, represented in Figure 2a, is characterized by a combustion front that moves with a burn velocity V. The equation governing the temperature (¿) is (mathematical equation)(available in full paper)
