ABSTRACT
The renewed research efforts on nu coal gasification in the United States stem from the rapidly emerging priorities on coal derived energy. This paper reviews some of the pertinent areas of investigation and considers the critical structural problems associated with underground coal gasification (UCG) of Eastern seams. UCG problems relating to thermo-mechanical characterization of Pittsburgh coal, fracture evaluation, and stress response solutions are investigated in this paper.
INTRODUCTION
Extensive laboratory and field tests coupled with modeling studies are being conducted in order to understand governing UCG process mechanisms and establish criteria for successful gasification [1]. The major UCG field programs underway in the United States are [2]: (a) the Linked Vertical Well (LVW) experiments on medium thickness sub-bituminous coal seams at Hanna, Wyoming(b ) the Packed Bed Process investigations on thick sub-bituminous coal beds near Gllette, Wyoming, and (c) the projected LVW and Long Wall Generator (LWG) experiments on thin bituminous seams at Pricetown, West Virginia. The supporting laboratory and modeling efforts primarily relate to chemical reaction kinetics studies on various types of coal with associated fluid flow and heat transfer effects. The importance of modeling structural responses has also gained recent recognition in view of the environmental and technical consequences resulting from subsidence and cavity roof collapse [3]. Other problem areas from a stress analysis and fracture mechanics standpoint include the sustained propagation of the combustion front in swelling coals and the dependence on temperature and stress of coal fracture permeabilities. This aspect has been illustrated in laboratory tests at the Morgantown Energy Research Center where a channel (simulating hydraulic fracturing or reverse combustion linking) can close due to swelling and plugging during forward combustion. These plugging effects are minimal in the reverse combustion mode. This paper examines basic considerations for characterizing failure thresholds and related thermomechanical behavior of Pittsburgh coal. Particular emphasis is placed on utilizing linear elastic fracture mechanics concepts for yielding fracture criteria for a cleated material such as coal. Since the thermal stresses and associated phenomena are fundamental to the advancement and stability of combustion, thermo-elastic/visco-elastic formulations and solutions for selected problems are given.
COAL FRACTURET HRESHOLD CHARACTERIZATION
The fracture evaluations for the cleated Pittsburgh coal are based primarily on the work of Griffith [4] and Paul and Mirandy [5] within the framework of the theories presented by McClintock and Walsh [6] and Brace [7]. Following Paul and Mirandy [5], the fracture threshold curve developed from the state of stress associated with a flat ellipsoidal cavity (a=b>>) subjected to a tensile stress s and a shear stress t (parallel and normal to the c axis respectively in Figure 1) for a: 0 is given by: (mathematical equation) (available in full paper)
where St is the uniaxial tensile strength and ß1 is related to the Poisson's ratio ¿ and complete elliptic integrals of the first (K) and second (E) kinds respectively by: (mathematical equation) (available in full paper)
The critical angles for the case in equation (1) are Expressbedy a = 0 and 2¿c,: tanl(-2t/ß1s).
