| Four two-dimensional cavity growth models are developed based on the permeation process concept in order to understand what has happened in previous field tests, and to learn better methods for future field tests and commercial scale operations. The first model considers only a wet coal region beyond the cavity. In the second model, dried coal is added to the first model, so that there are two different coal regions: the wet coal and dried coal regions. The third model includes the effect of links formed by reverse combustion, but the dried coal region is not included. Also in this model, plugging phenomenon, due to particulates carried by the injectant to the cavity wall, is introduced. The final model incorporates nonisotropic permeability, a dried coal region, links and plugging phenomenon.;Considerations of a wet coal region only or of wet coal and coal drying do not explain the high recovery of coal observed in field tests. However, plugging phenomenon prevents channeling of gases down high-permeability links, and thereby the recovery of coal is increased. Also link configurations have a marked effect on the coal source recovery. Computation results from the fourth model show that coal recovery is improved greatly if a large dried coal zone exists around the cavity. |
