Abstract
A linear stability analysis shows that reverse combustion in coal and tar sands is only conditionally stable for mobility ratios less than one. However, high air-flow rates and gas generation at the combustion front can be stabilizing influences. For unstable operation, an estimate of the size of the reverse combustion channel may be obtained from the curve for the most highly amplified wave length. This provides a method for calculating the air flux, combustion front velocity, and rate of progress of the burn front.
Recently the U.S. DOE Laramie Energy Technology Center (LETC) and Sandia Laboratories obtained experimental data about reverse combustion from a field test of in-situ coal gasification at Hanna, WY. These data show that 9.7 days were required for the development of a reverse combustion path 68 to 70 ft in length. The stability theory developed in this work predicts a length of 64 ft for this same 9.7-day period. In addition to quantitative predictions, stability theory provides an explanation of certain puzzling qualitative observations concerning reverse combustion.
Introduction
In-situ combustion is a potentially useful method for recovering fossil fuels from underground deposits. A number of in-situ combustion field tests have been conducted in oil reservoirs, tar sands, oil shale deposits, and coal seams. In-situ combustion can be classified into two broad categories: reverse combustion, in which the reaction front travels countercurrent to the flow of air, and forward combustion, in which the reaction zone travels in the same direction as the flow of air.
Reverse combustion is especially important for coal and tar sands. During forward combustion, tars vaporized at the flame front in either coal or tar sands travel by convection into cooler regions ahead of the reaction zone where they condense and subsequently reduce the natural permeability of the fuel bed.
In reverse combustion, vaporized tars or other high-molecular-weight compounds generated in the reaction zone travel toward the production well through a heated area already contacted by the high temperatures of the combustion front. As an added advantage, reverse combustion in tar sands substantially increases the relative permeability to gas. In lignite and subbituminous coal, drying and partial combustion typically increase the effective permeability to gas by four orders of magnitude. However, bituminous coal frequently swells on heating, and the net effect of reverse combustion on the permeability of swelling coals has not been investigated thoroughly. In coal and tar sands, reverse combustion is primarily a coking or carbonization process - i.e., the volatile components of the tar or coal are partially combusted while most of the carbon or coke is left unburned.
For these reasons, reverse combustion represents an important part of some in-situ combustion methods currently being investigated for tar sands and coal. In the linked vertical well process for in-situ coal gasification, reverse combustion is used first to develop a high-permeability path between the production and air injection wells, while in the second stage of the process forward gasification or combustion is used as the major gas production method. Both industrial companies and government laboratories have investigated the linked vertical well process. For tar sands, the LETC is considering the use of reverse combustion as a preparatory mechanism similar to that used in coal.
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