This invention relates to a process for retorting oil shale-coal mixtures and more particularly to a process for retorting mixtures of lean oil shale and coal.
Oil shale is a fine-grained, sedimentary rock which contains an organic material known as kerogen. Upon heating, kerogen decomposes to yield liquid oil, gases and residual carbon. The kerogen content varies with different geological formations, and some shale does not yield sufficient quantities of oil to economically justify its recovery. Unfortunately, some of the lower grades of oil shale must be mined in order to reach the richer deposits. Unless the oil can be extracted from these leaner resources, the overall costs of extraction will escalate needlessly.
Thermally efficient retorting systems use the energy of the residual carbonaceous material on the retorted shale for process heat. For example, the residual carbon may be burned to heat circulating solid heat carriers such as ceramic balls or particles, sand or spent shale. Alternatively, hot flue gases generated from the combustion can be used for direct or indirect heating of the raw shale.
The amount of carbonaceous residue remaining on the shale mineral structure after retorting is dependent upon various factors. At the temperatures required for commercial retorting, the primary factor is the grade or richness of the raw shale, with lower grades having proportionately less residue. For oil shales yielding less than about 0.13 liters of shale oil per kilogram of oil shale (30 gallons per ton), the quantity of organic residue in the retorted shale is insufficient to supply the total heat required for retorting the raw shale, when directly mixed in the preferred ratios of spent shale to raw shale.
It has been proposed to add supplementary carbonaceous material to the retorted shale in order to generate all or substantially all of the heat needed for heating the raw shale in the retorting zone. See, for example, U.S. Pat. Nos. 4,058,205; 3,939,057; 2,589,109; and the publication "Development of the Lurgi-Ruhr Gas Retort for the Distillation of Oil Shale," Lurgi Mineraloltechnik GMBH, Frankfurt (Main), October 1973, Page 11, Paragraph 5.
A particularly advantageous process for retorting oil shale and other similar materials is described in U.S. Pat. No. 4,199,432, issued Apr. 22, 1980 to Tamm et al, which is incorporated herein in its entirety by reference. In this process, fresh oil shale particles are passed into an upper section of a vertically-elongated retort and downwardly therethrough in the presence of hot heat carrier particles to heat the fresh oil shale particles to retorting temperatures sufficiently high to drive off hydrocarbonaceous materials, which are removed from the upper portion of the retort. A heated, nonoxidizing gas, e.g., recycled product gas, flue gas, nitrogen, or steam, is passed upwardly through the retort at a velocity of between about 1 to 5 feet per second. The size of the fresh oil shale particles and the heat carrier particles include particles which are fluidizable at the gas velocity and particles which are nonfluidizable at the gas velocity. The shale particles are passed downwardly through the retort at a rate providing a residence time for substantially complete retorting of the particles. The retort contains internals to substantially limit backmixing and slugging of the particles.
The retorted particles contain residual carbon. These particles are passed to a combustion zone where they are combusted with an oxygen-containing gas to heat the retorted particles along with any inert or spent shale particles present. The heated particles can then be recycled as heat carriers to the retort to provide process heat for retorting fresh shale particles. The combustion zone can be a liftpipe or an entrained bed reactor wherein the entrained particles are rapidly heated to combust residual carbonaceous material. To minimize the height of the liftpipe combustor, it is desirable to have a low combustor residence time of both gas and solids, e.g., less than about 4 seconds, preferably 1 to 2 seconds.