The ELFUEL coal drying facility is designed to utilize the high heat transfer characteristics of a moving packed bed counter-current heat exchanger to efficiently "hot-water dry" lignite coal. Past research into hot water drying of lignite indicates the process is energy inefficient, requiring greater energy input than what can be extracted from the treated lignite. The novel approach of the ELFUEL coal drying facility utilizes the high heat transfer characteristics of a counter-current solid/liquid packed bed to efficiently add and later remove heat to and from descending coal in a pressurized cylindrical vessel. This approach uses raw coal continuously descending in a vertical cylindrical refractory. Coal, upon entering the refractory at the top, gradually heats to process temperatures near the location of hot water injection through contact with hot water flowing upward. Below the point of hot water injection, cool water is forced upward past the descending coal to trap or conserve heat or energy in the system. Sufficient energy is conserved by this design to economically hot-water dry lignite coal. Design of a counter-current energy efficient system which adds and then removes heat has not been reported in the literature. Wonchala and Wynnyckyj (1986) reports counter-current packed bed processes are common in the metallurgical industries. Some important example include the iron blast furnace and iron-ore pelletizing shaft furnace which are very useful since they exhibit a very high potential heat transfer efficiency. However, the metallurgical counter-current gas-solid heat exchangers have not been found to be energy efficient due to channeling of hot gases (Wonchala and Wynnyckyj, 1986). It was the purpose of this study to determine whether the ELFUEL coal drying facility outlined in Minnesota Power's proposal "ELFUEL Demonstration of Low-Rank Coals" to the U. S. Department of Energy, Clean Coal Technology, Round #3 will perform as described and meet the objectives of the process, the economical hot-water drying of lignite coal.
|Published - Oct 1990