The present invention relates generally to a coal-water slurry pumping and drying system for feeding coal to a pressurized coal gasifier, and more particularly to a slurry atomizer for use in such a system for providing slurry droplets of a size capable of being "dried" without excessively heating the coal.
The conversion of the vast coal reserves of the world to usable fuels is becoming of increasing importance in the efforts to satisfy the ever growing energy demands. Coal gasification is a mechanism by which combustible gas may be readily generated for use as an energy source. Several gasification systems are presently utilized for such gas generation and function at pressures ranging from about atmospheric up to about 1500 psia.
In coal gasification systems operating at greater than atmospheric pressure, coal-transferring systems are required for introducing the coal into the high pressure vessel. One such coal-transferring system is a lock-hopper type apparatus formed of a plurality of valved hoppers disposed between the coal bin and the gasifier and includes appropriate valving and hopper-pressurizing systems for increasing the pressure of the coal within a selected hopper to a pressure corresponding to that of the gasifier. Such lock hopper apparatus are somewhat complex and suffer many problems, especially those due to erosion from the coal which considerably detracts from the life of the valves.
The use of pumps to pressurize coal for introduction of it into pressurized gasifiers has not been satisfactory due to the abrasive nature of dry coal which significantly reduces pump life. This erosion problem encountered in pumping coal has been somewhat relieved by mixing the coal with water prior to pumping the resulting slurry to the desired pressure. With the water present in the slurry, the combustion efficiency suffers. However, excess water has been successfully removed from the coal-water slurry by employing the coal-feeding and drying system set forth in assignee's U.S. Pat. No. 4,153,427 entitled "Apparatus and Method for Feeding Coal into a Coal Gasifier," which issued May 8, 1979. In this patented system a coal-water slurry is pumped to the desired pressure and then the water in the slurry is removed prior to the introduction of the coal into the gasifier by contacting the slurry with superheated steam in an entrained bed dryer. The steam is at a temperature sufficient to dry the coal by rapidly vaporizing the water in the slurry. At the exit end of the entrained bed dryer the steam is separated from the coal by employing a simple separator, e.g., a cyclone. The "dried" coal may then be introduced into the gasifier while the steam separated from the slurry may be recycled through a suitable filtering mechanism back into the dryer through a compressor and superheater. The steam, which is superheated to a temperature and is at a flow rate sufficient to vaporize essentially all of the water in the slurry and to superheat the vapor removed from the slurry, is admixed with the slurry at the entrance of the entrained bed dryer to effect the drying of the coal prior to overheating the coal particles which could lead to the agglomeration of the coal and/or to the devolatilization of the coal and the loss of evolved gases. In order to make the patented system function properly it is necessary that the coal slurry be rapidly admixed with the steam while being simultaneously formed into droplets of a size which will permit the necessary rapid drying. Inasmuch as the atomizer of the present invention is to be utilized in the system described and claimed in assignee's aforementioned patent, the patent teachings are incorporated herein by reference.