The present invention relates in general to atomizers, and in particular, to a new and useful slurry atomizer which mixes together and atomizes gaseous, liquid and solid components, while having good wear resistance and good resistance to pluggage.
Several types of atomizers are known. A text which discusses atomization as it is applied to spray drying is Spray Drying Handbook, Third Edition, K. Masters, published in 1979 by George Godwin Limited, London, England, especially chapter 6 thereof.
Atomizers for combining liquid with a high velocity gas stream have been known for many years. U.S. Pat. No. 1,373,525 to Payne discloses an atomizer for an oil burner which has a cylindrical mixing chamber that carries a supply of air and into which oil can be introduced by obliquely extending pipes. U.S. Pat. No. 2,931,580 to Johnson discloses the use of a small mixing chamber where air and fluid are brought together. A relatively long pipe carries the mixture to a nozzle. U.S. Pat. No. 3,010,660 to Barrett discloses an atomizer for use in a snow making machine where a length of pipe carries a water-air mixture to a nozzle.
U.S. Pat. No. 3,623,669 to Woods teaches the confluence of two fluid streams and one gas stream into a chamber for discharge through a disposable nozzle which is particularly suited for spraying paints or chemicals.
U.S. Pat. No. 3,712,681 to Marino et al teaches that a slurry can be propelled by pressurized water and air, through a pipe to a discharge nozzle. The length of the pipe is specified as being from one to ten feet long.
Liquid fuel atomizers for atomizing fuel oil and the like are known from U.S. Pat. Nos. 3,650,476 to Rackley et al and 3,929,290 to Tallarovic, assigned to the same assignee as that of the present invention.
Both of these patents teach the use of a central supply tube which is supplied with one of the fluids to be combined by the atomizer. The central tube is surrounded by an outer tube which defines an annular space with the central tube. This space conveys the other of the two fluids to be combined. Near the end of the tubes, a mixing chamber is formed where the two fluids are combined at a high velocity. A cap covers the mixing chamber. Apertures through the cap provide passages for the exiting atomized spray. Neither of these references, however, teach the use of an elongated cylindrical mixing chamber where the two fluids are combined together at a critical distance from the cap.
Prior art atomizers are known which have been used in the petrochemical industry to atomize pitch containing carbon particles up to 5/16 inch diameter utilizing a central mixing chamber into which atomizing steam is injected. An example of such an atomizer is shown in FIG. 1A.
A process involving the dry scrubbing of stack gases for the removal of sulphur dioxide requires atomizers for spraying a slurry of alkaline reactants into the stack gases. The assignee of the present application, having had considerable experience with atomizers for liquid fuels, has applied this technology to the atomization of alkaline reactants for the dry scrubbing of stack gases. The atomization of a slurry having both liquid and solid granular components, poses unique problems, however. Four technical papers presented by Babcock and Wilcox discuss the evolution of the dry scrubbing process and the use of specially designed atomizers for that process. The first of these entitled "DRY SCRUBBING ELIMINATES WET SLUDGE", by Hurst, presented Oct. 7-11, 1979 to Joint Power Generation Conference, Charlotte, N.C., discusses a demonstration plant which was built and operated using a Y-jet atomizer which was originally designed for liquid fuel. The paper entitled "CONTROL OF SO.sub.2 EMISSIONS BY DRY SCRUBBING" by Downs et al, presented Apr. 21-23, 1980, to American Power Conference, Chicago, Illinois, discusses some drawbacks of the atomizers used at that time for the dry scrubbing process. "DRY SCRUBBER DEMONSTRATION PLANT - OPERATING RESULTS", by Hurst et al, presented Oct. 28-31, 1980, to EPA Symposium on Flue Gas Desulfurization, Houston, Texas, discloses performance of the dry scrubbing process utilizing atomized slurry. "DRY SO.sub.2 SYSTEM DESIGN IN EARLY OPERATING EXPERIENCE AT BASIN ELECTRIC's LARAMIE RIVER STATION", by Anderson et al, presented Oct. 3-6, 1982, to Thirty-second Canadian Conference, Vancouver, British Columbia, Canada, discloses details of the flue gas distributer structure used around the atomizing nozzle for the dry scrubbing process. Finally a paper entitled "COMPARISON OF DRY SCRUBBING OPERATION OF LARAMIE RIVER AND CRAIG STATIONS", by Doyle et al, presented Nov. 16-21, 1986 to Symposium on Flue Gas Desulfurization, Atlanta, Georgia, describes two commercially operating dry sulfur removal (DSR) systems in which the present invention (as described on page 2 and in FIG. 4 thereof) has successfully operated to provide superior atomization while reducing pluggage potential, enhancing the operation of the DSR units.
In general, dual fluid atomizers can be sub-divided on the basis of location where the gas and liquid components are mixed together. An external mix dual fluid atomizer passes the gas and liquid streams through separate flow passages inside the atomizer. The two fluids are mixed together externally of the atomizer hardware by impinging jets of the two fluids against each other. An internal mix dual fluid atomizer mixes the fluids internally of the atomizer hardware and discharges them through a common flow passage.
When a liquid which is ladened with suspended solid particles (a slurry) is to be atomized, the choice of atomizer is limited by practical constraints. These constraints include flow capacity, the required size of droplets in the atomized spray, the size of the flow passages to pass the particles in the slurry, the physical durability of the atomizer parts, the sensitivity of the quality of the atomized spray to component dimensional changes, and commercially acceptable energy requirements to produce the atomized spray.
The atomizers disclosed in Rackley et al, and Tallarovic, identified above, are of the dual fluid internal mix type. These atomizer designs provide the finest droplets. They utilize a gaseous atomizing medium, such as air or steam, which is accelerated through a small diameter passage to establish a high velocity. The high velocity gas is mixed with the fluid, and the gas/fluid mixture is discharged through a flow passage as an atomized spray. The atomizer disclosed by Rackley et al is commonly referred to as the T-jet type. The slurry atomizers disclosed in the technical papers identified above, are commonly referred to as Y-jet atomizers. Slurry is sent through a central tube to a nozzle head having diverging nozzle passages. Air or steam is sent through an annular chamber around the central tube to entry ports in the nozzle passages positioned just downstream of the passage entry for the slurry. Up until now, the largest discharge port size that has been successfully tested is in the range of 0.1935 inches in diameter while the most consistent performance utilizes port sizes of only 0.1540 inches in diameter.
This limitation on port sizes lead to clogging problems. Atomization was poor in general and the wear rates were excessive, due to the highly abrasive properties of the slurry.
In response to these problems, the present inventors had designed an atomizer using discharge nozzles attached by a nine or ten foot barrel to a dual fluid mixing chamber, located at the opposite end. Problems in this design were also noted in the field, however. In particular, atomization was poor due to an inadvertent separation of the slurry. The slurry apparently became separated during its passage along the barrel from the rear mixing chamber to the nozzles located at the opposite end. The upwardly pointed nozzles produced extremely fine spray droplet sizes while the downwardly pointing nozzles had large droplet sizes which were unacceptable. Since droplet size is extremely important in dry scrubbing applications, and the desired droplet size is extremely small, in the range of 75-80 micrometers (75-80 millionths of a meter) mass median diameter or better, an atomizer design which had high capacity, fine atomization, and low energy requirements was needed.