PDF《Sulphur burning optimization》

500 micron droplet into several equally sized droplets of

100 microns each, the surface area can be increased by almost 500%. However, this is not the way spray nozzles operate. They do not create 100% equally sized droplets. The sprayed volume comprises many different size droplets that make up the drop size spectrum. The volume median diameter is a standard way of characterizing the size droplets that a particular nozzle will produce. However, this number is not useful when dealing with mass transfer applications such as evaporation and combustion. A more useful number is the Sauter mean diameter (D32), which is a means of relating the volume to surface area of a single droplet to the total volume to total surface area of all of the droplets. Another important drop size parameter for applications where residence time is a concern, such as the gas moving through a combustion furnace, is the maximum drop size, since it is this droplet that will take the longest to evaporate or combust. One must account for it as well. When T comparing different spray nozzles, it is important to clarify which drop size parameter you are using. Figure

1 shows the cross section of a furnace and the spray coverage of a hydraulic and pneumatic nozzle. It can be seen in Figure 1a that the drop size from the hydraulic nozzle is large enough and the spray pattern opens up enough that wetting of the furnace bottom is a concern. None of the spray droplets shown in Figure 1b of the pneumatic spray nozzle impact the wall and it appears that all of the sprayed sulphur is more able to be volatized. Further analysis would show that all of the sulphur was converted prior to the furnace exit. Atomization is key and is the critical first step as the sulphur is injected into the combustion furnace. Knowing what happens to these spray droplets and how it affects the furnace operation can be enhanced with CFD. Figure

2 shows three different furnace profiles. Figure 2a tracks the combustion gas as it moves through the furnace. This shows what it looks like without any sulphur injected and can provide valuable information about turbulent spots and low velocity areas. This information can then be used to analyze the spray gun placement. Figure 2b is the temperature profile with the spray guns turned on. And Figure 2c shows the particle tracking of the sulphur itself. All of these can be used to compare actual performance with any maintenance issues or in conjunction with studies to optimize performance of the furnace. Getting to a solution is typically quicker and less costly than repeated online tests, and benchmarks can be set for future analysis as well. Another problem that producers may encounter is spray nozzle turndown. Turndown of the nozzle refers to the effective operating range of the nozzle, or the ability to turn down the flow rate from peak flow conditions to low flow conditions. Proper atomization and consistent performance is required during start- up, low flow operation, as well as peak sulphur throughput. Methods used to adjust the flow rate are to use multiple sulphur guns or to adjust the operating pressure of the individual nozzles, or a combination of both. When changing the pressure to obtain different flow rates, it is important to realize that the performance of the respective spray nozzle changes as well. For instance, with a hydraulic nozzle, as you decrease the pressure in order to decrease the flow rate, drop size increases and the spray pattern or coverage collapses. Changes occur with air atomizing nozzles as well;

however, the changes are more subtle. This is due to the ability to alter the atomizing air pressure along with the sulphur feed pressure in order to help maintain a more consistent performance across a wider range of flow rates. Figure