Nozzles for gas scrubbing

Engineering consideration  - Wet and dry scrubbing

Wet scrubbing

As the name would suggest wet scrubbing involves saturating the gas flow with a spray.  The fluid being sprayed will react with the gas and remove the undesirable substances.  For example, ammonia being sprayed to remove NOx pollutants.  

When considering suitable nozzles fine atomisation is desirable as this increases the reactive surface area of the spray, but also reach and momentum of the spray is important especially if the gas flue is moving at a considerable velocity.  As smaller droplets have less momentum it is often the case that sprays consisting entirely of small droplets will get swept away by fast-moving gas flows and fail to reach certain areas.  For this reason, spiral nozzles, with their broad spectrum of droplets, are often useful.

Packed-bed scrubbers

A variant of wet scrubbers is the packed bed design. This type of system used a packed media bed filled with lots of small and geometrically complex-shaped media. The reacting liquid is sprayed on top of this media bed an trickles through it as the gas passes up through the bed. This increased the surface area of the liquid and the contact time between it and the gas.

With this type of scrubber, droplet size is not very important as we are not relying on spray atomisation to create the necessary surface area for reaction. Instead, we want spray nozzles that can evenly distribute fluid over the top of the media bed as quickly as possible. For this reason wide wide-angled spray nozzles like the TF spiral nozzles are often a good choice.  

Semi-dry scrubbers

In these scrubbers a wet slurry is sprayed into the gas flow and this reagent reacts with contaminants to form a solid so the gas flow remains dry. The most common type of this scrubber is flue gas desulphurisation where sulpher dioxide gas is removed by reacting a lime slurry with it to form gypsum. 

Dry scrubbing

A true dry scrubber will rely on a powdered reagent rather than a liquid to react with contaminants. Nozzles will still be used in many dry scrubbers to cool the gas to prevent heat damage to the scrubber and also to slow down the gas flow. As such, even in true dry scrubbers, spray nozzles are an important component in the system. To maintain the "dry" status of the scrubber the liquid used for cooling must be completely evaporated into the gas flow and not condense out when the gas cools. So tight control over droplet size (to speed up evaporation and heat transfer) and flow rate (to avoid over saturation) are required from the nozzle array.