Wet scrubbers enhance plant safety by offering inherent resistance to fire and explosion and tolerance to high moisture and/or high temperature gas streams. This is accomplished in a compact package with small overall footprint.
This article will address the causes of excess emissions from particulate scrubbers and potential corrective solutions. A later article will examine excess gaseous emissions from wet scrubbers.
The application of scrubber technology was very popular in the mid to late 20th century. When energy costs increased and more emphasis was placed on regulation of fine particulate, such as the current rules regarding PM10 and PM2.5 emission levels, scrubbers became less common. Environmental protection regulations for waterways and aquifers also proved to be barriers to use.
There are significant numbers of wet particulate scrubbers in operation with new projects being introduced that leverage the advantages of the technology. They are popular at plant sites where capturing dust in a water suspension offers material handling advantages for end uses. In cases with wet gas streams, scrubbers or wet electrostatic precipitators are necessary components of a correct capture strategy.
If an existing scrubber has been in compliance, but is currently exhibiting excess emissions OR the scrubber is only marginally out of compliance, a thorough inspection/ assessment is recommended. Trained and safety-certified personnel can determine if the scrubber can be brought back into compliance. Propelry experienced suppliers can also provide the designs, upgrades/repairs to the scrubber, monitoring and control devices and the maintenance services to reach that goal.
Wet scrubbers can be described as containing two main stages. The first is the “capture” stage, where the dust particles are “wetted” by contact with water droplets and/or a water bath, or through nucleation, so that the particles become contained in water droplets.
The second stage is “demisting”, where the droplets are removed from the gas stream and flushed by the wash water, so they eventually exit via a drain. In most scrubbers, the wash water is given some primary treatment to remove excess particulate. Some wash water is disposed of (blowdown) to maintain acceptable levels of dissolved and suspended solids, and fresh water is added to replace that lost in evaporation and blowdown. The wash water is constantly recycled to the scrubber to repeat the capture process and flushing of internal surfaces.
Among the causes of reduced capture efficiency with a scrubber is “fouling”, also called plugging and scaling. The reuse of wash water with even moderate levels of suspended and dissolved solids means that these solids will eventually buildup on the scrubber’s internal surfaces, in spray nozzles, and in wash water supply piping. Such buildup affects both the capture stage and demisting efficiency. Removing the fouling, improving the wash water treatment, and adding sensor equipment to monitor/warn of fouling will address this cause of excess emissions.
Another cause of diminished performance is in-leakage of excess air, over burdening the scrubber system. This is most often associated with corrosion of the scrubber or the connecting ducting. Corrosion is an inherent problem with scrubbers, in the wet environment and especially at wet-dry interfaces. Repairs, including changing to corrosion-resistant materials of construction or linings will remove the in-leakage and return the scrubber to original performance.
Inspection of the particular scrubber, its water circuit, its controls and its inlet gas conditions is critical to understanding what has led to emission performance decline or increase in pressure drop. The assessment based on this evaluation will guide the approach to solving the problem.
When existing scrubbers are emitting slightly above compliance levels, upgrades to the capture stage, demisting stage or both can bring the scrubber into compliance. Adding equipment to reduce particulate load on the scrubber, adding a preliminary stage to the scrubber to improve “wetting” of the particulate, mechanical changes to the capture stage to improve its performance and increasing wash water supply will improve capture.
Many existing scrubbers had more emphasis placed on the initial capture of the particles than on demisting. It is common for the demister component to be a simple vertical tube with a tangential gas entry from the capture stage. The inlet arrangement is designed to impart cyclonic flow to the gas and thus increase its contact with the inner, and hopefully flushed, wall of the tube. Adding enhanced demisting internals to the existing demisting section and keeping the internals clean will improve the collection of the droplets in the gas stream and the particulate captured within the droplets.
In some cases, scrubber upgrades will require additional pressure drop and/or additional water supply. Wash water pumps and main fan will require upgrades to accommodate these changes. Upgrades are a cost effective alternative in comparison to the design, permitting and installation of a completely new and different air pollution control devices.
Contact Nederman MikroPul if your wet particulate scrubbers are suffering from excess emissions or other operating difficulties.