How to Get More Out of Your Existing Wet Scrubber

In many existing incineration systems, wet scrubbers remain the workhorse for the capture and collection of particulates. Properly designed and operated wet scrubbers are also effective in removing many regulated heavy metal compounds and some acid gases. When venturi scrubbers are used to remove particulates, they are generally followed by an impingement tray or packed tower scrubber for additional acid gas collection and also to sub-cool the gases, thereby minimizing the horsepower required by the Induced Draft (I.D.) fan.To meet today’s MACT standards, wet scrubbers must capture a major portion of the fine (<2.5 µm) particulates, designated as PM2.5, which are the most difficult to capture. While existing installations have the option of installing additional particulate removal equipment, such as a wet electrostatic precipitator (WESP) or a baghouse (BH), to improve PM2.5 capture, the addition of these components will add significant capital cost and operational complexity. While it is well known (and documented) that venturi scrubber collection efficiency can be improved by increasing pressure drop through the venturi throat, the purpose of this paper is to present options for improving scrubber performance when a substantial increase in pressure drop cannot be accommodated with the existing I.D. fan.Although not as well-known as the benefits (inertial and diffusional deposition) of increased pressure drop, a substantial body of research exists that clearly demonstrates that water vapor condensation improves venturi scrubber collection efficiency. When water vapor condensation is induced upon the gas-borne particulates so as to increase their effective particle size, the collection efficiency due to inertial deposition will be increased. Additionally, scrubbing under condensing conditions adds one or more particle conditioning or collection mechanisms. This is analogous to the mechanism by which dispersing silver iodide into a rain cloud initiates the formation of rain drops on the silver iodide nuclei. These additional collection mechanisms include diffusiophoresis, thermophoresis and Stefan flow. These mechanisms have been individually researched over the past sixty years and were collectively referred to by Calvert et al. (1972) as “flux force/condensation” (FF/C).FF/C scrubbing has not been widely practiced, primarily because of the increased water usage required. However, in those cases where additional water can be made available, it can offer an attractive and economical advantage to replacing the existing I.D. fan or adding a WESP. On most venturi scrubbers, the throat section is flanged top and bottom, paving the way for addition of a reasonably priced replacement section, with features that will improve performance. However, before beginning the design of performance-improving changes to the venturi and tray or packed tower sections, it is necessary to completely understand their performance.Based on models developed by the authors, that include the simultaneous analysis of the thermodynamics, psychrometrics, and pressure drop characteristics of a scrubbing system, this paper analyzes the performance of a typical venturi/tray tower scrubber system and presents techniques and practices that can be used to: 1) apply FF/C scrubbing principles to existing scrubbers to improve particulate collection efficiency; and, 2) achieve maximum cooling benefit within the boundaries of an existing water supply.