Increasing the Fine Particulate Collection Efficiency of Existing Scrubbers without Increasing Pressure Drop: All it Takes is Water

In most sewage sludge incinerator (SSI) applications, wet scrubbers remain the workhorse of the industry for the capture and collection of particulates. Wet scrubbers are also effective in removing many regulated heavy metal compounds and some acid gases. SSI scrubbing systems typically consist of a venturi scrubber followed by an impingement tray tower scrubber. To meet today’s MACT standards, SSI air pollution control systems must capture a major portion of the fine (<2.5 µm) particulates, designated as PM2.5, which are the most difficult to remove. Many SSI installations have installed some additional particulate removal equipment, such as a wet electrostatic precipitator (WESP) or a baghouse (BH), to improve PM2.5 capture, but these components increase system capital and operating costs, along with system pressure drop, power consumption and operational complexity. While it is well known that venturi scrubber collection efficiency can be improved by increasing pressure drop through the venturi throat, there are practical limitations to how much the pressure drop can be increased at an existing SSI installation without requiring major changes to other equipment, such as the Fluidizing Air Blower or Induced Draft Fan. More importantly, this practice would also increase system power consumption.The effects of water vapor condensation have long been known to improve the collection efficiency of particulate scrubbing systems. Scrubbing under condensing conditions may add one or more particle conditioning or collection mechanisms to the two primary ones (inertial and diffusional deposition) normally present in scrubbing. Further, if water vapor condensation can be induced upon the gas-borne particulates so as to increase their effective particle size, the collection efficiency due to inertial deposition can be increased. 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. Collection mechanisms that are enhanced by scrubbing under condensing conditions 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).For a variety of reasons, FF/C scrubbing has not been widely practiced in most industrial settings, as the conditions present in most field systems are not appropriate for the economical application of these theories. This is largely because the process conditions necessary for the successful application of FF/C scrubbing are extremely restrictive.For FF/C scrubbing to be practical (both effective and economical), the incoming flue gas stream should have a relatively high (>150°F) dew point and the site must have an abundant supply of low-cost scrubbing water and a means to treat the attendant scrubber drainage. Wet scrubbers on SSI systems at wastewater treatment plants (WWTPs) are one of the very few applications where all three of these criteria are satisfied.Based on thermodynamic models developed by the authors, this paper analyzes the performance of a typical SSI venturi/tray tower scrubber system and presents techniques and practices that can be used to apply FF/C scrubbing principles to improve particulate collection efficiency and to achieve minimum total water consumption. This paper presents examples of wet scrubbing system performance and conclusions based on evaluation of the thermodynamic effectiveness of venturi and tray tower equipment in SSI applications. Recommendations are made as to how to optimally distribute water flows between each scrubbing stage to achieve better performance without increasing system pressure drop.