Critical Review Wet Electroscrubbers for State of the Art Gas Cleaning ANATOL JAWOREK,* ,† WAMADEVA BALACHANDRAN, ‡ ANDRZEJ KRUPA, † JANUSZ KULON, ‡,§ AND MARCIN LACKOWSKI † Institute of Fluid Flow Machinery, Polish Academy of Sciences, 80-952 Gdan ˜ sk, Fiszera 14, POLAND, Department of System Engineering, Brunel University, Uxbridge, MIDDX UB8 3PH, UK Current trends observed in air pollution control technology are closely related to the development of new, more efficient hybrid systems, i.e., those, which simultaneously utilize two or more physical mechanisms for dust or gaseous contaminants removal. These systems can operate more economically than conventional devices, especially in the removal of PM2.5 particles. The electrostatic scrubber (electroscrubber), discussed in this paper, is one of such types of devices, which combines advantages of electrostatic precipitators and inertial wet scrubbers, and removes many shortcomings inherent to both of these systems operating independently. The electroscrubber is a device in which Coulomb attraction or repulsion forces between electrically charged scrubbing droplets (collector) and dust particles are utilized for the removal of particles from a gas. Unlike wet electrostatic precipitators in which particles are precipitated only on the collection electrode, in electroscrubbers, the collection of dust particles takes place in the entire precipitator chamber. Compared to inertial scrubbers, the electroscrubbers can operate at lower droplet velocities, but the collection efficiency for a single droplet can be larger than 1. The paper reviews the state-of-the-art of wet electrostatic scrubbing (electroscrubbing) technique used for gas cleaning from dust or smoke particles. Three groups of problems are discussed: (1) The fundamental problems concerning the charged dust particle deposition on a charged collector, usually a drop, with a focus on different models describing the process. (2) The experimental works of fundamental importance to our knowledge referring to the scrubbing process, which can be used for validating the theory. (3) The laboratory demonstrations and industrial tests of different constructions of electroscrubbers designed for effective gas cleaning. The electroscrubber is not designed to replace wet or dry electrostatic precipitators but can be used as a complementary device following the last stage of conventional electrostatic precipitator, which helps to remove submicron particles. It was shown in the paper that a higher collection efficiency of an electroscrubber could be obtained for higher values of Coulomb number (i.e., higher electric charges on the collector and particle), and for a Stokes number lower than 5 (i.e., low particle- collector relative velocity). 1. Introduction Removal of particles smaller than a few micrometers from indoor or industrial gases presents a serious problem. Particles of this size, such as smoke, fine powders, or oil mist, which are usually hazardous to human health, are not easy to remove by conventional methods. Existing filters, cyclones, or inertial wet scrubbers, which employ inertial forces to remove particulate contaminants are ineffective in cleaning the gases from fine particles. This is because the motion of such particles is mainly governed by drag and molecular forces, and inertial force plays a diminishing role with decreasing particle size. Tighter fibrous filters can help in finer particles removal, but they operate at a high-pressure drop. Nozzle or Venturi scrubbers require liquid droplets of high velocity, but the pressure drop is also large in such devices. Water consumption in nozzle scrubbers is about 0.05 L/m 3 , and in Venturi scrubbers from 0.5 to 1.5 L/m 3 (1). Dry electrostatic precipitators use electrostatic forces, but charging of particles smaller than 1 μm is inefficient, and the collection efficiency sharply drops with decreasing particle size. The re-entrainment of fine particles from the collection electrode can also be observed (2 (chapter 22), 3, 4). Irrigated electrostatic precipitators can only partially solve the problem of particle re-entrainment. In such precipitators, the dust particles are charged similarly to conventional electrostatic precipitators, but the collection electrodes are washed instead of rapped. Washing removes problems with back-corona discharge, but the issue of fine particle charging still remains unsolved. Therefore, an effective control of particles in the size range from 0.01 to 2 μm, known in the literature as “Greenfield gap” (4), is still a great challenge for engineers. To solve these problems, wet electrostatic scrubber which combines advantages of dry and irrigated electrostatic precipitators, and conventional inertial scrubbers, was proposed by Penney (5). In electrostatic scrubbers, dust particles and scrubbing droplets are electrically charged to the same or opposite polarities. The charged droplets capture the oppositely charged dust particles due to Coulomb attraction forces, or, when the charges are of the same polarity, the particles and droplets are repelled to the chamber walls. Hereinafter in this paper, the scrubber using electro- static forces will be referred to as “electroscrubber” and the precipitation process as “electroscrubbing”. Since the wet electroscrubber is designed to remove fine particles, it cannot replace wet or dry electrostatic precipitators, but can be used as complementary device following the last stage of con- ventional electrostatic precipitators. * Corresponding author phone: +48 586995151; fax: +48 583416144; e-mail jaworek@imp.gda.pl. † Polish Academy of Sciences. ‡ Brunel University. § Present address: School of Electronics, University of Glamorgan, Pontypridd, Rhondda Cynon Taff, CF37 1DL, UK. 10.1021/es0605927 CCC: $33.50  2006 American Chemical Society VOL. 40, NO. 20, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 6197 Published on Web 09/19/2006