Developments of the Plasma-Enhanced Electrostatic Precipitator for Mercury Removal in Offgas

Abstract

This paper presents the latest results of testing using a scaled up 28-standard liter per minute (slpm) [1-standard cubic foot per minute (scfm)] version of the recently designed and constructed plasma-enhanced electrostatic precipitator (PEESP). Previous results were published at bench scale. The PEESP technology injects a selected reagent gas mixture through a corona discharge generated by central electrodes. By adapting the electrode for reagent gas injection directly into the active zone, the area surrounding the sharp discharge points, chemical radicals are immediately formed from the absorption of ozone at the corona discharge points. Experimental evidence suggests that reactive species and radicals are formed that selectively oxidize the elemental mercury vapor, which forms water-soluble compounds and fine particulate that are absorbed by the condensing water droplets. In the electric field, the water droplets acquire a negative charge and are attracted to the grounded positive electrode (reactor wall) where they are removed from the system. The aqueous mercuric ions, thus formed, are collected on the electrode walls and removed as a liquid slurry. The PEESP has no moving parts, minimizing maintenance. The PEESP process is expected to be economical. The PEESP electrode assembly is intended to replace the existing electrostatic precipitator central electrode and use the existing power supply. While oxidized forms are largely removed by scrubbing systems, highly volatile elemental mercury is not. The only known effective control technology for mercury is carbon or other types of sorbent injection followed by a baghouse. Since the PEESP uses gases as a reagent to oxidize the elemental mercury, it can greatly minimize the issues of storage, handling, and associated hazardous waste disposal typical of solid injection systems for mercury removal.