JV Task 117 - Impact of Lignite Properties on Powerspan's NOx Oxidation System

Tolbert, Scott ; Benson, Steven

University of North Dakota

关键词: 01 Coal, Lignite, And Peat;    36 Materials Science;    Oxidation;    Efficiency;    Scanning Electron Microscopy;   

DOI  :  10.2172/990811
RP-ID  :  None
RP-ID  :  FC26-98FT40321
RP-ID  :  990811

美国|英语

来源: UNT Digital Library

【 摘 要 】

Powerspan's multipollutant control process called electrocatalytic oxidation (ECO) technology is designed to simultaneously remove SO{sub 2}, NO{sub x}, PM{sub 2.5}, acid gases (such as hydrogen fluoride [HF], hydrochloric acid [HCl], and sulfur trioxide [SO{sub 3}]), Hg, and other metals from the flue gas of coal-fired power plants. The core of this technology is a dielectric barrier discharge reactor composed of cylindrical quartz electrodes residing in metal tubes. Electrical discharge through the flue gas, passing between the electrode and the tube, produces reactive O and OH radicals. The O and OH radicals react with flue gas components to oxidize NO to NO{sub 2} and HNO{sub 3} and a small portion of the SO{sub 2} to SO{sub 3} and H{sub 2}SO{sub 4}. The oxidized compounds are subsequently removed in a downstream scrubber and wet electrostatic precipitator. A challenging characteristic of selected North Dakota lignites is their high sodium content. During high-sodium lignite combustion and gas cooling, the sodium vaporizes and condenses to produce sodium- and sulfur-rich aerosols. Based on past work, it was hypothesized that the sodium aerosols would deposit on and react with the silica electrodes and react with the silica electrodes, resulting in the formation of sodium silicate. The deposit and reacted surface layer would then electrically alter the electrode, thus impacting its dielectric properties and NO{sub x} conversion capability. The purpose of this project was to determine the impact of lignite-derived flue gas containing sodium aerosols on Powerspan's dielectric barrier discharge (DBD) reactor with specific focus on the interaction with the quartz electrodes. Partners in the project were Minnkota Power Cooperative; Basin Electric Power Cooperative; Montana Dakota Utilities Co.; Minnesota Power; the North Dakota Industrial Commission, the Lignite Energy Council, and the Lignite Research Council; the Energy & Environmental Research Center (EERC); and the U.S. Department of Energy. An electrocatalytic oxidation (ECO) reactor slipstream system was designed by Powerspan and the EERC. The slipstream system was installed by the EERC at Minnkota Power Cooperative's Milton R. Young Station Unit 1 downstream of the electrostatic precipitator where the flue gas temperature ranged from 300 to 350 F. The system was commissioned on July 3, 2007, operated for 107 days, and then winterized upon completion of the testing campaign. Operational performance of the system was monitored, and data were archived for postprocessing. A pair of electrodes were extracted and replaced on a biweekly basis. Each pair of electrodes was shipped to Powerspan to determine NO conversion efficiency in Powerspan's laboratory reactor. Tested electrodes were then shipped to the EERC for scanning electron microscopy (SEM) and x-ray microanalysis. Measurement of NO{sub x} conversion online in operating the slipstream system was not possible because the nitric and sulfuric acid production by the DBD reactor results in conditioning corrosion challenges in the sample extraction system and NO measurement technologies. The operational observations, performance results, and lab testing showed that the system was adversely affected by accumulation of the aerosol materials on the electrode. NO{sub x} conversion by ash-covered electrodes was significantly reduced; however, with electrodes that were rinsed with water, the NOx conversion efficiency recovered to nearly that of a new electrode. In addition, the visual appearance of the electrode after washing did not show evidence of a cloudy reacted surface but appeared similar to an unexposed electrode. Examination of the electrodes using SEM x-ray microanalysis showed significant elemental sodium, sulfur, calcium, potassium, and silica in the ash coating the electrodes. There was no evidence of the reaction of the sodium with the silica electrodes to produce sodium silicate layers. All SEM images showed a clearly marked boundary between the ash and the silica. Sodium and sulfur are the main culprits in the fouling of the electrodes. Conclusions gained by this work are: (1) Sodium-rich aerosols and small ash particles accumulate and become bonded on the surface of the silica electrodes. (2) Ash accumulations adversely affected the NO{sub x} conversion. (3) The adverse impact occurs within a 2-week time period. (4) The ash accumulations are readily removed with a water wash, and the electrodes are not permanently affected by the ash. (5) The Powerspan ECO technology may prove to be viable if the ash accumulation can be kept to a minimum. Based on the results obtained in the work, it appears that the ECO technology has potential but is significantly impacted by the sodium-rich ash. Sodium reduction upstream of the reactor and aggressive ECO reactor cleaning methods are possible methods that will enable the ECO technology to maintain the expected 90% NOx removal efficiency.