Plasma-Catalytic Removal Of Formaldehyde From Atmospheric Pressure Gas Streams At Low Temperatures

Formaldehyde is a major indoor air pollutant and it is able to cause serious health disorders in residents. This work reports the removal of formaldehyde from gas streams via alumina-pellet-filled dielectric barrier discharge plasmas and plasma-catalytic oxidation of formaldehyde in gas streams via dielectric barrier discharges over Ag/CeO₂ pellets at atmospheric pressure have been investigated respectively. The primary destruction mechanisms of formaldehyde have been discussed. The main results presented in the dissertation have been summarized as follows:1. In the experiment of the removal of formaldehyde from gas streams via alumina-pellet-filled dielectric barrier discharge plasmas at atmospheric pressure and 70℃, with a feed gas mixture of 140 ppm HCHO, 21.0% O₂, 1.0% H₂O in N₂,-92% of formaldehyde can be effectively destructed at GHSV (gas flow volume per hour per discharge volume) of 16500 h^-1 and E_in = 108 J L^-1, the ratio of CO and CO₂ in outlet gas streams is about 1:1. An increase in the specific surface area of the alumina pellets enhances the HCHO removal, and this indicates that the adsorbed HCHO species may have a lower C-H bond breakage energy. The formaldehyde removal efficiency increases with the decreasing inlet HCHO concentration or rising discharge applied voltage, the increase of O₂ content in the inlet gas stream is able to enhance the selectivity of CO₂. Based on an examination of the influence of gas composition on the removal efficiency, the primary destruction pathways, besides the reactions initiated by discharge-generated radicals, such as O, H, OH and HO₂, may include the consecutive dissociations of HCHO molecules and HCO radicals through their collisions with vibrationally- and electronically-excited metastable N₂ species..2. In the experiment of plasma-catalytic oxidation of formaldehyde in gas streams via dielectric barrier discharges over Ag/CeO₂ pellets at atmospheric pressure and 70 C, with a feed gas mixture of 276 ppm HCHO, 21.0% O₂, 1.0% H₂O in N₂, ⁹9% of formaldehyde can be effectively destructed with an 86% oxidative conversion into CO₂ at GHSV of 16500 h^-1 and input discharge energy density of 108 J L^-1. At the same experimental conditions, the conversion percentages of HCHO to CO₂ from pure plasma-induced oxidation (discharges over fused silica pellets) and from pure catalytic oxidation over Ag/CeO₂ (without discharges) are 6% and 33% only. The above results and the CO plasma-catalytic oxidation experiments imply that the plasma-generated short-lived gas phase radicals, such as O and HO₂, play important roles in the catalytic redox circles of Ag/CeO₂ to oxidize HCHO and CO to CO₂.3. The energy consumption of removal formaldehyde from gas streams by plasma has been studied in this work. HCHO removal efficiency and the specific energy consumption of removal formaldehyde from gas streams decrease with increasing inlet HCHO concentration or GHSV or the diameter of reactor. With a feed gas mixture of 276 ppm HCHO, 21.0% O₂, 1.0% H₂O in N₂, the the specific energy consumption of removal formaldehyde from gas streams is only 92eV at GHSV of 16500 h^-1 and E_in = 108 J L^-1 when the reactor is filled with Ag/CeO₂ pellet. When inlet HCHO concentration is increased to 965 ppm, HCHO removal efficiency is decreased to 63%, HCHO specific energy consumption per HCHO moleculer converted decrease to 42eV.