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Institute
Vortrag “ICESP XIV. International Conference on Electrostatic Precipitation 2016”, Wroclaw, Polen, 19.-23.09.2016
Back corona and dust resistivity are well known topics in electrostatic precipitation. Back corona results from high dust resistivity and occurs when the field strength in the dust layer surpasses a critical value E crit, which is typically found to be in the order of 15 to 30 kV/cm. Besides the resistivity ρ, the current density is the main parameter: E = i ρ < E crit. Hence, possible actions against back corona include a reduction of ρ by dust conditioning, or a reduction of i, for example by pulsed corona operation.
Much work has been devoted to study the dependence of ρ on dust composition, temperature, humidity, adsorption layers and dust layer porosity, and a variety of different set-ups for dust resistivity measurements has been proposed. Even though some authors report a dependence of ρ on current density or field strength resp., dust resistivity is mostly seen as a material property.
However, in measurements on product dusts in the upper resistivity range, we found that dusts show extremely non-ohmic properties:
Most strikingly, resistivity may vary by several orders of magnitude with time.
Also, the experimental arrangement can change the resistivity results by orders of magnitude: When resistivity is measured with the dust layer exposed to a corona discharge (imitating the situation in a real ESP), ρ depends strongly on the layer thickness. Meanwhile the same dust does not show the strong layer thickness dependence when electrodes are placed on both sides of the dust layer, and also time effects are much less prominent.
Additional findings include that dusts that have been exposed to resistivity measurements show a high level of electrostatic charging afterwards.
An interpretation of these results may be found from the theory of semiconductors and electret materials. Accordingly, highly resistive dust layers do not contain mobile electrons. Current transport occurs only after charge carriers (free electrons or holes) have been injected from the electrodes or from a gas discharge adjacent to the surface.
When the injection process is non-symmetric (e.g. with corona discharge), the current transport through the dust layer can be dominated by charge carriers of one polarity, depending on the polarity of the corona. Some of the effects observed in experiment, namely the layer thickness effect and the current density influence, can indeed be ascribed to unipolar injection and space charge limited conduction.
The time effects can mainly be ascribed to the “deep trapping” of the charge carriers, leading to a high level of immobile space charge. As overall space charge is limited, this reduces the level of mobile space charge available for current transport. The space charge also leads to a deformation of the electric field inside the dust layer, and hence influences injection via the Schottky (thermionic field emission) mechanism.
By integrating the mechanisms of charge transport in dielectrics (injection, drift, immobilization, recombination) into the OpenFOAM® simulation package, rather realistic simulations of time-dependent resistivity are possible. The simulations also are helpful to identify the mechanisms responsible for certain types of resistivity behavior.
Dieser Tagungsband umfasst Beiträge, die im Rahmen des 5. Forums Agroforstsysteme, welches am 30. November und 1. Dezember in der Kaiserkrone zu Senftenberg (OT Brieske) stattfand, in Form von Vorträgen oder Postern präsentiert bzw. vorgestellt worden. Das Forum fand unter der Schirmherrschaft der Stadt Senftenberg statt und wurde durch die Innovationsgruppe AUFWERTEN (vom BMBF gefördertes Forschungsprojekt; Förderkennzeichen: 033L129 AN; www.agroforst-info.de) sowie durch das Ministerium für Ländliche Entwicklung, Umwelt und Landwirtschaft des Landes Brandenburg veranstaltet.
Das Forum stand unter der Überschrift „Bäume in der Land(wirt)schaft – von der Theorie in die Praxis“. Vor diesem Hintergrund ist die inhaltliche Vielfalt der Beiträge dieses Tagungsbandes zu verstehen. Diese befassen sich zum Teil mit konkreten wissenschaftlichen Fragestellungen, beleuchten andererseits jedoch auch Aspekte, die vor allem für die Praxis von großer Relevanz sind. Zudem werden in einigen Beiträgen Forschungsprojekte vorgestellt oder Planungsansätze für Agroforstflächen beschrieben.
Dieser Tagungsband enthält eine Vielzahl von Vorträgen und Veröffentlichungen mit Bezug zu Energieeffizienz.