page_title: Electrostatic precipitator
section_title: Low resistivity
hierarchical_section_title: Electrostatic precipitator / Collection efficiency (R) / Low resistivity
caption_attribution_description: English: Resistivity modeled as a function of temperature and sulfuric acid vapor (SO3) concentrations. Typical low sulfur eastern coal with moderate iron content in the ash.
context_page_description: An electrostatic precipitator is a filtration device that removes fine particles, like dust and smoke, from a flowing gas using the force of an induced electrostatic charge minimally impeding the flow of gases through the unit.
  In contrast to wet scrubbers, which apply energy directly to the flowing fluid medium, an ESP applies energy only to the particulate matter being collected and therefore is very efficient in its consumption of energy.
context_section_description: Particles that have low resistivity are difficult to collect because they are easily charged (very conductive) and rapidly lose their charge on arrival at the collection electrode. The particles take on the charge of the collection electrode, bounce off the plates, and become re-entrained in the gas stream. Thus, attractive and repulsive electrical forces that are normally at work at normal and higher resistivities are lacking, and the binding forces to the plate are considerably lessened. Examples of low-resistivity dusts are unburned carbon in fly ash and carbon black.
  If these conductive particles are coarse, they can be removed upstream of the precipitator by using a device such as a cyclone mechanical collector.
  The addition of liquid ammonia (NH
  ₃) into the gas stream as a conditioning agent has found wide use in recent years. It is theorized that ammonia reacts with H
  ₂SO
  ₄ contained in the flue gas to form an ammonium sulfate compound that increases the cohesivity of the dust.  This additional cohesivity makes up for the loss of electrical attraction forces.
  The table below summarizes the characteristics associated with low, normal and high resistivity dusts.
  The moisture content of the flue gas stream also affects particle resistivity. Increasing the moisture content of the gas stream by spraying water or injecting steam into the duct work preceding the ESP lowers the resistivity. In both temperature adjustment and moisture conditioning, one must maintain gas conditions above the dew point to prevent corrosion problems in the ESP or downstream equipment.  The figure to the right shows the effect of temperature and moisture on the resistivity of a cement dust. As the percentage of moisture in the gas stream increases from 6 to 20%, the resistivity of the dust dramatically decreases. Also, raising or lowering the temperature can decrease cement dust resistivity for all the moisture percentages represented.
  The presence of SO
  ₃ in the gas stream has been shown to favor the electrostatic precipitation process when problems with high resistivity occur. Most of the sulfur content in the coal burned for combustion sources converts to SO
  ₂. However, approximately 1% of the sulfur converts to SO
  ₃. The amount of SO
  ₃ in the flue gas normally increases with increasing sulfur content of the coal. The resistivity of the particles decreases as the sulfur content of the coal increases.
  Other conditioning agents, such as sulfuric acid, ammonia, sodium chloride, and soda ash (sometimes as raw trona), have also been used to reduce particle resistivity.  Therefore, the chemical composition of the flue gas stream is important with regard to the resistivity of the particles to be collected in the ESP.  The table below lists various conditioning agents and their mechanisms of operation.
  If injection of ammonium sulfate occurs at a temperature greater than about 600 °F (320 °C), dissociation into ammonia and sulfur trioxide results.  Depending on the ash, SO
  ₂ may preferentially interact with fly ash as SO
  ₃ conditioning.  The remainder recombines with ammonia to add to the space charge as well as increase cohesiveness of the ash.
  More recently, it has been recognized that a major reason for loss of efficiency of the electrostatic precipitator is due to particle buildup on the charging wires in addition to the collection plates (Davidson and McKinney, 1998).  This is easily remedied by making sure that the wires themselves are cleaned at the same time that the collecting plates are cleaned.
  Sulfuric acid vapor (SO
  ₃) enhances the effects of water vapor on surface conduction.  It is physically adsorbed within the layer of moisture on the particle  surfaces.  The effects of relatively small amounts of acid vapor can be seen in the figure below and to the right.
  The inherent resistivity of the sample at 300 °F (150 °C) is 5×10¹² ohm-cm. An equilibrium concentration of just 1.9 ppm sulfuric acid vapor lowers that value to about 7 x 10⁹ ohm-cm.
caption: Resistivity Modeled As A Function of Environmental Conditions - Especially Sulfuric Acid Vapor
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