Intermittent energization on electrostatic precipitators

It is generally accepted that dust resistivity in excess of $${10}^{11}\, \Omega {\rm cm}$$ leads to the formation of back corona and to a deterioration of ESP performance. Besides dust conditioning, the most popular approach for reducing problems with back corona is the application of pulsed corona systems. The idea behind pulsing is very simply to reduce the E-field within the dust layer by reducing the time-averaged current density. However, experience shows that other high resistivity cases are resolved successfully with an opposing approach, using especially smooth HV from three-phase rectifiers in combination with high current densities. A detailed investigation into this discrepancy has revealed that high values of dust resistivity can be traced back to at least two different mechanisms. “Standard” high resistivity dusts show the behaviour which is typical for dielectrics. High potential drop across the dust layer is produced by the injection and immobilization of charge carriers into the material. Resistivity is strongly dependent on time and corona polarity and inversely proportional to the root of current density. The dust layers reach a high level of space charge density. In a sample of an “untypical” high resistivity dust we found that resistivity is independent of layer thickness and of corona polarity and does not show time effects. The dust layer does not accumulate any significant amount of space charge. The potential drop across the dust layer is nearly independent from current density, resulting in a resistivity which is inversely proportional to current density. With very high E-fields, the dust layer shows a progressive increase of current uptake with time, ending in a breakthrough. All these are the typical characteristics of a varistor material. Our results show that different mechanisms of high dust resistivity do exist. Therefore an appropriate choice of the high voltage supply and its control should be based on a thorough analysis of the electrical properties of the dust. A clearly better separation efficiency will be achieved.

Effect of dust layer in electrostatic precipitators on discharge characteristics and particle removal

In this study, a flame-generated nm-range SiO2 aerosol (approx. 170 nm median aggregate diameter) is fed into an electrostatic precipitator with an operating temperature of 200 °C. While a highly porous layer of SiO2 nanoparticles (NPs) is deposited by electrostatic precipitation, a decrease of current uptake is observed initially, indicating exceptionally high values of the electric field within the layer (> 100 kV/mm) and of the layer resistivity (> 1013 Ω∙cm). Later a strong (13- to 17-fold) increase of current uptake is observed. Aerosol charge measurements show that charges of opposite polarity are emitted from the NP layer. Investigation of the NP layer by SEM shows that charge-emitting structures with a polarity-dependent morphology develop on an originally homogeneous NP layer. Based on the experimental evidence, the mechanisms of charge emission and structure formation are discussed. Charge emission from the precipitated dust layer is known as back corona in the field of electrostatic precipitation. It appears that the mechanisms of back corona observed with SiO2 NP layers are quite distinct from those observed with µm-range particles. While gas discharges inside the NP layer are suppressed due to small pore size, back corona inside the NP layers is apparently initiated by thermionic field emission of free electrons and secondary electron multiplication within the NP layer.

Electrostatic precipitators (ESP) serve as the primary air pollution control device for the majority of coal-fired utility boilers in the Eastern and Midwestern regions of the United States. Since most of these ESPs are collecting flyash generated from medium- and high-sulfur coal, they are not experiencing operational limitations which are common when treating high-resistivity particles and are performing at an efficiency that is as high as could be expected. However, there are indications that the collection efficiency could be improved with flue gas conditioning. Conditioning is commonly used for solving operational problems associated with high-resistivity dusts. The purpose of conditioning for low- and moderate-resistivity applications is to increase the adhesive characteristics of the dust. Flue gas conditioning that increases particle adhesion has the potential to improve collection efficiency because a large percentage of particulate emissions from a well-performing ESP is due to reentrainment. Improved ESP performance should result if particle reentrainment could be reduced by making the particles more adhesive. This could produce a significant reduction in emissions from and ESP from the Following mechanisms: reduced erosion-type reentrainment; reduced rapping emissions; reduced hopper reentrainment; increased agglomeration of fine particles.

Electrostatic precipitators (ESP) serve as the primary air pollution control device for the majority of coal-fired utility boilers in the Eastern and Midwestern regions of the United States. Since most of these ESPs are collecting flyash generated from medium- and high-sulfur coal, they are not experiencing operational limitations which are common when treating high-resistivity particles and are performing at an efficiency that is as high as could be expected. However, there are indications that the collection efficiency could be improved with flue gas conditioning. Conditioning is commonly used for solving operational problems associated with high-resistivity dusts. The purpose of conditioning for low- and moderate-resistivity applications is to increase the adhesive characteristics of the dust. Flue gas conditioning that increases particle adhesion has the potential to improve collection efficiency because a large percentage of particulate emissions from a well-performing ESP is due to reentrainment. Improved ESP performance should result if particle reentrainment could be reduced by making the particles more adhesive. This could produce a significant reduction in emissions from and ESP from the Following mechanisms: reduced erosion-type reentrainment; reduced rapping emissions; reduced hopper reentrainment; increased agglomeration of fine particles.

Characteristics of particulate matter from four coal–fired power plants with low–low temperature electrostatic precipitator in China

Dedusting unit is necessary in the circulating fluidized bed flue gas desulfurization (CFB-FGD) system, a semi-dry FGD technique which is widely used in air pollution control projects. The dedusting unit is located after the FGD absorber, to collect the desulfurization absorbent for recycle, and also to ensure to satisfy the dust emission standards. Electrostatic precipitator (ESP) is most popularly used in large-scale semi-dry FGD system. But the ESP operating conditions in FGD system are much different from those in general use, with high dust concentration, high humidity and high specific resistivity. According to the ESP operating cases in the semi-dry FGD projects, the effect factors on ESP performance are studied in this paper, such as dust specific resistivity, dust concentration, gas humidity, dust cleaning mode and power supply. The measures to improve the collection performance are also studied in this paper, from the stage of design, installation and operation.

Theory and application of cyclone impulse electrostatic precipitation

Experimental study on gaseous elemental mercury removal by wet electrostatic precipitators

Polarity dependence of applied voltage to deposition of charged particles for electrostatic precipitator

The Bechtel Confined Zone Dispersion (CZD) process for FGD retrofit situations was tested at two sites: one at a level of 5 MWe; the other at 70 MWe. The CZD process involves injecting a finely atomized slurry of hydrated lime into a straight run of duct between a boiler's air heater and its electrostatic precipitator (ESP). The effect of process variables on SO2 removal and ESP performance was investigated for dolomitic/calcitic lime. Removals of SO2 in excess of 50 percent were achieved for either lime type at the 5 MWe site. A very short duct length limited injection rate at the 70 MWe site, limiting sulfur removals to a maximum of 30 percent. SO2 removal data for both sites were successfully correlated on a common basis. ESP performance was not fully acceptable during lime injection at both sites, but it is felt that optimization of ESP operations should eliminate this problem. Additional testing is recommended to further explore ESP performance and to optimize lime injection parameters. The results obtained to date continue to indicate that the CZD process is an attractive and economical candidate for FGD retrofit situations.

Enhanced particle precipitation from flue gas containing ultrafine particles through precharging

The influence of design parameters on the occurrence of shielding in multi-electrode ESPs and its effect on performance

A study of aerosol filtration was conducted to improve U.S. Radionuclide Monitoring Station (RMS) performance for Nuclear Treaty Verification. The primary objectives of this study were to improve system operability and maintainability, reduce power consumption and operational cost, and reduce baseline radionuclide sensitivity. To meet these goals, Sandia National Laboratories (SNL) studied the performance of alternate filter materials and aerosol collection technologies that could be engineered into U.S. Radionuclide Monitoring Stations. Laboratory-scale filtration experiments were conducted with Filter Material 1, FM1 (current filter), Filter Material 2, FM2, and Filter Material 3, FM3. All three materials employ electrostatically charged filter fibers to enhance nanoparticle collection. FM2 and FM3 both had higher air permeability with respect to FM1 which is advantageous for high volume collection and power savings. Particle pre-charging, a well-established industrial technique used in electrostatic precipitators, was tested to see if electrostatically charging particles prior to filtration could enhance filter performance. We found that particle-pre-charging did enhance aerosol collection efficiencies for materials which would not have otherwise satisfied RMS performance requirements. Laboratory-scale testing indicated it may be possible to reduce the baseline radionuclide sensitivity to approximately 60% of its current value by increasing the volume of air sampled in 24 hours to 2.5 times the current air volume. Improvements to geolocation may also be possible with shorter air samples (e.g., 12 hours). A new methodology was developed at SNL to assess filter performance using established RMS certification procedures. We coined these tests “mid-scale” since they bridged the gap between laboratory-scale and full-scale RMS testing. Four filter specimens were drawn from the exact same atmospheric aerosol. Gamma spectroscopy was used to assess radiological backgrounds due to radon progeny and other naturally occurring radionuclides. Direct comparisons between the four filters allowed SNL to quantify the relative change in baseline sensitivity by altering air flow rate, filter material, and particle-pre charging. Mid-scale results agreed with laboratory-scale results: alterations to RMS configuration (filter, flow, and particle charge) may result in baseline sensitivities approximately 55-60% of their current values. Finally, an assessment of scalability was performed to determine if technical approaches used in laboratory-scale and mid-scale testing could be engineered into full-scale Radionuclide Monitoring Stations. Results suggested that particle-pre-charging is a viable technical approach if reductions in baseline sensitivity or power consumption are desired.

High resistivity dust is normally the major problem in electrostatic precipitation. Even though the corona current is only a fraction of a microampere per cm2 the resulting RI drop exceeds the breakdown voltage gradient of the dust layer. This breakdown or sparking through the collected dust causes the familiar back corona and premature sparkover. The Trielectrode Electrostatic Precipitator (TEP) is an effort to make precipitator performance insensitive to dust resistivity. The TEP (U. S. Patent 3,915,672) is based on the concept that the layer of collected dust behaves like capacitance and resistance in parallel so that the corona can occur in short pulses and yet maintain a moderately constant current through the dust layer. In between corona pulses the precipitating field is supplied by a third set of electrodes. Initial bench scale tests using high resistivity particulate indicate that the concept is sound. Recent testing has extended the effective operating range of the TEP for particulate resistivit...