Electrostatic precipitator with adaptive discharge electrode

An electrostatic precipitator having an adaptive discharge electrode is disclosed. In some embodiments, the discharge electrode may be formed of a non-ohmic material that exhibits a saturation velocity above a voltage threshold. The non-ohmic material may have a semiconductor with doping impurities or ceramics. In other embodiments, the discharge electrode is formed of an ohmic material characterized by increased resistance through the discharge electrode.

FIELD OF THE INVENTION

The invention relates generally to electrostatic precipitators for industrial use.

BACKGROUND ART

Electrostatic precipitators (“ESPs”) are commonly deployed in industrial applications to remove solid particles from gas flows by charging the particles and causing them to precipitate out of the gas flow. ESPs are useful in industrial and power generation applications to reduce pollution by collecting filterable dust or condensable particulate present in gasses. For example, ESPs are commonly used in fossil fuel power plants, oil and petrochemical refineries, cement plants, paper mills, various incinerators, industrial boilers, metallurgical processes, and other heavy industries to remove particulates from gas streams.

While there are multiple ESP geometries, discussed in further detail below, all ESPs have two primary components: a series of collecting electrodes and a series of discharge electrodes.FIG. 1depicts a typical prior art configuration of an ESP10used in power generation or major industrial applications. A number of large metallic collecting plates15are hung vertically and supported by at least two support members16. The plates15are spaced a set distance apart, with the spacing determined by the type of gas and particulates being cleaned. Typically, the plates15are anywhere from 9 to 16 inches apart. The plates15may be quite large, in some cases having heights exceeding 30 feet. Depending on the amount of particulate to be removed, additional plates15may be aligned behind the first row or field of plates15to create additional electric fields relative to the direction of gas flow. The collecting plates15are electrically grounded.

Between each pair of plates15is at least one discharge electrode wire or assembly20. Typically, there are multiple discharge electrode assemblies20. Where rigid discharge electrodes are used, as in the embodiment ofFIG. 1, each discharge electrode assembly20carries multiple discharge electrode points30. The discharge electrode assembly20may be a weighted wire or pipe and spike made of metal or other highly conductive material that carries a negative charge at a voltage above that necessary to achieve corona onset. A typical ESP may have thousands of discharge electrodes. When corona onset occurs (normally about 25 kV for 9″ gas passes), the gas around a discharge electrode and the particulates contained within it becomes ionized. The electrostatic field established between the discharge electrodes and collecting plates directs the negatively charged particles onto the grounded collecting plates.

FIGS. 2A, 2B, and 2Cdepict a typical prior art configuration for a discharge electrode assembly20known as a pipe-and-spike array. A metal pipe25passes vertically and halfway between two collecting plates15(as shown inFIG. 1). Each discharge electrode point30is a spike31arrayed horizontally about the pipe25. The spike31has a base33that is welded or otherwise secured to the pipe25. The body34of the spike extends out to a end or tip32. In some ESPs, a single spike31is directed in the upstream and downstream direction of the gas flow, such that each spike is parallel to the collecting plates15surrounding it, as depicted inFIG. 2C.

In other configurations, two spikes31are directed upstream and two spikes31are directed downstream. Each pair of spikes31may form a “V,” with each spike31directed slightly toward one of the two collecting plates15. In the “V-spike” configuration, depicted inFIG. 2Bas a cross-section, each spike31carries half the designed current capacity as compared to the single-spike configuration. Multiple sets of discharge electrode spikes30are spaced along the length of the metal pipe25, such that the entire cross-sectional area of gas40flowing past a pipe25can be ionized, carry charging current, and be scrubbed of particulates41. One set of spikes31is directed upstream, and the second set of spikes31is directed downstream. The size and angle of the spikes31is dependent upon the ESP's application and the gases40and particulates41composing the gas flow. For example, in an ESP for scrubbing gas40produced by an oil- or coal-fired boiler, the spikes31will be approximately 3 inches long and have a nominal diameter between ¼ inch and 3/16 inch. The base33of each spike31is welded to the pipe25. The end32of the spike31is a sharpened metallic point

Wire may also be used for a discharge electrode30in place of spikes. The wire may be round, square, twisted, barbed, or in other configurations. Round wire of 0.109″diameter is most common.

Other ESP configurations are also well-known and practiced to meet various design constraints. For example, in a vertical flow ESP, four collecting plates form a vertical, rectangular passage through which the gas flows. In this configuration the discharge electrode assembly has a single pipe dropped through the center of the vertical passage. Multiple spikes are arranged about the metal pipe at set distances. In this configuration, known as a “rod-and-star” array, the spike array for each discharge electrode is perpendicular to the gas flow. Multiple spikes may be arranged about a given point of the pipe.

As depicted inFIG. 3, during operation of a typical ESP, particulate-laden gas40is directed through the inlet region11, passes between the spaced collecting plates, known as gas passes15, and then exits through the outlet region12. The arrows represent the direction of gas flow, and the black dots represent the flow of electrons through the circuit. The discharge electrode assembly20is charged to a potential difference that causes the onset of negative coronal discharge and ionization of the gas40. The negatively charged discharge electrodes30and grounded collecting plates15produce an electrostatic field that electrostatically attracts negatively charged particles to the collecting plates15. During ionization, the gaseous atoms passing near the discharge electrodes30become ionized, as electrons associated with the atoms flow freely. Accordingly, the gas40becomes conductive. The negative ions in the gas40follow the field lines of the electrostatic field and flow toward the nearest collecting plate15. In so doing, they attach to particulates41carried by the gas40, which become charged and move to the collecting plates15as well. As the gas40flows through the gas passes15and past additional discharge electrodes20, particulates41build up on the plates15, forming a collected layer of ash that adheres to the plates15and is held there by clamping forces due to electrostatic pressure. The charging current incident on the ash layer is conducted through the ash layer to the grounded collection plate15. Periodically, a rapper raps the collecting plate15to loosen the collected ash layer by accelerating the plate. The separated ash layer then drops into a hopper or other collection device and is disposed of.

ESPs often exhibit sparking in the inlet field where particulate-laden gas begins flowing between the discharge and collecting electrodes. Electrostatic theory indicates that sparking occurs when small volumes of relatively clean gas is interposed between a discharge electrode and a collecting electrode. The resulting lack of particulates significantly reduces the space charge effect in this interelectrode space, which otherwise would be a relatively stable concentration of negatively charged particulate entrained in the area between the electrodes. This increases the magnitude of the electrostatic field at the surface of the collecting plate, which leads to a significant local increase in the intensity of the current discharge from the discharge electrode, which promotes spark initiation. Sparking collapses the electrostatic potential applied to the subject precipitator field, resulting in a temporary decrease of gas ionization and particle charging until the spark is quenched and the power supply is again brought up to voltage. This in turn significantly reduces the efficiency of the ESP.

While it is customary to use highly conductive metals to produce the make the and spikes of a discharge electrode assembly, metals are unable to resist the increased flow of current resulting from the increased gradient and strength of the electrostatic field that results in arcing. Most metals and metal alloys have a resistivity between 1-100 10−8ohm-meters, with very low dependence on temperature.

In addition to sparking caused by the non-uniform current density that results from varying space charge effects, warped collection plates result in a locally reduced distance between the discharge electrode and collection plate. This greatly reduces the allowable voltage that may be impressed on a discharge electrode array or in such a field before sparking is initiated. Warped collection plates result in significantly reduced efficiency and increased sparking.

Another issue in current ESPs concerns the efficiency of ESPs in applications having gas flows with high-resistivity dust and particles. Dust and particles exhibiting a collected layer resistivity in excess of 1*1012ohm-cm is considered highly resistive and is susceptible to both sparking and a phenomenon known as “back corona.” A back corona occurs when positive ions are generated by electrical breakdown internally within the collected ash layer. These positive ions migrate back towards the negatively charged discharge electrodes and can cause gas-borne particles to become positively charged or neutralized. The result is very high current flow and power dissipation within the ESP field, without proper dust charging or collection.

What is needed, then, is an ESP having individual electrodes capable of reducing sparking by locally limiting current density to a level that is supportable by the collected ash layer without sparking, while maintaining higher overall power supply and voltage and current.

SUMMARY OF THE INVENTION

In some aspects, the invention relates to a discharge electrode for use in an electrostatic precipitator and operating at an operating voltage and having a base configured to receive electrical current, a body formed of a material comprising a non-ohmic material, and a discharge tip, where the discharge electrode has a resistance of at least 100 megohms at the operating voltage.

In other aspects, the invention relates to an electrostatic precipitator having a collecting electrode, and a discharge electrode having a body and a discharge tip, where a material forming the body comprises a non-ohmic material.

In still other aspects, the invention relates to a discharge electrode for use in an electrostatic precipitator and operating at an operating voltage, the discharge electrode having a base configured to receive electrical current, a body formed of a material comprising an ohmic material, and a discharge tip, where the discharge electrode has a resistance of at least 100 megohms at the operating voltage.

In still other aspects, the invention relates to an electrostatic precipitator having a collecting electrode, and a discharge electrode having a body formed of a material comprising a doping impurity, where the resistance of the body is determined by the concentration of a doping impurity.

DETAILED DESCRIPTION

Described herein is a configuration for an adaptive discharge electrode that increases the efficiency of the ESP and reduces sparking. While the typical metals used to create a discharge electrode exhibit very low resistivity, other materials such as semiconductors have a higher resistivity, although not so high that the material is effectively an insulator. The resistivity of semiconductors is heavily dependent on the introduction of impurities into the material, a process known as doping. See Table 1 below for a listing of common metals and semiconductors and their resistivity.

The resistivity of a particular material, electrode, or other purely resistive material at varying voltages may be depicted as a V-I curve on a graph plotting current flow versus voltage. Current, voltage, and resistance are related according to Ohm's Law:
V=I*R

where V=Voltage (the potential difference across two contact points), I=current, and R=resistance. Ohm's Law is rearranged as I=V/R to plot a V-I curve. Accordingly, the slope at any given location along the curve is equal to 1/R. Materials having low resistivity exhibit a large slope on the V-I curve, whereas materials with high resistivity have a low slope.FIGS. 4A, 4B, and 4Cdepict the V-I curve of various materials. Metals, such as copper, have a very high slope, as copper provides practically no resistance to current flow over the length of a discharge electrode spike. Other materials, such as silicon, have a more moderate V-I slope. Elements and metal alloys typically have linear V-I curves, indicating that such materials have a constant resistivity. Materials characterized by constant resistivity with respect to current flow and voltage are known as “ohmic” materials.FIG. 4Ashows a V-I curve for a highly conductive ohmic material, such as copper.FIG. 4Bshows a V-I curve for a highly insulative ohmic material, such as most elemental nonmetal solids.

Some semiconductors exhibit non-linear resistivity, particularly when doped with certain impurities such as aluminum nitride or zinc oxide or produced with defects within the crystalline structure of the semiconductor. When a sufficiently strong electrical field is applied across this type of semiconductor, the electron drift velocity through the semiconductor reaches a maximum velocity (the saturation drift velocity), a state known as velocity saturation. Once the saturation drift velocity is reached, the current through the material remains relatively constant even as the voltage applied increases. Excess voltage is dissipated through the production of vibrational phonons, which vibrate the molecular structure and result in an increased the temperature of the material. Different impurities in a semiconductor can cause saturation at varying drift velocities. In particular, aluminum nitride and zinc oxide have been found to induce velocity saturation at current and voltage densities useful in ESPs. However, other semiconductor materials and doping impurities may also be used. For the resulting V-I curve, the curve flattens out as the current reaches a the limit determined by the saturation velocity, resulting in an asymptotic curve as depicted inFIG. 4C. Materials exhibiting these non-linear curves are “non-ohmic” materials and almost always semiconductors.

To reduce sparking, a discharge electrode30may be made of ohmic materials having moderate resistivity, such as certain semiconductor materials, or alternatively made of non-ohmic materials exhibiting asymptotic V-I curves, with the potential to completely eliminate sparking. In some embodiments using a pipe-and-spike array for example, the body34of the spike31may be made from these materials. This embodiment is depicted inFIG. 5.

In an alternative embodiment, the pipe25, spike tip32, and spike base33may be made of metals or metallic alloys, whereas just the body34of the spike31is lightly doped with atoms of semiconductors or doping impurities to produce a material that exhibits a significantly higher resistivity than metal.

In any embodiment characterized by a metallic pipe25, spike tip32, or spike base33, all components, except the discharge tip of the spike must be coated in an insulative material26. The insulative coating26eliminates the parallel electrical path represented by surface contamination, protects the pipe25and spike31from the corrosive effects of the gas40being scrubbed, and directs the electric current through the spike31.

In another embodiment, impurities used to dope semiconductors, such as zinc oxide or aluminum nitride, may be used to decrease the resistivity of a spike31or spike body34that is formed of an insulating material bonded with a metal. For example, as depicted inFIG. 6, the spike31may be formed of metal with a thin chip37of insulative material doped with zinc oxide, aluminum nitride, or some other doping impurity. For example, a chip37composed of zinc oxide may be only 10-20 microns thick to achieve the desired resistivity and saturation velocity. The concentration of impurities introduced into the body34may be proportional to the decreased resistivity across the chip37. In typical sparking conditions, when voltage increases due to the breakdown of space charge between the discharge electrode and collecting electrode, the current that can pass through the chip37is limited to a maximum amperage, as described above, and inhibits sparking.

In another embodiment, the entire spike body34may be composed of ohmic or non-ohmic poorly conducive materials having a resistivity substantially greater than metals, such as aluminum nitride, zinc oxide, or lightly doped semiconductors or ceramics. In this embodiment, rather than inserting a chip, the doping impurities would be introduced throughout the spike body34. The particular concentration of doping impurities would vary to achieve the desired resistivity and saturation velocity.

By fashioning a spike31or spike body34from materials exhibiting moderate or non-linear V-I curves, sparking and arcing in an ESP may be reduced or eliminated. When pockets of clean gas40are interposed between the discharge electrode30and collecting plate15and result in the increased electrostatic field strength, as described above, the current flow is expected to increase and result in a spark. However, by using semiconductor materials, or doping principally ceramic spikes31to become semiconductor materials, current flow is decreased through the spike31when compared to other spikes31in a given field, and into the gas40. This reduces sparking activity without having to directly control the voltage impressed on all discharge electrodes in a field, and the resulting current flow into each individual discharge electrode. Instead, the inherent resistance of the discharge electrode30will produce a voltage drop between the metal pipe25and the spike tip33of the discharge electrode30. The voltage drop will correspondingly decrease the current density, limiting current flow in the gas40flowing to the collecting plate15. By creating this voltage drop across the discharge electrode30, sparking is locally reduced or eliminated, depending on the resistive characteristics of the electrode and the ash layer.

Additionally, the use of the adaptive discharge electrode disclosed above can also locally limit the current density in areas where a warped collection plate results in reduced distances between the discharge electrodes and the collection plates. Warped collection plates significantly reduce the allowable impressed voltage limit before sparking occurs. Because the adaptive discharge electrode will increase resistivity when voltage increases, thereby limiting the current density, localized sparking due to warped collection plates can be reduced while the remainder of the electric field remains fully energized.

Yet another benefit is the optimization of current densities in applications involving highly resistive dust and the resulting back corona effect. During back corona, positive ions flow back from the collected dust layer towards the discharge electrode and interfere with the negative ion drift, resulting in very high current flow and low collection efficiency. By locally limiting current flows, the adaptive discharge electrode can maintain a relatively efficient level of current flow in localized areas where back corona may occur.

To achieve the necessary reduction in current to prevent or reduce sparking at an operating voltage of approximately 25 kV, the lowest measured internal resistance of the discharge electrode30should be at least 100 megohms (MΩ). For ESPs of various sizes and operating ranges, it is possible for the designed internal resistance, at saturation, of the discharge electrode30to meet or exceed 3000 MΩ.

The precise resistivity desired will depend on the particular application for which the ESP is used, and the local conditions, including the types of gases and particulates used, the operating temperature, and other factors known to those in the art. An example is provided to demonstrate how the particular resistivity for a given application may be determined.

A hypothetical ESP is used with gases produced from coal and oil boilers. The configuration is assumed to include the use of hanging vertical collecting plates15with multiple pipe-and-spike arrays for discharge electrode assemblies20arranged in the “V-spike” configuration, as depicted inFIG. 1. The gas pass width between each plate is 12 inches (30.5 cm). The collective plate area assumed to be covered by each tip will be 100 cm2(15.5 sq. in.) The allowable current density without sparking for this configuration is assumed to be 20 nanoamperes per cm2(20 nA/cm2). The operating temperature of the flue gas is assumed to be approximately 300° F. Under these conditions, the ESP will produce a current flow of approximate 10 microamperes (μA) through each discharge electrode30. The desired operating range of the ESP will be between 1 and 10 μA. To produce the necessary voltage drop across the discharge electrode30at the lower current flow rate, the spike ends33will need to exhibit a total resistance at or below 200 MΩ. At the high current flow rate, the individual spikes30will limit excess current by exhibiting a resistance of upwards of 3,000 MΩ.