EP2284442A2 - Electrostatic separator and heating system - Google Patents

Abstract

The precipitator (1) has a flow channel (3) comprising an inner space (5) through which a particle-containing exhaust gas flows in a flow direction (P). An electrode (6) is centrally arranged in the inner space of the channel, and forms a corona discharging zone by an electric field between the electrode and a channel wall (4). Discharge electrode sections (6a) are arranged at two positions along the electrode, where radial extensions of the electrode sections at one of the positions are same and differ from radial extensions of the sections at the other position. The discharge electrode sections are formed as discharge electrode top parts and/or discharge electrode edge parts. An independent claim is also included for a heating system comprising an electrostatic precipitator.

Description

The invention relates to an electrostatic precipitator, in particular for an exhaust pipe of an exhaust gas purification system, according to the preamble of claim 1.

Furthermore, the invention relates to a heating system for generating energy by means of combustion of an energy carrier with an electrostatic precipitator according to claim 7.

Due to emissions from heating systems and global efforts to reduce such emissions, heating systems use appropriate emission control systems. These are in particular to filter out the harmful substances and particles from exhaust gases, so that the remaining, purified exhaust gas can safely be released to the environment. In particular, such emission control systems are used in biomass heating systems, where in addition to otherwise economic and environmental benefits increased emissions of pollutants in the exhaust gases can occur. Especially the relatively high emission of particulate matter as a pollutant component is a problem in biomass heating systems.

From the EP 1 193 445 A2 An emission control system is known, which is used for biomass heating systems to reduce particulate matter emission. The device described therein can be installed in a flue gas channel and for this purpose has a lid which can be placed gas-tight on an associated opening on a flue gas channel. On the inside of the lid, a spray electrode, for example in the form of a tensioned wire or rod, is held over an insulating holder. A high-voltage transformer with rectifier function allows the construction of a high DC voltage between the spray and the lid, which is electrically connected to the furnace tube, so that it acts as a collector electrode.

Such an electrostatic filter with a spray electrode and a collector electrode is also known as an electrostatic precipitator. This is used for exhaust gas purification in an exhaust pipe of a heating system. It is through the spray, which runs approximately centrally through the exhaust pipe and therefore as the center electrode is designated, and a surrounding lateral surface of the exhaust pipe, a capacitor is formed, which is also referred to as a cylindrical capacitor in a cylindrical tube-shaped design of the exhaust pipe. The spray or center electrode generally has a circular cross section in the flow direction of the exhaust gas, wherein the diameter of the cross section or the radius of curvature is generally formed relatively small (for example, less than 0.4 mm). In order to deposit the pollutants, more precisely the particles of the exhaust gas which are not released into the environment, from the exhaust gas flow, an electrical field extending transversely to the direction of flow is formed by the center electrode and the collector electrode formed by the lateral surface with field lines from the center electrode to the collector electrode. For this purpose, a high voltage is applied to the center electrode, for example in the range of 15 kV. As a result, a corona discharge is formed, through which the particles flowing through the field in the exhaust gas are charged in a unipolar manner. Due to this charge, most of the particles migrate through the electrostatic Coulomb forces to the inner wall of the exhaust pipe, which serves as a collector electrode.

As mentioned above, the particles are electrostatically charged by the corona discharge which forms along the surface of the electrode. This is done at the molecular level by the following process: Is the electrode z. B. compared to the exhaust pipe to negative high voltage, so a large number of gas molecules is negatively charged. They move in the electric field applied by the electrode and the exhaust pipe in the direction of the exhaust pipe. If these meet on their way through the exhaust pipe to electrically neutral particles, they stick to these and charge the previously neutral particles also negative. The charged particles flow driven by electrostatic deflection forces to the inner wall of the exhaust pipe. Here the particles stick, lose their charge and are safely removed from the exhaust stream. This is the core process of an electrostatic precipitator and, depending on the geometry, height of the corona current, electrode shape, etc., leads to deposition rates of up to more than 90%.

The driving force of the corona discharge, which decisively influences the separation efficiency of the electrostatic fine dust filter (separator), is the strength of the electric field between the spray electrode and the collector electrode. The EP 1 193 445 A2 discloses a self-supporting rod electrode, this is made for example of a sheet metal strip produced by punching and has a plurality of protruding prongs. At these points, the electric field amplifies (concentrates) what the better Replacement of the electrons (corona discharge) is conducive. A separator with such a serration electrode therefore operates more effectively than one with a simple rod or wire electrode. As an alternative to the electrode design described above, a barbed wire-like electrode is also described.

The core process of electrostatic precipitation can be disturbed by the following effect: In the operation of electrostatic precipitators in exhaust gas contaminated with particulate matter, so-called corona quenching occurs. Corona quenching creates, if in the case of high particle concentration, about> 10 ¹⁴ particles / m ³ , the charged particles form a charge cloud, ie a space charge field, which surrounds the spray electrode. The charge cloud distorts the field distribution in the charger and weakens the electric field near the spray electrode. As a result, the field emission of free electrons from the surface of the spray wire is reduced, whereby less gas ions are available and consequently the charging of further particulate matter is reduced. At the same time, the movement of the charged fine dust particles towards the charging electrode is impaired. The charging of the dust particles does not extend to the entire cross section of the exhaust pipe, but takes place only in a small area around the spray around. The separation efficiency of the system drops accordingly. The importance of corona quenching for the separator operation can not or only insufficiently reduced by using the above-described wave electrodes, since the charge clouds also form in the vicinity of the teeth and isolate them.

The limited origin of new charge carriers is schematized as a cylinder (spatial view) or circle (plane cross-sectional view) can be represented around the spray electrode. Corona quenching is especially true for laminar flow fields, which are often present for the small flow rates of electrical precipitators.

The invention has for its object to provide an electrostatic precipitator, which is less prone to corona quenching.

Further, the invention has for its object to provide a heating system with a separator according to the invention, which guarantees reliable exhaust gas purification.

This is achieved by the objects with the features of claim 1 and of claim 7 according to the invention. Advantageous developments can be found in the dependent claims.

In the electrostatic precipitator according to the invention, in particular for an exhaust pipe of an exhaust gas purification system, with a flow channel having a channel wall and a channel inside, through which flows a particle-containing exhaust gas in a flow direction, and a centrally disposed in the channel interior, extending substantially in the direction of flow electrode to Formation of a corona discharge zone by means of an electric field between the electrode and the channel wall, wherein the electrode comprises means for generating at least one further corona discharge zone in the form of at least one of the electrode branching Srüelektrodenabschnittes, is provided to improve the separation efficiency of the electrostatic precipitator and to reduce the limiting influence of the corona quenching along the electrode are arranged at least two spray electrode sections at least two locations, wherein the radial Ers stretches (lengths in the radial direction between the spray electrode and the channel wall) of the spray electrode sections (6a) arranged at one location are substantially equal and different from the radial extensions (lengths) of the spray electrode sections (6a) located at another location. Thus, the geometry of such an electrode resembles that of a pine tree, in which the lengths of the branches (branch lengths of the various branch rings) decrease over the height.

In a further embodiment it is provided that the spray electrode section is designed as a spray electrode tip part and / or spray electrode edge part.

In yet another embodiment of the present invention it is provided that the spray electrode section branches off transversely, in particular radially, from the electrode.

An embodiment of the electrostatic precipitator further provides that the Sprühelektrodenabschnitt longitudinally, in particular axially branches off from the electrode.

A further exemplary embodiment of the present invention provides that the spray electrode sections are arranged distributed on the electrode in such a way that a uniform, in particular homogeneous arrangement of the conona discharge zones is realized.

Yet another embodiment of the present invention contemplates that the radial extents, e.g. the lengths of the Sprühelektrodenabschnitte are selected in the flow channel such that their free, the channel wall facing ends evenly distributed in adjacent cross-sectional areas, in particular annular areas of the flow channel, so that a uniform, in particular homogeneous arrangement of Cononaentladungszonen is realized over a flow channel cross-section.

The heating system according to the invention for generating thermal energy by burning an energy source such as biomass is characterized in that a fine dust emitting heating system such as a biomass heating system for burning the energy carrier, wherein particulate exhaust gases are formed, and an inventive electrostatic precipitator are provided.

• Die oben beschriebenen Mittel, insbesondere die sich daraus ergebende Geometrie und der Aufbau der Aufladungselektrode mit entsprechenden Verzweigungen unterschiedlicher Länge ermöglicht eine homogenere Verteilung des Aufladungsbereichs über den Querschnitt des Abgasrohres. Dadurch können trotz kurzer Reichweite der für die Aufladung zur Verfügung stehenden Gas-Ionen die Feinstaubpartikel in allen Querschnittsbereichen des Abgasrohres elektrisch geladen werden. Damit erhöht sich die Abscheideeffizienz des Systems auch in hoch kontaminierten Abgasströmen. Die Auswirkungen des Corona Quenching werden entsprechend aufgehoben.

With the electrostatic precipitator according to the invention and the heating system according to the invention, the following advantages are realized in particular:

• The means described above, in particular the resulting geometry and the structure of the charging electrode with corresponding branches of different lengths, allows a more homogeneous distribution of the charging region over the cross section of the exhaust pipe. As a result, the fine dust particles can be electrically charged in all cross-sectional areas of the exhaust pipe despite a short range of available for charging gas ions. This increases the separation efficiency of the system even in highly contaminated waste gas streams. The effects of corona quenching are reversed accordingly.

Fig. 1

schematisch in einem Längsquerschnitt und einer Draufsicht einen Ausschnitt eines elektrostatischen Abscheiders mit verzweigenden Sprühelektrodenabschnitten und

Fig. 2

schematisch zwei perspektivische Ansichten zweier Ausführungsbeispiele einer Elektrode mit mehreren Sprühelektrodenabschnitten.

The drawings illustrate several embodiments of the invention and show in the figures:

Fig. 1

schematically in a longitudinal cross section and a plan view of a section of an electrostatic precipitator with branching Sprühelektrodenabschnitten and

Fig. 2

schematically two perspective views of two embodiments of an electrode having a plurality of Sprühelektrodenabschnitten.

Fig. 1 shows schematically in a longitudinal section and a plan view (cross section) a section of an electrostatic precipitator 1 comprising an electrode 6 with branching spray electrode sections 6a. The electrostatic precipitator 1 is designed for a not shown here heating system for generating energy by burning an energy source such as biomass. In a preferred embodiment, the heating system comprises a heating system which is designed as a fine dust-emitting heating system such as a biomass heating system for burning a corresponding biomass energy carrier. In this combustion, particle-containing exhaust gases are produced, which are expelled through an exhaust pipe or an exhaust pipe 2. The electrostatic precipitator 1 is at least partially disposed in the exhaust pipe 2 of an exhaust gas purification system not shown here and includes a flow channel 3. The flow channel 3 is formed as a tubular portion of the exhaust pipe 2 and includes a channel wall 4 and a channel inside 5. flows through the flow channel 3 the particulate exhaust gas shown here by an arrow P in the flow direction also shown by the arrow P. In the interior 5 of the flow channel 3 extends in the flow direction P, the electrode 6, which is also referred to as a center electrode, spray electrode or corona electrode. The flow channel 3 is preferably formed in cross-section in the flow direction P rotationally symmetrical about a central axis (not shown here). The electrode 6 extends substantially along this central axis. The electrode 6 is in Fig. 1 formed in the section of the exhaust pipe 3 shown here vertically. The electrode 6 is fed via an electrode feed 7, which is covered with an insulator 8. Together with the channel wall 4, the electrode 6 forms a charging unit, in which particles can be charged electrically. For this purpose, the electrode 6 forms with the channel wall 4, applying a high voltage, an electric field whose field lines extend substantially radially to the electrode 6 and the channel wall 4, substantially transversely, more precisely at right angles to the flow direction P.

Between the spray electrode 6 and the exhaust pipe 2 formed as a precipitation electrode and surrounding the electrode 6, a high electric voltage is applied, whereby a corona discharge is formed at the spray electrode 6. In the electrostatic precipitator 1, free charge carriers in the form of free electrons and thus ionized gas molecules are injected into the charging region 9 by field emission or corona discharges at the spray electrode 6. The charge carriers then flow in the electric field according to their charge to the positive or negative Electrode. If the charge carriers hit dust particles, they are charged unipolar. A large part of the particles finally settles on the precipitation electrode and sticks there. Downstream of the charger also takes place a deposition of particles on the inner wall of the adjacent exhaust pipe 2 and a chimney, since the unipolar charged particles form a charge cloud 10 (shown here hatched around the Sprühelektrodenabschnitte) and flow by repulsion forces to the pipe wall 4.

A known technical difficulty in the operation of electrostatic precipitators 1 in highly contaminated with particulate matter exhaust gas is the so-called corona quenching. It arises if, in the case of high particle concentration (in particular greater than 10 ¹⁴ particles / m ³ ), the charged particles form the charge cloud 10 - also referred to as the space charge field - which surrounds the spray electrode 6. The charge cloud 10 distorts the field distribution and weakens the electric field in the vicinity of the spray electrode 6. This reduces the field emission of free electrons from the surface of the spray electrode, whereby less gas ions are available and consequently the charging of further particulate matter particles is reduced. At the same time, the movement of the charged fine dust particles to the collector electrode 4 (channel wall) is impaired. The charging of the dust particles no longer extends to the entire cross section of the exhaust pipe 2, but takes place only in a small area around the spray electrode 6 around. The separation efficiency of the system decreases accordingly when using a spray electrode according to the prior art. The limited origin of new charge carriers can be schematized as cylinders around the spray electrode. These considerations apply, in particular, to laminar flow fields, which are frequently present for the small flow velocities of electrical separators 1.

To increase the effectiveness of the electrostatic precipitator 1, the geometrically accessible charging region 9 is widened so that the separation efficiency of the system is increased. This is achieved by a multiplication and optimized distribution of the corona discharge zones. The required for this discharge 6 is formed with a plurality of Sprühelektrodenabschnitten 6 a, which in the embodiment according to Fig. 1 are formed as small peaks or edges. The field strength is particularly high here, since this is inversely proportional to the radius of curvature of a geometry. The tips or edges or the like are preferably distributed uniformly over the cross section of the charging unit. In this way, several small charging areas 9 which in total ensure the charging of the particles over the entire cross section of the charging unit, as shown clearly in the plan view. In the case of a laminar flow, all dust particles are detected and charged by one of these charging areas 9.

Fig. 2 schematically shows two perspective views of two embodiments of an electrode 6 with multiple Sprühelektrodenabschnitten 6a. The electrode 6 is z. B. in the manner of a barbed wire formed as spikes, spray electrode sections 6a executed (left figure in Fig. 2 ). In another embodiment (right figure after Fig. 2 ), the variable length spraying electrode portions 6a are realized in the manner of a Christmas tree. As in Fig. 2 illustrated, the various Sprühelektrodenabschnitte 6a are different, in particular of different lengths. In this case, in a preferred embodiment, over the length of the electrode 6, the sputtering electrode sections 6a are arranged from shorter sputtering electrode sections 6a to longer sputtering electrode sections 6a. In other embodiments, both shorter and longer Sprühelektrodenabschnitte 6a are arranged at individual nodes. In yet other embodiments, further spray electrode sections 6a, so-called sub-spray electrode sections, are formed on the spray electrode sections 6a.

In both embodiments, a plurality of spray electrode sections 6a branch off at branching regions spaced apart in the longitudinal direction of the electrode. In the illustrated embodiment, two spray electrode sections branch off radially from the electrode at the branch regions. The branching regions are arranged in the illustrated embodiment substantially equidistant from the spray electrode 6. In other embodiments, the distances vary. In the embodiment shown on the left with spines, the individual Sprühelektrodenabschnitte 6a do not intersect at the height of the electrode 6. In the embodiment shown on the right in the manner of a fir tree, the Sprühelektrodenabschnitte 6a intersect, however, substantially in the region of the electrode. 6

Claims (7)

Electrostatic separator (1), in particular for an exhaust pipe (2) of an emission control system, with
a flow channel (3) having a channel wall (4) and a channel interior (5), through which a particle-containing exhaust gas (P) flows in a flow direction, and arranged centrally in the channel interior (5), substantially in the flow direction (P) extending electrode (6), for forming a corona discharge zone by means of an electric field between the electrode (6) and the channel wall (4), the electrode (6) means for generating at least one further corona discharge zone in the form of at least one of the electrode (6) branching spray electrode section (6a) comprises,
characterized in that in order to improve the separation efficiency of the electrostatic precipitator along the electrode (6) at least two Sprühelektrodenabschnitte (6a) are arranged at least two locations, wherein the radial extensions of the arranged at a location Sprühelektrodenabschnitte (6a) are substantially equal to each other and from the radial extensions of the spray electrode sections (6a) arranged at another location. Electrostatic separator (1) according to claim 1 or 2,
characterized in that the spray electrode section (6a) is designed as a spray electrode tip part and / or spray electrode edge part. Electrostatic separator (1) according to one of claims 1 to 3,
characterized in that the spray electrode section (6a) branches off transversely, in particular radially, from the electrode (6). Electrostatic precipitator (1) according to one of claims 1 to 4,
characterized in that the spray electrode section (6a) branches off longitudinally, in particular axially from the electrode (6). Electrostatic separator (1) according to one of the preceding claims 1 to 6,
characterized in that the Sprühelektrodenabschnitte (6a) are arranged distributed on the electrode (6) that a uniform, in particular homogeneous arrangement of Cononaentladungszonen is realized. Electrostatic separator (1) according to one of the preceding claims 1 to 6,
characterized in that the radial extensions of the Sprühelektrodenabschnitte (6a) in the flow channel (3) are chosen such that their free, the channel wall (4) facing ends evenly distributed in adjacent cross-sectional areas, in particular annular areas of the flow channel (3) lie, so that a uniform, in particular homogeneous arrangement of Cononaentladungszonen is realized over a flow channel cross-section. Heating system for generating heat energy by burning an energy source such as biomass with
a particulate matter emitting heating system such as a biomass heating system for burning the energy carrier, wherein particle-containing exhaust gases are formed, and an electrostatic precipitator (1) according to one of the preceding claims 1 to 10.

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