JP3702230B2 - Method for agglomerating fine particles in exhaust gas, method for removing fine particles and apparatus therefor - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for agglomerating fine particles, a method for removing fine particles in an exhaust gas from an internal combustion engine, and an apparatus therefor, and more particularly, graphite contained in exhaust gas from an internal combustion engine such as diesel engine exhaust gas from light oil or heavy oil fuel or exhaust gas from city gas raw material. The present invention relates to a fine particle agglomeration method and a fine particle removal method suitable for agglomerating and separating fine particles such as those.
[0002]
[Prior art]
In recent years, as environmental pollution due to black smoke particulates (PM) and nitrogen oxides (NOx) emitted from internal combustion engines has become more serious, exhaust gas regulations have been gradually strengthened in many countries.
Among internal combustion engines, for example, diesel engines occupy a particularly important industrial position as a power source because they are excellent in fuel efficiency and durability. In terms of the environment, hydrocarbons (HC) and C0 contained in the exhaust gas are low, but there is a large amount of NOx that is a cause of harmful substances PM and air pollution.
[0003]
The use of a particulate removal filter for exhaust gas is one of the effective techniques for reducing PM, and a method for collecting graphite particulates using various filters has been developed. At that time, when fine particles adhere to the inner surface of the filter, many methods for removing fine particles such as soot by a combustion reaction in the filter have been proposed in order to eliminate clogging in the internal flow path.
However, in the method of removing particulates such as soot by burning, there is a problem that the load on the filter is large and the life of the filter is shortened due to deterioration or breakage.
Further, in order to burn the fine particles accumulated in the filter, a process of removing and regenerating the fine particles in the filter must be performed separately from the collecting process of the fine particles. Therefore, it is difficult to remove the fine particles from the continuously supplied exhaust gas, and it has been necessary to stop the flow of the exhaust gas and perform regeneration by combustion. For these reasons, there has been a long-awaited method for efficiently collecting fine particles in exhaust gas discharged continuously.
[0004]
[Problems to be solved by the invention]
The fine particles in the exhaust gas discharged from the internal combustion engine, for example, the fine particles in the diesel engine exhaust gas are mainly carbon. Carbon particles are low-resistance dust, and their electrical resistivity is about 10 ⁴ Ω · cm. On the other hand, the particle size of carbon particles (such as carbon black) that can be collected by the cyclone is about 10 μm.
Such a cyclone is difficult to separate and collect when the particles are very small because the inertial force is reduced and the gas flows on the gas stream. Therefore, when collecting the fine particles in the exhaust gas using a cyclone, the particles to be collected must be larger than a certain size. Therefore, as a pretreatment of the exhaust gas to be introduced into the cyclone, it is necessary to agglomerate fine particles having a small inertial force in a preceding stage to increase the particle size. More specifically, when a cyclone is used, it is necessary to agglomerate carbon particles discharged from the diesel engine to about 10 μm in the preceding stage.
[0005]
Therefore, in view of the above problems, the present inventors effectively agglomerated fine particles such as graphite contained in the exhaust gas, and a processing method capable of increasing the diameter to a particle size that can be efficiently recovered with a cyclone or the like, Furthermore, intensive studies were conducted to develop a method that can improve the processing efficiency and operation efficiency of the entire system for purifying particulates in exhaust gas.
As a result, the present inventors have achieved efficient separation and removal in the cyclone by enlarging the particles in the exhaust gas with a unique particle aggregation system that combines electrostatic aggregation and electrode plate aggregation in the upstream of the cyclone. As a result, it was found that the load on the downstream filter can be reduced and continuous operation for a long time can be realized, thereby solving the above-mentioned problems. The present invention has been completed from such a viewpoint.
[0006]
[Means for Solving the Problems]
That is, the present invention provides a method for agglomerating fine particles in exhaust gas including an electrostatic agglomeration step and an electrode plate agglomeration step. Here, the electrostatic aggregation (charge aggregation) step is a step of charging and agglomerating fine particles contained in the exhaust gas in the upstream direction with respect to the flow direction of the exhaust gas from the internal combustion engine. For example, positive pulse streamer discharge A method or a high frequency barrier discharge method can be used. The electrode plate agglomeration process is a downstream of the electrostatic agglomeration process, and the fine particles agglomerated between at least a pair of electrodes flow down along the flow of the exhaust gas, thereby aggregating the fine particles onto one electrode. In this step, re-scattering is repeated and the particle size of the fine particles or the aggregate thereof is further increased.
[0007]
Further, the present invention provides a method for removing particulates in exhaust gas by centrifuging the particulate component and gas component contained in the exhaust gas as a swirl flow after the electrostatic aggregation step and the electrode plate aggregation step. The method further includes a fine particle removal step of separating and collecting the fine particle component. Furthermore, it is possible to further include a step of adsorbing and removing fine particles using a filter that captures and accumulates unrecovered fine particles contained in the exhaust gas in the downstream of the fine particle removal step.
[0008]
Further, the present invention provides a ground electrode provided on an outer wall portion in the exhaust gas flow path, and a center electrode provided in a central portion of the exhaust gas flow path, upstream of the flow direction of the exhaust gas from the internal combustion engine. There is also provided a fine particle aggregating apparatus in exhaust gas, comprising: an electrostatic aggregating part provided; and an electrode plate aggregating part provided with at least a pair of electrode surfaces provided in the exhaust gas flow path at a subsequent stage of the electrostatic aggregating part. Is.
In such an apparatus, a metal plate is provided on each of the opposing outer wall portions in the exhaust gas flow path as a ground electrode in the electrostatic agglomeration portion, and from a metal wire such as a tungsten wire as a central electrode substantially parallel to the metal plate. The aspect by which the electrode which is formed is arrange | positioned is mentioned. Similarly, in the electrostatic aggregation part, a cylindrical metal part is provided on the outer periphery of the cylindrical exhaust gas channel as the ground electrode, and a tungsten wire or a stainless bolt or the like is provided in the cylindrical central part as the center electrode. An embodiment in which an electrode made of a metal wire made of or an electrode made of a metal rod having an uneven outer surface (for example, a bolt-shaped metal rod) is also included.
[0009]
On the other hand, in the electrode plate aggregating part of the fine particle aggregating apparatus, for example, at least a pair of electrode surfaces provided in the exhaust gas flow path are arranged with two metal plates facing each other along the outer wall, There is an embodiment in which the metal plate is a ground electrode and the other metal plate is a positive electrode. Similarly, in the electrode plate aggregating portion, at least a pair of electrode surfaces provided in the cylindrical exhaust gas flow path forms a coaxial electrode with a ground electrode made of a cylindrical metal installed on the outer periphery and the cylindrical outer periphery. An embodiment composed of a combination with a positive electrode disposed inside is also preferable.
Furthermore, in the fine particle aggregating apparatus of the present invention, a plurality of the electrostatic aggregating parts and electrode plate aggregating parts may be installed in parallel to the exhaust gas flow path.
[0010]
According to the present invention, fine particles such as graphite contained in combustion exhaust gas can be effectively agglomerated and efficiently separated and recovered by a cyclone or the like before being recovered by a filter. Therefore, the load on the filter, such as clogging of fine particles, is reduced, and long-time continuous operation using the filter can be realized.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
As a pre-stage for capturing and collecting the graphite fine particles (soot component, PM) in the exhaust gas by the porous filter, the fine particles in the exhaust gas are separated and recovered by a centrifugal separator such as a cyclone to reduce the amount of fine particles. In order to improve the ability to separate and collect fine particle components by this cyclone, it is necessary to aggregate the particles to about 10 μm. The present invention efficiently implements the aggregation of such fine particles.
Hereinafter, specific embodiments of the method according to the present invention will be described with reference to the accompanying drawings.
[0012]
In the present invention, using atmospheric pressure non-equilibrium plasma, (1) electrostatic aggregation process in the discharge space by charging carbon fine particles, (2) electrode plate by repeated aggregation and re-scattering on the dust collecting electrode By combining the aggregation step, the fine particles are enlarged.
The electrostatic aggregation process is performed by charging fine particles such as carbon in the presence of plasma under atmospheric pressure. Next, the process of agglomeration on the electrode and the process of separation and re-scattering from the surface of the electrode plate by the shear force of the gas flow are repeated. Enlarge. By such a combination of steps, fine particles such as carbon can be aggregated to 10 μm or more.
That is, in the case of low-resistance dust such as carbon particles, even if the dust is collected on the dust collecting electrode, the charge is immediately lost, so that the particles are separated and re-scattered by the gas flow. By repeating such electrode plate aggregation and re-scattering, the electrode has an action like a so-called high-pass filter.
[0013]
1 and 2 show examples of the fine particle aggregating apparatus of the present invention.
The apparatus of the present invention is composed of an electrostatic aggregating part that charges the fine particles in the previous stage and an electrode plate aggregating part in the subsequent stage. The shape is preferably a rectangular parallelepiped shape type shown in FIG. 1 or a concentric cylindrical shape type shown in FIG. Moreover, the structure which combined these arbitrarily is also possible.
In the rectangular parallelepiped type, the metal plate 2 is provided on the opposing outer wall portion in the exhaust gas flow path as a ground electrode in the electrostatic aggregation portion 10, and from the tungsten wire 1 substantially parallel to the metal plate 2 as a center electrode. The electrode which becomes is arrange | positioned. In the electrode plate aggregating portion 11, a pair of electrode surfaces provided in the exhaust gas flow path are arranged with two metal plates facing each other along the outer wall, and one metal plate 21 is a ground electrode. The other metal plate 22 is a positive electrode.
In the concentric cylindrical type, in the electrostatic aggregation part 10, a cylindrical metal part 31 is provided on the outer wall part which is the outer periphery in the cylindrical exhaust gas flow path as a ground electrode, and a cylindrical central part is used as a central electrode. The electrode which consists of a tungsten wire 1 or a stainless steel bolt is arrange | positioned. In the electrode plate aggregating portion, a pair of electrode surfaces provided in the cylindrical exhaust gas flow path are arranged so that a ground electrode 41 in which a cylindrical metal is disposed on the outer wall portion which is the outer periphery, and a coaxial circle with the cylindrical outer wall portion. It is composed of a positive electrode 42 that is formed and disposed therein.
[0014]
Next, a discharge method in the electrostatic aggregation process will be described. In order to generate atmospheric pressure non-equilibrium plasma, a streamer discharge which is a kind of corona discharge can be used.
As a first form, a positive pulse streamer discharge method can be mentioned. In this method, a positive bias voltage is applied in a pulse form to a linear electrode. Compared to the DC streamer, it is more efficient because there is no suppression of discharge due to space charge, and in the case of a negative streamer, positive ions generated in the vicinity of the linear electrode suppress the progress of the discharge and discharge. Is difficult to spread. In addition, it is considered that the positive polarity can generate a large number of positively charged fine particles and increase the effect of the subsequent electrode plate aggregation. Furthermore, if the pulse time is set before the streamer progresses and all-way dielectric breakdown occurs and the counter electrode is damaged, the damage can be prevented. If damage is completely prevented, the front surface of the opposing ground electrode can be covered with an insulator such as glass to perform barrier discharge.
[0015]
In the case of the pulse streamer discharge method, for example, in the rectangular parallelepiped type apparatus shown in FIG. 1, the metal plates 2 are provided on both sides in the exhaust gas flow path, and tungsten is provided at the center of the two metal plates. A central electrode such as wire 1 is provided. A high voltage is applied between the metal plate and the center electrode.
The pulse high voltage is generated by connecting a capacitor charged at a high voltage with a spark gap switch to a load. The shorter the voltage rise time, the larger the current and emission intensity, and the wider the emission region of discharge. In order to generate a pulse voltage with a short time width, there is a method using a pulse transmission circuit that combines a capacitor and an inductance and using the transient phenomenon. To improve the pulse power transfer efficiency, the impedance between the power supply circuit and the load It is important to consider matching. It is also effective to pressurize the spark gap switch in order to increase power transmission efficiency.
[0016]
As a second form, a high frequency barrier discharge method of about 50 Hz can be mentioned. In this case, since the fine particles pass through the bulk plasma and are positively and negatively charged, electrostatic aggregation in the space is accelerated. On the other hand, in the latter-stage electrode plate aggregation, dust is collected on both electrode plates. Therefore, it is considered that the effect of repeating the electrode plate aggregation is different from that of the positive pulse streamer.
[0017]
In the case of the high frequency barrier discharge method, for example, in a rectangular parallelepiped type apparatus as shown in FIG. 1, metal plates 2 are provided on both sides in the exhaust gas flow path, and a tungsten wire 1 or the like is provided at the center portion thereof. Center electrode. A high voltage is applied during this period, and a plate made of an insulator such as glass is placed between them.
Further, in the concentric cylinder type apparatus as shown in FIG. 2, a central electrode such as a tungsten wire 1 is provided in the central portion of the exhaust gas flow path, and an insulator such as glass is surrounded on the outer periphery thereof. Further, an electrode 31 is provided on the outer periphery thereof.
[0018]
In the case of the positive streamer discharge method, the fine particles contained in the exhaust gas are positively charged when passing through the discharge part. Many of the positive ions present in the streamer are trapped on the surface of the fine particles, so that many of them become positively charged fine particles. These fine particles mainly aggregate due to electrostatic aggregation (heteroaggregation) due to the difference in charge amount, while the potential barrier is lowered while maintaining the same charge. In addition, since it is considered that some of them are negatively charged due to adhesion of electrons, electrostatic aggregation with these also occurs.
On the other hand, in the case of high frequency barrier discharge, the passing fine particles are charged with both positive and negative polarities. Those charged negatively are those due to electron attachment, those charged positively are those due to secondary electron emission, and collision charges of positive ions. When the particle size is about 10 nm, a phenomenon that changes like a flip-flop that goes back and forth between positive and negative polarities appears, which is considered to be a factor of rapid aggregation of fine particles to a particle size of 100 nm to 1 μm.
For this reason, there is a possibility that the high-frequency barrier discharge method has a larger particle size of the fine particles at the rear of the charged / aggregated portion. In any of these methods, the fine particles that have undergone electrostatic aggregation in the front part are sent to the electrode plate aggregation part in the rear part to further increase the diameter so that the particle diameter is about 10 μm or more. Is done.
[0019]
Next, the electrode plate aggregation process will be described. FIG. 3 schematically shows the aggregation process of the fine particles.
By passing the fine particles after passing through the electrostatic aggregating portion through the electrode plate aggregating portion, the fine particles are enlarged by cumulative electrode plate aggregation. As described above, low-resistance dust such as carbon particles easily rescatters in the dust collector, and the dust collection performance of particles having a large particle size deteriorates. This is considered to be due to repetition of electrode plate aggregation and scattering as shown in FIG.
Fine particles positively charged in the streamer discharge are collected by the ground electrode. The positively charged fine particles that come one after another accumulate on the ground electrode and aggregate there. When the peeling force resulting from the gas flow exceeds the adhesion force with the electrode plate, the fine particles grown on the electrode plate return to the gas flow and re-scatter. At this time, the re-scattered particles are charged to the same polarity as the ground electrode by electrostatic induction, and are then collected on the positive electrode by the electric field between the electrode plates. Particles on the positive electrode are also agglomerated, separated, and collected on the ground electrode. By repeating this operation, the particle size of the particles becomes large, and is eventually discharged out of the system together with the gas flow.
When high-frequency barrier discharge is used at the front stage, fine particles are collected on both the positive polarity and the ground electrode in the dust collection section at the rear stage. In this case as well, in principle, electrode plate aggregation and scattering similar to those described above are repeated.
In this way, by combining the two-stage agglomeration action of the electrostatic agglomeration process and the electrode plate agglomeration process described above, only carbon particles larger than the particle size that can be collected by a subsequent cyclone or the like are discharged to the downstream of the aggregator. It becomes possible to make it.
[0020]
In the downstream of the electrostatic aggregation process and the electrode plate aggregation process, the exhaust gas containing the agglomerated fine particles is usually used as a swirl flow, and the fine particle component and the gas component contained in the exhaust gas are separated by centrifugation to collect the fine particle component. The fine particle removal step is performed. This fine particle removal process can be suitably performed by, for example, a cyclone dust collector. The cyclone used here is a kind of centrifugal separator. Fine particles in the exhaust gas rotate inside the conical shape, while heavy particle components are collected by gravity under the cyclone, and light components (gas etc.) are in the upper part. It is discharged outside from the piping.
In the electrostatic aggregating apparatus described above, there is a distance between an electrode to which a voltage that can be discharged under the magnitude of the obtained applied voltage and a ground electrode are applied. That is, since the gas of the electrostatic aggregating apparatus flows, the sectional area of the opening is determined by the distance. Also, in the posterior electrode plate aggregating device, the electric field strength between the electrodes is determined by the magnitude of the applied voltage for the fine particles, and the re-scatter distance of the fine particles changes. In this case, the distance between the electrodes, that is, the cross-sectional area of the opening of the device may be defined. In such a case, in order not to cause unnecessary pressure loss even at a desired gas flow rate, it is preferable to use a multi-cylinder type fine particle aggregating apparatus. In the multi-cylinder type fine particle aggregating apparatus, the electrostatic aggregating part and the electrode plate aggregating part may be bundled and installed in parallel with the gas flow path so that the cross-sectional area of the gas flow path has a desired size. Specifically, for example, as shown in FIG. 4, there is a multi-cylinder type particle aggregating apparatus 201 in which four particle aggregation parts 101 having a rectangular cross section are bundled into a four-cylinder type.
[0021]
Further, in the downstream of this fine particle removal step, a filter that normally captures and accumulates unrecovered fine particles contained in the exhaust gas is used to further remove and adsorb the fine particles. That is, even if the electrostatic aggregation step and the electrode plate aggregation step are combined, the particles that remain as fine particles without aggregation are not separated and removed by a cyclone or the like, and flow into the downstream. In this step, a filter is used to adsorb and remove and recover such unrecovered fine particles in the exhaust gas.
[0022]
As a material for the filter, for example, cordierite, silica, alumina or the like is used. In the present invention, since fine particles having a certain size or more are removed in the preceding stage up to the cyclone, it is preferable to use a filter having a pore diameter of, for example, about 50 μm to 200 μm. Part of the fine particles can be removed. In addition, you may use for this filter part combining the filter which has a 2 or more different pore diameter from a viewpoint which collects graphite fine particles more completely. When a filter is combined, large fine particles are trapped in the front stage, and small particles of different sizes are gradually trapped in the subsequent stage, thereby making it difficult to cause a sudden clogging in the filter section.
The shape of the filter is not particularly limited and can be arbitrarily determined depending on the shape of the exhaust gas flow path in each apparatus. For example, a cylindrical filter or the like can be used.
[0023]
【The invention's effect】
According to the method of the present invention, it is possible to efficiently agglomerate fine particles such as graphite contained in the exhaust gas of an internal combustion engine, and it is provided in the wake by increasing the particle diameter to about 10 μm or more. Fine particles can be effectively separated and recovered by a separation device such as a cyclone. And when installing a filter downstream of the cyclone, effective particulate removal is performed in the preceding stage, so the load on the filter such as clogging by particulates is greatly reduced, and the filter is used. Long continuous operation can be realized.
[Brief description of the drawings]
FIG. 1 is a perspective view showing a configuration of a rectangular parallelepiped device as an example of a fine particle aggregation device of the present invention.
FIG. 2 is a perspective view showing a concentric cylindrical device configuration as an example of the fine particle aggregating device of the present invention.
FIGS. 3A and 3B are schematic views sequentially showing (a) discharge in an electrostatic aggregation step, (b) electrostatic aggregation, and (c) electrode plate aggregation in the present invention.
FIG. 4 is a perspective view showing a multi-cylinder type particle aggregating apparatus which is an example of the particle aggregating apparatus of the present invention.
[Explanation of symbols]
1 Tungsten wire 2 Metal plate (electrode)
3 DC power supply 4 DC pulse 10 Electrostatic aggregation unit 11 Electrode plate aggregation unit 101 Single cylinder type particle aggregation device (particle aggregation unit)
201 Multi-cylinder type particle agglomeration device

Claims (10)

An electrostatic agglomeration step for charging and agglomerating fine particles contained in the exhaust gas in a upstream direction with respect to the flow direction of the exhaust gas from the internal combustion engine;
In the wake of the electrostatic aggregation step, the fine particles that have been electrostatically aggregated between at least a pair of electrodes flow down along the flow of the exhaust gas so that the fine particles are aggregated on one electrode and then re-scattered. Repeating the electrode plate agglomeration step to increase the particle diameter of the fine particles or the aggregate diameter to 10 μm or more ,
In the downstream of the electrostatic aggregation process and the electrode plate aggregation process,
A method for removing particulates in exhaust gas, comprising: a particulate removal step of separating the particulate components and gas components contained in the exhaust gas by centrifugation using the exhaust gas as a swirling flow, and collecting the particulate components. .   The method for agglomerating fine particles in exhaust gas according to claim 1, wherein a positive pulse streamer discharge method is used in the electrostatic agglomeration step.   The method for agglomerating fine particles in exhaust gas according to claim 1, wherein a high-frequency barrier discharge method is used in the electrostatic agglomeration step. In the downstream of the particulate removal process according to claim 1,
A method for removing particulates in exhaust gas, further comprising a step of adsorbing and removing particulates using a filter having a pore size of 50 μm to 200 μm that captures and accumulates unrecovered particulates contained in the exhaust gas. . An electrostatic agglomeration part comprising a ground electrode provided on an outer wall part in the exhaust gas flow path, and a central electrode provided in a central part of the exhaust gas flow path, upstream of the flow direction of the exhaust gas from the internal combustion engine When,
In the latter stage of the electrostatic aggregation portion, an electrode plate aggregation portion having at least a pair of electrode surfaces provided in the exhaust gas flow path;
A cyclone dust collector that collects the particulate component by separating the particulate component and the gas component contained in the exhaust gas by centrifugal separation, using an exhaust gas containing the aggregated particulate as a swirl flow after the electrode plate aggregating portion. When,
A device for removing particulates in exhaust gas, characterized by comprising: In the electrostatic aggregation portion, a metal plate is provided on each opposing outer wall portion in the exhaust gas flow path as the ground electrode, and an electrode made of a metal wire is disposed substantially parallel to the metal plate as the center electrode. The apparatus for removing particulate matter in exhaust gas according to claim 5. In the electrostatic agglomeration part, a cylindrical metal part is provided on the outer periphery of a cylindrical exhaust gas channel as the ground electrode, and a metal wire or an outer surface of the cylindrical central part as the center electrode is uneven. 6. An apparatus for removing particulates in exhaust gas according to claim 5, wherein an electrode made of a metal rod is disposed. In the electrode plate aggregating portion, at least the pair of electrode surfaces are arranged such that two metal plates face each other along the outer wall, one metal plate is a ground electrode, and the other metal plate is a positive electrode. The apparatus for removing particulates in exhaust gas according to claim 5, wherein: In the electrode plate aggregating portion, at least a pair of electrode surfaces provided in a cylindrical exhaust gas flow path forms a grounding electrode made of a cylindrical metal disposed on the outer periphery, and a coaxial circle with the cylindrical outer periphery. 6. The apparatus for removing particulates in exhaust gas according to claim 5, comprising a combination with a positive electrode disposed inside. The apparatus for removing particulates in exhaust gas according to claim 5, wherein a plurality of the electrostatic aggregating part and the electrode plate aggregating part are provided in parallel to the exhaust gas flow path.

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