Patent Application
20140020558
This application is based on and hereby claims priority to International Application No. PCT/EP2011/070251 filed on Nov. 16, 2011 and Indian Application No. 32/KOL/2011 filed on Jan. 11, 2011, the contents of which are hereby incorporated by reference.
The present invention relates to removal of particulate matter from a gas, and in particular.
Fuel and flue gas generated from the thermo-chemical conversion processes mostly contain dust particles having a wide range of sizes. These gases should be free of dust particles either to meet the end application or to meet the environmental norms.
Cyclone separators are well-known devices for removing particulates from a gas stream. In principle, a stream of particle-laden raw gas is introduced tangentially into a cyclonic separation zone so that the particles experience a centrifugal force in the ensuing swirling flow. The particles are collected on the outer wall of the separation zone and a resultant clean gas exits from a central exhaust duct. Cyclones are considered suitable for removing particles larger than 10 μm from a gas stream due to centrifugal force, which is responsible for particle separation. However, their low collection efficiency with respect to separation of particles smaller than 5 to 10 μm puts an additional requirement for further cleaning of the gas to the desired levels. Conceptually, cyclones can be designed to remove sub-micron particles but the associated pressure drop would be prohibitively high resulting considerable power consumption. Also, depending on the extent of dust loading i.e. concentration of dust particles, there is a possibility of choking at the entry of the cyclone. Hence, the best cyclone design is essentially a trade-off between performance i.e. collection efficiency and the allowable pressure drop.
To make a gas free of particulate matter, a series of pollution control equipment including cyclone, scrubbers, electrostatic precipitators (ESP) are generally employed. In this arrangement, the cyclone removes the larger particles from the gas to reduce load on the subsequent equipment like scrubber, ESP and filters where removal of the smaller particles takes place.
Due to strict environmental regulations, most of the industrial applications use dust filters, such as woven textile bag filter, or ceramic candle filter, to remove dust particles. However, the pressure drop across the filter, and problems related to the regeneration of filter, make this technology less attractive compared to other available options. Electrostatic precipitators are considered very effective in removal of smaller particles because of dominancy of electrical forces and have been used mainly in thermal power plants for fly ash removal. Pressure drop across an ESP is significantly lower than for any other pollution control equipment. Thus the resulting energy consumption is lower making the ESP a favorable option.
U.S. Pat. No. 4,352,681 discloses a combination of cyclone and electrostatic precipitator to improve the collection efficiency of the cyclone. In this arrangement, charging and collection of particle due to electrostatic separation and separation due to centrifugal action happens simultaneously inside cyclone. Unfortunately, the above disclosed type of hybrid apparatus for particle removal does not result in substantially increased particle collection efficiency because of several reasons. The first reason is that the disclosed apparatus provides inefficient particle charging, which is attributed to the fact that a large number of particles (especially of large sized particles) compete for electrical charges. The second reason is that low residence time inside of the cyclone results in poor efficiency of electrostatic assistance of particle collection.
One potential object is to improve the particle separation/collection efficiency of a hybrid particle removal apparatus involving a cyclone separator and an electrostatic separator.
The inventors propose combining a cyclonic separation with two stage electrostatic precipitation, to increase the overall collection efficiency by stepwise removal of particulate matter in a gas. Cyclonic separation is effective for removal of larger particles due to larger centrifugal force acting on the particles, while electrostatic precipitation is effective in removal of smaller particles because of dominancy of electrical forces. An important feature is to carry out electrostatic precipitation in two separate stages, namely, an ionization or particle charging stage, and a particle collection stage. In the proposed arrangement, the ionization stage has a higher flow cross-section than the particle collection stage. In this way, gas velocity is kept higher in the ionization stage to provide enhanced particle charging. On the other hand, gas velocities in the collection stage is kept lower to provide enough residence time for the ionized particles to get separated from the gas stream. The particle collection stage provides an electrical field across the flow-path to promote the separation and migration of the ionized particles.
The proposed technique of carrying out particle charging and particle collection in two separate stages provides significantly improved particle separation/collection efficiency with respect to the existing related art that includes single stage electrostatic precipitators.
In one proposed embodiment,
Advantageously, the above embodiment does not require any additional chamber for particle charging or ionization. The vortex finder duct of the cyclone is used as the ionization duct for particle charging. So, during normal cyclone operation when partially clean gas passes through this vortex finder duct, particles get charged. For particle collection, a separate duct (of higher flow cross-section) is arranged coaxially over the vortex finder duct.
In one embodiment, for facilitating corona discharge, the first portion of the electrode comprises a rod whose cross-section includes one or more sharp edges.
In an exemplary embodiment, the first portion of the electrode comprises a rod and a plurality of sharp-edged disks along the length of the rod. Advantageously, the above kind of electrode structure reduces the corona initiation voltage. This may be further advantageous in reducing electrical insulation problems at the voltage feed-through.
In an alternate embodiment, the first portion of the electrode comprises a rod having a single sharp-edged disk located at the first portion of the electrode. Advantageously, this provides concentration of ion current to a small region which results in increase of electric field and charge density, thus increasing the particle charging efficiency.
In a further embodiment, the proposed apparatus further comprises an insulated feed-through arrangement for passing the corona initiation voltage to the electrode.
In one example embodiment, the collection duct has a variable cross-sectional area that increases in the direction of flow along the third flow-path, and wherein the dimensions of electrode are configured such that the gap between collection duct and the second portion of the electrode is constant in the direction of flow along the third flow-path. Advantageously, this embodiment provides further increase in efficiency of charging and precipitation of ultrafine particles in the size range <200 nm and further advantageously incurs a lower voltage requirement.
In an alternate embodiment, wherein the dimensions of the first portion of the electrode are configured such that the gap between ionization duct and the first portion of the electrode increases in the direction of flow along the second flow-path. Advantageously, this embodiment leads to a reduction in the fraction of ionization current flowing in the second portion of the electrode without compromising collection efficiency.
In a further embodiment, the second portion of the electrode is covered by a metallic mesh. Advantageously, this embodiment provides a homogenous electric field in the particle collection stage such that no corona discharge initiation takes place at the particle collection stage. This ensures that no particle charging but only particle collection takes place at this stage. This configuration also reduces electric power requirements.
In a still further embodiment, the proposed apparatus further comprises an arrangement for cooling the feed-through arrangement.
These and other objects and advantages of the present invention will become more apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings of which:
a-b respectively illustrate an elevation view and plan view of a cyclone showing the dimensions used for calculation,
Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout.
Referring now to
The inlet 2 is designed to induce a swirl to the incoming particle-laden gas 6a as it is introduced tangentially to a cyclonic separation stage 3 (also referred to simply as “cyclone”). The cyclone 3 is disposed about an axis 15 and includes a flow-path 7. The swirl imparted to the gas 6a tends to concentrate the particulate matter, particularly large sized particles towards the outer wall 17 of the cyclone by centrifugal action, to produce a resultant partially clean gas 6b. The particulate matter that concentrates on the outer wall 17 of the cyclone may be removed, for example by rapping on the outer wall 17 and collected in a particle collection box 16. The partially clean gas 6b reverses its flow direction to exit the cyclone 3. While the cyclone 3 is effective for removing large sized particles, smaller and intermediate sized particles would experience lesser centrifugal force and therefore would remain with partially clean gas 6b. Removal of such remaining small and intermediate sized particulate matter from the partially clean gas 6b is carried out by two-stage electrostatic precipitation comprising a particle charging or ionization stage 4 and a particle collection stage 5, as illustrated below.
The ionization stage 4 is formed by a radially inwardly disposed ionization duct 10, also known as a vortex finder duct, disposed in the flow-path of the gas 6b exiting the cyclone. The ionization stage 4 includes a flow-path 8 formed by the duct 10 and further comprises a mechanism for producing a corona discharge in the flow-path 8 for charging or ionizing the particulate matter remaining in the partially clean gas 6b. To achieve this, an electrode 11 is disposed substantially coaxially with respect to the ionization duct 10, such that a first portion 11a of the electrode is within the ionization duct. The electrode 11 is connected to a high-voltage source, preferably of negative polarity, which is capable of producing a corona initiation voltage between the first portion 11a of the electrode and the ionization duct 10. As a result of the voltage applied between the ionization duct and the first portion 11a of the electrode, a corona discharge is produced in the flow-path 8 that ionizes the remaining particulate matter in the gas flowing through it. The electrode 11 may comprise a metallic rod having sharp edges, particularly at the location of the first portion 11a, to aid in the production of a corona discharge. Advantageously, the cross-sectional area of the flow-path 8, i.e., of the ionization duct 10 is designed to be sufficiently small such that the gas flowing therethrough has a high velocity (i.e., high energy), which, in turn increases the particle charging efficiency and reduces the possibility of particle collection within the ionization duct 10. In order to prevent particle collection in the ionization duct 10, a negative potential may be further applied to the ionization duct 10. Based on the charging requirement, the length of the ionization duct 10 and number of corona discharge producing electrodes can be varied to achieve best performance.
The gas flowing out of the ionization stage 4 thus comprises charged or ionized particulate matter. This charged or ionized particulate matter is separated from the gas subsequently at the particle collection stage 5. At the particle collection stage 5, the velocity of the gas is suitably reduced in order to provide enough residence time for particle separation. Accordingly, the particle collection stage 5 has a flow-path 9 whose cross-sectional area is increased relative to that of the flow-path 8. The separation of charged or ionized particles from the gas is effected by an electric field in the flow-path 9. In the illustrated embodiment, the particle collection stage 5 includes a collection duct 12 disposed coaxially with respect to the ionization duct 10, but having an increased cross-sectional area. The collection duct 12 surrounds a second portion 11b of the electrode that extends out of the ionization duct 10. The collection duct 12 is electrically grounded. An electric field is produced in the flow-path 9 due to the voltage applied between the second portion 11b of the electrode and the collection duct 12. This electric field causes migration of charged or ionized particles towards grounded collection duct. Once the charged particles are deposited at the collection duct 12 they may be removed either using some impulse/force in case of a dry system or liquid such as water in the case of a wet system. Since the velocity of particles in the ionization duct 10 is high, the deposition particles in the ionization duct 10 would be comparatively low and this can be cleaned by using some rapping mechanism at the end of operation.
The gas 6c flowing out of the particle collection stage 5 is thus a clean gas, substantially free of particulate matter, which exits the apparatus 1 through an outlet 13.
The feature of the apparatus 1 is thus to implement electrostatic precipitation in two separate stages, namely ionization and particle collection. This allows the gas velocity to be kept high at the ionization stage 4, which significantly increases the particle charging efficiency, while the gas velocity is kept low at the particle collection stage 5 to provide sufficient residence time for increased particle collection efficiency. The overall separation/collection efficiency is thereby increased in comparison to a single stage electrostatic precipitator (ESP). Further, the use of the cyclone 3 ensures that only small or intermediate sized particles need to be charged at the ionization stage, which advantageously incurs lower power requirements, thereby increasing charging efficiency.
Thus, in summary, in the arrangement disclosed in the illustrated embodiment, charging efficiency of particles in general and especially of small particles is improved due to (a) reduced number of particles to be charged, (b) lower number of large sized particles, which could be charged more easily, (c) reduced space charge effects in the ionization stage, and (d) increased ion density in the ionization stage for the same ESP current resulting in faster charging.
In most of the dry ESPs being currently used for fly ash removal, a dust conditioning chemical is mixed with gas before sending it to the ESP, mainly to avoid dust resistivity problem. In the proposal, this chemical can be added in the cyclone to provide better mixing of chemical with dust due to the existing vortex flow in the cyclone.
Further advantageously, the arrangement disclosed can be used for hot gas cleaning, which eliminates the need to install any other clean-up system. In particular, the apparatus is very useful of cleaning the producer gas. This is because in a conventional arrangement, tar present in the producer gas might condense on to the insulators (used to provide alignment to the discharge electrode) in the ESP. This deposition of tar on the insulator can result into corona collapse due to short circuiting of current. However, since the apparatus may be used for hot gas cleaning, the tar present in the producer gas will not condense on to the insulator and will not hinder the ESP operation.
The apparatus also provides a compact design that presents a solution to challenges related to the space and gas flow ducting and associated pressure drop faced by convention particulate control systems.
Several advantageous embodiments of the proposal may be considered, as illustrated referring to
In the embodiment shown in
The electrode 11 in this case includes an electrode rod whose cross-section has sharp edges, for example a rectangular or polygonal or star shaped cross-section. The corona initiation voltage is applied by a high-voltage power source 14 via an insulated feed-through arrangement 25. The sharp edges of the electrode facilitate production of corona discharge at the ionization stage 4. The gap between the first portion 11a of the electrode and the ionization duct 10 is indicated as di (also referred to as discharge gap) while gap between the second portion 11b of the electrode and the collection duct 12 is indicated as DC. In the embodiment of
In the embodiment illustrated in
In the embodiment illustrated in
In an alternate embodiment (not shown), the electrode dimensions of the first portion 11a may be configured such that the discharge gap di increases gradually in the direction of flow in the flow-path 8 at the ionization stage 4. This arrangement reduces the fraction of ionization current flowing in the electrode without compromising collection efficiency.
In the embodiment illustrated in
In the embodiment illustrated in
Example Calculations
In order to estimate the collection efficiency of the ESP in this the proposed arrangement, the following cyclone dimensions have been considered as shown in table 1 below:
a shows an elevation view 71 of the proposed arrangement illustrating the symbols/notations used for representing the various cyclone dimensions that are included in table 1.
Also, for the calculations, the following operating conditions for cyclone at 100% loading Conditions have been considered, namely a cyclone operational temperature of 500° C. and a gas flow rate of 93.24 kg/hr at ambient temperature.
The invention has been described in detail with particular reference to preferred embodiments thereof and examples, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention covered by the claims which may include the phrase “at least one of A, B and C” as an alternative expression that means one or more of A, B and C may be used, contrary to the holding in Superguide v. DIRECTV, 69 USPQ2d 1865 (Fed. Cir. 2004).
| Number | Date | Country | Kind |
|---|---|---|---|
| 32/KOL/2011 | Jan 2011 | IN | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2011/070251 | 11/16/2011 | WO | 00 | 9/26/2013 |
