CA2067388A1

CA2067388A1 - Process and device for cleaning exhaust gases

Info

Publication number: CA2067388A1
Application number: CA002067388A
Authority: CA
Inventors: Berndt Gellert; Michael Hirth; Oliver Riccius; Karlheinz Schmidle; Edmund Fleck; Harald Jodeit
Original assignee: Individual
Current assignee: ABB Asea Brown Boveri Ltd
Priority date: 1990-08-30
Filing date: 1991-08-08
Publication date: 1992-03-01
Also published as: DE4104923A1; CH683321A5; KR920702254A; WO1992004122A1; EP0498862A1; AU8288491A; JPH05501678A

Abstract

ABSTRACT OF THE DISCLOSURE
Process and device for cleaning exhaust gases For efficient generation of electrical power, de-dusted hot gas (10) is led to a gas turbine. Dust-laden hot gas (1) is generated by a pressurized fluidized-bed firing system or by coal gasification at a temperature of 850°C and a pressure of 16 bar and then cleaned in a de-dusting plant (4, 8, 9). The dedusting plant comprises a coagulator (4) for mutual deposition of dust particles and 2 downstream cyclones (8, 9) for separating the dust (13, 14). The coagulator (4) has a bundle of coagulator tubes (5) with attached vibrators (6). A 1st cyclone (8) is fitted with a cyclone electrode (11) for more effective dust separation. Granular bed filters or devices for chemical injection can be provided before, in, or after the coagulator (4) in order to reduce greenhouse gases and/or corrosive gases.
(Fig. 2)

Description

~ 2067'~8 TITLE OF THE_ INVENTION

Process and device for cleaning exhaust gases BACKGROUND OF THE INVENTION

Field of the Invention The invention starts from a process for cleaning a gas stream and from a cleaning plant for carrying out the process in accordance with the preamble of claims 1 and 7.

Discussion of the Background With the preamble to claims 1 and 7 the invention refers to a state of the art such as is familiar from G~-A 2 055 628. In this a polluted hot gas coming from a pulverized coal combustion system is fed to a gas turbine via an electrostatic separator and 2 cyclones connected in series. The polluted hot gas contains sodium and potassium particles with diameters in the range from 0.1 ym - 1 ym and ash particles in the range from 2 ~m -50 ym. Fairly ~mall particles are preferably separated in the electrostatic separator which has a plurality of ducts with wires at negative high-voltage potential. Aerosol particles are deposited on large ash particles rather than on the plates of the separator from which they are detached and entrained by vibration and the gas flow respectively. Fairly large ash particles are. mainly separated in the cyclones.
A process and an electrostatic filter, for flocculation of soot and conductive suspended particles of similar weight, with downstream settling chamber for electrically flocculated particles is known from DE-PS 844 593. In this case raw gas is fed from the top downward through a tube bundle with discharge electrodes.

-- 20673~8 The gas emerging from the lowwer ends of the tubes is deflected upward within the casing and, with an increase in cross-section, is fed to a spacious cyclone in which the remaining flocculated particles accumulate at the low flow velocity.
From US-A 4 478 613 it is known that the combustion gases from a diesel engine can be fed in sequence through a coagulator and a cyclone to remove particles and aerosols. A plurality of stacks with disk-shaped emission electrodes at negative potential arearranged in the grounded coagulator chamber. The coagulator can also be installed in the cyclone. In order to avoid deposits on the coagulator chamber wall, exhaust gas is passed over it at high velocity or else it is subjected to mechanical impacts or vibrations.
From the conference publication: AIAA-81-0393 by R.R. Boericke et al., Electrocyclone for High Temperature, High Pressure Dust Removal, AIAA l9th AEROSPACE SCIENCES
MEETING, January 12 - 15, 1981, St. Louis, Missouri, it is known that a plurality of electrocyclones can be connected ;~in series to separate dust from the hot gas stream from a power station with a pressurized fluidized-bed firing system. Rod-shaped high-voltage electrodes, which generate a maximum electrical field strength of 5 kV/cm, project into the upper part of the electrocyclones. One advantage of this device is that the electrical forces which act on the particle can be selected independently of the cyclone size and the volume flow in the cyclone. In contrast to this the inertial forces which are otherwise ~`30 active in the separation drop with falling volume flow and with increasing cyclone size. If the charge level of the particles is low then the advantages of the electrocyclones become ineffective. This is the case due to the very short residence time of the particles in the -- 2~38~

section of the first electrocyclone provided for charging the particles.
The pressurized fluidized-bed firing system is a promising new combustion technology for efficient environmentally friendly generation o~ current from coal.
The SO2 produced during the combustion can be combined directly by the addition of lime compounds to the coal.
Due to the low combustion temperature of 850C the NOX
formation is in fact slight, but not however so low that it is possible to dispense with secondary measures.
Current is normally generated by means of a steam process.
A substantial increase in the efficiency can however be achieved if the exhaust gase~, which at full load have a temperature of 850C and a pressure of 16 bar, are fed to a gas turbine. In order to protect the gas turbine blades from erosion the exhaust gases are cleaned of particles by means of 2- or multi-stage cyclones. With the known process it is mainly large particles (> 5 ~m), which could damage a turbine blade on impact, which are removed from the exhaust gas. There is still no satisfactory solution for removing smaller particles.
The separation of unstable coagulates is difficult. Coagulates of suspended particles are stable if cohesive forces between the particles are high when compared to inertial and aerodynamic forces. However, there are many applications with unstable coagulates which are distinguished only by the fact that the suspended particles appear at a much higher density than in the adjacent flow field. The advantage of separating coagulated particles more simply can be destroyed if the coagulates break apart during the separation in, for example, a cyclone.
For the relevant state of the art reference i9 also made to CH-A 673 411 from which is known an 2~7~g~

electrostatic filter device for continuous separation of solid and/or liquid particles from a gas stream.
A granular bed filter for separating dust and harmful gaseous substances is known from the German journal: Staub-Reinhaltung der Luft 48 (1988), pp. 379 - 386.

SUMMARY OF THE INVENTION

The invention as defined in claims 1 and 7 achieves the object of specifying a process for cleaning a gas stream and a cleaning plant for carrying out the process which make it possible to remove harmful substances more completely from a gas stream.
An advantage of the invention lies in the lower environmental pollution due to dust. An improved dust separation efficiency is achieved by relatively simple means.
Another advantage lies in the low pressure drop and temperature drop of the agglomerator or coagulator.
When used in power stations for generating current the service lives of gas turbine blades can be increased and the economic efficiency raised.
Another advantage lies in the exceptionally low space requirement for a given exhaust ga~ volume flow.
For applications in pressurized fluidized-bed power stations the low space requirement makes it possible to house the cleaning plant inside the pressure vessel for the combustion chamber.
In accordance with one advantageous embodiment of the invention it is proposed that small particles are removed in an electrostatic filter, modified as a coagulator and particle charging device, in combination with a downstream electrocyclone. On the one hand the modified electrostatic filter is operated in such a way 20~3~

that the fly-ash particles coagulate into large particles in an interplay between separation and re-entrainment, with the result that small particles in particular can be separated in a downstream cyclone. On the other hand the long residence time, caused by the interplay, of the particles in the electrostatic filter helps them reach a very hi~h electrostatic charge level so that the subsequent use of an electrocyclone is very effective.
The separating effect is increased as a whole, and to a substantial extent for small particles of < 5 ~m in particular. The size spectrum in the electrostatic filter is displaced towards large particles which simplifies subsequent inertial separation. This utilizes the advantages of the electrostatic filter, namely the simple and xobust design with low maintenance requirement, the very low pressure drop, the high precipitation rate of the particles at high temperature and high pressure, and the small constructional size, combined with high pressure.
At the same time, due to the long residence time of the particles in the electrostatic filter, their charge levels can be maximized, as a result of which the advantages of an electrocyclone can then be utilized.
In accordance with a further advantageous embodiment of the invention the corona discharge utilized intrinsically in the electrostatic filter can be utilized in such a way by suitable choice of electrodes and field strength that NOX or greenhouse gases are decomposed into harmless substances and hence removed.
The cleaning plant as specified in the invention can be combined advantageously with an additional chemical exhaust gas treatment system. It is particularly suitable with coal gasification, with a pressurized fluidized-bed firing system, and with refuse incineration plants. For this purpose various chemicals (additives, such a~ NH3) 2067~8~

are injected into the exhaust gas stream or directly into the combustion boiler.

BRIEF DESCRIPTION OF THE DRAWINGS
A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:
Fig. 1 shows a cleaning plant with a coagulator, which has a plurality of coagulator chambers separated by plates and positioned downstream of which are 2 cyclones connected in series, Fig. 2 shows a cleaning plant as shown in Fig. 1, but with a coagulator which is constructed of a plurality of coagulator tubes with vibrators and with which the 1st cyclone positioned downstream is an electrocyclone, Fig. 3 shows a diagram for illustrating the displace-ment of the particle size distribution through the coagulator under atmospheric condition~, Fig. 4 shows an electrostatic filter with dust separator and exhaust gas deflection system as a coagulator, Fig. 5 shows 2 serie~-connected electrostatic filters with dust separators, gas deflection system and a granular bed filter for chemical exhaust gas treatment, and Fig. 6 shows a coagulator which is connected to a cyclone via a granular bed filter.

. ' .
., ~0~73~g Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, in Fig. 1 a particle-or dust-laden gas or hot gas, which comes from a pressurized fluidized-bed firing system of a power station and which is fed via a de-dusting plant (4, 8, 9) to gas turbine blades (not shown) of a gas turbine, is designated by (1). At full load the hot gas (1) has a temperature of 850C and a pressure of 16 bar. The dust particles are electrically charged in a grounded plate electrostatic filter or a coagulator or coagulator particle charger (4) with grounded plates or coagulator chambers or coagulator particle charging chambers (5) on high-voltage electrodes or coagulator electrodes or coagulator particle charging electrodes (3), and are separated at the inner wall of the coagulator particle charger (4). The wire-shaped coagulator particle charging electrodes (3) are positioned in the center between the plates (5) or at the edge between the relevant edge plates and the inner wall of the coagulator particle charger (4). The agglomerates formed on the separating plates or coagulator particle charging chambers (5) and at the inner wall of the coagulator particle charger (4) are sporadically re-entrained by the gas flow which has an average velocity between 3 m/s and 10 m/s. If necessary the detachment can be assisted by sporadic rapping of the plates (5). In the cyclones (8, 9) connected downstream the agglomerates are separated as ash or dust (13) or (14) respectively, while de-dusted cyclone exhaust gas (10) is fed to a gas turbine (not shown).
The coagulator particle charging electrodes (3) are normally negatively charged; however, they can also be positively charged, or set alternately at negative and 2~733~

positive potential. In this way it is possible to utilize the bipolar coagulation in the gas space for particle enlargement in addition to the coagulation at the inner wall of the coagulator particle charger.
Fig. 2 shows the separation of dust from a hot gas (1) in a process diagram. (2) designates a high-voltage source which is connected to wire- or rod-shaped coagulator particle charging electrodes ~3) in the center of a multiplicity of tubular coagulator particle charging chambers or coagulator particle charging tubes (5) of the coagulator particle charger (4). Attached to each coagulator particle charging tube is a vibrator (6) in order to re-entrain in the hot air stream any agglomerates separated at the walls of the coagulator particle charging tubes (5) and to assist detachment. The coagulator particle charging chambers (5) are connected to ground potential (12); they have diameters in the range of 4 cm -cm, especially in the range from 5 cm - 25 cm, preferably of 25 cm, and lengths in the range from 3 m -7 m, particularly in the range from 4 m - 6 m, preferably of 5 m. The flow velocity of the gas in the coagulator particle charger (4) i~ high; it lies in the range between 1 m/s and 20 mts, preferably in the range between 3 m/s and 10 m/s. With an exhaust gas flow of 200 kg/s in a pressurized fluidized-bed power station under full load the tube bundle compri~es about 25 coagulator particle charging tubes (5) of 25 cm diameter, in which the flow velocity in the coagulator particle charger (4) is 8 m/s.
Coagulation of the dust particles occurs at the tube walls of the coagulator particle charging tubes (5) or in their immediate vicinity. An interplay arises between electrostatic separation and re-entrainment of the particles through turbulent shearing strain caused by the flow or by mechanical vibrations or impacts. The resulting long residence time of the particles in the 206~3~

coagulator particle charger (4) causes a very high particle charge which is utilized in the downstream electrocyclone (8).
(7) designates a coagulator exhaust gas or a coagulator particle charger exhaust gas, the particles of which are on average larger than those of the dust-laden hot gas (1) and in which the level of particle charging is higher than in a normal electrocyclone. The coagulator particle charger exhaust gas is fed to a 1st cyclone (8) with a central, rod-shaped, high-voltage -or cyclone electrode (11) and then to a 2nd cyclone (9) without high-voltage electrode. On the clean gas side the 2nd cyclone provides a cyclone exhaust gas (10) which is well cleaned of small and large particles and which is fed to a gas turbine. The coagulates in the clean gas or coagulator particle charger exhaust gas (7) from the coagulator particle charger (4) can be separated as ash (13, 14) in the downstream cyclones (8, 9) with little pressure drop.
Fig. 3 shows the effect of the coagulator particle charger (4) under atmospheric temperature and pressure conditions for a flow velocity of 8 m/s, a field strength of 4 kV/cm and a particle concentration of 17 g/m3. The cumu}ative ~ize distribution (V) in % is plotted on the ordinate and the particle diameter (d) in ~m on the abscissa. A curve designated by (15) represents the size distribution (V) when the coagulator particle charger (4) is switched off. (16) designates measured value band widths which are achieved when the coagulator particle charger (4) i~ switched on. When the coagulator particle charger (4) i8 switched off the percentage of particles of diameter smaller than 5.8 ~m is about 15 % of the total ma~s of all particles. When the coagulator particle charger (4) is switched on this percentage can be reduced to about 7 ~.

2~7~

It is ob~ious that a plurality of de-dusting plants (4, 8, 9) can be provided in parallel in a large power station. The gas temperature can lie in the range from 250C - 1400C, preferably in the range from 500C -1200C, and the pressure in the range from 5 bar - lO0 bar, preferably in the range from 5 bar - 20 bar.
Fig. 4 shows a vertical, electrostatic filter (17) as a coagulator with an impact catchpot or dust separator (18) essentially underneath the vertical, cylindrical, filter tube and with a horizontal deflection outlet (20) for a gas flow (22) deflected through 90C.
Parallel filter chambers or filter tubes (not shown), corresponding to the coagulator chambers (5) as shown in Figs. 1 and 2, are provided in the cylindrical filter tube. Coarse dust particles from the dust-laden hot gas (1) are separated as ash (13) in a cyclone (8) positioned upstream of the electrostatic filter (17).
With hot gas (1) containing fly-ash, coagulates (30) appear in the unipolar electrostatic filter (17) under certain operating conditions which depend on, among other things, the flow velocity of the hot gas (1) and the corona voltage. As a rule, coagulates (30) occur if particles come into contact with one another on a separating electrode (not shown) corresponding to the grounded plates or coagulator chambers (5) as shown in Figs. 1 and 2, so that cohesive surface forces such as, for example, liquid bridge~, electrostatic forces, etc., are effeative. When the particle~ are detached from the separating electrode again by, for example, ~hear strains, mechanical vibrations or the like, the coherent coagulates (30) are present in the gas stream. This often involves loose coherent flakes with little inherent stability. By suitable deflections of the flow, in which inertial forces are utilized, these coagulates (30) can nevertheless be separated continuously from the gas stream into, for 2~38~

example, the impact catchpot (18) without being destroyed.
The coagulates (30) precipitated in the impact catchpot (18) can be transported away continuously by means of a screw conveyor or a moving conveyor belt (19). A
pneumatic cleaning system (not shown) can also be provided instead of the conveyor belt (19).
This arrangement can be augmented by an electrostatic filter device as specified in CH-A 673 411 mentioned at the outset or by a belt filter (21) in the deflected flow (22) at the end of the deflection outlet (20). In this manner optimum use is made of the separating action of this electrostatic filter (17) for smaller particles.
Chemical injections (31, 32) can be provided at the inlet and/or outlet of the electrostatie filter (17) for ehemieal treatment of the exhaust gas in order to reduee unwanted eorrosive and/or greenhouse gases sueh as CO2, N2O, NOX from ineineration plants.
NOX moleeules are redueed and greenhouse gases are destroyed during the eorona diseharge oecurring in the eleetrostatie filter (17), with the result that harmless sub~tanees are produced. In discharges under high pressures of > 3 bar and high temperature~ of > 500C, sueh as oeeur in hot gases (1) from power stations, eleetrons are released from the eoagulator eleetrodes (3) by thermionie emission and thermionie field emission as well as by the known field emission. This effeet is reinforeed-if the eoagulator eleetrodes (3) are thieker than normal eorona eleetrodes. Sueh eleetrodes ean either be appropriately thiek through dust loading or ean be manufaetured appropriately from suitable materials. The ratio of the surfaee area of a eoagulator eleetrode (3) to the surface area of the 2nd eleetrode (5) surrounding it, ef. Figs. 1, 2 and 6, lies in the range from 1 : 400 'o 2 : 1, preferably in the range from 1 : 20 to 1 : 1. It 20~73$~

is preferable to use those materials for which tthe work function of the electrons is small. This is the case, for example, with metal electrodes above 750C when they are coated with fly-ash. In this case the work function is about 0.6 eV. Another advantage of this discharge is that denitrification can be carried out directly with relatively homogeneously distributed electrons, in contrast to pure corona discharges where electrons and ions are inhomogeneously distributed. In addition to this the energy of the charged particles is too high for optimum utilization. Electrons from thermionic field emission have less energy and are therefore more economical.
Chemical additives, such as NH3, methanol and other alcohols, can be injected into, before or between coagulators (4) to achieve denitrification. In fact, due to the prevailing temperature, even the purely thermal activation of the additives is sufficient for a de-NOx reaction. Such methods are familiar. However, the effect is substantially reinforced if they are activated by electrons in discharges. The charges from the corona of the electrostatic filter (field and thermionic field emissions) are optimally suited for this purpose.
Unstable intermediate substances, such as NH2, are then formed which carry out denitrification in the best possible way. A catalytic lining or doping of the walls of the electrodes of the coagulator (4) can also improve activation. Particularly advantageous is a chemical injection (32) after the coagulator (4) and before the cyclone (8), cf. Fig. 6. Another chemical injection can take place before or on the belt filter (21) and on a granular bed filter (28) as shown in Figs. 5 and 6. This chemical injection (32) can be carried out as a function of the load. Due to differing temperatures power stations ~0~7~

with pressurized fluidized-bed firing systems require more denitrification at partial load than at full load.
Fig. 5 shows a version o~ the optimized dust separation system with a different design. The flow conditions can be utili~ed better if the deflection of the exhaust gas takes place through an exhaust gas deflection angle ~ from 80, preferably in the range from 140 -< 180. A separation deviation angle designated by (~) can be > 90 so that there is less probability of the coagulates being re-entrained in an impact catchpot (25) of an electrostatic filter (23).
A tapered region (24) at the lower end of the coagulating section of the electrostatic filter (23) serves to accelerate the flow of the hot gas (1). In order that deposits from a deflected gas flow (29) do not form to an increased extent on the outer walls of the obliquely upward-angled deflection outlets (26), liquid films (27) which flush these regions can be provided in the deflection regions.
As is indicated in Fig. 5, 2 such electrostatic filters (23) can be operated in series one after the other. This has the advantage of substantially increasing the efficiency. The transition from the 1st to the 2nd stage takes place in tubes. Measures which, for example, reduce the unwanted chemical effects of the exhaust gas can be taken in the pipework between the stages. For example, a granular bed filter (28) with silicon and/or aluminum oxides, which acts as alkaIi getter through gas sublimation, can be provided in the vicinity of the inlet to the 2nd stage.
In gasification processes a chemical injection, e.g. of oxygen, which improves the calorific value of the synthesis gas or gives rise to other properties, can be provided instead of the granular bed filter (28).

2 0 ~

Other additives can be added in order to reduce greenhouse qases such as CO2 and N20. In gasification plants in particular it is possible to convert or reform C2 into CH4, methanol, or other usable fuels. This is important for applications in hot gases with pulverized coal firing systems.
The velocity of the hot gas in the electrostatic filters lies in the range from 3 m/s - 50 m/s, preferably in the range from 5 m/s - 10 m/s, so as to prevent deposits on the separating electrodes.
Fig. 6 shows a cleaning plant in which a coagulator (4) with coagulator chambers (5) running vertically from the top downward is connected on the outlet side to a cyclone (8) via a granular bed filter (28). The length of the coagulator tubes (5) lies in the range from 2.5 m - 28 m. Additionally or alternatively at least one cyclone can be provided before the coagulator (4).
With these coagulators and electrostatic filters it i8 possible to separate unstable coagulates. This separation takes place continuously and essentially utilizes inertial forces which require no additional expenditure of energy. The de-dusting plant is suitable not only for the separation of particles from atmospheric or combustion chamber flue gases but also, for example, for the separation of coagulates which are to be produced as a product.
Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.

20~7~u~

(Intended only for the examiner; not part of the application) LIST QF DESIGNATIONS

1 dust-laden hot gas 2 high-voltage source 3 coagulator electrodes, coagulator particle charging electrodes 4 coagulator, coagulator particle charger, agglomerator, plate electrostatic filter coagulator chambers, coagulator particle charging chambers or tubes, plates 6 vibrators 7 coagulator exhaust gas, coagulator particle charger exhaust gas 8, 9 1st cyclone or 2nd cyclone de-dusted cyclone exhaust gas 11 cyclone electrode 12 ground potential 13, 14 ash, dust coagulator off, coagulator particle charger off 16 coagulator on, coagulator particle charger on 17, 23 electrostatic filter 18 impact catchpot, dust separator 19 conveyor belt deflection outlet from 17 21 belt filter 22 deflected flow 24 tapered region impact Gatchpot from 21, dust separator 26 deflection outlet from 23 27 liquid film 28 granular bed filter 29 deflected flow in 23 coagulate~
31, 32 chemical injection d diameter 2~7~

V cumulative size distribution exhaust gas deflection angle separation deviation angle

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. A process for cleaning a gas stream, a) in which a dust-laden gas stream (1) to be cleaned is conveyed in parallel through a plurality of coagulator chambers; (5) of an electrostatic coagulator (4, 17, 23) for mutual deposition of dust particles, wherein b) the dust-laden gas stream (1) is conveyed at a velocity in the range of 3 m/s - 50 m/s through the at least coagulator (4, 17, 23), c) and wherein the dust-laden gas stream (1) is a hot gas stream with a temperature in the range of 250°C -1400°C, and d) with a pressure in the range of 5 bar - 100 bar.

2. The process as claimed in claim 1, wherein the dust-laden gas stream (1) is a hot gas stream with a temperature in the range of 500°C - 1200°C.

3. The process as claimed in claim 1 or 2, wherein the dust-laden gas stream (1) is a hot gas stream with a pressure in the range of 5 bar - 20 bar.

4. The process as claimed in one of claims 1 to 3, wherein, after an electrostatic coagulation of the dust particles, the dust-laden gas stream (1) a) is deflected through a pre-determinable deflection angle (.alpha.) in the range of 80° - < 180°, b) in particular through a deflection angle (.alpha.) in the range of 140° - 170°, c) wherein coagulates (30) in the gas stream (1) which, due to inertia, are not deflected in this way are separated continuously from the coagulator (17, 23) and d) the deflected part of the gas stream (1) is subjected to continuous electrostatic filtering.

5. The process as claimed in one of claims 1 to 4, wherein, before and/or after the coagulation in the electrostatic coagulator (4, 17, 23), the gas stream (1) to be cleaned is subjected to chemical treatment (31, 32) or cleaning (28).

6. A cleaning plant (4, 8, 9) for cleaning a gas stream (1) a) with at least one coagulator (4, 17, 23), which has at least 2 coagulator chambers (5), each with a coagulator electrode (3), wherein b) the length of the coagulator chamber (4) lies in the range between 2.50 m and 28 m, and c) the ratio of the surface area of the at least one coagulator electrode (3) to the surface area of a 2nd electrode (5) surrounding it lies in the range from 1 : 400 to 2 : 1.

7. The cleaning plant as claimed in claim 6, wherein the ratio of the surface area of the coagulator at least one electrode (3) to the surface area of a 2nd electrode (5) surrounding it lies in the range from 1 : 20 to 1 : 1.

8. The cleaning plant as claimed in claim 6 or 7, wherein a) at least one cyclone (8, 9) is connected upstream and/or downstream of the coagulator (4, 17, 23), b) and in particular wherein a 1st cyclone (8) connected downstream has at least one cyclone electrode (11).

9. The cleaning plant as claimed in one of claims 6 to 8, wherein a) the at least one coagulator has an electrostatic filter (17, 23) with an essentially linear vertical section for electrostatic coagulation of the particles in the dust-laden gas stream (1), b) an essentially linear dust separator or impact catchpot (18, 25) connected to it, c) in particular with a continuous separating device (19) for coagulates (30) separated in the impact catchpot (18, 25), and d) with at least one deflection outlet (20, 26) for the gas stream (1) in the region between the coagulation section and impact catchpot (18, 25), e) the direction of which in relation to that of the coagulation section forms an angle (.alpha.) in the range of 80° - < 180°, f) in particular in the range of 140° - 170°.

10. The cleaning plant as claimed in one of claims 6 to 9, wherein a) a device for chemical injection (31, 32) and/or a chemical filter (28) for separating and/or transforming gaseous harmful substances and compounds is provided before and/or in and/or after the coagulator (4, 17, 23), b) and in particular wherein the chemical filter is a granular bed filter (28).