IONISER FOR EFFLUENT GAS

Abstract: A method of treating effluent gases including the steps of providing a treatment zone (32, 120), causing effluent gases to flow to the zone, directing microwave energy to the zone and establishing a plasma (100) in the zone in which molecules in the effluent gas are ionised, permitting recombination of the ionised molecules into recombination products, and causing the recombination products to flow from the zone.

Description: IONISER FOR EFFLUENT GAS This invention relates to a method and apparatus for treating effluent gases. Treatment of effluent gases is necessary in order to prevent or reduce noxious substances being discharged into the atmosphere and also to prevent discharge of greenhouse gases. A common source of effluent gas is from internal combustion engines of various types including those used in automobiles. The object of the present invention is to provide a novel method and apparatus for treatment of effluent gas particularly, but not exclusively, treatment of exhaust gases from internal combustion engines. According to the present invention there is provided a method of treating effluent gases including the steps of providing a treatment zone, causing effluent gases to flow to said zone, directing microwave energy to said zone and establishing a plasma in said zone in which molecules in the effluent gas are ionised, permitting recombination of the ionised molecules into recombination products, and causing said recombination products to flow from said zone. The invention also provides apparatus for treating effluent gases including a gas ionisation chamber, an effluent gas inlet to the chamber, means for applying microwave energy to effluent gases in the chamber to thereby establish a plasma in the chamber in which molecules of the effluent gas are ionised, and a treated gas outlet from the chamber for discharge of recombination products of said ionised molecules. Preferably the gas inlet and gas outlet are adapted to be connected in the exhaust pipe of an automobile. In one embodiment the gas ionisation chamber is located in a housing which includes a water cooling means which, in use. receives cooling water from the internal combustion engine. The means for applying microwave energy may include a magnetron which is also arranged to be cooled by cooling water from the internal combustion engine. The magnetron may be coupled to the housing by means of a waveguide. A matching load may be coupled to the housing in order to absorb microwave energy which is not absorbed in the plasma process. The matching load may be also water cooled by cooling water from the internal combustion engine. The control means may be provided for controlling power output from the magnetron. Preferably further, the control means is sensitive to the amount of energy received by the matching load and the control means controls power output in response to this energy. The invention will now be further described with reference to the accompanying drawings, in which: FIGURE 1 is a schematic view of an ioniser of the invention coupled to an internal combustion engine; FIGURE 2 is a schematic view of one embodiment of an ioniser of the invention; FIGURE 3 is a perspective view of an ionisation chamber; FIGURE 4 is a schematic cross-sectional view through the chamber; FIGURE 4 A is a schematic cross-sectional view on the line 4A-4A in Figure 4; FIGURE 5 is a schematic view of a matching load; FIGURE 6 is a cross-sectional view through the line 6-6; FIGURE 7 is a schematic view of a preferred ioniser of the invention coupled to an internal combustion engine; FIGURE 8 is a fragmentary perspective view of the second embodiment of the invention; FIGURE 9 is a schematic cross-sectional view along the line 9-9; and FIGURE 10 is a schematic cross-sectional view along the line 10-10. Figure 1 illustrates schematically a water cooled internal combustion engine 2 having a radiator 4 for supplying cooling water to the engine. The engine also includes an exhaust manifold 6 which is coupled to an exhaust line 8. In an automobile, a battery 10 would be provided in the usual way. A gas ioniser 12 of the invention is coupled in the exhaust line 8, in accordance with the invention. It is preferably located as close to the exhaust manifold 6 as is practicable. Figure 2 diagrammatically illustrates the ioniser 12 of the invention. The ioniser 12 includes a magnetron 14 which may be of a commercially available type operating at 2.45GHz frequency and having the capability of pulsed and variable output power. It is connected to the battery 10 and is powered thereby subject to the process control unit 26, as described below. The nominal output power would be 500-1000 watts but this would depend upon the type of gas being treated and the type of internal combustion engine. Microwave energy from the magnetron 14 is transmitted to an ionisation chamber 16 by means of a waveguide 18, iris 20 and/or multiple stub tuner 22. The waveguide, iris and stub tuner are chosen in order to maximise the efficiency of transfer of microwave energy from the magnetron 14 to the ionisation chamber 16 in accordance with known principles. A wedge matching load 24 is coupled to the other side of the ionisation chamber 16 in order to absorb any microwave power which is not absorbed during the gas ionisation process, thus making any travelling reflections negligible. The process control unit 26 controls power output from the magnetron 14 in response to inputs received from other parts of the ioniser, as will be described in more detail below. Figures 3, 4 and 4 A illustrate the ionisation chamber 16 in more detail. The ionisation chamber 16 comprises a hollow metallic body 28 which is somewhat analogous to a rectangular waveguide and is preferably formed from aluminium. The body 28 includes a central portion 30, the interior of which constitutes an ionisation chamber 32. The ends of the central portion 30 are provided with flanges 34 and 36. The flange 34 enables coupling to the stub tuner 22 whereas the flange 36 enables coupling to the matching load 24. The 5 central part of the body 28 includes an inlet port 38 in an upper face 39 and an outlet port 40 in a lower face 41. Upper and lower mounting flanges 42 and 44 are mounted on the faces 39 and 41 adjacent to the ports. The flanges 42 and 44 enable coupling by means of bolts (not shown) to flanged pipes 46 and 48 which form part of the exhaust line 8. Heat resistant gaskets 50 and 52 are located between the flanges to prevent gas escape. Clamping flanges 10 54 and 56 may be provided to complete the coupling arrangement. Coolant tubes 60 and 62 may be bonded or braised to the faces 39 and 41 and/or flanges 42 and 44 to enable cooling of the body 28. Cooling water from the radiator 4 flows in the coolant tubes 60 and 62, as will be described below. 15 Figures 5 and 6 show a suitable arrangement for the wedge matching load 24. The load 24 is in the form of a rectangular waveguide body 70 having a flange 72 at one end and a closed face 74 at the other end. A microwave transparent wedge element 76 is disposed across the body, as shown in Figure 6. The body 70 may comprise a teflon or boron nitride plate and is preferably placed at an angle A where tan A =— h where h is the mternal height of 0 the waveguide and L is the wavelength relative to the axis of the waveguide body 70. In the illustrated arrangement the angle A is about 30° . It is sealed within the waveguide body so as to be in watertight relation with the inner faces. Preferably the plate is at least 2mm in thickness. The distance L as shown in Figure 6 is preferably one wavelength of the microwave radiation to be absorbed. As best seen in Figures 2 and 5, a side face 78 of the 5 waveguide body 70 is formed with water inlet and outlet tubes 80 and 82 to which are connected thermocouples 84 and 86 for sensing the temperature of water entering and leaving the interior of the waveguide body 70. As diagrammatically illustrated in Figure 1 , coolant water from the radiator 4 of the 0 internal combustion engine is circulated via coolant line 88 to the coolant tubes 60 and 62 of the ionisation chamber 16 and to the inlet tube 80 of the load 24. A return line 90 conveys heated water from these components back to the radiator. This provides a convenient way of cooling components of the ioniser device 12 utilising the available coolant water system of the internal combustion engine. Output signals from the thermocouples 84 and 86 can be inputted to the control unit 26, as diagrammatically shown in Figure 2. The basis of this control is to sense the temperature rise within the load 24. This is indicative of the amount of energy reaching the load which in turn is indicative of the energy not utilised in ionisation of gas passing through the ionisation chamber 32. Accordingly, if the temperature rise is above a predetermined limit, the control unit 26 causes the magnetron to reduce its power output until a predetermined level of temperature rise is sensed between the thermocouples 84 and 86. The control unit 26 may also be responsive to thermocouples (not shown) provided on other components in the system. The control unit 26 is preferably such that when the ignition to the internal combustion engine 2 is on, the magnetron 14 is also caused to be on. Its output, however, will initially be low under the control of the thermocouples 84 and 86 until effluent gases flow through the chamber 32. When this occurs a plasma 100, as diagrammatically illustrated in Figures 4 and 4A, will be established across the chamber 32 and extending into the ports 38 and 40. The effluent gases passing through the plasma 100 are ionised by virtue of absorption of microwave energy applied thereto from the magnetron. Thus the gases in the effluent are broken down into ions and will remain in an ionised state until they begin to cool down after the electromagnetic energy is no longer applied thereto, that is to say the ionised gases cool as they enter the outlet port 40. At this stage recombination can occur but, generally speaking, the recombination products will be different from the effluent material entering the ioniser 12. Ionisation starts either by means of a tungsten rod trigger or by means of an electric spark caused by discharge of a capacitor (not shown) discharging an electric spark in the chamber 32. In one arrangement a tungsten electrode 104 is caused to be introduced into the interior of the chamber 32 so that electric spark can be discharged between it and the hollow metallic body 30 of the waveguide. This assists in establishment of the ionisation process. After establishment, the electrode 104 is withdrawn and this can be accomplished by means of a simple linkage system 106 operated by a relay as diagrammatically illustrated in Figure 4A. Alternatively, the discharge may be initiated by discharge of the capacitor 108 shown in Figure 4A. It has been found that the plasma 100 tends to constrain the gases passing from inlet port 38 to the outlet port 40 in a column rather than flowing laterally into other parts of the chamber 32. A variety of chemical reactions can take place depending on the type of fuel upon which the internal combustion engine 2 is running and the operating conditions of the invention. For example, a stoichiometric combustion of octane would yield the following: C 8 H lg + 12.50 2 +47N 2 → 8CO 2 +9H 2 O+47N, Due to the fact that combustion is not always complete, CO, NO x , C X H X are usually present. The ionisation process taking place when microwaves interact with the gases causes the gas molecules to breakdown and some constituent reactions can be expressed by the following chemical equations: CO → C+O CO 2 → C+O 2 C x H x → C+H 2 O NO x +O → N+O 2 Typical results of this dissociation process for octane being burnt in an internal combustion engine (not fitted with a catalytic converter) are included in Table 1 below. "compared to exhaust products not treated by the ioniser It will be appreciated by those skilled in the art that the method and apparatus of the invention can very conveniently be utilised in internal combustion engines in cars because the apparatus is relatively compact, can be powered by the vehicle's battery and/or electrical system, and can be cooled by the normal cooling system of the engine. Preferably, the ioniser 12 is fitted in the exhaust line as close as possible to the exhaust manifold 6 and before any catalytic converter which may be present and also before a muffler 102, as shown in Figure 1. Figures 7 to 10 illustrate a preferred embodiment of an ioniser of the invention. In these drawings, the same reference numerals have been used to denote parts which are the same as or correspond to those of the first embodiment. Figure 7 shows the preferred ioniser 12 of the invention connected to the exhaust line 8 of the internal combustion engine 2. The ioniser 12 is located upstream of the muffler 102 and any catalytic converter (not shown), if fitted. In this arrangement cooling water from the radiator 4 is not required to cool the magnetron and ionisation chamber. If necessary, a cooling fan may be supplied to cool these components. In this arrangement, the ionisation chamber 16 has a different construction to that illustrated in Figures 8, 9 and 10. The ionisation chamber 16 includes an inner conduit 120 which is provided with flanges 122 and 124 for connecting the conduit in the exhaust line 8. The conduit 120 is preferably formed from aluminium and has a diameter which is the same or substantially the same as that of the diameter of the exhaust line 8. The inner conduit 120 will normally have an inner diameter in the range 40mm to 70mm depending on the capacity of the internal combustion engine 2. The ionisation chamber 16 further includes an outer cylindrical tube 126 which is formed from aluminium and is closed at one end by means of an end plate 128 and at the other by means of a tuning plug 130. The end plate 128 is preferably welded to the tube 126. In this embodiment, the magnetron 14 is mounted on a flange 132 which is connected to the cylindrical body 126 by means of a hollow cylindrical support tube 134, as best seen in Figure 10. A microwave antenna 136 extends from the magnetron 14 through an opening 138 in the flange 132 and then through the protecting body 134 and into the interior of the cylindrical body 126 adjacent to a microwave transparent window 140. The window 140 may be formed from ceramic or other microwave transparent material. As best seen in Figure 9, the ionising chamber 16 includes a tungsten rod trigger 142 which extends through an insulating support tube 144 so that the tip 146 of the trigger is located within the interior of the inner conduit 120. In this arrangement, a high voltage is applied to the rod 142 so that sparks are repeatedly struck between the tip 146 of the rod and the interior of the inner conduit 120. The sparks facilitate establishment and maintenance of a non-thermal microwave induced plasma in exhaust gases which flow into the inlet port 38, through the interior of the inner conduit 120, in the direction of arrow 148. The treated gas products pass out of outlet port 40. In this embodiment, the magnetron 14 is preferably powered from the battery 10 of the vehicle by means of a DC-AC converter (not shown). The output power from the magnetron, as determined by its duty cycle of operation is self- regulating, the average power usually being in the range 500 to 1000 watts. It has been found that the magnetron will inherently produce more power output in accordance with the amount of microwave absorption which takes place within the interior of the inner conduit 120. Thus, if there is a greater degree of ionisation of combustion products taking place in the ionisation chamber, the output of the magnetron will be correspondingly greater. Conversely, if the rate of ionisation is lower, the output power of the magnetron will be lower. The inner conduit 120, cylindrical body 126, end plate 128 and tuning plug 130 function as a circular waveguide 150 which can be tuned in order to concentrate the microwave energy from the antenna 136 in the interior of the tube 120 to thereby enhance establishment of plasma ionisation of the exhaust gases flowing through the inner conduit 120. Tuning of the circular waveguide 150 can be accomplished by axial adjustment of the tuning plug 130. In the illustrated arrangement, the tuning plug 130 had mail threads 152 which mesh with female threads 154 formed in the downstream end of the cylindrical body 126. A spur gear 156 is preferably integrally formed on the plug 130 and the control system 26 is operable to rotate the gear 156 so as to vary the axial position of the plug 130. Because of the presence of the gear 156 the plug 130 is preferably made from a durable metal such as steel or bronze. In this arrangement, the control system may include a thermocouple 158 or other temperature 5 sensor which senses the temperature of gases within the inner conduit 120 in the zone where the plasma 100 is typically located. If the thermocouple 158 detects a temperature of about 400°C, this indicates that the plasma has not been established or has been extinguished. If, however, the plasma is present, the thermocouple will detect a temperature of about 900 °C to 1000°C. The control system 26 responds to the temperature detected by the thermocouple 10 158, and if it is relatively low, say of the order of 400°C, it causes rotation of the gear 156 which in turn causes axial movement of the plug 130. The control system is arranged to move the plug first in one direction and then in the other and is arranged to stop movement of the plug when the thermocouple 158 detects the higher temperature, that is to say of the order of 1,000°C which indicates that the plasma 100 has been re-established. The tuning plug 130 15 varies the effective cavity length of the circular waveguide 150 and, when it is correctly tuned, it will better concentrate microwave energy from the antenna 136 on the exhaust gases flowing through the inner conduit 120. In the illustrated embodiment, the cylindrical body 126 has an internal diameter in the 0 range of say 80 to 110mm generally depending on the diameter of the inner conduit 120. The body 126 has a length in the range from 140 to 180mm. It has been found that the tuning plug 130 requires axial adjustment of about ± 10mm from a central position. The tuning plug 130 may be provided with a heat resistant gasket (not shown) adjacent to the inner conduit 120. Alternatively, the bore 131 through the plug 130 is carefully machined so that there is a very 5 small clearance between it and the conduit 120 so as to prevent microwave leakage. In the illustrated arrangement, part of the treated exhaust gas is fed back to a point upstream of the plasma 100 in order to have better control over the NO x emissions. This is accomplished by means of a recirculating tube 160, one end of which has an outlet port 162 0 located upstream of the microwave antenna 136. The downstream end of the tube 160 includes an inclined portion 164 which passes through the wall of the inner tube 120 and terminates with an inlet port 166. The recirculating tube 160 is preferably formed from aluminium and has an internal diameter which is in the range from 0.3 to 0.5 of the internal diameter of the inner tube 120. In this arrangement, the exhaust gases flowing in the conduit 120 will induce a low pressure at the port 162 which will cause recirculation of a predetermined percentage of the treated exhaust gases upstream of the plasma 100 where they are reionised to give better NO x control. Normally, about 30% to 50% of the treated exhaust gases will be recirculated through the recirculation tube 160. It has been found that the angle B between the inclined section 164 and a transverse plane 168 influences the percentage of exhaust gas which is recirculated. It has been found that optimal gas circulation for controlling NO x , the angle B should be in the range from 50° to 80° . A prototype of the embodiment illustrated in Figures 8 to 10 has been tested and the results are set out below. Two and four stroke engines were tested at idle speed with the magnetron 14 normally producing 600 watts of applied microwave energy. Table 1 shows typical results of exhaust gas treatment after a catalytic converter for a four stroke engine, the top row showing exhaust gas composition and the bottom row showing composition after microwave induced plasma (MIP) treatment in accordance with the invention. Table 1 - Four Stroke Engine Exhaust Gas Composition with Catalytic Converter The 0.095% in CO content level generated in the exhaust MIP discharge is due to the fact that at idle speed the catalytic converter controls more efficiently the CO content reducing to 0% content in the gas mixture, C0 2 was minimised by 58% . Experiments with a two stroke internal combustion engine exhaust gas and exhaust gas after treatment recirculation produced the results illustrated in Table 2. Table 2 - Two Stroke Engine Exhaust Gas Composition In this case the CO was reduced by 76%, CO 2 by 36%, and HC by 76.4% . These reductions indicate the efficiency of the method reducing the carbon content pollutant in the exhaust mixture. In the four stroke engine results, HC and CO in very low content were generated by the elementary free carbon resulting from the CO 2 dissociation. In the two stroke engine results, CO 2 presented lower dissociation for a small content level in the exhaust gas mixture compared with the four stroke case and at the same time CO and HC were greatly minimised. The NO x generation is related to the MIP discharge conditions. Further experiments were conducted with the two stroke engine with exhaust gas after treatment recirculation. The NO content in the MIP discharge is typically about 42% lower than without recirculation and this figure is further reduced when the microwave energy is applied in pulse mode, i.e. changing the duty cycle, the pulsing can be in the range 30kHz to 300kHz. The NO x formation may be reduced further and controlled by pulsing the microwave energy. Many modifications will be apparent to those skilled in the art without departing from the spirit and scope of the invention.

Claims: CLAIMS: 1. A method of treating effluent gases including the steps of providing a treatment zone (32,120), causing effluent gases to flow to said zone, directing microwave energy to said zone and establishing a plasma (100) in said zone in which molecules in the effluent gas are ionised, permitting recombination of the ionised molecules into recombination products, and causing said recombination products to flow from said zone. 2. A method as claimed in claim 1 including the step of causing discharge of an electric spark in said zone in order to initiate and/or maintain said plasma. 3. A method as claimed in claim 1 or 2 wherein the microwave energy is generated by a magnetron (14) and wherein the output power of said microwave energy is generally proportional to the rate of energy absorption during ionisation of molecules in said effluent gas. 4. A method as claimed in claim 1, 2 or 3, wherein the step of directing the microwave energy to said zone utilises a tunable wave guide (22,150) and tuning the waveguide in accordance with plasma conditions in said treatment zone. 5. A method as claimed in claim 4 wherein said tuning includes the step of varying the effective length of a circular waveguide (150) in accordance with temperature sensed in or downstream of said zone. 6. A method as claimed in any one of claims 1 to 5 including the step of recirculating a proportion of said recombination products to said zone. 7. A method as claimed in any one of claims 1 to 6 wherein said effluent gases are exhaust gases from an internal combustion engine (2). 8. Apparatus (112) for treating effluent gases including a gas ionisation chamber (116), an effluent gas inlet (38) to the chamber, means (14) for applying microwave energy to effluent gases in the chamber to thereby establish a plasma (100) in the chamber in which molecules of the effluent gas are ionised, and a treated gas outlet (40) from the chamber for discharge of recombination products of said ionised molecules. 5 9. Apparatus as claimed in claim 8 wherein said means for applying microwave energy includes a magnetron (14). 10. Apparatus as claimed in claim 9 wherein the magnetron operates at a frequency of 10 about 2.45GHz and with a continuous or pulsed power in the range 600 to 1000 watts. 11. Apparatus as claimed in claim 9 or 10 wherein the apparatus includes an electrode (104,146) for causing an electric spark in said chamber. 15 12. Apparatus as claimed in claim 9 to 11 wherein said gas ionisation chamber includes an effluent gas conduit (46,48,120) and said apparatus includes a waveguide (18, 150, [134?]) for applying said microwave energy transversely relative to the direction of said effluent gas flow in said conduit. 20 13. Apparatus as claimed in claim 12 wherein said conduit passes through an outer cylindrical body (126) which forms part of said waveguide (150). 14. Apparams as claimed in claim 13 including a microwave antenna (136) which extends from said magnetron through said cylindrical body (126) to a microwave transparent window 25 (140) in said effluent gas conduit (120). 15. Apparatus as claimed in claim 14 further including tuning means (130) for tuning said microwave energy into said ionisation chamber. 30 16. Apparams as claimed in claim 15 wherein the tuning means includes a tuning plug (130) which is mounted for axial movement relative to said outer cylindrical body. 17. Apparatus as claimed in claimed 16 wherein the plug has threads (152) which mesh with threads (154) in said outer cylindrical body (126) and the apparams includes control means (26) for causing relative rotation between the outer cylindrical body (126) and said plug (130) to thereby adjust the axial position of the plug relative to the outer cylindrical body. 5 18. Apparams as claimed in claim 17 wherein said plug (130) is provided with a gear (156). 19. Apparams as claimed in claim 18 wherein the control means includes sensing means 10 (158) for sensing whether the plasma is present or not, said control means being operable to move said plug until said plasma is re-established and then stop further movement of the plug. 20. Apparams as claimed in any one of claims 8 to 19 including a recirculating mbe (160) for recirculating a proportion of said recombination products back to said chamber for re- 15 treatment by said plasma. 21. Apparams as claimed in claim 20 wherein said proportion is about 30 to 50% of the effluent gases which enter the chamber. 20 22. Apparatus as claimed in claim 20 or 21 wherein said recirculation mbe includes an inlet port (166) in said effluent gas conduit (120) downstream of said window (140) and an outlet port (162) upstream of said window. 23. Apparatus as claimed in claim 21 or 22 wherein said effluent gas conduit (120) is 25 cylindrical and said recirculating mbe (160) is cylindrical and wherein the ratio of the latter to the former is in the range of 0.35 to 0.5. 24. Apparams as claimed in claim 22 or 23 wherein the recirculating mbe (160) includes an inclined portion (164) which passes through a sidewall of the effluent gas conduit (120), 30 said portion having an axis which is inclined at an angle of 50° to 80° relative to a transverse plane to said effluent gas conduit (120). 25. Apparams as claimed in any one of claims 12 to 24 wherein said effluent gas conduit includes means (122,124) for connecting the conduit in the exhaust line (8) of an internal combustion engine (2). 26. An internal combustion engine (2) having an exhaust line (8) including apparams as claimed in claim 24 connected in said exhaust line. 27. An internal combustion engine as claimed in claim 26 wherein the diameter or cross- sectional area of said exhaust line is equal to the diameter or cross-sectional area of said effluent gas conduit (120).

Inventors: DESTEFANI CARLOS ALBERTO, SIORES ELIAS, DESTEFANI, CARLOS, ALBERTO, SIORES, ELIAS
Applicants: DESTEFANI CARLOS ALBERTO, SIORES ELIAS, SWINBURNE LTD, DESTEFANI, CARLOS, ALBERTO, SIORES, ELIAS, SWINBURNE LIMITED
Application year: 2000
Main classification: B01D53
IPC codes: B01D53, F01N3