Process and device for plasma gas treatment

Abstract: A plasma enhanced gas reactor including a reaction chamber having a pair of field-enhancing electrodes each of which has an axial passage through it by one of which a reactant gas is admitted to the reaction chamber, and by the other of which reaction products are removed from the reaction chamber.

Description: The present invention relates to the use of a plasma to facilitate a reaction between gaseous substances. The use of plasmas for the processing of materials or facilitating reactions between gaseous substances is becoming an established technique and a number of forms of apparatus have been devised for these purposes. For example, our earlier patent application GB-A-2 282 738 discloses the use of an elongated cylindrical chamber having an inner rod electrode with helical radial projections thereon and an outer helical electrode, a silent discharge being established between the two electrodes. A gaseous mixture to be processed is passed through the silent discharge in the chamber. FR-A-2620436 A discloses a plasma reactor with means for causing a flow of a first reactant gas and a second reactant gas to intersect in the vicinity of a plasma. Also, our patent GB-A-2 273 027 discloses a microwave plasma generator comprising a chamber, means for establishing a microwave field in the chamber, a pair of conical field-enhancing electrodes in the chamber, means for admitting a gaseous medium to be excited to the plasma state, and an axial hole in one of the electrodes through which the excited gaseous medium can be withdrawn from the chamber. It is an object of the present invention to provide an improved plasma enhanced gas reactor. According to the invention there is provided a plasma enhanced gas reactor, comprising a reaction chamber, means for generating a localised plasma in the reaction chamber comprising means for coupling microwave radiation into the reaction chamber and means for concentrating microwave energy at a pre-determined position in the reaction chamber, means for causing a flow of a first reactant gas and a second reactant gas to intersect in the vicinity of the plasma and means for extracting reaction products from the reaction chamber. Preferably the means for producing a localised plasma comprises a pair of conical field-enhancing electrodes, each of which has an axial hole in it, through one of which one of the reactant gases is admitted to the chamber and through the other of which reaction products are removed from the reaction chamber. In one arrangement according to the invention the diameter of the axial hole or passage in one electrode conical assembly is greater than that of the other and preferably the electrode conical assembly having the smaller diameter axial passage is frusto-conical in form. In another arrangement according to the invention the axial passages formed in the electrode conical assemblies have at least approximately equal diameters and the upstream electrode conical assembly has a series of regularly spaced radial passages by means of which a proportion of the first reactant gas can be extracted and mixed in the second reactant gas prior to being subjected to the plasma. Alternatively the upstream electrode conical assembly has at least one radial passage by means of which the second reactant gas can be mixed with the first reactant gas immediately prior to being subjected to the plasma. Preferably the gap between the electrode conical assemblies is adjustable. The invention also provides a modular plasma processing apparatus comprising a plurality of plasma reaction chambers as aforesaid, means arranged to feed simultaneously a gaseous medium to be processed to each plasma reaction chamber, means for generating separately a plasma in each plasma reaction chamber, and outlet means for removing the processed gaseous medium from the plasma reaction chamber. Preferably the means for generating separately the plasma in the plasma reaction chambers comprises a microwave generator coupled directly to an associated plasma reaction chamber, and there is included means for controlling the action of the microwave generators comprising means for determining the partial pressure of a component of the gaseous medium and means for varying the power supplied to the microwave generators as a function of the partial pressure of the component of the gaseous medium. The means for varying the power supplied to the microwave generators may be adapted to vary equally the power supplied to each of the microwave power generators or alternatively may be adapted to vary the number of microwave generators to which power is supplied. The invention will now be described, by way of example, with reference to the accompanying drawings, in which Figure 1 is a section of a reaction chamber embodying the invention, and Figure 2 shows the plasma configurations produced by five different arrangements of electrode tips used in the embodiment of Figure 1. Figure 3 is a longitudinal sectionof a schematic plasma gas processing apparatus including a number of gas reactor units embodying the invention. Figure 4 is a cross-sectional view of a component of the apparatus shown in Figure 3, and Figure 5 is a schematic diagram of a system for controlling the apparatus of Figure 3. Referring to Figure 1 of the drawings, a plasma reaction chamber 100 consists of a cylinder 101 which is made of an insulating material such as borosilicate glass or quartz and two end-pieces 102 and 103, respectively. The cylinder 101 fits into annular grooves 4 which are machined in the end-pieces 102 and 103. The cylinder 101 and end-pieces 102 and 103 are sealed by means of O-ring seals 5. The assembly is held together by clamps, which are not shown in the figure. The end-pieces 102 and 103 are conical in form with the end-piece 102 cone angle smaller than that of the end-piece 103. Screwed into the end pieces 102 and 103 are replaceable electrode tips 106 and 107, respectively. The electrode tips 106 and 107 have smaller cone angles than the respective end-pieces 102 and 103 into which they are screwed. Again the electrode tip 106 has a smaller cone angle than that of the electrode tip 107. The electrode tip 106 is not screwed directly into the end-piece 102, but is screwed into a cylindrical block 108 which is a sliding fit in a bore 109 in the end-piece 102. The block 108 and bore 109 are rendered gas-tight by means of an O-ring seal 110. The position of the block 108 in the bore 109 in the end-piece 102, and hence the gap 111 between the electrode tips 106 and 107, can be adjusted by means of a screw 112 and nut 113. The electrode tips 106 and 107 each terminate in a sharp annular edge 114. The end-piece 102 has a cooling passage 115 formed in it and a number of entry ducts 116 to enable a first reactant gas to be admitted to the reaction chamber 100. The entry ducts 116 are fed from a plenum chamber 117 and are so shaped and disposed that the gaseous medium enters the reaction chamber 100 with a swirling motion. The end-piece 102, block 108 and electrode tip 106 all have an axial passage in them, by means of which a second reactant gas can be admitted to the chamber 100. The end-piece 103 and electrode tip 107 also have an axial passage 119 formed in them, by means of which reaction products can be extracted from the reaction chamber 100. The end-piece 103 has a coolant passage 120 formed in it. The end-pieces 102 and 103 are made of stainless steel. The tips 106 and 107 can be made of the same material or of a more corrosion and heat resistant metal such as tungsten. Means (not shown) are provided for coupling microwave radiation into the reaction chamber 100 from a source, which also is not shown. The tips 106 and 107 of the end-pieces 102 and 103, and in particular their sharp terminations, act as a pair of field-enhancing electrodes 121 which concentrate the microwave energy in the region of the gap 111 between the tips 106 and 107 of the electrodes 121 so that, in use, a plasma is formed in that region of the reaction chamber 100 only. Figure 2(a) illustrates the action of the reaction chamber as described above, Figures 2(b) and 2(c) illustrate the action of the reaction chamber with different electrode configurations and flow conditions for the two reactant gases. Referring to Figure 2(a), both the axial passages 118 and 119 are of the same diameter and the gap 111 between the electrodes 121 is about 2mm. A main reactant gas 201 is fed to the inlet of the passage 118 in the end-piece 102 assembly. An annular plasma 202 forms in the gap 111 between the electrodes 121. A second reactant gas 203, supplied to the reaction chamber 100 via the duct 116 is drawn into the exit passageway 119 in the tip 106 of the end-piece 103 assembly by the venturi effect of the main gas flow across the gap 111 between the electrode-tips 106 and 107 assisted by the swirling flow of the second reactant gas 203. As a result the second reactant gas 203 passes through the plasma, facilitating the transfer of energy to it. The two gases then react in a zone 204 at the entrance to the passageway 119 in the electrode tip 106 of the end-piece assembly 103. By way of example, if the first reactant gas is a gas contaminated with a relatively small proportion, for example (0.1 to 1%), of NO x , which it is desired to remove by reduction with NH containing radicals, the first reactant gas is passed through the passageway 118, and, although some of it will be excited in the plasma, most of it will bypass the plasma. All of the flow of the precursor for the generation of the NH containing radicals (NH 3 ), on the other hand, passes through the plasma and will be excited, both thermally and electronically, in so doing. Similar considerations apply if the main gas contains carbonaceous pollutants which it is desired to remove by oxidation reactions with O containing radicals. In this case, the second reactant gas can be air or oxygen. Figure 2(b) illustrates a different electrode configuration and gas flow pattern. In this case, the passage 118 in electrode tip 106 of the end piece assembly 102 is smaller in diameter than the passageway 119 in the electrode tip 106 of the end-piece assembly 103. The ratio of the diameters of the passages 118 and 119 is dictated by the desired proportions of the two reactant gases. The main gas feed 201 is via the entry ducts 116 to the reaction chamber 100, and the second reactant gas 203 is fed to the passage 118 in the end-piece 102 assembly. Unlike the previous arrangement, the tip 106 of the end-piece assembly 102 is flat, or even slightly hollowed, to facilitate the entry of the main reactant gas 203 into the gap 111 between the electrode tips 106 and 107. Again, an annular plasma 202 is formed with a reaction zone at the entrance to the passage 119 in the electrode tip 107 of the end-piece assembly 103, but in this case, it is the main reactant gas which passes through the plasma and is energised. Thus, this arrangement requires a higher microwave power input than does the previous arrangement. Figure 2(c) shows the same electrode configuration and gas flow pattern as does Figure 2(b), but in this case, the gap 111 between the electrode tips 106 and 107 is increased to about 5mm. As a result, a ball of plasma is formed in the gap 111 between the electrode tips 106 and 107 and both reactant gases are energised simultaneously, and the microwave power requirements are increased further. Figure 2(d) shows the same electrode configuration as Figure 2(a) but the upper electrode 106 is surrounded by a shroud 209 through which at least a proportion of the second reagent gas, 203 is fed. This arrangement also can be used with the electrode configurations of Figures 2(b) and 2(c). Figure 2(e) shows the same electrode configuration as Figure 2(a) but the upper electrode 106 has a number of regularly spaced radial holes 210 immediately upstream of the gap 111 between the electrodes 106 and 107. The venturi effect in the region of the gap 111 between the electrodes 106 and 107 causes a proportion of the first reactant gas 201 to be drawn through the holes 210 and mixed with the second reagent gas 203 prior to its passage through the plasma zone 202. As before, the configurations of Figures 2(b) to 2(e) can be used for gas purification by either reduction or oxidation reactions. Using the electrode configuration of Figure 2(a) it is possible to arrange for the second reactant gas to be injected into the main gas flow immediately upstream of the gap 111 between the electrodes 121, rather than rely on the venturi effect drawing the second reactant gas into the main gas flow. However, in this case, the advantage of passing the second gas flow actually through the plasma is lost. If desired, the effluent from the reaction chamber 100 can be passed though a second reactor containing, for example, a catalytic device for removing from the main gas pollutants other than those removed by the action of the plasma, or the end products of the reactions between the two gases in the reaction chamber 100. Other benefits which arise from such a combination of plasma generator and catalytic device are: a) free radicals existing in the effluent leaving the reaction chamber 100 and impinging on the catalyst material in the catalytic device can enhance the performance of the catalyst, and b) The thermal energy of the effluent from the reaction chamber 100 raises the temperature of the catalyst in the catalytic device, again enhancing its performance. The invention has been described in the context of the purification of gases. However, it is not limited to this application but can be used for other forms of plasma assisted reactions between gaseous media, or even aerosols. In order to increase the throughput of a plasma gas processing apparatus incorporating the invention a number of reactor chambers can be arranged to operate in parallel. Such an arrangement has the advantage that the failure of a single, or even more than one, reactor chamber does not render the entire apparatus ineffective. Referring to Figure 3, a modular plasma processing apparatus 300 consists of a cylindrical mounting 301 made of stainless steel which has a central flange 302 in which there is a ring of studs 303. In the mounting 301 are two radial arrays of housings 304 and axial passages 305 which extend through the mounting 301. Inserted into each housing 304 is a plasma reaction chamber 306 each of which has a magnetron 307 coupled directly to it. The magnetrons 307 are individually connected to a power supply unit, which is not shown in the drawing. The two arrays of housings 304 are staggered, as shown in Figure 4. Inside each plasma reaction chamber 306 there is a pair of conical field-enhancing electrodes 308 each of which has an axial hole 309 in it. Thus there is a continuous passage for a gaseous medium to be processed from one side of the apparatus to the other. A first section 310 of flared ducting has a standard industrial coupling flange 311 at one end and a flange 312 at the other, by means of which it is fastened to one plane surface 313 of the housing 301, the joint being sealed by an O-ring seal 314. A similar section 315 of flared ducting is attached to the other plane surface 316 of the housing 301. Components which correspond with similar components of the section of ducting 310 have the same reference numerals. An outer safety shroud 317 is attached to the flange 302 on the housing 304. As shown in the figure, the section of ducting 300 forms an inlet plenum 318 and the section of ducting315 forms an outlet plenum 319. The electrodes which are situated in the plasma reaction chambers 306 are provided with passages through which a colling medium can be circulated. For convenience, neither these nor the associated pipework are shown. Also, in the apparatus described a total of eight plasma reaction chambers 306 are shown. However, if desired, many more can be included. Referring to Figure 5, one of the plasma reaction chambers 306 is adapted to act as a sensor 501 for a control system 502, as shown in our co-pending application GB 96/10841.4 which is arranged to control the action of the remaining plasma reaction chambers 306. Optical data gathered from the sensor 501 is analysed in a microprocessor 503 to determine the presence and partial pressure of specified components of the gaseous medium passing into the plasma processing apparatus. The microprocessor 503 also is arranged to generate control signals which are applied to the power supply unit 504 which is connected to the magnetrons 307 of each plasma reaction chamber 306 (four of which are shown in the figure). The power supply unit 504 can be arranged either to vary the power supplied to all the magnetrons 307 as a function of the partial pressures of the specified components of the gaseous medium, or to vary the number of magnetrons 307 which are operational, again as a function of the partial pressure of the specified components of the gaseous medium. In the case where an inlet plenum chamber is used, the first system is preferred. In the case where separate inlet pipes to each plasma chamber 306 are used, the second system can be adopted. In this case, electrically operated inlet valves can be used so that the number of plasma reaction chambers 105 in circuit can be matched to the amount of gaseous medium to be processed. Alternatively, and particularly if the apparatus is to be used for the removal of a noxious component from a gaseous medium, a gas analyser can be positioned downstream of the plasma reactor chambers 105. There may be included means for supplying a reagent gas adapted to enhance the action of the plasma, either to the inlet plenum chamber into each of the plasma reaction chambers 306 separately. The reagent gas may be such as to enhance the formation of active species in the gaseous medium, or to facilitate oxidation or reduction reactions in the gaseous medium, as is known in the plasma processing.art. There may be included also, means for extracting some of the effluent from the apparatus and injecting it into the inlet to the apparatus.

Claims: A plasma enhanced gas reactor, comprising a reaction chamber, wherein there is included means (102,103) for generating a localised plasma in the reaction chamber comprising means for coupling microwave radiation into the reaction chamber and means (102, 103) for concentrating microwave energy at a pre-determined position in the reaction chamber, means (116, 118) for causing a flow of a first reactant gas and a second reactant gas to intersect in the vicinity of the plasma and means for extracting reaction products from the reaction chamber. A reactor according to Claim 1 wherein the means for concentrating microwave energy at the pre-determined position in the reaction chamber comprises a field-enhancing electrode assembly consisting of a pair of opposed conical assemblies (102, 103, 106, 107) each of which has an axial passage formed in it, the axial passage (118) in one conical assembly (102, 106) of the electrode constituting an inlet for a reactant gas and the axial passage (119) in the other (103, 107) constituting an outlet for reaction products. A reactor according to Claim 2 wherein there is included a shroud (209) surrounding the electrode conical assembly (102, 106) having the inlet passage (201) for a reactant gas and means for passing the second reactant gas through the annular gap between the shroud (209) and the said electrode conical assembly (102, 106). A reactor according to any of claims 1 to 3 further comprising a downstream reactor comprising a catalytic device. A modular plasma processing apparatus comprising a plurality of plasma reaction chambers (306) according to any of Claims 1 to 3, means (318) arranged to feed simultaneously a gaseous medium to be processed to each plasma reaction chamber (306), means (307) for generating separately a plasma in each plasma reaction chamber (306), and outlet means (319) for removing the processed gaseous medium from the plasma reaction chamber. An apparatus according to Claim 5 wherein the means (307) for generating separately the plasma in the plasma reaction chambers (306) comprises a microwave generator (307) coupled directly to an associated plasma reaction chamber (306), and there is included means for controlling the action of the microwave generators (307) comprising means (501) for determining the partial pressure of a component of the gaseous medium and means (502) for varying the power supplied to the microwave generators (307) as a function of the partial pressure of the component of the gaseous medium. Apparatus according to Claim 6 wherein one (501) of the plasma reaction chambers (306) is adapted to act as a sensor for the measurement of the partial pressure of the said component of the gaseous medium. Apparatus according to Claim 5 wherein there is included means for monitoring the constitution of the effluent from the plasma reaction chambers and means for varying the action of the plasma reaction chambers to maintain the partial pressure of a component of the effluent within pre-determined limits. Apparatus according to any of Claims 5 to 8 including means for extracting a proportion of the effluent from the plasma reaction chambers and adding it to the gaseous medium supplied to the plasma reaction chambers. Apparatus according to any preceding claim wherein there is included means for adding a reagent gaseous medium to the said gaseous medium. Apparatus according to Claim 10 wherein the reagent gaseous medium is added to the said gaseous medium prior to its admission to the plasma reaction chambers. A method of operating a plasma enhanced gas reactor according to any of Claims 2 to 5, wherein at least one of the reactant gases is passed through one of the axial passages (118) in the conical assemblies (102, 106) (103,107) and reaction products are removed from the reaction chamber through the axial passage (119) in the other conical assembly. A method according to Claim 12, wherein the gap (111) between the electrode conical assemblies (102, 106) (103, 107) is such that an annular plasma is formed in the gap (111) between the electrode conical assemblies (102, 106) (103, 1.07). A method according to Claim 12 or Claim 13, wherein the first reactant gas is a gas from which a pollutant is to be removed and the second reactant gas is adapted to be activated by the plasma to provide excited radicals which react with the pollutants in the first reactant gas. A method according to Claim 14 wherein the first reactant gas is contaminated with NO x compounds and the second reactant gas is a gas capable of being decomposed by the plasma to provide reducing free radicals.

Inventors: GILLESPIE ROBERT FREW, HALL STEPHEN IVOR, RAYBONE DAVID, WINTERBOTTOM FIONA, GILLESPIE, ROBERT FREW, HALL, STEPHEN IVOR, RAYBONE, DAVID, WINTERBOTTOM, FIONA
Applicants: ACCENTUS PLC
Application year: 1997
Main classification: B01D53
IPC codes: B01J19, H05H1, B01J12, B01D53