Patent Publication Number: US-2004042940-A1

Title: Plasma reactor gas processing

Description:
[0001] The present invention relates to the plasma-assisted processing of gaseous media and in particular to the reduction of the emission of carbonaceous and nitrogenous oxide combustion products from the exhausts of internal combustion engines.  
       [0002] One of the major problems associated with the development and use of internal combustion engines is the noxious exhaust emissions from such engines. Two of the most deleterious materials, particularly in the case of diesel engines, are particulate matter (primarily carbon) and oxides of nitrogen (NO x ). Increasingly severe emission control regulations are forcing internal combustion engine and vehicle manufacturers to find more efficient ways of removing these materials in particular from internal combustion engine exhaust emissions. Unfortunately, in practice, it is found that combustion modification techniques which improve the situation in relation to one of the above components of internal combustion engine exhaust emissions tend to worsen the situation in relation to the other. Even so, a variety of systems for trapping particulate emissions from internal combustion engine exhausts have been investigated, particularly in relation to making such particulate emission traps capable of being regenerated when they have become saturated with particulate material.  
       [0003] Examples of such diesel exhaust particulate filters are to be found in European patent applications EP 0 010 384; U.S. Pat. Nos. 4,505,107; 4,485,622; 4,427,418; and 4,276,066; EP 0 244 061; EP 0 112 634 and EP 0 132 166.  
       [0004] In all the above cases, the particulate matter is removed from diesel exhaust gases by a simple, physical trapping of particulate matter in the interstices of a porous, usually ceramic, filter body, which is then regenerated by heating the filter body to a temperature at which the-trapped diesel exhaust particulates are burnt off. In most cases the filter body is monolithic, although EP 0 010 384 does mention the use of ceramic beads, wire meshes or metal screens as well. U.S. Pat. No. 4,427,418 discloses the use of ceramic coated wire or ceramic fibres.  
       [0005] GB patent 2,274,412 discloses a method and apparatus for removing particulate and other pollutants from internal combustion engine exhaust gases, in which the exhaust gases are passed through a bed of charged pellets of material, preferably ferroelectric, having high dielectric constant. In addition to removing particulates by oxidation, especially electrical discharge assisted oxidation, there is disclosed the reduction of NO x  gases to nitrogen, by the use of pellets adapted to catalyse the NO x  reduction.  
       [0006] A problem which arises with plasma assisted gas reactors which include a bed of pellets of a high-dielectric constant material, such as that disclosed in specification GB 2 274 412, is that localised variations in the electric field in the pellet bed can occur, possibly leading to regions of the pellet bed in which the electric field is insufficient to enable a plasma to be established in a gaseous medium to be processed in the reactor.  
       [0007] It is an object of the present invention to provide an improved reactor for the plasma-assisted processing of gaseous media.  
       [0008] According to the present invention there is provided a reactor for the plasma assisted processing of a gaseous medium comprising a reactor chamber containing a gas permeable active material, means for constraining a gaseous medium to be processed in the reactor to pass through the chamber and the active material therein and a plurality of electrodes by means of which there can be established across the active material an electric field sufficient to establish a plasma in the gaseous medium passing through the interstices in the active material, wherein there is included at least one electrode connected to a high voltage input terminal and positioned within the active material between two parallel electrodes connected to an earth point.  
       [0009] The active material may comprise a bed of active material, preferably material having a high dielectric permittivity.  
       [0010] An advantage of the electrode arrangement of the present invention is that for a given width of a bed of pellets of active medium, the electric field is doubled, thus improving the efficiency of the production of a plasma in a gaseous medium passing through the bed of active material in the reactor.  
       [0011] Preferably the bed of active material has a hollow cylindrical configuration, and coaxial cylindrical electrodes form the inner and outer surfaces of the cylinder and the high voltage electrode is positioned halfway between the earthed electrodes and co-axial therewith.  
       [0012] In a preferred embodiment of the invention the bed of active material is in the form of individual pellets contained between two concentric cylindrical earthed electrodes, the high voltage electrode also is a cylindrical electrode concentric with the earthed electrodes and situated halfway between the earthed electrodes, the electrodes are gas permeable, the gaseous medium is constrained to flow radially through the bed of active material and the pellets on the upstream side of the high voltage electrode have a larger size than those on the downstream side of the high voltage electrode.  
       [0013] In a particular aspect of the present invention the plasma assisted gas reactor is adapted to be incorporated into the exhaust system of an internal combustion engine for the removal of nitrogenous and/or carbonaceous combustion products therefrom. In this form of the invention there may be included downstream of the plasma assisted gas reactor a catalytic converter for the further reduction of the concentration of noxious combustion products in the exhaust gases from the internal combustion engine.  
       [0014] The catalytic converter may be included in the same reactor chamber as the active material. In particular it can be included in the central region of a bed of active material, when that has a cylindrical configuration.  
       [0015] Examples of suitable materials for use as the active material are titanates, particularly barium titanate, titania, particularly in the anatase phase, zirconia and mixtures of these compounds, aluminas, metal-doped zeolites, and vanadia. 
     
    
    
     [0016] The invention will now be described, by way of example, with reference to the accompanying drawings, in which,  
     [0017]FIG. 1 is a longitudinal section of a first embodiment of the invention,  
     [0018]FIG. 2 is a longitudinal section of a second embodiment of the invention,  
     [0019]FIG. 3 is a longitudinal section of a third embodiment of the invention,  
     [0020]FIG. 4 is a longitudinal section of a fourth embodiment of the invention,  
     [0021]FIG. 5 is a longitudinal section of a fifth embodiment of the invention,  
     [0022]FIG. 6 is a longitudinal section of a sixth embodiment of the invention, and  
     [0023]FIG. 7 is a longitudinal section of a seventh embodiment of the invention, 
    
    
     [0024] Referring to FIG. 1, a reactor  100  for the plasma assisted treatment of the exhaust gases from an internal combustion engine to remove noxious combustion products therefrom consists of a cylindrical metal casing  101  which has an inlet stub  102  and an outlet stub  103  by means of which it can be incorporated into the exhaust system of an internal combustion engine. (Not shown in the figure). The reactor casing  101  is divided into two compartments  104 ,  105 . The upstream compartment  104  of the reactor casing  101  includes a bed  106  of pellets  107  of an active material, such as barium titanate, which is contained between two earthed concentric cylindrical electrodes  108 ,  109  made of perforated stainless steel. The upstream end of the inner electrode  108 , is closed by a thimble  110 , also made of stainless steel. Situated centrally within the bed  106  of pellets  107  of active material, concentric with the electrodes  108  and  109 , is a third cylindrical electrode  111 , also made of perforated stainless steel. The electrode  111  is connected to a high voltage terminal  112 . The electrodes  108 ,  109  and  111  are supported and located in the compartment  104  of the reactor casing  101  by two diaphragms  113  and  114  made of a ceramic insulating material which is resistant to thermal shock, such as alumina. The upstream electrode support  113  has a ring of regularly spaced axially oriented holes  115  around its periphery which open into a space  116  between the reactor casing  101  and the outer electrode  109 . The downstream electrode support  114  has a central aperture  117  the diameter of which is approximately equal to that of the inner electrode  108 . Thus, exhaust gases entering the reactor chamber  101  are constrained to pass radially through the bed  106  of pellets  107  before leaving the compartment  104  of the reactor casing  101  through the central aperture  117  in the inner earthed electrode  108 . The compartment  105  of the reactor  100  contains a conventional monolith catalyst body  118  for the reduction of exhaust emissions from internal combustion engines. Between the compartments  104 ,  105  of the reactor casing  101  there is a flow director plate  119  which is arranged to ensure that the flow of exhaust gases leaving the compartment  104  of the reactor casing  101  through the aperture  117  in the electrode  108  is expanded to encompass the whole upstream surface  120  of the catalyst body  118 .  
     [0025]FIG. 2 shows a longitudinal section of a second embodiment of the invention. Those components which correspond with similar components of the first embodiment have the same reference numerals. In this embodiment of the invention the reactor casing  101  has only one compartment  201  and a monolithic body of catalytic material  202  is contained within the inner electrode  108 . The remainder of the device is the same as the device of FIG. 1.  
     [0026]FIG. 3 shows a longitudinal section of a third embodiment of the invention. Again, components which are similar to those of previous embodiments have the same reference numerals. In this embodiment of the invention, again there is a monolithic catalyst within the inner electrode  108 , but in this case, the catalyst is in the form of a series of disks  301  instead of a single body and there is a second body  302  of catalyst situated between the aperture  117  in the second electrode  114  and the outlet stub  103  of the reactor casing  101 .  
     [0027]FIG. 4 shows another embodiment of the invention in which the space inside the inner electrode  108  is filled with beads  401  of catalytic material instead of a number of disks of monolithic catalyst. The remainder of the reactor is the same as for the FIG. 3 embodiment of the invention and bears the same reference numerals.  
     [0028]FIG. 5 shows an embodiment of the invention in which the high voltage electrode  111  is connected directly to a thermally protected high voltage transformer  501  which has a hollow cylindrical form and is contained in a second compartment  502  of the reactor casing  101 . Again the space within the inner electrode  108  is filled with a monolithic catalyst  202  for the further reduction of noxious components of internal combustion engine exhaust gases.  
     [0029]FIG. 6 shows another embodiment of the invention which has the high voltage electrode connected directly to the output from a high voltage transformer  601 . In this case, however, the space inside the inner electrode  108  has a plurality of disks  602  of monolithic catalytic material, as in the embodiment described with reference to FIG. 3, and the inside of the hollow cylindrical transformer  601  is filled with another body  603  of monolithic catalytic material.  
     [0030] The embodiments of the invention described so far are all configured to provide for a substantially radial flow of the gaseous medium through the bed  106  of pellets  107  of active material. However, plasma assisted gas processing reactors embodying the invention can be configured to provide an axial flow though the bed of active material of a gaseous medium to be processed in the reactor.  
     [0031]FIG. 7 shows one such arrangement. Again, those components which are similar to corresponding components of the first embodiment have the same reference numerals. Referring to FIG. 7, the inner electrode  108  is replaced by a central rod  701  and the electrode supports  113  and  114  are replaced by corresponding electrode supports  702 ,  703  which are either made of a readily gas permeable insulating ceramic material or which have an array of axial holes  704  in them. Also, the outer earthed electrode  109  and high voltage electrode  111  are made of unperforated stainless steel sheet. The baffle plate  119  is not required and is omitted. The remainder of the reactor is the same as for the FIG. 1 embodiment.  
     [0032] The configuration of the FIG. 7 embodiment lends itself to adaptation for dielectric barrier discharge operation. For this, at least the high voltage electrode is provided on all surfaces with a layer of dielectric material to form the dielectric barrier. Alternatively both earth electrodes are provided on all surfaces with a layer of dielectric material. However, a preferred arrangement for dielectric barrier discharge operation is for all the electrodes to be provided on all surfaces with a layer of dielectric material.  
     [0033] It will be appreciated that the radial flow embodiments of FIGS.  1  to  6  can be similarly adapted for dielectric barrier discharge operation, but, in that case, the design of the electrodes is more difficult since, in providing for gas passage through the electrodes, care is required to ensure that all metal surfaces are coated with a layer of dielectric barrier material.