Abstract:
The reduction of NO x  in diesel engine exhaust gas, typically at about 200° C. to 400° C., is accomplished using a dual bed NaY—CuY zeolite reduction catalyst. The effectiveness of the catalyst in reducing the nitrogen oxides is markedly increased by the separate and sequential additions of plasma reformed diesel fuel and ozone to the exhaust before it contacts the powdered catalyst. Reformed diesel fuel is obtained by withdrawing fuel from on-board storage, heating the withdrawn volume and stripping a more volatile fraction with air and passing the air/volatile diesel fuel fraction through a non-thermal plasma reactor. Ozone is obtained by blowing ambient air through a second non-thermal plasma reactor.

Description:
TECHNICAL FIELD 
     The present invention relates generally to reduction of nitrogen oxides in exhaust gas from a diesel engine. More specifically, this invention pertains to treating the NO x  content of the exhaust with the separate additions of reformed diesel fuel and ozone before passing the exhaust into contact with a base metal-exchanged zeolite reduction catalyst. 
     BACKGROUND OF THE INVENTION 
     Diesel engines are operated at higher than stoichiometric air to fuel mass ratios for improved fuel economy. Such lean-burning engines produce a hot exhaust with a relatively high content of oxygen and nitrogen oxides (NO x ). The temperature of the exhaust from a warmed up diesel engine is typically in the range of 200° to 400° C. and has a representative composition, by volume, of about 10–17% oxygen, 3% carbon dioxide, 0.1% carbon monoxide, 180 ppm hydrocarbons, 235 ppm NO x  and the balance nitrogen and water. 
     These NO x  gases, typically comprising nitric oxide (NO) and nitrogen dioxide (NO 2 ), are difficult to reduce to nitrogen (N 2 ) because of the high oxygen (O 2 ) content in the hot exhaust stream. It is, thus, an object of the present invention to provide an improved method of reducing NO x  in such gas mixtures. It is a more specific object of the present invention to provide a method of modifying diesel exhaust with reformed diesel fuel before the exhaust is treated with a zeolite type NO x  reduction catalyst. 
     SUMMARY OF THE INVENTION 
     This invention provides a method of reducing NO x  in a diesel engine exhaust stream using a dual bed reduction reactor containing base metal-exchanged Y zeolite catalysts. In accordance with the method, separate additions of plasma-reformed diesel fuel and ozone are made to the exhaust gas stream at locations upstream of the catalytic reduction reactor. These additions modify the exhaust composition to improve the performance of the NO x  reduction catalysts without degrading them. 
     In the present invention, the NO x  containing exhaust is ultimately passed into contact with a dual bed catalyst in which the upstream bed is sodium Y zeolite or barium Y zeolite and the downstream bed is copper Y zeolite. These base metal-exchanged Y-type zeolite catalysts will sometimes be referred to in this specification as NaY, BaY or CuY, respectively. The effectiveness of the dual bed catalyst is promoted by prior addition of plasma-reformed diesel fuel to the exhaust gas followed by the addition of ozone. The ozone addition converts NO to NO 2  before the exhaust reaches the reduction catalyst reactor. The reformed diesel fuel assists in the reduction of NO and NO 2  to N 2  over the base metal-exchanged Y zeolite catalysts. 
     Ozone for addition to the exhaust stream is suitably generated by passing ambient air through a suitable ozone generator. The ozone containing air is injected into the exhaust stream. Plasma reformed diesel fuel is suitably prepared using fuel withdrawn from the engine&#39;s fuel tank. The withdrawn volume of the low volatility diesel fuel is heated and fractionated by bubbling air through it to vaporize a low-boiling fraction of the diesel fuel hydrocarbons. The air-entrained, vaporized diesel fuel fraction is passed through a non-thermal plasma generator to reform the fuel for injection into the exhaust stream. The higher boiling fraction of the fuel is suitably returned either to the fuel tank or to the fuel delivery line for use in the engine. 
     The vaporized fraction of the diesel fuel contains its smaller hydrocarbon molecules. These hydrocarbon molecules are reformed (broken up into smaller radicals and oxidized by ozone) in the hyperplasma reactor. The reformed diesel fuel comprises effective reductant species for NO 2  and is introduced into the exhaust downstream of the ozone addition. As stated above, the ozone oxidizes NO in the exhaust gas to NO 2 . The NO 2  is then reduced to N 2  by reaction with reformed diesel fuel constituents over the dual bed base metal-exchanged Y zeolite catalysts. 
     An efficient non-thermal hyperplasma reactor is used to reform the fractionated fuel stream. The same type of plasma reactor may also be used for ozone generation. In a preferred embodiment, the plasma generator is a tube having a dielectric cylindrical wall defining a reactor space. A linear, high voltage electrode is disposed along the axis of the tube within this reactor space. An outer ground electrode, comprised of electrically conductive wire, is spirally wound around the cylindrical dielectric wall in a sequential pattern having a selected pitch that provides an axially discrete spacing between each turn of the wire. Application of a high frequency, AC voltage to the central electrode creates plasma in the ambient air passed through the reactor. The combination of the helical ground electrode having a discrete spacing between each turn and the linear axial electrode produces intertwined helical regions of active and passive electric fields. 
     The method of the present invention is capable of achieving an average of 95% conversion of NO x  to N 2 , at a catalyst temperature of 200° C., over prolonged operation of the dual bed base metal exchanged zeolite catalysts. The reductant species from the reformed diesel fuel do not degrade the catalyst. 
     The exhaust leaving a diesel engine contains unburned hydrocarbons, especially diesel particulates, and carbon monoxide that are preferably eliminated by catalytic oxidation and filtering of the exhaust prior to the ozone addition to the exhaust. 
     Other objects and advantages of the invention will be apparent from a description of a preferred embodiment which follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic flow diagram for exhaust from a diesel engine illustrating a preferred method for NO x  reduction in accordance with this invention; 
         FIG. 2  is a schematic view of an apparatus and method for fractionation of diesel fuel in the practice of this invention; 
         FIG. 3  is a side view, partly in cross section, of a non-thermal plasma reactor used for treating fractionated diesel fuel to produce a reductant for use in the practice of this invention; 
         FIG. 4  is a schematic view of a dual-bed base metal-exchanged Y zeolite catalytic reduction reactor as used in an embodiment of this invention; 
         FIG. 5  is a bar graph showing the concentrations in parts per million (ppm) of NO, NO 2 , NO x , acetaldehyde (AA), formaldehyde (FA), ethanol, propane, propylene, and carbon monoxide at locations A, B and C in the exhaust stream as designated in  FIG. 1 ; 
         FIG. 6  is a graph showing the NO x  conversion at the exit of the catalytic reactor versus time-on-stream of the dual bed catalyst over the first two hours of operation; and 
         FIG. 7  is a graph of the concentrations in ppm of NO, NO 2 , N 2 O and NO x  at the exit of the catalytic reactor versus time-on-stream of a dual bed catalyst over the first two hours of operation. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A practice of the invention is illustrated schematically in  FIG. 1 . Line  10  represents the flow of the exhaust gas from a diesel engine, not shown. Diesel engines are typically operated at air-to-fuel mass ratios that are considerably higher than the stoichiometric ratio of air to fuel and the exhaust gas contains an appreciable amount of unreacted O 2  as well as N 2  (from the air). The temperature of the exhaust from a warmed-up engine is typically in the range of about 200° C. to about 400° C. The practice of the invention will be illustrated in the case of a diesel engine but it is to be understood that the subject method could be used to treat the exhaust of other lean burn hydrocarbon fueled power sources if diesel fuel is available for the exhaust treatment. In diesel engine exhaust, in addition to O 2  and N 2 , the hot gas also contains CO, CO 2 , H 2 O and hydrocarbons (some in particulate form) that are not completely burned. But the constituent of the exhaust gas to which the subject invention is applicable is the mixture of nitrogen oxides (largely NO and NO 2  with a trace of N 2 O, collectively referred to as NO x ) that are formed by reaction of N 2  with O 2  in the combustion cylinders of the engine (or power plant). The content of NO x  in diesel exhaust is typically about 200–300 parts per million (ppm). So the purpose of this invention is to treat nitrogen oxides that constitute a very small fraction of the volume of the exhaust stream. 
     Exhaust stream  10  ultimately flows to a dual bed catalytic reduction reactor  12  for conversion of the NO x  content of the exhaust to N 2 . Although not shown in the  FIG. 1  exhaust flow diagram for illustration of an embodiment of this invention, the diesel engine exhaust may first be treated by catalytic oxidation and filtration for removal of diesel particulates and other unburned hydrocarbons. Following such oxidation and/or filtration, two important additions are made to exhaust stream  10  before it reaches reduction reactor  12 . 
     Reference is made to  FIGS. 1 and 2 . Diesel fuel, suitably from the fuel supply for the engine, is pumped, line  14  to a fuel fractionator  16 . The fuel enters inlet  18  ( FIG. 2 ) and is received in the aeration chamber  20  as fuel volume  28 . Aeration chamber  20  and fuel volume  28  are heated using external heating coil  22  to a suitable temperature, e.g. 200° C., for air vaporization of the low volatility hydrocarbon fuel. Ambient air is inducted, by blower means not shown, through air line  24  to vertical air feed tube  26  and, thus, into the bottom of chamber  20  below the surface of fuel volume  28 . The stream of air exits feed tube  26  through quartz frit  30  and bubbles up through fuel volume  28 . Thermocouple  32 , inserted through an otherwise closed end  34  of air feed tube  26 , extends down feed tube  26  to a suitable location below the surface of fuel volume  28 . Thermocouple  32  is used in a known manner for control of heater coil  22  in maintaining the temperature of fuel volume  28  at temperature suitable for fractionation of the diesel fuel. 
     The air stream bubbling through the heated fuel volume  28  of diesel fuel leaves the fractionator  16  through air/fuel outlet  36 . The ambient air bubbling through the heated fuel volume  28  strips out (vaporizes) a fraction of the fuel volume  28  to form an air stream carrying the more volatile, lower molecular weight hydrocarbons from the fuel. This hydrocarbon laden air stream flows through line  38  to a non-thermal, highly efficient plasma reactor, HP- 1 , for plasma reforming of the hydrocarbons. The structure and function of the efficient plasma reactor HP- 1 , termed a hyperplasma reactor, (and similar reactor HP- 2  for ozone generation from ambient air) will be described below in connection with the illustration of  FIG. 3 . In plasma reactor HP- 1 , the hydrocarbon molecules fractionated from the diesel fuel volume  28  are reformed and oxidized to form reactive NO x  reduction material, still carried in the air stream, through line  40  into exhaust stream  10 . The reformed fuel comprises hydrocarbons such as propane and propylene and oxygenated hydrocarbons such as formaldehyde, acetaldehyde and ethyl alcohol. 
     When fractionator  16  is used in combination with an operating engine the fractionation process is a continuous process. As the air stream, line  24 , strips out a relatively more volatile portion of fuel volume  28  the remainder of volume  28  becomes smaller and enriched with less volatile hydrocarbons. This portion of the withdrawn fuel is returned either to the fuel tank or to the fuel delivery line for combustion in the engine. Accordingly, it is preferred that diesel fuel be pumped continually to and from the fractionator  16  as follows. A measured volume of fuel is introduced into inlet  18  continuously or in suitable periodic batches. As fractionated fuel is removed in the flowing air stream, line  38 , residual fuel is drawn from fuel volume  28  through the bottom of fractionator  16  at outlet  42  and returned either to the fuel tank or to the fuel delivery line. The return flow of fuel is controlled by valve  48 , or other suitable means, to maintain a suitable fuel volume  28  in chamber  20 . Thus, in an operating engine embodiment, fuel and air are continuously delivered to fractionator  16  through fractionator inlets  18  and  24 , respectively, and streams of air/fractionated fuel and residual fuel are withdrawn through fractionator outlets  36  and  42 . 
     In  FIG. 3 , a non-thermal hyperplasma reactor  100  is illustrated that is suitable for use in reforming fractionated diesel fuel in a stream of air, and for generating ozone in a stream of air, both for use in the practice of this invention. The reactor  100  is sized and powered for its specific application. 
     Non-thermal plasma reactor  100  comprises a cylindrical tubular dielectric body  102 . The reactor  100  has two electrodes, a high voltage electrode  104  and a ground electrode  106 , separated by the tubular dielectric body  102  and an air gap  108 . The high voltage electrode  104  is a straight rod placed along the longitudinal axis of the tube  102 . The ground electrode  106  is a wire wound around the tubular dielectric body  102  in a helical pattern. The helical ground electrode  106  in combination with the axial high voltage electrode  104  provides intertwined helical regions of active  110  and passive  112  electric fields along the length of the reactor  100 . The helical active electric field  110  around the ground electrode  106  is highly focused for effective plasma generation for the reforming of diesel fuel and for ozone generation. 
     A high voltage, high frequency electrical potential is applied to the end leads  114 ,  116  to the center electrode. The helical outer ground electrode  106  is grounded as indicated at  118 . In the operation of the plasma reactor  100  as HP- 1  for reformation of the fractionated diesel fuel, a mixture of the fuel and air flows through the INLET of reactor  100  around center electrode  104  and within dielectric tube  102  and out EXIT end in the direction of the arrows seen in  FIG. 3 . The electrical potential applied to center electrode  104  generates the above described active  110  and passive  112  fields within the reactor  100 . These high potential, high frequency fields  110 ,  112  generate reactive hydrocarbon species and oxygen species within the flowing air/fuel stream in the air gap  108  which results in the production of oxygenated hydrocarbon radicals or other activated species. This oxygenated hydrocarbon-containing air stream leaves the reactor  100  (HP- 1 ), enters line  40 , and is immediately introduced into the exhaust stream  10  as indicated in  FIG. 1 . 
     As will be described in detail below, electrical power is applied to HP- 1  reactor at a level that is suitable to generate the reformed oxygenated hydrocarbon material. HP- 1  reactor is located close to, but away from, the hot exhaust pipe. HP- 1  plasma reactor is a non-thermal reactor but entering stream  38  may be above ambient temperature because ambient air was used to vaporize heated fuel volume  28  in fractionator  16 . 
     In addition to air/reformed diesel fuel stream  40 , ozone is generated in an ambient air stream and injected into exhaust stream  10 . Referring again to  FIG. 1 , ambient air is blown through a second plasma reactor, HP- 2 . Preferably, plasma reactor HP- 2  is a suitable adaptation of a non-thermal plasma reactor  100  as described with respect to  FIG. 3 . Alternatively, a commercial ozone generator may be used. When the ambient air is subjected to the high intensity alternating electric field in HP- 2  a fraction of the air is converted to ozone and the ozone/air mixture leaving HP- 2  through line  44  is injected into exhaust stream  10  downstream of line  40 , the reformed diesel fuel containing stream. 
     As seen in  FIG. 1 , the hot exhaust stream  10  containing suitable additions of reformed diesel fuel, stream  40  and ozone, stream  44 , enters the dual bed catalytic reduction reactor  12 . 
     As illustrated in  FIG. 4 , catalytic reduction reactor  12  houses a dual bed reduction catalyst. The upstream catalyst bed comprises a volume of sodium (or barium) Y zeolite, indicated as NaY (or BaY), and the downstream bed, usually a smaller volume, comprises copper Y zeolite (indicated as CuY). Y-type zeolites are aluminosilicate materials of rather specific alumina-to-silica ratio and crystal structure. They have ion-exchange capability and they are commercially available, often in their Na +  ion form. In the practice of this invention NaY may be converted to BaY or CuY by aqueous ion exchange. 
     The temperature at the reactor  12  outlet is used in controlling plasma power density in HP- 1  and HP- 2 , respectively and the volumetric feed ratios of reformed diesel fuel, line  40 , and ozone, line  44  for effective operation of the catalytic reduction reactor  12 . For example, the temperature at the outlet of the reduction catalyst may be monitored for effective exhaust gas treatment by thermocouple (indicated at T 1 ) or other suitable temperature sensor(s). Temperature data is transmitted to a digital controller (not shown) for controlling plasma power density and amount of stream additions through lines  40  and  44 . Stream  46  indicates the treated exhaust being discharged from the exhaust system. 
     The heat and hydrocarbon content of stream  46  may be utilized by using it to supplement or replace a portion of air stream  24  entering fuel fractionator  16  and/or the air stream entering ozone reactor HP- 2 . These recycled exhaust streams  50  (to fractionator  16 ) and  52  (to HP- 2 ) are shown schematically in  FIG. 1 . 
     In general, the requirement for reformed diesel fuel constituents increases with increased NO x  content in the exhaust and increased exhaust temperature (catalytic reactor temperature). For example, about 8 moles of reformed fuel, normalized as C 1  hydrocarbon per mole of normalized NO x  at a catalyst temperature of 200° C. Conversely, the ozone requirement is greatest at catalytic reactor temperatures of 150–200° C. and decreases to zero at reactor temperatures of 350–400° C. 
     The following experiments illustrate the practice and effectiveness of the invention. 
     Experimental 
     A simulated diesel exhaust gas composed, by volume, of 181.5 ppm NO, 24.5 ppm NO 2 , 17.6% O 2 , 2% H 2 O and the balance N 2  was used in the following laboratory scale tests. This simulated exhaust gas was used as stream  10  in  FIG. 1  for eventual catalytic reduction in a dual bed catalytic reactor as indicated at  12  in  FIG. 1 . 
     The dual bed catalytic reactor was made of a quartz tube with a ¼ inch (about 6.4 mm) outside diameter, 4 mm inside diameter, and containing NaY zeolite in an upstream bed and CuY in the downstream bed. CuY zeolite was made from NaY by aqueous ion-exchange of NaY obtained from Zeolyst Corp. The amounts of NaY and CuY used were 422 mg and 211 mg, respectively. The catalytic reactor was placed in an electric furnace whose temperature was controlled by a thermocouple located at the exit of the catalytic reactor. In these tests the catalytic reactor was maintained at 200° C. 
     A batch operation fractionator like that illustrated in  FIG. 2 , but without fuel exit line  42 , was made of a quartz bulb. Raw diesel fuel was contained in the bulb at a sufficient level. Air fed through the inlet tube and the vertical air feed tube flowed through the quartz frit making bubbles. The air bubbles generated a large surface area for diesel fuel evaporation while agitating the liquid fuel during their travel upward, resulting in an enhanced evaporation of diesel fuel. The temperature of the liquid fuel was controlled by adjusting the electric power supply to the heating element in response to the readings of a thermocouple. Though the preferred temperature range is 100–250° C. to fractionate a low-boiling portion of the fuel, a temperture of 200° C. was employed. The flow rate of ambient air to the fractionator was 34 cubic centimetes per minute at standard conditions (sccm). The air and low-boiling diesel fraction flowed through the exit to the hyperplasma reactor (HP- 1 ) for reforming. 
     A hyperplasma reactor for the fractionated diesel fuel was made in accordance with the reactor illustrated in  FIG. 3 . The reactor was made of a 8 mm o.d. (6 mm i.d.) quartz tube which served as a dielectric barrier. With the high voltage electrode in the center, it was made in a concentric cylindrical geometry. Air and vaporized diesl fuel at an unmeasured exhaust temperature from the fractionator entered HP- 1 . HP- 1  was unheated and the air/fractionated fuel mixture flowed through the annular space between the center elctrode and the quartz tube. The ground electrode was made of a Ni wire wound around the outer surface of the quartz tube in 20 turns at a pitch of 2 mm. The total length of the plasma generating area was 4 cm. An alternating high voltage of +/−7 kV was applied to the center electrode at a power level of 2.7 J/L. The reformed fuel was analyzed and contained propane, propylene, formaldehyde, acetaldehyde and ethanol. The carbon content of the reformed fuel may be normalized in terms of molar methane (C 1 ) content for purposes of simplifying process control. The amount of C 1  content of the reformed fuel is based on the NO x  content of the exhaust and the temerature of the exhaust or catalytic reactor. The reformed diesel fuel was fed to the simulated diesel engine exhaust gas stream before the stream reached the dual bed catalytic reactor. 
     A commercial ozone generator was used as HP- 2 . Air at room temperature was fed to the generator at 45 sccm and the air/ozone output of the generator containing 1200 ppm ozone was added to the simulated diesel exhaust downstream of the addition of reformed fuel and before the exhaust stream was passed through the catalytic reactor. This concentration of ozone in the air stream was suitable for the catalytic reactor operating at 200° C. and lower. The ozone requirement decreases, generally proportionately, as the temperature of the reactor increases. When the catalyst is at about 350° C. and higher, no ozone addition is required. 
     The simulated exhaust, reformed fuel, and ozone entered the dual bed catalyst reactor at a combined flow rate of 179 sccm and at a pressure of 101.3 kPa. The C 1 /NO x  ratio at the inlet of the catalyltic reactor (sample location B in  FIG. 1 ) was around 8. The space velocity in the 200° C. reactor was 11 k/h for the NaY bed and 22 k/h for the CuY bed. At higher catalyst temperatures the proportion of reformulated fuel, C1, increases. 
       FIG. 5  shows the product distribution measured by an FTIR at each sampling position (A, B and C in  FIG. 1 ) in the system. As specified above, the normalized exhaust composition with the hyperplasmas turned off was 181.5 ppm NO and 24.5 ppm NO 2 . Sampling position A shows the effect on exhaust gas composition of the addition of reformed diesel fuel. Sampling positon B shows the effect on exhaust gas compositon following the ozone addition. And sampling positon C shows the compositon of the gas leaving the catalytic reduction reactor. 
       FIG. 5  indicates that the major role of the first hyperplasma (HP- 1 ) is to produce acetaldehyde (AA), while the major role of the second hyperplasma (O 3  generator) is to oxidize NO to NO 2  in the exhaust gas stream. The data on NO x  concentrations in  FIG. 5  clearly demonstrates that the subject process can achieve 95% NO x  conversion at the catalyst temperature of 200° C. This is a remarkable performance better than anything reported in the literature using diesel fuel as the reductant. 
       FIG. 6  shows the transient NO x  conversion performance of the dual-bed catalytic reactor at 200° C. for the first two hours of operation. When the exhaust flow stream was fed to the catalytic reactor, the NO x  conversion reached above 90% and then decreased slightly, followed by a steady increase to steady-state conversion of 95%. There was no noticebale deactivation of catalyst activity. 
       FIG. 7  shows the transient evolution of N-containing species at the outlet of the catalytic reactor in the D/SCR system. The catalysts reached the steady state in about 90 minutes on stream without any indication of catalyst deactivation. The amounts of other N-containing species such as N 2 O, NO 2 , HCN and NH 3  were negligible. 
     The invention has been described by illustration of specific embodiments but the scope of the invention is not limited to them.