Abstract:
The present invention is a method for removing pollution from exhaust gas circulating in an exhaust line ( 12 ) of an internal-combustion engine, the exhaust line comprises an ammonia-sensitive catalysis means ( 46 ) with selective NOx catalytic reduction traversed by the gas and a means ( 56, 58 ) for injecting a reductant of the pollutants The decomposes the reductant into a hydrogen gas phase and an ammonia gas phase and, for a gas temperature below approximately 150° C. injects the hydrogen into the exhaust line in combination with a hydrogen-sensitive NOx catalysis means.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    Reference is made to PCT Application PCT/EP2014/073471 filed Oct. 31, 2014, and French Patent Application No. 13/61,158 filed Nov. 15, 2013, which are incorporated herein by reference in their entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a method for depolluting exhaust gas, notably from internal-combustion engines and in particular for motor vehicles. More particularly, the invention is a method for treating pollutants contained in the exhaust gas of an autoignition internal-combustion engine, notably of Diesel type, but it is also applicable to any means such as a method for spark-ignition engines, such as those running on gas fuel or on gasoline, and in particular with a lean mixture. 
         [0004]    The invention also concerns exhaust gas treatment using the method of the invention 
         [0005]    2. Description of Prior Art 
         [0006]    As it is well known, exhaust gases from engines contain many pollutants such as unburnt hydrocarbons, carbon monoxide, nitrogen oxides (NO and NO 2 ), which is more commonly referred to as NOx, for engines running on gasoline or gas, and additionally particles from Diesel type engines. 
         [0007]    It is widely understood that NOx emissions result from combustion occuring at high temperatures and with high oxygen content. These conditions are generally encountered in any type of combustion and in particular to those taking place under lean burn conditions, such as direct injection in lean burn mode, whatever the fuel being used. 
         [0008]    NOx emissions involve a major drawback in that they have a harmful effect directly on human health and indirectly through the secondary formation of tropospheric ozone. 
         [0009]    In order to comply with emissions standards and to preserve the environment and human health, it has become necessary to treat these pollutants prior to discharging the exhaust gas into the atmosphere. 
         [0010]    As it is generally well known, this is achieved by a treatment for removing pollutants in the exhaust gas circulating in the exhaust line of the engine. 
         [0011]    Thus, in order to treat the unburnt hydrocarbons and the carbon monoxide from engines running with a lean mixture, catalysis means such as an oxidation catalyst are arranged on the exhaust line. 
         [0012]    Regarding the exhaust gas of a Diesel engine, a particle filter is advantageously arranged on this line to capture and to remove the particles present in the exhaust gas, and thus to avoid discharging them to the atmosphere. 
         [0013]    This filter, which can also be a catalyzed filter, needs to be periodically regenerated in order to keep all of its filtration capacities by achieving combustion of the particles retained in this filter. These regeneration operations mainly by increasing the filter temperature, generally through exothermic oxidation, on a catalyst arranged upstream from the filter, for reducing chemical species resulting from the combustion or from an injection directly at the exhaust. 
         [0014]    Regarding the NOx emissions, the exhaust gas also flows through other catalysis means, notably catalysts of SCR (Selective Catalytic Reduction) type. This SCR catalyst allows selective reduction of the NOx to nitrogen through the action of a reductant. 
         [0015]    This reductant, which is generally injected upstream from the SCR catalyst, can be ammonia or a compound generating ammonia by decomposition, such as urea, or a hydrocarbon from a hydrocarbon-containing substance. 
         [0016]    Currently, the commonest technique for NOx depollution is SCR catalysis using ammonia. 
         [0017]    This ammonia is indirectly obtained by decomposition of a precursor injected in liquid form, generally an aqueous urea solution of 32.5 mass % urea, better known under the brand name “AdBlue” or “DEF”. 
         [0018]    Thus, the urea solution is injected into the exhaust line upstream from the SCR catalyst. The water contained in this solution is rapidly vaporized under the effect of the exhaust gas temperature, then each urea molecule decomposes in two stages into two ammonia molecules: 
         [0000]      (NH 2 ) 2 CO (urea)→NH 3  (ammonia)+HNCO (isocyanic acid)   (1)
 
         [0000]      HNCO+H 2 O→NH 3 +CO 2    (2)
 
         [0019]    Alternatively, ammonia can be directly injected into the exhaust line upstream from the SCR catalyst. 
         [0020]    As described in more detail in WO-2011/123,620 or WO-2012/027,368, this ammonia can come from an ammonia storage system or from a urea electrolysis operation. 
         [0021]    Indeed, this electrolysis produces ammonia and nitrogen at one electrode and hydrogen at the other. The ammonia and the nitrogen are fed into the exhaust line upstream from the SCR catalyst and the hydrogen is used as fuel for the internal-combustion engine to improve the energy efficiency thereof. 
         [0022]    Although these techniques are satisfactory, they however involve quite significant drawbacks. 
         [0023]    In fact, for a given SCR catalyst, the reaction efficiency mainly depends on the NO and NO 2  composition of the NOx, and on the gas temperature and flow rate. 
         [0024]    Thus, at temperatures below approximately 150° C., catalysis through Selective Catalytic Reduction of NOx with ammonia is inactive or hardly active. Furthermore, when using a urea-based precursor, for exhaust gas temperatures below approximately 180° C., vaporization of the water contained in the urea solution is difficult to obtain, as well as decomposition of the urea into ammonia and isocyanic acid. Deposits are then likely to form and eventually cause clogging of the exhaust line. Under such temperature conditions, injection of the urea solution into the exhaust line is generally avoided. 
         [0025]    The NOx are therefore discharged to the atmosphere without being treated by the SCR catalyst. 
         [0026]    The present invention aims to overcome the aforementioned drawbacks by means of a method and a plant allowing implementation of low-temperature SCR catalysis operations in a simple and inexpensive manner. 
       SUMMARY OF THE INVENTION 
       [0027]    The present invention thus relates to a method for removing pollutants in the exhaust gas circulating in an exhaust line, from an internal-combustion engine. The line comprises an ammonia-sensitive catalysis means with selective nitrogen oxides catalytic reduction traversed by the gas and means for injecting a reductant into the line in order to treat the pollutants upon passage of the gas through the catalysis means, comprising: 
         [0028]    decomposing the reductant into a compound with a hydrogen gas phase and a compound with an ammonia gas phase; and 
         [0029]    injecting the hydrogen gas phase into the exhaust line for an exhaust gas temperature below approximately 150° C. in combination with a hydrogen-sensitive NOx catalysis means for treating the pollutants in this gas. 
         [0030]    The method can use a catalysis means with selective catalytic reduction as the hydrogen-sensitive NOx catalysis means. 
         [0031]    The method can use an oxidation catalysis means as the hydrogen-sensitive NOx catalysis means. 
         [0032]    The method can comprise, for an exhaust gas temperature above approximately 150° C., inject the ammonia gas phase into the exhaust line in combination with the ammonia-sensitive catalysis means with NOx selective catalytic reduction for treating the pollutants in this gas. 
         [0033]    The method can comprise, for an exhaust gas temperature above approximately 180° C., inject the reductant into the exhaust line in combination with the ammonia-sensitive catalysis means with NOx selective catalytic reduction for treating the pollutants in this gas. 
         [0034]    The method can, for an exhaust gas temperature below approximately 150° C., inject the hydrogen gas phase into the exhaust line in combination with at least one additional catalysis means. 
         [0035]    The method can decompose the reductant by electrolysis. 
         [0036]    The method can comprise in placing at least one of the compounds in a tank. 
         [0037]    The method can control the injection of at least one of the compounds by means of a metering valve. 
         [0038]    The invention also relates to an apparatus for removing pollutants from the exhaust gas circulating in an exhaust line, notably from an internal-combustion engine, comprising an ammonia-sensitive catalysis means with selective NOx catalytic reduction disposed in the exhaust line, means for injecting a reductant into the line in order to treat the pollutants upon passage of the gas through the catalysis means and a means for electrolysis of the reductant, t wherein the exhaust line comprises a hydrogen-sensitive NOx catalysis means and an injector for a hydrogen gas phase compound coming from the electrolysis means. 
         [0039]    The plant can comprise a selective catalytic reduction catalyst as the hydrogen-sensitive NOx catalysis means. 
         [0040]    The plant can comprise an oxidation catalyst as the hydrogen-sensitive NOx catalysis means. 
         [0041]    The exhaust line can comprise an injector for an ammonia gas phase compound coming from the electrolysis means for injecting the ammonia upstream from the ammonia-sensitive catalysis means with NOx selective catalytic reduction. 
         [0042]    The exhaust line can comprise at least one additional catalysis means combined with an injector for a hydrogen gas phase compound coming from the electrolysis means. 
         [0043]    The apparatus can comprise a metering valve disposed in the pipe which connects the electrolysis means to the injector. 
         [0044]    The plant can comprise a tank disposed in the pipe connecting the electrolysis means to the injector. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0045]    Other features and advantages of the invention will be clear from reading the description given hereafter by way of non limitative example, with reference to the accompanying figures wherein: 
           [0046]      FIG. 1  shows an apparatus using the method according to the invention; 
           [0047]      FIG. 2  illustrates a first variant of  FIG. 1 ; and 
           [0048]      FIG. 3  illustrates another variant of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0049]    The exhaust gas treating apparatus for removing pollutants comprises an electrolysis means  10  for a urea-based precursor and an exhaust line  12  combined therewith. 
         [0050]    Exhaust gas is understood to be the exhaust gases coming from an internal-combustion engine, notably for a motor vehicle, but the invention is not limited to thereto and is useful for heating other gas types resulting from a combustion, such as flue gas from boilers. 
         [0051]    The electrolysis means for the urea-based precursor, which is described more in detail in WO-2011/123,620 and WO-2012/027,368, comprises a tank  14  containing precursor  16 , which preferably is an aqueous solution, and an electrolysis cell  18 . 
         [0052]    In order to simplify the rest of the description, the urea-based precursor is simply referred to hereafter as urea. 
         [0053]    The cell comprises a closed chamber  20  for reception of the urea  16  coming from the tank, a cathode  22  and an anode  24  housed within the chamber and immersed in the urea, an electric power source  26  supplying electrical power through electrical conductors  28  to the anode and the cathode, and discharge outlets  30  and  32  for the compounds resulting from the electrolysis. 
         [0054]    The electrical power source can have different origins, such as batteries, fuel cells, etc. 
         [0055]    As described in the aforementioned documents, the cell allows production by electrolysis a compound with an ammonia (NH 3 ) and nitrogen (N 2 ) gas phase, and as another compound having a hydrogen (H 2 ) gas phase. 
         [0056]    For simplification reasons, in the rest of the description, outlet  30  is considered to be the one that discharges the hydrogen gas phase and outlet  32  is considered to be the one allowing discharge of the ammonia and nitrogen gas phase. 
         [0057]    Advantageously, a partition wall  34  is arranged in the chamber which separates cathode  22  from anode  24  and outlet  30  from outlet  32  and isolates the hydrogen gas phase from the ammonia and nitrogen gas phase located in the upper part of the chamber. 
         [0058]    This chamber is supplied with urea in liquid form through a pipe  36  connecting the bottom of this chamber to tank  14 . Advantageously, this pipe comprises a metering pump  38  providing sufficient filling of the chamber for the anode and the cathode to be constantly immersed in the urea. 
         [0059]    As can be better seen in  FIG. 1 , exhaust line  12  comprises, in the direction of circulation of the exhaust gas from inlet  40  of this line to outlet  42 , at least one SCR means for NOx catalysis. More precisely, this line comprises two catalysts, a catalyst referred to as hydrogen catalyst  44  that reacts to form hydrogen which is located close to the exhaust gas inlet. This catalyst is followed in series by a SCR catalyst, referred to as ammonia catalyst  46 , which provides NOx reduction by ammonia. 
         [0060]    Of course, without departing from the scope of the invention, hydrogen catalyst  44  can be an oxidation catalyst or another SCR catalyst. 
         [0061]    In order to simplify the rest of the description below, the example chosen for the hydrogen catalyst is that of a SCR catalyst. 
         [0062]    In a manner known per se, the exhaust line carries a temperature detector (not shown) arranged at the exhaust line inlet providing knowledge at any time of the temperature of the exhaust gas circulating in the line. 
         [0063]    Alternatively, logic and/or computer can be provided, which allow an estimation at any time of the temperature of the exhaust gas circulating in the line. 
         [0064]    As is better visible in  FIG. 1 , hydrogen outlet  30  is connected to a hydrogen injector  48  disposed in the line upstream from hydrogen catalyst  44  by a pipe  50 . Similarly, the ammonia and nitrogen outlet is connected, by a pipe  52 , to an ammonia and nitrogen injector  54  located upstream from ammonia catalyst  46  between the hydrogen catalyst and this ammonia catalyst. Finally, line  12  carries, in a manner known per se, a urea injector  56  arranged upstream from ammonia catalyst  46  and connected by a pipe  58  to a urea injection circuit (not shown) to which tank  14  is connected. 
         [0065]    Advantageously, at least one of the pipes, and here both of the pipes, carry a metering valve  60  and  62  allowing controlling of the proportion of hydrogen (valve  60 ) and/or the proportion of ammonia and nitrogen (valve  62 ) that is injected into the exhaust line. 
         [0066]    Similarly, the pipes can carry a buffer tank  64  and  66  where the hydrogen, for tank  64 , and the ammonia, for tank  66 , produced by cell  18  will be stored. 
         [0067]    Of course, electric source  26 , pump  38  and valves  60 ,  62  are managed by any control means such as a calculator. 
         [0068]    During operation, and considering that at least one of the tanks does not contain a sufficient amount of compounds (hydrogen and/or ammonia) to ensure NOx reduction, and for exhaust gas temperatures below approximately 150° C., in particular upon start-up, electrolysis cell  18  is made operational by powering cathode  22  and anode  24 . 
         [0069]    Powering allows hydrogen to be generated at outlet  30  of the cell, and ammonia and nitrogen at outlet  32 . 
         [0070]    Simultaneously with the generation of hydrogen and ammonia by cell  18 , valve  62  for ammonia is set to closed position while valve  60  is set to open position. 
         [0071]    Hydrogen is thus injected upstream from hydrogen catalyst  44  by injector  48  via hydrogen tank  64  while ammonia is stored in ammonia tank  66 . 
         [0072]    Of course, if the amount of hydrogen and ammonia contained in tanks  64  and  66  is sufficient, cell  18  is not activated, and valves  60  and  62  are controlled as described above. 
         [0073]    This hydrogen injection thus allows treatment of the NOx contained in the exhaust gas that will flow through catalyst  44 . 
         [0074]    Indeed, the applicant has been able to highlight through various analyses that hydrogen is an excellent NOx reductant, and with temperatures of the order of just 100° C. 
         [0075]    By way of example, SCR catalysts using hydrogen with a composition based on Pt/SiO 2  or Pt/MgCeO or Pt/WO 3 /ZrO 2  have shown good activity and selectivity from 90° C. onwards. 
         [0076]    Ag/Al 2 O 3  type catalysts are also good candidates. 
         [0077]    Tests conducted with a SAPO-34 platinum zeolite-based catalyst obtained NO conversion ratios of 78% with a selectivity towards N 2  of 75% at a GHSV of 80,000 h −1  and a gas temperature of 120° C. 
         [0078]    Advantageously, the amounts of hydrogen required for SCR catalysis are very low. Considering the reaction 2H 2 +2NO→N 2 +2H 2 O and the need to reduce by, for example, 0.4 g NOx during the first 400 seconds of a NEDC cycle, 40 mg H 2  are necessary (reaction yield estimated at 66% here). 
         [0079]    As soon as the exhaust gas temperature is above 150° C., hydrogen valve  60  is set to closed position to stop the injection of hydrogen in line  12  while storing the hydrogen in hydrogen tank  64 . 
         [0080]    Simultaneously, ammonia valve  62  is set to open position and the ammonia contained in tank  66  is fed through ammonia injector  54  to the exhaust gas circulating in line  12  between catalysts  44  and  46 . 
         [0081]    The NOx present in the exhaust gas is then treated by catalysis using ammonia in catalyst  46 . 
         [0082]    Once the exhaust gas has reached a high temperature (of the order of 180° C. to 200° C.), ammonia injection is stopped by shutting valve  62  and cell  18  is made non-operational by cutting off the power supply from source  26 . 
         [0083]    Treatment of the NOx from the exhaust gas is thereafter performed in a conventional manner by injecting urea into line  12  through injector  56  arranged between catalysts  44  and  46 . 
         [0084]    The apparatus and the method of the invention allow the NOx treatment to be provided over a very wide temperature range from about 100° C. to over 450° C. 
         [0085]    Of course, the persons skilled in the art will consider all actions necessary and essential to control the metering valves (injection time, flow rate, etc.) to obtain the sufficient amount of compound upstream from the various catalysts to provide depollution of the exhaust gas after passage through SCR catalysts  44  and  46 . 
         [0086]    It should be noted that cell  18  can be active upon conventional NOx treatment with urea to ensure the production and storage of hydrogen and ammonia in tanks  64  and  66 . These compounds can thus be used for a future engine startup with exhaust gas temperatures of about 150° C. 
         [0087]    The examples of  FIGS. 2 and 3  illustrate the various possibilities of use of hydrogen for priming other catalysts, and with particle filter regeneration notably. 
         [0088]    The example of  FIG. 2  illustrates the possibility of placing an additional catalyst  70  in the exhaust line  12  between SCR hydrogen catalyst  44  and SCR ammonia catalyst  46 . Another additional catalyst  72  can also be arranged in this line after SCR ammonia catalyst  46 . 
         [0089]    In the example shown, the additional catalysts can be an oxidation catalyst or a three-way catalyst, or a particle filter, catalyzed or not. 
         [0090]    As better visible in  FIG. 2 , additional catalysts  70  and  72  are each associated with a hydrogen injector  74   a  and  74   b  arranged on the exhaust line upstream from these catalysts. Each one of these injectors is connected to hydrogen tank  64  by a pipe  76   a  and  76   b  carrying each a metering valve  78   a  and  78   b.    
         [0091]    Thus, when starting the vehicle and with exhaust gas temperatures below approximately 150° C., cell  18  is made operational as described above by generating hydrogen and ammonia. 
         [0092]    Upon starting the vehicle, ammonia valve  62  is set to closed position while valve  60  is set to open position for injecting hydrogen upstream from hydrogen catalyst  44  through injector  48 . Optionally, valves  78   a  and/or  78   b  are set to open position for injecting hydrogen upstream from additional catalysts  70  and/or  72  respectively through injectors  74   a  and/or  74   b,  or for thermal catalyst priming when using an oxidation catalyst. 
         [0093]    This hydrogen injection allows treatment of the NOx contained in the exhaust gas that flows through catalyst  44  and increases the exhaust gas temperature to initiate the catalysis operations of catalysts  70  and/or  72 . 
         [0094]    As soon as the temperature reaches a sufficient threshold value, of the order of 150° C., hydrogen injection is stopped at catalyst  44  and ammonia injection is performed at SCR catalyst  46 . 
         [0095]    Of course, hydrogen injection at catalysts  70  and/or  72  is stopped as soon as the operating temperature thereof is reached (oxidation catalyst). 
         [0096]    After this operation, the exhaust gas depollution method is continued as described above with ammonia injection stop and urea supply into line  12  through injector  56  for gas temperatures above the 180° C.-200° C. range. 
         [0097]    In cases where one of catalysts  70  or  72  is a catalyzed particle filter, hydrogen injection upstream from this filter, through injector  74   a  or  74   b,  can be controlled in order to assist with the combustion of particles contained in the filter. 
         [0098]    The example of  FIG. 3  differs from that of  FIG. 2  in the positioning of additional catalysts  70  and  72 . 
         [0099]    In  FIG. 3 , additional catalyst  70  is located between exhaust gas inlet  40  and SCR hydrogen catalyst  44 , and catalyst  72  is located after SCR ammonia catalyst  46  in the vicinity of exhaust gas outlet  42 . 
         [0100]    As mentioned in the description of for  FIG. 2 , catalysts  70  and  72  are linked to a hydrogen injector  74   a  and  74   b,  and to a pipe  76   a  and  76   b  carrying a metering valve  78   a  and  78   b  and connected to tank  64 . 
         [0101]    The principle of operation of the example of this  FIG. 3  is the same as in  FIG. 2 .