Method for depolluting exhaust gas, notably from internal-combustion engines, in particular for motor vehicles, and plant using same

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 catalyst (46) with selective NOx catalytic reduction traversed by the gas and an injector (56, 58) for injecting a reductant of the pollutants The catalyst 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 catalyst.

Claims

1. A method for removing pollutants from exhaust gas circulating in an exhaust line of an internal combustion engine, the line comprising an ammonia-sensitive catalyst providing selective catalytic reduction (SCR) of nitrogen oxides in the exhaust gas in the exhaust line and an injector for injecting a reductant into the exhaust line for treating the pollutants upon passage of the exhaust gas through the catalyst providing SCR of nitrogen oxides comprising: decomposing the reductant into hydrogen gas and ammonia gas phase; storing the hydrogen gas in a hydrogen gas tank and the ammonia gas phase in an ammonia gas tank; injecting the hydrogen gas from the hydrogen gas tank into the exhaust line in contact with a hydrogen sensitive SCR NOx reduction catalyst for treating pollutants in the exhaust gas for an exhaust gas temperature below 150 C.; injecting the ammonia gas from the ammonia gas tank into the exhaust line in contact with an ammonia SCR catalyst to provide NOx reduction for treating the pollutants in the exhaust line for exhaust gas temperatures between 150 C. and 180 C.; and injecting the reductant into the exhaust line in contact with the ammonia SCR catalyst to provide NOx reduction for treating exhaust gas at temperatures above 180 C.

2. The method as claimed in claim 1 comprising using an oxidation sensitive catalyst as a hydrogen sensitive SCR of NOx.

3. The method as claimed in claim 2 comprising injecting the hydrogen gas into the exhaust line in combination with at least one additional catalyst for treatment of exhaust gas temperatures below 150 C.

4. The method as claimed in claim 3 comprising decomposing the reductant with electrolysis.

5. The method as claimed in claim 2 comprising decomposing the reductant with electrolysis.

6. The method as claimed in claim 1 comprising injecting the hydrogen gas into the exhaust line in combination with at least one additional catalyst for treatment of exhaust gas temperatures below 150 C.

7. The method as claimed in claim 6 comprising decomposing the reductant with electrolysis.

8. The method as claimed in claim 1 comprising decomposing the reductant with electrolysis.

9. The method in accordance with claim 1 comprising controlling injection of at least one of the compounds with a metering valve.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) 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:

(2) FIG. 1 illustrates an apparatus using the method according to the invention;

(3) FIG. 2 illustrates a first variant of FIG. 1; and

(4) FIG. 3 illustrates another variant of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

(5) The exhaust gas treating apparatus for removing pollutants comprises an electrolysis system 10 for a urea-based precursor and an exhaust line 12 combined therewith.

(6) 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.

(7) The electrolysis system 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.

(8) In order to simplify the rest of the description, the urea-based precursor is simply referred to hereafter as urea.

(9) 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.

(10) The electrical power source can have different origins, such as batteries, fuel cells, etc.

(11) As described in the aforementioned documents, the cell allows production by electrolysis of a compound having an ammonia (NH.sub.3) and nitrogen (N.sub.2) gas phases, and as another compound having a hydrogen (H.sub.2) gas phase.

(12) 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.

(13) 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.

(14) 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.

(15) 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 catalyst 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.

(16) Of course, without departing from the scope of the invention, hydrogen catalyst 44 can be an oxidation catalyst or another SCR catalyst.

(17) In order to simplify the rest of the description below, the example chosen for the hydrogen catalyst is that of a SCR catalyst.

(18) 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.

(19) 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.

(20) 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.

(21) Advantageously, at least one of the pipes, and here both of the pipes, carry a metering valve 60 and 62 allowing controlling at least one of the proportion of hydrogen (valve 60) and the proportion of ammonia and nitrogen (valve 62) that is injected into the exhaust line.

(22) Similarly, the pipes can carry e buffer tanks 64 and 66 where respectively hydrogen is stored in tank 64, and ammonia in tank 66 which is produced by cell 18.

(23) Of course, electrical source 26, pump 38 and valves 60, 62 are managed by any control such as a calculator.

(24) 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.

(25) Powering allows hydrogen to be generated at outlet 30 of the cell, and ammonia and nitrogen at outlet 32.

(26) 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.

(27) Hydrogen is thus injected upstream from hydrogen catalyst 44 by injector 48 via hydrogen tank 64 while ammonia is stored in ammonia tank 66.

(28) 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.

(29) The hydrogen injection allows treatment of the NOx contained in the exhaust gas that will flow through catalyst 44.

(30) Indeed, the applicant has been able to highlight through various analyses that hydrogen is an excellent NOx reductant, with temperatures on the order of just 100 C.

(31) By way of example, SCR catalysts using hydrogen with a composition based on Pt/SiO.sub.2 or Pt/MgCeO or Pt/WO.sub.3/ZrO.sub.2 have shown good activity and selectivity from 90 C. onwards.

(32) Ag/Al.sub.2O.sub.3 type catalysts are also good candidates.

(33) Tests conducted with a SAPO-34 platinum zeolite-based catalyst obtained NO conversion ratios of 78% with a selectivity towards N.sub.2 of 75% at a GHSV of 80,000 h.sup.1 and a gas temperature of 120 C.

(34) Advantageously, the amounts of hydrogen required for SCR catalysis are very low. Considering the reaction 2H.sub.2+2NO.fwdarw.N.sub.2+2H.sub.2O and the need to reduce by, for example, 0.4 g NOx during the first 400 seconds of a NEDC cycle, 40 mg H.sub.2 are necessary (reaction yield estimated at 66% here).

(35) 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.

(36) 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.

(37) The NOx present in the exhaust gas is then treated by catalysis using ammonia in catalyst 46.

(38) 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.

(39) 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.

(40) This apparatus and the method linked of the invention allow the NOx treatment to be provided over a very wide temperature range between about 100 C. to over 450 C.

(41) Of course, a person 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 removal of pollutants from the exhaust gas after passage through SCR catalysts 44 and 46.

(42) 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.

(43) 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.

(44) 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.

(45) In the example shown, the additional catalysts can be an oxidation catalyst or a three-way catalyst, or a particle filter, catalyzed or not.

(46) As better visible in FIG. 2, additional catalysts 70 and 72 are each associated with a hydrogen injector 74a and 74b arranged on the exhaust line upstream from these catalysts. Each one of these injectors is connected to hydrogen tank 64 by a pipe 76a and 76b each carrying a metering valve 78a and 78b.

(47) Thus, when starting the vehicle at exhaust gas temperatures below approximately 150 C., cell 18 is made operational as described above by generating hydrogen and ammonia.

(48) 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, at least one of valves 78a and 78b are set to open position for injecting hydrogen upstream from at least one of additional catalysts 70 and/or 72 respectively through at least one of injectors 74a and 74b, or for thermal catalyst priming when using an oxidation catalyst.

(49) This hydrogen injection allows treatment of the NOx contained in the exhaust gas that flows through catalyst 44 and increasing the exhaust gas temperature to initiate the catalysis operations of at least one of catalysts 70 and/or 72.

(50) As soon as the temperature reaches a sufficient threshold value, on the order of 150 C., hydrogen injection is stopped at catalyst 44 and ammonia injection is performed at SCR catalyst 46.

(51) Of course, hydrogen injection at least one of catalysts 70 and 72 is stopped as soon as the operating temperature thereof is reached (oxidation catalyst).

(52) 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.

(53) In cases where one of catalysts 70 or 72 is a catalyzed particle filter, hydrogen injection upstream from this filter, through injector 74a or 74b, can be controlled in order to assist with the combustion of particles contained in the filter.

(54) The example of FIG. 3 differs from that of FIG. 2 in the positioning of additional catalysts 70 and 72.

(55) 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.

(56) As mentioned in the description of for FIG. 2, catalysts 70 and 72 are linked to a hydrogen injector 74a and 74b, and to a pipe 76a and 76b carrying a metering valve 78a and 78b and connected to tank 64.

(57) The principle of operation of the example of this FIG. 3 is the same as in FIG. 2.