Patent Application
20160298514
1. Field of the Invention
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.
The invention also concerns exhaust gas treatment using the method of the invention
2. Description of Prior Art
As it is well known, exhaust gases from engines contain many pollutants such as unburnt hydrocarbons, carbon monoxide, nitrogen oxides (NO and NO2), which is more commonly referred to as NOx, for engines running on gasoline or gas, and additionally particles from Diesel type engines.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Currently, the commonest technique for NOx depollution is SCR catalysis using ammonia.
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”.
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:
(NH2)2CO (urea)→NH3 (ammonia)+HNCO (isocyanic acid) (1)
HNCO+H2O→NH3+CO2 (2)
Alternatively, ammonia can be directly injected into the exhaust line upstream from the SCR catalyst.
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.
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.
Although these techniques are satisfactory, they however involve quite significant drawbacks.
In fact, for a given SCR catalyst, the reaction efficiency mainly depends on the NO and NO2 composition of the NOx, and on the gas temperature and flow rate.
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.
The NOx are therefore discharged to the atmosphere without being treated by the SCR catalyst.
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.
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:
decomposing the reductant into a compound with a hydrogen gas phase and a compound with an ammonia gas phase; and
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.
The method can use a catalysis means with selective catalytic reduction as the hydrogen-sensitive NOx catalysis means.
The method can use an oxidation catalysis means as the hydrogen-sensitive NOx catalysis means.
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.
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.
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.
The method can decompose the reductant by electrolysis.
The method can comprise in placing at least one of the compounds in a tank.
The method can control the injection of at least one of the compounds by means of a metering valve.
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.
The plant can comprise a selective catalytic reduction catalyst as the hydrogen-sensitive NOx catalysis means.
The plant can comprise an oxidation catalyst as the hydrogen-sensitive NOx catalysis means.
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.
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.
The apparatus can comprise a metering valve disposed in the pipe which connects the electrolysis means to the injector.
The plant can comprise a tank disposed in the pipe connecting the electrolysis means to the injector.
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:
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.
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.
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.
In order to simplify the rest of the description, the urea-based precursor is simply referred to hereafter as urea.
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.
The electrical power source can have different origins, such as batteries, fuel cells, etc.
As described in the aforementioned documents, the cell allows production by electrolysis a compound with an ammonia (NH3) and nitrogen (N2) gas phase, and as another compound having a hydrogen (H2) gas phase.
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.
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.
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.
As can be better seen in
Of course, without departing from the scope of the invention, hydrogen catalyst 44 can be an oxidation catalyst or another SCR catalyst.
In order to simplify the rest of the description below, the example chosen for the hydrogen catalyst is that of a SCR catalyst.
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.
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.
As is better visible in
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.
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.
Of course, electric source 26, pump 38 and valves 60, 62 are managed by any control means such as a calculator.
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.
Powering allows hydrogen to be generated at outlet 30 of the cell, and ammonia and nitrogen at outlet 32.
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.
Hydrogen is thus injected upstream from hydrogen catalyst 44 by injector 48 via hydrogen tank 64 while ammonia is stored in ammonia tank 66.
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.
This hydrogen injection thus allows treatment of the NOx contained in the exhaust gas that will flow through catalyst 44.
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.
By way of example, SCR catalysts using hydrogen with a composition based on Pt/SiO2 or Pt/MgCeO or Pt/WO3/ZrO2 have shown good activity and selectivity from 90° C. onwards.
Ag/Al2O3 type catalysts are also good candidates.
Tests conducted with a SAPO-34 platinum zeolite-based catalyst obtained NO conversion ratios of 78% with a selectivity towards N2 of 75% at a GHSV of 80,000 h−1 and a gas temperature of 120° C.
Advantageously, the amounts of hydrogen required for SCR catalysis are very low. Considering the reaction 2H2+2NO→N2+2H2O and the need to reduce by, for example, 0.4 g NOx during the first 400 seconds of a NEDC cycle, 40 mg H2 are necessary (reaction yield estimated at 66% here).
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.
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.
The NOx present in the exhaust gas is then treated by catalysis using ammonia in catalyst 46.
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.
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.
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.
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.
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.
The examples of
The example of
In the example shown, the additional catalysts can be an oxidation catalyst or a three-way catalyst, or a particle filter, catalyzed or not.
As better visible in
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.
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 78a and/or 78b are set to open position for injecting hydrogen upstream from additional catalysts 70 and/or 72 respectively through injectors 74a and/or 74b, or for thermal catalyst priming when using an oxidation catalyst.
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.
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.
Of course, hydrogen injection at catalysts 70 and/or 72 is stopped as soon as the operating temperature thereof is reached (oxidation catalyst).
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.
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.
The example of
In
As mentioned in the description of for
The principle of operation of the example of this
| Number | Date | Country | Kind |
|---|---|---|---|
| 13 61158 | Nov 2013 | FR | national |
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.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2014/073471 | 10/31/2014 | WO | 00 |
