WO2014044318A1 - Method and system for the removal of noxious compounds from engine exhaust gas - Google Patents

Method and system for the removal of noxious compounds from engine exhaust gas Download PDF

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Publication number
WO2014044318A1
WO2014044318A1 PCT/EP2012/068621 EP2012068621W WO2014044318A1 WO 2014044318 A1 WO2014044318 A1 WO 2014044318A1 EP 2012068621 W EP2012068621 W EP 2012068621W WO 2014044318 A1 WO2014044318 A1 WO 2014044318A1
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WO
WIPO (PCT)
Prior art keywords
catalyst
exhaust gas
ammonium nitrate
scr
upstream
Prior art date
Application number
PCT/EP2012/068621
Other languages
French (fr)
Inventor
Keld Johansen
Pio Forzatti
Isabella Nova
Enrico Tronconi
Original Assignee
Haldor Topsøe A/S
Politecnico Di Milano
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Publication date
Application filed by Haldor Topsøe A/S, Politecnico Di Milano filed Critical Haldor Topsøe A/S
Priority to PCT/EP2012/068621 priority Critical patent/WO2014044318A1/en
Publication of WO2014044318A1 publication Critical patent/WO2014044318A1/en

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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/74—General processes for purification of waste gases; Apparatus or devices specially adapted therefor
    • B01D53/86—Catalytic processes
    • B01D53/90—Injecting reactants
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/92—Chemical or biological purification of waste gases of engine exhaust gases
    • B01D53/94—Chemical or biological purification of waste gases of engine exhaust gases by catalytic processes
    • B01D53/9404—Removing only nitrogen compounds
    • B01D53/9409—Nitrogen oxides
    • B01D53/9431—Processes characterised by a specific device
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
    • F01N13/00—Exhaust or silencing apparatus characterised by constructional features ; Exhaust or silencing apparatus, or parts thereof, having pertinent characteristics not provided for in, or of interest apart from, groups F01N1/00 - F01N5/00, F01N9/00, F01N11/00
    • F01N13/009—Exhaust or silencing apparatus characterised by constructional features ; Exhaust or silencing apparatus, or parts thereof, having pertinent characteristics not provided for in, or of interest apart from, groups F01N1/00 - F01N5/00, F01N9/00, F01N11/00 having two or more separate purifying devices arranged in series
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
    • F01N13/00—Exhaust or silencing apparatus characterised by constructional features ; Exhaust or silencing apparatus, or parts thereof, having pertinent characteristics not provided for in, or of interest apart from, groups F01N1/00 - F01N5/00, F01N9/00, F01N11/00
    • F01N13/009—Exhaust or silencing apparatus characterised by constructional features ; Exhaust or silencing apparatus, or parts thereof, having pertinent characteristics not provided for in, or of interest apart from, groups F01N1/00 - F01N5/00, F01N9/00, F01N11/00 having two or more separate purifying devices arranged in series
    • F01N13/0093—Exhaust or silencing apparatus characterised by constructional features ; Exhaust or silencing apparatus, or parts thereof, having pertinent characteristics not provided for in, or of interest apart from, groups F01N1/00 - F01N5/00, F01N9/00, F01N11/00 having two or more separate purifying devices arranged in series the purifying devices are of the same type
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
    • F01N3/00—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust
    • F01N3/02—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for cooling, or for removing solid constituents of, exhaust
    • F01N3/021—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for cooling, or for removing solid constituents of, exhaust by means of filters
    • F01N3/033—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for cooling, or for removing solid constituents of, exhaust by means of filters in combination with other devices
    • F01N3/035—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for cooling, or for removing solid constituents of, exhaust by means of filters in combination with other devices with catalytic reactors, e.g. catalysed diesel particulate filters
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
    • F01N3/00—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust
    • F01N3/08—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous
    • F01N3/10—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by thermal or catalytic conversion of noxious components of exhaust
    • F01N3/105—General auxiliary catalysts, e.g. upstream or downstream of the main catalyst
    • F01N3/106—Auxiliary oxidation catalysts
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
    • F01N3/00—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust
    • F01N3/08—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous
    • F01N3/10—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by thermal or catalytic conversion of noxious components of exhaust
    • F01N3/18—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by thermal or catalytic conversion of noxious components of exhaust characterised by methods of operation; Control
    • F01N3/20—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by thermal or catalytic conversion of noxious components of exhaust characterised by methods of operation; Control specially adapted for catalytic conversion ; Methods of operation or control of catalytic converters
    • F01N3/2066—Selective catalytic reduction [SCR]
    • F01N3/208—Control of selective catalytic reduction [SCR], e.g. dosing of reducing agent
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2251/00—Reactants
    • B01D2251/20—Reductants
    • B01D2251/206—Ammonium compounds
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2251/00—Reactants
    • B01D2251/20—Reductants
    • B01D2251/206—Ammonium compounds
    • B01D2251/2067—Urea
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/90—Physical characteristics of catalysts
    • B01D2255/915—Catalyst supported on particulate filters
    • B01D2255/9155—Wall flow filters
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/92—Chemical or biological purification of waste gases of engine exhaust gases
    • B01D53/94—Chemical or biological purification of waste gases of engine exhaust gases by catalytic processes
    • B01D53/9404—Removing only nitrogen compounds
    • B01D53/9409—Nitrogen oxides
    • B01D53/9413—Processes characterised by a specific catalyst
    • B01D53/9418—Processes characterised by a specific catalyst for removing nitrogen oxides by selective catalytic reduction [SCR] using a reducing agent in a lean exhaust gas
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/92—Chemical or biological purification of waste gases of engine exhaust gases
    • B01D53/94—Chemical or biological purification of waste gases of engine exhaust gases by catalytic processes
    • B01D53/944—Simultaneously removing carbon monoxide, hydrocarbons or carbon making use of oxidation catalysts
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/92—Chemical or biological purification of waste gases of engine exhaust gases
    • B01D53/94—Chemical or biological purification of waste gases of engine exhaust gases by catalytic processes
    • B01D53/9459—Removing one or more of nitrogen oxides, carbon monoxide, or hydrocarbons by multiple successive catalytic functions; systems with more than one different function, e.g. zone coated catalysts
    • B01D53/9477—Removing one or more of nitrogen oxides, carbon monoxide, or hydrocarbons by multiple successive catalytic functions; systems with more than one different function, e.g. zone coated catalysts with catalysts positioned on separate bricks, e.g. exhaust systems
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
    • F01N2610/00—Adding substances to exhaust gases
    • F01N2610/02—Adding substances to exhaust gases the substance being ammonia or urea
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
    • F01N2900/00—Details of electrical control or of the monitoring of the exhaust gas treating apparatus
    • F01N2900/06—Parameters used for exhaust control or diagnosing
    • F01N2900/14—Parameters used for exhaust control or diagnosing said parameters being related to the exhaust gas
    • F01N2900/1404—Exhaust gas temperature
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02T—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO TRANSPORTATION
    • Y02T10/00—Road transport of goods or passengers
    • Y02T10/10—Internal combustion engine [ICE] based vehicles
    • Y02T10/12—Improving ICE efficiencies

Definitions

  • the present invention relates to a method and system for reducing emission of nitrogen oxides (NOx) and particulate matter being present in the exhaust from a lean burning internal compression ignition engine.
  • NOx nitrogen oxides
  • the method and system of the invention provides an improved re ⁇ duction of NOx during cold start of the engine.
  • the exhaust system of modern vehicles with lean burning engines is equipped with an oxidation catalyst, a particulate filter and a catalyst for the selective reduction of NOx (SCR) in the presence of a reducing agent.
  • SCR selective reduction of NOx
  • Oxidation catalysts being active in the oxidation of vola ⁇ tile organic compounds, nitrogen monoxide and carbon monox ⁇ ide and SCR catalysts are known in the art and disclosed in numerous publications.
  • particulate filters are the so called wall flow filters with a plurality of inlet and outlet channels.
  • the inlet channels are closed at their outlet and the out ⁇ let channels are closed at their inlet, so that the gas flowing into the filter is forced through porous walls de ⁇ fining the channels, whereby particulate matter is filtered off the gas.
  • ammonia is commonly employed as the reducing agent.
  • Ammonia is a noxious compound and it is preferred to generate ammonia in situ by thermal decomposi- tion of a urea solution being injected as ammonia precursor into the hot exhaust gas upstream the SCR catalyst.
  • the invention is based on using an SCR catalysed filter in combination with low temperature injection of ammonium nitrate into exhaust gas from a lean burning engine during the cold start phase of the engine when the exhaust gas temperature is below 250°C and a second SCR catalyst, wherein the necessary reducing agent is formed by decompo- sition of urea introduced into the exhaust gas at tempera ⁇ tures above 180°C in the cold start phase.
  • the necessary reducing agent is formed by decompo- sition of urea introduced into the exhaust gas at tempera ⁇ tures above 180°C in the cold start phase.
  • aqueous ammonium nitrate is injected into the exhaust gas upstream the catalysed particulate filter during a cold start phase of the engine when the gas has a temperature of below 250°C, and wherein - when the exhaust gas has reached a tem ⁇ perature of about 220-250°C - injection of ammonium nitrate is discontinued and urea is injected into the gas, either upstream the first SCR catalyst or downstream the first SCR catalyst and upstream a second SCR catalyst for the selective reduction of nitrogen oxides.
  • the method which additionally comprises the step of contacting the exhaust gas with a catalyst being active in oxidation of volatile organic com ⁇ pounds and carbon monoxide to carbon dioxide and water and nitrogen oxide to nitrogen dioxide, upstream of said particulate filter, wherein the ammonium nitrate may be in- jected into the exhaust gas stream prior to, or after, con ⁇ tact with the oxidation catalyst.
  • Injection of ammonium nitrate prior to contact with the oxidation catalyst near the engine manifold with short mixing distance has the poten ⁇ tial to the earliest injection during the cold start and thus improving the NOx removal.
  • the invention allows the oxidation catalyst (DOC) and the SCR catalysed filter (SCR/DPF) to be arranged in close cou ⁇ pled position.
  • the close coupled position together with a small volume of DOC and SCR/DPF will facilitate a fast heat up of these units and thus a sufficient catalyst activity in an early phase after cold start.
  • the DOC will early in the cold start phase form O2 from NO in exhaust and the close coupled filter SCR/DPF will have temperature condi ⁇ tions for passive soot regeneration with NO2 .
  • Ammonium nitrate is injected in the form of an aqueous so ⁇ lution. Injection can be started at an exhaust gas tempera ⁇ ture from 160 °C, preferably from 170 °C.
  • ammonium nitrate is injected into the exhaust gas prior to the contact with the DOC.
  • ammonium nitrate can be injected between the DOC and the SCR/DPF.
  • a close-coupled SCR catalyst is present up ⁇ stream the SCR catalyzed particulate filter, and both said urea and said aqueous ammonium nitrate are injected up ⁇ stream the close-coupled SCR catalyst.
  • Ammonium nitrate may be stored on board as such in a con ⁇ tainer, e.g. as an aqueous solution. Ammonium nitrate will convert NO to NO2 at the lower temperatures according to the reaction:
  • Ammonium nitrate injection is discontinued when the exhaust temperature is about 220-250 °C and urea injection into ex ⁇ haust gas leaving the catalysed filter is initiated at about 180°C. This implies that only a limited amount of stored ammonium nitrate is required for the total NOX reduction during the cold start phase.
  • ammonia is formed by decomposi- tion of a urea solution being injected into the hot exhaust gas between the SCR/DPF and the second SCR.
  • the NO in the exhaust gas is oxidised to O2 by contact with the DOC.
  • the formed O2 is used in the passive regeneration of the DPF.
  • all the amount of formed O2 can be used for passive soot regeneration of the filter and fast SCR from low temperatures .
  • the second SCR catalyst can be selected from cheaper vanadium or zeolite catalyst com- pounds.
  • the passive regeneration is more effective because ammonia is not present in the exhaust gas during the main driving cycle and the SCR function of the SCR/DPF is interrupted .
  • DOC oxidation catalyst
  • Urea - ammonium nitrate (UAN) solutions used as fertilizers, are commercially available
  • small amounts of ammonia may be present in the exhaust gas from the second SCR. It is thus preferred to pass the ex ⁇ haust gas from the second SCR through a selective ammonia oxidation catalyst (ASC) downstream the second SCR.
  • ASC selective ammonia oxidation catalyst
  • the se lective ammonia oxidation catalyst converts ammonia to ni ⁇ trogen .
  • the invention provides a system for use in the method according to the invention.
  • the system comprises within an engine exhaust gas channel connected to the engine, arranged in series
  • a particulate filter comprising a first catalyst for selective reduction of nitrogen oxides; optionally, a second catalyst unit for the selec ⁇ tive reduction of nitrogen oxides;
  • injection means for the injection of aqueous ammonium nitrate in aqueous solution into the engine exhaust gas chan- nel;
  • injection means for the injection of urea into the engine ex- haust gas channel.
  • the injection means for injection of ammonium nitrate is arranged between the engine and the inlet of the oxidation catalyst unit.
  • the injection means for injection of ammonium nitrate is connected to a container.
  • the first SCR catalyst integrated in the filter for use in the inventive method and system is based on thermostable copper and/or iron promoted zeolites or silica alumina phosphate compounds.
  • the second SCR catalyst for use in the inventive method and system is preferably selected from vanadium on titania, copper and/or iron promoted zeolites, copper and/or iron promoted silica alumina phosphates, optionally combined with cerium oxides with zirconium and aluminium oxides.
  • a close-coupled SCR catalyst is present upstream the SCR catalyzed particulate filter, and injection means for both urea and aqueous ammonium nitrate are present up ⁇ stream the close-coupled SCR catalyst.

Abstract

Method and system for the removal of noxious compounds from exhaust gas of a lean burning internal compression ignition engine, the method comprising passing the exhaust gas through a particulate filter being catalysed with a first SCR catalyst for selective reduction of nitrogen oxides; wherein aqueous ammonium nitrate is injected into the exhaust gas upstream the catalysed particulate filter during a cold start phase of the engine when the gas has a temperature of below 250°C, and wherein - when the exhaust gas has reached a temperature of about 220-250°C - injection of ammonium nitrate is discontinued and urea is injected into the gas, either upstream the first SCR catalyst or downstream the first SCR catalyst and upstream a second SCR catalyst for the selective reduction of nitrogen oxides.

Description

Method and system for the removal of noxious compounds from engine exhaust gas
The present invention relates to a method and system for reducing emission of nitrogen oxides (NOx) and particulate matter being present in the exhaust from a lean burning internal compression ignition engine. In particular, the method and system of the invention provides an improved re¬ duction of NOx during cold start of the engine.
The exhaust system of modern vehicles with lean burning engines is equipped with an oxidation catalyst, a particulate filter and a catalyst for the selective reduction of NOx (SCR) in the presence of a reducing agent.
Oxidation catalysts being active in the oxidation of vola¬ tile organic compounds, nitrogen monoxide and carbon monox¬ ide and SCR catalysts are known in the art and disclosed in numerous publications.
Typically used particulate filters are the so called wall flow filters with a plurality of inlet and outlet channels. The inlet channels are closed at their outlet and the out¬ let channels are closed at their inlet, so that the gas flowing into the filter is forced through porous walls de¬ fining the channels, whereby particulate matter is filtered off the gas.
In the SCR treatment, ammonia is commonly employed as the reducing agent. Ammonia is a noxious compound and it is preferred to generate ammonia in situ by thermal decomposi- tion of a urea solution being injected as ammonia precursor into the hot exhaust gas upstream the SCR catalyst.
Even if urea is innocuous and relatively easy to store on board of a car, use of a liquid solution of urea as a pre¬ cursor of ammonia reducing agent is problematic in particu¬ lar in the cold start phase of the engine, i.e. when the exhaust gas temperature is below 180°C. When injected as liquid solution in the exhaust gas, urea decomposes to ammonia in sufficient amounts for the SCR only at a temperature from about 180°C.
The invention is based on using an SCR catalysed filter in combination with low temperature injection of ammonium nitrate into exhaust gas from a lean burning engine during the cold start phase of the engine when the exhaust gas temperature is below 250°C and a second SCR catalyst, wherein the necessary reducing agent is formed by decompo- sition of urea introduced into the exhaust gas at tempera¬ tures above 180°C in the cold start phase. Thereby it is possible to obtain a NOx reduction rate of more than 99% in the engine exhaust gas in a complete driving cycle. Thus, the invention provides a method for the removal of noxious compounds from exhaust gas of a lean burning inter¬ nal compression ignition engine comprising in series the steps of
passing the exhaust gas through a particulate fil- ter being catalysed with a first SCR catalyst for selective reduction of nitrogen oxides; and wherein aqueous ammonium nitrate is injected into the exhaust gas upstream the catalysed particulate filter during a cold start phase of the engine when the gas has a temperature of below 250°C, and wherein - when the exhaust gas has reached a tem¬ perature of about 220-250°C - injection of ammonium nitrate is discontinued and urea is injected into the gas, either upstream the first SCR catalyst or downstream the first SCR catalyst and upstream a second SCR catalyst for the selective reduction of nitrogen oxides.
Of particular interest is the method which additionally comprises the step of contacting the exhaust gas with a catalyst being active in oxidation of volatile organic com¬ pounds and carbon monoxide to carbon dioxide and water and nitrogen oxide to nitrogen dioxide, upstream of said particulate filter, wherein the ammonium nitrate may be in- jected into the exhaust gas stream prior to, or after, con¬ tact with the oxidation catalyst. Injection of ammonium nitrate prior to contact with the oxidation catalyst near the engine manifold with short mixing distance has the poten¬ tial to the earliest injection during the cold start and thus improving the NOx removal.
The invention allows the oxidation catalyst (DOC) and the SCR catalysed filter (SCR/DPF) to be arranged in close cou¬ pled position. The close coupled position together with a small volume of DOC and SCR/DPF will facilitate a fast heat up of these units and thus a sufficient catalyst activity in an early phase after cold start. The DOC will early in the cold start phase form O2 from NO in exhaust and the close coupled filter SCR/DPF will have temperature condi¬ tions for passive soot regeneration with NO2 . Ammonium nitrate is injected in the form of an aqueous so¬ lution. Injection can be started at an exhaust gas tempera¬ ture from 160 °C, preferably from 170 °C.
Thus, in an embodiment of the invention ammonium nitrate is injected into the exhaust gas prior to the contact with the DOC.
Alternatively, ammonium nitrate can be injected between the DOC and the SCR/DPF.
In one aspect, a close-coupled SCR catalyst is present up¬ stream the SCR catalyzed particulate filter, and both said urea and said aqueous ammonium nitrate are injected up¬ stream the close-coupled SCR catalyst.
Ammonium nitrate may be stored on board as such in a con¬ tainer, e.g. as an aqueous solution. Ammonium nitrate will convert NO to NO2 at the lower temperatures according to the reaction:
NO + NH4NO3 → N02 + N2 + 2H20
Ammonium nitrate injection is discontinued when the exhaust temperature is about 220-250 °C and urea injection into ex¬ haust gas leaving the catalysed filter is initiated at about 180°C. This implies that only a limited amount of stored ammonium nitrate is required for the total NOX reduction during the cold start phase. In the main driving cycle when the ex¬ haust gas is above 220°C, ammonia is formed by decomposi- tion of a urea solution being injected into the hot exhaust gas between the SCR/DPF and the second SCR.
Above 200°C the NO in the exhaust gas is oxidised to O2 by contact with the DOC. The formed O2 is used in the passive regeneration of the DPF. Thus, above temperatures of 220°C all the amount of formed O2 can be used for passive soot regeneration of the filter and fast SCR from low temperatures . With modern low soot emission engines it is possible to rely on passive soot regeneration and the maximum inlet temperature to the second SCR catalyst can be kept below 550°C. This implies that the second SCR catalyst can be selected from cheaper vanadium or zeolite catalyst com- pounds.
As further an advantage of the method according to the in¬ vention the passive regeneration is more effective because ammonia is not present in the exhaust gas during the main driving cycle and the SCR function of the SCR/DPF is interrupted .
In addition, expensive oxidation catalyst (DOC) may even be fully omitted, and cold start facilitated in the following configurations:
Engine → NH4NO3 + urea → SCR/DPF → ASC → out or
Engine → NH4N03 + urea → SCR/DPF → urea → SCR → ASC → out or
Engine → NH4N03 + urea → SCR/DPF → SCR → ASC → out
or
Engine → NH4N03 → SCR/DPF → urea → SCR → ASC → out
or
Engine → NH4N03 + urea → ccSCR → SCR/DPF → ASC → out
(ASC = ammonium slip catalyst)
When a DOC is fully omitted, injection of ammonium nitrate before SCR/DPF will also assist soot combustion at low temperature above about 200 °C as O2 is directly formed.
Above this temperature is also an option to use blends of urea and ammonia solutions. Urea - ammonium nitrate (UAN) solutions, used as fertilizers, are commercially available
Small amounts of ammonia may be present in the exhaust gas from the second SCR. It is thus preferred to pass the ex¬ haust gas from the second SCR through a selective ammonia oxidation catalyst (ASC) downstream the second SCR. The se lective ammonia oxidation catalyst converts ammonia to ni¬ trogen . Additionally, the invention provides a system for use in the method according to the invention.
The system comprises within an engine exhaust gas channel connected to the engine, arranged in series
optionally, an oxidation catalyst unit for the oxi¬ dation of volatile organic compounds and carbon monoxide to carbon dioxide and water and nitrogen oxide to nitrogen dioxide;
a particulate filter comprising a first catalyst for selective reduction of nitrogen oxides; optionally, a second catalyst unit for the selec¬ tive reduction of nitrogen oxides;
upstream the particulate filter, injection means for the injection of aqueous ammonium nitrate in aqueous solution into the engine exhaust gas chan- nel; and
upstream the particulate filter, or between the particulate filter and the second catalyst for the selective reduction of nitrogen oxides, injection means for the injection of urea into the engine ex- haust gas channel.
In an embodiment of the invention, the injection means for injection of ammonium nitrate is arranged between the engine and the inlet of the oxidation catalyst unit.
In further an embodiment, the injection means for injection of ammonium nitrate is connected to a container.
When the DOC and SCR/DPF are arranged in close-coupled po- sition, temperature loss is limited, which facilitates higher temperatures and increased O2 formation over the DOC and higher temperatures in the filter resulting in an improved passive soot regeneration. To remove small amounts of ammonia having not been con¬ verted in the SCR catalysts, it is preferred to arrange an ammonia slip catalyst (ASC) downstream the second SCR unit. As already mentioned hereinbefore, suitable catalysts for use in the invention are known in the art and are not the subject of the present invention.
Preferably, the first SCR catalyst integrated in the filter for use in the inventive method and system is based on thermostable copper and/or iron promoted zeolites or silica alumina phosphate compounds.
The second SCR catalyst for use in the inventive method and system is preferably selected from vanadium on titania, copper and/or iron promoted zeolites, copper and/or iron promoted silica alumina phosphates, optionally combined with cerium oxides with zirconium and aluminium oxides.
Suitably, a close-coupled SCR catalyst is present upstream the SCR catalyzed particulate filter, and injection means for both urea and aqueous ammonium nitrate are present up¬ stream the close-coupled SCR catalyst.

Claims

Claims
1. A method for the removal of noxious compounds from exhaust gas of a lean burning internal compression ignition engine comprising in series the steps of passing the exhaust gas through a particulate fil¬ ter being catalysed with a first SCR catalyst for selective reduction of nitrogen oxides; and wherein aqueous ammonium nitrate is injected into the exhaust gas upstream the catalysed particulate filter during a cold start phase of the engine when the gas has a temperature of below 250°C, and wherein - when the exhaust gas has reached a tem¬ perature of about 220-250°C - injection of ammonium nitrate is discontinued and urea is injected into the gas, either upstream the first SCR catalyst or downstream the first SCR catalyst and upstream a second SCR catalyst for the selective reduction of nitrogen oxides.
2. The method according to claim 1, additionally com¬ prising the step of contacting the exhaust gas with a catalyst being active in oxidation of volatile organic compounds and carbon monoxide to carbon di- oxide and water and nitrogen oxide to nitrogen di¬ oxide, upstream of said particulate filter, wherein the ammonium nitrate may be injected into the ex¬ haust gas stream prior to, or after, contact with the oxidation catalyst.
3. The method according to any one of the preceding claims, wherein the ammonium nitrate is injected into the exhaust gas prior to the contact with the oxidation catalyst.
The method according to any one of claims 1-2, wherein the ammonium nitrate is injected into the exhaust gas between the oxidation catalyst and the SCR catalyzed particulate filter.
The method according to any one of claims 1 to 4, wherein the exhaust gas is further passed through an ammonia oxidation catalyst for selective oxida¬ tion of ammonia downstream the second SCR catalyst.
The method according to any one of the preceding claims, wherein the ammonium nitrate is comprised in an aqueous solution of urea and ammonium nitrate .
The method according to any one of the preceding claims, wherein a close-coupled SCR catalyst is present upstream the SCR catalyzed particulate fil¬ ter, and both said urea and said aqueous ammonium nitrate are injected upstream the close-coupled SCR catalyst .
System for use in the method according to any one of claims 1-7 comprising within an engine exhaust gas channel connected to the engine, arranged in series
optionally, an oxidation catalyst unit for the oxi¬ dation of volatile organic compounds and carbon monoxide to carbon dioxide and water and nitrogen oxide to nitrogen dioxide;
a particulate filter comprising a first catalyst for selective reduction of nitrogen oxides;
optionally, a second catalyst unit for the selec¬ tive reduction of nitrogen oxides;
upstream the particulate filter, injection means for the injection of aqueous ammonium nitrate in aqueous solution into the engine exhaust gas chan¬ nel; and
upstream the particulate filter, or between the particulate filter and the second catalyst for the selective reduction of nitrogen oxides, injection means for the injection of urea into the engine ex¬ haust gas channel.
The system according to claim 8, wherein the injection means for injection of ammonium nitrate as an aqueous solution is arranged between the engine and the inlet of the oxidation catalyst unit.
The system according to any one of claims 8-9, wherein the oxidation catalyst unit and the par¬ ticulate filter comprising a first catalyst for se¬ lective reduction of nitrogen oxides are arranged in close-coupled position.
The system according to anyone of claims 8-10 being further provided with a catalyst unit for the se¬ lective oxidation of ammonia to nitrogen downstream the second catalyst unit for the selective reduc¬ tion of nitrogen oxides. The system according to any one of claims 8-11, wherein a close-coupled SCR catalyst is present up¬ stream the SCR catalyzed particulate filter, and injection means for both urea and aqueous ammonium nitrate are present upstream the close-coupled SCR catalyst .
PCT/EP2012/068621 2012-09-21 2012-09-21 Method and system for the removal of noxious compounds from engine exhaust gas WO2014044318A1 (en)

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US10837338B2 (en) 2015-08-27 2020-11-17 Scania Cv Ab Method and exhaust treatment system for treatment of an exhaust gas stream
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CN109569293A (en) * 2018-12-10 2019-04-05 中国神华能源股份有限公司 A kind of equipment for denitrifying flue gas and denitration method for flue gas
CN109876796A (en) * 2019-04-04 2019-06-14 上海理工大学 A kind of cerium tungsten composite oxidant SCR (Selective Catalytic Reduction) denitrating denitrating catalyst and preparation method thereof

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