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11/13/2015
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IIT Kanpur Kanpur, India (208016)
www.iitk.ac.in/erl Selective Catalytic Reduction
Technique, NOx Storage Catalysts
Dr. Avinash Kumar Agarwal Professor
Engine Research Laboratory, Department of Mechanical Engineering,
Indian Institute of Technology, Kanpur INDIA [email protected]
Engine Research Laboratory, IIT Kanpur
Strategies for Future Emissions Legislation
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Engine Research Laboratory, IIT Kanpur
Conventional NOx Reduction Technologies
SAE, 2007-01-0239
Engine Research Laboratory, IIT Kanpur
NOx Control Technology Fundamental problem: Reductants that aid in NOx conversion prefer to react with oxygen
rather than NOx
Technology Performance Range NOx CO HC PM
Active Lean NOx 25-50 >70 >70 ~ 30
SCR Urea >70 >50 >70 > 30
NOx Adsorber 50-95 >70 >70 > 30
Plasma / NOx Cat. >60 >50 >50 ~ 30
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Engine Research Laboratory, IIT Kanpur
NOx Technologies Operating Experience
NOx Technology Concept Overview
Engine Research Laboratory, IIT Kanpur
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Engine Research Laboratory, IIT Kanpur
Engine Research Laboratory, IIT Kanpur
Catalyst Reaction type Emissions
Selective catalytic reduction (SCR): SCR by ammonia/urea
4NO + 4NH3 + O2 ↔ 4N2 + 6H2O
2NO + 2NO2+ 4NH3 ↔ 4N2 + 6H2O
NOX adsorbers (traps): NOX adsorption -lean exhaust, reduction -rich conditions
NO + 0.5O2 ↔ NO2
BaO+ 2NO2+ 0.5O2 ↔Ba(NO3)2
Soot filters Oxidation
C+0.5O2 ↔ C
NO2+ C ↔CO + NO
CO+0.5O2 ↔ CO2
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Engine Research Laboratory, IIT Kanpur
Engine Research Laboratory, IIT Kanpur
NOx Absorbers The NOx Adsorber Catalyst (NAC) is a new technology developed in the late 1990s.
NAC uses a combination of base metal oxide and precious metal coatings to effect control of NOx. The base metal component (for example, barium oxide) reacts with NOx to form barium nitrate – effectively storing the NOx on the surface of the catalyst.
When the available storage sites are occupied, the catalyst is operated briefly under fuel-rich, low-oxygen exhaust gas conditions.
This releases the NOx from the base metal storage sites and allows it to be converted over the precious metal components to nitrogen gas and water vapor.
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Engine Research Laboratory, IIT Kanpur
Sulfur poses challenges for NOx absorbers.
In addition to storing NOx, the NAC will also store sulfur, which reduces the capacity to store NOx.
Although 2011 and later model year non-road engines must use ultra-low sulfur fuel (15-ppm), sulfur at any level requires the engine design to provide for a periodic de-sulfation process – a process to remove sulfur from the catalyst.
This is similar to the NOx regeneration process, but at higher temperatures.
We expect NOx absorbers to appear first in light-duty automotive applications.
NOx Absorbers
Engine Research Laboratory, IIT Kanpur
Lean-NOx Catalysts A lean-NOx catalyst uses unburned hydrocarbons to reduce NOx over a catalyst.
The catalyst may contain precious metals such as platinum or other materials such as zeolite.
The NOx conversion efficiency depends on many factors – but typical values are 10%-25% in
use over practical duty cycles.
Lean-NOx catalysts do not have adequate NOx reduction capability for Tier 4 applications.
However, lean-NOx catalysts are often an excellent option for retrofits.
They are relatively easy to install and integrate with existing engine and equipment systems.
NOx Absorbing Catalysts (Lean Phase) NOx Absorbing Catalysts (Rich Phase)
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Engine Research Laboratory, IIT Kanpur
Process
Engine Research Laboratory, IIT Kanpur
Working Principle of LNT
Pt
NO, O2 SO2
NO2 SO3 CO2
Storage phase > 1
Pt, Rh
H2, CO, CO2
NOx+O2 CO2, N2
Regeneration < 1
CO
BaCO3 Ba(NO3)2 BaSO4
BaCO3
Ba(NO3)2
BaSO4
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Engine Research Laboratory, IIT Kanpur
NOx Absorber Catalyst
Engine Research Laboratory, IIT Kanpur
Chemistry of NOx Adsorber Catalyst
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Engine Research Laboratory, IIT Kanpur
Typical Reaction Scheme in LNT
Lean –NO oxidation and NOx trapping
Exothermic
Spread over a long period of time
Low reactant amounts (100’s ppm)
Regeneration –Nitrate reduction
Exothermic
Regeneration is typically short (~5 seconds)
Larger reactant amounts (concentrated nitrates on surface, larger % of reductant)
Engine Research Laboratory, IIT Kanpur
Lean NOX Trap Dramatic structural changes in LNT materials as NOx is adsorbed and desorbed.
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Engine Research Laboratory, IIT Kanpur
Real LNT composition & functions are complex 3-way Catalyst (Pt, Pd, Rh, CeO2, ZrO2, Al2O3) + NOx storage component (Ba, K)
Function in cyclic mode between fuel lean & rich conditions:
Phase 1: Normal lean phase : NOx storage
Phase 2: Short rich excursion: NOx release/reduction
Engine Research Laboratory, IIT Kanpur
Intrinsically transient,
gradient-rich
integral systems with temporally varying
chemistry & spatially varying chemistry
NOx Storage/Reduction (NSR);
Oxygen Storage Capacity (OSC)
Reductant evolution/consumption;
sulfation/desulfation
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Engine Research Laboratory, IIT Kanpur
NOx Adsorption Window
Engine Research Laboratory, IIT Kanpur
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Engine Research Laboratory, IIT Kanpur
NOx Absorber Technology
Cell geometry has positive impact on NOx storage.
Engine Research Laboratory, IIT Kanpur
LNT related Issues
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Engine Research Laboratory, IIT Kanpur
Impacts on NOx Efficiency for NOx Storage Catalyst
Engine Research Laboratory, IIT Kanpur
Major Obstacles
Sulphur absorbed on NOx trap reduces NOx conversion efficiency
Desulfurization process occurs at high temperature (~ 600 °C)
Aging/S poisoning
- NOx reduction/adsorption kinetics
- Desulfation chemistry (including heat and mass transfer effects
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Engine Research Laboratory, IIT Kanpur
LNT High Temperature Thermal Aging
Key concern for Lean NOx Trap Durability
− high temperature periodically required to desulfate LNTs
Exposure to lean and rich conditions is important characteristic of onboard de-sulfation
Expected deactivation mechanisms
− Precious metal sintering
− Surface area losses
− Solid-state reactions (barium aluminate formation)
− Storage medium migration
Engine Research Laboratory, IIT Kanpur
Mechanisms of LNT Deterioration at High Temperatures
Barium transformation (> 950 C)
Apparent Barium Agglomeration( 850-1000 C)
Potassium Migration and Loss ( 750-1000 C)
Platinum Sintering (700-1000 C)
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Engine Research Laboratory, IIT Kanpur
Deactivation through thermal aging
Pt
Al2O3/CeO2
BaCO3
Pt
Al2O3/CeO2 composite
loss of storage capacity through composite formation
composite
BaCO3
Engine Research Laboratory, IIT Kanpur
Impacts: Thermal Ageing and Sulphur Poisoning
Sulphur blocks NOx storage sites- Currently requires zero sulphur fuel
Sulphur can be purged from catalyst but this requires non work producing fuel consumption
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Engine Research Laboratory, IIT Kanpur
Deactivation through sulfur Poisoning
Pt
Al2O3/CeO2
BaSO4
BaCO3
loss of storage capacity through sulfate formation
Engine Research Laboratory, IIT Kanpur
Desulphation
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Engine Research Laboratory, IIT Kanpur
Sulfur Poisoning on a fresh LNT
Engine Research Laboratory, IIT Kanpur
Sulfation –Desulfation of LNT
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Engine Research Laboratory, IIT Kanpur
Sulfur poisoning vs. Thermal aging
NO
x co
nve
rsio
n [
%]
Temperature [°C]
Sulfur poisoning
Engine Research Laboratory, IIT Kanpur
Sulfur poisoning vs. Thermal aging
NO
x co
nve
rsio
n [
%]
Temperature [°C]
Thermal aging
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Engine Research Laboratory, IIT Kanpur
Effect on NOx Storage Capacity (NSC)
Loss of NSC after sulfation (32% after 60 min)
Desulfation improves NSC at short times, no effect at longer storage times
⇒bulk storage sites not fully desulfated?
Engine Research Laboratory, IIT Kanpur
Main Challenges of LNT Technology
DeNOx regeneration by engine internal measures in terms of drivability and driver transparency
Limited DeNOx regeneration operation area
Sulfur poisoning / desulfurization
Reliable desulfurization strategy
Long-term stability / thermal aging
DeNOx and DeSOx management / complexity of after-treatment control
Passive control in catalytic converter
Active control in engine fuel management
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Engine Research Laboratory, IIT Kanpur
• Passive control in catalytic converter • Active control in engine fuel
management
• Poor control of HC and CO emissions • High penalty of fuel economy • Power output fluctuations during rich excursions
• Active control in catalytic converter • Passive control in engine fuel management
• Energy efficient • Rich fuel pulses are generated within individual
catalysts • Engine optimization achieved without compromising individual catalyst
Engine Research Laboratory, IIT Kanpur
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Engine Research Laboratory, IIT Kanpur
LNT Temperature During Vehicle Operation
Engine Research Laboratory, IIT Kanpur
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Engine Research Laboratory, IIT Kanpur
Engine Research Laboratory, IIT Kanpur
Introduction
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Engine Research Laboratory, IIT Kanpur
Selective Catalytic Reduction: Urea
Engine Research Laboratory, IIT Kanpur
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Engine Research Laboratory, IIT Kanpur
Engine Research Laboratory, IIT Kanpur
The urea-SCR system basically consists of three elements:
Catalyst – The catalyst is mounted in the exhaust stream. It can be similar in outward appearance to a muffler, but depending on NOx reduction required
could be marginally larger. It contains chemical compounds which, in the presence of ammonia, help transform nitrogen
oxides into harmless chemicals. Urea – Urea quality and concentration in aqueous solution are important and must be controlled
and distributed properly. Urea is carried on board the equipment as a water solution in a storage tank with a typical
capacity of 5% of the diesel tank. The storage tank is sized to minimize operator filling, but within packaging and weight
constraints of the equipment. The storage tank and urea injection system must be protected from freezing or have a controlled
heating system, since the urea-water solution solidifies at approximately -11ºC. Urea injection and control system – A sophisticated injection system and controls (including
NOx and urea quality sensors) are required to deliver a precise amount of urea under all environmental conditions.
For each 1-g/hp-hr reduction in NOx, an SCR consumes urea at a rate of approx. 1.5% of the amount of fuel used.
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Engine Research Laboratory, IIT Kanpur
Selective Catalytic Reduction (SCR)
Selective Catalytic Reduction (SCR) systems use a chemical reluctant, urea, which converts to ammonia in the exhaust stream and reacts with NOx over a catalyst to form harmless nitrogen gas and water.
SCR systems are being proposed today for mobile on-highway applications and are expected to be introduced in Europe in October 2005.
In an SCR system, the urea injection rate must be tightly controlled. If the injection rate is too high, not all of the ammonia will react with the NOx, and some
ammonia will “slip” through the catalyst. If the rate is too low, the desired NOx reduction will not be achieved. Both situations are
undesirable and must be avoided.
Engine Research Laboratory, IIT Kanpur
Simplified SCR chemical Kinetics 6 Global Reactions
NH3 adsorption and desorption
Standard, No SCR
Fast, NO+NO2 SCR
NO2 SCR
NH3 oxidation
NH3+S->NH3*
NH3*->NH3+S
4NH3*+4NO+O2 >4N2+6H20
2NH3*+NO+NO2>2N2+3H20
4NH3*+3NO2->3.5N2+6H20
4NH3*+3O2->2N2+6H2O
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Engine Research Laboratory, IIT Kanpur
SCR Chemistry
Engine Research Laboratory, IIT Kanpur
SCR Temperature Window
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Engine Research Laboratory, IIT Kanpur
Effect of Fuel Sulfur Diesel fuel sulfur forms SO2 and Sulfates (PM) in exhaust.
Catalysts oxides SO2 to SO3 which further increase the PM. Higher the exhaust temperature,
higher is the effect.
Sulfur gets absorbed on the catalyst and reduce catalyst activity, hence efficiency.
Higher sulfur can de-activate catalyst and poison base metal.
Engine Research Laboratory, IIT Kanpur
Why low Sulfur in diesel fuel
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Engine Research Laboratory, IIT Kanpur
Lower Sulfur Diesel Issues Reduced Lubricity
Premature Injection Pump Failure
Addressed with Lubricity additives
Reduced Fuel Stability
Decreased Colour Stability
Formation of Insoluble Materials
Fuel Filter Plugging
Engine Research Laboratory, IIT Kanpur
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Engine Research Laboratory, IIT Kanpur
Engine Research Laboratory, IIT Kanpur
NO2 has a key role in SCR NO2 plays an important role in NOx reduction in an SCR catalyst and in passive regeneration of soot
in a particulate filter.
NO2 promotes high NOx conversion efficiency on vanadium catalysts through the fast SCR
(NO/NO2 = 1:1) and on base-metal exchanged zeolite catalysts through the fast (NO/NO2 = 1:1)
and NO2-SCR reactions.
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Engine Research Laboratory, IIT Kanpur
Engine Research Laboratory, IIT Kanpur
Potential for further improvement of emissions on cold-start cycles through thermal management
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Engine Research Laboratory, IIT Kanpur
NO\NH3–O2system: Std. SCR reaction
Engine Research Laboratory, IIT Kanpur
Concerns regarding SCR
Refilling of urea tank;
Use of urea being a standard quality;
Availability of urea;
Urea storage tanks large enough;
Tampering to save money;
Reliability and availability of sensors.
SCR systems rely on the dosing of a urea based reagent
Without reagent, NOx emissions of a Euro V vehicle could be as poor as a Euro II vehicle – completely unacceptable.
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Engine Research Laboratory, IIT Kanpur
Additional requirements In-service conformity.
The manufacturer will have to demonstrate to the Type- Approval Authority that its vehicles fulfill the established requirements during the whole useful life.
Durability of the after-treatment system.
To fulfill the limit values at Type Approval, the manufacturer will take into account the deterioration of
the after-treatment system during the useful life of the vehicle.
On Board Diagnostics (OBD).
The OBD system will monitor the components that have an influence on emissions to inform the driver about their failure so that correction measures would be taken.
Engine Research Laboratory, IIT Kanpur
SCR- General Issues (Lean-Burn Gasoline & Diesel) Second tank for urea
Urea injection system
Urea infrastructure
Customer compliance
Urea freezing, mixing, decomposing into NH3 at low Temp.
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Engine Research Laboratory, IIT Kanpur
SCR-Specific Issues for Lean-Burn Gasoline
No three-way activity at stoichiometry from SCR catalyst
Requires larger TWC
High NOx concentrations
More frequent refills or larger urea tank
High exhaust temperatures
- SCR catalyst loses NH3 storage capacity above 400oC
Need to inject urea to match NOx flux
Challenge for control system during transient driving
Hot rich exhaust conditions
- Durability of zeolite-based SCR catalysts
Engine Research Laboratory, IIT Kanpur
Sulfation Decreases Global NOx Conversion & Increases NH3 Selectivity
Before sulfation, NOxconv. was ~100%
• S decreased NOx conv. but significant impact only at 3.4 g L-1
• N2O was low & insensitive to S (or decreased under different conditions)
• NH3 increased significantly with each sulfur dosing
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Engine Research Laboratory, IIT Kanpur
Main Challenges of SCR Technology Reliable urea injection
Uniform ammonia distribution in the exhaust
NOx neutral SCR-catalyst heating-up strategy
Dosing strategy
Ammonia slip
Vehicle package
System costs
While the NH3-SCR technology addresses fuel consumption, the application of an additional reduction component is considered a drawback.
Combining DeNOx technologies with the application DOC/DPF requires an integrated approach at the very beginning of the engine development cycle.
Engine Research Laboratory, IIT Kanpur
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Engine Research Laboratory, IIT Kanpur
Tier 4/Stage IV Emissions Reduction Options
The final Tier 4/Stage IV emissions standards drive to very low NOx and PM limits.
While the primary focus for the Tier 3/Stage IIIA standard is on NOx reduction, the Tier 4/Stage IV standard drives both NOx and PM down to levels that will likely require after treatment
Engine Research Laboratory, IIT Kanpur
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Engine Research Laboratory, IIT Kanpur
Exhaust Gas After Treatment
NOx sensor
NOx storage cat.
Oxi- cat.
p sensor Temperature
Particle filter
NO2c
Kat.
EDC
sensor
Engine Research Laboratory, IIT Kanpur
Exhaust Gas After Treatment
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Engine Research Laboratory, IIT Kanpur
Future Needs
Particulate number measurement with continued particulate mass measurement;
Monitoring of CO2 emissions;
Inclusion of portable emission measurement systems (PEMS)
Sensors and Controllers
Emissions control technology priorities
1. Lean NOx traps
2. Diesel particulate filters
3. Urea/ammonia SCR
4. Sulfur traps
5. Engine exhaust heaters/conditioners
6. Fuel reformers
Engine Research Laboratory, IIT Kanpur
2010 Heavy Duty OBD Requirements
EGR System • EGR Valve • EGR Cooler
Air System • Turbocharger • Charge Cooler
Injection System • Fuel Flow • Pressure • Timing • Misfire
SCR System • SCR Efficiency • Urea Doser • Urea Quality
DOC/DPF • Filtration Efficiency • Incomplete Regeneration • HC Doser • HC Slip
Cooling System • Thermostat
Crankcase Ventilation Grid
Heater
Major monitors - Air system - EGR system - Injection system - Misfire - Cooling system - Crankcase ventilation - DOC - DPF - SCR
Rationality checks - Sensors - Actuators
Comprehensive component monitors - Circuit continuity checks
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Engine Research Laboratory, IIT Kanpur
Engine Research Laboratory, IIT Kanpur
System Configurations
System Configuration-1
System Configuration-2
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Engine Research Laboratory, IIT Kanpur
Diesel Oxidation Catalyst Combined with Electrically Powered Supercharger to Reduce PM Emission
Engine Research Laboratory, IIT Kanpur
SCR with DOC and DPF Performance
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Engine Research Laboratory, IIT Kanpur
SCR with DOC and DPF Performance
Engine Research Laboratory, IIT Kanpur
System Applicability