selective catalytic reduction - iit kanpur 24_25_scr and lnt.pdf · selective catalytic reduction...

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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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Conventional NOx Reduction Technologies

SAE, 2007-01-0239


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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NOx Technologies Operating Experience

NOx Technology Concept Overview


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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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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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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


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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Process


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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NOx Absorber Catalyst


Chemistry of NOx Adsorber Catalyst

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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)


Lean NOX Trap Dramatic structural changes in LNT materials as NOx is adsorbed and desorbed.

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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


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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NOx Adsorption Window


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NOx Absorber Technology

Cell geometry has positive impact on NOx storage.


LNT related Issues

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Impacts on NOx Efficiency for NOx Storage Catalyst


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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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


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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Deactivation through thermal aging

Pt

Al2O3/CeO2

BaCO3

Pt

Al2O3/CeO2 composite

loss of storage capacity through composite formation

composite

BaCO3


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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Deactivation through sulfur Poisoning

Pt

Al2O3/CeO2

BaSO4

BaCO3

loss of storage capacity through sulfate formation


Desulphation

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Sulfur Poisoning on a fresh LNT


Sulfation –Desulfation of LNT

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Sulfur poisoning vs. Thermal aging

NO

x co

nve

rsio

n [

%]

Temperature [°C]

Sulfur poisoning


Sulfur poisoning vs. Thermal aging

NO

x co

nve

rsio

n [

%]

Temperature [°C]

Thermal aging

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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?


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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• 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


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LNT Temperature During Vehicle Operation


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Introduction

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Selective Catalytic Reduction: Urea


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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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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.


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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SCR Chemistry


SCR Temperature Window

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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.


Why low Sulfur in diesel fuel

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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


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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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Potential for further improvement of emissions on cold-start cycles through thermal management

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NO\NH3–O2system: Std. SCR reaction


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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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.


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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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


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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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.


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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


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Exhaust Gas After Treatment

NOx sensor

NOx storage cat.

Oxi- cat.

p sensor Temperature

Particle filter

NO2c

Kat.

EDC

sensor


Exhaust Gas After Treatment

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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


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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System Configurations

System Configuration-1

System Configuration-2

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Diesel Oxidation Catalyst Combined with Electrically Powered Supercharger to Reduce PM Emission


SCR with DOC and DPF Performance

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SCR with DOC and DPF Performance


System Applicability

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selective catalytic reduction - iit kanpur 24_25_scr and lnt.pdf · selective catalytic reduction...

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