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2
Lidia Castoldi
Sorgenti di emissione e rischi connessi
Sorgenti:
fisse (impianti di generazione di potenza, boilers industriali, inceneritori, combustione di biomasse);
mobili (veicoli)
Rischi/danni:
problemi respiratori e cardiaci nell’uomo;
piogge acide;
produzione di ozono nella troposfera;
formazione di polveri sottili
3
Lidia Castoldi
Civile6%
Commercio4%
Industria18%
Trasporto32%
Energia elettrica40%
Civile6%
Commercio4%
Industria18%
Trasporto32%
Energia elettrica40%
2000
L’evoluzione dell’inquinamento
Civile8%
Commercio5%
Industria25%
Trasporto29%
Energia elettrica33%
1980
Calo delle emissioni dell’industria grazie al miglioramento delle tecnologie
Aumento delle emissioni da produzione di energia elettrica a causa del maggiore fabbisogno della stessa
Aumento delle emissioni da trasporto, a causa dell’aumento di diffusione degli automezzi, soprattutto in paesi emergenti
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Lidia Castoldi
sorgenti mobili,non diesel
26%
NOx
altro
processi dicombustione,altro
processi di combustione,industria
processi di combustione,utility
sorgenti mobili,diesel
6%5% 14%
23%26%
PM-10
altroaltri processi industriali
processi di combustione,altro
processi di combustione,industria
processi di combustione,utility
sorgenti mobili,non diesel
sorgenti mobili,diesel
24%17%
16%
10%9%6%
18%
L’evoluzione dell’inquinamento
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Lidia Castoldi
Power demand: future trend
Global primary energy demand is expected to increase 53% by 2030, during which time demand for electricity will more than double. 70% of this increase will come from developing nations, led by China and India.
By 2025, fossil fuels are expected to constitute 85% of the world’s primary energy mix
By 2025, the US’s consumption of electricity is projected to be 50% greater than in 2003. To meet this rising demand, while also retiring inefficient older plants, 281,000 megawatts of new power generationcapacity will be needed – the equivalent of almost 950 new power plants of 300 megawatts each.
However, America is not alone in its growing demand for energy. Japan imports 99% of its oil and 97% of its natural gas and has now overtaken Korea to become the world’s largest importer of ethanol. Two decades from now, China, the world’s fastest growing consumer of petroleum, could be importing 10 million barrels of oil per day.
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Lidia Castoldi
Power demand: electric power generation and energy consumption
OECD (Organisation for Economic Co-operation and Development)
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Lidia Castoldi
Previsioni sulla produzione di turbine a gas e impianti di potenza
Given the current need for new baseload capacity, as well as for power plant capacity additions, we believe that the worldwide demand for the latest technology gas turbine-based power plants will result in modest production of the super-large gas turbine machines, those of 180 MW and larger.
Production of those machines could grow from 48 machines in 2003 to 150-160 machines per year in the period 2011-2014. Those machines can be expected to be procured by China, North Korea, Vietnam, Indonesia, Thailand, Brazil, and the Middle East.
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Lidia Castoldi
Previsioni sulla produzione di turbine a gas e impianti di potenza
Worldwide orders for gas turbine machines for electrical generation of 1 MW and larger grew from about 800 machines in 1986/1987 to 875 in 1998/1999.
The trend continued into 1999/2000, when 1,200 machines were ordered, and 2000/2001, when about 1,540 machines were ordered.
Orders then fell off over the next two years to about 840 and 600 machines, respectively.
Orders in the 2003/2004 period totaled about 700 machines, about equal to Forecast International's projection for the 2004/2005 period.
Of the 7,550 machines projected to be manufactured during the next 10 years, machines of 125 MW and larger should account for more than 30% of unit production and over 70% of value of production.
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Lidia Castoldi
Limiti di emissione: sorgenti fisse
35Inceneritori (11% O2)
25Turbine a gas (15% O2)
50Centrali termoelettriche a gas naturale (3% O2)
75Centrali termoelettriche ad olio combustibile (3% O2)
100Centrali termoelettriche a carbone (6% O2)
EULimiti di emissione NO X (ppm)
USUSUSUSNOx emission budgets set by EPA require thatUtility Generation Stations must achieve in 22 States100 ppm by 2003 in the Ozone seasons (May-September)
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Lidia Castoldi
Tecniche di riduzione degli NO x
� Tecniche primarie� Water/Steam Injection� Lean Premix Combustion (DLN)� Catalytic Combustion
� Tecniche di post-trattamento:
� SCR (Selective Catalytic Reduction)
�urea- or ammonia- SCR
�HC-SCR
� NSR (Nitrogen Storage Reduction)
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Lidia Castoldi
Ammonia- or urea-SCR:- used in stationary applications- when used in vehicles the catalyst is operated under fast transient conditions
- transient kinetics and dynamic model of the catalytic reactor required
NSR:- less mature technology- associated with a number of complications
Tecniche secondarie di riduzione degli NO x
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Lidia Castoldi
LNT & SCR
We have: NOx NO NO2 ONO- NO3-
We want: N2
We add: H2 (CO, HC) or NH 3
O2 H2O CO2
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Lidia Castoldi
NOx Emission Control Technologies
Secondary Methods:Conventional SCR 9 ppmHigh-T SCR 9 ppmLow-T SCR 9 ppmSCONOx 2 ppm
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Lidia Castoldi
Common Applications : coal-fired power plantsoil-fired power plantsgas-fired power plantsindustrial boilerscogeneration units
Overall capacity of the utility sector: around 300.000 MWe
Other Applications: waste incineratorscombined NOx-dioxin abatementchemical plants (e.g. HNO3 tail gas, FCC Units)steel industries glass industries cement industries
They account roughly for 10-15% of the total catalyst volume.
Applications of SCR technology
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Lidia Castoldi
ENEL - SCR deNOx reference list
SCR DENOx PLANTS: 13.850 MWe IN OPERATION
POWER PLANT SIZE(MWe)
FUELPRIMARY
ABATEMENTSYSTEM
SCRCONFIGUR.
SCRSTART-UP
MONTALTO DICASTRO 4 x 660 O / G OFA / LNOxB (TEA) HIGH DUST 1996 / 98
TORRE VALDALIGANORD 4 x 660 O / G BOOS HIGH DUST 1997 / 98
BRINDISI SUD 4 x 660 C / O / ORIM NOx PORTS / LNOxB HIGH DUST 1997 / 98
FIUME SANTO 2 x 320 C / O / ORIM OFA HIGH DUST 1997 / 98
FUSINA 2 x 320 C / O OFA HIGH DUST 1997 / 98
TURBIGO 250-320 O / G BOOS / LNOxB (TEA) HIGH DUST 1998
ROSSANO CALABRO 4 x 320 O / G BOOS HIGH DUST 1998 / 99
VADO LIGURE 2 x 320 C / O LNOxB (TEA) /REBURNING HIGH DUST 1998 / 99
TERMINI IMERESE 2 x 320 O / G BOOS HIGH DUST 1999
S. FILIPPO DEL MELA 2 x 320 ORIM / O BOOS SIDE STREAM 2000
LA SPEZIA 1 x 600 C / O NOx PORTS / LNOxB HIGH DUST 2000
SULCIS 1 x 240 C / O LNOxB (TEA) TAIL END 2000
FUEL: C: COAL O: HEAVY FUEL OIL ORIM: ORIMULSION
LNOxB: LOW NOx BURNERS
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Lidia Castoldi
Selective Catalytic Reduction of NO x with NH 3
4 NH3 + 4 NO + O2 → 4 N2 + 6 H2O Standard-SCR
6 NO + 4 NH3 → 5 N2 + 6 H2O Slow6 NO2 + 8 NH3 → 7 N2 + 12 H2O NO2-SCRNO + NO2 + 2NH3 → 2 N2 + 3 H2O Fast-SCR
Undesired reactions : unselective reactions, ammonia oxidation reactions, oxidation of SO2
Formation of other pollutants consumption of a reactant
Chemistry of the DeNO x SCR
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Lidia Castoldi
Undesired oxidation of SO 2 to SO 3
SO2 + ½ O2 → SO3
SO3 + H2O → H2SO4
NH3 + SO3 + H2O → (NH4) HSO4
2 NH3 + SO3 + H2O → (NH4)2 SO4
SO3 can react with H2O and NH3 to form ammonium sulphates, which
can deposit onto the catalyst and onto the Air Pre Heater downstream
of the SCR reactor.
Chemistry of the DeNO x SCR
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Lidia Castoldi
Deposition of ammonium sulfates
� The deposition of ammonium sulfates is controlled by thermodynamics.
� Temperature above 250-300°C are typically required for stable catalyst operation and ammonia slips of 1-3 ppm are required in the reactor design
� The conversion of SO2 to SO3over the catalyst must be lower than 1%
0.1
1.0
10
100
1000
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Lidia Castoldi
Efficiency for NOx removal versus temperature
A. Noble metals catalystsB. Metal oxides catalystsC. Zeolite catalysts
150 200 250 300 350 400 450 500 55020
30
40
50
60
70
80
90
cat. Ccat. B
cat. A
Con
vers
ione
NO
x (%
)
Temperatura (°C)
DeNOx SCR commercial catalysts
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Lidia Castoldi
Metal based catalysts made of homogeneous mixtures o f
TiO2 (≈ 80% w/w)
V2O5 (< 1-2% w/w)
WO3 (≈ 10% w/w) or MoO3 (≈ 6% w/w)
Silico aluminates
Glass fibers
Employed in form of
DeNOx SCR conventional commercial catalysts (300-400 °C)
honeycomb plates
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Lidia Castoldi
TiO2 (≈ 80% wt%) high surface area and S-resistant carrier
WO3 (≈ 10% wt%) surface acidity andthermal stability
V2O5 (< 1-2% w/w) active phase
Silico aluminates mechanical promoters
Glass fibers mechanical promoters
DeNOx SCR conventional commercial catalysts (300-400 °C)
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Lidia Castoldi
Monolithic materials
It is a unitary structure composed of inorganic oxides or metals in the form of a honeycomb with uniform sized and parallel channels that may be square, triangular, hexagonal, round.
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Lidia Castoldi
Monoliths are preferred to pellet shaped catalysts in environmental applications: low pressure drop, excellent attrition resistant, good mechanical properties…
Straight parallel channels Large open frontal area
High external surface/volume ratio
Low pressure dropsLow tendency to plugging
High activity
Thin layer of coated active material Lower intra-phase diffusional resistance
High thermal conductivityMetallic substrate
Monolithic materials
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Lidia Castoldi
Why monoliths in the SCR?
� NO reduction is very fast and is controlled by external and internal diffusion.� SO2 oxidation is very slow and controlled by chemical kinetics.
The SCR activity is increased by increasing the catalyst external surface area (i.e. cell density) whereas the S02 oxidation activity is reduced by decreasing the wall thickness of the catalyst.A good balance between macropores to speed up diffusion of reagents and micropores to provide high specific surface area can lead to optimal catalyst performances (in the limit of the required mechanical specifications).
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Lidia Castoldi
Geometric data forhoneycomb and plate type catalysts
Honeycomb catalysts Plate-type catalysts
Geometry of the element,
mm x mm x mm
150 x 150 x (500-1300) 500 x (500-600)
Number of cells 15 x (15-40)x 40 -
Channel width, mm 8.5-3 -
Pitch, mm 10-3.7 7-3.8
Wall thickness, mm 1.5-0.6 1.2-0.8
Specific surface area, m2/m3 340-860 280-500
Void fraction, % 64-72 ~ 80
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Lidia Castoldi
Monolith matrix made of cordierite or thin metal foils coated with SCR materials
High T catalysts: zeolite-type (high Si/Al ratio)(up to 600°C)
Low T catalysts: noble metal-based(down to 200°C) high vanadium-content
DeNOx SCR non-conventional commercial catalysts
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Lidia Castoldi
Catalyst optimization has significantly reduced the size and the cost of the SCR reactor and has greatly increased the economics of the SCR process.
Catalyst life of 16.000 – 24.000 h is typically guaranteed by catalyst suppliers but longer catalyst life is observed in practice and expected in reality.
Major causes of catalyst deactivation are sintering for gas-fired units, poisoning by alkaline metals for oil-fired units and pore blocking by calcium sulfates for coal-fired units.
Catalyst optimization
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Lidia Castoldi
Effect of non uniform distribution
To ensure high NOx removal efficiency and low ammonia slip during industrial operation a uniform distribution of NOx/NH3 mole ratio, temperature and velocity over the entire cross section of the catalytic converter must be approached.
This is achieved by:- use of guide vanes and of dummy layer before the catalyst layers;- use of cold models at a reduced scale and CFD calculations;- proper design of the ammonia distribution grid;- precise tuning of the ammonia distribution grid during plant start up.
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Lidia Castoldi
ESP
Boiler FGD
DeNOx
Tail end
Boiler DeNOx FGD
ESP High dust
ESP
Boiler DeNOx FGD
Low dust
SCR configurations for boilers applications
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Lidia Castoldi
Inceneritori
� I fumi di combustione, in uscita dal filtro a manica, si trovano a T ~ 200°Ce con conc. SOx ~ 0.
� Il reattore SCR è operato a bassa T per minimizzare i consumi energetici.
240-250°CDry-adsorber
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Lidia Castoldi
Il reattore SCR è integrato nel sistema HRSG.
SCR configurationsfor Gas Turbine applications
HRSG
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Lidia Castoldi
Inomata et al.
NH3 adsorbed at a Brønsted V site adjacent to a vanadyl reacts with gas-phase NO (Eley-Rideal mechanism ) to form N2 and H2O and a reduced V species.The reduced V species is then re-oxidized by gaseous oxygen
Mechanism of the DeNO x Reaction
M. Inomata et al, J. Catal. 62 (1980) 140
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Lidia Castoldi
V
O
OO O
N
H
HH
V
O
OO O
N
H
HH
..
N O.
V
O
OO O
N
H
H
N O
.
V
O
OO OH.
H
N N
H2OV
O
OO O
NH3
/ O21 4/ H2O1 2
Ramis et al.“amide-nitrosamide” mechanism
Mechanism of the DeNO x Reaction
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Lidia Castoldi
Topsøe et al, J. Catal. 151 (1995) 241
Topsøe et al.
NH3 is adsorbed at a V(5+)-OH Brønsted acid site and is activated by a near-by V(5+)=O group, that is then reduced to V(4+)-OH. Gaseous NO reacts with adsorbed NH3 to form an intermediate which decomposes to N2 and H2O.V(4+)-OH is re-oxidized to V(5+)=O by gas-phase oxygen
Mechanism of the DeNO x Reaction
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Lidia Castoldi
� Both mechanisms proposed by Inomata and by Topsoe require the participation of dymeric vanadyl species.� Mechanism proposed by Ramis et al. require the participation of isolated vanadyl and is consistent with the linear dependence of the rate constant of NOx reduction on the vanadia content.�Other catalyst components in addition to vanadia (i.e. tungsta and titania) do adsorb ammonia and participate in the reaction as “reservoir” of adsorbed ammonia species.�A key step of the mechanism is represented by the formation of areaction intermediate that decomposes selectively and quantitatively to nitrogen and water.
Mechanism of the DeNO x Reaction
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Lidia Castoldi
Condizioniricche
Condizionimagre Ba(NO2)2
Ba(NO3)2
NOx
Ba PtO
Ba(NO2)2
Ba(NO3)2
HC
N2
metalli nobili
ossidazione/riduzione
ossidi di metalli alcalini-terrosiaccumulo
Pt-Ba/γγγγ-Al2O3
Tecnologia NSR
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Lidia Castoldi
Tecnologia SCONOX TM
(Goal Line and Süd Chemie)
Rimozione di NOx (SCONOX TM) nelle Turbine a Gas in USA (unità di
cogenerazione Sunlaw Federal di 32 MW,impianto Wyeth Biopharmadi 5 MW, unità di cogenerazione presso University of California).
Catalyst: Pt-K2CO3/Al2O3
Oxidation/adsorption cycle:CO + ½ O2 → CO2
NO + ½ O2 → NO2
2NO2 + K2CO3 → CO2 + KNO2 + KNO3
Regeneration cycle:KNO2 + KNO3 + 4H2 + CO2 → K2CO3 + 4 H2O(g) + N2
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Lidia Castoldi
Regeneration gas generator
1040°CCH4 + ½ O2 + 1.88 N2 → CO + 2 H2 + 1.88 N2
Cat.
(or steam reforming)
shiftCO + 2H2 + H2O + 1.88 N2 → CO2 + 3H2 + 1.88 N2
Tecnologia SCONOX TM
(Goal Line and Süd Chemie)
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Lidia Castoldi
Layout of SCONOx TM catalyst
Turbine package
Exhaust transition
Open isolation louvres
Closed isolation louvresSconox catalyst blocks
High pressure heatrecovery steam generators
Exhaust stack
Louvres closedduring regeneration
Regenerationgas outlet (2)
Regenerationgas inlet (1)
Low pressure heat recovery steamgenerator
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Lidia Castoldi
Typical design arrangement
Several catalyst sections
80% of these are in the oxidation/adsorption cycle ⇒ 15-20 minutes
20% of these are in the regeneration cycle ⇒ 3 - 4 minutes
GHSV = 20.000 h-1 (25ppm → 1ppm) 15.000 h-1 (50ppm → 1ppm)
Temperature = 150°- 370°C
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Lidia Castoldi
The SCOSOx sulphur removal system
CO + ½ O2 → CO2
SO2 + ½ O2 → SO3
SO3 + SORBER → [SO3 + SORBER]
[SO3 + SORBER] + 4H2 → H2S + 3H2O
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Lidia Castoldi
Sunlaw’s Federal Plant and NO x emissions rateon a rolling 15 minutes average from the SCONOx TM unit
Applied to Gas Turbines SCONOX has been successfully applied downstream of a 34 MW GT.
<1 vppm @ 15% O2
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Lidia Castoldi
Cost impact factors for selected NO x control technologies (1999)
Turbine output (class) 5 MW 25 MW 150 MW
NOx emission control
technology $/ton ¢/kWh $/ton ¢/kWh $/ton ¢/kWh
DLN (25 ppm) 260 0.075 210 n.d. 122 0.054
Water/Steam injection (42 ppm) 1652 0.410 984 0.240 476 0.152
Catalytic combustion (3 ppm) 957 0.317 692 0.215 371 0.146
Conventional SCR (9 ppm) 6274 0.469 3541 0.204 1938 0.117
High temperature SCR (9 ppm) 7148 0.530 3841 0.221 2359 0.134
Low temperature SCR (9 ppm) 5894 1.060 2202 0.429 n.d. n.d.
SCONOx (2 ppm) 16327 0.847 11554 0.462 6938 0.289
1. The $/ton value is a useful comparative indicator when the inlet and outlet concentrations are the same2. The ¢/KWh value provides electricity cost impact of a particular technology. The comparison is most meaningful for an equivalent ppm outlet concentration3. Both values are based on 8.000 full-load operating hours
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Lidia Castoldi
Comments (based on ¢/KWh values)
1. High-T SCR is slightly more costly than conventional SCR
2. Low-T SCR and SCONOx are 2 times more costly thanconventional SCR
3. Each SCR technology fills a unique niche; cost impact may beof secondary significance
4. SCONOx is the only secondary control technology that doesnot require NH3 injection
5. The cost of catalytic combusor is 2-3 times higher a DLN combustor alone. However, to rich the same NOx levels DLN must be equipped with SCR or SCONOx
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Lidia Castoldi
1. The cost impact is highest when emission control technologies are applied tosmall turbines (5MW)
2. DLN technology and catalytic combustion exhibit lower cost impacts thanpost-combustion tecnologies for both small and large GT
3. Catalytic combustion is very promising in view of low cost impact and NOxlevel.
Comparison of NO x control technologies
0.0
0.2
0.4
0.6
0.8
1.0
1.2
DLN
(9-25ppm)
Catalytic
(3ppm)
W/S Inj
(42ppm)
Conven.
SCR
(9ppm)
SCONOx
(2ppm)
Low-T
SCR
(9ppm)
High-T
SCR
(9ppm)
5 MW 25 MW 150 MW
Cos
t of P
ower
Impa
ct (
cent
s/K
Wh)
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Lidia Castoldi
Selective Catalytic Reduction
� Improved catalyst design (higher DeNOx effectiveness factor)
� Higher NOx reduction efficiency
� Study of the SCR process for GT
�Applicazione in inceneritori e turbine a gas a bassa T
� Better understanding of several fundamental issues (e.g. catalyst reactivity, mechanism, kinetics)
SCONOx�Study of the process and of the available commercial catalysts� Develop cheaper and S-resistant catalysts
RESEARCH OPPORTUNITIES
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Lidia Castoldi
How do we get there?
Improvements in the combustion engine technologies c an help…but they are not sufficient… DPF and deNO x systems are mandatory!
BOSCH, 1st MinNOx conf., 2/2007
DENSO, 2nd MinNOx conf., 6/2008
BMW, 2nd MinNOx conf., 6/2008
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Lidia Castoldi
Why diesel engines?
DAIMLER, CAPOC meeting, 4/2009
Make the gasoline engine asefficient as the Diesel engineand the Diesel engine as cleanas the gasoline engine
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Lidia Castoldi
NOx control techniques
BASF
BMW
DENSO
DAIMLER, CAPOC meeting, 4/2009
2nd MinNOx conf., 6/2008
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Lidia Castoldi
LNT & SCR
SCR
4NH3 + 4NO + O2 → 4N2 + 6H2O Standard SCRStandard SCR
2NH3 + NO + NO2 → 2N2 + 3H2O Fast SCRFast SCR
4NH3 + 3 NO2 → 3.5 N2 + 6H2O NONO22 –– SCRSCR
• DOC upstream of the SCR (NO � NO2)
• Continuous process with urea/NH3discontinuous dosage
• Commercial catalysts: V2O5–WO3/TiO2 &Fe- or Cu-zeolites (ZSM5, Beta)
LNT
NOx NOx storagestorage
NOx NOx reductionreduction
• PM component
• Cyclic conditions: long lean phases, short rich phases
• Commercial catalysts: Pt-Ba/Al 2O3 & Pt-Al 2O3/BaO/CeO2/TiO2
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Lidia Castoldi
SCR: linee di tendenza
Motivazioni forti per:
� sviluppare un modello dinamico del reattore monolitico SCR;
� estendere la finestra di lavoro verso la regione delle basse T (~ 300°C ���� ~ 200°C) per esempio con l’uso di catalizzatori a base di zeoliti (scambiate con Fe, C u…).
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Lidia Castoldi
Tecnologia di abbattimento di NOx daesausti di mezzi pesanti Diesel
Reazione della standard SCR4NH3 + 4NO + O2 → 4N2 + 6H2O
Urea-SCR
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Lidia Castoldi
La reazione SCR che coinvolge consumi equimolari di NO e NO2 è nota come “fast”SCR di interesse per applicazioni a bassa T :
2 NH3 + NO + NO2 → 2 N2 + 3 H2O
In questo caso è necessario un catalizzatore di pre-ossidazione per convertire parte di NO a NO2
Urea-SCR: reazione fast-SCR
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Lidia Castoldi
Principali caratteristiche:
� disproporzione veloce di NO2 a nitriti e nitrati
2 NO2 ↔ N2O4 (+H2O) ↔ HNO2 + HNO3
� decomposizione dei nitriti in presenza di ammoniaca:
HNO2 + NH3 → [NH4NO2] → N2 + 2 H2O
Fast SCR: meccanismo
C. Ciardelli et al., Chem. Comm., 2004;I. Nova et al., Catal. Today, 2006;E. Tronconi et al., J. Catal., 2006;P. Forzatti et al., MI 2007 A 742 del 12.4.2007
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Lidia Castoldi
0
200
400
600
800
experimental langmuir temkin
time
T=280°C
Con
cent
razi
one
(ppm
)
)1(3
θ−= NHadsads Ckr θαθ ⋅
−−=
°
)1(expRT
Ekr deso
desdes
Dynamic modelling of SCR
NH3 adsorption-desorptionStep changes of NH3 inlet concentration over V2O5-WO3/TiO2 catalyst at T=280°C
Large amounts of ammonia are adsorbed, the adsorption of ammonia is fast and the desorption iscompleted only at high T
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Lidia Castoldi
0 200 400 600 800 1000 1200
0
100
200
300
400
500
600
700
800
Time (s)
Theor. Exp.C
once
ntra
tion
(ppm
)
NO adsorption-desorptionStep changes of NO inlet concentration over V2O5-WO3/TiO2 catalyst at T=280°C
NO does not appreciably adsorb on the catalyst surface
Dynamic modelling of SCR
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Lidia Castoldi
0 500 1000 1500 2000 2500
0
200
400
600
800
INNH3
N2
NO
NH3
Time (s)
Con
cent
ratio
n (p
pm)
rNO is almost unaffected by changes in the ammonia surface coverage at high coverage
Dynamics of the surface reactionStep changes of the NH3 inlet concentration in flowing He + NO + O2 (1%) at 220°C
Dynamic modelling of SCR
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Lidia Castoldi
0.0 0.2 0.4 0.6 0.8 1.00.00
0.01
0.02
0.03
0.04
0.05
0.06
r NO /[
CN
O]
θNH3
a “reservoir” of NH3 species (adsorbed onto W and Ti sites) is present and available for the reaction upon desorption followed by readsorption at reactive V sites.
ra (NH3) ≈ rNO >> rd(NH3) => ⇒ assumption of equilibrated adsorption incorrect
SCR Rate equation
0 500 1000 1500 2000 2500
0
200
400
600
800
INNH3
N2
NO
NH3
Time (s)
Con
cent
ratio
n (p
pm)
−−−°=*
* 3exp1expθ
θθ NH
NONO
NONO CRT
Ekr
Dynamic modelling of SCR
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Lidia Castoldi
2000 3000 4000 5000 6000
0
200
400
600
800
1000
Exp NH3
Exp N2
Exp NO Calc NH
3
Calc N2
Calc NO
Con
cent
ratio
n, p
pm
β
θθ
θ
2OP
11
exp
⋅
−+
⋅⋅
−
=
°
LHK
CRT
Ek
rNO
NOoNO
NO
Step changes of the NH3 inlet concentration in He + NO + O2 (1%) + H2O (1%) at 200°C
The conversion of NO goesthrough a maximum(reaction rate lower inpresence of excess NH3)
This has been described bya dual site LHHW mechanismthat assumes competitionbetween NO and NH3 foradsorption onto the catalyst
Dynamic modelling of SCR
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Lidia Castoldi
0 100 200 300 400 500
0.0
0.2
0.4
0.6
0.8
1.0 ETC
NO
x
Time (s)
Validato presso DC suCatalizzatore SCR full scale(25-43 L) e con gas discarico realiCommercializzato da DC per Heavy Duty Vehicles
D. Chatterjee et al., SAE technical paper 2005;D. Chatterjee et al., SAE technical paper 2006;E. Tronconi et al., Catal. Today, 2006
Dynamic modelling of SCR
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Lidia Castoldi
“NO x storage-reduction” Catalytic Systems
Pt-Ba/γγγγ-Al2O3
noble metal
oxidation/reduction
alkaline – earth metal
storage
NOx abatement
N. Takahashi et al., Cat. Today, 27 (1996) 63
Rich conditions
Lean conditions Ba(NO2)2
Ba(NO3)2
NOx
Ba PtO
Ba(NO2)2
Ba(NO3)2
HC
N2
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Lidia Castoldi
NOx Storage-Reduction Catalysts or Lean NOx Traps: Pt-Ba/γγγγ-Al 2O3
S. Matsumoto, CATTECH, 4 , 2000, 102-109
“NO x storage-reduction” Catalytic Systems
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O2 ↔ 2 O*
BaO + O* + 2 NO ↔ Ba (NO2)2
Ba(NO2)2 + 2 O* ↔ Ba(NO3)2
NO + ½ O2 � NO2
BaO + 2 NO2 + O* ↔ Ba(NO3)2
BaO + 3 NO2 ↔ Ba(NO3)2 + NO
Ba(NO3)2 + 5H2 ↔ N2 + 5H2O + BaO Ba(NO3)2 + HC ↔ N2 + BaO (+H2O+ CO2)
� Adsorption of oxygen species associated with Pt sites.
� Nitrites are formed first and then are transformed into nitrates.
� NO is oxidized to NO2 and then NO2 is adsorbed to form nitrates.
� Nitrates are reduced to nitrogen.
“NO x storage-reduction” Catalytic Systems: fundamental chemistry
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Lean NO x Trap materials
The catalytic material adopted is basically alumina containing primarilybarium (Ba) and platinum (Pt), which are the key players in the storage and reduction of NOx.Among alkali and alkaline earth elements, Ba is the most effective element to store NOx in the LNT catalyst.
Other components include:�cerium oxide (CeO2) is used as tank of oxygen;� lanthanum oxide (La2O3) is used to stabilize the alumina support;� rhodium (Rh), and zirconium oxide (ZrO2), both of which are used to promote the formation of hydrogen which efficiently removes sulphates from the catalyst;� titanium oxide (TiO2), which suppresses the absorption of sulphate.
The alumina is coated on a ceramic support. The coating is made as an ultra-thin layer only about 100 µµµµm (0.1 mm) thick, and is very porous, resulting in a high surface-to-volume ratio. One gram of the catalyst provides more than 100 square meters of surface area.
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NO NO2
NO3-
NO
NO2-
Porous Supportas g-aluminarP= 100ÅVP = 0.82cm3/gby BET;dC= 70Åby XRD
Alkaline earth metalas bariumdC= 70Å as barium carbonate monocl.dC= 150Å as barium carbonate orthoromb.by XRD
Noble metalas platinum%Pt= 60%by chemisorption
Ba
Pt
Al2O3
Lean NO x Trap materials: Pt-Ba/ γγγγ-Al2O3
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Ceramic monoliths present large pores and low surface area (0,3 m2/g). It is therefore necessary to deposit a carrier + active catalyst onto the channel walls.This catalytic layer is called washcoat
Lean NO x Trap materials: monolithic materials
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SubstratePrimer
Washcoat
Substrate : provides mechanicals and geometrical characteristics to the catalyst (ceramic or metallic).
Primer : favours the adhesion of the washcoat to the substrate (affinity with both substrate and washcoat).
Washcoat : provides high surface area to support active components (e.g. noble metals).
Lean NO x Trap materials: monolithic materials
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The substrate provides geometric, physical and mechanical characteristics to the catalyst.
Desired properties : • Shaped in a structured form.• Resistant at the reaction temperature• Resistant to thermal shock• Low thermal expansion coefficient• Chemical inertia with respect to active washcoat
Lean NO x Trap materials: monolithic materials
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Catalytic Muffler
Standard Cordierite Monoliths
Advanced ConceptMetallicMonoliths
Lean NO x Trap materials: monolithic materials
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Ceramic Substrates : Cordierite (2MgO* 5SiO2*2Al2O3), Mullite(3Al2O3*SiO2), Alumina (α-Al2O3), Titania (TiO2), Carbon Silica (SiC).
High thermal shock resistance High mechanical strengthHigh melting point (T<1300°C)Good chemical and mechanical bonding of the washcoat
Sw = washcoat thickness
L = cell spacing
d=cordierite wall thickness
Lean NO x Trap materials: monolithic materials
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Lidia Castoldi
The washcoat is a layer ofporous ceramic material(5% to 20% w/w ).
Desired properties:
• adhesion to the support
• uniformity in thickness
• high surface area and appropriate
pore distribution
• affinity with the supported
active elements
• thermal and chemical stability
under reaction conditions
Lean NO x Trap materials: monolithic materials
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Lidia Castoldi
Desired properties :
• High Activity at Low temperature
• High thermal stability
• Low Vapour Pressure
• High resistance to poisoning
In the washcoat active elements are contained
Lean NO x Trap materials: monolithic materials
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Basic layout of LNT catalytic systems
*Exhaust Gas Recirculation (EGR) introduces exhaust gas into the intake of the engine replacing some of the air. This has the effect of reducing NOx emissions by reducing the in-cylinder gas temperatures: NOx production is very temperature sensitive
*
by Ricardo Inc.
Lidia Castoldi
LNT performances
NOx storage behaviour of LNT catalyst at 673 K in engine bench evaluation and definition of NOx
storage amount. The air–fuel mixture feed was switched to a lean mixture (A/F=23.5) from a rich mixture (A/F=10) for 1 s. After 10 min, the air–fuel mixture was switched to a rich mixture for 1 s, and then to a lean mixture again.
S. Matsumoto, CATTECH, 4 , 2000, 102-109
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Sulphur poisoning
Relationship between the efficiency of NOx
conversion and the amount of sulphur deposit on the Pt-Ba/Al2O3 catalyst after durability test
S. Matsumoto, CATTECH, 4, 2000, 102-109
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S poisoning mechanism: hypothesis
γ-Al 2O3
BaSO4
BaOPt
SO2 SO3 Lean phase
γ-Al 2O3
BaOPt
SO2
Rich phaseS S S
Formation of aluminumsulphate Al2(SO4)3 which covers the surface of γ-Al2O3 or plugs the micro pores of γ-Al2O3.
Formation of BaSO4 under lean conditions
Formation of Pt-sulphur compounds (sulphides) under rich conditions
Al 2(SO4)3 γ-Al 2O3
BaOPt
SO2 SO3
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Open problems
S resistance :� deactivation/regeneration � catalyst durability� new materials (TiO2-, Rh-, ZrO2-added catalyst)� new structure (the geometrical structure of the catalyst would also function to minimize the size of the sulfate particles and make them easier to be removed from the catalyst, i.e. square cells vs hexagonal cell)� reduction of sulphur in the fuel
Thermal stability (more stable support)
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storage :Transient Response Method
(TRM)
regeneration :
Temperature ProgrammedDesorption (TPD)
Techniques: FT-IR, Transient Response Method (TRM) @ 350°CMolecules: NO, NO2, NO+O2, NO2+O2
0 1 0 0 0 2 0 0 0 3 0 0 0 4 0 0 0 5 0 0 0 6 0 0 0
0
2 0 0
4 0 0
6 0 0
8 0 0
1 0 0 0
3 0 0
3 5 0
4 0 0
4 5 0
5 0 0
5 5 0
6 0 0
conc
entr
atio
n, p
pm
T, °C
time, s
NO, NO2 (+O2)
temperature
GHSV=105 Ncc/gcat·h
Methods @POLIMI
Model catalysts : Pt-Ba/γ-Al2O3, Pt/γ-Al2O3 , Ba/γ-Al2O3 (Pt=1% w/w, Ba=16% w/w)
60-701000.82160Pt-Ba/γγγγ-Al 2O3
Pt dispersion(%)
Pore radius(Å)
Pore volume(cm3/g)
Surface area(m2/g)
Catalyst
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NO/O2 adsorption on Ba/Al 2O3 @ 350°C
NO
NO in
concentration, ppm
time, s
0
1000
500
0 500 1500 25001000
NO2
2000-500
Non-negligible adsorption of NOx (1.6*10-4 mol/gcat).
NOx storage initially occurs as nitrites (and nitrates).
2000 1800 1600 1400 1200 10000.0
0.2
0.4
Abs
orba
nce
Wavenumbers (cm-1)
ionic nitrates(1410, 1320, 1030 cm-1)
bridged nitrates(1545 cm-1)
20 min
15 min
2000 1800 1600 1400 1200 10000.0
0.2
0.4
Abs
orba
nce
Wavenumbers (cm-1)
ionic nitrites
(1330,1210 cm-1)
10 min
3 min5 min
1 min
bridged nitrates(1545 cm-1)
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NO
NO in
concentration, ppm
time, s
0
1000
500
0 500 1500 25001000
NO2
2000-5002000 1800 1600 1400 1200 1000
0.0
0.2
0.4
Abs
orba
nce
Wavenumbers (cm-1)
ionic nitrates(1410, 1320, 1030 cm-1)
bridged nitrates(1545 cm-1)
20 min
15 min
NO/O2 adsorption on Ba/Al 2O3 @ 350°C
Non-negligible adsorption of NOx (1.6*10-4 mol/gcat).
NOx storage initially occurs as nitrites (and nitrates).
Nitrites are transformed into nitrates ad-species.
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NO
NO2 in
concentration, ppm
time, s
0
1000
500
0 2000 4000 500030001000
NO2
NOx
Extensive NOx adsorption (8.9*10-4 mol/gcat), no NOx dead time.
NO2 adsorption with NO release, detection of nitrates
Disproportionation route:
3 NO3 NO22 + BaO + BaO →→→→→→→→ Ba(NOBa(NO33))22 + NO + NO ↑↑↑↑↑↑↑↑
2 NO2 NO22 + BaO → Ba(NO+ BaO → Ba(NO33--NONO22))NONO22 + Ba(NO+ Ba(NO33--NONO22) → Ba(NO) → Ba(NO33))22 + NO+ NO
time, s
0
0.3
0 500 1000 1500
0.1
2000-500
0.2
0.4NO evolved/NO2 consumed
1800 1600 1400 1200 1000
0.0
0.4
0.8
1.2
Abs
orba
nce
Wavenumbers(cm-1)
3 min
5 min10 min
1 min
bridgingnitrates(1545 cm-1)
ionic nitrates(1410, 1320 cm-1)
1800 1600 1400 1200 1000
0.0
0.4
0.8
1.2
Abs
orba
nce
Wavenumbers(cm-1)
1800 1600 1400 1200 1000
0.0
0.4
0.8
1.2
Abs
orba
nce
Wavenumbers(cm-1)
3 min
5 min10 min
1 min
bridgingnitrates(1545 cm-1)
bridgingnitrates(1545 cm-1)
ionic nitrates(1410, 1320 cm-1)
ionic nitrates(1410, 1320 cm-1)
NO2 adsorption on Ba/Al 2O3 @ 350°C
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NO2 adsorption via the NO2 disproportionation route.
No NOx dead time.
NO2 decomposition to NO and O2.
Detection of only nitrates ad-species.
NO
NO2 in
time, s
0 2000 4000 600030001000
NO2
NOx
O2
5000
NO2 adsorption on Pt-Ba/Al 2O3 @ 350°C
concentration, ppm
0
1000
500
1800 1600 1400 1200 10000.0
0.2
0.4
0.6
Abs
orba
nce
Wavenumbers (cm-1)
10 sec
10 min
5 min
3 min
1 min
bridging nitrates(1545 cm-1)
ionic nitrates(1410, 1320 cm-1)
1800 1600 1400 1200 10000.0
0.2
0.4
0.6
Abs
orba
nce
Wavenumbers (cm-1)
10 sec
10 min
5 min
3 min
1 min
bridging nitrates(1545 cm-1)
bridging nitrates(1545 cm-1)
ionic nitrates(1410, 1320 cm-1)
ionic nitrates(1410, 1320 cm-1)
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Presence of dead time for NO and NO2 breakthrough.
Oxidation of NO to NO2.
NOx storage occurs initially as nitrites (and nitrates).
Nitrites are readily transformed into nitrates.
NO
NO in
concentration, ppm
time, s
NO2
NOx
2000 1800 1600 1400 1200 10000.0
0.2
0.4
0.6
0.8
1.0
1.2
ionic nitrates(1410, 1320, 1030 cm-1)
bridged nitrates(1545, 1030 cm-1)
10 min
5 min
3 min
1 min
Wavenumbers (cm-1)
1 min
ionic nitrites(1330,1210 cm-1)
NO/O2 adsorption on Pt-Ba/Al 2O3 @ 350°C
0 500 1000 1500 2000 2500 3000
0
200
400
600
800
1000
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Nova et al., Top.Catal., 30/31 (2004) 181Nova et al., J.Catal., 222 (2004) 377
NOx adsorption mechanism on Pt-Ba/Al 2O3
NO + O2
NO2
Pt
Pt BaO
NONO33--
Al 2O3 Nitrate speciesNitrate species
Pt BaO
NONO22--
Al 2O3 Nitrite speciesNitrite speciesPt + Ba
NO
O2NO2Ba
Literature agreement on:• NO2 disproportionation reaction• storage of nitrite & nitrate ad-species
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H2/H2O-TRM at different T after NO/O2 adsorption @ 350°C
� N2 selectivity increases with T at the expenses of NH3
NH3 is likely an intermediate in N2 formation
0
500
1000
1500
2000H
2 in
150°C
NH3
H2
200°C
NH3
N2
0 300 600 900
0
500
1000
1500
2000
Time (s)
H2
N2
350°C
0
500
1000
1500
2000
H2
N2
NH3
Con
cent
ratio
n (p
pm)
The reduction of NO x stored on PtBa/Al 2O3
1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 00
2 0
4 0
6 0
8 0
1 0 0
5 0
6 0
7 0
8 0
9 0
1 0 0
N 2 s e le c t iv i ty , %
T e m p e ra tu re [°C ]
N O x re m o v a l e ff ic ie n c y , %
The reduction of stored NOx is complete only at high TBoth N2 and ammonia are observed with N2 preceding NH3 Lietti et al., J.Catal., 257 (2008)
270
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Ba(NOBa(NO33))22 + 8H+ 8H22 →→ BaO + 2NHBaO + 2NH33 + 5H+ 5H22OO
@ lower T: nitrates are selectively reduced by H 2 to NH 3 :
@ increasing T: NH 3 formation ↓↓↓↓ & N2 formation ↑↑↑↑
Ba(NOBa(NO33))22 + 5H+ 5H22 →→ BaO + NBaO + N22 + 5H+ 5H22OO
@ higher T: nitrates are mostly reduced to N 2, but NH 3 is detected
Ba(NOBa(NO33))22 + 8H+ 8H22 →→ BaO + 2NHBaO + 2NH33 + 5H+ 5H22OO
BUT: nitrates are also reduced by NH 3 to N 2
3Ba(NO3)2 + 10NH3 → 3BaO + 8N2 + 15H2O
Is ammonia an intermediate in N2 formation?
The reduction of NO x stored on PtBa/Al 2O3
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100 200 300 400
0
200
400
600
800
0
500
1000
1500
2000
H2
N2
NH3
NO
Con
cent
ratio
n, p
pm
Temperature °C
100 200 300 400
0
400
800
1200
H2
N2
NH3
Con
cent
ratio
n, p
pm
Temperature °C
NH3 reduces stored nitrates selectively to N2, but its reactivity is lower than that of H2
NH3/H2O
H2/H2O
The reduction of NO x stored on PtBa/Al 2O3: H2- & NH3-TPRS experiments
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Nitrate reduction with H 2 to nitrogen occurs via a two steps in series proces s:
Step 1) Fast reaction of H2 with nitrates to form NH3:
Step 2) Slower reaction of NH3 with nitrates to form N2:
Step 1 + 2 account for the overall stoichiometry of N 2 formation:
Ba(NOBa(NO33))22 + 8H+ 8H22 →→ BaO + 2NHBaO + 2NH33 + 5H+ 5H22OO
3Ba(NO3Ba(NO33))22 + 10NH+ 10NH33 →→ 3BaO + 8N3BaO + 8N22 + 15H+ 15H22OO
Ba(NOBa(NO33))22 + 5 H+ 5 H22 →→ BaO + NBaO + N22 + 5 H+ 5 H22OO
Nitrates reduction by H 2
Lietti et al., J.Catal., 257 (2008) 270
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Lidia Castoldi
Similar amounts of N2 are obtained by reducing of nitrates with H2 or NH3
150 200 250 300 3500,0
5,0x10-5
1,0x10-4
1,5x10-4
2,0x10-4
2,5x10-4
3,0x10-4
N2
[mol
/gca
t]N
2 formation
Temperature [°C]
reduction with H2
reduction with NH3
Reaction of NHReaction of NH 33 with nitrates with nitrates is rateis rate --determiningdetermining
Nitrates reduction by H 2 & NH3
Step 1) Fast reaction of H2 with nitrates to form NH3:
Step 2) Slower reaction of NH3 with nitrates to form N2 (RDS):
Ba(NOBa(NO33))22 + 8H+ 8H22 →→ BaO + 2NHBaO + 2NH33 + 5H+ 5H22OO
3Ba(NO3Ba(NO33))22 + 10NH+ 10NH33 →→ 3BaO + 8N3BaO + 8N22 + 15H+ 15H22OO
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Lidia Castoldi
Zone I: regenerated trap
Zone II: reaction of H2 with nitrates
Zone III: reaction of ammonia with nitrates
Zone IV: spent trap
0
100
200
300
400
160018002000
H2
N2
NH3
0
100
200
300
400
160018002000
N2
NH3
H2
Time (s)C
once
ntra
tion
(ppm
)
Low-T (150 °C)
High-T (300 °C)
Lietti et al., J.Catal., 257 (2008) 270
The H2 front model for the reduction of the stored NO x
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Cumaranatunge et al., J.Catal, 246 (2007) 29
NH3 formation over LNTs
Pihl et al., SAE 2006-01-3441
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� Funzionamento in continuo
� Sfrutta le reazioni di riduzione selettiva ad N2 tra NOX e NH3
� Necessità di un accurato controllo nella strategia di alimentazione di NH3
� Necessità di avere a bordo Urea (precursore NH3)
DeNOx catalytic systems: SCR
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Lidia Castoldi
� Ossidazione NO/NO2� Accumulo NOX come nitriti/nitrati
� Riduzione dei nitriti/nitrati� Produzione di N2 e NH3
Funzionamento ciclico non stazionario
DeNOx catalytic systems: LNT
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Lidia Castoldi
Meccanismo di riduzione:
Ba(NO3)2 + 8 H2 →→→→ 2 NH3 + BaO + 5 H2O
3 Ba(NO3)2 + 10 NH3 →→→→ 8 N2 + 3 BaO + 15 H2O
Possibilità di uso combinato e sinergico delle due tecniche
L.CumaranatungeL.CumaranatungeL.CumaranatungeL.Cumaranatunge etetetet al. J. al. J. al. J. al. J. CatalCatalCatalCatal 2007.2007.2007.2007.Lietti Lietti Lietti Lietti etetetet al., J. al., J. al., J. al., J. CatalCatalCatalCatal 2008200820082008
DeNOx catalytic systems: SCR + LNT
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Combinati (Bluetec)
� Attualmente installato su autovetture
� Funzionamento ciclico
� Sfrutta l’ammoniaca di slip del letto LNT per la rimozione di NOX via SCR
� Emissione di N2 e H2O come “unici” prodotti finali
DeNOx catalytic systems: SCR + LNT
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Lidia Castoldi
NH3 NH3 NH3NOx
NOX
Pt-Ba/ γγγγAl 2O3 LNT
Fe-ZSM-5 SCR
Fase Lean(I)
FLOW
NOx
NOX
N2
Pt-Ba/ γγγγAl 2O3 LNT
Fase Lean(II)
FLOW
NOx NOxNH3 NH3
Fe-ZSM-5 SCR
� Produzione di N 2
Pt-Ba/ γγγγAl 2O3 LNT
Fe-ZSM-5 SCR
Fase Rich(I)
FLOW
Pt-Ba/ γγγγAl 2O3 LNT
Fase Rich(II)
FLOW
NOxNH3
Fe-ZSM-5 SCR
NOx NOx
NH3H2
N2
NOx
H2
NH3
� Slip di NH 3 contenuto
DeNOx catalytic systems: SCR + LNT
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Lidia Castoldi
Diesel Particulate Filter (DPF)
Wall-flow filters: are made of ceramic monolith and are based on a shallow-bed filtration mechanism;
PSA system: employs a SiC wall-flow monolith, a catalytic pre-oxidiser and Ce-fuel additive;
Continuously regenerating DPF (CRT): employs an oxidation catalyst upstream to convert exhaust NO to NO2 and the NO2 is the primary oxidant for the stored PM.
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Lidia Castoldi
Wall-flow trap: ceramic monolith
Different types of diesel particulate filters have been developed; the most efficient remove more than 99 % by number of the exhaust-gas particulates
The channels are blocked at alternative ends. To pass through the monolith the exhaust gas is forced to flow through the channel walls, which retain the contained particulate matter in the form of soot and allow gaseous components to exit.
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Lidia Castoldi
Abatement efficiencies:
Particulate:90%
CO: 90%
HC: 90%
NOx: 3%
Continuously regenerating Trap System(CRT by Johnson Matthey)
NO →→→→ NO2Oxydising catalyst CO →→→→ CO2
HC →→→→ CO2 and H 2O
Non-catalytic
Wall-flow trap
NO2 + C →→→→ NO + CO2
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Lidia Castoldi
Reduce PM and NOx simultaneously and continuously
PM is oxidized by active oxygen released in the NOx storage process and by excess oxygen in exhaust gas, or by active oxygen released in the process of reducing the stored NOx
Conversion efficiency of greater-than-80% in both PM and NOx in the initial stage of operation
Requires fuel with low sulfur content to maintain a high conversion efficiency for a long duration
Diesel Particulate NO x Removal (DPNR) features
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Lidia Castoldi
D-CAT Concept For Clean Diesel Technology
Clean power diesel engines produces levels of NOx and PM emissions that are respectively around 50% and 80% below Euro-4 standards.
The Clean Power engine uses Toyota’s D-CAT (Diesel Clean Advanced Technology): its heart is the DPNR (Diesel Particulate NOx
Reduction system)
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Lidia Castoldi
Diesel Particulate NO x Removal (DPNR) concept
�A catalytic wall-flow filter coated with a NSR catalyst is used to accomplish the simultaneous removal of soot and NOx
K. Nakatani et al., SAE paper 2002-01-0957
Enlarged view of exhaust gas flowing substrate wall
NOx storage reduction catalyst
Fine porous ceramic structure
NOx storage reduction catalyst
Exhaust gas flow
Fine porous ceramic structure
Exhaust gas
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Lidia Castoldi
� Investigations on the De-NOx and De-soot reactivity of DPNR catalysts are still scarce
K. Nakatani et al., SAE Paper SP-1674, 2002-01-0957
Diesel Particulate NO x Removal (DPNR) concept