Catalyst 101
Technology and Practice
Outline
•
•
•
•
•
•
•
•
Historical Background
Basic Chemistry
Building a Catalyst
Application Engineering
Catalyst Sizing Process
Engine Specific Details
% Hits the Fan
When *@#!&%
Keeping It Working
1
This is not going to be a . . .
Historical Background
•
The Environmental Protection
Agency (EPA) was created in
December
att th
the
D
b 1970 by
b Congress
C
request of President Nixon
•
This was driven by public outcry
over the ever increasing levels of
air, water and solid waste pollution
•
EPA was chartered to establish
programs to protect human health
and prevent damage to the
environment as a result of pollution
2
Excerpt from the Memo to the President
Recommending the Formation of the EPA
The enormous future needs for land, minerals, and energy require that
the protection of our environment receive a powerful new impetus. In
this, the nation will be on the "horns of a dilemma." The economic
progress which we have come to expect, or even demand, has almost
invariably been at some cost to the environment.
Pesticides have increased the yield of our crops and made it possible for
less land to produce more food. They have also polluted the streams and
lakes. Automobiles have broadened our economic and social
i i even as they
h have
h
di i d the
h air
i andd jammed
j
d our
opportunities,
dirtied
highways. Some means must be found by which our economic and
social aspirations are balanced against the finite capacity of the
environment to absorb society's wastes.
Which Pollutants are of Concern
• All engines produce
• Oxides of nitrogen
• Nitrogen Oxide (NO)
• Nitrogen Dioxide (NO2)
• Nitrous Oxide (N2O)
Collectively called NOx
Currently not controlled, but is a greenhouse gas
• Carbon Monoxide (CO)
• Hydrocarbons (CxHy) from unburned or partially burned fuel
• Does not include Methane, may not include Ethane
3
Why Catalysts?
•
Catalytic converters were eventually chosen as the control method
for automobiles
• Demonstrated better performance than competing technologies
• Easily scalable manufacturing processes
• Cost effective for the amount of pollution converted
•
Stationary industrial engines borrow heavily from the automotive
industry for the same reasons
Basic Chemistry
4
How a Catalyst Works
“Common Understanding”
g
ff pp
Magic Stuff Happens
Lots of Pollution
Much Less Pollution
Big Picture Overview
3-Way Catalyst on a Rich Burn Engine
NOx
H2 O
CO
N2
C2H6 + C3H8 , etc
The Bad Guys
CO2
The Good Guys
5
What is a Catalyst?
• A catalyst is a substance which affects the rate of a chemical reaction
without being consumed or altered by the reaction.
A+B
C+D
• Catalysts are a significant portion of the field of study in the world of
chemistry.
• Catalysts are used to make the majority of materials and products we use
everyday.
• Gasoline, plastics, synthetic materials, chemicals, pharmaceuticals
M
d solid
l d fats
f
• Margarine
and
• The American Chemical Society estimates that 90-95% of the products we
use on a daily basis have at least one catalytic reaction somewhere in its
manufacturing process.
What is a Catalyst?
• All chemical reactions are an exchange of energy from the reactants to the
p
products.
• Energy, usually in the form of heat, is put into the reactants to get the
reaction started.
• A catalyst lowers the amount of energy required for the reaction to begin.
• Exactly how a catalyst does this is still not completely understood.
6
Catalyzed vs Non-Catalyzed Reaction
Energy
Ea
Ea
(Catalyst)
Reactants
Reaction Pathway
Products
Chemical Reactions Across Engine
Catalysts
•
Reduction Reactions
NOx + CO
NOx + H2
NOx + CyHn
N2 + CO2
N 2 + H2 O
N2 + CO2 + H2O
•
Oxidation Reactions
CO + O2
CO2
CO + H2O
CO2 + H2
CyHn + O2
CO2 + H2O
CH2O + O2
CO2 + H2O
The reactions we do for engine
g
exhaust all involve moving
g oxygen
yg
either into or out of molecules.
Following the oxygen can help you troubleshoot the operation of
your catalyst.
7
The one key point to remember about a catalyst
is this:
A catalyst does not destroy pollutants in the exhaust, it rearranges the atoms that make up the pollutant into new molecules
that have less harmful effects on the environment and/or human
health.
Building a Catalyst
8
Catalyst Composition
• A catalyst is composed of the following three items:
• Substrate
• Washcoat
• Active Components – Tailored to the engine type (Rich Burn or
Lean Burn)
Catalyst Manufacturing Process
Foil Patterning
Substrate
Forming
Surface
Preparation
Finishing
Operations
Precious
Metals Coating
Washcoating
9
Substrate
• Acts as the skeleton of the
catalyst.
• Metal foil is the preferred choice
for industrial engine
applications.
• Ceramics used for majority of
cars and trucks.
• Foil for high performance cars
and motorcycles & scooters
• The foil is a stainless steel alloy
that contains ~5% aluminum.
• Aluminum content is key for
coating adhesion.
Substrate – Corrugation Options
• Cell structure
• Cell Density
• Expressed as Cells/in2 (cpsi)
• The higher the number the smaller the cells
• 200 cpsi and 300 cpsi are common
• 400 cpsi+ are used for cars
• Cell Geometry
• Corrugation patterns in the foil
• Straight is the most common
• Herringbone
•
Formed by rolling through gear-like tooling or
by die stamping
10
Substrate – Forming the Element
• Once corrugated the element is constructed
by:
• Co
Co-winding
winding corrugated and flat foil to form a
round element
• Pieces cut to length and stacked with flat foil to
form a rectangular element
• Patterned foils can be folded back and forth
on themselves to form rectangular elements
• Brazing or other techniques can be used to
create a monolithic structure that prevents
telescoping
Heat Treatment Process
This is a view of the raw
foil’s surface after the
initial surface
preparation process. The
roughened, spiky
looking areas are crystals
of aluminum oxide
growing out of the foil.
These form the anchors
for the washcoat.
The crystals
Th
l are grown
by exposing the foil to
1,700oF for several hours.
11
Washcoat
•
The washcoat increases the available surface area.
• 100 m2/g or approximately 490,000 ft2/lb
•
•
Provides more locations to place active components.
Typically various forms of aluminum oxide.
•
Contains other trace components to enhance performance.
• The highly porous Gamma crystal state is most common
• Cerium, Lanthanum, Zirconium, etc
Washcoating Process
•
Washcoat can be applied to the substrate in several ways
• By dipping in a vat of washcoat slurry
• Being sprayed with washcoat
• Being passed through a waterfall-like curtain of slurry
•
After the washcoat is applied the substrate is air knifed to clear the
cells of excess slurry.
•
The substrate is then dried and cured for several hours at 800oF –
1,000 oF to fix the washcoat to the foil.
12
This is a view of a
washcoated surface.
Notice all the bumps and
protrusions. Each of
them has an unseen
porous structure where
the precious metals will
be deposited.
Active Components
• Typically a combination of platinum group metals:
• Platinum (Pt), Palladium (Pd), Rhodium (Rh)
• Pt and Pd work to convert CO and Hydrocarbons
• Rh converts NOx
• Widely dispersed as very small clusters of metal crystals.
• 10-100 metal atoms per crystal
13
Sources of Platinum Group Metals
Major PGM Sources
•
Southern Africa
p is usually
y tied up
p in long-term
g
pp y contracts
• Output
supply
• Labor issues
• Nationalization of mines in Zimbabwe
• Electricity shortages
• Refining capacity
•
Russia
• PGM’s
PGM’ are found
f
d in
i the
h deposits
d
i off nickel
i k l and
d copper
• Market forces for these base metals influence Russian supply
• High grade ores becoming depleted
• State controlled export permits
14
2011 PGM Market Overview
Platinum
6,480,000
Palladium
7,360,000
Rhodium
765,000
74.9%
12.9%
12.2%
34.8%
47.3%
17.9%
87.9%
9.5%
2.6%
Recycling
Total Supply
2,045,000
8,525,000
2,345,000
9,705,000
280,000
1,045,000
Demand
8,095,000
8,430,000
906,000
38.4%
30.6%
25.3%
5.7%
71.4%
6.0%
29.3%
-6.7%
78.6%
Mining
Southern Africa
Russia
Other
Auto Catalyst
Jewelry
Industrial Catalyst
Investment
Chemicals
Glass
Electrical
Surplus/Deficit
4.3%
7.9%
8.6%
0.6%
430,000
1,275,000
139,000
PGM Market Prices
Jan 2000 to April 2012
$/Troy Ounce
$11,000
$10,000
$9,000
$8,000
$7,000
$6,000
Pt
$5,000
Pd
Rh
$4,000
$3,000
$2,000
$1,000
$0
15
Applying the Precious Metals
• Precious metals are dissolved to form a chemical solution
• The pores of the washcoat are flooded with solution until saturated
• Either by dipping, spraying or waterfall application
• The coated part is dried and cured again at 800oF -1,000oF
• This breaks down the dissolved precious metal compound
• Forms the precious metal crystals
• Binds them to the surface of the washcoat
• Sometimes the precious metals are pre-mixed with the washcoat
slurry and applied in a single step.
Visible light
view of a
finished catalyst
surface in an
electron
l t
microscope.
16
This is an X-ray
illuminated view
of the same
surface. By
varying the X-ray
wavelength the
various elements
can be made to
fluoresce. Each
bright spot is the
location of a Pt
containing crystal.
Notice how widely
distributed they
are.
The same surface
now under a
different X-ray
wavelength
l
h that
h
reveals the locations
of Rh containing
crystals.
17
Precious Metal Content Decisions
• PM Species
• Which ones
• What ratios between them
• PM Loading
• Boundary conditions
• Minimum to initiate reaction
• Point of diminishing return
• Economic balance
Precious Metal Species
• For 3-way catalyst
y technologies
g
• Derived from automotive catalyst
• Traditional is Pt/Rh
• Ratios range from 3/1 to 7/1
• Pd/Rh formulations are appearing in the field
• Ratios range from 5/1 to 12/1
• Oxidation
• Can be either Pt or Pd only or mixture of Pt/Pd
• For mixtures the ratios range from 4/1 to 1/2
18
Precious Metal Loading
Effect of PM Loading on Catalyst Performance
100
90
80
% Conversion
70
60
50
40
30
20
10
0
200
300
400
500
600
700
800
Temperature (oF)
1X
3X
6X
30X
What am I getting?
• Limited number of companies have the ability to apply the coatings
y rely
y on Toll Coating
g or Private Labeling
g
• Many
• Heavily influenced or controlled by the automotive catalyst
manufacturers
• Formulations are considered Proprietary or Top Secret and are not
disclosed by most suppliers to the industry
• Why?
19
Precious Metal Coatings – What’s Out There
Application Engineering
20
Factors Affecting Catalyst Performance
• Catalyst Temperature
• Supplies the energy for the chemical reaction
• Space Velocity (aka – Residence Time)
• Sets the overall performance of the catalyst
• Cell density and geometry effects
Effect of Temperature on a Catalyst
Catalysts are specified so
that they operate at or further
to the right of this point so
that changes in temperature
do not cause large changes
in performance.
Mass Transfer Limited Region
Here the ability of the VOCs to
diffuse to the surface of the catalyst
controls the performance.
Kinetic Rate Limited Region
Here the rate of the chemical reaction
controls the performance.
This graph shows the effect of temperature on the performance of a catalyst at for a given space
velocity. While this graph is for a hydrocarbon the pattern is similar for NOx, CO and other
hydrocarbons.
21
Reactivity of Different Molecules
Across a Catalyst
Conversion Efficiency Comparison 100
90
80
Conversion Efficiency %
70
60
50
40
30
20
10
0
200
300
400
500
600
700
800
Catalyst Inlet Temperature (oF)
Methane ‐ C1
Propane ‐ C3
Hexane ‐ C6
Octane ‐ C8
Decane ‐ C10
Hydrogen
Factors Affecting Catalyst Performance
• Residence time in the catalyst is defined by the industry with the
term:
Gas Hourly Space Velocity or GHSV
• Ratio of Flow rate (std-ft3/hr) to catalyst volume (ft3).
• Lower GHSV means longer residence time (i.e.: a bigger catalyst)
and better performance.
• Specified by the catalyst manufacturer in order to meet
performance requirements.
22
Space Velocity Example
• 33.5” diameter x 3.5” catalyst
= 1.785 ft3 of catalyst
• Waukesha 7044 GSI
• 7,773 acfm at 1,181oF
= 2,511 scfm
• GHSV = ((2,511
scfm x 60 min/hr)
,
/ ) 1.785 ft3
= 84,391 hr-1
• This means it takes .043 seconds for exhaust to pass
through the catalyst.
Factors Affecting Catalyst Performance
• Substrate Influences
• When exhaust enters the cell a flow pattern develops
• NOx, CO and HC’s have to diffuse through the boundary layer to reach
the catalyst surface
Faster in the center of the channel
Boundary Layer of Nearly Stagnant Exhaust
23
Factors Affecting Catalyst Performance
• Substrate Influences
• The slower the flow the thicker the boundary layer grows.
• This is due to the loss of turbulence
• A thicker boundary layer means the longer it takes NOx, etc to
reach the catalyst’s surface
Higher Cell Density
Lower Cell Density
Remember: A higher cell density means a smaller individual cell.
Factors Affecting Catalyst Performance
• The higher the cell density the longer the flow keeps its turbulence
• Eventually does become non-turbulent or laminar
• Non-straight cell geometries work even better
• When the flow has to make a turn it becomes turbulent again
• Keeping the boundary layer thinner helps performance
• Hi
Higher
h cell
ll d
densities
iti and
d non-straight
t i ht cell
ll geometries
t i can b
be used
d tto
improve performance in an existing housing
•
Provided the additional pressure drop is acceptable.
24
Mass Transfer Limitations - Cell Density Effects
Mass Transfer Limited Conversion as a Function of Space Velocity and Cell Density Propane at 900oF
100%
M
Mass Transfer Limited Conversion Efficiency
95%
90%
85%
80%
75%
70%
0
50,000
100,000
150,000
200,000
250,000
300,000
Space Velocity (hr ‐1 )
100 cpsi
200 cpsi
300 cpsi
400 cpsi
Mass Transfer Limitations - Cell Density Effects
Mass Transfer Limited Conversion as a Function of Space Velocity and Cell Density Propane at 900oF
100%
M
Mass Transfer Limited Conversion Efficiency
95%
90%
85%
80%
75%
70%
0
50,000
100,000
150,000
200,000
250,000
300,000
Space Velocity (hr ‐1 )
100 cpsi
200 cpsi
300 cpsi
400 cpsi
25
Effect of Space Velocity on Catalyst Performance
Catalytic Activity as a Function of Space Velocity
100%
Decreasing GHSV Shifts the Performance in this the Performance in this
Direction
90%
80%
Conversion Efficiency
70%
60%
50%
40%
Increasing GHSV Shifts the Performance in this Direction
30%
20%
10%
0%
0
200
400
600
800
1,000
1,200
o
Temperature ( F)
30,000 60,000 90,000 120,000 150,000 Factors Affecting Catalyst Performance –
Space Velocity
• Think of a catalyst as a group of sequential segments
• For a given GHSV and temperature each segment converts a certain % a
NOx, CO and HC’s that enter it.
Y” of Flow Depth
X%
X%
X%
X%
X%
26
Factors Affecting Catalyst Performance –
Space Velocity
•
Let’s say that for a specified GHSV at a high enough temperature each segment can convert 50%
of NOx. Then the progression would look like this if 1000 ppm of NOx enters the catalyst
1000 ppm
31.25 ppm
50%
1000 ppm
•
50%
500 ppm
50%
250 ppm
50%
125 ppm
50%
62.5
ppm
31.25
ppm
So the overall performance of the catalyst would be:
%CE = 1- Cout/Cin
= 1-31.25ppm/1000ppm
= 96.9%
Mechanism of Reaction
• The conversion process across the catalyst
progress along a multi-step
multi step path
• Each step must occur in order for the catalyst to
successfully convert the pollutants
27
Mechanism of Reaction
Step 1 – Diffusion and Adsorption
Mechanism of Reaction
Step 2 – Chemical Reaction
28
Mechanism of Reaction
Step 3 – Desorption and Diffusion
Catalyst Sizing Process
29
Catalyst Sizing Process - Overview
•
Step 1 – Review engine data and calculate conversion efficiency
required to meet the operating permit
•
Step 2 – Calculate GHSV for “Exact Size” Catalyst for selected
formulation and cell pattern
•
Step 3 – Calculate “Exact Size” Catalyst for selected flow depth
•
Step 4 – Make compensations for internal housing blockages and
performance safety factor, then select the next largest standard
size element
Sizing Process – Step 1
• Using this formula calculate the required conversion efficiency
%CE = (1- Cout/Cin ) x 100%
Model
Flow (acfm)
Temperature (oF)
Brake HP
NOx (g/bHP‐hr)
CO (g/bHP‐hr)
Caterpillar
G3406TA
1,282
1,038
325
11.6
11.3
Waukesha
7042GSI
6,997
1,125
1,478
13.0
9.0
Superior
8G825
4,957
1,330
800
15.0
10.0
Permit Level NOx Efficiency
CO Efficiency
NOx ‐ 1 g/bhp‐hr, CO ‐ 2 g/bhp‐hr
g p
g p
91.4%
92.3%
93.3%
82.3%
77.8%
80.0%
Permit Level NOx Efficiency
CO Efficiency
NOx ‐ 0.5 g/bhp‐hr, CO ‐ 1 g/bhp‐hr
95.7%
96.2%
96.7%
91.2%
88.9%
90.0%
30
Step 2 – Calculate the GHSV
Performance data, such as seen here, is used by a computer program to
calculate the space velocity required
Conversion Efficiency vs Space Velocity (GHSV)
3‐Way Catalyst Formulation "A" on 200 cpsi Foil Pattern
3
Way Catalyst Formulation A on 200 cpsi Foil Pattern
100
98
96
94
Conversion Efficiency
•
92
NOx
90
88
86
84
CO
82
80
50,000
70,000
90,000
110,000
130,000
150,000
170,000
GHSV (hr ‐1 )
Step 2 – Calculation Output
Space Velocity Results from computer modeling program
Caterpillar
G3406TA
Waukesha
7042GSI
Superior
8G825
1 g NOx/ 2 g CO
NOx
CO
157,010
159,141
GHSV Required
150,035
183,220
142,106
171,226
0.5g NOx/ 1 g CO
NOx
CO
122,396
113,649
GHSV Required
118,115
125,421
113,146
119,682
Model
31
Step 3 – Calculate “Exact Size” Element
• Factor in a safety factor to account for:
• Flow inconsistencies in the housing
• Variation in raw emissions from the engine
• Ash accumulation
• Factor in blockages to flow that are internal to the
housing such as:
• Crossbars
• Retaining spiders
• Gasket overlap of catalyst face
• Generally an overdesign of 10-20% accounts for these
items
Steps 3 and 4 – Resulting Element Sizes
32
Always keep this in mind
A catalyst is a molecular conversion machine that has
been designed to operate:
•At a certain range of temperatures
•At a specific exhaust flow rate and composition
•Processing a specific concentration of pollutants
Trying to operate outside of the design parameters risk being out
of compliance.
Catalyst Details Specific for
Engines
33
Engines and the Types of Catalyst They Use
• Gas Fired Rich Burn
3-Way Catalyst
• Gas Fired Lean Burn
Oxidation Catalyst
3-Way Catalyst Specifics
•
3-Way catalyst controls
NOx, CO and
Hydrocarbons.
Effect of Lambda on 3‐Way Catalyst Performance
100%
A 3-Way catalyst for a rich
burn engine needs an AFR
system because, as seen
from the chemical
reactions, the oxygen atom
is removed from the NOx
and given to the CO and
hydrocarbons.
90%
80%
70%
Conversion Efficiency
•
60%
50%
40%
30%
20%
•
If there is more than 0.5%
oxygen in the
h exhaust
h
the
h
catalyst will take oxygen
from the air and your
emissions will not be in
control.
Common Operating Range for 3‐Way Catalyst
10%
0%
0.9750
0.9800
0.9850
0.9900
0.9950
1.0000
1.0050
Lambda Setting
NOx
CO
NMHC
34
Engine Exhaust Profile
Why AFR Setting is Vital
Engine Emissions and Catalyst Conversion Efficiency as a Function of Lambda ()
14
100%
CO Conversion
13
90%
12
80%
11
NOx Emissions
70%
9
60%
8
7
50%
CO Emissions
6
40%
Conversion Efficiency
Raw Engine Emisions (g/bhp‐hr)
10
5
30%
4
3
20%
NOx Conversion
2
10%
1
0
0.97
0%
0.975
0.98
0.985
0.99
0.995
1
1.005
1.01
Lambda () Setting
NOx
CO
NOx Conv.
CO Conv.
35
Oxidation Catalyst Specifics
• A lean burn engine, which has more than 0.5% oxygen in the
exhaust uses a catalyst that only controls CO and Hydrocarbons.
•
•
Formaldehyde
Air
F
ld h d and
d other
th Hazardous
H
d
Ai Pollutants
P ll t t (HAPs)
(HAP ) coming
i under
d greater
t
control with implementation of ZZZZ Rules.
Depending upon Operating Permit limits a different formulation of catalyst may
be required to control Formaldehyde.
• Because of the high oxygen content NOx is not controlled.
• If NOx control is need for a lean burn engine then an SCR system is
dd d
added.
• SCR systems use a special catalyst and add either ammonia or urea to
the exhaust as additional reactants that convert the NOx.
When *@#!&% Hits the Fan
36
Causes of Catalyst Failure
• Overheating - Temperatures above 1,350oF.
• Masking
by di
dirt,
M ki - Sites
Sit covered
d over b
t char,
h sulfur,
lf etc.
t
• Poisoning - Chemical attack on the catalyst by phosphorus, heavy
metals, silicones.
• Misfires – Damages catalyst structure.
• Bypass Leakage – How much is too much?
Overheating
•
Excessive temperatures trigger a physical change in the structure of the washcoat.
•
•
•
Collapses the washcoat’s porous structure trapping the active components so that they are
inaccessible to the air flow
Temperatures
at the
off the
are hotter
than
T
h surface
f
h catalyst
l
h
h what
h the
h thermocouples
h
l read
d ffor
the air.
It takes time for the thermocouples to read the increase in temperature and shut off the
engine.
•
The damage to the washcoat is time and temperature dependent.
•
Irreversible damage
g to the catalyst.
y
•
•
•
1,375oF to 1,400oF
1,400oF to 1,500oF
1,500oF +
Hours
Minutes
Seconds
37
Overheating
Masking
• Accumulation of dirt and debris
on the catalyst.
• Blocks airflow through the cells
or to the pores.
• Changes the effective GHSV
• Flow has to go through the
remaining open cells.
• Increases GHSV which lowers
performance.
performance
• Does not cause a permanent
change in the catalyst.
38
Masking
Space Velocity Revisited
•
Recall the earlier example showing that the efficiency of a catalyst depends upon the GHSV
through the cells.
1000 ppm
31.25 ppm
50%
1000 ppm
50%
500 ppm
50%
250 ppm
50%
125 ppm
50%
62.5
ppm
31.25
ppm
%DRE = 1- Cout/Cin
= 1-31.25ppm/1000ppm
= 96.9%
39
Ashing Effects on Space Velocity
If cells become blocked then the GHSV will increase for the remaining open cells and the effect on
the performance could look like this.
1000 ppm
77.8 ppm
40%
1000 ppm
40%
600 ppm
40%
360 ppm
40%
216 ppm
40%
129.6 ppm
77.8 ppm
%DRE = 1- Cout/Cin
= 1-77.8ppm/1000ppm
= 92.2%
Example of the Effect of Ashing on
Performance
Conversion Efficiency and Space Velocity as a Function of Masking
30.25" Diameter Catalyst for a 7042GSI
99%
180,000
98.1% is the starting point for a fresh element
98%
170,000
97%
160,000
96.2% is the minimum conversion efficiency for 0.5 g/bhp‐hr area
96%
150,000
140,000
94%
130,000
93%
Space Velocity
95%
Conversion Efficiency
•
120,000
92%
110,000
91%
100,000
90%
89%
90,000
0%
5%
10%
15%
GHSV
20%
25%
Efficiency
30%
35%
40%
45%
7042 GSI
40
Masking and Precious Metal Content
Performance Loss Due to Ash Build‐up Effect of Precious Metal Loading
0%
‐1%
‐2%
‐3%
‐4%
‐5%
‐6%
‐7%
0%
5%
10%
15%
20%
25%
% of Catalyst Face Blocked Due to Ash
50 g/ft3
25 g/ft3
12.5 g/ft3
With increased PM content comes increasing tolerance to masking
Misfires
•
Misfires damage the structure of the
catalyst
• Pressure waves distort the cell
pattern.
• Broken
B k engine
i components
t fly
fl
down the piping and hit the
catalyst.
•
Changes the flow of exhaust through
the catalyst.
• Can cause bypass openings to
appear.
• Exhaust then does not come in
contact with the catalyst so no
conversion happens.
• It doesn’t take very much
bypass flow to throw the
system out of compliance.
41
Misfires
•
Worst Case Scenario For the Catalyst
• Ignition failure
• Dumps fuel and air into
exhaust.
• This mixture reaches the hot
catalyst.
• Catalyst then reacts this
air/fuel mixture with a
resulting spike in
temperature rise.
•
Result
• Before the control system has time
to sense and react the catalyst is
destroyed
• Foil has softened to the point
where it is deformed by the
pressure of the flow
• Complete failure of the
substrate
Bypass Leakage
• Allows uncontrolled exhaust to go
around the catalyst.
• Amount of flow through the
bypass points will be in
proportion to the total pressure
drop through the catalyst.
• Cumulative effect of all bypass
points can q
p
quickly
y put
p the engine
g
out of compliance.
42
Why Leakage Can Ruin Your Day
Engine 1 with a new catalyst
25.5” diameter catalyst to meet 0.5 g/bhp‐hr permit limit.
Catalyst is expected to have a 98% conversion efficiency.
Leakage pathway is 1/8” wide gap around 10” of the 80” total circumference.
30 scfm
13.5 g/bhp‐hr
0.244 g/bhp‐hr
13.5 g/bhp‐hr
1,193 scfm
1,223 scfm
Y g/bhp‐hr
1,223 scfm
Mass Balance Calculation (1,993 scfm * 0.244 g/bhp‐hr) + (30 scfm * 13.5 g/bhp‐hr) = (1,223 scfm * Y g/bhp‐hr)
Rearranging and solving for Y gives
Y = (1,193 * 0.244)+(30 * 13.5)
1,223
Y = 0.57 g/bhp‐hr
How Big a Gap Can I Have?
• 7042 GSI with a 33.5” diameter catalyst
• Catalyst has a circumference of 105
105”
• 1 g/bhp-hr NOx Permit
• Total opening can be equivalent to a 2.375” diameter hole
• 1/8” wide gap would be 35.4” long (33.7% of circumference)
• 1/4” wide gap would be 17.7” long (16.9% of circumference)
• 0.5 g/bhp-hr NOx Permit
• Total opening can be equivalent to a 1.375” diameter hole
• 1/8” wide gap would be 11.8” long (11.2% of circumference)
• 1/4” wide gap would be 5.9” long (5.6 % of circumference)
•
The tighter your permit is the less leakage you can tolerate!
43
Preventing Bypass Leakage
• Gasketing is vital to
preventing bypassing.
• Always use a gasket.
• Never re-use an old
gasket.
• Check to see if the housing is
warped.
• Double up gasketing, if
possible, until the housing
can be repaired or
replaced.
•
Inspect the element for
voids, loose foil sections or
gaps between the foil and
the outer rim.
• If found the element should
be replaced
Poisoning
• Permanent deactivation of the catalyst.
• Poisoning agents interact chemically with either the washcoat or the
precious metal.
• Catalyst formulation can tolerate some poisons in air stream, but the
limit is pretty low.
• Examples:
L d
Lead
Arsenic
Phosphorus
Silicones
S
l
Zinc
Mercury
44
Poisoning
Specific Poisoning Concerns for Engines
• Lubrication oil can be a source of catalyst masking or
poisoning agents.
• Engine oil blow-by needs to be kept to a minimum.
• Engine oils need to be low ash varieties (less than 0.6 wt%)
• Phosphorus and zinc containing anti-wear or detergent
additives.
• Anti-freeze or other coolant mixtures.
• Silicon containing
g compounds
p
• Phosphorus compounds
45
How Damage Effects Control Efficiency
•
When a catalyst is damaged you loose effectiveness in the segments from the inlet face towards
the outlet face.
1000 ppm
131.2 ppm
5%
1000 ppm
•
15%
950 ppm
35%
807.5 ppm
50%
524.9 ppm
50%
262.4
ppm
131.2 ppm
So the overall performance of the catalyst would be:
%DRE = 1- Cout/Cin
= 1-131.2ppm/1000ppm
= 86.9%
Keeping it Working
46
Catalyst Maintenance
• Proper catalyst maintenance requires:
• Proper oil selection
• Minimizing oil blow-by
• Eliminating or minimizing the number of misfires
• Even with these steps
• A catalyst will eventually become dirty and need to be cleaned.
• Even in a perfect world thermal aging effects will eventually deteriorate
the performance.
Catalyst Cleaning
•
When a catalyst becomes dirty or ashed up it can usually be cleaned to restore
performance.
•
This is a chemical cleaning process done either at the factory or at a designated
facility with proper equipment and trained technicians.
•
Cleaning is a multi-step process
•
Cleaning will not restore a catalyst to brand new levels, but it can extend the life of a
catalyst.
47
Cleaning Process Outline
A
A. Bathed in a constantly agitated
caustic solution
B
B. Rinsed in a DI water bath
C. Bathed in a constantly agitated
acidic solution
C
D
D. Rinsed in a DI water bath
E. Bathed in a final DI water bath to
remove final traces of cleaning
g
solution
E
F
F. Placed in a industrial convection
dryer until bone dry
Catalyst Cleaning – What Not to Do!
• Do Not Take the catalyst to the car wash!
• High pressure wands can strip off the coating or damage foil
cells.
ll
• Detergent may contain Phosphorus.
• Uses water that contains Chlorine and Fluorine.
• Dissolved solids in the water are deposited into the washcoat
48
Catalyst Cleaning – What Not to Do!
• If you do wash in DI water, make sure the catalyst
is Bone Dry before re-installing it!
• Letting it air dry in the sun for a few hours is
inadequate!
• At minimum place the catalyst in front of a fan with
the air blowing through the cells for 48 hours.
• If not then this is what happens
• Water adsorbed by the coating turns to steam when
hit by hot engine exhaust.
• 1 lb of water at 211oF occupies 0.017 ft3 of volume
• 1 lb of steam at 212oF occupies
p 26.88 ft3 of volume
• The escaping steam fractures the washcoat and
breaks it free from the foil.
Limitations of the Cleaning Process
• Will not remove
• Heavy metals – Lead, iron, tin, etc.
• Catalyst poisons – Phosphorus,
Phosphorus arsenic
• Will not restore a catalyst that has seen high temperature excursions.
• High temperatures again cause a change in the structure of the washcoat.
• If the coating has been fractured due to backfires and other pressure events
it may strip sections off of the substrate.
49
Cleaning Process Expectations
General Catalyst Recovery Profile from Repeated Washings
100%
98%
Conversion Efficiency
96%
94%
92%
90%
88%
Efficiency Needed to Meet Permit
86%
84%
0
5
10
15
20
25
30
35
40
45
50
Time
Washing Limitations
50
How it would look if we could see it
In Conclusion
•
Catalysts are not “Black Magic” nor do you need a
“Secret Decoder Ring” to understand them and use
them properly.
•
The keys to good catalyst performance can be summed
up as:
• Well maintained engine.
• Properly sized catalyst for the engine and the
regulations.
• Regular monitoring of catalyst and engine system.
• Routine cleaning of the catalyst.
• Rigorous attention to the gasketing to prevent
bypassing.
51
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