Engine Catalyst 101
How a Catalyst Works – Common Understanding
Magic Stuff Happens
Lots of Pollution
Much Less Pollution
Big Picture Overview
3‐Way Catalyst on a Rich Burn Engine
NOx
H2O
CO
N2
HMHC
The Bad Guys
CO2
The Good Guys
Building a Catalyst
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 used to make the majority of materials and products we use everyday. „
„
Gasoline, plastics, synthetic materials, chemicals, pharmaceuticals
Margarine and solid fats
„
All chemical reactions are an exchange of energy from the reactants to the products
„
It does this by lowering the energy level required for the reaction to proceed.
Chemical Reactions Across Engine Catalysts
„
Reduction Reactions
NOx + CO NOx + H2
NOx + CyHn
N2 + CO2
N2 + H2O
N2 + CO2 + H2O
„
Oxidation Reactions
CyHn + O2
CO + O2
CO + H2O CO2 + H2O
CO2
CO2 + H2
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)
Substrate
„
Acts as the skeleton of the catalyst.
Metal foil is the preferred choice for engine applications.
The foil is a stainless steel alloy that contains aluminum.
„
Has a roughened surface for adhesion of the washcoat.
„
„
Substrate
„
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
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 are grown
by exposing the foil to
1,700oF for several
hours.
Washcoat
„
„
„
„
The washcoat increases the surface area.
Provides more locations to place active components.
Typically various forms of aluminum oxide.
Contains other trace components to enhance performance.
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
Visible light
view of a
finished catalyst
surface in an
electron
microscope.
This is an X-ray
illuminated view of
the same surface.
By varying the Xray 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 that
reveals the locations
of Rh containing
crystals.
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
… Derived from automotive catalyst technologies
… 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
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
Temperature (oF)
1X
3X
6X
30X
800
Application Engineering
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.
Factors Affecting Catalyst Performance
„
Residence time in the catalyst: 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.
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
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
Factors Affecting Catalyst Performance
„
The higher the cell density the longer the flow keeps its turbulence
…
„
Non‐straight cell geometries work even better
…
„
Eventually does become non‐turbulent or laminar
When the flow has to make a turn it becomes turbulent again
Keeping the boundary layer thinner helps performance
Effect of Space Velocity on Catalyst Performance
Catalytic Activity as a Function of Space Velocity
100%
Decreasing GHSV Shifts 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
Temperature ( oF)
30,000 60,000 90,000 120,000 150,000 1,200
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%
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
So the overall performance of the catalyst would be:
%DRE = 1‐ Cout/Cin
= 1‐31.25ppm/1000ppm
= 96.88%
31.25
ppm
Factors Affecting Catalyst Performance – Space Velocity
„
Now if that catalyst is moved to another engine that has a higher exhaust flow rate the space velocity will increase 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
So the overall performance of the catalyst would be:
%DRE = 1‐ Cout/Cin
= 1‐77.8ppm/1000ppm
= 92.2%
77.8 ppm
How does this all come together?
Engine Catalyst Application Sheet
Client Information
Company
Contact(s)
Title
Address
City
State
Phone
Fax
Zip
E-mail
Raw Emission Data and Performance Targets
Engine Manufacturer
Engine Model
Engine Type
Rich Burn
Fuel Type
Natural Gas
Lean Burn
Diesel
Gasoline
Exhaust Flow Rate
scfm
acfm
Exhaust Temperature
o
o
F
Propane
Other
lb/hr
C
Engine Brake HP
Annual Run Time:
hrs/day
x
days/wk x
Raw Emissions
Basis
g/bhp-hr
wks/yr
Performance Targets
lb/hr
ppmv
Basis
NOx
NOx
CO
CO
NMHC
NMHC
NMNEHC
NMNEHC
Formaldehyde
g/bhp-hr
lb/hr
ppmv
Formaldehyde
Oxygen Content
vol %
Water Content (if known)
vol%
Acessories, Special Features or Other Requirements
Reference Oxygen Content
vol %
% Destruction
Application Data Review Engine 1
Engine 2
XP99‐007
3,540
6Z945GQ
4,035
Std Flow (scfm)
Brake HP
1,075
1,223
725
1,290
1,222
900
NOx (g/bHP‐hr)
CO (g/bHP‐hr)
13.5
11.0
8.0
9.0
Model
Flow (acfm)
o
Temperature ( F)
Task: Calculate how much catalyst is needed to meet the required performance targets
Performance Data – Used to select the GHSV
NOx Performance
40/5:0:1
100%
DRE %
95%
90%
85%
80%
50,000
75,000
100,000
125,000
GHSV
150,000
175,000
200,000
225,000
Design Calculation Results
Scenario 1
NOx (g/bHP‐hr)
CO (g/bHP‐hr)
2
4
Scenario 2
1
2
Scenario 3
0.5
1
NOx DRE Required
CO DRE Required
Engine 1
85.2%
63.6%
Engine 2
75.0%
55.6%
Engine 1
92.6%
81.8%
Engine 2
87.5%
77.8%
Engine 1
96.3%
90.9%
Engine 2
93.8%
88.9%
GHSV
Calculated Diameter
Actual Minimum Diameter
176,413
16.17
19.50
243,000
13.77
17.00
129,431
18.87
23.50
162,000
16.87
21.50
102,210
21.24
25.50
121,500
19.48
23.50
Actual Minimum Diameter takes into account internal blockages inside the housing and then rounds up the next standard size element.
How it would look if we could see it
Catalyst Details Specific for Engines
Engines and the Types of Catalyst They Use
„
„
Gas Fired Rich Burn
Gas Fired Lean Burn
…
3‐Way Catalyst
Oxidation Catalyst
Gas fired engines emit NOx, CO and Hydrocarbons
„ NOx is a major contributor to smog formation.
„ Hydrocarbons are unburned fuel components and formaldehyde.
3‐Way Catalyst Specifics
„
3‐Way catalyst controls NOx, CO and Hydrocarbons.
„
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. „
If there is more than 0.5% oxygen in the exhaust the catalyst will take oxygen from the air and your CO and hydrocarbon emissions will not be in control.
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.
„
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 added.
…
SCR systems use a special catalyst and add either ammonia or urea to the exhaust as additional reactants that convert the NOx.
When it Hits the Fan
Causes of Catalyst Failure
„
Overheating ‐ Temperatures above 1,350oF.
„
Masking ‐ Sites covered over by dirt, char, sulfur, etc.
„
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 surface of the catalyst are hotter than what the thermocouples read for 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.
…
…
…
„
1,375oF to 1,400oF 1,400oF to 1,500oF 1,500oF + Irreversible damage to the catalyst.
Hours
Minutes
Seconds
Masking
„
Accumulation of dirt and debris on the catalyst.
„
Blocks airflow through the cells or to the pores.
…
„
Changes the effective GHSV
Does not cause a permanent change in the catalyst.
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.
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.
Misfires
„
Misfires damage the structure of the catalyst
… Pressure waves distort the cell pattern.
… Broken engine components fly 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.
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 quickly put the engine out of compliance.
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.
Bypass Leakage Effect
Effect of Leakage on the Overall Performance of the Converter
as a Function of Cumulative Hole Diameter
(Catalyst Sized for a 0.5 g/hp‐hr Permit Limit)
3.0
Stack NOx Concentration (g/hp‐hr)
2.5
2.0
1.5
1.0
0.5
0.0
0.00
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
2.25
2.50
2.75
3.00
3.25
Cumulative Hole Diameter (inches)
Cat 3306 TA (14.5 in Diameter)
Waukesha 7042 GSI (33.5 in Diameter)
3.50
3.75
4.00
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% destruction efficiency.
Leakage pathway is 1/8” wide gap around 10” of the 80” total circumference.
30 scfm
0.27 g/bhp‐hr
13.5 g/bhp‐hr
1,223 scfm
13.5 g/bhp‐hr
1,993 scfm
Y g/bhp‐hr
1,223 scfm
Mass Balance Calculation (1,993 scfm * 0.27 g/bhp‐hr) + (30 scfm * 13.5 g/bhp‐hr) = (1,223 scfm * Y g/bhp‐hr)
Rearranging and solving for Y gives
Y = (1,993 * 0.27)+(30 * 13.5)
1,223
Y = 0.77 g/bhp‐hr
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
So the overall performance of the catalyst would be:
%DRE = 1‐ Cout/Cin
= 1‐131.2ppm/1000ppm
= 86.9%
131.2 ppm
Keeping it Working
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
… Caustic wash to remove organic materials
… Acidic wash to remove inorganic debris.
… Proper rinsing with de‐ionized water and adequate drying before reinstalling.
„
Cleaning will not restore a catalyst to brand new levels, but it can extend the life of a catalyst.
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.
… Detergent may contain Phosphorus.
… Uses water that contains Chlorine and Fluorine.
…
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 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, 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.
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.
709 21st Avenue  Bloomer, WI 54724 715‐568‐2882 phone  715‐568‐2884 fax www.catalyticcombustion.com