Engine Catalyst 101 - Gill Instrumentsgillinstruments.com/data/Engine-Catalyst.pdf · 3‐Way...

Engine Catalyst 101

How a Catalyst Works

–

Common Understanding

Lots of Pollution Much Less Pollution

Magic Stuff Happens

Big Picture Overview 3‐Way Catalyst on a Rich Burn Engine

HMHC

NOx

CO

H2

O

N2

CO2

The Bad Guys 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 + DCatalysts are used to make the majority of materials and products we use everyday.

Gasoline, plastics, synthetic materials, chemicals, pharmaceuticalsMargarine 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

Oxidation Reactions

Cy

Hn

+ O2

CO2

+ H2

O

CO + O2

CO2

CO + H2

O

CO2

+ H2

Reduction Reactions

NOx

+ CO

N2

+ CO2

NOx

+ H2

N2

+ H2

O

NOx

+ Cy

Hn

N2

+ CO2

+ H2

O

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 DensityExpressed as Cells/in2 (cpsi)

The higher the number the smaller the cells200 cpsi and 300 cpsi are common

400 cpsi+ are used for cars

Cell GeometryCorrugation 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 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 that reveals the locations of Rh containing crystals.

Precious Metal Content Decisions

PM SpeciesWhich ones

What ratios between them

PM LoadingBoundary 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

0

10

20

30

40

50

60

70

80

90

100

200 300 400 500 600 700 800

Temperature (oF)

% C

onve

rsio

n

1X 3X 6X 30X

Application Engineering

Factors Affecting Catalyst Performance

Catalyst TemperatureSupplies the energy for the chemical reaction

Space Velocity (aka – Residence Time)Sets the overall performance of the catalyst

Cell density and geometry effects

Mass Transfer Limited RegionHere the ability of the VOCs to diffuse to the surface of the catalyst controls the performance.

Kinetic Rate Limited RegionHere the rate of the chemical reaction controls the performance.

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.

Effect of Temperature on a Catalyst

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.


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.


Substrate InfluencesWhen 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

Boundary Layer of

Nearly Stagnant

Exhaust

Faster in the center of the channel


Substrate InfluencesThe slower the flow the thicker the boundary layer grows.

This is due to the loss of turbulenceA thicker boundary layer means the longer it takes NOx, etc to reach the catalyst’s surface


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 betterWhen 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

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 200 400 600 800 1,000 1,200

Temperature (oF)

Conversion Efficiency

30,000 60,000 90,000 120,000 150,000

Increasing GHSV Shifts

the Performance in this

Direction

Decreasing GHSV Shifts

the Performance in this

Direction

Factors Affecting Catalyst Performance – Space Velocity

Think of a catalyst as a group of sequential segmentsFor a given GHSV and temperature each segment converts a certain % a NOx, CO and HC’s that enter it.

X% X% X% X% X%

Y”

of Flow Depth


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

So the overall performance of the catalyst would be:%DRE = 1‐

Cout

/Cin= 1‐31.25ppm/1000ppm= 96.88%

1000

ppm500

ppm250

ppm125

ppm62.5

ppm31.25

ppm

50% 50% 50% 50% 50%

1000 ppm 31.25 ppm


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.


Cout

/Cin= 1‐77.8ppm/1000ppm= 92.2%

1000

ppm600

ppm360

ppm216

ppm129.6

ppm77.8

ppm

40% 40% 40% 40% 40%

1000 ppm 77.8 ppm

How does this all come together?Engine Catalyst Application Sheet

Client Information

CompanyContact(s)

Title

AddressCity State ZipPhone FaxE-mail

Raw Emission Data and Performance Targets

Engine Manufacturer

Engine Model

Engine Type Rich Burn Lean Burn

Fuel Type

Exhaust Flow Rate scfm acfm lb/hr

Exhaust Temperature oF oC

Engine Brake HP

Annual Run Time: hrs/day x days/wk x wks/yr

Raw Emissions Performance Targets

Basis Basis

NOxCO

NMHCNMNEHC

Formaldehyde

Oxygen Content vol % Reference Oxygen Content vol %Water Content (if known) vol%

Acessories, Special Features or Other Requirements

NOx

Formaldehyde

NMNEHCNMHC

CO

ppmvg/bhp-hr lb/hr g/bhp-hr

Natural Gas Gasoline

% Destructionlb/hr ppmv

Propane OtherDiesel

Application Data Review

Task: Calculate how much catalyst is needed to meet the required performance targets

Engine 1 Engine 2

Model XP99‐007 6Z945GQ

Flow (acfm) 3,540 4,035

Temperature (oF) 1,075 1,290

Std Flow (scfm) 1,223 1,222

Brake HP 725 900

NOx (g/bHP‐hr) 13.5 8.0

CO (g/bHP‐hr) 11.0 9.0

Performance Data – Used to select the GHSV

NOx Performance40/5:0:1

80%

85%

90%

95%

100%

50,000 75,000 100,000 125,000 150,000 175,000 200,000 225,000

GHSV

DRE

%

Design Calculation Results

Actual Minimum Diameter takes into account internal blockages inside the

housing and then rounds up the next standard size element.

NOx (g/bHP‐hr) 2 1 0.5

CO (g/bHP‐hr) 4 2 1

Engine 1 Engine 2 Engine 1 Engine 2 Engine 1 Engine 2

NOx DRE Required 85.2% 75.0% 92.6% 87.5% 96.3% 93.8%

CO DRE Required 63.6% 55.6% 81.8% 77.8% 90.9% 88.9%

GHSV 176,413 243,000 129,431 162,000 102,210 121,500

Calculated Diameter 16.17 13.77 18.87 16.87 21.24 19.48Actual Minimum Diameter 19.50 17.00 23.50 21.50 25.50 23.50

Scenario 1 Scenario 2 Scenario 3

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 3‐Way Catalyst

Gas Fired Lean Burn 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 flowTemperatures 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 Hours1,400oF to 1,500oF Minutes1,500oF + Seconds

Irreversible damage to the catalyst.

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.

MisfiresMisfires 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.

MisfiresWorst Case Scenario For the Catalyst

Ignition failureDumps 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.

ResultBefore 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 flowComplete 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 EffectEffect 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)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

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 3.50 3.75 4.00

Cumulative Hole Diameter (inches)

Stack NOx Co

ncen

tration (g/hp‐hr)

Cat 3306 TA (14.5 in Diameter) Waukesha 7042 GSI (33.5 in Diameter)

Why Leakage Can Ruin Your Day

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 givesY = (1,993 * 0.27)+(30 * 13.5)

1,223

Y = 0.77 g/bhp‐hr

Engine 1 with a new catalyst25.5”

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.

0.27 g/bhp‐hr

13.5 g/bhp‐hr

Y g/bhp‐hr

1,993 scfm

30 scfm

1,223 scfm

13.5 g/bhp‐hr

1,223 scfm

How Damage Effects Control Efficiency

When a catalyst is damaged you loose effectiveness in the segments from the inlet face towards the outlet face.


Cout

/Cin= 1‐131.2ppm/1000ppm= 86.9%

1000

ppm950

ppm807.5

ppm524.9

ppm262.4

ppm131.2

ppm

5% 15% 35% 50% 50%

1000 ppm 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 processCaustic wash to remove organic materialsAcidic 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 happensWater 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 removeHeavy 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

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