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 H 2 O CO N 2 HMHC CO 2 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 + 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 NO x + CO N 2 + CO 2 NO x + H 2 N 2 + H 2 O NO x + C y H n N 2 + CO 2 + H 2 O Oxidation Reactions C y H n + O 2 CO 2 + H 2 O CO + O 2 CO 2 CO + H 2 O CO 2 + H 2
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/in 2 (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 1,700 o F 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 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 70 % Conversion 60 50 40 30 20 10 0 200 300 400 500 600 700 800 Temperature ( o F) 1X 3X 6X 30X
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 ft 3 /hr) to catalyst volume (ft 3 ). 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 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
Effect of Space Velocity on Catalyst Performance Catalytic Activity as a Function of Space Velocity 100% 90% 80% Decreasing GHSV Shifts the Performance in this Direction Conversion Efficiency 70% 60% 50% 40% 30% 20% Increasing GHSV Shifts the Performance in this Direction 10% 0% 0 200 400 600 800 1,000 1,200 Temperature ( o 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%
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% 50% 50% 50% 50% 1000 ppm 500 ppm 250 ppm 125 ppm 62.5 ppm So the overall performance of the catalyst would be: %DRE = 1 C out /C in = 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% 40% 40% 40% 40% 1000 ppm 600 ppm 360 ppm 216 ppm 129.6 ppm 77.8 ppm So the overall performance of the catalyst would be: %DRE = 1 C out /C in = 1 77.8ppm/1000ppm = 92.2%
How does this all come together? Engine Catalyst Application Sheet Client Information Company Contact(s) Title Address City State Zip Phone Fax E-mail Raw Emission Data and Performance Targets Engine Manufacturer Engine Model Engine Type Rich Burn Lean Burn Fuel Type Natural Gas Diesel Gasoline Propane Other Exhaust Flow Rate scfm acfm lb/hr Exhaust Temperature Engine Brake HP o F o C Annual Run Time: hrs/day x days/wk x wks/yr Raw Emissions Performance Targets Basis NOx CO NMHC NMNEHC g/bhp-hr lb/hr ppmv Basis NOx CO NMHC NMNEHC g/bhp-hr lb/hr ppmv Formaldehyde Formaldehyde Oxygen Content vol % Reference Oxygen Content vol % Water Content (if known) vol% % Destruction Acessories, Special Features or Other Requirements
Application Data Review Engine 1 Engine 2 Model XP99 007 6Z945GQ Flow (acfm) 3,540 4,035 Temperature ( o F) 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 Task: Calculate how much catalyst is needed to meet the required performance targets
Performance Data Used to select the GHSV 100% NOx Performance 40/5:0:1 95% DRE % 90% 85% 80% 50,000 75,000 100,000 125,000 150,000 175,000 200,000 225,000 GHSV
Design Calculation Results Scenario 1 Scenario 2 Scenario 3 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 19.48 Actual Minimum Diameter 19.50 17.00 23.50 21.50 25.50 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 1,350 o F. 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. 1,375 o F to 1,400 o F Hours 1,400 o F to 1,500 o F Minutes 1,500 o F + 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.
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 3.0 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) 2.5 Stack NOx Concentration (g/hp hr) 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 3.50 3.75 4.00 Cumulative Hole Diameter (inches) Cat 3306 TA (14.5 in Diameter) Waukesha 7042 GSI (33.5 in Diameter)
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 13.5 g/bhp hr 13.5 g/bhp hr 0.27 g/bhp hr Y g/bhp hr 1,223 scfm 1,993 scfm 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% 15% 35% 50% 50% 1000 ppm 950 ppm 807.5 ppm 524.9 ppm 262.4 ppm 131.2 ppm So the overall performance of the catalyst would be: %DRE = 1 C out /C in = 1 131.2ppm/1000ppm = 86.9%
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 211 o F occupies 0.017 ft 3 of volume 1 lb of steam at 212 o F occupies 26.88 ft 3 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.
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