All About Catalyst Contaminants in Reciprocating Engines
By Steven DeCicco and Dr. Tracy Staller
GMC 2016
Executive Summary
This paper examines how a catalyst works and how ash contaminants interfere. It
describes the sources of ash in a reciprocating engine and the quantities reaching the
catalyst. The paper concludes with principles for catalyst design and maintenance that
promote long service life.
Exhaust gas catalysts have an enormous internal surface area within the nano-pore
structure of its ceramic wash coat. That surface is covered with precious metals such as
platinum and rhodium – catalytic active ingredients. Strong performance depends upon
those nano-scale pores being open to exhaust gas, and the precious metals being
available to react with the exhaust gas. Ash can clog pores and coat precious metals.
Reciprocating engine exhaust pushes a lot of ash at the catalyst. Primary sources are
lubricating oil and coolant. Ash-producing additives make up 20-30% of lube oil –
detergents, dispersants, anti-wear additives, viscosity improvers, friction modifiers, rust
and corrosion inhibitors, etc. The ash content of natural gas engine oils comes in four
levels from ashless (<0.1% sulphated ash) to high ash (>2%). In the cylinders lube oil
vaporizes and liberates “ash” containing sulfur, zinc, phosphorus, and calcium.
Consider a Cat G3616 consuming oil at the OEM’s brake specific oil consumption
(BSOC) rate (>2 lb/hr). If it’s low ash oil the catalyst would be exposed to >114,000
mg/d of ash, including >7,000 mg/d of zinc, >43,000 mg/d sulfur, and >6,000 mg/d of
phosphorous. From the standpoint of catalytic surface, these are large quantities.
Antifreeze coolants also generate catalyst contaminants – silicon, potassium,
phosphorous, sodium, molybdenum, and iron. Coolant is normally isolated from the
combustion system and does not have a path to the exhaust system. Leak-paths from
the coolant recirculation loop to the combustion cylinders are created by catastrophic
gasket failures and by chronic leaking pre-chambers. In the cylinders coolant
evaporates, burns, and sends the additive ingredients into the exhaust system as ash.
The design of an exhaust gas catalyst anticipates this operating environment. Design
features increase catalyst tolerance, and routine maintenance procedures increase
service life.
Page 1 of 16
All About Catalyst Contaminants in Reciprocating Engines
By Steven DeCicco and Dr. Tracy Staller
GMC 2016
Overview – How A Catalyst Works and How It Becomes Deactivated
An exhaust gas catalyst has an enormous internal surface area. For example, one
cubic foot of a nonselective catalytic reduction (NSCR) catalyst has on the order of 55
football fields of internal surface area. That surface area exists inside the pore structure
of the catalyst’s ceramic wash coat – more specifically within the wash coat’s nanoscale pores. The surfaces of those pores are covered with the catalyst’s active
ingredients – precious metals such as platinum and rhodium. Strong catalyst
performance is dependent upon these nano-scale pores being open to exhaust gas,
and the precious metals on those internal surfaces being available to react with the
exhaust gas.
Catalyst become deactivated when:
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
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The wash coat pores become blocked by the accumulation of contaminants at
the entrance to the pores – preventing the exhaust gas from entering the pores
where most of the catalytic surface area exists
The internal surfaces of the wash coat’s pore structure become covered by the
accumulation of contaminants – reducing the amount of “active” surface area,
i.e., “masking” the precious metals and preventing exhaust gas from coming
in contact with the catalyst’s active ingredients
The precious metals become deactivated by reaction with contaminants to
form non-catalytic forms of the precious metals
Some of these forms of deactivation can be reversed using EmeraChem’s precise
chemical washing process. However, during the washing process there is little to no
mechanical agitation deep within the wash coat’s nano-scale pores, and some of the
chemical changes brought about by the contaminants are irreversible. When these
types of permanent deactivation occur the catalyst must be replaced.
Where Catalyst Contaminants Come From And How They Are Introduced Into The
System
Exhaust gas catalysts are poisoned and/or inhibited by impurities carried in the engine
exhaust. The sources and identities of these impurities include:



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Lubricating oils: sulfur (S), phosphorous (P), zinc (Zn), calcium (Ca), magnesium
(Mg)
Antifreeze coolants: silicon (Si), potassium (K), phosphorous (P), sodium (Na),
molybdenum (Mo), iron (Fe)
Fuel: sulfur (S), arsenic (As), possibly silcon (Si)
Combustion air: silicon (Si), sulfur (S), calcium (Ca)
Of all poisons and inhibitors, sulfur (S), phosphorus (P), zinc (Zn), and calcium (Ca) are
the most common contaminants found on exhaust gas catalysts. These substances
Page 2 of 16
All About Catalyst Contaminants in Reciprocating Engines
By Steven DeCicco and Dr. Tracy Staller
GMC 2016
accumulate on the catalyst’s surface and compete with the exhaust pollutants for
access to the surface area and the catalyst’s active ingredients – the precious metals.
Accumulation of these contaminants on catalytic surfaces reduces a catalyst’s
effectiveness and even low levels of impurities are enough to completely cover a
catalyst’s active sites. These substances are generally regarded as catalyst inhibitors
except for phosphorus which can react with and poison NSCR formulations. All of them
however, decrease catalytic efficiency.
Poisoning is defined as a loss of catalytic activity due to the chemisorptions of impurities
on the catalyst’s active precious metal sites. Normally, a distinction is made between
poisons and inhibitors. Poisons are substances that interact very strongly and
irreversibly with the catalyst’s active precious metal sites, whereas the adsorption of
inhibitors on the catalytic surface is weak and most-often reversible using EmeraChem’s
chemical washing process.
Engine Lubricating Oil as a Source of Contaminants
Lube oils are intended to coat the cylinder walls and thus have a pathway to the
catalyst. Excessive quantities of lube oil enter into the exhaust system by leaking
through worn out piston rings, faulty valve seals, failed gaskets and/or warped engine
components. In the cylinders the intense high-temperature combustion process burns
some of this lube oil off the cylinder walls and sends the ash and partially combusted
hydrocarbons into the exhaust system. This shows up on the catalyst as an
accumulation of ash and carbon soot or “coke.”
All engine oils have two components: additives and base oil. The total volume of
additives in motor oil can range from 20 to 30 percent, depending on brand, formulation
and application. These additives can enhance, suppress or add properties to the base
oil.
 Detergents
 Dispersants
 Anti-wear additives
 Viscosity index improvers
 Friction modifiers
 Extreme pressure additives
 Rust and corrosion inhibitors
 Anti-oxidants
 Pour point depressants
 Anti-foaming agents
Natural gas engine oils have special formulations that differ from diesel and gasoline
engine oil formulations. This is because natural gas engines:
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All About Catalyst Contaminants in Reciprocating Engines
By Steven DeCicco and Dr. Tracy Staller
GMC 2016




Burn cleaner, with no soot contamination of the crankcase engine oil. This
requires less detergents and dispersants and allows these lubricants to be
formulated with lower ash levels.
Burn gaseous fuel. Unlike diesel engines, there is no dilution of the lube oil by the
fuel which makes it more critical to prevent viscosity increase of the oil.
Burn hotter, with an exhaust temperature 300°F to 400°F (165°C to 235°C)
higher than diesel. Therefore, oxidation and nitration of the oil are increased, as
is valve wear.
Sometimes operate at constant speed. Therefore, the engine is more prone to
retaining deposits and plug fouling.
Purpose of Ash in Engine Lubricating Oil
“Ash” is a common catalyst contaminant found in reciprocating engine exhaust (see
Figure 1). The primary source of ash is engine lubricating oils. The inorganic
compounds that produce ash are added to hydrocarbon-based lubricating oils to
achieve the special properties listed above.
Most equipment manufacturers specify engine oil based
on its ash content and viscosity grade. The ash is the
portion of the lubricant that is left behind as a deposit
after complete burning of the oil. It is whitish-gray and
comes from the metallic detergents and antiwear
additives. The ash content of natural gas engine oils is
available in four general levels:




ashless (less than 0.1 percent sulphated ash),
low ash (0.2 to 0.6 percent),
medium ash (0.7 to 1.2 percent) and
high ash (greater than 2.0 percent).
Figure 1 - Catalyst
Heavily Coated with Ash
The ash directly provides valve protection in four-stroke
engines. The detergent additives (which are bases) neutralize acids (see discussion of
Detergents, below). Therefore, the oil with the lowest ash content that will provide the
necessary valve protection and acid neutralization is desired. The use of higher ash oils
may cause more deposits to accumulate in the engine and on the catalyst. Excessive
ash deposits may be caused by using an oil with too high of an ash content, overlubrication or many other mechanical factors. This may result in reduced heat transfer,
preignition and/or detonation, ring sticking or breaking, plug fouling, valve burning and
premature catalyst deactivation.
Four-stroke engines typically require either a low or medium ash oil to provide the
sacrificial protective layer of ash on the exhaust valves and seats to prevent valve
recession. Some older Waukesha four-stroke engines require higher ash oils due to the
Page 4 of 16
All About Catalyst Contaminants in Reciprocating Engines
By Steven DeCicco and Dr. Tracy Staller
GMC 2016
high valve angles that were used. Higher ash oils, with higher base numbers are also
used to neutralize highly corrosive fuels.
Two-stroke engines do not have intake or exhaust valves and generally require an
ashless or very low ash oil to minimize exhaust port blockage. These two-stroke
engines typically have large bore cylinders and have oil injection ports feeding oil
directly into each cylinder. This provides a direct pathway for raw and partially burned
lubricating oil to reach the catalyst.
Composition of Ash in Engine Lubricating Oil
Ash additives are sources of the zinc (Zn), sulfur (S), phosphorous (P), and calcium
(Ca) found on catalysts. These catalyst contaminants prevent the catalytic converter
from reducing emissions, and ash accumulation could eventually restrict and reduce
exhaust gas flow and increase backpressure.
A common lube oil additive would be an anti-wear formula. Anti-wear additives have
particles that are shaped similar to detergents and dispersants (see below), but the
polar heads of these molecules are attracted to metal surfaces. Once attached to a
metal surface (such as piston rings), anti-wear additives form a sacrificial layer that
protects the surfaces beneath them from degradation under boundary conditions. Antiwear ingredients are also known as anti-scuffing agents.
A common form of inorganic anti-wear ingredient added to lube oils is zinc di-alkyl-dithio-phosphate (ZDDP, Zn[(S2P(OR)2]2 where “R” is an hydrocarbon like C2H4). From
the name and formula we see that ZDDP includes zinc (Zn), sulfur (S, “thio”), and
phosphorous (P). The ash that comes from burning ZDDP off the cylinder walls is
sometimes referred to as “sulfated ash” – a common form is zinc sulfate.
Calcium (Ca) is also a common additive to hydrocarbon-based lubricating oils. Like
ZDDP, its purpose is to coat and protect internal engine surfaces. The ash that comes
from burning calcium-containing lube oil off the cylinder walls is also referred to as
“sulfated ash” and takes the form of calcium sulfate.
Magnesium (Mg) is another common additive to hydrocarbon-based lubricating oils and
its ash is often found on catalysts.
Purpose and Composition of Detergents in Engine Lubricating Oil
Detergent and dispersant additives are sources of the calcium, magnesium and sulfur
found on catalysts.
A typical additive package found in engine oil would include a detergent and a
dispersant. These two additives work together to help rid the engine system of deposits
caused by the burning of fuel and contributed to by blow-by gases.
Page 5 of 16
All About Catalyst Contaminants in Reciprocating Engines
By Steven DeCicco and Dr. Tracy Staller
GMC 2016
Dispersants and detergents are small particles that have a polar head and an oleophilic
tail. The polar heads are attracted to contaminants within the oil and surround them,
forming a structure called a micelle.
Detergents are typically calcium (Ca) or magnesium (Mg) salts of alkaryl sulfonic acids,
commonly referred to as “sulfonates” or organic sulfonates – hydrocarbon molecules
containing sulfur (S). When lube oil is burned off the cylinder walls the detergents
decompose and release sulfur, calcium and magnesium into the exhaust stream.
Those elements contribute to the creation of sulfated ash discussed above and
accumulate on the catalyst.
Antifoaming agents in lube oil may contain silicon (Si).
Quantity of Ash Generated by Lube Oil
Table 1 at the end of this paper calculates the ash generated from a Cat G3616TA
engine. The engine is assumed to be consuming lube oil at the factory specified “brake
specific oil consumption rate” (BSOC). In practical terms, that equates to consuming
2.1 pounds per hour of lubricating oil. This lubricating oil and its constituents enter the
exhaust gas and flow through the catalyst.
The table examines four grades of oil: Mobil Pegasus Special CF oil, “low ash” oil,
“medium ash” oil, and “high ash” oil and the amount of ash generated by consuming 2.1
lb/hr of oil. Consider the low ash oil, i.e., oil in the range of 0.2 to 0.6% ash. The table
uses the commonly seen value of 0.5% ash. Catalyst downstream of this Cat3616
engine would be exposed to over 114,000 mg of ash per day. More specifically, the
exhaust gas would deliver to the catalyst >7,000 mg/day of zinc, >43,000 mg/day of
sulfur, >6,000 mg/day of phosphorous, and >36,000 mg/day of calcium. In a 1000 hour
operating period every cubic foot of catalyst in the exhaust stream would have passing
through it over 620,000 mg of ash (assuming 8 cubic feet of catalyst are installed).
Soot
Another form of catalyst fouling occurs when lube oil emissions coat the catalyst surface
with carbon soot or “coke.” In a natural gas engine soot is produced from the partial
combustion of hydrocarbons in the lube oil. Carbon deposits prevent the catalytic
converter from reducing emissions, and could eventually restrict and reduce exhaust
gas flow.
The process of a catalyst being coated with carbon soot is technically referred to as
“coke formation” – a process by which carbonaceous residues cover the catalyst’s
active sites and decrease the catalyst’s active surface area. A primary cause of “pore
blockage” is caused when coke formations are so large that carbon blocks the entrance
to internal pores of the catalyst, thereby prohibiting exhaust gas flow from reaching the
enormous catalytic surface area.
Page 6 of 16
All About Catalyst Contaminants in Reciprocating Engines
By Steven DeCicco and Dr. Tracy Staller
GMC 2016
Antifreeze Coolants as a Source of Contaminants
Antifreeze coolants are a common source of catalyst contaminants. Antifreeze is a
potential source of silicon (Si), potassium (K), phosphorous (P), sodium (Na),
molybdenum (Mo), and iron (Fe) that could enter the exhaust and contaminate the
catalyst.
Antifreeze is used in the cooling circuit of internal combustion engines to reduce the
freezing point of the recirculating coolant fluid and to prevent the engine from
overheating due to the high temperatures. Antifreeze also possesses corrosionresistant properties. It contains chemicals that inhibit scale formation within the engine,
cooler/radiator and accompanying components like piping and hoses.
During normal operation, the coolant recirculation system is isolated from the
combustion system and coolant does not have a path to the engine exhaust system.
However, when engine gaskets fail (e.g., head gaskets) a leak-path is created from the
coolant recirculation loop to the combustion cylinders. The intense high-temperature
combustion in the cylinders evaporates the coolant, burns the glycol, and sends the
coolant additive ingredients into the exhaust system as ash. These coolant-based ash
ingredients show up on the catalyst as potassium silicates (K combined with Si) and
sodium silicates (Na combined with Si). Iron shows up as iron oxide.
Industrial engines use coolants with base fluids consisting of mixtures of ethylene glycol
(C2H6O2) and water, or other base fluids such as a mixture of propylene glycol (C3H8O2)
and water (propylene glycol is safer for the environment and less toxic to humans).
Propylene glycol oxidizes when exposed to air and heat, forming lactic acid. If not
properly inhibited, this fluid can be very corrosive, so pH buffering agents such as
dipotassium phosphate (K2HPO4) and potassium bicarbonate (KHCO3) are often added
to propylene glycol, to prevent acidic corrosion of metal components. These are
potential sources of potassium (K) and phosphorous (P) that could enter the exhaust
and accumulate on the catalyst.
In addition to the base fluid, there are a small amount of other ingredients including
corrosion inhibitors, antifoams, dyes and other additives such as iron that reduces
engine wear. These ingredients are what differentiate one coolant from another. These
ingredients include compounds that contain silicon (Si), iron (Fe), potassium (K),
phosphorus (P), sodium (Na), and molybdenum (Mo). Some new blends of industrial
engine antifreeze coolants are low-silicate and phosphate-free.
Traditionally, there were two major corrosion inhibitors used in antifreeze coolants:
silicates and phosphates. Common additives include sodium silicate (Na2SiO3),
disodium phosphate (Na2HPO4), sodium molybdate (Na2MoO4), sodium borate
(Na2[B4O5(OH)4]·8H2O), and dextrin (hydroxyethyl starch). These are potential sources
of silicon (Si), phosphorous (P), sodium (Na), molybdenum (Mo), that could enter the
exhaust and accumulate on the catalyst. Disodium fluorescein dyes are added to
Page 7 of 16
All About Catalyst Contaminants in Reciprocating Engines
By Steven DeCicco and Dr. Tracy Staller
GMC 2016
antifreeze to help trace the source of leaks – another source of sodium (Na)
contamination found on catalysts.
In the absence of leaks, antifreeze chemicals such as ethylene
glycol or propylene glycol may retain their basic properties
indefinitely. By contrast, corrosion inhibitors are gradually used
up, and must be replenished from time to time.
According to one of EmeraChem’s large gas compression
customers Cat G3616 pre-chambers have been known to “leak”
antifreeze, some more than others depending upon whether it’s
an original OEM or aftermarket pre-chamber, whether it’s a onepiece or two piece unit, and the number of operating hours on the
pre-chamber. This coolant leakage is the probable source of
silicon and potassium on the catalyst when the engine has never
experienced a major antifreeze leak. The photo on the right
(Figure 2) shows evidence of coolant leakage around the O-ring
Figure 2 Pre-Chamber
seals on a G3616 pre-chamber. Notice the colored stains and
Showing Coolant Leak
notice the silicon dioxide crust on the firing end (top of the photo).
Fuel and Combustion Air as a Source of Contaminants
Reduced sulfur species enter the system primarily in the natural gas fuel. Reduced
sulfur compounds such as hydrogen sulfide (H2S) are a naturally occurring component
of “sour gas.” In addition, odorants are added to natural gas, such as hydrogen sulfide
and mercaptans [(R–SH) group where R represents an alkane, alkene, or other carboncontaining group of atoms]. Sulfur compounds might also enter a system through the
combustion air and evaporative cooling water for combustion air. Some gas fields in the
Southwest contain enough arsenic to prematurely deactivate emission control catalysts.
Combustion air drawn into the engine has been found to be a source of hydrocarbons,
and “dust” containing silicon, calcium, potassium, sodium, and even lead.
Biogases and Siloxanes “Biogas” fuels such as landfill gas and wastewater treatment
digester gas are becoming increasingly popular for providing “green energy.” Once
considered an undesirable waste byproduct and greenhouse gas, biogas fuels are now
being burned as fuel in reciprocating internal combustion engines and combustion
turbines to generate electricity. Many of the biogas fuels contain “siloxanes.” When
burned in an engine siloxanes decompose and release ultra-fine particles of silicon
dioxide (SiO2). These ultra-fine particles are known to be harmful to the engines
themselves and permanently clog and deactivate catalysts.
Siloxanes start with silicone. Many commercial products including detergents, soaps,
shampoos, deodorants, and lotions contain silicone in one form or another. Because of
the widespread use of these products, our landfills and wastewater contain silicone
compounds. During anaerobic decomposition, these compounds volatize to form
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All About Catalyst Contaminants in Reciprocating Engines
By Steven DeCicco and Dr. Tracy Staller
GMC 2016
siloxane compounds in the landfill gas or digester gas. Siloxanes are hydrocarbon
gases that contain silicon such as octamethylcyclotetrasiloxane (C H O Si ).
8
24
4
4
When siloxane-laden fuel gas is burned, the siloxanes form silicon dioxide or “silica”
(SiO2) vapor. The SiO2 vapor condenses and nucleates to primary nanometer sized
particles (0.000000001 meter) that are carried in the turbulent exhaust gas mixture.
These primary silica particles can accumulate with each other and group into a larger
nanometer particles 10 to 100x as big but still consisting of primary particles (like
grapes). From the standpoint of a catalyst surface this is ultrafine, far smaller than
“PM10”, which is 10,000 nanometers in size.
Silicon dioxide forms hard, crusty deposits on internal engine components (see Figure
3). These deposits collect in small passages and in areas of tight clearances restricting
the flow of air, fuel, and lubricants, and increasing engine wear. Silica particles also
make their way into recirculating engine oil where their abrasive character wears away
at engine internals.
The reciprocating engines and turbines in the early biogas applications experienced
accelerated engine wear due to the effects of siloxanes in the fuel. Life expectancies
for the emission control catalysts were measured in days instead of years for
conventional natural gas fuels. Process technologies to remove siloxanes from biogas
fuels are commercially available and continually improving. While these fuel cleanup
technologies can achieve impressively low siloxane levels (e.g., 50 to 100 ppb), the
remaining siloxanes will accumulate on the catalyst and eventually cause premature
and permanent deactivation.
Figure 3 - Silica Deposition on Microturbine
Engine and turbine manufacturers have published guidelines for the maximum amount
of siloxanes allowed in the fuel supplied to their systems. The engine manufacturer’s
siloxane removal spec is designed to protect the engine but is not sufficient to protect
the catalyst. Industry experience consistently shows that when siloxanes are present in
the engine fuel, even at levels below the range of detectability (30 – 70 ppb), the life of
oxidation catalyst and NSCR catalyst will be shortened. As shown in Table 2 below,
even these low levels can add up to thousands of milligrams per day of silicon dioxide
reaching every cubic foot of catalyst. That is regarded as a very high exposure rate.
Page 9 of 16
All About Catalyst Contaminants in Reciprocating Engines
By Steven DeCicco and Dr. Tracy Staller
GMC 2016
Figure 4 - Silica Deposition on Catalyst
The ultrafine silicon dioxide particles are carried along in the exhaust gas and reach the
catalytic converter (see Figure 4). Accumulation begins, and is greatest, near the
catalyst inlet. The inlet channel surfaces are quickly covered by SiO2 thereby blocking
or masking gas mass transfer to the catalyst sites within the catalyst layer. SiO2 cannot
be dissolved off or mechanically washed off the catalyst surface.
Mechanisms of Catalyst Deactivation
Ash and Particulates. The primary mechanism of deactivation for particulates of all
types is masking. Depending on the size of the particulate material and the size of the
pores in the wash coat, the particulates either cover the active components of the
catalyst (large particulates) or block the wash coat pores (ultra-fine particulates). Of
these two mechanisms the most devastating to catalyst activity is the latter. Masking by
covering the catalyst surface does inhibit activity, however, because most ash has a low
bulk density and contaminants can still diffuse through the layer albeit at a significantly
reduced rate. These surface deposits can be easily dislodged and removed by
vacuuming or by gently blowing air through the catalyst. In addition, these surface
layers of ash are easily and effectively removed by chemical washing processes that
utilize recirculation of clean reagents.
Masking by either a dense impervious layer of ash or by blocking the pores is the most
devastating because it excludes all access of pollutants to catalytically active precious
metal sites either under the impervious layer or beyond the blockage. Masking by these
particulates are difficult to reverse for a number of reasons. First, in both instances the
ability of the chemical wash solutions to contact them is limited by the exposed face and
their impervious nature. For dense surface deposits extended time can be effective in
their removal, however, for ultra-fine particulates, which penetrate deep in the pore
structure, there is little if any effect from extending the wash time. Because the
transport of wash chemicals into the pores in sufficient amounts to react with
particulates is extremely slow these types of deposits are nearly impossible to remove
and are considered permanent deactivators.
Sulfur. Sulfur contaminants come to the catalyst in two forms: gaseous and condensed.
Condensed sulfur compounds in the form of calcium and zinc sulfates (ZDDP/detergent
Page 10 of 16
All About Catalyst Contaminants in Reciprocating Engines
By Steven DeCicco and Dr. Tracy Staller
GMC 2016
sourced) typically are non-reactive with wash coats and are deposited as ash.
However, gaseous sulfur (typically SO2) from odorants or reduced sulfur compounds in
the fuel do have significant impacts on catalyst performance. Sulfur dioxide is oxidized
to a limited extent to SO3 on the active catalyst precious metals. The SO3 that is
produced subsequently reacts with the alumina wash coat. Generally this happens in
the area directly around the active catalyst precious metal crystallites creating areas of
the alumina surface that have enhanced susceptibility to sulfur attack. Over time
additional aluminum sulfate is formed and eventually (a) forms shells which either
encase the catalyst precious metal crystallites or hinder the approach of pollutant
molecules; or (b) blocks the pore from further penetration of pollutants deeper into the
wash coat pores.
Poisons. As mentioned previously catalyst poisons are contaminants which react
strongly with the catalyst constituents. For lean-burn engines, the most common poison
encountered is arsenic (fuel sourced) which reacts directly with the active catalyst
metals forming compounds and/or alloys which cannot be washed or removed. Arsenic
is also common for NSCR or Three-Way catalyst, however, the most prevalent poison is
phosphorus (lube oil sourced). Phosphorus reacts with the ceria to effectively reduce
the oxygen storage capacity. While the phosphorus can be removed by washing, the
cerium associated with it is removed as well resulting in a permanent decrease in
oxygen storage capacity, which hinders NSCR chemistry and performance.
Mechanical Deactivation of Catalyst. Mechanical deactivation of a catalyst can be
caused when engine malfunction produces conditions that physically alter the catalyst
and damage its components. Key causes of damage are pressure waves caused by
deflagrations (“backfire” events), and over-temperature conditions.
Catalysts can be reduced to incinerated char
when raw fuel is discharged from the
combustion chamber into the exhaust flow, and
then ignited on a catalyst’s high temperature
platinum-rich surface (see Figure 5). If the
amount of fuel is high enough and the situation
continues long enough the catalyst could
disintegrate. But before reaching that
catastrophic condition the catalyst’s ceramic
wash coat becomes damaged – and often not
visible to the eye. This damage occurs when
the heat released in the catalyst begins to melt
the ceramic wash coat and causes its nanoscale pore structure to collapse and
permanently cut off its internal surface area.
Catalyst over-temperature destruction can be
Figure 5 - Thermally Charred Catalyst from
Combusting Fuel on Catalyst
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All About Catalyst Contaminants in Reciprocating Engines
By Steven DeCicco and Dr. Tracy Staller
GMC 2016
caused by engine malfunctions including: faulty oxygen sensors, incorrect fuel mixtures,
worn spark plugs or plug wires, faulty check valves, incorrect ignition timing, faulty fuel
valves and other ignition malfunctions.



Deteriorated Spark Plugs or Spark Plug Wires: spark plugs that don’t fire, or
misfire, can cause unburned fuel to be discharged into the exhaust system.
Improperly Operating Oxygen Sensor: an oxygen sensor failure can lead to
incorrect readings of exhaust gasses. A faulty sensor can cause air / fuel ratios to
be either too rich or too lean. A rich mixture can cause unburned fuel to be
discharged into the exhaust system. Lean mixtures produce conditions which
diminish the rate at which hydrocarbons are oxidized
Catalyst plugging and fouling: plugging the catalyst can obstruct exhaust flow.
Exhaust flow obstruction creates backpressure and increases heat in the exhaust
system, which can ultimately lead to overheating.
Effectiveness of Regeneration (Chemical Washing)
Most Catalyst manufacturers recommend some form of washing for the removal of
contaminates from exhaust catalysts. While these vary in the chemicals and
concentrations utilized and the order of application, most consist of two primary wash
processes, one employing an alkaline solution and the other employing an acid solution.
Between the primary wash processes and rinses, and secondary wash processes using
chelating agents, reductants or other chemical treatments may be utilized. In general a
washing processes can remove 75-90% of contaminates from the surface region of the
catalyst monolith when the correct chemicals are used in the correct sequence for the
correct amount of time, when the chemical solutions are always fresh, and when the
chemical solutions are recirculated through the catalyst.
Alkaline Wash Process In the alkaline wash process a strong caustic solution (e.g
sodium hydroxide) is utilized to solubilize the majority of the ash as well as many of the
other inorganic species. The primary contaminants removed by the caustic wash
process are zinc, sulfur, phosphorus and silica. However, it must be noted that the
majority of the ash, silica and other contaminates removed are taken from the surface
layers of the catalyst wash coat. The removal rates of ash and silica deposits which
penetrate deep into the pores of the wash coat are relatively low as the wash solution
which penetrates into the pores of the wash coat is quickly depleted. Since diffusion
rates are extremely slow both into (reagents) and out of (contaminants) the pores, the
amounts of contaminants removed from the pore structure is small. Studies have
shown that multiple wash cycles can be effective in the removal of additional sulfur and
other soluble contaminants (e.g. zinc).
Acid Wash Process The acid wash process utilizes a strong acid solution, typically an
organic acid, to solubilize those contaminants which are not directly soluble in the
caustic wash. These include calcium containing ash, some sulfur and phosphorus
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All About Catalyst Contaminants in Reciprocating Engines
By Steven DeCicco and Dr. Tracy Staller
GMC 2016
containing contaminants. The selection of the organic acid is an important
consideration as it influences not only the removal rates of the contaminants but must
be chosen to ensure that it does not remove catalyst precious metals. Under proper
conditions the acid wash process can remove 80-95% of calcium containing
contaminants in addition to much of the remaining zinc species (10-20% of the total zinc
present). It is worth noting that little if any sulfur and only small amounts < 15% of the
phosphorus is removed during the acid washing process.
Implications for Catalyst Design – Building for Long Service Life
Catalyst Substrate. Not all metal substrates behave toward ash in the same way. One
type of cell geometry is referred to as “discrete cell” substrate where each cell is
constructed as an open, straight channel from inlet to outlet. Another type of substrate
is referred to as a “tortuous path” substrate where layers of corrugated foil are stacked
at an angle to provide an irregular path from inlet to outlet. Forensic examination of
both types after thousands of hours of service typically shows heavier ash deposition on
the tortuous path catalyst surface than on discrete cell substrates. This is an expected
result because particles flow through a discrete cell in laminar flow(see Figures 6a and
6b), whereas the changing flow direction in a tortuous path cause particles to collide
with the walls and adhere to them (see Figures 7a and 7b). Tortuous path substrates
resemble various mist eliminator technologies which are good at separating two phases
(gas/liquid or gas/solid).
Figure 6a - “Discrete Cell”
Substrate Geometry
Figure 7a - “Tortuous Path”
Substrate Geometry
Figure 6b - Ash Particles Moving
in Laminar Flow Through
Discrete Cell Substrate
Figure 7b - Ash Particles Colliding
With Cell Walls and Adhering
In heavy ash applications, more open cell structure is an advantage. For example, a
200 cell per square inch (cpsi) cell density presents a larger more open channel for
particle-laden gas flow than a 300 cpsi cell density. In extremely heavy applications,
Page 13 of 16
All About Catalyst Contaminants in Reciprocating Engines
By Steven DeCicco and Dr. Tracy Staller
GMC 2016
such as 2-stroke engines, guard beds upstream of the catalyst capture some of the ash
and keep the catalyst surface cleaner for a longer period of time.
Catalyst Coating. The substrate surface is coated with a high surface area ceramic
alumina wash coat. The wash coat surface area of a 24.5” diameter x 3.5” deep NSCR
element (or a 24”x24”x3”) is approximately equal to 55 football fields. The surface of
that alumina wash coat is covered with platinum (Pt) crystals for oxidation catalyst;
platinum and rhodium (Rh) for NSCR catalyst. Pt and Rh are quite tolerant of
contaminants and respond well to chemical washing. Palladium (Pd) is used by some
catalyst suppliers, but it is vulnerable in that it reacts with common contaminants,
becomes deactivated by sulfur and other common contaminants (often permanently),
and does not respond well to chemical washing.
The precious metal loading should be conservative but not wasteful. The precious
metal loading is only one of many factors in manufacturing a highly active catalyst. The
chemical form and purity of the precious metals on surface is important, as is the
precious metal crystallite size and dispersion. Very small nano-scale metal crystallite
sizes, highly dispersed on the wash coat surface makes for a more active catalyst than
a higher loading of precious metals but consisting of larger crystallite sizes with
correspondingly poorer dispersion on the wash coat surface.
The purpose of a conservative precious metal loading is to provide a “sacrificial” safety
factor, i.e., to withstand some loss of platinum/rhodium due to contaminant masking
while still providing enough fresh platinum/rhodium availability to achieve the
performance. The ultimate purpose is to extend the service life of the catalyst between
chemical washings.
Catalyst Volume. The catalyst volume (cubic feet) should also be conservative for
higher ash applications. Like the higher platinum loading, the purpose of the additional
catalyst volume is to provide a “sacrificial” safety factor that anticipates masking. The
intent is to tolerate masking on the inlet section while continuing to have enough
unmasked active catalyst volume to sustain catalytic performance. The increased
catalyst depth also promotes laminar flow for keeping particles entrained, and provides
additional platinum-covered surface area to extend the service life of the catalyst.
Page 14 of 16
All About Catalyst Contaminants in Reciprocating Engines
By Steven DeCicco and Dr. Tracy Staller
GMC 2016
Table 1 - Calculation of Oil Consumption and Catalyst Ash Exposure in a Recip Engine
Table 1 - Calculation of Oil Consumption and Catalyst Ash Exposure in a Recip Engine
Engine Description
Cat G3616
Brake Horsepower (BHP)
4035
Brake Specific Oil Consumption (BSOC) (lb/BHP-hr)
0.000521
Oil Consumption (lb/hr)
2.10
Mobil
"Medium
Pegasus
"Low Ash"
Ash"
"High Ash"
Lube Oil Description
Special CF
0.2-0.6%
0.7-1.2%
>2%
Ash Content of Lube Oil
0.1%
0.5%
1.0%
2.0%
Ash Production Rate Passing Thru Catalyst (lb/hr)
0.00210
0.01051
0.02102
0.04204
Ash Production Rate Passing Thru Catalyst (mg/d)
22,906
114,530
229,060
458,119
Zinc Production Rate (mg/d as Zn)
7,330
20,157
Calcium Production Rate (mg/d as Ca)
36,650
36,650
Sulfur Production Rate (mg/d as S)
43,979
56,807
Phosphorus Production rate (mg/d as P)
6,414
18,325
Other Ash Constituents (Mg, Ba, O) (mg/d)
20,157
97,121
Catalyst Quantity
Catalyst Size (cuft each)
Catalyst Volume (cuft total)
Catalyst Ash Exposure - Daily (mg/ft3 of catalyst)
Catalyst Ash Exposure - Per 1000 Hrs (mg/ft3)
8
0.958
7.664
8
0.958
7.664
8
0.958
7.664
8
0.958
7.664
2,989
124,532
14,944
622,661
29,888
1,245,322
59,775
2,490,644
956
4,782
5,738
837
2,630
4,782
7,412
2,391
Average Zinc Exposure Daily (mg/ft3 of catalyst)
Average Calcium Exposure Daily (mg/ft3)
Average Sulfur Exposure Daily (mg/ft3)
Average Phosphorus Exposure Daily (mg/ft3)
Page 15 of 16
All About Catalyst Contaminants in Reciprocating Engines
By Steven DeCicco and Dr. Tracy Staller
GMC 2016
Table 2 - Estimate of Catalyst Exposure to Silica (SiO2) from
Combustion of Siloxane-Contaminated Fuel in a Recip Engine Application
Case 1
Case 2
Siloxane-contaminated fuel flow rate (lb/hr)
30,682
30,682
Siloxane Fuel Concentration and Incoming Flow Rate
D3
D3
Assumed siloxane compound
222
222
Molecular weight siloxane
Siloxane concentration (ppbw)
10.0
50.0
139
696
Siloxane incoming flow rate (mg/hr)
53
264
Silicon incoming flow rate (mg/hr)
Silica (SiO2) Exhaust Flow Rate
60
60
Molecular weight silica (SiO2)
2.71
13.5
Silica (SiO2) exhaust flow rate (g/day)
Catalyst Volume
2.0
2.0
Hypothetical catalyst volume (ft3)
Catalyst Exposure to Silica (SiO2) Contamination
Catalyst exposure (mg/cubic foot/day)
1,355
6,777
Exposure During Warranty Period
3
3
years of operation
96.5%
100.0%
avg plant availability
8,453
8,760
hours per year of operation
Catalyst exposure (mg/cubic foot)
1,432,128
7,420,350
Jenbacher J616GS E22, 2677 BHP, 1500 rpm, 4SLB (Data From Actual Landfill)
Page 16 of 16