The effect of high sulfur fuel to 2 way catalyst

[Azmio]

Guys,

I need to get an answer to a technical problem that I am having. I know that high sulfur fuel is very detrimental to 3 way catalyst and lean burn aftertreatment. Will the high sulfur fuel be detrimental to 2 way catalyst which removes only CO and HC? Will the formation of sulphate acts negatively over the surface area?

 

[ivymike]

yes. I don't know the details of the chemstry, but an oxidation catalyst (as for lean burn gas engines) is negatively impacted by fuel sulfur. I'm told that getting the catalyst very hot for a while can reverse some of the effect.

 

[JSteve2]

Sulfur binds to most metals used as catalysts and takes up sites that are then not available for other uses. It has more of an effect on full adsorption activities (e.g. NOx adsorbers) than on faster catalyst activities such as oxidation, but it reduces both effects. A percentage of the sulfur can be driven off at high temperatures which must be balanced against physical degradation of the catalyst. High sulfur levels can in some circumstances degrade catalyst activity very rapidly.

I don't know how much response you will get on this question beyond the above, because most of the specifics (e.g. activity reduction levels, recovery temperatures, and catalyst formulations) will be proprietary information.

 

[Azmio]

thanks a lot. What confuses me is the issue of high sulfur fuel for lean burn engine. I know that NOx is a big issue for lean burn engine but somehow other non lean burn engine works well and meet the emission target even with above 250 ppm sulfur gasoline.

This makes me wonder on whether high sulfur fuel will react negatively to 2 way catalyst too.

 

[JSteve2]

Two things at work here:
1) lean burn engines use diesel (typically) which is higher in sulfur relative to gasoline and natural gas in the unmodified state.
2) Lean engines require NOx control while stoichiometric engines largely require CO and unburned HC control. NOx control is more susceptible to sulfur poisoning than CO or unburned HC control.

 

[Azmio]

JSteve,

Bingo, I think you hit the spot with "NOx control is more susceptible to sulfur poisoning than CO or unburned HC control. ". I tend to agree very much with you especially when Lean NOx trap is used.

Few years ago, many of the latest GDI engine from German makers could not be introduced in China due to high sulfur fuel used. The normal port injection had no problem at all.

The next question that I have in mind, if the 2 way catalyst is affected by high sulfur fuel, how bad will it be?

 

[franzh]

We did some tests last year on propane with ethyl mercaptan, primary made from a sulfur compound. We found that if high levels of sulfur (>120-160 ppm) were introduced, the cat showed signs of reduced effectiveness, BUT, when the sulfur was removed and returned to Stoich (not sure about lean burn but I suspect the same result), the effects of sulfur were reversed, totally.

We didnt try to poison the cat with unrealistic levels of sulfur, just the extremes one might encounter with commercial grade propane. For what its worth, if unreasonable amounts of sulfur were introduced for long periods of time, I suspect a deterioration would result.

Franz

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[JSteve2]

franzh,
Keep in mind that "unrealistic levels" are relative. Diesel in the natural state can be 2000 ppm plus, which could be relevant for example if someone filled an on-road tank with off-road diesel. However, I think even off-road has standards below 250 ppm in many countries now, and certainly will have lower standards in the future. Engine oil can exceed 5000 ppm, and there are various conditions where at least some oil can get into the fuel, combustion chambers, and/or exhaust.

Also - the regeneration does reverse the sulfur damage as you indicate, it's the catalyst damage at high temperature that is the primary irreversible issue.

 

[turbocohen]

Low sulphur diesel will deactivate a 2way rapidly. Over the life of a typical diesel even ulsd 15ppm max (yeah right) cat life is still a big issue in the real world.

 

[ivymike]

must depend on catalyst formulation. I've seen some test results showing notable degradation at 50ppm S in natural gas. (H2S form, maybe?)

With or without the S, the oxidation catalysts degrade over time and have a relatively short (compared to the engine) useful life.