Selective Catalytic Reduction Catalyst Regeneration

[TugboatEng]

I'm new to this SCR business with DEF for NOx reduction. It's proving to be quite the annoyance to my very low lifetime load cycle engines in my fleet. My engine manufacturer is asking me to operate my engines for hours at a time st full power to burn out deposits in the catalyst. This consumes thousand of dollars an hour of diesel fuel. The consensus is that heat is the necessary ingredient to clean the catalyst.

Since I don't do science by consensus, I have some questions. My understanding is that the SCR catalyst requires an operating temperature of 750°F to become functional. Is this temperature also sufficient to remove fouling from carbon?

At full power, my exhaust temperatures are usually in the 1100°F region. Does this truly clean things out faster?

At full power I suspect there is less excess O2 in the exhaust. This seems like it would prevent cleaning of the catalyst as deposits cannot burn without O2. Am I wasting even more fuel and time trying to generate higher than necessary temperatures?

 

[BrianPetersen]

Is this for SCR, or is it the DPF? DPF requires regeneration. The SCR function technically does not, but...

Don't know about your application, but automotive went to an integrated design that performs both functions in one unit. Driving at highway speed for 20ish minutes from time to time is usually enough to regenerate the DPF. High enough load to get exhaust temp up but still in lean operation.

The lean-NOx catalyst system (no "diesel exhaust fluid") is the one that requires periodic slightly-rich operation to regenerate first the NOx storage catalyst then high-load but lean operation to regenerate the (separate) DPF. Besides turning out to be ineffective, this was a lot of trouble.

 

[BrianPetersen]

Minor digging to refresh my memory found 600 C to be the required temperature for DPF regen. That may have to be induced (e.g. by delaying injection timing). But it shouldn't take "hours".

 

[TugboatEng]

These units are SCR only. The manufacturer uses the term deposition to describe the problem. Everyone assumes this is carbon soot from the engine. Our back pressure is instrumented and does not show high. I wonder what parameter is being used to measure deposition?

 

[BrianPetersen]

It's probably a mathematical model, which inherently means guesswork. I will guess in return that the manufacturer wants as little soot coating the catalyst as possible so that the catalyst area is as effective as possible, and that the amount of soot coating the catalyst that would lead to NOx emission violations is well short of the amount of soot that would cause a back-pressure increase. But, that's just a guess.

Given that you have back pressure instrumented, I'd just run the thing as normal until that back pressure starts showing some sort of increase (how much? your guess is as good as mine) and then do a DPF-burnoff procedure for (guess) 10 minutes and see where you are for back pressure, and adjust procedures from there. If you have NOx monitored, maybe that would show some interesting trends.

For DPF burnoff, you want 600-650 C EGT and lean. Delaying injection timing and turning off EGR (if so equipped) and rendering intercooling ineffective by whatever means (if possible) should send things in that direction. Presumably your EGT is monitored also. In the automotive applications, all these regen procedures are dealt with by the OEM ECU so that the end user isn't supposed to have to do anything special, although driving on the motorway at 120+ km/h for 20+ minutes on frequent occasions helps, and short-trip city driving kills them.

 

[TugboatEng]

Remember, this is SCR only. We have no DPF.

Reducing intercooling to boost EGT without burning extra fuel is an interesting idea.

 

[BrianPetersen]

Yeah, no DPF, but igniting and burning off carbon requires the same things: reaching ignition temperature with at least some oxygen present.

 

[TugboatEng]

I'm starting to think deposition refers to poisoning of the catalyst. We're getting high NOx and high ammonia alarms post SCR. No wonder heat isn't curing the problem.

 

[BrianPetersen]

Ooooh, that's different. High NOx + high ammonia = catalyst not doing enough (or anything).

Ultra-low sulfur fuel?

 

[TugboatEng]

I can't answer that question. It's most likely ULSD but could also have R95 or R99 mixed in.