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Urea- or aqueous ammonia- based SCR?

Urea- or aqueous ammonia- based SCR?

Urea- or aqueous ammonia- based SCR?

I am working on a natural gas gen-set and am specifying a urea-based SCR to meet the local NOx limitations. My client has asked about consideration for aqueous ammonia in lieu. Does anybody have any experience as to the differences between the two in capital cost, operating cost, level of storage, HSE considerations?

I have found an old article on-line that claims that aqueous ammonia is about 40% more on lifecycle cost, but it is about 15 years old.

Many thanks,


RE: Urea- or aqueous ammonia- based SCR?

Why do you need urea at all for natural gas? All the natural gas systems I'm familial with use normal 3-way cats.


The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

RE: Urea- or aqueous ammonia- based SCR?

I tHink that modern lean burn gas engines are not suitable for use with a three way cat, because only rich burn gas engines are suitable, which results in a higher fuel consumption.

I never installed a SCR but looked at them several times. Ammonia or urea are reputed to both work, but clients want to use urea because of the safety issues with ammonia. Just google it.

Depends which regulatory regime that you work under and what the legal and safety issues are.

As i once said to someone: that's an interesting decision, which prison do you want to go to?

RE: Urea- or aqueous ammonia- based SCR?

Virtually all the systems I've worked with in the last several years have been urea systems. Mainly due to safety and regulatory concerns. When SCR systems for lean burn gas engines came out several years ago, we did have a few that went the aqueous ammonia route, mostly some cold storage facilities that already had ammonia storage onsite. But those systems seemed to be problematic in the injection and metering systems, mostly due to corrosion and clogging.

From a long term operation and maintenance aspect, I think you'll find Urea to be a much better choice. Biggest issue I have had with urea systems is suppliers delivering poor grade urea, dirty and not at proper concentration levels.

Hope that helps, MikeL.

RE: Urea- or aqueous ammonia- based SCR?

These natural gas engines are rich-burning and there is up to 9% oxygen in the exhaust, making 3-way catalysts unusable for NOx.

Thanks for all the good comments, I agree with urea as a better choice from a storage risk perspective and have tried to make a case to end the conversation.


RE: Urea- or aqueous ammonia- based SCR?

Actually at 9% O2 these would be considered "lean burn" engines. Rich burn engines have exhaust oxygen in the .5 to 1% range for use with 3 way non-selective catalysts.

The quickest way to end the argument may be a call to your local fire marshall, in many areas I work in they are the AHJ who has nixed ammonia storage and many of the sites.


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