Ammonia Flare design 4

[Briveros]

Hello:

I am working in a project, that requires an Ammonia
Flare system which has to work as a process flare and as emergency system
for atmospheric storage tanks. I´d like to ask us for information about:

1.- Different technologies avalaible for this system and its typical
drawings or PFD.
2.- Ask you about if it is possible to connect atmospheric storage tanks
(1,5 psig design pressure) to a common flare or if is mandatory to have an
emergency flare just for the storage.
3.- Technical or investigation papers about this subject.
4.- Is auxiliar gas necessary for improve the ammonia combustion?


Thank you,
Best regards

 

[Flareman]

Briveros

Ammonia flaring has a lot of confusing "heresay" associated with it so I'll try to give you the short version of the facts as I see them.

First of all, ammonia is a very stable molecule so it takes a lot of activation energy to get it to burn. When it burns it has no solid intermediates (like carbon) which can help the internal heat transfer so, although it burns well in a closed environment, there is a big difference between the way it burns upward through a gas stream when it can utilise convective heat transfer and the way it burns downward when it has to rely on radiative heat transfer.

In consequnce, if you want to do a good job, you always need to have a good solid base flame from pilots, or a ring of supplementary gas nozzles which can keep reigniting the flame. Hydrocarbon mixed into the gas does a similar job.

Complications
- EPA regulation 40CFR60.18 would suggest that, because the heating value exceeds 200 Btu/scf, ammonia doesn't need support. That's a dangerous assumption.
- most ammonia turns up with water or CO2 which makes the flame stability problem worse.
- cold ammonia is worse than hot ammonia. At temperatures below 70 degC ammonia mixtures have zero ability to propagate downward so any flare flames are totally reliant on the pilots or base flame. This is why a lot of ammonia flames generate odor, because the conversion efficiency can be low, particularly in a wind.

The reason that many ammonia flares are stand-alone is frequently because the storage tanks are too far away from the main flare system or it is otherwise impractical to connect the systems from a process consideration. You mention a pressure limitation and that could be the overriding sizing concern in your case.

IF there are no process or pressure interactions (watch out for water seals), in my personal view, it would be better to use the hydrocarbon to swamp the ammonia and get it to burn fully. Putting a small ammonia tip alongside a hydrocarbon flare tip also give a beneficial stability.

There's not a lot of helpful technical information on the market and the more you find, the more confused you'll be.

Good luck

David

http://www.geocities.com/flareman_xs

 

[Montemayor]

I can add little - if anything to what David has so well explained about the challenge of flaring ammonia fumes or vapors. My experience reflects exactly what he has described. My only addition to this thread is that I would describe an ammonia flare as being an ammonia "incinerator". I've always felt the term incineration befitted the oxidation of ammonia more properly - since the pesky molecule is hard to erradicate with a flame, even though it is technically supposed to "burn" as a fuel.

Before I consider an ammonia flare, I would inquire about the feasibility of conserving the ammonia vapors - either with recompression, refrigeration, or with conversion to aqua ammonia as a stored fertilizer. Flaring ammonia can be very expensive in capital and operating expenses.

 

[BenThayer]

i worked at a plant that produced ammonia and had atmospheric storage tanks.

the tanks had their own dedicated flare. it was not part of the emergency flare systems related to the ammonia plant.

be extremely careful when considering connecting these devices since you have very low pressures at these tanks.

our systems were basically the following:

refrigeration compressors with a spare compressors (one low stage/booster and one high stage) and spare condenser.

we had switching between two substations in case of a power failure. the site was also fed from two sets of high tension lines from the power company.

with that said, we had that part of the system pretty well backed up.

the flare was also one of several states of release capacity.

the compressors would cycle on based on pressure, typically ran from 6-10 "wc.
the flare would open at about 16" and be wide open at about 20"wc.
their was a control valve that would open and vent to the atmosphere at about 24"wc.
and then there was the weighted manway cover that would lift at 27"wc.

now these are from memory and are meant as general approximations but you get the idea.

everything flareman said is true. it burns but not great and you plan on having a good pilot system.

and dont listen to the environmental types telling you about NOx. there might be some but not much.

and pay close attention to the flare-ignition equipment. it really stinks to have to re-lighting the pilots in a downpower. they are not all created equal.

and read over things like

http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=9757

and also pay attention to the radiant heat zone of the flare.