10 replies to this topic

[Shahzad Munir]

I am working on a project of ammonia Flare.Can any body help me regarding the listed clarifications with the standard references:
1. Can ammonia ignite properly without assist gas requirement. The temperature of ammonia vapors are -33 degree celcius and it is anhydrous liquid ammonia.if yes than hat should be the exit velocity of the ammonia vapors from Flare Tip.
2. Can a velocity seal be used fo...r ammonia service and hydrogen service as waste gas in flare.
3. What is the approximate pressure drop and cost differece between velocity and molecular seal.
4. in a flare case the waste gas contains 79% H2 and 2% CH4 and remaining is Nitrogen and Argon. Is there any possibility of smoke at Flare Tip.

Thanks and regards

[ankur2061]

Shahzad,

If you had used the search feature of the forum you would have found another thread with the same title as Ammonia Flare. Refer the link below:

http://www.cheresour...__fromsearch__1

1. Ammonia is a gas with low calorific value. It will require an assist gas (steam or air) for proper combustion. I had made a blog entry regarding "Flare Tip Exit Velocity" and "Flare Tip Diameter". Refer the link below:

http://www.cheresour...e-tip-diameter/

2. Most recognized operating companies do not approve of velocity or molecular seals due to reasons of flow obstruction and mechanical integrity of the seal. However, velocity and molecular seals are still used in many of the older flares that are operating. API STD 521 describes these seals (molecular and velocity) in Section 6.4.3.6.2 with their function and their drawbacks.

3. Also refer Section 5.5 -"Purge Gas Conservation Seals" of API STD 537.

4. Flare gas composed predominantly of Hydrogen provides smokeless flaring. In my opinion, if your flare gas contains 79 % H2, it should provid a clean smokeless flame. However, do ensure that the falre gas does not contain aromatics such as Benzene, Toluene, Xylenes because they contribute to smoke.

Hope this helps.

Regards,
Ankur.

[Art Montemayor]

Ammonia combustion will not sustain itself. Another, higher heating value fuel is required to continuously flare / burn ammonia gas. You can ignite the ammonia gas, but it will extinguish if left to itself. An ammonia flare is actually an "incinerator" for the ammonia because the ammonia cannot burn by itself and another, independent fuel is required to flare it.

An assist gas (steam or air) is required to atomize the ammonia - as any other fuel - but does not contribute to its continuous burning as does the required additional fuel that keeps the total flame alive.

[S.AHMAD]

1. You need to choose the right assist gas.
2. If you are using the H2 riched gas, probably you are on the right track. H2 has wide range of flammability limit
3. I have come across an ammonia plant using natural gas as the assist gas - The flare flame shown encauraging but the ammonia reached ground level and resulted in public complain.
4. The flammable limit of NG is between 2 to 15% and ammonia is between 15 to 28%. The two flammable limit probably meet within a very narrow range. That may explain why combustion of ammonia using NG as the assist gas is ineffective.
5. I am not very sure whether the incombustible of ammonia is due to its low heatt of combustion or due to OUTSIDE the range of combustion limit or both.
6. If the reason is due to low heat then the big question is how did the flanmmable limit was determined and the literature cited between 15 to 28%??
7. Probably the type of burner required for complete combustion of ammonia is different from that of NG, due to different flammability limit range.
8. Please take note that, good combustion of ammonia could be achieved with catalyst as shown in the nitric acid plant without using any assist gas.
9. If I could remember correctly, and correct me if I am wrong, the catalyst is to catalyze the reaction between NO to NO2, not to assist the combustion of ammonia.
10. Probably more intensive research is required for effective combution of ammonia flare and the type and quantity of assist gas required

Edited by S.AHMAD, 05 February 2012 - 04:12 AM.

[ankur2061]

4. The flammable limit of NG is between 2 to 15% and ammonia is between 15 to 28%. The two flammable limit probably meet within a very narrow range. That may explain why combustion of ammonia using NG as the assist gas is ineffective.
6. If the reason is due to low heat then the big question is how did the flanmmable limit was determined and the literature cited between 15 to 28%??

Ahmed,

Can you explain what is meant by flammability limit?

Regards,
Ankur.

[S.AHMAD]

Dear Ankur
1. Flammibility limit is the volume % of combustible materals in air. If thr Vol% is higher than The upper limit (UFL), the mixture is too rich, Combustion could not take place. If the vol% is lower than the lower limit (LFL), the mixture is too lean and combutsion cannot take place.
2. For methane, the literature cited between 5.3% and 15%. For hydrogen between 4 to 75% for ammonia between 15 to 28%.
3. The UFL and LFL figures are also a function of temperature.

[ankur2061]

Ahmed,

The UFL / LFL / LEL / UEL values are used for determining the flammability / explosivity of gas to prevent a fire or explosion endangering human life. I have never heard of these terms being used for flares because a flame is lit by an external source such as a pilot flame whereas gas below its LFL exhibits an oxygen rich gas or hydrocarbon deficient gas and above UFL exhibits an oxygen deficient or hydrocarbon rich gas. Below LFL and above UFL combustion cannot take place. This means that any gas exiting the flare tip has to fall between the LFL and the UFL for a flame to sustain itself. LFL / UFL is certainly data which is required for estimating the dangers of a fire in case of a flame-out and build-up of ground level concentration of the unburnt flammable gas.

When we talk of flare, no air or in other words oxygen is permitted to ingress the flare. The flare gas exits the flare tip and is ignited by a pilot flare which is continuosly operating. Even for assisted flares the mixing is done at the flare tip for the purpose of atomization or dispersion. The LHV or LCV of the gas determines the requirement of the auxiliary fuel gas that needs to be mixed with the main flare gas in order to sustain a flame and as mentioned by Art Montemayor, Ammonia LHV or LCV is not sufficient to sustain a flame without an auxiliary gas.

So my question remains that how does the LFL and UFL relate to combustion at the flare tip?

Regards,
Ankur.

[Shahzad Munir]

Ankur/Ahmad,
Thanks for your responses. I have one more question,
Does Assist gas and auxilary gas is different from each other.
Please correct if I am wrong: Assist gas will used to automize the Ammonia and Auxilary Gas will be required to sustain combustion.
Does Ammonia Need an Auxilary fuel for sustainable combustion.
Also can you help me regarding the pressure Drop difference Between Velocity and density seal?

[S.AHMAD]

Dear Ankur
1. Any combustion process to take place, be it at the lare tip or inside the petrol/diesel car engine, the mixture must falls within the flammibility limit. Outside the limit of flammability, no combustion wll take place.
2. For combustion at flare tip, air is induced "naturally" by the burner using the flared gas as the motive fluid. This is where I believe for effective combustion of ammonia, requires specific type of burner such that correct amount of air is induced for complete combustion to take place.
3. If we are using the same type of burner for methane, the amount of air nduced is probably too much resulting in lean mixture such that combustion cannot take place.
4. If we mix methane and ammonia, due the difference in flammibility limit, when methane combustion takes place, ammonia combustion does not occur. Similarly, if combustion of ammnbia is possible, combustion of methane is unable to proceed.
5. The right type of assist/auxilliary gas is prime important.

[S.AHMAD]

Dear Munir
1. Please take note that, atomization is only required for combustion of LIQUID.
2. Are you sending liquid ammonia to flare?
3. Please take note, liquid MUST no be allowed to go to flare it may result in "rain of fires".
4. If you have liquid to flare, K.O drum is required.
5. In flare, what we have is only pilot gas and the flared gas.
6. Assist gas or auxilliary gas, is premix with the flare gas to ensure sustainable ammonia combustion. Whether it is being called assist gas or auxilliary gas, the purpose is the same.

Edited by S.AHMAD, 05 February 2012 - 07:47 PM.

[S.AHMAD]

Dear Munir
1. Instead of sending the ammonia to flare, you should also evaluate the feasibility of recovering the ammonia. Especially, if the plant is located within residential area, releasing of ammonia is a serious problem for asmatic people.
2. Flaring seems to be an easy and cheaper solution, but it may not the right long term solution.