Sour gas flare stack 3

[Renoyd]

Hello,

After SO2 dispersion modeling, the flare stack has to be 500 ft to meet the regulations, in addition to adding fuel gas, what are the other options? If adding fuel gas, how to operate? Thanks.

 

[mbeychok]

Renoyd:

If your sour gas is in an oil refinery (or close to a refinery) that has a sulfur recovery unit, then why not use an amine unit to remove the H2S from the gas and send the H2S to the sulfur plant ... instead of burning the sour gas in a flare stack?

Milton Beychok
(Visit me at www.air-dispersion.com)
.

 

[JLSeagull]

Consider an incinerator, a.k.a. thermal oxidizer.

 

[Flareman]

Renoyd
Unless you have a tremendous flow, 500 ft seems very tall based only on SO2, which can only be produced by burning the H2S and, itself, creating a large plume rise. I suspect that this height may arise from consideration of flame out when you have to disperse the unburned H2S which is heavier than air and can fall to grade downwind of the stack in certain ambient conditions.

H2S normally burns well, even though the heating value is relatively low. However, it does not have a strong solid transient with a radiant component and so it is always a good idea to maintain a hydrocarbon flow along with the H2S. If the H2S comes with CO2, or H2O, these are inert and will reduce the flammability of the mixture and further affect the flame stability and visibility. Get the paper "Improving Flare Design" from

http://www.geocities.com/flareman_xs

and navigate to MainIndex|Downloads. (There is also a flame and plume calculation paper) This will give you a method for estimating the true flammability of the final mixture with support fuel. If you have to do this visually, just add enough hydrocarbon so that you can see some yellow/orange color in the visible flame.
Depending on where you are in the World, and the local temperature and humidity, you may see a downwind plume which looks like steam (with a slightly blue color) This forms as the atmosphere mixes with the flue gases and the H2SO3 begins to condense. You will find that this is reduced as the hydrocarbon content of the flame increases.

Make sure that you have good pilots with flame monitoring and reliable automatic reignition. I prefer High Energy reignition to the flame front method in critical applications like this.

David

 

[Renoyd]

Milton,
There is no SRU present. It is just a flare for a wellsite PSV's, Blowdowns...

JLSeagull,
Yeah, you are right. Incinerator is definately an option.

David,
You've got a good website and lots of useful information there.
I did the modeling basically using EPA screen3 method. 55% heat is assumed to be released to atmosphere and 45% left for plume rise. The pseudo parameters are then calculated based: the equivalent stack diameter, the adjusted stack height, exit velocty 20 m/s, temp 1273K. These parameters are input into the program togather with terrain data and meteorological data for the modeling. The H2S content in the sour gas is about 13 mol%, and the flaring gas flow is about 3MMSCFD. Is it reasonable to have the result of 500 ft flare? Did I do it wrong?

 

[mbeychok]

JLSeagull:

Am I missing something? The combustion flue gases from the incinerator will contain SO2 if the incinerator combusts H2S, will it not?

Milton Beychok
(Visit me at www.air-dispersion.com)
.

 

[JLSeagull]

The hydrogen sulfide in the sour gas or acid gas is converted to sulfur dioxide. The conversion level in a thermal oxidizer is far better than in a flare. I understand that the SO2 levels can be acceptable. Others can likely provide better detail.

 

[mbeychok]

JLSeagull:

The U.S. EPA (and the environmental agencies of many other governments) have strict regulations on SO2 emissions. If the feed gas contains H2S at a level that will burn to create more SO2 emissions than allowed (whether burned in a flare or an incinerator or a process furnace or whatever), then dispersion modeling is required to find the flue gas stack height that produces ground-level SO2 concentrations which comply with the environmental regulations ... or the amount of SO2 emissions must be reduced by some means (for example, by using a Flue Gas Desulfurization process).

There is no basis whatsoever for saying that: "... the SO2 levels can be acceptable" without knowing how much H2S is being burned and without doing air-dispersion modeling on a case-by-case basis.

Milton Beychok
(Visit me at www.air-dispersion.com)
.

 

[JLSeagull]

Mbeychok you seem rather offended that some possibility nmay exist that incinerators may comply with the EPA requirements. Sorry. Initially I attached some EPA links pertaining to the incinerator temperature control etc. However, as a chemical engineer you understand the conversion from H2S to SO2 well enough to do your own additional research.

The basis of the statement is that many sulfur conversion units have acid gas incinerators. These may be thermal oxidizers, catalytic oxidizers or other such devices. I prefer to label them incinerators. The regulations and the incinerators both exist. Thus, either "... the SO2 levels can be acceptable" or perhaps they just don't comply.

Regarding air dispersion modeling, I have seen sour and acid gas flares offshore (international locations not the USA) with a nicely packed white plume of sulfur dioxide laying down horizontally onto the next occupied platform. I don't see the white smoke from the incinerators, Thus, incinerators just might be be better than flares in that regard and the plume does not appear to disperse much in the prevailing westerly winds of the Caribbean. I would prefer to be upstream.

 

[KenA]

Renoyd - you could just make the flare 500ft. It's been done before. We've just completed a flare which is a little over 600ft. Talk to John Zink.

 

[mbeychok]

JLSeagull:

I am not offended, nor do I wish to offend you ... I'm just trying to keep readers from possibly being mislead. The original poster has now told us that his sour gas from an oil wellsite has 13 mol% H2S in it (which means that the H2S is 13 volume % of the gas). That is much higher than you are going to find in most sulfur recovery unit tail gases.

Your observation that plumes from incinerators may be less visible than from a flare is no indication that it does or does not comply with regulatory requirements. As you said yourself, you "would prefer to be upstream" of the incinerator plumes. Our duty and responsibility as engineers is to avoid emitting hazardous air pollutants be they visible or invisble .... and be they in the United States or offshore in the Caribbean or elsewhere. Would you not agree with that?

Please don't take offense, but I repeat again (be it a flare, a furnace or an incinerator), there is no basis whatsoever for saying that: "... the SO2 levels can be acceptable" without knowing how much H2S is being burned and without doing air-dispersion modeling on a case-by-case basis.

Milton Beychok
(Visit me at www.air-dispersion.com)
.

 

[vivekmari]

where you able to find an answer. just out of curiosity, what was your original stack height and diameter. to me 500ft stack is pretty high.

 

[786392]

13% value for H2S(Hydrogen Sulfide)seems definitely too high and most probably either caustic scrubbing or other methodologies must be employed upstream of venting these to the flares

Since the produced SO2(sulfur dioxide)must be objectionable/ capable of producing acid rains in close vicinity areas affecting any eco-systems of vegetables or even animals.

Best Regards
Qalander(Chem)

 

[25362]

As 786392 indicates, the proposed procedures to catch SO₂ from dilute gas streams are based on using dry or wet caustic systems.

Noel de Nevers Air Pollution Control Engineering (McGraw-Hill) has a chapter dealing with the subject.

 

[Flareman]

Gentlemen
Been away. Just getting back into this.
Of course, if you burn a given quantity of H2S you get a given quantity of SO2. The only difference between an incinerator and an elevated flare is usually the total amount that you can manage in an incinerator (much less than an open flame). Also Incinerators control the air inflow and the outlet temperature of the flue gases. Open flames do not, so the downwind visible plume is a function of the developing concentration of acidic vapor subjected to ambient diffusion whereas the incinerator flue gas is already well mixed. Also, this feature is very dependent on local humidity which is generally higher for platform flares than it is for a small incinerator in Oshkosh. It is also a function of the other hydrocarbons and or inerts which are part of the flue gas contributions. Wind speed and local stability categories are also a factor.

Just because I have nothing better to do today I ran my own calc on a supposed flow of 13000 lb/h of 13%vol H2S + 70% C3 + N2, H2O and CO2. MW = 39.5, @ 80 degF. This is approx 3 MMscfd and has an LCV approx 16,300 Btu/b.

Fully converting all the H2S to SO2 we get 2741 lb/h in flue gas from the flame. Using the heat and momentum for plume rise, gets me to a low discharge height (< 50 ft) and a downwind GLC of SO2 < 2 ppm. Stability A or F give the worst results (low winds)

Supposing flame out, and dumping 1458 lb/h of unburned H2S is a different story. All the heavy H2S falls to grade at the bottom of the flare (in stability A low wind) and we need a greater height (> 350 ft) to get to 10 ppm of H2S.

I have a small issue with using chimney calculation for open flames.
My calculation method is fully explained in the paper "A proposed comprehemsive model for flare flames and plumes" at the website

http://www.geocities.com/flareman_xs

navigate to main index|downloads.

You are probably stuck with a chimney model which you have to fudge a litttle to get it to pretend it's an open flame.

Regards
David