15 replies to this topic

[Attiq Ahmad]

Hello everybody,
Can anyone explain that "Why Temperature of Stack is kept so high??"
I am working as Assistant Process Engineer in Ammonia unit of a Urea manufacturing plant. There we have a stack of Primary Reformer where we control its temperature at above 340 F. I've not found any reason that why it cannot be lower??
Thank you

[gvdlans]

The lowest temperature to which flue gases can be cooled depends on the type of fuel used: 250°F for natural gas, 300°F for coal and low sulphur content fuel oils, and 350°F for high sulphur fuel oils. These limits are set to prevent condensation and possible corrosion of the stack.

So probably your Primary Reformer uses a fuel with a high sulfur content...

[Zauberberg]

Hello Attiq,

If acid condensation is the main concern at your plant, there is a standard procedure for flue gas dew point estimation, if sulfur content and C/H ratio of your fuels are known.
Please reffer to the following article:
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Also, simple "Google" search for terms "Flue gas dew point" or "Cold-end corrosion" will bring you many interesting results.

[Attiq Ahmad]

The lowest temperature to which flue gases can be cooled depends on the type of fuel used: 250°F for natural gas, 300°F for coal and low sulphur content fuel oils, and 350°F for high sulphur fuel oils. These limits are set to prevent condensation and possible corrosion of the stack.

So probably your Primary Reformer uses a fuel with a high sulfur content...

Thanx for reply Gvdlan,
We use natural gas as fuel for Primary Reformer which contains about 10 ppm of total sulphur but the design basis is 300 ppm of total sulphur. Actually, initially sulphur contents in 1980s were high but now it is always below 15 ppm.

Can we cool these flue gases down to 250 F safely??
Do you have any relationship between sulphur contents in fuel gas and stack temperature??

[Zauberberg]

Try this one:

http://www.hrsgdesign.com/design0.htm

Go to Process link on the top left side of the page. Click on Acid Dew Point of Flue Gas menu and follow the instructions.
Many of these diagrams are available at engineering handbooks.

[engware]

Hi guys:

I would like to add that with a higher exit gas temperature, free gas lift is provided reducing the power requirements for moving/pushing the exhaust gas to the atmosphere.

Thanks,

Gordan

[Attiq Ahmad]

Hi guys:

I would like to add that with a higher exit gas temperature, free gas lift is provided reducing the power requirements for moving/pushing the exhaust gas to the atmosphere.

Thanks,

Gordan

I think with the higher exit gas temperature, heat losses will increase more quickly as compared to reduction in Power requirement for moving/pushing the exhaust gas to the atmosphere.
Thanx Gordan,

I think gvdlans is right.

[engware]

Hi Attiq:

There has to be an optimum exit gas temperature in order to provide sufficient exhaust gas free lift and not to increase significantly any heat losses while maintining the exhaust gas temperature high enough preventing anu undesired condesation and corrosion.

For the purpose of good engineering discussions, Engineering Software web site -- http://www.engineering-4e.com -- provides an MS Excel file dealing with combustion of coal and oil. For the case of complete combustion, this spreadsheet allows one to calculate and predict the gas composition and find out how low the exhaust temperature can be, with some additional quick steam table look ups and calculations, having no condensation take place.

Since we are dealing with multiple engineering issues, an optimum working point needs to be found for viable and safe operation of the technical equipment.

Thanks,

Gordan

Below are a few plots related to combustion -- complete combustion of coal, oil and gas with air at standard conditions (298 [K] and 1 [atm]) with no heat loss and stoichiometric conditions.

[Technocrat]

dear attiq,

i am a chemical engineer from mumbai having a four year experience in methanol plant. you may ask more queries to me regarding primary reformer and the entire methanol plant.

regards.

[Attiq Ahmad]

Hello Gordan,
Thanx for sending this spreadsheet. I'll check this out very soon and also I'll apply it to our case.

Best Regards

Attiq



Hello Dhirajkumar,
nice to see u here. I've a number of un-awnsered questions regarding primary reformer as well as secondary reformer. As Soon as I'll find some time, I'll put them on web.

Best Regards,
Attiq

[engware]

Attiq:

I am glad to be of some help.

Thanks,

Gordan

[steamreformer]

plz mail me at sahilspm@yahoo.co.in

[Andrei]

Hello,

I do not want to offend anybody but my understanding about how a SMR works is a little different. Everybody is concerned about acid corrosion caused by Sulphur. I do not consider this concern founded.
From what I know, a reformer uses a combined fuel, about 1/3 fresh fuel, and about 2/3 tail gas from the hydrogen separation, I assume it's a PSA. Is that your configuration Attiq?
Now, the source of the tail gas is exactly the SMR feed, and as I know that has very stringent sulphur requirements. Any sulphur in the feed will poison the catalyst; usually people are removing very deep the sulphur before entering the reformer. You should have a feed desulphurization upstream of your SMR. The consequence is that 2/3 of the fuel used is virtually sulphur free.
Now, there is some sulphur, very low (10 ppm as you say) coming with the fresh fuel, and diluted with the tail gas, and generously diluted with combustion air. What would be the stack sulphur content? I can tell you: extremely low.
I think the reason for keeping a certain stack temperature resides in the thermal configuration of the reformer. There are some steam coils there, certain temperature approach maybe have to be kept.
Do not judge the reformer as a normal fire heater, because it is not. Look elsewhere; sulphur is not the culprit this time.

Regards

[pawan]

Let me clarify Andrei,
Generally Fuel is not 2/3 tail gas & 1/3 fresh. Its normally 95% fresh & only 5% tail gas,
bcoz in most of the cases a PGR purge gas recovery unit is installed for the production of
additional ammonia.

Also Now a days materials are available where U can reduce flue gas temp to a level
~90 Deg C for maximizing heat recovery. You can go thru an article on my blog related to this.

Here is the link - Improve your Boiler Efficiency

[Andrei]

Thank you very much Pawan for the clarifications. I confess you managed to raise me at a higher level of confusion with your supporting article.
I do not want to engage in a dispute, the article is right, you may be right. My understanding was different, I was thinking that we are discussing about a Methane fired furnace, as the original poster confirmed, and not a Coal fired one.

[pawan]

Thanks Andrei

I don't think that a healthy discussion is a dispute....

U r right that my article is for boilers where generally coal is fired. However it is applicable to Methane or NG fired furnace also. You will be surprised to note that even in my plant in steam reformer furnace which was NG fired we reduced the temperature to ~100 DegC using a duplex MOC for tubes bcoz of low concentration of Sulphur & its running for last 7 years.

Actually, generally people keep too much margin on temp to be on safer side due to
high uncertainity on exact level of sulphur which is in ppm level. Also, some quality variation may lead to higher Dew Point occassionally. This is the reason of high stack temp in most of the cases.