10 replies to this topic

[sreekanthtm]

FUEL GAS COMPOSITION(Mol%):

CH4 - 97.49

C2H6 - 1.5

N2 - 1

CO2 -0.01

FUEL GAS FLOW RATE: 712 Nm3/HR

 

FLUE GAS COMPOSTION(mg/Nm3):

NOX - 150

CH4 - 11

CO - 175

 

If we are providing 20% excess air ,calculate flue gas flow rate(Nm3/hr)?

 

Please help me.....

[thorium90]

Post 15 of the following thread might be what you need

http://www.cheresour...fired-reboiler/

 

Im curious, 20% excess air and you still have 11mg/Nm3 CH4? With 150mg/Nm3 NOx, your flame temperature would be like >900C, so together with 20% excess air, you still have unburnt hydrocarbons as well as 175mg/Nm3 CO?

Edited by thorium90, 20 March 2013 - 02:55 AM.

[Shivshankar]

Hi,

 

 

To solve your query, you should know fundamentals in combustion, stochiometry, heat transfer and material balance.

 

Go to below link for further information.

 

http://www.cheresour...te-of-flue-gas/

http://www.cheresour...e-gas-analysis/

 

 

Regards

Shivshankar

Edited by Shivshankar, 20 March 2013 - 03:58 AM.

[sreekanthtm]

Thorium90:

 

Dear sir,

 

           I have calculated by using format given in the link.I got 8,863 Nm³/hr against an actual flow rate of 11,700 Nm³/hr....

[thorium90]

Oh? Maybe you could post it here along with your actual flow screenshot? Would be interesting to see what could be the reason.

 

Perhaps the reason is the humidity. Seems you left it at 0kmol right?

 

Assuming it exits at about 70C, from the following chart,

http://www.engineeri...-air-d_281.html

 

I get about 0.26kg water per kg dry air. From the spread sheet, 303kmol of stoichiometric air and taking air MW as 28.9, I get 8753kg air. Multiply by 0.26 gives 2279kg water which gives 126.5kmol water. When put into the spreadsheet gives about 11700Nm3/hr flue gas.

 

Not sure if im doing it correct though.... It could be other temperatures, then it wont be 100% saturated, but it seems 126.5kmol water corresponds to your actual. So perhaps if you have the exit temperature you would be able to calculate how saturated it is too

Edited by thorium90, 20 March 2013 - 09:58 AM.

[Art Montemayor]

Thorium90:

 

This is a good discussion of some basic stoichiometric principles.  Are you also using a spreadsheet to do your calculations (which I highly recommend for any stoichiometric exercise)?  If so, could you and the OP both submit your spreadsheets to follow the stoichiometric logic and reasoning to arrive at the results?

 

You are absolutely correct in stating that a basic priority is to establish the moisture in the combustion air (atmospheric humidity).  This is a prerequisite before beginning any combustion calculation.  Also, I would alert all readers that although I am a fan of “The Engineering Toolbox”, there is a lot to be wary of and care to be taken when using their submitted data.  Like many internet websites, Engineering Toolbox at times has information that tends to be suspicious and care and checking should be used.  For example, please note that what I assume we engineers are looking at in the cited hyperlink is a group of curves showing the amount of air held in ATMOSPHERIC air at SEA LEVEL.  The cited curves all state that the data is for “Air moisture holding capacity” – it is never stated or identified at WHAT conditions or where.  This may – or may not – be a trivial point.  We have to “trust” and make “assumptions” of the data.  I would recommend (from prior experience) that young engineers check this type of data with other sources when relying on serious calculations.  I learned many years ago not to simply "trust" or make assumptions on serious calculations.

 

This stoichiometric problem seems to be a student’s assigned homework.  That’s OK, but if so then it should have been posted in the Student Forum.  The logic seems to be a reiterative type of calculation:

 

We are given:

• The Fuel composition (in mol %) and rate of combustion;
• The partial flue gas composition (in mass/flue gas volume) and the amount of excess air fed to the combustion.

From the above basic data, one can calculate the combustion process to yield the products, using 20% excess air.  That is why we have to identify the combustion air and its water content – in order to have a way to calculate the total composition of the flue gas (which has water vapor in it).  Additionally, we have to allow for the amount of air to produce the NOX and CO and take that into consideration.  We only know the amount of NOX and CO from the flue gas analysis and so must reiterate between that analysis and the amount of combustion air supplied to yield the total flue analysis.  Does anyone have a different idea on the algorithm to be established for the required spreadsheet?

[ankur2061]

To all,

 

The combustion calculations spreadsheet available in the download section provided combustion reactions for some basic hydrocarbons as well as Carbon, Hydrogen, sulfur and sulfur dioxide.

 

Here is the link:

 

http://www.cheresour...n-calculations/

 

Regards,

Ankur.

[thorium90]

Hi, as suggested by Art, made a compilation.

Attached Files

•  burnNG1.xlsx   109.93KB   414 downloads

Edited by thorium90, 20 March 2013 - 10:22 PM.

[sreekanthtm]

Dear sir, 

            Thank you very much.....

Hi, as suggested by Art, made a compilation.

[sreekanthtm]

Dear sir,
         
           In the above calculation the amount of air to produce the NOX and CO is not taken into consideration.So how will we calculate that also?

Edited by sreekanthtm, 23 March 2013 - 10:20 PM.

[thorium90]

Unfortunately, NOx and CO formation are equilibrium reactions dependent on temperature. Furthermore, its just in the ppm range, so in comparison to the flue gas flowrate, it really seems pretty minor. You could try calculating them separately.