15 replies to this topic

[keby9wtw]

One question:

Dewpoint equation of SO2 according to Kiang:

Td=1000/{3.9526 - 0.1863*ln(PH2O) + 0.000867*ln(PSO2) - 0.00091*ln(PH2O*PSO2)}
Pressures (P) in the equation is given in mm Hg


I am planning to use the equation above to find the flue gas outlet temperature from waste heat boiler ( above the acid dew point ) . Initially, I used the online acid dew point calculator and obtained a value of 125 degree Celsius ( seems reasonable) . Then, I assume the total pressure of the flue gas is at atmospheric pressure. Finding the partial pressure(pi=yi.P) then substitute into the equation and I obtained a value of 53 degree Celsius. Is my assumption for the total pressure right ? Or should I use PV=nRT (however temperature is unknown too, iteration is needed ? )

[ankur2061]

keby9wtw,

I had downloaded an excel calcualtor for flue gas dew point using both "Verhoff and Banchero" and "Kiang & Yen Hsiung" which I am attaching for your reference.

The default value for the pressure for flue gases is considered as 0 kg/cm2g or 0 barg in the attached excel calculator.

Additionally have a look at the link below which provides the equation by "Okkes" and "Verhoff & Bancher".

http://www.cheresour...acid-dew-point/

Hope this helps.

Regards,
Ankur.

Attached Files

•  cal._dew_point.xls   206KB   1268 downloads

Edited by ankur2061, 10 December 2011 - 10:17 AM.

[keby9wtw]

Thank you ankur for the excel. However ,i would like to know whether my approach to the problem is correct or not. I would like to solve this manually.

[ankur2061]

keby9wtw,

Considering the total pressure as atmospheric pressure is correct. As you may have noticed that the dewpoint equations for Sulfuric acid dewpoint and Hydrochloric acid dewpoint are different in the equations provided in the excel sheet.

Regards,
Ankur.

[keby9wtw]

however there is no input for SO3 in the excel spreadsheet.But first I need to get the conversion of so2 to so3. Given that:

N2 = 73.08%
CO2 = 7.59%
O2 = 4.474%
SO2 = 2.68E-4 %
H20 = 0.149 %

By using manual calculation, im getting the temperature of <100 oC assuming that the conversion of S02 to SO3 is 3%. Any mistake here ?

[ankur2061]

keby9wtw,

SO2 conversion to SO3 is considered as 3.5% based on the Otsuka equation.

Regards,
Ankur

[kkala]

however there is no input for SO3 in the excel spreadsheet.But first I need to get the conversion of so2 to so3. Given that:
N2 = 73.08%
CO2 = 7.59%
O2 = 4.474%
SO2 = 2.68E-4 %
H20 = 0.149 %
By using manual calculation, im getting the temperature of <100 oC assuming that the conversion of S02 to SO3 is 3%. Any mistake here ?

I guess that H2O=14.9% (not 0.149% due to apparent error), right?
Flue gas composition indicates natural gas, or fuel gas with very litle content of S compounds.
Locally supplied natural gas has an estimated acid dew point less than 100 oC (roughly remembered 90 oC).

http://www.jehar.com/dewpnt.htm would give 105.5 oC for your case, assuming equilibrium SO3/SO2 at 1000 oC (conversion 3.46%). If you assume 3% conversion, acid dew point would go to 104.4 oC. If 2%, 102.2 oC. Verification through hand calculation or another calculator is indicated, to assess the results. It seems the correct value is being approached.

Estimates of acid dew point are not very precise, mainly due to assumptions incorporated in each piece of software. For boiler fuel oil of 0.6 % S, flue gas acid dew point was estimated from (say) 123 to (say)133 oC, differing from vendor to vendor. Even for gaseous fuels, flame temperature and SO2 /SO3 formation along its length differs from case to case.

[keby9wtw]

Oh sorry my mistake, it's 14.9%. Thanks sir, I found flue gas temperature to be 120-150 C in references, That's y I wasnt confident with my manual calculation

[kkala]

...Thanks sir, I found flue gas temperature to be 120-150 C in references, That's y I wasnt confident with my manual calculation

I believe your recent manual calculation approached true value, flue gas temperature in references is OK, since it also contains a safety margin.
This margin can be 25 oC over the acid dew point estimated for the sulphur content of fuel.

[gilooou]

Good morning,
How can I calculate the partial pressure of SO2 used in these formulas from its volume concentration (ppm in flu gases)?

[ankur2061]

At standard conditions of 0 deg C and 101.325 kPa (abs), one kg-mole of any gas occupies a volume of 22.414 m3.

Let us assume that you have 100 ppmv of SO2 in your flue gas. It means that 1,000,000 m3 of flue gas contains 100 m3 of SO2.

From the first statement I made, 22.414 m3 of flue gas will correspond to 1 kg-mole. Thus 1,000,000 m3 of flue gas will be:

1,000,000 / 22.414 = 446,145 kg-moles of flue gas

Similarly, 100 m3 of SO2 will correspond to:

100 / 22.414 = 4.46 kg-moles of SO2

Thus:

446,145 kg-moles of flue gas contains 4.46 kg-moles of SO2

So 100 kg-moles of flue gas will contain:

100*4.46 / 446,145 kg-moles of SO2 = 0.001 kg-moles of SO2

Converting this into mole fraction we get:

0.00001 mole fraction of SO2 in flue gas

Now using Raoult's law

partial pressure of a gas = mole fraction of gas * total pressure

where total pressure = 101.325 kPa (abs)

so for the example given above:

partial pressure of SO2 = 0.00001*101.325 = 0.00101325 kPa (abs)

All of the above gives the very known conclusion that

mole percent = volume percent

I hope I have been able to give you some clear fundamental understanding.

Regards,
Ankur.

[kkala]

Good morning, How can I calculate the partial pressure of SO2 used in these formulas from its volume concentration (ppm in flu gases)?

You will assume that total flue gas pressure is 1 Atm, which is quite close to reality. For instace, if the fan of combustion air develops a discharge head of 500 mm H2O, flue gas pressure would be 500/10000=0.05 kgf/cm2 g at the discharge and 0.0 kgf/cm2 g at the exit of the stack.
Consider now the previous analysis of flue gas, where SO2 content is 2.68E-4% v/v, so it is 2.68E-4% mol/mol or 2.68E-6 mol/mol or 2.68 ppm Since total gas pressure is 1 Atm, partial pressure of SO2 (Dalton's law) is 1*2.68/1E6 =2.68E-6 Atm=2.77E-6 kgf/cm2 in absolute pressure (not gauge).
In General 1Atm*a ppm=aE-6 Atm a.
ppm in gases is expressed in molar basis (same as volume basis)

Edited by kkala, 12 December 2011 - 02:09 PM.

[ankur2061]

You will assume that total flue gas pressure is 1 Atm, which is quite close to reality. For instace, if the fan of combustion air develops a discharge head of 500 mm H2O, flue gas pressure would be 500/10000=0.05 kgf/cm2 g at the discharge and 0.0 kgf/cm2 g at the exit of the stack.
Consider now the previous analysis of flue gas, where SO2 content is 2.68E-4% v/v, so it is 2.68E-4% mol/mol or 2.68E-6 mol/mol or 2.68 ppm Since total gas pressure is 1 Atm, partial pressure of SO2 (Dalton's law) is 1*2.68/1E6 =2.68E-6 Atm=2.77E-6 kgf/cm2 in absolute pressure (not gauge).
In General 1Atm*a ppm=aE-6 Atm a.
ppm in gases is expressed in molar basis (same as volume basis)

For combustion air fan sizing and discharge pressures refer:

http://www.cheresour...eaters-boilers/

Regards,
Ankur.

[RAJIV JAIN]

thanks

[tacojoi]

Dear Ankur,

Could you tell me what Otsuka equation is? And which paper or book can find it?

Thanks a lot.

Regards,
Taco.

[ankur2061]

Taco,

Ostsuka Equation:

Dew Point (°C) = 20*log(% SO3 in flue gas) + A
where:
A = 201 when H2O is about 15% in flue gas
A = 198 when H2O is about 12.5% in flue gas
A = 194 when H2O is baout 10% in flue gas

Note:

Otsuka's equation assumes 3.5% conversion of SO2 in flue gas to SO3

Hope this helps.

Regards,
Ankur.