CO2 effect on flue gas dew point

[roaling]

The question is simple and clear.
How does CO2 content effect the dew point of flue gas exhausted from natural gas fuelled gas turbine towards a HRSG?
Thank a lot for your help.

 

[Guidoo]

The water dew point is a function of the partial water vapor pressure. As you probably know, the partial water vapor pressure is equal to moles of water in flue gas divided by total moles of flue gas times the total system pressure. The total system pressure will generally be 1 atm (=101.325 kPa).

You can plot water dew point as function of partial vapor pressure using table below:

Water vapor pressure [kPa] Water dew point [°C]
2 17
5 33
10 46
15 54
20 60
25 64

 

[25362]

To roaling, do you mean CO2 or SO2 ?

 

[Guidoo]

Estimation of sulfur trioxide (SO3) dew point is described on page 330 of Carl Branan's Rules of Thumb for Chemical Engineers, 2nd edition. It also contains a reference to:
Okkes, A.G., "Get Acid Dew Point of Flue Gas, " Hydrocarbon Processing, July 1987, p.53.

 

[rmw]

Several good threads with good links to useful information on this website. Have you searched the site??

rmw

 

[roaling]

Thanks Guidoo.
I know the relation between dew point and partial water vapor pressure. What is unknow to me is the relation with the H20-CO2 system that beeing an acid is a condensing agent.
Good day, Roaling

 

[Guidoo]

Roaling,

My first reply was based on text section "water dew point and water content" of book "physical properties of natural gases", published by Dutch company called Gasunie. Since virtually all flue gases (and certainly flue gases resulting from combustion of natural gases) contain CO2, I am quite sure it will work for flue gases containing CO2.

From your last reply, I understand that you are indeed interested in the water dewpoint (and not SO3), please confirm.

 

[25362]

As far as I am aware of, carbonic acid may form when CO2 dissolves in liquid water. Most of the CO2 molecules in water exist as hydrates, only a fraction react with water to form carbonic acid.

In the reaction CO2(aq)+ H2O(l) <=> H2CO3(aq), the equilibrium strongly favors the left-hand reactants in such a manner that in saturated, ~0.033 M, aqueous (solvated) CO2, the ratio [CO2(aq)]/[CO3H2(aq)] is about 600:1. Heating reverses the reaction.

BTW, it is widely recognized that as result of this reaction, the sea contains about 60 times more CO2 than the atmosphere.

When it comes to the ability to neutralize base, all dissolved CO2, not only carbonic acid, is available because CO2 can react directly with aqueous hydroxide ion.

In short, it is my belief that CO2 is not supposed to affect water's dew point in combustion gases at atmospheric pressure, by forming carbonic acid as postulated by Roaling.

 

[grampi1]

CO2 corrosion is a potential issue if and only if the pp is over 15psi. Not relevant for stack gas. Reviewed stack gas corrosion info lately and no one worries about the CO2 concentration, just SO2, SO3, percent S in fuel and water vapor pressure.