CHP stack flow via O2%

[Natz1235]

Hi,

I hope someone can help me please....

I need to calculate the flow through a stack from a CHP (Gas Turbine and fired HRSG), for all possible conditions (GT only firing (gas or oil), HRSG only firing, both firing). The previous person working on this seems to have picked a general number from the identical unit and stuck with that for the GT firing but I can't find out where this came from.

The measurements I have are:
GT gas flow (in Sm3)
GT oil flow
Gas flow to the HRSG (Nm3/h)
%O2 in stack
Temperature of the stack.
What mode the CHP is in (GT firing/GT+HRSG firing)


The GT doesn't often run on oil so if I can find a solution for gas only at this point I'd stll be happy.

I know that the %O2 in the stack represents the excess air and I should be able to work out from the fuel flow what the Stoichiometric air volume needed is, but after that my calculations seem to go to pot...

If anyone has any ideas I'd be really greatful thanks.

Nat

 

[25362]

From very old notes I gathered the volume [nm³] at NTP (0^oC and 1 atm) of the flue gases from perfect combustion of hydrocarbons can also be approximated from their low calorific values P_i as follows:

Refinery gases with P_i between 11000 and 16000 kcal/nm³: (1.1 P_i/1000) + 0.85 nm³/nm³
Fuel oil with P_i between 9500 and 10500 kcal/kg: (1.1P_i/1000) + 0.6 nm³/kg

When burning hydrocarbons (gases, liquids or a mixture) the stoichiometry of perfect combustion enables estimating % xs air from the % CO₂ and % O₂ present in the flue gases on a dry basis (Orsat-type measure).

Having these data in hand you could estimate the NTP volume of gases in the stack.

To estimate the actual volume given the temperature shouldn't then be a problem.

 

[Natz1235]

Thank you for your help 25362.

Unfortunately the LHV of my gas is ~8500kcal/m3, is there an equation for this level of CV?

Also, I'd like to check that the excess air equation would be:
Ex(%)= 100 * %O2/(20.9 - %O2)

For a gas turbine what kind of excess air levels would be expected?

 

[25362]

All I'm saying refers to the burning of hydrocarbons.
Say your gas is a mixture of 93% methane and 7% ethane.
The amount of air A_o needed for a stoichiometric perfect combustion would be:

0.93[×]9.52 + 0.07[×]16.66 = 10.02 nm³/nm³​

The volume of flue gases V_o would be:

0.93[×]10.52 + 0.07[×]18.16 = 11.05 nm³/nm³​

The "xs" air is sometimes expressed as n = A/A_o
The volume of fue gases with a perfect combustion with xs air would be:

V = V_o + (n-1) A_o​

Knowing %O₂ and %CO₂ in the "dry" flue gases, n can be estimated as:

n = (100-%CO₂-%O₂)/(100-%CO₂-4.76%O₂)​

If you have the elementary composition of your fuels you can estimate the volume of air needed and the volume of combustion gases produced.

For liquid fuel oil with net calorific values [P_i]between 9500 and 10,500 kcal/kg, the volume of theoretical air is approximated by:

A_o = 0.9P_i/1000 + 2.0 nm³/kg​

With this information I assume you can estimate the volume of flue gases, but you need to know the % CO₂ to estimate the "xs" air factor n.