6 replies to this topic

[kunju1234]

Hi,


For a frame 6 DLN-1 gas turbine,
Fuel composition by% Vol is
CH4 = 94.46
C2H6 = 0.80
C3H8 = 0.02
N2 = 4.23
CO2 = 0.49

Flue gas composition by % vol is
Argon = 0.87
N2 = 73.20
O2 = 13.46
CO2 = 3.12
H2O = 9.35

Ambient pressure = 101.35 KPa
Ambient temp = 35 degree Centigrade
Exhaust temp = 565 degree Centigrade
Exhaust pressure loss = 2.94 KPa

The questions i have are
A. How to have the LHV and HHV of fuel ? Please i do not need calculators- just the manual way from tables
B. How to take the mean specific heat of the flue gas from the composition of exhaust ?(To know the exhaust energy loss to atmosphere)
C. What is the best way to calculate the efficiency of the Turbine (Expander) part of the Gas Turbine, without knowing the temperature inside the combustion chamber (temperature at the inlet of the nozzle).?
D. Is there a way to have the composition of flue gas from the composition of fuel assuming 100 % combustion and knowing the % of O2 in the flue gas?
E. Is there a method to find the temperature at the inlet of first stage nozzle if the Compressoe exhaust pressure (1200 KPa) and temperature (399 degree centigrade) are known
Thanks<br clear="all">

[breizh]

Let you consider this resource :

http://hydrogen.pnl....ting_values.xls

Breizh

[kunju1234]

Breizh,

Thank you for the response, the spreadsheet has the LHV and HHV of most fuels and chemicals. Here we have the composition of fuel and from that how to calculate the LHV and HHV. What about the remainig queries any ideas are welcome


Thanks again

[Zauberberg]

1. Heating values of various mixed gaseous fuels can be obtained directly from LHV/HHV values of individual components, by applying common mixing rule:

LHV,mix = X1*LHV1 + X2*LHV2 +... + Xn*LHVn

where Xi is mole fraction of the component, and LHVi is corresponding lower heating value of the component. Heating values can be obtained on the internet. For example: http://www.engineeri...lues-d_420.html

2. Again, for known composition of flue gases and known pressure/temperature ranges, you can introduce the same mixing rule - multiplying component mole fractions with their corresponding heat capacities at inlet and outlet conditions. If you want something more exotic, refer to Prausnitz's "The properties of gases and liquids" handbook or other thermodynamics textbooks.

3. Without knowing inlet conditions, it is not possible to calculate efficiency of the turbine. You can Google the term "adiabatic flame temperature" to get a feeling what might be the combustion chamber temperature, or contact the equipment manufacturer for obtaining this data. Perhaps it is already available in the equipment databooks. Another resource: http://www.chemecalc... Fl Temp v2.xls

4. Do we miss something here? You know the fuel gas composition and % excess air, and calculation of the flue gas composition is straightforward. 1 mole CH4 produces 1 mole CO2 and 2 moles H2O, 1 mole C2H6 produces 2 moles CO2 and 3 moles H2O... As easy as that.

Not sure if I understood your last question. If you refer to the turbine compressor (axial air compressor), inlet air temperature can be assumed to be equal to the ambient air temperature.

[kunju1234]

Zauberberg,

Thankyou for your detailed response,
1.Calculation of LHV and HHV from your explanation is straight forward. (Pl clear my guess, the Gross Heating Values mentioned are the HHV and the Net Heating values are the LHV)
2. The mean specific heat of flue gas is clarified.
3. Please allow me to explain the compressor part of the gas turbine so that i can clarify my question

a. The inlet conditions are as mentioned the ambient pressure and temperature
P1=Ambient pressure = 101.35 KPa
T1=Ambient temp = 35 degree Centigrade
b. Let the compressor dischage conditions be

P2=Compressor discharge pressure = 1115 KPa
T2=Compressor discharge temp = 373 degree Centigrade

Let the Ratio of Specific heat of air--Cp/Cv = k = 1.4

The isentropic temperature T2' at compressor discharge can be calculated
by T2'/T1=(P2/P1)^(k-1/k) or T2'=338.2418333 degree Centigrade

and the the isentropic efficiency of compressor = (T2'-T1/T2-T1)*100 = 89.71651873 %

And
Polytropic efficiency can be calculated by T2/T1= (P2/P1)^(k-1)/k *poly
where poly is the polytropic efficiency of compressor
when calculated polytropic efficiency of compressor=92.53237942 %

Now coming to my question how the efficiency of the Turbine part of the gas turbine, Is it calculated in the same way ?? since the exact temperature T3 (Inlet temperature to the turbine nozzle after the combustion chamber) is not measured in actual practices.

As you mentioned (thanks) the excess amount of air can be calculated from the O2 % of exhaust, the amount of air inlet to the compressor inlet can also be calculated by



O2 % in Exh =13.46

O2 in Air = 20.95

% excess air =179.70

Air inlet to compressor= M=1+Excess air-mass of fuel

Is it a practice to do a mass balance or an enthalpy balance (if so please share some ideas) and then reverse calculate the Temperature T3 ?
The last question is related to T3

Am I making myself clear

Thanks

[Zauberberg]

For calculating T3 (expansion turbine inlet temperature), you can use the spreadsheet I have posted in my point No. 3 - it is a spreadsheet that calculates adiabatic flame temperature based on fuel gas composition, fuel gas flow, and combustion air flow. I had a chance to cross-check this spreadsheet for Frame 5 and Frame 9 machines that operate at fairly fixed thermal efficiency (if they are brand new or well maintained), and by using calculated adiabatic flame temperature and measured flue gas outlet temperature, I used to obtain efficiency values quite close to those published by the equipment manufacturer.

[kunju1234]

Thank you for the explanations.