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The importance to know the real natural gas properties

Fuel consumption and fuel heat input belong to the most sensitive parameters in today's gas turbine operation. For large consumers and power plants 0.1% difference in heat rate can have hundreds of thousands of dollars impact on fuel costs within just one year. Because of this penalties to manufacturers are extraordinary high when not meeting the heat rate guarantees.

Gas turbine operators are well advised to have their own calculation tool, being able to crosscheck thermal performance, but also monthly bills coming from gas suppliers. Of course there are enough other reasons for engineers to stay up to date with natural gas properties.

Let's discuss the effects calculating real while using as an example gas# 4 from ISO 20765-1-2005 (BS ISO 20765-1-2009) with 100 bar and 76.85°C at the »limit of supply«. To be consistent with reference conditions across chemical energies and enthalpies for air and flue gas flows let's take also for the gas the reference conditions of 0°C and 1.01325 bar.

Gas example

Gas 4 ISO 20765-1 2005(E)
p         =  10.00 MPa  absolute gas pressure
T         = 330.00 K    gas temperature
H2        =   9.50 Mol% hydrogen
He        =   0.02 Mol% helium
H2O       =   0.01 Mol% water vapor
CO        =   1.00 Mol% carbon monoxide
N2        =  10.00 Mol% nitrogen
O2        =   0.01 Mol% oxygen
H2S       =   0.01 Mol% hydrogen sulfide
Ar        =   0.01 Mol% argon
CO2       =   1.60 Mol% carbon dioxide
CH4       =  73.50 Mol% ethane
C2H6      =   3.30 Mol% ethane
C3H8      =   0.74 Mol% propane
i-C4H10   =   0.08 Mol% iso-butane
n-C4H10   =   0.08 Mol% n-butane
neo-C5H12 =   0.00 Mol% neo-pentane
i-C5H12   =   0.04 Mol% iso-pentane
n-C5H12   =   0.04 Mol% n-pentane
n-C6H14   =   0.02 Mol% n-hexane
n-C7H16   =   0.01 Mol% n-heptane
n-C8H18   =   0.01 Mol% n-octane
n-C9H20   =   0.01 Mol% n-nonane
n-C10H22  =   0.01 Mol% n-decane

Results

Molar   Mass    = 17.3170 kg/kmol
Gas  Constant   = 480.135 J/(kg*K)
Low  Heat Value =  39.729 MJ/kg = 17081. BTU/lb
High Heat Value =  44.185 MJ/kg = 18996. BTU/lb

Pressure                    1.013bar  1.013bar  1.013bar  14.73psi    Actual
Temperature                 0.  degC  15. degC  25. degC  60. degF    Actual
------------------------------------+---------+---------+---------+---------+
Wobbe Lower Index    MJ/m3    39.747    37.668
Wobbe Upper Index    MJ/m3    44.204    41.892
Low Heat Value       MJ/m3    30.755    29.144    28.161    29.155  2480.543
Low Heat Value     BTU/ft3     825.4     782.2     755.8     782.5   66575.3
High Heat Value      MJ/m3    34.204    32.412    31.319    32.425  2758.736
High Heat Value    BTU/ft3     918.0     869.9     840.6     870.2   74041.7
Density              kg/m3    0.7741    0.7336    0.7088    0.7338   62.4360
Density             lb/ft3    0.0483    0.0458    0.0443    0.0458    3.8979
Relative Density       [-]    0.5987    0.5986
Compression Factor     [-]   0.99803   0.99839   0.99859   0.99840   0.95309
Internal Energy      kJ/kg  -182.385  -159.342  -143.749  -158.483  -106.063
Enthalpy             kJ/kg   -51.494   -21.214    -0.799   -20.087    54.101
Entropy          kJ/(kg*K)    0.2860    0.3939    0.4636    0.3967   -1.5311
Heat Capacity cp kJ/(kg*K)    2.0058    2.0321    2.0512    2.0331    2.5087
Heat Capacity cv kJ/(kg*K)    1.5206    1.5475    1.5671    1.5486    1.7374
Isentropic Exponent    [-]    1.3165    1.3110    1.3071    1.3108    1.4123
Joule Thomson Coeff. K/bar    0.4627    0.4137    0.3845    0.4120    0.1996
Speed of Sound         m/s   415.105   425.537   432.263   425.915   475.610
Dynamic Viscosity Pa*s*E-6    10.954    11.477    11.821    11.496    15.329
HeatCircuitCapacityW/(m*K)    0.0286    0.0306    0.0320    0.0307    0.0462

ISO 6976-1995 for Molar Mass and Calorific Values at 0degC
ISO 20765-1 2005 for Thermodynamic Properties

Real gas density is higher than ideal gas density

The effect of compression factor tells that the real gas density is higher than the ideal gas density, typically by 1...5% at operating conditions. Ideal gas density neglects the compression factor. The compression factor for the example is z = 0.95309, therefore the real natural gas density in this example is 4.69% higher.

The real gas density is

ρ = M/R · p/z · T = p/Rs · z · T     (1)

ρ
real gas density [kg/m³]
M
molar mass [kg/mol]
p
absolute gas pressure [Pa = N/m²]
R
universal gas constant: 8.31451 J/(mol·K) = 8.31451 (N·m)/(mol·K)
Rs
specific gas constant [J/(kg·K)]
z
compression factor: typical values 0.95...0.99, in ideal gas z = 1.0
T
absolute temperature [K]

Real gas enthalpy is lower than ideal gas enthalpy

Next to the chemical energy, which is expressed by the heat value, the real gas enthalpy is part of the overall heat input. The real natural gas enthalpy is smaller than the ideal gas enthalpy, and the overall heat input is

HI = m · (LHV + Δh)     (2)

HI = m · [39729 + 54.101 - (-51.494)] kJ/kg = m · 39835 kJ/kg     (3)

The ideal gas enthalpy would be calculated with just the temperature difference and a typical heat capacity of 2.3 kJ/(kg·K).

HI = m · (LHV + cpΔT)     (4)

HI = m · [39729 + 2.3 · (76.85 - 0.0)] kJ/kg = m · 39906 kJ/kg     (5)

HI
heat input [kW = kJ/s]
m
fuel gas mass flow [kg/s]
LHV
Low heat value combustion at 0 °C [kJ/kg]
Δh
real gas enthalpy difference between metering pressure and temperature and reference conditions [kJ/kg]
cp
heat capacity for the ideal gas (depending only on temperature) [kJ/(kg·K)]
ΔT
difference between metering temperature and reference temperature [K]

The real heat input in this example is 0.18% lower.

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