determining natural gas parameters expressed in volume

DETERMINING NATURAL GAS QUALITY PARAMETERS EXPRESSED IN VOLUME m³:

In compliance to ISO 13686 Requirements: Natural Gas Quality Designation

Jessol M. Salvo, BSME

Science Research Specialist

Introduction:

Natural Gas is composed of hydrocarbon gases such as Methane (CH4), Ethane (C2H6), Propane (C3H8)

and Butane (C4H10), with some inert gases such as Carbon Dioxide (CO2) and Nitrogen (N2). It is relatively

easy to determine the Superior Calorific Value or High Heating Value of any natural gas mixture. Just

remember that the total energy (heat) content of a natural gas mixture is equal to the sum of the energy

(heat) content of its components, this law follows the First Law of Thermodynamics or the Law of

Conservation of Energy and can be expressed in an Energy Balance Equation.

Energy Balance:

HHVnatgas = ΣHHVcomponents = HHVCH4(X CH4) + HHVC2H6(XC2H6)+HHVC3H8(XC3H8) + HHVC4H10 (XC4H10)…

This is relatively easy if the unit of measurement of energy (heat) is in terms of mass such as pounds

(lbs.) or kilos (kg). Most Engineering Handbooks have prescribed the Calorific Values of natural gas

components in terms or lbs. or kg. These Calorific Values are constant regardless of their pressure and

temperature.

From Engineering Handbooks:

Natural Gas Component Kcal/kg MJ/kg Methane CH4 13,284 55.6 Ethane C2H6 12,400 51.92 Propane C3H8 12,030 50.37 Butane C4H10 11,830 49.53

The Challenge – Express Parameters in Volume rather than Mass:

However, the challenge lies in determining the High Heating Value of various natural gas mixtures

measured in terms of volume such as m³ rather than mass such as lbs. or kg. Traditionally, natural gas

parameters are expressed in mass such as Btu/lb. or MJ/kg. Note that gases behave differently at

different temperatures and pressures, i.e., gas densities vary with respect to changes in pressure and

temperature.

ISO Standards such as ISO 13686 entitled “International Standard – Natural Gas Quality Designation”

require natural gas parameters to be expressed in volume (m³) rather than the traditional way of

expressing them in mass such as lbs. or kilos.

The Need for a Density Equation:

In order to comply with ISO requirements, the work around would be to develop a density equation to

enable us to work with units expressed in mass and convert them to units expressed in volume. The

density equation would have to be expressed as a function of pressure and temperature since ISO 13443

require natural gas measurements be anchored in standard temperatures and pressures of 288.15°K

(15°C) and 1.01325 bars (1 atm) as reference conditions.

The density equation can be derived from the Ideal Gas Law:

; Where =

, R =

(universal gas constant)

Substituting, we arrive at the following density equation:

=

RT = density (ρ)

Density equation = f (P,T):

This equation is now a function of pressure and temperature, thus we can account for gas behavior at

standard temperatures and pressures.

The Need for a Molecular Weight Equation:

The equation also requires that we know the molecular weight of the natural gas mixture. Since natural

gas mixtures can vary depending on the concentration of each of its components, thus natural gas

molecular weight will also vary.

It can be stated that the molecular weight of a natural gas mixture is equal to the sum of the molecular

weight of its components. This principle is derived from Dalton’s Law of Partial Pressures and the Molar

Fraction Law:

MWnatgas = ΣMW components = MWCH4(X CH4) + MWC2H6(XC2H6) + MWC3H8(XC3H8) + MWC4H10 (XC4H10)…

The Molecular Weight of natural gas components and air is as follows:

Periodic Tables: Molecular Weights Gases Molecular Weights

Methane 16.043 g/mole Ethane 30.07 g/mole Propane 44.09 g/mole Butane 58.12 g/mole Nitrogen 14.00 g/mole Air 28.97 g/mole

With these derived formulas, we can now proceed in determining natural gas parameters expressed in

volume m³. Now let’s try using these formulas based on data of known natural gas fields as follows:

Methane Ethane Propane Butane Nitrogen

Richest Mix: Nigeria LNG (Nigeria) 87.9% 5.5% 4% 2.5% 0.1%

Leanest Mix: Kenai LNG (Alaska) 99.8% 0.1% - - 0.1%

Richest Natural Gas Mix: Nigeria LNG

Molecular Weight:

MWnatgas = ΣMW components = MWCH4(X CH4) + MWC2H6(XC2H6) + MWC3H8(XC3H8) + MWC4H10 (XC4H10)…

= 16.043g/mole(.879) + 30.07g/more(.055) + 44.09g/mole(.04) + 58.12g/mole(.025) +

14.0g/mole(.001)

Density (ρ): at standard temperature and pressure (STP)

Now that we know the Molecular Weight and Density of the richest natural gas mix, we can proceed

determining other parameters required in the ISO quality standards, such as Relative Density (d),

Superior Calorific Value or High Heating Value (HHV), and Wobbe Index (WI).

Determining Superior Calorific Value expressed in Volume m³:

Going back to the equation and applying this to the richest natural gas mix:


Note again that in the equation, Superior Calorific Value is a function of the sum of the energy (heat)

contributed by each natural gas component. Since available data on calorific values are based on mass

such as Btu/lbs. or MJ/kg, we would need to apply the density equation on each natural gas component

before we can come up with the Superior Calorific Value of the entire natural gas mix.

Going back to the Molecular Weights of each natural gas component and the density equation, we come

up with these computations:

Molecular Weight Nigeria gas = 19.00g/mole

Density (ρ) Nigeria gas = 0.803 gram/liter or kg/m³

Density of Methane at STP:

ρCH4 =

=

(

)

= 0.678 kg/m³

Density of Ethane at STP:

ρC2H6 =

=

(

)

= 1.271 kg/m³

Density of Propane at STP:

ρC3H8 =

=

(

)

= 1.864 kg/m³

Density of Butane at STP:

ρC4H10 =

=

(

)

= 2.457 kg/m³

Density of Air at STP:

ρair =

=

(

)

= 1.224 kg/m³

Note that we have accounted for the density of each natural gas component with standard temperature

and pressure as reference condition in accordance to ISO requirements. With these values, we can now

determine the Superior Calorific Value of each natural gas component by multiplying these values with

their known Calorific values expressed in mass such as Btu/lbs. or MJ/kg

Given High Heating Values (SCV) in kg from Engineering Handbook:

Natural Gas Component Kcal/kg MJ/kg

Methane CH4 13,284 55.6

Ethane C2H6 12,400 51.92

Propane C3H8 12,030 50.37

Butane C4H10 11,830 49.53

High Heating Values (SCV) in m³ at STP conditions given derived densities (ρ):

Natural Gas Components Density(ρ) x MJ/kg MJ/m³

Methane CH4 (0.678 kg/m³)(55.6 MJ/kg) 37.697

Ethane C2H6 (1.271 kg/m³)(51.92 MJ/kg) 65.990

Propane C3H8 (1.864 kg/m³)(50.37 MJ/kg) 93.890

Butane C4H10 (2.457 kg/m³)(58.12 MJ/kg) 142.800

Applying the Energy Balance Equation and given the % share of each natural gas component, we can

now determine the Superior Calorific Value as follows:

Given composition of richest natural gas mix:



Apply this on the Energy Balance Equation:


HHVnatgas =37.7Mj/m³(.879) + 65.99MJ/m³(.055) + 93.89Mj/m³(.04) + 142.80Mj/m³(.025) + 0(.001)


Determining Wobbe Index (WI) in m³:

Another major requirement in the ISO for natural gas quality standards is the determination of Wobbe

Index (WI). Wobbe Index is defined as the Superior Calorific Value or High Heating Value divided by the

square root of the relative density of the natural gas mixture.

√

The equation requires that we derive first the relative density of the natural gas mixture. Relative

density is defined as the density of the natural gas mixture divided by the density of air at standard

temperature and pressure.

Since density (ρ) of air and the richest natural gas mixture has already been determined earlier, relative

density is thus computed as follows:

Thus Wobbe Index is determined as follows:

√

√

The same process can be repeated to determine the Superior Calorific Value of ANY natural gas mix. All that is

needed is to specify the % share of each natural gas component in the total volume mix.

HHVnatgas = 43.75MJ/m³

Determining Methane Number (MN):

Methane Number (MN) is a rating indicating the knocking characteristics of natural gas as fuel for

internal combustion engines. It is comparable to the Octane Number for petroleum products such as

gasoline; i.e., the higher the Methane Number, the better it is for internal combustion engines.

There are three ways to determine Methane Number, However for this discussion, we will use the GRI

method since this is the method where we can manually compute for the Methane Number given the %

volume share of each natural gas component.

From ISO 15403, the equation used for determining Methane Number via GI Method is given as follows:

Linear Coefficient Relation:

MON = (137.78Xmethane) + (29.948Xethane) + (-18.193Xpropane) + (-167.062Xbutane) + (181.233XCO2) + (26.994XN2)

Correlation between MON and MN:

MN = 1.445 x (MON) – 103.42

Where:

MON = Motor Octane Number

MN = Methane Number

Note that “X” represents the % volume share of each natural gas component. Methane Number unit

less, however it is a function of % volume share of each natural gas component, not by quantity of mass.

Again, given the composition of the richest natural gas mix:



MON = (137.78Xmethane)+(29.948Xethane)+(-18.193Xpropane)+(-167.062Xbutane)+(181.233XCO2)+(26.994XN2)

MON = (137.78(.879))+(29.948(.055))+(-18.193(.04))+(-167.062(.025))+(181.233(0))+(26.994(.001))

MN = 1.445 x (MON) -103.42

MN = 1.445 x (117.88) -103.42

MON = 117.88

MN = 66.91

Significance of Natural Gas Parameters:

The following table summarized important natural gas parameters and its significance

Natural Gas Parameter Significance

Superior Calorific Value (HHV) Amount of heat which would be released by the complete combustion in air

of a specified quantity of gas, in such a way that the pressure at which the

reaction takes place remains constant, and all the products of combustion are

returned to the same specified temperature as that of the reactants.

In commercial transactions, this is the most important parameter as prices of

LNG are hinged on its heat content, such as $/Btu.

Wobbe Index (WI) “Volumetric-basis1” Superior Calorific Value, at specified reference conditions,

divided by the square root of the relative density at the same specified

metering reference conditions.

The Wobbe Index is used to compare the combustion energy output of

different composition fuel gases, i.e., if two fuels have identical Wobbe

Indices, they will have the same energy output given the same pressure and

valve settings.

The Wobbe Index is a critical factor to minimize the impact of the changeover

when analyzing the use of substitute natural gas (SNG) fuels such as propane-

air mixtures.

Density (ρ) Mass of a natural gas mixture divided by its volume at a specified pressure

and temperature.

For this discussion, the reference condition is set at standard temperature

and pressure of 288.15°K (15°C) and 1.0125 bars (1 atm.). This means that the

higher the density, the heavier the natural gas mix at a specified reference

volume.

Relative Density (d) Density of a gas divided by the density of dry air of standard composition at

the same specified conditions of pressure and temperature.

Relative density compares the density of natural gas with respect to air, i.e.,

relative density < 1 means the natural gas mixture is lighter than air, while a

relative density > 1 means the natural gas mixture is heavier than air. Relative

density plays an important role in determining the Wobbe Index of a natural

gas mix.

1 Superior Calorific Value expressed in heat per unit volume such as MJ/m³, rather than common practice of heat per unit mass such as Btu/lb.

or MJ/kg.

determining natural gas parameters expressed in volume

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