How To Determine Natural Gas Heating Value

Natural gas prices are something that the average person does not really spend a

lot of time thinking about unless they are heating their house with it in the wintertime and they live where it is cold. However, natural gas prices are something that affect all of us to a very great degree. How so? Well, not only does it determine how much we pay to heat our homes if they are heated using natural gas, but it also dictates how much raw materials cost for many of the products we use due to the fact that natural gas is a raw material in itself as well as one of the main energy sources that powers the basic petrochemical industry. The price of natural gas both directly and indirectly influences the prices of things we buy. So, what determines the price of natural gas? Two things. The supply to demand ratio and the heating value of the gas itself. This article discusses how to determine the heating value of natural gas using gas chromatography.

The heating value of a sample of natural gas is very easy to determine if you have the right equipment and standards in place. Generally speaking, natural gas is composed of the following components which are typical but can vary greatly:

Ethane 5%

Nitrogen 2.5%

Carbon Dioxide 1%

Propane 1%

Isobutane 0.3%

n-Butane 0.3%

Neopentane 0.1%

Isopentane 0.1%

n-Pentane 0.1 %

n-Hexane and Heavier 0.1%

Methane (composes the remainder of the gas volume)

The overall BTU or heating value of the natural gas in sample in question is analyzed using a gas chromatograph. A gas chromatograph takes a sample of gas and separates and quantifies each component in the sample. This is done by first sampling a BTU reference standard on the gas chromatograph that has known components at know concentrations. After several runs or samples of the reference standard are completed, a calibration table is set up that notes the peak identification by retention or elution time, the concentration (which is added to the table manually) and a response factor that is used to calculate unknowns.

Next the unknown sample us run and the corresponding chromatogram is analyzed by either human eyes or software. Peaks at specific elution times determine the identities of the components in the sample and peak size or area multiplied by the corresponding response factor reveal the concentration of the identified components in the sample. Concentrations are referred to in the world of analytical chemistry as molar concentrations and the fraction for of this number is known as the molar fraction. For example, if the methane concentration of a natural gas sample is 90% by volume, then the molar fraction of the methane in the sample is 0.9 molar fraction.

Now, to determine the heating value of the overall sample, the molar fraction of each identified and quantified component in the mixture is multiplied by the corresponding and empirically determined Heating Value. All of the results are added together and the resulting total is the heating value of the natural gas sample. The heating value of the natural gas sample is essentially a number that states how much energy the gas has or how much work it can do, thus determining it's value. The determination of heating value for natural gas is a career in itself for an enormous number of chemists in the energy industry.

by: Mark T He

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