Fri. Sep 24th, 2021

City gas distribution

Transporting future fuel

#5 Measurement of Natural Gas

5 min read

The first thing to remember: natural gas is measured by volume (cubic meter) but is sold based on its heating content (Btus).

First of all, we need to understand what is SCM & BTU or MMBTU.

Standard cubic meter: A standard cubic meter defines an amount of gas contained in a volume of one cubic meter at standard temperature 15°C and pressure of 101.325 kilopascals.

BTU: MMBtu is an acronym for Metric Million British Thermal Unit, and it is a unit traditionally used to measure heat content or energy value. It is widely associated with the measurement of natural gas in energy terms globally. A Btu is defined as the amount of heat required to raise the temperature of one 1 pound (0.454 kg) of liquid water by 1 °F (0.556 °C) at a constant pressure of one atmosphere. When 1 Btu is represented in a unit of million, it is termed as Metric Million British Thermal Unit (MMBtu).

1MMBTU = 252000 Kcal, therefore if we assume GCV as 10000 Kcal/SCM we will get 1MMBTU = 25.2 SCM

Example: Convert 550 MMBTU in SCM assuming GCV as 9650 Kcal/SCM

= 550*252000/9650 = 14362 SCM

Here, GCV is Gross Calorific value.

The calorific value for natural gas varies between 8500 kcal/scm to 9600 kcal/scm.

Also, here Calorie is the amount of heat required to raise the temperature of one gram of water through one degree Centigrade and “Kilocalorie” is equal to 1000 calories. It may be defined as ‘the quantity of heat required to raise the temperature of one kilogram of water through one degree Centigrade. Thus, 1 kcal = 1000 cal

Calorific Value: The calorific value of gas can be defined as the amount of heat produced on combusting a unit volume of gas and can be expressed in kcal/m3. Calorific value depends directly on the methane content of natural gas, i.e., the higher the methane content, the greater the calorific value. the composition of natural gas varies with the age of the landfill, therefore calorific value also varies along with its composition. It can be the Gross calorific value or Net calorific value.

Gross calorific value: The Calorific value is called GCV if the water of combustion is entirely condensed and the heat contained in the water vapor is recovered.

Lower/Net calorific value: The Calorific value is called LCV or NCV if the products of combustion contain the water vapor and the heat in the water vapor is not recovered.

Note that, for gaseous fuels, two Calorific Values are defined – the ‘Net Calorific Value,’ which excludes the heat energy present in the water formed by combustion, and the ‘Gross Calorific Value,’ which includes it. This because, in addition to heat energy, fuels that contain hydrogen atoms create hot water vapor as a by-product of combustion.

According to the dulong’s formula,

GCV = 1/100[8080C + 34500(H-O/8) + 22400S], here the C, S, O , H are the percentages of carbon, sulphur, oxygen and hydrogen

NCV=(GCV-0.09H*587), here 587 cal/g is the latent heat of steam

Note: In natural gas, we can assume NCV = 90% of GCV.

Knowledge of the CV of natural gas is an essential part of our day-to-day activities, as this information is used to determine the amount of energy we transport. CV information is provided daily to gas shippers and suppliers, which is then used to bill gas consumers. We also use this data to determine transportation charges for gas shippers and suppliers.

The CV of natural gas is measured continually, using process gas chromatographs. Process gas chromatographs separate natural gas into its constituent compounds (i.e. methane, ethane, carbon dioxide, etc.) and measures the amount of each in the gas. The physical characteristics of each component, as defined by ISO 6976, are programmed into the chromatograph and an overall CV is derived from the measured composition.

Some other information regarding natural gas include the following;

Btu—British thermal unit(s)
Ccf
—the volume of 100 cubic feet (cf)
M—one thousand (1,000)
MM—one million (1,000,000)
Mcf—the volume of 1,000 cubic feet
MMBtu—1,000,000 British thermal units
Therm—One therm equals 100,000 Btu or 0.10 MMBtu

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