Natural gas mainly consists of methane (CH4 70–90 %). There are a number of other components (ethane, propane, butane, nitrogen, hydrogen sulfide and others) including carbon dioxide (CO2) which may be present in concentrations from several ppm up to 8%. Pipeline transmission legislation sets limits for the CO2 content of natural gas. Therefore, CO2 has to be measured and, if necessary, reduced by technical processes such as amine washers, polymeric membranes, or pressure swing absorbers (PSAs).
Measuring the CO2 content of natural gas can be accomplished by various means including gas chromatography and optical non-dispersive infrared measurement. Often a simpler and more cost-effective solution is the use of a process gas analyzer setup.
Methane, ethane, propane and other hydrocarbons in natural gas influence the CO2 measurement. This cross interference is low for methane (approx. 1:700), but larger for ethane (approx. 1:100) and higher hydrocarbons. If CO2 has to be measured in the percent range, the error from cross interferences by the background gases is negligible. Calibration can be performed with the mean background concentration of the natural gas components. Even calibration in nitrogen might be possible.
For ppm CO2 ranges, calibration with mixed background gases or a nitrogen background is not possible. Varying gas composition in natural gas would lead to varying errors in CO2 measurement, which are too high for ppm measurement ranges. To overcome the problem of varying background in natural gas, a special gas analyzer setup with a CO2 absorbing agent can be used. In an external sample handling system the sample gas stream is split and the CO2 in one stream is removed with a scrubber. This stream is flowing through the reference side. The other stream which still contains the CO2 is applied to the measurement side of the analyzer cell. With this configuration, variations in the background of natural gas will affect both sides of the analysis cell and will therefore be cancelled out. In the external sample handling system, the left vessel contains the CO2 absorbing material, whereas the right vessel is a blank vessel filled with glass beads to equalize the flow.
Click to read the Application Note by Emerson.