Carbon Dioxide Measurement in Natural Gas0 pages

Application Note

Oil & Gas

Carbon Dioxide (CO2) Measurement in Natural Gas
Application

High CO2 Content

Concentrations of carbon dioxide (CO2) across gas pipelines and
distribution networks is strictly enforced and kept at low levels. If
CO2 concentrations are higher than desired, removal techniques
such as amine washers, polymeric membranes, or pressure
swing absorptions (PSAs) are installed. To monitor CO2 levels,
the Rosemount Analytical X-STREAM flameproof analyzer can be
equipped to provide continuous analysis of CO2 for natural gas
applications.

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.

Low CO2 Content

For ppm CO2 ranges, calibration with mixed background gases or in
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.

Background
Natural gas mainly consist of methane (CH4 70–90 %). Other
components are ethane (C2H6), propane (C3H8), butane (C4H10) and
higher hydrocarbons (up to 20 %). Carbon dioxide (CO2) can go
from several ppm up to 8 %. Further components are nitrogen (N2),
hydrogen sulfide (H2S) and noble gases as helium (He), argon (Ar),
and neon (Ne).

Solution for ppm CO2 Measurement in
Natural Gas
To overcome the problem of varying background in natural gas,
a special gas analyzer setup with a CO2 absorbing agent can be used.

Pipeline transmission legislation sets limits for the CO2 con­ ent of
t
natural gas. Therefore, CO2 has to be measured and, if necessary,
reduced by technical processes.
CO2 measurement in natural gas can be done with complex gas
chromatographs (GC), as well as with optical non-disper­ ive infrared
s
(NDIR) measurement, which allows a continuous measurement.

Gas Analyzer Application
Methane, ethane, propane and other hydrocarbons in natural gas are
influencing the CO2 measurement. This cross interfer­ nce is low for
e
methane (approx. 1:700), but larger for ethane (approx. 1:100) and
higher hydrocarbons.
Natural Gas Composition

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 measure­ ent side of the analyzer
m
cell.
With this configuration, variations in the background of natu­
ral gas will affect both sides of the analysis cell and will there­fore
be cancelled out. The external sample handling system is shown
in Figure 1. The left vessel contains the CO2 absorbing material,
whereas the right vessel is a blank vessel filled with glass beads to
equalize the flow.

Interferences on the CO2 Measurement

C1

90 %

20 ppm

C2

4%

10 ppm

C3– C5

4%

15 ppm

N2

2%

No interference

H2O

0.02 %

No interference

CO2

10 ppm

No interference

Figure 1 - External Sample Handling System

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