Precise measurement of moisture in natural gas

Despite the push towards renewables, natural gas is still an urgently needed fuel for industrial and power-generating applications and private supply for years to come. To reduce the share of fossil fuels, the injection of both hydrogen and biomethane into the distribution grids is currently being ramped up further. To compensate for fluctuating demand, energy companies store natural gas in depleted oil and gas fields or salt caverns, among other places. However, this saturates it with moisture and it has to be dried before it is suitable to be fed into the grids.

Moisture content in natural gas

Excessively high moisture content in the natural gas, when fed into downstream transport pipelines, promotes hydrate formation, which can lead to clogging of the pipeline and damage to the compressors. In addition, the contractual specification for the maximum moisture content, imposed by the transmission system operator, must be met. Accurate control of moisture concentration through precise measurement is, thus, essential for safe and efficient operation.  

If, for example, measurement shows an inaccurate value that is too high, this could mean unnecessary dehydration processes are carried out or, worst still, a shut-in. An underestimation of the measurement, on the other hand, leads to insufficient dehydration, with the aforementioned consequences for the pipeline and compressors. A number of technologies are available to energy companies for measuring moisture. These include impedance and capacitive sensors, chilled mirrors, quartz crystal microbalance analyzers (QCM), and tunable diode laser absorption spectrometers (TDLAS) – each with advantages and disadvantages in terms of accuracy, speed, and cost.  

Contamination through glycol

The most common and cost-effective method of removing water from natural gas is to use glycol as a liquid desiccant in a glycol absorber or contactor process. This approach is well established, but also carries risks, as glycol can be forced out of the upper part of the column together with the dry natural-gas stream, due to excessively high gas velocities. Glycol, however, has a high dielectric constant, so it will be detected by any downstream moisture sensor with capacitive metal oxide/impedance sensors, causing them to deliver inaccurate readings. 

In comparison, tunable diode laser absorption spectrometers (TDLAS) work independently of glycol contamination. With this non-contact measurement method, the concentration or density of the gas or gas components to be examined is determined from a measured absorption. Existing TDLAS analyzers, therefore, seem at first glance to offer a good solution for determining the moisture content. However, natural gas is a gas mixture of a wide range of hydrocarbons – from light, short-chain aliphatics (non-aromatic compounds) to heavy, long-chain molecules. These very different compositions of natural gas can not only distort measurements but also affect detection limits and accuracy.

Usually, an attempt is made to cushion the effects of these interactions by calibrating the analyzer for the gas composition used. However, this does not always work as desired with the often strongly fluctuating components of the natural gas, and thus leads to errors that lie outside the performance specifications of the manufacturers. For example, a specified TDLAS performance limit of ±4 ppm suggests a so-called confidence band of about 2 °C dew point. Realistically, this deviation could be as much as 20 ppmV and could occur at any measurement point where the composition of the natural gas changes. Typical examples are long-distance gas pipelines fed by several gas sources, and possibly also downstream of a fuel gas injection such as with biomethane. The overall impact is enormous, as the additional error increases the confidence band to about 14 °C dew point.

Measure humidity with an accuracy of 1ppmv

A solution to the problem is promised by a next-generation analyzer that, in addition to being impervious to impurities, automatically compensates for variations in gas composition through special algorithms in spectroscopy. Michell's OptiPEAK TDL600 is capable of determining moisture content in natural gas with an accuracy of 1ppmV (<100 ppmV ) – even in the presence of acidic components such as carbon dioxide and hydrogen sulfide, as well as varying hydrogen levels. The non-contact technology requires minimal maintenance, even in demanding applications such as changing methane concentrations or sour gas. The analyzer is also fully certified for hazardous areas and offers class-leading measurement performance, stability, and detection sensitivity.

The high accuracy is particularly relevant because not only is the feed-in of biomethane increasing, hydrogen feed-in is also being ramped up to reduce the share of fossil fuels in the natural-gas supply. In the transmission grids of EU countries, increases in H2 feed-in of up to 20 % are planned, to be produced by electrolysis of water from surplus solar and wind power. Increased hydrogen levels have been shown to have no effect on the measurement accuracy of Michell's OptiPEAK TDL600 This is confirmed by independent tests conducted by DBI Gas- und Umwelttechnik in Leipzig, Germany. Even the addition of 10 mol % H2 to natural gas showed no negative effect on the accuracy and overall measurement performance of the analyzer. 

Conclusion

The Michell's OptiPEAK TDL600 enables the moisture content in real natural-gas compositions to be determined with an accuracy of ±1ppm_V – much more precise than comparable TDLAS analyzers. This protects against both unnecessary process costs due to over-drying and shutdowns due to overly pessimistic measurements.  

Author: Rolf Kolass