How novel optical moisture detection can reduce risk and save costs in natural gas storage?

Recent steep rises in European gas prices have been driven by increased demand together with reduced levels of supply. Despite of the push towards renewables, natural gas remains an attractive fuel for industrial and power generating applications because it is relatively efficient and burns cleaner than both coal and petroleum products. In addition, governments and energy suppliers worldwide are looking to hydrogen injection into the natural gas transmission systems to reduce both fossil fuel consumption and emissions.

While gas production is relatively predictable, its consumption is marked by seasonal and daily weather variations. Energy companies compensate for variable demand by storing gas and this becomes even more important as normal supplies are reduced. Natural gas may be stored in several ways, but it is most commonly held underground at pressure, ideally close to where it is consumed. Energy companies reuse depleted hydrocarbon fields (oil and gas), aquifers and salt caverns to store natural gas inventory, which enables them to meet peak loads more easily.

Each method of storage has specific physical characteristics including porosity, permeability, and retention capability. The store’s capacity and deliverability rate – the speed at which gas can be withdrawn – are particularly important because they impact the economics of the storage facility.

Why are moisture measurements in natural gas important?

One of the consequences of storing gas underground is that inevitably, it becomes impure. Pumping gas into what is essentially a ‘wet hole in the ground’ will increase its moisture content. It is well known that strict control of moisture concentration is essential for safe and efficient operation of the transmission network. To that end, energy companies require a fast, accurate and reliable means of measuring moisture which is made when gas is exported from storage. This is to judge if dehydration processing is required to fulfil the tariff limit/contractual specification for moisture content required by the transmission pipeline operator receiving the gas.

Energy companies measure moisture in natural gas using a range of techniques, each of which has its advantages and drawbacks in terms of accuracy, speed and cost of measurement. Technologies available for moisture measurement include impedance and capacitive sensors, chilled mirror, quartz crystal micro-balance and tunable diode laser spectroscopy.

What are the potential issues with moisture measurements in natural gas?

If the analysis is inaccurate, there are two possible outcomes. The consequences depend on whether the analysis returns an over- or under-estimate of the true moisture content.

Over-reading

An analysis that reports higher-than-actual moisture content in the gas is known as ‘over-reading’. Over-readings increase costs and add delays to the release of the gas because the operator performs more moisture removal processing than is necessary. These incremental processing costs can quickly become significant as the gas volumes increase.

Under-reading

Under-reading is reporting lower moisture levels than reality. This incurs the risk that the operator will not remove sufficient moisture from the gas. This could lead to hydrate formation in the downstream transmission pipeline with the potential for pipeline blockage and compressor damage. In the worst case, breaching contractual specifications or tariff limits can lead to shut-off of the transmission pipeline incurring commercial losses and fines, increased risk of pipeline corrosion and ultimately catastrophic pressure failure.

What are the recommended measurement techniques for measuring moisture in stored natural gas?

Tunable diode laser absorption spectrometers (TDLAS) are especially suited to measuring moisture content in natural gas. The Michell OptiPEAK TDL600 is a next-generation analyzer that automates online measurement of moisture in variable compositions of natural gas and biomethane. It offers class-leading accuracy, with an operating range down to 1ppmV and a fast response time. Its low maintenance, simple installation and setup and built-in self-verification ensures its low overall cost of ownership.

Michell OptiPEAK TDL600 moisture in natural gas analyzer

Achieving reliable moisture measurements in changing natural gas compositions

TDLAS analyzers suffer from significant interference when interacting with Methane, Ethane, Carbon Dioxide and Hydrogen Sulfide: all of which are present in natural gas. Our earlier article, ‘What is Hydrocarbon Dew Point and how is it measured?’ has a detailed analysis of the composition of natural gas.

The components of natural gas – discover more

The level of interference governs detection limits and accuracy, but these errors can be mitigated by calibrating the analyzer for the gas composition in use. However, real-world natural gas composition varies dramatically, which results in errors outside of manufacturers’ performance claims. A specified TDLAS performance limit of ±4ppm suggests a confidence band of approximately 2 °C dew point. Realistically, this variation could be as high as 20ppmV and could occur at any point of measurement where the composition of natural gas is changing. A typical example of this would be transmission gas pipelines which are supplied from multiple gas wells, fed with regassified LPG or downstream of non-conventional fuel gas injection, such as biomethane.

The effect of this is dramatic since the additional error increases the confidence band to around 14 °C dew point.

The Michell OptiPEAK TDL600 offers class-leading accuracy of ±1ppmV over real-world gas compositional ranges. This level of accuracy guards against both unnecessary process costs from over-drying and shut-off due to overly pessimistic measurements. We estimate that improving the accuracy to this level has the potential to deliver efficiency savings of up to 20% when processing real-world gas.

Non-contact measurement technique protects from contamination from glycol in natural gas

The most common and cost-effective way to remove water from natural gas is to use glycol as a liquid desiccant within a glycol absorber or contactor process. Although this approach is well established, it does carry some risks. One potential issue is that excessive gas velocities can force glycol out of the top of the column along with the dry natural gas stream. Glycol has a high-dielectric constant, so any downstream moisture sensor that uses metal-oxide capacitive/impedance sensor technology will detect the glycol and return inaccurate or full-scale wet readings. The use of TDLAS technology, which is a non-contact measurement, is immune to the effects of glycol contamination.

Ready for hydrogen and natural gas mixtures

In addition to its immunity to contamination, the Michell OptiPEAK TDL600 uses advanced spectroscopy algorithms to automatically compensate for variation in background gas composition. This is becoming especially useful given various government environmental initiatives across Europe require 20% of EU energy from renewables. Biomethane injection into natural gas distribution networks is also commonplace in Europe and North America. Hydrogen injection is a further phase in reducing the proportion of fossil fuel within natural gas supplies. Progress towards up to 20% H2 injection, produced via electrolysis of water powered from excess solar and wind generated electricity, is envisaged within networks across EU countries.

It has been proven that these increased Hydrogen levels have no effects on the measurement accuracy of the Michell TDLAS analyzer. This is supported within independent testing conducted by DBI Gas- und Umwelttechnik in Leipzig where 10 mol% H2 added into natural gas showed no adverse effect on the accuracy and overall measurement performance of the TDL600.

As this sector continues to change by extracting natural gas from diverse sources like shale, and injection of non-conventional fuel gas such as biomethane and hydrogen becomes more commonplace, operators need faster, more accurate and robust measurement technologies.

Existing TDLAS analyzers appear to offer a solution but often fail to deliver in the real-world as, in situations where gas composition changes, background interference can result in significant errors.

The Michell OptiPEAK TDL600 is capable of quantifying moisture levels in natural gas with single figure ppmV precision, even in the presence of acidic and sour components such as Carbon Dioxide and Hydrogen Sulfide and with varying levels of Hydrogen. This level of accuracy unlocks potential efficiency savings of up to 20% when applied to the processing and removal of moisture from natural gas.

Want to read more about natural gas storage? The value of gas storage – Questions and Answers by Gas Infrastructure Europe is an excellent place to start.

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