Natural gas is sourced from various gas fields and offers a major leap towards a renewable-based energy future. Colourless, odourless and non-toxic, it is the cleanest burning hydrocarbon on the market today, with global demand on the rise. It is used for both heat and power, and the automotive market has also embraced it to keep vehicles on the road.
“The majority of natural gas is transported via onshore or offshore gas pipelines,” says Carmine Canale, business development manager – Gas Analytics at Endress+Hauser South Africa. “For stranded gas reserves where there is little or no local demand, or for long transport distances – more than 1000 km offshore or 3000 km onshore – it becomes more economical to liquefy the natural gas prior to transportation, especially as it ensures safe and easy storage to its various locations. In addition, due to the nearly 600-fold reduction in volume for liquid natural gas (LNG), it is often the preferred method for local storage.
Canale points out that for continuous uninterrupted operation – the gas pre-treatment, dehydration and liquefaction processes – it is essential to ensure on-time loading and shipment of the LNG. In addition, processing feed gas to remove contaminants is critical to the operation of LNG plants.
Raw natural gas from different geological formations contains varying amounts of H2S and CO2). These contaminants must be removed from LNG feed gas to prevent CO2 from freezing at cryogenic processing temperatures and H2S from exceeding gas quality specifications.
Measuring the amount of energy transferred is critical
“Once LNG has been liquefied and stored in tanks, it is ready for transport to other markets, usually via LNG carriers. These custody transfers from the storage tank to the ship, or from the ship to a receiving terminal, often involve millions of Rands of product in a single shipment. As such, it is essential that both the deliverer and recipient know exactly how much energy has been transacted, and an uncertainty of as little as one percent in energy transferred can cost the buyer or seller dearly,” explains Canale.
The compositional measurements of natural gas in gaseous and liquefied states, and of mixed refrigerants, help LNG plants operate efficiently and determine the energy value of LNG.
According to the Group International des Importateurs de Gaz Naturel Liquéfié (GIIGNL), a key factor in properly determining energy transfer is the measurement of gross calorific value (GCV), which is calculated from LNG composition.
For traditional GC-vaporiser systems, it is essential to eliminate partial and pre-vaporisation for all LNG flow rates of the LNG sample, which requires careful installation and proper maintenance to ensure good insulation and no hot spots in the sample vaporisation and transport paths. In addition, the GIIGNL states that LNG must be sampled during stable flow. Data collected during flow variations and interruptions is not used in the calculation of the energy transferred, and can result in a significant amount of LNG being transferred without known GCV.
Laser spectroscopy provides the solution
Laser spectroscopy analysers from SpectraSensors, the leaders in gas analysis and a company of the Endress+Hauser Group, have the technology to perform critical measurements throughout the LNG value chain – from pre-treatment and liquefaction through to custody transfer and regasification – to support on-time shipments.
Its tuneable diode laser absorption spectroscopy analysers (TDLAS) monitor H2O, H2S and CO2 concentrations in natural gas as it undergoes treatment to remove and control these contaminants prior to liquefaction.
Furthermore, SpectraSensors Raman Optograf LNG analysers perform on-line composition measurements of feed gas, LNG as a cryogenic liquid, mixed refrigerant, and gas, following LNG regasification.
“A unique feature of the Optograf LNG analyser is that it measures the composition of LNG in the cryogenic liquid state, resulting in faster update times because it does not require any vaporisation, sample conditioning, or sample transport,” continues Canale. “The Optograf LNG analyser is essentially immune to LNG flow variations, and so can provide a more complete measurement of the GCV of the entire shipload. Maintenance is also significantly lower with the Optograf LNG analyser, since there are no moving parts and no insulation to degrade.”
LNG that has been delivered to an import terminal may however be too rich or too lean to meet the local specifications of the downstream markets to which the gas will be delivered. There are several ways to modify the CV of the natural gas. It can be reduced by adding N2 or CO2, or by selectively removing C3/C4 components from the sample. “The CV can be increased by mixing with liquefied petroleum gas (LPG) or natural gas liquids (NGL). And, in some cases, LNG with different CVs can be blended to achieve target quality,” explains Canale.
Measurement of the CV of the final mixture is essential to confirm that the product quality meets local regulations to avoid significant tariffs.
The Wobbe Index (WI), which is calculated from the gas composition, is the most widely accepted measure of gas quality. The Optograf LNG analyser provides composition measurement of the LNG and WI, and simultaneously provides the concentration of nitrogen, which can be used to ensure that nitrogen levels do not exceed pipeline specifications.
WI modification of natural gas at import terminals is often measured with a Process GC. Prior to analysis, the LNG sample must be vaporised.
Poor repeatability in vaporisation leads to large variations in WI, adding uncertainty to the ballasting process. In addition, vaporisation and sample transport lag time, coupled with typical Process GC update times of four to five minutes, means that this approach provides only sporadic updates during ballasting, increasing the likelihood of overshooting the WI target, and incurring additional cost. Because the Optograf LNG analyser is able to measure the sample in the cryogenic liquid, response times are much faster and repeatability is much higher than with legacy GC/vaporiser systems.
As demand grows, LNG export facilities will need to handle natural gas feeds of varying quality from multiple sources, and so adjust refrigerant composition to ensure the lowest energy consumption during liquefaction. In addition, these plants will need to compensate for refrigerant losses with make-up refrigerant. The technology from SpectraSensors measures both natural gas and make-up refrigerant composition in both the gas and liquid phases, so plant operators can measure the quality of the feed gas and thus adjust their make-up refrigerant for optimum plant operating efficiency.
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