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As the emphasis on decarbonisation and minimising greenhouse gas (GHG) emissions increases in many countries and markets, LNG is continuing to maintain and grow its share of the world’s changing energy portfolio due to its lower intrinsic carbon content than oil and coal. Green energy sources like wind, solar, hydro, and nuclear present an opportunity for the LNG industry to push decarbonisation further throughout the LNG value chain.
The LNG value chain consists of three main sections:
- Upstream – exploration, production, and processing of natural gas.
- Midstream – liquefaction and transport by LNG carriers and bunkering vessels.
- Downstream – storage, regasification, distribution, and end use.
GHG emissions occur in all three sections of the value chain. Upstream emissions occur due to leakage, flaring, and the generation and utilisation of energy for pipeline compression. Midstream processes contribute to emissions from liquefaction processes and LNG transportation. The downstream section of the value chain is responsible for the most GHG emissions, as carbon dioxide (CO2) and other greenhouse gases are released when the LNG is regasified and combusted. To carry out decarbonisation of the LNG supply chain, a detailed understanding of the emissions produced by each stage is required to select the proper strategy. As a leading liquefaction technology licensor and equipment supplier, Air Products is developing solutions for decarbonisation of the liquefaction process.
Prior to liquefaction, natural gas requires pre-treatment to remove impurities such as mercury, CO2, sulfur compounds, water, and heavy hydrocarbons. The high-pressure natural gas is cooled by heat exchange with one or more refrigerants to approximately -150°C before it is reduced in pressure to remove nitrogen and helium and generate methane flash gas for fuel. It is then stored at atmospheric pressure for shipment. During LNG production, greenhouse gas emissions are produced from the following sources:
- Venting of CO2 removed from the natural gas feedstock during pretreatment.
- Combustion of fuel to generate power to drive refrigerant compressors in the liquefaction process.
- Combustion of fuel to provide ancillary power and process heat for the facility.
- Flaring of natural gas during plant operation.
- Fugitive methane emissions.
Reducing the GHG emissions of LNG production requires consideration of these sources to reduce CO2 and hydrocarbon emissions to the atmosphere. While the ultimate goal of decarbonisation is to achieve zero carbon, there is value in partial decarbonisation. Some strategies are lower in installed cost and may provide positive financial returns to the liquefaction project, while others are higher cost and may not be adopted without financial incentives or regulation that bring the reduction of carbon emissions within the project scope.
Pretreatment decarbonisation
Natural gas may contain from 1 – 10% or more CO2, and this must be reduced to about 50 ppm prior to liquefaction to prevent freeze-out of the CO2 and subsequent blockage of equipment in the cryogenic liquefaction process. For a natural gas feed with 6% CO2 in the natural gas, the CO2 content is about 0.14 t CO2e/t of LNG. CO2 is removed from the natural gas in an acid gas removal unit (AGRU) using an adsorbent or solvent and may be recovered for commercial use or vented to the atmosphere. Recovery of CO2 for commercial purposes such as the manufacture of urea fertilizer, production of dry ice, or carbonation of beverages results in only temporary prevention of carbon emissions. Capture of the CO2 followed by underground sequestration is being used to permanently reduce some of these emissions.1
Energy efficient liquefaction technology
The clean natural gas from the pretreatment system is cooled in the liquefaction unit by heat exchange with a circulating refrigerant. A discussion of the many refrigeration process cycles that are available is beyond the scope of this article, and references are included below.2,3,4 The liquefaction phase incorporates refrigerant compressors which consume a large amount of power, on the order of 275 KWh – 375 kWh/t of LNG produced, depending on the process cycle selected and project details. The compressors are typically driven by gas turbines, with fuel supplied by the methane flash gas generated in the LNG pressure reduction step prior to storage. With simple cycle gas turbine drivers (about 35% thermal efficiency) the corresponding CO2 emissions are about 0.15 t – 0.21 t CO2e/t of LNG.
A straightforward way to reduce carbon emissions is to select a process cycle with high efficiency to reduce the power requirement and consequent fuel consumption. High process efficiency also provides a financial benefit: reduced auto-consumption of feedstock for fuel and/or reduced power import costs. Guidelines for choosing a higher efficiency process include consideration of:
- Vapour compression refrigeration cycles, such as mixed refrigerant processes and pure component cascade processes, use liquid refrigerants and generally have higher efficiency than gas expansion cycles with vapour refrigerants, due to more favourable thermodynamics and lower refrigerant circulation rates.
- Mixed refrigerant cycles such as single mixed refrigerant (SMR) can provide higher process efficiency than pure component cycles due to smaller temperature differences between refrigerant and natural gas.
- Processes with separate precooling and liquefaction refrigerants such as propane precooled mixed refrigerant (C3MR) and dual mixed refrigerant (DMR) generally have higher process efficiency than single refrigerant processes such as SMR due to additional flexibility in optimising the process to meet specific cooling requirements.
A C3MR or DMR process typically has a 5 – 15% efficiency advantage over SMR or pure component cascade, and therefore 5 – 15% lower emissions. A similar production benefit provides a significant financial advantage to the owner as well.
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Read the article online at: https://www.lngindustry.com/special-reports/12072024/lng-decarbonisation-strategies-for-a-cleaner-future/
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