Best practices in decarbonisation for LNG export facilities
Reducing emissions from LNG plants can be technically feasible and economically viable as well as environmentally desirable.
Peter Zhang, Gulf LNG Solutions
Saeid Mokhatab, LNG Consultant
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Recently, all business organisations have been under greater pressure from both the public and investors to manage their operations associated with reducing carbon footprints. As the US liquefied natural gas (LNG) market continues to grow due to globally increased demand for cleaner energy, reducing greenhouse gas (GHG) emissions from each stage of the LNG value chain has become particularly important to LNG project developers and facility owners.
Carbon dioxide (CO₂) emissions associated with the liquefaction process account for approximately 6 to 10% of overall GHG emissions of the entire LNG value chain. Decarbonising current LNG export facilities is a difficult challenge for facility owners. Not only do they need to justify the increased cost associated with deploying various decarbonisation options, but they also must find those options that have demonstrated unmitigated success and can meet owners’ unique decarbonisation goals.
This article reviews some of the best practices in addressing decarbonisation for LNG export facilities from an engineering design point of view. Not all LNG projects are designed equally and with the same level of emissions. As a result, the authors believe there are opportunities for LNG facility owners to address the unique decarbonisation issues associated with their existing facilities, and for LNG project developers to do it right in the first place during the development of new projects.
Process overview of LNG export facilities
An LNG export facility (see Figure 1) is typically comprised of natural gas treating facilities, one or more liquefaction trains, LNG storage tanks, one or more LNG ship-loading facilities, as well as supporting infrastructure and utilities. In the natural gas liquefaction plant, LNG is produced from a cryogenic liquefaction process where natural gas (mainly methane) is cooled down to approximately -160oC (-256oF) at 103 kPa (15 psi) to liquefy it for easy storage and transportation. The LNG production process flow is as follows: natural gas from raw gas transmission pipelines, typically at relatively high pressure, is fed to the LNG plant (see Figure 2), where it first goes through a series of processing steps to remove the undesirable components. These include heavy hydrocarbon liquids (condensate), free water, acid gases (carbon dioxide and hydrogen sulphide), water vapour, mercaptans, mercury, and other hydrocarbons heavier than methane called natural gas liquids (NGLs) contained in the natural gas feed stream, to prevent freezing issues in the cryogenic process, and to meet final LNG product specifications. Nitrogen, a potential natural gas contaminant, will be removed in the later cryogenic process through fractionation or fuel gas purge. After the NGLs are removed, the residual natural gas stream gets liquefied (using an external refrigeration system) and then sent to the end-flash nitrogen removal unit to meet the required specification to improve its calorific value and to avoid storage problems. The flash gas stream from nitrogen removal unit can be used as fuel gas. However, to meet fuel gas requirements, excess nitrogen needs to be rejected from this flash gas stream.
Emission sources in LNG export facilities
To understand the decarbonisation options in an LNG export facility, one must first locate the main GHG emission sources. There are primarily two major GHG emission sources in an LNG plant facility. One is CO₂ and methane contained in the feed gas, and the other is the flue gas produced from fuel gas combustion devices, such as gas turbines, thermal oxidiser, and other process fired heaters.
CO₂ in the feed gas is typically removed by amine absorption during feed gas treatment in the CO₂ absorber in the acid gas removal unit (AGRU). The off-gas from the amine regeneration column, which typically contains CO₂ and a small amount of hydrogen sulphide (H₂S) and other light hydrocarbons, is sent to a thermal oxidiser to destroy the hydrocarbons and other hazardous components by combustion or oxidation. The thermal oxidiser is often operated with some augmented fuel gas due to the low heat value of CO₂ off-gas from the AGRU. The flue gas from the oxidiser mainly contains nitrogen, unreacted oxygen, CO₂, water vapour, nitrogen oxides (NOx), sulphur oxides (SOx), and tiny amounts of uncombusted hydrocarbons depending on the combustion efficiency of the burner. These components can be vented to the atmosphere (to the extent allowed by local regulations) at a safe location. Alternatively, a carbon capture unit for CO₂ recovery can be added if needed.
The amount of flue gas produced from a gas turbine is proportional to its fuel gas consumption, which in turn is a function of the duty required (or power output), fuel gas composition, and thermal efficiency.
In addition, there are other emission sources in an LNG export facility. They can be intermittent or continuous and together can sometimes make significant contributions to the overall annual emissions of an LNG facility. Those emission sources include fugitive leaks from process equipment/turbomachines, piping and valves, vents from pressure control, compressor seals, emergency relief (flaring), and venting during commissioning, start-up, maintenance, and shutdowns. They may also include fuel gas purge for nitrogen removal (usually flared), nitrogen-rich venting from the nitrogen rejection unit, boil-off gas venting when boil-off gas compressor is down (usually flared), and ballasting vents during LNG storing and ship loading/unloading (also normally flared). Methane is a far more potent GHG than CO₂, so flaring those emissions (rather than venting them) can reduce their impact by a factor of 35 or more.
Means to decarbonise LNG export facilities
While decarbonising an LNG export facility is a challenge, various efforts have been taken in the LNG sector to reduce emissions, which basically can be divided into the three following categories:
Elimination This category represents the choice of energy forms that power an LNG facility which can eliminate GHG emissions. Due to a proportional relationship between fossil energy or fossil fuel usage and GHG emissions, using a non-fossil fuel energy source will be the ultimate solution to achieve zero emissions. This includes using electrical motors based on electricity generated from renewable energy sources for refrigerant compressors. While partial substitution of fossil fuel by non-fossil fuel is possible for an LNG facility, with current technologies and energy mix, completely substituting fossil fuel is unlikely. It is particularly challenging for those existing LNG facilities that have already installed gas turbines for their refrigerant compressors.
Mitigation On the basis of utilising fossil fuel energy, decarbonisation means reducing emissions from fuel gas combustion or methane leaks from piping and instrument leaks. This category includes improving energy efficiency through process design and equipment modifications. This includes utilising more energy-efficient equipment and advanced technologies associated with minimising emissions. It should be noted that, even if all of these mitigation methods are applied, not all GHG emissions associated with operating the facility may be eliminated.
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