Decarbonisation Technology August 2022 issue

eliminate the use of dedicated fired heaters, which means no fuel gas consumption and no emissions for these necessary utilities. Energy recovery In several areas of an LNG plant, high-pressure fluids are sometimes let down to lower pressure for process reasons. For instance, in the feed gas circuit, the high pressure of feed gas is often let down to a lower pressure as required for NGL recovery. And in the rich amine circuit, the high-pressure, rich amine stream exiting from the bottom of a high- pressure amine absorber is typically let down to a lower pressure for amine flashing. In those instances, pressure letdowns are commonly realised by throttling valves. An alternative is to use turbine/expanders and turbochargers, respectively, to recover some of the lost energy. Note that the cost of equipment and maintenance of said equipment is not trivial and must be considered in the overall analysis.  Mitigation of emissions In addition to improvement in energy efficiency, there are other areas where decarbonisation opportunities exist in a liquefaction export facility. Many have been successfully practised in operating LNG facilities and are accepted as standard features, designs, or procedures. The following is a short list: Fugitive emissions prevention Prevention of hydrocarbon leaks from static equipment and turbomachines, piping connections and valves, plus associated safety and environmental regulation compliance are ongoing tasks of LNG facilities during design and operations. A good fugitive emission management programme should address quantifying and eliminating fugitive emissions in the first place, i.e., during the design phase. This includes compressor type selection (including seal type) and valve selection (i.e., low [external] leakage valves versus high leakage valves). Abnormal venting and emergency relief Emissions through venting/flaring during emergency shutdowns can sometimes be enormous. Implementing predictive maintenance programmes and improving equipment reliability can help prevent

unplanned shutdowns and thus reduce lifetime emissions from the facility. Overpressure protection Overpressure protection through relief and depressurisation systems in LNG process facilities is the standard means of protecting people, an owner’s assets, and the environment. However, in reviewing emission- mitigating methods for overpressure protection, atmospheric release (usually via flaring) and the associated negative environmental impact, as well as the potential lost revenue due to relief valve opening, must be addressed. It is common practice to consider solutions such as inherent safer design through high-integrated pressure protection systems (HIPPS) that can address both potentially hazardous emissions and the costly release while still providing overpressure protection. Commissioning and start-up Venting during process commissioning and start-up is unavoidable. However, emissions can be mitigated through proper planning and reviewing of commissioning and start-up procedures. For instance, utilising nitrogen for initial dry-out of mercury removal beds (placed downstream of the dehydration vessels) may have advantages over using fuel gas in minimising CO₂ emissions. Similarly, during AGRU commissioning, or turndown operation at start-up, the over- circulating lean amine may be unnecessary as it will increase the amount of dissolved hydrocarbons in the rich amine and potentially increase emissions, depending on the disposition of the flash gas.  Carbon capture and storage Chevron Australia’s Gorgon LNG facility installed the world’s largest CCS system to capture carbon emissions. After treatment, CO₂ captured from the AGRU in the feed gas pretreatment section of the LNG plant is injected into a giant sandstone formation about 2000 meters deep from the surface, where it remains permanently trapped.

Peter Zhang Saeid Mokhatab


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