As natural gas production continues to surge in the U.S., so has the demand on the nation’s pipelines and liquefied natural gas (LNG) facilities. There are currently four main markets for LNG in the U.S. — LNG export, peak shaving, industrial fuel and transportation fuel — and each end use has product specifications that must be met.
Impurities such as carbon dioxide, heavy hydrocarbons and nitrogen can exist in the natural gas feeding into liquefaction facilities, and those impurities need to be removed or otherwise managed properly.
To deliver an effective final product, the liquefaction facility designer must consider the specifications for the intended end use, identify potential impurities in the feed gas supply and implement a pretreatment approach specific to the application.
Specification considerations — Based on how its LNG will ultimately be used, liquefaction facilities will need to meet required specifications to maintain a consistent product. For example, if the LNG will be used as a fuel source for a combustion turbine, the quality of the fuel when vaporized has to match acceptable heating value levels for the turbine to operate efficiently for power generation. For peak shaving use, the LNG must meet tariff requirements of the receiving pipeline for that region. The gas for the LNG export market has to meet the specifications of the destination market as required by the off-take agreements. And in the transportation LNG market, quality specifications exist based on the end user type, whether trucks, buses or marine applications.
Treatment considerations — The presence of impurities can raise several challenges in the liquefaction process. For example, carbon dioxide in feed gas from a pipeline will freeze in liquefaction systems if not properly removed. In addition, special consideration should be given to how to handle the carbon dioxide that has been removed. If it is returned back to the pipeline, without proper treatment, the resulting natural gas produced may not meet pipeline tariff limits for carbon dioxide. Natural gas product with elevated carbon dioxide levels, if used in natural gas-fueled trucks or buses, could also corrode parts and drastically reduce vehicle efficiency. In fuel source applications, the heating value of the resulting gas must be managed by using pretreatment to monitor heavy hydrocarbon components.
No matter the differences that exist for each end use, the overall process design development remains the same. Leveraging the specification and pretreatment considerations will help effectively design a new LNG liquefaction facility or update an aging facility’s treatment system based on current feed gas compositions that may have changed over time from the original design. For either a greenfield or brownfield site, the process would require sampling of the feed gas that the facility will use, identifying impurity levels and developing a pretreatment system to appropriately remove or reduce them.
Following this process to design an adequate pretreatment system will instill confidence that the LNG produced, no matter the end use, will meet contractual pipeline tariffs or LNG quality specifications.
The treatment possibilities of natural gas to deliver quality LNG are vast and complex. Learn more about the process.