Jennifer Adams, Siemens, USA, introduces the concept of all-electric LNG, which can provide greater control of LNG-plants, as well as lower OPEX and lower emissions.
Today, most LNG plants around the world (and those currently under development) feature a liquefaction island with refrigeration compressors driven by directly coupled industrial gas turbines. In recent years, however, as train sizes have fluctuated and operators have come under increased pressure to reduce facility lifecycle costs and lower greenhouse gas (GHG) emissions, the concept of all-electric LNG (E-LNG) has garnered significant interest.
E-LNG opens new possibilities for controlling the liquefaction process, while at the same time making plant operation cheaper, cleaner, and more reliable. Nevertheless, executing the concept and achieving a lower plant emissions profile is not a matter of simply replacing gas turbines with electric drives. It requires an integrated approach in which there is collaboration between the LNG and power generation disciplines during project development. Historically, the two have operated in a siloed fashion. A comprehensive digital strategy is also needed to manage the long-term performance of facilities once they are in operation.
It is only through the adoption of a holistic project approach that stakeholders can optimise plant performance and capture the full range of benefits that E-LNG has to offer.
Electric drives vs gas turbines
Variable speed drives (VSDs) can offer numerous advantages over gas turbines for natural gas liquefaction, largely due to the higher efficiency of electric motors. A single-shaft industrial gas turbine operating in an open-cycle configuration has an efficiency of approximately 25 - 30%. State-of-the-art electric motors, on the other hand, offer efficiencies up to 95%.
The ability of VSDs to maintain high efficiency over a wide operating range allows operators to decouple the capacity of the compression train from the driver. For the development of a gas turbine-driven liquefaction plant, if there is a requirement for production flexibility – for example, to meet the growing demands of the LNG spot market – the gas turbine will likely have to be larger than what is optimal (i.e. oversized), given the baseload of the plant. This is not an issue with VSDs, as they can be rapidly tuned to meet specific LNG production targets. Motors with capacities of 75 MW are already in operation in many industrial facilities across the world. Siemens is confident that even larger VSDs of up to 100 MW will be a reality in the coming years.
Electric motors are also able to provide significant advantages over gas turbines regarding availability and maintenance. For example, a typical gas turbine driving an 8 million tpy compression train has availability of approximately 95%, and, after two years in operation, requires anywhere from 10 days to three weeks for scheduled maintenance. Electric drives, on the other hand, can achieve 99% availability, and it is not uncommon for large motors to go as many as six years without requiring scheduled maintenance.
This is an abridged version of an article that was originally published in the July 2020 issue of LNG Industry. The full version can be read here.
Read the article online at: https://www.lngindustry.com/special-reports/08072020/the-future-is-electric/