According to the McKinsey Global Gas and LNG Outlook to 2050 report, demand for LNG is expected to grow 3.4 percent annually to 2035 — with approximately 100 million metric tons of additional capacity needed to meet demand growth as well as decline from existing projects1.
The liquefaction process introduces a large contraction in volume to the product, making it much more economical to store and transport over extended distances from the source. This helps to establish new customers and underscores the LNG market's strength.
Raw natural gas (the feed gas) is extracted from the reservoir in its wet state. Once water, CO2, H2S and Hg are removed, it typically contains 85 – 95 percent methane. It also contains heavier hydrocarbons, such as ethane, propane and butane. Conditioning of the natural gas to remove some or all of the non-methane components is a common requirement of LNG production. These components are then often sold separately as natural gas liquids (NGLs) or natural gas condensate. Methane needs to be cooled to -259.6°F/-162°C to liquefy it, a temperature considered as cryogenic. Storage and transport of the LNG then needs to ensure this cryogenic state remains.
For LNG production, compressors are used to compress refrigerants, not to compress the actual unprocessed natural gas. These cooled, liquid refrigerants are then subsequently used as the cooling medium within heat exchangers, which cool the natural gas and convert it into LNG.
The loss of a compressor that may ultimately lead to an operational shutdown (planned or otherwise) could cost millions of dollars per day in lost production, so the reliability of operations for extended intervals is essential.
A compressor is a rotating piece of machinery and requires something to mechanically drive (rotate) it. There are four basic options for this: a reciprocating engine, a steam turbine, a gas turbine or an electric motor. The most common options in use today are the gas turbine and the electric motor.
Proven LNG-applied performance, cost and wide range of available sizes make the gas turbine the most popular choice by far for LNG refrigerant compressor drivers.
There are two dominant options in today’s marketplace for the refrigeration cycle process for large capacity LNG trains: Air Products’ propane precooled mixed refrigerant process, AP-C3MR™, and the ConocoPhillips Optimized Cascade® process (a number of variations on these two processes have been developed and implemented in recent years). When a process has been selected, the number of refrigerant compressors and mechanical drivers can be determined.
Typical gas turbines used may be heavy duty frame engines (currently the frame 7E is a popular choice) that generate approximately 90 MW (equivalent), or a higher number of smaller aeroderivative units, which tend to generate approximately 30 – 35 MW (equivalent), such as the LM2500+. Whichever architecture is selected, keeping the plant running for extended periods is always the top priority.
Frame engines have bigger output, are heavier and more robust. Economy of scale also provides for a lower dollar per kW ratio with frame engines.
Aeroderivative engines are smaller but with higher efficiency and easier maintenance/changeouts of components. They are less robust and typically require more frequent maintenance. They do, however, offer significant operational flexibility by allowing some gas turbines to be taken offline for periods of time (with reduced LNG output), without having to take the entire liquefaction train down and flare all gas being processed. Frame engine driven refrigerant compressor trains do not typically have this capability – when the gas turbine goes down, the entire train goes down. The size and weight of the aeroderivative engine also makes it more suited for FLNG applications.
In an LNG train, a single turbine shutdown could result in the process being taken offline with lengthy shutdown and startup procedures leading to huge losses. To maximize uptime and ensure continued, consistent performance of the gas turbines a carefully designed air intake filtration system is required. Typically, gas turbine compressor issues relating to the ingestion of ambient air particulate, salts and hydrocarbons account for 60 – 80 percent of overall gas turbine losses2. Reducing or eliminating this contamination correctly is key to reliable plant operations and maximizing LNG output.
Gas turbines are subjected to a wide variety of contaminants, which if not addressed, can cause corrosion, erosion and fouling, leading to reduced performance, or even complete and catastrophic failure of internal components. A filtration system should be designed based on specific installation and environmental conditions on site, as well as the operational needs of the end-user.
For LNG compressor drivers, this typically means extended uptime with very high efficiency (EPA+ levels of filtration), minimizing compressor fouling. LNG plants are typically situated in coastal (salt laden) environments, so wet and dry salt mitigation and the use of hydrophobic filtration are also key requirements.
A filtration system is needed to protect the gas turbine from multiple contaminants present in the air — contaminants that will vary significantly day-to-day and season-to season. For example, in a dusty coastal location, there will be high levels of both dust and salt. If an area is prone to high humidity or fog events, moisture will periodically also be a crucial factor. To handle different challenges, multiple stages of filtration are required.
In high dust areas, as levels of captured contaminant build up, the differential pressure across the filter will increase sharply. In these areas, self-cleaning filters, which use reverse pulses of compressed air to periodically remove layers of captured dust, are often selected.
To handle moisture, coalescing filters are often added upstream of the main filters to agglomerate droplets and help stop the captured dust becoming sticky/muddy (with a resultant pressure drop increase). If there are very high levels of dust, however, there is also a risk that the coalescer itself becomes blocked and is either forced out of place, or the large pressure spike causes the unit to alarm and shut down.
The TS1000 is a technology from Parker which uses a mesh designed to deliberately allow dust and sand to pass through while coalescing free moisture. This leaves the final filters to handle relatively dry dust and sand; something they are perfectly well designed to address. Treated Microfibre glass is the preferred choice of filtration media for very high efficiency final stage filters, providing high-level dry particulate removal and hydrophobic (wet salt removal) performance. Microglass is approximately 10 times thicker than some other high efficiency medias on the market. This ultimately means it is more resistant to blockages, and any pressure increases occur slowly and predictably over long periods of time, avoiding the infamous ‘hockey stick’ effect, when pressure drop (Delta P) increases rapidly with very little warning.
Having multiple filtration stages for LNG intakes also provides the ability to change filters without the need to shut down the turbine, commonly a key requirement. This requires careful design to ensure the stages offer the right incremental levels of filtration to avoid frequent blockage, and to ensure contaminants do not affect turbine performance. As changeout of the prefilter stages, which are designed to capture larger particulate and coalesce moisture, can occur without shutting down the turbine, the air intake system designer needs to consider and design for ease of online changeout from the outset.
For the very high efficiency final filtration stages, filters should be selected that offer extended service life. Using an extended 24 inch deep filter, rather than a traditional 12 – 17 inch filter, is one option which increases the surface area for particulate collection and, therefore, requires less frequent changeout. In general, prefilters should require changing no more than around once per year; second stage filters once every two years; and third or fourth stage filters (if present) approximately once every four years. If a filtration system requires more regular maintenance than this, a review of its design is recommended.
The demand for LNG is set to continue to rise and the reliability of the systems used to liquefy the gas is critical. Any unscheduled shutdown can lead to significant costs and a huge revenue hit from lost production. Although the gas turbine filtration system may appear a small concern for the overall LNG train performance, having the right filters in place will have an enormous impact on the short and long-term performance of the plant.
Read more about the importance of good filtration systems to overall LNG production train performance. Download the Whitepaper: Why LNG Needs the Right Filters.
This post was contributed by Pete McGuigan, Parker Hannifin Ltd, UK.
This blog was adapted from an article originally published in a recent issue of LNG Industry.
2. GEA18414 – offline water wash optimisation for F-class gas turbine pulsed wash systems.