Insight
Some key ongoing developments can reduce the cost of liquefied natural gas (LNG) projects. Offshore liquefaction will enable cost-effective use of remote fields, and economical mid-sized single machine LNG trains could improve project viability onshore. Improved design methods - the increased use of exergy analysis in conceptual design, modular engineering where appropriate and computing techniques such as dynamic simulation - can be used along with appropriate design margins and engineering standards to reduce capital costs.

Uncertainty over the viability of large base-load LNG projects has created interest in economical LNG production in mid-sized plants. Optimising these plants, requires a re-appraisal of accepted process technologies and traditional design practices. A new mixed-refrigerant process is suitable for train capacities of up to 1.5 million metric tons per year (MMtpy) using a single machine/driver and a high-efficiency plate-fin heat exchangers. Power requirement is close to that of a cascade refrigerant cycle. The relatively simple machinery configuration compared to a cascade cycle makes it an attractive option for a wide range of plant sizes. Using multiple trains can give high capacity, with higher reliability than single-train plants.

Currently, there is interest in offshore LNG plants for developing remote gas fields. This article shows that proven refrigeration cycles using expanders are the best choice for offshore developments. Such projects are approaching commercial development, and some of the key design issues are discussed.

Background
For remote gas fields where pipeline transportation is expensive, natural gas must be either liquefied or converted to high-value liquid products. LNG has the advantages that it contains about 40% more heating value than liquid fuels derived from chemical conversion of natural gas and is produced using well-established technology. Although gas-to-liquids conversion technology is attracting attention, existing plants are small and many processes are still in a developmental stage. Liquid products from gas conversion are aimed at different markets than LNG. It is unlikely this technology will have a large impact on LNG demand in the near future.

LNG trade has grown steadily since the mid-1960s. Current annual global demand is about 80-million metric tons. Japan imports approximately two-thirds of this. Although the Asian economic downturn has created uncertainty over the future demand in Japan and South Korea, LNG demand is expected to grow in other countries, notably China and India. The emergence of a new market in LNG for independent power producers (IPPs) has helped to justify new projects in Nigeria, Trinidad, Qatar and Oman. For the IPPs, LNG can represent a reliable and secure supply of environmentally friendly fuel for combined-cycle gas turbine plants. These plants give high overall efficiency for relatively low capital cost.

Fig. 1: Typical breakdown of liquification plant capital costs

LNG projects are inherently capital-intensive, with the liquefier making up around 25% to 50% of the total cost (Fig.1). The balance is for storage, send-out terminals, jetties and ships (base load) or vaporisers (peak shave) (Fig. 2). The liquefier is where a process designer can bring about the largest cost savings, even affecting overall project viability.

Since their advent in the 1960s, liquefaction plants have increased in capacity to take advantage of economies of scale. The overall investment in single projects (including the whole production and supply chain) is extremely large; and project development can take many years. Medium- and small-scale projects are becoming more attractive because they are usually easier to promote and implement, and can subsequently be expanded in capacity. As a result, technology developments pertinent to smaller-scale projects are posing challenges to process designers.

Fig. 2:Features of the LNG chain

Liquefaction plants are generally classed as either peak-shave or base-load plants, depending on their size / role. Peak-shave facilities are usually relatively small (typically up to 100,000 tpy) and are used to overcome mis-matches between supply and demand. They liquefy and store excess natural gas during periods of low demand and vaporise it at times of peak demand. Many peak-shave plants were built in the 1970s and '80s, primarily in Europe and North America.

Base-load plants supply several thousand tpd of LNG, usually for marine transportation. The number of base-load trains operating or under construction world-wide is now approaching 70, at 15 sites. The maximum achievable liquefaction train size has increased over the last 30 years, with maximum train capacities now over 3 MMtpy of LNG.

 

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