Published December 2003
Movement of natural gas from remote locations with abundant supply of natural gas to the consuming countries can be economically achieved only via the liquefaction route with shipping by ocean tankers. About 160 million metric tons/y of new liquefied natural gas (LNG) capacity is being implemented or in various planning stages in addition to the existing 100 million mt/y of LNG capacity of about 20 global facilities.
Until recently, cascade refrigeration systems for LNG production accounted for about 3.5% of the LNG global market with significant production at ConocoPhillips plant in Kenai, Alaska. Almost all the balance of the LNG market, about 90%, is predominantly propane pre-cooled, mixed refrigerant systems. Recent marketing efforts by a collaboration of ConocoPhillips and Bechtel are increasing the market share of cascade refrigeration technology.
This Review evaluates the economics of a base loaded, generic cascade refrigeration LNG plant nominally producing at least 600 million scf per stream day (4.375 million mt/y at 0.95 on stream factor) of LNG using two 50% capacity refrigeration and liquefaction trains. The feed gas is lean, containing less than 8 vol% (17 wt%) C2+, and also has low nitrogen and CO2 contents (less than 1 vol% and 1.2 vol% respectively). A generic LPG (liquefied petroleum gas) recovery process is used and the nitrogen stripping step is avoided. The refrigeration systems use a combined cycle mode of propylene and ethylene cycles driven by gas turbines and methane refrigeration driven by steam turbines. A closed loop methane refrigeration system is used for this lean gas plant, however, an open loop methane cycle could be considered as an alternate. The competing mixed refrigerant cycle technology is given a cursory review.
The proposed design represents a relatively low greenhouse gas emission plant (0.20 ton CO2/ton of LNG, as opposed to a typically reported 0.25-0.35 ton CO2/ton LNG) with low NOx emission.