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.
