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The role of real gas Brayton cycles for the use of liquid natural gas physical exergy.

Author(s) : ANGELINO G., INVERNIZZI C. M.

Type of article: Article

Summary

When using the cooling capacity of LNG several thermodynamic schemes are proposable employing conventional and non conventional conversion cycles. All conventional systems make use of organic working fluids such as methane or propane in series of Rankine cycles used in a cascading mode. A simpler system is available, using a single cycle and a single fluid in a Brayton cycle. However ordinary Brayton cycles exhibit a modest efficiency. Resorting to Brayton cycles with strong real gas effects (which is possible selecting the base parameters of pressure and temperature in the vicinity of the critical point) improves considerable cycle performance. Since the level of cold in a LNG flow is thermodynamically predetermined, working fluids must be selected with a critical point which fit the LNG thermal capacity, i.e. some 5-15°C higher than the usual LNG temperature which is around -160°C. Nitrogen was found as the best fluid to exploit real gas effects with efficiencies above 63% while perfect gas cycles give efficiencies around 56%. However, in real gas cycles the cooling capacity of LNG is only partially exploited: a better exploitation is obtained from perfect gas cycles or for more complex cascading Rankine cycle. Selecting working fluids with a higher critical temperature than nitrogen, as for example argon, the efficiency decreases to 58% respect to 63% for nitrogen, but the utilization of the cold of LNG improves from 0.30 MW/(kg/s) to 0.75 MW/(kg/s). Obviously as heat rejection temperature increases a larger fraction of cold in the LNG flow can be utilized. Combined cycles making use of a gas turbine offer also a good performance. The merits of real gas effect Brayton cycles also in this case remains evident. Finally, it is theoretically possible to use real gas effect Brayton cycles at low temperatures, which are typical of waste heat (say 100-150°C: in this case cycle efficiency remain good, but power obtainable from a unit flow of LNG is modest. [Reprinted with permission of Elsevier. Copyright 2011.]

Details

  • Original title: The role of real gas Brayton cycles for the use of liquid natural gas physical exergy.
  • Record ID : 30003613
  • Languages: English
  • Source: Applied Thermal Engineering - vol. 31 - n. 5
  • Publication date: 2011/04
  • DOI: http://dx.doi.org/10.1016/j.applthermaleng.2010.10.032

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