Keywords: motor cycle LNG electrical power plant cold hot temperature<\/p>\n\n\n\n
This document has to be reviewed in relation with the gas turbine exhaust temperature considering different gas turbine types and including or not cogeneration<\/p>\n\n\n\n
The production of liquefied natural gas (LNG) requires, over the entire treatment chain, a very large consumption of energy. Ones mention often a consumption of 15% of the equivalent energy which may be produced by LNG. Indeed, after its extraction, the gas is treated, cooled to ambient temperature, then cooled down to -160 \u00b0 C for transport, in LNG carriers<\/strong>, in cryogenic tanks at atmospheric pressure. The liquefaction of natural gas is carried out via cold cryogenic refrigeration loops (with decreasing operating temperature) comprising cryogenic compressors driven by gas turbines of hundreds of MW. During transport in the LNG carriers, part of the gas vaporizes which must be suctioned (to avoid pressurization of the cryogenic tanks) and re-condensed to avoid their loss. For that purpose, these gases are sometimes used to supply gas engines propelling the LNG carrier.<\/p>\n\n\n\n On land, the gas is stored in other cryogenic tanks while waiting for a new gasification of the liquid according to the demand for natural gas. The gas is introduced into the natural gas network by compressing it to a pressure of the order of 70 bar. The gasification of liquefied natural gas is obtained using heat exchangers heated by sea water<\/strong> or by a heat supply resulting from gas combustion in case of high demand.<\/p>\n\n\n\n Most\nof thermo mechanical cycles includes in the end of their cycle a\nso-called \u201cPower\u201d or \u201cFree\u201d turbine for mechanical or\nelectrical production and delivering hot gases at an average\ntemperature of 100 \u00b0 C (Value given as a matter of example) without\ngoing in the details of the different types of turbine.<\/p>\n\n\n\n Gas\nturbines include four main sections: an air compression section, a\ncombustion chamber providing an air flow at high temperature and high\npressure, a first expansion of the hot gas for the driving of the\ncompression section and a second expansion section (“free”\nturbine) for the driving of a mechanical machine or an electric\ngenerator. There are many variants of gas turbines some types being\ndesigned for a considerable improvement in thermal efficiency of the\ncycle, in particular, the use of combined cycle or co generation\ncycle without forgetting the differences linked to the construction\nof the machines: aero derivative or industrial.<\/p>\n\n\n\n The\nsteam turbines used for mechanical or electrical drive include from\nupstream to downstream expansion stages with high pressure and small\ndiameter, medium pressure with medium diameter and low pressure with\nlarge diameter. The operation of the last wheel (very low pressure)\nof a steam turbine presents many constraints: significant centrifugal\nforces, erosion by droplets and micro particles and a condensation\nchamber of very low pressure and very large volume.<\/p>\n\n\n\n For all these machines, obtaining a high Carnot efficiency imposes a high temperature at the inlet of the first expansion section that is to say a high temperature in the combustion chamber or the steam boiler. This results in a relatively high outlet temperature in the last expansion stage, often of the order of a hundred degrees.<\/strong><\/p>\n\n\n\n The\nheat, currently dispersed into the atmosphere, can be recovered in\nseveral forms:<\/p>\n\n\n\n –\nDistribution in a heat network of water at a fairly high temperature\n(for example, between 70 and 100 \u00b0 C). The heat is distributed\nwithout requiring any thermodynamic cycle.<\/p>\n\n\n\n – Distribution in a heat network of water at a medium temperature<\/strong> (for example, between 30 and 50 \u00b0 C). A very large part of the heat can be recovered using a thermodynamic cycle (a heat pump) permitting the transfer of heat with medium temperature to another fluid operating at a higher temperature<\/strong>, for example, between 60 and 70 \u00b0 C. The overall system operates with a high coefficient of performance considering the absolute temperature ratio. Note: absolute temperature in degree Kelvin is obtained in adding 273 to the relative temperature in degree Celsius. As an example, a temperature ratio of (273+40)\/(273+65)=0,926 between the low and the high absolute temperatures provides a high coefficient of performance meaning that little energy is required to transfer heat from the cold to the hot sources.<\/p>\n\n\n\n –\nTransformation of heat into mechanical energy if the temperature is\nrelatively high, for example, above 100 \u00b0 C. \n<\/p>\n\n\n\n At\nthis last level of temperature (100 \u00b0C), the conversion of heat into\nmechanical energy (motor cycle) requires relatively large equipment\nwhich would take a long time for money return to the investment. One\nmay however wonder that considering a nuclear power plant comprising\nfour 1 300MW units it would not be preferable sometimes to build only\nthree units and to invest in a downstream installation recovering the\nresidual energy (heat) discharged into the atmosphere. This would\npresent several advantages, less nuclear combustible requirement and\ntreatment, increased safety and less impact to the environment. As a\nreminder, the Carnot efficiency of a unit with water steam at 100 \u00b0\nC which will ultimately be discharged into an atmosphere at 10 \u00b0 C\nis 24 percent.<\/p>\n\n\n\n The Carnot efficiency increasing with the temperature difference between the cold and the hot sources (actually, the ratio of absolute temperatures_\u00b0K), it would appear useful to associate, in an motor cycle, a thermal power plant rejecting steam at 100 \u00b0 C and the gasification of liquefied natural gas carried out at -160 \u00b0 C<\/strong>. The Carnot efficiency of the engine cycle would be 70%. <\/strong>It would be worth to study the benefir of such an association. It should be noted that during cold periods (winter), thermal (steam) power stations and gas terminals are equally required.<\/p>\n\n\n\n This\ncould also concern the association of an electrical power plant\ncomprising several gas turbines and of an LNG terminal. The\nassociation of the two production units is even more immediate when\nthe gas turbines are supplied by the natural gas provided by the gas\nterminal.<\/p>\n\n\n\n The\ninstallations could be secured by separating them in a distance of\nthe order of a few kilometres without significant impact on the\nthermal properties of the fluids circulating in insulated pipes. In\naddition, there is a strong synergy in the operation of the various\nunits, electrical demands and increased gas flow rate generally\noccurring at the same time, for example, according to seasonal\nvariations. It should be pointed out that the gas network is a very\nbig gas volumetric storage (several thousands of kilometres in 800 mm\npipe diameter) authorizing great flexibility in the gasification\nprocess of the LNG and also considering the large interval of\noperation between the maximum and minimum pressures. Another factor\nincreasing the flexibility is the gas storage in ground reservoirs\n(reconverted saline mines). \n<\/p>\n\n\n\n If\nthe association between a thermal electrical plant and a LNG terminal\npresents too many difficulties, a more simple motor cycle may be\nconsidered using the hot source of the sea water usually at an\naverage temperature of 10 \u00b0C. The Carnot efficiency is lowered to 60\n% but it is still relatively high and moreover more simple, safer and\nwithout any interdependency between two production units carrying\ndifferent objectives and belonging to different companies.<\/p>\n\n\n\n Summary\nof the thermal associations with corresponding Carnot efficiency:<\/p>\n\n\n\n a) Thermal unit providing a fluid (water steam or gas fumes) at 100\u00b0C associated with a cold source at 10 \u00b0C <\/strong>(ambient air or river water). The Carnot efficiency is only 24%.<\/strong> It is relatively simple but requires huge equipment providing relatively little energy.<\/p>\n\n\n\n b) Thermal unit providing a fluid at 100\u00b0C associated with a LNG plant at -160\u00b0C<\/strong>. The Carnot efficiency is 70%<\/strong>. Despite a relatively high Carnot efficiency, the association is very complex and presents many constraints of operation including safety.<\/p>\n\n\n\n c) Water at 10\u00b0C associated with a LNG plant at -160\u00b0C<\/strong> (terminal usually installed at sea shore therefore, sea water considered here). The Carnot efficiency is 60%.<\/strong> The schematic is relatively simple but could provide a significant amount of energy.<\/strong> It should be noted however that an intermediate circuit needs to be implemented requiring cryogenic equipment: heat exchanger, tubing, maniflod and turbo expander.<\/p>\n","protected":false},"excerpt":{"rendered":" Keywords: motor cycle LNG electrical power plant cold hot temperature This document has to be reviewed in relation with the gas turbine exhaust temperature considering different gas turbine types and including or not cogeneration LNG terminal The production of liquefied natural gas (LNG) requires, over the entire treatment chain, a very large consumption of energy. […]<\/p>\n","protected":false},"author":1,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-751","page","type-page","status-publish","hentry"],"yoast_head":"\nGas and steam turbines<\/h2>\n\n\n\n
Energy recovery from hot gases<\/h2>\n\n\n\n
Motor cycles<\/h2>\n\n\n\n