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
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 ° C for transport, in LNG carriers, 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.
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 or by a heat supply resulting from gas combustion in case of high demand.
Gas and steam turbines
Most of thermo mechanical cycles includes in the end of their cycle a so-called “Power” or “Free” turbine for mechanical or electrical production and delivering hot gases at an average temperature of 100 ° C (Value given as a matter of example) without going in the details of the different types of turbine.
Gas turbines include four main sections: an air compression section, a combustion chamber providing an air flow at high temperature and high pressure, a first expansion of the hot gas for the driving of the compression section and a second expansion section (“free” turbine) for the driving of a mechanical machine or an electric generator. There are many variants of gas turbines some types being designed for a considerable improvement in thermal efficiency of the cycle, in particular, the use of combined cycle or co generation cycle without forgetting the differences linked to the construction of the machines: aero derivative or industrial.
The steam turbines used for mechanical or electrical drive include from upstream to downstream expansion stages with high pressure and small diameter, medium pressure with medium diameter and low pressure with large diameter. The operation of the last wheel (very low pressure) of a steam turbine presents many constraints: significant centrifugal forces, erosion by droplets and micro particles and a condensation chamber of very low pressure and very large volume.
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.
Energy recovery from hot gases
The heat, currently dispersed into the atmosphere, can be recovered in several forms:
– Distribution in a heat network of water at a fairly high temperature (for example, between 70 and 100 ° C). The heat is distributed without requiring any thermodynamic cycle.
– Distribution in a heat network of water at a medium temperature (for example, between 30 and 50 ° 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, for example, between 60 and 70 ° 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.
– Transformation of heat into mechanical energy if the temperature is relatively high, for example, above 100 ° C.
At this last level of temperature (100 °C), the conversion of heat into mechanical energy (motor cycle) requires relatively large equipment which would take a long time for money return to the investment. One may however wonder that considering a nuclear power plant comprising four 1 300MW units it would not be preferable sometimes to build only three units and to invest in a downstream installation recovering the residual energy (heat) discharged into the atmosphere. This would present several advantages, less nuclear combustible requirement and treatment, increased safety and less impact to the environment. As a reminder, the Carnot efficiency of a unit with water steam at 100 ° C which will ultimately be discharged into an atmosphere at 10 ° C is 24 percent.
The Carnot efficiency increasing with the temperature difference between the cold and the hot sources (actually, the ratio of absolute temperatures_°K), it would appear useful to associate, in an motor cycle, a thermal power plant rejecting steam at 100 ° C and the gasification of liquefied natural gas carried out at -160 ° C. The Carnot efficiency of the engine cycle would be 70%. 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.
This could also concern the association of an electrical power plant comprising several gas turbines and of an LNG terminal. The association of the two production units is even more immediate when the gas turbines are supplied by the natural gas provided by the gas terminal.
The installations could be secured by separating them in a distance of the order of a few kilometres without significant impact on the thermal properties of the fluids circulating in insulated pipes. In addition, there is a strong synergy in the operation of the various units, electrical demands and increased gas flow rate generally occurring at the same time, for example, according to seasonal variations. It should be pointed out that the gas network is a very big gas volumetric storage (several thousands of kilometres in 800 mm pipe diameter) authorizing great flexibility in the gasification process of the LNG and also considering the large interval of operation between the maximum and minimum pressures. Another factor increasing the flexibility is the gas storage in ground reservoirs (reconverted saline mines).
If the association between a thermal electrical plant and a LNG terminal presents too many difficulties, a more simple motor cycle may be considered using the hot source of the sea water usually at an average temperature of 10 °C. The Carnot efficiency is lowered to 60 % but it is still relatively high and moreover more simple, safer and without any interdependency between two production units carrying different objectives and belonging to different companies.
Summary of the thermal associations with corresponding Carnot efficiency:
a) Thermal unit providing a fluid (water steam or gas fumes) at 100°C associated with a cold source at 10 °C (ambient air or river water). The Carnot efficiency is only 24%. It is relatively simple but requires huge equipment providing relatively little energy.
b) Thermal unit providing a fluid at 100°C associated with a LNG plant at -160°C. The Carnot efficiency is 70%. Despite a relatively high Carnot efficiency, the association is very complex and presents many constraints of operation including safety.
c) Water at 10°C associated with a LNG plant at -160°C (terminal usually installed at sea shore therefore, sea water considered here). The Carnot efficiency is 60%. The schematic is relatively simple but could provide a significant amount of energy. It should be noted however that an intermediate circuit needs to be implemented requiring cryogenic equipment: heat exchanger, tubing, maniflod and turbo expander.