More Isothermal Compression
The advantage of isothermal compression is not only that it makes reheat and recuperation possible but also that it requires less energy than adiabatic compression, as is illustrated by the following formulae for the isothermal and adiabatic compression of an ideal gas:
Isothermal
where E is the energy in J/mol, R is the universal gas constant of 8.314 J/mol. K, k is the isentropic exponent Cp/Cv of the isobaric heat capacity divided by the isochoric heat capacity, Tin is the inlet temperature of the compressor in K, and Pug^Piow *s pressure ratio of the compressor.
Taking air of 300 K, for which к is about 1.4 and a pressure ratio of 10, it is easily shown that for this case adiabatic compression requires 1.41 times the energy of isothermal compression. One should not be surprised by this large difference, as it must be kept in mind that the energy for heating of the gas during compression is coming from shaft power that otherwise could have generated power or electricity. This heating is thus equivalent to electric resistance heating! In case of isothermal compression the air will be heated with additional fuel in the combustor, but there the heating takes place with virtually 100% efficiency, whereas via the shaft the heating takes place with an efficiency of about 40% based on fuel.
Many attempts have been made over the years to accomplish a more isothermal compression. The most obvious solution is to split the compressor in various parts and to apply indirect intercooling between the various stages. Although this will lower the energy required for compression, it has the disadvantage that the compressor is split up in various parts and that the heat exchangers result in additional pressure drop for the air flow.
An example of a more positive approach to a more isothermal compression is the Sprint gas turbine that features a water spray injection between the two compressor stages of a General Electric LM6000 aero-derivative gas turbine (McNeely 1998). This so-called wet compression as a means to accomplish a more isothermal compression has often been proposed in the past but was never applied (Milo AB 1936; Brown, Boveri & Cie 1968; Beyrard 1966; Societe Rateau 1952). The main purpose of the water spray in the Sprint gas turbine is to increase the capacity of the turbine for power generation, but at ambient temperatures above 5°С it also increases the efficiency of the power plant.
Combinations of More Isothermal Compression and Recuperation
All major improvements in gas turbine-based cycles concern the use of a more isothermal compression and recuperation. The highest efficiencies are achieved with so-called humid air turbines (HAT) in combination with heat recuperation from the turbine exhaust gases. The cycles involved are called HAT cycles. Two HAT cycles will be discussed: the HAT cycle and the Tophat cycle.
In the HAT cycle a flue gas heat recuperator replaces the heat-recovery steam generator (HRSG). In the recuperator the sensible heat in the hot exhaust gases leaving the turbine are used to preheat humidified combustion air and water (Schipper 1993). The combustion turbine air compressor is also intercooled and cooled after final compression (aftercooling). The heat recovered in these cooling steps preheats additional water, and the hot water humidifies the pressurized combustion air in a multistage, countercurrent saturator. The major disadvantage of this scheme is that instead of a compressor and a turbine as in a normal gas turbine, many more pieces of rotating equipment are required in the form of an additional compressor and pumps. Moreover, large spray columns are required for humidifying the water to be injected into the air, and finally, the heat in the hot gases leaving the turbine is used for the low-temperature service of preheating and evaporating water. Various modifications of the HAT cycle have been proposed, such as the cascaded HAT (CHAT) cycle (Nabhamkin 1995), but all suffer from one or more of the disadvantages mentioned above. The only exception is the Tophat cycle discussed in the next section.
The reason why wet compression, that is humidifying the air during compression inside the compressor, has found so little application is most likely due to the fact that most atomizing devices available today can only produce water droplets with a diameter of 30 |i or larger. Smaller droplets can be made, but this requires generally complex or bulky equipment. An alternative for small droplets are spray towers as used in HAT cycles, which result in additional pressure drops. This is a pity, as in wet compression good use is made of the unique high heat of evaporation of water, whereas this same quality is a disadvantage for the Rankine cycle.
Only recently an elegant and compact method has been proposed for making small droplets. This has made it possible to inject an extremely large amount of water into the atmospheric air entering the air compressor or in the compressor itself.
The flow scheme of the Tophat cycle is shown in Figure 7-20 (van der Burgt and van Liere 1996). The water is injected in the air A entering the compressor in such
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FLUE GAS NET WATER PURGE COMPRESSOR GAS GENERATOR TURBINE Figure 7-20. Tophat Cycle |
a way that the compressor does not suffer from a parasitic pressure drop. It is injected in the form of very fine droplets of a mean diameter of about 1-3 p. These droplets, that can be made by combining flash evaporation with efficient atomizers as in the “swirl flash technology” (van Paassen and van Liere 1980), are so small that the droplets will (a) evaporate in the milliseconds available in the compressor, (b) will not cause erosion problems, and (c) follow the path of the gas stream without being centrifuged out. The humidified air В leaving the compressor at the required pressure is essentially saturated with water. In a recuperator, the humidified air is heated with the hot exhaust gases leaving the turbine to a temperature of, say, 50- 100°C below the turbine outlet temperature before being routed to the combustor, where the hot air is used for the combustion of the fuel. The hot pressurized flue gas C then enters the turbine. The exhaust gas D leaving the turbine preheats the humidified air as well as the water used for humidifying the air and, if required, the fuel. After leaving the recuperator the water in the exhaust gas is routed to a condenser, after which the dry exhaust gas leaves via the stack. The condensate is partly recycled, and the surplus is purged from the system.
The compression as proposed for the Tophat cycle is not completely isothermal but quasi-isothermal. In practice it results in a compressor outlet temperature for the humidified air, which varies from about 100 to 175°С for discharge pressures of 8 to 32 bar, respectively, when starting with ISO air (15°C and a moisture content of 1.19mol%) and using injection water of 200 °С. This is clearly illustrated in Figure 7-21A (data for an isentropic efficiency of the compressor of 87%).
Quasi-isothermal compression hence requires less energy per unit (kmol/s)/ISO-air than adiabatic compression. This advantage increases with the pressure ratio, as illustrated in Figure 7-21B.
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(В) Pressure ratio Figure 7-21. (A) Compressor Discharge Temperatures as Function of Compression Ratio (B) Compression Energy for Adiabatic and Isothermal Compression |
The greatest advantage of a more isothermal compression is that now it becomes advantageous to have a recuperator in which the sensible heat in the gases leaving the turbine are used to preheat the air leaving the compressor. This heat, which in a combined cycle is used to drive an additional steam cycle, is now used in the more efficient and less costly Joule cycle itself.
The Recuperator
Recuperators—or flue gas heated air pre-heaters—play an important part in some synthesis gas technologies such as steam reforming but have not found favour in connection with gas turbines, whether in IGCC or standard applications. This is a logical outcome of the concentration on adiabatic compression and the fact that the outlet temperatures of air compressor and gas turbine are too close for a recuperator to have any important effect. Also the use of a recuperator with conventional quasi-isothermal compression with the use intercoolers as used in process gas compressors does not have any beneficial effect, since even if the heat removed via the intercoolers is used for say boiler feedwater preheat, it transfers heat from the gas turbine cycle to the less efficient steam cycle. This is different in the Tophat cycle since the heat is used to increase the mass entering the gas turbine by evaporating water into the combustion air.
Arguments are sometimes raised against recuperators because of the poor heat transfer and large surface area involved. These arguments are however generally superficial. The steam superheater in the HRSG is also a gas-gas exchanger with similar heat transfer coefficients as is the air preheater in a steam reformer and both are successful components in their respective environments.
Furthermore, the construction of the headers and so on is much lighter than the equivalent HRSG steam superheater, which reduces problems related to thermal shock. Assuming that the Tophat stations will be started up and shut down as frequently as the alternative of combined cycles, the point of thermal shock is not very relevant. The reason is that the metal temperatures and temperature cycles are about the same when the preheat temperature of the humidified air is restricted to the superheat temperature of the steam in a CC.
Moreover, the recuperator has a very smooth temperature profile in the steady state. Because the heat exchange is restricted to the exchange of sensible heat (gas- gas and gas-water) the enthalpy supply and demand lines are almost parallel, as is illustrated in the typical example in Figure 7-22. In this example the humidified air and the fuel gas are both preheated to 500°C, and the water used for evaporation during compression is preheated to 200°C. As can be seen, there is hardly any pinch. For the case in question, the hot exhaust gas, after having preheated the humidified air, the natural gas, and the water, has a temperature of about 145°C.
The Water Cycle
The distillate quality water required for injection can be obtained by condensing the water in the exhaust gas. This gas has then to be further cooled after it leaves the recuperator. This can advantageously be accomplished in a two-stage direct contact condenser. The first condensate, comprising 5-10% of the water present in the exhaust gas, contains virtually all the solids contained in the combustion air and the fuel that have acted as condensation nuclei. This water can be used as a purge in order to avoid build-up of solid contaminants in the system. The pure condensate from the second stage can then be used for humidifying the air.
The use of indirectly cooled condensers does not look attractive because of the large amount of inert gases in exhaust gas, as this results in very large heat exchange surfaces and hence in costly equipment. Sometimes indirect cooling may be economical though, for example, when the heat utilized in a combined heat and power system involving, for example district heating or seawater distillation.
An important point is, of course, whether cooling water is available. On ships and for offshore applications, this will never present a problem. In arid areas, air-cooling or a cooling tower must be used. As all fuels contain hydrogen, there is always a net production of water. In arid areas this is advantageously used for irrigation. In the case of natural gas the mass of net water produced is about equal to the mass of the fuel.
The efficiency of the Tophat cycle is very dependent on the temperature difference between the hot turbine exhaust gases entering the recuperator and the humidified air leaving the recuperator. Typically for a 30°C decrease in the recuperator temperature difference there will be an increase of about one percentage point on the overall cycle efficiency.
Also, in the Tophat cycle the turbine inlet temperature is a factor in relation to efficiency, although it should be realized that raising the turbine inlet temperature of the turbine is not so important for the Tophat cycle as for a CC. The reason is that because of higher inlet temperatures, both a high pressure ratio is required and the temperature of the gases leaving the turbine is generally increased. Hotter exhaust gases would lead to higher maximum temperatures in the recuperator and hence imply the use of more expensive steels for this service. For this reason the maximum preheat temperatures of the humidified air was limited to 500°C so as to keep the maximum metal temperatures in virtually all cases to below 550°C. With these restrictions there is not much effect in raising the inlet temperatures above 1300°C.
Station Efficiency and N0X Control
The biggest advantage regarding NOx control of the Tophat cycle is the fact that the stoichiometric adiabatic flame temperatures (SAFTs) are so low. As is well known, lower SAFTs result in lower NOx emissions. In the standard Joule cycle, higher station efficiencies are obtained by increasing both the pressure ratio and the turbine inlet temperatures, resulting in higher SAFTs. Using quasi-isothermal compression as applied in the Tophat cycle generally leads to lower SAFTs for stations with a higher efficiency. This is clearly illustrated in Figure 7-23, where SAFTs are plotted against station efficiencies for various cases: a Joule cycle, a Tophat cycle, and a case where only quasi-isothermal compression (without recuperator) is used. The reason for the low SAFTs of the quasi-isothermal compression cases is the low oxygen content and the higher moisture content of the air (see Figure 7-24).
Applications
The high efficiency of the Tophat cycle of 60% or more makes it attractive for many applications apart from as a replacement for combined cycle stations. The fast
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Figure 7-23. Stoichiometric Adiabatic Flame Temperature as a Function of Station Efficiency |
start-up and the absence of a steam cycle make it attractive for many applications where now open cycles are used. Examples include peak shaving, the use of gas turbines in ships, offshore applications and liquid natural gas (LNG) plants, combined heat and power schemes, and so on. The fact that Tophat cycles can be applied for duties from, say, 500 kW onwards, means that they can even be considered for trucks, locomotives, off-the-road vehicles, and mining equipment.