STIG cycle

Lloyd carried out a range of similar calculations, for differing thermodynamic parameters the results are presented in Fig. 8.12 in comparison with those for a basic STIG cycle with the same parameters of pressure ratio and maximum temperature. There is indeed similarity between the two sets, with the TCR plant having a higher efficiency. It is noteworthy that both cycles obtain high thermal efficiency at quite low pressure ratios as one would expect for what are essentially CBTX recuperative gas turbine cycles. 

A second type of CRGT plant involving modification of the fuel before combustion (Cycle B3) is shown in Fig. 8.14. Now some part of the exhaust from the turbine (which contains water vapour) is recirculated to the reformer where the fuel is modified. Thus this FG/TCR cycle has an element of the semi-closed cycle plus modification of the combustion process. The chemical process involved in this cycle has been described in Section 8.5.4, but there is now no simple comparison that can be made between the FG/ TCR cycle and the basic STIG cycle, as de.scribed in Section 8.6.2.1. 

In another example Newby et al. [6] calculated a cycle with the reformer operating at comparable pressure and temperature but with a higher recycling rate of 1.7, leading to a conversion rate of a = 0.56 (this is closer to the conversion rate of Lloyd s steam/TCR cycle, a = 0.373, described in the last section). A thermal efficiency of 38.7% is claimed for this FG/TCR cycle, slightly greater than the simple CBT cycle efficiency of 35.7% but much less than the calculated efficiency for the steam/TCR cycle (48.7%) and a comparable STIG cycle (45.6%). 

There are many variations on these two basic cycles which will be considered later. But first we discuss the basic thermodynamics of the STIG and EGT plants. 

Fig. 6.2 shows a simplified diagram of the basic STIG plant with steam injection S per unit air flow into the combustion chamber the state points are numbered. Lloyd 2 presented a simple analysis for such a STIG plant based on heat input, work output and heat rejected (as though it were a closed cycle air and water/steam plant, with external heat supplied instead of combustion and the exhaust steam and air restored to their entry conditions by heat rejection). His analysis is adapted here to deal with an open cycle plant with a fuel input/to the combustion chamber per unit air flow, at ambient temperature To, i.e. a fuel enthalpy flux of/7i,o. For the combustion chamber, we may write... 

In the search for higher plant thermal efficiency, the simplicity of the two basic STIG and EGT cycles, as described by Frutschi and Plancherel, has to some extent been lost in the substantial modifications described above. But there have been other less complex proposals for water injection into the simple unrecuperated open cycle gas turbine one simply involves water injection at entry to the compressor, and is usually known as inlet fog boosting (IFB) the other involves the front part of an RWI cycle, i.e. water injection in an evaporative intercooler, usually in a high pressure ratio aero-derivative gas turbine plant. 

Newby et al. 6 also studied a steam/TCR cycle with similar parameters and steam/air ratio. They calculated an efficiency of 48.7%, compared with 35.7% for a comparable CBT plant, 45.6% for a STIG plant and 56.8% for a CCGT plant, all for similar pressure ratios and top temperatures. 

Fig. 2 In situHRTEM image sequence of the growing CNTs (Scale bar = 5 nm). Images (a-h) show one cycle in the elongation/contraction process (Stig et al. 2004)...

Humid Air Turbine Cycle. One innovative improvement and modification of the I-STIG turbine cycle is the humid air turbine (HAT) cycle (37), which eliminates the use of the HRSG to generate steam. In the HAT cycle, a flue gas heat recuperator, replaces the HRSG and is used to preheat both humidified combustion air and water. 

---