Recuperative gas turbine

The new marketplace of energy conversion will have many new and novel concepts in combined cycle power plants. Figure 1-1 shows the heat rates of these plants, present and future, and Figure 1-2 shows the efficiencies of the same plants. The plants referenced are the Simple Cycle Gas Turbine (SCGT) with firing temperatures of 2400 °F (1315 °C), Recuperative Gas Turbine (RGT), the Steam Turbine Plant (ST), the Combined Cycle Power Plant (CCPP), and the Advanced Combined Cycle Power Plants (ACCP) such as combined cycle power plants using Advanced Gas Turbine Cycles, and finally the ITybrid Power Plants (HPP). 

However in practice, for the same states 1-5 the steam raised S will be less hence there is no advantage in operating a STIG plant in this variation of the basic CBTX recuperative gas turbine plant. Nonetheless, this form of analysis as developed by Lloyd will prove to be useful in the discussion of the chemical recuperation plant in Chapter 8. 

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. 

The second issue is the improvement of the low-temperature performance of combustion catalysts, i.e., the activity at combustor inlet conditions. All the proposed catalytic combustor designs available today need a pilot flame, or a heat exchanger in the case of recuperative gas turbines, to heat the compressed combustion air to a temperature sufficient for ignition of the catalyst. The possibility of avoiding this pilot flame is considered very important, since it would further reduce NO emissions. The catalyst surface area and washcoat loading are very important for the low-tempcraturc activity. 

The proposed General Atomics GT-MHR," with a direct recuperative gas-turbine cycle, has an efficiency of 48% with an exit gas temperature of 850°C. The AHTR, with an indirect recuperative multireheat gas-turbine cycle (Fig. 2.11), has an efficiency of 54%—assuming the same temperatures and turbomachinery parameters. Current materials may allow molten salt temperatures of 750°C. At these temperatures, the AHTR matches the efficiency of the GT-MHR with its exit helium temperature of 850°C. At 1000°C turbine inlet temperature that might be obtained with advanced materials, using the same fuel that currently limits the GT-MHR to an exit heliiun gas temperature of 850°C, and taking advantage of the improved heat transfer properties of the molten salt (see above), the efficiency of the AHTR can exceed 59%. 

The Hawthorne and Davis approach thus aids considerably our understanding of a/s plant performance. The main point brought out by their graphical construction is that the maximum efficiency for the simple [CHT]i cycle occurs at high pressure ratio (above that for maximum specific work) whereas the maximum efficiency for the recuperative cycle [CHTX]i occurs at low pressure ratio (below that for maximum specific work). This is a fundamental point in gas turbine design. 

Consider next a recuperative STIG plant (Fig. 6.5, again after Lloyd [2]). Heat is again recovered from the gas turbine exhaust but firstly in a recuperator to heat the compressed air, to state 2A before combustion and secondly in an HRSG, to raise steam S for injection into the combustion chamber. 

Fig. 9.2 shows how a simple open circuit gas turbine can be used as a cogeneration plant (a) with a waste heat recuperator (WHR) and (b) with a waste heat boiler (WHB). Since the products from combustion have excess air, supplementary fuel may be burnt downstream of the turbine in the second case. In these illustrations, the overall efficiency of the gas turbine is taken to be quite low ((tjo)cg = ccJf ca 0.25), where the subscript CG indicates that the gas turbine is used as a recuperative cogeneration plant. 

A gas turbine CHP. scheme which operates at Liverpool University, UK, consists of a Centrax 4 MW (nominal) gas turbine with an overall efficiency of about 0.27, exhausting to a WHB. The plant meets a major part of the University s heat load of about 7 MW on a mild winter s day. Supplementary firing of the WHB (to about 15 MW) is possible on a cold day. Provision is al.so made for by-passing the WHB when the heat load is light, in spring and autumn,. so that the plant can operate very flexibly, in three modes viz., power only, recuperative and supplementary firing. 

Improvements also have been made to the gas turbine for naval applications. An intercooled recuperative (ICR) gas turbine has been designed to improve the fuel consumption of naval power plants. The engine has a recuperator to take the heat that would otherwise be wasted m the exhaust and transfers it to the air entering the combustor. The new engine is expected to save about 30 percent ot the fuel consumed, compared to the simple gas turbine. The ICR engine is, however, larger and more expensive than the simple gas turbine. 

The results of the performance calculations are summarized in Table 9-24. The efficiency of the overall power system, work output divided by the lower heating value (LHV) of the CH4 fuel, is increased from 57% for the fuel cell alone to 82% for the overall system with a 30 F difference in the recuperative exchanger and to 76% for an 80 F difference. This regenerative Brayton cycle heat rejection and heat-fuel recovery arrangement is perhaps the simplest approach to heat recovery. It makes minimal demands on fuel cell heat removal and gas turbine arrangements, has minimal number of system components, and makes the most of the inherent high efficiency of the fuel cell. 

Power conversion cycle Gas-turbine, direct, recuperative, with intermediate cooling... 

It is not the purpose of this paper to evaluate the suitability of methanol as a fuel for gas turbines. Consequently, no attention will be given to such factors as the cost of methanol fuel, safety considerations of exchanging heat between hot exhaust gases and fuel, and the dynamics of the complex cycle with recuperative chemical reactions. The purpose of this paper is to outline the thermodynamic Implications of chemical recuperation using methanol fuel as an example. 

Further, Kraemer et al. describe tests with a catalytic lean-premixed prevaporized gas turbine combustor for recuperative turbogenerators. The catalyst was... 

Anon, (1997a). US Patent 5640840. Recuperative steam cooled gas turbine method and apparatus. Assigned to Westinghouse Electric Corporation, Pittsburgh, PA, 24 June. 

A Stand-alone gas turbine system in the configuration shown in Fig. 5.52 has a relatively low value of generating efficiency ( 20%). This value can be raised (to 30%) by adding a recuperative heat exchanger, which is usually placed between the air compressor and combustion chamber and fed by turbine exhaust. 

Fig. 3.16 showed carpet plots of efficiency and specific work for several dry cycles, including the recuperative [CBTX] cycle, the intercooled [CICBTX] cycle, the reheated [CBTBTX] cycle and the intercooled reheated [CICBTBTX] cycle. These are replotted in Fig. 6.17. The ratio of maximum to minimum temperature is 5 1 (i.e. T nx 1500 K) the polytropic efficiencies are 0.90 (compressor), 0.88 (turbine) the recuperator effectiveness is 0.75. The fuel assumed was methane and real gas effects were included, but no allowance was made for turbine cooling.

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