CBT

From the potential energy, calculate the Boltzmann factor, exp(—iC(r )/cBT). 

MET, mercaptobenzothiazole TMTM, tetramethylthiuram mono sulfide TMTD, tetramethylthiuram disulfide and CBTS, Al-cyclohexyl-2-benzothiazole... 

Fig. 2.8. Exergy fluxes in actual CBT ga.s turbine plant with combustion.

Fig. 2.9. Work output and exergy losses in CBT ga.s turbine plant (all. as I ractions of fuel exergy).

The Hawthorne and Davis analysis is first generalised for the [CBT]i open circuit plant, with fuel addition for combustion,/ per unit air flow, changing the working fluid from air in the compressor to gas products in the turbine, as indicated in Fig. 3.11. Real gas effects are present in this open gas turbine plant specific heats and their ratio are functions off and T, and allowance is also made for pressure losses. 

Fig. 3.13. Overall efficiency of CBT)ig cycle as a function of pressure ratio r with (and temperature ratio 6) as...

The [CBT]ig efficiency is replotted in Fig. 3.14, against (Tt,ITx) with pressure ratio as a parameter. There is an indication in Fig. 3.14 that there may be a limiting maximum temperature for the highest thermal efficiency, and this was observed earlier by Horlock et al. [8] and Guha [9]. It is argued by the latter and by Wilcock et al. [10] that this is a real gas effect not apparent in the a/s calculations such as those shown in Fig. 3.9. This point will be dealt with later in Chapter 4 while discussing the turbine cooling effects. 

A set of calculations using real gas tables illustrates the performance of the several types of gas turbine plants discussed previously, the [CBT]ig, [CBTX]ig, [CBTBTX]ig, [CICBTXIig and [CICBTBTX]ig plants. Fig. 3.15 shows the overall efficiency of the five plants, plotted against the overall pressure ratio (r) for = 1200°C. These calculations have been made with assumptions similar to those made for Figs. 3.13 and 3.14. In addition (where applicable), equal pressure ratios are assumed in the LP and HP turbomachinery, reheating is set to the maximum temperature and the heat exchanger effectiveness is 0.75. 

The first point to note is that the classic Hawthorne and Davis argument is reinforced— that the optimum pressure ratio for the [CBT]ig plant (r = 45) is very much higher than that for the [CBTX]ig plant (r = 9). (The optimum r for the latter would decrease if the effectiveness (s) of the heat exchanger were increased, but it would increase towards that of the [CBT]ig plant if e fell towards zero.)... 

The nomenclature introduced by Hawthorne and Davis [4] is adopted and gas turbine cycles are referred to as follows CHT, CBT, CHTX, CBTX, where C denotes compressor H, air heater B, burner (combustion) T, turbine X, heat exchanger. R and I indicate reversible and irreversible. The subscripts U and C refer to uncooled and cooled turbines in a cycle, and subscripts 1,2, M indicate the number of cooling steps (one, two or multi-step cooling). Thus, for example, [CHT] C2 indicates an irreversible cooled simple cycle with two steps of turbine cooling. The subscript T is also used to indicate that the cooling air has been throttled from the compressor delivery pres.sure. 

The arguments of this section are developed sequentially, starting with internally reversible cycles and then considering irreversibilities. Here we concentrate on the gas turbine with simple closed or open cycle (CHT, CBT). 

Fig. 4.9. Calcula(ion of efficiency of simple [CBT] plants—single-stop cooled (CBTIk j and uncooled CBT]n —as a function of maximum lemp>eraiurc (T ai) with pressure ratio (r) as a parameter.

Fig. 4.10 shows more fully calculated overall efficiencies (for turbine cooling only) replotted against isentropic temperature ratio for various selected values of Tj = T,.,. This figure may be compared directly with Fig. 3.9 (the a/s calculations for the corresponding CHT cycle) and Fig. 3.13 (the real gas calculations of efficiency for the uncoooled CBT cycle). The optimum pressure ratio for maximum efficiency again increases with maximum cycle temperature T. ... 

Fig. 4.10. Calculation of efficiency of. -iimple CBT plant—single-step cooled [CBT n ] as a function of iseniropic temperature ratio with maximum temperature (7 ) as a parameter.

Fig. 4.11. Calculation of efficiency of simple CBT plants—single-.step cooled ICBTlica uncooled [CBT ]ii—a.s a function of specific work with pressure ratio (r) and maximum temperature as parameters and with r)p< = t), = 0.9. 7hi = 1073 K (after Ref. 5 ).

Fig. 4.12. Calculation of efficiency of ICBT] plant uncoolcd CBT uj a.s a function of combustion lemperalurc (7eoi) single-step cooled (CBT)k i as a function of rotor inlet temperature (TVi,). Pres,sure ratio r = 30. t)c — 0.8.

In Chapter 4 calculations were made on the overall efficiency of CBT plants with turbine cooling, the fraction of cooling air (tp) being assumed arbitrarily. In this chapter, we outline more realistic calculations, with the cooling air fraction i/r being estimated from heat transfer analysis and experiments. 

Subsequently, we refer briefly to other comparable studies, including the calculations of exergy losses and rational efficiency. Finally, we show the real gas exergy calculations for two practical plants—[CBT]i and [CBTX]i. 

The results of a set of computer calculations for a CBT plant with single-step cooling (i.e. of the first stage nozzle guide vanes) are illustrated in Fig. 5.2, in the form of (arbitrary) overall thermal efficiency (tjq) against pressure ratio (r) with the combustion temperature T. oi as a parameter, and in Fig. 5.3 as tjq against with r as a parameter. 

Fig. 5.2 shows that for the single-step cooled CBT plant at a given combustion temperature, the overall efficiency of the cooled gas turbine efficiency increases with pressure ratio initially but, compared with an uncooled cycle, reaches a maximum at a lower optimum pressure ratio. Fig. 5.3 shows that for a given pressure ratio the efficiency generally increases with the combustion temperature even though the required cooling fraction increases. 

Fig. 5.4 shows a carpet plot of overall efficiency against specific work for the cooled [CBTJici plant (single step) with pre.ssure ratio and combustion temperature as parameters. As shown earlier, by the preliminary air standard analysis and the subsequent calculations in Chapter 4, there are relatively minor changes of thermal efficiency compared with the uncooled plant [CBT]iuc, but there is a major effect in the reduction of specific work. 

Fig. 5.3. Overall efficiency of [CBT]ici plant with single-step cooling of NGVs. as a function of combustion temperature with pressure ratio as a parameter.

Fig. 5.8. Contours of overall efficiency for [CBT c.i plant with three step cooling, again.st combustion...

Fig. 5.10 shows the exergy losses as a fraction of the fuel exergy (including the partial pressure terms referred to in Section 2.4) for the General Electric LM 2500 [CBT]ic plant. 

Fig. 5.10. Calculated exergy losses as fractions of fuel exergy for the General Electric LM 2500 CBT plant, for varying combustion temperatures (K) (after Ref. [13 ).

Before eonsidering the effects of water injection in an EGT type plant, it is worthwhile to refer to the earlier studies on the performanee of some dry recuperative cycles. Fig. 6.6 shows the T..s diagram of a [CBT i X r cyele, with a heat exchanger effectiveness of unity. It is implied that the surface area for heat transfer is very large, so that the outlet temperature on the cold side is the same as the inlet temperature on the hot side. However, due to the higher specific heat of the hot gas, its outlet temperature is higher than the inlet temperature of the cold air. 

Rufli s calculations (Fig. 7.7a, b), indicated that the optimum pressure ratio for a CCGT plant is relatively low compared with that of a simple gas turbine (CBT) plant. In both cases, the optimum pressure ratio increa.ses with maximum temperature. Davidson and Keeley [6] have given a comparative plot of the efficiencies of the two plants (Fig. 7.9), showing that the optimum pressure ratio for a CCGT plant is about the same as that giving maximum specific work for a CBT plant. 

Fig. 7.9. Overall efficiency of CCGT plant compared with overall efficiency and specific work of CBT plant...

These cycles allow sequestration and disposal of CO2 as a liquid, rather than allowing it to enter the atmosphere. They involve the introduction of additional equipment for the CO2 removal but little or no modification of the basic CBT or CBTX plant itself. 

Use of similar removal equipment in a simple CBT cycle is also possible but the exhaust gas from the turbine would require cooling before sequestration. 

B1 Steam/TCR Open/CBT CRj/steam reforming Natural gas/air None Attractive simplicity and efficiency... 

B3 FG/TCR SC/CBT CHVsIcam reforming Natural gas/air None Little efficiency gain... 

Obviously the availability of a non-carbon fuel, usually hydrogen, would obviate the need for carbon dioxide extraction and disposal, and a plant with combustion of such a fuel becomes a simple solution (Cycle Cl, a hydrogen burning CBT plant, and Cycles C2 and C3, hydrogen burning CCGT plants). 

Cl Hydrogen or hydrogen/nitrogen Open/CBT None Hydrogen/air None Nitrogen compression required... 

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