Page 184 - Advanced Gas Turbine Cycles

150 Advanced gas turbine cycles

temperature 1 250°C (after turbine cooling), which with the selected turbomachinery
efficiencies leads to a recuperation temperature and a pressure level of about 600°C and
15 bar, respectively. These enable the molal concentrations after reforming to be
calculated, as explained in Section 8.5.2. a (the conversion rate) is determined as 0.373
and p as 0.190, so the concentrations after reforming are as follows: CI-L,, 8.1 %; CO, 0.4%;
H2, 19%; C02, 4.5%; H20, 68%. Thus, with 37.3% of the CH4 converted, it follows that
the heat transferred from the exhaust gas is about 110 kJ and the heating value of the
resultant reformed syngas is 0.164 [CV], = 1.15 MJ, where [CV], = 7.02 MJkg is the
syngas calorific value. Calculation of the remaining part of the cycle is straightforward.
The heating value of the gas supplied for combustion is enhanced by about 10%
(although the calorific value is substantially reduced compared to the methane supplied,
from some 50 to 7 MJkg). This is mainly due to the large concentration of hydrogen, as
indicated in the equilibrium concentrations of the gases following the reforming.
However, the thermal efficiency of the cycle is given by the work output divided by the
calorific value of the original methane fuel supplied and is 47.6%.
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.
Newby et al. [6] also studied a steaflCR cycle with similar parameters and steadair
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. 8.13 shows Cycle B2, a development of Lloyd’s simple steam!TCR cycle for C02
removal, as proposed by Lozza and Chiesa [7]. However, this is a CCGT plant in which the
syngas produced by the steam reformer is cooled and then fed to a chemical absorption
process. This enables both water and C02 in the syngas to be removed and a hydrogen rich
syngas to be fed to the combustion chamber.
After allowing for the performance penalties arising from the C02 removal, Lozza and
Chiesa estimated an efficiency of 46.1%, for a maximum gas turbine temperature of
1641 K and a pressure ratio of 15 (compared with the basic CCGT plant efficiency of
56.1 %). They concluded that the plant cannot compete, in terms of electricity price, with a
semi-closed combined cycle with C02 removal (Cycle A2).

8.6.2.2. The Jlue gas thermo-chemically recuperated (FGITCR) cycle
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 described in Section 8.6.2. I.