Page 177 - Advanced Gas Turbine Cycles

Chapter 8. Novel gas mrbine cycles I43

carbon monoxide, is
1
[ AH1 SyN =P[AHH~ T +( 1 - a)[AHc& 1~+(4a- P)[Affcol~= [AHIcH~ + [AHITcR,
This is thus greater than the heating value of the original unit mole of methane supplied but
is contained in a larger number of moles of syngas (N).

8.5.3. Partial oxidation

In the second chemical reaction to be considered, insufficient oxygen is supplied to the
fuel for stoichiometric combustion (50%), but steam is also supplied (Fig. 8.5~). Now the
chemical reactions involved in the partial combustion are:
A : CH4 + H20 * CO + 3H2,
the steam reforming reaction;
B: CO+H20*C02+HZ,

the water shift reaction; and
C : CH, + 0.502 H CO + 2H2,
the PO reaction.
As in the steam/TCR analysis the Boudouard reaction is ignored here, together with
direct methane decomposition.
The PO reaction, leading to five constituents, is now
2CH4 + $2 + nH20

=+(I -y)CH4+6C02+(y-6+1)CO+(3y+6+2)H2+(n- y-6)H20
The solution then follows along the same lines as for TCR; if the temperature and pressure
are known then y, 6 and the resulting mole fractions can be determined from the
equilibrium constants. The temperature change between inlet and outlet is now likely to be
higher than in the TCR reactions, so the determination of the Kps as functions of a single
mean temperature for the reaction is more difficult.

8.5.4. Thermo-chemical recuperation using flue gases @ue gas/TCR)

Another approach which has been suggested for thenno-chemical reforming can now
be considered. It involves recirculation of exhaust gas from the turbine, which already
contains some C02 and H20, to mix with the fuel in a reformer; the resulting syngas is then
supplied to the main combustion chamber. The combustion process producing the flue gas
is assumed to be virtually stoichiometric, with a small amount of excess air. The flue gas
thus contains a small amount of oxygen and Po of the fuel (CH4) may take place, together
with the steam reforming and water shift reactions.
The ‘Van’t Hoff box’ for this process will produce five components+arbon dioxide,
carbon monoxide, water vapour and hydrogen, and unconverted methane. Again if