Page 176 - Advanced Gas Turbine Cycles

I42 Advanced gas turbine cycles

the ‘heating value’ of the fuel before it is burnt in the combustion chamber. This does not
necessarily mean that the calorific value is increased, but that the mass of the new fuel
(syngas) may be increased so that the overall ‘heating value’ is also increased.
For the steam-TCR process, within a so-called ‘Van’t Hoff box’ containing the total
reaction process (Fig. 8.5b). there are two stages:
A : CH4 + H20 w CO + 3H2;
and

B: CO+H20*CO2+H2.
The so-called Boudouard reaction involving solid carbon is ignored here.
Stage A, the steam reforming reaction, is highly endothermic and stage B, usually
known as the water gas shift reaction, is exothermic, so the overall reaction (A + B)
requires heat to be supplied. If this overall reaction is in equilibrium then the resulting
mixture is made up of carbon monoxide, carbon dioxide, hydrogen, water vapour and
remaining methane. Thus, if a moles of methane are converted (per mole supplied), and P
moles of hydrogen are formed then the overall reaction may be written as
CH4 + nH2O * (4a - P)CO + (P - 3a)C02 + PH2

+ (n + 2a - P)H20 + (1 - a)CH4,
where the total moles of the mixture are N = (n + 1 + 2a).
The net heat input that is required depends on the pressurep and the temperature T, and
hence the equilibrium constants KPA(T) and KPB(T), respectively, which can be calculated
as

With (&)A and (K,), known from tables of chemical data, then the various mole fractions,
a, P, etc. may be determined if T and p are known.
Assuming that C& and H20 are supplied at T, the temperature at which TCR takes
place, the heat required to produce the overall change (AHTCR) is given by

=(4a-
-
)T
>T
[WTCR P)(hco )T + (P- 3 a)(hco, )T + (P~H, +@a- P)(~H~o)T Q(~CH~
=(4Q-P)[hco+O.5ho2 -k02 IT+PihHz +0*5hO, -hHzOl

The ‘heating value’ of the resultant syngas mixture per mole of methane supplied, but
now containing (1 - a) moles of C&, /3 moles of hydrogen and (4a - P) moles of