Page 188 - Advanced Gas Turbine Cycles

I54 Advanced gas turbine cycles

Bannister et al. [ 101 made a study of the hydrogen fuelled CCGT plant (Cycle C2),
with a closed upper ‘gas turbine’ cycle (Fig. 8.16). A number of different working
fluids were used in the latter, the water produced in combustion being separated and
extracted downstream of the HRSG. Again there is relatively little variation of
efficiency with choice of the upper cycle working fluid, each of which has some
practical limitations, but that with steam as the working fluid offers highest efficiency,
approaching 60% (HHV).
Bannister et al. then considered a novel ‘Rankine’ type hydrogen fired cycle (Cycle
C3), as shown in Fig. 8.17. Low pressure wet steam leaving the turbine in the ‘gas
turbine’ upper cycle then enters the hot side of the HRSG. After leaving the HRSG as
wetter steam this mainstream flow enters the condenser. After condensation, some
water, equal in mass flow to that produced in combustion (m per unit flow at entry), is
then discharged. The rest (unit) flow is pumped back into the cold side of the HRSG to
receive heat from the (1 + m) wet steam stream. Within the HRSG, this unit water flow
passes through
(a) an economiser,
(b) an evaporator to leave as saturated steam, and
(c) a superheater to impart a margin of superheat before entry to the combustion
chamber.
This superheated steam then acts as a moderator for the hydrogedoxygen combustion,
which takes place at high pressure, 166 bar in the original study. Two subsequent
reconfigurations of the cycle changed this high pressure to 365 and 250 bar, respectively,
the same general cycle approach being followed but with some added cooling streams. The
Westinghouse group concluded that a cycle efficiency of 60% (HHV) could be achieved
with this Rankine type cycle.
Further detailed studies of several complex hydrogen fuelled cycles, including the
‘Rankine’ cycle C3, have been made by Japanese authors, e.g. Sugisita et al. [l I].
Their preference is for a ‘topping/extraction’ cycle. In this cycle, the mainstream flow
from the combustor in the upper cycle, after passing through an HP steam turbine, gets
cooled in the first of the two heat exchangers, from a superheated state to the saturation
condition. The flow is then split, one stream expanding further to condenser pressure,
with the combustion product water flow (m) being discharged. The remainder of this
stream is pumped up, recuperated by the second of the two heat exchangers, expanded
again in another turbine and then mixed with the remaining topping cycle flow. Sugisita
et a]. claim over 60% efficiency for this so-called Jericha cycle.

8.6.4. Cycles with modiJcation of the oxidant in Combustion
We next consider a number of plants in which the combustion process is modified by
changing the oxidation of the fuel, Table 8.ID and Figs. 8.18-8.20. The first group (DI,
D2 and D3) are plants with Ginsufficient air is supplied to the Po reactor, less than that
required to produce stoichiometric combustion. The second group (D4, D5 and D6) are
plants where air is replaced as the oxidant by pure oxygen which is assumed to be available
from an air separation plant.