Page 151 - Advanced Gas Turbine Cycles

122 Advanced gas turbine cycles

7.5.2. Regenerative feed heating

For a comprehensive discussion on feed heating in a CCGT plant, readers may refer to
Kehlhofer’s excellent practical book on CCGTs [2]; a summary of this discussion is given
below.
Kehlhofer takes the gas turbine as a ‘given’ plant and then concentrates on the
optimisation of the steam plant. He discusses the question of the limitation on the stack and
water entry temperatures in some detail, their interaction with the choice of p, in a single
pressure steam cycle, and the choice of two values of pc in a dual pressure steam cycle.
Considering the economiser of the HRSG he also argues that the dewpoint of the gases at
exhaust from the HRSG must be less than the feed-water entry temperature; for sulphur
free fuels the water dewpoint controls, whereas for fuels with sulphur a ‘sulphuric acid’
dewpoint (at a higher temperature) controls. Through these limitations on the exhaust gas
temperature, the choice of fuel with or without sulphur content (distillate oil or natural gas,
respectively) has a critical influence ab initio on the choice of the thermodynamic system.
For the simple single pressure system with feed heating, Kehlhofer first points out that
the amount of steam production (M,) is controlled by the pinch point condition if the steam
pressure (p,) is selected, as indicated earlier (Eq. (7.21)). However, with fuel oil
containing sulphur, the feed-water temperature at entry to the HRSG is set quite high (Tb is
about 130°C), so the heat that can be extracted from the exhaust gases beyond the pinch
point [M,(h, - hb)] is limited. As shown by Rufli, the condensate can be brought up to Tb
by a single stage of bled steam heating, in a direct contact heater, the steam tapping
pressure being set approximately by the temperature Tb.
Kehlhofer then suggests that more heat can be extracted from the exhaust gases, even if
there is a high limiting value of Tb (imposed by use of fuel oil with a high sulphur content).
It is thermodynamically better to do this without regenerative feed heating, which leads to
less work output from the steam turbine. For a single pressure system with a pre-heating
loop, the extra heat is extracted from the exhaust gases by steam raised in a low pressure
evaporator in the loop (as shown in Fig. 7.8, after Wunsch [ll]). The evaporation
temperature will be set by the ‘sulphuric acid’ dewpoint (and feed water entry temperature
Tb = 130°C). The irreversibility involved in raising the feed water to temperature Tb is
split between that arising from the heat transfer from gas to the evaporation (pre-heater)
loop and that in the deaeratodfeed heater. It is shown in Ref. [I] that the total
irreversibility is just the same as that which would have occurred if the water had been
heated from condenser temperature entirely in the HRSG. Thus, the simple method of
calculation described at the beginning of Section 7.5.1 (with no feed water heating and
Tb = T,) is valid.
Kehlhofer explains that the pre-heating loop must be designed so that the heat extracted
is sufficient to raise the temperature of the feed water flow from condenser temperature T,
to T,! (see Fig. 7.6). The available heat increases with live steam pressure (pc), for selected
Tb(= T,) and given gas turbine conditions, but the heat required to preheat the feed water is
set by (T,! - T,). The live steam pressure is thus determined from the heat balance in the
pre-heater if the heating of the feed water by bled steam is to be avoided; but the optimum
(low) live steam pressure may not be achievable because of the requirement set by this
heat balance.