Page 112 - Advanced Gas Turbine Cycles

Chapter 6

‘WET’ GAS TURBINE PLANTS

6.1. Introduction

As Frutschi and Plancherel [I] have explained, there are two basic gas turbine plants
with water injection; they are illustrated in Fig. 6.1.
Fig. 6. la shows diagrammatically the steam injection gas turbine (STIG) plant; steam,
raised in a heat recovery steam generator (HRSG) downstream of the turbine, is injected
into the combustion chamber or into the turbine nozzle guide vanes.
Fig. 6. I b shows diagrammatically the evaporative gas turbine (EGT) in which water is
injected into the compressor outlet and is evaporated there; the mixture may then be
further heated in the ‘cold’ side of a heat exchanger. It enters the combustion chamber and
then passes through the turbine and the ‘hot’ side of the heat exchanger.
There are many variations on these two basic cycles which will be considered later. But
first we discuss the basic thermodynamics of the STIG and EGT plants.

6.2. Simple analyses of STIG type plants
6.2.1. The basic STIG plant

Fig. 6.2 shows a simplified diagram of the basic STIG plant with steam injection S per
unit air flow into the combustion chamber; the state points are numbered. Lloyd [2]
presented a simple analysis for such a STIG plant based on ‘heat input’, work output and
‘heat rejected’ (as though it were a closed cycle air and watedsteam plant, with external
heat supplied instead of combustion and the exhaust steam and air restored to their entry
conditions by heat rejection). His analysis is adapted here to deal with an open cycle plant
with a fuel inputfto the combustion chamber per unit air flow, at ambient temperature To,
i.e. a fuel enthalpy flux of.fifo. For the combustion chamber, we may write
ha2 +fhfO + SA,, = ( 1 +f)hg3 + Sk,, (6.1)

where subscripts a, g and s refer to air, gas (products of combustion) and steam. The
enthalpy of the steam quantity (h,) is at the same temperature as the gas, and for
convenience is carried separately through the analysis, i.e. the total enthalpy is H =
(I +f)h, + Sh,. In reality, the steam and gas are fully mixed at all stations downstream of
the combustion process.