Page 167 - Advanced Gas Turbine Cycles

Chapter 8. Novel gas turbine cycles 135

8.2.4. Plants (0) with modijication of the oxidant in combustion

In conventional cycles, combustion is the major source of irreversibility, leading to
reduction in thermal efficiency. Some novel plants involve partial oxidation (PO) of the
fuel in two or more stages, with the temperature increased before each stage of
combustion, and the combustion irreversibility consequently reduced. In other plants full
oxidation is employed which makes C02 removal easier.
Six cycles with oxidant modification are listed as
D1 the simple PO open CBT cycle-involving staged combustion of the fuel;
D2 the Po open CCGT cycle-involving staged combustion of the fuel and low pressure
C02 removal;
D3 the semi-closed CICBTBTX cycle-involving staged partial combustion of the fuel,
intercooling, recuperation and low pressure C02 removal;
D4 the ‘semi-closed’ CBT or CCGT plant with full oxidation-oxygen supplied to the
combustion chamber instead of air, with C02 removal at low pressure level;
D5 the ‘semi-closed’ CBT plant with full oxidation-oxygen supplied to the combustion
chamber instead of air, with C02 removal at high pressure level;
D6 the Matiant cycle-an almost closed CICBTBTX cycle using full oxidation and full
COz removal.

8.2.5. Outline of discussion of novel cycles

Below we describe
(i) the additional equipment that is required for plants with C02 sequestration and
liquefaction, at high or low pressure (in Section 8.3);
(ii) the concept of the ‘semi-closed’ cycle which features in some of the proposed plants
(in Section 8.4); and
(iii) the various chemical reactions involved in combustion modification, through
chemical recuperation, PO, etc. (in Section 8.5).
We then discuss in more detail the individual cycles listed above (in Section 8.6).
We also give calculations of the performance of some of these various gas turbine
plants. Comparison between such calculations is often difficult, even ‘spot’ calculations at
a single condition with state points specified in the cycle, because of the thermodynamic
assumptions that have to be made (e.g. how closely conditions in a chemical reformer
approach equilibrium). Performance calculations by different inventodauthors are also
dependent upon assumed levels of component performance such as turbomachinery
polytropic efficiency, required turbine cooling air flows and heat exchanger effectiveness;
if these are not identical in the cases compared then such comparisons of overall
performance become invalid. However, we attempt to provide some performance
calculations where appropriate in the rest of the chapter.
Finally, in Section 8.7, we describe some modifications of the integrated gasification
combined cycle (IGCC) which enable COz to be removed (Cycles E).