Thermodynamic Analysis of Supplementary-Fired Gas Turbine Cycles

This paper presents an analysis of the possibilities for improving the efficiency of an indi-rectly biomass-fired gas turbine (IBFGT) by supplementary direct gas-firing. The supple-mentary firing may be based on natural gas, biogas or pyrolysis gas. Intuitively, sup-plementary firing is expected to result in a high marginal efficiency.

The paper shows that depending on the application, this is not always the case. The interest in this cycle arises from a recent demonstration of the feasibility of a two-stage gasification process through construction of several plants. The gas from this process could be divided into two streams, one for primary and one for supplementary firing.

A preliminary analysis of the ideal, recuperated Brayton cycle shows that for this cycle any supplementary firing will have a marginal efficiency of unity per extra unit of fuel. The same result is obtained for the indirectly fired gas turbine (IFGT) and for the supplementary-fired IFGT. These results show that the combination of external firing and internal firing have the potential of reducing or solving some problems associated with the use of biomass both in the recuperated and the indirectly fired gas turbine: The former requires a clean, expensive fuel.

The latter is limited in efficiency due to limita-tions in material temperature of the heat exchanger. Thus, in the case of an IBFGT, it would appear be very appropriate to use a cheap biomass or waste fuel for low tempera-ture combustion and external firing and use natural gas at a high marginal efficiency for high temperature heating.

However, it is shown that this is not the case for a simple IBFGT supplementary-fired with natural gas. The marginal efficiency of the natural gas is in this case found to be independent of temperature ratio and lower than for the recu-perated gas turbine. Instead, other process changes may be considered in order to obtain a high marginal efficiency on natural gas.

Two possibilities are discussed: Integration between an IFGT and pyrolysis of the biofuel which will result in a highly efficient utilization of the biomass, and integration between external biomass firing, and com-bined internal biomass and natural gas firing.