Gas turbines for polygeneration? A thermodynamic investigation of a fuel rich gas turbine cycle
Gas turbines as used nowadays are working far in the fuel lean regime, which is most reasonable for mobile applications, since the formation of pollutants and soot are avoided while the temperatures remain low enough to avoid damage of the turbine. However, from a thermodynamic point of view the exergy utilization is far from optimum at such conditions.
For stationary conditions a different approach may be worth a second thought: the use of gas turbines as chemical reactors for hydrogen and carbon monoxide production in combination with power generation and the utilization of the exhaust enthalpy stream. A gas turbine model cycle is analyzed using complex equilibria including radicals and chemical exergies. Chemical exergies were calculated from equilibrating the gas mixtures at different points in each process with a large excess of moist air. Methane was studied as an exemplary fuel.
Comparing the exergy losses of the idealized gas turbine process, the losses for the fuel rich stoichiometry are lower than at the lean stoichiometry used in gas turbines nowadays. The exact values of the exergetic efficiency depend on the pressure ratio, which was studied in the range of 10 to 30. The hydrogen to carbon monoxide ratio would be typically near 2.2, while the adiabatic flame temperature would be in a range which either would cause no damage to typical gas turbines or could be handled with carbon fiber reinforced carbon. The composition of the gases is likely to change within the turbine, where temperature and enthalpy drops. This was considered in additional calculations where chemical equilibration of the gas mixture in the turbine is considered. The possibility to combine a partial oxidation with an energy conversion process and thus produce syngas mixtures would add an additional flexibility to the gas turbine process, which is worth consideration.