New high efficiency mixed cycles with air-blown combustion for CO2 emission abatement

In this paper a new advanced mixed cycle (AMC) for CO2 emission abatement with high conversion efficiency is presented. The AMC plant lay-out consists of a reheat gas turbine with steam injection in the first combustion chamber, a steam turbine for steam expansion before its injection, a heat recovery boiler for superheated and resuperheated steam generation and an atmospheric separator for water recovery from exhaust gas mixture. The steam recirculation in the cycle allows to reduce the excess of air to limit the turbine inlet temperature and then to enrich the exhaust gas by CO2, as it occurs in combined cycle provided with exhaust gas recirculation at the compressor inlet. This involves a stack flow rate much lower than in conventional cycle configuration so that exhaust gas treatment for CO2 removal may be useful applied. In this work the chemical absorption technique for CO2 removal has been considered. The thermodynamic performance of the proposed AMC plant has been investigated in comparison with that attainable by combined cycle power plants (CC). This comparison has been developed pointing out the efficiency decrease involved by the CO2 removal systems and by the unit for the liquefaction of the removed carbon dioxide. The main result of the performed investigation is that while the two plants attain the same efficiency level without CO2 removal (about 56% for AMC and 55.8% for CC) the AMC plant achieves a net electric efficiency of about 50% with CO2 removal and liquefaction units: it's over 2 points higher than the efficiency evaluated for the CC equipped with the same CO2 units (about 47.7%). The final carbon dioxide emissions are resulted of about 0.04 kg/kWh for AMC and CC, while the emissions of the plants without CO2 removal systems are of about 0.36 kg/kWh.