Three Reactors Chemical Looping Combustion for High Efficiency Electricity Generation With CO₂ Capture From Natural Gas

Chemical-Looping Combustion (CLC) is a process where fuel oxidation is accomplished by the oxygen carried by a metal oxide, circulating across two reactors: a reduction reactor (reducing the metal oxide by oxidizing the natural gas fuel) and an oxidation reactor (re-oxidizing the metal by reacting with air, a strongly exothermic reaction). The system produces: (i) a stream of oxidation products (CO₂ and H₂O), ready for carbon sequestration after water separation and CO₂ liquefaction; (ii) a stream of hot air (deprived of some oxygen) used as working fluid of a gas turbine cycle. Due to the moderate temperature (∼850°C) of this stream, sensibly lower than those adopted in commercial gas turbines, the combined cycle arranged around this concept suffers from poor conversion efficiency and, therefore, economics. In the present paper, the basic CLC arrangement is modified by inserting a third reactor in the loop. This reactor, by exploiting an intermediate oxidation state of the circulating metal, produces H₂ used as decarbonized fuel to raise the temperature of the air coming from the oxidation reactor, up to the highest value allowed by the modern gas turbine technology (∼1350°C), thus achieving elevated efficiency and specific power output. This paper is aimed to assess the potential of power cycles based on the three reactors (CLC3) arrangement. More specifically, we will discuss the plant configuration, the process optimization and the performance prediction. Results show that the CLC3 system is very promising: the net LHV efficiency of the best configuration exceeds 51%, an outstanding figure for a natural gas power cycle producing liquid, disposal-ready CO₂ and negligible NO_x emissions. Commercial gas turbines can be easily adapted to operate in the specific conditions of the CLC3 arrangement which, apart from the reactors system, does not require the development of novel technologies and/or high-risk components. The paper also reports a final comparison with a rival technology based on natural gas partial oxidation, water-gas shift reaction and CO₂ separation by MDEA absorption. This work has been performed within the research on the Italian Electrical System “Ricerca di Sistema”, Ministerial Decrees of January 26 – 2000, and April 17 – 2001.