Exergy Analisys of a Syngas-Fueled Combined Cycle with Chemical-Looping Combustion and CO₂ sequestration

Fossil fuels are still widely used for power generation. Nevertheless, it is possible to attain a short and medium term substantial reduction of greenhouse gas emissions to atmosphere through a sequestration of the CO₂ produced in fuels oxidation.

Chemical-looping combustion (CLC) is a thermochemical process where fuel oxidation is carried out through an intermediate agent that actuates as oxygen carrier between two separated reactors: i) a reduction reactor or fuel reactor, where the oxygen carrier is reduced oxidizing the fuel, and ii) an oxidation reactor or air reactor, where the oxygen carrier is oxidized in air. Overall, the system carries out the same chemical transformation as a conventional combustion, with the fundamental advantage of segregating the oxidation products CO₂ and H₂O into an output flow not diluted in air, where the only non-condensable gas is CO₂. Therefore, CLC allows an integration of CO₂ capture in the power plant without energy penalty. In addition, a lower exergy destruction in the combustion chemical transformation is achieved, leading to a greater thermal efficiency of the power generation process. Most efforts have been devoted to CLC systems based on methane as a fuel.

This paper focus on a second-law analysis of a combined cycle power plant with CO₂ sequestration using syngas as fuel and a CLC combustion system, considering the CO₂ sequestration and storage stage. A comprehensive analysis of the performance of such a power plant from a Second Law point of view is carried out. The exergy flows along the power plant are evaluated and compared with a similar system based in a conventional combustion, finding a notable increase of the power plant efficiency.  In addition, the power plant behaviour is simulated in a range of operating conditions, leading to an optimization of the key parameters of the cycle.  Also, in order to investigate the influence of syngas composition on the results, different H₂-content fuels are considered.