Oxy-fuel combustion feasibility of compression ignition engines using oxygen separation membranes for enabling carbon dioxide capture

Highlights: • An oxy-fuel combustion is studied in a compression ignition engine. • A membrane is used to separate O2 from the air, powered by waste heat. • The CO₂-fuel mixture is diluted with exhaust gas recirculation using two different control systems. • Similar brake powers and brake specific fuel consumptions are obtained at high speeds. • At low speeds, the brake power increases 30 • A CO₂ capture system can be easily added to the engine, leading to an emissionless operation. Oxy-fuel combustion concept is studied in a compression ignition engine (CIE) using Mixed Ionic-electronic Conducting Membranes (MIECs) to separate oxygen (O₂) from air in order to achieve a clean combustion eliminating completely nitrogen oxides (NO_x) emissions and enabling upcoming carbon dioxide (CO₂) capture. Exhaust gas recirculation (EGR), composed mainly by CO₂ and water, is used to control the in-cylinder temperature and exhaust gas wasted energy is recovered for producing the O₂ required by the engine by heating up the MIEC. For this purpose, different engine configurations are analyzed in order to find out an optimum one in terms of energy efficiency and engine performance. Basically, two different EGR control systems (CS) are studied: one with a variable geometry turbine (VGT) on EGR line and other with a VGT on cylinder exhaust line. A simulation software, so-called Virtual Engine Model (VEMOD), is employed in this study to build and analyze the proposed oxy-fuel engine model which is calibrated with experimental data. The engine and its auxiliary components (turbochargers and heat exchangers) are assessed under oxy-fuel combustion conditions for the engine full load operation points from 1250 rpm to 3500 rpm. If compared to a conventional CIE, at high engine speeds the proposed oxy-fuel combustion engine provides similar brake power and indicated efficiency, whereas at low engine speeds, despite of it yields higher brake-specific fuel consumption (BSFC on average, more than 10%), a brake power enhancement (on average, more than 30%) is observed. In any way the breakthrough result is the feasible end of correlation between BSFC and CO₂ emissions due to CO₂ capture.