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To capture as much carbon dioxide as possible while minimizing the performance degradation, this study proposes two solid oxide fuel cell (SOFC)/polymer electrolyte membrane fuel cell (PEMFC) hybrid systems. One is a hybrid system in which the PEMFC not only generates power, but also performs the role of CC (hybrid system 1). The other is a hybrid system in which pressure swing adaption (PSA) is incorporated to capture carbon dioxide, and consumption of hydrogen in the PEMFC is maximized (hybrid system 2). In addition, an organic Rankine cycle (ORC) was added to further improve the performance of the hybrid systems by utilizing the waste heat of exhaust gas. The cascade effects between SOFC and PEMFC contribute to achieving nearly complete CC and high efficiency. Especially, hybrid system 2 achieved 10% and 5%p improvements over the stand-alone SOFC system in net power and efficiency. It is promising in that the proposed systems utilizing cascade effects can achieve nearly zero carbon emission and high efficiency despite the use of natural gas without hydrogen supply. In other words, the proposed systems are expected to contribute to the realization of carbon neutrality without waiting for the technological maturation associated with hydrogen.

The effect of hydrogen co-firing on the performance and operating conditions of gas turbines (GTs) and gas turbine combined cycles (GTCCs) was investigated. Two operating scenarios were applied, and off-design analyses were conducted for three GTs with different turbine inlet temperatures (TITs). In the first scenario, the TIT was kept constant. As the hydrogen co-firing ratio increases, the pressure ratio of GT increased, and the power outputs of both GT and GTCC increased. With 100 % hydrogen in the 1650 °C GT, power output and efficiency improved by 5.36 % and 1.55 %p for GT, and 2.90 % and 0.72 %p for GTCC. In the second scenario, the power output was kept constant. The operating point moves toward lower pressure ratio and TIT regimes, and the efficiency of both GT and GTCC increased. With 100 % hydrogen in the 1650 °C GT, TIT decreased by 50.3K, and efficiency increased by 1.42 %p for GT, and 24.9K and 0.42 %p for GTCC. In both scenarios, the higher the TIT, the impact of hydrogen co-firing was more pronounced. It is expected that the second scenario might reduce NOx emissions while still increasing system efficiency in comparison to the NG fired case by operating the GT at a lower TIT and equivalence ratio.