Efficiency enhancement and NOx emission reduction of a turbo-compressor gas engine by mass and heat recirculations of flue gases

• Heat and mass recirculation of flue gas in a turbo-compressor is proposed.
• A plate-fin heat exchanger (PFHE) as an air pre-heater used for heat recirculation.
• 34.7% reduction in NOx and 5.8% improvement in exergy efficiency were obtained.
• Balance between different objectives is made in NSGA-II algorithm.
• LINMAP, TOPSIS and fuzzy decision making methods used to select optimal solution.

A simple Turbo-Compressor model-GE MS 6001B PLTG-PLN-Sektor Tello Makassar with 30 MW power generation, 27.71% thermal efficiency and 26.07 exergy efficiency (at ISO condition) was considered for the efficiency enhancement and emission reduction. A cross flow plate-fin heat exchanger (PFHE) as an air pre-heater (heat recirculation) along with direct recirculation of a part of flue gases into the combustion chamber (mass recirculation) were considered as modifications of original turbo-compressor gas engine to increase thermal efficiency and reduce NOx emission. In a multi-objective optimization process, geometric and thermal specifications of the plate-fin heat exchanger as well as the percentage of the recirculated flue gas were obtained. The payback time for the capital investment of the heat exchanger and NOx emission were minimized simultaneously while the exergetic efficiency of the gas cycle was maximized and a frontier of optimal solution called as the Pareto frontier was obtained in objective space. The final optimal solution was selected from the Pareto frontier using three different decision-making methods, including the fuzzy Bellman-Zadeh, TOPSIS and LINMAP methods. It was shown that the best results in comparison to the simple cycle led to 34.7% reduction in NOx emission and 5.8% improvement in exergy efficiency (as difference).