Fuel consumption and NOx emissions optimization of a marine natural gas engine with lean-burn and reformed exhaust gas recirculation strategies

Abstract

Marine natural gas (NG) engines have received extensive attention in the shipping industry. To further improve the performance of the lean-burn engine, the reformed exhaust gas recirculation (REGR) technology which can realize hydrogen-rich combustion and exhaust gas recirculation is considered. In the present study, a comprehensive investigation on the performance of the marine NG engine with the exhaust gas-fuel reformer (the closed-loop system) was performed by GT-power software. Firstly, the characteristics of the reformer under different excess air ratios (λ) and reformate addition rates (Rre) were analyzed at specific M/O (the volume ratio of CH4 to O2 in the reformer inlet). Then, effects of the lean-burn strategy (the increase of λ) and REGR strategy (the increase of Rre) on combustion, performance and emission of the engine were studied. Moreover, the multi-objective genetic algorithm was utilized to obtain the optimal operating parameters (λ, Rre and M/O) of the closed-loop system. The results showed that the combustion duration is prolonged, the peak in-cylinder pressure and the NOx emissions are reduced with the increase of λ and Rre, but the brake specific fuel consumption (BSFC) is slightly increased. Despite the energy loss during the reforming process, BSFC of the engine with REGR strategy is lower than that of the lean-burn strategy due to the improvement of the engine thermal efficiency. Taking IMO’s Tier Ⅲ NOx standards and lowest BSFC as the optimization objects, the closed-loop system is recommended to work at λ = 1.41–1.49, Rre = 12–14%, and M/O = 1.0–1.2.