Hydrogen production and energy efficiency optimization of exhaust reformer for marine NG engines: A view of surface reaction kinetics

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In the present study, for an insight into the catalytic reforming process of the exhaust reformer and its optimization strategy, a one-dimensional inhomogeneous reforming model and a thermodynamic model were implemented in a Ni/Al2O3 catalytic packed-bed description of single reforming tube inside the reformer. The models were evaluated by comparison of numerical simulations with data derived from reforming experiments in a flow reactor over a columnar nickel-based catalyst. Meanwhile, the effects on reforming characteristics and energetics of the process of exhaust gas-methane reforming inside the reformer was investigated from a view of surface reaction kinetics under the variation of temperature and O2 concentration of the exhaust gas from the engines, as well as the supply of CH4 and H2O into reformer. The results showed that the hydrogen production characteristics of exhaust gas-methane reforming is mainly controlled by the exhaust temperature and various inlet feedstock of the reformer. There is a critical exhaust temperature of 610 K to activate the process of exhaust gas-methane reforming. For the efficient operation of the exhaust reformer, it is necessary to increase the O2 concentration of the exhaust gas via adjusting the excess air coefficient of the engine, because in this case the dehydrogenation process of CHi(s) is enhanced and more hydrogen production on the catalyst surface with an atmosphere of more H(s) through the desorption way. Compared with the thermodynamic values, the used catalyst still has some space for improvement in the catalytic properties to approach the thermodynamic hydrogen production characteristics of the reforming system at different initial exhaust temperatures and M/O values. Summarily, the optimal operation conditions for the exhaust reformer were determined: M/O = 0.8–1.5 and S/M = 0.5–1.5, which can achieve efficient hydrogen production (about 13–23 %) and maintain high reforming energy efficiency (≥60 %).