Numerical Investigation Of Exhaust Gas Reformer On Hydrogen Production For A LNG Engine

The reformed exhaust gas recirculation can not only improve power and emission characteristics of the natural gas engine,but also raise the overall thermal efficiency of the engine through recovering some waste heat.As a result,it has gotten widespread attention.The reforming process of exhaust gas and fuel directly affects combustion characteristics of the engine,and so the fixed-bed reactor for exhaust reforming is set as the research object.Coupled with the detailed catalytic reaction mechanism,the numerical simulation of the exhaust reforming characteristics within the porous catalytic reaction zone was carried out in the present study.Firstly,the geometric model of the random stacking structure was built through the discrete element method,and the three-dimensional reaction behavior of the mixed reforming gas in the reactor was also presented.The results indicate that the axial and radial distribution of the flow,temperature and species concentration shows large non-uniformity in a fixed-bed reactor for exhaust reforming due to the inhomogeneity distribution of the local porosity in the porous catalytic reaction zone.After the mixture of gas enters the reaction zone,the reforming process mainly involves oxidation reforming,steam reforming and water gas shift reaction.As the axial distance increases,the oxygen concentration in the reactor decreases gradually and the proportion of the oxidation reforming reaction decreases continuously.In addition,the active sites of C（s）increase gradually with the increase of axial distance indicating the carbon deposition increases consistently in the reforming process.Subsequently,the reactor was represented with the porous media model,and the influence of initial conditions on the hydrogen production characteristics from exhaust reforming process was explored.It shows that the methane conversion rate and mass fraction of hydrogen in the outlet decreases with the increase of the gas hourly space velocity,and the molar ratio of H₂ to CO increases first and then decreases.The increase of methane proportion in the gas mixture promotes the hydrogen production ratio per unit mass of methane,while the molar ratio of H₂ to CO decreases.When the proportion of water vapor in the gas mixture increases,the methane conversion rate increases accordingly while the molar fraction of hydrogen in the outlet of the reactor changes slightly.When the tube wall temperature increases,the methane conversion rate increases exponentially,yet the molar ratio of H₂ to CO decreases.Moreover,the change of porosity has a limited effect on the methane conversion rate,while the pressure drop is increasing when the gas mixture flows through the fixed-bed reactor for exhaust reforming.