Numerical Investigation On The Reacting Characteristics Of Exhaust Gas-Fuel Reformer For An LNG Engine

The on-board hydrogen addition in LNG engines and waste heat recovery can be achieved through the technology of reformed exhaust gas recirculation,which improves the emission of LNG engines.In the closed-loop system of reformed exhaust gas recirculation,the reaction process of exhaust gas and fuel in the reformer is the key to realize the flexible and adjustable hydrogen production of the engine.Hence,the catalytic reaction characteristics in the porous zone of the fixed bed reformer were studied by a numerical model coupled with a detailed methane catalytic reaction mechanism,which provides a theoretical basis for the design and optimization of the reformer.First,the porous media model was used to simplify the porous zone formed by particle accumulation,and a two-dimensional numerical model of single-tube reforming reaction was constructed.The model was validated by the fixed-bed reforming reaction test data,on this basis,the catalytic reaction characteristics of exhaust gas-fuel in the reformer under different initial conditions were studied.The simulation results show that the methane reforming rate and the hydrogen molar fraction at the outlet of the reactor first increase and then decrease with the increase of the mass flow rate of the inlet gas,while the molar ratio of H₂ to CO decreases continuously.When the proportion of methane in the mixed gas increases,the methane reforming rate decreases,the hydrogen production ratio per unit mass of methane decreases continuously.The addition of water vapor promotes the steam reforming process of methane,which makes the methane reforming rate increase continuously.When the inlet temperature increases,the methane conversion rate increases greatly,but the molar ratio of H₂ to CO decreases continuously.When the diameter of catalyst pellets increases,the void fraction in the reaction zone of fixed bed exhaust gas-fuel reforming reactor increases but the pressure drop in the tube of fixed bed reformer decreases.As the catalyst stacking rises,methane reforming rate,hydrogen molar fraction at reactor outlet and the molar ratio of H₂ to CO increase rapidly at first,then remained unchanged,respectively.The outlet temperature of reactor decreases with the increase of particle packing length,but the maximum temperature in the reactor zone remains constant with the increase of the pellet accumulation length.In this paper,the distribution of three-dimensional temperature field,flow field and concentration field in the shell side and tube side of the tube of counter-current fixed bed reformer was further studied.The reaction process of hydrogen production was analyzed by the distribution of mass fraction of each component in the reformer along the axial direction.At the same time,the effects of shell-side temperature and different ratio of exhaust on the reactivity characteristics of tubular reformer were investigated.The results show that increasing shell-side exhaust temperature can not only improve the methane reforming rate,but also avoid the local overheating in the tube which causes catalyst deactivation.However,with the increase of shell side temperature,the molar ratio of H₂ to CO tends to decrease.Increasing the amount of exhaust gas entering the pipeline will result in the ascending of the maximum temperature of the pipeline,the reduction of methane reforming rate and hydrogen mass fraction,but the total amount of hydrogen production will increase,and the molar ratio of H₂ to CO will decrease with the increase of the amount of exhaust gas entering the pipeline.