Thermodynamic and kinetic analyses for hydrogen production via methane autothermal reforming

A systemic thermodynamic analysis was conducted for the autothermal reforming of natural gas to produce hydrogen. A kinetic modeling was made for the autothermal reforming by employing the pre-mixed laminar model included in CHEMKIN package that incorporates the mechanisms of CH₄ oxidation, steam reforming and dry reforming; kinetic parameters for methane reforming over Ni-based catalyst, widely used in the natural gas reforming industry, were adopted. The results show that the equilibrium composition of outflow gases is depended on the reaction temperature, the air-fuel ratio and water-fuel ratio. The pressure has a limited impact on the product distribution but has a remarkable impact on the time needed to reach the equilibrium. Under 715℃~730℃, 0.7MPa~1.0MPa, O₂/CH₄ of 0.60~0.70, and H₂O/CH₄ of 3.15~3.25, the fraction of H₂ in the product is higher than 68%, while CO does not exceed 10% (dry basis) and the carbon deposition is close to 0. The kinetic analysis suggests two distinct stages during the autothermal reforming process. The initial stage is a rapid oxidation zone, where H₂O and CO₂ are the main products. The second stage is a slow conversion zone, where methane steam reforming reaction as well as water gas shift reaction and slight dry reforming take place, with H₂, CO and CO₂ being the main products. Adding H₂O in the feed gas is a visible measure for avoiding hot spot in the reactor and controlling the formation of CO.