Syngas methanation in fluidized bed for an advanced two-stage process of SNG production

Process simulation was conducted to identify the technical feasibility and optimal reactor combination for a simple two-stage methanation process producing substitute natural gas (SNG). With two adiabatic fixed bed reactors in series the reaction temperature control requires high product gas recycle ratio which is much energy-consuming. An isothermal fluidized bed at 723–823K combining a tail-end fixed bed was shown to be a high efficient one-pass methanation process. Over a Ni–Mg/Al2O3 catalyst methanation by laboratory tests in a fluidized bed exhibited obvious technical superiority over that in a fixed bed in terms of activity and carbon deposition on the spent catalyst. By noting further that the low-velocity bubbling fluidized bed leads to extremely high cross-sectional areas for large-scale industrial applications, the optimal reactor combination was proposed to be a fast fluidized bed and a clean-up fixed bed, which not only simplifies the methanation process but also reduces the reactor size and catalyst amount required. The two-stage methanation process combining a fast fluidized bed as the main reactor and a tail-end clean-up fixed bed is potentially a highly competitive advanced technology for SNG production from syngas. For the high heat-carrying capacity of catalyst particles, the heat carried by catalyst is about 12.1 times of that carried by the product gas at a reactor exit temperature of 723K, and the required circulation rate of catalyst was 90.8kg·m−2.s−1, a value that is practically possible for the fast fluidized bed operation. Meanwhile, process simulation shows that the CO conversion and selectivity to CH4 are all close to the hydrodynamic equilibrium and indeed not dominated by kinetics. [Display omitted] •Process simulation was applied to identify optimal reactors for a two-stage methanation process.•Two adiabatic fixed beds in series require high product gas recycle ratio to control temperature.•Methanation in fluidized bed showed higher activity and less carbon deposition on spent catalyst.•Methanation could be completed through the fast reactions occurring on the catalyst surface.•Methanation in a fast fluidized bed is closed to the equilibrium and non-kinetic limited.