Numerical simulation on the effect of operating conditions and syngas compositions for synthetic natural gas production via methanation reaction

Abstract

The synthetic natural gas (SNG) production via methanation reaction was investigated in this study. A fixed-bed tubular reactor was used as the physical model and axisymmetric non-isothermal governing equations for the gas flow, energy transfer and species transport were solved numerically. The operating conditions (inlet pressure, inlet temperature, feed rate, and heat transfer) and the reactant gas composition effects on the reaction were studied. CO conversion, CH4 yield, H2 efficiency and CO2 yield were used to characterize the reaction performance. It was found that CO methanation is sensitive to the reaction temperature. Based on the operating conditions used in this study, CO conversion from 0% at reactant inlet temperature (Tin) of 300°C abruptly increases to 100% at Tin=380°C. As Tin is greater than 380°C, a hot spot exists in the reactor, leading to decreases in CO conversion and CH4 yield. It was also found that under the same operating condition, 60% decrease in CO conversion could result from an adiabatic reactor compared with the isothermal reactor. CO conversion and CH4 yield can be enhanced by increasing the reaction pressure and H2/CO ratio, and decreasing the space velocity. With the addition of product species (CH4, CO2, and H2O) as the reactants, a decrease in CO conversion was found except for the addition of H2O. It was found that the CH4 yield was decreased by product species addition.Low CO conversion and CH4 yield were obtained when coal- or biomass-derived syngas was used as the feedstock for methanation reaction due to low H2/CO ratio. To enhance SNG production via syngas methanation, it is suggested that the water-gas shift reaction be employed to increase the H2/CO ratio before carrying out methanation reaction.