Methanation of syngas from biomass gasification: Small-scale plant design in Aspen Plus

The objective of this study is to investigate the upgrading of low-quality nitrogen-diluted syngas derived from biomass air gasification processes into a methane-rich gas stream. Both the thermodynamic and the kinetic aspects are addressed in the paper. Using the Aspen Plus software, a thermodynamic analysis was conducted; then, different plant designs are simulated and compared, including reactor sizing and performance. The results demonstrate that the upgrading of diluted syngas poses challenges which limit its application to small-scale decentralized systems. It was found that a system comprising of four adiabatic fixed-bed reactors, inter-cooling, and efficient water removal achieves a favorable balance between performance and cost. Operating the system at a pressure of 5 bar is deemed adequate as it reduces the required catalyst mass and prevents solid carbon deposition. Notably, this configuration achieved good results, including a 99.4 % CO conversion, 89.3 % CO2 conversion, and 95.6 % CH4 yield. The final methane molar content reached 26.4 %, with a calorific value of 8.62 MJ/Nm3(STP).