Methanation of Milena product gas for the production of bio-SNG

Synthetic Natural Gas (SNG) from biomass, also called “Green Gas”, is a very promising product, because it can be used for all applications that are already known for natural gas. Compared to other biomass conversion routes, the major advantage of SNG production is the potential to use the existing gas infrastructure. ECN is developing the Milena indirect gasifier, which is very suitable for SNG production, because it produces already high amounts of methane in the gasification. The methane content in the product gas accounts for already a large amount of the total energy content. This is beneficial to the overall process efficiency and is also good for the methanation process, where the conversion from syngas to methane is a strongly exothermic reaction and where it is difficult to control the process temperature. The methanation process has been studied in the past, but application has been scarce. In order to determine the optimal methanation conditions for the Milena product gas, thermodynamic equilibrium calculations were done with AspenPlus. It was found that in the methanation process there is a risk of carbon formation, especially at lower temperatures and also at higher pressures. Adding additional steam to the product gas can reduce the risk of carbon formation. If carbon formation can be suppressed, the methane yield increases with lower temperatures and higher pressures. At lower temperatures (below 300 °C), the effect of the pressure on the methane yield is not as strong as at higher temperatures. The maximum energy efficiency of the methanation of the Milena product gas is 88%. This gives a maximum biomass to SNG yield of 70% with the Milena gasifier. This is a high efficiency, because the Milena gasifier produces already a large amount of methane. The overall yield could be increased if conditions can be found for the gasifier to produce even more methane.