Table 1. Overview of Different Processes Capable of Converting Renewable Biomass to Methane Gas.

	Anaerobic digestion	Syngas biomethanation	Syngas methanation
Pretreatment	Mechanical, Chemical, Biological	Gasification	Gasification
Conversion mechanism	Saccharolysis, acidogenesis, methanogenesis	Hydrogenogenesis, methanogenesis	Chemical methanation
Catalyst	Biological (undefined mixed culture)	Biological (defined (co)culture)	Metal catalysts (e.g., nickel)
Disadvantages	Low substrate conversion efficiency	Low production rates compared to chemical conversion	Sensitive to different syngas compositions
	High CO2 content in outlet (up to 50%)		Sensitive to syngas impurities
	Relatively low production rates	Outlet gas not completely free of CO2, CO and H2	Relatively expensive catalysts
			Outlet gas not completely free of CO2, CO and H2
Advantages	Robust and cheap process	Cheap, self-replicating catalysts	High production rates
	Can convert dilute organic wastes (e.g., wastewater)	Good production rate for a biological system	High methane content in a single step
		High methane content in a single step
		Robust for different syngas compositions
		Tolerance to syngas pollutants
Gas composition	CH4, CO2	CH4, H2, CO2, CO	CH4, H2, CO2, CO
Chemical efficiencya	20–40%26	N.D.	50–70% (wood to SNG)35,36

^aEnergy of the product compared to the energy content of the original feedstock.