Decarbonization of the European natural gas grid using hydrogen and methane biologically produced from organic waste: A critical overview

The global CO2 emissions are above 35 GtCO2/year. Recently, the European Commission approved the Green Deal with the aim to reduce the greenhouse gases emissions. Hydrogen, because of its capacity to burn without CO2 formation, is considered one of principal energy option for the future to obtain decarbonization. In 2020, total hydrogen demand was estimated at 8.6 Mt but over 90% of all hydrogen production plants use fossil fuels as feedstock. In perspective, a relatively small part of this hydrogen can be of biological origin. Anaerobic digestion, in particular two stages processes, can assure biological hydrogen and methane productions using organic waste and waste effluents as feedstock. Carbon dioxide in biogas can be removed to obtain a gas blend of methane and hydrogen with typical concentrations of 90:10-70:30% v/v. This review aims to provide an overview on bio-hythane production in Europe at Technology Readiness Level greater or equal to 5 (technology validated in relevant environment). At the moment, there are few examples of biohythane plants at pilot or higher scales, mainly located in Italy, France and United Kingdom. Here reactors with volumes in the range 20-350 L and 300-1000 L were able to assure a stable hydrogen and methane, respectively, production. The highest volumes of 700 and 3800 L for acidogenic and methanogenic phases, respectively, were successfully tested close Milan (Italy).