Metabolic heat production impacts industrial upscaling of ex situ biomethanation trickle-bed reactors

Abstract

Conversion of electrical energy and CO2 to a chemical energy carrier in the form of biomethane, can be carried out through the process chain of biomethanation. Although this process provides a promising source of fossil-free natural gas, limited experience with upscaling still hampers its industrial maturation. Through methanation of raw biogas in a 225 L pilot-scale trickle-bed reactor by use of electrolysis-derived H2, this work uniquely elucidates how metabolic heat from the exothermic biomethanation process present both a challenge and opportunity for upscaling the biomethanation technology. For the first time, a trickle-bed reactor subject to periodic trickling is characterised based on axial temperature profiles as a result of exothermic heat release, in up-flow and down-flow gas configurations. At maximum, reactor core temperatures in the upper thermophilic range (78 and 83 °C) were recorded in certain axial locations, respectively, for substrate gas flow rates of 3.0 and 7.5 NLCO2/Lreactor/d. During a long-term continuous period at 3.0 NLCO2/Lreactor/d, biomethane production with a CH4 content of ≥95 vol% was measured, in both the reactor’s up-flow and down-flow gas configurations. The alternating flow directions and periodic trickling affected both hydrodynamic factors and thermal stability, showing the importance of preheating and humidifying gaseous feed streams and the implementation of a metabolic heat management strategy, by way of cooling, to provide thermal and reaction stability for future upscaling of trickle-bed biomethanation reactors for biological power-to-gas.