Circular Economy Model Applied to an Anaerobic Digester Cohabiting With an Electrolyser and a Biological Methanation Unit to Produce Additional Biomethane That Can Be Injected Into a Gas Network

The general aim of the research is to increase the biomethane production of a methanization platform using a combined aerobic digester (AD) system in conjunction with a hydrogren production unit and a biological methanation unit. Today, the CH₄ from conventional AD is mainly produced by acetoclastic methanogenesis. However, the CO₂ fraction of the biogas is generally not used and is released directly into the atmosphere.

There is a growing body of literature on the conversion of CO₂ from digester gas to produce additional CH₄, thereby reducing GHG emissions from the biogas plant. Ultimately, the addition of the proposed technologies to the biomethanization project would enable a theoretical conversion of 75-90% of the mass of CO₂ present in the biogas, thereby increasing biomethane production by 52-71%, from 5 M Nm³ of CH₄/year (without the proposed technologies) to 7.6-8.5 M Nm³ (with the proposed technologies).

With regard to the annual quantification of GHG emissions reduced or avoided by implementing the project with the technologies, the solution's emissions are negative, since the CO2 that would have been emitted into the atmosphere is instead used by the biomethanizer to be converted into CH₄; and the reference scenario's emissions are related to the GHGs that would normally have been emitted if conventional natural gas had been combusted (based on a cradle-to-combustion) lifecycle analysis approach). As a result, the difference in GHG emissions reduced or avoided between the two scenarios is estimated at -24,647 tCO₂e/year thanks to the implementation of the proposed technologies. Using the biomethane from the AD and the synthetic methane from the biomethanizer as a substitute for fossil natural gas would therefore make it possible to avoid these emissions and create carbon credits.

Two scenarios have been developed to estimate the cost per tonne of the CO₂e reduced or avoided in Quebec. In the optimistic scenario, with a natural gas sales rate applied to the residential sector ($18.57/GJ; $0.71/m³), the cost per tonne of CO₂e reduced or avoided is $96.91/tCO₂e. On the other hand, in the pessimistic scenario, with a natural gas sales rate applied to industry ($6.80/GJ; $0.26/m³), the cost per tonne of CO₂e reduced or avoided is $243.42/tCO₂e. The average cost of these two scenarios is $170.17/tCO₂e.