Enhancing Methane Yield through Biological Biogas Upgrading

Raw biogas has a relatively low calorific value because methane is the primary combustible component; therefore, it is often upgraded to biomethane. The calorific value of methane is approximately 11.06 kWh/m³ (Jørgensen 2009). Biomethane has characteristics that are very similar to those of natural gas and is considered a promising substitute for fossil natural gas and its use does not require major new infrastructure investments. Various upgrading technologies, including physical and chemical absorption, cryogenic separation, membrane separation, and adsorption, can be used, and most of these technologies are already technologically mature. CO₂ produced during the upgrading process is often vented to the atmosphere.

Conventional biogas upgrading requires significant capital investment in upgrading units, compressors, pumps, heat exchangers, and other auxiliary equipment. In addition, this conversion pathway is energy intensive. In this context, biological biogas upgrading (BBU), also known as hydrogenotrophs-based biological methanation (HBM), has emerged as a promising alternative for biomethane production, as it converts CO₂ into additional methane rather than releasing it into the atmosphere.

There are three main types of BBU configurations: in-situ, ex-situ, and hybrid. In the in-situ conversion process, hydrogen is fed directly into the AD, where it reacts with the CO₂ present in the reactor. One of the main advantages of this approach is that it does not require the construction of new facilities. However, it also has several drawbacks. Injecting excessive hydrogen can disrupt the entire AD process, while the consumption of CO₂ can increase the pH level, potentially inhibiting microbial activity. In addition, limited gas-liquid mass transfer of hydrogen can reduce the overall efficiency of the process. (Kougias, Treu, Benavente, Boe, Campanaro & Angelidaki 2017.)

In ex-situ BBU, an additional anaerobic reactor is used alongside the AD.  Hydrogen produced from an external source and biogas produced by the AD are fed to the anaerobic reactor, which contains hydrogenotrophic cultures (microorganisms that convert CO₂ and H₂ to CH₄).  In the hybrid process, the in-situ and ex-situ configurations are combined together in an integrated manner. This integration enables enrichment of biomethane to more than 98% (Kougias et al. 2017.)

Where  represents the volume of hydrogen,  represents CO₂ fraction, and  is the biogas flow rate. In this calculation, a CO₂ fraction of 40% is assumed, corresponding to a typical biogas composition of 60% CH₄ and 40% CO₂.

In practice, gas–liquid mass transfer of hydrogen is limited. Under favourable conditions, a hydrogen utilization efficiency of up to 84% has been reported (Kougias et al. 2017). In this context, the hydrogen demand for the process would be 45.7 L/h. Similarly, methane conversion efficiencies of up to 92% can be achieved. Based on these assumptions, the additional methane produced corresponds to an increase in total methane yield of approximately 61.3%. (Kougias et al. 2017; Antukh et al. 2022.)  

The cost of methane upgrading in BBU is strongly influenced by the cost of hydrogen. This relationship can be estimated theoretically using the methanation reaction presented in Equation 1. According to Equation 1, 4 mol of H₂ are required to produce 1 mol of CH₄. On a mass basis, this corresponds to 1 kg of H₂ producing approximately 2 kg of CH₄ under ideal conditions. Therefore, the hydrogen-related methane production cost can be approximated as half of the hydrogen price.

When hydrogen utilization efficiency and methane conversion efficiency are taken into account, the hydrogen-related cost contribution increases to approximately 0.65 times the hydrogen price. Figure 2 illustrates the effect of hydrogen-related cost on methane production in BBU at different hydrogen price levels.

The market price of liquefied biogas (LBG) in Finland in 2025 ranges from approximately 1.67 to 1.90 €/kg (Tilastokeskus). Although this value provides an indicative market benchmark, it is not directly comparable to the hydrogen-derived methane production cost estimated in this study, as the latter considers only the hydrogen-related cost contribution and excludes capital costs, reactor operation, gas upgrading, liquefaction, and distribution. Nevertheless, the analysis suggests that biomethane produced via BBU would only approach economic competitiveness if renewable hydrogen costs fall below approximately €3/kg. Since current renewable hydrogen prices in Europe range from €5.3/kg to €13.5/kg, they remain well above this indicative break-even level (Espitalier-Noël, Fonseca, Fraile, Muron, Pawelec, Santos & Staudenmayer 2024).