## 4 Emerging Biogas Upgrading Technologies
### Separation via Phase Addition
#### 4.1.3 Catalytic Methanation  
Methanation consists of converting the CO\({}_{2}\) present in the biogas into methane by continuously adding H\({}_{2}\) to the process, thereby increasing carbon utilization [54]. There are two different types of methanation: catalytic and biological. They are based on the Sabatier reaction (Equation (10)), in which CO\({}_{2}\) reacts with H\({}_{2}\) to produce CH\({}_{4}\)[102] through a very exothermic reaction:  
\[\text{CO}_{2}+4\text{ H}_{2}\rightleftharpoons\text{CH}_{4}+2\text{H}_{2}\text{O} \tag{10}\]  
Catalysts that have been widely reported include noble metal-based catalysts and non-noble metal-based catalysts [103]. Ruthenium (Ru) is highly reactive for methanation, but its effectiveness and selectivity depend on the chosen support, the dispersion of metallic Ru on the support, and the inclusion of promoters that enhance Ru activity [104]. Nickel (Ni) is extensively investigated as a metal for CO\({}_{2}\) methanation, given its relatively high activity, outstanding CH\({}_{4}\) selectivity, and lower cost compared to noble metals [105] (cataly at 250-350 \({}^{\circ}\)C and 10-20 bar) [79]. CO\({}_{2}\) can be separated from the CH\({}_{4}\) in raw biogas, but the latest research shows that feeding the reactor with pre-treated biogas (CH\({}_{4}\) and CO\({}_{2}\) after the removal of H\({}_{2}\)S and other pollutants) allows better thermic control of the reactor [106].  
Hydrogen is obtained via water electrolysis using surplus energy from renewable sources [52; 78]. For this reason, water electrolysis to obtain hydrogen is studied as a form of energy storage because of the fluctuation in electricity production from renewable sources, creating a new technology called Power-to-Gas [75; 102]. However, this process requires a high energy demand, while trace compounds from the biogas degenerate the catalyst, resulting in a need for periodical replacement, and the costs are high, especially those derived from hydrogen production [75; 107]. There are several demonstration plants in Europe to prove the viability of the technology, and it is forecasted that catalytic methanation will be very interesting for biogas plants in the medium-term future [79].  
[PERSON] et al. [108] prepared a NiRh/CeO\({}_{2}\) catalyst. This was tested in a simulated biogas methanation reaction, achieving CO\({}_{2}\) conversion values as high as 70%. The effect of the H\({}_{2}\)/CO\({}_{2}\) molar ratio on the simulated biogas feed was evaluated, demonstrating that substoichiometric H\({}_{2}\)/CO\({}_{2}\) molar ratios are suitable for obtaining high-quality upgraded biogas. CO\({}_{2}\) conversion ranged between 62.5 and 66.7% at a reaction temperature of 500-600 \({}^{\circ}\)C.  
The carbon emissions associated with producing synthetic NG via the catalytic methanation of biogas were assessed [109]. This method is considered viable for a partial reduction of carbon emissions in gas network infrastructure. As a result, synthetic NG can facilitate the intermittent integration of renewable sources by enabling seasonal storage and offering a gas alternative with lower carbon emissions. The resulting gas from catalytic methanation may achieve high CH\({}_{4}\) content (up to 99%) with nearly 100% conversion of CO\({}_{2}\)[110].