## 4 Emerging Biogas Upgrading Technologies
### Separation via Phase Addition
#### 4.1.1 Inorganic Solvent Scrubbing  
ISS is a type of chemical absorption still in the development phase for biogas upgrading, but it is mature for CO2 capture and removal in post-combustion gas flows [68]. The used solvents are an aqueous solution of alkaline salts such as NaOH, KOH, Ca(OH)2, and NH4 OH [61; 68]. The main advantage of inorganic solvents compared to chemical absorption using amines is that they are usually cheaper and widely available [68; 82]. Furthermore, NaOH and NH4 OH have a higher theoretical CO2 capture capacity compared \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Parameters** & **PSA** & **WS** & **OPS** & **AS** & **MS** & **CS** \\ \hline \multirow{6}{*}{1000 Nm\({}^{3}\)/h biogas} & 1750 [77] & 1000 [77] & 1000 [77] & 2000 [77] & 1782 [61] & \multirow{6}{*}{-} \\  & 1782 [61] & 1620 [61] & 1944 [61] & 2106 [61] & & 2000 [65; 77] & - \\  & 2000 [65; 66] & 1700 [65] & 2000 [65] & 2500 [65] & 1300-2400 [66] & & \\  & & 2592 [61] & 2187 [61] & 2187 [61] & 216 [61] & & \\ \multirow{6}{*}{Specific CAPEX for different flows (\(\ell\)/(Nm\({}^{3}\)/h))} & 2800 [65] & 2700 [65] & 3078 [61] & 2916 [61] & 2750 [65] & 600 Nm\({}^{3}\)/h: \\  & 2900 [66] & 2500 [66] & 3300 [65] & 3300 [66] & 3500 [62] & 3500–3700 [62] & 2300 [66] \\ \cline{1-1}  & 3700 [62] & 3500 [62] & 3500 [62] & 3500 [62] & 3500 [62] & & \\ \cline{1-1}  & & & & 3890 [61] & & & \\ \cline{1-1}  & 250 Nm\({}^{3}\)/h biogas & 4455 [61] & 4000 [65] & 4400 [66] & 4450 [61] & 3000–3500 [65] & \multirow{6}{*}{-} \\  & 1 biogas & 5400 [62; 66] & 5500 [62] & 5000 & & & \\ \cline{1-1}  & & & & 5000 & & & \\ \cline{1-1}  & 100 Nm\({}^{3}\)/h biogas & 3000 [79] & 5000 [66; 79] & - & - & 5000 [79] & \multirow{6}{*}{-} \\ \cline{1-1}  & 100 (\% of CAPEX) [65] & - & 2–3 & 2–3 & 3 & 3–4 & - \\ \cline{1-1} \cline{2-5}  & Maintenance costs (\% of CAPEX) [65] & \multirow{2}{*}{56} & \multirow{2}{*}{15} & \multirow{2}{*}{39} & \multirow{2}{*}{59} & \multirow{2}{*}{25} & \multirow{2}{*}{-} \\ \cline{1-1}  & & & & & & & \\ \cline{1-1} \cline{2-5}  & Maintenance costs for 1000 m\({}^{3}\)/h biogas (m\(\ell\)/y) [61] & & & & & & \\ \hline \multirow{6}{*}{Availability (\%)} & 94 [61; 78] & 96 [61; 78] & 96 [61; 78] & 91 [61; 78] & 98 [61; 78] & \multirow{6}{*}{-} \\  & 94–96 [52] & 95–96 [65] & 96–98 [65] & 96 [65] & 95–98 [65] & & \\ \hline \multirow{6}{*}{Number of plants in Europe (2019) [80]} & \multicolumn{3}{c}{175} & \multirow{2}{*}{103} & \multirow{2}{*}{173} & \multirow{2}{*}{10} \\  & & & & & & & 1 (MS + PSA) & \\ \cline{1-1} \cline{2-5}  & & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison of developed upgrading technologies regarding capital expenditure (CAPEX), maintenance costs, availability, and number of working plants; 1 k USD = 810 \(\ell\). to MEA [68, 82]. KOH is more expensive than NaOH, but K\({}_{2}\)CO\({}_{3}\) is formed, which is a compound with many industrial applications (e.g., the crystal industry, special glass production, potassium salts, inks and pigments, detergents, the food industry, and waste gas treatment) [68, 83]. NH\({}_{4}\)OH is less corrosive than MEA and requires less energy for its regeneration [82, 84], but it is highly volatile, and some traces can be present in the upgraded gas [85]. The process includes, like regular chemical absorption, a first step in which the CO\({}_{2}\) from the biogas is chemically removed via the alkali compound in an absorption column. The chemical reactions with CO\({}_{2}\) for the alkali solvents are shown in Equations (1)-(4) [68, 82]. Afterwards, regeneration or replacement of the solvent takes place.