## 4 Emerging Biogas Upgrading Technologies
### Separation via Phase Addition
#### 4.1.1 Inorganic Solvent Scrubbing
& & & & & \\ \hline \multirow{2}{*}{12\% vol NaOH} & \(p=0.1\) MPa & & & 85.99\% CH\({}_{4}\) & & \\  & Packed column & T = 298 K & & 10.8\% CO\({}_{2}\) & & \\  & (H = 2 m; D = 0.1 m) & Liquid flow & 59.8\% CH\({}_{4}\) & & 3.3\% H\({}_{2}\)O & & \\  & Packing plastic biochals & 72 kg/h & 40.1\% CO\({}_{2}\) & & 76.0\% CH\({}_{4}\) & & adding a WS & \\
5\% vol NH\({}_{3}\) & & Gas flow: 6 kg/h & & 8.9\% CO\({}_{2}\) & & & \\  & & Gas flow: 6 kg/h & & 3.4\% H\({}_{2}\)O & & & \\  & & & & 7.2\% NH\({}_{3}\) & & & \\ \hline \multirow{2}{*}{10\% NaOH} & Packed column & & & & & \\  & (H = 18 m; D = 0.3 m) & \(p=0.1\) MPa & 60.59\% CH\({}_{4}\) & 93.62\% CH\({}_{4}\) & & \\  & Packing: polyethylene & T = 303 K & 36.03\% CO\({}_{2}\) & 2.05\% CO\({}_{2}\) & & & \\  & balls & & & & & \\ \hline \multirow{2}{*}{0.1 M NaOH} & \multirow{2}{*}{Glass cylinder (H = 1 m; \(p=0.1\) MPa} & & 95.5\% CH\({}_{4}\) & & \\  & & & & 3.2\% CO\({}_{2}\) & & \\  & & & 46.8\% CO\({}_{2}\) & & \\ \hline \multirow{2}{*}{0.1 M Ca(OH)\({}_{2}\)} & \multirow{2}{*}{plastic biochals} & \(p=0.1\) MPa & & 95.0\% CH\({}_{4}\) & & \\  & & & & & \\ \hline \multirow{2}{*}{0.1 M NaOH} & \multirow{2}{*}{Glass cylinder (H = 1 m; \(p=0.07\) m) Packing:} & \multirow{2}{*}{\(p=0.1\) MPa} & & 95.5\% CH\({}_{4}\) & & \\  & & & & 3.2\% CO\({}_{2}\) & & \\  & & & & & 100 min & \\ \hline \multirow{2}{*}{0.1 M Ca(OH)\({}_{2}\)} & \multirow{2}{*}{plastic biochals} & \(p=0.1\) MPa & & 95.0\% CH\({}_{4}\) & & \\  & & & & 4.0\% CO\({}_{2}\) & & \\ \hline \multirow{2}{*}{0.1 M NaOH} & \multirow{2}{*}{Glass cylinder (H = 1 m; \(p=0.1\) MPa} & & 95.0\% CH\({}_{4}\) \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Solvent and** & **Apparatus** & **Operating** & **Biogas Composition** & **Product Purity** & **Loading Capacity** & **Comments** & **Case/** \\
**Concentration** & **Approximate** & **Conditions** & **(\% Vol)** & **(\% Vol)** & **(Highest CH\({}_{4}\), **\% Co\({}_{2}\)/kg Solvent Solution)** & **Comments** & **Reference** \\ \hline Pig manure: 1 g NH\({}_{4}\)+/L & & & - & 0.0031 & & & \\ \hline Effluent from a digester & Lab-scale stirrer bath & \(p=0.1\) Mpa & 70\% CH\({}_{4}\) & 87.5\% CH\({}_{4}\) & 15 min; fertilizer & 5/[90] (ab-scale tests) \\ treating pig manure: 1.1 g NH\({}_{4}\)+/L & & T = 33 K & 30\% CO\({}_{2}\) & 12.5\% CO\({}_{2}\) & 0.0045 & & \\  & & & & (theoretical) & & \\ \hline
5\% mass concentration & Column filled with & \(p=0.1\) Mpa & 60\% CH\({}_{4}\) & 97\% CH\({}_{4}\) & - & 6/[83] (plot-scale tests) \\ KOH & Laboratory packing & 313-318 K & 40\% CO\({}_{2}\) & 2\% CO\({}_{2}\) & - & - & (plot-scale tests) \\ KOH & (H = 0.99 m; D = 0.08 m) & T (gas) = 288-293 K & 51\% CH\({}_{4}\) & & & \\ \hline  & & Biogas flow: & 51\% CH\({}_{4}\) & & & & \\
0.2 M Ca(OH)\({}_{2}\) & - & 5 L/min Solution & 39.3\% CO\({}_{2}\) & 89.3\% CH\({}_{4}\) & - & - & 7/[91] (ab-scale tests) \\  & & & flow: 30 L/min & 2940.61 ppm Hg\({}_{5}\) & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: _Cont._The process of solvent regeneration through lime (Ca(OH)\({}_{2}\) solution) treatment is called causticization. The product from NaOH's reaction with CO\({}_{2}\) is Na\({}_{2}\)CO\({}_{3}\) (Equation (1)), which can react with Ca(OH)\({}_{2}\), according to Equation (5). The solvent is regenerated and can be recirculated to the absorption column, but now Ca(OH)\({}_{2}\) has to be regenerated (Equations (6) and (7)). The formation of CaO and CO\({}_{2}\) from CaCO\({}_{3}\) occurs under high temperatures, making this process very energy-extensive, even higher than MEA regeneration [85]. Similar reactions for KOH, Ca(OH)\({}_{2}\), and NH\({}_{4}\)OH regeneration take place [68; 92]. Since raw lime production is environmentally costly (it releases CO\({}_{2}\)), several efforts have been made to find other lime sources. It has been stated that waste material rich in calcium can regenerate the alkaline solvent or even permanently store CO\({}_{2}\)[68].