## 4 Emerging Biogas Upgrading Technologies
### Separation via a Solid Agent
#### 4.2.1 Ash Filters  
Mineral carbonation is a process for CO\({}_{2}\) sequestration that happens spontaneously in nature [90]. In this process, CO\({}_{2}\) reacts with CaO and forms CaCO\({}_{3}\)[68], according to Equation (11). It is a slow process at ambient temperature and pressure, but the rate of reaction can be increased by running the process at the best liquid-to-solid ratio (the aqueous mineral carbonation of limestone) and by increasing both the temperature and the pressure [68]. This process is called accelerated carbonation:  
\[\text{CaO}+\text{CO}_{2}\rightleftharpoons\text{CaCO}_{3}. \tag{11}\]New sustainable filter materials, such as ash filters, can also be used in biomethane technologies [111]. A source of calcium can be found in ash from different processes, and it can be used to react with the CO\({}_{2}\) present in raw biogas. Also, ash is rich in other compounds that can contribute to the separation of H\({}_{2}\)S [90]. [PERSON] [112] used a wood ash filter and obtained a concentration of 0% for both CO\({}_{2}\) and H\({}_{2}\)S in the outlet gas. The loading capacity of the ash was 0.1-0.2 g CO\({}_{2}\)/g of dry ash. Another experiment using ash from mussel shell found a loading capacity of only 1.4 g CO\({}_{2}\)/kg of ash and a saturation time of 30 min (defined as the time that the concentration of CO\({}_{2}\) in the outlet gas was below 5%), while ash from a biomass incineration plant had a capacity of 15.4 g CO\({}_{2}\)/kg of ash and a saturation time of 300 min [90]. [PERSON] et al., [113] used bottom ash (BA) from municipal solid waste incineration (MSWI) for biogas upgrading. They called this process BABIU, standing for BA for biogas Upgrading. The maximum obtained purity of the upgraded biogas, in a pilot-scale test, was 99% methane for an inlet concentration of 58%, obtaining a capacity of 13.9 kg CO\({}_{2}\)/t of BA. Also, the content of H\({}_{2}\)S decreased from 95 ppm in the inlet to 1.4 ppm. Capacity values of 23 kg CO\({}_{2}\)/t of BA were obtained in a test with an inlet concentration of 54.7% methane and an outlet of 95% but using less BA than what was used to achieve the previous results [113]. Other wastes that have been studied as a source of calcium are fly ash from coal burning, slag from ground steel (derived from the steel manufacturing process, it is an industrial by-product that constitutes a calcium-rich waste material [114,115]), dust from a cement kiln, waste combustion residues, BA from municipal solid waste incinerator, and paper mill effluent [68]. Table 6 presents some results concerning the use of ash filters.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \multirow{2}{*}{**Ash Filter Type**} & **Biogas** & **Upgraded Gas** & **Loading** & **Coments** & **Case/Reference** \\  & **Composition** & **Composition** & **Capacity** & **Comments** & **Case/Reference** \\  & **(\%Vol)** & **(\%vol)** & **(kg CO\({}_{2}\)/t Ash)** & & & \\ \hline \multirow{4}{*}{BA from MSWI} & 58\% CH\({}_{4}\) & 99\% CH\({}_{4}\) & 13.9 & \multirow{4}{*}{\begin{tabular}{c} This process is called \\ BABIU (Bottom Ash for \\ Biogas Upgrading) \\ \end{tabular} } & \multirow{4}{*}{\begin{tabular}{c} 1/[113] \\ (pilot plant) \\ \end{tabular} } \\  & 54.7\% CH\({}_{4}\) & 95\% CH\({}_{4}\) & \multirow{4}{*}{\begin{tabular}{c} 23 (less BA \\ than in the \\ previous result) \\ \end{tabular} } & \multirow{4}{*}{\begin{tabular}{c} This process is called \\ BABIU (Bottom Ash for \\ Biogas Upgrading) \\ \end{tabular} } & \multirow{4}{*}{\
\begin{tabular}{c} 1/[113] \\ (pilot plant) \\ \end{tabular} } \\  & & & & \\ \cline{1-1}  & 54.7\% CH\({}_{4}\) & 95\% CH\({}_{4}\) & \multirow{4}{*}{
\begin{tabular}{c} 23 (less BA \\ than in the \\ previous result) \\ \end{tabular} } & & & \\