Simultaneous Absorption and Adsorption Processes for Biogas Purification using Ca(OH)2 Solution and Activated Clinoptilolite Zeolite/Chitosan Composites

This study examined the effects of acid/base activation and chitosan coating on clinoptilolite zeolite as an adsorbent for biogas purification from palm oil mill effluent (POME) using simultaneous absorption–adsorption methods. The effects of chitosan concentration in the clinoptilolite zeolite/chitosan (ZAC) composites were studied to determine the best type of adsorbent for purifying biogas to obtain the highest methane (CH₄) concentration: the biogas produced from POME via an anaerobic digestion process had a CH₄ concentration of 87% and a carbon dioxide (CO₂) concentration of 13%. In this study, the Ca(OH)₂ solution was used for the absorption process, and the ZAC composite was used as the adsorbent in the adsorption process. To enhance the adsorption efficiency of the adsorbent when purifying biogas, clinoptilolite zeolite (ZA) was activated using strong acid (HCl) and base (NaOH) in various concentrations (ranging from 1–3 M), calcination at 450°C for 2 h, and coating with chitosan concentrations (ranging from 0.25–1 v/v%). The ZA was coated with chitosan to increase its adsorption efficiency, as chitosan contains high levels of amine and hydroxyl groups that interact with CO₂ impurities and form carbamic acid, ultimately producing carbamate salt. The composition of biogas before and after treatment was analyzed using gas chromatography. Overall, the final content of the biogas after the purification process with absorption using the Ca(OH)₂ solution and adsorption in a fixed-bed column using the ZAC2-0.5 composite was 0.42% CO₂ and 99.58% CH₄. The purified biogas had a very high methane gas content; thus, this study’s findings suggest that purified biogas can be used as a clean energy source for wider industrial applications.

Biogas purification; Chitosan; Clinoptilolite zeolite; Composite; Methane content; Simultaneous absorption–adsorption

The production and utilization of biogas for green energy in wider industrial applications and cleaner fuels has attracted a great deal of attention for many countries. It is necessary to purify raw biogas to enhance its energy content; this is done by removing impurities such as CO₂, H₂S, and water vapor. This increases the methane purity of the biogas, making it  possible to inhibit corrosion when used in pipelines and other instruments such as column reactor and machine. This also has implications for new green energy, reducing the economic losses from maintenance and operational costs for this devices and also pipelines (Bak et al., 2019).

In contrast, enriching biogas has the potential to replace natural gas in future altogether, as biogas can be used in electricity production in co-generation with heat and power (Kadam & Panwar, 2017). The amount of CH₄ is increased and concentrated in enriched biogas to achieve a similar composition and the same standards as a natural gas, allowing it to be used as transportation fuel and in pipeline systems for household use. Biogas can also be converted into a liquid form using cryogenic freezing, chilling the gas to -80°C and then further chilling it to -162°C (Kadam & Panwar, 2017). This method makes it more economical to compress and transport the biogas over longer distances for further applications.

Huge increases in the price of fossil fuels since 2008 due to the economic crisis have prompted researchers to study methods of upgrading biogas since 19^th century (Osman et al., 2019). Enriching biogas involves removing unwanted gases such as CO₂, H₂S, and water vapor to increase its calorific value and specific heat and minimize its corrosive nature caused by the acidic gases it contains (Leonzio, 2016; Kusrini et al., 2017). Its high CO₂ content results in low calorific value, and further purification of the resultant gas to remove CO₂ is required. Some methods for purification include pressure swing adsorption, adsorption, and chemical absorption (Ackley et al., 2003; Alonso-Vicario et al., 2010; Leonzio, 2016). Masyhuri et al. (2013) purified biogas using a Ca(OH)₂ solution. Kusrini et al. (2018) captured CO₂ using graphite waste composites and ceria. In terms of the adsorptive purification of biogas, researchers have used adsorbents containing physisorption, such as activated carbon and zeolite. Chemisorption efforts have utilized iron oxide and iron oxide hydroxide (Bak et al., 2019). The most-used substances for biogas purification are activated carbon and zeolite (Alonso-Vicario et al., 2010; Peluso et al., 2019).

This study successfully modified clinoptilolite zeolites via acid/base activation, calcination, and coating with chitosan and applied them as an adsorbent to purify biogas from POME using simultaneous absorption and adsorption methods. The modification changed the structure of the clinoptilolite zeolites, making them highly effective as an adsorbent to remove CO₂ from biogas. The final content of the biogas after the purification process with absorption with the Ca(CO)₂ solution and adsorption in a fixed-bed column using the ZAC2-0.5 composite was the most effective, with a composition of CH₄ (99.58%) and CO₂ (0.42%). The resultant purified biogas had a very high methane gas content (99.58%) and very low concentration of impurities. This study recommends that biogas produced via purification using a Ca(OH)₂ solution and a ZAC-0.5 composite be used as a clean energy source for wider industrial applications.

We thank the Ministry of Agriculture, Republic of Indonesia through Grant of Kerjasama Kemitraan Penelitian dan Pengembangan Pertanian Nasional (KKP3N) No. 54.30/HM.240/I.1/3/2016.

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