A review of biogas upgrading technologies: key emphasis on electrochemical systems

AD

anaerobic digestion

AEM

anion exchange membrane

AWE

alkaline water electrolyser

CA

chemical absorption

CHP

combined heat and power

CS

cryogenic separation

ES

electrochemical separation

HER

hydrogen evolution reaction

HOR

hydrogen oxidation reaction

HPWS

high-pressure water scrubbing

MCFC

molten carbonate fuel cell

MS

membrane separation

OER

oxygen evolution reaction

PSA

pressure swing adsorption

SOFC

solid oxide fuel cell

WS

water scrubbing

Rapidly growing energy demand owing to rapid and unplanned urbanisation and industrialisation has led to increased consumption of fossil fuel-based energy. The growth of global fossil fuel consumption is about 2% per annum (Mason 2007; Jackson et al. 2019). Non-renewable energy sources have been contributing to 81% of global energy demand including natural gas (21.6%), coal (28.1%), and oil (31.7%) (Chen et al. 2011; Welsby et al. 2021). While global primary energy resource is dominated by non-renewable fossil fuels, fossil fuels reserves are estimated to be depleted over 200 years (Shafiee & Topal 2009). In addition, a large amount of CO₂ pollution released from the combustion of fossil fuels causes environmental problems such as climate change. For example, CO₂ emission was estimated to be 36.3 billion tons in 2021 (Duan et al. 2022). To limit the global temperature increase to less than 2 °C by 2100, the CO₂ emission must be reduced to less than 5 Gton/year before 2050, while the current total emission is 48 Gton/year (Rogelj et al. 2016; Valluri et al. 2022). Given the severity of the threat posed by climate change, which is driven by fossil fuel-driven emissions, global energy transformation is looking for renewable energy resources to replace fossil fuels to address these challenges. Accordingly, researchers have been focused on looking for a renewable and cost-effective alternative to fossil fuels.

Lately, biogas has started to arouse great interest as a sustainable energy source for renewable fuel production (Baena-Moreno et al. 2020; Zhang et al. 2020). Biogas is generated through a sequence of processes (hydrolysis, acidogenesis, acetogenesis, and methanation) in an anaerobic digestion (AD) process (Weiland 2010). AD is a biologically mediated process through which microorganisms break down biodegradable materials such as biomass, manure, sewage, and municipal waste in the absence of oxygen (Bhatia 2014). Although the composition of biogas is mainly dependent on the substrate type being digested and the pH of the AD reactor, the gas is generally composed of methane (CH₄) (55–65%), carbon dioxide (CO₂) (35–45%), nitrogen (N₂) (0–3%), oxygen (O₂) (0–2%), hydrogen (H₂) (0–1%), hydrogen sulphide (H₂S) (0–1%), and ammonia (NH₃) (0–1%) (Balat & Balat 2009). The real methane content in biogas is generally higher than the mentioned values because some of the CO₂ gas is dissolved in digestate which is a mixture of solid and liquid (Weiland 2010; Awiszus et al. 2018).

Due to its methane content, biogas represents an attractive gaseous biofuel. Power generation from biogas is currently the most popular market in the world (Abanades et al. 2021). Biogas contributes to about 20% of the total biopower production and 4% of global heat generation (Scarlat et al. 2018).

A review of the biogas global market shows that biogas utilisation to produce renewable electricity is underdeveloped even though the biogas industry is emerging worldwide. This underdevelopment raises a technical question – what are the current issues challenging the broader use of biogas as a renewable resource?

At present, the primary biogas usage is in combined heat and power (CHP) generators for heat and electricity generation, and the excess is flared before being released into the atmosphere. However, the efficiency of these cogeneration engines is about 33% for electricity and 45% for heat generation (Appels et al. 2011). Microgas turbines and fuel cells are alternatives to CHPs. Microgas turbines have lower energy efficiency (250–31%) compared to CHPs but require longer servicing intervals. High-temperature fuel cells, such as solid oxide fuel cells (SOFC) and molten carbonate fuel cells (MCFC), can use biogas directly as a source of H₂. However, coke formation during internal biogas reforming to produce hydrogen represents the main problem commonly faced (Assabumrungrat et al. 2006; Xuan et al. 2009). Biogas can also be upgraded to biomethane by CO₂ removal and injected into the natural gas pipeline (Khan et al. 2021b).

Improving the quality of biogas is a key to enhance the utilisation of biogas (Divya et al. 2015). The combustibility of biogas is determined by its CH₄ content, and other components are considered contaminants. The large CO₂ content in the biogas is a major obstacle that hinders broad biogas utilisation as it lowers the calorific value of biogas and its relative density (Persson 2003). The calorific value of raw biogas is about half of that of natural gas on a mass basis (Table 1). Also, CO₂ solubilised in the moisture of biogas causes severe corrosion of the infrastructure due to the formation of carbonic acid (Tang et al. 2021). Additionally, CO₂ removal from biogas is becoming more important owing to global warming (Mulu et al. 2021).

To upgrade the biogas, CO₂ must be either removed or converted to a value-added product such as CH₄ by reaction with H₂ (Kougias et al. 2017). Biomethane is the final product that contains CH₄ (950–99%) and CO₂ (10–5%), with a trace of H₂S (Abatzoglou & Boivin 2009).

Assuming the system's temperature of 298 K and a CO₂ mole fraction of 0.45 in the raw biogas (⁠⁠), and a CO₂ mole fraction of 0.02 in the upgraded biogas, and n_x,in/n_y,in = 1, biogas upgrading would require minimum thermodynamic energy of 0.00136 kWh/m³ raw biogas.

Water is the most widely used solvent in biogas upgrading, accounting for approximately 41% of global biogas upgrading plants. Its widespread use is attributed to its low sensitivity to biogas trace contaminants (Awe et al. 2017). The principle of this separation technique is based on the solubility difference of various gas components in the water. The solubility of CO₂ in water is about 24 times higher than CH₄ (the solubility of CO₂ and CH₄ in water at standard conditions are 0.034 and 0.0014 mol/L) (Santamarina & Jang 2010). The absorbed gas is physically bound to the water. To increase the solubility of CO₂ in the water, this technology is usually operated at a pressure ranging between 0.8 and 1.2 MPa (Wylock & Budzianowski 2017) which is referred to as high-pressure water scrubbing (HPWS). The biogas is compressed to about 0.8 MPa and fed into the bottom of a packed bed column containing Rasching or Pall support to ensure intensive mass transfer, while water is counter-currently introduced to the top of the column (Huertas et al. 2011; Wantz et al. 2022).

Alternatively, the WS process can be operated near atmospheric condition where the stripper column is set at near atmospheric pressure. High-pressure plants are proved to be slightly more energy efficient if compressed biogas is required unless low-cost water is available (Budzianowski et al. 2017).

To regenerate the spent water, either the rich-CO₂ water is pumped to a low-pressure stripper (regenerative absorption), or freshwater can be replaced with low-cost freshwater if available (single pass). In single-pass technology, the water usually comes from a wastewater treatment plant and after scrubbing it is usually depressurised and then released into the environment (Angelidaki et al. 2018). In the single-pass technique, the water-raw biogas ratio (L/G) typically ranges between 0.1 and 0.2 (Bauer et al. 2013a; Muñoz et al. 2015).

The energy requirement for the WS depends on the design parameters and operating conditions such as temperature and pressure. The main energy requirements of the WS technology include pumping water, compressing biogas, and cooling the compressed gas (Nock et al. 2014).

Cryogenic treatment is based on the different condensation and distillation properties of gaseous components. The boiling point of CO₂ is −78 °C, while CH₄ has a boiling point of −161.5 °C at atmospheric pressure (Jonsson & Westman 2011). This difference in condensation properties facilitates the separation of CO₂ from liquefied CH₄. This technique includes four major stages for low-temperature CO₂/CH₄ separation (Aryal et al. 2021):

• 1. Increasing the pressure up to 4,900 kPa and biogas temperature to about 74 °C.

• 2. Reducing the temperature to −40 °C, while keeping the same pressure by using either a pre-cooler heat exchanger and cooler operating on a refrigeration cycle (direct cooling: for large scale) or using liquid nitrogen as the cooling agent (indirect: small scales).

• 3. Distillation and liquefaction with a distillation column, producing gaseous CH₄ and liquefied CO₂.

• 4. Flashing the gas outlet to reach CH₄ purity of above 97%.

It is important to maintain high pressure throughout the process to avoid CO₂ freezing out at temperatures below −78 °C (Ahmed et al. 2021).

Based on the product state, cryogenic upgrading techniques can be classified into two types based on anti-sublimation-based technology (solid CO₂) or distillation-based technology (liquid CO₂). In the anti-sublimation-based technology, CO₂ is directly transformed from the gaseous phase to the solid phase, whereas in the distillation-based technique, CO₂ is extracted in a liquid state. CH₄ is liquefied in both approaches (Naquash et al. 2022). In the anti-sublimation process, solidified CO₂ is removed from the reactor through melting or vaporisation (Clodic & Younes 2006). The advantage of this technique is that the upgraded biogas does not need further compression and the produced solid CO₂ is a valuable by-product.

A ubiquitous obstacle to this technology is clogging derived from CO₂ freezing out along the gas refrigeration process and other biogas impurities (Ali et al. 2010; Yousef et al. 2018; Nguyen et al. 2021).

An early attempt to separate CO₂ from CH₄ using polyimide (PI) membrane was made by an Air Liquide company (MEDAL) in 1994 (Baker & Lokhandwala 2008). The principle of this technology relies on the different permeability rates of the gaseous component through a porous membrane. There are two stages for mass transport across the membrane. First, the gas diffuses along the dense zone, and then diffusion of gas into the porous area (Scholz et al. 2013). Since the driving force of the membrane technology is the partial pressure difference across the permeable barrier, the separation method could be more efficient for gas mixtures with high CO₂ content. In the biogas upgrading context, CO₂ and H₂S diffuse across the membrane to the permeate side while CH₄ stays inside the membrane achieving a high-purity CH₄.

MS technology can be categorised into two types: (a) gas–gas high-pressure operation (20–40 bar) (dry) and (b) gas–liquid low-pressure operation (wet) (Thrän et al. 2014). In the gas–gas operation, both sides of the membrane are dry, thus the separation efficiency only depends on the difference in permeability between CO₂ and CH₄. The typical membrane module for this type of operation is hollow fibre membranes (Kadam & Panwar 2017). In the gas–liquid module, however, the permeate dissolves into a solvent, and then it passes through the porous membrane to the other side due to concentration gradient and convective flow induced by the pressure difference (Kim et al. 2021). Amine-based solutions such as alkanolamines are the most used absorbent for CO₂ removal in the set approach (Ochedi et al. 2021).

Among all potential materials for the membrane technology, those that are adequately resistant to the biogas impurities and fouling could be used for biogas upgrading. Inorganic, polymeric, and composite membranes are three types of membranes employed in biogas upgrading plants (Chen et al. 2015). The main material used in inorganic membranes are alumina, cobalt, copper, iron, nickel, palladium, and platinum. The main disadvantage of the inorganic membrane lies in their poor mechanical properties (fragile). This characteristic poses a significant challenge in the production of high surface area module (Nunes & Peinemann 2001). In contrast, organic polymers offer a promising alternative due to their excellent manufacturability, as well as acceptable chemical and thermal stability, overcoming the limitations associated with inorganic metals. The frequently employed polymers include polycarbonate (PC), cellulose acetate (CA), polyesters (PE), and PI (Sabee et al. 2022). The composite membrane approach combines the advantages of inorganic materials, such as notable chemical and thermal stability, while capitalising on the cost-effective manufacturability of organic polymer (Nik et al. 2011; Asadi et al. 2022; Li et al. 2022). However, it is worth noting that this method is currently limited to laboratory-scale applications.

The key concerns of the membrane technology involve the pre-compression of biogas and the membrane's lifespan, typically ranging from 5 to 10 years (Bauer et al. 2013b).

The principle of PSA is based on the selective adsorption of gas molecule onto the solid surface of molecular sieve material, determined by its molecular size (Khan et al. 2017). Adsorbents with pores of a uniform size of 3.7 Å allow the CO₂ molecules with a kinetic diameter of 3.4 Å to retain their structure through physical, van der Waals, or electrostatic interaction (Galante et al. 2012). CH₄ molecules with a bigger size (3.8 Å) remain in the gas phase and the CH₄-rich stream exit from the column head (Nakao et al. 2019). As such, the core of the PSA technique lies in the adsorbent material.

Different types of adsorbent material are used for biogas upgrading, namely zeolites, carbon-based adsorbents, and innovative adsorbents like metal-organic frameworks (MOFs). Among these materials, MOFs show the highest CO₂ capture capacity owing to their ultrahigh porosity (Trickett et al. 2017). In the PSA systems, these adsorbents are stacked in a vertical packed column under 4–10 bar pressure to enhance CO₂ adsorption (Wahono et al. 2020).

The PSA systems constitute four stages in a cyclical mode, namely pressurisation, adsorption, blow-down, and purge (Aryal et al. 2021):

• (I) Pressurisation: Pre-treated biogas is fed to the bottom of a packed column filled with adsorbent until the desired pressure (4–10 bar) is reached.

• (II) Adsorption: In the pressurised column, CO₂ molecules which are smaller than CH₄ are selectively retained in the column while a CH₄-rich stream passes through the column.

• (III) Blow-down (evacuation): After the saturation with CO₂, the CH₄-rich product gas is released which results in a pressure drop inside the column to the atmospheric pressure.

• (IV) Purge: The feed biogas is injected into the column at atmospheric pressure for further regeneration of the adsorbent bed. At this stage, the entrapped gases are released leading to the regeneration of adsorbent material.

A notable advantage of the PSA system is its low maintenance, as the system is dry and does not require additional cost for aqueous solvents. However, the yield and purity of the upgraded gas are usually lower compared to other biogas upgrading technologies. Some reports have indicated a maximum CH₄ yield of 91% achieved by their systems (De Hullu et al. 2008).

Chemical absorption relies on the reversible reaction between CO₂ and reactive solvent, leading to the formation of a weakly bonded carbon compound that can be regenerated. Commonly used commercial solvents for this purpose include amine solution and caustic solvents.

In amine scrubbing, the CO₂ gas is absorbed by an organic solution (amine) through both chemical and physical reactions. The most commonly employed amine-based solutions in commercial-scale amine scrubbing are monoethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA), N-methyldiethanolamine (MDEA), and piperazine (PZ) (Deublein & Steinhauser 2011; Rasi et al. 2011; Xu et al. 2015; Golmakani et al. 2022). The CO₂ molecules are physically transferred from the gas phase to the liquid phase, followed by a chemical reaction that consumes the dissolved CO₂. To maintain an optimised mass transfer rate for CO₂ in amine scrubbing, the molar flow rate of the amine solution should be supplied at least four times higher than the CO₂ molar rate (Bauer et al. 2013b).

Amine scrubbing systems typically comprise two columns: absorber units and stripper units. Compressed biogas (1–2 bar) is introduced at the bottom of the column, whereas the amine solution is fed in a counter-current manner from the top. CO₂ gas is absorbed by the aqueous amine solvent through an exothermic chemical reaction, causing a temperature increase in the column from 20–40 to 45–65 °C (Privalova et al. 2013). Various column designs, including packed column, tray column, bubble column, spray column, and membrane contactors are employed to enhance the gas–liquid interface area (Gruenewald & Radnjanski 2016).

Subsequently, the CO₂-rich solution is pumped to the stripper column, operating at a pressure of 1.5–3 bar and a temperature of 120–160 °C. The regeneration process can also be accomplished by reducing the pressure (Abdeen et al. 2016). Amine demonstrates high selectivity towards CO₂, enabling a CH₄ recovery of 99% (Vickers 2017).

Common limitations associated with amine scrubbing include high energy consumption for the regeneration process, corrosion, and amine degradation (Chang et al. 2022). Amine scrubbing for CO₂ capture is energy-intensive (1.1–1.6 kWh/kg CO₂) (Brunetti et al. 2010). Amine degradation, caused by irreversible reaction between amine CO₂, O₂, or other gases, leads to solvent losses and foaming (Abdeen et al. 2016). Further, amine solution can become corrosive when reacting with CO₂, particularly at elevated temperatures. A study by Kittel et al. (2009) showed that an absorption column using a 30% by weight MEA solution to treat a flue gas with an 8% CO₂ content has a corrosion rate of about 1 mm/year at a temperature of 110 °C.

NaOH is the predominant caustic solvent for CO₂ absorption to clean biogas, primarily due to its strong ionic bonds allowing it to completely dissolve in water (Hikita et al. 1976; Baena-Moreno et al. 2019; Mulu et al. 2021). NaOH concentration ranging from 5 to 12% is commonly used in the CO₂ absorption column (Mahmoudkhani & Keith 2009).

The operation of the caustic solvent scrubbing follows the same principles as the other chemical absorption process. The absorption unit involves an absorption column and a regeneration column to regenerate the spent alkaline solution. The regeneration of the spent NaOH can be accomplished by exchanging calcium and sodium ions through the addition of calcium hydroxide, resulting in calcium carbonate (CaCO₃) precipitation. Subsequently, CaCO₃ is thermally decomposed at 700 °C to produce CO₂ and calcium oxide. Finally, the hydration reaction between calcium oxide and water regenerates calcium hydroxide (Ghosh et al. 2019).

A key advantage of caustic solutions is their higher CO₂ absorption capacity compared to amine-based solutions (Kittel et al. 2009). However, the regeneration of caustic solvents is more energy-intensive compared to amine-based solutions owing to the formation of thermally stable carbonated substances (Abdeen et al. 2016).

Electrochemical CO₂ capture has garnered significant attention due to its energy efficiency and environmental compatibility, particularly in the separation of CO₂ from dilute gas mixtures (e.g. flue gas and air). As a part of the broader field of CO₂ capture, the literature has been surveyed within the context of biogas upgrading techniques to provide a broad overview of development in the CO₂ capture field.

The concept of electrochemical CO₂ separation from biogas involves electrochemical absorption and desorption of CO₂ via pH gradient induced by an electric field which is driven by electrochemical reactions. This method is commonly performed using alkaline membrane systems, where anion exchange membranes (AEM) allow ion migration while preventing the mixing of anolyte and catholyte. The CO₂ capture and recovery through pH gradient involve a complex interplay of phase equilibria and reaction kinetics. The CO₂ absorption involves CO₂ dissolution in the aqueous media and then a chemical reaction with the liquid dissociating to different ions depending on the pH. The rate of chemical absorption is much faster compared to purely physical absorption mechanisms due to the involvement of chemical reactions. Overall, CO₂ absorption and recovery comprise a sequence of elementary reactions as outlined in Table 2.

Table 2

Elementary reactions involved in chemical CO₂ absorption

Reaction no. .	Reaction .	Reaction constant .	Reference .
(8)	CO2(g) CO2(aq)
(9)	CO2(aq) + H2O H2CO3	K1 = 6.6 × 10−4 M−1s−1	Wang et al. (2010)
(10)	CO2(aq) + OH−	K2 = 12.1 × 103 M−1 s−1	Wang et al. (2010)
(11)	+ OH− + H2O	Ka1 = 4.69 × 1011 s−1	Harned & Scholes (1941)
(12)	+ H+ H2CO3	Ka2 = 1.7 × 1011 s−1	Adamczyk et al. (2009)
(13)	H2O ⇋ OH− + H+	Kw = 2.21 × 1014	Maeda et al. (1987)

Reaction no. .	Reaction .	Reaction constant .	Reference .
(8)	CO2(g) CO2(aq)
(9)	CO2(aq) + H2O H2CO3	K1 = 6.6 × 10−4 M−1s−1	Wang et al. (2010)
(10)	CO2(aq) + OH−	K2 = 12.1 × 103 M−1 s−1	Wang et al. (2010)
(11)	+ OH− + H2O	Ka1 = 4.69 × 1011 s−1	Harned & Scholes (1941)
(12)	+ H+ H2CO3	Ka2 = 1.7 × 1011 s−1	Adamczyk et al. (2009)
(13)	H2O ⇋ OH− + H+	Kw = 2.21 × 1014	Maeda et al. (1987)

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It is widely recognised that capturing CO₂ at high pH levels is significantly faster than that at moderate pH levels, primarily due to faster hydration of CO₂ with OH⁻ compared to water (Wang et al. 2010; Ye & Lu 2014). Considering the second order for CO₂ reaction with OH⁻, it is expected that CO₂ capture at pH = 14 is 10⁵ times faster than that at pH = 9. However, these reaction rates are a function of the temperature and ionic strength of the system. Carbonic anhydrase (CA) is commonly used to promote the hydration rate of CO₂ (Chang et al. 2021; Rasouli et al. 2022; Talekar et al. 2022). Gladis et al. (2018) reported the absorption rate of CO₂ in aqueous MDEA increased by three times when the concentration of CA increased from 0 to 3 g/L. However, CA is unstable in some particular conditions. Their stability is dependent on the pH, temperature, and salt concentration (Santos et al. 2009).

Typically, an electrochemical system is made of anode and cathode compartments which are separated by an ion-exchange membrane (IEM). However, to enhance the energy efficiency of the electrochemically driven CO₂ capture system, various cell configurations have been reported. In this section, we introduce and analyse each method separately.

Although high-purity oxygen finds various industrial applications, such as in the activated sludge process in wastewater treatment plants (Mohammadpour et al. 2021), the OER remains a major bottleneck in this method due to its sluggish kinetics, limiting the energy efficiency of the AWE (Shi et al. 2019). From a thermodynamic perspective, water electrolysis requires a thermodynamic minimum energy of 237 kJ (equivalent to a cell voltage of 1.23 V) (Yu et al. 2022). An additional cell voltage must also be applied to overcome activation overpotential, mainly arising from intrinsic kinetic barriers associated with half-reactions at each electrode, particularly at the anode. Taking all overpotentials into account, the cell voltage of a typical alkaline electrolyser varies between 1.8 and 2.4 V (Kaninski et al. 2011; Carmo et al. 2013).

As an alternative to the water electrolysis-assisted CO₂ capture, the electrochemical systems using redox cycling can be employed for biogas upgrading. In these systems, a specific redox-active carrier can be employed to separate CO₂ from biogas through either a pH-gradient generation approach or a binding mechanism.

pH gradient. In the pH-gradient generation mechanism, the electrochemical cell undergoes cycling oxidation and reduction reactions at the anode and cathode, respectively, to generate a pH gradient for the absorption and desorption of CO₂ (Landon & Kitchin 2010; Rigdon et al. 2017; Shu et al. 2020; Muroyama et al. 2021). Thermodynamically, this mechanism requires a net potential of zero. However, the generated pH gradient and overpotentials involved in the process increase the energy input. To capitalise on reduced energy demand through cycling oxidation and reduction reactions, various cell configurations have been reported. Utilising the by-product oxygen during water electrolysis is one approach to minimise the total cell voltage requirement. Pennline et al. (2010) used an O₂ cycling cell to separate CO₂ from flue gas. The pure oxygen produced in the anode was utilised at the cathode allowing pH-gradient generation at cell potentials where water electrolysis does not occur. The half-reactions of this setup are expressed as follows:

Binding mechanism. On the other hand, in the binding mechanism, the electrochemically activated redox carrier selectively binds CO₂ at one electrode to separate CO₂ from a gas mixture and is deactivated at the opposite electrode to desorb the CO₂ and regenerate the carrier. Thus, the CO₂ binding route consists of the following steps (Rakowski Dubois & Dubois 2009):

• The redox-active compound is initially reduced to the ionic form.

• The generated anions bind CO₂ facilitating transportation to the other side of the membrane.

• The carrier-CO₂ complex is oxidised to purge CO₂.

In the earliest attempt, Ward (1970) used ferrous ions to transport nitric oxide across a membrane. Common redox carriers in this system include quinones (Gurkan et al. 2015), 4,4′-bipyridine (Ranjan et al. 2015), and thiolates (Singh et al. 2017). Huang et al. (2019) reported an energy-efficient electrochemical design relying on using quinone redox reactions to generate a pH gradient for CO₂ capture (Figure 11(b)). A mixture of Tiron and NaOH solutions, termed Na₂Q, was used as an absorbent medium for CO₂ capture. However, the quinone reduction reaction is difficult to occur at very high catholyte pH values, which result in poor regeneration of alkaline absorbents.

However, it is worth nothing that the use of amine-based absorbents in the EMAR technique poses challenges related to corrosion and degradation.

A comparative economic study between five well-established technology for CO₂ separation from biogas is presented in Table 3. To remove CO₂ from raw biogas with a flow rate of less than 1,000 m³ raw biogas/h, WS technology, and PSA have comparable energy costs and appear to consume about 30% less energy than other major competing biogas upgrading technologies. However, the WS technology requires a high demand for water due to its low CO₂ absorption capacity (20 L per m³ of CO₂) (Bodbodak & Moshfeghifar 2016). From an economic perspective, this technology is technically viable if cheap water supplies such as treated wastewater from wastewater treatment plants are available. The major components in the energy consumption of this technology are gas compression, water compression, and water cooling (Golmakani et al. 2022). The performance of PSA was demonstrated to be competitive with HPWS with PSA displaying slightly higher energy consumption. The energy demand for the feed gas compression accounts for the major contributor to the energy penalty of this technique (Ho et al. 2008). In MS technology, the net energy input per unit mass of CO₂ is 0.65 kWh/kg CO₂. Replacement of membrane which usually has a lifetime of 5–10 years and pressurisation of the feed gas are considered the major expenses in this technology (Kapoor et al. 2019).

The CS technology is an energy-intensive method with a net energy requirement of 1.33 kWh/kg CO₂ due to the high pressure and cooling source employed in this system. Liquified natural gas (LNG) is the most common cold source used in this technique. Alternatively, the use of a refrigerator is a viable option to provide the cooling energy required. The main energy-intensive units of this technology consist of compressors, pumps, and multi-stream heat exchangers for CO₂ liquefaction (Song et al. 2019). The series of both cryogenic and mechanical cycles of this technology makes the CS difficult to scale. Andersson et al. (2009) reported an energy requirement of 1.96 kWh/kg CO₂ for a commercial CS unit.

Chemical scrubbing is another key gas separation technique that requires the least electricity demand among other existing technologies. However, a high capture cost arose as a result of the high thermal energy requirement for absorbent regeneration in the desorption column (1.8 kWh/kg CO₂) makes this technology the most energy-intensive gas separation approach for biogas upgrading (Vo et al. 2018). In the regeneration column, a temperature of 160 °C is required to dissociate NaHCO₃ to Na₂CO₃, H₂O, and CO₂. To further decompose Na₂CO₃ to Na₂O which can be converted to NaOH, a high temperature of 800 °C is required (Maile et al. 2017).

Overall, it is worth mentioning that the preferred technique for biogas upgrading also depends on the availability of utilities. WS is preferred when a cheap source of water is available. The chemical scrubbing requires low-pressure steam to regenerate the spent solvent, whereas the cryogenic approach requires a low-cost cooling source such as the evaporation of LNG. Membrane technology is a promising candidate when the required utilities are expensive.

Table 4 summarised the energetic performance of the various electrochemically driven CO₂ separation technologies. Among the electrochemical-induced pH-swing methods for CO₂ separation, the H₂ cycling cell exhibited the best energy efficiency outcome because of the elimination of sluggish OER in this design. The H₂ cycling cell is able to regenerate the spent alkaline solution for CO₂ capture with a minimum energy input of 0.13 kWh/kg CO₂, in contrast to the water electrolysis device, which requires 2.27 kWh/kg CO₂. This difference is attributed to the low cell voltage (0.2 V) required to achieve a CO₂ recovery rate of about 1.8 mol/m²/h via the H₂ cycling cell (Muroyama et al. 2021), whereas the water electrolysis cell required a 10 times higher cell voltage (over 2 V) to achieve the same CO₂ recovery rate (Verbeeck et al. 2019). However, Shu et al. (2020) reported a high gross energy requirement of over 2 kWh/kg CO₂ to recover spent caustic solution using an H₂ cycling cell. Considering the CO₂ emission factor of 0.55 kg CO₂/kWh for electricity generation, this design becomes infeasible when using a non-renewable energy source. The primary contributor to the energy penalty in their systems was transport losses due to gas bubble formation in the middle chamber, which was separated from the anode and cathode compartment by two cation exchange membranes.

O₂ cycling cell represents an attractive alternative where there is a mixture of CO₂ and O₂ such as the product of combustion containing CO₂ and unreacted O₂. However, the applied cell potential in those reports was in excess of about 800 mV corresponding to a CO₂ separation energy requirement of 0.65 kWh/kg CO₂ (Pennline et al. 2010) and 1.4 kWh/kg CO₂ (Rigdon et al. 2017). (The difference in reported energy requirement is due to difference in faradaic efficiency.)

Although the binding mechanism has yet to achieve industrial utility, it holds promise for CO₂ separation with low energy requirements. With this method, Rahimi et al. (2020) conducted bench-scale experiments that focused on continuous CO₂ recovery from amine solution. Thermodynamic analysis of this study indicated that the energy requirement for regeneration was approximately 0.25–0.37 kWh/kg CO₂, comparable to that of the H₂ cycling cell.

In contrast, the electrochemically reversible quinone-based solution for reactive CO₂ separation from flue gas incurs a higher energy expenditure (1.03 kWh/kg CO₂) as reported by Huang et al. (2019). The energy penalty involved in this process was mainly due to the challenging regeneration of Na₂Q via quinone regeneration under extremely alkaline conditions.

From an economic standpoint, the electrochemical CO₂ separation techniques showed a minimum achievable net energy requirement of 0.25 kWh/kg CO₂. This is notably lower than the net energy requirement of conventional CO₂ capture technologies (approximately 0.56 kWh/kgCO₂). Therefore, this technology has the potential to reduce the energy cost of CO₂ separation to meet the acceptable net energy requirement of <0.53 kWh/kg CO₂ for CO₂ capture (Clausse et al. 2011; Sharifian et al. 2021). However, the energy required for the electrochemical methods is dependent on the applied cell potential which is correlated to the operating current density. The current densities studied in the literature that reflects the CO₂ recovery rate are very low compared to the reported energy requirements for industrial applications. At these CO₂ recovery rates, a very high surface area is necessary to process typical biogas inflow rates (100–1,000 m³/h) (Shen et al. 2015). For example, at a CO₂ recovery rate of 2 mol/m²/h, an electrochemical cell stack with a surface area of approximately 900 m² would be required to capture CO₂ from biogas with a flow rate of 100 m³/h, and CO₂ content of 40% (equivalent to about 1,800 mol CO₂/h). In comparison, a typical electrochemical cell consists of approximately 50 anodes and cathodes with a surface area of about 1 m² (total surface area of 50 m²) (Havlík 2008).

Although electrochemical methods show great potential with regards to viability and technical feasibility, they are generally developed at the laboratory scale for flue gas treatment and require further improvements for biogas upgrading applications. Recommendations for future optimisation research to improve the economic aspect of this technique based on this review are as follows:

• Develop and test electrochemical CO₂ capture technique for biogas upgrading application as there is currently insufficient data on the performance of electrochemical CO₂ capture with biogas as the CO₂ source.

• Operate and evaluate the electrochemical CO₂ separation systems at controlled mild catholyte pH values (e.g. pH = 9) where the bicarbonate ion constitutes the primary charge carrier across AEM as opposed to the strongly alkaline pH values where carbonate or hydroxyl ions are the major charge carrier. To date, the electrochemical CO₂ separation systems are often operated at uncontrolled high pH values where electron per CO₂ migrated through AEM (e/CO₂) is not optimised.

• Enhance the current efficiency of the electrochemical CO₂ separation systems. To maximise the energy efficiency of this technique, it is required to achieve the e/CO₂ ratio of 1 to avoid current efficiency losses.

• Separate O₂ production stream and CO₂ recovery compartment. Although electrolytic O₂ production is an energy-intensive reaction, the high-purity oxygen stream produced during water electrolysis could potentially be utilised in many industrial applications such as the activated sludge process. However, the CO₂ evolution in the anode compartment contaminates the O₂ production stream. This necessitates a new design to avoid pure O₂ contamination with CO₂.

• Evaluate technical feasibility of H₂ cycling cell as a means of CO₂ capture technology that requires the least energy demand. In this technique, the H₂ generated at the cathode should be effectively transferred to the anode side for utilisation. The operation challenges should be identified and addressed for this newly developed technology.

This study offers a critical overview on the production of biomethane through CO₂ separation from biogas, with particular focus on the potential application of electrochemical methods. The five key state-of-the-art biogas upgrading technologies including WS, CS, MS, PSA, and chemical absorption, were reviewed and compared with the novel electrochemical CO₂ separation technique. The electrochemical CO₂ separation technology emerges as a promising CO₂ separation, albeit not yet widely popularised and remaining primarily at the bench scale. Among the available electrochemical CO₂ separation technologies, the pH-gradient mechanism is predominantely employed due to its simplified operation, utilisation of low-cost electrode materials, and absence of toxic chemicals. This mechanism exhibited a minimum net energy requirement of about 0.25 kWh/kg CO₂ to separate CO₂ from a gas mixture which is half of the energy input of the conventional technologies. The comprehensive analysis of this technology recommends that future research endeavours should prioritise process optimisation, including enhancement of current efficiency, augmentation of CO₂ recovery rates, and advancement of operational and configurational design for potential large-scale implementation.

Conceptualisation: H.M., G.H.; Data curation: H.M.; Formal analysis: H.M., G.H.; Funding acquisition: A.P., G.H.; Investigation: H.M., G.H., A.P.; Resources: H.M.; Supervision: A.P., G.H.; Writing – original draft: H.M.; Writing – review and editing: H.M., G.H., A.P., K.Y.C. All authors have read and agreed to the published version of the manuscript.

This project was supported by Murdoch University through a PhD scholarship to H.M. We wish to acknowledge Water Corporation of Western Australia for its funding and a keen interest in a biological biogas upgrading research project and for providing a top-up scholarship to H.M. We would also like to thank Dr Ralf Cord-Ruwisch for his dedicated support and guidance.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.