Recent Development in Physical, Chemical, Biological and Hybrid Biogas Upgradation Techniques

Abstract

Energy driven technologies and enhanced per-capita waste production have led to the establishment of novel technologies to simultaneously produce fuels as well as treat the wastes. Anaerobic digestion is cost-effective and sustainable process to produce biogas. Biogas is a mixture of CO₂, CH₄, H₂S, is an eco-friendly and inexpensive renewable biofuel. This mixture of gases restricts biogas utilization in vehicular fuel, CHPs, therefore, biogas upgradation becomes a necessary step. Conventional upgradation technologies for example water scrubbing, physical adsorption, chemical adsorption, amine scrubbing, etc. are cost intensive and require high maintenance. Novel technologies like biological methods of biogas upgradation are being investigated and new improvements are made in the conventional methods. This review aims to give a close insight about various technologies of upgradation including, pressure swing, amine scrubbing, membrane separation, cryogenic separation, biological methods, etc., along with the major challenges and limitations. The study also intends to provide an overview about the future perspective and scope of these technologies.

1. Introduction

Enhancing the energy technology is the need of hour. Exploding population and urge for higher energy consumption have ignited the need for the new and sustainable methods to meet this demand. As the world is exposed to the climate change and food scarcity, environmentalists have shifted their focus towards the greener and cleaner technologies. Anaerobic digestion (AD) is promising method to produce biogas and deal with the increasing energy crisis. Renaissance of AD technology is being done by concentrating on waste to energy concept [1,2]. The product of AD i.e., biogas contains majorly methane (50–60%), 30–40% carbon dioxide (CO₂) and some other traces. The quality of biogas depends on the type of substrate fed into the anaerobic digester. The pH of the digester also affects the content of the biogas. Other than these gases, some other impurities are also present such as hydrogen sulfide (H₂S), water vapors (H₂O), nitrogen gas (N₂) and oxygen (O₂). These impurities depend on different parameters and sources such as, water vapors (H₂O) which are in 5–10% in concentration, arises from the evaporation of liquid media present in the digester especially which are running on thermophilic condition. Similarly, N₂ of concentration 0–3% may originate from the air inflow. H₂S (0–3%), O₂ (0–1%) and Siloxane (0–41 mgm⁻³) are some other contaminants produced from different sources like presence of sulfate produces H₂S, high nitrogen waste or proteinaceous waste produces NH₃ [3,4]. Additionally, bio-CH₄ has numerous applications shown in Figure 1 as in combined heat and power (CHPs) for heating and electricity production, as a vehicular fuel and cooking gas if the above-mentioned contaminants are properly removed [5,6]. Also, calorific value (CV) methane is 50.4 MJ/kg CH₄ or 36 MJ/m³ CH₄, whereas CV of biogas is 20–25 MJ/m³ due to these contaminations. H₂S and ammonia are poisonous and corrosive in nature, causing damage to metallurgical part of the grid unit and sulfur oxide (SO_X) emissions after burning. The occurrence of H₂S in raw biogas can cause pipelines of gas, IC engines and boilers to corrode. When biogas is deployed in IC engines as a fuel, Si particles formed on the walls of the engine may cause all exhaust pipe and catalytic converter to deteriorate. Biogas with ammonia and halogenated hydrocarbons has poor ignition qualities and can cause corrosion in engines and pipelines of gas after burning [7,8]. Table 1 shows the impurities which are essential to treat according to the usage of the biogas.

Due to these problems, the purification of biogas has become the important aspect for energy utilization. Nowadays, there are several methods to remove the contaminants from the biogas. The word “Biogas-upgradation” denotes the enhancement of calorific value of biogas by removing CO₂ and other traces thus increasing the overall value of fuel [9]. Bio-CH₄ is designated to the upgraded biogas when the fuel standard matches to the natural gas. [10]. However, a minimum of 95% Bio-CH₄ concentration with constituents is acceptable for compensation of natural gas as per European Commission standards [11,12,13,14]. Biogas energy use is expected to expand two times in a period of 10 years, from ~15 GW in 2012 to ~30 GW in 2022 [3,8,15,16]. To achieve this target of 95% bio-CH₄, raw biogas must go through a number of treatments dependent on the biogas’s intended use [17,18]. Absorption, adsorption, and gas permeation techniques can improve raw biogas to natural gas standards. Further, pressure swing absorption (PSA), water, organic solvent, chemical scrubbing and separation employed through membrane and cryogens are few of the commercially available biogas purification systems [19]. However, the systems used to upgrade raw biogas are influenced by the intended use of biomethane, finance associated and efficiency of the upgrading process [20].

Table 1. Recommendation of impurity removal according to the usage.

Impurities	Boiler	Stationary Engine	Kitchen Stove	Vehicle Fuel	Natural Gas Grid	Liquid Biomethane
Hydrogen Sulphide	<103 ppm	543–1740 ppm	8–9 ppm	3–4 ppm	2–3 ppm	2–3 ppmv
Carbon Dioxide	Not recommended	Not recommended	Not recommended	Recommended	Highly recommended	23–24 ppmv
Water Vapour	Not recommended	Not recommended	Not recommended	Highly recommended	Highly recommended	0.1–0.9 ppmv
Siloxanes	Not recommended	8–40 ppm	Not recommended	Not recommended	Highly recommended	Not recommended
Reference	[3]	[9]	[3,9]	[3,9]	[19,21]	[22]

Table 1. Recommendation of impurity removal according to the usage.

Impurities	Boiler	Stationary Engine	Kitchen Stove	Vehicle Fuel	Natural Gas Grid	Liquid Biomethane
Hydrogen Sulphide	<103 ppm	543–1740 ppm	8–9 ppm	3–4 ppm	2–3 ppm	2–3 ppmv
Carbon Dioxide	Not recommended	Not recommended	Not recommended	Recommended	Highly recommended	23–24 ppmv
Water Vapour	Not recommended	Not recommended	Not recommended	Highly recommended	Highly recommended	0.1–0.9 ppmv
Siloxanes	Not recommended	8–40 ppm	Not recommended	Not recommended	Highly recommended	Not recommended
Reference	[3]	[9]	[3,9]	[3,9]	[19,21]	[22]

This review aims to offer a broad overview of current upgrading technologies, significant advancements in upgrading technologies and innovative solutions currently being developed. The novelty of this review is to explore the hybrid systems of biogas upgradation. Also, it has been discussed in detail all kinds of physical, chemical and developing biological methods. New ways of CO₂ utilization have been discussed which could help for reducing the CO₂ emissions and convert into value added products. Additionally, challenges in current upgrading technologies and effect of these technologies on economy have also been reported. Figure 2 is the network visualization consists of 2 clusters and 577 links, depicting the occurrence of the terminologies of abstract and titles of the reviewed article.

2. Technologies for Biogas Upgradation

Biogas purification and treatment have been the focus of substantial research in past few years. The frequency of biogas plants, their biomethane output, electric capacity and raw biogas upgrading plants have been risen dramatically in last few years which is shown in Figure 3a–f).

Figure 4 depicts a flow chart showing different types of biogas purification technologies and Figure 5 shows schematics of different type of commercially used biogas purification techniques.

2.1. Physicochemical Methods

The earliest techniques for absorbing pollutants from raw biogas to segregate the waste are physicochemical procedures. Physical techniques are often inexpensive but inefficient, and which need extensive post-processing to recover pollutants or renew absorbents [23].

2.1.1. Physical Method of Adsorption

Physical adsorption is a technique for removing pollutants from biogas using physical techniques such as water washing. The most widely used approach for removing H₂S and CO₂ from biogas is physical method of adsorption by scrubbing using water. Water scrubbing technique is employed in around 41% of the world’s biogas upgrading facilities [23]. The water scrubbing procedure depends on the solubility of H₂S and CO₂ i.e., their solubility in water is more than CH₄. At 26 °C, CO₂ has a 26-folds greater solubility in water than methane. At a pressure of 6–10 bars, physical water scrubbing is performed [20]. The mass transfer between gas and liquid can be boosted through packaging material used for packing the column of absorption showing a coefficient of higher mass transfer. When compared to the water in the column, raw biogas travels uphill in a counter-current direction, and the H₂S and CO₂ are miscible in the water. The purified gas exits from the top of the absorption column, while water holding CO₂, H₂S, and minute quantities of CH₄ exits from the bottom to a flash tank. The flash tank’s pressure is decreased to 3–4 bars, and the Bio-CH₄ is collected. However, the large amount of CH₄ loss during water scrubbing is due to greater pressure differential between the absorption and desorption columns. Other parameters that impact on Bio-CH₄ loss during water scrubbing include desorption column pressure, raw biogas methane concentration and water flow in the scrubbing column [24]. Besides, single-pass water scrubbing and regenerative absorption are the two forms of water scrubbing. Former method uses processed water from waste water treatment plants absorb CO₂ and H₂S, and then returns the water to the treatment facilities after absorption [25]. In regenerative absorption, excellent grade freshwater is utilized to replenish the desorption column after absorbing contaminants from raw biogas by air stripping. For improving raw biogas, water scrubbing methods need a substantial volume of water. As a result, water regeneration is critical to the technology’s economic feasibility [14,26]. When employed in waste water treatment facilities, water scrubbing technology is more cost effective as without the need for regeneration, waste water treatment plants’ secondary and tertiary effluent can be used in place of freshwater [14,27]. Further, it is impossible to avoid drying the biomethane obtained after water scrubbing due to its high-water content [28]. Prior to being stored and used, the refined biomethane is pressurised at a high pressure (approximately 200 bars) [29,30]. Furthermore, in the water scrubbing biogas upgrading process, Jin et al. [31] have used ultrasound and vacuum to enhance CO₂ dissolution. The study’s findings revealed that CO₂ desorption has an impact on the high pressure water scrubbing process’ efficiency. By using vacuum, the amount of stripping air was greatly reduced, and CO₂ desorption was significantly improved. The use of ultrasonography had a significant impact on the static CO₂ desorption [31].

2.1.2. Chemical Method of Adsorption

Chemical method of adsorption is a technique for removing pollutants from biogas by chemical techniques, generally requiring a solvent. Without any chemical reactions, contact between biogas and organic solvents may selectively collect CO₂. There are several organic solvents such as N-methyl pyrrolidone, methanol, and polyethylene glycol ether (PEG) are often used to absorb CO₂ and H₂S from raw biogas. CO₂ is more miscible in organic solvents than in water for instance, PEG can capture five times more CO₂ than water [32]. Quantity of Organic solvent required in this process are lower than those for water due to their higher adsorption capacity and require less pumping energy [32]. This method may also be used to remove trace impurities including N₂, H₂S, O₂, and H₂O [24]. PEG solutions such as Genosorb^® and Selexol^® are usually employed in organic solvent scrubbing of raw biogas [33]. Additionally, H₂S is more soluble in Selexol^® than CO₂, and during the regeneration process removing H₂S from organic solvents, necessitates higher temperatures. However, higher energy input will be required due to the greater compression and pressure demand costs. In order to absorb solvents, raw biogas is pressurised to a pressure of 8 bar, then the pressured gas is cooled to 20 °C [34]. The absorption column is then supplied with this cooled pressured gas [14]. The organic solvent is likewise chilled to 19–20 °C before being inserted from the top into the column for absorption, where it flows in the opposite path of the biogas. The solvent of organic nature, which is saturated with CO₂ and H₂S, is removed from the absorption column. Then, it is heated to 75–80 °C before being injected into the bottom. It is decompressed to atmospheric pressure in the desorption column [20]. Because in organic scrubbing, the absorption process occurs at low pressure ~8 bars than in water scrubbing ~10 bars, the organic scrubbing process requires less energy. Organic solvents, on the other hand, have a complicated regeneration mechanism. For the regeneration of organic solvents, release of pressure and stripping of air methods are ineffective. Furthermore, the organic solvents must be heated between 39 and 82 °C for regeneration, which necessitates an additional energy input of ~0.15 kWh/Nm³ biogas. However, heat integration to the process of regeneration could reduce the energy consumption [27,35]. Chemical method of absorption techniques based on amines, have been used to get rid of H₂S and CO₂ from gases since the 1950s. As an intermediate, amines of primary and secondary nature react with CO₂ to create zwitterions [36]. Also, most widely utilised solutions during the amine scrubbing of raw biogas are “methyldiethanolamine”, “diethanolamines”, and “aminoethoxyethanol”. A stripping unit and an absorber make up the amine scrubbing upgrading plant. At a pressure of 1–2 bars, raw biogas is delivered through the absorption column’s intake. Against the flow of raw biogas, amine solution enters the absorption column at its very top. Because of the exothermic reaction with CO₂, Temperature rises from 20 °C to 45 °C to 65 °C for an amine aqueous solution. The amine solution’s capacity to absorb CO₂ rises as the reaction speed quickens which as a result temperature of the reaction increase. The CO₂ and H₂S-saturated aqueous amine solution exit from the bottom of the absorption column, travels through a column used to transfer heat, and enters the stripper column. A reboiler provides heat to the stripping column, raising the temperature to 120–150 °C and 1.5–3 bar [37]. The stripping column’s greater temperature helps in the revival of the amine solution by breaking the connection between CO₂ and the waste amine solution. The primary disadvantage of the amine scrubbing procedure is the huge amount of heat required to replenish the amine solution [33]. To lower the greater energy consumption during the regeneration of the amine solution, raw biogas should be pretreated [38]. The use of liquid or solid lime to desulfurize biogas is an ancient and eminent approach that is no longer extensively employed. Because CO₂ interacts fast with ground lime, a larger concentration of CO₂ lowers the rate of H₂S elimination [11]. Ca (HCO₃)₂ is generated in this process, which can react with Ca (SH)₂. Sodium hydrogen sulfide is formed when hydrogen sulphide reacts with sodium carbonate in solution. Natural soil or ferrous material in its unprocessed state can be used to eliminate H₂S. The level of methane in the flue gases rapidly reduces when using the chemical absorption method. The chemical absorbent must be replenished regularly to retain a significant methane level, resulting in a high running cost [39]. Cavaignac et al. recently conducted a techno-economic and environmental assessment of the amine scrubbing biogas upgrading process. This study’s findings revealed that di-glycolamine upgrading may remove up to ~95 percent CO₂, with improved biogas containing up to 92% methane [40]. During the amine scrubbing procedure, amine creates effective bonds with gas molecules, necessitating a greater level of energy for amine solution regeneration [41]. The focus of amine scrubbing technology development these days is to reduce the amount of energy required for amine solution regeneration. Variations in gas flow rate, temperature, the development of novel amine solutions, and the use of efficient heat exchangers can all help to minimize energy use [42,43]. The rusting of equipment and the discharge of volatile chemicals into the environment are both caused by the breakdown of amine solution. Furthermore, the breakdown of amine solutions can produce nitramines and nitroamines, which are dangerous to human well-being and the natural world [44]. Similarly, deep eutectic solvents (DES) have been used for biogas upgradation process and it has proved to be a promising substitute to traditional amine-based liquids and ionic solvents. In a study, aqueous urea (70 wt%) was used to upgrade raw biogas that included 60% CH₄, 39% CO₂, and 1% H₂S and it was found that enhanced biogas contained 98.5% CH₄ and had a 97.5% recovery rate. With a liquefaction rate of 91%, the enhanced biogas was liquefied. The economic research revealed that a 72% DES-based integrated process may save 3.5% of the total investment cost, 24% of the operational investment, and 16% of the total annualised investment instead of an integrated upgrade based on MEA [45,46]. A recent study by Supek et al. found that upgrading 1 m³ of raw biogas with DES costs 0.40 EUR, which is in line with other biogas purification systems [47]. DES can also remove siloxanes, CO₂, and H₂S from biogas in one step.

2.1.3. Pressure Swing Method of Adsorption

The pressure swing method of adsorption (PSA) technique is based on the premise that gas molecules are adsorbed on an adsorbent material dependent on their molecular sizes [9]. CH₄ can be isolated from CO₂ due to its bigger size [48]. The compressed biogas at a pressure of 5–10 bars is passed into a column of adsorption where contaminants such as H₂S, CO₂, H₂O, O₂ and N₂ are adsorbed and removed from the mixture. Because of its greater molecular size, Bio-CH₄ gas at low pressure is collected at the upper side of the adsorption column and does not adhere to the adsorbent. In most cases, four adsorption columns are installed together to make the process continuous [20]. In order to maintain continuous operation before the adsorbent material is completely saturated, biogas is transferred to another vessel [49]. Following the inundation of the adsorbent, the pressure is lowered, resulting in the removal of the adsorbed contaminants and the regeneration of the adsorbing material [47,50]. Some of the most common PSA sorbents include silica gels and activated carbon, as well as synthetic zeolites [48].

It is possible to regenerate adsorbents in 3 different ways. When the revival of adsorbent happens as a result of providing a vacuum after shutting the input valve, the procedure is referred to as “vacuum swing adsorption”. It is also known as PSA if the revival of adsorbent takes place as a result pressure is lowered to the level of the surrounding atmosphere. When the temperature is raised between 31 and 121 °C while the pressure stays constant, the process is referred to as “temperature swing adsorption” [29]. As a result, because the adsorption material employed in this method for the refinement of biogas adsorbs H₂S permanently, H₂S is regarded a harmful contaminant for this procedure [51]. It must be eradicated from the biogas before it can be utilised for the upgrading of biogas. PSA is a biogas processing system that is both efficient and effective. However, the primary disadvantage of the PSA method is that an additional gas treatment step is necessary in order to prevent emissions into the surrounding environment. A hybrid stage two PSA unit has been developed to purify the output gas from the first stage [52]. Zeolite 5A has been employed in this unit. The development of an effective and economical adsorbent material, such as MOFs (Metal-Organic Frameworks) and ZIFs (Zeolitic imidazolate frameworks), is currently being investigated for building a reliable, inexpensive, and competent PSA system in order to lower the operating expenses. According to Chidambaram et al., two porous MOFs consisting of tetra-carboxylate ligands and aluminium metal ions Al³⁺ were utilized in their experiments [53]. The findings of the investigation indicated that both materials were resistant to CO₂ and Bio-CH₄ and were permeable to these gases and not permeable to water vapours. Both materials were discovered to be suitable for use commercially as well. This requires a multistage PSA technique in order to effectively eradicate CO₂. Another adsorbent namely carbon molecular sieve has been studied in vacuum pressure swing adsorption where the micropore size of the molecular sieve was in the rage of 0.25–0.6 nm. Raw biogas containing ~50% methane when passed through it, upgrades biogas with 98% pure Bio-CH₄ gas [54] by silica gel employed as an adsorbent [55]. Absorbents with significant absorption potentials, enhanced selectivity, and presumably high thermal resistance may be likely to be used for improved separation efficiency as well as a greater level of process integration. Consequently, there is reason to believe PSA could be a successful absorption method for CO₂ in the years ahead [56,57]. The primary difficulty facing PSA at the moment is its higher energy costs, higher electricity consumption which account for 77 percent of the overall cost, as well as its higher initial investment. This can be alleviated by using alternative energy sources, decreasing the flow of unwanted materials to the upgrading unit, and optimising the biogas upgrading plant [58]. The designing of the PSA is also complex process, if it involves various types of adsorbents. Therefore, a study was done both experimental as well as numerical which compared the performance of three activated carbons for separation of CO₂ from the raw biogas on the basis of adsorption data [59]. There is need of further research study to improve the design and optimum parameters so that process could be efficient and economical for biogas upgradation.

2.1.4. Technique of Membrane Separation

The size of the particle and distinct compounds of different chemical affinity are taken into consideration throughout the membrane separation upgrade process [60]. Notably, membrane separation technology for treating biogas was validated to other methods, and it was found to be a relatively simple procedure to carry out. Despite the fact that membrane separation does not necessitate a high level of operating requirements, the most significant disadvantage is the substantial upfront investment. [61,62] Removal of the contaminants for example, H₂S and H₂O from raw biogas is important before it can be compressed in order to enhance the pressure of the resulting gas. Next, through the membrane separator pressurised gas is injected, where CO₂ is separated from the mixture. The high-pressure side of the membrane separator is where the improved biogas exits, while the low-pressure side is where the pollutants are removed from the enhanced biogas. When it comes to upgrade raw biogas, polymer or membranes of inorganic nature are frequently utilised in the process. There are three main types of membrane fibre materials are utilised in the gas separation process, which are classified as follows: membranes of inorganic in nature, mixed polymer membranes, and mixed-form membranes [63]. Most often used membrane for commercial gas separation is the polymer type membrane as these are economical, easy to manufacture and do not change shape and size or the structure at high pressure. While, ceramic and metal film (Inorganic in nature), are still in the early stages of development and are not yet ready to be used on a large scale in industry. Ultimately, a polymer and chemicals of inorganic in nature chemicals are combined to form a mixed membrane. Aromatic rings and functional groups derived from aromatic polyimides work as molecular sieve, allowing for the passage of molecules across them. Mixed membranes can also be found that have an aromatic ring attached to the polymer chain and a sulfol group linked to the sulfol group, as in, polyethylenimide, polyether amides, or polysulfones. Polysulfone membranes have the ability to tolerate water, weak acids, as well as alkali. [61,63]. Carbon dioxide selectivity (CO₂/CH₄) ratio of 1000/1 allows for the passage of particulates like carbon dioxide, hydrogen sulfide, and water vapour, while sustaining methane or nitrogen gas [27]. H₂S, CO₂, H₂O, and O₂ are allowed to pass through while redriving nitrogen and methane gas. Polymer, cellulose and acetate are less expensive, have greater stability, and are easier to scale up than non-polymeric materials [60]. As a result, these are chosen over materials of non-polymeric. Nanotechnology has been used to produce new membranes with high selectivity factors, which have demonstrated a better methane recovery rate than previous membranes [48,60]. Two separate systems, namely high-pressure membrane systems and low-pressure which are both based on membrane technology., make up the membrane biogas upgrading technique that is widely used in the commercial production of raw biogas. The raw biogas is upgraded in the system with high-pressure membrane at a pressure greater than 20 bar [11], but other systems operate at a low pressure of 8–10 bar. More than 96 percent of methane concentration is achieved by the system of multi-phased high pressure biomethane production [3]. Raw biogas is separated from liquid absorbent on one side of the membrane in a low-pressure membrane system. Sodium hydroxide and ammonium solutions are widely used as liquid absorbent in low-pressure membrane systems. The low-pressure system is used to produce bio-methane with a CH₄ concentration of more than 97% and a CO₂ fraction of high purity. It is possible to sell the reasonably pure CO₂ that is created during the upgrading process for a variety of uses [48]. Although the method for membrane upgrading is straightforward, new membranes are being developed that are capable of upgrading biogas in more harsh situations, as well as working efficiently under lower pressures and with humid raw biogas [64]. For biogas upgrading, there are currently expensive membranes currently in use, as well as further study is needed to improve the membrane process’s profitability and efficiency. It is also necessary to do research in order to build membranes that have improved permeability without losing their selectivity. When it comes to separating combinations of methane and carbon dioxide, ionic liquids have shown to be the most effective method. Furthermore, the membranes also are very costly at the present time, and further study is necessary to enhance their economics. Furthermore, the economic feasibility of membrane separation procedures has been shown to be difficult to achieve for lower scale facilities (biogas flowrates of less than 100 Nm³/h) [65]. The success of a ~95 Nm³/h biogas digester using a membrane separation facility was examined by Peppers et al. [66], which was published recently. According to the findings of the study, pre-treatment of the gas is required in order to achieve maximum methane quality as well as the preservation of the membrane.

Inorganic membranes have received a great deal of interest because of their excellent selectivity, permeability, and tolerance to extreme environmental conditions. Thermal stability, mechanical strength and chemical resistance are improved in these membranes, resulting in increased separation efficiency. Additionally, these membranes are simple to clean anytime, if required. Inorganic membranes may be made from zirconia, activated carbon, silver, alumina, nickel, silica, MOFs, or carbon nanotubes [67,68]. They can also be made from other materials such as silver, nickel, or zirconia. The disadvantage of these membranes is that they are fragile, expensive, and have a poor surface area per module volume. Furthermore, the permeation of membranes of inorganic in nature cannot be easily bolstered, that is why they have not been widely employed [61].

Because biogas purification is accomplished using polymeric membranes, these materials are characterised as either rubbery or glassy depending on how biogas is scrubbed. Gas molecules’ miscibility and dispersion are the fundamental principles that these membranes employ in order to separate them. Among the materials typically utilised in the construction of these polymeric membranes [69], there are polydimethyl siloxane, polycarbonate, polyimide, cellulose acetate and polysulfone. Great selectivity in permeation as well as high mechanical strength are characteristics of polymeric membranes [70,71]. The manufacturing process is less expensive, and the cleaning procedure is less complicated. Processes such as compaction, ageing, and plasticization, on the other hand, might degrade the performance of these materials [72].

Essentially, mixed matrix are the membranes of composite nature that are constructed by combining a dispersed phase of inorganic material and a continuous phase of polymeric material in order to obtain improved selection and permeation while maintaining a lesser rate [73]. Membranes of polymeric and inorganic nature are produced [74,75], and they are capable of overcoming the constraints of either polymer or inorganic membranes. Chlorine/carbon dioxide separation is most commonly accomplished using zeolites (zl)and organo-metallic filters. Because of the homogeneous pore size of these membranes, it is possible to separate the gas molecules with relative ease. Because of their cheaper manufacturing costs, their versatility in application, and how well it adsorbs and how much it can pack into a given space, elasticity and definite surface area [29], these membranes are becoming increasingly popular. Because of the availability of the polymeric material, they also have increased temperature resistance as well as tensile strength, and they may be employed in hard environmental conditions [75,76]. Producing a homogenous combination of polymers and solvents is the typical approach for creating a flat mixed matrix membrane. On the other hand, another solution is generated by sonication from an inorganic-solvent mixture, and the solution is then vaporised at a certain temp. after which the coating is softened in order to eliminate any leftover solvents A frequent application for this technology is the production of thick polymer films [61].

2.1.5. Separation through Cryogens

When it comes to liquefaction, different components of biogas have different melting points and pressures, which makes it ideal for cryogenic biogas upgrading. The upgrading of raw biogas is carried out in four steps at 80 bar pressure and 170 °C temperature [38] under pressure and temperature control. In the first step, contaminants such as siloxanes, hydrogen sulphide (H₂S), moisture, and halogens are eliminated from the impure biogas. A heat exchanger is used to cool the impurities-free gas to a temperature of 25 degrees Celsius in the second stage after it has been pressurised to 1000 kilopascal. It is necessary to cool down this chilled gas further to 55 degrees Celsius in phase three, where carbon di-oxide is transformed to a liquid state. In fourth phase, the temp. of gas is lowered 85 degrees Celsius, at which time the CO₂ freezes and is separated from the rest of the gas. In the course of the cryogenic upgrading process, pure carbon dioxide is created as a by-product, which may then be sold to generate additional profit [48]. The cryogenic upgrading technique may also be used to produce liquid natural gas (LNG) and liquid biomethane [38], which are both valuable fuels. It is possible to eliminate the pollutants present in biogas by using a cryogenic separation (CS) technique with a constant pressure of 10 bar, as described in [77]. Low temperature chemical synthesis is possible due to the great purity of the carbon dioxide produced as a result of this process. In contrast to other separation approaches, the drawback of employing low-temperature technology to handle biogas is that the expense of carbon dioxide separation remains higher when compared to other separation techniques [78].

Low-temperature hybrid systems for biogas upgradation are currently in the early stages of research and development. The biggest challenge in absorbing at low temperatures is the removal of moisture, which necessitates the use of a significant amount of energy. Additionally, cryogenic membranes are susceptible to difficulties with oxygen enrichment units and membrane contamination, whereas low-temperature hydrates need a greater degree of technological maturity as well as continuous process development [78,79]. Developing new commercial designs with reduced energy usage will be necessary in the future to increase the efficiency of low-temperature processes [80,81]. The compression and refrigeration of raw biogas demand a significant amount of energy, which is the primary difficulty for CS. The energy required by the CS technology is approximately 10% of the energy required by the biomethane generated. Furthermore, another issue associated with this technique is freeze carbon di-oxide created throughout the procedure has the potential to block the machinery. As a result, the pollutants that have been eradicated from the unprocessed biogas throughout the upgradation process must be removed carefully. The development of methods for recovering the energy used in the condensation of raw biogas through the liquefaction of biomethane is one option for strengthening cryogenic technology. The liquefaction of biomethane at 15 bar and 125 degrees Celsius can help to reduce the amount of energy consumed throughout both processes. Aside from that, the frozen CO₂ produced during the upgradation process might be used in industrial applications [82].

2.1.6. Upgradation by Adsorption: Solid Surface

Adsorbing the CO₂ molecules on a solid surface can be used as an alternate way of separating CO₂ from CH₄. Because of van der Waals or physical forces, gas molecules are selectively attached to a solid surface during the process of adsorption. This approach makes use of either distinct adsorption balances or variable adsorption kinetics, depending on the situation. The adsorbed molecules of CH₄, for example, are separated from one another more slowly than those of CO₂. Certain porous solids are commonly used as adsorbent materials, such as clay. There are many classic adsorbents such zeolites, titanium silicate, carbon molecular sieves, activated carbon, and silica gel; biochar is a relatively new and environmentally beneficial option [83]. Carbon dioxide, moisture, hydrogen sulphide (H₂S), and other contaminants can be extracted from biogas depending on the qualities of the adsorbent material employed. However, it is possible that H₂S will be co-separated with carbon dioxide, which would result in a considerable reduction in the lifetime of the adsorbents [14]. The simultaneous removal of carbon dioxide, nitrous oxide, and hydrogen sulphide from raw biogas, resulting in methane yields ranging from 96 to 98 percent (by volume), is one of the many advantages of this method. According to [14,84], adsorbent active holes become blocked when exposed to gases such as H₂S or ammonia, which is a major downside of the adsorption method. Biochar is created via the thermochemical conversion of organic molecules under oxygen-depleted circumstances [83] and is considered to be an environmentally beneficial adsorbent [84]. Carbon dioxide adsorption in biochar is dependent on the biochar’s physio-chemical properties, which change according to the feedstock type and operation conditions [81,85]. Linville et al. [85] observed that biochar created from digested food waste from an anaerobic digester operating on shredded fine, small particle size walnut shell material removed 61% of CO₂, whereas coarse walnut shell feedstock removed only 51% of CO₂.The fine walnut shell biochar has a higher surface area than coarse biochar because of the smaller particle size.

Additionally, biochar’s high porosity leads in a higher surface area, which aids in CO₂ adsorption [86]. Numerous studies have established that biochar’s high specific surface area significantly contributes to CO₂ adsorption [81,83,87]. According to [88], biochar with a restricted micropore distribution is an excellent material for CO₂/CH₄ separation. Moreover, because biochar contains alkalis and alkali earth metals, a slightly alkaline pH in the biochar given to digesters is advantageous because it facilitates in the conversion of CO₂ to bicarbonate or carbonate [86]. This enhances methane synthesis by hydrogenotrophic methanogens, which reduce CO₂ as a result of the decrease of CO₂. The absence of lower oxygen functional groups on the biochar surface prevents the formation of water clusters, which allows for the formation of hydrophobic sites on the biochar surface that attract non-polar CO₂ [89]. In the presence of water, increased surface hydrophobicity has been recognized as a promising technique for enhancing CO₂ collection efficiency [89]. Another factor is that acidic functional groups are decreased while basic functional groups are raised, which can occur naturally or as a result of biochar alteration. As a result, basic biochar surfaces that are more favorable to CO₂ adsorption are produced [83,86].

Table 2. Specifications of different conventional biogas purification techniques.

Specifications	Membrane Separation	PSA	WS	AS	OSS	References
Flue gas Purification	Present	Present	Present	Absent	Absent	[90,91]
Desulfurize Recommended	Present	Present	Absent	Present	Absent	[90,91]
H2O Separation	Absent	Absent	Present	Present	Absent	[90,91]
Nitrogen and oxygen exclusion	Present Partially	Present	Absent	Absent	Absent	[21,92]
Chemicals Required	Absent	Absent	Absent	Present	Present	[92,93]
Operational Press.	Less than 8; more than 6 bar	Less than 10; more than 4 bar	Less than 10; more than 4 bar	1 bar	Less than 8; more than 6 bar	[31,94]
Outlet Press.	Less than 8; more than 6	~5 bar	~10 bar	~5	1.3–7.5	[21,31]
Thermal energy requirement	NA	NA	NA	95–184 °C	54–79 °C	[31,94]
Electricity Requirement (kWh/m3 biomethane)	0.24–0.42	0.45	0.45	0.26	0.48–0.66	[91,95]
Operational Cost (€)	~82000	~187000	~110500	~135000	-	[51,91]
Capital Cost (€)	~232500	~680500	~264500	~352500	-	[51,91]
Servicing cost (€/year)	~25500	~55500	~14500	58500	38500	[48,91]
Loss of Methane (%)	<1.4	<1.8	<1.8	<0.1	<1.8	[91,96]
Recovery percentage	95–97	>95	>96	99.7	>98.8	[91,97]
Technical Availability	96.8	93.8	95.6	90.8	-	[90,91]

shows the specifications of different conventional biogas purification techniques.

2.2. Biological Methods

2.2.1. Biological Upgrading: Chemoautotrophic

It is the conversion of CO₂ to CH₄ by the action of hydrogenotrophic methanogenic bacteria that serves as the fundamental concept of chemoautotrophic biogas upgrade.

The basic chemical reaction of this process is as follows:

CO₂ + 4H₂ → 2H₂O + CH₄ (ΔG = 131 kJ),

(1)

In order to transform CO₂ into CH₄, hydrogen (H₂) is necessary during the upgrading of raw biogas [14], and throughout the process, each mole of CO₂ would create an additional mole of CH₄. As a result, additional value is generated throughout the biogas upgrading process. Additionally, this technique is employed to minimise CO₂ emissions in the electronics sector, where the H₂ necessary for the process is created by electrochemical treatment of water containing hydrofluoric acid [98]. Using groups of methanogens, which are capable of converting CO₂ into CH₄, syngas generated by the gasification of biomass or coal may also be converted to methane and used as a fuel. Among the frequent methanogens are Methanospirillum sp., Methanococcus sp., Methanosarcina sp., Methanobacterium sp., Methanoculleus sp., and Methanothermobacter sp. [99,100,101,102,103]. Methanogens are bacteria that can convert CO₂ into CH₄. Methanogens are members of the Archaea phylum, and some survive in mesophilic circumstances, while others flourish in thermophilic conditions in the pH range of 6.5–8. Methanogens are found in soil, water, and air. Renewable energy sources should also provide the hydrogen necessary for upgrading raw biogas, in order to make the process as environmentally friendly as possible. Power to gas [14] is a term that refers to the use of wind or solar energy for the electrolysis of water in order to create hydrogen, which is advocated in this context [104]. It is possible to convert CO₂ to methane via hydrogenotrophic methanogenesis, either in a separate bioreactor or by injecting hydrogen directly into the biogas reactor. One issue with adding H₂ to a conventional biogas reactor is that it raises the pH of the mixture, which can cause the process to fail. Aside from that, the standard biogas reactor is not intended to overcome gas-liquid mass transfer, and intense mixing can interfere with the normal operation of anaerobic consortia. This has resulted in the procedure being employed on a limited scale [4]. In biogas upgrading, there are three types of biological processes to consider: in-situ, ex-situ, and hybrid processes, which will be addressed more below. Figure 6 shows schematics of different biological upgrading systems.

Biological Upgrading: In-Situ

During this procedure, hydrogen gas is fed into the anaerobic vessel, where it reacts with the CO₂ already present inside the reactor to form CH₄ through the action of hydrogenotrophic methanogens. Using this procedure, it is possible to recover almost 99 percent of the potential methane under required pH and temperature conditions. Using in-situ bio-electrochemical biogas upgrading technology, [92] created a system that is very efficient. Under steady working circumstances, the cathode potential applied was just 0.5 V at the maximum. The study’s findings indicated that the enhanced biogas contained 97 percent methane, which was more than expected. It was discovered that the CO₂ reduction rate was 318.5 mol/d/m². In addition, it was discovered that the rate of electro-methanogenesis increased further with the amount of proton used, which increased the pH of the system, resulting in a larger solubility of the CO₂. The procedure is straightforward and cost-effective, and it offers infrastructure for the storage of renewable energy [105]. The pH increase caused by the loss of bicarbonate, which functions as a buffer, is currently the most serious problem associated with the in-situ biological process [14]. Co-digestion with an acidic waste, on the other hand, can alleviate this problem [103]. According to the results of the study, the in-situ biogas upgrading process outperformed the ex-situ biogas upgrading process, and the performance of this system may be further enhanced by continuous operation [106].

Biological Upgrading: Ex-Situ

The method involves the addition of H₂ from outside source and raw biogas from AD into another reactor where hydrogenotrophic bacteria are already present so that carbon di-oxide is converted to methane [10]. Because the purification of unprocessed biogas is carried out in another reactor, the procedure will have no effect on the real AD process. Furthermore, the procedure is straightforward, adaptable, and not dependent on the inclusion of biomass resources [14]. This technique of upgrading may also be used to handle large volumes of biogas with hydrogen addition since it reduces the amount of time necessary for the gas to be converted during the retention period. Compared to the in-situ technique [107], this upgrading procedure is gaining more attention since it is a significantly more regulated means of upgrading the biogas [107].

Biological Upgrading: Photoautotrophic

When the H₂ gas comes from outside, it is injected into the AD, where it interacts with carbon di-oxide to generate methane as a result of the action of methane forming bacteria, which is then recycled. A second anaerobic reactor containing hydrogenotrophic methanogens is used to remove the leftover carbon di-oxide from the purified gas [108]. The purified gas, consisting of leftover carbon di-oxide, injected in the reactor. A portion of the carbon di-oxide alteration could be done during the first AD process, and the final AD process can be used just for final purification and purification.

2.2.2. Biological Upgrading: Hybrid

Microalgae and prokaryotic cyanobacteria are used to extract CO₂ from raw biogas in either open or closed photobioreactors, depending on the kind of biogas being treated. When compared to an open system, closed photobioreactors have the benefit of using less water and land than an open system. [109] The fundamental advantage of this method is that it removes CO₂ and H₂S from raw biogas in a single step, which is the primary advantage of this procedure. As a result, the use of microalgae for the upgradation of raw biogas offers a number of advantages, including (a) CO₂ reduction while simultaneously increasing the CH₄ content, (b) the production of useful biomass, and (c) the removal of nutrients from wastewater [110]. The biomass generated by CO₂ valorization using this technology can be utilised as a feedstock for the manufacturing of additional value-added goods, such as pharmaceuticals. The algal biogas upgrading process, in addition, decontaminates the digestate produced by the AD process [111]. Using a micro-algal system, Yan et al. discovered that a micro-algal system eliminated pollutants such as COD (85.5 percent), total phosphorus (92.4 percent), and nitrogen (87.1 percent) from raw biogas, with an additional 85 percent CO₂ removal [112]. The solubility of CO₂ and the transfer of mass in microalgae are the most difficult problems to solve in this technology [113,114]. Other drawbacks of this technique include the difficulty in extracting biogas and the solubility of methane in micro-algae environments, both of which are problematic. Using an indirect biogas upgrading method [65], these drawbacks can be mitigated to some extent.

The fundamental disadvantage of a closed system is that it has higher energy requirements for light generation, as well as higher initial capital expenses, than an open system. Open photoautotrophic systems, on the other hand, demand less initial investment and energy, but their CO₂ removal efficiency is lower as a result. It is possible to recover 97 percent of the methane produced by the photoautotrophic biogas upgrading process when H₂S removal is done at the same time. It is possible to feed biogas from the anaerobic reactors directly into the photobioreactors or to feed it outside to an absorption column in this system. Afterward, the carbon dioxide emitted by the raw biogas is absorbed by the micro-algal broth, which is recirculated in an absorption column separate from the photobioreactor [115]. Eukaryotic microalgae and prokaryotic cyanobacteria remove CO₂ from raw biogas by utilising water, nutrients, and sun radiation as food sources.

It is created byproducts like as biomass, heat, and oxygen as a result of this process. It has been found that by increasing the CH₄ percentage of biogas, the CO₂ content of the final output gas may be reduced to 2–6% [116]. Because algae can metabolise enormous amounts of carbon dioxide when exposed to sufficient light, they can develop at an incredibly fast rate and are very adaptable to a variety of diverse environmental situations [100,117]. In addition to the generation of active biomass as a byproduct, photoautotrophic biogas upgrading has the potential to produce methane through the use of an AD reactor [118] or to be employed in the development of value-added goods such as pigments [119]. It is possible to employ this procedure for both upgrading biogas and treating wastewater at the same time [120,121]. Marine et al. investigated various novel photosynthetic biogas upgrading approaches, including the use of greenhouses in the winter, the stripping of CO₂ from photobioreactors by air stripping in the winter, and the use of digestate in the summer [122]. Following the study’s conclusion, it was discovered that the H₂S was completely removed under all conditions tested, and that the methane concentration in the upgraded biogas ranged from 89.5 percent to 98.2 percent during the first strategy, 93.0 percent to 98.2 percent when using the second strategy, and from 96.3 percent to 97.9 percent when using the third strategy.

2.2.3. Biological H₂S Removal

Because of its corrosive character, H₂S is regarded to be one of the most damaging pollutants in biogas [123]. In biogas purification, biological H₂S removal is a relatively novel concept, and it has piqued the interest of academics and industry alike. Due to the requirement for extra chemicals, typical chemical H₂S removal procedures are expensive. Additionally, these processes frequently generate waste, which must be handled and disposed of properly to avoid environmental contamination. The biological elimination of H₂S is both cost-effective and environmentally benign. Furthermore, biological desulfurization may be carried out at lower temperatures and pressures without the use of chemicals [124], resulting in reduced operating costs.

2.2.4. Bio Trickling Filtration

The H₂S is evaded from the unprocessed biogas using this approach, which involves placing it in a humid packed bed containing sulfur oxidising bacteria. These bacteria are aerobic in nature and forms biofilm. Then tested on an industrial scale, this method has shown promising outcomes, especially at low H₂S concentrations of ~13,000 parts per million. However, some concerns are still with biological desulfur, including the generation of elemental sulphur, which causes clogging of the filter [125] and the creation of sulphur dioxide. Sulfuric acid, in addition to elemental sulphur, can be created by either a complete or partial oxidation of the sulphur compound. During the bio-trickling filtering process, sulfite and thiosulfate are also occasionally formed in small amounts. There have been several tests conducted on various bed materials, including “polypropylene HD Q-PAC^®”, plastic strengthened with glass-fiber, and “polyurethane foam” [126], all of which have been proven to be effective.

2.2.5. Desulfuration through Microareation

In addition to microaeration, or microaerobic desulfurization or restricted aeration, or micro-oxygenation, is a novel technology for desulfurization of biogas [124,125]. A little quantity of O₂ is introduced in the AD to encourage the growth of sulfur oxidizing bacteria, which results in the creation of sulfur oxide [127]. The “microaeration” system has successfully checked in a full-size AD running on sewage sludge. [128,129] The term “micro-aeration” is coined when the air is pumped in AD, whereas air which is in pure state is pumped in AD pumped then the word “micro-oxygenation” is coined. The desulfurize of unprocessed biogas takes place in AD, which must be cleaned on a regular basis in order to evade blockage. If the regular maintenance is not done, it is possible that the H₂S removal effectiveness will decrease. In addition, the cleaning procedure raises costs associated with H₂S removal [128].

2.2.6. Desulfuration through Microareation

Carbon dioxide from raw biogas may be utilised to produce high-value compounds, which can be used to power vehicles. The chemical can be transformed into other products such as butanol and ethanol by the use of biological upgrading processes. Carbon dioxide and hydrogen are converted into soluble organic compounds by bacteria such as Clostridium scatologenes, Acetobacterium woodii, and Butyribacterium methylotrophicumn [116], among others. To upgrade biogas through the conversion of CO₂ to H₂, it is necessary to have access to inexpensive hydrogen sources. A number of fermentation systems are capable of directly converting CO₂ and carbohydrates into carboxylic acids. Through the fermentation of glucose with the Actinobacillus succinogenes bacteria, the CO₂ in biogas is transformed to succinic acid [117]. As a result of this upgradation procedure, the biogas may be cleaned to include around 96% methane, and there are no concerns with gas-liquid mass transfer [100]. The study of microbial electrolytic cells, which capture carbon dioxide and convert it into methane, has gained interest along with hydrogenotrophic methanogens, which convert carbon dioxide into methane. The process of eliminating carbon dioxide from the atmosphere by converting it into methane is referred to as electro-methanogenesis, as shown in the following equation.

8 e⁻ + 8 H⁺ + CO₂ → 2 H₂O + CH₄,

(2)

The majority of carbon dioxide removal systems rely on the addition of hydrogen to the biogas in order to create additional methane through the conversion of CO₂. However, according to recent research, some methanogens, such as Methanosaeta, are capable of participating in direct interspecies electron transfer (DIET). As the creation of hydrogen necessitates the use of energy for electrolysis [118,119], DIET is a “cheaper” option as compared to the transfer of electrons between species via H₂. Conventional biogas facilities that use outdated technology should be replaced with biological technologies that have just come into existence. It has not yet been possible to apply biological technology in large-scale or pilot-plant applications. Because of their complexity, upscaling biological procedures from lab size to full scale is currently the most difficult problem. Various abiotic and biotic aspects need to be considered when scaling up biological processes to full scale. These procedures are also time-consuming and necessitate the purchase of additional equipment [111]. Another study was done to screen the best material for the maximum mass transfer coefficient of the H₂ gas during bio methanation process in trickle-bed digester. It was found in the study that powdered clay matter eased the process of maximum mass transfer coefficient of H₂ gas to 10/min which further enhanced the production rate of methane by removing 99% of hydrogen [130].

Aside from this, biological technologies necessitate the collection of additional experimental data in order to establish a complete energy-mass balance, which is required for a credible TEA as well as an LCA. This is especially true since these processes depend on the carbon di-oxide and hydrogen mass transfer and the kinetics of these processes are substantially different [106] from those of conventional processes.

2.3. Comparison

Table 3. A comparison between advantages and disadvantage of various purification methods.

Technology	Advantages	Disadvantages
Membrane Separation	Reduced Maintenance Costs, Easy and compact built, Easy to use and safe, Light-weight, Increased dependability, Reduced Costs of Operation, Low energy Consumption, Selectivity is good. Flexibility on a larger scale, Chemicals are not necessary, Environment-Friendly, Less space is required, Simple Installation, No harmful emissions.	High cost of membranes, H2S need to be removed, Demand of energy is high, Methane loss is high, Membrane degradation
PSA	No Chemicals needed, Extremely Effective, Customizable, Low energy Consumption, Increased Pressure, Convenient Procedure, Impurities-tolerant, Useful for smaller scale, Reduced Emissions, Simple installation, N2 and O2 adsorption, Rapid regeneration of sorbents	Costly Procedure, CH4 loss is high. Before upgradation hydrogen sulphide and H2O need to be removed, Fouling susceptibility, Costly investments, Costly maintenance, Process control is tedious
Water Scrubbing	Easy Procedure, Lesser purification cost Adaptable to pressure and temperature, changes in the system, Remove H2S and NH3 at the same time, Tolerate Contaminants, Chemical not needed, Regeneration of water is simple.	More water is required, Efficacy is less, Slow Procedure, Bacterial growth, Corrosivity, High power requirement, Foam formation, Water wastage is more
Amine Scrubbing	Increased Productivity, Reduced Methane Discharge, Speedy Process, Removing of H2S completely, Initial cost of the amine solvents is high, there are no moving parts, CO2 adsorption capacity	Costly investment, Treatment Required for waste chemicals, Toxic solvents, High demand of energy for solvent regeneration, Precipitate formation, Foaming problems, Carcinogens formation
Physical absorption using organic solvents	Enhanced Productivity, Reduced CH4 Losses, Regeneration. Removal of H2S, H2O, and NH3 at the same time, Organic solvents have an anticorrosion property, Increased CO2 Solubility	Expensive Procedure, High energy requirement for solvent renewal, Solvent is expensive
Cryogenic	Chemicals are not needed, CH4 losses are reduced, Scalability is simple, Pure CH4, Reduced cost of Energy, High Separation	Energy demand is high. Efficacy is dependent on temperature
Biological Separation	Easy procedure, Capital investment is low. Less money required for maintenance, No undesired by products	Use of H2 gas. Not for commercial use. Cannot remove high H2S concentration

shows the comparison between the advantage and disadvantage of all the purification method.

3. Challenges with Current Upgrading Technologies

Currently, traditional biogas upgrading techniques are widely employed, accounting for 98% of all upgrading facilities. These traditional systems, on the other hand, have issues that might raise the cost of purifying raw biogas. When it comes to raw biogas upgrading techniques, water scrubbing is the most widely utilised conventional process. It is employed in 41 percent of the upgrading facilities. Nonetheless, this procedure uses a significant amount of water, and the need for water renewal will significantly raise the cost of water treatment as well as the process [131]. Utilizing developing biological processes, such as those that do not need the use of water during the upgrading of raw biogas, may help to reduce the water shortage [132].

A second frequently used traditional technique for the purification of unprocessed biogas is amine scrubbing [133]. However, a significant level of energy is needed to regenerate the chemicals that are employed, which might raise the process’s operating costs. Furthermore, the possibility for corrosion and amine leaching are two issues that have been raised in connection with this method [134]. Additionally, during chemical renewal in organic scrubbing, an enormous level of energy is needed, making the procedure prohibitively expensive. It is also necessary to increase treatment expenses in the case of pressure swing adsorption due to the extra pre-treatment of outcoming gas which is needed while utilizing different zeolites.

Lastly, the membrane techniques that are currently employed for biogas purification are prohibitively costly. As a general rule, pre-treatment is a frequent step performed prior to upgrading raw biogas, and it is deemed necessary to boost the effectiveness of the upgrading process while also avoiding equipment damage caused by contaminants [135]. In addition, an extra apparatus is necessary for standard biogas upgrading procedures in order to generate higher temperatures or higher pressures, which enhances the expense of these approaches even more. More than that, the CO₂ that is extracted from unprocessed biogas by these procedures is frequently released in the environment, this enhances the GHG emission as well as the byproduct is wasted that can be converted into value added product [136].

4. Combined Upgrading Technologies

The pros and cons of each biogas upgrading process are different, and none of them is currently regarded optimal. Hybrid technologies, which combine two or more technologies to create a single product, may be able to overcome some of these drawbacks. With the help of 7 distinct mixed membrane upgrading techniques, Scholz and colleagues explored the upgrading of raw biogas. In comparison to the conventional water scrubbing technology, the results showed that utilising the membrane+water scrubbing mixed technology has high capital investment [63]. In contrast, it was discovered that the membrane+cryogenic mixed technology has been advantageous than the simple cryogenic technology in terms of cost and performance.

Aside from that, to compare performance in terms of energy of combined technique of cryogenic and conventional method Belaissaoui and colleagues used computer simulations. The combined system was then contrasted with a traditional chemical absorption technology to see which one performed better (namely monoethanolamine MEA). According to the study’s findings, the former’s energy performance improved by 12–15 percent when compared to the latter [137].

Song conducted a study showing a comparison of functioning and financial viability of combined system of cryogens and membrane with that of 3-phased membrane system of upgradation method throughout CO₂ capturing from raw biogas, and discovered that the 3-phased membrane system shows improved functioning during optimum conditions. The total power expenditure of this 3-phased membrane system was also significantly lower than that of the standard chemical absorption technology which was 3–4 MJ/kg CO₂ in comparison [80]

5. Effect of Upgradation Techniques on Economy and Environment

There has been a plethora of research published on the “life cycle costing” (LCC) and “life cycle assessment” (LCA) of various upgradation systems. Few researches reveal that generation of biomethane produces lesser amount of greenhouse gases as compared to the production of natural gas and power production from natural gas [5,129]. According to some reports, biogas-to-biomethane systems have higher environmental performance than biogas-to-power systems [138], however other research have demonstrated that cogeneration has greater eco-friendly performance [139]. Apart from that, the economic feasibility of the biogas to biomethane systems has been demonstrated to be superior to that of alternative energy processes such as cogeneration.

According to the findings of Starr et al., the CO₂ emissions from high pressure water scrubbing and amine scrubbing (AS) are the least when compared to other traditional upgrading methods. The emissions from amine scrubbing and high-pressure water scrubbing are 5% lower and 16% lower respectively in comparison of MS (membrane scrubbing) and OFS (organic physical scrubbing) [140]. The membrane separation has reduced implication on environment when compared to other standard biogas upgrading methods [139]. The CO₂ emissions from high pressure water scrubbing, pressure swing adsorption, and amine scrubbing techniques were found to be 5 percent, 4 percent, and 27 percent greater, respectively while comparing to MP (membrane permeation) technology. Hauser analysed the life cycle assessment and cost study of 5 biogas upgrading systems, including aeration, high-pressure water treatment, photocatalytic oxidation, chemical sorption, and membrane permeation (MP). The study’s findings indicated that, when compared to other technologies, cryogenic separation technology had the lowest environmental effect, according to the finding’s membrane permeation, amine scrubbing, pressure swing adsorption and high-pressure water scrubbing technology all had much higher environmental consequences than cryogenic separation [141]. However, cryogenic separation had significantly lower environmental impacts than any of these technologies [141]. Several biogas upgrading systems, including amine scrubbing, high pressure water scrubbing, pressure swing adsorption, cryogenic separation, and membrane permeation, were examined in detail by Brendelkken, who analysed the costs of five different biogas upgrading technologies. The study’s findings revealed that pressure swing adsorption and high-pressure water scrubbing techniques are more economical compared to other upgradation techniques [142]. It was discovered that both methods were 138 percent, 61 percent, and 29 percent economical with respect to cryogenic separation, amine scrubbing, membrane permeation techniques, respectively.

Lombardi and Francini conducted a comparison of the implications on environment and economy membrane permeation, high-pressure water scrubbing, potassium carbonate scrubbing, amine scrubbing, and pressure swing adsorption. Amine scrubbing technique has higher eco-friendly effect with low environmental impacts that were 24, 14, 4, and 2% lower than those of pressure swing adsorption, high-pressure water scrubbing, membrane permeation, and potassium carbonate scrubbing, respectively, compared to the other technologies. According to the results of the study, high-pressure water scrubbing techniques has been most economical due to its compact parts, whilst potassium carbonate scrubbing has been economical in big sized upgrading techniques. The environment and economic perspective of amine scrubbing, pressure swing scrubbing, membrane separation and water scrubbing for liquifying, distributing and upgrading of biogas. The study’s findings indicated that there are very minimal differences between liquifying and upgrading of biogas, especially when the gas has to move through pipelines before being used in the process. It was discovered that amine scrubbing technology was better than many other techniques owing to its lesser energy consumption, purer methane quality, and reduced consequences on the environment, and for liquifying gas, mixed-refrigerant technology has been found better [143].

6. Industrial Scale of Biogas Upgrading Technologies

Rise in the environmental concerns and government subsidies have increased in biogas plants number has in turn increased the number of biogas upgradation units attached with the plant itself. It is estimated that the worldwide biomethane market has a value of USD 0.62 billion in 2017, and it is increasing at a rate of 25% each year. The biomethane industry is estimated to be worth $4.96 billion in 2026, according to projections. For domestic uses, many nations have set a target of replacing natural gas with biomethane by the year 2026 [144]. France is aiming to create 8 TWh of energy from biomethane by 2023 [145], according to its plans. Study predicts that biomethane will become a significant green and clean renewable energy source in the upcoming years. Sweden has set a target of using biomethane as a transportation fuel at a 100 percent rate by 2028 [65].

Table 4. Estimated operating and capital investment of different upgradation technology.

Cost	Water Scrubbing Technology	Organic Solvent Scrubbing Technology	Amine Scrubbing Technology	PSA Technology	Membrane Technology
Estimated investment cost (Euro/(m3/h) of biomethane)
<100 m3/h	~10,000	9500	9500	10,500	7500
<200 m3/h	~5200	~5100	~5100	~5200	~4600
<300 m3/h	~3200	~3200	~3200	~3600	~3200
Estimated operating cost (Euro/(m3/h) of biomethane)
<100 m3/h	~13.0	~13.6	~14.2	~10.8	~10.5
<200 m3/h	~11.3	~11.2	~10.0	~11.1	~9.6
<300 m3/h	~8.1	~10.0	~10.2	~8.2	~6.5

shows the approximate investment and capital cost of the various biogas upgradation technology [91].

Currently, there are 181 plants using water scrubbing technology, which is widely used technique. Water scrubbing technique is quite straightforward when compared to other methods. But the increased water needs of water scrubbing biogas upgrading technologies pose a significant barrier. Therefore, by recycling secondary and tertiary effluent, it is possible to further lower the cost of the water cleansing procedure. Over the previous five years, there has also been a rise in the employment of membrane separation technology, which has gone from 92 plants in 2015 to 173 plants in 2019. There are several advantages to membrane separation technology such as flexible design, a lesser movable fragment, a sturdy architecture, and a minimum footprint.

In addition, with advancements in material science and engineering, the performance of membranes has been further enhanced, and also it is anticipated that the application of membrane separation technologies will continue to grow in the upcoming years. Additionally, other biogas upgrading methods such as pressure swing adsorption, chemical scrubbing, organic physical cleaning, and cryogenic separation have seen a decrease in commercial use during the previous years. Because of its ability to extract pure CO₂, CS technology has the potential to be a promising approach in the future [82]. It can also be used to produce dry ice.

7. Challenges and Future Scope of Recent Upgrading Techniques

The hybrid technologies (biological-cryogenic) are among the new upgrading technologies that are being considered. Still, greater operational and investment costs related to cryogenic separation are difficulties that are faced in the process. Furthermore, the clogging of pipelines caused by greater concentrations of pollutants, as well as the carbon di-oxide and methane losses, are also the primary obstacles to the widespread use of this developing biogas upgrading technology [4,14,16].

Approx. 84% CH₄ recovery is obtained from in-situ biological biogas upgrading technique done in biogas digester itself [146]. However, the key difficulty in this procedure is the suppression of methane forming bacteria caused by alkaline pH over 8.4. It is believed that this pH increase is related to the removal of bicarbonate during production of biogas. It may be possible to remove this impediment through co-digesting substrate with another feedstock of acidic nature or by maintaining externally through pH sensors during the process [100]. The process of upgradation also depends upon the breaking down of VFA i.e., volatile fatty acids and alcohols. When H₂ is present in small quantities, oxidation will take place. Increased H₂ concentration (>10 Pa) results in the build-up of various VFAs (propionate, lactate, and ethanol) [147] in the solution. This will result in a disbalanced anaerobic digestion system and the accumulation of VFAs, which will eventually cause the AD system to collapse while the concentration of volatile fatty acid increases and the pH decreases. The H₂ gas is partially soluble in the aqueous solution owing to the loss in the liquid-gas mass transfer and, as a result, a reduction in the conversion of CO₂ to CH₄ efficiency. In comparison to the in-situ process, ex-situ biogas upgradation is better adaptable. However, this technique faces a problem, of lower liquid -gas mass transfer [14]. The liquid-gas mass transfer is influenced by a variety of factors, including the reactor’s configuration [10], the recirculation flow of gas [10], the stir force, which influences the size of the bubble [100], and the device for diffusion [148]. According to the photo-autotrophic process, about 98% of the methane is recovered, and the primary problem associated with this system is the larger demand of energy and capital investment [149].

While an open system is less expensive to install than a closed system, it requires lighter and has a lower CO₂ removal effectiveness. Additionally, in a photo- bioreactors with pure microalgae, 1 mol of CO₂ is fixed to produce 1 mol of oxygen, which has an impact on how well the end product performs [14,150]. A greater concentration of CO₂ has been shown to have an effect on the development of microalgae because of the reduction in pH. Aside from that, the available light’s strength and wavelength have an impact on the number of photoautotrophs that may be found. Artificial light sources, such as various LEDs, have proven to be more advantageous than natural sunshine when compared to the latter, owing to the exact wavelengths and intensities that give the best circumstances for the growth of photoautotrophs [151].

All biological upgrading processes, including the gas fermentation system where carbon di-oxide from biogas is to useful compounds, require a low-cost hydrogen source, that has been the utmost pressing issue. Furthermore, the effect of H₂S on the synthesis of compounds is yet unclear [14]. It is a technologically advanced technique that could be used in the “circular economy” and biogas power generation is a good example of this. A rising population are becoming interested in utilising biogas to generate heat and power instead of fossil fuels. In contrast, the utilization of biomethane instead of natural gas has become prominent round the globe and hence discouraging the use of fossil fuels. Biomethane may also be used to replace diesel as a fuel for vehicles, reducing the amount of greenhouse gases emitted by the heavy transportation industry. Numerous commercial ways for biogas upgrading based on physical and chemical processes are already available. Developing novel and low-cost biological methods will allow greater efficiency in biogas upgrading. The biogas upgrading sector relies greatly on capital and operational expenses, and also on government policies and incentives, making it impossible to anticipate that which technologically advanced technique will lead the market in the near future. It is anticipated that the production cost of new and renewable energy, cost of storage of energy and CO₂ capturing cost will continue to decline, resulting in a significant reduction in overall capital and operating costs in the future, as well as the identification of clear winning upgrading technologies. For the individual technologies under consideration, analysis of sustainability and economics will be required to increase their practicability at commercial scale. Advanced sustainability evaluation techniques like “exergy”, “exergo-economics”, and “exergo-environmental” analysis may be used to offer a more comprehensive instance for selecting the most appropriate biogas upgradation technology for a given situation.

8. Conclusions

Membrane separation, water scrubbing, organic solvent scrubbing and amine scrubbing are some of the biogas upgrading technologies which have several limitations that must be overcome in order to use commercially viable technology. Many of these procedures need a large energy input and/or the use of expensive chemicals in order to convert biogas to pure CH₄, raising the expenses of utilising CH₄ produced from purified biogas as a natural CH₄ gas alternative. The use of “biological biogas upgrading” methods can become important in this respect because they use less energy and do not require the use of expensive chemicals.

When it comes to upgrading biogas, biological CO₂ capture and conversions procedures help to increase the amount of methane produced and, as a result, generate money, whereas physical and chemical processes will do the opposite. Currently, these techniques have been evaluated at the laboratory scale, and further research is required to get the techniques to a developed state suitable for commercialization. Hydrogen is required for the operation of biological technology, and it may be created as a sustainable energy source from electrolysis. This (electrolysis) is a process that is now undergoing extensive growth, with the expectation that both capital and operating costs will be lowered by orders of magnitude in the next years. When tiny, standardized electrolysers become accessible, it is envisaged that the field of biological biogas upgrading would make significant advancements.