Keywords

1 Introduction

Fossil fuel exhaustion and increasing concern in environmental pollution at alarming rate have encouraged the researchers to look for environment friendly as well as cost-effective sources of energy [10]. Thus, various forms of renewable energy, for instance wind, tidal wave, solar and biomass, are getting attention globally during the last few decades [22]. Solar photovoltaic systems are limited due to high cost of production while use of wind, tidal and wave energy is site specific. One of the options to convert biomass to energy is the production of biogas through anaerobic digestion. The advantage of biogas technology is ease in production and sustainability. Harnessing energy from biomass is gaining popularity in developing countries due to the high availability of biomass and bio-waste. Landfills and wastewater treatment plants serve as the natural basis for enormous amount of biogas production. Many arable farmlands universally have now implemented in constructing anaerobic digesters to produce small quantities of biogas from organic waste such as kitchen waste, sludge and manure. Widespread availability of the organic materials is required for biogas synthesis, and it is considered as a potentially effective and sustainable energy source [24]. World Bioenergy Association reported that around 18.6% of the total global energy consumption was contributed by renewable energy, in which approximately 14% comprise of bioenergy Fig. 1 [42].

Fig. 1
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Global energy consumption in 2014

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Internal combustion (IC) engines provide outstanding drivability and durability, due to its comparatively compact size, high power-to-weight ratio, high compression ratio and safe to operate with less time required to start the engine. A series of fossil fuels comprising gasoline or diesel and alternative fuels such as natural gas, biomethane gas, biodiesel or ethanol have been studied on IC engines [64]. The engine serves as the major automobile mover in today’s generation and is expected to continue for years to come. Although there are alterations, the basic operational principles of the IC engines have not shifted significantly [60]. Using of biogas in IC engine has certain advantages, being an eco-friendly carbon-neutral fuel, it has the potential to solve problems related to waste management. It has high compression ratio and comparatively lower proportion of sulfur than diesel engine. This has an impact on the cost, performance and environment with high emission, loud noise and low comfortability. The fuel can be used in both light- and heavy-duty vehicles with no or slight modification. However, high self-ignition temperature (650 °C) of biogas prevents its direct usage in CI engine and involves utilizing a dual-fuel engine constituting of biogas and other fuel in combination. Investigation on the combustion characteristic and their effects on exhaust emission with different fuel combination mode has been reported. Dual-fuel approach demonstrated lesser emission of NOx and particulate matter but sharply increased unburnt hydrocarbon emissions in contrast to diesel fueling mode [67]. The presence of CO2 lowers its calorific value, flame and burning speed, traces of H2S in raw biogas may corrode the metal parts, these are identified as additional lacunae associated with biogas utilization and need to be addressed. There are several reports on enrichment of CH4 by removal of CO2 and H2S gas by scrubbing [32].

1.1 Process of Biogas Production: Anaerobic Digestion

Biogas is a mixture of combustible gases obtained from anaerobic digestion (AD) of organic matter by a community of microbial consortia. The digestion process takes place through various reactions and interactions among the methanogens and substrates which are fed into the digester as input [46]. The anaerobic digestion process generally consists of four steps—hydrolysis, acidogenesis, acetogenesis and methanogenesis [26], as shown in Fig. 2.

Fig. 2
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Breakup of anaerobic digestion process

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Biogas typically is a mixture of methane (CH4) and carbon dioxide (CO2) ranging from 45 to 60% each and a small amount of other gases like hydrogen (H2), nitrogen (N2), hydrogen sulfide (H2S) and ammonia (NH3) (ranges in between 100 and 1000 ppm), respectively [47]. The variable composition of biogas is due to the variety of materials that can be used for its production (Fig. 3). Temperature plays a key role in biogas production. Hydraulic retention time (HRT) is normally inversely proportional to the process temperature and varies from place to place. The feed materials are generally animal by-products, which are fed to the digester. In some cases, the feed materials first go through a controlled pre-sanitation process for deactivation of pathogens and for breaking their proliferation cycles. The digested residue after the AD process is transferred automatically to the outlet tanks, which are typically covered with a concrete to prevent methane leakage to the atmosphere. The digested residue has an added advantage of high nutrient value and can be reprocessed directly as fertilizer for the agricultural fields.

1.2 Biogas Production from Co-digestion

The availability of various types of feed materials in the same geographical area facilitates integrated residue management, posing substantial environmental benefits, like recycling of nutrients back to the agricultural land, energy savings, cost benefits and reduction of CO2 emissions [37]. In recent times, co-digestion has gained much attention in many countries since it increases the yield of AD. Co-digestion helps in managing different types of organic waste at one place in production of biogas. Due to a balanced mixture composition of waste materials, digestion of more than one substrate in the same digester can establish a positive synergism in the digestion medium, and the added nutrients can support microbial growth for enhancing biogas production [48, 56]. Co-digestion helps in accomplishing a better NPK ratio attributed to mixing of multiple organic wastes, thereby enhancing the digestate as a fertilizer. Co-digestion of cattle manure and fruit and vegetable wastes (FVW) under mesophilic condition (35 °C) in a continuous stirred tank reactor increased the percentage of FVW from 20 to 50%, and the methane yield enhanced from 230 to 450 L/kg VS added [18]. FVW are rapidly degraded by contaminating microorganisms, and this takes place even faster when they exhibit signs of excessive ripening or are subjected to mechanical damage. An effective co-digestion is not simply a blend of multiple feed materials treated at the equal time. Biogas production and stability of the process are fully dependent on waste composition like C:N ratio (25:1 to 30:1), process conditions like pH (6.8–7.2), operation temperature (35–37 °C) and the population of microbial community in the system. If not mixed in a proper ratio, co-digestion may also cause antagonistic interactions, leading to lower methane yields than expected [55].

Fig. 3
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Schematic representation of anaerobic co-digestion

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1.3 Enhanced Biogas Production Using Pre-treatment

For AD, generally animal dung (cattle, pig, horse, mule and donkey) has been the most preferable feedstock. However, because of limited feedstock availability, it has become essential for researchers to explore new substrate suitable for utilization in AD while simultaneously contributing to the growing energy needs. Lignocellulosic biomasses are abundantly available worldwide as well as their high carbohydrate content make these materials an attractive feedstock for biofuel production [77]. Unfortunately, the challenge associated with utilization of lignocellulosic biomasses as feedstock for biogas production is their recalcitrant structure [31]. The recalcitrant structure of lignocellulosic biomass makes it fairly impervious to bacterial attack. The rigid outer layer of lignin makes it very difficult for microorganisms to access the cellulose and hemicellulose components inside it. Hence, the hydrolysis step is often considered as the rate-limiting step when utilizing these kind of substrates [71]. An initial pre-treatment step prior to AD process is of utmost importance in order to rupture the recalcitrant structure of the lignocellulosic biomass to release the cellulose and hemicellulose to the microbial consortia present in the digester, which in turn increases the rate of biomass degradation along with an increase in the biogas yield [82]. However, in some cases, the chemical agent used for the pre-treatment process can act as a budding inhibitor for the microorganisms involved in the AD. Generally, the pre-treatment should yield a polysaccharidic-rich substrate with minimum amounts of inhibitory by-products and also be cost effective. A number of authors have reported different pre-treatment methods using different types of lignocellulosic biomass for enhancing biogas production [31, 52, 78]. Some of them are mentioned below:

  • Physical (liquid hot water)

  • Physico-chemical

  • Chemical (acidic and alkaline processes)

  • Microbial Processes (fungi, bacterial consortium, etc.)

  • Ionic liquid process

  • Irradiation (Microwave, gamma, X-ray, etc.).

Using alcohols or weak organic acids as a pre-treatment agent for degradation of lignocellulosic biomass seems to be an interesting process. During the process of AD, alcohols or weak organic acids generally form as intermediary products. So the above-mentioned inhibitory problems could be avoided as after the pre-treatment these solvents can be converted into additional methane production. Pre-treatment of forest residues using methanol, ethanol or acetic acid before AD resulted in higher methane yields in a batch experimental process. The techno-economic calculations presented that treatment with methanol was more viable economically due to its lower price and easy recovery after the treatment [36].

1.4 Challenges of the Current Processes and Need for Biogas Upgradation

The methane in biogas is a high-valued energy source, although other constituents are impurities that pose key obstacles to the viable use of biogas [1]. CO2 through combustion has no energy yield and greatly diminish the heating value per volume of biogas due to its high content. Calorific value of biogas with CO2 varies from 18.7 to 26 MJ/m3 and that of without CO2 varies from 33.5 to 35.3 MJ/m3 [51]. Apart from CO2, another major impurity is H2S. It is always present in biogas, while concentrations differ with the feedstock [76]. It is toxic and highly corrosive in nature, often damaging gas burners, gas storage tanks, compressors, engines and pipelines to transport. Upon combustion, it also forms a risky pollutant sulfur dioxide [39]. That is why to use the biogas effectively, it is very important to remove the CO2 and H2S from it. Thus, removal of CO2 and H2S from biogas will enhance the fuel efficiency which could serve as a source of immense energy that can be used effectively for different purposes like main power source for transport vehicles and also used for powering of generators for electrical energy [3]. This possibility of use is justified by biogas properties, which makes it convenient for IC engines. The actual calorific value of biogas mainly depends on CH4 percentage, temperature and absolute pressure and is a vital parameter for the performance of an engine. The fuel consumption of IC engine using biogas is often specified in m3n/h or m3n/kWh. After scrubbing, biogas can be compressed and stored in gas cylinder and transported wherever it is required. Also, this scrubbed biogas decreases greenhouse gas emissions [63]. The properties of biogas are given in Table 1.

Table 1 Properties of biogas [33, 68]
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2 Biogas Upgradation Technologies

Biogas upgrading to biomethane is a novel degree of gas purification. The splitting of minor impurities (moisture, H2S, etc.) and especially CO2 is essential and critical operation. The removal of these impurities is necessary for all generally used gas applications like CHP engines, boilers, vehicles or injection in the natural gas grid. The qualitative requirements for removal of key constituents from biogas fitting to its uses are given in Table 2 [65]. Currently, an amount of altered biogas upgradation technologies to fulfill the job of producing biomethane of sufficient quality are commercially available and have proven to be technically and economically feasible. The major step comprises drying of the raw biogas and the removal of CO2 and thus enhancement of the heating value of the produced gas. Several essential mechanisms are involved to achieve selective separation of gas components. These may include physical or chemical absorption, adsorption on a solid surface, membrane separation, cryogenic separation and chemical hydrogenation [54]. All biogas upgradation technologies have their own specific advantages and disadvantages and are different for different biogas upgrading sites. The accurate choice of the economically feasible technology is strongly rely on the quality and quantity of the raw biogas to be raised the biomethane quality to be attained and its utilization.

Table 2 Requirement of purification
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2.1 Physical Absorption Method Using Water and Organic Solvents

Physical absorption employing water as a solvent for dissolving CO2 and H2S is considered as the simplest, requiring fewer infrastructure, eco-friendly, cost-effective and widely accepted method for biogas upgradation [69]. For biogas scrubbing, physical absorption method is generally applied as they are effective even at low flow rates that the biogas plants are normally operated at. In this case, the absorbed gas components are physically bound to the scrubbing agent. The absorption in water is employed because CO2 and H2S have higher solubility in water than CH4 and will therefore be dissolved to a higher extend, particularly at lower temperatures and higher pressures. This method can tolerate H2S concentrations of around 300–2500 ppm, but higher H2S concentrations are detrimental to the scrubbing system as it lowers the pH of the scrubbing liquid [53]. In this process, the raw biogas is compressed and fed into a packed-bed column from bottom, and pressurized water is sprayed from the top which flows downward. The process is thus a counter-current one. Purified biogas (biomethane) leaves column at the top and dissolves CO2 as well as H2S in water, which are collected at the bottom of the tower. In order to maintain the absorption performance, the scrubbing liquid has to be replaced by fresh liquid or regenerated in a separated step (desorption or regeneration step). The regenerated water is then pumped back to the absorber as fresh scrubbing liquid. CO2 is released into atmosphere as an off-gas in case of water recirculation system or stays in water in case of a single-pass system [58]. Any CH4 dissolved in water is captured and recycled in the absorption column in order to alleviate methane losses. Viyaj et al. [73] designed a water scrubbing system in which the CO2 content was reduced by 99%. The setup used a gas inlet pressure and flow rate of 1.0 MPa and 1.5 m3/h, respectively, while the corresponding water flow rate was 1.8 m3/h. The limitation of this technique is that the air constituents, oxygen and nitrogen, are dissolved in the water stream during regeneration and thus elated to the upgraded biomethane. Therefore, biomethane produced with this technology always contains small amount of oxygen and nitrogen. This technology also uses an organic solvent solution (e.g., polyethylene glycol) instead of water as a scrubbing agent. Solubility of CO2 in these solvents is found higher than in water. As a result, less scrubbing liquid circulation is required for higher CO2 absorption, and thereby, smaller apparatus is needed for the same raw biogas upgradation. Some of the commercially available organic physical scrubbing agents used for biogas upgrading technologies are Genosorb®, Selexol® and Rektisol® [6].

2.2 Chemical Absorption Method

The mechanism of chemical absorption is quite similar as water scrubbing process where a chemical reaction takes place connecting scrubbing agent components and absorbed gas components within the liquid phase. But the conformation of chemical absorption is much simpler with improved performance because of higher CO2 solubility at low pressure in highly reactive chemical absorbents [7]. The separation principle of absorption is established on different solubility of various gas constituents in a liquid-scrubbing solution. As the process includes development of reversible chemical bonds among the solute and the solvent, regeneration of the solvent, thus, involves breaking of these bonds and requires significantly a very great energy input [59]. The most employed chemical solvents are generally diglycolamine (DGA), monoethanolamine (MEA), triethanolamine (TEA), diethanolamine (DEA), methyldiethanolamine (MDEA) and sterically hindered amines, such as 2-amino-2-methyl-1-propanol (AMP) and piperazine (PZ) which in comparison with water can dissolve significantly much more CO2 per unit volume of biogas [38]. Usually, amine scrubbing plants are operated at slightly high pressure which is already available in the raw biogas, and no extra compression of the gas is required. The high selectivity and absorptivity of the amine solution has an advantage during absorption but turns out to be a hindrance during regeneration of the scrubbing solution. MEA is the cheapest amine, and it is the most commonly used as a scrubbing agent due to its high absorption capacities for CO2. The amine scrubbing agent can also absorb H2S from the raw biogas, but higher temperature is needed during regeneration of the solution. Hence, it is advisable to remove this component prior to the amine scrubber. Inorganic solvents generally employed for this process are sodium hydroxide (NaOH), potassium hydroxide (KOH) and calcium hydroxide Ca(OH)2. The solubility of CO2 in NaOH is higher in comparison with amines [2]. Theoretically, to absorb 1 ton of CO2, 1.39 tons of MEA will be required, and when compared with NaOH, only 0.9 tons is required [8]. Alkali hydroxides are more efficient, cost effective and easily available in the market as compared to amines. But regeneration of these hydroxides is complex and challenging because of the formation of thermally stable products such as Na2CO3, K2CO3 and CaCO3 salts. Yeh et al. [79] studied a chemical absorption and desorption process for the removal of CO2. They used MEA as scrubbing agent in a conventional and structured packed column. Significant improvements in CO2 removal above 90% were obtained with the structured packing. Tippayawong and Thanompongchart [72] investigated a method for biogas scrubbing and CH4 enrichment where they used aqueous solutions of NaOH, Ca(OH)2 and MEA for the chemical absorption of CO2 and H2S in a packed column. Test results revealed that the aqueous solutions used were effective in reacting with CO2 in biogas (over 90% removal efficiency), making CH4 an enriched fuel.

2.3 Pressure Swing Adsorption (PSA)

Pressure swing adsorption (PSA) is an established and developed technology for biogas upgradation. The method is based on specific adsorption behavior of at least one gaseous component (adsorbate) on a solid surface (adsorbent) under elevated pressure mainly as a result of physical or Van der Waals or electrostatic forces [28]. Adsorbent materials are able to capture some gaseous component of a mixture by adsorption affinity and molecular size of the adsorbent. The physico-chemical properties of gaseous components present in biogas are given in Table 3. The size of CO2 molecules is smaller than the size of CH4 molecules. Consequently, in the instance of biogas, the molecules of CO2 easily adsorbed on a selective adsorbent material and later enriching CH4 content of the gas. The efficiency of adsorption process depends mainly on temperature, pore size of adsorbent and partial pressure of adsorbate. Normally, solids with a large surface area per unit volume adsorbents are commercially available. Gas purification can also be performed using some form of activated carbon, alumina, silica or silicates, which are also known as molecular sieves. Usually adsorbents with molecular sieve, a regular pore size of 3.7 Å is used to capture CO2 (molecular size of 3.4 Å) into the pores, while rejecting CH4 molecules (molecular size of 3.8 Å). By an appropriate choice of adsorbent, the process is able to eliminate CO2, H2S, moisture and other impurities either simultaneously or selectively from biogas [70]. Conventional PSA usually consist of four adsorption columns packed with adsorbents. One cycle has typically four basic steps: pressure build-up, adsorption, depressurization and regeneration. After building pressure, CO2 is captured from raw biogas in adsorbent and consequently leaves the adsorber as off-gas by a stepwise decrease in pressure. Enriched CH4 leaves the column as a biomethane stream. Afterward, the pressure is increased again with raw biogas or biomethane, and the adsorber is ready for the next sequence of loading. Biogas feeding and column pressurization are usually carried out at 4–10 bars to increase CO2 adsorption inside the pores [14]. The CH4 concentration in the raw biogas can be upgraded up to 96–98%; however, up to 4% CH4 can be lost in the off-gas stream [65]. Since the adsorption of H2S is irreversible and harm the adsorbent material, these components have to be removed before reusing the adsorption column.

Table 3 Physico-chemical properties of biogas components [40]
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2.4 Membrane Separation

Separation by membrane is a mature commercialized technology with a market stake of 10%. It has emerged as an attractive process for biogas upgradation [57]. The process established on the principle of selective permeation of different components through a semi-permeable membrane. The carriage of each component is driven by the change in partial pressure over the membrane and highly relies on the permeability of the membrane material in the component. It is additionally resolute by other factors such as changes in temperature, concentration and electric charges of different gases. Membranes for biogas upgradation are made of materials that are mainly permeable for CO2, H2S and water while CH4 passes only to a very low extent [14]. For high methane purity, the difference in permeability of CH4 and CO2 must be high. Basically, for gas separation, there are three types of membranes use, inorganic, polymeric and mixed matrix membranes (MMMs). Some of the polymeric and non-polymeric membrane materials for biogas upgrading are given in Table 4. Polymeric membranes are the most widely used membranes in biogas sector. The polymeric membranes are preferred for the separation of biogas because of their low cost, high selective permeability, easy production and stability at high pressures. To offer adequate membrane surface area in compact biogas plants, these membranes are used in the form of hollow fibers combined to a number of parallel membrane modules [12]. Solid membrane fabricated from acetate–cellulose polymer has permeability for CO2 and H2S up to 20 and 60 times, respectively, higher than CH4. However, a pressure of 25–40 bar is required for the process [30]. Using high-pressure gas separation, the raw biogas can be filtered up to 94% CH4 in one-stage performance, and it can be improved up to 96% CH4 by using two- or three-stage performance [75].

Table 4 Membrane constituents for biogas upgrading [7, 12]
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2.5 Cryogenic Separation

The cryogenic method of purification is based on the principle of separation of different gases by fractional condensations and distillations at low temperatures. As different gases liquefy at different temperature and pressure domains, it is possible to separate gas components by cooling and compressing the biogas. The process has an advantage that it allows recovery of pure component in the form of a liquid, which can be transported conveniently [27]. The boiling point of CH4 at 1 atm pressure is −161.5 °C which is quite lower than the boiling point of CO2 (−78.2 °C), thus facilitating the separation of CO2 from biogas by liquefying CO2 at very low temperatures [29].

Removal of water and H2S from biogas is necessary in order to avoid freezing and other problems; however, the gases like N2 and O2 can be condensed during CH4 separation [20]. The cryogenic separation is carried out by initially drying followed by multistage compression of the raw biogas with intermediate cooling up to 80 bar. The pressurized biogas is stepwise cooled to −55 °C to achieve the liquefaction of CO2 and finally expanded to 8–10 bar in a flash tank at −110 °C to facilitate biomethane purification via CO2 solidification [65]. This method can upgrade the raw biogas up to 97% and CH4 loss lower than 2%. The process requires large number of equipment facilities and thus consumes high investment and a large amount of energy which increases the final cost of biomethane production [23]. The operating cost and practical problems of clogging and freezing due to high concentration of solid CO2 or presence of rest impurities limit the wider implementation of this technique. Table 5 summarizes the advantages and disadvantages of biogas upgradation techniques for CO2 removal.

Table 5 Summary of advantages and disadvantages of biogas upgradation processes [65, 81]
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3 Emerging Technologies for Biogas Upgradation

Though conventional technologies are promising in upgradation of biogas, the market for biogas upgrading is often characterized by harder competition with the establishment of new upgrading technologies. Biological technologies are one of the recent developments in biogas upgrading technology. Both conventional and emerging biogas upgradation technologies are currently being developed for enhancing performance, for increasing CO2 reduction efficiency and for rendering them cost effective to get a wider implementation network.

3.1 Biological CO2 Removal Technologies

Biological biogas upgradation involves the employment of microbes for conversion of CO2 and H2 into CH4. On comparison with the conventional technologies, the major advantage of biological technology is that the CO2 is converted into other value-added products at atmospheric pressure and moderate temperature contributing significantly to a sustainable bio-based and circular economy [62]. Biological biogas upgrading consists of two types of metabolic pathways. The first metabolic pathway involves the role of hydrogenotrophic methanogens which converts CO2 directly to CH4. The other metabolic pathway chooses an indirect route where homoacetogenic bacteria first convert CO2 to acetate and then converted to CH4 by the acetoclastic methanogens [8]. The former pathway is more preferable as hydrogenotrophic methanogens (Methanobacterium, Methanoculleus and Methanomicrobium) are more abundant than acetoclastic methanogens such as Methanosarcina [4]. The biological biogas upgrading process is classified into two types:

  1. (a)

    In situ biological methane enrichment and

  2. (b)

    Ex situ biological methane enrichment

3.1.1 In Situ Biological Methane Enrichment

Biological in situ biogas upgradation implicates the injection of H2 inside the biogas digester in order to couple with the endogenous CO2 produced in the anaerobic digester to convert into CH4 by the action of autochthonous hydrogenotrophic methanogens [41]. The process can enrich CH4 up to nearly 99% if the operational parameters (e.g., pH, temperature, H2 flow rate, etc.) are fully monitored and controlled [74]. Though in situ biogas upgrading is a cost-effective method, it has also some technical challenge with the increase of pH values above 8.5 due to the elimination of the key buffering agent, i.e., bicarbonate, thus leading to inhibition of methanogenesis [44]. Therefore, 8.5 is measured as the threshold pH for ideal biomethanation process in both conventional and emerging biogas production systems. In order to overcome this technical challenge, co-digestion of manure with acidic waste was anticipated to arrest the pH in an optimal range during the biogas upgrading process [45]. Another challenge is that the oxidation of volatile fatty acids (VFA) and alcohols are thermodynamically viable only if the partial pressure of H2 is very low [25]. On the other hand, addition of H2 in the biogas digester increases the concentration of H2 inside the digester which inhibits the process of VFA oxidation [13]. Due to this concern, the in situ methane enrichment process is limited to laboratory-scale studies only and more process optimization is required to get a wider implementation worldwide.

3.1.2 Ex Situ Biological Methane Enrichment

The concept of ex situ biogas upgradation came into existence to overcome the challenges incurred in in situ biogas upgradation. The ex situ biogas upgradation implicates the accumulation of CO2 from biogas and H2 from external sources in a separate anaerobic reactor having the hydrogenotrophic methanogenic archaea, resulting in their subsequent conversion to CH4. In this process, CO2 is utilized as carbon source and H2 as reducing agent for the production of CH4 [41].

Some advantages of this method compared to the in situ process [9] are:

  • Simple biochemical process since no biomass is required for degradation.

  • The conversion occurs in a separate unit which ensures the stability of existing biogas plant.

  • External source of waste makes the process more flexible (e.g., CO2 from syngas and H2 from hydrolysis of water).

  • The process takes less retention time as it can handle high volumes of influent gases.

The efficiency of biogas upgradation strongly depends on the reactor type, partial pressure of H2 and operating temperature inside the bioreactor, which can result in methane conversion from 79 to 98%. On comparison with mesophilic culture, the enriched thermophilic culture resulted in >60% higher H2 and CO2 bioconversion in batch assay [43]. The types of reactors that can address the challenges of gas–liquid mass transfer of H2 are fixed-bed reactors, anaerobic trickle-bed reactors (ATBR), continuous stirred tank reactor (CSTR) and series up-flow reactors. A methane upgradation of 95.4% was achieved in a thermophilic CSTR functioned with mixed methanogenic culture [45]. In a recent study, it was shown that 96% CH4 production was achieved by operating the process in a mesophilic ATBR using immobilized hydrogenotrophic culture [62]. An assessment of different in situ and ex situ methane enrichment processes is given in Table 6.

Table 6 Assessment of in situ and ex situ biogas advancement processes
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4 Application of Biogas in IC Engines

With increasing pollutant exhaust emissions, regulations and policies are being tightened worldwide to combat the problem of poor ambient air quality. Therefore, fostering use of clean alternative fuels as substitutes for IC engines is the need of the hour [34, 66]. There are various fuels that can be used in IC engines, but they should meet certain physical and chemical properties. Usually fuels used in IC engines are re-designed to satisfy requirements of engine performance effectively. The physico-chemical properties of biogas justify the applicability of biogas as an alternate fuel for IC engines. The actual calorific value of biogas is the key factor for the performance of an engine. For both spark ignition (SI) and compression ignition (CI) engines, biogas provides a clean fuel. To use biogas effectively in SI engines, a higher compression ratio (CR) engine with magneto ignition and modification of piston and carburetor is required, while in CI engine it can be employed efficiently in dual-fuel mode with diesel as a pilot fuel. For use of biogas as a vehicle fuel, upgradation of biogas is necessary which has been discussed in Sects. 2 and 3. After removal of impurities like CO2, H2S and water, it can be compressed in a three- or four-stage compressor up to a pressure of 20 MPa and stored in a gas cascade for enabling quick refueling of cylinders. Compression of biogas is required to put more volume of gas in a small cylinder, so that the engine will run for a longer time [19, 21].

4.1 Biogas in SI Engines

The use of biogas in SI engines is gaining popularity because of higher auto ignition temperature, ability to form homogeneous mixture and higher hydrocarbon ratio. Being a clean fuel with a high auto-ignition temperature and high octane number, biogas can resist knocking in SI engines. Due to higher anti-knock index, SI engines fueled by biogas have relatively higher CR than that of petrol which enhances the thermal efficiency of the engine. In the case of operation using simulated biogas, SI engine CR can even be raised in a range from 11:1 to 13:1 because of the high self-ignition temperature and has been found to be appropriate for operation without knock [21]. The presence of impurities such as CO2 and N2 affects in characteristics of combustion, calorific value, flammability limit and flame speed. Also, increase in impurities percentage aggravates cyclic variation and also lowers power output. In order to achieve low emission and the best fuel conversion efficiency for a biogas-fueled SI engine, it is highly recommended to maintain the air–fuel ratio, accurate CR and ignition timing [35, 61]. The modification of an SI engine to operate with biogas is simple and quite easier than CI engine because the engine is designed to operate on an air–fuel mixture with spark ignition. The basic alteration is to provide a biogas–air mixture instead of a carburetor. The engine is controlled by variation of the mixture supplied to the engine through operating the butterfly valve placed between the biogas mixer and engine intake system. Adequate design of the mixing device along with the precise control of butterfly valve can ensure the provision of constant air/fuel ratio irrespective of the actual amount sucked into the engine [49].

4.2 Biogas in CI Engines

Due to high self-ignition temperature, biogas cannot be utilized as a stand-alone fuel in CI engine; however, it could be applied in CI engine in dual-fuel mode. The dual-fuel engine is a modified diesel engine includes an initial supply of a mixture of fresh air and gaseous fuels to a cylinder and injection of a small amount of diesel, usually termed pilot fuel for ignition of the combustible mixture. The pilot fuels are generally of high cetane value. The amount of diesel required for sufficient ignition is between 10 to 20% of the amount required for operation on diesel fuel alone. Gaseous fuel quickly mixes with air to form a standardized air–fuel mixture that improves the emission characteristics and retain high efficiency, leading to the widespread utilization of biomass energy [15, 16]. The use of biogas fuel in dual CI engine shows less emission of NOx and particulate matter when compared with diesel mode engine. A dual-fuel CI engine can attain higher efficiency owing to its high CR as compared to a sole biogas-fueled SI engine. Faster and complete combustion takes place in the combustion chamber as the diesel fuel injected provides multi-point ignition sources, which increases the stability of the engine performance [19, 50]. Additionally, recent inclusions of hydrogen and LPG with simultaneous lowering of CO2 emissions are considered as major developments in the performance of biogas dual-fuel engines [21]. However, the process parameters such as fuel composition, air–fuel ratio, inlet temperature as well as emission characteristics have to be monitored considerably for effective application of biogas in IC engines.

5 Conclusions

The biogas production from different organic wastes through anaerobic digestion and physico-chemical properties is discussed for use as an alternative fuel in IC engines. The changes that have to be made in the existing engines to use biogas as a fuel are being discussed. The biogas purification is necessary as it contains not only CH4 but also other gases, considered as impurities, such as CO2 and H2S. Different existing and emerging biogas upgradation technologies for CO2 and H2S removal are discussed, among which water scrubbing is found to be a simple and cost-effective method for purification. Reduction of CO2 in biogas for both SI and CI engines increases the calorific value of the fuel, thermal efficiency and power output. Use of biogas as a fuel could decrease carbon emissions compared to other fossil fuels. Another advantage of using upgraded biogas in vehicles is that it leads to lower emissions of NOx and particulate matter. Enriched methane from biogas is a potential source of energy and can be used as an alternative fuel for IC engines. However, scale-up of this highly potential technology and appropriate R&D measures is required to fulfill the demand of alternate energy for IC engines worldwide.