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Chapter

Biomethane Production and

Applications
Moses Jeremiah Barasa Kabeyi
and Oludolapo Akanni Olanrewaju

Abstract

Biomethane production generally involves the cleaning to remove minor unwanted

components of biogases such as hydrogen sulfide (H2S) and moisture (H2O) and
upgrading in a process that involves the removal of carbon dioxide (CO2) to increase
the concentration of CH4 to 95–99% and reduce CO2 concentration to 1–5%, with little
or no hydrogen sulfide (H2S). Biomethane gas is a flexible and easy to store fuel
having similar properties and applications as natural gas with no need to modify the
settings for natural gas devices and equipment. Biomethane can be used for industrial
and domestic applications ranging from thermal and power generation and feedstock
for processes like the Fischer-Tropsch (FT) for fuel manufacturer and direct power
generation in hydrogen or biogas fuel cells like production of green hydrogen. There-
fore, biomethane promises to play a leading role in the energy transition through
hydrogen, electricity, and other renewable fuels production. Biomethane production
by biogas upgrading methods include the pressure swing adsorption, which has an
option of temperature swing adsorption, absorption technics based on amine, mem-
brane separation, cryogenic separation, and biological separation. The technology
adopted may depend on factors such as costs, quality of products, location, and
technology maturity and requirements.

Keywords: biogas, biogas cleaning, biogas purification, biomethane, anaerobic

digestion, biogas enrichment

1. Introduction

The production of biogas has been growing and so is the demand for upgraded
biogas for applications like vehicle fuel or injection to the natural gas grid. Biogas has
to be upgraded to facilitate efficient use in these applications by removal of carbon
dioxide which is inert yet it constitutes a significant portion of raw biogas at the
expense of methane [1, 2]. Biogas is a mixture of gases produced by action of micro-
organisms through anaerobic digestion which is a complex process made up of four
stages i.e.: hydrolysis, acidogenesis, acetogenesis and methanogenesis leading to bio-
gas. The composition of biogas is influenced by the type of feedstock used and
anaerobic digestion process control [3]. Other than production in anaerobic digesters,
biogas can also be produced from landfills and through biomass thermal pyrolysis and
1
Anaerobic Digestion – Biotechnology for Reactor Performance and Environmental Sustainability

gasification processes. The mixture produced generally consists of 30–75% methane

(CH4), 25–55% carbon dioxide (CO2) and other constituents or impurities like hydro-
gen (H2), oxygen (O2), nitrogen (N2), hydrogen sulfide (H2S), water (H2O) and
ammonia (NH3), dust particles, siloxanes, aromatic and halogenated compounds,
which are often in tiny quantities [4–6].
Anaerobic digestion is a sustainable process used for simultaneous treatment and
production of biogas energy resource. Biogas is a renewable energy resource produced
by anaerobic digestion and has methane as the main component with impurities like
hydrogen sulfide, carbon dioxide water vapor, siloxanes, hydrocarbons, oxygen,
ammonia, carbon monoxide and nitrogen. The energy content of biogas is reduced by
the impurities while others cause operational challenges to combustion systems like
corrosion [7, 8]. This makes it important to apply different technologies to remove
these harmful and undesirable impurities [9]. There are many established and devel-
oping physicochemical technologies for biogas operation and maintenance costs,
energy requirements, efficiency of removal and other parameters [9, 10].
Biomethane or upgraded biogas is made by qualitative processing of raw biogas
through several steps to remove impurities mainly carbon dioxide. Biomethane as a
fuel provides new opportunities at different levels for the society. In this chapter,
various methods of biomethane production are described and compared [11, 12].
There is growing interest in the use of biomethane as a renewable substitute of
natural gas in applications like transport fuel which has created demand for biogas
upgrading. This chapter is a critical review that summarizes the state-of-the-art tech-
nologies used in cleaning and upgrading. Covered in the review are the description of
biomethane production methodologies, scientific and technical outcomes related to
them, bio-methanation efficiency, challenges and feasibility of the technologies
[8, 13].
Biogas is a renewable energy carrier that can be exploited directly as a fuel or as a
feedstock for production of hydrogen or synthesis gas. The main constituents of
biogas are carbon dioxide (CO2) and Methane (CH4), but there are quantities con-
taminants like such as hydrogen sulfide (H2S), ammonia (NH3), moisture and silox-
anes whose existence and composition is a function of the source of biogas like
landfills, anaerobic fermentation of manure [14, 15]. The contaminants have the
following undesirable effects to biogas applications a fuel;

i. They can be detrimental to any biogas thermal or thermo-catalytic conversion

device (e.g., corrosion, erosion, fouling); and

ii. Generation harmful environmental emissions. It is therefore important to

include biogas purification steps upstream of its final use processes [9].

Other than methane which is the main energy source in biogas, raw biogas has
impurities that are noncombustible while others are harmful to the equipment and
environment and should therefore be removed to make it suitable for wide range of
applications in heat and power generation [8]. The treatment and purification/
upgrading pathways and applications are summarized in Figure 1 below.
From Figure 1, it is shown that biogas treatment mainly involves desulphurization
and drying of raw biogas. Making it an ideal feedstock for applications like boiler
fuel, cogeneration (CHP) and biogas reforming for production of hydrogen and other
fuels and as s fuel for direct combustion processes like boilers for heat and power
production.
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Figure 1.
Main biogas potential applications.

Purification on the other hand leads to the production of biomethane which is an

ideal feedstock for biomethane reforming for hydrogen, production of compressed
liquid bio-natural gas and liquefied bio natural gas as well a direct substitute for
natural gas fuel applications including injection to natural gas pipelines [2, 6, 16].

2. Technologies for biogas cleaning, drying and purification

Controlling the level of impurities in biogas is essential for success of its recovery.
The implementation of treatment and purification technologies must consider the
requirements of each specific application of biogas. These technologies aim to adjust
the calorific value and remove contaminants that affect the quality of biogas and the
useful life of the equipment. The most demanding techniques aim to purify biogas to
obtain biomethane. Currently, different techniques that allow the treatment and
purification of biogas are commercially available [12, 17, 18].
There are many technologies available at commercial and laboratory scale for the
treatment and purification of biogas. These methods include condensation, absorption
and adsorption processes for raw biogas. Hydrogen sulfide (H2S) removal can be done
by biological in situ, ex situ, biofilter and biological gas scrubbing techniques. Purifi-
cation which involves mainly removal of CO2 can be done by absorption i.e. amine
and pressurized water scrubbing and amine scrubbing, membrane permeation and
cryogenic method [2, 19–21]. Figure 2 summarizes the various methods for treat-
ment/cleaning and purification of raw biogas.
From Figure 2, it is noted that treatment or cleaning mainly involves removal of
moisture and H2S while upgrading or purifications mainly targets the CO2 for
removal. The choice of biogas treatment and purification technology is a function of
factors like the amount of biogas produced, its composition, the level of purification
required, and process costs in terms of capital, energy consumption and operational
expenditure (CAPEX and OPEX). In biomethane requirements, a combination of
processes is used, as no technology can remove all contaminants from biogas [1, 2].
There are simpler and cheaper technologies available for treatment of biogas with
the objective of cleaning biogas for sensitive applications [2]. The degree of biogas
3
Anaerobic Digestion – Biotechnology for Reactor Performance and Environmental Sustainability

Figure 2.
Main technologies to biogas treatment and purification.

treatment depends on the intended application and the initial composition of raw
biogas. Most common treatment involves the removal of H2O and H2S [4, 12].

2.1 Moisture (H2O) removal

Biogas may contain moisture concentration of between 3 to 10%. Water removal is

usually carried out at an early stage of the treatment, to protect the downstream
equipment against corrosion and allow the biogas feedstock to fulfill the requirements
of subsequent purification steps. Table 1 is a summary of advantages and disadvan-
tages of processes for H2O removal from biogas.
From Table 1, the strengths and weaknesses of moisture removal techniques from
biogas are presented. Moisture can be removed from raw biogas by condensation,
adsorption and absorption. Condensation has high energy consumption and is expen-
sive in terms of investment and maintenance, but the process is simple and effectively
removes hydrocarbons and oil particles. The adsorption process has low operating
costs, has high removal rate and the adsorbent materials are regenerated as main
advantages, although it has high investment costs and requires the prior removal of
oil. The Absorption process of moisture removal from raw biogas effectively elimi-
nates hydrocarbons, and like the adsorption method has high removal rates and
absorption materials can be regenerated. However, the method has high investment
costs, making it only viable at high biogas flow rates. The regeneration of adsorption
materials is done at high pressure and temperature making operations and mainte-
nance expensive [7, 22].

i. Condensation
In condensation, separation of steam and water from biogas is affected
through by use of cyclone separators. Condensation of water can be improved
further by cooling biogas below the dew point of the gas. For this purpose,
cooling pipes are installed with a slope and a purging system to collect the
condensate collected [1, 6].
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Process Advantages Disadvantages

Condensation • Simple process, suitable for any biogas • High energy consumption;
drying flow; • Requires installation of long tubes with
• Elimination of hydrocarbons and oils slope and freeze-resistant;
particles; • High investment and maintenance
• Application as pre-treatment in all costs.
systems.

Adsorption • Adsorbent materials can be regenerated; • Requires prior removal of particles and
drying • High removal rate, which allows the oil;
process to be applied to any type of biogas • High investment cost;
use; • Suitable for small or medium biogas
• Low operating cost. flows.

Absorption • Materials can be regenerated; • High investment cost;

drying • High removal rate, which allows the • Economic viability only for high biogas
process to be applied to any type of biogas flow rates;
use; • Absorbent material regeneration
• Elimination of hydrocarbons particles. carried out at high pressure and
temperature.

Table 1.
Advantages and disadvantages of the main H2O removal processes from biogas.

ii. Adsorption
Cylindrical reactors containing adsorbent materials are used in adsorption
drying process. Commonly used adsorption materials are silica gel, activated
carbon, aluminum oxides, magnesium oxides and zeolites. The adsorption
materials are installed in a fixed bed, that can be exchanged and regenerated
when it gets saturated. The system can also operate alternately with two
columns, where one has the adsorption material at room temperature and
pressure between 6 bars and 10 bars, and the other column is a standby unit
where regeneration is done [1, 23].

iii. Absorption
In absorption drying biogas flows through an absorption tower, in
countercurrent with a solution of glycol or other hygroscopic materials. In the
process moisture or steam and hydrocarbons are chemically absorbed. This
method was originally used to dry natural gas. Absorption operations take
place at high pressure of between 20 and 40 bars, while regeneration occurs at
around 200°C [1, 2].

2.2 H2S removal process

Hydrogen sulfide is (H2S) is a gaseous chemical found in many fuel gases, biogas,
natural gas, syngas, coke oven gas, landfill gas, refinery gas, and wastewater steams
among others etc. [24]. Hydrogen sulfide is flammable, toxic, and extremely hazard-
ous and should therefore be captured and removed from biogas. The challenge and
need to H2S has led to the development of different materials and methods over the
years for its removal. Some alkanolamines are used as absorbents and while metal
oxides are used as adsorbents [17, 18, 25]. The removal of H2S from fuels is imperative
in terms of both safety and economics. The main challenge of H2S in application of

5
Anaerobic Digestion – Biotechnology for Reactor Performance and Environmental Sustainability

biogas is its inherent tendency to form an acidic solution in the presence of water
which leads to pipelines and equipment corrosion [8, 25].
H2S removing process is divided into two broad levels or categories on the basis of
intended application for produced biogas. Hydrogen sulfide (H2S) removal is nor-
mally done by means of biological or physical- chemical processes, and can be classi-
fied as external or internal depending on whether it is done outside or inside the
anaerobic bio-digester. The first level involves production of biogas with, H2S con-
centration of below 500 ppm, and can reach as low as 100 ppm. The second level
involves reduction in H2S concentrations less than 0.005 ppm, which are typical
specifications and requirements for biomethane gas [4, 24].

2.2.1 Biological removal of H2S

There are various established methods for biological removal of H2S from raw
biogas. They include in situ, biofilter, biological scrubber, and ex-situ techniques.

i. In Situ methane enrichment

Situ is a biotechnology based on the direct injection of pure air or oxygen and
in the process, the bacteria that oxidize H2S develop with the presence
oxygen, leading to the biological removal process of H2S, to produce sulfur
(S) which leaves the digester via the digested. The microorganisms are widely
found in the anaerobic environment present in bio-digesters [4]. In situ
desorption technology is yet to be fully developed even though it has been
around for over 20 years. In-situ is based on the greater solubility of CO2 over
CH4 in water. The process set up includes an anaerobic digester linked or
connected to an external desorption unit. Sludge transported to an aerated
desorption column from the digester. Nitrogen or air flowing in counter-
current mode and dissolves the CO2 from the sludge in the desorption unit.
The sludge desorbed sludge is pumped back into the digester to reabsorb
amore CO2, and the sludge as the sludge is continuously recycled in the
desorption column. It is possible to strip out H2S with dissolved CH4 and CO2
from the recirculating sludge by applying large quantities of air or N2, causing
reduction in the H2S and CO2 concentration [26, 27].
Therefore, in Situ is a biotechnology which works by direct injection of pure
air or oxygen and causing bacteria to oxidize H2S leading to biological
removal H2S, to produce sulfur (S) which exits the digester via the digested.
These microorganisms are widely present in anaerobic environment in bio-
digesters [4].

ii. Biofilter
In the bio filter technology biogas is passed through a column having a
synthetic material, in the form of a biofilm. The parallel or
countercurrent flow system is used to maintain the humidity and nutrients,
that are essential for the microorganisms that degrade of H2S [4]. The
purification system consists of a bioreactor where sulfur-oxidizing bacteria
like the Thiobacillus, Pseudomonas and Acidothiobacillus are immobilized
on a carrier. In the process, moisturized biogas is injected from the bottom
of the bio filter and forced through a moist, packed bed with microbial
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biofilm which purifies biogas. The bed material is used to supply nutrients or
nutrient solution added from the top occasionally. Oxygen whose
concentration is 5–10% of volume is supplied by injecting air directly into the
gas stream [3].

iii. Biological gas scrubber

In biological gas scrubber, a two-stage system is used to remove H2S. In the
first stage H2S scrubbing column, applies sodium hydroxide solution while
activated sludge is used in the second stage which is injected with injected
with air, because the microorganisms used are aerobic, leading to the solution
regeneration [4]. Bio scrubber system is applied in the removal of compounds
like ammonia, amines, hydrocarbons, hydrogen sulfide and odorous
contaminants. Bio scrubbing system consists of two reactor units with the
first reactor as absorption tower where the pollutants are absorbed in a liquid
phase before it goes to the second reactor which is activated sludge.
Degradation occurs in the activated sludge reactor where microorganisms like
Thiobacillus and Thioalkalivibrio) grow in suspended flocks. The effluent
generated is recirculated back to the absorption tower. In the removal of H2S,
a sedimentation tank is installed after the second reactor for collection of
elemental Sulfur with O2 being used as the oxidant. Optimal microbial growth
and activity are maintained addition of oxygen, nutrients and pH regulation
together with continual purging of by-products and excess biomass out of the
system [3].

iv. Ex-situ
Ex- situ biogas cleaning and upgradation relies on supply of carbon dioxide
from external sources and hydrogen in an anaerobic reactor, which eventually
contributes to their conversion to methane. The ability of ex situ process to
manage high concentrations of influent gases, reduces retention time to about
1 hour leading to a smaller device for upgrading. Depending mainly on the
reactor used, the ex-situ technology can produce methane with final purity of
79–98%, the main challenge facing this technology is low gas–liquid mass
transfer rate [26]. Therefore, ex-situ is more of an upgrading than cleaning
method although it can do both by design. The advantages and
disadvantages of the biological H2S removal processes are summarized in
Table 2 below.
From Table 2, the three discussed biological methods for H2S removal have
significant differences in terms of use of chemicals, operation and
maintenance costs and product quality. Biofilter and biological gas scrubber
techniques need external oxygen injection. The main advantage of in-situ
method is that it has low investments and maintenance costs and does not
require chemicals.

2.2.2 Physical-chemical removal of H2S

These techniques involve use of salts or iron oxides or sulfide precipitation is used
to remove H2S inside the digester. Iron oxides or salts are added that react with H2S, to
produce non-soluble compounds, like iron sulfides, that precipitate and are removed
7
Anaerobic Digestion – Biotechnology for Reactor Performance and Environmental Sustainability

Process Advantages Disadvantages

In situ • A simple process • Oxygen injection my affect the anaerobic

• Low investment and maintenance cost digestion and can oxidize methane
• Does not need chemicals • Potentially explosive mixtures may occur
• Cannot achieve biomethane purification level

Biofilter • Enables the removal of ammonia • Requires nutrients renewal hence more
• Does not require chemicals operation and maintenance cost
• The injection of oxygen is external to the • Only suitable for small biogas flows
digester hence no negative effect to the • Injection of air at high levels through the
digestion biofilter is not suitable for biomethane
production

Biological • Oxygen introduced is external to the • Uses chemicals

gas process and has no negative impact to • High operations and maintenance costs
scrubber the digestion • The process needs fresh water introduction
• Good for high biogas flow rates
• The process can attain purity
requirement for biomethane

Ex citu • Can be used to attain high methane • The process relies on carbon dioxide supplied
purity levels needed for biomethane from external sources hence an extra cost
• Requires smaller devices due to lower • The process has low gas–liquid mass transfer
retention time rate

Table 2.
Biological process for removal of H2S (summary by the author).

together with effluents from the biodigester. Through direct dosing, chemicals are
added to a reactor installed in the biogas line. The H2S adsorption process is achieved
by retention in a solid form having a large surface area or in materials with high
internal porosity. Activated carbon and iron oxides are the common adsorbent mate-
rials applied in the process. Activated carbon enable production of low concentrations
of H2S based on the catalytic oxidation of H2S on the surface of the activated carbon,
which is easy to impregnate with catalysts that speed up the reaction and improve the
process capacity [4]. The advantages and disadvantages of each physical- chemical
H2S removal process is summarized in Table 3.
From Table 3, it is noted that there are broadly two main approaches of physical-
chemical methods of H2S i.e. addition of salts or iron oxides in situ and adsorption.
The addition of chemicals does not be used to attain biomethane quality although the
process is simple and cheap. The adsorption method is moderate in cost, attains high
removal rate for H2S to attain biomethane level of purity but incurs high energy costs
and high operation and maintenance cost related to replacement of the adsorbent.

3. Biogas upgrading methods

Production of biogas and use has several environmental, social and economic
benefits. It is a source of renewable energy, and its production is also considered as a
manure production factory. Biomethane has wider industrial applications hence bio-
gas up-gradation is desirable [2]. The main drivers of biogas up-gradation is rapid
increment in the price of fossil fuels and growing concerns over global climate change
due to greenhouse gas emissions. Biomethane has opened a new window for the
replacement of natural gas from the energy mix. There are multiple biogas
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DOI: http://dx.doi.org/10.5772/intechopen.112059

Process Advantages Disadvantages

Addition of salts or ✓ The process is simple and cheaper ✓ Cannot attain biomethane purity levels
Iron oxides in situ ✓ No need for oxygen injection ✓ Forms precipitates within the digestor
✓ Low maintenance costs and hence handling issues
✓ The process uses chemicals

Adsorption ✓ Has moderate investment costs ✓ The process has high energy
requirements consumption
✓ Has got high rate of removal ✓ Extra cost incurred to renew absorbent
✓ Can attain biomethane quality and in form of operation and maintenance
standards ✓ Extracted sulfur cannot be used
✓ Oxygen injection does not affect
the use of doped activated carbon

Table 3.
Advantages and disadvantages of physical-chemical methods for H2S removal (summary by the author).

up-gradation technologies which are available on commercial scale while others are
still developing and are at laboratory scale. The technologies that are widely accepted
have attained prominence on their operational efficiency and reliability, merits and
demerits and future outlook [10, 18, 21].
Biomethane production requires more complex and expensive techniques com-
pared to biogas treatment methods aimed at attaining high degree of purity for biogas.
Biogas upgrading combines biogas treatment and purification processes to remove
other gases from biogas, thence separate methane (CH4) and effectively increasing its
heating value. Purification of biogas involves removal of carbon dioxide which is
mandatory for biomethane to substitute natural gas in pipeline system for natural gas
distribution and use as a fuel for applications like vehicle fuel [28].

3.1 Chemical scrubbing

Chemical scrubbing systems use aqueous organic or inorganic compounds to bind

the CO2 or H2S molecules existing in biogas. The most commonly used scrubbing
systems use organic compounds, namely aqueous amine solutions like diethanolamine
(DEA), monoethanolamine (MEA) or methyl diethanolamine (MDEA). Most scrub-
bing systems using amine solutions have an absorber unit maintained between biogas
pressure of 1–2 bars which is injected to the tank bottom with amine solution flowing
from the top and a stripper. The solute and solvent (CO2) undergo a reversible
exothermic chemical reaction with the product amine solution which is rich in
CO2 and H2S which proceeds to the stripper for regeneration operating at a pressure of
1.5–3 bars and temperature of 120-160°C [6].
The heat in the stripper disrupts the chemical bonds formed in absorption phase
and which creates steam having CO2. Upon cooling this steam, the CO2 is released
while the condensate is recirculated back to the stripping column. Some commercial
systems can cope with biogas with H2S content of up to 300 ppm/v, but H2S poisons
the amine, cause corrosion and increase the system energy requirements hence the
need to remove H2S before amine scrubbing [3, 25].
The advantages of the chemical scrubbing using amine solutions include high
selectivity of the amines by CO2 and the substantial extraction compared to other
methods e.g. two times more CO2 per unit volume is absorbed compared with water.
9
Anaerobic Digestion – Biotechnology for Reactor Performance and Environmental Sustainability

Figure 3.
Chemical scrubbing with amine solution [29].

The process has significantly low energy requirements mainly because of exothermic
reactions and low process pressure operations i.e. 1–2 bar for absorption column and
1.5–3 bar in the stripping column. The draw backs include high energy requirements
for solvent regeneration, expensive amine solvents and losses of solvents to evapora-
tion which increase operation costs [3]. Figure 3 shows the process of chemical
scrubbing using amine solution.
From Figure 3, it is noted that the main system elements for the chemical scrub-
bing with amine solution are the adsorption column, heater, a cooler, a stripping
column, and a heating medium for the stripping column which may be hot water, oil
or steam.
In chemical scrubbing (CSC), there is reversible reactions between absorbed sub-
stances and solvent used. The commonly used biogas upgrading absorption solutions
is based on amines i.e. methyl diethanolamine diethanolamine, monoethanolamine,
and piperazine. For amine scrubber an absorber tank is used in which carbon dioxide
is absorbed from the biogas operating at 20–65°C and 1–2 bar, then followed by a
stripper where carbon dioxide is released by heating the stream. Chemical scrubbing
with amine facilitates production of high concentration of methane concentration in
biomethane greater than CH4 > 99%. The limitation of chemical scrubbing needs
pre-treatment stage, to remove H2S and has got high operational and investment
costs [30].
The process is similar to pressurized water scrubbing, but is a chemical absorption
technique. The solution absorbs CO2 in biogas, by chemical reaction between amine
and CO2. The absorber is maintained at operating pressure 1–2 bar while the stripper
maintained 1.5–3 bar. The process is exothermic, causing temperature rise of amine
solution and higher efficiency since the reaction between amine and CO2 increases
with increase in temperature. Hydrogen sulfide (H2S) should be removed prior to the
reaction to avoid poisoning the amine solution [1, 7].

3.2 Organic physical scrubbing

Organic physical scrubbing work on the same principle with water scrubbing with
the difference being the use of an organic solvent with higher affinity for H2S and
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DOI: http://dx.doi.org/10.5772/intechopen.112059

CO2. Methanol and dimethyl ethers of polyethene glycol (DMPEG) mixtures are all
used in biogas upgrading. The process simultaneously absorbs hydrogen sulfide, car-
bon dioxide, and water due to their higher solubility in polyethene glycol than meth-
ane. Examples of commercially available organic physical scrubbing products
Selexol® and Genosorb®. These products exhibit high hi solubility of NH3 and CO2
compared to H2O. Selexol® can absorb three times more CO2 than water hence lower
liquid requirements which requires a smaller upgrading [3, 25].
The challenge associated with high solubility of carbon dioxide in organic
solvents is difficulty to regenerate organic solvents. Higher solubility of H2S compared
to CO2 in Selexol® leads to increased separation temperatures during the regeneration
of the solvent hence higher energy consumption. It is therefore advice able to remove
H2S before the gas is treated with the solvent. The Selexol process may also be
configured to remove H2S selectively, or non-selectively in order to remove both CO2
and (H2S) [25].
In the first stage, raw biogas is compression and cooled to (7–8 bar, 20°C), before
injection to the bottom of the absorption column. Since temperature affects Henry’s
constant the organic solvent is cooled down before it is fed to the column. The
desorption column is used to regenerate the organic solvent by heating it to 80°C and
reducing pressure to 1 bar. This leads to final methane content of 96–98.5% and less
than 2% CH4 losses, in an optimized full-scale plant [3]. Figure 4 shows that the
organic physical scrubbing method.
From Figure 4, it is noted that the main elements of the organic scrubbing
method are Sulfur absorber, CO2 absorber, H2S concentrator, H2S stripper, stripper
reboiler reflux pump and reflux accumulator. In organic physical scrubbing, CO2 in
raw biogas is absorbed in an organic solvent e.g. a mix of dimethyl ethers of polyeth-
ylene glycol [29].
Concerns over the environment has motivated a shift from the use of conventional
solvents to green solvents. This includes the use of deep eutectic solvents (DESs),
consisting of two or more components, which are mainly hydrogen bond donors and
acceptors [9, 25]. It desirable for the solvents to have a lower melting point, very low

Figure 4.
Organic physical scrubbing [1].

11
Anaerobic Digestion – Biotechnology for Reactor Performance and Environmental Sustainability

vapor pressure and preferably be biodegradable. It is through selection of best fit

hydrogen bond donors and hydrogen bond acceptors and donors, that the DESs can be
appropriately engineered to yield desired thermodynamic and physical characteristics.
It is also possible to remove other biogas contaminants by appropriate process modi-
fications [3, 9].

3.3 Pressure swing adsorption (PSA) and vacuum swing adsorption (VSA)

The principle upon which the pressure swing adsorption (PSA) is the adsorption of
CO2 compared to CH4 under conditions of high pressure due to differences in molec-
ular characteristics and the affinity of the adsorbent material used. The vacuum swing
adsorption (VSA) is based on the same principle is except that it operates under
vacuum during the desorption step. Materials used as adsorbent matter PSA is
required to have high surface area, e.g. alumina, silica gel, activated carbon, zeolite,
polymeric sorbents and carbon molecular sieves [10, 31].
Pressure swing adsorption is a technique that works by selective adhesion of one or
more components of the mixture, on the surface of a micro-porous solid. In this case
the material for biogas upgrading is typically equilibrium-base adsorbents. The adsor-
bent pores allow an easy penetration of the carbon dioxide molecules but filters the
larger methane molecules. The molecular sieve materials used include zeolites and
activated carbon which act as the adsorptive materials for biogas upgrading. The
process requires a pretreatment step because the materials used in PSA plants foul in
the presence of raw biogas impurities. The pressure swing technology can achieve 95–
99% methane purity for upgraded biogas which meats the typical technical specifica-
tions for the grid injection [25]. The main limitations of the pressure swing process are
the pre-treatment requirement and extensive process control making the process
expensive. To reduce operational costs, the temperature swing adsorption (TSA) is
used instead. Temperature swing adsorption works at constant pressure and needs
thermal energy to regenerate the adsorbent material making it suitable in
applications having cheap heat source [30, 31]. The pressure swing method is illus-
trated in Figure 5.
From Figure 5, it is noted that the main processes and elements of the compressor
for raw biogas, chambers for absorption, depressurization, desorption, pressurization,
and vacuum pump for extraction of vent gases [10]. The characteristics of PSA unit
include feeding pressure, cycle time, purging pressure, adsorbent, and column inter-
connectedness among other things [29].
Pressure swing adsorption (PSA) is a dry method used to separate gases on basis of
their properties. Raw biogas is compressed to an elevated pressure and supplied to an
adsorption column that retains CO2 but leaves CH4. Once the column material is
saturated with CO2, pressure is released hence CO2 is desorbed and fed to the off-gas
stream. Multiple columns can be applied columns are needed for continuous operation
allowing them to be closed and opened consecutively [29].
The Pressure Swing Adsorption (PSA) technology makes use of the ability of
porous adsorbent medium to adsorb specific molecules out of raw biogas and then
release through the application of different pressure levels. For the case of raw biogas
upgrading [1, 29]. For the biogas upgrading process, the operation is based on the
different molecular dimensions of CO2 which is 0.34 nm, methane CH4 with 0.38 nm.
The application of adsorbent material with cavities of 0.37 nm facilitate retention of
CO2 in the pores, as methane flows out with no retention. The most utilized adsorbent
materials are the Zeolites and activated carbons due to their high efficiency [1].
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Figure 5.
Pressure swing adsorption system [29].

The pressure swing adsorption process takes place in vertical columns that are
packed with absorbents. The process has four steps; adsorption, depressurization,
desorption and pressurization in the listed order. Biogas passes through in the pres-
surized column, while CO2, N2, O2 and H2S are adsorbed by selected material. Hydro-
gen sulfide and siloxanes are irreversible adsorbed onto adsorption material and
should therefore be removed, together with moisture before injection into the PSA
system. It is recommended to use multiple adsorption columns to ensure continuous
operation. Once the saturation of adsorbent material is saturated, biogas is allowed
into the next column, as regeneration is done for the saturated column. The adsorp-
tion column is depressurized to about atmospheric pressure (PSA) or kept under
vacuum (VSA). A mixture of CH4 and CO2 with high content of CH4 methane content
is released and recycled to the PSA inlet. Biomethane produced can attain purity of
96–98%; but up to 4% CH4 can be lost within the off-gas stream [10, 25].
The Pressure Swing Adsorption (PSA) technology makes use of the ability of
porous adsorbent medium to adsorb specific molecules out of raw biogas and then
release through the application of different pressure levels. For the case of raw biogas
upgrading [1, 29]. For the biogas upgrading process, the operation is based on the
different molecular dimensions of CO2 which is 0.34 nm, methane CH4 with
0.38 nm. The application of adsorbent material with cavities of 0.37 nm facilitate
retention of CO2 in the pores, as methane flows out with no retention. The most
utilized adsorbent materials are the Zeolites and activated carbons due to their high
efficiency [1].
Recent development of the PSA/VSA focus on optimization of adsorption materials
and technology. New methods include vacuum swing adsorption system that applies
amine-containing nanogel particles supported by carbon fiber having a honeycomb
shape whose primary application is the capture of CO2 from flue gas with potential use
in biogas upgrading. In this method, the size of the column and operational costs are
reduced by using a rotating design and honeycomb carbon fibers as supportive mate-
rial, while the combination with amine-containing nanogel particles, increases the
recovery of CO2 [10, 31]. The Amine-containing nanogel particles also reversibly
uptake and release CO2 at lower regeneration temperature of about 75°C which limit
the degradation and volatility of amine used [31].

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Anaerobic Digestion – Biotechnology for Reactor Performance and Environmental Sustainability

3.4 Absorption techniques

In the absorption techniques, purification and enrichment processes are based on

the solubility of constituent gases in biogas in a selected liquid. The commonly used
liquids are water or organic solvent like methanol, N-methyl pyrrolidone, and poly-
ethylene and glycol ethers are for absorption of CO in physical absorption plants
installations. Amine scrubbing is widely applied for chemical absorption. Water
scrubbing has two main applications, namely pre-treatment e.g. before PSA and for
the removal of H2S in actual upgrading. The main limitation of these technique is that
it requires significant plant size to achieve high final concentration of methane [30].

3.4.1 Pressurized water scrubbing

Water scrubbing is the most common technology for both biogas cleaning and
upgrading. Pressurized water scrubbing depends on the separation of CO2 and H2S
from raw biogas as a result of increased solubility of CO2 compared to CH4. Based on
Henry’s law, CO2 solubility in water at 25°C is about 26 times greater than the solubil-
ity of methane [13]. Raw biogas is first compressed to 6–10 bar, ND up to 40°C then
injected into the absorption column from the bottom side of the tank, while water is
supplied from the top while water is supplied from the top side of the column then it
flows in the counter-current flow of the gas. The absorption column of the system is
filled with random packing material for increased gas-liquid mass transfer [25].
Biomethane is released from the top of the scrubber, the water phase containing
the CO2 and H2S are circulated to the flush column, in which the pressure is degreased
to 2.5–3.5 bar and while traces of CH4 dissolved in the water is recovered. On the basis
of water re-use single pass scrubbing is often employed when water is from sewage
treatment plants and “regenerative absorption”. Water can be regenerated in a
desorption column by decompression at pressure, leading to the removal of CO2 and
H2S. Water decompression is done by air stripping but where biogas has high concen-
trations of H2S, steam or inert are consumed on desorption process to prevent forma-
tion of elemental Sulfur by means of air stripping, which leads to operational
problems. The regeneration is desirable of huge water requirement by the system e.g.
water flow to upgrade 1000 Nm3 /h of raw biogas needs 180 and 200 m3/h based on
pressure and water temperature. Upon drying, in drying stage, the purity of methane
formed can reach 99% purity [13, 25].
Pressurized water scrubbing process works on the basis of the fact that carbon
dioxide is more soluble in water than methane. It is the simplest and most popular
upgrading technology for biogas. It is necessary to remove H2S from biogas prior to
scrubbing due to its high solubility in water, making its removal difficult. Hence the
need to previously remove hydrogen sulfide (H2S) from biogas, to avoid corrosion
and process efficiency reduction. Biogas is first compressed and fed to the absorption
column (scrubber), for cooling to (5°C) and pressurized (4–10 bar) to allow water to
absorb CO2 and other impurities. The flash tank is used for water regeneration in the
first phase with the recovery of the absorbed biogas, being recycled by injecting at the
biogas inlet. The second phase of regeneration takes place in a second column called a
stripper through a countercurrent with air, operating under atmospheric pressure
[2, 4]. Figure 6 shows the water scrubbing system.
The main parts of the water scrubbing system as shown in Figure 6 are the water
separator, a compressor a flash tank, desorption column, a cooler, filter, water and an
upgraded biogas dryer for upgraded biogas. A water scrubber is a physical scrubber
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Figure 6.
Water scrubbing system [29].

which exploits the fact that CO2 is more soluble in water than methane. The CO2 is
separated from the raw biogas and dissolved into the water in the absorption column
by application of high pressure of 6–10 bar. The CO2 is then released from the water
in the desorption column, by addition of air at atmospheric pressure [29].

3.4.2 Chemical scrubbing

Chemical scrubbing systems use aqueous organic or inorganic compounds to bind

the CO2 or H2S molecules existing in biogas. The most commonly used scrubbing
systems use organic compounds, namely aqueous amine solutions like diethanolamine
(DEA), monoethanolamine (MEA) or methyl diethanolamine (MDEA) [25]. Most
scrubbing systems using amine solutions have an absorber unit maintained between
biogas pressure of 1–2 bars which is injected to the tank bottom with amine solution
flowing from the top and a stripper. The solute and solvent (CO2) undergo a reversible
exothermic chemical reaction with the product amine solution which is rich in CO2
and H2S which proceeds to the stripper for regeneration operating at a pressure of
1.5–3 bars and temperature of 120-160°C [6].
The heat in the stripper disrupts the chemical bonds formed in absorption phase
and which creates steam having CO2. Upon cooling this steam, the CO2 is released
while the condensate is recirculated back to the stripping column. Some commercial
systems can cope with biogas with H2S content of up to 300 ppm/v, but H2S poisons
the amine, cause corrosion and increase the system energy requirements hence the
need to remove H2S before amine scrubbing [3].
The advantages of the chemical scrubbing using amine solutions include high
selectivity of the amines by CO2 and the substantial extraction compared to other
methods e.g. two times more CO2 per unit volume is absorbed compared with water.
The process has significantly low energy requirements mainly because of exothermic
reactions and low process pressure operations i.e. 1–2 bar for absorption column and
1.5–3 bar in the stripping column. The draw backs include high energy requirements
for solvent regeneration, expensive amine solvents and losses of solvents to evapora-
tion which increase operation costs [3, 25]. Figure 7 shows that chemical scrubbing
using amine solution.
Figure 7 demonstrates a chemical scrubbing system using amine solution. It is
equipped with the absorption column a heater cooler and a stripping column with a
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Anaerobic Digestion – Biotechnology for Reactor Performance and Environmental Sustainability

Figure 7.
Chemical scrubbing with amine solution [1].

heating medium being hot water, oil or steam. The removal of CO2 using reactive
systems is not new, but it is less common compared to other technologies like PSA and
water scrubbing. The synopsis of features of the chemical scrubbing technology is to
use a reagent which chemically binds carbon dioxide molecules for removal from the
gas [29].
In chemical scrubbing (CSC), there is reversible reactions between absorbed sub-
stances and solvent used. The commonly used biogas upgrading absorption solutions is
based on amines i.e. methyl diethanolamine diethanolamine, monoethanolamine, and
piperazine. For amine scrubber an absorber tank is used in which carbon dioxide is
absorbed from the biogas operating at 20–65°C and 1–2 bar, then followed by a stripper
where carbon dioxide is released by heating the stream. Chemical scrubbing with
amine facilitates production of high concentration of methane concentration in
biomethane greater than CH4 > 99%. The limitation of chemical scrubbing needs pre-
treatment stage, to remove H2S and has got high operational and investment costs [30].
The process is similar to pressurized water scrubbing, but is a chemical absorption
technique. The solution absorbs CO2 in biogas, by chemical reaction between amine
and CO2. The absorber is maintained at operating pressure 1–2 bar while the stripper
maintained 1.5–3 bar. The process is exothermic, causing temperature rise of amine
solution and higher efficiency since the reaction between amine and CO2 increases
with increase in temperature. Hydrogen sulfide (H2S) should be removed prior to the
reaction to avoid poisoning the amine solution [7].

3.4.3 Organic physical scrubbing

Organic physical scrubbing work on the same principle with water scrubbing with
the difference being the use of an organic solvent with higher affinity for H2S and
CO2. Methanol and dimethyl ethers of polyethene glycol (DMPEG) mixtures are all
used in biogas upgrading. The process simultaneously absorbs hydrogen sulfide, car-
bon dioxide, and water due to their higher solubility in polyethene glycol than meth-
ane. Examples of commercially available organic physical scrubbing products
Selexol® and Genosorb®. These products exhibit high hi solubility of NH3 and CO2
compared to H2O. Selexol® can absorb three times more CO2 than water hence lower
liquid requirements which requires a smaller upgrading [3].
The challenge associated with high solubility of carbon dioxide in organic solvents
is difficulty to regenerate organic solvents. Higher solubility of H2S compared to CO2
in Selexol® leads to increased separation temperatures during the regeneration of the
solvent hence higher energy consumption. It is therefore advice able to remove H2S
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before the gas is treated with the solvent. The Selexol process may also be configured
to remove H2S selectively, or non-selectively in order to remove both CO2 and
(H2S [25]).
In the first stage, raw biogas is compression and cooled to (7–8 bar, 20°C), before
injection to the bottom of the absorption column. Since temperature affects Henry’s
constant the organic solvent is cooled down before it is fed to the column. The
desorption column is used to regenerate the organic solvent by heating it to 80°C and
reducing pressure to 1 bar. This leads to final methane content of 96–98.5% and less
than 2% CH4 losses, in an optimized full-scale plant [3, 25]. The organic physical
scrubbing method is shown in Figure 8 below.
The main elements of an organic physical scrubbing system as shown in Figure 8
are the Sulfur absorber, CO2 absorber, a compressor H2S concentrator, a reflux pump
and accumulator, and stripper reboiler. An organic solvent is used to absorb the CO2
in raw biogas organic in physical scrubbing method in a process that is theoretically
similar to water scrubbing, based on the Henry’s law. These solvents include a mix of
dimethyl ethers of polyethylene glycol. The relative solubility of the biogas compo-
nents depends on the solvent used e.g. the solubility of carbon dioxide is much higher
in the organic solvent than in water, meaning that the Henry’s constant for carbon
dioxide is higher. CO2 has a solubility of 0.18 M/atm in Selexol which is about 3 times
higher than in water. CO2 is about 17 times more soluble than methane in the
Genosorb solvent which is a smaller difference than for water, in which CO2 is 26
times more soluble than methane [29]. These differences in solubility have technical
and economic implications.
Concerns over the environment has motivated a shift from the use of conventional
solvents to green solvents. This includes the use of deep eutectic solvents (DESs),
consisting of two or more components, which are mainly hydrogen bond donors and
acceptors [9]. It desirable for the solvents to have a lower melting point, very low
vapor pressure and preferably be biodegradable. It is through selection of best fit
hydrogen bond donors and hydrogen bond acceptors and donors, that the DESs can be
appropriately engineered to yield desired thermodynamic and physical characteristics.
It is also possible to remove other biogas contaminants by appropriate process modi-
fications [3, 9].

Figure 8.
Organic physical scrubbing [3].

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Anaerobic Digestion – Biotechnology for Reactor Performance and Environmental Sustainability

3.5 Membrane separation

One alternative to the conventional absorption in biogas upgrading is membrane

technology which make use of membranes which can be made from polymeric mate-
rials like cellulose acetate [3]. The membrane is a filter with ability to separate com-
ponents in raw biogas to the molecular level. Membranes came into use in the 1990s
and were initially built with less selective membranes and were applied in applications
with lower recovery demand for the methane [29]. The membrane separation tech-
nology is a viable alternative to the conventional absorption-based biogas technology.
The process relies on the selective permeability properties of membranes which
effectively enable separation of biogas components. The various components of raw
biogas have different relative permeation rates which can be ordered hierarchically
from the slowest to fastest permeation as follows; C3H8, CH4, N2, H2S, CO2 then H2O
[13, 25]. This is demonstrated in Figure 9 below.
The membrane separation system is demonstrated in Figure 9 showing stage wise
removal of CO2, O2, H2O from raw biogas leaving methane in purified form leaving
the membrane. This process can also remove CO2, H2O and hydrogen and parts of the
oxygen from biogas. The permeation rate depends on the size of molecules hydro-
philic through a typical membrane typically made of a glassy polymer [29].
The membranes used in separation are selectively permeable barriers that are
designed allow some molecules to pass through but stop or block other with process
drivers being the pressure, relative concentration, temperature, and electric charges of
the molecules. The membranes used are of three major types, namely polymeric
membranes, inorganic membranes, and mixed matrix membranes [2]. Inorganic
membranes have got higher mechanical strength, chemical resistance and thermal
stability making them more popular. Mixed matrix membranes are the mostly used
type of membrane separation in industrial applications. The polymeric and inorganic
membrane separation technologies need pre-treatment since H2S negatively affect
medium –term performance. As a strategy to recover up to 99.5% methane, a multi-
stage membrane strategy is adopted. The penetration of the membrane technology of
separation is high costs and low reliability [2, 30].
The membrane permeation technique works on the basis of the difference in
permeability of between the various constituents of biogas. The action of a membrane
facilitate separation in which methane is retained, while carbon dioxide and other

Figure 9.
Operation of membrane separation [29].

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constituents penetrate through the membrane. Three types of membranes are cur-
rently used for biogas purification; polymeric, inorganic and mixed matrix mem-
branes. The process is not meant to remove H2S and H2O, however, they should not be
allowed to affect the performance of the membrane. By introducing multiple stages of
membranes, CH4 concentration above 98% and with low operational cost can be
attained [3, 4].
From Figure 10, it is noted that a gas is fed into a membrane separator where the
impurities mainly H2S, CO2, are isolated from raw biogas to exceed as permeates while
larger methane molecules exceed as retentate i.e. biomethane.
The membrane separation is divided into wet (gas–liquid) and dry (gas–gas)
techniques. Biogas is usually pressurized to 20–40 bars or 6–20 resulting in CH4
abundant gas which will on one side of the membrane with the higher pressure. The
CO2, some H2S and a significant amount of methane of 10–15% diffuses to the lower
pressure side. Contaminants like water, siloxanes, NH3, VOCs and H2S are removed
before membrane separation to avoid corrosion and clogging. There are different
configurations of gas-gas units i.e. single-pass membrane unit or multiple stage mem-
brane units with internal recirculation of permeates and retentates. For one system,
about 92% purity of biomethane can be attained while multiple stages can attain 96%
or more methane purity [3, 25].
The cold-membrane and cryogenic technologies combination is an interesting
approach that can be used in upgrading of biogas. Polyimide and polysulfone mem-
branes can be used in biogas upgrading process to attain up to 98% methane purity,
based on simulation results. The process has relatively lower energy requirement of
about 1.6 MJ/kg CH4 which is lower than energy requirements of a standard mem-
brane process which is about 2.4 MJ/kg CH4. The process can further lower energy
requirements to 0.8 MJ/kg CH4 if the process is coupled with liquefied methane
regasification [3, 8].

3.6 Cryogenic separation

Cryogenic separation technology is done by a gradual reduction in the temperature

of raw biogas causing liquefaction of CH4, CO2 and other constituent parts to ensure
methane meets quality standards for Liquefied Natural Gas (LNG). Raw biogas is
initially dried and compressed to 80 bars followed by stepwise cooling to 110°C
leading to gradual removal of impurities like., siloxanes, H2O, H2S, halogens etc. and

Figure 10.
Principle of membrane separation [6].

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Anaerobic Digestion – Biotechnology for Reactor Performance and Environmental Sustainability

CO2 which is the main impurity in biogas to obtain almost pure biomethane with
purity (> 97%) [13, 32].
The physical principle behind cryogenic technique is based on the fact that the
gases like carbon dioxide, hydrogen sulfide liquefy and solidify under different pres-
sure and temperature conditions. Therefore, the cryogenic plants operate at very low
temperature ( 170°C) and high pressure (80 bar). Biogas purification is done by
cryogenic technology, with lower methane losses but the process is expensive. The
cryogenic process can be used in the production of liquefied natural gas (Bio-LNG)
[6, 30].
The cryogenic separation process involves the separation of different gas compo-
nents based on the basis of their different boiling points by gradually reducing the
temperature. The process begins by compressing biogas to 80 bars then reducing the
temperature to 25°C. At this temperature and pressure, components that are
removed from raw biogas are moisture, halogens, siloxanes and H2S. Reducing tem-
perature to 55°C, liquefies most of the present CO2, then further reduction to 85°C
is the last step which removes the remaining CO2 in solid form. However, to avoid
operational challenges like pipe clogging, ice formation and heat exchanger clogging,
the impurities like H2S, water, siloxanes and halogens are by practice removed prior to
cryogenic separation [6].
The purity of biomethane produced with cryogenic upgrading be over 97% with
methane losses of less than 2%. The limitation of cryogenic upgrading the high
investment and operation costs, methane losses and need for pretreatment to remove
impurities. There are additional variants and configurations like cryogenic distillation
or cryogenic adsorption [2, 3]. The cryogenic process is shown in Figure 11.
From Figure 11, we note that the cryogenic separation method is characterized by
successive compression and cooling at different pressure to liquefy and isolate the
different components of raw biogas. Water is removed in the distillation Colum of the
process.
Although the cryogenic separation process is quite promising with interesting
performance and results, the method is still under development with just few facilities
operating at commercial scale. The process limitations so far are high costs of invest-
ment and operation costs, methane losses and clogging derived from increased con-
centration of solid CO or and presence of other impurities [13, 28].
The cryogenic processes take advantage of the low temperatures to achieve their
goals. By allowing component gases in raw biogas to liquify. A process does not have
to operate below a fixed temperature level for it to be considered “cryogenic”., but
since the However, since the processes involved in this process are done well below

Figure 11.
Cryogenic separation system [6].

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55°C, the common gases in raw biogas can be liquefied and separated which forms
the basis for cryogenic biogas upgrading [29].

3.7 Biological upgrade techniques

These processes apply biological separation via hydrogenotrophic methanogenesis

consisting of hydrogenotrophic-methanogens to convert CO2 and H2 in to CH4 but
commercialization of the processes is limited by several challenges [30]. The positive
side is that it requires low investment and operating costs particularly in terms of
electricity and head demand. It does not require any chemical products or additional
equipment. Easy operation and maintenance. The process leads to high concentrations
of hydrogen sulfide while O2/N2 excess necessitates additional cleaning while air
overload creates an explosive mixture [33].
Although the physicochemical techniques dominate the biogas upgrading market,
biological methods have been riding for the last 20 years. Biological technologies for
Sulfur treatment in biogas are classified into chemotrophic and photosynthetic types.
The advantage of biological techniques is that end products are non-hazardous i.e.
sulfur or sulfate with efficiency being same or higher than that of physicochemical
technologies while the Sulfur recovered can be used as a raw material for production
of sulfuric acid, fungicides and Sulfur fertilizers [33, 34].

3.7.1 Chemotrophic removal of hydrogen sulfide

In this process, the Chemotrophic Sulfur oxidizing bacteria, also known as color-
less Sulfur bacteria, is the ideal microbial group used for biodegradation of H2S. The
bacteria are used to oxidize the reduced Sulfur compounds like. Sulfide, polysulfide,
elemental Sulfur, thiosulfate, and sulfite to gain chemical energy and utilize CO2 as a
carbon source. Biodegradation of Hydrogen sulfide is done aerobically using where
the electron acceptor is O2 and anaerobically where the electron acceptor is NO3. The
bacteria include genera Thiobacillus, Acidithiobacillus, Sulfolobus, Thiovulum,
Thiothrix and Thiospira [3, 24].

3.7.2 Biofiltering

Biofilters remain the simplest type of gas desulphurization systems. The purifica-
tion system consists of a bioreactor where sulfur-oxidizing bacteria like the
Thiobacillus, Pseudomonas and Acidothiobacillus are immobilized on a carrier. In the
process, moisturized biogas is injected from the bottom of the biofilter and forced
through a moist, packed bed with microbial biofilm which purifies biogas. The bed
material is used to supply nutrients or nutrient solution added from the top occasion-
ally. Oxygen whose concentration is 5–10% of volume is supplied by injecting air
directly into the gas stream [3, 24]. Figure 12 shows the biofiltering system.
The biofiltering system as demonstrated in Figure 12 consists of peristatic pumps
for biogas and air, a bioreactor and biogas storage.
Factors influencing the operation of biofilters include the bed medium, moisture
content of biogas, gas temperature, the pH, nutrient and oxygen levels, and the
development of biofilm. A good or suitable bed material should have large specific
area and porosity, create small pressure loss, light in specific weight and cheap. The
bed material should absorb gas odor and but retain its nutrients, contain indigenous
microorganisms and water i.e. moisture content between 40 and 60%. Suitable bio
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Anaerobic Digestion – Biotechnology for Reactor Performance and Environmental Sustainability

Figure 12.
Biofiltering system [29].

filter materials include natural organic materials like composts, coconut fiber,
woodchips/bark, and peats mainly because of their native microorganism consortia
and good level of performance [3, 35].
The benefits of using biofilters are reduced operating costs, no chemical require-
ments. The main limitations in use of bio filters in purification of biogas are media
acidification by the sulfuric acid formed from H2S, degradation and inefficient
mixing. Solutions to these challenges include using a carrier with alkaline properties,
adding alkaline. Biofilters are also not suitable for high loading rates due to limited
buffering capacity and limited control capability for moisture, and pH during high
airflows [3, 25].

3.7.3 Bio trickling filters

The general mechanism of biotrickling filtration is same as bio filters, except for
the use of inert packing bed material hence the need for continuous supply of the
nutrient solution. Plastic supports, activated granular carbon or porous ceramics, are
materials commonly used to provide support for biofilm formation. Advantages of bio
trickling over traditional filters include better process stability, better control and
regulation of the pH and temperature, low flow resistance, less space and continuous
nutrient supply. The continuous washout of products of acidic reactions solves the
problem of buffering and acidification common in bio filters. However, the challenge
of continuous nutrient supply leads to excessive growth of biomass and clogging of
anaerobic zones. Commercially available bio trickling systems include BioSulfurex®
(DMT Environmental Technology), Biopuric process (Biothane Corporation),
Bidox® (Colsen B.V.) and BiogasCleaner® (BioGasclean) [3, 5].

3.7.4 Bio scrubbing

Bioscrubber system is applied in the removal of compounds like ammonia, amines,

hydrocarbons, hydrogen sulfide and odorous contaminants. Bio scrubbing system
consists of two reactor units with the first reactor as absorption tower where the
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pollutants are absorbed in a liquid phase before it goes to the second reactor which is
activated sludge. Degradation occurs in the activated sludge reactor where microor-
ganisms like Thiobacillus and Thioalkalivibrio) grow in suspended flocks. The effluent
generated is recirculated back to the absorption tower. In the removal of H2S, a
sedimentation tank is installed after the second reactor for collection of elemental
Sulfur with O2 being used as the oxidant. Optimal microbial growth and activity are
maintained by addition of oxygen, nutrients and pH regulation together with contin-
ual purging of by-products and excess biomass out of the system [3, 36].
No injection of N2 and O2 is required by bio scrubbers. They are used to handle
load fluctuation loads better as well as stable performance due to easy control condi-
tions. The main disadvantage of the bio scrubbing process is high initial costs. Com-
mercially available bio scrubbing systems used for H2S removal from are THIOPAQ®
process and Sulfothane™ which are very similar. In the process, the gas is injected to
the absorption tower where counter flow of alkaline solution absorbs H2S from raw
biogas. The formed sulfide-containing goes to a micro-oxygenated reactor. The
chemotrophic sulfur oxidizing bacteria, are dominated by haloalkaliphilic Thioalka-
livibrio which convert absorbed sulfide to elemental Sulfur [3, 37].

3.7.5 Phototrophic Sulfur removal with anoxygenic bacteria

Phototrophic Sulfur removal uses bacteria with ability to utilize light as an energy
source to remove Sulfur compounds from the environment e.g. anoxygenic
phototrophic sulfur bacteria, purple non-Sulfur bacteria, cyanobacteria, and
phototrophic members of phylum Chloroflexi and Heliobacteria. Some bacteria like
the anoxygenic phototrophic sulfur bacteria can oxidize hydrogen sulfide to elemental
Sulfur through an oxygenic photosynthesis. Anoxygenic phototrophic Sulfur bacteria
consist of two families namely Chlorobiaceae (green sulfur bacteria) and
Chromatiaceae (purple Sulfur bacteria). The purple and green sulfur bacteria use light
as an energy source, and use reduced Sulfur compounds as electron donors for photo-
synthetic CO2 reduction. Sulfide oxidation produces globules of elemental Sulfur. The
Chromatiaceae store Sulfur outside of their cells while the Chlorobiaceae store Sulfur
inside. The green Sulfur bacteria utilize bacteriochlorophyll c, d, or e found in special
light-harvesting organelles (chlorosomes) that allow the growth under the lower
intensity light (25–80 lx). The photosynthetic pigments in purple Sulfur bacteria, are
bacteriochlorophyll a or b and various carotenoids i.e. spirilloxanthin, rhodopinal,
spheroidene, and okenone) [25].

3.7.6 Chemoautotrophic methods

The chemoautotrophic biogas upgrading methods rely on the hydrogenotrophic

methanogens which use H2 to convert CO2 to CH4 based on the following Eq. (1):

4H2 þ CO2 ! CH4 þ H2 OΔ0 ¼ 130:7KJ=mol (1)

To make this reaction renewable requires that the source of hydrogen used should
be derived from renewable sources hence the need to apply renewable electricity to
hydrolyze water for H2 generation. This facilitate storage of the surplus energy gen-
erated by solar and wind to create a new technology called power to gas (P2G).
Variable renewable sources need buffering to enable energy delivery when it is dark
with no solar and the wind is still. Storage batteries are widely used to store electricity
23
Anaerobic Digestion – Biotechnology for Reactor Performance and Environmental Sustainability

but have drawbacks like capacity limitations, high production cost, and use of toxic
materials. Hydrogen (H2) is a clean energy resource that can be produced by electrol-
ysis of water. As a renewable energy carrier, H2 has some disadvantages like very low
volumetric energy density about 10.88 MJ/m3 compared to CH4 which has 36 MJ/m3
making hydrogen storage a challenge in terms of space requirement. This makes
integration of P2G technology for conversion of H2 to CH4 really attractive as it
integrates wind or solar energy technology as well as biogas technology [25, 37].
Biogas upgrading makes use of existing facilities of the biogas plants which reduces
the initial investment cost. The process of chemoautotrophic does not separate or
absorb the CO2, instead it is converted to methane (CH4) leading to significant
increase in the final energy value of the output “wind gas” wind gas is methane
produced using the surplus energy from wind turbines) or “solar gas” which is meth-
ane produced using surplus solar energy. This technology acts as a precondition for
the sustainability of the ambitious biogas development strategy of decoupling the
biogas production from the biomass availability. Hydrogen assisted biogas upgrading
configurations are classified into in-situ, ex-situ and hybrid designs. The in-situ and
ex-situ processes have been experimentally proven with several research undertaken
unlike the hybrid concept which is still under development [13, 36].

3.8 In-situ biological biogas upgrading

This method was earlier own presented as one of the raw biogas cleaning technique
not aimed at producing biomethane grade biogas. Situ is a biotechnology based on the
direct injection of pure air or oxygen and in the process, the bacteria that oxidize H2S
develop with the presence oxygen, leading to the biological removal process of H2S, to
produce sulfur (S) which leaves the digester via the digested. The microorganisms are
widely found in the anaerobic environment present in bio-digesters [4, 37]. In situ
desorption technology is yet to be fully developed even though it has been around for
over 20 years. In-situ is based on the greater solubility of CO2 over CH4 in water. The
process set up includes an anaerobic digester linked or connected to an external
desorption unit. Sludge transported to an aerated desorption column from the
digester. Nitrogen or air flowing in counter-current mode dissolves the CO2 from the
sludge in the desorption unit. The sludge desorbed sludge is pumped back into the
digester to reabsorb amore CO2, and the sludge as the sludge is continuously recycled
in the desorption column. It is possible to strip out H2S with dissolved CH4 and CO2
from the recirculating sludge by applying large quantities of air or N2, causing reduc-
tion in the H2S and CO2 concentration [26, 27].
In the in-situ concept, H2 is injected into a biogas reactor so that it is coupled with
the endogenous CO2 from anaerobic digestion in the digester for conversion into CH4
by autochthonous methanogenic archaea. The process can yield methane with purity
of 99% if operational parameters like the pH are fully monitored to values above 8.5,
as a result of the removal of bicarbonates which inhibits of methanogenesis [13]. CO2
dissolved in the liquid phase of the reactor dissociates to H+ and HCO3 ions Utiliza-
tion of carbon dioxide (CO2) leads to reduction in H+, which causes concomitant
increase in the fermentation pH [13]. The reaction is summarized in Eq. (2) below.

H2 O þ CO2 $ Hþ þ HCO3 (2)

Studies on in-situ biogas upgrading reactors show some process inhibition of

methanogenesis from bicarbonate consumption which proves the argument that
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conventional biogas production systems need a pH of 8.5 as the threshold for optimum
bio-methanation process for mesophilic and thermophilic activities. Co-digestion with
acidic waste can be applied to mitigate the pH increase and alleviate the technical
limitations. Another solution to the challenge is application of pH control which
enable upgrading to almost pure biomethane [13].
Oxidation of the Volatile Fatty Acids (VFA) and alcohols is thermodynamically
feasible in where H2 concentration is very low. High H2 levels (> 10 Pa) inhibit bio-
digestion and, and promote accumulation of electron sinks like lactate, ethanol, pro-
pionate, and butyrate. VFA degradation will cease if there is introduction of sudden
and high H2 concentrations in the reactor which causes system imbalanced or even,
fatal deteriorated as a result of excess acidification caused by VFA accumulation.
Injection of H2 in batch reactors at a concentration more than stoichiometric amount
for hydrogenotrophic methanogenesis leads to accumulation of acetate, due to stimu-
lated homoacetogenic pathway, and/or decreased methanogenic activity of
acetoclastic archaea. However, upon longer term H2 exposure, there is increase in
hydrogenotrophic population which improves the utilization capacity of H2 and
reverts the inhibition [13, 25].
Solubilization of H2 to the liquid phase is another important parameter since it
must cross the interface between the liquid and gas for it to be available for the
microorganisms. Hence aqueous solubility of most gasses is rather low, limiting the
gas–liquid mass transfer and which retards performance the bioreactor. Therefore, the
material and module type used to inject H2, use of gas recirculation flows and the
reactor designs are important aspects of the implementation of sufficient in-situ
biogas upgrading. Studies in Batch experiments showed that the rate of uptake of H2
decreases rapidly at CO2 concentrations <12% and maximum CH4 purity attained was
89%. Studies in continuously fed reactors using hollow fiber membranes for H2 injec-
tion in a reactor treating cattle manure and cheese realized 96% CH4 purity of final
gas. Studies in up flow anaerobic sludge blanket reactor, using a hollow fiber mem-
brane in an external degassing unit and realized 94% methane purity [13, 36].

3.9 Ex-situ biological biogas upgrading

Ex- situ biogas upgradation relies on supply of carbon dioxide from external
sources and hydrogen in an anaerobic reactor, which eventually contributes to their
conversion to methane. The ability of ex situ process to manage high concentrations of
influent gases, reduces retention time about 1 hour leading to a smaller device for
upgrading. Depending mainly on the reactor used, the ex-situ technology can produce
methane with final purity of 79–98%, the main challenge facing this technology is low
gas–liquid mass transfer rate [26].
Through studies, it has been established that the operating temperature signifi-
cantly affects bio-methanation efficiency e.g. enriched thermophilic culture resulted
in >60% higher H2 and CO2 bioconversion when compared to mesophilic culture in
batch. In q typical study, increase of operating temperature from 55 to 65°C showed
significant increase in efficient of bio-methanation operation Other than temperature,
an adaptation period is needed for microorganisms to efficiently ferment the CO2 and
H2 gasses e.g. it was established that operating a mesophilic trickle-bed reactor with
immobilized hydrogenotrophic culture for 8 months, improved output to CH4 content
of over 96%. Similar results are experienced for bio-methanation efficiency under
thermophilic conditions [25].

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Anaerobic Digestion – Biotechnology for Reactor Performance and Environmental Sustainability

The reactor type and application of gas recirculation or liquid mixing are important
design parameters for biogas upgrading system. Up flow in series or bubble column
reactors realize over 98% methane purity, even when H2 is injected through conven-
tional spargers instead of advanced membrane modules. The trickle bed reactor sys-
tems yield higher CO2 and H2 conversion efficiency to achieve as high as 98–99%
methane purity, due to the formation of biofilm of mixed anaerobic consortia which
act as good biocatalyst for the process. High stirring speed or diffusion devices with
pore sizes generate gas-bubbles which are able to mix the reactor yield better kinetics
and gas quality [13].
The bio filter technology involves passing the biogas through a column having a
synthetic material, in the form of a biofilm. The parallel or countercurrent flow
maintains the humidity and nutrients, that are essential for the microorganisms that
degrade of H2S [4].
In biological gas scrubber, a two-stage system is used to remove H2S. In the first
stage H2S scrubbing column, applies sodium hydroxide solution while activated
sludge is used in the second stage which is injected with injected with air, because the
microorganisms used are aerobic, leading to the solution regeneration [4].

4. Applications of biomethane

Biomethane has superior properties compared to biogas and is attractive substitute

of natural gas. It has both industrial and domestic application like use as cooking gas,
cogeneration can be packed in containers/cylinders as compressed biomethane and
can be injected to natural gas mains for distribution. The main challenge is the cost of
processing which is a function of technology used. Bio-CNG, which is a methane-rich
compressed fuel in form of biomethane. Bio-CNG is made from pure biogas with more
than 97% methane composition pressurized to 20–25 MPa. Compressed bio-CNC is
similar in properties to regular CNG in terms of its fuel properties, economy, engine
performance, and emissions. Like regular CNG, bio CNG has high octane number, and
yields high thermal efficiency. It can therefore substitute the regular compressed
natural gas in gas pipelines and other applications including fuel for natural gas power
plants [28, 36, 38, 39].

4.1 Hydrogen production

Hydrogen is an ideal raw material for a sustainable energy transformation, but

with the challenge being where and how to get hydrogen from renewable sources.
Renewable hydrogen can be produced using renewable energy sources and usually
produced via water electrolysis [40]. Biogas has applications beyond electricity and
biomethane production, as because, through steam reforming, it can be used to man-
ufacture green hydrogen, in a process where a catalyst refines and separates the
hydrogen from the gas stream [6, 40]. The most common method used to manufac-
ture hydrogen is by steam-reforming of natural gas, followed by pressure-swing
adsorption to remove impurities. However, small reformers like those used for com-
bined heat and power with biogas plants are in commercial operation [40]. The biogas
has low heating coefficient due to high composition of carbon dioxide and water vapor
interfere with the combustion process [2, 40]. Upon removal of carbon dioxide and
water molecules, the methane (CH4) can be used for hydrogen synthesis and bio-fuel
production. Methane can be split to hydrogen molecules (H2) in a process that can be
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done in a steam/methane reformer. In the process, high pressure and temperature

steam is combined with the methane (CH4) to produces flow of hydrogen molecules
and CO molecules [2, 6, 33].
It is through thermochemical processes of hydrocarbons that large-scale hydro-
gen production is manufactured through the reforming process. Biomethane has
significant potential application in hydrogen manufacture as a substitute of fossil
natural gas as a raw material for reforming processes. The demand for renewable
hydrogen production is set to grow significantly due to concerns over fossil fuels
depletion and greenhouse gas emissions, and associated concerns over global cli-
mate change. The availability of capital, desired hydrogen amount and purity
hydrogen and the composition of available biogas will influence the selection of
reforming processes [41].
Biomethane as significant application in fuel cell technologies for applications like
power generation Fuel cells can use hydrogen to generate electric power just like
batteries as well as fuel for the fuel for powering fuel cars. Fuel cell technology in
power generation is emission free and hence attractive [42]. Biomethane can be used
as a source for renewable hydrogen, for stationary fuel cells and power fuel cell
electric vehicles (FCEVs). The Hydrogen-powered FCEVs are environmentally
attractive since they have no tailpipe emissions other than making them extremely
clean as transport option to fossil fuel powered vehicles [37, 41].
Use of biomethane for hydrogen production can increase energy sustainability can
be for energy applications like fossil fuels. Hydrogen can be manufactured by
autothermal reforming (ATR), electrolysis or methane reforming (SMR) [43].
Biomethane can be used as a substitute of natural gas which will provide a hedge
against growing demand for natural gas [41].
Hydrogen fuel can be used to reduce emissions from engines which are widely
used in transportation. Hydrogen fuel cells promise to provide an alternative to inter-
nal combustion (IC) engines particularly due to the clean exhaust emissions, renewal
nature of the fuel and higher efficiencies. Hydrogen fuel cell vehicles can achieve
widespread acceptance except for existing challenges like waste heat removal in
mobile applications [44].

4.2 Production of biofuels from biomethane

The transport sector is important since it accounts for about 14% of the global
greenhouse gas emissions [45]. Liquefied biomethane is a feasible fuel for power
plants and heavy tracks and can also be used as a raw material for production of other
fuels and chemicals like methanol, dimethyl ether, and hydrogen fuel. Biomethane is
currently used as a transport fuel many countries with benefits of lower environmen-
tal impact compared to fossil fuels and several other processed transport fuels [46].
Biofuels include the Bio-CNG which is compressed biomethane similar to (CNG)
in properties with industrial, automotive and domestic applications. The process
needs removal of impurities likes water, N2, O2, H2S, NH3 and CO2 to achieve com-
position of >97% CH4, <2% O2 at 20–25 MPa. Bio-CNG occupies less than 1% of the
volume at standard conditions [46, 47].
Biomethane can also be used in the industry as transport fuel by liquefying it to at a
high pressure re ranging from 0.5 to 15 MP [4]. In the biological or chemical pathways,
biomethane can be converted to methanol, diesel, liquefied petroleum (LPG) and
gasoline. Methanol is produced by partial oxidation of methane as shown below;
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Anaerobic Digestion – Biotechnology for Reactor Performance and Environmental Sustainability

CH4 þ 0:5O2 ! CH3 OH ∆H0 ¼ 128 kJ=mol (3)

In another method, methane is biologically converted from to methanol by using

methanotrophic bacteria used in methanol production through the action of methane
monooxygenase (MMO) enzyme [5].
Methanol can also be produced by reforming methane to syngas then followed by
catalytic conversion of syngas to methanol as shown below [6].
2H2 þ CO ! CH3 OH ∆H0 ¼ 91 kJ=mol (4)
3H2 þ CO2 ! CH3 OH þ H2 O ∆H0 ¼ 49 kJ=mol (5)

Methanol can then be converted to gasoline through methanol-to-gasoline process.

Biogas or biomethane can be processed to methanol through dry reforming, steam
reforming, partial oxidation reforming, autothermal Reforming (ATR) and the
Fischer-Tropsch (FT) Process. Synthesis gas (syngas) is the main product of
biomethane reforming process. Syngas is a raw material for production of many long
chain hydrocarbons [48].

i. Dry reforming
In dry reforming, CO and H2 are produced by reaction of methane (CH4) and
carbon dioxide (CO2). The process utilizes two greenhouse gases (CH4 and
CO2) making it very attractive. Unfortunately, the endothermic reaction
reduces the of CO2 emissions, since CO2 emitted generate the heat required
for the reaction has to be accounted for. Dry reforming is an efficient route
for producing synthesis gas yielding a H2/CO ratio close to 1 [49]. The
disadvantage of dry reforming compared with steam reforming it produces
lower syngas ratio (H2/CO = 1), The ratio of H2/CO ratio is influenced by
water gas shift reaction (WGS), which reduces the ratio due to reverse
reaction that oxidizes hydrogen to water. In this process, the H2/CO ratio is
kept between 1 and 2 by partial oxidation of methane through feeding water.
This enhances forward water gas shift reaction. The energy demand by the
process is lower since partial oxidation is exothermic [48]. The temperature
range for dry reforming process 700–1000°C [48].
CH4 þ CO2 ! 2CO þ 2H2 ∆H0 ¼ 247 kJ=mol (6)

ii. steam reforming and water shift reaction

This process combines methane in biomethane with water vapor generate CO
and H2 in the in the presence of a catalyst. The process is endothermic and
takes place between 650 and 850°C, to produce hydrogen yield of 60–70%
[49]. Steam reforming takes place between 700 and 900°C. The two-step
chemical reaction is shown below;
CH4 þ H2 O ! CO þ 3H2 ∆H0 ¼ 206 kJ=mol (7)
CO þ H2 O ! CO2 þ H2 ∆H0 ¼ 41 kJ=mol (8)

The process of steam reforming is often followed by a water shift reaction to

improve hydrogen generation.

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iii. Partial oxidation reforming (POR)

This process is used to produce hydrogen at reduced energy cost because the
process is moderately exothermic compared to steam reforming which is
highly endothermic. H2 and CO are produced by the partial oxidation at
atmospheric pressure and between 700 and 900°C partial oxidation
reforming. The H2/CO ratio of 2 yield is achieved in full conversion with
reduced soot formation. Methane react with oxygen to form carbon dioxide
(CO2) due to decrease in CO selectivity. The combustion is strongly
exothermic leading to formation of hot-spots in the reactor bed and coke
deposition on the catalyst [49]. In this process, methane is oxidized to syngas
as demonstrated below

CH4 þ 0:5O2 ! CO þ 2H2 ∆H0 ¼ 25:2 kJ=mol (9)

iv. Autothermal Reforming (ATR)

Autothermal Reforming is a combination of two processes i.e., POR and SR in
the presence of carbon dioxide. In Autothermal Reforming (ATR) partial
oxidation takes place in the reactor to produce heat needed for steam
reforming in the catalytic zone. The process does not need external heating
and the reactor is easy to stop and restart. Compared to partial oxidation
reaction, the hydrogen yield is higher and consumes less oxygen
compared [49].

4.2.1 Upgrading syngas

The Syngas produced from the dry reforming has carbon dioxide which should
be removed before it is supplied to the Fischer-Tropsch reactor. Amine absorption
has high selectivity for carbon dioxide. Other applications of this technology are
separation of CO2 from flue gases, natural gas cleaning and largescale upgrading
of biogas. Common solvents used in the process are alkanolamines like
monoethanolamine (MEA), diethanolamine (DEA) or methyldiethanolamine
(MDEA) [50, 51].

4.2.2 Fischer-Tropsch (FT) process

The Fischer-Tropsch (FT) synthesis, was name after the German inventors Franz
Fischer and Hans Tropsch is a process used to manufacture liquid hydrocarbon fuels
like coal-to-liquids (CTL) and/or gas-to-liquids (GTL) based on source of syngas.
[50, 51]. The biomethane and natural gas conversion to fuels via Fischer-Tropsch
synthesis (FT-synthesis) is feasible at industrial scale [48]. The Fischer-Tropsch (FT)
process is coverts syngas to products like LPG, diesel, and jet fuels [24, 50].
The Fischer-Tropsch synthesis (FT-synthesis) polymerizes the carbon and hydro-
gen atoms in syngas or biomethane to create long chain molecules. The process is run
over iron or cobalt catalyst at 20–30 bars [14] in an overall exothermic process leading
to polymerization of CH2 to hydrocarbons with long chains called syncrude. The
various reactions in Fischer-Tropsch process are summarized below;

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Anaerobic Digestion – Biotechnology for Reactor Performance and Environmental Sustainability

ð2n þ 1Þ H2 þ nCO ! CnH2nþ2 þ nH2 O (10)

2nH2 þ nCO ! Cn H2n þ nH2 O (11)

Reactors used are include multi-tubular fixed bed, circulating fluidized bed,
fixed fluidized bed, and slurry reactor. The reactions for the slurry reactor conditions
are 20–30 bar, ands 200–300°C while the syngas H2/CO ratio of 1–1.8 [48, 50]. For high
temperature synthesis, fluidized-bed FT reactors are used to generate light hydrocar-
bons in form of gaseous hydrocarbons and gasoline and generally have higher output.
The catalysts used are Fe and Co which are sensitive to sulfur compounds in syngas [50].

4.2.3 Biofuels from biomethane

Various biofuels can be made from biomethane for the transport sector e.g. methanol,
compressed biogas (CBG), hydrogen, liquid biogas (LBG), dimethyl ether and Fischer-
Tropsch (FT) fuels [52]. Fuels can be produced by biogas upgrading to biomethane then
compressing to make (CBG) or liquefying to make (LBG), or gasification to produce
syngas for use in manufacture of hydrogen, methanol, DME and FT diesel [52].

4.3 Biomethane for gas and power grids

In many countries, governments have come up with national support schemes to

promote biomethane market. Support mechanisms include feed-in support schemes,
green gas products and quota obligations as market drivers in Europe. Biomethane
production for many countries is based on organic waste as feedstock, but for
Germany which dominates Europe’s feed-in market is based on energy crops. In
Germany, the main driver is the feed-in tariff for renewable electricity through
the Renewable Energy Sources Act (EEG). Biomethane support schemes mainly
relay on mass balancing systems or ‘book and claim’-certificates. Existing mass
balancing systems, can contribute to international market development through
creation of common standards. As biomethane becomes popular and relevant in
the energy systems, integration into power and gas grids and shift away from subsidies
to markets and competition with natural gas will become major issues [53].
Biomethane should meet some standard specifications with respect to storage and
transport before it can be practical injection into existing natural gas networks. The
presence of various components in different concentrations make it difficult to inject
biogas to the grid hence the need for upgrading. Pipeline designers should know the
exact he thermodynamic properties of a gas mixture are, particularly in terms of
density, and heating value which may tend to vary greatly in biogas [54].
Biomethane has a very important role to play in the transition to renewable sources
of energy. Demand based production of biomethane for power generation directly links
the gas grid and the electricity grids which can help in balancing the power grid. The gas
grid will shift from fossil fuel distribution to provide energy balancing service provider
with short-term as well as seasonal storage options. There is increasing integration of
decentralized biomethane feed-in into the gas grid for the gas grid infrastructure thus
introducing new challenges. There are examples in Germany of facilities that feed in
more biomethane to the local distribution network than the total discharge which leads
to the need to compress the excess gas and transfer it to a higher level [24, 53].
Production of biomethane from energy crops has a negative impact on agriculture.
On the other hand, use of digestion residues as fertilizers close to biogas production
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site improves local nutrient cycles. Production of energy intensive nitrogen fertilizers
and use of declining global phosphorous reserves can be avoided by use of bio-
fertilizer from the digesters [8, 18].
Biomethane is the most efficient biofuel in terms of fuel production equivalent per
area of crop land needed and is therefore expected to a larger role in the fuel/energy
market because of government support, growing use in NGVs and reduction in GHG
emissions. There is growing awareness of biomethane and a shift in perception from
regarding biomethane as a sub-branch of biomass production to an independent
renewable energy resource. And legislation and strategies are recognizing biomethane
as an independent energy resource [53].
The main sustainability challenge facing biomethane market is cost of subsidies
and need for free market competition with fossil natural gas which can be accelerated
if the market price of natural gas rises. The European cap-and-trade for greenhouse
gas emissions GHGs is another driving factor for the future. Since use of biomethane
omits GHG emissions, there will not be compensation or penalties in form of GHG
certificates [37, 53].
The evolution of biomethane markets is expected to create their own demand and
supply and also enable and exchange between different countries since the green gas
product market open to international trade. Each country has tended to create its own
set of biomethane support schemes to address individual situations and are therefore
designed with to address the priorities and challenges of specific countries. For
biomethane market to grow, countries should open up their support schemes to
biomethane imported from neighboring countries to encourage international trade in
biomethane [53].

4.4 Electricity from biomethane

Biomethane can be used as fuel for power generation in various prime movers.
They include internal combustion engines, gas turbines of varying sizes, fuel cells,
among other. The efficiency can be improved through combustion and conversion in
set ups like cogeneration and tri-generation schemes [6, 37].
Diesel engines can run on biomethane as a direct substitute of natural gas. Biomethane
used can made from biogas upgrading or gasification and methanation schemes [55].
Diesel engines would perform efficiently whether using pure diesel or when running in
dual fuel mode as long as the calorific value of fuel is controlled [55, 56].
Electricity from biomethane can be used directly onsite to avoid or limits electric-
ity imports from the grid while excess generated electricity within the design of
decentralized power generation systems using a wide range of prime movers for the
electric generators e.g. turbines, internal combustion engines, fuel cells, etc.
Biomethane can converted to hydrogen fuel for wide renewable applications or used
in fuel cells for direct conversion. Various pathways for use of biomethane for power
generation are summarized in Table 4 below.
From Table 4, it is biomethane can be used through various conversion technolo-
gies with varying characteristics in thermal and electricity generation. The conversion
can be done in cogeneration, trigeneration and open conversion systems. Prime
movers that can use biomethane include internal combustion engines, gas turbines,
fuel cells, and Stirling engines as well as production of fuels for application in trans-
port, heat and electricity generation.
In the transport sector, biomethane has a double role to play in emissions reduction
i.e. as a direct fuel substitute of fossil fuels and as feedstock for production of biofuels/
31
Anaerobic Digestion – Biotechnology for Reactor Performance and Environmental Sustainability

Device/ Application Remarks

technology

1 Fuel cell Generation of electricity Very efficient, reliable but expensive

2 Hydrogen Biomethane can be converted to Renewable hydrogen process if its biogas

production hydrogen for use as combustion fuel or or methane. Hydrogen is manufactured
electricity and process chemicals. by dry reforming, steam reforming, or
hydrolysis

3 Bio-methanation Biomethane can be fed to natural gas Renewable replacement of fossil natural
supply as substitute for natural gas gas is feasible with use of biomethane

4 Diesel engine Biomethane can be used as a diesel Diesel engines have more fuel flexibility
engine fuel in either dual fuel mode or and efficiency and can easily use easily
pure gas engines use biofuels as fossil fuel substitutes

5 Gas/petrol Biomethane can be used as a fuel for Less efficient than diesel engines but are
engine petrol or gasoline engines with little or easier to convert to biogas fueled engines.
no modification

6 Stirling engine Stirling engines are also called hot air Stirling engines have fuel flexibility and
engines. can run on a wider range of fuels

7 Gas turbine Based on size, gas turbines can be Turbines are simple in construction, are
micro, small, and large gas turbines in versatile and can use raw biogas as well as
open, closed or combined cycle biomethane and easy to operate.
configuration

8 Cogeneration In cogeneration, biomethane is burn to Cogeneration with biomethane as a fuel

simultaneously produce useful heat and can be applied on various conversion
electricity. systems like Stirling engines, diesel
engines, gas turbines, hydrogen and fuel
cells to increase system efficiency

9 Trigeneration Trigeneration refers to generation of Trigeneration is the most efficient

electricity and both heating and cooling conversion system but more complex and
from same fuel/energy resource expensive
simultaneously

Table 4.
Summary of biogas to electricity conversion systems and technologies [2, 6].

chemicals through the Fischer-Tropsch (FT) Process e.g. diesel, jet fuel, and gasoline,
and through reforming processes to produce hydrogen and methanol [6, 28].
Biomethane production and use has less environmental impact, but is still associated
with some greenhouse gas emissions like CO2, CH4 and N2O whose quantities depend
on the technology applied and the source of biogas or feedstock used. Biomethane use
can reduce the negative environmental impact and pollution potential while anaerobic
digestion and gasification used to produce biogas and syngas can be used for hygiene
and of bio wastes further keeping the environment clean and healthy [57–59].

5. Results and discussion

5.1 Summary

Biogas is a product of the anaerobic digestion process with many applications as in

generation of renewable energy. The main component of biogas with energy value is
methane, but has impurities like moisture, carbon dioxide, siloxanes, hydrogen
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sulfide, siloxanes, hydrocarbons, oxygen, ammonia, oxygen, carbon monoxide and

nitrogen whose presence is undesirable as they reduce the calorific value of biogas and
create operational problems in the energy systems. This necessitate biogas cleaning
and application of multi-stage technologies to produce upgraded biogas called
biomethane. Biomethane gas is a flexible and easy to store fuel with similar properties
and applications as natural gas with no need to modify any equipment settings for
natural gas devices and equipment.
Technologies that are commercially available for operating biogas upgrading bio-
gas today include amine scrubbers, water scrubbers, PSA units, organic scrubbers and
membrane units. Cryogenic upgrading technology though interesting has some
important operational challenges that have to be resolved. For medium scale
upgrading schemes all the most common upgrading options are feasible. The scrub-
bing technologies have proved to be effective and efficient and have similar costs of
investment and operation. The water scrubber is a preferred choice for many applica-
tions due to the simplicity and reliability, but the high purity and very low methane
slip from amine scrubbers are notable characteristics. The pressure swing adsorption
(PSA) and membrane units, have similar investment costs as the scrubbers. Advances
in the membrane technology has also led to low methane slips with this technology,
which is a notable progress [1, 6].
The physical and chemical biogas upgrading technologies are the technologies that
are currently mature and have reached near optimum technical and economic feasi-
bility. There main limitation is huge energy consumption which limit expansion of the
biogas market. Upgrading technology selection may depend on factors like costs,
quality of products, location, and technology maturity and requirements [26]. It is
possible to attain 98% methane content with a water scrubber but the content of
oxygen and nitrogen in the raw biogas cannot be separated in the water scrubber and
therefore should be controlled. Oxygen and nitrogen can also be transported with
water from the aerated desorption column to the absorption column. With methane
composition of 50% in raw biogas, composition of oxygen and nitrogen can double in
the final product after upgrading [29, 32].
Biological methods are the less explored methods for industrial-scale testing and
optimization scenarios, even show they promise significant potential in terms of
techno-economic feasibility with new horizons in hybrid renewable energy applica-
tions. There also exist large gaps between pilot scale and commercial scale technology
applications like hydrate separation, cryogenic separation, biotechnologies, and
chemo lithotrophic-based bioreactors. The main limitation to wide scale use of
biomethane is high cost remains a major limitation to widespread biogas
application [26].

5.2 Production pathways for biogas

Biomethane is produced by processing raw biogas through multiple steps and by of

methane from other components. The various pathways for biogas and biomethane
production and applications are summarized in Figure 13.
Figure 13, shows various pathways for production of biogas and related by-
products, biogas purification, upgrading and various energy applications. The raw
materials for biogas production undergo pre-treatment and then fed to the biodigester
to undergo anaerobic digestion to produce biogas and digestate. Digestate is applied as
green manure, animal bedding, and application like crop irrigation to enhance agri-
cultural production. Raw biogas is purified mainly by desulphurization for
33
Anaerobic Digestion – Biotechnology for Reactor Performance and Environmental Sustainability

Figure 13.
Pathways for production and application f biogas and biomethane [6].

applications like boilers, cogeneration and engines. Raw biogas can also be upgraded
and applied in catalytic reforming for production of hydrogen and biofuels, liquefied
to produce bio-liquified natural gas (bio-LNG) or compressed to produce bio-
compressed natural gas (bio-CNG) [2, 6, 35].

5.3 Processes comparisons

Biomethane has become an important renewable energy resource for heat and
power generation and industrial applications as a feedstock. Applications of
biomethane include a transport fuel as a substitute for natural gas, diesel and liquid
natural gas, thermal applications i.e. steam and heat generation, combined heat and
power, tri-generation, and injection into the natural gas grid upon meeting certain
requirements. Biomethane can be manufactured through upgrading of biogas or by
gasification followed by methanation process. The approaches in biomethane produc-
tion have similar efficiencies in biomethane production from the energy output and
conservation perspective but since the two technologies have fundamental differences
in process and equipment, the cost of the output varies. The main limitations facing
biomethane production are the high costs of the process while many biological pro-
cesses are still under research and development and are yet to be fully commercialized.
Biomethane technology market is a promising venture globally mainly due to
existence of mature production, conversion technologies and applications Biomethane
remains viable and as a result of abundance in cheap feed stocks supply for anaerobic
digestion and gasification. Biomethane has significant flexibility for domestic and
industrial scale production and use and is promising to be a leading economical
alternative to produce renewable bioenergy.
There are five main biomethane production processes through biogas upgrading.
The techniques are pressure swing adsorption which has an option of temperature
34
Biomethane Production and Applications
DOI: http://dx.doi.org/10.5772/intechopen.112059

swing adsorption, absorption technics based on amine, membrane separation, cryo-

genic separation and biological separation. Biogas upgrading can be significantly
improved by combining a wide range of methods ranging from biological and physi-
cochemical processes and adaptation of technologies in the field of advanced oxida-
tion or anaerobic phototrophs. The treatment of biogas can theoretically apply
biological methods like chemotrophic or phototrophic to remove H2S then start
upgrading using more efficient physicochemical processes. Purification of biogas is
generally a high energy intensive process. But through appropriate choice of a combi-
nation of cleaning and upgrading methods based on the methane purity demand saves
energy as well as minimize methane loss, in large scale operations. The physicochem-
ical processes are more developed and widely used compared to many biological
methods which are still new and not yet commercialized, but they offer significant
huge potential in terms of efficiency, feasibility, and technological easiness. Biological
methods of upgrading biogas open new horizons for integration of different forms of
renewable energy besides electricity, storage advances and decoupling bioenergy pro-
duction from the availability of biomass resources. The various processes have differ-
ent benefits and limitation. The water scrubbing can simultaneously remove CO₂, H₂S,
NH₃ and dust but consumes a lot of water. On the other hand, the pressure swing
adsorption technique requires biogas pretreatment since water and hydrogen sulfide
can damage the adsorbents but, has low energy requirement. The advantage of Amine
absorption is low methane loss and produces high quality CO₂, although it has high
energy consumption. The advantage of the membrane permeation systems is there
compactness and ease of operation but yields a relatively low methane (CH₄) purity.
Biomethane can be used in power generation using prime movers gas turbines, micro
turbines, diesel engines, petrol engines, Stirling engines. Other applications are hydro-
gen production, manufacture of transport fuels, fuel for cogeneration and trigeneration,
compression to bio-CNG and LPG, syngas production, methanol production.

5.4 Applications

Biogas has multiple applications which include heat and electricity generation.
Electricity can be produced from biogas at sewage works, by means of a combined
heat and power (CHP) engine, gas turbines, petrol engines, modified diesel engines of
dual engines, among others. Biogas and biomethane can also be used as a fuel in
automobiles to power an internal combustion engine or a fuel cell in cleaner processes
compared to use of fossil fuels [2, 5, 6, 60].
Upgraded biogas or biomethane can attain same properties as natural gas and
hence be used as a substitute fuel for natural gas as green natural gas. Biomethane can
be injected to natural gas pipelines for use in applications domestic heating and
cooking, power generation and feedstock for many industrial processes [6, 35, 61].

6. Conclusions

Biogas is a product of the anaerobic digestion process with many applications as in

generation of renewable energy. The main component of biogas with energy value is
methane, but has impurities like moisture, carbon dioxide, siloxanes, hydrogen sul-
fide, siloxanes, hydrocarbons, oxygen, ammonia, oxygen, carbon monoxide, and
nitrogen whose presence is undesirable as they reduce the calorific value of biogas and
create operational problems in the energy systems. This necessitates biogas cleaning
35
Anaerobic Digestion – Biotechnology for Reactor Performance and Environmental Sustainability

and upgrading to biomethane. Biomethane gas is a flexible and easy to store as a fuel
having similar properties and applications as natural gas with little or no modifications
to the natural gas equipment.
Biogas upgrading methods can be classified into physical, chemical and biological
methods like water scrubbing, physical absorption, pressure swing adsorption, cryo-
genic separation, membrane separation, chemical scrubbing, chemoautotrophic
methods, photosynthetic upgrading and desorption. The physical and chemical
upgrading technologies have almost reached optimal level but still have high energy
requirements. High-pressure water scrubbing is more economic for small-sized plants,
but potassium carbonate scrubbing has high net value for large-sized plants. There-
fore, physicochemical methods are technologically ready compared to biological
methods which are still new and not yet commercially available, although they offer
huge potential in respect to feasibility, technological easiness, and potential. Through
biological upgrading new opportunities for integrating different forms of renewable
energy are availed besides upgrading including electricity storage advances and
decoupling bioenergy production from availability of biomass.
Biogas can be cleaned or purified to remove harmful components like moisture and
H2S without necessarily upgrading to biomethane which is mainly about the removal
of Biogas. removal of CO2. Some upgrading methods remove other impurities in
addition to CO2, while others require upfront removal of H2O and H2S. Raw biogas
cleaning/treatment and upgrading which enables the use of biogas in applications like
vehicles fuel or for injection into the natural gas grid as a substitute for natural gas.
There have been significant developments over the last few years, in the field of
biogas cleaning and upgrading through process improvements and development of
new technologies although water scrubbing, PSA and amine scrubbing currently
dominate the market. Membrane separation is a technology is while organic physical
scrubbers have limited share of biogas upgrading market. Cryogenic upgrading tech-
nologies, which are potentially the best choice for combination with liquefaction of
biomethane, still face operational challenges that may be resolved.
The market for biomethane globally is promising mainly due to existence of
mature production, energy conversion technologies and applications and abundance
of cheap feed stocks for anaerobic digestion and gasification. Biomethane has signifi-
cant flexibility for domestic and industrial scale production and use and is promising
to be a leading economical alternative to natural gas.
The main limitations facing biomethane production are high costs of the process
while others like biological techniques are still under development. Biogas upgrading
can be significantly improved by combining a wide range of methods ranging from
biological and physicochemical processes and adaptation of technologies in the field of
advanced oxidation or anaerobic phototrophs. The treatment of biogas can theoreti-
cally apply biological methods like chemotrophic or phototrophic to remove H2S then
start upgrading using more efficient physicochemical processes. Purification of biogas
is generally a high energy intensive process. But through appropriate choice of a
combination of cleaning and upgrading methods based on the methane purity demand
saves energy as well as minimize methane loss, in large scale operations. The physico-
chemical processes are more developed and widely used compared to many biological
methods which are still new and not yet commercialized, but they offer significant
huge potential in terms of efficiency, feasibility, and technological easiness. Biological
methods of upgrading biogas open new horizons for integration of different forms of
renewable energy besides electricity, storage advances and decoupling bioenergy pro-
duction from the availability of biomass resources.
36
Biomethane Production and Applications
DOI: http://dx.doi.org/10.5772/intechopen.112059

Biomethane can be used in power generation using various available uses like the
use of gas turbines, micro-turbines, diesel engines, petrol engines, Stirling engines
besides thermal applications as a biofuel for transport and industrial applications. The
tracks for production of fuels biomethane are compressing to produce (CBG) or
liquefying to make (LBG), hydrogen production, methanol, production, DME, and FT
diesel.
Biomethane can be manufactured through upgrading of biogas or by gasification
followed by methanation process. The approaches in biomethane production have
similar efficiencies in biomethane production from the energy output and conserva-
tion perspective but since the two technologies have fundamental differences in
process and equipment, the cost of the output varies. High initial and operating costs
remain the limiting factor facing the biomethane technology market, while several
promising technologies are still under research and development.

Acknowledgements

The authors wish to appreciate researchers and scholars in the field of geothermal
energy and electricity for providing significant, credible, and reliable information in
all aspects of geothermal energy with ready access. This made the production of this
study successful.

Funding

There is no funding provided for this research and the whole exercise was fully
funded the researchers.

Author’s contribution

The first author conceptualized the manuscript and produced the draft for
review by the second author who also facilitated funding for publication of the
manuscript.

Availability of data

The research has provided all data and information used and did not use any
undeclared data and information. However, any datasets used and/or analyzed
during the current study are available from the corresponding author on reason-
able request.

Conflict interest

The authors declare that they have no conflict of interest.

37
Anaerobic Digestion – Biotechnology for Reactor Performance and Environmental Sustainability

Consent for publication

The authors have authority to publish the research work.

Ethical approval and consent to participate

Not applicable.

Author details

Moses Jeremiah Barasa Kabeyi* and Oludolapo Akanni Olanrewaju

Industrial Engineering Department, Durban University of Technology,
Durban, South Africa

*Address all correspondence to: 2206469@dut4life.ac.ke; moseskabeyi@yahoo.com

© 2023 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of
the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0),
which permits unrestricted use, distribution, and reproduction in any medium, provided
the original work is properly cited.
38
Biomethane Production and Applications
DOI: http://dx.doi.org/10.5772/intechopen.112059

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