of these methods is high bio-methane recovery (typically above 97%). The drawbacks associated with
liquid solutions for CO 2 and H 2 S removal are the
high energy requirement, especially in regeneration
of adsorbents, selectivity of chemicals used, negative
environmental impact from waste liquids, and high
corrosion rate (Awe et al., 2017).
Membrane separation relies on the principle of
selective permeability of membranes allowing the
separation of the biogas components. The technology is effective for the removal of CO 2 , H 2 S, and
moisture from raw biogas. The advantages of membrane separation are the process is compact, light
in weight (thin membranes used), has low energy
and maintenance requirements, and easy to process.
The drawbacks are high membrane costs and maintenance costs since commercial membranes are fragile
(Ryckebosch, Drouillon, & Vervaeren, 2011).
Cryogenic separation exploits the fact that different gases liquefy under different temperature–pressure
conditions. The difference in boiling points of biogas constituents can be exploited to separate other
gases like CO 2 , H 2 S, N 2 , O 2 , and siloxanes from
methane at high pressure above 80 bar and low
temperatures of up to −160
◦ C (Awe et al., 2017).
This technique can purify raw biogas to produce a
high-purity product with a methane concentration of
90%–99%. The limitations of this process are that it
requires high capital and operating costs due to cooling and compression by a large amount of equipment
and instruments such as compressors, turbines, heat
exchangers, and distillation columns (Xiao, Avalos,
Vinh, & Kaliaguine, 2015).
Adsorption is a relatively low-cost and effective process for biogas purification. Adsorption is deemed the
most economical and feasible of all biogas purification
techniques (Abdullah, Mat, Aziz, & Roslan, 2017).
Adsorbent materials are able to selectively retain some
compounds of a mixture by molecular size. Adsorption using processes such as pressure swing adsorption
(PSA) separates the different gases from biogas based
on their molecular characteristics and the affinity of
the adsorbent material. This method is advantageous
in that, high bio-methane recovery (95%–99%) is
achieved and the gas can be directly delivered at high
pressures. However, disadvantages associated with it
are high investment and operational costs and an extensive process control is needed, hence it cannot be used
for small-scale application. In fact, the most difficult
aspect of PSA operation is controlling the high temperature and pressure, which has limited the application
of this method on a wider scale (Noorain, Kindaichi,
Ozaki, Aoi, & Ohashi, 2019).
Adsorption under ambient conditions using a variety of micro-porous materials, such as activated carbons, zeolites, and metal–organic frameworks have
been considered to carry out CO 2 separation (Durán,
Álvarez-Gutiérrez, Rubiera, & Pevida, 2018). The utilization of activated carbon offers advantages due to
its high adsorption capacity at ambient conditions,
low regeneration cost, long-term stability, and fast
kinetics. The utilization of activated carbon for biogas
upgrading is extensive; it can be used for the removal
of CO 2 and H 2 S from biogas (Vivo-Vilches et al.,
2017). The production of carbon adsorbents from
biomass feedstocks can involve physical or chemical activation to develop the porosity. Generally prior
cleaning, washing, and drying of carbon-rich materials are required. There are two main steps for the
preparation of activated carbon: the first step is the carbonization of carbonaceous raw material below 800
◦ C
under an inert atmosphere. The next step is activation
of the carbonized product by a physical or chemical
method (Mdoe, 2014). Activation chemicals include
potassium iodide, zinc acetate, potassium hydroxide,
potassium carbonate, and sodium hydroxide (Zulkefli
et al., 2019).
Hydrogen sulfide can be removed using a catalyst of
iron oxide in the form of oxidized steel wool or chips of
iron cut from the lathe operation of any workshop. Iron
chips are often disposed of from workshops as they are
of no great value. These can therefore be utilized in
biogas cleaning after being exposed in air. When raw
biogas comes into contact with steel wool/chips, iron
oxide gets converted to iron sulfide which forms elemental sulfur when exposed to air (Shah & Nagarseth,
2015). This review evaluates the performance obtained
from various studies using activated carbon and iron
oxide for the removal of moisture, H 2 S, and CO 2 from
raw biogas.
3 LOW-COST ADSORBENTS FOR BIOGAS
PURIFICATION
3.1 Activated carbon
Agricultural wastes and by-products such as wood
residues, fruit peels, hulls of rice, and coconut shells
are potential sources of activated carbon for biogas
purification (Durán et al., 2018). Coconut shells are
appropriate for the preparation of activated carbon due
to their high carbon content and low ash content. During the activation process, the spaces between the elementary crystallites become cleared of less organized,
loosely bound carbonaceous material. The resulting
channels through the graphitic regions, the spaces
between the elementary crystallites, together with the
tissues within and parallel to the graphitic planes constitute the porous structure, with large surface area
(Marshall, Ahmedna, Rao, & Johns, 2000).
3.1.1 Carbon(IV) oxide removal using activated
carbon
Activated carbon can adsorb CO 2 from raw biogas due
to its high porosity. The changes in molecular dipoles
of CO 2 through an asymmetric bond stretching motion
results in some permanent polarity in CO 2 molecules
at room temperature and pressure. The charged CO 2
molecules undergo adhesion and Van der Waals attraction towards the high surface area of activated carbon
(Mamun, Karim, Rahman, Asiri, & Torii, 2016). The
presence of water vapor in raw biogas affects the
CO 2 capture process design. CO 2 adsorption capacity
226
liquid solutions for CO 2 and H 2 S removal are the
high energy requirement, especially in regeneration
of adsorbents, selectivity of chemicals used, negative
environmental impact from waste liquids, and high
corrosion rate (Awe et al., 2017).
Membrane separation relies on the principle of
selective permeability of membranes allowing the
separation of the biogas components. The technology is effective for the removal of CO 2 , H 2 S, and
moisture from raw biogas. The advantages of membrane separation are the process is compact, light
in weight (thin membranes used), has low energy
and maintenance requirements, and easy to process.
The drawbacks are high membrane costs and maintenance costs since commercial membranes are fragile
(Ryckebosch, Drouillon, & Vervaeren, 2011).
Cryogenic separation exploits the fact that different gases liquefy under different temperature–pressure
conditions. The difference in boiling points of biogas constituents can be exploited to separate other
gases like CO 2 , H 2 S, N 2 , O 2 , and siloxanes from
methane at high pressure above 80 bar and low
temperatures of up to −160
◦ C (Awe et al., 2017).
This technique can purify raw biogas to produce a
high-purity product with a methane concentration of
90%–99%. The limitations of this process are that it
requires high capital and operating costs due to cooling and compression by a large amount of equipment
and instruments such as compressors, turbines, heat
exchangers, and distillation columns (Xiao, Avalos,
Vinh, & Kaliaguine, 2015).
Adsorption is a relatively low-cost and effective process for biogas purification. Adsorption is deemed the
most economical and feasible of all biogas purification
techniques (Abdullah, Mat, Aziz, & Roslan, 2017).
Adsorbent materials are able to selectively retain some
compounds of a mixture by molecular size. Adsorption using processes such as pressure swing adsorption
(PSA) separates the different gases from biogas based
on their molecular characteristics and the affinity of
the adsorbent material. This method is advantageous
in that, high bio-methane recovery (95%–99%) is
achieved and the gas can be directly delivered at high
pressures. However, disadvantages associated with it
are high investment and operational costs and an extensive process control is needed, hence it cannot be used
for small-scale application. In fact, the most difficult
aspect of PSA operation is controlling the high temperature and pressure, which has limited the application
of this method on a wider scale (Noorain, Kindaichi,
Ozaki, Aoi, & Ohashi, 2019).
Adsorption under ambient conditions using a variety of micro-porous materials, such as activated carbons, zeolites, and metal–organic frameworks have
been considered to carry out CO 2 separation (Durán,
Álvarez-Gutiérrez, Rubiera, & Pevida, 2018). The utilization of activated carbon offers advantages due to
its high adsorption capacity at ambient conditions,
low regeneration cost, long-term stability, and fast
kinetics. The utilization of activated carbon for biogas
upgrading is extensive; it can be used for the removal
of CO 2 and H 2 S from biogas (Vivo-Vilches et al.,
2017). The production of carbon adsorbents from
biomass feedstocks can involve physical or chemical activation to develop the porosity. Generally prior
cleaning, washing, and drying of carbon-rich materials are required. There are two main steps for the
preparation of activated carbon: the first step is the carbonization of carbonaceous raw material below 800
◦ C
under an inert atmosphere. The next step is activation
of the carbonized product by a physical or chemical
method (Mdoe, 2014). Activation chemicals include
potassium iodide, zinc acetate, potassium hydroxide,
potassium carbonate, and sodium hydroxide (Zulkefli
et al., 2019).
Hydrogen sulfide can be removed using a catalyst of
iron oxide in the form of oxidized steel wool or chips of
iron cut from the lathe operation of any workshop. Iron
chips are often disposed of from workshops as they are
of no great value. These can therefore be utilized in
biogas cleaning after being exposed in air. When raw
biogas comes into contact with steel wool/chips, iron
oxide gets converted to iron sulfide which forms elemental sulfur when exposed to air (Shah & Nagarseth,
2015). This review evaluates the performance obtained
from various studies using activated carbon and iron
oxide for the removal of moisture, H 2 S, and CO 2 from
raw biogas.
3 LOW-COST ADSORBENTS FOR BIOGAS
PURIFICATION
3.1 Activated carbon
Agricultural wastes and by-products such as wood
residues, fruit peels, hulls of rice, and coconut shells
are potential sources of activated carbon for biogas
purification (Durán et al., 2018). Coconut shells are
appropriate for the preparation of activated carbon due
to their high carbon content and low ash content. During the activation process, the spaces between the elementary crystallites become cleared of less organized,
loosely bound carbonaceous material. The resulting
channels through the graphitic regions, the spaces
between the elementary crystallites, together with the
tissues within and parallel to the graphitic planes constitute the porous structure, with large surface area
(Marshall, Ahmedna, Rao, & Johns, 2000).
3.1.1 Carbon(IV) oxide removal using activated
carbon
Activated carbon can adsorb CO 2 from raw biogas due
to its high porosity. The changes in molecular dipoles
of CO 2 through an asymmetric bond stretching motion
results in some permanent polarity in CO 2 molecules
at room temperature and pressure. The charged CO 2
molecules undergo adhesion and Van der Waals attraction towards the high surface area of activated carbon
(Mamun, Karim, Rahman, Asiri, & Torii, 2016). The
presence of water vapor in raw biogas affects the
CO 2 capture process design. CO 2 adsorption capacity
226