as engine fuel. The high content of carbon dioxide
contaminants reduces the energy density of biogas.
Furthermore, the contaminants like hydrogen sulfide affect the internal parts of engine by causing
corrosion (Magomnang et al. 2014) In addition, biogas contaminants affect the environment and human
health.
The significance of biogas as a bioenergy can be
increased by purification and upgrading in a process
that entails the removal of contaminants by various
methods. Commercial processes for the upgrade of
biogas include membrane separation, cryogenic separation, pressure swing adsorption, water scrubbing,
physical scrubbing, chemical absorption, hydrate formation, and biological conversion process (Khan et al.
2017; Q. Sun et al. 2015;. Though these methods have
high adsorption capacity, they have limitations, which
include high-energy requirements and initial investment cost. Biogas can also be purified using natural
minerals like clay (Chen et al. 2014), zeolite (Paolini
et al. 2015), and coal fly ash (Ferella et al. 2017).
They are low cost and have simple operation methods.
However, natural minerals adsorbents are not available
everywhere. Therefore, biomass adsorbent materials
have become an alternative choice for biogas cleaning.
Biomass is the most abundant material on the Earth’s
surface. In addition, it is environmentally friendly and
cheap. Furthermore, biomass materials can be converted to adsorbents, such as activated carbon and
biochar.
Activated carbon can be derived from any rich carbon organic materials like agricultural waste, plant
biomass, industrial waste, and household wastes
(Rodriguez-reinoso et al. 2008). It can also be prepared from fossil carbon (Hsu & Teng 2000). However,
fossil sources are the main cause of global warming.
Therefore, the preparation of activated carbon from
plants, agriculture waste, or industrial waste is a more
sustainable and preferable method of preparing carbon adsorbents. Biochar is another cheap and widely
available material from biomass that can be used for
biogas cleaning. It can be produced from carbon-rich
materials like agricultural wastes. Biomass adsorbents
have been used for various applications, including air
pollution control (Nor et al. 2013), battery electrodes
and capacitors (Kalyani & Anitha 2013), water and
wastewater treatment (Bhatnagar et al. 2015; Wong
et al. 2018), remediation of heavy metals like arsenic
(Asadullah, Jahan, & Boshir 2014), and adsorbent for
carbon dioxide capture (Rashidi et al. 2013).
The process of producing adsorbents from biomass
materials may entail physical or chemical treatments.
The characteristics of biomass adsorbents, which
include the surface area and pore size, are determined
by the substrates used and the process conditions (Jung
et al. 2019).This paper reviews the documented studies
on the removal of biogas contaminations; carbon and
hydrogen sulfide using biochar and activated carbon
derived from biomass. The summary of characteristics and potential of biomass adsorbents in cleaning of
biogas are also given.
2 EFFECT OF BIOGAS CONTAMINATIONS
Carbon dioxide is a greenhouse gas which is emitted in
large scale from industries, households, vehicles, and
other fuel machines. Furthermore, it is the main contaminant in biogas and has the highest contribution to
global warming (Na et al. 2002). The process of global
warming starts with the short-wave length solar radiations hitting the Earth’s surface. Some radiations are
absorbed by the Earth while others are reflected to the
atmosphere. Those reflected are in infrared form and
are absorbed by greenhouse gases like carbon dioxide and cause an increase in atmospheric temperature,
thereby causing global warming. Further, the carbon
dioxide content in biogas affects the internal metallic parts when used as fuel for the engine. Basically,
the high content of carbon dioxide reduces biogas
calorific value, flammability range, and flame velocity
(Porpatham et al. 2008).
The hydrogen sulfide contaminants in biogas affect
the internal engine parts. A low content of hydrogen
sulfide, 50–10,000 ppm is sufficient to cause corrosion and wear on metallic components (Al Mamun et
al. 2015; Sevimo˘ glu & Tansel 2013). Hydrogen sulfide
reacts with engine metal components and forms sulfur
dioxide and water. Sulfur dioxide and sulfur trioxide
react with water to form corrosive sulfurous acid and
sulfuric acid, respectively. Therefore, traces of hydrogen sulfide in biogas are undesirable and should be
eliminated before usage as engine fuel. In addition,
the corrosion caused reduces the effectiveness of the
heat exchanger (Razbani et al. 2011). Also, hydrogen
sulfide affects human health by causing headaches,
dizziness, respiratory illness, and poor memory (Bates
et al. 2002). Hence, biogas should be purified before
application in engines.
3 BIOMASS ADSORBENTS FOR REMOVAL OF
CARBON DIOXIDE FROM BIOGAS
3.1 Biochar
Biochar is a cheap abundant material with high carbon
content. It is produced from biomass materials and has
wide applications in wastewater treatment (Qambrani
et al. 2017), soil remediation (Saifullah et al. 2018),
and the absorption of carbon dioxide (Madzaki et al.
2016). The main elements in biochar include carbon,
hydrogen, oxygen, and ash. It may also contain traces
of sulfur and hydrogen. However, the actual composition may vary depending on the substrate type and
production process. Biochar can be produced by various processes including pyrolysis, gasification, and
hydrothermal carbonization (HTC). In the pyrolysis
process, biomass materials are decomposed at temperatures between 350–900
◦ C in the absence of oxygen
(Méndez et al. 2014). The main product of the process is syngas (CO, H 2 , and hydrocarbon gases), while
biochar is a by-product. Biochar production through
gasification entails partial oxidation of biomass by
287
contaminants reduces the energy density of biogas.
Furthermore, the contaminants like hydrogen sulfide affect the internal parts of engine by causing
corrosion (Magomnang et al. 2014) In addition, biogas contaminants affect the environment and human
health.
The significance of biogas as a bioenergy can be
increased by purification and upgrading in a process
that entails the removal of contaminants by various
methods. Commercial processes for the upgrade of
biogas include membrane separation, cryogenic separation, pressure swing adsorption, water scrubbing,
physical scrubbing, chemical absorption, hydrate formation, and biological conversion process (Khan et al.
2017; Q. Sun et al. 2015;. Though these methods have
high adsorption capacity, they have limitations, which
include high-energy requirements and initial investment cost. Biogas can also be purified using natural
minerals like clay (Chen et al. 2014), zeolite (Paolini
et al. 2015), and coal fly ash (Ferella et al. 2017).
They are low cost and have simple operation methods.
However, natural minerals adsorbents are not available
everywhere. Therefore, biomass adsorbent materials
have become an alternative choice for biogas cleaning.
Biomass is the most abundant material on the Earth’s
surface. In addition, it is environmentally friendly and
cheap. Furthermore, biomass materials can be converted to adsorbents, such as activated carbon and
biochar.
Activated carbon can be derived from any rich carbon organic materials like agricultural waste, plant
biomass, industrial waste, and household wastes
(Rodriguez-reinoso et al. 2008). It can also be prepared from fossil carbon (Hsu & Teng 2000). However,
fossil sources are the main cause of global warming.
Therefore, the preparation of activated carbon from
plants, agriculture waste, or industrial waste is a more
sustainable and preferable method of preparing carbon adsorbents. Biochar is another cheap and widely
available material from biomass that can be used for
biogas cleaning. It can be produced from carbon-rich
materials like agricultural wastes. Biomass adsorbents
have been used for various applications, including air
pollution control (Nor et al. 2013), battery electrodes
and capacitors (Kalyani & Anitha 2013), water and
wastewater treatment (Bhatnagar et al. 2015; Wong
et al. 2018), remediation of heavy metals like arsenic
(Asadullah, Jahan, & Boshir 2014), and adsorbent for
carbon dioxide capture (Rashidi et al. 2013).
The process of producing adsorbents from biomass
materials may entail physical or chemical treatments.
The characteristics of biomass adsorbents, which
include the surface area and pore size, are determined
by the substrates used and the process conditions (Jung
et al. 2019).This paper reviews the documented studies
on the removal of biogas contaminations; carbon and
hydrogen sulfide using biochar and activated carbon
derived from biomass. The summary of characteristics and potential of biomass adsorbents in cleaning of
biogas are also given.
2 EFFECT OF BIOGAS CONTAMINATIONS
Carbon dioxide is a greenhouse gas which is emitted in
large scale from industries, households, vehicles, and
other fuel machines. Furthermore, it is the main contaminant in biogas and has the highest contribution to
global warming (Na et al. 2002). The process of global
warming starts with the short-wave length solar radiations hitting the Earth’s surface. Some radiations are
absorbed by the Earth while others are reflected to the
atmosphere. Those reflected are in infrared form and
are absorbed by greenhouse gases like carbon dioxide and cause an increase in atmospheric temperature,
thereby causing global warming. Further, the carbon
dioxide content in biogas affects the internal metallic parts when used as fuel for the engine. Basically,
the high content of carbon dioxide reduces biogas
calorific value, flammability range, and flame velocity
(Porpatham et al. 2008).
The hydrogen sulfide contaminants in biogas affect
the internal engine parts. A low content of hydrogen
sulfide, 50–10,000 ppm is sufficient to cause corrosion and wear on metallic components (Al Mamun et
al. 2015; Sevimo˘ glu & Tansel 2013). Hydrogen sulfide
reacts with engine metal components and forms sulfur
dioxide and water. Sulfur dioxide and sulfur trioxide
react with water to form corrosive sulfurous acid and
sulfuric acid, respectively. Therefore, traces of hydrogen sulfide in biogas are undesirable and should be
eliminated before usage as engine fuel. In addition,
the corrosion caused reduces the effectiveness of the
heat exchanger (Razbani et al. 2011). Also, hydrogen
sulfide affects human health by causing headaches,
dizziness, respiratory illness, and poor memory (Bates
et al. 2002). Hence, biogas should be purified before
application in engines.
3 BIOMASS ADSORBENTS FOR REMOVAL OF
CARBON DIOXIDE FROM BIOGAS
3.1 Biochar
Biochar is a cheap abundant material with high carbon
content. It is produced from biomass materials and has
wide applications in wastewater treatment (Qambrani
et al. 2017), soil remediation (Saifullah et al. 2018),
and the absorption of carbon dioxide (Madzaki et al.
2016). The main elements in biochar include carbon,
hydrogen, oxygen, and ash. It may also contain traces
of sulfur and hydrogen. However, the actual composition may vary depending on the substrate type and
production process. Biochar can be produced by various processes including pyrolysis, gasification, and
hydrothermal carbonization (HTC). In the pyrolysis
process, biomass materials are decomposed at temperatures between 350–900
◦ C in the absence of oxygen
(Méndez et al. 2014). The main product of the process is syngas (CO, H 2 , and hydrocarbon gases), while
biochar is a by-product. Biochar production through
gasification entails partial oxidation of biomass by
287