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Biofuels
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Biogas purification processes: review and
prospects
J. E. Castellanos-Sánchez, F. A. Aguilar-Aguilar, R. Hernández‐Altamirano,
José Apolonio Venegas Venegas & Deb Raj Aryal
To cite this article: J. E. Castellanos-Sánchez, F. A. Aguilar-Aguilar, R. Hernández‐Altamirano,
José Apolonio Venegas Venegas & Deb Raj Aryal (2023): Biogas purification processes: review
and prospects, Biofuels, DOI: 10.1080/17597269.2023.2223801
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Biogas purification processes: review and prospects
J. E. Castellanos-S
anchez
a
, F. A. Aguilar-Aguilar
b,c
, R. Hern
andez-Altamirano
b,c
, Jos
e Apolonio Venegas
Venegas
d
and Deb Raj Aryal
d
a
Facultad de Ciencias Agron
omicas de la Universidad Aut
onoma de Chiapas, Chiapas, M
exico;
b
Centro Mexicano para la Producci
on
m
as Limpia, Instituto Polit
ecnico Nacional, Ciudad de M
exico, M
exico;
c
Laboratorio Nacional de Desarrollo y Aseguramiento de la
Calidad de Biocombustibles (LaNDACBio), Instituto Polit
ecnico Nacional, Ciudad de M
exico, M
exico;
d
Catedr
aticos CONAHCYT-UNACH,
M
exico
ABSTRACT
Biogas is generated by controlled anaerobic digestion from animal manure, wastewater, landfills,
industry, agroindustry, or other organic residues. Biogas is mainly composed of 60–70% methane,
30–40% carbon dioxide, and hydrogen sulfide <1%. Pure CH
4
generates approximately 30.67 to
36.68 MJ m
3
or 10 Kw m
3
of energy, being able to compete with conventional fuels. The carbon
dioxide and hydrogen sulfide from biogas decrease the energy potential of methane and damage
equipment. The biogas purified could be a competitive biofuel with conventional fuels. Currently,
there are many physical, chemical, and biological biogas upgrading methods; efficient, promising,
and yield greater than 90% of recovered CH
4
. The biogas upgrading requires expensive infrastruc-
ture and chemical reagents and generates by-products that can cause long-term environmental
problems. For this reason, purification methods must be comprehensive and should contribute to
improving the purity of CH
4
from biogas. This study reviewed the most relevant methods, the
operating conditions to remove CO
2
and H
2
S, as well as the advantages and disadvantages of bio-
gas purification processes.
Abbreviations: CH
4
: methane; IPCC: intergovernmental panel on climate change; CO
2
: carbon
dioxide; PSA: pressure swing adsorption; NOM: motor octane number; AC: alternating current; CaO:
calcium oxide; CV: charcoal; MEA: monoethanolamine; ppm: parts per million; GHG: greenhouse
gases; Ca(OH)
2
: calcium hydroxide; NaOH: sodium hydroxide; KOH: potassium hydroxide; CaCO
3
:
calcium carbonate; H
2
S: hydrogen sulfide; NaHCO
3
: baking soda; AT: ambient temperature; Fe
2
O
3
:
iron oxide; HCl: hydrochloric acid; H
2
O
2
: hydrogen peroxide.
ARTICLE HISTORY
Received 15 March 2023
Accepted 7 June 2023
KEYWORDS
Biogas; purification method;
biomethane
Introduction
Raw biogas (CH
4
,CO
2,
and H
2
S) is generated from the
anaerobic digestion of different organic biomass like agri-
cultural (farmers, animal, residues, organic residues, by-
products, or residues), agro-industrial (residues or by-prod-
ucts from the food chain), or organic wastes from urban
solids [1]. Nevertheless, the intergovernmental panel on cli-
mate change (IPCC) refers that CH
4
and CO
2
are considered
the main greenhouse gases (GHG), methane with 25 times
greater global warming potential (GWP) than CO
2
[2]. One
of the alternatives to reduce GHG emissions is biogas pro-
duction from controlled biodigesters to produce energy.
Biogas purified can be used as a fuel, methane is a hydro-
carbon that can be burnt due to its chemical properties [3–
5]. However, CO
2
and H
2
S in biogas generate problems
during and after combustion. For example, CO
2
decreases
the heating value, and hydrogen sulfide (H
2
S) is highly cor-
rosive, which causes damage to the equipment and instru-
ments [6]. Many authors suggest removing H
2
S and CO
2
from biogas to improve the combustion quality of biogas,
higher energy efficiency, and avoid corrosion of materials.
The biogas purified (CH
4
>95%) could be competitive
mainly with gasoline, diesel, firewood, hydrogen, and nat-
ural gas. This would allow the diversification of quality
biogas, mixing with conventional gases for its commercial-
ization and improving transportation [7]. The main problem
that the industry or rural areas have is the poor quality of
raw biogas from biodigesters which cannot be mar-
keted [8].
Biogas upgrading technologies have focused on CO
2
and H
2
S removal, the upgrading methods have been classi-
fied into physical, chemical, and biological or combined
processes. Studies have shown the efficiency of some
methods in CO
2
removal, through membrane separation,
chemical absorption, aqueous solutions, biological purifica-
tion, pressure swing adsorption (PSA), and cryogenic separ-
ation [9–12]. Chemical adsorption, absorption by iron, and
microbiological separation have been used for the removal
of hydrogen sulfide [13–17]. It is important that purification
processes are efficient, low cost, and easily manipulated,
allowing the end user to have access to these purification
media. In turn, it is required that methane production can
be an integral process, that is, that the use of organic
waste can be efficient and completely sustainable and
reduce the carbon footprint.
This study has the purpose of carrying out a review of
the most used technologies for biogas upgrading, such as
the physical, chemical, and biological processes to remove
CO
2
(carbon dioxide), and H
2
s (hydrogen sulfide),
CONTACT F. A. Aguilar-Aguilar faguilar@ier.unam.mx Centro Mexicano para la Producci
on m
as Limpia, Instituto Polit
ecnico Nacional, Av. Acueducto
s/n, Col. La Laguna Ticom
an, 07340 Ciudad de M
exico, M
exico
ß2023 Informa UK Limited, trading as Taylor & Francis Group
BIOFUELS
https://doi.org/10.1080/17597269.2023.2223801
comparing removal efficiency, the advantages, and the dis-
advantages of the processes.
Physicochemical properties and applications of
methane
Nowadays, there has been a lot of interest in the produc-
tion, use, and applications of biogas worldwide, which seek
to solve environmental problems related to disposing of
waste, reducing GHG, and generating added value to
waste. Therefore, raw biogas, or quality biogas, which is
the last link in this waste recovery process, is a flammable
gas that has a high calorific value [18,19]. Quality biogas
(100% CH
4
) has a heating value of 46 to 55 MJ kg
1
or
30.67 to 36.68 MJ m
3
. But raw biogas with 60% of CH
4
decreases the heating value to 18.38-22 MJ m
3
and is not
competitive with other fuels. At present, there are some
methods to generate biomethane, by anaerobic digestion
or electrolytic (a novel process). In the research carried out
by Gonc¸alves et al. [20], evaluated electrochemistry proc-
esses to generate synthetic biofuel (biomethane) from
liquefied biomass. The operational parameters were ligno-
cellulosic materials, temperature, and varying concentra-
tions of acidified zeolite catalyst. The results show that the
high yield was obtained from biomass evaluated at 300 C,
4% acidified zeolite Y catalyst, and was achieved 35% CH
4
by syngas/biomethane electrolytic production.
According to Cornejo [21] and Gonz
alez [22], methane
has no odor or color, it is made up of a central carbon
atom and four hydrogens (Figure 1)[23], known as a tetra-
hedron or triangle, besides being the shortest of short-
chain hydrocarbons.
Methane generates a complete combustion reaction
with oxygen, as a natural oxidizer or oxidant, during com-
bustion it generates energy (Table 1), one mole of CO
2,
and two moles of water vapor [24,25], as presented in
Equation (1):
CH4þ2O2!CO2g
ðÞ
þ2H2Og
ðÞ
þenergy 890 KJ
mol
(1)
From this combustion reaction (Equation (1)), one mole of
methane can generate up to 890 KJ mol
1
, like natural gas
(900 KJ mol
1
), and greater than hydrogen (286 KJ mol
1
).
It is important mention that CO
2
and H
2
O, products of
combustion, are less polluting than the release of methane
into the environment. Nevertheless, using conventional
fuels (LP gas, biodiesel, gasoline, among others) contributes
to GHG generation and they are not renewable. Hydrogen
can be an alternative to fuels since it is generated from
renewable and non-renewable sources from reforming or
hydrolysis [26,27]. Though, hydrogen has certain disadvan-
tages such as the production process, difficult to store, and
expensive to obtain; that has yet to be further explored for
distribution and commercial use. Also, firewood is used as
fuel in developing countries since its heating value is 16–
22.5 MJ kg
1
(Table 1)[26–29]. The irrational use of fire-
wood has led to deforestation with serious ecological dam-
age, and during combustion generates fine particles (PM
2.5
)
that have caused irreversible problems for users; therefore,
firewood must be replaced by an ecological fuel that is not
harmful to the environment and humans. In this way, the
biogas from anaerobic digestion can be a strategic alterna-
tive for the valuation of organic waste, and be a source of
renewable energy. The comprehensive assessment of
industrial and agro-industrial waste can bring socioeco-
nomic and environmental benefits and local development.
According to Tobares and Tanigawa [29,31], biogas
applications are focused on electricity (8.15-10.18 kW m
3
)
and heat (30.67-36.68 MJ m
3
), or purified methane could
be used as fuel and gas grid, as shown in Figure 2.[32].
According to Saunders [33], one cubic meter of biogas
(60% of methane) generates 6.35 kW of electrical energy.
Likewise, V
azquez [34] demonstrated that one cubic meter
of biogas (>95% of CH
4
) produces 10.18 kW m
3
of nominal
alternating current (AC) power at 110 V, generating energy
for 1.15 h. Biogas production by biodigesters is a promising
alternative [35,36]. As already seen in this study, the
impurities (CO
2
and H
2
S) from biogas decrease its profit-
ability and do not take full advantage of the heating value.
Biomethane purification is one of the current challenges
since it must be a feasible, low-cost, easy-to-handle process
with high methane recovery. Likewise, studies have focused
on finding efficient methods for biogas purification,
through physical, chemical, and biological purification
processes.
CO
2
and H
2
S removal methods
Figure 3 shows the biogas upgrading methods like
Pressure Swing Adsorption (PSA), solvents, chemical
absorption with the use of strong bases, membrane separ-
ation, biological process, and cryogenic with high pressures
and low temperatures. However, this study focuses on
methods that require less infrastructure (physical, chemical,
and biological) to remove CO
2
and H
2
S from biogas, effi-
cient upgrading methods, low cost, and generating fewer
by-products [37–39]. It must be noted during this review it
has been observed that many authors suggest in the purifi-
cation process, H
2
S must first be eliminated to avoid corro-
sion of the materials and then eliminate carbon dioxide.
Physical method
CO
2
removal
Water is a universal solvent, this property is due to the
ability to form hydrogen bonds with other substances,
mainly with molecules (gases or liquids) that will have an
affinity for it (polar), for this reason, it has been studied for
biogas purification. According to Couvert et al. [40], carbon
dioxide has the capacity to be solubilized in water with
1.65 g kg
1
and methane with 0.023 g kg-
1
, water canFigure 1. Molecular structure of methane, modified from Moron [23].
2 J. E. CASTELLANOS-SÁNCHEZ ET AL.
solubilize CO
2
and efficient methane recovery. In methane
purification (scrubber) processes with water, at the begin-
ning of the process the dissolved CO
2
is surrounded by
water molecules, where carbonic acid is generated
(Equation (2)):
CO2þH2O$H2CO3(2)
Once carbonic acid is formed, it dissociates to give
bicarbonate and protons (H
þ
)(Equation (3)):
H2CO3$H2CO
3þHþ(3)
The CO
2
dissolved is a Lewis acid that generate H2CO
3
and H
þ
ions, while CH
4
and water generate the following
reaction (Equation (4)):
CH4þH2O!CO þ3H2(4)
When methane dissolves in water generates carbon
monoxide (CO) and hydrogen (H
2
), so the CO could gener-
ate problems, such as decreasing the heating value, low
methane concentration, and highly toxic gas.
According to Barrera-Cardoso et al. [41], in the pressur-
ized water purification method, biogas was fed from the
bottom to a packed bed column, and water pressurized is
sprayed from the top of the column (Figure 4), the biogas
Table 1. Physical and chemical properties of liquid and gaseous fuels [26–29].
Property Unit Methane LP Gas Natural Gas Ethanol Gasoline Hydrogen Firewood (dry)
State of aggregation –Gas Gas Gas Liquid Liquid Gas Solid
Chemical formula –CH
4
C
3
H
8
Gas mixture C
2
H
6
C
7.3
H
14
,2O
0.15
H
2
–
Density Kg m
-3
0.67 0.61 0.80 0.79 719.30 0.09 650.91
Heating value MJ kg
-1
46-55 46-51 42-55 26.8 44-46 120-142 16-22.5
MJ m
-3
30.67-36.68 28.06-31.11 33.6-44 21.14 31649.20-33087.80 10.68-12.64 10414.56-14645.47
Specific heat KJ mol
-1
890 –900 1366.9 5400 286 300
Energy KWm
-3
8.15-10.18 7.79-8.64 9.33-12.22 5.87 8791.44-9191.05 2.96-3.51 2892.93-4068.18
Figure 2. Uses of biogas and methane, modified from Ely et al. [30].
Figure 3. Purification methods for removal of CO
2
and H
2
S by physical,
chemical, or biological.
BIOFUELS 3
upgrading was a counter-current process. In this way, the
CO
2
dissolves in the water and exits through the bottom of
the tower, allowing an efficient process with 95% CO
2
removed. Sometimes the water can be recirculated to the
purification tower itself. Likewise, Morero [42] and Morgado
et al. [43] point out that using pressurized water to purify
biogas is an efficient physical method since the purification
of approximately 98% methane was achieved. In the same
way, most methods use the same spray principle, only the
biogas flow rate, water flow rate, temperature, concentra-
tion of reagent, reagent type, and pressure, among other
factors, have been studied [44–47].
The study conducted by Vijay [48], showed that biogas
purification depends mainly on the pressure in the scrub-
bing column, the flow rate of water, and the flow rate of
biogas at the inlet of the tower. The initial biogas compos-
ition was 60-65% CH
4
, 35-40% CO
2
, and 0.5-1.0% H
2
S. The
biogas purification system was designed to evaluate differ-
ent conditions, varying the biogas inlet flow rate from 1 to
3m
3
h
1
, 0.6 to 1 MPa of pressure, and 2.5 m
3
h
1
of pres-
surized water at ambient temperature. The results show
different values of CO
2
removed of 87.6, 99, 83.9, 73.8, and
53.7% with 1, 1.5, 2, 2.5, and 3 m
3
h
1
biogas flow rates,
respectively, and 1 MPa inlet biogas pressure. Therefore, at
inlet biogas flow 1.5 and 2.5 m
3
h
1
of pressurized water
and 1 MPa of pressure removes 99% CO
2
from the biogas.
Water scrubbing is a simple process, an economical and
suitable process to purify biogas. Pe~
na and D
avila-del-
Carpio [49] built and evaluated a biogas purification system
with pressurized water (Scrubber) to remove CO
2
. The ini-
tial biogas composition was 45.8% CH
4
and 21% CO
2
.In
this study, 6, 9, and 10 bar pressures were evaluated at 10,
15, and 20 C temperatures. The results show that at 10 C
and 10 bar and 79.5% of CO
2
was removed. This managed
to reduce biogas impurities to acceptable proportions
(Table 2). Leit
on [46] in his study used pressurized water in
a purification tower at 200 kPa, a water flow rate of 25 L
min
1
at 26 C, and a biogas flow rate of 20 L min
1
(Table
2). In this process, the biogas encountered the water coun-
tercurrent and finally, a drying column was used for mois-
ture removal with 2 kg of calcium oxide (CaO) and 1 kg of
charcoal (CV). The initial methane concentration was 69%
(CO
2
and H
2
S not reported) and the final purification pro-
cess achieved 93% methane recovered. This study demon-
strated that biogas can be purified and compressed using
materials and equipment available in the region at a low
cost. Ofori-Boateng and Kwofie [47] evaluated the purifica-
tion and compression of biogas from a 50 m
3
biodigester
and evaluated a bed packed with scrubbing water. The bio-
gas was composed of 55-65% methane and 30–45% car-
bon dioxide. The results showed that 92% CO
2
was
removed and up to 95% methane recovered (Table 2). The
research concluded that biogas should be purified for stor-
age and to prolong the efficiency of energy production.
Islamiyah et al. [50] purposed designing a biogas purifica-
tion of water scrubber to remove CO
2
from biogas. In the
study varied water levels in the tower: 50, 60, and 70 cm
high; 50 L min
1
of water flow rate. The initial composition
of biogas was 66.6% CH
4
, 15% CO
2,
and 18.4 mg L
1
H
2
S,
the results show a removal efficiency of 21.2% CO
2
. The
author concluded that the biogas purification with a water
scrubber was not so efficient. Gantina et al. [51] studied
the water scrubber method to remove CO
2
and eliminate it
from biogas
.
The inlet factors were two, three, and four bar
of biogas pressures and 0.1 and 0.15 L s
1
water flow in
60 s. The initial biogas composition was 51.1% CH
4
, 20.1%
CO
2,
and 20.8% N
2
. This study reported that CO
2
was
removed effectively with 99.5% and CH
4
increased by
38.18% in biogas, using 4 bar of pressure and a water flow
rate of 0.15 L s
1
. Nie et al. [52] compared water scrubbing
and propylene carbonate to remove CO
2
from biogas. The
operation condition using water scrubbing was 1300 to
1800 mL min
1
of biogas flow rate, 200 mL min
1
of water,
and 800 kPa of adsorption pressure. The operation condi-
tion using propylene carbonate was 5000 to 6500 mL
min
1
of biogas flow rate, 100 mL min
1
of water, and
800 kPa of adsorption pressure at ambient temperature.
The initial biogas composition was 54.63 to 57.29% CH
4,
42.71 to 45.37% CO
2,
and 0 to 400 ppm H
2
S. Propylene car-
bonate was the best in CO
2
removal with 22 L
1
while
Scrubbing water removed 4.98 L
1
. This study demon-
strated that propylene carbonate in CO
2
removal was effi-
ciently applied for biogas upgrading.
In the physical method, pressurized water and solvents
have been used in different conditions that, although the
PSA method has allowed the efficient elimination of CO
2
,
could present many drawbacks in its implementation on a
real scale, although the water absorption capacity is
Figure 4. General scheme of the physical absorption of CO
2
with the use of
water, modified from Barrera-Cardoso et al. [41].
Table 2. Comparison of the physical absorption method for CO
2
removal.
Reagents Biogas inflow Reagent inflow Temperature (C) Initial CO
2
(%) Final CO
2
(%) Initial CH
4
(%) CH
4
recovered (%) Reference
H
2
O 1.5 m
3
h
-1
1.8 m
3
h
-1
AT40 1 60 85 [48]
H
2
O 10 bar 15 L s
-1
10 21 6.9 45 85 [49]
H
2
O 25 L min
-1
20 L min
-1
26 ––69 93 [46]
H
2
O––AT–8 55-65 75-95 [47]
H
2
O–50 L min
-1
AT15 11.62 66.6 –[50]
H
2
O60Ls
-1
0.15 L s
-1
AT20.1 0.11 51.1 70.6 [51]
H
2
O 1.3-1.8 L min
-1
0.2 L min
-1
AT42.71 4.98 L 54.63 90 [52]
AT-Ambient Temperature.
4 J. E. CASTELLANOS-SÁNCHEZ ET AL.
relatively high for CO
2
, it could require large amounts of
water to purify several cubic meters of biogas. Studies
have shown that pressurized water in the biogas purifica-
tion method is effective at low flow rates, is easy to oper-
ate, requires little infrastructure, and is economically viable.
The main problem in biogas upgrading with water scrub-
bing is carbon monoxide (CO) production, a highly toxic
gas, which problems in human health. Also, water, in bio-
gas upgrading, changes its physical and chemical proper-
ties completely, which would lead to the search for an
additional process for its treatment. These drawbacks could
increase the costs of biogas purification, the pH of the
water changes, wastewater, pollution, expensive and
unprofitable.
H
2
S removal
Nowadays, various technologies have been developed to
remove H
2
S by physical adsorption, pressurized water has
been alternative in biogas upgrading. According to
Campuzano [53], hydrogen sulfide is soluble in water at
the rate of 0.41 g per 100 ml at 20 C, to dissociate into
hydronium and sulfur ions as shown in the reactions
(Equations (5–7)):
H2SþH2OÀH2S (5)
H2Saq
ðÞ
ÀHþþHS(6)
HSÀHþþS2(7)
In turn, Burgos [54] and Camiz
an [55] mention that the
solubility of H
2
S is influenced by temperature, that is, the
higher temperature and the better effectiveness. M
ojica
et al. [56] investigated a purification process with pressur-
ized water for the removal of carbon dioxide and hydrogen
sulfide, in different conditions as the biogas flow rate of
20 L min
1
, 54.5, and 72 kPa adsorption pressure and 27
and 37 C water temperature. The initial biogas presented a
concentration of 68.57% CH
4
, 30.56% CO
2,
and 0.88% H
2
S,
Table 3, at the end of the process 93.15% CH
4
, 6.84% CO
2,
and H
2
S 0.01% were obtained, both operating conditions
were efficient, removing 77% CO
2
and 98% H
2
S. The
authors suggest exploring drying methods for biogas to
improve its quality. Cheng-Chang et al. [57] studied water
scrubbing for biogas purification from a pig farm. The
assays of water pressurized were conducted with an inlet
of the biogas flow rate of 50, 100, and 140 L min
1
,a
retention time of 30 and 90 s of biogas, and different water
levels of 50, 60, and 70 cm, the initial H
2
S in biogas was
around 6000 ppm. The results show that the H
2
S was
removed significantly with 51% in 30 s and a water level of
70 cm, decreasing the biogas flow rate at 50 cm. The pro-
cess to remove H
2
S was efficient in a short operating time,
but the water absorption capacity decreased rapidly, and
water replacement was constant in the process.
L€
antela et al. [58] studied the effect of recycling water
for biogas purification from a landfill. the assays were con-
ducted on the effect of the pressure of 20-25 bar, tempera-
ture of 10-25 C, and water flow rate of 5.5 to 11 L min
1
.
The initial biogas was 50.8-57.9% CH
4
, 37.8- 43.6% CO
2,
and 100-166 ppm H
2
S. The results showed that the pressur-
ized water, at pH 4.6, had a high absorption capacity at a
biogas flow rate of 11 L min
1
and a pressure of 20 bar at
15 C. Likewise, it was shown that the best results were
generated at 25 bar, 11 L min
1
, and 10 to 15 C with
90.2% of methane recovered, 88.9% CO
2,
and 90% H
2
S
removed. Improving the ratio between the width and
height of the column could be efficient in CO
2
and H
2
S
removal, compared to commercial biogas purification
plants. Islamiyah et al. [50] purposed designing a biogas
purification of water scrubber to remove CO
2
and H
2
S from
biogas. In the study varied water levels in the tower: 50,
60, and 70 cm high; water flow rate of 50 L min
1
. The ini-
tial composition of biogas was of 66.6% CH
4
, 15% CO
2,
and
18.4 mg L
1
of H
2
S, the results show a removal efficiency
of 32.8% H
2
S eliminated. The author concluded that biogas
purification with a water scrubber was not so efficient.
The water scrubber like the physical method can be
considered efficient to remove CO
2
and H
2
S, however, it
requires large volumes of water for the purification process,
and temperature, water pressurized, water recirculation,
and wastewater treatment are necessary. Likewise, electrical
energy is required for the sprinkling of water and for its
treatment. Most studies for physical treatment have
focused on the use of laboratory-scale water purification
systems, though, in a full-scale process a specialized infra-
structure would be needed due to the large amount of
water used and a treatment plant for the wastewater
generated.
Chemical method
CO
2
removal
The chemical method has been one of the methods
studied to remove CO
2
and H
2
S from biogas, mainly with
strong bases such as calcium hydroxide sodium hydroxide
(NaOH), (Ca (OH)
2
), and, potassium hydroxide (KOH), in
aqueous solution or granules [4,8,59]. In this case,
Kismurtono et al. [60] studied the mass transfer in CO
2
removal from biogas using NaOH solution. It evaluated a
packed column, 1 M NaOH solution, the biogas flow of
600 mL s
1
, the solution flow rate of 50 mL s
1
, and 350 to
700 mm H
2
O. The raw biogas was composed of 63.2% CH
4
,
25.29% CO
2
, 11.10% CO
2,
and impurities. Using a biogas
flow rate of 600 mL s
1
, a NaOH (1 M) solution flow rate of
Table 3. Comparison of the method by physical absorption for the elimination of H
2
S.
Reagents Biogas inflow Reagent inflow Temperature (C) Initial H
2
S Final H
2
S
CH
4
initial
(%)
CH
4
recovered
(%) Reference
H
2
O/sawdust/
activated carbon
0.65 m
3
h
-1
20 L min
-1
27 8800 ppm (0.88%) 100 ppm (0.01%) 68.57 93.15 [56]
H
2
O 140 L min
-1
–AT6000 ppm (6%) 2933.3 ppm (0.29%) ––[57]
H
2
O 7.41 Nm
3
h
-1
5.5 a 11 L min
-1
20-25 100-166 ppm
(0.01- 0.0166%)
13.2 ppm (0.0013%) 50.8-57.9 90.2 [58]
H
2
O–50 L min
-1
–18.4 mg L
-1
–66.6 –[50]
AT-Ambient Temperature.
BIOFUELS 5
50 mL s
1,
and at 350 mm H
2
Oat30˚C, 80% CO
2
was
removed and 88.94% CH
4
recovered. Maile et al. [61] eval-
uated the chemical absorption of CO
2
in biogas purifica-
tion. The study evaluated three concentrations of NaOH
with 1 (M1), 2 (M2), and 3 (M3) mol L
1
. The raw biogas
was composed of 56% methane, 42% carbon dioxide, and
traces. This study revealed that the most efficient was
observed with 3 mol L
1
of NaOH with 66% CO
2
removed
and an increased methane concentration with 80%. Ghatak
and Mahanta [4] evaluated a biogas purification system
using soda-lime (composed of Ca (OH)
2
(94%), NaOH (2-
4%), KOH (1-3%) and H
2
O (14-19%)) to remove CO
2
from
biogas. The study evaluated the inlet biogas compressed
flow rate of 1 to 5 L min
1
and 1 to 5 bars of pressure,
then biogas passed through the purification tower, and
biogas was composed of 55 to 77% CH
4
, 30 to 45% CO
2
,
and traces. The results show that a biogas flow rate of 1 L
min
1
and 5 bar pressure removed over 95% (1.344%) CO
2
and recovers 97.7% CH
4
from biogas. According to the
author, although there are many biogas purification meth-
ods, not all of them can be implemented in rural areas.
In the biogas purification process, soda-lime generates
three consecutive reactions (Equations (8–10)) with CO
2
,
and as a by-products calcium carbonate (CaCO
3
), sodium
bicarbonate (NaHCO
3
), and NaOH [4]:
CO2þH2O!CO2aq
ðÞ (8)
NaOH þCO2!NaHCO3(9)
CaðOHÞ2þNaHCO3!CaCO3þH2OþNaOH (10)
As a by-product of the purification reaction, CaCO
3
is
generated, which can be proposed as a restorer of agricul-
tural soils [4].
Srichat et al. [8] evaluated the elimination of CO
2
from
biogas with calcium hydroxide (Ca (OH)
2
) and amine solu-
tion. The study evaluated different conditions, 0.1 and
0.2 mol of Ca (OH)
2
and 0.1 and 0.2 mol of Mono Ethanol
Amine (MEA); the biogas flow rate was 5, 10, and 15 L
min
1
and the solution flow rate was 10, 20 and 30 L
min
1
and a retention time of 30 min (Table 4). The raw
biogas composition was 51% CH
4
and 39.36% CO
2
. The
results show that the best results were with 0.2 mol of Ca
(OH)
2
, a solution flow rate of 10 L min
1
, and a biogas flow
of 5 L min
1
. This study shows that the factors involved in
biogas purification are the concentration and type of
reagent, the solution flow rate, and the biogas flow rate.
Also, Maile et al. [59] evaluated the potential of Mono
Ethanol Amine (MEA) in biogas purification. The study var-
ied the concentrations of MEA from 10 to 30% and three
temperature was used (ambient temperature, 30 and 40 C)
for the adsorption of CO
2
from biogas. The raw biogas was
composed of 52% CH
4
and 46% CO
2
and traces. The results
show that 30% MEA solution and 40 C removed 76% CO
2
and 88% CH
4
was recovered. MEA removes the impurities
of the biogas efficiently. However, the amines in biogas
upgrading tend to be expensive on a laboratory scale, the
final disposal of amines could generate an extra cost in the
process and increase purification costs.
Although there are a wide variety of chemical reagents
used for biogas upgrading, the MEA is efficient in the bio-
gas purification process. However, the reagents have the
disadvantage of being expensive and difficult to access for
full-scale treatment. Otherwise, chemical bases tend to be
easy to find, economical, and environmentally friendly, for
example, salts generated after purification can be used to
restore the soil. It is important to note that, diluted strong
bases are useful for removing CO
2
from biogas, but at the
end of its reactivity it must be discarded or neutralized,
generating extra costs for the purification of biogas. On the
other hand, strong bases concentrated in the solid base
could be promising, since calcium hydroxide with carbon
dioxide generates CaCO
3
as a final product, instead of
being discarded it can be used as a soil restorer.
H
2
S removal
In biogas upgrading, the iron (III) oxide (Fe
2
O
3
) is the most
used, which is relatively easy to prepare, at a low cost, and
efficient in removing H
2
S. In the preparation of iron oxide
(Fe
2
O
3
), it is generated by an oxidation reaction using iron
(Fe) and oxygen (O
2
) as reagents, as illustrated in Equation
(11):
4Fe þ3O2!2Fe2O3(11)
Once obtained the Fe
2
O
3
is evaluated under different
conditions to remove the H
2
S, generally, this elimination is
carried out following the reaction (Equation (12)), forming
a molecule of iron sulfide or ferric type III and three moles
of water [62,63].
Fe2O3þ3H2S!Fe2S3þ3H2O (12)
Similarly, type III iron sulfide is very easy to convert to
the state of Fe
2
O
3
active (Equation (13)), in the presence of
oxygen, which again the Fe
2
O
3
can be used as a substrate
to remove H
2
S.
2Fe2S3þ3O2 >Fe2O3þ6S (13)
Elizondo and Herrera [64] evaluated purification proc-
esses for H
2
S removal to improve the quality of the biogas.
The filters were composed of, I) coconut shell/gravel 2:1; II)
sawdust/sand 5:1, charcoal or bokashi (with high iron con-
centrations of 5.593 ppm). The initial composition of biogas
was 60-70% CH
4
, 30-40% CO
2,
and impurities. It was
observed that the charcoal system removed 97.08% H
2
S,
while the bokashi filter removed 96.01%, with results very
similar (Table 5). Therefore, both systems are considered
efficient to eliminate H
2
S, environmentally friendly and eco-
nomical systems. It is interesting to use organic waste as
an alternative for biogas treatment. Instead, Torres-
Calder
on et al. [65] evaluated two fixed-bed towers for
Table 4. Comparison of the chemical absorption method for CO
2
removal.
Reagents Biogas inflow Reagent Inflow
Temperature
(C)
Initial CO
2
(%)
Final CO
2
(%)
Initial CH
4
(%)
CH
4
recovered
(%) Reference
Soda lime 1 L min
-1
5 bar AT–1.34 41.5 97.7 [4]
Ca (OH)
2
5 L min
-1
10 L min
-1
AT39.6 –51 89.30 [8]
NaOH ––AT46 16 52 80 [61]
MEA ––AT46 15 52 85 [59]
AT- ambient temperature.
6 J. E. CASTELLANOS-SÁNCHEZ ET AL.
hydrogen sulfide removal. Tower (1) with iron chip pre-
treated with HCl and NaOH, Tower (2) with hydrogen per-
oxide (H
2
O
2
) and sodium chloride (NaCl) and the control
used only iron. The raw biogas was composed of 180 ppm
H
2
S, which was introduced with a flow rate of 111.6 L h
1
to the fixed bed towers at 19 C. The results show that the
highest H
2
S removed occurred in Tower 2 with 99, 58%
H
2
S removed and 96.36% removed in Tower 1 (Table 5).
Therefore, both purification towers are efficient in remov-
ing H
2
S. Mendoza [66] evaluated the removal of H
2
S from
biogas, this research evaluated four substrates; iron filing,
activated carbon, compost, and air injection, in different
concentrations in H
2
S removal, cost, and ease of operation
of the filters. In the results was observed that the best sub-
strate was the iron filing in H
2
S removal, with an efficiency
of 94.08% H
2
S eliminated (Table 5). Therefore, iron filings
could be an efficient alternative in the elimination of H
2
S,
easy to obtain, and a safe and low-cost process.
Manrique et al. [67] evaluated rice husk (Oryza sativa)as
an adsorbent material for the elimination of H
2
S from bio-
gas. Rice husk was evaluated as activated rice charcoal
(ACrice), iron oxide-impregnated rice activated carbon
(ACFeO), and commercial activated carbon (CAC). The initial
biogas composition was 60% CH
4
, 30% CO
2
,1-2%ofH
2
S
and traces. The H
2
S removal was 39% from ACrice, 48.5%
from ACFeO, and 31.25% from CAC. Activated carbon
impregnated with FeO (ferric oxide) was the best material
for purifying biogas. However, obtaining the absorbent
material based on rice husks requires a complex process,
which increases the cost and makes it difficult to access.
The research by Valencia et al. [34] studied the design, con-
struction, and evaluation of a purification system and com-
pression for biogas. This research aimed to remove CO
2
and H
2
S from biogas, the first purification filter had Fe
2
O
3,
pretreated with HCL and NaOH, and a secondary filter with
pressurized water recirculated. The raw biogas composition
was 50-60% CH
4
, 30-40% CO
2,
and 0.5 to 3% H
2
S. Results
show that the purification system reduces the CO
2
values
in the biogas from 30% of its initial value to 9%,70% CO
2
removed, and from 1.5% of H
2
S to 0.0008%, 99% H
2
S
removed, practically negligible (Table 5).
Quesada et al. [68] studied the use of calcium oxide and
Fe
2
O
3
to remove H
2
S from biogas from a dairy farm. The
results indicated that initial hydrogen sulfide was found in
concentrations of 370 ppm within the produced biogas.
After the biogas purification through the calcium oxide fil-
ter, a concentration of H
2
S was 225 ppm, achieving a 40%
reduction. These values were below the results presented
in other purification studies.
The removal of H
2
S with CaO, ferrous substances called
red blood cells, activated carbon, iron (Fe) filings, and
organic matter can be a viable alternative in biogas
upgrading, available in any region and inexpensive. The
use of organic materials in the form of activated carbon
has been a novel material for biogas, upgrading it is impor-
tant to study different conditions to optimize its use and
make the process efficient. However, Fe
2
O
3
is promising in
the removal of H
2
S, since it is a cheap material, easy to
generate, safe to use, with great effectiveness, and can be
reused.
Biological method
CO
2
removal
Biological methods are ecological and interesting in biogas
upgrading, studies have mainly involved chemotrophic or
photosynthetic microorganisms. The purification processes
by biological processes do not generate waste, are eco-
nomical, do not require chemical products, require low
energy consumption, high degradation rates, systems sup-
ported by biofilms and native species from each region can
be used. However, biological processes are slow and iso-
lated microorganisms tend to be easily contaminated [69].
For this reason, many studies have proposed the CO
2
from
biogas to study growth, harvesting of microalgae, and
upgrading biogas.
In photobioreactors, microalgae assimilate carbon diox-
ide (C
inorganic
) by the CO
2
concentration mechanism (CCM),
mainly to reduce CO
2
and increase O
2
in the environment
[70]. In the process to remove CO
2
with microalgae and
cyanobacteria, its main source of growth is sunlight,
nutrients, CO
2
from biogas
,
and water as illustrated in
Figure 5, and the final products are microalgal biomass,
biogas purified (>90% of methane) and O
2
gas [72–74].
Table 5. Comparison of the chemical adsorption method for the removal of H
2
S.
Reagents Biogas inflow Initial H
2
S (ppm) Final H
2
S (ppm) Removal H
2
S (%) Reference
Charcoal 7.94 m
3
(28 L min
-1
) 68.09 1.98 97.08 [64]
Bokashi 1.70 m
3
(6 L min
-1
) 92.61 3.71 96.01 [64]
Iron filings ––250 94.08 [66]
Iron shaving 111.6 L h
-1
180 9.37 94.79 [65]
Iron shaving 111.6 L h
-1
180 0.75 99.58 [65]
Activated carbon –1000 610 39 [67]
FeO/ activated carbon –1000 515 48.5 [67]
Commercial activated carbon –1000 687.5 31.25 [67]
Fe
2
O
3
/H
2
O–1500 8 99.46 [34]
CaO 7 kPa 370 225 40 [68]
Figure 5. Illustration of involvements for microalgae growth, modified from
Płaczek et al. [71].
BIOFUELS 7
Chuanchai and Ramaraj [13] evaluated the biogas pro-
duction from buffalo grass with dung and biogas purifica-
tion. The experimental process was carried out through
photoautotrophic microalgae (Chlorella vulgaris), for 8h
retention time, was studied inlet two biogas flow rate of
0.9 and 1.9 L min
1
. The raw biogas was composed of 71%
CH
4
, 28% CO
2,
and 0.013% H
2
S. The results show that the
biogas flow rate of 0.9 L min
1
achieved a concentration of
91% CH
4
recovered (Table 6) and 80% CO
2
removed; while
the biogas flow rate of 1.9 L min
1
recovered 83% CH
4
,
and 46.4% CO
2
removed. It is important to have a balance
in the N:P ratio that allows for maximizing the total
removal of CO
2
from biogas.
Converti et al. [72] investigated biogas purification by a
biological process using the cyanobacterium Arthrospira
platensis. The initial composition of biogas was 70.5–76.0%
CH
4
and 13.2–19.5% CO
2
. The CO
2
from biogas was used
as a CO
2
source in a biological photoreactor with
Arthrospira platensis with a retention time of 50 days. The
results show that there is a direct relationship between bio-
mass growth and CO
2
consumption (95%) (Table 6). The
operation of a biogas production and purification plant can
generate problems such as monitoring CO
2
removal, the
incidence of light, and the monitoring of physicochemical
parameters.
Panduro-Pisco et al. [73] evaluated the biological purifi-
cation of biogas from sludge using the microalgae Chlorella
vulgaris. In this study, the efficiency of CO
2
removal from
biogas was evaluated by different concentrations of micro-
algae (10, 30, and 60%) and biol as substrate (50, 100, and
150 mL). The initial biogas composition was 67.75% CH
4
,
32.43% CO
2
, and 0.64% (6394.33 ppm) H
2
S. The results
show that the treatment with 30% microalgae, 60 L of
water, and 100 mL of Biol, removed 25.5% CO
2
, 74.5% CH
4
recovered, and a heating value of 7049 kcal. The CO
2
removal efficiency could be improved by recirculating the
same biogas to the bioreactor or coupling a physical or
chemical method to improve yields.
Guo et al. [74] studied the best light, photoperiod, and
microalgae co-cultivation process in removing CO
2
from
biogas. It was evaluated the CO
2
removal efficiency with
Culture 1: Chlorella vulgaris (FACHB-31); Culture 2: Chlorella
vulgaris/Ganoderma lucidum (G. lucidum, 5.765), and Culture
3: Chlorella vulgaris/activated sludge. These were subjected
to three photoperiods (light: darkness), 12h: 12h, 14h: 10h,
and 16h: 8h in 10 days, cool white light with a light of
200 lmol m
2
s
1
at 25C and raw biogas had 64.59% CH
4
and 33.7% CO
2
. Results show that C. vulgaris/G. lucidum
removed 59.37% CO
2
under a photoperiod of 14h light:10h
darkness and 450 lmol m
2
s
1
light. The cultures Chlorella
vulgaris/Ganoderma lucidum and Chlorella vulgaris/activated
sludge systems did not have significant differences in CO
2
removal. Also, Xu et al. [75] studied endophytic bacteria
(endogenous bacteria S395-1 and S395-2) isolated from
Chlorella vulgaris and co-cultured to determine a suitable
biogas purification. Chlorella vulgaris and Chlorella vulgaris/-
bacteria (Chlorella v./S395-1 and Chlorella v./S395-2 co-cul-
ture were evaluated for 10 days, 12h light/12h darkness,
200 lmol m
2
s
1
light intensity at 25 C. Initial biogas was
composed of 62.17% CH
4
, 34% CO
2
, 0.54, and impurities
[76]. The results show that the symbiotic system had
removal efficiencies of 68.13% CO
2
. The microalgal-bacterial
could improve wastewater treatment and remove CO
2
from
biogas successfully. Ramaraj et al. [13] utilized Chlorella vul-
garis for biogas upgrading, and results achieved >95%
CO
2
and H
2
S removed. The biological process is a friendly
alternative to the environment and is feasible to produce
quality biogas.
Miyawaki et al. [77] studied the production and purifica-
tion of biogas from the microalga Tetradesmus obliquus.In
the research, pig manure, cattle manure, and domestic
wastewater were used to produce biogas. An airlift photo-
bioreactor inoculated with Tetradesmus obliquus microalgae
was used for biogas upgrading, at the inlet a biogas flow
rate was of 1 L min
1
, samples were collected at 5, 30, and
60 min before and after biogas purification at room tem-
perature (35 C). The initial composition of biogas was of
63% CH
4
,37% CO
2
and <100 ppm H
2
S. The microalga
Tetradesmus obliquus efficiently absorbs CO
2
with 86%
removed. Thus, biogas injection showed to be effective for
large-scale microalgae biomass production and biogas
upgrading.
In some cases, both physical and chemical biogas purifi-
cation processes generate irreversible environmental
impact, which could be less harmful after treatment once
their useful life ends. Likewise, chemical processes have
been the most efficient for biogas upgrading but must pay
attention to some considerations, which could be the
cheapest and least polluting. The biological process has
been and has generated promising results in biogas
upgrading since it has certain advantages over other treat-
ments since several products are obtained (microalgal bio-
mass and purified quality biogas) and it is environmentally
friendly. In addition, microalgae can be used as raw mater-
ial for bioproduct generation [78], as well as biogas pro-
duction and purification, which could be considered as an
integral process or under biorefinery concept.
H
2
S removal
As has already been seen in this study, there are physical
and chemical processes for the removal of H
2
S and bio-
logical processes, which have been evaluated on a labora-
tory and pilot scale. These methods use some
microorganisms that could metabolize their liquid or gas-
eous substances. Specifically, microorganisms that can
remove H
2
S from biogas, which is based on the oxidation
Table 6. Comparison of the biological method for CO
2
removal.
Species Biogas inflow (L min
-1
) Initial CO
2
(%) Final CO
2
(%) Initial CH
4
(%) Final CH
4
(%) Reference
Chlorella vulgaris 0.9 28 8.56 71 91 [13]
Chlorella vulgaris 1.8 28 15 71 85 [13]
Arthrospira platensis –19.5 –76 –[72]
Chlorella vulgaris (10%w L
-1
)–39.6 32.2 60.4 67.6 [73]
Chlorella vulgaris (30%w L
-1
)–39.6 25.5 60.4 74.5 [73]
Chlorella vulgaris (60%w L
-1
)–39.6 39 60.4 60.5 [73]
Chlorella vulgaris /G. lucidum –33.7 23.5 64.59 –[74]
8 J. E. CASTELLANOS-SÁNCHEZ ET AL.
of hydrogen sulfide to easily eliminate sulfur compounds,
such as elemental sulfur (S) or sulfates (SO
42
), fixing CO
2
as a stoichiometric function of sulfur oxidation (Equations
14–17). Through this process, it is possible to purify the
biogas, removing CO
2
and H
2
S. Pure or mixed cultures of
microorganisms are used in purification studies. Depending
on the type of microorganisms, H
2
S could be converted
from sulfides to sulfur or alternatively to sulfate, as shown
in reactions [79,80].
H2S$HSþHþ(14)
0:5O2þHS!OHþS0(15)
1:5O2þS0þH2O!2HþþSO2
4(16)
2O2þHS!HþþSO2
4(17)
There are a wide variety of microorganisms that can oxi-
dize H
2
S, that grow in different temperature and pH envi-
ronments, that grow in aerobic or aerobic systems, and
have the potential to purify biogas, Table 7.
Cerr
on and Matos [85] studied the removal of H
2
S from
a compost-based biofilter (bacterial consortium) and
organic substrate in a fixed-bed reactor. In this research
the flow rate of biogas was 4 L min
1
, first, the biogas flow
through a humidifier filter to maintain humidity and then
to the fixed bed filter with compost, the residence time of
biogas was 10.80, 27, and 43.20 s at ambient temperature.
Before the biogas filtering process, the initial biogas was
composed of 55-70% CH
4
, 30- 45% CO
2,
and 3% ppm. This
study shows that the highest removal e of H
2
S was with a
longer residence time of 43.20 s and was eliminated 100%
H
2
S. This process was efficient in biogas upgrading, the use
of compost can be a viable alternative.
Kuo-Ling et al. [14] evaluated the removal of hydrogen
sulfide from biogas using a biological-chemical process
with Acidithiobacillus ferrooxidans CP9 species, metabolizing
sulfur and was provided sufficient ferric iron level for a sta-
ble system. A laboratory-scale bioreactor was operated for
311 days, 0- 20 g L
1
of ferric iron level, and 18 to 42 Cof
bioreactor temperature. The biogas composition was 61%
CH
4
, 26% CO
2
, and 1645 ppm of H
2
S. The results show that
98% of H
2
S was eliminated in an operating period of
311 days (Table 8). The study demonstrated the feasibility
of using Acidithiobacillus ferrooxidans CP9 for biogas
upgrading.
Wei-Chih et al. [82] used a chemical-biological process
to remove H
2
S from biogas on a pilot scale. This process
was carried out with biogas from cow manure, bioreactor
was inoculated with Acidithiobacillus ferrooxidans, the
second reactor with a chemical reagent, and varying the
concentration of H
2
S inlet. The initial composition of biogas
was 59% CH
4
, 21% CO
2,
and 3542 ppm H
2
S, the biogas
flow rate of 50 to 100 L min
1
, the bioreactor was operated
at 30 min h
1
and 12 h d
1
. The study achieved 90–95%
removal efficiency of H
2
S, between 288 to 144 s, in 16 days
of monitoring. These results demonstrate that the chem-
ical-biological process is efficient in biogas purification.
Zdeb [83] studied the removal efficiency of hydrogen
sulfide from biogas with bog iron ore, incorporated into
the biological desulfurization in the wastewater treatment
plant ‘Hajd
ow’. The study used sulfur-oxidizing bacteria
Thiobacillus (thiooxidans and thioparus) and Sulfolobus. The
raw biogas had between 55 to 80% methane, <40% car-
bon dioxide, and up to 1110 ppm H
2
S. Comparing the
results of H
2
S removal, it was observed that using only bog
iron ore eliminates between 75.1 and 89.9%, while the bio-
logical process improves efficiency between 97.6 and
99.5% (Table 8). Therefore, using sulfur-oxidizing bacteria is
efficient for biogas upgrading.
Ibrahim et al. [86] evaluated the biological elimination
of H
2
S in a pilot-scale biofilter system from biogas. The
bio-trickling filter was inoculated with an H
2
S-oxidizing
consortium and was designed to process one ft
3
min
1
(SCFM) of biogas, inlet biogas was supplied with 0.5- 1 L
min
1
, the retention time of 60 to 120 s at ambient tem-
perature. The initial biogas was composed of 45.5% CH
4
,
35% CO
2
, and 730 to 2000 ppm H
2
S. Results show the H
2
S
maximum removal efficiency of 94% and the capacity of
the biofilter to remove 24 g H
2
Sm
3
h
1
. Also, Ibrahim
et al. [80] study biogas upgrading by chemical and bio-
logical H
2
S removal. A synthetic biogas (95.69% CO
2
, 4.3%
compressed air, and 0.005% H
2
S) was utilized and the ferric
sulfate as an oxidizing agent for H
2
S, 6, 10, 12, and 17 g
Fe
3þ.
L
1
concentrations in the ferric iron solution were
used at ambient temperature. This purification process was
efficient for H
2
S removal at 15 g L
1
ferric iron and a resi-
dence time of seven minutes.
The bacterial consortium or specific bacteria has been
an adequate treatment to eliminate H
2
S from biogas, how-
ever, it important to have the optimal conditions to maxi-
mize the H
2
S removal. It could be one of the most
important challenges in the purification of biogas. Also,
native microorganisms or microorganism consortiums could
be another viable alternative to biogas upgrading.
Advantages and disadvantages of processes
As already seen throughout this study, there is a wide var-
iety of physical, chemical, and biological alternatives to
remove CO
2
and H
2
S under different conditions. From the
studies, different advantages and disadvantages have been
observed that could be a turning point in terms of the use
of technologies, as presented in Table 9. It is important to
mention that the processes wanted to be accessible, that
is, with little infrastructure, reactive materials, low cost,
easy to operate, does not generate waste, and a sustain-
able process.
In this study, it has been observed that most of the bio-
gas purification processes have been carried out on a
laboratory scale, very few on a full scale, in a semi-continu-
ous process, and few in a continuous process. Laboratory-
scale processes can be easily controlled, however, in real
scale processes the factors that influence biogas purifica-
tion can no longer be controlled. In addition, most proc-
esses may require a complex infrastructure, chemical
reagents expensive and the use of pure species would be
expensive. In addition, the energy operation, the operating
Table 7. Aerobic and anaerobic bacteria in biogas purification.
Bacteria Genus References
Aerobic Acidianus, Acidithiobacillus,
Aquaspirilum, Aquifex, Bacillus, Beggiota,
Methylobacterium, Paracoccus Pseudomonas,
Starkeya, Sulfolobus, Thermitiobacillus,
Thiobacillus y Xanthobacter.
[14,81–83]
Anaerobic Allochromatium, Chlorobium, Rhodopseudomonas,
Rhodovulum
Thiocapsa
[84]
BIOFUELS 9
expenses, and the disposal of waste could generate
another problem. In the market there could be some
chemical reagents that are available and at a low cost that
can be regenerated or can be recycled as iron residues,
biocarbon, quicklime, or native species (microalgae, bac-
teria, or fungi), which could be viable alternatives in biogas
upgrading on a real scale and in continuous processes, in
industry or rural biodigesters.
Future perspectives
In physical, chemical, and biological processes, many purifi-
cation processes have been found promising to remove
CO
2
and H
2
S, considering that these methods were eval-
uated under different conditions. We observed that most
of the studies have been tested on a laboratory scale and
not on a real scale. It is important that when a purification
study is carried out, the substrates to be used must be
considered, which are low cost, accessible, post-treatment,
environmentally friendly, safe, and above all that the proc-
esses are easy to operate.
In physical methods, the use of water as a natural
absorbent has been proposed, but this could not be con-
sidered a completely viable process on a real scale, due to
the amount of water used. Although an efficiency of over
90% has been obtained at a laboratory or pilot scale, it
Table 8. Comparison of the biological method for the elimination of H
2
S.
Species
Biogas inflow
(L m
-1
)Initial H
2
S (ppm)
Final H
2
S
(ppm)
Removal
H
2
S
(%)Reference
Microbial consortium 0.92 141-190 0 100 [85]
Acidithiobacillus ferrooxidans CP9 –1645 49.35 97 [14]
Acidithiobacillus ferrooxidans –3542 177.1 95 [82]
Thiobacillus/Sulfolobus –1110 5.55 99.5 [83]
Table 9. Advantages and disadvantages of different biogas purification technologies.
Method Reagent Advantages Disadvantage
Physical H
2
O Elimination of 99% CO
2
and H
2
S.
95-98% of methane recovered.
Water is available.
Simple operation.
It needs a lot of water.
It requires a pumping system.
Temperature increase.
Wastewater treatment.
Infrastructure.
It generates CO as a toxic gas.
Risk of bacterial growth.
Drying is required before using
any application
Chemical Soda- lime High CO
2
selectivity.
Rapid absorption.
Efficient in the removal of CO
2
.
Dependence and high costs on the
chemical.
At the conclusion its
effectiveness should be discarded
or an appropriate treatment.
Complex infrastructure.
Chemical MEA A higher concentration improves
CO
2
absorption.
Highly efficient (95–98% CH
4
)
No bacterial growth risk
(high pH).
Its use is expensive.
The concentration and flow of
biogas are factors that influence
purification.
Incomplete regeneration.
Corrosion.
Disposal
Chemical Ca (OH)
2
High selectivity to CO
2
Low cost.
Market availability.
Once its reactivity is finished,
It has agricultural applications as
a soil improver.
Absorbs moisture easily.
Chemical Iron oxide Availability.
Easy and low-cost pretreatment
Low cost
100% H
2
S removed
Recycling
It requires treatment prior to use.
Biological Microalgae Generates biomass.
>85% methane recovered.
Integral process.
Endemic species may be used.
Ecofriendly.
Native species can be used
Slow process
Requires proper installations.
By having the presence of CH
4
and O
2
,
It can generate fire.
The expensive process if isolated
species are used.
Biological Microbial consortium Generates sulfur fertilizer.
No pollution
Low cost
Native species can be used
It requires the adaptation of
microorganisms.
Sensitive to environmental
changes.
Biological Cyanobacteria Generates biomass
Absorption of 95% of CO
2
Slow process.
Isolation and adaptability of the
species.
Sensitive to environmental
changes.
10 J. E. CASTELLANOS-SÁNCHEZ ET AL.
would be worthwhile to propose an alternative treatment
for wastewater or its incorporation into biodigesters. For
this, it would be important to characterize the wastewater
and determine its physical and chemical properties.
Chemical processes (strong bases) have been the most
efficient in removing CO
2
and H
2
S above 90%, but using
diluted chemical substances could generate extra residues,
which could require treatment or neutralization. In add-
ition, it has been observed that using solutions at low con-
centrations is not feasible, since its effectiveness would
reduce quickly and act as a limiting reagent. For this rea-
son, many authors have proposed using strong bases to
increase the useful life of the reagent. Likewise, it would be
interesting to continue evaluating organic substrates in the
form of activated biocarbon or iron residues CO
2
and H
2
S
removal.
Biological methods are novel in the purification of bio-
gas, many studies have sought the possibility of incorporat-
ing microalgae and cyanobacteria, demonstrating their
effectiveness with greater than 90%. However, studies have
been conducted on a laboratory scale or a pilot scale.
Incorporating these purification systems on a real scale
would be interesting for the integral use of organic waste.
The biogas upgrading process is intended to be com-
pletely sustainable, that is, to treat organic waste and pro-
duce biofertilizer and methane (>95%), to generate a
profitable process and zero waste. This allows the end user
(company and/or farmer) to produce biogas simply, with
cheap and available materials. The biogas by anaerobic
digestion and biogas purification contributes to reducing
the emission of greenhouse gases, recovering wastes, gen-
erating affordable and non-polluting energy, and achieving
the 2030 agenda for sustainable development.
Acknowledgements
The National Council of Humanities, Sciences and Technologies
(CONAHCYT), the Master of Science in Tropical Agricultural Production
of the Autonomous University of Chiapas, and the Academic Body of
Livestock Agroforestry, Centro Mexicano para la Producci
on m
as
Limpia of the Instituto Polit
ecnico Nacional for the support granted.
Disclosure statement
No potential conflict of interest was reported by the authors.
ORCID
F. A. Aguilar-Aguilar http://orcid.org/0000-0003-3021-1186
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