Renewable and Sustainable Energy Reviews 154 (2022) 111826

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Renewable and Sustainable Energy Reviews

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State-of-the-art assessment of cryogenic technologies for biogas upgrading:

Energy, economic, and environmental perspectives
Ahmad Naquash a, Muhammad Abdul Qyyum b, *, Junaid Haider c, Awais Bokhari d,
Hankwon Lim c, Moonyong Lee a, **
a
School of Chemical Engineering, Yeungnam University, Gyeongsan, 712-749, Republic of Korea
b
Department of Petroleum & Chemical Engineering, Sultan Qaboos University, Muscat, Oman
c
School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology, 50 UNIST-gil, Eonyang-eup, Ulju-gun, Ulsan, 44919, Republic of Korea
d
Department of Chemical Engineering, COMSATS University Islamabad (CUI), Lahore Campus, Punjab, 54000, Pakistan

A R T I C L E I N F O A B S T R A C T

Keywords: In this review, conventional biogas upgrading technologies, including physical absorption, chemical absorption,
Biogas upgrading adsorption, and membrane technologies, are evaluated in terms of their production cost, energy consumption,
Biomethane production and number of installed plants. Amongst these technologies, cryogenic technology is the most energy- and cost-
Technical challenges
intensive. Considering the advantages and disadvantages of upgrading technologies and transportation re­
Future prospects
Life cycle assessment
quirements, cryogenic technology can provide dual benefits if integrated with liquefaction, as both require low
Process systems engineering temperatures for operation. In recent years, various standalone or integrated studies have been conducted to
analyze the performance of cryogenic processes based on energy consumption, economic benefits, and opera­
tional feasibility for a range of biogas compositions. These studies, which include technical, economic, and
environmental analyses, are examined in this review paper. Based on these assessments, cryogenic distillation-
based biogas upgrading was found to be economical in terms of energy consumption and product purity.
Nevertheless, other emerging cryogenic technologies, such as controlled freeze zone and anti-sublimation, must
be explored further from technical and economic perspectives. Furthermore, in this review, technical challenges
are discussed and future directions for academic and industrial applications are suggested along with the
practical implications of this study.

primarily influenced by energy production and consumption [1]. The

majority of the world’s energy generation is driven by fossil fuels [1],
1. Introduction
which are also the major sources of carbon emissions. Coal is a major
source of global carbon emissions, contributing ~38% of the total CO2
The world currently faces the dual challenges of a high energy de­
emissions (2.33 kt/GWh) [5]. A comparison of CO2 emissions by fuel
mand and the need to reduce carbon emissions [1]. The high energy
type as of 2011 [5] is shown in Fig. 1.
demand has increased the consumption of fossil fuel-based energy
Fig. 1 shows that the major source of carbon emissions from fossil
sources, such as crude oil, and natural gas [2]. The global energy con­
fuels is coal, followed by oil and natural gas. The challenge of reducing
sumption of crude oil and natural gas exhibited an annual increase of
carbon emissions may be overcome by utilizing alternative green energy
0.9% and 2.0%, respectively, in 2019 [3]. Since the increasing demand
sources. Substantial developments have been made in the energy sector
for fossil fuels severely impacts the environment through global warm­
for reducing carbon emissions through the successful utilization of
ing, the global energy market is gradually shifting towards green, clean
renewable sources, such as solar photovoltaic (PV) and wind energy [6].
energy sources, such as renewable fuels [4]. The global consumption of
The carbon emissions released from renewable fuels are significantly
renewable fuels, comprising traditional biofuels, such as wood and
lower than those released from the combustion of fossil fuels.
agricultural bio-waste, exhibited a significant growth rate of 12.2% in
From 1990 to 2018, the average annual renewable energy sources
2019 compared to their consumption in 2018 i.e., 25.83 EJ [3].
exhibited a growth rate of 2.0%, led by solar PV and wind energy; solar
The latter challenge, i.e., the need to reduce carbon emissions, is

* Corresponding author. Department of Petroleum & Chemical Engineering, Sultan Qaboos University, Muscat, Oman
** Corresponding author. School of Chemical Engineering, Yeungnam University, Gyeongsan, 712-749, Republic of Korea
E-mail addresses: m.qyyum@squ.edu.om (M.A. Qyyum), mynlee@yu.ac.kr (M. Lee).

https://doi.org/10.1016/j.rser.2021.111826
Received 7 June 2021; Received in revised form 10 September 2021; Accepted 26 October 2021
1364-0321/© 2021 Elsevier Ltd. All rights reserved.
A. Naquash et al. Renewable and Sustainable Energy Reviews 154 (2022) 111826

upgrade (remove CO2) biogas have been adopted in different processes

Nomenclature and abbreviations and include physical absorption [11], chemical absorption [12], mem­
brane [13], adsorption [13], and cryogenic processes [14]. Non-reactive
LBM Liquefied biomethane solvents, such as water or organic solvents, are used in physical ab­
TWh Tera Watt-hours sorption for the absorption of gases [11]. Water scrubbing is the process
MEA Monoethanolamine that utilizes water as the solvent, whereas the process that uses organic
DEA Diethanolamine solvents, such as methanol and dimethyl ethers of polyethylene glycol, is
MR Mixed refrigerant known as physical scrubbing [12]. Chemical absorption processes use
LCA Life cycle analysis chemical solvents, such as monoethanolamine (MEA) and diethanol­
GWP Global warming potential amine (DEA), which react with gases to separate them from the gas
AWR Alkaline with regeneration mixture [13]. The solvent is then regenerated in a stripping unit and
ODP Ozone depletion potential recycled to the absorption unit. A large amount of heat is required in
EBA European biogas association chemical absorption compared to that in the physical absorption process
AD Anaerobic digestion [12]. The adsorption process is classified as physisorption or chemi­
WS Water scrubbing sorption based on the working principle [13]. In the physisorption
GWh Giga watt-hours process, adsorption and desorption depend on weak van der Waals
ORC Organic Rankine cycle forces. The chemisorption process involves the formation of strong
ARC Absorption refrigeration cycle chemical bonds between the adsorbate and adsorbent [15]. Membrane
GHG Greenhouse gas separation is another technology used for biogas upgrading via selective
BABIU Bottom ash upgrading gas penetration through the membrane, where solubility and diffusion
HPWS High-pressure water scrubbing are the key drivers of separation. Gas permeation is dependent on the
CHP Combined heat and power generation solubility and diffusion coefficients of the membrane process [13].
Cryogenics is a relatively new and emerging technology compared to
other biogas upgrading technologies. It involves the separation of gases
based on their condensation or sublimation temperatures. This separa­
tion is conducted through either distillation or anti-sublimation [16]
based on the phase of CO2 separated. Cryogenic distillation requires a
high pressure [14] and low temperature [14] for operation, and a large
amount of energy is required for compression and cooling. Additionally,
the distillation process requires a multi-stage compression system,
which increases the production cost [17]. In the anti-sublimation [18]
process, CO2 is separated from CH4 through solidification on the sur­
faces of a heat exchanger or a specially designed cold box and extracted
later from the surface in a liquid or vapor form with a high product
purity [18]. Biomethane can be liquified (also known as LBM) at a
significantly lower cost by the anti-sublimation process than by the
cryogenic distillation process [17].
The environmental effect of biogas is an important aspect that should
be considered. Undertaking a life cycle assessment (LCA) is useful for
studying environmental impacts. Several LCA studies have been per­
formed to date considering the biogas supply chain, including the biogas
upgrading processes. An LCA of biogas upgrading technologies was

Table 1
Fig. 1. CO2 emissions by fuel type as of 2011 [5]. Typical composition and characteristics of biogas from different sources [10].
Parameter Unit Composition
PV and wind energy showed an annual growth rate of 36.5% and 23.0% Landfill gas Biogas from AD
per annum, respectively, followed by biogas, which showed a growth
Lower Heating Value MJ/kg 12.3 20.2
rate of 11.5% per annum [7]. Biogas is a major biomass-based renewable Density kg/Nm3 1.3 1.2
fuel because of its potential as a promising electricity generation source Wobbe index MJ/Nm3 18 27
[8], transportation fuel [9], and cooking fuel [8]. Biogas can be pro­ Methane number >130 >135
duced from municipal waste, landfills, and sewage sludge. The compo­ Methane % 35–65 53–70
Carbon dioxide % 15–50 30–47
sition and parameters of biogas [10] vary depending on the source, as
Nitrogen % 5–40 –
shown in Table 1. Oxygen % 0–5 –
According to Table 1, the major constituents of biogas are CH4 and Hydrogen Sulphide ppm 0–100 0-10,000
CO2. To obtain high-purity CH4 as an alternative to natural gas, CO2 and Ammonia ppm 5 <100
other impurities (N2, H2S, and O2) need to be removed from biogas. Total Chlorine (as Cl⁻) mg/Nm3 20–200 0–5

Different methods to clean (remove H2S and other impurities) and

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A. Naquash et al. Renewable and Sustainable Energy Reviews 154 (2022) 111826

conducted by Starr et al. [19], which included water scrubbing, alkaline The practical implications and the conclusions of this study are provided
with regeneration, and bottom ash upgrading processes. The bottom ash in Sections 8 and 9, respectively.
upgrading process was found to be the most environmentally friendly
biogas upgrading process [19]. In another study by Aziz and Hanafiah 2. Methods and scope
[20], the LCA of a biogas supply chain, spanning the raw material to
biogas production, was performed; however, they did not consider the The present review was conducted to assess cryogenic technologies
biogas upgrading process. During an LCA of biogas upgrading, different for biogas upgrading. Initially, a brief yet comprehensive introduction of
upgrading technologies, including cryogenic technology, were consid­ biomethane production is provided, after which the cryogenic technol­
ered by Florio et al. [21]. The global warming potential of the cryogenic ogies, including standalone (cryogenic distillation and anti-sublimation)
process (1.16 kgCO2 eq.) is lower than that of the chemical scrubbing and hybrid technologies, are discussed. Other conventional biogas
process (1.27 kgCO2 eq.) but higher than that of other processes (e.g., upgrading technologies are out of the scope of this study. The cryogenic
membrane: 1.09 kgCO2 eq.; pressure swing adsorption (PSA): 1.11 technologies are evaluated in terms of energy and economics. Further,
kgCO2 eq.; and high pressure water scrubbing (HPWS): 1.11 kgCO2 eq.) LCA studies on the biogas supply chain involving biogas upgrading are
[21]. considered and its impact on the environment is evaluated. On the basis
The cryogenic biogas upgrading technologies explored in the existing of these analyses, technical challenges, future recommendations, and
literature have been comprehensively reviewed and evaluated by practical implications are presented.
different authors. The cryogenic technologies, including cryogenic
distillation and anti-sublimation processes, have been evaluated by Song 3. Biomethane production: a brief outlook
et al. [22] in terms of product purity, recovery, and energy consumption.
Furthermore, technology-related challenges and possible solutions to The biomethane sector is currently expanding, with an increasing
overcome these challenges have been discussed. However, environ­ production trend globally (as shown in Fig. 2). This is particularly true
mental or economic policy-based perspectives were not considered [22]. for Europe, which produced 2.4 bcm of biomethane in 2019 [24].
In another study by Baena-Moreno et al. [23], cryogenic distillation, This high biomethane production rate in Europe helps meet its own
cryogenic packed bed, and anti-sublimation technologies were dis­ energy requirements, and also allows European countries to trade with
cussed, and their major advantages and limitations were included along other countries. Although biomethane is only used in national markets,
with a brief introduction to these technologies. However, the biomethane trade occurs among European countries in the form of
literature-based advancements in these technologies were not consid­ biomethane certificates. For example, in 2015, 162 GWh/y biomethane
ered and the techno-economic and environmental comparisons were was exported and 216 GWh/y was imported [25] within Europe, as
also not investigated [23]. shown in Fig. 3.
Several advancements have been made in cryogenic biogas upgrad­ Denmark exported 165 GWh/y of biomethane in 2016 [25]. Simi­
ing technologies in recent years. The potential of these technologies has larly, the Netherlands and Sweden imported 243 GWh/y and 155
not yet been fully explored owing to the communication gap between GWh/y of biomethane in 2016, respectively [25]. Switzerland increased
academic researchers and process engineers. To bridge this gap, a the amount of biomethane imported to 308 GWh/y in 2016 [25] and
detailed assessment of cryogenic technologies is essential for the design 460 GWh/y in 2018 [26]. This increasing trade demonstrates the
and development of an economical and environment-friendly biogas
upgrading process. This type of assessment provides a useful platform
for new academic researchers who intend to work in this field as it
clarifies the scope for future research. In this context, a brief and
comprehensive assessment-based review is required to consider three
major aspects: energy, economics, and the environment. To the best of
the authors’ knowledge, no such study has yet been reported in the
literature. Therefore, an assessment-based study that considers the 3Es
(namely, the energy, economic, and environmental perspectives) of
cryogenic biogas upgrading technologies is proposed. The present study
aimed to provide a technical and economic assessment of recent con­
ceptual studies as well as the assessment of LCA studies. Conceptual
studies based on biogas feed conditions, product conditions, process
simulation tools, energy consumption, and economic evaluation are
examined in this study. Furthermore, the LCA studies considering the
biogas upgrading is evaluated in this study. The technical challenges
associated with cryogenic technologies are analyzed, and the future
prospects and practical implications are discussed. The paper is
compiled as follows: a brief overview of biogas production is provided in
Section 2, an overview of cryogenic biogas upgrading technologies is
provided in Section 3, and a table of recent cryogenic biogas upgrading
processes and a technical–economic assessment is presented in Section
4. The biogas supply chain is investigated based on an LCA in Section 5,
and the technical challenges associated with cryogenic technologies and Fig. 2. Yearly trend of global biomethane production [24].
future research prospects are presented in Sections 6 and 7, respectively.

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A. Naquash et al. Renewable and Sustainable Energy Reviews 154 (2022) 111826

Table 2
Biomethane quality standards for transportation through a pipeline and in the
liquefied form.
Components Pipeline transportation [28] LBM transportation [29]

CO2 <3% <25 ppmv

H2S <5 mg/m3 <4 ppmv
H2O <32 mg/m3 <1 ppmv

Fig. 3. Biomethane certificates exported and imported in 2015 in Europe [25].

importance of biomethane as a clean fuel, the use of which must be

expanded globally to achieve future emission targets by reducing the use
of fossil-based fuels. Nevertheless, the major challenge in interconti­
nental or international trade is the uneconomical mode of trans­
portation. Generally, biomethane is transported by road in a compressed
state or through tankers in a liquefied form. The transport of liquefied
biomethane over long distances (>25–250 km) [27] can be economical
depending on the shipment size. To transport biomethane in the com­
Fig. 4. Comparison of biogas upgrading technologies [17].
pressed form through pipelines or in liquefied form via ships, certain
quality standards need to be considered, some of which are listed in
Table 2. 4. Cryogenic biogas upgrading
Different upgrading technologies, such as water scrubbing (WS),
amine scrubbing, membrane technology, adsorption, and cryogenic Cryogenic technology can be categorized into two types based on the
separation, are commercially available. The biogas upgrading technol­ required product state: anti-sublimation-based technology (solid CO2) or
ogies are summarized in Fig. 4 in terms of the associated specific energy distillation-based technology (liquid CO2). In an anti-sublimation pro­
consumption (SEC), production costs, and number of installed upgrad­ cess, CO2 is directly converted from the vapor phase to the solid phase,
ing plants [17]. bypassing the liquid phase. Methane is extracted in the vapor phase, and
According to Fig. 4, WS accounts for the maximum number of then liquefied for end use. In the cryogenic distillation process, both CO2
installed plants (175) due to its lowest energy consumption of 1.42 and CH4 are separated and extracted in liquefied forms.
kWh/kg [17]. The worldwide SEC for membrane technology is 1.59
kWh/kg with 148 biogas upgrading plants, while those for physical and 4.1. Cryogenic distillation process
chemical scrubbing plants is 1.46 kWh/kg and 2.27 kWh/kg, respec­
tively. Among these technologies, cryogenic technology is applied in the Cryogenic distillation is a technique used for upgrading biogas and
fewest biogas upgrading plants (≤9) [17]. extracting CO2 in a liquefied form by carefully considering the CO2
The application of cryogenic technology for biogas upgrading is freezing temperature and pressure. A study was conducted by Yousef
relatively new. The major limitations of this technology are its high et al. [33] to upgrade biogas cryogenically in a distillation column by
energy consumption (3.28 kWh/kg) and production cost (0.45 €/kg) due avoiding CO2 freeze-out. The raw biogas was pressurized to 4.9 MPa and
to the required operating conditions, i.e. high pressure (~80 bar) and cooled to − 61 ◦ C before it entered the distillation column [33]. The CO2
low temperature (~− 110 ◦ C) [17]. Therefore, cryogenic could be extracted from the bottom in a liquefied form and biomethane
technology-based commercial biogas upgrading plants account for only (with 94.4% purity) was extracted from the top with an SEC of 0.31
2% of all plants [30]. The major benefit of cryogenic technology is the kWh/Nm3 raw biogas [33]. To avoid CO2 freezing on the trays, the effect
high product purity (99.9%) obtained [31]. Thus, cryogenic biogas of CO2 freezing with respect to tray temperatures was also studied [33].
upgrading technology has promising potential owing to its benefits To improve product purity, a dual-column low-temperature distillation
when integrated with liquefaction [32] due to the lower operational process was further proposed [34]. Using the proposed process [34],
temperature requirements. CH4 at a purity of 97.12% was obtained with an energy consumption of
0.35 kWh/Nm3 raw biogas. In another study [35], a distillation column

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A. Naquash et al. Renewable and Sustainable Energy Reviews 154 (2022) 111826

Fig. 6. Process flow diagram of an anti-sublimation process for biogas

upgrading [17].
Fig. 5. Flow diagram of the cryogenic distillation process for biogas upgrad­
ing [35].
were investigated using a phase diagram of the CH4–CO2 binary system
and a process was proposed to recover liquid CO2 and biomethane. The
was used to obtain a high product purity with reduced power con­ CO2 extracted from the cold box simultaneously provided precooling in
sumption (0.24 kWh/Nm3). The cold energy of biomethane was utilized the heat exchanger and allowed the extraction of frozen CO2 from the
to cool biogas prior to distillation, which in turn reduced the SEC to cold box at a high pressure. N2 was used to provide additional cooling
0.236 kWh/Nm3 raw biogas and improved the CH4 purity to 97.16% for biogas and biomethane liquefaction in this process [17]. The SEC of
[35]. Therefore, further studies are required to reduce the energy con­ the process was 1.3 kWh/kg with a specific production cost of 0.1 €/kg
sumption and increase the CH4 purity. The process mechanism is illus­ [17]. The process mechanism is illustrated in Fig. 6.
trated in Fig. 5.
4.3. Controlled freeze zone technology
4.2. Anti-sublimation process
The controlled freeze zone (CFZ) is a cryogenic single distillation
In the anti-sublimation process, cleaned biogas is upgraded by column technology that was introduced by ExxonMobil in 1983 [37] to
passing it through a specially designed chamber operating at cryogenic remove impurities, including CO2, from natural gas [38]. Exxon built a
temperatures. The pressure and temperature are adjusted according to pilot plant to process 0.02 MNm3/day of feed natural gas with varying
the phase diagram of the CH4–CO2 binary system [17]. CO2 solidifies in CO2 compositions (15–65%) [38]. The CO2 content could be reduced to
this specially designed chamber [16]. After a certain period, solid CO2 is the order of 100 ppm in the product gas [38]. The commercial plant had
removed from the chamber through melting or vaporization. Alterna­ minimum and maximum operating capacities of 0.4 and 28.3
tively, the vaporization and solidification of CO2 can be performed in MNm3/day of natural gas, respectively [38].
separate chambers [16]. The anti-sublimation of CO2, i.e., the solidifi­ Using this technology, CO2 is removed from natural gas by freezing
cation process for flue gas, has been patented by Clodic et al. [16]. In this and melting in a specially designed chamber that is installed in the
process, CO2 is solidified after heat exchange with refrigerants in a middle of a distillation column (Fig. 7). The upper and lower sections of
specially designed cold chamber. The pressure and temperature are the distillation column are conventional packed beds, and the middle
adjusted according to the vapor–solid equilibrium. After a certain time, section is an open chamber consisting of three subsections (spray noz­
solid CO2 is removed from the chamber by adjusting its temperature and zles, freeze zone, and melting trays). Dehydrated and cooled natural gas
pressure to the CO2 triple point for the conversion of CO2 into the liquid is introduced as a feed into the upper section of the column. The CH4 is
phase for extraction. This system is operated simultaneously with vaporized in the upper section as a result of an increase in the residual
another system to maintain a continuous product supply. In another CO2 concentration because the temperature is higher than that below.
process developed by Schach et al. [36], CO2 was removed from flue gas Liquid CO2 is sprayed through nozzles (in the middle section), which
through an anti-sublimation process. The process was simulated in leads to solidification. Pure solid CO2 is collected on the melt trays by
Aspen Plus using the RGibbs block by considering anti-sublimation maintaining a temperature above the solidification temperature. The
vapor–solid equilibria (CO2 (solid) ↔ CO2 (vapor)) [36]. In this process, solid melts into a liquid and is removed from the bottom (stripping)
the feed gas temperature was reduced to the CO2 solidification tem­ section. The stream leaving the bottom is high-purity liquid CO2, while
perature (155 K) at 100 kPa, and 90% pure CO2 was extracted. The that leaving the top is high-purity CH4. According to Exxon [39], the CFZ
temperature and pressure conditions were calculated based on the phase technology is more economical in terms of its capital costs (10–27%
diagram. The SEC of the process was 0.286 kWh/kgCO2 [36]. A similar lower) than other gas treatment options (Selexol, Ryan-Holmes). How­
process mechanism was applied by Spitoni et al. [17] for biogas ever, the CFZ technology is not economical when the gas contains a
upgrading, where the solidification temperature and pressure of CO2 significant amount of ethane or higher hydrocarbons [40].

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Scholz et al. [44]. The results showed that the membrane-integrated

conventional upgrading processes showed low upgrading costs
compared to membrane-integrated cryogenic processes. However, the
hybrid cryogenic processes were competitive in terms of plant profit,
when compared with conventional processes [44].

5. Assessment of cryogenic biogas upgrading systems

A state-of-the-art assessment of cryogenic technologies for biogas

upgrading has been presented in this section with respect to energy,
economic, and environmental aspects. This assessment evaluated the
SEC, specific production cost, process simulation tool, equation of state
model, economic analysis, and product conditions, as shown in Table 3.
The findings of recent conceptual studies related to cryogenic biogas
upgrading systems are summarized in Table 3, and the energy and
economic evaluations are presented based on these findings. The envi­
ronmental aspect was considered with respect to the LCA of the biogas
supply chain. According to the literature [45], the overall supply chain
of biogas is generally considered when an LCA is conducted. Therefore,
in this study, LCA studies of the biogas supply chain were investigated,
particularly for biogas or biomethane production. A comparison of
biogas LCA studies is presented in Table 5 in terms of the functional unit,
methodology, and output.
According to Table 3, the cryogenic distillation process is most
commonly applied for cryogenic upgrading of biogas. The major limi­
tations of the cryogenic distillation process are a low CH4 purity and CO2
freezing in the distillation column. CO2 freeze-out at different pressures
in a distillation column was analyzed by Yousef et al. [14] to determine
the optimal temperature and pressure ranges. These temperature and
pressure ranges limit the purity of the product. To overcome these
Fig. 7. Process flow diagram of controlled freeze zone technology for biogas limitations, the dual-column distillation process [34] may be a feasible
upgrading [38].
approach for obtaining a high product purity. Using a dual-column
distillation process, Hashemi et al. [32] obtained 99.99% pure LBM.
4.4. Hybrid cryogenic technologies Another benefit of using a dual-column distillation process is the low
energy consumption. According to Yousef et al. [34], the same CH4
Standalone cryogenic technologies have several bottlenecks purity (97%) is obtained, but the energy consumption significantly re­
including their high energy consumption [17], high production costs duces when two columns are utilized (<1.8 MJ/kgCO2) instead of one
[17], and availability of cold energy sources [23]. A hybrid process (i.e., (>3 MJ/kgCO2).
cryogenic adsorption with a membrane process) is a viable option to The freezing of CO2 has also been exploited by some authors to
overcome these challenges. An experimental and simulation study was achieve separation of CO2 from CH4 [17]. This approach is relatively less
conducted by Song et al. [41] to analyze the energy-saving opportunities explored than the distillation process because of the lack of a dedicated
using the membrane–cryogenic hybrid process for biogas upgrading. An unit operation block for the anti-sublimation process in simulation
experimental study [41] was conducted using polyimide and poly­ software i.e., Aspen Plus and HYSYS. Another major limitation of the
sulfone membrane modules at different temperature ranges anti-sublimation process is the high energy consumption (0.77
(− 30–40 ◦ C), and the simulation of the integrated scheme was per­ kWh/Nm3 raw biogas) [52]. Therefore, efforts should be made to reduce
formed using the results. The results of this experimental study revealed the energy consumption by optimizing either the selected CO2 freezing
that the process had good performance in terms of product purity (CH4: temperature and pressure or the refrigeration cycle.
98%, CO2: 99.8%) and energy consumption (0.8 MJ/kgCH4) at a low Cryogenic hybrid processes are considered to be viable options for
membrane unit temperature [41]. In another simulation study [42], an biogas upgrading. An experimental and simulation study of an inte­
adsorption–cryogenic hybrid process was investigated for CO2 capture grated membrane–cryogenic hybrid process was conducted by Song
using vacuum swing adsorption followed by CO2 liquefaction. The re­ et al. [22], wherein they presented a performance comparison of the
sults showed that the specific shaft work of 1.4 GJ/t CO2 was required to hybrid process with other standalone and hybrid processes. Their [22]
obtain 98.2% CO2 purity. A techno-economic study was conducted by process was energy efficient, with an SEC of 0.8 MJ/kgCH4 at a high
Anantharaman et al. [43] for carbon capture using hybrid mem­ product purity (CH4: 98%) and recovery (CH4: 98.2%).
brane–cryogenic technology. According to their analysis, the cost (48 Among the reviewed processes, the cryogenic distillation process is
€2008/t CO2 avoided) of CO2 avoided to achieve a CO2 capture ratio of 85% considered to be energy efficient. Owing to the CO2 freeze-out problem,
was 9% more than that for the MEA absorption process [43]. A other cryogenic processes (such as anti-sublimation and cryogenic
comparative techno-economic study of the hybrid processes for biogas hybrid processes) should be further explored. However, studies dealing
upgrading, including the cryogenic hybrid processes, was conducted by with economic analyses are limited. Thus, economic analysis and

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Table 3
Technical and economic assessment of cryogenic biogas upgrading technologies.
Year Process Technology Biogas Feed Sp. Energy Sp. Product Economic Simulator/ Remarks Ref
composition Conditions consumption Production Specs analysis EOS
Cost

2011 BM- Cryogenic CH4: 0.55, T: 25 ◦ C, P: 2.9 MJ/kg N/A Purity: CH4: N/A N/A The proposed [46]
CO2 Adsorption CO2: 0.45 2 bar, CH4 99.1% process used
(Packed bed) Flowrate: Recovery: cryogenic
0.312 kg/s CH4: 94.3% adsorption
(packed bed) to
upgrade biogas.
The proposed
process’s
productivity was
reported as 350.2
kg CH4/hm3
compared to the
base case
(vacuum pressure
swing adsorption)
value (43.1 kg
CH4/hm3) with
improved product
purity. The energy
consumption was
22% lower as
compared to
VPSA.
2014 BM- Cryogenic CH4: 0.985, T: N/A N/A N/A N/A gPROMS Two different [47]
CO2 Adsorption CO2: 0.15 198–279 K, commercial
(TSA) P: 1–10 adsorbents
bar, (zeolite 4 A and
Flowrate: 13X) have been
67–260 tested against the
cm3/min cryogenic
separation of
methane and
carbon dioxide in
the TSA process.
An experiment
was performed on
a cryogenic
temperature
range of 198
K–279 K and a
pressure range of
1–10. Methane
adsorption on
zeolite 13X was
idealized in
contrast with
zeolite 4 A,
preferable for
carbon dioxide
adsorption.
2016 BM- Distillation CH4: 0.60, T: 35 ◦ C, P: 0.26 kWh/ N/A Purity: CH4: N/A Aspen A single column [48]
LCO2 CO2: 0.40 120 kPa, Nm3 raw 94.5%, Hysys/PR low temperature
Flowrate: biogas CO2: distillation
1000 99.7%. scheme was
kmol/h Recovery: applied to remove
CH4: CO2 from biogas
99.85%, in liquid form by
CO2: avoiding CO2
91.22% freeze out. The
specific energy
consumption per
CO2 captured was
reported as 1377
MJ/ton CO2 and
the CO2 capture
rate was reported
as 91.3%.
2017 BM- Dual column CH4: 0.60, T: 35 ◦ C, P: 0.25 kWh/ N/A Purity: CH4: Investment Aspen A cryogenic [49]
LCO2 low CO2: 0.40 120 kPa, Nm3 raw 97.12%, cost (M€) = Hysys/PR distillation
temperature Flowrate: biogas CO2: 1–2.7 column-based
distillation 1000 99.92%, technology was
kmol/h Recovery: applied to remove
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A. Naquash et al. Renewable and Sustainable Energy Reviews 154 (2022) 111826

Table 3 (continued )
Year Process Technology Biogas Feed Sp. Energy Sp. Product Economic Simulator/ Remarks Ref
composition Conditions consumption Production Specs analysis EOS
Cost

CH4: CO2 and CH4 from

99.85%, biogas in liquid
CO2: form. The
95.65% proposed process
uses the cryogenic
energy of
liquefied
biomethane to
precool raw
biogas, which
reduces energy
consumption. The
proposed process
shows that the
specific energy
consumption was
also the same for
increased capacity
(i.e., 2000 Nm3/
h)
2018 LCO2- Distillation CH4: 0.60, T: 35 ◦ C, P: 0.31 kWh/ N/A Purity: CH4: N/A Aspen A distillation [33]
BM CO2: 0.40 102 kPa, Nm3 raw 94.4%, Hysys/PR column is used to
Flowrate: biogas CO2: separate CO2 from
100 kmol/ 99.5%. CH4 in liquid form
h Recovery: with 99.5%
CH4: 99.7% purity. The
optimum process
conditions were
sorted out to
achieve maximum
recovery by
avoiding CO2
freeze out in the
distillation
column. The
refrigeration was
provided by
propane and
ethane based
cascaded vapor
compression
refrigeration
cycle.
2018 LBM- Distillation CH4: 0.60, T: 35 ◦ C, P: N/A N/A Purity: CH4: N/A Aspen CH4 and CO2 [50]
LCO2 CO2: 0.40 200 kPa, 99.7%, Hysys/PR separation was
Flowrate: CO2: carried out in a
63.44 99.98% single distillation
kmol/h column. Nitrogen
is used as a
refrigerant to
provide cold duty.
They have not
stated the SEC and
not evaluated the
CO2 freezing
behavior in the
distillation
column as because
at their product
conditions
(− 56.6 ◦ C/2620
kPa), CO2 freezing
occurs.
2018 LBM- Ryan Holmes CH4: 0.60, T: 35 ◦ C, P: N/A N/A N/A N/A Aspen Three cryogenic [51]
LCO2 extractive CO2: 0.40 1 atm Hysys/SRK biogas upgrading
distillation, processes (Ryan
Dual pressure Holmes extractive
low distillation, Dual
temperature pressure low
distillation and temperature
anti- distillation and
sublimation anti-sublimation)
has been studied
and compared
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A. Naquash et al. Renewable and Sustainable Energy Reviews 154 (2022) 111826

Table 3 (continued )
Year Process Technology Biogas Feed Sp. Energy Sp. Product Economic Simulator/ Remarks Ref
composition Conditions consumption Production Specs analysis EOS
Cost

with MEA based

chemical
absorption
process as a base
case. The
processes were
compared based
on the energy
consumption in
terms of produced
biomethane
required for LBM
production such
that the low
temperature
distillation
process has the
least percentage
of produced
biomethane for
LBM production
(14%), which
means this process
is least energy
intensive.
Moreover, the
operation type
wise energy
consumption
analysis was also
conducted, which
shows that the
maximum energy
consumption in
the Ryan Holmes
process takes
place for biogas
upgrading
(39.74%) step. In
the distillation
process, the
maximum
percentage of
energy
consumption took
place at the biogas
compression
(38.19%) step,
whereas in the
anti-sublimation
process, the
biogas upgrading
step consumes
major energy
(73.99%).
2018 BM- Membrane - CH4: 0.60, T: 35 ◦ C, P: 0.8 MJ/kg N/A Purity: CH4: N/A PROII/PR A membrane- [41]
LCO2 Cryogenic CO2: 0.40 120 kPa, CH4 98%, CO2: cryogenic hybrid
hybrid Flowrate: 99.8%, process was
1000 Recovery: studied through
kmol/s CH4: 98.2% experiment and
simulation. The
experimental
study was
conducted using
polyimide (PI)
and polysulfone
(PSF) membrane
and then the
process was
simulated using
the experimental
data. The PSF
membrane was
found was found
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A. Naquash et al. Renewable and Sustainable Energy Reviews 154 (2022) 111826

Table 3 (continued )
Year Process Technology Biogas Feed Sp. Energy Sp. Product Economic Simulator/ Remarks Ref
composition Conditions consumption Production Specs analysis EOS
Cost

more economical
in terms of
product purity
and energy
consumption.
2018 LBM- Anti- CH4: 0.40, T: 30 ◦ C, P: 0.53 kWh/kg N/A Purity: CH4: N/A Aspen A cryogenic (anti- [52]
LCO2 sublimation CO2: 0.59, 1.5 bar, raw biogas 97% Hysys/ sublimation)
H2S: 0.005, Flowrate: PRSV process was
H2O: 0.005 10 t/d simulated and
compared with
conventional
biogas upgrading
and liquefaction
process. The
results of
cryogenic
upgrading showed
that the proposed
technology is
economical (0.61
kWh/Stm3 raw
biogas) in terms of
energy
consumption for
small scale plant.
2018 BM- Dual column CH4: 0.60, T: 35 C, P: 0.35 kWh/ N/A Purity: CH4: Investment Aspen A dual column [34]
LCO2 low CO2: 0.40 120 kPa, Nm3 raw 97.12%, cost (1000 Hysys/PR low temperature
temperature Flowrate: biogas CO2: €/(m3/h)) distillation
distillation 100 kmol/ 99.92%, = 2.3–8.6 process was
h Recovery: proposed for the
CH4: separation of CO2
99.95% from CH4 in liquid
form. The CO2
freeze out the
temperature was
carefully
monitored to
avoid freeze out
and analysis was
provided against
tray temperatures
in both distillation
columns. The
impact of varying
feed pressure and
CO2 mole fraction
on CO2 freeze out
was also studied
in the proposed
process.
Moreover, the
effect of the reflux
ratio on product
purity was also
analyzed.
2019 LCO2- Anti- CH4: 0.5, T: 35 ◦ C, P: 1.297 kWh/ 0.18 €/kg Recovery: Operating Aspen CO2 is separated [17]
LBM sublimation CO2:0.5 1 bar, kg LBM CH4: 100% cost (1000 Hysys/PR in solid form from
Flowrate: €/y): >300 CH4 in an HX.
308.2 kg/h Solid CO2 is then
extracted from HX
by direct contact
with CO2 gas to
liquefy and
remove
2019 LCO2- Dual column CH4: 0.6, T: 35 ◦ C, P: 2.07 kWh/kg N/A Purity: CO2: N/A Aspen The cryogenic [32]
LBM low CO2: 0.399, 1 atm, LBM 97.97%, Hysys/SRK separation process
temperature H2S: 0.001 Flowrate: CH4: is compared with
distillation 1000 99.9%. the conventional
kmol/h Recovery: amine absorption
CH4: process. The
98.7%, cryogenic process
CO2: is economical in
99.93% terms of energy
consumption. The
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A. Naquash et al. Renewable and Sustainable Energy Reviews 154 (2022) 111826

Table 3 (continued )
Year Process Technology Biogas Feed Sp. Energy Sp. Product Economic Simulator/ Remarks Ref
composition Conditions consumption Production Specs analysis EOS
Cost

amine absorption
process has a
specific energy
consumption of
3.35 kWh/kg
LBM.
2019 LCO2- Distillation CH4: 0.611, T: 25 ◦ C, P: 3.574 kWh/ N/A Purity: CO2: N/A Aspen Amine scrubbing, [53]
BM CO2: 0.3693, 2 bar, kmol 98.48%, Hysys/ water scrubbing,
O2: 0.0098, Flowrate: CH4: Sour SRK caustic wash and
N2: 0.0098, 20 kmol/h 95.46%. cryogenic process
H2S: 124 Recover: for biogas
ppm, CH4: 94% upgrading was
NH3: 3 ppm simulated and the
results were
compared. The
cryogenic process
has the highest
electricity
demand of 71.5
kW, followed by
the water
scrubbing process
(38.8 kW).
2019 LCO2- Distillation CH4: 0.60, T: 35 ◦ C, P: 0.38 kwh/ N/A Purity: CH4: Investment Aspen Cryogenic [35]
BM CO2: 0.40 125 kPa, Nm3 cleaned 97.16%, cost (1000 Hysys/PR distillation-based
Flowrate: biogas CO2: €/(m3/h)) biogas upgrading
120 kmol/ 99.26% = 2.27–8.49 process was
h proposed. In this
scheme, the
cryogenic energy
of liquid biogas
(extracted from
separator) was
used to precool
raw biogas to
reduce the cooling
load on the
refrigeration
cycle.
2020 CO2- Distillation CH4: 0.611, T: 25 ◦ C, P: 11.2 kWh/ N/A Purity: CO2: N/A Aspen The exergy [54]
BM CO2:0.369, 200 kPa, kmol 98.48%, Hysys analysis-based
H2S: 0.0001, Flowrate: CH4: study is conducted
N2: 0.0098, 320 kmol/ 92.67%. for biogas
O2:0.0098 h upgrading in an
integrated ORC-
ARC-MR process.
The ORC cycle is
supported by solar
power to provide
the required
power using R113
as a working fluid.
The NH3 water-
based ARC system
is applied to
provide required
precooling and
MR is utilized for
liquefaction. A
detailed exergy
analysis is
conducted. The
overall exergy
efficiency is
71.62%.
However, the
product purity of
methane and
recovery is low.

N/A = Not Available.

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A. Naquash et al. Renewable and Sustainable Energy Reviews 154 (2022) 111826

Table 4 phase change. Therefore, the greatest amount of energy (approxi­

Cost comparison of cryogenic biogas upgrading technologies with conventional mately 99.8% [51]) is utilized in this step to provide the cooling duty for
technologies. the phase change.
Technology Specific Investment Costs Plant capacity References Upgrading technologies can also be compared based on the bio­
(€/Nm3/h) (Nm3/h) methane purity with respect to energy consumption, as shown in Fig. 9.
Water scrubbing 5500 100 [28] In cryogenic distillation, CO2 freezing is a major bottleneck for
Organic 4500 250 [28] achieving a high biomethane purity [33] and it occurs at the top of the
scrubbing distillation column at low temperatures, restricting the further purifi­
Chemical 3200 600 [28]
cation of biomethane [33]. To mitigate this problem, the upgrading
scrubbing
PSA 2700 600 [28] process can be performed in two distillation columns operating at
Membrane 2500 400 [28] different pressures [34]. A dual-pressure low-temperature distillation
Cryogenic 5600 152 [35] configuration was simulated in Aspen HYSYS by Hashemi et al. [32].
distillation The high-pressure column operated at 50.5 bar, while the low-pressure
column operated at 39.5 bar. The first column yielded biomethane with
process optimization to reduce energy consumption should be con­ a purity of approximately 90.8%, and the purity of this biomethane was
ducted to determine the most energy-efficient and economical biogas increased to up to 99% in the second column [32]. Both columns
upgrading process. operated at low temperatures (− 70 and − 88 ◦ C), which is beneficial for
obtaining the final product in a liquefied form. Moreover, a
dual-pressure distillation configuration for biogas upgrading was stud­
5.1. Energy analysis ied by Yousef et al. [34]. According to their results, the first and second
columns produced 94.5% and 99% pure biomethane, respectively, with
This section evaluates cryogenic technologies (distillation and anti- an SEC of 0.35 kWh/Nm3 raw biogas [34]. They comprehensively
sublimation) based on energy consumption. Cryogenic biogas upgrad­ studied the freezing of CO2 by considering multiple factors, such as the
ing processes are energy intensive owing to the high compression energy reflux ratio, percentage of CO2 in the feed, and column tray tempera­
and cooling requirements for biogas upgrading [51]. The energy con­ tures [34]. Single column distillation was simulated in HYSYS for biogas
sumption by three well-known biogas upgrading cryogenic technologies upgrading by Ahmad et al. [50] to analyze the biomethane purity that
were compared herein by operation type (compression, upgrading, and can be obtained using a single distillation column. They achieved 99.7%
liquefaction) and quality type (mechanical power, heating duty, and biomethane purity and 99% CO2 purity [50]. However, they did not
cooling duty), as shown in Fig. 8. According to Fig. 8, compression en­ analyze the CO2 freezing phenomenon on the distillation column trays.
ergy required in the dual-pressure low-temperature distillation accounts The CO2 freezing problem in the low-temperature single distillation
for 38% of the total energy consumed, owing to the high-pressure of column was analyzed by Yousef et al. [33], wherein up to 97% bio­
40–50 bar required for distillation [51]. The cryogenic duty required for methane purity was achieved with an SEC of 0.31 kWh/Nm3 raw biogas
a low-temperature distillation column accounts for approximately 62% [33]. It is apparent from the foregoing discussion that the single column
of the total energy required to produce liquefied biomethane [51]. The distillation process is preferable if the CO2 purity is not a constraint. If
energy required for the upgrading step and that for the heating load the cost is not a constraint, the dual column distillation process is the
account for 32% and 12% of the total energy utilized by the preferable option to achieve the maximum biomethane purity.
Ryan-Holmes process, owing to the regenerative step involved in Owing to the fact that CO2 freezes at low temperatures during
extractive distillation [51]. The anti-sublimation process is different in upgrading, multiple researchers have studied the solid–vapor phase
terms of energy consumption compared to other processes. The most behavior of CO2–CH4 and proposed a process to extract CO2 from CH4 in
energy-intensive step involved in anti-sublimation is the upgrading step, the solid form (anti-sublimation). The anti-sublimation process was
as it requires approximately 74% of the total energy consumed [51]. In compared with conventional processes by Baccioli et al. [52]. They
the upgrading step, CO2 is removed from CH4 through a solid-to-vapor

Fig. 8. Energy distribution for cryogenic biogas upgrading by operation type Fig. 9. Comparison of biogas upgrading cryogenic technologies with respect to
(Bottom X, Left Y) and quality type (Top X, Right Y). biomethane purity and energy consumption.

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A. Naquash et al. Renewable and Sustainable Energy Reviews 154 (2022) 111826

reported 97% biomethane purity with an SEC of 1.45 kWh/kg bio-LNG

[52]. The CO2 freezing phenomenon was also exploited by Spitoni et al.
[17], who proposed a scheme to extract CO2 in the liquid form, preceded
by CO2 solidification with a 100% biomethane recovery. The high SEC
obtained for the anti-sublimation process was due to the cryogenic en­
ergy required for CO2 solidification, combined with the compression
energy (which is dependent on the CO2 fraction present in the feed gas
and the operating pressure) [17].

5.2. Economic aspects

In this section, the economic evaluation of cryogenic technologies is

presented based on investment and production costs. An economic
comparison of biogas upgrading through cryogenic single column and
dual-column distillation processes at two different capacities is pre­
sented in Fig. 10. An increase in capacity leads to a decrease in invest­
ment cost by maintaining a constant feed composition and the same
operating conditions for both single and dual-column distillation, as
shown in Fig. 10. It is evident from this figure that a shift from a single
column to a dual-column distillation process results in an increase in the
investment cost by 1.30% and 2.94% for capacities of 1400 and 600 m3/
Fig. 11. Specific production cost of LBM for cryogenic distillation and desu­
h, respectively. This increase in investment cost is lower for large- blimation technologies.
capacity plants and higher for small-capacity plants.
A cost comparison of the two cryogenic technologies, i.e., distillation
The cryogenic distillation process shows a high investment cost
and anti-sublimation, is presented in Fig. 11. The costs were presented
compared to conventional technologies. One possible reason is the high
by Spitoni et al. [17] using the terms “Traditional Cryogenic” and
cost of distillation columns. However, it should be noted that these costs
“Desublimation” for the cryogenic technologies. In this study, “Tradi­
are strictly dependent on the plant capacity. Increasing the capacity of a
tional Cryogenic” was assumed to imply cryogenic distillation
plant may decrease the investment cost [28].
technology.
As shown in Fig. 11, cryogenic distillation has a higher production
6. LCA perspective
cost than desublimation. This is because cryogenic distillation requires a
higher pressure for biogas, which increases the cost of LBM production.
LCA is an important approach to study the environmental impacts of
In contrast, the desublimation process is performed below the triple
biogas and its supply chain management. It is a cradle-to-grave approach
point of CO2 to avoid the CO2 liquid phase. Therefore, it does not require
to assess the environmental impacts of raw material extraction, pro­
a very high pressure. The production cost of the anti-sublimation process
duction, processing, transportation, and end use [19]. The raw material
(0.2 €/kg) is lower than that of the distillation process (0.45 €/kg). The
for biogas is obtained from organic sources, and it is crucial to examine
specific investment cost to upgrade biogas through the cryogenic pro­
the environmental impacts of biogas production and upgrading.
cess is compared with that of other conventional technologies in Table 4.
Therefore, many existing studies have analyzed different aspects of the
biogas supply chain through LCAs to determine methods for reducing
environmental impacts [55]. A brief literature review on the LCA of
biogas is presented in Table 5.
In Table 5, a detailed comparison of LCA studies on biomethane
production is presented. A brief discussion is presented to evaluate the
environmental impact determined in each study. Starr et al. [19] con­
ducted an LCA on three biogas upgrading technologies, namely HPWS,
AWR and BABIU. The GWPs of BABIU and AWR were significantly lower
than that of HPWS, mainly because of the utilization of carbon miner­
alization technology by the AWR and BABIU processes. In another study,
conducted by Fusi et al. [58], an LCA was conducted on five different
plants with feedstocks including animal and food wastes. The GWP for
Plant 5 (cow slurry) was the lowest i.e., − 395 kg CO2 eq./MWh elec­
tricity, illustrating the sequestration of CO2 emissions [58]. Similarly,
Patterson et al. [56] conducted an LCA for biomethane production with
biogas upgrading through PSA. The system boundary was delineated
from feedstock (food waste and wheat feed) collection to biomethane
utilization as a vehicle fuel. The results showed that biomethane derived
from food waste is beneficial for reducing vehicular emissions.
Fig. 12 presents a comparison of the GHG emissions of biomethane
production from biowaste (municipal organic and agro-industrial) and
Fig. 10. Economic comparison of cryogenic biogas upgrading technologies.
biomass sources (mainly maize crop). These bio feedstocks and natural

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Table 5
Literature review of LCA studies conducted on biogas supply chain.
Year LCA Technologies Software/ The functional unit Output Remarks Ref.
Methodology (FU)

2011 Biomethane Amine gas treatment ISO Standard GHG emissions/kWh GHG emissions: 44.6 The LCA of the biomethane [45]
production (2006a) and ISO biomethane gCO2eq/kWh production process was conducted
Standard (2006b) and the findings were compared with
natural gas plant emissions. The GHG
emissions were 82% lower than
natural gas, with a total cumulative
energy demand of 180.4%.
2012 Biogas High-pressure water GaBi/CML 2001 Removal of 1 ton BABIU: 9.1 t of CO2 The AWR process has the lowest [19]
upgrading scrubbing (HPWS), impact assessment CO2 from Biogas equivalent/FU. impact in GWP due to its ability to
technologies Alkaline with method (ISO HPWS: 9.1 t of CO2 store CO2. BABIU process has the
regeneration (AWR), 14040) equivalent/FU. highest impact on ODP because of
Bottom ash upgrading AWR: 8.0 t of CO2 transportation through diesel and the
(BABIU) equivalent/FU. lowest impact in GWP. Overall, the
BABIU process has the lowest
environmental impact.
2013 Bio-hydrogen Anaerobic digestion- Sima Pro/Eco- Production of fuel N/A Two stages (biohydrogen- [56]
and biomethane based hydrogen/methane indicator 99 (biomethane/ biomethane) and single stage
production production using PSA Hierarchist/ biohydrogen- (biomethane) experimental study
technology Average biomethane) were conducted to study hydrogen
and methane yield using two types of
feedstocks (food waste and wheat
feed). The two-stage process for
wheat feed has a lower impact on the
environment than a single stage
process, whereas the food waste two-
stage process results in an increased
environmental burden.
2015 Biomethane Absorption process openLCA/CML 1 kW of energy/1 m3 N/A The LCA study was conducted on [57]
2001 (ISO 14040) of biomethane biogas upgrading through an
absorption process using water,
diglycolamine, and polyethylene
glycol dimethyl ether as solvents.
Among these, water-based absorption
process was the most economical.
2016 Biogas Anaerobic digestion- GaBi/ISO 14040/ Generation of 1 Electricity generation: The LCA was conducted on electricity [58]
electricity based biogas 44 methodology MWh of electricity 781–7972 MWh/year generation produced through biogas
and CML 2001 (plant 1 to plant 5) in an integrated AD-CHP plant. Five
method plants are considered for this study
based on the feedstock. The results
show that plant 5 (feedstock: cow
slurry) was the most viable option
environment point (GWP: 395 kgCO2
eq.).
2019 Biomethane as a Anaerobic digestion SimaPro Environmental GWP: Biogas (food Biomethane is assessed as an [59]
transport fuel impact (GWP, ODP, waste) = 0.28, Biogas alternate fuel for transportation
POCP, AP, EP) (Manure) = 1.4. compared to natural gas, fossil diesel,
ODP: Biogas (food biodiesel, and electricity. The GWP
waste) = 0.02, Biogas analysis shows that electricity
(Manure) = 0.05. through coal has maximum GWP
POCP: Biogas (food with a major share of fuel production.
waste) = 0.11, Biogas Fossil diesel captures maximum
(Manure) = 0.13. percentage in ODP. The biogas
AP: Biogas (food system is economical in comparison
waste) = 1.12, Biogas to other fuels considering parameters
(Manure) = 3.76. (global warming, ozone depletion,
EP: Biogas (food acidification potential,
waste) = 0.42, Biogas eutrophication potential) after
(Manure) = 0.49. environmental impact analysis.
2020 Biogas Anaerobic digestion SimaPro/ReCiPe 1 ton of POME Total characterization An LCA study was conducted on [20]
production 2016 factor: biogas production from Palm Oil Mill
GWP (kgCO2 eq.) = Effluent (POME) considering system
2795.73 boundary from oil palm nursery to
LUC (m2a crop eq.) = the biogas production plant. A
1084.89. process wise product and emissions
WCP (m3) = 3024.6 were calculated in impact
assessment. Maximum CO2 emissions
(GWP) were from palm oil followed
by oil palm plantation.

N/A = Not Available.

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A. Naquash et al. Renewable and Sustainable Energy Reviews 154 (2022) 111826

The aforementioned challenges have not yet been overcome and

must be addressed for smooth, continuous, and energy- and cost-
efficient cryogenic upgrading of biogas.

8. Future recommendations

Several technologies are available for both biomethane production

and liquefaction. The selection of these technologies depends on factors
such as the percentage of CO2 in biomethane, energy consumption of the
liquefaction process, and economic feasibility. Among the biomethane
production processes, the cryogenic process is the most viable. It could
be integrated with the liquefaction process, thus ensuring less energy
consumption. The economic feasibility of integrating liquefaction
technology with biogas upgrading must be considered owing to the
limited number of biogas plants. Therefore, policymakers require a
comprehensive framework to enhance the production capacity of bio­
methane upgrading technologies and integrating them with liquefaction
technologies for advanced applications of LBM. Incentive-based policies
should be implemented for small-scale biogas and LBM plants to develop
and support local infrastructure. These policies may include government
or joint government–private sector funding, especially for small-scale
Fig. 12. Comparison of GHG emissions derived from bio feedstocks and natural biogas projects. Further, tax exemptions from the government for
gas [45,60]. biogas projects can pave the way for developing interest of industrialists
to invest in renewable fuels. A report by the EPA [62] was published that
gas sources are Europe-based, while the quality of the biomethane discussed the role of incentivizing biomethane production, with Ireland
produced meets the standards required for use in the natural gas grid being assessed for the case study. The authors recommended incentives
[60]. The results are compared with those of natural gas (German mix), to support and promote biomethane production, and similar policies can
indicating a significant reduction in GHG emissions by utilizing bio be devised in other countries to promote the biogas sector [62]. The
feedstocks. The greatest reduction (82%) is found at the biomethane other prospective recommendations for future are listed below:
production plant in Einbeck, Germany [60]. Fig. 12 also depicts the
potential of replacing natural gas with biomethane and its environ­ • Cryogenic upgrading of biogas provides an additional benefit if in­
mental impact in terms of reduced GHG emissions. tegrated with the liquefaction process, as both processes require a
low temperature for operation. Thus, the cryogenic upgrading pro­
7. Technical challenges cess should be integrated with the liquefaction of biomethane for
energy efficiency. A study was presented by Hashemi et al. [32] in
The main technical challenges associated with cryogenic biogas which cryogenic upgrading and absorption-based biogas upgrading
upgrading technologies are the high energy consumption and costs that followed by LBM production were compared and analyzed.
can be attributed to compression and equipment requirements, partic­ • Cryogenic biogas upgrading followed by LBM production requires
ularly for the cryogenic distillation process. Cryogenic processes require enormous amount of energy. The LBM production process could be
a low temperature, which is provided by the refrigeration cycle. The integrated with other processes, e.g., LNG regasification, to provide
refrigeration cycle consists of a multi-compression system that requires a the required cold energy.
substantial amount of energy for operation. The major challenges • New cryogenic upgrading technologies for CO2 anti-sublimation
associated with cryogenic technologies are listed below. must be explored to improve the technical and economic efficiency
of biogas upgrading. An alternative configuration-based thermody­
• The high energy consumption associated with the compression of namically feasible process must be designed for the efficient
biogas or refrigerants is a major challenge. handling of solid CO2.
• The high capital investment is attributed to the large amount of • Owing to new emerging technologies and recent developments, the
equipment involved, particularly in cryogenic distillation evaluation of optimal process economics and energy-saving oppor­
technology. tunities is vital. Rigorous multi-objective process optimization could
• The freezing of CO2 in cryogenic distillation columns is a challenge help reduce energy consumption and plant operating costs.
that should be overcome to ensure smooth operation. • Conventional and advanced exergy analysis is a proven thermody­
• In the anti-sublimation process, the handling of solid CO2 is complex namic approach for understanding process thermodynamics and
and challenging. evaluating the irreversibility of a process. The source and nature of
• Biogas compositions and conditions vary widely depending on their the irreversibilities could be quantitatively determined through
source. An optimal process design, that consider these variations, is advanced exergy analysis, so that such irreversibilities could be
important. The sensitivity analysis considering a wide range of reduced or optimized. This could facilitate the development of an
biogas composition is a viable option to design an optimal process. energy- and cost-efficient biogas upgrading process.
This analysis can help predict the effect of varying composition on • A comparative LCA study must be conducted to consider conven­
process conditions. In this way, an optimal and efficient process can tional and non-conventional biogas upgrading technologies so that a
be designed which can account for a range of biogas composition and fair comparison amongst these technologies can be made. A study on
conditions. A similar study was presented by Baccioli et al. [61] in this topic was conducted by Starr et al. [19]. However, the scope of
which they conducted a sensitivity analysis of varying methane their study was limited. A comprehensive discussion in this context
contents in biogas to study its effect on energy consumption. will be a matter of interest for academic researchers and industry
• Biogas is generally produced at a small scale. The economical pro­ practitioners.
duction of LBM from large-scale production processes is challenging.

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A. Naquash et al. Renewable and Sustainable Energy Reviews 154 (2022) 111826

9. Practical implications Acknowledgement

Renewable energy sources comprise approximately 19% of the This work was supported by the National Research Foundation of
global energy mix, and their contribution to the global energy mix is Korea (NRF) grant funded by the Korea government (MSIT)
estimated to increase to up to 50% by 2050 [63]. This increase in re­ (2021R1A2C1092152) and by Priority Research Centers Program
newables will have a dual impact: reduction in dependence on fossil through the National Research Foundation of Korea (NRF) funded by the
fuels and reduction in GHG emissions [63]. The European biogas asso­ Ministry of Education (2014R1A6A1031189).
ciation (EBA) [64] aims to introduce renewable energy in fossil
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