www.Life cycle assessment of microbial electro synthesis for 1
commercial product generation 2
Tobechi Okoroafor, Ph.D.1; Sue Haile, Ph.D.2; and Sharon Velasquez-Orta, Ph.D.3 3
1 Ph.D. Scholar, School of Engineering, Faculty of Science, Agriculture and Engineering, Newcastle 4
University, Newcastle upon Tyne, NE1 7RU, United Kingdom. Email: [email protected] 5
2 Senior Lecturer, School of Engineering, Faculty of Science, Agriculture and Engineering, Newcastle 6
University, Newcastle upon Tyne, NE1 7RU, United Kingdom. Email: [email protected] 7
3 Lecturer, School of Engineering, Faculty of Science, Agriculture and Engineering, Newcastle 8
University, Newcastle upon Tyne, NE1 7RU, United Kingdom (corresponding author). Email: 9
Abstract 11
Microbial electrosynthesis (MES) uses microbes and electricity to convert CO2 to high grade chemicals 12
alleviating greenhouse gas emissions. Little is known on the environmental loads associated with the 13
scale up of the technology. Initially, the MES environmental impacts of synthesizing acetic, formic or 14
propionic acids, methanol or ethanol were assessed using the LCA software GaBi and electricity 15
produced 60% from fossil fuels. Results showed that formic acid production had the lowest 16
environmental impact in all eco indicators due to comparatively low energy requirements of its reactor 17
and rectification unit. Second, three different formic acid production methods were compared with 18
MES. Hydrolysis of methyl formate, the main conventional method was shown to be less 19
environmentally harmful than the other three CO2 utilizing technologies analysed when electricity used 20
was generated from fossil fuels except for the impact on climate change. Producing electricity from 21
renewable sources (Hydro, biogas, wind and photovoltaic) made MES able to mitigate climate change 22
having the lowest negative impact on the environment compared to others. Synthesis of products 23
through MES using wind generated electricity could provide considerable benefits and should be 24
considered when MES is industrially applied. 25
Author Keywords: Microbial electrosynthesis, Life cycle assessment, Global warming 26
Introduction 27
Bioelectrochemistry involves the transfer of electrons between a solid electrode and immobilised 28
bacteria. Immobilisation helps reduce the distance between the bacteria and electrode in order to 29
preserve activity (Gooding and Gonçales, 2017). Interest in this science has increased exponentially 30
over the years as researchers become aware of its enormous potential. Bioelectrochemical systems 31
(BES) is a technology that was originally developed for the conversion of wastewater to bioelectricity. 32
The technique uses an anode and cathode electrode, usually separated by a proton exchange 33
membrane (Logan et al., 2006). Oxidation and reduction reactions occur at the anode and cathode, 34
respectively. These redox reactions are driven by catalysts interacting with electrodes connected via 35
an electrical circuit (Das et al., 2019). 36
BES have numerous application and depending on their operation can be classified as microbial fuel 37
cells (MFCs), microbial electrolysis cells (MECs), enzymatic fuel cells (EFCs), microbial solar cells 38
(MSCs) or microbial desalination cells (MDCs) (Santoro et al., 2017). The electron transferred from 39
the electrode to the bacteria could either be direct or indirect through the use of mediators (Rabaey et 40
al., 2010). Microbial electrosynthesis (MES) works similar to MEC which requires external energy to 41
be supplied for the desired bioelectrochemical reaction to occur and has attracted recently a lot of 42
research attention (Jiang and Jianxiong Zeng, 2018; Jiang et al., 2019). MES can be used to produce 43
methane, acetate, formic acid and other higher biofuels from CO2 (Cheng et al., 2009; Nevin et al., 44
2011; Jourdin et al., 2016; Vassilev et al., 2018). It has also been shown recently to have the ability to 45
upgrade biogas by increasing its methane content (Das et al., 2018; Das and Ghangrekar, 2018). 46
Synthesis is usually done using bacteria or enzymes as biocatalyst in the cathode chamber of a BES. 47
However, cell configurations having more than one cathode chamber have been employed with 48
conditions in subsequent chambers set to promote synthesis of different targeted products (Vassilev et 49
al., 2019). As MES uses CO2, technology development can contribute to the European Union 2050 50
emission reduction targets. 51
MES convert CO2 to value added chemicals leading to a reduction in carbon emissions (Finn et al., 52
2012; Bajracharya et al., 2017). However, the MES technology has to be correctly applied to ensure it 53
does not lead to environmental burdens that increase negative environmental impacts. Having this last 54
scenario may mitigate the positive effects of using the technology. For this reason, a comprehensive 55
assessment of other environmental burdens apart from GHGs is required. Researchers have been able 56
to improve productivity and resilience of biocatalysts used for MES. However, after almost a decade, 57
commercial application of the technology has not be exploited (Prévoteau et al., 2020). This however 58
could be in the horizon, as reported chemical yields increase. This current paper evaluates the 59
potential environmental impacts of using MES on an industry scale for the production of chemicals. 60
Life cycle assessment (LCA) was used to analyse the potential environmental impacts of producing 61
acetic, propionic and formic acid, methanol and ethanol using MES. A modelled full-scale production 62
MES process was assessed from cradle to gate. Environmental burdens were characterised using GaBi 63
LCA software. Additionally, MES for the synthesis of formic acid was compared with two CO2 64
utilizing technologies and the main conventional formic acid production route. 65
Methodology 66
Process description, assumptions and products assessed in this paper follow the same general 67
guidelines used in Christodoulou et al. (2017), where the energy requirement, global warming 68
potential and economic feasibility of MES applied on an industrial scale were evaluated for acetic, 69
propionic and formic acids, plus methanol and ethanol production using only natural gas as the energy 70
source. 71
Goal and scope definition 72
This LCA study aimed to analyse and understand the potential environmental impacts of MES 73
through a process based LCA model. The environmental impacts of the process for the synthesis of 74
acetic, propionic and formic acid, methanol and ethanol from cradle to gate was assessed and 75
environmental burdens characterised using the International Life Cycle Data System (ILCD) method 76
in GaBi LCA software. The software was chosen because it has established competency in LCA 77
research (Speck et al., 2015; Speck et al., 2016). The functional unit used was 1,000 tonnes per year 78
(t/yr) of products synthesized from CO2 for a ten year plant life. Environmental burdens associated 79
with decommissioning of the plants were not considered as this was a cradle to gate study. The gas 80
(CO2) was assumed to be captured from a coal fired plant fitted with post combustion capture system. 81
Environmental credit from the CO2 captured is accounted for in the overall calculations. Electricity 82
supplied to the plant is assumed to be from the 2014 estimate of the UK national grid (coal and oil 83
30.5%, natural gas 30.1%, renewables 19.2%, nuclear 19.0%, other 1.2%), of course now the UK grid 84
has decarbonised considerably (Thinkstep, 2016; DBEIS, 2017; DBEIS, 2019). This was selected to 85
illustrate the effect of a fossil fuel dominated grid power production system on the environmental 86
burdens of MES. Potential environmental impacts in this analysis is the net climate change impact 87
calculated by deducting the environmental credit gained from CO2 used by the process. Additionally, 88
this study aims to compare the environmental effects of producing formic acid using MES with that of 89
both abiotic electrochemical reduction and conventional routes. Formic acid was selected because it 90
has been shown to have relatively low global warming potential and high economic value when 91
compared to other chemicals that can be synthesized by the process (Christodoulou et al., 2017). 92
Fig. 1 shows the system boundary of the LCA model. The process is divided into phases to ease 93
analysis including CO2 capture, purification and transportation, product formation, separation and 94
packaging. All MES plant unit operations (mixer, MES reactor, gas separator and rectification unit) 95
and parameters that make up the MES plant remained the same as those described in (Christodoulou 96
et al., 2017). 97
Life Cycle Inventory 98
A conceptual design is implemented in the LCA software GaBi according to a 1,000t/yr commercial 99
plant size (See Appendix Fig.A1-A7 for GaBi flowsheets). Gabi was selected because it is fully 100
compliant with the ISO 14010 and 14044 standards and can efficiently estimate the output of 101
particular processes (in terms of GHG and other environmental burdens produced) (Thinkstep, 2017). 102
Commented [SVO1]: This should be in the past tense isn’t it? Please check all the paper.
Table 1 summarizes the water and energy consumed each year for the products evaluated. Process 103
description and associated assumptions remain the same and have been previously described 104
elsewhere therefore would not be stated here (Christodoulou et al., 2017). The life cycle inventory 105
(LCI) data for energy consumption of each process in MES plant modelling was calculated based on 106
energy data and technical information available from contractors, open literature and the GaBi 107
software (Thinkstep, 2016; Christodoulou et al., 2017). Energy data for the MES reactor took into 108
account both cathode and anode energy load unlike Christodoulou and co-worker where only cathode 109
energy load was evaluated (Christodoulou et al., 2017). Water oxidation (see equation 1) is assumed 110
to occur at the anode for the required protons and electrons while the MES reaction to products at the 111
cathode (Rabaey and Rozendal, 2010; Christodoulou et al., 2017). 112
4𝐻2𝑂 → 2𝑂2 + 8𝐻+ + 8𝑒− Eo = 0.817V vs SHE at pH=7 (1) 113
Table 2 shows the reaction balances and selected MES reactor potentials. The energy values for acetic 114
and formic acid were taken from experimental data while the theoretical electrochemical data was 115
used for propionic acid, methanol and ethanol (Nevin et al., 2011; Marshall et al., 2013). The work of 116
Marshall and co-workers was selected for acetic acid because it showed the long term viability of 117
producing the chemical using MES (Marshall et al., 2013). This indicated that MES can be deployed 118
commercially on a large scale. However, formic acid unlike acetic acid is not widely reported as being 119
synthesized by whole cell biocatalysts. This is because formic acid is a main intermediate in the 120
Wood-Ljungdahl pathway for the synthesis of other chemicals (Oswald et al., 2018). Therefore, the 121
chemical could be used as substrate by other formate consuming bacteria species attached to the 122
biocathode after generation. However, using enzymatic electro-synthesis which uses CO2 like MES, 123
high formic acid productivity has been achieved (Chiranjeevi et al., 2019). Nevin and co-worker 124
showed direct synthesis of formic acid from CO2 in MES at potentials close to its theoretical value (-125
0.430 vs SHE) (Nevin et al., 2011). This work was therefore chosen because detectable formic acid 126
yield making use of whole cell biocatalysts instead of extracted enzymes was achieved. 127
Alongside energy inputs, materials used in manufacturing the MES reactor and other unit operations 128
are accounted for in the assessment. Electrodes for the MES reactors are biotic, similar to the design 129
analysed by Christodoulou and co-workers (Christodoulou et al., 2017). Most MES studies make use 130
of biotic cathodes and noble metals such as Pt as counter electrodes (Jiang and Jianxiong Zeng, 2018). 131
However, biotic anode and cathode MES reactor design have proved effective for electro catalytic 132
CO2 reduction by (Marshall et al., 2013) and (Giddings et al., 2015). Carbon fibre made from 133
polyarcylonitrile was considered as the MES reactor electrode material. Inventory data for the carbon 134
fibre was collated from the work of Shemfe and co-researchers (Shemfe et al., 2018). Electrolytes are 135
needed for electro catalytic reduction of CO2 because it aids the transfer of electrons and protons to 136
and from the electrodes. The MES reactor aqueous medium consisted of the following (per litre of 137
water); 2.5g NaHCO3; 0.25g NH4CL; 0.21g MgCl2.6H2O; 0.03g CaCl2; 0.1g KCl; 0.6g NaH2PO4.H2O 138
and trace quantities of Wolfe vitamin solution and modified Wolfe’s mineral solution (Christodoulou 139
et al., 2017).The database of GaBi and Ecoinvent 3.0 accounted for the chemicals used in the 140
electrolytes. Sodium carbonate, sodium phosphate and potassium carbonate were used as proxies for 141
sodium bicarbonate, sodium dihydrogen phosphate and potassium bicarbonate due to lack of life cycle 142
data. Conversion rates and faradaic efficiencies for the MES reactors were assumed to be 58.8% and 143
69% respectively (Marshall et al., 2013; Christodoulou et al., 2017). 144
Comparison of formic acid production routes 145
Formic acid is a colourless corrosive acid which is totally miscible in water and other numerous polar 146
solvents. It is mainly used in the textile, pharmaceuticals and food industry with the leather and 147
tanning industry being it largest user in 2003 (Reutemann and Kieczka, 2000; Bulushev and Ross, 148
2018).This has however been overtaken recently by its use as an additive and preservative in animal 149
feed which accounts for as at 2019 around 20% of global formic acid usage (Mordor Intelligence, 150
2019). The total amount of formic acid manufactured worldwide is estimated to be 0.95 mega tonnes 151
per year with the price set at between $0.60 and $0.70 per kg in the first half of 2014. Demand for the 152
chemical is set to increase by around 6% in 2019 because of its ever expanding use cases (Bulushev 153
and Ross, 2018). Formic acid can be used to store hydrogen due to its good properties and simple 154
dehydrogenation (Bulushev and Ross, 2018). 155
Conventionally formic acid can be manufactured through oxidation of hydrocarbons, hydrolysis of 156
formamide, production from formates and hydrolysis of methyl formate (Reutemann and Kieczka, 157
2000). In the oxidation of hydrocarbons to produce formic acid, methane and methanol can be 158
employed. Methanol oxidation yield formaldehyde which in turn is oxidized to formic acid in a two 159
step process (see equation 2 and 3) (Andrushkevich et al., 2014). Methane on the one hand is oxidized 160
to formic acid using heterogeneous catalysts. Using methane is advantageous because oxidation 161
occurs at low temperatures (60oC) even though yields are low (Hutchings, 2016). Formic acid 162
production through hydrolysis of formamide was prominent in Europe until the end of the 19 th 163
century. The consumption of ammonia and sulfuric acid coupled with ammonium sulphate generation 164
made the process less economically competitive (Reutemann and Kieczka, 2000). Production from 165
formates usually uses sodium formate (see equation 4) and calcium formate (see equation 5) with 166
acidolysis done with sulfuric or phosphoric acid (Reutemann and Kieczka, 2000). Comparing the 167
different ways formic acid can be produced conventionally, Hydrolysis of methyl formate is currently 168
the main way the chemical is manufactured. The route accounts for around 90% of all formic acid 169
installation production facilities. It occurs in a two stage process where 95% carbon monoxide and 170
30% methanol are initially reacted to produce methyl formate which is then hydrolysed to synthesize 171
formic acid (see equation 6 and 7) (Saavalainen et al., 2017). 172
Oxidation of hydrocarbons: 173
𝐶𝐻3𝑂𝐻 + 0.5𝑂2 → 𝐻𝐶𝐻𝑂 + 𝐻2𝑂 (2) 174
𝐻𝐶𝐻𝑂 + 0.5𝑂2 → 𝐻𝐶𝑂𝑂𝐻 (3) 175
Production from formates: 176
2𝐻𝐶𝑂𝑂𝑁𝑎 + 𝐻2𝑆𝑂4 → 𝑁𝑎2𝑆𝑂4 + 2𝐻𝐶𝑂𝑂𝐻 (4) 177
(𝐻𝐶𝑂𝑂)2𝐶𝑎 + 𝐻2𝑆𝑂4 → 𝐶𝑎2𝑆𝑂4 + 2𝐻𝐶𝑂𝑂𝐻 (5) 178
Hydrolysis of methyl formate; 179
𝐶𝐻3𝑂𝐻 + 𝐶𝑂 → 𝐻𝐶𝑂𝑂𝐶𝐻3 (6) 180
𝐻𝐶𝑂𝑂𝐶𝐻3 + 𝐻2𝑂 → 𝐶𝐻3𝑂𝐻 + 𝐻𝐶𝑂𝑂𝐻 (7) 181
182
183
Electrocatalytically formic acid has been shown to be produced through biotic (Reda et al., 2008; 184
Srikanth et al., 2014) and abiotic catalysts (see Fig. 2) (Gupta et al., 2016). In the case of biotic 185
catalyst the chemical is produced through MES by supplying suitable bacteria or biomolecules 186
electrons and CO2 for synthesis (Srikanth et al., 2017). Abiotically, the chemical can be produced 187
either through homogenous and heterogeneous abiotic catalysts (Gupta et al., 2016). Four ways of 188
manufacturing the chemical are compared in this study using the ILCD (International Life Cycle Data 189
System) method in GaBi. The options consisted of three processes that utilises CO2 (Microbial 190
electrosynthesis (MES), Abiotic electrochemical reduction (AER), Hydrogenation of carbon dioxide 191
(HCD)) and a conventional benchmark (Hydrolysis of methyl formate (HMF)). Comparative analysis 192
between the three CO2 utilizing formic acid production routes listed above and a conventional route 193
was carried out. The functional unit used for this comparison remained 1,000 t/yr of formic acid 194
generated at a commercial grade concentration of between 90% and 99%. The database of Ecoinvent 195
3.0 and Gabi LCA software was used to conduct this comparison with electricity supplied for each 196
plant assumed to be from the 2014 estimate of the UK national grid (coal and oil 30.5%, natural gas 197
30.1%, renewables 19.2%, nuclear 19.0%, other 1.2%) (DBEIS, 2017). Assumptions for the MES 198
plant are the same as those described above while those associated with the AER, HCD and HMF 199
plants are described in the subsections below; 200
Abiotic electrochemical reduction plant 201
Abiotic electrochemical reduction of CO2 follows the same principles as MES without the use of 202
micro-organisms as catalyst. As with MES, CO2 and protons are synthesized to products at the 203
cathode with water oxidized to oxygen and protons at the anode (Tao et al., 2017). It was assumed 204
that the AER plant used similar unit operations (mixer, AER reactor, gas separator and rectification 205
unit) as the MES plant. The anode of the AER reactor was assumed to be platinum while the cathode 206
copper. Platinum was selected because it is the most widely used anode for water oxidation in AER 207
(Endrődi et al., 2017; Evangelisti et al., 2017). As stated previously, copper was chosen as the 208
cathode and therefore the abiotic electro catalyst. Selection was based on its unique ability to produce 209
a wide variety of products specifically hydrocarbons similar to those obtained through MES (Kuhl et 210
al., 2012). Catalytic stability problems leading to gradual degradation due to carbon deposits and 211
other toxic elements was not taken account in this study as it was assumed that catalyst performance 212
does not depreciate over time (Qiao et al., 2014). 213
Electrolytes are needed for electro catalytic reduction of CO2 because it aids the transfer of electrons 214
and protons to and from the electrodes. Electrolytes used for CO2 reduction in AER reactors could be 215
aqueous, ionic or organic (Zhang et al., 2017). This study assumes an aqueous electrolyte (0.1M 216
KHCO3) as it is comparable to those used in MES reactors. Aqueous electrolyte are also cheap due to 217
water usage but has the disadvantage of low CO2 solubility (0.03M in 25oC ) and competing hydrogen 218
evolution reaction reducing faradaic efficiency (Tao et al., 2017). Conversion rates were assumed to 219
be 58.8% for the plant reactor with the unreacted gas recycled back. Cathodic potential (-0.860 vs 220
SHE) and Faradaic efficiency (30%) for product formation in the AER reactor was assumed using 221
experimental results obtained by (Kuhl et al., 2012) and modelled using GaBi (See Appendix Fig.A5. 222
for flowsheet). 223
Hydrogenation of carbon dioxide plant 224
Formic acid production from a HCD plant were evaluated and compared with other ways of 225
generating the chemical. The plant was assumed to be located at Newcastle Upon Tyne, UK. CO2 226
needed for plant operation was assumed to be supplied at 0.1758 GJ per tonne from a coal fired plant 227
30km away (Bhown and Freeman, 2011). The transportation of the gas to the plant site was assumed 228
to be by a diesel powered Euro type truck using GaBi data. Energy required to compress CO2 from 229
atmospheric pressure to that required by the plant is allocated to the HMF plant overall energy duty. It 230
is assumed that the required hydrogen needed to operate the main reactor is supplied by an 231
electrolyser situated onsite. Inventory data needed for modelling of the HCD plant was obtained from 232
the work of (Pérez-Fortes et al., 2016) and the database of Gabi (See Appendix Fig.A6. for 233
flowsheet). 234
Hydrolysis of methyl formate plant 235
Methyl formate hydrolysis was used as the bench mark conventional process because of its wide 236
industrial application (Saavalainen et al., 2017). The process involves two stages with the first being 237
methanol carbonylation for the formation of methyl formate. Methyl formate is then hydrolysed to 238
produce formic acid in the second stage. Methanol and carbon monoxide which are the main raw 239
materials needed by the plant is assumed to be supplied from a production facilities 30 Km from the 240
plant site. Inventory data for the HMF plant was collated from the databases of Ecoinvent 3.0 and 241
Gabi (See Appendix Fig.A7. for flowsheet). 242
Table 3 shows a summary of the LCI of the four formic acid production routes assuming 1,000 t/yr of 243
formic acid is synthesized. The values summarized in the table are grouped into raw materials, 244
product, output and energy consumed by the plants. For the three CO2 utilizing plants unreacted CO2 245
and water are assumed to be perfectly recycled back into the plant with the analysis done for a ten 246
year time frame. 247
It can be seen from Table 3 that the main contributors to climate change and other environmental 248
burdens is expected to be plant electricity and steam usage. Electricity consumed by the three CO2 249
utilizing plants ranged between 14,652 GJ/yr and 7,183 GJ/yr while that of the HMF plant was 1,044 250
GJ/yr. The relative difference is mainly due to electricity being used to undergo formic acid synthesis 251
in the reactors of the MES, AER and HCD plants. The HMF plant (19,500 GJ/yr) showed the highest 252
consumption of steam for the production of 1,000 t/yr of formic acid while the HCD plant (10,030 253
GJ/yr) was the highest for the CO2 utilizing routes. Analysing raw material used it can be observed 254
that both the MES and AER plants (1,094 t/yr) used the most CO2 obtaining the maximum allocated 255
environmental credit associated with CO2 use. The HMF plant does not use CO2 and therefore no 256
environmental credit was given to the process. All plants analysed used comparable process water 257
with the HCD plant using water in an electrolyser to produce H2 which would be feed into its main 258
reactor. 259
Results and discussion 260
MES Plant 261
Climate change 262
The assessment of the MES plant gives an increase in GHG emissions for all the products (acetic acid, 263
formic acid, propionic acid, ethanol and methanol) analysed (see Table 4). This is similar to the values 264
Commented [SVO2]: Check again between past and present tense.
observed by Christodoulou and co-workers for acetic (61,638 t CO2 eqv) and propionic acid (48,797 t 265
CO2 eqv) when natural gas was assumed as the energy source (Christodoulou et al., 2017). However, 266
ethanol (42,100 t CO2 eqv), methanol (33,200 t CO2 eqv) and formic acid (2,200 t CO2 eqv) which 267
previously had a negative value has now increased. This is because total energy load (anode and 268
cathode) was used for this assessment and generation of electricity from the 2014 estimate of the UK 269
national grid produces more than three times as much GHG emissions per GJ of electricity produced 270
than natural gas (0.05 t CO2 eqv per GJ). Table 4 also shows the amount of contribution to climate 271
change by the GHGs. The most prevalent GHG in all the different products is CO2 accounting for up 272
to 99% of emissions with the next potent greenhouse contribution being made by methane due to its 273
high CO2 equivalent value. 274
Fig. 3 shows the climate change contributions of the different unit operations of the MES plant for 275
acetic, propionic acid and formic acid, methanol and ethanol. Acetic and propionic acids relatively 276
high amount of contribution to climate change is mainly due to their energy intensive rectification 277
unit. Rectification of acetic (50,700 t CO2 eqv) and propionic acid (44,234 t CO2 eqv) contributed 278
75% and 65% respectively to climate change. Formic acid rectification contributed 55% while that of 279
methanol and ethanol contributed less than 6% to climate change. Evaluating the five MES reactors it 280
was observed that only formic acid (-170.77 t CO2 eqv) had a negative climate change impact. The 281
MES reactors of ethanol and methanol contributed significantly to climate change accounting for 92 282
and 90% respectively. This can be attributed to the large amount of energy needed for MES (380,371 283
GJ for ethanol and 286,479 GJ for methanol) compared to other unit operations. Acetic acid (16,000 t 284
CO2 eqv) and propionic acid (22,100 t CO2 eqv) MES reactors contributed less than 30% to climate 285
change. Results indicates that without supporting unit operations standalone MES reactors (16,000 t 286
CO2 eqv for acetic acid, 22,100 t CO2 eqv for propionic acid, 39,095 t CO2 eqv for ethanol and 30,200 287
t CO2 eqv for methanol) still has a net positive global warming potential. Formic acid (-170.77 t CO2 288
eqv) is a notable exception as it was observed to be the best performing reactor having a negative 289
global warming potential. 290
Other Environmental Burdens 291
Eight midpoint indicators were selected for a more detailed analysis from thirteen indicators of the 292
ILCD methods. The indicators chosen included ozone depletion (OD, in kg R11-Eqv), human toxicity 293
cancer effects (HT, in CTUh), particulate matter (PM, in PM2.5 Eqv), ionising radiation (IR, in U235 294
Eqv), photochemical ozone formation (POF, in kg NMVOC Eqv), acidification (AC, in Mole of H+ 295
Eqv), freshwater europhication (FE, in kg P Eqv) and ecotoxicity (EC, in CTUe). The remaining 296
midpoint indicator results and raw data for the MES plants can be seen in Appendix Fig. A8 and 297
Table A1 respectively. The results are displayed relative to the maximum value in each of the 298
midpoint impact category. 299
Data obtained from the analysis shows that all MES plants has overall negative impact on the 300
environment in all the impact category. This indicates that the environmental benefits of CO2 301
utilization cannot compensate for the generation of other environmental burdens. Fig. 4 and Fig. 5 302
shows that formic acid contributes the lowest in the selected impact categories as its relative values to 303
the maximum is always less than 30%. Ethanol was observed to have the maximum value in all of the 304
selected impact categories differing from results obtained for climate change (See Fig. 3). Propionic 305
and acetic acid gave very similar results in all the selected impact categories as they were found to 306
always have a relative value more than 50% to the maximum (ethanol). 307
The cradle to gate environmental impacts are further assessed in terms of plant unit operations. Grid 308
electricity used for synthesis of products in the MES reactors had relatively large environmental 309
impact contributions in most impact categories. Assessing ethanol which had the most energy 310
intensive MES reactor it was observed that synthesis of the chemical takes up between 94.5% and 311
98.2% in the selected impact categories. Similar results were seen in methanol and formic acid which 312
alongside ethanol had relatively low rectification energy duty. Assessing formic acid specifically it 313
was observed that its MES reactor takes up 88.9% of AC, 81.8% of EC, 76.7% of FE, 80.2% of HT, 314
92.3% of IR, 76.1% of OD, 86.3% of PM and 87.7% of POF. Process steam needed to heat the 315
reboilers of acetic and propionic producing MES plants contributed on average 15% in the selected 316
impact categories. Analysing acetic acid which uses the most energy for rectification it was observed 317
that the unit operation takes up 20.4% of AC, 9.7% of EC, 4.9% of FE, 14.9% of HT, 1.0 % of IR, 318
16.1% of OD, 14.1% of PM and 34.6% of POF. This was found to be always less than its MES 319
reactor in all impact categories except climate change. Rectification of formic acid, ethanol and 320
methanol contributed less than 4% in all impact categories apart from climate change. All other unit 321
operations (CO2 capture, mixer and gas separator) contribute less than 5% in all selected impact 322
categories for each product analysed. Therefore, it can be suggested that the hot spot of the modelled 323
MES plants in terms of unit operation are the MES reactors. 324
Flue gas from the gas separator and incineration of filtered biomass could also contribute to the 325
environmental burdens of the MES plants. As the selectivity of CO2 to products (88%) and faradaic 326
efficiency (69%) of the MES reactors are not 100% there may be undesired by-products formed. This 327
would have to be vented off to the atmosphere alongside oxygen as CO2 is being recycled back to the 328
MES reactor. The impurities in the flue gas would most likely be composed of hydrogen (over 95%) 329
and methane (below 5%). The use of 2-bromoethanesulfonate in biofilm development should help 330
suppress methanogenic bacteria growth hence its expected minimal quantity (Christodoulou et al., 331
2017). Vented Hydrogen gas can affect the upper atmosphere due to a build-up of water vapour. This 332
can cause ozone depletion and its undesired consequences (Tromp et al., 2003). Methane on the other 333
hand contributes to climate change and POF environmental burdens (Sadhukhan et al., 2014). 334
Incineration of biomass would take place twice each year and it is away from the plants; hence 335
environmental burdens associated with its transportation needs to be taken into account 336
(Christodoulou et al., 2017). Municipal waste incineration is known to contribute about 0.36 t CO2 337
eqv/ton to global warming (Havukainen et al., 2017). This value should be what is expected when the 338
filtered bacteria cells are incinerated. As the MES plants would be incinerating just a few grams of 339
bacteria cells per year, this waste treatment method should not significantly affect climate change and 340
other environmental burdens results of the MES plants. 341
Sensitivity Analysis 342
MES plant 343
A sensitivity analysis has been carried out by exploring two MES plant parameters changes. CO2 344
conversion and faradaic efficiency were examined to see the environmental impact variations 345
resulting from the changes in these parameters. Alternative scenarios different to the base case where 346
these parameters are set at 40% and 100% respectively were assessed. The scenario analysis results of 347
the different CO2 conversion and efficiency are shown in Fig. 6 for climate change and Appendix 348
Fig.A9 for other environmental burdens. 349
From Fig. 6, it was observed that when the CO2 conversion and faradaic efficiency are reduced a 350
higher environmental impact in terms of climate change is seen. This can be explained by the change 351
in the energy requirements for the MES reactor and gas separator. An energy requirement change of 352
between 25-30% is seen in the MES reactor for each of the products analysed when the CO2 353
conversion and faradaic efficiency is changed to the selected percentages. In terms of the gas 354
separator a CO2 conversion of 100% eliminates the need for the unit operation as oxygen can be 355
vented off to the atmosphere directly from the MES reactor. A reduction to 40% increases the energy 356
requirement by more than two times in the case of all products analysed. It can be seen from Fig. 6 357
that only the global warming potential of formic acid (-734 t CO2 eqv) becomes negative when the 358
CO2 conversion and faradaic efficiency is set at 100%. Comparatively low rectification and MES 359
reactor energy requirements are the main reason for this change yielding a positive result. This 360
indicates that low synthesis and rectification energy requirements are necessary for good conversion 361
and efficiency values seen in experimental research to yield positive global warming reduction for a 362
commercial grade MES plant. Sensitivity analysis results for other environmental impact burdens 363
(Appendix Fig. A9.) indicated that for all selected impact categories a variation of between 5% and 364
40% occurred. Ethanol at 40% CO2 conversion and efficiency as with the base scenario was observed 365
to have the maximum value in all (HT,IR,OD,PM,AC,EC FE, and POF) of the selected impact 366
categories. The base scenario was surpassed marginally by acetic acid and methanol at 40% CO2 367
conversion and faradaic efficiency. Formic acid was observed to still have the lowest effect on the 368
environment for all impact categories even though the parameters were adjusted to the worst-case 369
scenario. 370
Comparison of formic acid production routes 371
Climate change 372
An assessment of four formic acid producing plants was done using GaBi. The assessment gives a 373
positive impact in terms of GHG emissions for the all the formic acid producing plants analysed (see 374
Table 5). However, the MES plant (2,120 t CO2 eqv) was found to emit the least amount of GHGs 375
about eight times lower than the HMF plant (18,400 t CO2 eqv). Interestingly a CO2 utilizing plant 376
had a higher global warming potential than the conventional plant analysed. This shows that the 377
environmental credit associated with CO2 use may not be sufficient to rival conventional processes. 378
Table 5 also shows the amounts of GHGs emitted by each of the formic acid producing routes. The 379
most prevalent GHG in all the four plants analysed was CO2 with the next potent GHG being 380
methane. The HCD plant emitted the most CO2 (28,806 t CO2 eqv) alongside the other GHGs 381
assessed. Emission amounts were between 44% and 83% that of the HMF plant. It was observed that 382
the HMF plant emitted the least amount of sulphur hexafluoride (1.02E-11 t CO2 eqv) and nitrous 383
oxide (0.15 t CO2 eqv) while the MES plant carbon dioxide (12,221 t CO2 eqv) and methane (28 t 384
CO2 eqv). Comparing the three carbon utilizing routes, MES released the least amount of all GHGs, 385
significantly lower than the AER and HCD plants. As MES had the lowest global warming potential 386
amongst the plants analysed and released comparably low amounts of each GHGs, producing formic 387
acid through this route in terms of climate change is beneficial over the routes analysed. 388
Other Environmental Burdens 389
Eight midpoint indicators were selected for a more detailed analysis from thirteen indicators of the 390
ILCD methods. The indicators chosen included ozone depletion (OD, in kg R11-Eqv), human toxicity 391
cancer effects (HT, in CTUh), particulate matter (PM, in PM2.5 Eqv), ionising radiation (IR, in U235 392
Eqv), photochemical ozone formation (POF, in kg NMVOC Eqv), acidification (AC, in Mole of H+ 393
Eqv), freshwater europhication (FE, in kg P Eqv) and ecotoxicity (EC, in CTUe). The remaining 394
midpoint indicator results and raw data for the formic acid producing plants can be seen in Appendix 395
Fig. A10 and Table A3 respectively. The results are displayed relative to the maximum value in each 396
of the midpoint impact category. 397
Data obtained from the analysis shown in Fig. 7 and Fig. 8 indicates that the CO2 utilizing HCD plant 398
contributes the most in all the selected impact categories. It was on average more than 20% higher 399
than the next worst plant for all the impact categories analysed. The conventional formic acid 400
production plant (HMF) performed better than all assessed plants in six (AC,EC, POF,HT,IR and PM) 401
of the selected ILCD impact categories. This may be due to the use of mostly steam (19,500 GJ/yr) 402
instead of electricity (1,044 GJ/yr) for energy. Steam usage in the HMF contributes 42.5% to AC, 403
17.1% to EC, 45.2% to POF, 27.9% to HT, 4.1% to IR and 34.7% to PM. The HMF plant was also 404
seen to be marginally better than the HCD plant in the EF environmental impacts category. Using 405
MES for the generation of formic acid had the lowest environmental impact in the EF and OD impact 406
categories. The plant was observed to always have a relative value in each impact category less than 407
60% of the maximum which was the HCD plant. Results presented here showed than the use of CO2 408
as raw material does not guarantee environmental benefit as other factors such as amount of energy 409
needed for production should be considered. The use of an electrolyser in the HCD plant places large 410
energy burden on the plant and if eliminated could reduce energy consumption by around 92% (Pérez-411
Fortes et al., 2016). This makes the use of MES particularly attractive over other CO2 utilizing routes 412
as it used comparatively low amounts of energy (7,183 GJ/yr) hence the relatively low environmental 413
impact. However in most impact categories this was not sufficient to show value over the 414
conventional process. 415
Environmental impacts are further assessed in terms of raw materials. For the HMF plant, carbon 416
monoxide production using synthetic gas contributes 42% to EC, 84.2% to EF, 39.7% to HT and 417
69.4% to OD. It is the highest contributor in these impact categories and in all the impact category 418
analysed was consistently above 15%. Comparing this with the environmental burdens of capturing 419
CO2 from a coal fired plant for MES. It was observed that the environmental burdens did not exceed 420
2.6% in all impact categories other than climate change. Overall in MES, electricity for synthesis 421
contributed the highest in all the impact categories. This shows that CO2 capture energy is not the 422
main environmental hot spot for implementation of MES technology in the manufacturing of formic 423
acid on a commercial scale. 424
Looking at the market, formic acid is sold at different concentrations. These concentration vary 425
between 85 - 99 weight% with 85% formic acid concentration being the most traded (Pérez-Fortes et 426
al., 2016). Assessing the MES plant, a change in formic acid concentration delivered to the end users 427
would specifically affect the rectification unit of the plant. The concentration of formic acid after 428
synthesis in the MES reactor is 45% hence the need for rectification. Energy required for the 429
rectification of the chemical contributes 55% to climate change but only less than 4% in the other 430
selected impact categories. This is because natural gas is used to supply the needed energy instead of 431
the grid. Any change in the formic acid concentration delivered to the end user by the plant would 432
therefore affect mainly climate change. Comparative results shown here already indicates that the 433
MES plant is more beneficial than all the other plants analysed in this environmental category. 434
Therefore, a concentration change should not affect results presented here. In the case of other 435
environmental burdens due to the relatively insignificant effect of the rectification unit a change in 436
formic acid concentration to 85% would not be able to make the MES plant rival the conventional 437
plant in the AC,EC, POF,HT,IR and PM impact categories. 438
Production using different energy sources 439
Different electricity sources were evaluated for the production of formic acid in MES, AER, HCD and 440
HMF plants. A mix of fossil fuel (coal, oil and gas) and renewable sources (Hydro, biogas, wind and 441
photovoltaic) were chosen. Environmental impacts in terms of climate change and other burdens are 442
analysed in the subsections below. 443
Climate Change 444
Table 6 shows the global warming potential associated with using fossil fuel and renewable sources to 445
generate electricity for the MES, AER, HCD and HMF plants. The MES plant global warming 446
potential remained positive when powered by coal (9,940 t CO2 eqv) and oil (9,367 t CO2 eqv) but 447
turned negative when natural gas (-615 t CO2 eqv) was used. This is consistent with results obtained 448
by Christodoulou and co-researchers as natural gas is often seen as a cleaner form of fossil fuel based 449
electricity source and has been shown to emit less GHGs than coal and oil (Jaramillo et al., 2007; 450
Burnham et al., 2012; Christodoulou et al., 2017).. Negative global warming potential are also 451
recorded when renewable energy sources were used. This shows that the choice of energy source for 452
the synthesis of formic acid using biocatalysts is important. However, renewable energy should be 453
favoured as it consistently had a lower global warming potential than natural gas. As mentioned 454
previously the UK grid is decarbonising quickly (DBEIS, 2019). Using renewable energy (hydro, 455
biogas, wind and photovoltaic) decreased the global warming potential in the base scenario (2014 UK 456
national grid) by on average more than 9,000 t CO2 eqv while that of natural gas by 2,735 t CO2 eqv. 457
Comparing coal (9,940 t CO2 eqv) and oil (9,367 t CO2 eqv) with the base scenario (2,120 t CO2 eqv) 458
it was observed that there is no climate change benefit to their use. The global warming potential of 459
the conventional route analysed (HMF plant) remained positive even though renewable energy 460
sources are used. This is because 71.1% of its global warming potential value is accounted to the 461
steam (10,030 GJ/yr) used by the plant. 462
Looking at the HCD plant it was observed to have negative global warming potential when nuclear (-463
1,272 t CO2 eqv), hydro (-1,178 t CO2 eqv) and wind (-1,240 t CO2 eqv) are used. However, it had a 464
positive global warming potential when biogas (6,390 t CO2 eqv) and photovoltaic (5,590 t CO2 eqv) 465
are used as electricity sources. This was however between 60% and 98% lower than values seen for 466
fossil fuels usage. Results outlined here show that there is climate change benefit of using renewable 467
energy source to power formic acid synthesis plants that utilizes CO2. 468
Other Environmental Burdens 469
For the assessment of other environmental burdens associated with producing formic acid from MES 470
using other sources of electricity, the same impact categories as used above was employed. The 471
complete midpoint results for the MES are shown in Appendix Fig. A11. 472
Fig. 9A and Fig. 9B shows the results for other environmental impact burdens when electricity source 473
differed from that of the base scenario (2014 UK national grid). It was observed that for all the impact 474
categories using coal, oil, nuclear, biogas and photovoltaic means of electricity generation was higher 475
than the base scenario for the MES plant. Coal and oil had more negative environmental impact in 476
five (AC, EC, FE, HT, PM and POF) of the eight selected categories. Nuclear, biogas and 477
photovoltaic also had more negative impact in three (EC, HT and PM), one (FE) and three (EC, HT 478
and OD) midpoint indicators respectively. Natural gas and the remaining renewable energy sources 479
(Hydro and wind) consistently had lower negative environmental impact than the base scenario. 480
Results obtained here are comparable to the other CO2 utilizing plant (HCD plant) analysed (see 481
Appendix Fig. A12.). Electricity generation by wind turbines shows good promise when both climate 482
change and other environmental burdens are assessed. The technology was consistently lower than the 483
base scenario for the CO2 utilizing plants in all ILCD impact categories. 484
The same impact category was used to assess the other environment burdens of using different 485
electricity sources in the conventional plant (see Appendix Fig. A13.). It was observed that coal and 486
oil was higher than the base case in five of the eight selected impact categories. Environmental 487
burdens associated with the use of nuclear, biogas and photovoltaic were higher in up to three impact 488
categories. This showed similar trend to what was observed for the CO2 utilizing plants. However in 489
the AC, EC, FC and POF impact categories environmental burdens did not go lower than 60% the 490
value of the base scenario differing from what was observed in the MES plant. Generation of 491
electricity was also found to be lower than the base scenario in all ILCD impact categories. Wind 492
energy usage for formic acid production has been shown to be environmentally better than the 2014 493
estimate of the UK national grid for both CO2 utilizing and conventional plants. UK is an island 494
nation therefore energy from both onshore and offshore wind turbines can be relatively easy to 495
harness when compared to landlocked countries although cost have to also be considered. However 496
offshore wind farms should be favoured as it has been proven marginally beneficial in terms of global 497
warming (Kaldellis and Apostolou, 2017). The UK government has made great strides in its low 498
carbon generation initiative as energy production from wind turbines and other renewables now 499
account for the majority of the national grid power (DBEIS, 2019). 500
Fig. 10 compares the four plants analysed if electricity used was generated using only wind turbines. 501
Values displayed are relative to the maximum value in each impact category. It was observed that 502
wind energy usage relegated the HMF plant to having comparatively the worst impact on the 503
environment than all CO2 utilizing routes. This could be because the impact of a change to a more 504
environmentally friendly source of electricity is relatively small for the HMF plant as steam is mostly 505
used. The HCD plant which previously had the highest impact in most categories is on average 55% 506
better when wind energy is used instead of the base scenario. These analysis and results suggest that 507
MES should be deployed on an industrial level favouring renewable electricity generation. 508
509
Future Outlook 510
MES is being considered as a conversion route of CO2 to chemicals that can alleviate greenhouse gas 511
emissions alongside other environmental burdens (Das et al., 2019). However, the current industrial 512
application of MES is still constrained by low chemical synthesis rates, high energy demand and high 513
capital expenditures (Prévoteau et al., 2020). 514
During the decade long research into MES, the maximum synthesis rate of acetic acid, the most 515
frequently produced chemical, is still in the region of 685 g/m2/d (Jourdin et al., 2015). Industrial 516
production will require this value to increase at least 100 times to reach commercial viability. Another 517
problem associated with industrialization is product selectivity, as currently targeted chemicals are 518
usually manufactured along with other side products. This is especially the case when a range of 519
chemicals are synthesized using homoacetogens (Prévoteau et al., 2020) or mixed cultures. Good 520
selectivity of products can be obtained by carefully managing the pH of the cathodic medium and the 521
use of inhibitors such as 2-bromoethanesulfonate to prevent loss of electrons to methanogenic bacteria 522
(Christodoulou et al., 2017). However, this may be impractical in a full scale industrial plant as it adds 523
environmental and financial burdens to the system.. 524
The source and amount of energy used in MES is of utmost importance for the sustainability and 525
economic viability of the technology. Environmental burdens associated with electricity usage are 526
highly dependent on the faradaic efficiency and applied voltage of the process (Jung et al., 2020). Cell 527
design would need to be improved to enhance these parameters and reduce energy loss. The use of 528
renewable energy instead of fossil fuels may increase the sustainability of the system. Different 529
sources of intermediate energy supply could be a way of dealing with interruptions from renewable 530
energy sources (del Pilar Anzola Rojas et al., 2018). For example by combining solar, wind and/or 531
biomass energy sources. Having a combined renewable system, could significantly increase the 532
reliability and sustainability of the technology when scaling up. Also industrial MES reactors may 533
require improvements in the ionic membranes used and a more stable anode electrode than the biotic 534
electrodes used in this study. However, the use of noble metals as anodes will add to environmental 535
burdens and cost. 536
Environmental assessment of MES done in this study made use of operating parameters and reaction 537
efficiencies from experimental results using lab scale reactors. These values may be too optimistic as 538
they are based on the parameters not deviating from those of expected future commercial scale up of 539
the technology. As there are numerous unsolved challenges which have been outline above, the long 540
term performance in a real industrial environment needs to be evaluated. This would enable data from 541
practical large scale operation to be used in future sustainability assessments. As MES have had a 542
relative brief time to technologically mature, limited pilot scale systems are available. 543
Notwithstanding, over the last decade the process has made great stride towards commercialization 544
and with continuous research industrial scale plants that can mitigate climate change and other 545
environmental burdens can be achieved. 546
Conclusions 547
To facilitate the commercial application of microbial electrosynthesis (MES) a cradle to gate life 548
cycle assessment was performed in this study. Environmental impact variations were performed for 549
MES plants capable of producing acetic, propionic and formic acid, ethanol and methanol. The results 550
showed that formic acid production was always more than 70% lower than the maximum product in 551
the climate change, ozone depletion (OD), human toxicity cancer effects (HT), particulate matter 552
(PM), ionising radiation (IR), photochemical ozone formation (POF), acidification (AC), freshwater 553
europhication (FE) and ecotoxicity (EC) environmental impact categories. The low environmental 554
impacts were mainly due to the lower energy requirement of its reactor and rectification unit. The 555
reduction of greenhouse gas emissions due to MES of formic acid can only be achieved at high 556
conversion and faradaic efficiencies using electricity from fossil fuel dominated grid power. 557
Depending on the product generated, conversion and faradaic efficiencies there can be climate change 558
benefit of using MES for synthesis of chemicals. 559
Life cycle assessment (LCA) of four types of formic acid production routes, a MES, an abiotic 560
electrochemical reduction (AER), a hydrogenation of carbon dioxide (HCD) and hydrolysis of methyl 561
formate (HMF) plant were also developed in this study. The use of HMF to produce formic acid was 562
shown to be environmentally beneficial than the three CO2 utilizing technologies in six (AC,EC, 563
POF,HT,IR and PM) of the nine impact categories analysed when electricity used was generated from 564
the fossil fuel dominant 2014 estimate of the UK national grid. MES had the lowest environmental 565
impact in the climate change, FE and OD impact categories always having a relative value in each 566
impact category less than 60% of the maximum. Assessment of other sources of energy showed that 567
renewable energy helps reduce climate change in both CO2 utilizing and conventional plants. 568
However, in the case of biogas and photovoltaic energy generation environmental burdens shifted to 569
the EC and OD impact categories. Generation of electricity through wind turbines is of particular 570
interest as it had the ability to reduce environmental burdens in all impact categories analysed when 571
compared with the grid. This enabled the MES, AER and HCD to be more environmental friendly 572
than the conventional plant (HMF) in all impact categories. Synthesis of formic acid through MES 573
using wind generated electricity provides benefits and should be employed when MES is industrially 574
applied. 575
As there is still need for more research on the industrial application of MES, this LCA study provided 576
an initial assessment serving as a basis for future LCA studies on large scale application of MES and 577
other types of bioelectrochemical systems. Based on the conclusions, the production of formic acid is 578
of interest and is the best suited product for MES to provide environmental benefits if applied 579
industrially. 580
Data Availability 581
All data, models, or code generated or used during the study appear in the submitted article. 582
Acknowledgements 583
The authors are thankful to the Petroleum Technology Development Fund (Nigeria) for financial 584
funding. 585
References 586
Andrushkevich, T.V., Popova, G.Y., Danilevich, E.V., Zolotarskii, I.A., Nakrokhin, V.B., Nikoro, 587
T.A., Stompel, S.I. and Parmon, V.N. (2014) 'A new gas-phase method for formic acid production: 588
Tests on a pilot plant', Catalysis in Industry, 6(1), pp. 17-24. 589
Bajracharya, S., Srikanth, S., Mohanakrishna, G., Zacharia, R., Strik, D.P. and Pant, D. (2017) 590
'Biotransformation of carbon dioxide in bioelectrochemical systems: State of the art and future 591
prospects', Journal of Power Sources. 592
Bhown, A.S. and Freeman, B.C. (2011) 'Analysis and Status of Post-Combustion Carbon Dioxide 593
Capture Technologies', Environmental Science & Technology, 45(20), pp. 8624-8632. 594
Blanchet, E., Duquenne, F., Rafrafi, Y., Etcheverry, L., Erable, B. and Bergel, A. (2015) 'Importance 595
of the hydrogen route in up-scaling electrosynthesis for microbial CO2 reduction', Energy & 596
Environmental Science, 8(12), pp. 3731-3744. 597
Bulushev, D.A. and Ross, J.R. (2018) 'Towards Sustainable Production of Formic Acid', 598
ChemSusChem, 11(5), pp. 821-836. 599
Burnham, A., Han, J., Clark, C.E., Wang, M., Dunn, J.B. and Palou-Rivera, I. (2012) 'Life-Cycle 600
Greenhouse Gas Emissions of Shale Gas, Natural Gas, Coal, and Petroleum', Environmental Science 601
& Technology, 46(2), pp. 619-627. 602
CEAE (2014) Redox half reaction reductions potentials and free energies (Accessed: 20/07/2016). 603
Cheng, S., Xing, D., Call, D.F. and Logan, B.E. (2009) 'Direct biological conversion of electrical 604
current into methane by electromethanogenesis', Environmental Science and Technology, 43(10), pp. 605
3953-3958. 606
Chiranjeevi, P., Bulut, M., Breugelmans, T., Patil, S.A. and Pant, D. (2019) 'Current trends in 607
enzymatic electrosynthesis for CO2 reduction', Current Opinion in Green and Sustainable Chemistry, 608
16, pp. 65-70. 609
Christodoulou, X., Okoroafor, T., Parry, S. and Velasquez-Orta, S.B. (2017) 'The use of carbon 610
dioxide in microbial electrosynthesis: Advancements, sustainability and economic feasibility', Journal 611
of CO2 Utilization, 18, pp. 390-399. 612
Das, S., Chatterjee, P. and Ghangrekar, M.M. (2018) 'Increasing methane content in biogas and 613
simultaneous value added product recovery using microbial electrosynthesis', Water Science and 614
Technology, 77(5), pp. 1293-1302. 615
Das, S., Das, S., Das, I. and Ghangrekar, M.M. (2019) 'Application of bioelectrochemical systems for 616
carbon dioxide sequestration and concomitant valuable recovery: A review', Materials Science for 617
Energy Technologies, 2(3), pp. 687-696. 618
Das, S. and Ghangrekar, M. (2018) 'Value added product recovery and carbon dioxide sequestration 619
from biogas using microbial electrosynthesis', Indian Journal of Experimental Biology, 56(1), pp. 620
470-478. 621
DBEIS, D.f.B., Energy and Industrial Strategy) (2017) 'Digest of United Kingdom energy statistics, 622
2017'. 623
DBEIS, D.f.B., Energy and Industrial Strategy) (2019) 'Digest of United Kingdom energy statistics, 624
2019'. 625
del Pilar Anzola Rojas, M., Zaiat, M., Gonzalez, E.R., De Wever, H. and Pant, D. (2018) 'Effect of the 626
electric supply interruption on a microbial electrosynthesis system converting inorganic carbon into 627
acetate', Bioresource Technology, 266, pp. 203-210. 628
Endrődi, B., Bencsik, G., Darvas, F., Jones, R., Rajeshwar, K. and Janáky, C. (2017) 'Continuous-629
flow electroreduction of carbon dioxide', Progress in Energy and Combustion Science, 62, pp. 133-630
154. 631
Evangelisti, S., Tagliaferri, C., Brett, D.J.L. and Lettieri, P. (2017) 'Life cycle assessment of a 632
polymer electrolyte membrane fuel cell system for passenger vehicles', Journal of Cleaner Production, 633
142, pp. 4339-4355. 634
Finn, C., Schnittger, S., Yellowlees, L.J. and Love, J.B. (2012) 'Molecular approaches to the 635
electrochemical reduction of carbon dioxide', Chemical Communications, 48(10), pp. 1392-1399. 636
Giddings, C.G.S., Nevin, K.P., Woodward, T., Lovley, D.R. and Butler, C.S. (2015) 'Simplifying 637
microbial electrosynthesis reactor design', Frontiers in microbiology, 6, pp. 468-468. 638
Gooding, J.J. and Gonçales, V.R. (2017) 'Recent advances in the molecular level modification of 639
electrodes for bioelectrochemistry', Current Opinion in Electrochemistry, 5(1), pp. 203-210. 640
Gupta, K., Bersani, M. and Darr, J.A. (2016) 'Highly efficient electro-reduction of CO2 to formic acid 641
by nano-copper', Journal of Materials Chemistry A, 4(36), pp. 13786-13794. 642
Havukainen, J., Zhan, M., Dong, J., Liikanen, M., Deviatkin, I., Li, X. and Horttanainen, M. (2017) 643
'Environmental impact assessment of municipal solid waste management incorporating mechanical 644
treatment of waste and incineration in Hangzhou, China', Journal of Cleaner Production, 141, pp. 453-645
461. 646
Hutchings, G.J. (2016) 'Methane Activation by Selective Oxidation', Topics in Catalysis, 59(8), pp. 647
658-662. 648
Jaramillo, P., Griffin, W.M. and Matthews, H.S. (2007) 'Comparative Life-Cycle Air Emissions of 649
Coal, Domestic Natural Gas, LNG, and SNG for Electricity Generation', Environmental Science & 650
Technology, 41(17), pp. 6290-6296. 651
Jiang, Y. and Jianxiong Zeng, R. (2018) 'Expanding the product spectrum of value added chemicals in 652
microbial electrosynthesis through integrated process design—A review', Bioresource Technology, 653
269, pp. 503-512. 654
Jiang, Y., May, H.D., Lu, L., Liang, P., Huang, X. and Ren, Z.J. (2019) 'Carbon dioxide and organic 655
waste valorization by microbial electrosynthesis and electro-fermentation', Water Research, 149, pp. 656
42-55. 657
Jourdin, L., Freguia, S., Flexer, V. and Keller, J. (2016) 'Bringing High-Rate, CO2-Based Microbial 658
Electrosynthesis Closer to Practical Implementation through Improved Electrode Design and 659
Operating Conditions', Environmental Science & Technology, 50(4), pp. 1982-1989. 660
Jourdin, L., Grieger, T., Monetti, J., Flexer, V., Freguia, S., Lu, Y., Chen, J., Romano, M., Wallace, 661
G.G. and Keller, J. (2015) 'High Acetic Acid Production Rate Obtained by Microbial Electrosynthesis 662
from Carbon Dioxide', Environmental Science & Technology, 49(22), pp. 13566-13574. 663
Jung, S., Lee, J., Park, Y.-K. and Kwon, E.E. (2020) 'Bioelectrochemical systems for a circular 664
bioeconomy', Bioresource Technology, p. 122748. 665
Kaldellis, J.K. and Apostolou, D. (2017) 'Life cycle energy and carbon footprint of offshore wind 666
energy. Comparison with onshore counterpart', Renewable Energy, 108, pp. 72-84. 667
Kuhl, K.P., Cave, E.R., Abram, D.N. and Jaramillo, T.F. (2012) 'New insights into the 668
electrochemical reduction of carbon dioxide on metallic copper surfaces', Energy & Environmental 669
Science, 5(5), pp. 7050-7059. 670
Logan, B.E., Hamelers, B., Rozendal, R., Schröder, U., Keller, J., Freguia, S., Aelterman, P., 671
Verstraete, W. and Rabaey, K. (2006) 'Microbial Fuel Cells: Methodology and Technology', 672
Environmental Science & Technology, 40(17), pp. 5181-5192. 673
Marshall, C.W., Ross, D.E., Fichot, E.B., Norman, R.S. and May, H.D. (2013) 'Long-term operation 674
of microbial electrosynthesis systems improves acetate production by autotrophic microbiomes', 675
Environmental science & technology, 47(11), pp. 6023-6029. 676
Mordor Intelligence (2019) Formic acid market - growth, trends, and forecast (2020 - 2025). India. 677
[Online]. Available at: https://www.mordorintelligence.com/industry-reports/formic-acid-market. 678
Nevin, K.P., Hensley, S.A., Franks, A.E., Summers, Z.M., Ou, J., Woodard, T.L., Snoeyenbos-West, 679
O.L. and Lovley, D.R. (2011) 'Electrosynthesis of Organic Compounds from Carbon Dioxide Is 680
Catalyzed by a Diversity of Acetogenic Microorganisms', Applied and Environmental Microbiology, 681
77(9), pp. 2882-2886. 682
Oswald, F., Stoll, I.K., Zwick, M., Herbig, S., Sauer, J., Boukis, N. and Neumann, A. (2018) 'Formic 683
Acid Formation by Clostridium ljungdahlii at Elevated Pressures of Carbon Dioxide and Hydrogen', 684
Frontiers in Bioengineering and Biotechnology, 6(6). 685
Pérez-Fortes, M., Schöneberger, J.C., Boulamanti, A., Harrison, G. and Tzimas, E. (2016) 'Formic 686
acid synthesis using CO2 as raw material: Techno-economic and environmental evaluation and 687
market potential', International Journal of Hydrogen Energy, 41(37), pp. 16444-16462. 688
Prévoteau, A., Carvajal-Arroyo, J.M., Ganigué, R. and Rabaey, K. (2020) 'Microbial electrosynthesis 689
from CO2: forever a promise?', Current Opinion in Biotechnology, 62, pp. 48-57. 690
Qiao, J., Liu, Y., Hong, F. and Zhang, J. (2014) 'A review of catalysts for the electroreduction of 691
carbon dioxide to produce low-carbon fuels', Chemical Society Reviews, 43(2), pp. 631-675. 692
Rabaey, K., Largus Angenent, Uwe Schroder and Keller, J. (eds.) (2010) Bioelectrochemical Systems: 693
from extracellular electron transfer to biotechnological application. London, UK: IWA Publishing. 694
Rabaey, K. and Rozendal, R.A. (2010) 'Microbial electrosynthesis — revisiting the electrical route for 695
microbial production', Nat Rev Micro, 8(10), pp. 706-716. 696
Reda, T., Plugge, C.M., Abram, N.J. and Hirst, J. (2008) 'Reversible interconversion of carbon 697
dioxide and formate by an electroactive enzyme', Proceedings of the National Academy of Sciences, 698
105(31), pp. 10654-10658. 699
Reutemann, W. and Kieczka, H. (2000) 'Formic Acid', in Ullmann's Encyclopedia of Industrial 700
Chemistry. Wiley-VCH Verlag GmbH & Co. KGaA. 701
Saavalainen, P., Turpeinen, E., Omodara, L., Kabra, S., Oravisjärvi, K., Yadav, G.D., Keiski, R.L. 702
and Pongrácz, E. (2017) 'Developing and testing a tool for sustainability assessment in an early 703
process design phase – Case study of formic acid production by conventional and carbon dioxide-704
based routes', Journal of Cleaner Production, 168, pp. 1636-1651. 705
Sadhukhan, J., Ng, K.S. and Hernandez, E.M. (2014) Biorefineries and chemical processes: design, 706
integration and sustainability analysis. John Wiley & Sons. 707
Santoro, C., Arbizzani, C., Erable, B. and Ieropoulos, I. (2017) 'Microbial fuel cells: From 708
fundamentals to applications. A review', Journal of Power Sources, 356, pp. 225-244. 709
Shemfe, M., Gadkari, S., Yu, E., Rasul, S., Scott, K., Head, I.M., Gu, S. and Sadhukhan, J. (2018) 710
'Life cycle, techno-economic and dynamic simulation assessment of bioelectrochemical systems: A 711
case of formic acid synthesis', Bioresource Technology, 255, pp. 39-49. 712
Speck, R., Selke, S., Auras, R. and Fitzsimmons, J. (2015) 'Choice of Life Cycle Assessment 713
Software Can Impact Packaging System Decisions', Packaging Technology and Science, 28(7), pp. 714
579-588. 715
Speck, R., Selke, S., Auras, R. and Fitzsimmons, J. (2016) 'Life Cycle Assessment Software: 716
Selection Can Impact Results', Journal of Industrial Ecology, 20(1), pp. 18-28. 717
Srikanth, S., Alvarez‐Gallego, Y., Vanbroekhoven, K. and Pant, D. (2017) 'Enzymatic 718
electrosynthesis of formic acid through carbon dioxide reduction in a bioelectrochemical system: 719
effect of immobilization and carbonic anhydrase addition', ChemPhysChem, 18(22), pp. 3174-3181. 720
Srikanth, S., Maesen, M., Dominguez-Benetton, X., Vanbroekhoven, K. and Pant, D. (2014) 721
'Enzymatic electrosynthesis of formate through CO2 sequestration/reduction in a bioelectrochemical 722
system (BES)', Bioresource Technology, 165, pp. 350-354. 723
Sutter, J. (2007) Life Cycle Inventories of chemicals. Swiss Centre for Life Cycle Inventories. 724
Tao, M., Qun, F., Hengcong, T., Zishan, H., Mingwen, J., Yunnan, G., Wangjing, M. and Zhenyu, S. 725
(2017) 'Heterogeneous electrochemical CO 2 reduction using nonmetallic carbon-based catalysts: 726
current status and future challenges', Nanotechnology, 28(47), p. 472001. 727
Thinkstep (2016) GaBi - Product Sustainability Software [Computer program]. 728
Thinkstep (2017) Overview of GaBi LCA Software. Available at: http://www.gabi-software.com/uk-729
ireland/index/ (Accessed: 27/09/2017). 730
Tromp, T.K., Shia, R.-L., Allen, M., Eiler, J.M. and Yung, Y.L. (2003) 'Potential Environmental 731
Impact of a Hydrogen Economy on the Stratosphere', Science, 300(5626), p. 1740. 732
Vassilev, I., Hernandez, P.A., Batlle Vilanova, P., Freguia, S., Krömer, J.O., Keller, J., Ledezma, P. 733
and Virdis, B. (2018) 'Microbial Electrosynthesis of Isobutyric, Butyric, Caproic acids and 734
corresponding Alcohols from Carbon dioxide', ACS Sustainable Chemistry & Engineering. 735
Vassilev, I., Kracke, F., Freguia, S., Keller, J., Krömer, J.O., Ledezma, P. and Virdis, B. (2019) 736
'Microbial electrosynthesis system with dual biocathode arrangement for simultaneous acetogenesis, 737
solventogenesis and carbon chain elongation', Chemical Communications, 55(30), pp. 4351-4354. 738
Zhang, L., Zhao, Z.-J. and Gong, J. (2017) 'Nanostructured Materials for Heterogeneous 739
Electrocatalytic CO2 Reduction and their Related Reaction Mechanisms', Angewandte Chemie 740
International Edition, 56(38), pp. 11326-11353. 741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
Table 1. Energy and water consumption of assessed acetic, formic, propionic acids, methanol and ethanol MES plants 758
Parameter Acetic
acid
Formic
acid
Propionic
acid
Methanol Ethanol
Energy consumed capturing and
processing CO2 (GJ/yr)
379.68 247.68 461.65 355.80 494.89
Process water consumed (m3/yr) 740.81 483.28 930.52 1319.66 1448.42
Energy consumed by MES plant
(rectification unit, bioreactor and
gas separator) (GJ/yr)
95457.09 8844.97 92211.75 31779.49 41051.51
Energy consumed for packaging
(GJ/yr)
126.70 109.00 134.67 167.92 168.58
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
Table 2. Reaction balances and cell potential for CO2 reduction into acetic, formic and propionic acids, methanol and 778 ethanol MES plants 779
Product Overall reaction Anode
potential
(V vs.
SHE)
Cathode
potential
(V vs.
SHE)
MES
reactor
Potential
(V vs.
SHE)
References
Acetic acid 2𝐶𝑂2 + 4𝐻2𝑂𝐵𝑖𝑜𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑡→ 𝐶𝐻3𝐶𝑂𝑂𝐻 + 2𝐻2𝑂 + 2𝑂2 0.817 -0.393 -1.210 (Marshall et
al., 2013)
Formic acid 𝐶𝑂2 + 𝐻2𝑂𝐵𝑖𝑜𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑡→ 𝐻𝐶𝑂𝑂𝐻 + 0.5𝑂2
0.817 -0.400 -1.217 (Nevin et
al., 2011;
CEAE,
2014)
Propionic
acid
3𝐶𝑂2 + 7𝐻2𝑂𝐵𝑖𝑜𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑡→ 𝐶𝐻3𝐶𝐻2𝐶𝑂𝑂𝐻 + 4𝐻2𝑂 + 3.5𝑂2
0.817 -0.290 -1.107 (CEAE,
2014)
Methanol 𝐶𝑂2 + 3𝐻2𝑂𝐵𝑖𝑜𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑡→ 𝐶𝐻3𝑂𝐻 + 𝐻2𝑂 + 1.5𝑂2 0.817 -0.390 -1.207 (CEAE,
2014)
Ethanol 2𝐶𝑂2 + 6𝐻2𝑂𝐵𝑖𝑜𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑡→ 𝐶𝐻3𝐶𝐻2𝑂𝐻 + 3𝐻2𝑂 + 3𝑂2
0.817 -0.335 -1.152 (Blanchet et
al., 2015)
780
781
782
783
784
785
786
787
Table 3. Life cycle inventory of the formic acid production routes analysed 788
Unit MES Plant AER Plant HCD Plant HMF
Plant
Reference (Pérez-
Fortes et
al., 2016)
(Sutter,
2007)
Raw
material
CO2 t/yr 1,094 1,094 830 -
H2O t/yr 483 483 560 600
CO t/yr - - - 614
CH3OH t/yr - - - 40
Product
HCOOH t/yr 1,000 1,000 1,000 1,000
Output
O2 t/yr 348 348 480 -
Energy
CO2 capture
energy
GJ/yr 192 192 146 -
Plant
electricity
GJ/yr 7,183 11,989 14,652 1,044
Steam GJ/yr 1,771 1,771 10,030 19,500
Total energy GJ/yr 8,954 13,761 24,682 20,544
789
790
791
792
793
794
795
796
Table 4. Greenhouse gases emissions from acetic acid, propionic acid, formic acid, ethanol and methanol MES plant (1,000 797 t/yr) modelled using GaBi (Electricity from 2014 UK national grid) compared with emissions using natural gas as electricity 798
Products
Greenhouse gases GWP (Electricity
from UK national
Grid) (t CO2 eqv)
GWP (Electricity
from burning
natural gas) (t CO2
eqv)
(Christodoulou et
al., 2017)
Carbon
dioxide (t)
Methane
(t)
Nitrous
Oxide
(t)
Sulfur
Hexafluoride
(t)
1 t CO2
eqv
25 t CO2
eqv
298 t
CO2
eqv
22800 t CO2
eqv (x 10-7)
Acetic
Acid
81,203 118 0.90 1.03 67,800 61,638
Propionic
Acid
84,171 135 1.11 1.08 67,700 48,797
Ethanol 59,703 144 1.42 1.52 42,100 -7,526
Methanol 45,636 109 1.32 1.15 33,200 -5,577
Formic
Acid
12,305 28 0.27 0.32 2,200 -4,428
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
Table 5. Greenhouse gases emissions from MES, AER, HCD and HMF plants (1,000t/yr) modelled using GaBi (Electricity 817 from 2014 UK national grid) 818
Plant
Greenhouse gases GWP (Electricity
from UK
national Grid) (t
CO2 eqv)
Carbon
dioxide (t)
Methane
(t)
Nitrous
Oxide
(t)
Sulfur
Hexafluoride
(t)
1 t CO2
eqv
25 t CO2
eqv
298 t
CO2
eqv
22800 t CO2
eqv (x 10-7)
MES 12,221 28 0.27 0.32 2,120
AER 19,397 46 0.45 0.51 9,820
HCD 28,806 60 0.56 0.61 22,200
HMF 17,378 33 0.15 0.10 18,400
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
Table 6. Global warming potential from the eight different electricity sources analysed for production of 1,000t/yr formic 839 acid using MES, HCD and HMF plants 840
Plant
GWP (t CO2 eqv)
Coal Gas Oil Nuclear Hydro Biogas Wind Photo
voltaic
MES 9,940 -615 9,367 -9,600 -9,550 -5,780 -9,581 -8,687
AER 22,700 5,309 21,797 -9,530 -9,453 -3,220 -9,500 -8,024
HCD 37,932 16,755 36,786 -1,271 -1,178 6,390 -1,240 559
HMF 19,500 18,032 19,448 16,759 16,766 17,300 16,800 16,900
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
Fig. 1. System boundary for 1,000t/yr MES plant for formic, acetic and propionic acids, methanol and ethanol production 861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
Fig. 2. Biotic and abiotic electro catalytic formic acid production 881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
Fig. 3. Climate change contributions of MES plant for the production of acetic, propionic and formic acids, methanol and 896 ethanol 897
898
899
900
901
902
903
904
905
906
907
908
909
910
Fig. 4. Life cycle environmental burdens (Human toxicity, ionizing radiation, ozone depletion and particulate matter) of the 911 MES plant for the production of acetic, propionic and formic acids, ethanol and methanol using ILCD method. Results are 912 displayed relative to the maximum value in each impact category. 913
914
915
916
917
918
919
920
921
922
923
924
925
926
Fig. 5. Life cycle environmental burdens (Acidification, ecotoxixity, eutrophication and photochemical ozone formation) of 927 the MES plant for the production of acetic, propionic and formic acids, ethanol and methanol using ILCD method. Results 928 are displayed relative to the maximum value in each impact category. 929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
Fig. 6. Scenario analysis of CO2 conversion and faradaic efficiency 945
946
947
948
949
950
951
952
953
954
955
956
957
958
Fig. 7. Life cycle environmental burdens (Acidification, ecotoxixity, eutrophication and photochemical ozone formation) of 959 MES, AER, HCD and HMF plants using ILCD method. Results are displayed relative to the maximum value in each impact 960 category. 961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
Fig. 8. Life cycle environmental burdens (Human toxicity, ionizing radiation, ozone depletion and particulate matter) of 976 MES, HCD and HMF Plants using ILCD method. Results are displayed relative to the maximum value in each impact 977 category. 978
979
980
Fig. 9. Life cycle environmental burdens of the MES plant for eight different electricity sources using ILCD method. A) 981 Acidification, ecotoxixity, eutrophication and photochemical ozone formation B) Human toxicity, ionizing radiation, ozone 982 depletion and particulate matter. Results are displayed relative to the value of the UK national grid in each impact category. 983
984
Fig. 10. Life cycle environmental burdens of the MES, AER, HCD and HMF plant using wind as the electricity source. A) 985 Acidification, ecotoxixity, eutrophication and photochemical ozone formation B) Human toxicity, ionizing radiation, ozone 986 depletion and particulate matter. Results are displayed relative to the maximum value in each impact category. 987
988
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