Life Cycle Assessment of Power-to-Gas: Syngas vs Methane
Life Cycle Assessment of Power-to-Gas: Syngas vs Methane André Sternberg and André Bardow Chair of Technical Thermodynamics, RWTH Aachen University [email protected]
Supporting Information Contents S1. LCA data S2. Sensitivity analysis for global warming and fossil depletion threshold values for electricity supply S3. Threshold values for further environmental impact categories S4. Global warming impact of SNG production S5. Environmental threshold values for renewable electricity S6. References
S1
S1 LCA data Table S1. Inputs and outputs of considered chemical conversion processes per kg product Product Reaction
CO
Inputs
reverse Water-Gas-Shift
CO
Dry Reforming
CO
Steam Methane Reforming
CH4
Sabatier a
H2
NG
[kg] 0.072
Outputs
Emissions
Reference
CO2 Electricity Heat
H2
Steam
Heat
CO2
[kg]
[kg]
[kWh]
[kWh]
[kg]
[kg]
[kWh]
[kg]
-
1.581
1.34b
0.61
-
-
0.016
-
1
-
0.260 0.910
b
1.98
0.336
-
1
-
0.723
-
0.87
2.38 0.216 0.149
-
0.009
1
0.506
-
2.939
0.33
0.009a
3.008
0.194
2
-
-
0.050 0.201
-
b
mixed with main product, electricity is also used for reactor heating
1, 2
Table S2. Data for construction of electrolysis and hydrogen storage Value PEM electrolysis
Comment -
life time electrolysis3 life time hydrogen storage
3
steel demand hydrogen storage
4
inventory is considered equivalent to PEM fuel cell based on rated capacity
15 a
value adapted from PEM fuel cell
50 a
value adapted from liquid storage tank
422.7 t/t H2
per hydrogen capacity; chromium steel 18/8 considered
Table S3. Considered LCA data sets Product
Name of data set
Year
Database
Heat
Thermal energy from natural gas (efficiency 95 %) [DE]
2012
GaBi ts
Natural gas
Natural gas mix [DE]
2012
GaBi ts
Coal
Hard coal mix [DE]
2012
GaBi ts
Steam
Steam production from natural gas (efficiency 90 %) [DE]
2012
GaBi ts
MEA
Market for monoethanolamine [GLO]
2015
ecoinvent 3.1
Steel
Steel production, electric, chromium steel 18/8 [RER]
2015
ecoinvent 3.1
Electrolysis
PEM fuel cell 2kWe, future*
2015
ecoinvent 3.1
Electricity
Electricity from biogas [DE]
2012
GaBi ts
Electricity
Electricity from biomass (solid) [DE]
2012
GaBi ts
Electricity
Electricity from geothermal [IT]
2012
GaBi ts
Electricity
Electricity from hard coal [DE]
2012
GaBi ts
Electricity
Electricity from hydro power [DE]
2012
GaBi ts
Electricity
Electricity from lignite [DE]
2012
GaBi ts
Electricity
Electricity from natural gas [DE]
2012
GaBi ts
Electricity
Electricity from nuclear [DE]
2012
GaBi ts
Electricity
Electricity from photovoltaic
2012
GaBi ts
Electricity
Electricity from wind power
2012
GaBi ts
*original data set refers to fuel cell for combined heat and power (CHP); for inventory of electrolysis heat distribution system is not considered
S2
S2 Sensitivity analysis for global warming and fossil depletion threshold values for electricity supply In this section, a sensitivity analysis is presented for the aspects summarized in Table S4. Table S4. Aspects considered in sensitivity analysis Base case in main text I ) Utilization of by-products
no environmental credit for utilization of by-products
II) CO2 supply (power plant)
Power-to-Gas pathway: power plant with CO2 capture (η = 27.25 %) conventional processes: power plant without CO2 capture (η = 38.27 %)
III) Electricity demand of electrolysis
50 kWh per kg hydrogen
IV) Methane emissions from natural gas supply
8 g CH4 per kg natural gas
V) Operation of electrolysis and sizing of hydrogen storage
electrolysis: 2,500 full load hours per year hydrogen storage: sized to cover 10 days without renewable electricity supply
I) Utilization of by-products In contrast to the main text, the environmental credit for the by-products is here based on the avoided burden principle. For this purpose, the avoided burden processes are determined. Steam. For the steam produced in conventional syngas process, DRM process and SNG process, a credit is given for steam generation from natural gas with an efficiency of 90 %. Heat The purge gas produced in the DRM and rWGS process contains CO, hydrogen and methane. We assume thermal utilization of the purge gas. Thus, a credit is given for heat supply by natural gas. The CO2 emissions of the combustion of the purge gas are determined assuming full combustion, i.e., CO and methane are converted to CO2. Oxygen. Oxygen is co-produced in the electrolyzer. The rWGS, DRM and SNG process receive a credit for the production of oxygen by an air separation unit. The environmental impacts of air separation units are dominated by the demand for electricity. For reasons of consistency, we do not use LCA data based on the current grid electricity mix. Rather, the Power-to-Gas routes receive a credit on their electricity demand. The reduced electricity demand of the Power-to-Gas routes increases the threshold value. The electricity demand of the avoided air separation process is 0.25 kWh / kg O2.5 S3
II) CO2 supply The scenarios considered in the sensitivity analysis affect either the power plant of the Powerto-Gas pathways or the power plant of the conventional processes. The changes compared to the base case are written in italics. II a) Power plant with CO2 capture without energy penalty In this case, the power plant with CO2 capture has the same efficiency as the power plant without CO2 capture. This can be considered as the best case for CO2 supply.
Power-to-Gas pathway: Coal-fired power plant with CO2 capture (η = 38.27 %)
Conventional processes: Coal-fired power plant without CO2 capture (η = 38.27 %)
II b) Power plant with CO2 capture vs. power plant with CO2 capture and storage (CCS) In this case, the conventional process also uses a power plant with CO2 capture. The captured CO2 is stored in the underground. We assume that no additional energy is required for the CO2 storage.
Power-to-Gas pathway: Coal-fired power plant with CO2 capture (η = 27.25 %)
Conventional processes: Coal-fired power plant with CO2 capture (η = 27.25 %) and subsequent CO2 storage in underground
II c) Power plant with CO2 capture vs. renewable electricity: In this case, the conventional process uses a wind power plant to satisfy electricity demand of functional unit.
Power-to-Gas pathway: Coal-fired power plant with CO2 capture (η = 27.25 %)
Conventional processes: wind power plant
II d) Power plant with CO2 capture supplies electricity to electrolysis: In this case, the electricity of the power plant with CO2 capture is supplied to electrolysis. Thus, the functional unit only contains the production of syngas and SNG, respectively.
Power-to-Gas pathway: Coal-fired power plant with CO2 capture (η = 27.25 %)
Conventional processes: No power plant required
S4
III) Electricity demand of electrolysis III a) 45 kWh per kg hydrogen (10 % lower than in main text) III b) 55 kWh per kg hydrogen (10 % higher than in main text) IV) Methane emissions from natural gas supply In the sensitivity analysis, methane emissions of 17 g per kg natural gas are considered. This value corresponds to the methane emissions for the natural gas supply in Slovakia, the country with the highest methane emissions in Europe according to the GaBi database.6 This scenario is only considered for global warming impacts not for fossil-depletion impacts. V) Operation of electrolysis and sizing of hydrogen storage The operation of electrolysis and the sizing of hydrogen storage have an influence on the environmental impacts of construction. V a) 900 full load hours per year for electrolysis V b) 5,000 full load hours per year for electrolysis V c) Hydrogen storage is sized to cover 5 days without renewable electricity supply V d) Hydrogen storage is sized to cover 20 days without renewable electricity supply
S5
S2.1 Steady-state operation of electrolysis (use of grid electricity) In this case, the electrolysis is operated in 8,000 h per year and no hydrogen storage is required. Thus, construction of electrolysis and hydrogen storage (IV) is not considered. Power-to-Syngas (rWGS)
Power-to-Syngas (DRM)
Power-to-SNG
Threshold value GW electricity / (kg CO2-eq / kWh)
0.25 0.20 0.15 0.10 0.05 0.00 I)
-0.05
II) Power plant with CO2 capture ...
III) η PEM
IV)
Figure S1. Sensitivity analysis for global warming (GW) threshold value for electricity supply. The dashed lines indicate the threshold values for the base case presented in the main text. Power-to-Syngas (rWGS)
Power-to-Syngas (DRM)
Power-to-SNG
Threshold value FD electricity / (kg Oil-eq / kWh)
0.10 0.08 0.06
0.04 0.02
0.00 -0.02
I)
II) Power plant with CO2 capture ...
III) η PEM
Figure S2. Sensitivity analysis for fossil depletion (FD threshold value for electricity supply. The dashed lines indicate the threshold values for the base case presented in the main text.
S6
S2.2 Part-load operation of electrolysis (use of 100 %renewable electricity) In this case, the electrolysis unit is operated in part-load and hydrogen storage is required. Power-to-Syngas (rWGS)
Power-to-Syngas (DRM)
Power-to-SNG
Threshold value GW electricity / (kg CO2-eq / kWh)
0.20 0.15 0.10 0.05
0.00 -0.05 I)
-0.10
II) Power plant with CO2 capture ...
III) η PEM
IV)
V) Construction electrolysis unit and storage
Figure S3. Sensitivity analysis for global warming (GW) threshold value for electricity supply. The threshold values refer to a share of 100 % renewable electricity in electrolysis. The dashed lines indicate the threshold value for the base case presented in the main text.
Power-to-Syngas (rWGS)
Power-to-Syngas (DRM)
Power-to-SNG
Threshold value FD electricity / (kg Oil-eq / kWh)
0.08 0.06 0.04
0.02 0.00 I)
-0.02
II) Power plant with CO2 capture ...
III) η PEM
V) Construction electrolysis unit and storage
Figure S4. Sensitivity analysis for fossil depletion (FD) threshold value for electricity supply. The threshold values refer to a share of 100 % renewable electricity in electrolysis. The dashed lines indicate the threshold value for the base case presented in the main text.
S7
S3 Threshold values for further environmental impact categories In this section, threshold values for electricity supply are analyzed for 12 further environmental impact categories which are available in ReCiPe. In Table S5, all environmental impact categories with positive threshold values are summarized. Only in these environmental impact categories, the Power-to-Gas pathways can achieve lower environmental impacts than the conventional process if an environmentally suitable electricity supply process is chosen. If the forecasted German grid electricity mix for 2050 is applied, lower environmental impacts compared to conventional processes are only achieved for global warming and fossil depletion. S3.1 Steady-state operation of electrolysis Table S5. Environmental impact categories with positive threshold values for electricity supply. Environmental impact category
rWGS
DRM
SNG
Global warming (in kg CO2-eq/kWh)
1.4E-01
1.9E-01
8.2E-02
Photochemical oxidant formation (in kg NMVOC/kWh)
2.3E-06
4.4E-05
-
Fossil depletion (in kg Oil-eq/kWh)
5.6E-02
7.3E-02
4.0E-02
Comparison of Power-to-Gas pathways to conventional processes 100000 10000
rWGS
DRM
SNG
1000
100 10 1 0.1
Figure S5. Environmental impacts (without electricity supply) of Power-to-Gas pathways normalized to corresponding conventional process. If the value for the Power-to-Gas pathways is lower than 1, the corresponding environmental threshold value for electricity supply is positive.
S8
Environmental hot spots of Power-to-Gas pathways (without electricity supply) Power plant (MEA supply)
Power plant (Coal supply)
Natural gas as feedstock
Construction electrolysis
Heat from natural gas
100% 80%
60% 40% 20% 0% 100% 80%
60% 40% 20%
0%
100% 80% 60% 40% 20% 0%
Figure S6. Hot spots for rWGS (top), DRM (middle) and SNG (bottom) process. Coal supply also includes CO2 emissions due to combustion of coal in power plant.
S9
S3.2 Part-load operation of electrolysis In this case, positive threshold values are only achieved in the impact categories global warming and fossil depletion. Environmental hot spots of Power-to-Gas pathways(without renewable electricity supply) Coal-fired power plant
Heat from natural gas
Natural gas as feedstock
Construction electrolysis
Construction H2 storage
Grid electricity
100% 80% 60%
40% 20% 0%
100% 80% 60% 40% 20% 0% 100% 80% 60% 40%
20% 0%
Figure S7. Hot spots for rWGS (top), DRM (middle) and SNG (bottom) process. Power plant contains coal supply, MEA supply and CO2 emissions due to combustion of coal in power plant.
S10
S4 Global warming impact of SNG production
Global warming impact / (kg CO2-eq / FUSNG)
Wind
Solar
2050
2040
2030
2020
0.20
0.15
0.10
0.05
0.00 0
0.1
0.2
0.3
0.4
Global warming impact electricity / (kg CO2-eq/kWh)
Natural gas
SNG
Figure S8. Global warming impact of SNG process and conventional natural gas production as a function of global warming impact of electricity supply. The functional unit (FUSNG) is the production of 1 MJ natural gas and 0.049 kWh electricity. The vertical lines represent the global warming impacts of electricity supply from wind and solar, and forecasted German electricity mixes.
S11
S5 Environmental threshold values for renewable electricity For the utilization of a mix of intermittent renewable and base-load grid electricity with a given share of renewable electricity (X) and environmental impact of grid electricity a threshold value for renewable electricity can be determined by solving the inequality presented in the main text (eq 3): TV 𝐸𝐼electricity supply ≥ 𝐸𝐼renewable electricity ∙ 𝑋 + 𝐸𝐼grid electricity ∙ (1 − 𝑋).
(S1)
Transforming eq S1 yields:
TV 𝐸𝐼renewable electricity
=
TV 𝐸𝐼elctricity supply − 𝐸𝐼grid electricity ∙ (1 − 𝑋)
𝑋
.
(S2)
The environment benefit per kWh renewable electricity is the difference of the threshold value for renewable electricity and the environmental impact of the chosen renewable electricity technology. TV 𝐸𝐼benefit = 𝐸𝐼renewable electrictiy − 𝐸𝐼renewable electricity
S12
(S3)
S6 References (1) CO2RRECT (ref. no. 033RC1006B), CO2-Reaction using Regenerative Energies and Catalytic Technologies; 2014 (in German). (2) Müller, B.; Müller, K.; Teichmann, D.; Arlt, W. Chem. Ing. Tech. 2011, 83, 2002– 2013 (in German). (3) ecoinvent Data V 3.1, Swiss Centre for Life Cycle Inventories, http://ecoinvent.org/, (accessed March 2016). (4) Mori, M.; Jensterle, M.; Mržljak, T.; Drobnic, B. Int. J. Life Cycle Ass. 2014, 19, 1810–1822. (5) Hong, J.; Chaudhry, G.; Brisson, J.; Field, R.; Gazzino, M.; Ghoniem, A. F. Energy 2009, 34, 1332 – 1340. (6) GaBi ts, Software-System and Database for Life Cycle Engineering. thinkstep AG, Germany, 2016.
S13
Supporting Information Contents S1. LCA data S2. Sensitivity analysis for global warming and fossil depletion threshold values for electricity supply S3. Threshold values for further environmental impact categories S4. Global warming impact of SNG production S5. Environmental threshold values for renewable electricity S6. References
S1
S1 LCA data Table S1. Inputs and outputs of considered chemical conversion processes per kg product Product Reaction
CO
Inputs
reverse Water-Gas-Shift
CO
Dry Reforming
CO
Steam Methane Reforming
CH4
Sabatier a
H2
NG
[kg] 0.072
Outputs
Emissions
Reference
CO2 Electricity Heat
H2
Steam
Heat
CO2
[kg]
[kg]
[kWh]
[kWh]
[kg]
[kg]
[kWh]
[kg]
-
1.581
1.34b
0.61
-
-
0.016
-
1
-
0.260 0.910
b
1.98
0.336
-
1
-
0.723
-
0.87
2.38 0.216 0.149
-
0.009
1
0.506
-
2.939
0.33
0.009a
3.008
0.194
2
-
-
0.050 0.201
-
b
mixed with main product, electricity is also used for reactor heating
1, 2
Table S2. Data for construction of electrolysis and hydrogen storage Value PEM electrolysis
Comment -
life time electrolysis3 life time hydrogen storage
3
steel demand hydrogen storage
4
inventory is considered equivalent to PEM fuel cell based on rated capacity
15 a
value adapted from PEM fuel cell
50 a
value adapted from liquid storage tank
422.7 t/t H2
per hydrogen capacity; chromium steel 18/8 considered
Table S3. Considered LCA data sets Product
Name of data set
Year
Database
Heat
Thermal energy from natural gas (efficiency 95 %) [DE]
2012
GaBi ts
Natural gas
Natural gas mix [DE]
2012
GaBi ts
Coal
Hard coal mix [DE]
2012
GaBi ts
Steam
Steam production from natural gas (efficiency 90 %) [DE]
2012
GaBi ts
MEA
Market for monoethanolamine [GLO]
2015
ecoinvent 3.1
Steel
Steel production, electric, chromium steel 18/8 [RER]
2015
ecoinvent 3.1
Electrolysis
PEM fuel cell 2kWe, future*
2015
ecoinvent 3.1
Electricity
Electricity from biogas [DE]
2012
GaBi ts
Electricity
Electricity from biomass (solid) [DE]
2012
GaBi ts
Electricity
Electricity from geothermal [IT]
2012
GaBi ts
Electricity
Electricity from hard coal [DE]
2012
GaBi ts
Electricity
Electricity from hydro power [DE]
2012
GaBi ts
Electricity
Electricity from lignite [DE]
2012
GaBi ts
Electricity
Electricity from natural gas [DE]
2012
GaBi ts
Electricity
Electricity from nuclear [DE]
2012
GaBi ts
Electricity
Electricity from photovoltaic
2012
GaBi ts
Electricity
Electricity from wind power
2012
GaBi ts
*original data set refers to fuel cell for combined heat and power (CHP); for inventory of electrolysis heat distribution system is not considered
S2
S2 Sensitivity analysis for global warming and fossil depletion threshold values for electricity supply In this section, a sensitivity analysis is presented for the aspects summarized in Table S4. Table S4. Aspects considered in sensitivity analysis Base case in main text I ) Utilization of by-products
no environmental credit for utilization of by-products
II) CO2 supply (power plant)
Power-to-Gas pathway: power plant with CO2 capture (η = 27.25 %) conventional processes: power plant without CO2 capture (η = 38.27 %)
III) Electricity demand of electrolysis
50 kWh per kg hydrogen
IV) Methane emissions from natural gas supply
8 g CH4 per kg natural gas
V) Operation of electrolysis and sizing of hydrogen storage
electrolysis: 2,500 full load hours per year hydrogen storage: sized to cover 10 days without renewable electricity supply
I) Utilization of by-products In contrast to the main text, the environmental credit for the by-products is here based on the avoided burden principle. For this purpose, the avoided burden processes are determined. Steam. For the steam produced in conventional syngas process, DRM process and SNG process, a credit is given for steam generation from natural gas with an efficiency of 90 %. Heat The purge gas produced in the DRM and rWGS process contains CO, hydrogen and methane. We assume thermal utilization of the purge gas. Thus, a credit is given for heat supply by natural gas. The CO2 emissions of the combustion of the purge gas are determined assuming full combustion, i.e., CO and methane are converted to CO2. Oxygen. Oxygen is co-produced in the electrolyzer. The rWGS, DRM and SNG process receive a credit for the production of oxygen by an air separation unit. The environmental impacts of air separation units are dominated by the demand for electricity. For reasons of consistency, we do not use LCA data based on the current grid electricity mix. Rather, the Power-to-Gas routes receive a credit on their electricity demand. The reduced electricity demand of the Power-to-Gas routes increases the threshold value. The electricity demand of the avoided air separation process is 0.25 kWh / kg O2.5 S3
II) CO2 supply The scenarios considered in the sensitivity analysis affect either the power plant of the Powerto-Gas pathways or the power plant of the conventional processes. The changes compared to the base case are written in italics. II a) Power plant with CO2 capture without energy penalty In this case, the power plant with CO2 capture has the same efficiency as the power plant without CO2 capture. This can be considered as the best case for CO2 supply.
Power-to-Gas pathway: Coal-fired power plant with CO2 capture (η = 38.27 %)
Conventional processes: Coal-fired power plant without CO2 capture (η = 38.27 %)
II b) Power plant with CO2 capture vs. power plant with CO2 capture and storage (CCS) In this case, the conventional process also uses a power plant with CO2 capture. The captured CO2 is stored in the underground. We assume that no additional energy is required for the CO2 storage.
Power-to-Gas pathway: Coal-fired power plant with CO2 capture (η = 27.25 %)
Conventional processes: Coal-fired power plant with CO2 capture (η = 27.25 %) and subsequent CO2 storage in underground
II c) Power plant with CO2 capture vs. renewable electricity: In this case, the conventional process uses a wind power plant to satisfy electricity demand of functional unit.
Power-to-Gas pathway: Coal-fired power plant with CO2 capture (η = 27.25 %)
Conventional processes: wind power plant
II d) Power plant with CO2 capture supplies electricity to electrolysis: In this case, the electricity of the power plant with CO2 capture is supplied to electrolysis. Thus, the functional unit only contains the production of syngas and SNG, respectively.
Power-to-Gas pathway: Coal-fired power plant with CO2 capture (η = 27.25 %)
Conventional processes: No power plant required
S4
III) Electricity demand of electrolysis III a) 45 kWh per kg hydrogen (10 % lower than in main text) III b) 55 kWh per kg hydrogen (10 % higher than in main text) IV) Methane emissions from natural gas supply In the sensitivity analysis, methane emissions of 17 g per kg natural gas are considered. This value corresponds to the methane emissions for the natural gas supply in Slovakia, the country with the highest methane emissions in Europe according to the GaBi database.6 This scenario is only considered for global warming impacts not for fossil-depletion impacts. V) Operation of electrolysis and sizing of hydrogen storage The operation of electrolysis and the sizing of hydrogen storage have an influence on the environmental impacts of construction. V a) 900 full load hours per year for electrolysis V b) 5,000 full load hours per year for electrolysis V c) Hydrogen storage is sized to cover 5 days without renewable electricity supply V d) Hydrogen storage is sized to cover 20 days without renewable electricity supply
S5
S2.1 Steady-state operation of electrolysis (use of grid electricity) In this case, the electrolysis is operated in 8,000 h per year and no hydrogen storage is required. Thus, construction of electrolysis and hydrogen storage (IV) is not considered. Power-to-Syngas (rWGS)
Power-to-Syngas (DRM)
Power-to-SNG
Threshold value GW electricity / (kg CO2-eq / kWh)
0.25 0.20 0.15 0.10 0.05 0.00 I)
-0.05
II) Power plant with CO2 capture ...
III) η PEM
IV)
Figure S1. Sensitivity analysis for global warming (GW) threshold value for electricity supply. The dashed lines indicate the threshold values for the base case presented in the main text. Power-to-Syngas (rWGS)
Power-to-Syngas (DRM)
Power-to-SNG
Threshold value FD electricity / (kg Oil-eq / kWh)
0.10 0.08 0.06
0.04 0.02
0.00 -0.02
I)
II) Power plant with CO2 capture ...
III) η PEM
Figure S2. Sensitivity analysis for fossil depletion (FD threshold value for electricity supply. The dashed lines indicate the threshold values for the base case presented in the main text.
S6
S2.2 Part-load operation of electrolysis (use of 100 %renewable electricity) In this case, the electrolysis unit is operated in part-load and hydrogen storage is required. Power-to-Syngas (rWGS)
Power-to-Syngas (DRM)
Power-to-SNG
Threshold value GW electricity / (kg CO2-eq / kWh)
0.20 0.15 0.10 0.05
0.00 -0.05 I)
-0.10
II) Power plant with CO2 capture ...
III) η PEM
IV)
V) Construction electrolysis unit and storage
Figure S3. Sensitivity analysis for global warming (GW) threshold value for electricity supply. The threshold values refer to a share of 100 % renewable electricity in electrolysis. The dashed lines indicate the threshold value for the base case presented in the main text.
Power-to-Syngas (rWGS)
Power-to-Syngas (DRM)
Power-to-SNG
Threshold value FD electricity / (kg Oil-eq / kWh)
0.08 0.06 0.04
0.02 0.00 I)
-0.02
II) Power plant with CO2 capture ...
III) η PEM
V) Construction electrolysis unit and storage
Figure S4. Sensitivity analysis for fossil depletion (FD) threshold value for electricity supply. The threshold values refer to a share of 100 % renewable electricity in electrolysis. The dashed lines indicate the threshold value for the base case presented in the main text.
S7
S3 Threshold values for further environmental impact categories In this section, threshold values for electricity supply are analyzed for 12 further environmental impact categories which are available in ReCiPe. In Table S5, all environmental impact categories with positive threshold values are summarized. Only in these environmental impact categories, the Power-to-Gas pathways can achieve lower environmental impacts than the conventional process if an environmentally suitable electricity supply process is chosen. If the forecasted German grid electricity mix for 2050 is applied, lower environmental impacts compared to conventional processes are only achieved for global warming and fossil depletion. S3.1 Steady-state operation of electrolysis Table S5. Environmental impact categories with positive threshold values for electricity supply. Environmental impact category
rWGS
DRM
SNG
Global warming (in kg CO2-eq/kWh)
1.4E-01
1.9E-01
8.2E-02
Photochemical oxidant formation (in kg NMVOC/kWh)
2.3E-06
4.4E-05
-
Fossil depletion (in kg Oil-eq/kWh)
5.6E-02
7.3E-02
4.0E-02
Comparison of Power-to-Gas pathways to conventional processes 100000 10000
rWGS
DRM
SNG
1000
100 10 1 0.1
Figure S5. Environmental impacts (without electricity supply) of Power-to-Gas pathways normalized to corresponding conventional process. If the value for the Power-to-Gas pathways is lower than 1, the corresponding environmental threshold value for electricity supply is positive.
S8
Environmental hot spots of Power-to-Gas pathways (without electricity supply) Power plant (MEA supply)
Power plant (Coal supply)
Natural gas as feedstock
Construction electrolysis
Heat from natural gas
100% 80%
60% 40% 20% 0% 100% 80%
60% 40% 20%
0%
100% 80% 60% 40% 20% 0%
Figure S6. Hot spots for rWGS (top), DRM (middle) and SNG (bottom) process. Coal supply also includes CO2 emissions due to combustion of coal in power plant.
S9
S3.2 Part-load operation of electrolysis In this case, positive threshold values are only achieved in the impact categories global warming and fossil depletion. Environmental hot spots of Power-to-Gas pathways(without renewable electricity supply) Coal-fired power plant
Heat from natural gas
Natural gas as feedstock
Construction electrolysis
Construction H2 storage
Grid electricity
100% 80% 60%
40% 20% 0%
100% 80% 60% 40% 20% 0% 100% 80% 60% 40%
20% 0%
Figure S7. Hot spots for rWGS (top), DRM (middle) and SNG (bottom) process. Power plant contains coal supply, MEA supply and CO2 emissions due to combustion of coal in power plant.
S10
S4 Global warming impact of SNG production
Global warming impact / (kg CO2-eq / FUSNG)
Wind
Solar
2050
2040
2030
2020
0.20
0.15
0.10
0.05
0.00 0
0.1
0.2
0.3
0.4
Global warming impact electricity / (kg CO2-eq/kWh)
Natural gas
SNG
Figure S8. Global warming impact of SNG process and conventional natural gas production as a function of global warming impact of electricity supply. The functional unit (FUSNG) is the production of 1 MJ natural gas and 0.049 kWh electricity. The vertical lines represent the global warming impacts of electricity supply from wind and solar, and forecasted German electricity mixes.
S11
S5 Environmental threshold values for renewable electricity For the utilization of a mix of intermittent renewable and base-load grid electricity with a given share of renewable electricity (X) and environmental impact of grid electricity a threshold value for renewable electricity can be determined by solving the inequality presented in the main text (eq 3): TV 𝐸𝐼electricity supply ≥ 𝐸𝐼renewable electricity ∙ 𝑋 + 𝐸𝐼grid electricity ∙ (1 − 𝑋).
(S1)
Transforming eq S1 yields:
TV 𝐸𝐼renewable electricity
=
TV 𝐸𝐼elctricity supply − 𝐸𝐼grid electricity ∙ (1 − 𝑋)
𝑋
.
(S2)
The environment benefit per kWh renewable electricity is the difference of the threshold value for renewable electricity and the environmental impact of the chosen renewable electricity technology. TV 𝐸𝐼benefit = 𝐸𝐼renewable electrictiy − 𝐸𝐼renewable electricity
S12
(S3)
S6 References (1) CO2RRECT (ref. no. 033RC1006B), CO2-Reaction using Regenerative Energies and Catalytic Technologies; 2014 (in German). (2) Müller, B.; Müller, K.; Teichmann, D.; Arlt, W. Chem. Ing. Tech. 2011, 83, 2002– 2013 (in German). (3) ecoinvent Data V 3.1, Swiss Centre for Life Cycle Inventories, http://ecoinvent.org/, (accessed March 2016). (4) Mori, M.; Jensterle, M.; Mržljak, T.; Drobnic, B. Int. J. Life Cycle Ass. 2014, 19, 1810–1822. (5) Hong, J.; Chaudhry, G.; Brisson, J.; Field, R.; Gazzino, M.; Ghoniem, A. F. Energy 2009, 34, 1332 – 1340. (6) GaBi ts, Software-System and Database for Life Cycle Engineering. thinkstep AG, Germany, 2016.
S13
