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An Electrochemical Haber-Bosch Process*
V. Kyriakou, I. Garagounis, A. Vourros, E. Vasileiou, and M. Stoukides
Aristotle University of Thessaloniki (AUTH) and
Center for Research & Technology Hellas (CERTH)
Financial Support: CoorsTek Membrane Sciences
* JOULE, 4(1)pp. 142-158 (2020).
T. Eggeman, in “Kirk- Othmer Encyclopedia of Chemical Technology,” 4th edition, John Wiley (2008)
Industrial Ammonia Production
▪ Supports >50% of earth’s population
▪ High T, P → High capital cost
▪ Responsible for 1-2% energy consumption and CO2 emissions worldwide
➢ Hydrogen production (highly endothermic)
Methane steam reforming: CH4 + H2O ➔ CO + 3 H2
Water gas shift: CO + H2O ➔ CO2 + H2
➢ Preparation of synthesis gas (extreme purification)
➢ Pressurization (150-250 bar)
➢ Ammonia synthesis (exothermic): N2 + 3H2 ➔ 2 NH3
Overall Reaction: 3 CH4 + 6 Η2O + 4 Ν2 <==> 3 CO2 + 8 NH3
In industrial practice, the CO2/NH3 molar ratio is about 1.1 (instead of 0.4)
The main steps in the NH3 synthesis plant
Electrolyte: BaZr0.7Ce0.2Y0.1O2.9 (BZCY72), thickness: 30 μm
Counter Electrode: Ni-BZCY72, area: 20-25 cm-2
Working Electrode: VN-Fe, area : 8 cm-2
Tube length: 250 mm
* H. Iwahara, et al, Solid State Ionics 4, 359–363 (1981).
* * S. Robinson, et al, J. Membr. Sci. 446 (2013) 99–105; W.G. Coors, A. Manerbino, J. Membr. Sci. 376 (2011) 50–55.
Plants and bacteria produce NH3 at ambient conditions (nitrogenase, N2 , H+, e- )
Discovery* and development of high temperature H+ conductors
Development * * of ceramic membrane reactors with high H+ conductivity
✓ High H+ conductivity (10-6 moles H2 cm-2 s-1) at 450-700 oC
✓ The hydrogen source can be a hydrocarbon (e.g. steam reforming)
NH3
electrode
9.99 mm
H+
H+
H+
H+
CH4 , H2O
N2 + 6H+ +6e- 2 NH3
CO, CO2
N2
N2, H2, NH3
CH4 + H2O CO + 3H2
CO + H2O CO2 + H2
H2 2H+ + 2e-
➢ Methane conversion increases by H2 removal (operation at lower T)➢ Purification of hydrogen is eliminated
Combined SSAS and CH4-H2O reforming
Overall: 3 CH4 + 6 H2O + 4 N2➔ 8 NH3 + 3 CO2
2H+ +2e- H2
Anode side
0 30 60 90 120 150
40
50
60
70
80
90
100
550 oC
600 oC
650 oC
CH
4 c
on
ve
rsio
n,
%
Current, mA
0 30 60 90 120 15090
92
94
96
98
100
CO
2 s
ele
ctivity,
%
Current, mA
550 oC
600 oC
650 oC
Results of the combined process
0 30 60 90 120 1500
15
30
45
60
75
90
NH
3 r
ate
, m
mo
l. h-1. m
-2
Current, mA
550 oC
600 oC
650 oC
0 30 60 90 120 1500
3
6
9
12
15
18
NH
3 f
ara
daic
effic
iency
, %
Current, mA
550 oC
600 oC
650 oC
Cathode side
At the anode:
▪ Open-circuit: methane conversion > 60%
▪ Closed-circuit: methane conversion up to 85%
▪ Open-circuit: CO2 selectivity up to 90%.
▪ Closed-circuit: CO2 selectivity > 99%.
At the cathode:
▪ NH3 is formed at a rate of up to 1.95 x 10-9 mol.s-1.cm-2 with a
corresponding Faradaic Efficiency of 5.5%.
▪ FEs up to 14% at lower current densities
Summary of Experimental Results
Visualizing an electrochemical HB
SSAS Reactor: CH4 + Η2O ↔ CO2 + 6Η+ + 6e- (anode)
N2 + 6H+ + 6e- ↔ 2NH3 (cathode)
2H+ + 2e- ↔ H2 (cathode)
Protonic Ceramic Fuel Cell: H2 ↔ 2Η+ + 2e- (anode)
1/2O2 + 2H+ + 2e- ↔ H2O (cathode)
(The PCFC is assumed to operate at a 45% efficiency)
NH3, N2, H2
0 20 40 60 80 1000
2
4
6
8
10
12
EHB process
HB process
CO
2 e
mis
sio
ns,
kg
CO
2/k
g N
H3
Faradaic efficiency to NH3, %
PCMR operation window
FENH3 = 35%
0 20 40 60 80 1000
500
1000
1500
2000
PCMR operation window
Electrical
Thermal
Total
HB process
PCMR voltage = 0.3 V
Ener
gy, k
J.m
ol-1 N
H3
Faradaic efficiency to NH3, %
FENH3 = 35%
Electrochemical vs Industrial HB
➢ >35% FE is required to exhibit lower energy demands and CO2 emissions
than the industrial HB
The “Giddey” Requirements 1
➢ current density: 0.25-0.5 A cm-2
➢ current efficiency : >50% ➢ NH3 is production rates: 4.3 - 8.7 x 10-7 mol cm-2 s-1
The DOE (REFUEL) Requirements 2
➢ current density: 0.3 A cm-2
➢ Faradaic efficiency : 90% ➢ NH3 is production rates: 9.3 x10-7 mol cm-2 s-1
1 S..Giddey et al, Int'l J. Hydrogen Energy 38, pp. 14576-14594 (2013)2 I.McPherson, J. Zhang, JOULE, 4, pp. 12-14 (2020)]
What is the target
Increasing the Faradaic Efficiency to a 40-50% and the NH3
production rate to 5 x 10-7 mol cm-2 s-1 is a real challenge. It
will require intense collaboration among researchers in the
fields of electrochemistry, heterogeneous catalysis and
materials science.
But it is worth the effort…
Thank you for your attention!
Electrical energy demands
SSASREACTOR
CO2capture
CH4, H2O
N2,H2O
PCFC NH3 storage
Condenser2
Anode:CH4 +2H2Oà CO2 +8H+ +8e-
Condenser1
H2O
Condenser3
Cathode:N2 +8H+ +8e-à 2NH3 +H2
N2,H2
N2
H2O
N2 (H2)
N2 (H2)
Anode:2H2à 4H+ +4e-
Cathode:O2+4H+ +4e-
à 2H2O
Air(N2,O2)