SULFIDE EFFECT ON BIOGAS UPGRADING WITH A BIOELECTROCHEMICAL SYSTEM
C H R I S T Y D Y K S T R AS P Y R O S G . P A V L O S T A T H I S
ATHENS 2017 5th International Conference on Sustainable Solid Waste ManagementAthens, Greece, June 22, 2017
School of Civil & Environmental EngineeringGeorgia Institute of TechnologyAtlanta, GA 30332‐0512, [email protected]
INTRODUCTION & BACKGROUND
AnaerobicDigestion
MethaneCH4
Carbon DioxideCO2
Substrate
Biogas Yield(L/kg volatile
solids)aMethane
Content (%, v/v)
Fat 1,000 – 1,250 70 – 75
Protein 600 – 700 68 – 73
Carbohydrate 700 – 800 50 – 55
a At 25°C, 1 atm; Petersson and Wellinger, 2009. IEA Bioenergy.
2
Trace Gases
(e.g., H2S, H2, N2)
BIOGAS UPGRADING3
• Energy intensive• Carbon waste product• Expensive consumables
Biogas Upgrading
AbsorptionPhysical
Chemical
Adsorption
Activated Carbon
Alumina
Zeolite
MembranesGas separation
Gas adsorptionCryogenics
BiologicalBiomass Production (e.g., algae)
Bioelectrochemical Systems
Direct conversion of CO2 to CH4
BIOELECTROCHEMICAL SYSTEMS4
Oxidation
Organics CO2/ oxidized organics, H+, e‐
Reduction
CO2, H+, e‐CH4
• Microbes are an inexpensive, self‐renewing catalyst• The potential applied at A (< 1 V) can be supplied by photovoltaics/renewables• Optional proton exchange membrane, B
COCO2,CH4
>>CH4
HYDROGEN SULFIDE
• Corrosive, toxic (NIOSH, IDLH = 100 ppm)
• Produced by sulfate‐reducing bacteria during anaerobic digestion
• Inhibitory to methanogenesis during anaerobic digestion [1]
• Feedstock C:S ratio predicts biogas H2S [2]
Desulfovibrio vulgaris
[1] Chen, Y., et al., 2008. Biores. Technol. 99(10), 4044‐4064.
[2] Peu, P., et al., 2012. BioresourceTechnol. 121, 419‐424.
FeedstockC/S (g/g)
Theoretical Biogas H2S (%, range)
Grease trap waste 798 0.0 – 0.1
Biological sludge 59 0.6 – 1.9
Industrial WW biological sludge 46 0.8 – 2.0
Pig bristles 19 2.0 – 4.9
Harvested green seaweed 7 5.5 – 17.7
5
RESEARCH OBJECTIVEDetermine how the presence of hydrogen sulfide (H2S), a common contaminant in anaerobic digester biogas, affects the conversion of carbon dioxide (CO2) to methane (CH4) in the cathode of a bioelectrochemical system (BES).
RESEARCH APPROACH
6
7 cycles 3 cycles 3 cycles 3 cycles 1 cycle 1 cycle 1 cycle
TIME (d)
0 2 4 6 8 10
ME
THA
NE
(mm
ol)
0
1
2
3
R2 = 0.997
R2 = 0.999
100% CO2,no H2S
99% CO2, 1% H2S
98% CO2, 2% H2S
97% CO2, 3% H2S
96% CO2, 4% H2S
95% CO2, 5% H2S
94% CO2, 6% H2S
• Compare the mean initial 3‐day CH4production rate following feeding
• Assess the effect of H2S on the full BES performance
Figure 1. Linear biocathode CH4 production during the first 3 days of a feeding cycle
METHODS: BIOELECTROCHEMICAL SYSTEM7
Anode• 300 mL total, 250 mL liquid volume• Carbon felt electrode with
exoelectrogens• Batch‐fed acetate (4 g COD/L)
weekly
Cathode• 300 mL total, 250 mL liquid volume• Carbon felt electrode with
methanogens and SS collector• Batch‐fed CO2 (g) (92 mL at 22°C, 1
atm) weekly• Applied potential ‐0.8 V vs. SHE with
Gamry Interface 1000 potentiostat• Continuously mixed with magnetic
bars and stir plates at 22°C• Nafion 117 proton exchange
membrane (PEM)
RESULTS: BIOCATHODE CH4 PRODUCTION8
INITIAL CATHODE HEADSPACE H2S (%)
0 1 2 3 4 5 6
INIT
IAL
3-D
AY
CH
4 PR
OD
UC
TIO
N R
ATE
(mm
ol/d
)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
n = 7
n = 3
n = 3n = 3
n = 1
n = 1n = 1
• H2S improves biocathode CH4production rate up to 2‐3% initial H2S
• Initial H2S concentrations of 4‐6% result in a decreased biocathode CH4 production rate
• Two competing effects:
• Depress CH4 production (≥4% H2S): Inhibition of methanogens?
• Improve CH4 production (≤3% H2S): What is/are the process(es) involved?
Figure 2. Mean initial 3‐day biocathode CH4production rates following feeding with an initial headspace concentration of 0‐6% H2S (n, number of feeding cycles).
ELECTROCHEMICAL PERFORMANCE
‐0,03‐0,02‐0,010,000,010,020,030,040,050,060,07
‐1,2 ‐1,0 ‐0,8 ‐0,6 ‐0,4 ‐0,2 0,0
Curren
t (mA)
Voltage (V vs. Ag/AgCl)
0% H2S 4% H2S
5% H2S 6% H2S
H2S (%)
CE (%)
CCE (%)
0 11 100
4 19 99
5 58 13
6 58 15
CE, Coulombic Efficiency: The ratio of total Coulombs actually transferred to the anode from the substrate, to maximum possible Coulombs if all substrate removal produced current. [1]
CCE, Cathode Capture Efficiency: The ratio of total Coulombs actually transferred to the CH4 from the cathode, to maximum possible Coulombs if all current produced CH4. [2]
[1] Logan et al., 2006. ES&T[2] Villano et al., 2013. Bioresource Technol.
9
2
2
2
2
H2S WITHIN A METHANOGENIC BES
Anode Cathode
Potentiostat
PEM
CO2 H2S CO2N2 N2 N2
Acetate
CO2
CO2
CO2
CH4
CH4
N2 CO2 CH4
H+ H+H+H+
e‐ e‐
H2SH2S
Henry’s Law constant in catholyte medium• CO2: 32.7 mM/atm• H2S: 82.0 mM/atm
10
Ag/AgCl reference electrode
H2S IN THE CATHODE – INHIBITORY EFFECT
CO2
CH4 H2SH2S
CH4
CO2
CO2
H2SH2S HS‐HS‐
H2SHS‐H2SHS‐
S2‐S2‐
e‐
e‐
H+
11
H2S is the most toxic of the sulfide species
80%
20%
H2S IN THE CATHODE – INHIBITORY EFFECT
CO2
CH4 H2SH2S
The methanogenic biocathode is semi‐protected from sulfide inhibition by biofilm formation and a local high pH at the cathode surface.
CH4
CO2
CO2
H2SH2S HS‐HS‐
H2SHS‐H2SHS‐
S2‐S2‐
e‐
e‐
High local pHN
eutral pH
12
80%
20%H2S is the most toxic of the sulfide species
H+
H2S IN THE ANODE – ENHANCEMENT EFFECT
N2
Acetate
CO2
CO2
H2S / HS‐
SO42‐
?Acetate
CO2SRB
• Low H2S → more electrons donated to the anode → more biocathode CH4 production• High H2S → s mulate SRB cycle → divert acetate eeq from the anode → less biocathode CH4 production
Potential anode H2S oxidation productsS0 Sx2‐ S4O6
2‐
S2O32‐ SO4
2‐
Sun et al., 2009. ES&T
13
N2
e‐
H2S IN THE ANODE – ENHANCEMENT EFFECT
N2
Acetate
CO2
CO2
H2S / HS‐
SO42‐
?Acetate
CO2SRB
• Low H2S → more electrons donated to the anode → more biocathode CH4 production• High H2S → s mulate SRB cycle → divert acetate eeq from the anode → less biocathode CH4 production
Potential anode H2S oxidation productsS0 Sx2‐ S4O6
2‐
S2O32‐ SO4
2‐
Sun et al., 2009. ES&T
14
N2
e‐
InitialCathode
H2S (%)
AcetateRemoval
(%)
Final Anode SO4
2‐
(mM)
H2S Recovery as
Anode SO4
2‐ (%)
0 91.4 0.00 0
4 99.6 0.18 24
5 91.7 0.17 18
6 91.0 0.20 18
InitialCathode H2S
(%)
AcetateRemoval
(%)
Final AnodeSO4
2‐
(mM)
H2S Recovery as Anode SO4
2‐
(%)
Final H2S in Catholyte(mM)
Final H2S in Cathode Gas
(%)
H2S Removal Efficiency
(%)
0 91.4 ‐ ‐ ‐ ‐ ‐
4 99.6 0.18 24 0.47 0.5 84.6
5 91.7 0.17 18 0.63 0.7 83.4
6 91.0 0.20 18 0.81 1.0 83.2
15BIOCATHODE H2S REMOVAL
CONCLUSIONS
• Up to 3‐4% H2S in biogas can enhance biocathode CH4production by contributing electrons to the anode
• Above 4% H2S, biocathode CH4 production decreasesdue to: i) inhibition of methanogens at the cathode; ii) sulfide oxidation cycling in the anode, which diverts electron equivalents away from CH4 production
16
ACKNOWLEDGEMENTS17
This material is based in part upon work supported by the US National Science Foundation Graduate Research Fellowship under Grant No. DGE‐1148903 (2012 to 2017) awarded to Christy M. Dykstra, who was also awarded the Canham Graduate Studies Scholarship by the Water Environment Federation (2016) and the Georgia Power Fellowship (2017).
METHODS: ANALYTICAL18
• Gas pressure
• Gas composition
• Acetate
• Dissolved CO2, H2S
• Voltage
• Current
• Cyclic Voltammetry
Pressure transducer
Gas chromatography (GC) with Thermal Conductivity Detector (TCD)
GC with Flame Ionization Detector (FID)
Sample acidification (6 N H2SO4) followed by composition analysis of evolved gas (conditional calibration)
Handheld multimeter and GamryInterface 1000 potentiostat
RESULTS: SERUM BOTTLE TESTS
• Cathode inoculum: hydrogenotrophic, methanogenic, suspended growth culture fed with H2/CO2 (80:20) and catholyte medium with vitamins and trace metals
• Similar methane production at all initial gaseous H2S concentrations up to 3% H2S
19
0,00
0,05
0,10
0,15
0,20
0,25
0,30
0,35
0,40
0,45
0 2 4 6 8
METHAN
E (m
mol)
TIME (d)
0% H2S0.38% H2S0.75% H2S1.50% H2S3.0% H2S
