Nanostructured ZnO loaded on ceramic honeycomb syngas desulfurization Wen-Da Oh*, Junxi Lei, Andrei Veksha, Apostolos Giannis, Grzegorz Lisak, Teik-Thye Lim 19-21 February 2018
2nd International Conference on Catalysis and Chemical Engineering *Residues and Resource Reclamation Centre, Nanyang Environment and Water Research Institute, Nanyang Technological University, 50 Nanyang Ave, 639798 Singapore. Email address:
[email protected] 1
Introduction • The gasification process converts solid waste (e.g., municipal solid waste, horticultural) into raw syngas.
• Desulfurization for corrosion control of downstream gas turbine and other equipment.
Gasifier
• The raw syngas contains corrosive gases (particularly sulfur compounds such as H2S and COS) which need to be removed prior to downstream application.
Solid Waste
Syngas + impurities
Recyclable slag/metals
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Introduction • Conventional methods for corrosive gas removal are scrubbing using chemical or physical sorbents at low temperature (below 40°C). • High temperature desulfurization (200-500°C) for higher thermal efficiency in the downstream power generation process compared to conventional wet method (< 40°C) by scrubbing using chemical or physical sorbents. MxOy + xH2S + (y–x)H2 ↔ xMS + yH2O
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Selection of metal oxides
Equilibrium concentration of H2S (ppmv)
• Thermodynamic calculation was conducted using HSC software to identify the best sorbent. 100
20 ppmv
•
Stable form of metal oxide/metal in syngas are ZnO, CuO, FeO, MnO.
•
Equilibrium concentration of H2S increased with increasing temperature.
•
Syngas: 50 ppmv H2S, 8 vol% CO, 15 vol% CO2, 10 vol% H2, 26 vol% H2O, and N2 balance (Temperature & pressure: 200400 ºC, 1 atm).
10
1
0.1
0.01
ZnO CuO FeO MnO
1E-3
1E-4 150
200
250
300
350
400
450
Temperature (ºC)
ZnO > CuO ≈ MnO > FeO
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Sorbent preparation ZnO immobilized on ceramic (cordierite– mullite honeycomb) honeycomb support.
ZnO nanorods
ZnO nanosheets
ZnO-nR1-nanorods ZnO-nR2-nanorods (smaller) ZnO-nS-nanosheets 5
Experimental Experimental setup of the desulfurization process. 1 – Cylinder, 2 – Valve, 3 – Mass flow controller, 4 – Syringe pump, 5 – Three–way valve, 6 – Furnace, 7 – Tubular reactor, 8 – Honeycomb sorbent, 9 – P2O5 trap, 10 – Alkaline solution trap, 11 – GC–FPD–NPD, 12 – Controller, 13 – Computer, ––– Heated gas line, TC – Thermocouple.
Syngas composition H2S: 50 ppmv H2: 10 vol% CO: 8 vol% CO2: 15 vol% Steam: 26 vol% N2: Balance
Temperature: 400 ºC Pressure: 1 atm Space velocity: 9230 & 4615 h-1 Total gas flow: 200 & 100 mL/min Sorbents with 0.6% ZnO loading: Commercial ZnO (0.212-0.56mm) ZnO-nR1, ZnO-nR2, ZnO-nS
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Results and discussion
FESEM micrographs of ZnO loaded on the surface of the honeycomb support prepared with Zn(AC)2·2H2O at different HMTA concentration, and proposed mechanism of ZnO nanosheets formation. The FESEM micrographs shows top view of the ZnO nanosheets. [Zn(NO3)2·6H2O] = 0.10 M.
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Results and discussion
XRD patterns of ZnO–nR2, ZnO–nS, ZnO–nS–3, and commercial ZnO sorbent.
Cross–section FESEM micrographs of (a) ZnO–nR2, and (b) ZnO–nS; (c) and (d) EDX elemental distribution of ZnO–nS; (e) top–view and (f) cross section of ZnO– nS–3.
Results and discussion 45
2
S (ppmv) H2SH(ppmv)
35 30 25 20
ZnO-nR1 ZnO-nR2 Commercial ZnO ZnO-nS
15 10 5 0
35 30 25 20
10
20
30
40
50
60
70
80
90 100 110 120 130
ZnO-nR1 ZnO-nR2 Commercial ZnO ZnO-nS
15 10 5 0
0
-1
SV = 9230 h (c)
40
HTotal sulfur(ppmv) (ppmv) 2S+COS
SV = 9230 (a)
40
45
h-1
0
10
20
30
40
50
60
70
80
90 100 110 120 130
Time(min) (min) Time
Time Time(min) (min)
Sorbent
Sulfur capacity at H2S breakthrough time (mg S / g ZnO)
Sulfur capacity at total sulfur breakthrough time (mg S / g ZnO)
Commercial ZnO
5.0 ± 0.1
4.6 ± 0.3
ZnO-nR1
8.6 ± 1.6
9.0 ± 1.9
ZnO-nR2
12.9 ± 5.5
12.0 ± 5.1
ZnO-nS
49.1 ± 8.9
48.7 ± 11.1
ZnO-nS exhibited the best desulfurization performance
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Effects of operating parameter
Breakthrough curve for (a) H2S and (b) total sulfur removal at different space velocities using ZnO–nS and commercial sorbent. Conditions: T = 400°C, and 𝐶𝐻2 𝑆 = 50 ppmv.
Breakthrough curve for (a) H2S and (b) total sulfur removal at different H2S concentrations using ZnO– nS and commercial sorbent. Conditions: T = 400°C and GHSV = 9230 h–1.
Reusability
Breakthrough curve for (a) H2S and (b) total sulfur removal at various cycles. Conditions: T = 400°C, 𝐶𝐻2 𝑆 = 50 ppmv, and GHSV = 9230 h–1.
XRD, FESEM and EDX mapping of ZnO–nS. The inset shows H2S sorption capacity of ZnO–nS at t = 4 h. Regeneration conditions: 10% air and nitrogen (balance) at 650°C and GHSV of 3415 h–1. 11
Conclusions • ZnO with nanorods and nanosheets morphologies were successfully immobilized on the cordierite–mullite honeycomb support via a facile seeding–growth protocol. • The ZnO nanosheets (ZnO–nS) presented the best performance for total sulfur (H2S and COS) removal from syngas compared to the ZnO nanorods and commercial ZnO sorbents due to its significantly better surface coverage and higher crystallinity on honeycomb. • The ZnO–nS also showed better regenerability, lower mass transfer resistance, and higher SC compared to the commercial ZnO sorbent. • The results of this study indicate that the nanostructured ZnO loaded on honeycomb has promising potential for syngas desulfurization. 12
Thank You
This research is supported by the National Research Foundation, Prime
Minister’s Office, Singapore and the National Environment Agency, Ministry of the Environment and Water Resources, Singapore, under the Waste–to–Energy Competitive Research Programme (WTE CRP 1501 105). 13
Thermodynamic Calculation Theory: Gibbs free energy minimization for chemical reactions Minimize the Gibbs free energy of the sorbent-syngas system to achieve equilibrium of the system
Calculate the equilibrium concentration of H2S based on:
Metal oxide and syngas applied in the calculation Metal oxide: ZnO, CuO, Fe2O3, MnO2 Syngas: 50 ppmv H2S, 8 vol% CO, 15 vol% CO2, 10 vol% H2, 26 vol% H2O N2 balance (source: JFE) Temperature & pressure: 200-400 ºC, 1 atm
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