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1TU Bergakademie Freiberg · Institute of Energy Process Engineering and Chemical Engineering · Chair of Energy Process Engineering and Thermal Waste
Treatment · Reiche Zeche · Fuchsmuehlenweg 9 · 09599 Freiberg, Germany · Phone: +49 3731 39-4511 · Fax: +49 3731 39-4555 · www.iec.tu-freiberg.de
Coal ash sintering characterization by
means of impedance spectroscopy
Ronny Schimpke, Stefan Thiel, Steffen Krzack, Bernd Meyer
7th International Freiberg/Inner Mongolia conference
Tuesday, June 9th, 2015
I. Background and motivation
II. Fundamentals of impedance spectroscopy
III. Experimental procedure
IV. Validation by the Na2O-SiO2-System
V. Sintering temperature of Rhenish lignite ash
− Dwell time experiments
− Experiments with varying heating rates
VI. Conclusion
2
Electric conductance /
impedance
Ash fusibility test
(ASTM, ISO, DIN)
Bed
temperature
Source: Mason & Patel: Chemistry of ash
agglomeration in the U-GAS® process, Fuel
Processing Technology, vol.3, 1980
I. Background and motivation
3
Electrostatic
forces
Van der
Waals forcesLiquid phases
Sintering
Particle interaction Agglomeration
Particle size
Bed ash
fraction
Gas velocity
Process conditions
Bed height
Detection of ash sintering temperature
Combined
DTA and TGA
DilatometryThermal conductivity
analysis
Thermo-chemical
equilibrium
Compression strength
…
Crystalli-
zation… …
Electronic properties of solid materials
Solid materials of mineral origin cover 4 different charge transport
mechanisms:1)
• Electrolytic conduction
• Electrolyte in pore system
• Electronic conduction by high conducting phases, e.g.:
• Iron and iron oxides
• Graphite (not carbon!)
• Thermally induced semiconduction
• Ion transport in partial melts at high temperatures
Statement:
Rapid increase in capacity of a system indicates oriented charge
carriers in melts or in the solid state at temperatures close to the
beginning of melting → sintering2)
I. Background and motivation
4
1) Nover, G.: Electrical properties of crustal and mantle rocks. In: Surveys in Geophysics, 2005, vol. 25, Nr. 5, S. 593–651
2) Simmat, R.; Jahn, D; Neuroth, M.; Nover, G.: Comparison of methods for the detection of sintering and melting processes in lignite
ashes. In: 52nd Int. Coll. Refractories, 2009
Impedance - the alternating current resistance
Voltage: 𝑈 𝑡 = 𝑈0 ∙ 𝑠𝑖𝑛 𝜔𝑡
Current: 𝐼 𝑡 = 𝐼0 ∙ 𝑠𝑖𝑛(𝜔𝑡 + 𝜑)
II. Fundamentals of impedance spectroscopy
5
Impedance: 𝑍(𝜔) =𝑈(𝑡)
𝐼(𝑡)(unit: Ω)
Complex plane: 𝑍 𝜔 = 𝑍′ + 𝑖𝑍′′
𝑍 = 𝑍′ 2 + 𝑍′′ 2
𝑍
φ
𝑍′′
𝑍′
𝐼(𝑡)
𝑈(𝑡)
φ
1
𝑓=2𝜋
𝜔
𝑡
𝑈/𝐼
𝐼0𝑈0
Measuring principle and analysis
II. Fundamentals of impedance spectroscopy
6
𝐶1
𝑅1𝑅𝑝𝑟𝑒
𝑍′′
𝑍′
𝑅1 𝐶1
𝑅1
𝑍′′
𝑍′
𝑅1 𝐿1
𝑅1
−𝑍′′
𝑍′
𝑅𝑝𝑟𝑒 𝑅1
𝜑𝑚𝑎𝑥
Real Capacities:
Constant Phase Element
(CPE)
𝐶𝑃𝐸1
𝑅1𝑅𝑝𝑟𝑒
Typical spectra for ashes
Problem: Unknown order of semicircles (e.g., RCPE3 might
represent grain boundary effects)
II. Fundamentals of impedance spectroscopy
7
Resistances and capacities of:
• RCPE1: Bulk (extensive values)
• RCPE2: grain boundary
(intensive values)
• RCPE3: charge transfer
resistance and double layer
capacity at the electrode/grain
boundary (intensive values)
Inhomogeneous material like ash
• RCPE4,5,…i: different phases as
contact material at grain
boundaries
𝐶𝑃𝐸1
𝑅1
𝐶𝑃𝐸2
𝑅2
𝐶𝑃𝐸3
𝑅3
−𝑍′′
𝑍′
Sample preparation
Test facility
• T < 1300°C
• Gas: N2, air, CO, CO2
• Ambient pressure
Potentiostat:
Gamry Series G 300
• 150 mV
• 0.1 … 300,000 Hz
• 11 points per decade
III. Experimental procedure
8
Pellet
Compression
with 6 N/mm2
Ashed at
450°C
Stored in an
exsiccatorCoal
Cyl. Pellets
Ø 15 mm
H = 2 - 6 mm
Electrode
(graphite or Pt-
coated alumina)
Sample
Sample holder
III
Two validation cases selected
Case I:
• Na2O: 51.0 Ma.-%
• SiO2: 49.0 Ma.-%
• FactSage:
• 3.6 wt.-% melt
• Slag atlas: TSolidus = 1005°C
Case II:
• Na2O: 49.7 Ma.-%
• SiO2: 50.3 Ma.-%
• FactSage:
• 8.0 wt.-% melt
• Slag atlas: TSolidus = 837°C
IV. Validation by the Na2O-SiO2-System
9
Source: Verein Deutscher Eisenhüttenleute – slag atlas, 2nd edition, Verlag
Stahleisen GmbH, 1995
IV. Validation by the Na2O-SiO2-System
10
25°C
950°C
1010 °CCase I: TSolidus = 1005°C
Bulk (RCPE1):
Resistance drop: 950 – 1000°C
No capacity information
TSinter = ~950°C
Grain boundaries (RCPE2):
Capacity increase: 920 – 970°C
Resistance drop: 950 – 970°C
IV. Validation by the Na2O-SiO2-System
11
25°C
775°C
850°CCase II: TSolidus = 837°C
Bulk (RCPE1):
Resistance drop: 830°C
No capacity information
TSinter = ~790°C
Grain boundary (RCPE2):
Capacity increase: 790 – 820°C
Continuous resistance drop
Ash characterisation
• X-ray fluorescence (main components):
• Ash fusibility test (DIN 51730)
Dwell time experiments
• Dwell time: 6 h
• Temperature increments: 25 K
• Atmosphere: N2
Results:
• Capacity C2 increase: ~770°C
→ Beginning of Sintering
V. Sintering temperature of Rhenish lignite ash
12
oxide Na2O MgO Al2O3 SiO2 SO3 CaO Fe2O3
wt.-% 3,6 18,2 4,5 1 20,3 35,4 14,7
Initial shrinking Initial deformation (A) Softening (B) Hemispherical temp. (C) Fluid temp. (D)
840°C 1285°C 1355°C 1448°C > 1500°C
Experiments with varying heating rates
• Heating rates: 0,1; 1; 2; 5; 10 K/min
• Atmosphere: N2
Capacity increase at ~ 770°C in both cases → Beginning of Sintering
0.1 K/min:
• Good raw data for approximation
V. Sintering temperature of Rhenish lignite coke ash (3059)
13
1.0 K/min:
• Above 780°C difficult to
separate RCPE2 from RCPE3
Measurement technique
• Capacity increase at grain boundaries
Beginning of sintering
• Bulk resistance drop can support sintering detection
• Electrode/grain boundaries usually give too less information about
sintering
• Maximum heating rate: 1 K/min
Sintering results
• Sintering can be observed at temperatures below any occurrences in
ash fusibility tests
First mobile phases can be detected
• Comparison with other measurement techniques have been done
• Next step: feasibility for different ash compositions
VI. Conclusion
14
Acknowledgement
15
Thanks to:
• German Federal Ministry of
Economics and Energy and the
Poerner Group in the framework
of the COORVED project
Thank you for your attention – Questions?
Contact:
Ronny Schimpke
Phone: + 49 (0)3731 / 39 4499
Mail: [email protected]