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: ronny.schimpke@iec.tu-freiberg.de