Coal ash sintering characterization by means of impedance spectroscopy

Transcription

1 Coal ash sintering characterization by means of impedance spectroscopy Ronny Schimpke, Stefan Thiel, Steffen Krzack, Bernd Meyer 7 th International Freiberg/Inner Mongolia conference Tuesday, June 9 th, 2015 TU Bergakademie Freiberg Institute of Energy Process Engineering and Chemical Engineering Chair of Energy Process Engineering and Thermal Waste Treatment Reiche Zeche Fuchsmuehlenweg Freiberg, Germany Phone: Fax:

2 I. Background and motivation II. Fundamentals of impedance spectroscopy III. Experimental procedure IV. Validation by the Na 2 O-SiO 2 -System V. Sintering temperature of Rhenish lignite ash Dwell time experiments Experiments with varying heating rates VI. Conclusion 2

3 I. Background and motivation Particle interaction Agglomeration Process conditions Van der Waals forces Liquid phases Bed temperature Bed ash fraction Electrostatic forces Sintering Particle size Gas velocity Crystallization Bed height Source: Mason & Patel: Chemistry of ash agglomeration in the U-GAS process, Fuel Processing Technology, vol.3, 1980 Detection of ash sintering temperature Dilatometry Thermal conductivity analysis Ash fusibility test (ASTM, ISO, DIN) Compression strength Combined DTA and TGA Thermo-chemical equilibrium Electric conductance / impedance 3

4 I. Background and motivation 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 sintering 2) 1) Nover, G.: Electrical properties of crustal and mantle rocks. In: Surveys in Geophysics, 2005, vol. 25, Nr. 5, S ) Simmat, R.; Jahn, D; Neuroth, M.; Nover, G.: Comparison of methods for the detection of sintering and melting processes in lignite ashes. In: 52 nd Int. Coll. Refractories,

5 II. Fundamentals of impedance spectroscopy Impedance - the alternating current resistance Voltage: U t = U 0 sin ωt Impedance: Z(ω) = U(t) I(t) (unit: Ω) Current: I t = I 0 sin(ωt + φ) Complex plane: Z ω = Z + iz 1 Z = Z 2 + Z 2 U/I f = 2π ω Z I(t) U 0 I 0 φ U(t) t Z φ Z 5

6 II. Fundamentals of impedance spectroscopy Measuring principle and analysis R 1 L 1 R 1 C 1 Z Z R 1 Z R 1 Z R pre R 1 Z R pre R 1 Real Capacities: Constant Phase Element (CPE) R pre R 1 C 1 φ max Z CPE 1 6

7 II. Fundamentals of impedance spectroscopy Typical spectra for ashes Resistances and capacities of: R 1 R 2 R 3 RCPE 1 : Bulk (extensive values) RCPE 2 : grain boundary (intensive values) Z CPE 1 CPE 2 CPE 3 RCPE 3 : charge transfer resistance and double layer capacity at the electrode/grain boundary (intensive values) Inhomogeneous material like ash Z RCPE 4,5, i : different phases as contact material at grain boundaries Problem: Unknown order of semicircles (e.g., RCPE 3 might represent grain boundary effects) 7

8 III. Experimental procedure Sample preparation Coal Ashed at 450 C Pellet Compression with 6 N/mm 2 Cyl. Pellets Ø 15 mm H = 2-6 mm Stored in an exsiccator Test facility T < 1300 C Gas: N 2, air, CO, CO 2 Ambient pressure Potentiostat: Gamry Series G mv ,000 Hz 11 points per decade Electrode (graphite or Ptcoated alumina) Sample Sample holder 8

9 IV. Validation by the Na 2 O-SiO 2 -System Two validation cases selected Case I: Na 2 O: 51.0 Ma.-% SiO 2 : 49.0 Ma.-% FactSage: 3.6 wt.-% melt Slag atlas: T Solidus = 1005 C I II Case II: Na 2 O: 49.7 Ma.-% SiO 2 : 50.3 Ma.-% FactSage: 8.0 wt.-% melt Slag atlas: T Solidus = 837 C Source: Verein Deutscher Eisenhüttenleute slag atlas, 2 nd edition, Verlag Stahleisen GmbH,

10 IV. Validation by the Na 2 O-SiO 2 -System Case I: T Solidus = 1005 C 950 C Bulk (RCPE 1 ): Resistance drop: C No capacity information 1010 C Grain boundaries (RCPE 2 ): Capacity increase: C Resistance drop: C T Sinter = ~950 C 25 C 10

11 IV. Validation by the Na 2 O-SiO 2 -System Case II: T Solidus = 837 C 775 C Bulk (RCPE 1 ): Resistance drop: 830 C No capacity information 850 C Grain boundary (RCPE 2 ): Capacity increase: C Continuous resistance drop T Sinter = ~790 C 25 C 11

12 V. Sintering temperature of Rhenish lignite ash Ash characterisation X-ray fluorescence (main components): oxide Na 2 O MgO Al 2 O 3 SiO 2 SO 3 CaO Fe 2 O 3 wt.-% 3,6 18,2 4,5 1 20,3 35,4 14,7 Ash fusibility test (DIN 51730) Initial shrinking Initial deformation (A) Softening (B) Hemispherical temp. (C) Fluid temp. (D) 840 C 1285 C 1355 C 1448 C > 1500 C Dwell time experiments Dwell time: 6 h Temperature increments: 25 K Atmosphere: N 2 Results: Capacity C 2 increase: ~770 C Beginning of Sintering 12

13 V. Sintering temperature of Rhenish lignite coke ash (3059) Experiments with varying heating rates Heating rates: 0,1; 1; 2; 5; 10 K/min Atmosphere: N 2 Capacity increase at ~ 770 C in both cases Beginning of Sintering 0.1 K/min: Good raw data for approximation 1.0 K/min: Above 780 C difficult to separate RCPE 2 from RCPE 3 13

14 VI. Conclusion 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 14

15 Acknowledgement 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 / Mail: ronny.schimpke@iec.tu-freiberg.de 15